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The Future of Layer-by-Layer Assembly: A Tribute to ACS Nano Associate Editor Helmuth Möhwald

Cite this: ACS Nano 2019, 13, 6, 6151–6169
Publication Date (Web):May 24, 2019
https://doi.org/10.1021/acsnano.9b03326
Copyright © 2019 American Chemical Society
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Abstract

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Layer-by-layer (LbL) assembly is a widely used tool for engineering materials and coatings. In this Perspective, dedicated to the memory of ACS Nano associate editor Prof. Dr. Helmuth Möhwald, we discuss the developments and applications that are to come in LbL assembly, focusing on coatings, bulk materials, membranes, nanocomposites, and delivery vehicles.

The classic realization of layer-by-layer (LbL) assembly was introduced three decades ago, (1−7) with significant contributions from our colleague, the late ACS Nano associate editor Helmuth Möhwald. (8−33) Research on Langmuir–Blodgett deposition and later LbL assembly carried out by Helmuth Möhwald (34) created a critically important foundation for development of multilayer composites based on hybrid organic–inorganic nanostructures and numerous related technologies. Early studies in this area involved self-assembly of multilayers from graphite oxide, (35) clay sheets, (36) nanoparticles, (8,37−42) and other materials, serving as conceptual growth points for the evolution of the fields of biomimetic composites, energy materials, and self-assembly. Numerous studies inspired and authored by Helmuth Möhwald not only paved the way for rapid expansion of nanoparticle-based design of nanocomposites, but also led to understanding biomineralization processes in Nature and their utilization in diverse areas of technology. (43) This field has since undergone massive expansion that continues to this day, and LbL is now an established and widely used technique for coating and encapsulation. With several publications per day, LbL assembly has matured from a scientific oddity to an accessible and useful tool for the preparation of nanoscale functional films. It continues to be used to create new commercial products, making it as interesting for various industries now as chemical vapor deposition (CVD) and physical vapor deposition (PVD) were in the 1960s. Whereas the past and the present of LbL have been extensively reviewed, (11,44−50) in this Perspective, we focus on future opportunities using this exciting technique.

Numerous studies authored and inspired by Helmuth Möhwald not only paved the way for rapid expansion of nanoparticle-based design of nanocomposites, but also led to understanding biomineralization processes in Nature and their utilization in diverse areas of technology.

The classic realization of layer-by-layer assembly as a dip-and-rinse process has several conceptual advantages over other methods of materials preparation that predicated its wide use in science and technology. First, compared to other techniques, for instance, sequential spin-coating, it enables preparation of nearly ideal conformal coatings on surfaces of any topography. Thus, it has been applied to planar surfaces, spherical particles, inside pores, and onto other more complex geometries. Second, LbL is universal and flexible. It is compatible with other chemistries, meaning that a wide variety of different surfaces can be coated, not only charged substrates. The sequential assembly of the layers involves a washing step after the addition of each layer, which reduces the excess non-assembled materials or molecules. Because of the large variety of materials out of which layers can be formed, LbL enables convenient surface chemistry tailoring. Another important characteristic of LbL technology is the broad and independent variability of each double layer, which, in contrast to many other coating and encapsulation technologies, enables the modular construction of multifunctional devices like a box of bricks that have different properties and can be combined in different ways. These properties, combined with the availability of various stimuli (51−55) to control responsiveness of polyelectrolyte assemblies, make LbL an extremely versatile technology platform. Third, the LbL method replicates the essential aspects of physics and chemistry of materials engineering in living organisms, and therefore, it leads to the amazing spectrum of biomimetic materials. Importantly, they may or may not be based on biomacromolecules pertaining to a specific biological process. Replicating the molecular-scale adaptation of the different structural components at the interfaces taking place in, for instance, biomineralization, one can attain structures and properties equal to or better than those of materials found in biology. (56) Note, however, that as with biology, many LbL processes require time for atomistic relaxation at the interfaces. Although the formation of multilayers approaching thermodynamic equilibrium is a rather time-consuming process, many future technologies will require such highly complex structures. This complexity will be illustrated below for some especially promising developments in this field. Of note are several successful reports of accelerating LbL processes using automated procedures on both planar and colloidal templates. (57−61) In parallel, endeavors have also been undertaken to produce coatings with properties similar to those of LbL films but using single-step approaches. (62−64)
In this Perspective, we highlight future directions with a focus on three areas: (1) functional coatings on planar and highly curved surfaces, (2) free-standing membranes and bulk materials, and (3) delivery vehicles based on encapsulation for biomedical applications.

Layer-by-Layer-Based Functional Surface Coatings

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Versatility of Coatings

Layer-by-layer coatings are extremely versatile. They enable variability in (1) composition, i.e., the integration of different materials; (2) vertical structuring normal to the surface, i.e., the possibility to create defined sequences of layers; (65) and (3) anisotropic alignment, i.e., to orient anisotropic materials within layers.

(1) Toward Multinanocomposites

Among all methods for functionalizing surfaces, LbL assembly arguably has the largest choice of deployable components (inorganic salts, organic molecules, polymers, DNA, (66,67) graphene oxide, biomolecules, lipids, nanoparticles, or biological objects including cells (68)). In polyelectrolyte multilayers, one can bring tens (if not hundreds) of materials together in ordered ways, whereas the number of components in most current nanocomposites is less than ∼20. The precision of LbL in controlling the structures of materials bridging the molecular, nano-, meso-, and microscales (69−72) makes it possible to create conformal coatings with exceptionally high curvatures, including functionalized particles, (73−76) and to design self-assembled nanocomposites with previously unexpected combinations of macroscale properties. (36,69,77) Along these lines, LbL makes it possible to demonstrate the transition between nano- and macroscale optical effects in gold films experimentally by precisely tuning interparticle distances in multilayers of silica-coated gold nanoparticles, (78,79) leading to extremely efficient substrates for surface-enhanced Raman scattering (SERS) detection. (80) The fundamental findings regarding rare combinations of properties made using LbL-based composites were confirmed using other techniques, such as vacuum-assisted filtration (81−83) and spin-coating. (84) Layer-by-layer assembly enables the design and preparation of materials with adjustable multifunctionality, which is difficult, if not impossible, using other formulation technologies. Thus, LbL offers the tools to fabricate advanced materials by combining heterogeneous components with potential applications in optoelectronic devices, smart surfaces, solar cells, etc.

(2) Toward Three-Dimensional (3D) Coatings

The LbL technique also offers multiple approaches for the fabrication of composite materials from heterogeneous components, where the compositions of the materials are varied in the direction normal to the substrate. In addition to gradients in chemical composition, one can also vary the mechanical, optical, and electronic properties of composites in the vertical direction. The LbL concept can also be integrated with other nano- and microfabrication techniques. In particular, the use of printing strategies and combinations with other 3D coatings with variable vertical composition are possible. Combining LbL assembly with other modern strategies (e.g., roll-to-roll, lithography, and 3D printing, etc.) as well as high-throughput production methods (57,58,85−88) is beneficial for the preparation of novel functional LbL composites. Note that multilayer nanocomposites, made by LbL and Langmuir–Blodgett deposition, are extensively used in industry already, albeit produced using closely related, derivative methods. Some representative examples of these multilayer composites are those based on various forms of nanocarbons (graphene, graphene oxides, graphene, nanotubes, nanoribbons, graphene carbon quantum dots, etc. (81,82,89−91)) and those based on various forms of ceramic nanoplatelets (clay, metal oxides, MXenes, etc. (89,92−94)). The former are employed in energy technologies, whereas the latter are used in membrane and coating technologies. In each case, the composite multilayer production is reliable, scalable, and low cost due to self-assembly of anisotropic colloids. Future directions in multilayer biomimetic composites are likely to include computational design of the multilayers starting from molecular dynamics (95−97) and coarse-grained models of the multilayers. (92)

(3) Toward Materials with Complex Anisotropies

Most of the current materials are isotropic. Materials with anisotropic properties are, in general, more difficult to prepare and to characterize. For example, grazing incidence spraying (98) enables alignment of nanowires, nanorods, and nanofibers in plane (99,100) during the deposition of individual layers in LbL films. With unidirectionally oriented multilayers, one can fabricate films containing ultrathin polarizers. (100) This approach, however, is capable of producing more complex anisotropies, even over large surface areas, by changing the direction of alignment in each individual layer of a multilayer film. We are just starting to realize materials with crisscross and even helical superstructures. Materials with such anisotropies are likely to be interesting for various applications in mechanics, photonics, and other areas.

Protective Coatings

Layer-by-layer assembly provides a convenient coating strategy for the protection of consumer products, such as paints for corrosion protection or antigraffiti coatings. (101−104) Here, among the spectrum of technological advances based on LbL materials, one should mention anticorrosion coatings investigated by Möhwald and co-workers. Andreeva et al. deposited oppositely charged (PEI/PSS)n polyelectrolyte (PE) multilayers on aluminum surfaces. (102) The corrosion processes on the aluminum surfaces were blocked due to the pH-buffering ability of polyelectrolyte-based LbL coatings (Figure 1a). Another representative advance in this area is halloysite nanocontainers for anticorrosion coatings. Shchukin et al. first deposited LbL-assembled polyelectrolyte multilayers of (PAH/PSS)n on the surfaces of inhibitor-loaded halloysite nanotubes. (105) After the halloysite nanocontainers were embedded, the sol–gel SiOx/ZrOy active composite coatings with the nanocontainers showed long-term anticorrosion performance. The LbL composite multilayers provide effective storage and prolonged release of the inhibitor. Similarly, SiO2 particles coated with LbL multilayers entrapping inhibitors were used as nanocontainers to achieve self-healing and anticorrosion composite coating simultaneously. (18,103) Li et al. also designed a silica/polymer double-walled hybrid nanotube loaded with active molecules for metal corrosion protection. (106) A new generation of anticorrosion coatings that possess passive matrix functionality and that actively respond to changes in the local environment has been introduced. (107) Active corrosion protection aims to restore the properties of the material when the passive coating matrix is broken and corrosion of the substrate has started. The main component of the self-healing anticorrosion coatings are capsules in flat layers, which provide controlled release of the corrosion inhibitor on demand and only inside the corroded area (see Figure 1a). This release acts as a local trigger for the mechanism that heals the defects. The LbL assembly approach is an effective tool for the fabrication of the capsule shells, controlling release of the corrosion inhibitor on demand. Layer-by-layer assembly enables the use of various materials as shell components, utilizing weak, mostly electrostatic forces for their assembly. Depending on the nature of the “smart” materials (e.g., polymers, nanoparticles) introduced into the container shell, different stimuli can induce reversible and irreversible shell modifications: pH, ionic strength, temperature, ultrasonic treatment, and electromagnetic fields. The different responses that can be observed vary from fine effects, such as tunable permeability, to more profound ones, such as total rupture of the container shell. These different behaviors depend on the composition of the polyelectrolyte multilayers (e.g., weak polyanion–weak polycation or strong polyanion–weak polycation-based interactions).

Figure 1

Figure 1. (a) Layer-by-layer assembly can be used in protective coatings in several ways, e.g., as nanoreservoirs of corrosion inhibitors and in multicomponent coatings. (b) Layer-by-layer assembly can be used for biomolecule immobilization in sensing devices and biofuel cells. (c) Layer-by-layer assembly can be used in photoelectrochemical devices to create 3D structures; in medical devices, different biological materials can be assembled in each layer independently. (d) Tailored coatings for better control of cell–surface interactions. (e) Layer-by-layer assembly can be used in antibacterial coatings of implants.

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Coatings for Photonics and Energy-Related Applications

There are numerous energy applications that can take advantage of the tunable mechanical, electrical, and chemical properties of LbL composites. (108) In fact, the first implementation of graphene composites on electrodes currently used in a variety of batteries, supercapacitors, conductive inks, and fuel cells was demonstrated in LbL composites referring to these materials as graphite oxide in 1996. (35) The excellent laminar organization of the films also afforded demonstration of the transition from the nonconductive state of graphite oxide to reduced graphene and their utilization in lithium batteries. (109) Composite materials with identical layered design were later produced by other techniques, such as vacuum-assisted filtration, are widely used in the technology. (81) There are also many other energy-conversion devices that employ LbL multilayers from electroconductive materials which include batteries, supercapacitors, catalysts, solar cells, and fuel cells, and these modern applications frequently place higher demands on the performance of the composites. (110) Layer-by-layer multilayers can integrate the properties of different constituent materials, and judicious design enables them to take on multiple roles and functionalities, for instance, as battery anodes or ion-transporting membranes. (111−113) Layer-by-layer assembly also facilitates the creation of controlled assemblies to study photonic properties of materials. Layer-by-layer assembly enables composite functional materials that combine polymers with oppositely charged nanoparticles. Such structures can easily be created on planar substrates (114−117) and on colloidal microspheres. (23,118−120) The organized superstructures from semiconductor nanoparticles (NPs), also known as quantum dots (QDs), can also be made using LbL as was demonstrated for CdS, PbS, and TiO2. (121) The advantage of QDs compared to graphite/graphene oxide is that they are capable of emitting in the visible (114,122) and near-infrared (122) parts of the spectrum; this property has been used to fabricate luminescent self-assembled films and to study energy transfer in such composites. (115) Directed energy transfer from specific layers of QDs toward an interface or electrode was made possible exactly due to the possibility of arranging the LbL layer in the order of decreasing or increasing band gaps in graded semiconductor nanostructures. (123) Excitation recycling in LbL-grown graded band gap QD structures has been demonstrated (116) and ascribed to superefficient exciton funneling to the layer containing the largest QDs. (117) Besides graphene, layer-by-layer assembly is also compatible with other emerging two-dimensional (2D) materials such as hexagonal boron nitride whose LbL coatings yield exceptional performance as gate dielectrics in graphene field-effect transistors. (124) Last but not least, LbL has been used to build coatings for electromagnetic shielding. Flexible and electrically conductive thin films are required for electromagnetic interference (EMI) shielding of portable and wearable electronic devices. (93) The LbL technique enables combinations of nanoparticles and polymers, providing a platform for developing hierarchical architectures with a combination of properties including mechanical strength, transparency, and conductivity. (89) Spin-spray LbL enables rapid assembly of 2D Ti3C2 MXene–carbon nanotube (CNT) composite films for EMI shielding. These semitransparent LbL MXene–CNT composite films showed high conductivities and high specific shielding effectiveness, which are among the highest reported values for flexible and semitransparent composite thin films.

Biomolecule Immobilization for Sensing and Biofuel Cells

The LbL technique has found widespread applications in the fixation of biomolecules to surfaces (49,125) because it enables (1) engineering of man-made materials with structural analogy to biomaterials; (2) the controlled deposition of biomolecules because deposition can be governed not only by the number of layers but also by adjusting pH, ion concentration, temperature, and polyelectrolyte and biomolecule concentrations in each layer; (3) the defined integration of different biomolecules in different layers and, thus, the creation of sequential signal chains; and (4) the incorporation of other functional components, such as mediators, which can facilitate electron transfer between immobilized molecules and electrodes or lipids, which, in turn, facilitates integration of more hydrophobic membrane proteins. (126) Furthermore, additional layers on top of the biomolecular assembly enhance the stability of the coatings and ensure efficient discrimination against unwanted species when the multilayer structure is used for sensing purposes. Interestingly, biomolecules cannot simply be passively incorporated into LbL architectures, but because they often carry charges, they can be used as separate building blocks in the assembly process. In this context, alternating polymer/biomolecule or NP/biomolecule structures can be formed, (127,128) as well as pure biomolecular LbL assemblies, such as DNA/protein or protein/protein multilayers. (129) The beauty of the technique can also be demonstrated by immobilizing different biomolecules in different layers on the sensing surface. This localization enables the construction of defined signal pathways by exploiting sequential reaction schemes. Here, reaction products formed in one layer can be further converted in a subsequent layer, as shown in Figure 1b. (13) These artificial architectures can mimic biological functions, such as sequential electron transfer reactions or switchable pathways. (130) This capability enabled, for instance, the first implementation of tissue-adapted neuroprosthetic implants from conductive composites (131) and light-induced excitation of neurons. (132) Further steps in this directions can be based on direct electron transfer between the immobilized protein molecules. The LbL technique enables the artificial arrangement of redox centers, while keeping them in or close to their natural states. Here, developments are still at early stages—more advanced structures appear to be feasible though, such as the arrangement of enzymes into complex cascades to create artificial metabolome structures with high efficiency. (133−135)

The layer-by-layer technique enables combinations of nanoparticles and polymers, providing a platform for developing hierarchical architectures with a combination of properties including mechanical strength, transparency, and conductivity.

Switchable Coatings for Photoelectrochemistry

Electrochemical devices can be controlled by light based on photosensitive switches, such as QDs. (136−139) Light-generated charge carriers can create photo currents, which enables both control and monitoring of electrochemical reactions (see Figure 1c). (140) Inorganic, photoactive materials such as QDs are commonly used in such applications, but light-sensitive biomolecules have also gained considerable interest for the conversion of light into electrical or chemical energy. (141) An example is the protein supercomplex photosystem I, which can be assembled with the help of negatively charged DNA and the positively charged redox-protein cytochrome c. This system generates well-defined photocurrents, the magnitude of which depends on the number of deposited layers. (142) Charge transfer in LbL structures has been well-studied by numerous groups. (14,17,127,143−145) Layer-by-layer assembly can be used to increase the coverage of redox active molecules by assembling 3D structures, thereby dramatically increasing the analytical signal, but also improving the signal-to-noise ratio (SNR). (146,147) The response from multilayer structures is significantly enhanced compared to the response from single monolayer-based structures. In addition, different kinds of biological modifications can be introduced to the LbL structures of the photoelectrochemical devices. For example, LbL offers the convenient possibility to immobilize enzymes, thereby controlling redox reactions close to the light switches as fixed on the surface of the electrodes. The porous structures of LbL films enable substrates to reach the enzymes and cosubstrates or reaction products, such as O2 or H2O2, to reach the light switches. (148−150) In the future, we expect antibodies and DNA to be incorporated as recognition elements into LbL-based structures. Creating defined sequences of antibodies or oligonucleotides within the 3D assemblies is also an important goal. (151,152) There is great potential for photoelectrochemical devices to be developed that can sense multiple analytes in parallel. Layer-by-layer structures can also introduce good biocompatibility by modifying working electrodes in such a way that applications in cell-based detection become possible. The detection of several metabolites will enable more specific studies of cellular activities. (152) Moreover, the biocompatibility and the ease of preparation of LbL-based systems will enable the fabrication of miniature sensors for human uses such as wearable health monitors and portable environmental monitoring devices. In addition, electrochromic coatings can be produced by self-assembly of 2D titanium carbide (Ti3C2Tx) MXene and gel electrolyte with a visible absorption peak shift from 770 to 670 nm and a 12% reversible change in transmittance with a switching rate of <1 s when cycled in an acidic electrolyte under applied potentials of less than 1 V. (153) The LbL film can act as both transparent conductive coating and active material in an electrochromic device, opening avenues for a number of optoelectronic, sensing, and photonic applications. Hybrid systems prepared by LbL assembly of polyoxometalate clusters and poly(4-vinylpyridine) also show reversible electro- and photochromic behavior. (154,155)

Tailored Coatings for Better Control of Cell–Surface Interactions

For many applications, detailed understanding of the interface between cells and underlying substrates is critical. (156,157) It is well established that LbL assembly offers a means to immobilize different biomolecules on surfaces using mild deposition conditions (see Figure 1d). (158) Layer-by-layer assemblies can integrate plasmids, (159) growth factors, (160) proteins, genetic material, antibodies, and antibiotics directly into the layers or the components can be precomplexed with polyelectrolytes and then assembled as complexes. (161) For such biological components, e.g., for growth factors, their action can be extended in time (162) or triggered by external stimuli, whereas their controlled release can be regulated by barrier layers. Multilayers can be prepared from biocompatible polyelectrolytes and their mechanical properties; wettability, and interactions with proteins and cells, can be fine-tuned by chemical cross-linking, thermal annealing, (163) or the addition of nanoparticles into the assembly (see Figure 2). (164) This strategy enables the availability of biomolecules on surfaces to be controlled. (162,165,166) Imagine chemically identical surfaces (composition, roughness, etc.), below which nanoreinforced strata are hidden (i.e., deposited) that enable control of the tensile strength of the interface. Other combinations of surface properties can be deposited on top of cell-culture gels. Such surface engineering would be extremely useful for implants and scaffolds as a means to enhance cell adhesion, mobility, and differentiation. In the long term, there are numerous different ways for the LbL technique to be implemented. For example, they can be used to modify scaffolds and implants to create customized environments and interfaces in tissue engineering. Layer-by-layer assembled surfaces can also be laterally patterned as substrates for the growth of cells. Micropatterned deposition of LbL films has been used to generate architecturally organized cellular structures that better mimic the complex microstructures of tissues in the body. For example, patterned cocultures were generated by sequentially depositing micropatterned LbL films made of hyaluronic acid and polylysine or collagen that could be used to render regions of a surface adhesive to cells. (167,168) In such cultures, patterned cocultures of liver cells and fibroblasts showed increased functionality compared to various controls.

Figure 2

Figure 2. Scheme of the protein adhesion mechanism and the effects on cell adhesion for non-annealed poly(l-lysine/alginate) (PLL/Alg) and annealed-PLL/Alg. (157) Results from the exchangeability assays are schematically described. On annealed PLL/Alg layer-by-layer (LbL) surfaces, proteins exhibit augmented interactions with the substrate, the exchangeability is reduced, and fibronectin (FN), either alone or in cooperation with bovine serum albumin (BSA), has stronger interactions with the LbL surface coating. The effect on cell adhesion is also illustrated. (169) The objects depicted in the scheme are not to scale, and for FN, only the FN III fragment is represented. Adapted with permission from ref (150). Copyright 2019 John Wiley & Sons, Inc.

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Figure 3

Figure 3. Changes in cell adhesion and in the physicochemical properties of layer-by-layer (LbL) multilayer coatings induced by thermal annealing. (157) (a) Scheme of the assembly and annealing protocols. (b) Phase contrast images of C2C12 cells adhered on glass, poly-l-lysine/alginate (n-PLL/Alg, a-PLL/Alg), n-chitosan/hyaluronic acid (n-Chi/HA), or a-Chi/HA as indicated. (c) Average cell adhesion spreading area from cells seeded on glass, n-PLL/Alg, a-PLL/Alg, n-Chi/HA, or a-Chi/HA polyelectrolyte multilayers. (d) Changes in physicochemical properties of polyelectrolyte multilayers upon annealing. Adapted with permission from ref (150). Copyright 2019 John Wiley & Sons, Inc.

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Micropatterned deposition of layer-by-layer films has been used to generate architecturally organized cellular structures that better mimic the complex microstructures of tissues in the body.

This strategy ultimately results in a multitude biomimetic composites including those made in bulk form. The diverse composite structures replicated using LbL assembly (170−173) made possible ex vivo replication of nacre, (36) enamel, (170) extracellular matrix, (174,175) and models of cellular organelles. (176−178) By combining LbL assembly with other fabrication techniques at the micrometer and millimeter scale, tissue replicas with complex geometries have been obtained, such as for bone marrow. (179,180) The exceptional materials properties of the multilayer composites and the generality of the approach have also made possible the design, fabrication, and implementation of implantable devices, (132,181,182) sensors, (53,183,184) drug-delivery vehicles, (185,186) and optical devices, (187,188) exceeding the performance of existing technologies.

Antibacterial Surface Coatings

The advent of LbL films has led to several new strategies for the development of antibacterial coatings, from the fabrication of multilayers with cationic polymers that disrupt bacterial membranes, (189) to the assembly of antibacterial nanomaterials such as silver nanoparticles or graphene oxide, (190) to the encapsulation of antibiotics in the multilayers, to combinations of all these elements. (191) The LbL assembly can include several layers of nanomaterials, combine layers of different nanomaterials in a film, or facilitate inclusion of antibiotics in the films by complexing with the polymers (see Figure 1e). Many antibiotics have charged groups that can be used to form complexes with polyelectrolytes in the LbL films or assembled in films replacing polyelectrolyte layers. The LbL technique has the advantage that it can be applied straightforwardly on almost any charged surface, and antibacterial coatings could be developed for medical devices as well as for implants. In particular, the LbL technique has significant potential in the design of antibacterial coatings that can inhibit nosocomial infections during implant surgery. An optimal antibacterial coating for bone implants based on release of an antibiotic should involve an initial burst release at the time of surgery, followed by prolonged release over the weeks following the surgical intervention to ensure bone tissue regeneration. (192) The LbL technique can be used to design films capable of fully or partially degrading and releasing antibiotics at different times and rates. Examples in the literature show that aminoglycans, such as gentamicin, can be released from LbL films, combining burst and steady releases that would be particularly suitable for implant surgery. (193) Moreover, the LbL technique enables the additional assembly of growth factors on the coating that can counteract negative effects on cell growth and differentiation caused by a high localized dose of antibiotics. (194) These combinations can result in films with enhanced antibacterial properties and in the design of coatings suitable for different environments in multiple medical settings or for antifouling applications.

Layer-by-Layer-Based Membranes

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Purification Technologies

Another future for LbL coatings lies in separation technologies, such as liquid or gas permeation membranes (see Figure 4). (69,195,196) Significant pioneering work has already been carried out, (197−201) starting with gas separation membranes, (69) but recently, LbL membrane modification for fresh water production has been explored further. Nanofiltration membranes for the removal of particles down to virus sizes of ∼35 nm are not able to retain dissolved materials, such as ions, leading to issues with water hardness, low molecular weight pharmaceutical agents, etc., which become increasingly problematic in fresh water preparation. However, reverse osmosis (RO) membranes consume a great deal of energy and retain all salts, which is not useful for drinking water. In contrast, a few LbL-assembled layers of poly(diallyldimethylammonium chloride) (PDADMAC)/PSS on top of tubular filtration membranes of pore size 20 nm are able to increase the retention of magnesium sulfate from 5% to over 90% and for several endocrines above 50–90% depending on the endocrine type (see Figure 5). (202) In contrast to RO membranes, these LbL membranes allow permeation of sodium chloride, maintain high fluxes, and require much less pressure and energy. Up to now, the LbL coating of membranes has been evaluated only for films based on the combination of one polycation with one polyanion. However, one can imagine that a multifunctional coating could improve the membranes further. The first layer on the membrane has to ensure a good connection of the LbL film to the membrane in order to resist sufficiently high-pressure back-flushing cycles. Furthermore, the first polyelectrolyte has to be assembled exclusively on top of the pores and should not penetrate into the pores, as otherwise these would be blocked. The intermediate layers should utilize a design in which the mesh size controls the retention of the analyte and also ensures the removal of specific pollutants. (91) Finally, the outermost layer should reduce the fouling behavior of the membranes by controlling its hydrophilic properties and electrostatic repulsion. (203−205)

Figure 4

Figure 4. Layer-by-layer (LbL) assembly can be used for membranes for gases and liquids.

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Figure 5

Figure 5. Retention of different endocrines by an uncoated poly(ether sulfone) membrane (red) and by a layer-by-layer (LbL)-coated (PDADMAC/PSS)4 membrane (green). Unpublished data by the group of Lars Dähne.

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Introducing Channels in Biological Membranes

Biomimetic nature of LbL materials opens the possibility to replicate biological membranes. Cell membranes comprise not only lipids but also high protein content, (206) for example, transmembrane proteins that form channels for molecular transport into/out of cells. Lateral inhomogeneity is important. Here lies one big challenge for the future. To date, LbL structuring has predominantly only been possible perpendicular to the surface, i.e., by variation of the compositions of the different layers. However, in order to create LbL-assembled membranes mimicking the function of biological membranes—for example, with integrated protein-based channels—lateral structuring would also be required. In the simplest case, “channels” in the form of holes could be introduced, for example, by nanoplasmonic heating. (207) Another option lies in tethered membranes. In recent work, dense membranes with limited defects and high resistivity were assembled on top of multilayers. (208−211) These membranes can contain channels with selective ion permeability. (212) For electronic sensing, the multilayers provide a means to control the distance of the lipid bilayer from the electrodes, which is particularly useful for membranes incorporating channels and transmembrane proteins, avoiding undesired effects from the electrode on channel and protein behavior. (212) Another method for lateral structuring might be based on the fusion of microcapsules. (213) Still, despite the numerous ideas outlined here, lateral structuring of LbL films remains a challenge.

Layer-by-Layer-Based Encapsulation for Delivery Vehicles

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Layer-by-layer Assembly for Encapsulation

Layer-by-layer technology for micro- and nanoencapsulation was introduced ∼20 years ago and initially looked extremely promising (see Figure 6). (10,11,214−219) The key advantage was considered to be the simplicity with which one could construct multifunctional delivery systems. In fact, LbL capsules can combine multiple functions and external responsiveness. However, LbL-based encapsulation suffers from high permeability of small, water-soluble molecules (i.e., leaching) and rather time-consuming processes for fabrication. Some of these problems have been solved, such as expanding the class of molecules that can be encapsulated (e.g., doxorubicin, paclitaxel, liquid crystals, siRNA) without severe leaching, (44,45,220−225) and inroads have been made into the problem of scale-up. (61,226,227) This approach has also been made possible by extending the initial capsule geometries to more sophisticated structures, such as capsosomes, etc. (228−235) The capsule shells can also be labeled with different types of nanoparticles, providing contrast for imaging (118−120) or enabling magnetic targeting. (236,237) Currently, the technology still has potential, particularly in areas where other technologies are not available. A number of studies on various cell types, including macrophages, dendritic cells, neurons, and stem cells, have demonstrated that incubation with cells results in internalization of capsules by cells without significant effects on cell viability. (238−240) The elastic properties facilitate their uptake as the capsules can easily be deformed during internalization. (241−244) In other words, cells were found to tolerate capsule internalization, which is not always the case for other delivery systems. Detailed studies on the tissue response after subcutaneous (245) and pulmonary (246) administration of degradable LbL capsules composed of polypeptide and polysaccharide building blocks have also demonstrated that LbL capsules exhibit a moderate foreign body response and are easily internalized by immune cells, such as macrophages and dendritic cells. This advance should pave the way for further development of such carriers in advanced vaccine technologies. In summary, LbL offers a good platform for delivery of encapsulated cargo inside cells, which is discussed below in terms of drug delivery and imaging/sensing. Because the capsules remain in endosomes/lysosomes after internalization, endosomal escape and translocation of encapsulated compounds to the cytosol remains a significant hurdle.

Figure 6

Figure 6. Layer-by-layer (LbL) assembly can be used to fabricate encapsulation platforms for nanodelivery.

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Delivery of Therapeutic Agents

The LbL technique opens the possibility of assembling therapeutics in between layers of polyelectrolytes, on top of nano/microparticles that protect a certain cargo, while, at the same time, making multiple functional groups available in the polyelectrolyte, which can be engineered to generate stealth coatings for targeting delivery. For the delivery of encapsulated therapeutics in polyelectrolyte multilayers, the assemblies must degrade, liberating the material entrapped between the layers. However, as we noted above, LbL assemblies have intrinsically semipermeable properties that can be tuned by means of layer numbers and thicknesses as well as by the type of the interacting polyelectrolyte pairs, resulting in leaching even before intended degradation and subsequent release. Thus, the initial euphoria in scientific articles to encapsulate low molecular weight drugs and to release them in a controlled manner on demand has not yet translated into real-world applications. As previously noted, the most critical reason for this difficulty in translation is the high permeability of the films for small molecules (see Figure 7). Even for the very dense polyelectrolyte system poly(allylamine hydrochloride)/polystyrenesulfonate (PAH/PSS), researchers recorded release rates ranging from minutes to a few hours for water-soluble molecules having molecular weights below 5 kDa. (247) In contrast, large molecules with molecular weights above 10 kDa can be permanently immobilized, either in the polyelectrolyte layers or in capsules comprising polyelectrolyte walls. This same conclusion has been shown over the past decade for a variety of biomolecules, including proteins, such as antibodies; (248) growth factors; (249) hormones; (250) enzymes; (251) nucleic acids, such as DNA plasmids; (237) different types of RNA molecules, such as silencing RNAs; (252) and polysaccharides such as alginate, carrageenan, chitosan, and hyaluronic acid. (253)

Figure 7

Figure 7. Permeability of layer-by-layer (LbL) membranes consisting of different polyelectrolyte combinations (8 layers) for small molecules (fluorescein). Unpublished data from the group of Lars Dähne.

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In parallel, new therapeutic avenues based on the delivery of high molecular weight drugs, for instance, at the site of the implantation of LbL material. (159) Plasmids, specific antibodies, RNA, or DNA can be delivered by incorporation in LbL films and can be utilized as personalized medicines. However, due to the sensitive and specialized recognition of such molecules by our immune system, it is hard to deliver them efficiently in vivo to the intended targets. For this purpose, LbL assemblies could have a bright future in the form of capsule formulations, because the necessary multifunctionality can be delivered by LbL technology. For example, an ideal capsule should have an inner surface that is not interacting with the biomolecule in order to retain its functionality. The intermediate layers determine the release behavior, which could be controlled slow release, immediate release caused by an internal trigger (e.g., by the lower pH value in cancer cells or by specific enzymatic surroundings (254,255)) or release activated by an external trigger (e.g., NIR light, (22,237,256−258) X-ray radiation, ultrasound (US), (259−261) or magnetic fields (262,263)). Internal triggers can readily be created by combining polycations and polyanions in such a way that their degradation will be fast or slow. The sequential assembly of polyelectrolytes in LbL enables control over the composition of the layers in the vertical direction and could be used to deliver different therapeutics progressively. For example, two siRNA molecules with complementary actions could be assembled in different positions within the LbL film, so that they are released sequentially. Degradation of the multilayers in biological fluids or intracellularly can be selected because matrices can be programmed by varying the assembly conditions, the number of assembled layers and the combination of polycations and polyanions. The surface layer is also important: It should not be recognizable by the immune system in order to realize high circulation times, but it should nonetheless bind specifically to a defined target. In the case of systemic delivery, there is the problem of targeting, i.e., to produce locally enhanced concentrations of the pharmaceutic agent at the desired target site. One interesting approach, which has not yet been fully exploited, is cell-mediated delivery, where cells are used as natural transporters to carry the encapsulated materials to a targeted site. (264) Here, externally driven cell navigation could be used. (265)In vitro studies have demonstrated that cell motion is possible in magnetic field gradients if the cell has internalized magnetic capsules. (266) Magnetic capsules can bring genetic materials inside the cells and reprogram the cells in such a way that the follow-up sorting of altered and nonmodified cells can easily be done by magnetic sorting. (267) Such magnetic targeting is biocompatible. The ability of mesenchymal stem cells (MSCs) to differentiate was not affected by magnetic manipulation. (268) This is important, as MSCs impregnated with capsules could be used as natural cargo transporters. Also, whereas many applications focus on systemic delivery, local delivery may offer new approaches, which deliberately avoid the “targeting” issue. One interesting example is LbL particles that were designed for transdermal delivery of vaccine and adjuvant peptides via hair follicles. In contrast to dissolved molecules, particles in sizes ranging between 300 and 900 nm can be inserted in hair follicles by intense massage. (269) The diffusion of vaccines to the Langerhans cells in the skin is much easier through the follicle membrane than through the epidermis. In order to transport the vaccine peptides into the hair follicle, 600 nm silica particles were coated with polymethacrylate with LbL assembly, with an outermost layer having a pKa value of 6.2. The pH difference between skin (pH 5–5.5) and follicle center (pH 7.4) was selected for the delivery of the peptides, which were tagged with four glutamic amino acids bearing negative total charge. The vaccines were efficiently adsorbed at pH 4.5 onto the partly positively charged particles and kept stably attached during the skin massage. After arriving at the follicle center where the pH was 7.4, the zeta-potential of the particles switched and became highly negative and the vaccines were released due to electrostatic repulsion. In general, LbL capsules have made the step from in vitro demonstration to in vivo experiments. For example, these particle systems can induce bone formation in vivo (when loaded with growth factors), (249) target atherosclerotic plaques in vivo, (270) and generate a significant immune response in vivo (when loaded with immunogenic peptides). (271) Both peptide- and protein-antigen-loaded LbL capsules generate a significant immune response in vitro and in vivo. (254) It was demonstrated that ovalbumin (OVA) (a model vaccine)-specific CD4 and CD8 T cells were activated to proliferate in vivo following intravenous (271) and subcutaneous (272) vaccination of mice with OVA protein- and OVA peptide-loaded LbL capsules. The OVA encapsulated within the capsules resulted in greatly enhanced antigen presentation and proliferation of antigen-specific CD4 and CD8 T cells that provided enhanced protection against viral infection and tumor growth. Furthermore, LbL capsules could be further engineered on their surface with immune-stimulatory molecules to boost the antigen-specific immune responses against encapsulated antigen. (273) The latter work was carried out with the idea of using LbL-coated microneedles for transdermal vaccination. Indeed, several groups have investigated codelivery of antigen and immune stimuli, both on colloidal and planar substrates. (273−275)

Imaging and Sensing

In diagnostic imaging, highly developed methods/modalities are applied, such as ultrasound imaging (US), X-ray computed tomography (CT), magnetic resonance imaging (MRI), near-infrared imaging (NIR), photoacoustic imaging (PAI) and nuclear imaging methods such positron emission tomography (PET) or single photon emission computed tomography (SPECT). Each of these imaging methods has advantages, but also drawbacks, such as limited spatial or time resolution, sensitivity, impairment of the patient, etc. Therefore, several methods must be combined in order to optimize the images and information obtained. (276) Instruments for this purpose are already under development, but suitable contrast agents providing contrast for different imaging modalities and methods are also necessary. These agents can be based on molecular materials or on particles. Solid particles should be in the nanometer range, whereas flexible particles could be used at the micron scale of erythrocytes. By means of LbL technology, such multifunctional contrast agents can be produced in a controlled way. One example was recently developed, which is simultaneously applicable for US, MRI, SPECT, and NIR imaging. (277) The core of the flexible 3 μm particles consisted of an air bubble, which is stably encapsulated by cross-linked poly(vinyl alcohol) (PVA) for US imaging. Positive charges were introduced in the PVA matrix in order to achieve controlled LbL coating. Two double layers of PSS/PAH-1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) were assembled. The NOTA label complexes technetium for SPECT imaging. On top, double layers of citrate-stabilized iron oxide nanoparticles (SPION)/PAH were assembled for dark contrast in MRI imaging. (278) For NIR imaging, further fluorescent layers of PAH-Cy5/PSS were assembled. Finally, targeting was demonstrated by biotinylated antibodies coupled to an outermost PAH/streptavidin layer. (279,280) Thus, LbL enables convenient integration of different contrast modalities in one single particle. Apart from simple imaging, where contrast depends on the local concentration of contrast agent, functional imaging, i.e., sensing, is possible. In this case, the signal of the contrast agent also depends on the local environment. There are several examples of encapsulated, analyte-sensitive fluorophores, (281−283) which enable the detection of local ion concentrations. The changes in environment must be taken into account when designing these multimodal particles. For example, many ion-sensitive fluorophores also respond to local pH, so one severe challenge concerning future in vivo applications is that particles will undergo massive local pH changes along their trajectories in the body, for example, upon endocytosis by macrophages. One solution might be to use more complex systems, such as sensors with distance-dependent quenching of optical or magnetic signals. In order to protect these systems from agglomeration, they could be encapsulated. The LbL shell around the sensors would then enable analytes to diffuse in and out, whereas it would retain and protect the actual sensor system. One developed glucose microsensor is depicted in Figure 8. Due to the possibility of multicompartment encapsulation of different molecules in different locations within one particle by LbL, (284) even feedback-controlled systems might be developed. A drug could be encapsulated for delivery in one compartment, whereas a sensor monitoring the action of the drug could be placed in an adjacent compartment. (285,286)

Figure 8

Figure 8. Glucose sensing in layer-by-layer (LbL) capsules, containing ConcanavalinA (ConA) and Dextran (Dex), labeled with fluorescence resonance energy transfer (FRET) pair. Unpublished data from the group of Lars Dähne.

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By means of layer-by-layer technology, multifunctional contrast agents for diagnostic imaging can be produced in a controlled way.

Challenges for Layer-by-Layer Coated Particles Intended for in Vivo Use

Following the above-outlined possibilities for applying LbL-based particles to in vivo delivery and imaging/sensing, one can summarize a number of key challenges for the future. (1) Highly biocompatible and biodegradable materials need to be developed and used. (2) Further fundamental studies need to be undertaken to understand the interactions of LbL particles and biological systems in order to probe parameters such as elasticity and shape and how these influence biological responses. (3) Automation of the preparation of LbL particles should be further developed, as this capability is critical to reproducibility and streamlining preparation. (4) More focus should be placed on such particles for local delivery applications (e.g., their use as depots) and their interactions with the local cellular and protein environment, not only limiting their studies to systemic delivery applications. Thus, the development of LbL-based vehicles continues and important breakthroughs lie ahead.

Author Information

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  • Corresponding Author
  • Authors
    • Shuang Zhao - Fachbereich Physik, CHyN, Universität Hamburg, 22607 Hamburg, Germany
    • Frank Caruso - ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, and the Department of Chemical Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia;  Orcidhttp://orcid.org/0000-0002-0197-497X
    • Lars Dähne - Surflay Nanotec GmbH, 12489 Berlin, Germany
    • Gero Decher - CNRS Institut Charles Sadron, Faculté de Chimie, Université de Strasbourg, Int. Center for Frontier Research in Chemistry, Strasbourg F-67034, France;  Int. Center for Materials Nanoarchitectonics, Ibaraki 305-0044, Japan;  Orcidhttp://orcid.org/0000-0002-7976-8818
    • Bruno G. De Geest - Department of Pharmaceutics, Ghent University, 9000 Ghent, Belgium;  Orcidhttp://orcid.org/0000-0001-9826-6170
    • Jinchen Fan - Department of Chemical Engineering and Biointerfaces Institute, University of Michigan, Ann Arbor, Michigan 48105, United States;  Orcidhttp://orcid.org/0000-0002-8905-1693
    • Neus Feliu - Fachbereich Physik, CHyN, Universität Hamburg, 22607 Hamburg, Germany;  Orcidhttp://orcid.org/0000-0002-7886-1711
    • Yury Gogotsi - Department of Materials Science and Engineering and A. J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, Pennsylvania 19104, United States;  Orcidhttp://orcid.org/0000-0001-9423-4032
    • Paula T. Hammond - Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02459, United States;  Orcidhttp://orcid.org/0000-0002-9835-192X
    • Mark C. Hersam - Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208-3108, United States;  Orcidhttp://orcid.org/0000-0003-4120-1426
    • Ali Khademhosseini - Department of Bioengineering, Center for Minimally Invasive Therapeutics (C-MIT), California NanoSystems Institute (CNSI), University of California, Los Angeles, Los Angeles, California 90095, United States;  Orcidhttp://orcid.org/0000-0002-2692-1524
    • Nicholas Kotov - Department of Chemical Engineering and Biointerfaces Institute, University of Michigan, Ann Arbor, Michigan 48105, United States;  Michigan Institute for Translational Nanotechnology, Ypsilanti, Michigan 48198, United States;  Orcidhttp://orcid.org/0000-0002-6864-5804
    • Stefano Leporatti - CNR Nanotec-Istituto di Nanotecnologia, Italian National Research Council, Lecce 73100, Italy;  Orcidhttp://orcid.org/0000-0001-5912-7565
    • Yan Li - College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China;  Orcidhttp://orcid.org/0000-0002-3828-8340
    • Fred Lisdat - Biosystems Technology, Institute for Applied Life Sciences, Technical University, D-15745 Wildau, Germany
    • Luis M. Liz-Marzán - CIC biomaGUNE, San Sebastian 20009, Spain;  Ikerbasque, Basque Foundation for Science, Bilbao 48013, Spain;  Orcidhttp://orcid.org/0000-0002-6647-1353
    • Sergio Moya - CIC biomaGUNE, San Sebastian 20009, Spain;  Orcidhttp://orcid.org/0000-0002-7174-1960
    • Paul Mulvaney - ARC Centre of Excellence in Exciton Science, School of Chemistry, University of Melbourne, Parkville, Victoria 3010, Australia;  Orcidhttp://orcid.org/0000-0002-8007-3247
    • Andrey L. Rogach - Department of Materials Science and Engineering, and Centre for Functional Photonics (CFP), City University of Hong Kong, Kowloon Tong, Hong Kong SAR;  Orcidhttp://orcid.org/0000-0002-8263-8141
    • Sathi Roy - Fachbereich Physik, CHyN, Universität Hamburg, 22607 Hamburg, Germany;  Orcidhttp://orcid.org/0000-0002-0645-5386
    • Dmitry G. Shchukin - Stephenson Institute for Renewable Energy, Department of Chemistry, University of Liverpool, Liverpool L69 7ZF, United Kingdom;  Orcidhttp://orcid.org/0000-0002-2936-804X
    • Andre G. Skirtach - Nano-BioTechnology group, Department of Biotechnology, Faculty of Bioscience Engineering, Ghent University, 9000 Ghent, Belgium;  Orcidhttp://orcid.org/0000-0002-4468-7620
    • Molly M. Stevens - Department of Materials, Department of Bioengineering and Institute for Biomedical Engineering, Imperial College London, London SW7 2AZ, United Kingdom;  Orcidhttp://orcid.org/0000-0002-7335-266X
    • Gleb B. Sukhorukov - School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, United Kingdom;  Orcidhttp://orcid.org/0000-0001-6213-0562
    • Paul S. Weiss - Department of Bioengineering, Center for Minimally Invasive Therapeutics (C-MIT), California NanoSystems Institute (CNSI), University of California, Los Angeles, Los Angeles, California 90095, United States;  Department of Chemistry and Biochemistry and Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States;  Orcidhttp://orcid.org/0000-0001-5527-6248
    • Zhao Yue - Department of Microelectronics, Nankai University, Tianjin 300350, China
    • Dingcheng Zhu - Fachbereich Physik, CHyN, Universität Hamburg, 22607 Hamburg, Germany
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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S.Z. was supported by the Chinese Scholarship Council (CSC). F.C. acknowledges the award of a National Health and Medical Research Council (NHMRC) Senior Principal Research Fellowship (APP1135806) and support from the Australian Research Council Centre of Excellence in Convergent Bio-Nano Science and Technology (Project No. CE140100036). L.D. thanks the BMBF (topical vaccination) and the EU FP6 and FP7 (LbL Brane, 3M and Sight) for supporting the research. S.E.M. thanks the MAT2017-88752-R Retos project from the Ministerio de Economia, Industria y Competitividad, gobierno de España. M.C.H. acknowledges support from the Materials Research Science and Engineering Center (MRSEC) of Northwestern University (NSF DMR-1720139). N.A.K. acknowledges the Alexander von Humboldt Foundation for a visiting professorship to Hamburg University. S.L. is supported by Progetto FISR-CNR “Tecnopolo di Nanotecnologia e Fotonica per la Medicina di Precisione”- CUP B83B17000010001. P.M. acknowledges support through ARC Grant CE170100026. A.L.R. was supported by a grant from the Germany/Hong Kong Joint Research Scheme sponsored by the Research Grants Council of Hong Kong and the Germany Academic Exchange Service of Germany (Reference No. G-CityU106/18). S.R. was supported by Fazit Stiftung. D.S. acknowledges an ERC Consolidator Grant No. 647969. A.G.S. acknowledges support of BOF-UGent (01IO3618, BAS094-18, BOF14/IOP/003) and FWO-Vlaanderen (G043219). W.J.P. received funding from the Deutsche Forschungsgemeinschaft (DFG PA 794/21-1). Z.Y. was supported by the National Natural Science Foundation of China (61871240).

References

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    Sukhorukov, G. B.; Moehwald, H.; Decher, G.; Lvov, Y. M.
    Thin Solid Films (1996), 284-285 (), 220-223CODEN: THSFAP; ISSN:0040-6090. (Elsevier)
    Multilayers films of nucleic acids (DNA, polyuridylic and polyadenylic acids) and polycations (polyallylamine, polyethylenimine, polylysine and polyarginine) were fabricated by consecutive adsorption of anionic and cationic polyelectrolytes. The film growth process was controlled by x-ray reflectivity and UV spectroscopy. The penetration of DNA-specific dye was examd. for DNA-polyallylamine self-assembled films.
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  7. 7
    Decher, G. Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites. Science 1997, 277, 1232– 1237,  DOI: 10.1126/science.277.5330.1232
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    Fuzzy nanoassemblies: toward layered polymeric multicomposites
    Decher, Gero
    Science (Washington, D. C.) (1997), 277 (5330), 1232-1237CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    A review with 105 refs. Multilayer films of org. compds. on solid surfaces have been studied for more than 60 yr because they allow fabrication of multicomposite mol. assemblies of tailored architecture. However, both the Langmuir-Blodgett technique and chemisorption from soln. can be used only with certain classes of mols. An alternative approach (fabrication of multilayers by consecutive adsorption of polyanions and polycations) is far more general and has been extended to other materials such as proteins or colloids. Because polymers are typically flexible mols., the resulting superlattice architectures are somewhat fuzzy structures, but the absence of crystallinity in these films is expected to be beneficial for many potential applications.
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  8. 8
    Caruso, F.; Caruso, R. A.; Möhwald, H. Nanoengineering of Inorganic and Hybrid Hollow Spheres by Colloidal Templating. Science. 1998, 282, 1111– 1114,  DOI: 10.1126/science.282.5391.1111
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    8
    Nanoengineering of inorganic and hybrid hollow spheres by colloidal templating
    Caruso, Frank; Caruso, Rachel A.; Moehwald, Helmuth
    Science (Washington, D. C.) (1998), 282 (5391), 1111-1114CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Hollow silica and silica-polymer spheres with diams. 720-1000 nm were fabricated by consecutively assembling silica nanoparticles and polymer onto colloids and subsequently removing the templated colloid either by calcination or decompn. upon exposure to solvents. SEM and TEM images demonstrate that the wall thickness of the hollow spheres can be readily controlled by varying the no. of nanoparticle-polymer deposition cycles, and the size and shape are detd. by the morphol. of the templating colloid. The hollow spheres produced are envisioned to have applications in areas ranging from medicine to pharmaceutics to materials science.
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  9. 9
    Caruso, F.; Lichtenfeld, H.; Giersig, M.; Möhwald, H. Electrostatic Self-Assembly of Silica Nanoparticle-Polyelectrolyte Multilayers on Polystyrene Latex Particles. J. Am. Chem. Soc. 1998, 120, 8523– 8524,  DOI: 10.1021/ja9815024
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    9
    Electrostatic Self-Assembly of Silica Nanoparticle-Polyelectrolyte Multilayers on Polystyrene Latex Particles
    Caruso, Frank; Lichtenfeld, Heinz; Giersig, Michael; Moehwald, Helmuth
    Journal of the American Chemical Society (1998), 120 (33), 8523-8524CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The construction of composite multilayers of 25 nm diam. silica particles and poly(diallyldimethylammonium chloride) was carried out on submicrometer-sized polystyrene latex particles via the sequential electrostatic adsorption of SiO2 and PDADMAC from dil. soln.
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  10. 10
    Zhang, R.; Shang, J.; Xin, J.; Xie, B.; Li, Y.; Möhwald, H. Self-Assemblies of Luminescent Rare Earth Compounds in Capsules and Multilayers. Adv. Colloid Interface Sci. 2014, 207, 361– 375,  DOI: 10.1016/j.cis.2013.12.012
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    10
    Self-assemblies of luminescent rare earth compounds in capsules and multilayers
    Zhang, Renjie; Shang, Juanjuan; Xin, Jing; Xie, Beibei; Li, Ya; Mohwald, Helmuth
    Advances in Colloid and Interface Science (2014), 207 (), 361-375CODEN: ACISB9; ISSN:0001-8686. (Elsevier B.V.)
    A review. This review addresses luminescent rare earth compds. assembled in microcapsules as well as in planar films fabricated by the layer-by-layer (LbL) technique, the Langmuir-Blodgett (LB) method and in self-assembled monolayers. Chem. pptn., electrostatic, van der Waals interactions and covalent bonds are involved in the assembly of these compds. Self-organized ring patterns of rare earth complexes in Langmuir monolayers and on planar surfaces with stripe patterns, as well as fluorescence enhancement due to donor-acceptor pairs, microcavities, enrichment of rare earth compds., and shell protection against water are described. Recent information on the tuning of luminescence intensity and multicolors by the excitation wavelength and the ratio of rare earth ions, resp., are also reviewed. Potential applications of luminescent rare earth complex assemblies serving as biol. probes, temp. and gas sensors are pointed out.
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  11. 11
    Kolesnikova, T. A.; Skirtach, A. G.; Möhwald, H. Red Blood Cells and Polyelectrolyte Multilayer Capsules: Natural Carriers versus Polymer-Based Drug Delivery Vehicles. Expert Opin. Drug Delivery 2013, 10, 47– 58,  DOI: 10.1517/17425247.2013.730516
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    11
    Red blood cells and polyelectrolyte multilayer capsules: natural carriers versus polymer-based drug delivery vehicles
    Kolesnikova, Tatiana A.; Skirtach, Andre G.; Moehwald, Helmuth
    Expert Opinion on Drug Delivery (2013), 10 (1), 47-58CODEN: EODDAW; ISSN:1742-5247. (Informa Healthcare)
    A review. Introduction: Red blood cells (RBCs) and lipid-based carriers on the one hand and polymeric capsules on the other hand represent two of the most widely used carriers in drug delivery. Each class of these carriers has its own set of properties, specificity and advantages. Thorough comparative studies of such systems are reported here for the first time.Areas covered: In this review, RBCs are described in comparison with synthetic polymeric drug delivery vehicles using polyelectrolyte multilayer capsules as an example. Lipid-based compn. of the shell in the former case is particularly attractive due to their inherent biocompatibility and flexibility of the carriers. On the other hand, synthetic approaches to fabrication of polyelectrolyte multilayer capsules permit manipulation of the permeability of their shell as well as tuning their compn., mech. properties, release methods and targeting.Expert Opinion: In conclusion, properties of RBCs and polyelectrolyte multilayer capsules are reported here highlighting similarities and differences in their prepn. and applications. In addn., their advantages and disadvantages are discussed.
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  12. 12
    Shchukin, D. G.; Möhwald, H. Multilayer Polyelectrolyte Assembly in Feedback Active Coatings and Films. In Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials, 2nd ed.; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, Germany, 2012; Vol. 2, pp 1039– 1050.
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    Dronov, R.; Kurth, D. G.; Möhwald, H.; Scheller, F. W.; Lisdat, F. A Self-Assembled Cytochrome c/Xanthine Oxidase Multilayer Arrangement on Gold. Electrochim. Acta 2007, 53, 1107– 1113,  DOI: 10.1016/j.electacta.2007.02.044
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    13
    A self-assembled cytochrome c/xanthine oxidase multilayer arrangement on gold
    Dronov, Roman; Kurth, Dirk G.; Moehwald, Helmuth; Scheller, Frieder W.; Lisdat, Fred
    Electrochimica Acta (2007), 53 (3), 1107-1113CODEN: ELCAAV; ISSN:0013-4686. (Elsevier B.V.)
    A polyelectrolyte multilayer combining cytochrome c (cyt. c) and xanthine oxidase (XOD) is assembled on a gold electrode and investigated with respect to a signal chain formation from a xanthine oxidase substrate in soln. The multilayer assembly is prepd. by means of an electrostatic self-assembly technique and consists of two parts each comprising one type of protein, which is responsible for a specific function. The outer part of the film contains XOD immobilized within poly(ethylenimine) layers and is responsible for selectivity towards hypoxanthine (HX) by its enzymic conversion to uric acid. The inner layers contain cyt. c mols. embedded into a sulfonated polyaniline (PASA) matrix. The signal transfer mechanism within the assembly is proposed to be a mediated one. Thus, cyt. c plays a role of an internal transducer translating a HX concn. into an amperometric electrode response. Formation of the multilayer structure is confirmed by surface plasmon resonance (SPR), electrochem. expts. and UV-vis spectrophotometry. Influence of the multilayer compn. on sensor performance is discussed.
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    Tang, F.; Cui, Q.; Wu, F.; Li, L.; Möhwald, H. Photoinduced Long-Range Charge Transfer in Polyelectrolyte Multilayers. J. Photopolym. Sci. Technol. 2008, 21, 729– 731,  DOI: 10.2494/photopolymer.21.729
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    14
    Photoinduced long-range charge transfer in polyelectrolyte multilayers
    Tang, Fu; Cui, Qianling; Wu, Feipeng; Li, Lidong; Mohwald, Helmuth
    Journal of Photopolymer Science and Technology (2008), 21 (6), 729-731CODEN: JSTEEW; ISSN:0914-9244. (The Conference of Photopolymer Science and Technology)
    Fluorescence quenching expts. prove that electron transfer occurs inside the films even in the absence of a high d. of conjugated bonds. In addn. the arrangement inhibited back transfer thus leading to a long-lived intermediate that is optically addressable. The ultimate electron acceptor is free in soln. and sepd. phys. from the donor, providing ready access to the polyviologen redox products. Hence these films or capsules made in analogous way may also serve as information storage.
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  15. 15
    Li, Z.; Xu, F.; Li, Q.; Liu, S.; Wang, H.; Möhwald, H.; Cui, X. Synthesis of Multifunctional Bovine Serum Albumin Microcapsules by the Sonochemical Method for Targeted Drug Delivery and Controlled Drug Release. Colloids Surf., B 2015, 136, 470– 478,  DOI: 10.1016/j.colsurfb.2015.09.056
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    15
    Synthesis of multifunctional bovine serum albumin microcapsules by the sonochemical method for targeted drug delivery and controlled drug release
    Li, Zhanfeng; Xu, Fengzhi; Li, Quanshun; Liu, Songfeng; Wang, Hongyan; Mohwald, Helmuth; Cui, Xuejun
    Colloids and Surfaces, B: Biointerfaces (2015), 136 (), 470-478CODEN: CSBBEQ; ISSN:0927-7765. (Elsevier B.V.)
    Multifunctional folic acid conjugated [email protected] microcapsules ([email protected] MCs) have been prepd. successfully based on the sonochem. method. The as-synthesized [email protected] MCs have a suitable size range for biomedical applications, and a high loading capacity for water-insol. drugs. Furthermore, [email protected] MCs show excellent magnetic properties. Cytotoxicity tests indicate that [email protected] MCs are non-toxic. Cellular uptake and flow cytometric assay illustrate together that [email protected] MCs can target tumor cells selectively through mol. targeted endocytosis. As carriers for water-insol. drugs, [email protected] MCs are also proved to possess superior redox- and thermo- dual responsiveness for controlled drug release.
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  16. 16
    Malcher, M.; Volodkin, D.; Heurtault, B.; André, P.; Schaaf, P.; Möhwald, H.; Voegel, J. C.; Sokolowski, A.; Ball, V.; Boulmedais, F.; Frisch, B. Embedded Silver Ions-Containing Liposomes in Polyelectrolyte Multilayers: Cargos Films for Antibacterial Agents. Langmuir 2008, 24, 10209– 10215,  DOI: 10.1021/la8014755
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    16
    Embedded Silver Ions-Containing Liposomes in Polyelectrolyte Multilayers: Cargos Films for Antibacterial Agents
    Malcher, Marta; Volodkin, Dmitry; Heurtault, Beatrice; Andre, Philippe; Schaaf, Pierre; Mohwald, Helmuth; Voegel, Jean-Claude; Sokolowski, Adam; Ball, Vincent; Boulmedais, Fouzia; Frisch, Benoit
    Langmuir (2008), 24 (18), 10209-10215CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    A new antibacterial coating made of poly(L-lysine)/hyaluronic acid (PLL/HA) multilayer films and liposome aggregates loaded with silver ions was designed. Liposomes filled with an AgNO3 soln. were first aggregated by the addn. of PLL in soln. The obtained micrometer-sized aggregates were then deposited on a PLL/HA multilayer film, playing the role of a spacer with the support. Finally, HA/PLL/HA capping layers were deposited on top of the architecture to form a composite AgNO3 coating. Release of encapsulated AgNO3 from this composite coating was followed and triggered upon temp. increase over the transition temp. of vesicles, found to be equal to 34 °C. After detn. of the minimal inhibitory concn. (MIC) of AgNO3 in soln., the antibacterial activity of the AgNO3 coating was investigated against Escherichia coli. A 4-log redn. in the no. of viable E. coli cells was obsd. after contact for 120 min with a 120 ng/cm2 AgNO3 coating. In comparison, no bactericidal activity was found for PLL/HA films previously dipped in an AgNO3 soln. and for PLL/HA films with liposome aggregates contg. no AgNO3 soln. The strong bactericidal effect could be linked to the diffusion of silver ions out of the AgNO3 coating, leading to an important bactericidal concn. close to the membrane of the bacteria. A simple method to prep. antibacterial coatings loaded with a high and controlled amt. of AgNO3 is therefore proposed. This procedure is far superior to that soaking AgNO3 or Ag nanoparticles into a coating. In principle, other small bactericidal chems. like antibiotics could be encapsulated by this method. This study opens a new route to modify surfaces with small solutes that are not permeating phospholipid membranes below the phase transition temp.
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  17. 17
    Sezer, M.; Feng, J. J.; Khoa Ly, H.; Shen, Y.; akanishi, T.; Kuhlmann, U.; Hildebrandt, P.; Möhwald, H.; Weidinger, I. M. Multi-Layer Electron Transfer Across Nanostructured Ag-SAM-Au-SAM Junctions Probed by Surface Enhanced Raman Spectroscopy. Phys. Chem. Chem. Phys. 2010, 12, 9822– 9829,  DOI: 10.1039/c003082a
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    17
    Multi-layer electron transfer across nanostructured Ag-SAM-Au-SAM junctions probed by surface enhanced Raman spectroscopy
    Sezer, Murat; Feng, Jiu-Ju; Khoa Ly, H.; Shen, Yanfei; Nakanishi, Takashi; Kuhlmann, Uwe; Hildebrandt, Peter; Moehwald, Helmuth; Weidinger, Inez M.
    Physical Chemistry Chemical Physics (2010), 12 (33), 9822-9829CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The authors have developed a new layered Au-Ag electrode for studying interfacial electron transfer processes by surface enhanced resonance Raman (SERR) spectroscopy. The device consists of a nanostructured Ag support which is sepd. from a Au film via a thin self-assembled monolayer (SAM) of amino-terminated mercaptanes (Cy-NH2, with y = 6, 8, 11). The Au film is biocompatibly coated to allow for binding of redox-active proteins. The authors have explored the performance of this device for analyzing interfacial electron transfer processes by stationary and time-resolved SERR spectroscopy, using the heme protein cytochrome c (Cyt-c) as a benchmark protein. The SERRS intensity of Cyt-c on Ag-(Cy-NH2)-Au electrodes and Ag electrodes was comparable when the protein was electrostatically attached to the metal coated by a SAM of carboxyl-terminated mercaptanes (Cx-COOH) surface but 25 times higher upon covalent attachment via Cys102 to the bare Au surface. In the case of electrostatic adsorption the protein remained exclusively in its native state. Electron transfer between the protein and the Ag electrode occurred in an almost ideal Nernstian behavior with a no. of transferred electrons close to one (n = 0.8-0.9). Conversely, the covalent attached Cyt c showed two broad redox transitions (n = 0.3) and a partial conversion to a nonnative species which remained redox inactive in the potential range from +0.1 to -0.3 V For the electrostatically immobilized Cyt, apparent electron transfer rates of 0.8 and 49 s-1 were obtained for y = 11 and x = 15 and 10, resp., indicating a fast long-distance electron transfer through the multilayer with the electron tunneling through the Cx-COOH SAM being the rate limiting step.
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  18. 18
    Shchukin, D. G.; Zheludkevich, M.; Yasakau, K.; Lamaka, S.; Ferreira, M. G. S.; Möhwald, H. Layer-by-Layer Assembled Nanocontainers for Self-Healing Corrosion Protection. Adv. Mater. 2006, 18, 1672– 1678,  DOI: 10.1002/adma.200502053
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    18
    Layer-by-layer assembled nanocontainers for self-healing corrosion protection
    Shchukin, Dmitry G.; Zheludkevich, Mikhail; Yasakau, Kiryl; Lamaka, Sviatlana; Ferreira, Mario G. S.; Moehwald, Helmuth
    Advanced Materials (Weinheim, Germany) (2006), 18 (13), 1672-1678CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The self-healing anticorrosion effect of layer-by-layer (LbL) assembled nano-reservoirs (polyelectrolyte-coated nano-particles) embedded in a hybrid coating deposited onto an Al alloy is investigated (see figure). The corrosion inhibitor, benzotriazole, is entrapped in the polyelectrolyte at the LbL assembly step; its release is initiated by local pH changes near the corrosion-damaged zones in the alloy. The nanoreservoirs increase long-term corrosion protection of the substrate and provide effective inhibitor storage and its prolonged release on demand.
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  19. 19
    Tong, W. J.; Gao, C. Y.; Möhwald, H. Single Polyelectrolyte Microcapsules Fabricated by Glutaraldehyde-Mediated Covalent Layer-by-Layer Assembly. Macromol. Rapid Commun. 2006, 27, 2078– 2083,  DOI: 10.1002/marc.200600533
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    19
    Single polyelectrolyte microcapsules fabricated by glutaraldehyde-mediated covalent layer-by-layer assembly
    Tong, Weijun; Gao, Changyou; Mohwald, Helmuth
    Macromolecular Rapid Communications (2006), 27 (24), 2078-2083CODEN: MRCOE3; ISSN:1022-1336. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Single polyelectrolyte component microcapsules and multilayers, exemplified by poly(allylamine hydrochloride) (PAH), have been prepd. using a method of glutaraldehyde (GA)-mediated covalent layer-by-layer (LbL) assembly. The GA crosslinking of the adsorbed PAH results in surfaces covered by reactive aldehyde groups, which can then react with PAH to result in another layer of covalently linked PAH. The repeated assembly of single polyelectrolyte in an LbL manner can be thus achieved. The PAH multilayers can grow linearly along with the layer no., and their thickness can be controlled at the nanometer scale, as verified by UV-vis absorption spectrometry and ellipsometry. Single polyelectrolyte microcapsules are obtained after removal of the template cores at low pH. The morphol. and integrity are confirmed by scanning force microscopy and confocal laser scanning microscopy.
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    Volodkin, D.; Skirtach, A.; Möhwald, H. Bioapplications of Light-Sensitive Polymer Films and Capsules Assembled Using the Layer-by-Layer Technique. Polym. Int. 2012, 61, 673– 679,  DOI: 10.1002/pi.4182
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    20
    Bioapplications of light-sensitive polymer films and capsules assembled using the layer-by-layer technique
    Volodkin, Dmitry; Skirtach, Andre; Moehwald, Helmuth
    Polymer International (2012), 61 (5), 673-679CODEN: PLYIEI; ISSN:0959-8103. (John Wiley & Sons Ltd.)
    A review. This review covers the area of bioapplications of layer-by-layer assembled polymer planar films and microcapsules-biol. relevant responses stimulated by light. We present recent progress in light-stimulated release of bioactive mols. from the assembled structures. Functionalization of the films with metal nanoparticles and bioactive mols. that provide the films with light sensitivity and reservoir properties is also described. The use of these films in the delivery of bioactive mols. is closely related to light-induced cargo release from capsules; this promising method of intracellular delivery is also addressed. Copyright © 2012 Society of Chem. Industry.
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    Ibarz, G.; Dähne, L.; Donath, E.; Möhwald, H. Smart Micro- and Nanocontainers for Storage, Transport, and Release. Adv. Mater. 2001, 13, 1324– 1327,  DOI: 10.1002/1521-4095(200109)13:17<1324::AID-ADMA1324>3.0.CO;2-L
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    21
    Smart micro- and nanocontainers for storage, transport, and release
    Ibarz, Gemma; Dahne, Lars; Donath, Edwin; Mohwald, Helmut
    Advanced Materials (Weinheim, Germany) (2001), 13 (17), 1324-1327CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH)
    There is no expanded citation for this reference.
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  22. 22
    Skirtach, A. G.; Munoz Javier, A.; Kreft, O.; Köhler, K.; Piera Alberola, A.; Möhwald, H.; Parak, W. J.; Sukhorukov, G. B. Laser-Induced Release of Encapsulated Materials Inside Living Cells. Angew. Chem., Int. Ed. 2006, 45, 4612– 4617,  DOI: 10.1002/anie.200504599
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    22
    Laser-induced release of encapsulated materials inside living cells
    Skirtach, Andre G.; Munoz Javier, Almudena; Kreft, Oliver; Kehler, Karen; Piera Alberola, Alicia; Moewald, Helmuth; Parak, Wolfgang J.; Sukhorukov, Gleb B.
    Angewandte Chemie, International Edition (2006), 45 (28), 4612-4617CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Escape route: The laser-initiated release of fluorescently labeled polymers from polyelectrolyte-multilayer microcapsules is demonstrated inside living cancer cells. A polymer is incorporated in capsules with metal nanoparticles in their walls, which serve as light-absorbing centers. The capsules are internalized by cells and near-IR light ruptures the walls of the capsules, thus releasing the content into the cells.
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    Skirtach, A. G.; Dejugnat, C.; Braun, D.; Susha, A. S.; Rogach, A. L.; Parak, W. J.; Möhwald, H.; Sukhorukov, G. B. The Role of Metal Nanoparticles in Remote Release of Encapsulated Materials. Nano Lett. 2005, 5, 1371– 1377,  DOI: 10.1021/nl050693n
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    23
    The role of metal nanoparticles in remote release of encapsulated materials
    Skirtach, Andre G.; Dejugnat, Christophe; Braun, Dieter; Susha, Andrei S.; Rogach, Andrey L.; Parak, Wolfgang J.; Moehwald, Helmuth; Sukhorukov, Gleb B.
    Nano Letters (2005), 5 (7), 1371-1377CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Laser mediated remote release of encapsulated fluorescently labeled polymers from nanoengineered polyelectrolyte multilayer capsules contg. gold sulfide core/gold shell nanoparticles in their walls is obsd. in real time on a single capsule level. We have developed a method for measuring the temp. increase and have quant. investigated the influence of absorption, size, and surface d. of metal nanoparticles using an anal. model. Exptl. measurements and numerical simulations agree with the model. The treatment presented in this work is of general nature, and it is applicable to any system where nanoparticles are used as absorbing centers. Potential biomedical applications are highlighted.
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  24. 24
    Beissenhirtz, M. K.; Scheller, F. W.; Stocklein, W. F. M.; Kurth, D. G.; Möhwald, H.; Lisdat, F. Electroactive Cytochrome c Multilayers within a Polyelectrolyte Assembly. Angew. Chem., Int. Ed. 2004, 43, 4357– 4360,  DOI: 10.1002/anie.200352804
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    Multilayer electrodes: Electroactive cytochrome c multilayers within a polyelectrolyte assembly
    Beissenhirtz, Moritz K.; Scheller, Frieder W.; Stocklein, Walter F. M.; Kurth, Dirk G.; Mohwald, Helmuth; Lisdat, Fred
    Angewandte Chemie, International Edition (2004), 43 (33), 4357-4360CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Electroactive cytochrome c multilayer assemblies were constructed within a polyelectrolyte network on a gold electrode. Up to 15 layers were electroactive, with a linear increase in redox active protein per layer. Evidence for a protein-protein electron transfer through the assembly was found (pathway marked by arrows in the picture).
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  25. 25
    Sukhorukov, G.; Dähne, L.; Hartmann, J.; Donath, E.; Möhwald, H. Controlled Precipitation of Dyes into Hollow Polyelectrolyte Capsules Based on Colloids and Biocolloids. Adv. Mater. 2000, 12, 112– 115,  DOI: 10.1002/(SICI)1521-4095(200001)12:2<112::AID-ADMA112>3.0.CO;2-P
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    Controlled precipitation of dyes into hollow polyelectrolyte capsules based on colloids and biocolloids
    Sukhorukov, Gleb; Dahne, Lars; Hartmann, Jurgen; Donath, Edwin; Mohwald, Helmuth
    Advanced Materials (Weinheim, Germany) (2000), 12 (2), 112-115CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH)
    Hollow polyelectrolyte capsules were recently introduced as a novel type of artificial smart material. Here the successful filling of polyelectrolyte capsules based on human erythrocytes with various dye compds. is reported using a novel method of selective pptn. Control of surface properties and conditions leads to preferential crystn. in the interior of the capsules.
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    Ibarz, G.; Dahne, L.; Donath, E.; Möhwald, H. Controlled Permeability of Polyelectrolyte Capsules via Defined Annealing. Chem. Mater. 2002, 14, 4059– 4062,  DOI: 10.1021/cm011300y
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    Controlled Permeability of Polyelectrolyte Capsules via Defined Annealing
    Ibarz, Gemma; Daehne, L.; Donath, Edwin; Moehwald, Helmut
    Chemistry of Materials (2002), 14 (10), 4059-4062CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The influence of thermal treatment on the permeability of polyelectrolyte multilayer capsules is studied via confocal fluorescence microscopy of individual microcapsules. For the specific most frequently used example, capsules made of poly(styrenesulfonic acid) and poly(allylamine hydrochloride), the penetration of fluorescein is shown to be reduced by 3 orders of magnitude on heating at 80 °C. The data show that holes formed during prepn. may be annealed in a predictable way.
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    Dähne, L.; Leporatti, S.; Donath, E.; Möhwald, H. Fabrication of Micro Reaction Cages with Tailored Properties. J. Am. Chem. Soc. 2001, 123, 5431– 5436,  DOI: 10.1021/ja002911e
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    Fabrication of micro reaction cages with tailored properties
    Dahne L; Leporatti S; Donath E; Mohwald H
    Journal of the American Chemical Society (2001), 123 (23), 5431-6 ISSN:0002-7863.
    Hollow polyelectrolyte capsules in micro- and submicrometer size were prepared. Their interior was functionalized by a "ship in bottle" synthesis of copolymers. While the monomers permeated the capsule wall easily, the formed polymers remained in the capsule cage. The physicochemical properties of the capsule interior such as ion strength, pH, light absorption, and fluorescence could be controlled independently from the surrounding solvent by means of the chemical nature of the captured polymer. In case of polyelectrolytes the osmotic pressure of the counterions led to a swelling of the capsules which can be important for micromechanics. The functionalization with light-sensitive materials allowed selective photoreactions inside the capsules. Synthesis of polyelectrolytes at high concentration resulted in an intertwining of the capsule wall with the polymer. The modified walls behaved like ion exchange membranes and showed selectivity toward adsorption and permeation of organic ions. The modified capsules offer many possibilities for novel applications as containers for controlled precipitation, as nanoreactors for catalyzed reactions, or as sensors.
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    Dai, Z.; Voigt, A.; Leporatti, S.; Donath, E.; Dähne, L.; Möhwald, H. Layer-by-Layer Self-Assembly of Polyelectrolyte and Low Molecular Weight Species into Capsules. Adv. Mater. 2001, 13, 1339– 1342,  DOI: 10.1002/1521-4095(200109)13:17<1339::AID-ADMA1339>3.0.CO;2-Q
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    Layer-by-layer self-assembly of polyelectrolyte and low molecular weight species into capsules
    Dai, Zhifei; Voigt, Andreas; Leporatti, Stefano; Donath, Edwin; Dahne, Lars; Mohwald, Helmuth
    Advanced Materials (Weinheim, Germany) (2001), 13 (17), 1339-1342CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH)
    There is no expanded citation for this reference.
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    Dai, Z. F.; Dähne, L.; Donath, E.; Möhwald, H. Mimicking Photosynthetic Two-Step Energy Transfer in Cyanine Triads Assembled into Capsules. Langmuir 2002, 18, 4553– 4555,  DOI: 10.1021/la0255222
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    Mimicking photosynthetic two-step energy transfer in cyanine triads assembled into capsules
    Dai, Zhifei; Daehne, Lars; Donath, Edwin; Moehwald, Helmuth
    Langmuir (2002), 18 (12), 4553-4555CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    A two-step energy transfer cascade was constructed in the wall of hollow microcapsules by means of defined assembly of three cyanine dyes, thiacyanine (antenna), thiacarbocyanine (receiver and antenna), and indodicarbocyanine (receiver), in the wall plane and perpendicular to it. The efficiency of the energy transfer steps has been measured by fluorescence spectroscopy. The unidirectional energy transfer perpendicular to the surface was smaller than for the lateral one. The antenna dyes on the outer surface of the capsules harvest the light energy and conduct it downhill by Forster resonance energy transfer to the capsule interior in analogy to chloroplasts in nature. However, the obtained efficiency is much smaller, though some improvement was reached by using J-aggregate-forming dyes.
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    Dai, Z.; Dähne, L.; Donath, E.; Möhwald, H. Downhill Energy Transfer via Ordered Multichromophores in Light-Harvesting Capsules. J. Phys. Chem. B 2002, 106, 11501– 11508,  DOI: 10.1021/jp0256490
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    Downhill Energy Transfer via Ordered Multichromophores in Light-Harvesting Capsules
    Dai, Zhifei; Daehne, Lars; Donath, Edwin; Moehwald, Helmuth
    Journal of Physical Chemistry B (2002), 106 (44), 11501-11508CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    A variety of light-harvesting capsules were successfully constructed by assembling polyelectrolyte complexes and antenna dyes of graduated absorption energy into nanoscale walls (thickness of a single layer around 2 nm). Two-step downhill energy transfer was demonstrated with absorption and fluorescence spectroscopy for the first time on light-harvesting capsules. Energy transfer occurs parallel to the capsule surface if the dyes are coadsorbed to the preassembled capsule wall. Energy transfer perpendicular to the surface was achieved by sequentially assembling the dyes in different layers of the wall. Although perpendicular energy transfer is less efficient because of longer intermol. distances, utilization of J-aggregates and optimization of dye combinations and dye ratios yielded an efficient energy cascade along three components. The light-harvesting capsules are analogous to the light reactions of photosynthesis in chloroplasts.
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    Dai, Z. F.; Möhwald, H.; Tiersch, B.; Dähne, L. Nanoengineering of Polymeric Capsules with a Shell-in-Shell Structure. Langmuir 2002, 18, 9533– 9538,  DOI: 10.1021/la026491d
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    Nanoengineering of Polymeric Capsules with a Shell-in-Shell Structure
    Dai, Zhifei; Moehwald, Helmuth; Tiersch, Brigitte; Daehne, Lars
    Langmuir (2002), 18 (24), 9533-9538CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Polymeric capsules possessing a shell-in-shell structure were fabricated by assembling layers of polyelectrolytes with interlayers of silica nanoparticles onto microspheres of melamine formaldehyde. Subsequently the cores were removed with hydrochloric acid and the silica layers with hydrofluoric acid. Removal of the silica layers leaves an aq. phase of a poly(allylamine hydrochloride) soln. between an outer and an inner polyelectrolyte shell. The interlayer thickness could be varied by the diam. of silica particles or by the no. of deposition steps. The double-shell structure of the microcapsules was detd. by scanning force microscopy, confocal laser scanning microscopy, and transmission electron microscopy on ultramicrotome sections in detail. In contrast to one-shell capsules of polyelectrolytes, the permeability for small mols. does not change with time and temp. In addn., the shell-in-shell capsules exhibit a remarkably higher mech. stability as shown by expts. of osmotically induced deformations.
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    Dai, Z.; Dähne, L.; Möhwald, H.; Tiersch, B. Novel Capsules with High Stability and Controlled Permeability by Hierarchic Templating. Angew. Chem., Int. Ed. 2002, 41, 4019– 4022,  DOI: 10.1002/1521-3773(20021104)41:21<4019::AID-ANIE4019>3.0.CO;2-Z
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    Novel capsules with high stability and controlled permeability by hierarchic templating
    Dai, Zhifei; Dahne, Lars; Mohwald, Helmuth; Tiersch, Brigitte
    Angewandte Chemie, International Edition (2002), 41 (21), 4019-4022CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Hollow capsules possessing either nanopores or sphere-in-sphere structures were prepd. by consecutive alternating adsorption of poly(styrenesulfonic acid) (PSS), polyallylamine.HCl (PAH), and silica on melamine formaldehyde resin cores followed by removal of the MF resin cores (HCl) and dissolving of SiO2 with HF. Obtained PSS/PAH capsules were characterized by scanning force microscopy (wall thickness), ultra-microtom transmission electron microscopy (morphol., pore size distribution), and confocal laser scanning microscopy (permeability).
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    Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Möhwald, H. Novel Hollow Polymer Shells by Colloid-Templated Assembly of Polyelectrolytes. Angew. Chem., Int. Ed. 1998, 37, 2201– 2205,  DOI: 10.1002/(SICI)1521-3773(19980904)37:16<2201::AID-ANIE2201>3.3.CO;2-5
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    Novel Hollow Polymer Shells by Colloid-Templated Assembly of Polyelectrolytes
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    Exact control of the film thickness of polyelectrolyte shells (a transmission electron microscopy image is shown) is achieved by colloid-templated consecutive adsorption of polyanions and polycations followed by decomposition of the templating core. Possible areas of application for these shells range from the pharmaceutical, food, cosmetic, and paint industries to catalysis and microcrystallization.
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    Möbius, D.; Möhwald, H. Structural Characterization of Monolayers at the Air-Water Interface. Adv. Mater. 1991, 3, 19– 24,  DOI: 10.1002/adma.19910030104
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    Kotov, N. A.; Dekany, I.; Fendler, J. H. Ultrathin Graphite Oxide-Polyelectrolyte Composites Prepared by Self-Assembly: Transition Between Conductive and Non-Conductive States. Adv. Mater. 1996, 8, 637– 641,  DOI: 10.1002/adma.19960080806
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    Ultrathin graphite oxide-polyelectrolyte composites prepared by self-assembly. Transition between conductive and non-conductive states
    Kotov, Nicholas A.; Dekany, Imre; Fendler, Janos H.
    Advanced Materials (Weinheim, Germany) (1996), 8 (8), 637-641CODEN: ADVMEW; ISSN:0935-9648. (VCH)
    The prepn. and characterization of ultrathin films composed of self-assembled graphite oxide (GO) platelets and polyelectrolytes (P = poly(diallyldimethylammonium chloride)) and their subsequent in-situ redn. to a graphite-like state (G) by chem. and electrochem. methods was investigated. First the construction of a P/GO self-assembled sandwich unit is described. The process of self assembly of the successive P/GO layers onto a quartz slide was monitored by absorption spectroscopy. Surface plasmon spectroscopy allowed layer-by-layer monitoring of the build-up of the (P/GO) sandwich units. At. force microscopy images revealed the height of each GO sheet to be between 18 and 28 Å, the self-assembled film being composed of 2-3 GO sheets. The x-ray diffraction data of the (P/GO)30 film are given. The chem. redn. of the GO in the (P/GO)n films was accomplished by aq. N2H4 or by a Zn/HCl mixt. The electrochem. redn. was performed by scanning the potential from +0.7 to -1.5 V vs. SCE monitored by cyclic voltammetry. The peak at 0.95 ± 0.05 V corresponded to the redn. of GO to G. The lateral resistivity of the (P/GO)10 film was investigated. The effect of the non-conductive state transition was also obsd. in photoelectrochem. measurements of films into which CdS semiconductor nanoparticles were incorporated.
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    Tang, Z.; Kotov, N. A.; Magonov, S.; Ozturk, B. Nanostructured Artificial Nacre. Nat. Mater. 2003, 2, 413– 418,  DOI: 10.1038/nmat906
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    Nanostructured artificial nacre
    Tang, Zhiyong; Kotov, Nicholas A.; Magonov, Sergei; Ozturk, Birol
    Nature Materials (2003), 2 (6), 413-418CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Finding a synthetic pathway to artificial analogs of nacre and bones represents a fundamental milestone in the development of composite materials. The ordered brick-and-mortar arrangement of org. and inorg. layers is believed to be the most essential strength- and toughness-detg. structural feature of nacre. It has also been found that the ionic crosslinking of tightly folded macromols. is equally important. Here, we demonstrate that both structural features can be reproduced by sequential deposition of polyelectrolytes and clays. This simple process results in a nanoscale version of nacre with alternating org. and inorg. layers. The macromol. folding effect reveals itself in the unique saw-tooth pattern of differential stretching curves attributed to the gradual breakage of ionic crosslinks in polyelectrolyte chains. The tensile strength of the prepd. multilayers approached that of nacre, whereas their ultimate Young modulus was similar to that of lamellar bones. Structural and functional resemblance makes clay- polyelectrolyte multilayers a close replica of natural biocomposites. Their nanoscale nature enables elucidation of mol. processes occurring under stress.
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    Kotov, N. A.; Putyera, K.; Fendler, J. H.; Tombácz, E.; Dékány, I. Cadmium Sulfide Particles in Organomontmorillonite Complexes. Colloids Surf., A 1993, 71, 317– 326,  DOI: 10.1016/0927-7757(93)80047-I
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    Cadmium sulfide particles in organomontmorillonite complexes
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    Colloids and Surfaces, A: Physicochemical and Engineering Aspects (1993), 71 (3), 317-26CODEN: CPEAEH; ISSN:0927-7757.
    CdS particles were generated by the infusion of H2S into Cd-salt-contg. organomontmorillonite complexes dispersed in EtOH-H2O, -C6H6, and -cyclohexane mixts. Montmorillonites ion exchanged to different extents by octadecylammonium and hexadecylpyridinium salts were used as organomontmorillonite complexes. CdS formation was examd. as a function of the extent of surfactants present in the organomontmorillonite complexes, the compn. of the solvents, and the concn. and type of Cd salts used. Absorption and emission spectrophotometry, DTA, dynamic laser light scattering and x-ray diffractometry were used for monitoring CdS formation.
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    Kotov, N. A.; Meldrum, F. C.; Fendler, J. H. Monoparticulate Layers of Titanium Dioxide Nanocrystallites with Controllable Interparticle Distances. J. Phys. Chem. 1994, 98, 8827– 8830,  DOI: 10.1021/j100087a002
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    Monoparticulate Layers of Titanium Dioxide Nanocrystallites with Controllable Interparticle Distances
    Kotov, Nicholas A.; Meldrum, Fiona C.; Fendler, Janos H.
    Journal of Physical Chemistry (1994), 98 (36), 8827-30CODEN: JPCHAX; ISSN:0022-3654.
    The arrested hydrolysis of Ti(iso-PrO)4 by millimolar concns. of H2O in a mixt. of CHCl3 and PrOH in the presence of CTAB and Me4NOH resulted in the formation of stable dispersions (sols) of nanocryst. (18-22 Å diam.) TiO2. Spreading the sols on H2O surfaces in a Langmuir film balance produced monoparticulate TiO2 films. Heat treatment of a given sol prior to dispersion on the aq. subphase resulted in nanocryst. monoparticulate TiO2 films with reduced interparticulate distances.
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    Kotov, N. A.; Meldrum, F. C.; Wu, C.; Fendler, J. H. Monoparticulate Layer and Langmuir-Blodgett-Type Multiparticulate Layers of Size-Quantized Cadmium Sulfide Clusters: A Colloid-Chemical Approach to Superlattice Construction. J. Phys. Chem. 1994, 98, 2735– 2738,  DOI: 10.1021/j100062a006
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    Monoparticulate Layer and Langmuir-Blodgett-Type Multiparticulate Layers of Size-Quantized Cadmium Sulfide Clusters: A Colloid-Chemical Approach to Superlattice Construction
    Kotov, N. A.; Meldrum, F. C.; Wu, C.; Fendler, J. H.
    Journal of Physical Chemistry (1994), 98 (11), 2735-8CODEN: JPCHAX; ISSN:0022-3654.
    Monoparticulate layers of dodecylbenzenesulfonic acid-stabilized, 34.0, 29.4, and 26.5 Å diam. CdS clusters were formed at air-H2O interfaces in a Langmuir film balance. Surface pressure vs. surface area isotherms and Brewster angle microscopy demonstrated that increasing surface pressure resulted in transitions from well-sepd. to well-compressed monoparticulate layers and, ultimately, to multiparticulate layers. The CdS particles with diams. of 34.0, 29.4, and 26.5 Å form films with crit. areas per particle (conceptually analogous to headgroup areas) of 1100, 800, and 650 Å2, resp., which correspond well to values calcd. by assuming hexagonal close packing of hard spheres (887, 749, and 608 Å2). Sequential transfer to solid substrates (by the Langmuir-Blodgett (LB) technique) led to the formation of multiparticulate layers of size-quantized CdS clusters. Absorbances of CdS in the LB film increase linearly with the no. of layers transferred, substantiating the uniformity of deposition. The position of the absorption edge was invariant between the CdS particles in CHCl3 soln. and in LB films. Fluorescence max. of the CdS particles, however, shifted from 510 nm in CHCl3 to 455 nm in LB films.
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    Motte, L.; Billoudet, F.; Lacaze, E.; Pileni, M. P. Self-Organization of Size-Selected, Nanoparticles into Three-Dimensional Superlattices. Adv. Mater. 1996, 8, 1018– 1020,  DOI: 10.1002/adma.19960081218
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    Self-organization of size-selected nanoparticles into three-dimensional superlattices
    Motte, Laurence; Billoudet, Francoise; Lacaze, Emanuelle; Pileni, Marie Paule
    Advanced Materials (Weinheim, Germany) (1996), 8 (12), 1018-1020CODEN: ADVMEW; ISSN:0935-9648. (VCH)
    The formation, using reverse micelles, and self-organization of Ag2S nanocrystallites differing in their size (3, 4, 6 nm) is reported. Examn. of the crystallites by TEM and AFM in tapping mode revealed 3-dimensional quantum dot superlattices with a face-centered cubic structure. The superlattice was still present after several months. The size distribution could be narrowed by chem. treatment and particle extn. from the micelles.
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    Yonezawa, T.; Matsune, H.; Kunitake, T. Layered Nanocomposite of Close-Packed Gold Nanoparticles and TiO2 Gel Layers. Chem. Mater. 1999, 11, 33– 35,  DOI: 10.1021/cm980687a
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    Layered nanocomposite of close-packed gold nanoparticles and TiO2 gel layers
    Yonezawa, Tetsu; Matsune, Hideki; Kunitake, Toyoki
    Chemistry of Materials (1999), 11 (1), 33-35CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Nanocomposites consisting of Au nanoparticles and TiO2 ultrathin layers were prepd. using a combination of layer-by-layer assembly and a surface sol-gel process. The Au nanoparticles stabilized by a monolayer of bis(11-hydroxyundecyl) disulfide were adsorbed on the surface of the TiO2 gel and close-packed monoparticulate layers were formed. Multilayer films of TiO2/Au nanoparticles were also prepd. The samples were characterized by SEM and UV-vis spectroscopy.
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    Berndt, P.; Kurihara, K.; Kunitake, T. Adsorption of Poly(styrenesulfonate) onto an Ammonium Monolayer on Mica: A Surface Forces Study. Langmuir 1992, 8, 2486– 2490,  DOI: 10.1021/la00046a022
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    Adsorption of poly(styrenesulfonate) onto an ammonium monolayer on mica: a surface forces study
    Berndt, Peter; Kurihara, Kazue; Kunitake, Toyoki
    Langmuir (1992), 8 (10), 2486-90CODEN: LANGD5; ISSN:0743-7463.
    The adsorption of Na poly(styrenesulfonate) to a cationic monolayer surface was studied by means of direct surface forces measurement and surface pressure-area isotherms. The expanded state of the monolayer at the air-water interface is considerably reduced in the presence of the polymer in the subphase. Max. contraction of the surface monolayer is attained at a polymer concn. ∼1 mg/L, or 5 × 10-6 M. The surface forces between the charged surfaces decreases by >1 order of magnitude at this polymer concn. This indicates nearly complete neutralization of the surface by adsorption of poly(styrenesulfonate). This fact and the absence of structural forces in the vicinity of the surface point to flat adsorption of the polymer onto the surface.
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    Kilper, S.; Facey, S. J.; Burghard, Z.; Hauer, B.; Rothenstein, D.; Bill, J. Macroscopic Properties of Biomimetic Ceramics Are Governed by the Molecular Recognition at the Bioorganic-Inorganic Interface. Adv. Funct. Mater. 2018, 28, 1705842,  DOI: 10.1002/adfm.201705842
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    Cui, J.; Richardson, J. J.; Bjornmalm, M.; Faria, M.; Caruso, F. Nanoengineered Templated Polymer Particles: Navigating the Biological Realm. Acc. Chem. Res. 2016, 49, 1139– 1148,  DOI: 10.1021/acs.accounts.6b00088
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    Nanoengineered Templated Polymer Particles: Navigating the Biological Realm
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    Accounts of Chemical Research (2016), 49 (6), 1139-1148CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    Nanoengineered materials offer tremendous promise for developing the next generation of therapeutics. We are transitioning from simple research questions, such as "can this particle eradicate cancer cells" to more sophisticated ones like "can we design a particle to preferentially deliver cargo to a specific cancer cell type" These developments are poised to usher in a new era of nanoengineered drug delivery systems.We primarily work with templating methods for engineering polymer particles and investigate their biol. interactions. Templates are scaffolds that facilitate the formation of particles with well-controlled size, shape, structure, stiffness, stability, and surface chem. In the past decade, breakthroughs in engineering new templates, combined with advances in coating techniques, including layer-by-layer (LbL) assembly, surface polymn., and metal-phenolic network (MPN) coordination chem., have enabled particles with specific physicochem. properties to be engineered. While materials science offers an ever-growing no. of new synthesis techniques, a central challenge of therapeutic delivery has become understanding how nanoengineered materials interact with biol. systems. Increased collaboration between chemists, biologists, and clinicians has resulted in a vast research output on bio-nano interactions. Our understanding of cell-particle interactions has grown considerably, but conventional in vitro experimentation provides limited information, and understanding how to bridge the in vitro/in vivo gap is a continuing challenge. As has been demonstrated in other fields, there is now a growing interest in applying computational approaches to advance this area. A considerable knowledge base is now emerging, and with it comes new and exciting opportunities that are already being capitalized on through the translation of materials into the clinic.In this Account, we outline our perspectives gained from a decade of work at the interface between polymer particle engineering and bio-nano interactions. We divide our research into three areas: (i) biotrafficking, including cellular assocn., intracellular transport, and biodistribution; (ii) biodegrdn. and how to achieve controlled, responsive release of therapeutics; and (iii) applications, including drug delivery, controlling immunostimulatory responses, biosensing, and microreactors. There are common challenges in these areas for groups developing nanoengineered therapeutics.A key "lesson-learned" has been the considerable challenge of staying informed about the developments relevant to this field. There are a no. of reasons for this, most notably the interdisciplinary nature of the work, the large nos. of researchers and research outputs, and the limited standardization in technique nomenclature. Addnl., a large body of work is being generated with limited central archiving, other than vast general databases. To help address these points, we have created a web-based tool to organize our past, present, and future work [Bio-nano research knowledgebase, http://bionano.eng.unimelb.edu.au/knowledge_base/ (accessed May 2, 2016)]. This tool is intended to serve as a first step toward organizing results in this large, complex area. We hope that this will inspire researchers, both in generating new ideas and also in collecting, collating, and sharing their experiences to guide future research.
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    Richardson, J. J.; Björnmalm, M.; Caruso, F. Technology-Driven Layer-by-Layer Assembly of Nanofilms. Science 2015, 348, aaa2491  DOI: 10.1126/science.aaa2491
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    Peyratout, C. S.; Dähne, L. Tailor-Made Polyelectrolyte Microcapsules: From Multilayers to Smart Containers. Angew. Chem., Int. Ed. 2004, 43, 3762– 3783,  DOI: 10.1002/anie.200300568
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    Tailor-made polyelectrolyte microcapsules: From multilayers to smart containers
    Peyratout, Claire S.; Daehne, Lars
    Angewandte Chemie, International Edition (2004), 43 (29), 3762-3783CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. This review addresses the fabrication and properties of novel polyelectrolyte microcapsules, with an emphasis on their mech. and permeability properties. Ease of prepn. through layer-by-layer self assembly, accurate control over wall thickness as well as flexibility in the choice of constituents make these capsules very promising for numerous applications in materials and life science. Moreover, by engineering the inner and outer interfaces, these capsules can be used as microreactors for pptn., crystn., and polymn. reactions, as well as enzymic, and heterogeneous catalysis.
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    Dähne, L.; Peyratout, C. Nanoengineered Capsules with Specific Layer Structures. Dekker Encyclopedia of Nanoscience and Nanotechnology, 2nd ed. six volume set (print version); CRC Press: Boca Raton, FL, 2004; pp 2355– 2367.
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    Möhwald, H.; Lichtenfeld, H.; Moya, S.; Voigt, A.; Sukhorukov, G.; Leporatti, S.; Dähne, L.; Antipov, A.; Gao, C. Y.; Donath, E. From Polymeric Films to Nanocapsules. In Adsorption and Aggregation of Surfactants in Solution; Mittal, K. L., Shah, D. O., Eds.; Marcel Dekker: New York, 2002; Vol. 132, pp 485– 490.
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    Lisdat, F. Trends in the Layer-by-Layer Assembly of Redox Proteins and Enzymes in Bioelectrochemistry. Curr. Opin. Electrochem. 2017, 5, 165– 172,  DOI: 10.1016/j.coelec.2017.09.002
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    Trends in the layer-by-layer assembly of redox proteins and enzymes in bioelectrochemistry
    Lisdat, Fred
    Current Opinion in Electrochemistry (2017), 5 (1), 165-172CODEN: COEUCY; ISSN:2451-9111. (Elsevier B.V.)
    The mini-review will shortly introduce into the different principles of layer-by-layer deposition and the approaches used for the communication of biomols. within such architectures with electrodes. Recent developments in combining the different basic deposition principles will be shown. Here very often nanomaterials such as carbon nanotubes or graphene have increasingly been used. In order to ensure communication of enzymes in several layers with the electrode often redox polymers and, to a less extent, conducting polymers have been applied, but also the redox protein cyt c has been exploited as electron shuttle-deposited in a layered fashion. Communication to membrane proteins can be achieved via hydrophobic shuttle mols., such as quinones, when lipid pathways are created within the structure. Besides enzymic catalysis also photobiocatalysis has become increasingly in the focus.
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    De Geest, B. G.; Sukhorukov, G. B.; Möhwald, H. The Pros and Cons of Polyelectrolyte Capsules in Drug Delivery. Expert Opin. Drug Delivery 2009, 6, 613– 624,  DOI: 10.1517/17425240902980162
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    The pros and cons of polyelectrolyte capsules in drug delivery
    De Geest, Bruno G.; Sukhorukov, Gleb B.; Moehwald, Helmuth
    Expert Opinion on Drug Delivery (2009), 6 (6), 613-624CODEN: EODDAW; ISSN:1742-5247. (Informa Healthcare)
    A review. Polyelectrolyte multilayer microcapsules and nanocapsules are under review as multifunctional delivery systems. Tailoring functions in the entity of a single capsule is done by incorporation of functional polyelectrolytes or nanoparticles in between the layers during electrostatic self-assembly. The resulting capsules possess different properties such as controlled and triggered release, responsiveness to temp., pH and light and could be navigated with a magnetic field. A variety of substances can be encapsulated and delivered to cells and tissues. Potential applications as well as in vivo expts. have recently been explored. Capsules made of biodegradable polymers showed low toxicity in vivo. Perspectives on and obstacles to a way of broader application are discussed.
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    Delcea, M.; Möhwald, H.; Skirtach, A. G. Stimuli-Responsive LbL Capsules and Nanoshells for Drug Delivery. Adv. Drug Delivery Rev. 2011, 63, 730– 747,  DOI: 10.1016/j.addr.2011.03.010
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    Stimuli-responsive LbL capsules and nanoshells for drug delivery
    Delcea, Mihaela; Moehwald, Helmuth; Skirtach, Andre G.
    Advanced Drug Delivery Reviews (2011), 63 (9), 730-747CODEN: ADDREP; ISSN:0169-409X. (Elsevier B.V.)
    Review of basic principles and recent developments in the area of stimuli responsive polymeric capsules and nanoshells formed via layer-by-layer (LbL) is presented. The most essential attributes of the LbL approach are multifunctionality and responsiveness to a multitude of stimuli. The stimuli can be logically divided into 3 categories: phys. (light, elec., magnetic, ultrasound, mech., and temp.), chem. (pH, ionic strength, solvent, and electrochem.) and biol. (enzymes and receptors). Using these stimuli, numerous functionalities of nanoshells were demonstrated: encapsulation, release including that inside living cells or in tissue, sensors, enzymic reactions, enhancement of mech. properties, and fusion. This review describes mechanisms and basic principles of stimuli effects, describes progress in the area, and gives an outlook on emerging trends such as theranostics and nanomedicine.
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    Zhuk, A.; Sukhishvili, S. A. Stimuli-Responsive Layer-by-Layer Nanocomposites. Soft Matter 2013, 9, 5149– 5154,  DOI: 10.1039/c3sm00071k
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    Stimuli-responsive layer-by-layer nanocomposites
    Zhuk, Aliaksandr; Sukhishvili, Svetlana A.
    Soft Matter (2013), 9 (21), 5149-5154CODEN: SMOABF; ISSN:1744-683X. (Royal Society of Chemistry)
    A review. The fast-growing area of materials prepd. with the layer-by-layer (LbL) technique has become firmly established over the last three decades. Responsive LbL assemblies present one attractive type of LbL materials, useful in sensing, controlled delivery and actuation. However, stimuli-responsive LbL films made exclusively from polymers or org. mols. are somewhat restricted in the types of responses that can be applied to a material, and often suffer from poor mech. strength and limited durability. Assembly of inorg. nanoparticles (INPs), including spherical nanoparticles, nanorods, nanotubes, and nanosheets, among others, within multilayer films brings addnl. benefits by not only improving mech., electronic, magnetic and optical properties of these materials, but also endowing them with the capability to transform external signals and transfer them to other film components. Combining INPs with temp.-, pH-, or light-responsive polymers within LbL films yields novel materials with unique characteristics and properties easily controllable by external stimuli. In this paper, we discuss strategies for designing various types of INP-org. LbL assemblies, the nature of their stimuli response and potential applications.
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    Hua, F.; Cui, T.; Lvov, Y. M. Ultrathin Cantilevers Based on Polymer-Ceramic Nanocomposite Assembled through Layer-by-Layer Adsorption. Nano Lett. 2004, 4, 823– 825,  DOI: 10.1021/nl0498031
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    Ultrathin cantilevers based on polymer-ceramic nanocomposite assembled through layer-by-layer adsorption
    Hua, Feng; Cui, Tianhong; Lvov, Yuri M.
    Nano Letters (2004), 4 (5), 823-825CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    An ultrathin cantilever based on nanoorganized film from six alternating monolayers of clay plates, polycations, and magnetite nanoparticles was developed. The 170-nm thick clay-polymer-magnetite cantilever is a magnetic free-standing microstrip with a root anchored on a substrate. Hundreds of such cantilevers can be produced on a solid substrate within one processing cycle and, therefore, to provide sensor arrays or an adjustable surface with synchronized cantilever movements using an external magnetic field.
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    Kang, S.; Wang, L.; Zhang, J.; Du, J.; Li, M.; Chen, Q. Electroreductive Coupling Layer-by-Layer Assembly. ACS Appl. Mater. Interfaces 2017, 9, 32179– 32183,  DOI: 10.1021/acsami.7b10917
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    Electroreductive coupling layer-by-layer assembly
    Kang, Shusen; Wang, Lei; Zhang, Jian; Du, Jia; Li, Mao; Chen, Qidai
    ACS Applied Materials & Interfaces (2017), 9 (37), 32179-32183CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Rapid and covalent layer-by-layer (LbL) assembly is of significance for practical applications because of superior chem. and mech. stability. The electrochem. LbL assembly via an accelerating trigger can be automated and programmable in response to elec. signals to in situ fabricate covalently layered thin films with chem. and mech. stability. In this paper, electroreductive coupling layer-by-layer assembly is introduced as both covalent and rapid methodol. for prepg. layered thin films. This assembly is triggered by C-C coupling of peripheral alkynyls, which have own absorption below 300 nm and can transform to optical and elec. inert double/single or triple/single alternative bonding formations significantly without optical or elec. alternations of desirable photoelec. building blocks, superior to other linkers among covalent LbL assemblies. Not limited to fabrication of optical thin films, this assembly is readily available for oxygen sensitive substrates or materials and also a powerful addn. to electrooxidative coupling LbL assembly for developing the economically dynamoelec. LbL machines without moving or changing exptl. gears.
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    Ma, Y.; Zhang, Y.; Wu, B.; Sun, W.; Li, Z.; Sun, J. Polyelectrolyte Multilayer Films for Building Energetic Walking Devices. Angew. Chem. 2011, 123, 6378– 6381,  DOI: 10.1002/ange.201101054
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    Podsiadlo, P.; Kaushik, A. K.; Shim, B. S.; Agarwal, A.; Tang, Z.; Waas, A. M.; Arruda, E. M.; Kotov, N. A. Can Nature’s Design be Improved Upon? High Strength, Transparent Nacre-Like Nanocomposites with Double Network of Sacrificial Cross Links. J. Phys. Chem. B 2008, 112, 14359– 14363,  DOI: 10.1021/jp801492n
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    Can Nature's Design be Improved Upon? High Strength, Transparent Nacre-Like Nanocomposites with Double Network of Sacrificial Cross Links
    Podsiadlo, Paul; Kaushik, Amit K.; Shim, Bong Sup; Agarwal, Ashish; Tang, Zhiyong; Waas, Anthony M.; Arruda, Ellen M.; Kotov, Nicholas A.
    Journal of Physical Chemistry B (2008), 112 (46), 14359-14363CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    The prepn. of a high-strength and highly transparent nacre-like nanocomposite via layer-by-layer assembly technique from poly(vinyl alc.) (PVA) and Na+-montmorillonite clay nanosheets is reported in this article. We show that a high d. of weak bonding interactions between the polymer and the clay particles: hydrogen, dipole-induced dipole, and van der Waals undergoing break-reform deformations, can lead to high strength nanocomposites: σUTS ∼ 150 MPa and E' ∼ 13 GPa. Further introduction of ionic bonds into the polymeric matrix creates a double network of sacrificial bonds which dramatically increases the mech. properties: σUTS ∼ 320 MPa and E' ∼ 60 GPa.
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    Schlenoff, J. B.; Dubas, S. T.; Farhat, T. Sprayed Polyelectrolyte Multilayers. Langmuir 2000, 16, 9968– 9969,  DOI: 10.1021/la001312i
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    Sprayed Polyelectrolyte Multilayers
    Schlenoff, Joseph B.; Dubas, Stephan T.; Farhat, Tarek
    Langmuir (2000), 16 (26), 9968-9969CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Polyelectrolyte multilayers deposited by sequential spraying of poly(styrene sulfonate) and poly(diallyldimethylammonium) solns. are described. A highly uniform thin film is rapidly obtained over a large area. The morphol., uniformity, and chem. compn. of sprayed multilayers, and the selective membrane properties, are virtually identical to those prepd. by dip-immersion.
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    Krogman, K. C.; Zacharia, N. S.; Schroeder, S.; Hammond, P. T. Automated Process for Improved Uniformity and Versatility of Layer-by-Layer Deposition. Langmuir 2007, 23, 3137– 3141,  DOI: 10.1021/la063085b
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    Automated process for improved uniformity and versatility of layer-by-layer deposition
    Krogman, K. C.; Zacharia, N. S.; Schroeder, S.; Hammond, P. T.
    Langmuir (2007), 23 (6), 3137-3141CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The recently developed practice of spraying polyelectrolyte solns. onto a substrate in order to construct thin films via the layer-by-layer technique has been further investigated and extended. Here we describe a fully automated system capable of depositing thin polymer films from atomized mists of solns. contg. species of complementary functionality. Film growth is shown to be similar to that in conventional "dipped" LbL assembly, whereas the reported technol. allows us to realize 25-fold decreases in process times. Furthermore, complete automation removes human interaction and the possibility of operator-induced nonuniformities. We extend the versatility of the spray LbL technol. by depositing both weak and strong polyelectrolyte films, hydrogen-bonded films, and dendritic compds. and nanoparticles, broadening its range of future applications. Finally, the technol. is used to uniformly coat an otherwise hydrophobic substrate from aq. solns. ESEM images indicate that the atomization process produces a conformal coating of individual nanofibers within the substrate, dramatically changing the hydrophilicity of the macroscopic surface. Such an automated system is easily converted to an array of nozzle banks and could find application in the rapid, uniform coating of large areas of textile materials.
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    Krogman, K. C.; Lowery, J. L.; Zacharia, N. S.; Rutledge, G. C.; Hammond, P. T. Spraying Asymmetry into Functional Membranes Layer-by-Layer. Nat. Mater. 2009, 8, 512– 518,  DOI: 10.1038/nmat2430
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    Spraying asymmetry into functional membranes layer-by-layer
    Krogman, Kevin C.; Lowery, Joseph L.; Zacharia, Nicole S.; Rutledge, Gregory C.; Hammond, Paula T.
    Nature Materials (2009), 8 (6), 512-518CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    As engineers strive to mimic the form and function of naturally occurring materials with synthetic alternatives, the challenges and costs of processing often limit creative innovation. Here we describe a powerful yet economical technique for developing multiple coatings of different morphologies and functions within a single textile membrane, enabling scientists to engineer the properties of a material from the nanoscopic level in com. viable quantities. By simply varying the flow rate of charged species passing through an electrospun material during spray-assisted layer-by-layer deposition, individual fibers within the matrix can be conformally functionalized for ultrahigh-surface-area catalysis, or bridged to form a networked sublayer with complimentary properties. Exemplified here by the creation of selectively reactive gas purifn. membranes, the myriad applications of this technol. also include self-cleaning fabrics, water purifn. and protein functionalization of scaffolds for tissue engineering.
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    Lefort, M.; Popa, G.; Seyrek, E.; Szamocki, R.; Felix, O.; Hemmerlé, J.; Vidal, L.; Voegel, J. C.; Boulmedais, F.; Decher, G.; Schaaf, P. Spray-on Organic/Inorganic Films: a General Method for the Formation of Functional Nano- to Microscale Coatings. Angew. Chem., Int. Ed. 2010, 49, 10110– 10113,  DOI: 10.1002/anie.201002729
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    Spray-On Organic/Inorganic Films: A General Method for the Formation of Functional Nano- to Microscale Coatings
    Lefort, Mathias; Popa, Gabriela; Seyrek, Emek; Szamocki, Rafael; Felix, Olivier; Hemmerle, Joseph; Vidal, Loic; Voegel, Jean-Claude; Boulmedais, Fouzia; Decher, Gero; Schaaf, Pierre
    Angewandte Chemie, International Edition (2010), 49 (52), 10110-10113, S10110/1-S10110/21CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    This paper presents surface coatings prepd. by "simultaneous spray coating of interacting species" (SSClS). This process results in a fast reaction between the complementary components on a macroscopic surface, hence leading to a continuous and gradual buildup of a coating, the thickness of which is controlled by the spraying time, while the solvent and excess material are removed by drainage. This one-step deposition process is extremely user-friendly and allows the prepn. of very homogeneous films, which show optical interference colors over large areas. This property is a prerequisite for the fabrication of for example, optical devices.
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    Richardson, J. J.; Ejima, H.; Lorcher, S. L.; Liang, K.; Senn, P.; Cui, J.; Caruso, F. Preparation of Nano- and Microcapsules by Electrophoretic Polymer Assembly. Angew. Chem., Int. Ed. 2013, 52, 6455– 6458,  DOI: 10.1002/anie.201302092
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    Preparation of Nano- and Microcapsules by Electrophoretic Polymer Assembly
    Richardson, Joseph J.; Ejima, Hirotaka; Loercher, Samuel L.; Liang, Kang; Senn, Philipp; Cui, Jiwei; Caruso, Frank
    Angewandte Chemie, International Edition (2013), 52 (25), 6455-6458CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We have introduced an Electrophoretic Polymer Assembly (EPA) method for the coating of particles with polymer multilayers and for the prepn. of polymer nano- and microcapsules. Agarose acts as a natural, simple to use, thermally responsive, and safe immobilizing microfluidic hydrogel crucial for the prepn. of core-shell particles and capsules by EPA. EPA is a versatile technique that holds promise as an efficient, potentially automatable method compatible with a diverse range of polymers and particle sizes. Charged biopolymers, covalently coupled polymers, and hydrogen-bonding polymers yielded stable capsules when layered; thus, this system preserves the versatility inherent to conventional LbL assembly with respect to the polymers that can be used. Importantly, layering on immobilized particles is not necessarily limited to electrophoretic deposition or the polymers, buffers, and particle-recovery method used in the reported expts.
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    Ejima, H.; Richardson, J. J.; Liang, K.; Best, J. P.; Van Koeverden, M. P.; Such, G. K.; Cui, J.; Caruso, F. One-Step Assembly of Coordination Complexes for Versatile Film and Particle Engineering. Science 2013, 341, 154– 157,  DOI: 10.1126/science.1237265
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    One-Step Assembly of Coordination Complexes for Versatile Film and Particle Engineering
    Ejima, Hirotaka; Richardson, Joseph J.; Liang, Kang; Best, James P.; van Koeverden, Martin P.; Such, Georgina K.; Cui, Jiwei; Caruso, Frank
    Science (Washington, DC, United States) (2013), 341 (6142), 154-157CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    The development of facile and versatile strategies for thin-film and particle engineering is of immense scientific interest. However, few methods can conformally coat substrates of different compn., size, shape, and structure. We report the one-step coating of various interfaces using coordination complexes of natural polyphenols and Fe(III) ions. Film formation is initiated by the adsorption of the polyphenol and directed by pH-dependent, multivalent coordination bonding. Aq. deposition is performed on a range of planar as well as inorg., org., and biol. particle templates, demonstrating an extremely rapid technique for producing structurally diverse, thin films and capsules that can disassemble. The ease, low cost, and scalability of the assembly process, combined with pH responsiveness and negligible cytotoxicity, makes these films potential candidates for biomedical and environmental applications.
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    Dierendonck, M.; De Koker, S.; Cuvelier, C.; Grooten, J.; Vervaet, C.; Remon, J. P.; De Geest, B. G. Facile Two-Step Synthesis of Porous Antigen-Loaded Degradable Polyelectrolyte Microspheres. Angew. Chem., Int. Ed. 2010, 49, 8620– 8624,  DOI: 10.1002/anie.201001046
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    Facile Two-Step Synthesis of Porous Antigen-Loaded Degradable Polyelectrolyte Microspheres
    Dierendonck, Marijke; De Koker, Stefaan; Cuvelier, Claude; Grooten, Johan; Vervaet, Chris; Remon, Jean-Paul; De Geest, Bruno G.
    Angewandte Chemie, International Edition (2010), 49 (46), 8620-8624, S8620/1-S8620/2CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Herein, we report on the synthesis of porous antigen-loaded degradable polyelectrolyte microspheres using spray drying as a simple, yet efficient, and scalable prodn. technique. Key in our present work is the use as a sacrificial component of calcium carbonate nanoparticles, which are directly spray-dried in combination with antigen and oppositely charged polyelectrolytes to form a polyelectrolyte framework that entraps the antigen and the calcium carbonate nanoparticles. Extn. of calcium carbonate subsequently leads to porous microspheres. Results demonstrated encapsulation of antigen into the porous, degradable polyelectrolyte microspheres which were efficiently taken up by dendritic cells, the most potent antigen-presenting cells. These types of microspheres could be promising vaccine delivery systems.
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    Porcel, C. H.; Izquierdo, A.; Ball, V.; Decher, G.; Voegel, J. C.; Schaaf, P. Ultrathin Coatings and (Poly(glutamic acid)/Polyallylamine) Films Deposited by Continuous and Simultaneous Spraying. Langmuir 2005, 21, 800– 802,  DOI: 10.1021/la047570n
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    Ultrathin Coatings and (Poly(glutamic acid)/Polyallylamine) Films Deposited by Continuous and Simultaneous Spraying
    Porcel, C. H.; Izquierdo, A.; Ball, V.; Decher, G.; Voegel, J.-C.; Schaaf, P.
    Langmuir (2005), 21 (2), 800-802CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Polyelectrolyte films can be obtained by simultaneous polyanions and polycations spraying onto a solid surface. The most rapid film growth is found for polyanion-polycation spraying rate ratios close to 1. The film appear smooth and uniform for a sufficiently long spraying time. This continuous buildup process of nanocomposite multicomponent polyelectrolyte films should find numerous applications in material science such as biomaterial coatings or new membrane designs.
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    Muzzio, N. E.; Pasquale, M. A.; Gregurec, D.; Diamanti, E.; Kosutic, M.; Azzaroni, O.; Moya, S. E. Polyelectrolytes Multilayers To Modulate Cell Adhesion: A Study of the Influence of Film Composition and Polyelectrolyte Interdigitation on the Adhesion of the A549 Cell Line. Macromol. Biosci. 2016, 16, 482– 495,  DOI: 10.1002/mabi.201500275
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    Polyelectrolytes Multilayers to Modulate Cell Adhesion: A Study of the Influence of Film Composition and Polyelectrolyte Interdigitation on the Adhesion of the A549 Cell Line
    Muzzio, Nicolas E.; Pasquale, Miguel A.; Gregurec, Danijela; Diamanti, Eleftheria; Kosutic, Marija; Azzaroni, Omar; Moya, Sergio E.
    Macromolecular Bioscience (2016), 16 (4), 482-495CODEN: MBAIBU; ISSN:1616-5187. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Polyelectrolyte multilayers (PEMs) with different polycation/polyanion pairs are fabricated by the layer-by-layer technique employing synthetic, natural, and both types of polyelectrolytes. The impact of the chem. compn. of PEMs on cell adhesion is assessed by studying cell shape, spreading area, focal contacts, and cell proliferation for the A549 cell line. Cells exhibit good adhesion on PEMs contg. natural polycations and poly(sodium 4-styrenesulfonate) (PSS) as polyanion, but limited adhesion is obsd. on PEMs fabricated from both natural polyelectrolytes. PEMs are then assembled, depositing a block of natural polyelectrolytes on top of a stiffer block with PSS as polyanion. Cell adhesion is enhanced on top of the diblock PEMs compared to purely natural PEMs. This fact could be explained by the interdigitation between polyelectrolytes from the two blocks. Diblock PEM assembly provides a simple means to tune cell adhesion on biocompatible PEMs.
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    Liao, W. C.; Sohn, Y. S.; Riutin, M.; Cecconello, A.; Parak, W. J.; Nechushtai, R.; Willner, I. The Application of Stimuli-Responsive VEGF- and ATP-Aptamer-Based Microcapsules for the Controlled Release of an Anticancer Drug, and the Selective Targeted Cytotoxicity toward Cancer Cells. Adv. Funct. Mater. 2016, 26, 4262– 4273,  DOI: 10.1002/adfm.201600069
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    The Application of Stimuli-Responsive VEGF- and ATP-Aptamer-Based Microcapsules for the Controlled Release of an Anticancer Drug, and the Selective Targeted Cytotoxicity toward Cancer Cells
    Liao, Wei-Ching; Sohn, Yang Sung; Riutin, Marianna; Cecconello, Alessandro; Parak, Wolfgang J.; Nechushtai, Rachel; Willner, Itamar
    Advanced Functional Materials (2016), 26 (24), 4262-4273CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The synthesis of microcapsules consisting of DNA shells crosslinked by anti-VEGF (vascular epithelial growth factor) or anti-ATP (ATP) aptamers and loaded with tetramethylrhodamine-modified dextran, TMR-D, and Texas Red-modified dextran, TR-D, resp., as fluorescence labels acting as models for drug loads, is described. The aptamer-functionalized microcapsules act as stimuli-responsive carriers for the triggered release of the fluorescent labels in the presence of the overexpressed cancer cell biomarkers VEGF or ATP. The VEGF- and ATP-responsive microcapsules are, also, loaded with the anticancer drug doxorubicin (DOX), in the form of DOX-functionalized dextran, DOX-D. The release of DOX-D from the resp. microcapsules proceeds in the presence of VEGF or ATP as triggers. Preliminary cell expts. reveal that the ATP-responsive DOX-D-loaded microcapsules undergo effective endocytosis into MDA-MB-231 cancer cells. The ATP-responsive DOX-D-loaded microcapsules incorporated into the MDA-MB-231 cancer cells reveal impressive cytotoxicity as compared to normal epithelial MCF-10A breast cells (50% vs 0% cell death after 24 h, resp.). The cytotoxicity of the ATP-responsive DOX-D-loaded microcapsules toward the cancer cells is attributed to the effective unlocking of the microcapsules by overexpressed ATP, and to the subsequent release of DOX from the dextran backbone under acidic conditions present in cancer cells (pH = 6.2).
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    Liao, W.; Lu, C.; Hartmann, R.; Wang, F.; Sohn, Y. S.; Parak, W. J.; Willner, I. Adenosine Triphosphate-Triggered Release of Macromolecular and Nanoparticle Loads from Aptamer/DNA-Cross-Linked Microcapsules. ACS Nano 2015, 9, 9078– 9086,  DOI: 10.1021/acsnano.5b03223
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    Adenosine Triphosphate-Triggered Release of Macromolecular and Nanoparticle Loads from Aptamer/DNA-Cross-Linked Microcapsules
    Liao, Wei-Ching; Lu, Chun-Hua; Hartmann, Raimo; Wang, Fuan; Sohn, Yang Sung; Parak, Wolfgang J.; Willner, Itamar
    ACS Nano (2015), 9 (9), 9078-9086CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The synthesis of stimuli-responsive DNA microcapsules acting as carriers for different payloads, and being dissocd. through the formation of aptamer-ligand complexes is described. Specifically, stimuli-responsive anti-ATP aptamer-crosslinked DNA-stabilized microcapsules loaded with tetramethylrhodamine-modified dextran (TMR-D), CdSe/ZnS quantum dots (QDs), or microperoxidase-11 (MP-11) are presented. In the presence of ATP as trigger, the microcapsules are dissocd. through the formation of aptamer-ATP complexes, resulting in the release of the resp. loads. Selective unlocking of the capsules is demonstrated, and CTP, GTP, or TTP do not unlock the pores. The ATP-triggered release of MP-11 from the microcapsules enables the MP-11-catalyzed oxidn. of Amplex UltraRed by H2O2 to the fluorescent product resorufin.
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    Seyrek, E.; Decher, G. Layer-by-Layer Assembly of Multifunctional Hybrid Materials and Nanoscale Devices. In Polymer Science: A Comprehensive Reference; Matyjaszewski, K., Müller, M., Eds.; Elsevier: Amsterdam, 2012; Vol. 7, pp 159– 185.
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    Kotov, N. A.; Magonov, S.; Tropsha, E. Layer-by-Layer Self-Assembly of Alumosilicate-Polyelectrolyte Composites: Mechanism of Deposition, Crack Resistance, and Perspectives for Novel Membrane Materials. Chem. Mater. 1998, 10, 886– 895,  DOI: 10.1021/cm970649b
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    Layer-by-layer self-assembly of aluminosilicate-polyelectrolyte composites: mechanism of deposition, crack resistance, and perspectives for novel membrane materials
    Kotov, N. A.; Magonov, S.; Tropsha, E.
    Chemistry of Materials (1998), 10 (3), 886-895CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The morphol. and gas permeation properties of montmorillonite-polyelectrolyte self-assembled multilayer systems have been investigated. Without any special pretreatment, a stable film of montmorillonite-polymer composite was assembled on a hydrophobic poly(ethylene terephthalate) support by means of layer-by-layer deposition. All individual aluminosilicate platelets were found to be oriented in parallel to the substrate, while their surface d. strongly depended on the nature of the polyelectrolyte (charge and mol. wt.). The org.-inorg. films were found to be very flexible and crack-resistant even under a considerable mech. stress. This opened a possibility to preclude gas diffusion through defects and to design highly selective ultrathin membranes. A permeation rate of oxygen was found to decrease 6.6 times for ca. 200 nm montmorillonite/poly(diallyldimethylammonium) chloride film, while a permeation rate of aq. vapors did not change at all. This effect was attributed to the dominance of soln./adsorption permeation mechanism over the Knudsen diffusion. This fact made the montmorillonite-polyelectrolyte multilayers stand out among other thin films for which the diffusion through defects was found to be the primary mechanism of gas permeation. This finding demonstrated both practical and fundamental importance of hybrid thin films that combined the properties of both org. and inorg. materials.
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    Kotov, N. A. Ordered Layered Assemblies of Nanoparticles. MRS Bull. 2001, 26, 992– 997,  DOI: 10.1557/mrs2001.255
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    MRS Bulletin (2001), 26 (12), 992-997CODEN: MRSBEA; ISSN:0883-7694. (Materials Research Society)
    A review. Layer-by-layer (LBL) assembly of nanoparticles (NPs) and nanocolloids is presented as a technique for thin-film growth of such materials. In contrast to vol.-processing methods, the control of the 2D lateral packing of the NPs and the sequence of the deposited layers can be controlled. Selective strengthening of NP/NP and NP/polyelectrolyte intermol. interactions via surface modification leads to densely packed layers of NPs, while the phase segregation of polymer/NP composites is avoided. Various application examples are presented, including stratified LBL assemblies, freestanding LBL-assembled films, and graded semiconductor films. The microstructure, magnetical and optical properties of the LBL assemblies are described in relation to the prepn. conditions.
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    Yashchenok, A. M.; Gorin, D. A.; Badylevich, M.; Serdobintsev, A. A.; Bedard, M.; Fedorenko, Y. G.; Khomutov, G. B.; Grigoriev, D. O.; Möhwald, H. Impact of Magnetite Nanoparticle Incorporation on Optical and Electrical Properties of Nanocomposite LbL Assemblies. Phys. Chem. Chem. Phys. 2010, 12, 10469– 10475,  DOI: 10.1039/c004242k
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    Impact of magnetite nanoparticle incorporation on optical and electrical properties of nanocomposite LbL assemblies
    Yashchenok, Alexey M.; Gorin, Dmitry A.; Badylevich, Mikhail; Serdobintsev, Alexey A.; Bedard, Matthieu; Fedorenko, Yanina G.; Khomutov, Gennady B.; Grigoriev, Dmitri O.; Moehwald, Helmuth
    Physical Chemistry Chemical Physics (2010), 12 (35), 10469-10475CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Optical and elec. properties of polyelectrolyte/Fe oxide nanocomposite planar films on Si substrates were studied for different amt. of Fe3O4 nanoparticles incorporated in the films. The nanocomposite assemblies prepd. by the layer-by-layer assembly technique were characterized by ellipsometry, at. force microscopy, and secondary ion mass-spectrometry. Absorption spectra of the films reveal a shift of the optical absorption edge to higher energy when the no. of deposited layers decreases. Capacitance-voltage and current-voltage measurements were applied to study the elec. properties of metal-oxide-semiconductor structures prepd. by thermal evapn. of Au electrodes on nanocomposite films. The capacitance-voltage measurements show that the dielec. const. of the film increases with the no. of deposited layers and the fixed charge and the trapped charge densities have a neg. sign.
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    Malikova, N.; Pastoriza-Santos, I.; Schierhorn, M.; Kotov, N. A.; Liz-Marzán, L. M. Layer-by-Layer Assembled Mixed Spherical and Planar Gold Nanoparticles: Control of Interparticle Interactions. Langmuir 2002, 18, 3694– 3697,  DOI: 10.1021/la025563y
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    Layer-by-Layer Assembled Mixed Spherical and Planar Gold Nanoparticles: Control of Interparticle Interactions
    Malikova, Natalie; Pastoriza-Santos, Isabel; Schierhorn, Martin; Kotov, Nicholas A.; Liz-Marzan, Luis M.
    Langmuir (2002), 18 (9), 3694-3697CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Au nanoparticles (NPs) were prepd. by redn. with salicylic acid in aq. soln. The resulting dispersions contained a mixt. of flat triangular/hexagonal and smaller close-to-spherical NPs. As expected from theor. considerations, such nanocolloids display two clearly differentiated surface plasmon bands at 540 and 860 nm assocd. with spherical and anisotropic triangular/hexagonal NPs, resp. Layer-by-layer (LBL) assembly was used to deposit thin films of the Au colloids. UV-visible data indicate preferential adsorption of the flat particles on polyelectrolyte films. Importantly, a new band developed at 650 nm as the no. of the Au NPs bilayers increased. This finding indicates that there exists a strong interaction between the NPs in adjacent layers, resulting in the surface plasmon absorption at a new wavelength. The insertion of extra polyelectrolyte or montmorillonite layers between the Au bilayers gradually reduces the interlayer interaction and resulted in the NP composite films with absorption spectra virtually identical to those of the original dispersion. The bilayer deposition sequence in LBL assembly, i.e. multilayer architecture, can be used to control the strength of NP-NP coupling in the layered composites.
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    Salgueirino-Maceira, V.; Caruso, F.; Liz-Marzan, L. M. Coated Colloids with Tailored Optical Properties. J. Phys. Chem. B 2003, 107, 10990– 10994,  DOI: 10.1021/jp034302+
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    Coated Colloids with Tailored Optical Properties
    Salgueirino-Maceira, Veronica; Caruso, Frank; Liz-Marzan, Luis M.
    Journal of Physical Chemistry B (2003), 107 (40), 10990-10994CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    Tailored optical properties were imparted to polystyrene (PS) colloids of various sizes by the layer-by-layer (LbL) assembly of SiO2-coated Au ([email protected]) or SiO2-coated Ag ([email protected]) nanoparticles with different shell thicknesses and an electrostatically bridging polyelectrolyte, poly(diallyldimethylammonium chloride) (PDADMAC). The spectral position of the surface plasmon band of the composite colloid spheres is tailored through (i) the type of SiO2-coated nanoparticles used, (ii) the thickness of the SiO2 shell, and (iii) the no. of nanoparticle layers deposited. For thin SiO2 shells (<10 nm), the no. of nanoparticle layers deposited on the PS particles influences the plasmon band position due to electromagnetic coupling between neighboring metal nanoparticle cores. However, in the case of thick SiO2 shells (>18 nm), nanoparticle core-core interparticle interactions are fully prevented and the plasmon band position is unaffected. Addnl., SiO2 dissoln. with HF leads to compact nanoparticulate metal layers on the PS spheres. The particles prepd. are expected to find use as novel building blocks with unique optical properties for integration into advanced materials.
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    Schneider, G.; Decher, G.; Nerambourg, N.; Praho, R.; Werts, M. H.; Blanchard-Desce, M. Distance-Dependent Fluorescence Quenching on Gold Nanoparticles Ensheathed with Layer-by-Layer Assembled Polyelectrolytes. Nano Lett. 2006, 6, 530– 536,  DOI: 10.1021/nl052441s
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    Distance-dependent fluorescence quenching on gold nanoparticles ensheathed with layer-by-layer assembled polyelectrolytes
    Schneider, Gregory; Decher, Gero; Nerambourg, Nicolas; Praho, Raiessa; Werts, Martinus H. V.; Blanchard-Desce, Mireille
    Nano Letters (2006), 6 (3), 530-536CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The authors report on the prepn., characterization, and photophys. study of new fluorescent core/shell nanoparticles fabricated by electrostatic layer-by-layer assembly. On the basis of gold cores with a diam. of 13 nm, these nanocolloids possess different fluorescently labeled polymer corona layers at various distances from the surface of the core metal using nonfluorescent polyelectrolytes as spacer layers. UV-visible spectroscopy and transmission electron microscopy confirm that the particle suspensions of fluorescently labeled core/shell nanoparticles are stable at all stages of their construction. Photophys. investigations reveal strongly distance-dependent fluorescence quenching in these particle systems. The contribution of the metal core to this quenching can be assessed precisely after the gentle dissoln. of the gold cores by potassium cyanide. The photophys. measurements reveal clearly that the gold nanoparticles decrease the transition probability for radiative transitions.
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    Dykman, L.; Khlebtsov, N. Gold Nanoparticles in Biomedical Applications: Recent Advances and Perspectives. Chem. Soc. Rev. 2012, 41, 2256– 2282,  DOI: 10.1039/C1CS15166E
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    Gold nanoparticles in biomedical applications: recent advances and perspectives
    Dykman, Lev; Khlebtsov, Nikolai
    Chemical Society Reviews (2012), 41 (6), 2256-2282CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Gold nanoparticles (GNPs) with controlled geometrical, optical, and surface chem. properties are the subject of intensive studies and applications in biol. and medicine. To date, the ever increasing diversity of published examples has included genomics and biosensorics, immunoassays and clin. chem., photothermolysis of cancer cells and tumors, targeted delivery of drugs and antigens, and optical bioimaging of cells and tissues with state-of-the-art nanophotonic detection systems. This crit. review is focused on the application of GNP conjugates to biomedical diagnostics and analytics, photothermal and photodynamic therapies, and delivery of target mols. Distinct from other published reviews, we present a summary of the immunol. properties of GNPs. For each of the above topics, the basic principles, recent advances, and current challenges are discussed (508 refs.).
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    Del Pino, P.; Pelaz, B.; Zhang, Q.; Maffre, P.; Nienhaus, G. U.; Parak, W. J. Protein Corona Formation around Nanoparticles—From the Past to the Future. Mater. Horiz. 2014, 1, 301– 313,  DOI: 10.1039/C3MH00106G
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    Protein corona formation around nanoparticles - from the past to the future
    del Pino, Pablo; Pelaz, Beatriz; Zhang, Qian; Maffre, Pauline; Nienhaus, G. Ulrich; Parak, Wolfgang J.
    Materials Horizons (2014), 1 (3), 301-313CODEN: MHAOBM; ISSN:2051-6355. (Royal Society of Chemistry)
    A review. The protein adsorption layer (a.k.a. the "protein corona") that forms on the surface of colloidal nanoparticles plays an important role in their interaction with living matter. Thus, characterization of the protein corona is of utmost importance for understanding how exposure to nanoparticles affects the biol. responses of cells and organisms. Although a lot of exptl. studies have been reported in this direction, a comprehensive picture is still missing, in particular due to the multitude of different scenarios under which expts. have been performed. In this review an anal. of existing exptl. data about the protein corona, and an outline for required future work will be given. In particular the authors review how existing simple anal. models such as the adopted Hill model may help to ext. quant. data from such expts. such as equil. dissocn. and kinetic coeffs. Careful quant. assessment of equil. and kinetic properties would allow for a comparison of protein binding data from the vast array of engineered nanoparticles, so that basic principles could be revealed. This review outlines that the field is in dire need of more quant. studies to further our understanding of protein corona formation and its biol. consequences.
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    Ariga, K.; Hill, J. P.; Ji, Q. Layer-by-Layer Assembly as a Versatile Bottom-Up Nanofabrication Technique for Exploratory Research and Realistic Application. Phys. Chem. Chem. Phys. 2007, 9, 2319– 2340,  DOI: 10.1039/b700410a
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    Layer-by-layer assembly as a versatile bottom-up nanofabrication technique for exploratory research and realistic application
    Ariga, Katsuhiko; Hill, Jonathan P.; Ji, Qingmin
    Physical Chemistry Chemical Physics (2007), 9 (19), 2319-2340CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    A review. The layer-by-layer (LbL) adsorption technique offers an easy and inexpensive process for multilayer formation and allows a variety of materials to be incorporated within the film structures. Therefore, the LbL assembly method can be regarded as a versatile bottom-up nanofabrication technique. Research fields concerned with LbL assembly have developed rapidly but some important physicochem. aspects remain uninvestigated. In this review, we will introduce several examples from physicochem. investigations regarding the basics of this method to advanced research aimed at practical applications. These are selected mostly from recent reports and should stimulate many phys. chemists and chem. physicists in the further development of LbL assembly. In order to further understand the mechanism of the LbL assembly process, theor. work, including thermodn. calcns., has been conducted. Addnl., the use of mol. dynamics simulation has been proposed. Recently, many kinds of physicochem. mol. interactions, including hydrogen bonding, charge transfer interactions, and stereo-complex formation, have been used. The combination of the LbL method with other fabrication techniques such as spin-coating, spraying, and photolithog. has also been extensively researched. These improvements have enabled prepn. of LbL films composed of various materials contained in well-designed nanostructures. The resulting structures can be used to investigate basic physicochem. phenomena where relative distances between interacting groups is of great importance. Similarly, LbL structures prepd. by such advanced techniques are used widely for development of functional systems for phys. applications from photovoltaic devices and field effect transistors to biochem. applications including nano-sized reactors and drug delivery systems.
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  78. 78
    Ung, T.; Liz-Marzán, L. M.; Mulvaney, P. Optical Properties of Thin Films of [email protected]2 Particles. J. Phys. Chem. B 2001, 105, 3441– 3452,  DOI: 10.1021/jp003500n
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    78
    Optical Properties of Thin Films of [email protected] Particles
    Ung, Thearith; Liz-Marzan, Luis M.; Mulvaney, Paul
    Journal of Physical Chemistry B (2001), 105 (17), 3441-3452CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    Homogeneous films of [email protected] particles were deposited on glass as a prototype 3-dimensional artificial solid using the LBL method. The film thickness is controlled by the no. of dipping cycles and is measured by AFM. Each cycle results in approx. one monolayer of particles being deposited. The particle films are dense, but disordered. The optical properties of the resulting thin films were analyzed as a function of the particle vol. fraction, which is controlled through the SiO2 shell thickness. The surface plasmon peak position in films with vol. fractions up to φ > 0.5 is accurately predicted by the Maxwell-Garnett model. The films exhibit remarkably uniform, transmitted colors and display metallic reflection at low angles of incidence, even at low vol. fractions. The films can be annealed at T > 500 K to provide extremely stable, optical films.
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    Vial, S.; Pastoriza-Santos, I.; Perez-Juste, J.; Liz-Marzan, L. M. Plasmon Coupling in Layer-by-Layer Assembled Gold Nanorod Films. Langmuir 2007, 23, 4606– 4611,  DOI: 10.1021/la063753t
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    79
    Plasmon Coupling in Layer-by-Layer Assembled Gold Nanorod Films
    Vial, Stephanie; Pastoriza-Santos, Isabel; Perez-Juste, Jorge; Liz-Marzan, Luis M.
    Langmuir (2007), 23 (8), 4606-4611CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    A systematic study of the optical effects derived from plasmon coupling in mono- and multilayers of gold nanorods is presented. The monolayers were prepd. using the std. polyelectrolyte-assisted layer-by-layer (LbL) method and gold nanorods coated with either poly(N-vinyl pyrrolidone) or homogeneous silica shells. Such plasmon coupling leads in general to extensive red-shift and broadening of the longitudinal plasmon bands, which are discussed on the basis of recently reported theor. modeling. Whereas for PVP-coated rods, strong interactions were obsd. for high-d. monolayers and closely spaced multilayers, increasingly efficient screening is obsd. for thicker silica shells.
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    Abalde-Cela, S.; Ho, S.; Rodriguez-Gonzalez, B.; Correa-Duarte, M. A.; Alvarez-Puebla, R. A.; Liz-Marzan, L. M.; Kotov, N. A. Loading of Exponentially Grown LbL Films with Silver Nanoparticles and Their Application to Generalized SERS Detection. Angew. Chem., Int. Ed. 2009, 48, 5326– 5329,  DOI: 10.1002/anie.200901807
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    Loading of Exponentially Grown LBL Films with Silver Nanoparticles and Their Application to Generalized SERS Detection
    Abalde-Cela, Sara; Ho, Szushen; Rodriguez-Gonzalez, Benito; Correa-Duarte, Miguel A.; Alvarez-Puebla, Ramon A.; Liz-Marzan, Luis M.; Kotov, Nicholas A.
    Angewandte Chemie, International Edition (2009), 48 (29), 5326-5329, S5326/1-S5326/5CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Thin films made by exponential layer-by-layer (LBL) growth display high diffusivity and can be readily infiltrated with inorg. nanoparticles. They can sequestrate mol. systems from soln. as a function of the compn. of their layers, while providing intense surface-enhanced Raman scattering (SERS) signals (see picture).
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    Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-Based Composite Materials. Nature 2006, 442, 282– 286,  DOI: 10.1038/nature04969
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    Stankovich, Sasha; Dikin, Dmitriy A.; Dommett, Geoffrey H. B.; Kohlhaas, Kevin M.; Zimney, Eric J.; Stach, Eric A.; Piner, Richard D.; Nguyen, SonBinh T.; Ruoff, Rodney S.
    Nature (London, United Kingdom) (2006), 442 (7100), 282-286CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Graphene sheets (one-atom-thick two-dimensional layers of sp2-bonded carbon) are predicted to have a range of unusual properties. Their thermal cond. and mech. stiffness may rival the remarkable in-plane values for graphite (∼3000 W.m-1.K-1 and 1060 GPa, resp.); their fracture strength should be comparable to that of carbon nanotubes for similar types of defects; and recent studies have shown that individual graphene sheets have extraordinary electronic transport properties. One possible route to harnessing these properties for applications would be to incorporate graphene sheets in a composite material. The manufg. of such composites requires not only that graphene sheets be produced on a sufficient scale but that they also be incorporated, and homogeneously distributed, into various matrixes. Graphite, inexpensive and available in large quantity, unfortunately does not readily exfoliate to yield individual graphene sheets. Here we present a general approach for the prepn. of graphene-polymer composites via complete exfoliation of graphite and mol.-level dispersion of individual, chem. modified graphene sheets within polymer hosts. A polystyrene-graphene composite formed by this route exhibits a percolation threshold of ∼0.1 vol.% for room-temp. elec. cond., the lowest reported value for any carbon-based composite except for those involving carbon nanotubes; at only 1 vol.%, this composite has a cond. of ∼0.1 S m-1, sufficient for many elec. applications. Our bottom-up chem. approach of tuning the graphene sheet properties provides a path to a broad new class of graphene-based materials and their use in a variety of applications.
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    Wan, S.; Li, Y.; Mu, J.; Aliev, A. E.; Fang, S.; Kotov, N. A.; Jiang, L.; Cheng, Q.; Baughman, R. H. Sequentially Bridged Graphene Sheets with High Strength, Toughness, and Electrical Conductivity. Proc. Natl. Acad. Sci. U. S. A. 2018, 115, 5359– 5364,  DOI: 10.1073/pnas.1719111115
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    Sequentially bridged graphene sheets with high strength, toughness, and electrical conductivity
    Wan, Sijie; Li, Yuchen; Mu, Jiuke; Aliev, Ali E.; Fang, Shaoli; Kotov, Nicholas A.; Jiang, Lei; Cheng, Qunfeng; Baughman, Ray H.
    Proceedings of the National Academy of Sciences of the United States of America (2018), 115 (21), 5359-5364CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    We here show that infiltrated bridging agents can convert inexpensively fabricated graphene platelet sheets into high-performance materials, thereby avoiding the need for a polymer matrix. Two types of bridging agents were investigated for interconnecting graphene sheets, which attach to sheets by either π-π bonding or covalent bonding. When applied alone, the π-π bonding agent is most effective. However, successive application of the optimized ratio of π-π bonding and covalent bonding agents provides graphene sheets with the highest strength, toughness, fatigue resistance, elec. cond., electromagnetic interference shielding efficiency, and resistance to ultrasonic dissoln. Raman spectroscopy measurements of stress transfer to graphene platelets allow us to decipher the mechanisms of property improvement. In addn., the degree of orientation of graphene platelets increases with increasing effectiveness of the bonding agents, and the interlayer spacing increases. Compared with other materials that are strong in all directions within a sheet, the realized tensile strength (945 MPa) of the resin-free graphene platelet sheets was higher than for carbon nanotube or graphene platelet composites, and comparable to that of com. available carbon fiber composites. The toughness of these composites, contg. the combination of π-π bonding and covalent bonding, was much higher than for these other materials having high strengths for all in-plane directions, thereby opening the path to materials design of layered nanocomposites using multiple types of quant. engineered chem. bonds between nanoscale building blocks.
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    Mao, L. B.; Gao, H. L.; Yao, H. B.; Liu, L.; Cölfen, H.; Liu, G.; Chen, S. M.; Li, S. K.; Yan, Y. X.; Liu, Y. Y.; Yu, S. H. Synthetic Nacre by Predesigned Matrix-Directed Mineralization. Science 2016, 354, 107– 110,  DOI: 10.1126/science.aaf8991
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    Synthetic nacre by predesigned matrix-directed mineralization
    Mao, Li-Bo; Gao, Huai-Ling; Yao, Hong-Bin; Liu, Lei; Coelfen, Helmut; Liu, Gang; Chen, Si-Ming; Li, Shi-Kuo; Yan, You-Xian; Liu, Yang-Yi; Yu, Shu-Hong
    Science (Washington, DC, United States) (2016), 354 (6308), 107-110CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Although biomimetic designs are expected to play a key role in exploring future structural materials, facile fabrication of bulk biomimetic materials under ambient conditions remains a major challenge. Here, we describe a mesoscale "assembly-and-mineralization" approach inspired by the natural process in mollusks to fabricate bulk synthetic nacre that highly resembles both the chem. compn. and the hierarchical structure of natural nacre. The millimeter-thick synthetic nacre consists of alternating org. layers and aragonite platelet layers (91 wt. percent) and exhibits good ultimate strength and fracture toughness. This predesigned matrix-directed mineralization method represents a rational strategy for the prepn. of robust composite materials with hierarchically ordered structures, where various constituents are adaptable, including brittle and heat-labile materials.
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    Jiang, C.; Markutsya, S.; Pikus, Y.; Tsukruk, V. V. Freely Suspended Nanocomposite Membranes as Highly Sensitive Sensors. Nat. Mater. 2004, 3, 721– 728,  DOI: 10.1038/nmat1212
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    Freely suspended nanocomposite membranes as highly sensitive sensors
    Jiang, Chaoyang; Markutsya, Sergiy; Pikus, Yuri; Tsukruk, Vladimir V.
    Nature Materials (2004), 3 (10), 721-728CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Highly sensitive sensor arrays are in high demand for prospective applications in remote sensing and imaging. Measuring microscopic deflections of compliant micromembranes and cantilevers is developing into one of the most versatile approaches for thermal, acoustic and chem. sensing. Here, we report on an innovative fabrication of compliant nanocomposite membranes with nanoscale thickness showing extraordinary sensitivity and dynamic range, which makes them candidates for a new generation of membrane-based sensor arrays. These nanomembranes with a thickness of 25-70 nm, which can be freely suspended over large (hundred micrometers) openings are fabricated with mol. precision by time-efficient, spin-assisted layer-by-layer assembly. They are designed as multilayered mol. composites made of a combination of polymeric monolayers and a metal nanoparticle intralayer. We demonstrate that these nanocomposite membranes possess unparalleled sensitivity and a unique autorecovering ability. The membrane nanostructure that is responsible for these outstanding properties combines multilayered polymer/nanoparticle organization, high polymer-chain orientation, and a pre-stretched state.
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    Li, Y.; Wang, X.; Sun, J. Layer-by-Layer Assembly for Rapid Fabrication of Thick Polymeric Films. Chem. Soc. Rev. 2012, 41, 5998– 6009,  DOI: 10.1039/c2cs35107b
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    Layer-by-layer assembly for rapid fabrication of thick polymeric films
    Li, Yang; Wang, Xu; Sun, Junqi
    Chemical Society Reviews (2012), 41 (18), 5998-6009CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. In the past two decades, layer-by-layer (LbL) assembly has been proven to be a convenient and versatile method to fabricate functional films. However, using traditional dipping LbL assembly to fabricate micrometer-thick films is time consuming. Compared with ultrathin films, micrometer-thick films prepd. by LbL assembly possess enhanced mech. stability, and allow deposition of a significantly increased amt. of materials and the integration of multiple functions. These merits of thick films produced by LbL assembly can result in new functions and allow the functions of ultrathin films fabricated by LbL assembly to be optimized. In this tutorial review, the methods for rapid fabrication of thick polymeric films involving LbL assembly are reviewed. The functions of such films that are relevant to their micrometer thickness are discussed.
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    Mannarino, M. M.; Liu, D. S.; Hammond, P. T.; Rutledge, G. C. Mechanical and Transport Properties of Layer-by-Layer Electrospun Composite Proton Exchange Membranes for Fuel Cell Applications. ACS Appl. Mater. Interfaces 2013, 5, 8155– 8164,  DOI: 10.1021/am402204v
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    86
    Mechanical and Transport Properties of Layer-by-Layer Electrospun Composite Proton Exchange Membranes for Fuel Cell Applications
    Mannarino, Matthew M.; Liu, David S.; Hammond, Paula T.; Rutledge, Gregory C.
    ACS Applied Materials & Interfaces (2013), 5 (16), 8155-8164CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Composite membranes composed of highly conductive and selective layer-by-layer (LbL) films and electrospun fiber mats were fabricated and characterized for mech. strength and electrochem. selectivity. The LbL component consists of a proton-conducting, methanol-blocking poly(diallyl di-Me ammonium chloride)/sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (PDAC/sPPO) thin film. The electrospun fiber component consists of poly(tri-Me hexamethylene terephthalamide) (PA6(3)T) fibers in a nonwoven mat of 60-90% porosity. The bare mats were annealed to improve their mech. properties, which improvements are shown to be retained in the composite membranes. Spray LbL assembly was used as a means for the rapid formation of proton-conducting films that fill the void space throughout the porous electrospun matrix and create a fuel-blocking layer. Coated mats as thin as 15 μm were fabricated, and viable composite membranes with methanol permeabilities 20 times lower than Nafion and through-plane proton selectivity five and a half times greater than Nafion are demonstrated. The mech. properties of the spray coated electrospun mats are shown to be superior to the LbL-only system and possess intrinsically greater dimensional stability and lower mech. hysteresis than Nafion under hydrated conditions. The composite proton exchange membranes fabricated here were tested in an operational direct methanol fuel cell. The results show the potential for higher open circuit voltages (OCV) and comparable cell resistances when compared to fuel cells based on Nafion.
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    Jiang, C.; Tsukruk, V. V. Freestanding Nanostructures via Layer-by-Layer Assembly. Adv. Mater. 2006, 18, 829– 840,  DOI: 10.1002/adma.200502444
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    Freestanding nanostructures via layer-by-layer assembly
    Jiang, Chaoyang; Tsukruk, Vladimir V.
    Advanced Materials (Weinheim, Germany) (2006), 18 (7), 829-840CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Freestanding flexible nanocomposite structures fabricated by layer-by-layer (LbL) assembly are promising candidates for many potential applications, such as in the fields of thermomech. sensing, controlled release, optical detection, and drug delivery. In this article, we review recent advances in the fabrication and characterization of different types of freestanding LbL structures in air and at air/liq. and liq./liq. interfaces, including micro- and nanocapsules, microcantilevers, freely suspended membranes, encapsulated nanoparticle arrays, and sealed-cavity arrays. Several recently developed fabrication techniques, such as spin-assisted coating, dipping, and micropatterning, make the assembly process more efficient and impart novel phys. properties to the freestanding films.
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    Zhang, X.; Chen, H.; Zhang, H. Layer-by-Layer Assembly: From Conventional to Unconventional Methods. Chem. Commun. 2007, 0, 1395– 1405,  DOI: 10.1039/B615590A
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    Layer-by-layer assembly: from conventional to unconventional methods
    Zhang, Xi; Chen, Huan; Zhang, Hongyu
    Chemical Communications (Cambridge, United Kingdom) (2007), (14), 1395-1405CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    A review. Layer-by-layer (LbL) assembly is a powerful means for fabricating multilayer thin films with controlled architecture and compn. This feature article discusses different types of methods for LbL assembly. On the one hand, some of the conventional LbL methods are introduced, which are driven by electrostatic interactions, hydrogen bonds, step-by-step reactions, sol-gel processes, mol. recognition, charge-transfer, stepwise stereocomplex assembly, and electrochem. On the other hand, some of the unconventional methods for fabricating of the building blocks which can not be assembled by conventional methods are also summarized. These unconventional methods usually involve the formation of supramol. structures via one type of self-assembly. These structures can subsequently be used as building blocks in another type of self-assembly. To take advantage of these conventional and unconventional methods, a great no. of building blocks can be fabricated into multilayer thin films with a defined sequence structure in a designed way. It has been demonstrated that LbL methods provide new horizons for surface mol. engineering.
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    Weng, G. M.; Li, J.; Alhabeb, M.; Karpovich, C.; Wang, H.; Lipton, J.; Maleski, K.; Kong, J.; Shaulsky, E.; Elimelech, M.; Gogotsi, Y.; Taylor, A. D. Layer-by-Layer Assembly of Cross-Functional Semi-Transparent MXene-Carbon Nanotubes Composite Films for Next-Generation Electromagnetic Interference Shielding. Adv. Funct. Mater. 2018, 28, 1803360,  DOI: 10.1002/adfm.201803360
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    Feifel, S. C.; Lisdat, F. Silica Nanoparticles for the Layer-by-Layer Assembly of Fully Electro-Active Cytochrome c Multilayers. J. Nanobiotechnol. 2011, 9, 59,  DOI: 10.1186/1477-3155-9-59
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    Silica nanoparticles for the layer-by-layer assembly of fully electro-active cytochrome c multilayers
    Feifel, Sven C.; Lisdat, Fred
    Journal of Nanobiotechnology (2011), 9 (), 59CODEN: JNOAAO; ISSN:1477-3155. (BioMed Central Ltd.)
    For bioanal. systems sensitivity and biomol. activity are crit. issues. The immobilization of proteins into multilayer systems by the layer-by-layer deposition has become one of the favorite methods with this respect. Moreover, the combination of nanoparticles with biomols. on electrodes is a matter of particular interest since several examples with high activities and direct electron transfer have been found. Our study describes the investigation on silica nanoparticles and the redox protein cytochrome c for the construction of electro-active multilayer architectures, and the electron transfer within such systems. The novelty of this work is the construction of such artificial architectures with a non-conducting building block. Furthermore a detailed study of the size influence of silica nanoparticles is performed with regard to formation and electrochem. behavior of these systems. We report on interprotein electron transfer (IET) reaction cascades of cytochrome c (cyt c) immobilized by the use of modified silica nanoparticles (SiNPs) to act as an artificial matrix. The layer-by-layer deposition technique has been used for the formation of silica particles/cytochrome c multilayer assemblies on electrodes. The silica particles are characterized by dynamic light scattering (DLS), Fourier transformed IR spectroscopy (FT-IR), Zeta-potential and transmission electron microscopy (TEM). The modified particles have been studied with respect to act as an artificial network for cytochrome c and to allow efficient interprotein electron transfer reactions. We demonstrate that it is possible to form electro-active assemblies with these non-conducting particles. The electrochem. response is increasing linearly with the no. of layers deposited, reaching a cyt c surface concn. of about 80 pmol/cm2 with a 5 layer architecture. The interprotein electron transfer through the layer system and the influence of particle size are discussed. This study demonstrates the ability to construct fully electro-active cyt c multilayer assemblies by using carboxy-modified silica nanoparticles. Thus it can be shown that functional, artificial systems can be build up following natural examples of protein arrangements. The absence of any conductive properties in the second building block clearly demonstrates that mechanisms for electron transfer through such protein multilayer assemblies is based on interprotein electron exchange, rather than on wiring of the protein to the electrode. The construction strategy of this multilayer system provides a new controllable route to immobilize proteins in multiple layers featuring direct electrochem. without mediating shuttle mols. and controlling the electro-active amt. by the no. of deposition steps.
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    Irigoyen, J.; Laakso, T.; Politakos, N.; Dahne, L.; Pihlajamäki, A.; Mänttäri, M.; Moya, S. E. Design and Performance Evaluation of Hybrid Nanofiltration Membranes Based on Multiwalled Carbon Nanotubes and Polyelectrolyte Multilayers for Larger Ion Rejection and Separation. Macromol. Chem. Phys. 2016, 217, 804– 811,  DOI: 10.1002/macp.201500433
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    Design and Performance Evaluation of Hybrid Nanofiltration Membranes Based on Multiwalled Carbon Nanotubes and Polyelectrolyte Multilayers for Larger Ion Rejection and Separation
    Irigoyen, Joseba; Laakso, Timo; Politakos, Nikolaos; Dahne, Lars; Pihlajamaeki, Artho; Maenttaeri, Mika; Moya, Sergio Enrique
    Macromolecular Chemistry and Physics (2016), 217 (6), 804-811CODEN: MCHPES; ISSN:1022-1352. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A polyelectrolyte multilayer (PEM) membrane of poly(diallyldimethylammonium chloride) (PDADMAC) and poly(sodium 4-styrenesulfonate) (PSS) is deposited on top of a thick-layer micrometer range of multiwalled carbon nanotubes (MWCNTs) assembled on a porous silicon carbide (SiC) tubular membrane support. MWCNTs are assembled "layer-by-layer" alternating oxidized CNTs and poly(allylamine hydrochloride)-modified CNTs. The MWCNTs layer is crosslinked by annealing after the assembly. The MWCNT layer acts as a spacer between the PDADMAC/PSS PEM and the SiC support. The MWCNT support increases water permeability in 42% compared with the PEMs deposited without MWCNTs. Hybrid MWCNT-PEM membranes show high rejection for divalent ions, which increases directly with flux. A rejection up to 92% is measured for MgSO4 and there is up to a 60% rejection difference between MgCl2 and NaCl, making the hybrid MWCNT-PEMs highly appealing for nanofiltration and monovalent and divalent ion sepns.
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    Kaushik, A. K.; Podsiadlo, P.; Qin, M.; Shaw, C. M.; Waas, A. M.; Kotov, N. A.; Arruda, E. M. The Role of Nanoparticle Layer Separation in the Finite Deformation Response of Layered Polyurethane-Clay Nanocomposites. Macromolecules 2009, 42, 6588– 6595,  DOI: 10.1021/ma901048g
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    92
    The Role of Nanoparticle Layer Separation in the Finite Deformation Response of Layered Polyurethane-Clay Nanocomposites
    Kaushik, Amit K.; Podsiadlo, Paul; Qin, Ming; Shaw, Charles M.; Waas, Anthony M.; Kotov, Nicholas A.; Arruda, Ellen M.
    Macromolecules (Washington, DC, United States) (2009), 42 (17), 6588-6595CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    Nanoscale control of structure in polymer nanocomposites is crit. for their performance but has been difficult to investigate systematically due to the lack of a suitable exptl. model. In this work, we investigated the role of nanoparticle layer sepn. in the finite deformation response of layered polyurethane- (PU-) montmorillonite (MTM) nanocomposites. A series of multilayered nanocomposites was manufd., with alternating PU and MTM nanolayers, using a layer-by-layer manufg. technique. The systematic variation in MTM nanoparticle vol. fraction was achieved by varying the thickness of the PU nanolayer and therefore the MTM layer sepn. Traditional polymer nanocomposite blending techniques result in a wide variation in nanoparticle sepn. for a given nanocomposite. In this investigation, we controlled the MTM nanoparticle layer sepn., which allowed us to examine its effect on the nanocomposite response over a broad range in nanoparticle vol. fraction. The PU-MTM nanocomposites demonstrated an increasing yield strength and stiffness with increased MTM vol. fraction or reduced nanoparticle layer sepn. A transition from ductile to brittle behavior in the stress-strain constitutive response was obsd. at a high vol. fraction of MTM nanoparticles. We demonstrate that a crit. nanoparticle sepn. exists, below which brittle behavior dominates the response of PU-MTM nanocomposites.
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  93. 93
    Shahzad, F.; Alhabeb, M.; Hatter, C. B.; Anasori, B.; Hong, S. M.; Koo, C. M.; Gogotsi, Y. Electromagnetic Interference Shielding with 2D Transition Metal Carbides (MXenes). Science 2016, 353, 1137– 1140,  DOI: 10.1126/science.aag2421
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    93
    Electromagnetic interference shielding with 2D transition metal carbides (MXenes)
    Shahzad, Faisal; Alhabeb, Mohamed; Hatter, Christine B.; Anasori, Babak; Hong, Soon Man; Koo, Chong Min; Gogotsi, Yury
    Science (Washington, DC, United States) (2016), 353 (6304), 1137-1140CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Materials with good flexibility and high cond. that can provide electromagnetic interference (EMI) shielding with minimal thickness are highly desirable, esp. if they can be easily processed into films. Two-dimensional metal carbides and nitrides, known as MXenes, combine metallic cond. and hydrophilic surfaces. Here, we demonstrate the potential of several MXenes and their polymer composites for EMI shielding. A 45-μm-thick Ti3C2Tx film exhibited EMI shielding effectiveness of 92 decibels (>50 decibels for a 2.5-μm film), which is the highest among synthetic materials of comparable thickness produced to date. This performance originates from the excellent elec. cond. of Ti3C2Tx films (4600 S per cm) and multiple internal reflections from Ti3C2Tx flakes in free-standing films. The mech. flexibility and easy coating capability offered by MXenes and their composites enable them to shield surfaces of any shape while providing high EMI shielding efficiency.
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  94. 94
    Zhou, Z.; Panatdasirisuk, W.; Mathis, T. S.; Anasori, B.; Lu, C.; Zhang, X.; Liao, Z.; Gogotsi, Y.; Yang, S. Layer-by-Layer Assembly of MXene and Carbon Nanotubes on Electrospun Polymer Films for Flexible Energy Storage. Nanoscale 2018, 10, 6005– 6013,  DOI: 10.1039/C8NR00313K
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    94
    Layer-by-layer assembly of MXene and carbon nanotubes on electrospun polymer films for flexible energy storage
    Zhou, Zehang; Panatdasirisuk, Weerapha; Mathis, Tyler S.; Anasori, Babak; Lu, Canhui; Zhang, Xinxing; Liao, Zhiwei; Gogotsi, Yury; Yang, Shu
    Nanoscale (2018), 10 (13), 6005-6013CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Free-standing, highly flexible and foldable supercapacitor electrodes were fabricated through the spray-coating assisted layer-by-layer assembly of Ti3C2Tx (MXene) nanoflakes together with multi-walled carbon nanotubes (MWCNTs) on electrospun polycaprolactone (PCL) fiber networks. The open structure of the PCL network and the use of MWCNTs as spacers not only limit the restacking of Ti3C2Tx flakes but also increase the accessible surface of the active materials, facilitating fast diffusion of electrolyte ions within the electrode. Composite electrodes have areal capacitance (30-50 mF cm-2) comparable to other templated electrodes reported in the literature, but showed significantly improved rate performance (14-16% capacitance retention at a scan rate of 100 V s-1). Furthermore, the composite electrodes are flexible and foldable, demonstrating good tolerance against repeated mech. deformation, including twisting and folding. Therefore, these tens of micron thick fiber electrodes will be attractive for applications in energy storage, electroanal. chem., brain electrodes, electrocatalysis and other fields, where flexible freestanding electrodes with an open and accessible surface are highly desired.
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  95. 95
    Hoda, N.; Larson, R. G. Explicit- and Implicit-Solvent Molecular Dynamics Simulations of Complex Formation between Polycations and Polyanions. Macromolecules 2009, 42, 8851– 8863,  DOI: 10.1021/ma901632c
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    95
    Explicit- and Implicit-Solvent Molecular Dynamics Simulations of Complex Formation between Polycations and Polyanions
    Hoda, Nazish; Larson, Ronald G.
    Macromolecules (Washington, DC, United States) (2009), 42 (22), 8851-8863CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    Explicit- and implicit-solvent mol. dynamics simulations were performed to study complexation in two polyelectrolyte systems: poly(styrene sulfonate)/poly(allylamine hydrochloride) (PSS/PAH) and poly(acrylic acid)/poly(allylamine hydrochloride) (PAA/PAH). Both systems were used in layer-by-layer assembly of polyelectrolyte films, with the first of these typically yielding linear growth in film thickness with increasing no. of layers and the second yielding exponential growth. In both systems, the polyelectrolytes have the same no. of monomers and are present in stoichiometric proportion, and water is used as solvent. Simulations provided insights into the structure and compn. of the complexes. The PSS/PAH complex is more compact and has a smaller fraction of water than the PAA/PAH complex due to Ph rings in the PSS chain and the hydrophilic nature of charged groups in PAA. Either an increase in salt concn. or a decrease in partial charge fraction increases swelling and water content in the complex. To overcome the computational limitations assocd. with the explicit-solvent simulation, a simple strategy to develop the force field for an implicit-solvent simulation is proposed. In the absence of salt and for fully charged polyelectrolytes, the radius-of-gyration and the various radial distribution functions predicted by the implicit-solvent simulation match well with those predicted by the explicit-solvent simulation, while reasonable agreement is obtained in the other cases. The implicit-solvent simulation was performed for bigger system sizes, and trends were found similar to those obsd. for smaller system sizes, suggesting that the simulation results are independent of system size.
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  96. 96
    Patel, P. A.; Jeon, J.; Mather, P. T.; Dobrynin, A. V. Molecular Dynamics Simulations of Layer-by-Layer Assembly of Polyelectrolytes at Charged Surfaces: Effects of Chain Degree of Polymerization and Fraction of Charged Monomers. Langmuir 2005, 21, 6113– 6122,  DOI: 10.1021/la050432t
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    96
    Molecular Dynamics Simulations of Layer-by-Layer Assembly of Polyelectrolytes at Charged Surfaces: Effects of Chain Degree of Polymerization and Fraction of Charged Monomers
    Patel, Pritesh A.; Jeon, Junhwan; Mather, Patrick T.; Dobrynin, Andrey V.
    Langmuir (2005), 21 (13), 6113-6122CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    We performed mol. dynamics simulations of the electrostatic assembly of multilayers of flexible polyelectrolytes at a charged surface. The multilayer build-up was achieved through sequential adsorption of oppositely charged polymers in a layer-by-layer fashion from dil. polyelectrolyte solns. The steady-state multilayer growth proceeds through a charge reversal of the adsorbed polymeric film which leads to a linear increase in the polymer surface coverage after completion of the first few deposition steps. Moreover, substantial intermixing between chains adsorbed during different deposition steps is obsd. This intermixing is consistent with the obsd. requirement for several deposition steps to transpire for completion of a single layer. However, despite chain intermixing, there are almost perfect periodic oscillations of the d. difference between monomers belonging to pos. and neg. charged macromols. in the adsorbed film. Weakly charged chains show higher polymer surface coverage than strongly charged ones.
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  97. 97
    Lacevic, N. M.; Joshi, S. P. Energy Dissipation Mechanism in Nanocomposites Studied via Molecular Dynamics Simulation. In Challenges in Mechanics of Time-Dependent Materials and Processes in Conventional and Multifunctional Materials. In Conference Proceedings of the Society for Experimental Mechanics Series; Antoun, B., Qi, H., Hall, R., Randon, G., Lu, H., Lu, C., Eds.; Springer: New York, 2013; Vol. 2, pp 23– 27.
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  98. 98
    Blell, R.; Lin, X.; Lindström, T.; Ankerfors, M.; Pauly, M.; Felix, O.; Decher, G. Generating In-Plane Orientational Order in Multilayer Films Prepared by Spray-Assisted Layer-by-Layer Assembly. ACS Nano 2017, 11, 84– 94,  DOI: 10.1021/acsnano.6b04191
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    98
    Generating in-Plane Orientational Order in Multilayer Films Prepared by Spray-Assisted Layer-by-Layer Assembly
    Blell, Rebecca; Lin, Xiaofeng; Lindstrom, Tom; Ankerfors, Mikael; Pauly, Matthias; Felix, Olivier; Decher, Gero
    ACS Nano (2017), 11 (1), 84-94CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The authors present a simple yet efficient method for orienting cellulose nanofibrils in layer-by-layer assembled films through spray-assisted alignment. While spraying at 90° against a receiving surface produces films with homogeneous in-plane orientation, spraying at smaller angles causes a macroscopic directional surface flow of liq. on the receiving surface and leads to films with substantial in-plane anisotropy when nanoscale objects with anisotropic shapes are used as components. First results with cellulose nanofibrils demonstrate that such fibrils are easily aligned by grazing incidence spraying to yield optically birefringent films over large surface areas. The cellulosic nanofibrils are oriented parallel to the spraying direction and that the orientational order depends for example on the distance of the receiving surface from the spray nozzle. The alignment of the nanofibrils and the in-plane anisotropy of the films were independently confirmed by at. force microscopy, optical microscopy between crossed polarizers, and the ellipsometric detn. of the apparent refractive index of the film as a function of the in-plane rotation of the sample with respect to the plane of incidence of the ellipsometer.
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  99. 99
    Sekar, S.; Lemaire, V.; Hu, H.; Decher, G.; Pauly, M. Anisotropic Optical and Conductive Properties of Oriented 1D-Nanoparticle Thin Films Made by Spray-Assisted Self-Assembly. Faraday Discuss. 2016, 191, 373– 389,  DOI: 10.1039/C6FD00017G
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    99
    Anisotropic optical and conductive properties of oriented 1D-nanoparticle thin films made by spray-assisted self-assembly
    Sekar, S.; Lemaire, V.; Hu, H.; Decher, G.; Pauly, M.
    Faraday Discussions (2016), 191 (Nanoparticles with Morphological and Functional Anisotropy), 373-389CODEN: FDISE6; ISSN:1359-6640. (Royal Society of Chemistry)
    We report on the fabrication of oriented anisotropic metal nanoparticle thin films made by Grazing Incidence Spraying (GIS) and on the anisotropic plasmonic properties of the resulting thin films. Gold nanorods of two different aspect ratios and silver nanowires were self-assembled as a uniaxially aligned monolayer with the GIS approach. In particular, we examine the influence of the nanowire/nanorod length and diam. on the degree of ordering detd. by electron microscopy pictures. Furthermore, we show that the anisotropy of the optical properties (probed by polarized UV-visible-near IR spectroscopy) strongly depend on the quality of alignment. The prepd. monolayer thin films have an orientation order parameter of up to 0.83 for silver nanowires, which is reflected in an optical anisotropy of 0.57 in the UV-visible and 0.76 in the near IR through the selective excitation of transverse and longitudinal surface plasmon resonance modes. The electronic transport in oriented silver nanowire monolayers is also shown to be highly directional, with the sheet resistance varying over almost an order of magnitude depending on the transport direction. Such anisotropic conductive plasmonic thin films may find applications in various fields like biochem. sensing, energy transport and harvesting or optoelectronic devices.
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  100. 100
    Hu, H.; Pauly, M.; Felix, O.; Decher, G. Spray-Assisted Alignment of Layer-by-Layer Assembled Silver Nanowires: A General Approach for the Preparation of Highly Anisotropic Nano-Composite Films. Nanoscale 2017, 9, 1307– 1314,  DOI: 10.1039/C6NR08045F
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    100
    Spray-assisted alignment of Layer-by-Layer assembled silver nanowires: a general approach for the preparation of highly anisotropic nano-composite films
    Hu, H.; Pauly, M.; Felix, O.; Decher, G.
    Nanoscale (2017), 9 (3), 1307-1314CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    The present article focuses on the build-up and the properties of oriented silver nanowire monolayer films and Layer-by-Layer assembled multilayer films. We describe the template-free oriented spray-assisted assembly of silver nanowires at solid/air-interfaces using Grazing Incidence Spraying, a simple and versatile approach that allows the formation of highly oriented thin films with a tunable d. and in-plane orientation. Depending on the spraying conditions the nematic order parameter, which describes the angular spread of misaligned nanowires, can be as high as 0.98 (a value of 1.00 corresponding to a perfectly parallel alignment). The combination with the Layer-by-Layer assembly allows building multilayer thin films possessing in-plane anisotropy. In order to demonstrate that the local alignment does not cancel out on the macroscopic scale but leads to direction-dependent properties, we use linearly polarized UV-Vis-NIR spectroscopy to probe the selective excitation of the transverse and longitudinal localized plasmon resonances of the nanowires. The polarization efficiency of the thin films increases strongly with the in-plane d., the degree of orientation, and the no. of silver nanowire layers. Multilayer films contg. 4 layers of nanowires oriented in the same direction reach a polarization efficiency of up to 97% in the near-IR region.
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  101. 101
    Shchukin, D.; Möhwald, H. A Coat of Many Functions. Science 2013, 341, 1458– 1459,  DOI: 10.1126/science.1242895
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    101
    A coat of many functions
    Shchukin, Dmitry; Mohwald, Helmuth
    Science (Washington, DC, United States) (2013), 341 (6153), 1458-1459CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    There is no expanded citation for this reference.
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    Andreeva, D. V.; Fix, D.; Möhwald, H.; Shchukin, D. G. Buffering Polyelectrolyte Multilayers for Active Corrosion Protection. J. Mater. Chem. 2008, 18, 1738– 1740,  DOI: 10.1039/b801314d
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    102
    Buffering polyelectrolyte multilayers for active corrosion protection
    Andreeva, Daria V.; Fix, Dmitri; Moehwald, Helmuth; Shchukin, Dmitry G.
    Journal of Materials Chemistry (2008), 18 (15), 1738-1740CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    Polyelectrolyte multilayers successively deposited on an ultrasonically pre-treated metal surface demonstrate anticorrosion self-healing behavior by the blocking of the corrosion processes without addn. of any other inhibiting species due to their pH-buffering ability.
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  103. 103
    Zheludkevich, M. L.; Shchukin, D. G.; Yasakau, K. A.; Möhwald, H.; Ferreira, M. G. S. Anticorrosion Coatings with Self-Healing Effect Based on Nanocontainers Impregnated with Corrosion Inhibitor. Chem. Mater. 2007, 19, 402– 411,  DOI: 10.1021/cm062066k
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    103
    Anticorrosion Coatings with Self-Healing Effect Based on Nanocontainers Impregnated with Corrosion Inhibitor
    Zheludkevich, Mikhail L.; Shchukin, Dmitry G.; Yasakau, Kiryl A.; Moehwald, Helmut; Ferreira, Mario G. S.
    Chemistry of Materials (2007), 19 (3), 402-411CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The present work shows a new contribution to the development of a new protective system with self-healing ability composed of hybrid sol-gel films doped with nanocontainers that release entrapped corrosion inhibitor in response to pH changes caused by corrosion process. A silica-zirconia based hybrid film was used in this work as an anticorrosion coating deposited on 2024 aluminum alloy. Silica nanoparticles covered layer-by-layer with polyelectrolyte layers and layers of inhibitor (benzotriazole) were randomly introduced into the hybrid films. The hybrid film with the nanocontainers reveals enhanced long-term corrosion protection in comparison with the undoped hybrid film. The scanning vibrating electrode technique also shows an effective self-healing ability of the defects. This effect is obtained due to regulated release of the corrosion inhibitor triggered by the corrosion processes started in the cavities. The approach described herein can be used in many applications where active corrosion protection of materials is required.
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    Andreeva, D. V.; Skorb, E. V.; Shchukin, D. G. Layer-by-Layer Polyelectrolyte/Inhibitor Nanostructures for Metal Corrosion Protection. ACS Appl. Mater. Interfaces 2010, 2, 1954– 1962,  DOI: 10.1021/am1002712
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    104
    Layer-by-Layer Polyelectrolyte/Inhibitor Nanostructures for Metal Corrosion Protection
    Andreeva, Daria V.; Skorb, Ekaterina V.; Shchukin, Dmitry G.
    ACS Applied Materials & Interfaces (2010), 2 (7), 1954-1962CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Multicomponent coating formed by polyelectrolyte multilayers opens new opportunities for anticorrosion protection. Here the authors demonstrate a novel method of corrosion protection based on formation and deposition of polyelectrolyte multilayers on Al and steel alloys, anal. of different polyelectrolyte compns. (strong-strong, strong-weak, weak-weak) as candidates for corrosion protective layers. The multilayer nanonetwork exhibits very high corrosion protection because of the nature and versatility of the polyelectrolyte complex. The anticorrosion activity of the coating is based on the following mechanisms: (1) pH buffer formed by polybase and polyacid complex suppress pH changes caused by corrosion degrdn.; (2) coating regeneration and defect elimination due to relative mobility of polymer chains in swollen state; (3) polyelectrolyte layers form a carrier for inhibitor allowing its release on demand; (4) polyelectrolyte nanonetwork provides a barrier between surface and environment. The authors optimize the coating prepn. conditions in a rational way by applying various polyacid-polybase combinations. The authors use the scanning vibration electrode technique to characterize corrosion protection of the novel coating.
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  105. 105
    Shchukin, D. G.; Lamaka, S. V.; Yasakau, K. A.; Zheludkevich, M. L.; Ferreira, M. G. S.; Möhwald, H. Active Anticorrosion Coatings with Halloysite Nanocontainers. J. Phys. Chem. C 2008, 112, 958– 964,  DOI: 10.1021/jp076188r
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    105
    Active Anticorrosion Coatings with Halloysite Nanocontainers
    Shchukin, Dmitry G.; Lamaka, S. V.; Yasakau, K. A.; Zheludkevich, M. L.; Ferreira, M. G. S.; Moehwald, H.
    Journal of Physical Chemistry C (2008), 112 (4), 958-964CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    This work contributes to the development of a new generation of active corrosion protection coatings composed of hybrid sol-gel films doped with halloysite nanotubes able to release entrapped corrosion inhibitors in a controllable way. A silica-zirconia-based hybrid film was used in this work as an anticorrosion coating deposited on 2024 aluminum alloy. Halloysite nanotubes with inner voids loaded by corrosion inhibitors (2-mercaptobenzothiazole) and outer surfaces layer-by-layer covered with polyelectrolyte multilayers were introduced into the hybrid films. The sol-gel film with the nanocontainers reveals enhanced long-term corrosion protection in comparison with the undoped sol-gel film. This effect is obtained because of the self-controlled release of the corrosion inhibitor triggered by the corrosion processes. Utilization of the inner halloysite nanotube lumen as a storage medium for the corrosion inhibitor offers a novel way of fabricating composite core-shell type nanomaterials with their further application as a main component of feedback-active coatings.
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  106. 106
    Li, G. L.; Zheng, Z.; Möhwald, H.; Shchukin, D. G. Silica/Polymer Double-Walled Hybrid Nanotubes: Synthesis and Application as Stimuli-Responsive Nanocontainers in Self-Healing Coatings. ACS Nano 2013, 7, 2470– 2478,  DOI: 10.1021/nn305814q
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    106
    Silica/Polymer Double-Walled Hybrid Nanotubes: Synthesis and Application as Stimuli-Responsive Nanocontainers in Self-Healing Coatings
    Li, Guo Liang; Zheng, Zhaoliang; Moehwald, Helmuth; Shchukin, Dmitry G.
    ACS Nano (2013), 7 (3), 2470-2478CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    We report the development of silica/polymer double-walled hybrid nanotubes, which consist of a hollow cavity, a porous silica inner wall, and a stimuli-responsive (pH, temp., and redox) polymeric outer wall, as a novel nanocontainer system. The length, diam., wall thickness, and aspect ratio of the hybrid nanotubes are precisely controlled in the range of 48-506 nm, 41-68 nm, 3-24 nm, and 1.2-7.6, resp. The hybrid nanotubes loaded with active mols. exhibit morphol.-dependent release and pH-, temp.-, redox-responsive release, which enable a wide range of applications from energy storage to drug delivery and self-healing coatings for metal corrosion protection.
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    Shchukin, D. G.; Shchukina, E. Capsules with External Navigation and Triggered Release. Curr. Opin. Pharmacol. 2014, 18, 42– 46,  DOI: 10.1016/j.coph.2014.09.002
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    Capsules with external navigation and triggered release
    Shchukin, Dmitry G.; Shchukina, Elena
    Current Opinion in Pharmacology (2014), 18 (), 42-46CODEN: COPUBK; ISSN:1471-4892. (Elsevier Ltd.)
    A review. Encapsulation is an important technol. for pharmaceutical industry, food prodn., et cetera. Its current level of development requires capsule functionalization. One of the interesting ideas to provide new functionality to the microcapsule and nanocapsule is layer-by-layer deposition of functional species. This technique provides step-by-step adsorption of various species (polyelectrolytes, nanoparticles, proteins) when the layer growth is controlled by electrostatic, hydrogen bonding, hydrophobic forces and forming multilayer shells with nanometer precision. This review article introduces recent achievements of layer-by-layer technique attaining external navigation ability and release properties the capsule shell.
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    Pomerantseva, E.; Gogotsi, Y. Two-Dimensional Heterostructures for Energy Storage. Nat. Energy. 2017, 2, 17089,  DOI: 10.1038/nenergy.2017.89
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    Two-dimensional heterostructures for energy storage
    Pomerantseva, Ekaterina; Gogotsi, Yury
    Nature Energy (2017), 2 (7), 17089CODEN: NEANFD; ISSN:2058-7546. (Nature Research)
    Two-dimensional (2D) materials provide slit-shaped ion diffusion channels that enable fast movement of lithium and other ions. However, electronic cond., the no. of intercalation sites, and stability during extended cycling are also crucial for building high-performance energy storage devices. While individual 2D materials, such as graphene, show some of the required properties, none of them can offer all properties needed to maximize energy d., power d., and cycle life. Here we argue that stacking different 2D materials into heterostructured architectures opens an opportunity to construct electrodes that would combine the advantages of the individual building blocks while eliminating the assocd. shortcomings. We discuss characteristics of common 2D materials and provide examples of 2D heterostructured electrodes that showed new phenomena leading to superior electrochem. performance. We also consider electrode fabrication approaches and finally outline future steps to create 2D heterostructured electrodes that could greatly expand current energy storage technologies.
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    Fendler, J. H. Colloid Chemical Approach to the Construction of High Energy Density Rechargeable, Lithium-Ion Batteries. J. Dispersion Sci. Technol. 1999, 20, 13– 25,  DOI: 10.1080/01932699908943776
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    Colloid chemical approach to the construction of high energy density rechargeable lithium-ion batteries
    Fendler, Janos H.
    Journal of Dispersion Science and Technology (1999), 20 (1 & 2), 13-25CODEN: JDTEDS; ISSN:0193-2691. (Marcel Dekker, Inc.)
    The layer-by-layer self-assembly of poly(diallyldimethylammonium) chloride, PDDA; graphite oxide, G, nanoplatelets; and polyethylene oxide, PEO, onto transparent indium tin oxide substrates, S, have provided cationic working electrodes which, with lithium wires as ref. and counter electrodes, functioned as high energy d. rechargeable lithium-ion batteries. Specifically, very high specific capacities (1232 and 1134 mAh.g-1) have been detd. for batteries which had pos. electrodes of ten sandwich layers of self-assembled films: S-(PDDA/GO/PEO)10 and S-(PDDA/PEO)10. The importance of the exfoliating GO into nanoplatelets and the colloid chem. of self-assembly are discussed.
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    Xiao, F. X.; Pagliaro, M.; Xu, Y. J.; Liu, B. Layer-by-Layer Assembly of Versatile Nanoarchitectures with Diverse Dimensionality: A New Perspective for Rational Construction of Multilayer Assemblies. Chem. Soc. Rev. 2016, 45, 3088– 3121,  DOI: 10.1039/C5CS00781J
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    110
    Layer-by-layer assembly of versatile nanoarchitectures with diverse dimensionality: a new perspective for rational construction of multilayer assemblies
    Xiao, Fang-Xing; Pagliaro, Mario; Xu, Yi-Jun; Liu, Bin
    Chemical Society Reviews (2016), 45 (11), 3088-3121CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    Over the past few decades, layer-by-layer (LbL) assembly of multilayer thin films has garnered considerable interest on account of its ability to modulate nanometer control over film thickness and its extensive choice of usable materials for coating planar and particulate substrates, thus allowing for the fabrication of responsive and functional thin films for their potential applications in a myriad of fields. Herein, we provide elaborate information on the current developments of LbL assembly techniques including different properties, mol. interactions, and assembly methods assocd. with this promising bottom-up strategy. In particular, we highlight the principle for rational design and fabrication of a large variety of multilayer thin film systems including multi-dimensional capsules or spatially hierarchical nanostructures based on the LbL assembly technique. Moreover, we discuss how to judiciously choose the building block pairs when exerting the LbL assembly buildup which enables the engineering of multilayer thin films with tailor-made physicochem. properties. Furthermore, versatile applications of the diverse LbL-assembled nanomaterials are itemized and elucidated in light of specific technol. fields. Finally, we provide a brief perspective and potential future challenges of the LbL assembly technol. It is anticipated that our current review could provide a wealth of guided information on the LbL assembly technique and furnish firm grounds for rational design of LbL assembled multilayer assemblies toward tangible applications.
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  111. 111
    Liu, L.; Choi, B. G.; Tung, S. O.; Lyu, J.; Li, T.; Zhao, T.; Kotov, N. A. Materials Engineering of High-Performance Anodes as Layered Composites with Self-Assembled Conductive Networks. J. Phys. Chem. C 2018, 122, 14014– 14028,  DOI: 10.1021/acs.jpcc.8b01105
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    111
    Materials Engineering of High-Performance Anodes as Layered Composites with Self-Assembled Conductive Networks
    Liu, Lehao; Choi, Bong Gill; Tung, Siu On; Lyu, Jing; Li, Tiehu; Zhao, Tingkai; Kotov, Nicholas A.
    Journal of Physical Chemistry C (2018), 122 (25), 14014-14028CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The practical implementation of nanomaterials in high capacity batteries has been hindered by the large mech. stresses during ion insertion/extn. processes that lead to the loss of phys. integrity of the active layers. The challenge of combining the high ion storage capacity with resilience to deformations and efficient charge transport is common for nearly all battery technologies. Layer-by-layer (LBL/LbL) engineered nanocomposites are able to mitigate structural design challenges for materials requiring the combination of contrarian properties. Here, we show that materials engineering capabilities of LBL augmented by self-organization of nanoparticles (NPs) can be exploited for constructing multiscale composites for high capacity Li ion anodes that mitigate the contrarian nature of 3 central parameters most relevant for advanced batteries: large intercalation capacity, high conductance, and robust mechanics. The LBL multilayers were made from 3 function-detg. components, namely, polyurethane (PU), Cu nanoscale particles, and Si mesoscale particles responsible for the high nanoscale toughness, efficient electron transport, and high Li storage capacity, resp. The nanocomposite anodes optimized in respect to the layer sequence and compn. exhibited capacities as high as 1284 and 687 mAh/g at the 1st and 300th cycle, resp., with a fading rate of 0.15% per cycle. Av. Coulombic efficiencies were as high as 99.0-99.4% for 300 cycles at 1.0 C rate (4000 mA/g). Self-organization of Cu NPs into 3-dimensional (3D) networks with lattice-to-lattice connectivity taking place during LBL assembly enabled high electron transport efficiency responsible for high battery performance of these Si-based anodes. This study paves the way to finding a method for resoln. of the general property conflict for materials utilized in for energy technologies.
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  112. 112
    Tung, S. O.; Ho, S.; Yang, M.; Zhang, R.; Kotov, N. A. A Dendrite-Suppressing Composite Ion Conductor from Aramid Nanofibres. Nat. Commun. 2015, 6, 6152,  DOI: 10.1038/ncomms7152
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    112
    A dendrite-suppressing composite ion conductor from aramid nanofibres
    Tung, Siu-On; Ho, Szushen; Yang, Ming; Zhang, Ruilin; Kotov, Nicholas A.
    Nature Communications (2015), 6 (), 6152CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Dendrite growth threatens the safety of batteries by piercing the ion-transporting separators between the cathode and anode. Finding a dendrite-suppressing material that combines high modulus and high ionic conductance has long been considered a major technol. and materials science challenge. Here we demonstrate that these properties can be attained in a composite made from Kevlar-derived aramid nanofibres assembled in a layer-by-layer manner with poly(ethylene oxide). Importantly, the porosity of the membranes is smaller than the growth area of the dendrites so that aramid nanofibres eliminate 'weak links' where the dendrites pierce the membranes. The aramid nanofibre network suppresses poly(ethylene oxide) crystn. detrimental for ion transport, giving a composite that exhibits high modulus, ionic cond., flexibility, ion flux rates and thermal stability. Successful suppression of hard copper dendrites by the composite ion conductor at extreme discharge conditions is demonstrated, thereby providing a new approach for the materials engineering of solid ion conductors.
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  113. 113
    Wang, M.; Emre, A.; Tung, S.; Gerber, A.; Wang, D.; Huang, Y.; Cecen, V.; Kotov, N. A. Biomimetic Solid-State Zn2+ Electrolyte for Corrugated Structural Batteries. ACS Nano 2019, 13, 1107– 1115,  DOI: 10.1021/acsnano.8b05068
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    113
    Biomimetic Solid-State Zn2+ Electrolyte for Corrugated Structural Batteries
    Wang, Mingqiang; Emre, Ahmet; Tung, Siuon; Gerber, Alycia; Wang, Dandan; Huang, Yudong; Cecen, Volkan; Kotov, Nicholas A.
    ACS Nano (2019), 13 (2), 1107-1115CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Batteries based on divalent metals, such as the Zn/Zn2+ pair, represent attractive alternatives to Li-ion chem. due to their high safety, reliability, earth-abundance, and energy d. However, archetypal Zn batteries are bulky, inflexible, non-rechargeable, and contain a corrosive electrolyte. Suppression of the anodic growth of Zn dendrites is essential for resoln. of these problems and requires materials with nanoscale mechanics sufficient to withstand mech. deformation from stiff Zn dendrites. Such materials must also support rapid transport Zn2+ ions necessary for high Coulombic efficiency and energy d., which makes the structural design of such materials a difficult fundamental problem. Here, we show that it is possible to engineer a solid Zn2+ electrolyte as a composite of branched aramid nanofibers (BANFs) and poly(ethylene oxide) by using the nanoscale organization of articular cartilage as a blueprint for its design. The high stiffness of the BANF network combined with the high ionic cond. of soft poly(ethylene oxide) enable effective suppression of dendrites and fast Zn2+ transport. The cartilage-inspired composite displays the ionic conductance 10× higher than the original polymer. The batteries constructed using the nanocomposite electrolyte are rechargeable and have Coulombic efficiency of 96-100% after 50-100 charge-discharge cycles. Furthermore, the biomimetic solid-state electrolyte enables the batteries to withstand not only elastic deformation during bending but also plastic deformation. This capability make them resilient to different type of damage and enables shape modification of the assembled battery to improve the ability of the battery stack to carry a structural load. The corrugated batteries can be integrated into body elements of unmanned aerial vehicles as auxiliary charge-storage devices. This functionality was demonstrated by replacing the covers of several small drones with corrugated Zn/BANF/MnO2 cells, resulting in the extension of the total flight time. These findings open a pathway to the design and utilization of corrugated structural batteries in the future transportation industry and other fields of use.
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    Gao, M.; Lesser, C.; Kirstein, S.; Möhwald, H.; Rogach, A. L.; Weller, H. Electroluminescence of Different Colors from Polycation/CdTe Nanocrystal Self-Assembled Films. J. Appl. Phys. 2000, 87, 2297– 2302,  DOI: 10.1063/1.372177
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    114
    Electroluminescence of different colors from polycation/CdTe nanocrystal self-assembled films
    Gao, Mingyuan; Lesser, Constanze; Kirstein, Stefan; Mohwald, Helmuth; Rogach, Andrey L.; Weller, Horst
    Journal of Applied Physics (2000), 87 (5), 2297-2302CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Water-sol. thiol capped CdTe nanocrystals were assembled into ultrathin films in combination with poly(diallyldimethylammonium chloride) (PDDA) by the self-assembly method of layer-by-layer adsorption of oppositely charged polyelectrolytes. Electroluminescent devices, which produce different color emissions, were fabricated by sandwiching CdTe/PDDA films between ITO and aluminum electrodes using CdTe nanocrystals of different sizes. It is shown that the electroluminescence (EL) spectra of the CdTe/polymer films are nearly identical to the photoluminescence spectra of the corresponding CdTe nanocrystals in aq. solns. The devices produce a room-light visible light output with an external quantum efficiency up to 0.1%. Light emission is obsd. at current densities of 10 mA/cm2 and at low onset voltages of 2.5-3.5 V, which depends on the thickness of the film indicating field-dependent current injection. A variation of the EL efficiency with the size of the CdTe particles is obsd. and explained by the size-dependent shift of the CdTe energy levels with respect to the work function of the electron injecting Al electrode. This is confirmed by the behavior of two-layer devices prepd. from two differently sized CdTe particles being spatially sepd., i.e., one size CdTe near ITO and the other size CdTe near Al by using the self-assembly method.
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    Franzl, T.; Koktysh, D. S.; Klar, T. A.; Rogach, A. L.; Feldmann, J.; Gaponik, N. Fast Energy Transfer in Layer-by-Layer Assembled CdTe Nanocrystal Bilayers. Appl. Phys. Lett. 2004, 84, 2904– 2906,  DOI: 10.1063/1.1702136
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    Fast energy transfer in layer-by-layer assembled CdTe nanocrystal bilayers
    Franzl, T.; Koktysh, D. S.; Klar, T. A.; Rogach, A. L.; Feldmann, J.; Gaponik, N.
    Applied Physics Letters (2004), 84 (15), 2904-2906CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The authors report on efficient resonant energy transfer in bilayers of H2O-sol. CdTe quantum dots. The bilayers of CdTe nanocrystals of 2 different sizes capped by short-chain thiols were formed by layer-by-layer assembly. Temporally and spectrally resolved fluorescence spectroscopy reveals spectral diffusion of the fluorescence signal for quantum dots within 1 layer as well as rapid (254 ps) energy transfer from layers of small dots to layers of larger dots, which is fast for nanocrystal pairs. Subspecies within the inhomogeneous distribution of donor nanocrystals even show energy transfer rates of (134 ps)-1 due to a large spectral overlap with acceptor nanocrystals.
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    Franzl, T.; Klar, T. A.; Schietinger, S.; Rogach, A. L.; Feldmann, J. Exciton Recycling in Graded Gap Nanocrystal Structures. Nano Lett. 2004, 4, 1599– 1603,  DOI: 10.1021/nl049322h
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    116
    Exciton Recycling in Graded Gap Nanocrystal Structures
    Franzl, Thomas; Klar, Thomas A.; Schietinger, Stefan; Rogach, Andrey L.; Feldmann, Jochen
    Nano Letters (2004), 4 (9), 1599-1603CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We propose a cascaded energy transfer structure made of semiconductor nanocrystals. Funnel-like band gap profiles are realized applying layer-by-layer assembly to CdTe nanocrystals of distinct sizes. Optical excitations efficiently transfer along the band gap gradient and are finally captured by the largest nanocrystals. The photoluminescence yield from the center layer is surprisingly high. The stepwise passing on of excitation energy avoids hot carriers, and the funnel structure recycles otherwise trapped and lost electron-hole pairs.
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  117. 117
    Klar, T. A.; Franzl, T.; Rogach, A. L.; Feldmann, J. Super-Efficient Exciton Funneling in Layer-by-Layer Semiconductor Nanocrystal Structures. Adv. Mater. 2005, 17, 769– 773,  DOI: 10.1002/adma.200401675
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    117
    Super-efficient exciton funneling in layer-by-layer semiconductor nanocrystal structures
    Klar, Thomas A.; Franzl, Thomas; Rogach, Andrey L.; Feldmann, Jochen
    Advanced Materials (Weinheim, Germany) (2005), 17 (6), 769-773CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    In semiconductor nanocrystals the electronic energy gap is detd. not only by the material but also by the size of the nanocrystals. This allows the construction of an energy-gap gradient normal to multiple layers of nanocrystals where the diams. of the nanocrystals are monotonically increasing or decreasing in subsequent layers. In such devices the authors observe a highly efficient funneling of excitation energy from layers comprising smaller nanocrystals towards the layer with the largest nanocrystals in the center of the funnel. Most importantly, not only are excitons in radiative states transferred, but also excitons from trapped states, usually lost for luminescence, can be effectively recycled, hence increasing the overall luminescence yield.
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  118. 118
    Susha, A. S.; Caruso, F.; Rogach, A. L.; Sukhorukov, G. B.; Kornowski, A.; Möhwald, H.; Giersig, M.; Eychmüller, A.; Weller, H. Formation of Luminescent Spherical Core-Shell Particles by the Consecutive Adsorption of Polyelectrolyte and CdTe(S) Nanocrystals on Latex Colloids. Colloids Surf., A 2000, 163, 39– 44,  DOI: 10.1016/S0927-7757(99)00428-8
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    118
    Formation of luminescent spherical core-shell particles by the consecutive adsorption of polyelectrolyte and CdTe(S) nanocrystals on latex colloids
    Susha, A. S.; Caruso, F.; Rogach, A. L.; Sukhorukov, G. B.; Kornowski, A.; Mohwald, H.; Giersig, M.; Eychmuller, A.; Weller, H.
    Colloids and Surfaces, A: Physicochemical and Engineering Aspects (2000), 163 (1), 39-44CODEN: CPEAEH; ISSN:0927-7757. (Elsevier Science B.V.)
    Functional core-shell particles were prepd. by assembling a composite multilayer shell of charged polyelectrolytes and luminescent CdTe(S) nanocrystals (Cd telluride with a certain content of sulfide) via their consecutive electrostatic adsorption from soln. onto micron-sized latex particles. The formation of the composite shell was confirmed by optical spectroscopy, confocal microscopy and high-resoln. TEM. Variation of the size and surface chem. of the semiconductor nanoparticles, the size and shape of the colloid templates, and the nature of the polyelectrolyte opens new avenues for the prodn. of a variety of novel core-shell materials.
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    Gaponik, N.; Radtchenko, I. L.; Gerstenberger, M. R.; Fedutik, Y. A.; Sukhorukov, G. B.; Rogach, A. L. Labeling of Biocompatible Polymer Microcapsules with Near-Infrared Emitting Nanocrystals. Nano Lett. 2003, 3, 369– 372,  DOI: 10.1021/nl0259333
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    119
    Labeling of Biocompatible Polymer Microcapsules with Near-Infrared Emitting Nanocrystals
    Gaponik, Nikolai; Radtchenko, Igor L.; Gerstenberger, Maria R.; Fedutik, Yuri A.; Sukhorukov, Gleb B.; Rogach, Andrey L.
    Nano Letters (2003), 3 (3), 369-372CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Microcapsules consisting solely of biocompatible components were prepd. in water by a colloidal templating technique on sol. carbonate cores using alginic acid sodium salt, protamine sulfate, dextran sulfate, and chitosan. These microcapsules, as well as capsules made from synthetic polyelectrolytes, were labeled with water-sol. CdTe nanocrystals (NCs) emitting in the visible and, for the first time, with CdxHg1-xTe or HgTe NCs emitting in the near-IR. The luminescence efficiency of NCs at physiol. conditions remained stable for two weeks in the case of CdTe and at least for a month for CdHgTe and dropped by 80% for HgTe because of the shift of the luminescence band outside the water transmission window. Biocompatible microcapsules labeled with CdxHg1-xTe NCs emitting at 750-1200 nm might be of special interest for monitoring the drug delivery processes.
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  120. 120
    Gaponik, N.; Radtchenko, I. L.; Sukhorukov, G. B.; Rogach, A. L. Luminescent Polymer Microcapsules Addressable by a Magnetic Field. Langmuir 2004, 20, 1449– 1452,  DOI: 10.1021/la035914o
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    120
    Luminescent Polymer Microcapsules Addressable by a Magnetic Field
    Gaponik, Nikolai; Radtchenko, Igor L.; Sukhorukov, Gleb B.; Rogach, Andrey L.
    Langmuir (2004), 20 (4), 1449-1452CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The simultaneous encapsulation of both luminescent semiconductor and magnetic oxide nanoparticles in polymer microcapsules is demonstrated for the first time. Highly luminescent CdTe semiconductor nanocrystals serve as luminescent markers, while magnetic Fe3O4 nanoparticles allow external manipulation of the capsules by magnetic field. The method introduced is general enough to allow the fabrication of different types of multifunctional capsules in a similar way. The use of multifunctional water-compatible capsules introduced in this paper for the controlled release and directed drug delivery in biol. systems is envisaged.
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    Kotov, N. A.; Dekany, I.; Fendler, J. H. Layer-by-Layer Self-Assembly of Polyelectrolyte-Semiconductor Nanoparticle Composite Films. J. Phys. Chem. 1995, 99, 13065– 13069,  DOI: 10.1021/j100035a005
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    121
    Layer-by-Layer Self-Assembly of Polyelectrolyte-Semiconductor Nanoparticle Composite Films
    Kotov, Nicholas A.; Dekany, Imre; Fendler, Janos H.
    Journal of Physical Chemistry (1995), 99 (35), 13065-9CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
    Spectroscopic (absorption and emission), microscopic (transmission electron and at. force), X-ray diffraction and photocurrent measurements have provided evidence for the formation of stable ultrathin films with regular periodicities, by the layer-by-layer self-assembly of polycations and cadmium sulfide, lead sulfide and titanium dioxide nanoparticles.
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  122. 122
    Rogach, A. L.; Koktysh, D. S.; Harrison, M.; Kotov, N. A. Layer-by-Layer Assembled Films of HgTe Nanocrystals with Strong Infrared Emission. Chem. Mater. 2000, 12, 1526– 1528,  DOI: 10.1021/cm0000649
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    122
    Layer-by-layer assembled films of HgTe nanocrystals with strong infrared emission
    Rogach, A. L.; Koktysh, D. S.; Harrison, M.; Kotov, N. A.
    Chemistry of Materials (2000), 12 (6), 1526-1528CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Thin films of HgTe/polymer composite were prepd. by the layer-by-layer assembly (LBL) method using colloidal solns. of thioglycerol-capped HgTe nanocrystals as starting material. For the LBL deposition, the substrates (ITO-covered glass, Au-coated quartz crystals, and Si wafers) were dipped into a soln. of poly(diallyldimethylammonium chloride) (PDDA) and into the dispersion of the HgTe nanocrystals. In this way, on each exposed surface a HgTe/PDDA bilayer is formed. The films were characterized by STM, measurements of the adsorption kinetics, and UV-Vis absorption spectroscopy. A strong photoluminescence emission in the near-IR was obsd. around 1600 nm for a multilayer (PDDA/HgTe)20 LBL film.
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    Mamedov, A. A.; Belov, A.; Giersig, M.; Mamedova, N. N.; Kotov, N. A. Nanorainbows: Graded Semiconductor Films from Quantum Dots. J. Am. Chem. Soc. 2001, 123, 7738– 7739,  DOI: 10.1021/ja015857q
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    123
    Nanorainbows. Graded semiconductor films from quantum dots
    Mamedov, Arif A.; Belov, Artem; Giersig, Michael; Mamedova, Nataliya N.; Kotov, Nicholas A.
    Journal of the American Chemical Society (2001), 123 (31), 7738-7739CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    One dimensionally graded, semiconducting films were grown by layer-by-layer assembly. The films were prepd. from different dispersions contg. nanoparticles of CdTe stabilized by thioglycolic acid and the polyelectrolyte poly(diallyldimethylammonium chloride). The LBL assembly was carried out on glass and plastic substrates. The 4 CdTe dispersions had different luminescence maxima at 495-505, 530-545, 570-585, and 605-620 nm resulting in green, yellow, orange, and red luminescence, resp. They were characterized by AFM, TEM, cross-sectional, confocal, optical microscopy, and luminescence spectroscopy.
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    Zhu, J.; Kang, J.; Kang, J.; Jariwala, D.; Wood, J. D.; Seo, J. W. T.; Chen, K. S.; Marks, T. J.; Hersam, M. C. Solution-Processed Dielectrics Based on Thickness-Sorted Two-Dimensional Hexagonal Boron Nitride Nanosheets. Nano Lett. 2015, 15, 7029– 7036,  DOI: 10.1021/acs.nanolett.5b03075
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    124
    Solution-Processed Dielectrics Based on Thickness-Sorted Two-Dimensional Hexagonal Boron Nitride Nanosheets
    Zhu Jian; Kang Joohoon; Kang Junmo; Jariwala Deep; Wood Joshua D; Seo Jung-Woo T; Chen Kan-Sheng; Marks Tobin J; Hersam Mark C
    Nano letters (2015), 15 (10), 7029-36 ISSN:.
    Gate dielectrics directly affect the mobility, hysteresis, power consumption, and other critical device metrics in high-performance nanoelectronics. With atomically flat and dangling bond-free surfaces, hexagonal boron nitride (h-BN) has emerged as an ideal dielectric for graphene and related two-dimensional semiconductors. While high-quality, atomically thin h-BN has been realized via micromechanical cleavage and chemical vapor deposition, existing liquid exfoliation methods lack sufficient control over h-BN thickness and large-area film quality, thus limiting its use in solution-processed electronics. Here, we employ isopycnic density gradient ultracentrifugation for the preparation of monodisperse, thickness-sorted h-BN inks, which are subsequently layer-by-layer assembled into ultrathin dielectrics with low leakage currents of 3 × 10(-9) A/cm(2) at 2 MV/cm and high capacitances of 245 nF/cm(2). The resulting solution-processed h-BN dielectric films enable the fabrication of graphene field-effect transistors with negligible hysteresis and high mobilities up to 7100 cm(2) V(-1) s(-1) at room temperature. These h-BN inks can also be used as coatings on conventional dielectrics to minimize the effects of underlying traps, resulting in improvements in overall device performance. Overall, this approach for producing and assembling h-BN dielectric inks holds significant promise for translating the superlative performance of two-dimensional heterostructure devices to large-area, solution-processed nanoelectronics.
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    Feifel, S. C.; Kapp, A.; Lisdat, F. Protein Multilayer Architectures on Electrodes for Analyte Detection. In Biosensors Based on Aptamers and Enzymes; Gu, M. B., Kim, H. S., Eds.; Springer: Berlin, 2013; Vol. 140, pp 253– 298.
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    Heath, G. R.; Li, M.; Rong, H.; Radu, V.; Frielingsdorf, S.; Lenz, O.; Butt, J. N.; Jeuken, L. J. C. Multilayered Lipid Membrane Stacks for Biocatalysis Using Membrane Enzymes. Adv. Funct. Mater. 2017, 27, 1606265,  DOI: 10.1002/adfm.201606265
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    Dronov, R.; Kurth, D. G.; Möhwald, H.; Scheller, F. W.; Lisdat, F. Communication in a Protein Stack: Electron Transfer between Cytochrome c and Bilirubin Oxidase within a Polyelectrolyte Multilayer. Angew. Chem., Int. Ed. 2008, 47, 3000– 3003,  DOI: 10.1002/anie.200704049
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    Communication in a protein stack: electron transfer between cytochrome c and bilirubin oxidase within a polyelectrolyte multilayer
    Dronov, Roman; Kurth, Dirk G.; Moehwald, Helmuth; Scheller, Frieder W.; Lisdat, Fred
    Angewandte Chemie, International Edition (2008), 47 (16), 3000-3003CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Assemblies of proteins and enzymes can serve as a model for mimicking biol. signal transfer. A polyelectrolyte multilayer of an electron-transfer protein (cytochrome c, red) and a copper enzyme (bilirubin oxidase, blue) has catalytic properties for four-electron oxygen redn. The proposed architecture guarantees efficient protein-protein interaction without an addnl. redox mediator.
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    Grant, G. G. S.; Koktysh, D. S.; Yun, B.; Matts, R. L.; Kotov, N. A. Layer-by-Layer Assembly of Collagen Thin Films: Controlled Thickness and Biocompatibility. Biomed. Microdevices 2001, 3, 301– 306,  DOI: 10.1023/A:1012456714628
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    Dronov, R.; Kurth, D. G.; Möhwald, H.; Spricigo, R.; Leimkuehler, S.; Wollenberger, U.; Rajagopalan, K. V.; Scheller, F. W.; Lisdat, F. Layer-by-Layer Arrangement by Protein-Protein Interaction of Sulfite Oxidase and Cytochrome c Catalyzing Oxidation of Sulfite. J. Am. Chem. Soc. 2008, 130, 1122– 1123,  DOI: 10.1021/ja0768690
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    Layer-by-Layer Arrangement by Protein-Protein Interaction of Sulfite Oxidase and Cytochrome c Catalyzing Oxidation of Sulfite
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    Journal of the American Chemical Society (2008), 130 (4), 1122-1123CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Layer-by-layer self-assembly of sulfite oxidase and cytochrome c was carried out without addnl. polymeric polyelectrolytes. The arrangement shows a linear increase in the immobilized protein mass after each deposition cycle. The modified electrodes demonstrate electrocatalytic activity for sulfite oxidn. generating catalytic current with a linear increase with the no. of layers. This effect shows that, in the protein assembly without a polyelectrolyte, electron transfer occurs, thus supporting the concept for direct interprotein electron exchange.
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    Feifel, S. C.; Kapp, A.; Ludwig, R.; Lisdat, F. Nanobiomolecular Multiprotein Clusters on Electrodes for the Formation of a Switchable Cascadic Reaction Scheme. Angew. Chem., Int. Ed. 2014, 53, 5676– 5679,  DOI: 10.1002/anie.201310437
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    Nanobiomolecular Multiprotein Clusters on Electrodes for the Formation of a Switchable Cascadic Reaction Scheme
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    A supramol. multicomponent protein architecture on electrodes is developed that allows the establishment of bidirectional electron transfer cascades based on interprotein electron exchange. The architecture is formed by embedding two different enzymes - laccase (Lac) and cellobiose dehydrogenase (CDH) - and a redox protein - cytochrome c (cyt c) - by means of carboxy-modified silica nanoparticles in a multiple layer format. The construct is designed as a switchable dual analyte detection device allowing the measurement of lactose and oxygen, resp. As the switching force we apply the electrode potential, which ensures control of the redox state of cytochrome c. The two signal chains are operating in a non-sepd. matrix and are not disturbed by the other biocatalyst.
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    Gheith, M. K.; Sinani, V. A.; Wicksted, J. P.; Matts, R. L.; Kotov, N. A. Single-Walled Carbon Nanotube Polyelectrolyte Multilayers and Freestanding Films as a Biocompatible Platform for Neuroprosthetic Implants. Adv. Mater. 2005, 17, 2663– 2670,  DOI: 10.1002/adma.200500366
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    Advanced Materials (Weinheim, Germany) (2005), 17 (22), 2663-2670CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Layer-by-layer (LBL) assembly has been used to prep. single-walled carbon nanotube (SWNT) freestanding structures that can be used for implantable devices with unique mech. and elec. properties (see Figure). The thin LBL membranes prepd. are biocompatible and support extensive neurite outgrowth. The stretched fiber-like LBL freestanding films exhibit neuronal guiding. The SWNT structures described may potentially enable connectivity between neurons.
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    Pappas, T. C.; Wickramanyake, W. M. S.; Jan, E.; Motamedi, M.; Brodwick, M.; Kotov, N. A. Nanoscale Engineering of a Cellular Interface with Semiconductor Nanoparticle Films for Photoelectric Stimulation of Neurons. Nano Lett. 2007, 7, 513– 519,  DOI: 10.1021/nl062513v
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    Nano Letters (2007), 7 (2), 513-519CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The remarkable optical and elec. properties of nanostructured materials are considered now as a source for a variety of biomaterials, biosensing, and cell interface applications. In this study, the authors report the first example of hybrid bionanodevice where absorption of light by thin films of quantum confined semiconductor nanoparticles of HgTe produced by the layer-by-layer assembly stimulate adherent neural cells via a sequence of photochem. and charge-transfer reactions. The authors also demonstrate an example of nanoscale engineering of the material driven by biol. functionalities.
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    Rabe, K. S.; Müller, J.; Skoupi, M.; Niemeyer, C. M. Cascades in Compartments: En Route to Machine-Assisted Biotechnology. Angew. Chem., Int. Ed. 2017, 56, 13574– 13589,  DOI: 10.1002/anie.201703806
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    Angewandte Chemie, International Edition (2017), 56 (44), 13574-13589CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Biol. compartmentalization is a fundamental principle of life that allows cells to metabolize, propagate, or communicate with their environment. Much research is devoted to understanding this basic principle and to harness biomimetic compartments and catalytic cascades as tools for technol. processes. This Review summarizes the current state-of-the-art of these developments, with a special emphasis on length scales, mass transport phenomena, and mol. scaffolding approaches, ranging from small crosslinkers over proteins and nucleic acids to colloids and patterned surfaces. The future exploration and exploitation of these complex systems will largely benefit from tech. solns. for the integrated, machine-assisted development and maintenance of a next generation of biotechnol. processes. These goals should be achievable by implementing microfluidics, robotics, and added manufg. techniques supplemented by theor. simulations as well as computer-aided process modeling based on big data obtained from multiscale exptl. analyses.
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    Kreft, O.; Skirtach, A. G.; Sukhorukov, G. B.; Möhwald, H. Remote Control of Bioreactions in Multicompartment Capsules. Adv. Mater. 2007, 19, 3142– 3145,  DOI: 10.1002/adma.200701977
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    Advanced Materials (Weinheim, Germany) (2007), 19 (20), 3142-3145CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Remote control over reactions in multicompartment capsules is investigated. Dual-compartment capsules were constructed by consecutive fabrication of the inner and the outer parts of CaCO3 microcapsules sepd. by a polyelectrolyte multilayer IR-sensitized by nanoparticle doping. Upon near-IR laser illumination the contents of the inner capsules are mixed with the outer one. This research paves the way to a simple method for performing sophisticated bioreactions in confined vols.
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    Bäumler, H.; Georgieva, R. Coupled Enzyme Reactions in Multicompartment Microparticles. Biomacromolecules 2010, 11, 1480– 1487,  DOI: 10.1021/bm1001125
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    Coupled enzyme reactions in multicompartment microparticles
    Baumler Hans; Georgieva Radostina
    Biomacromolecules (2010), 11 (6), 1480-7 ISSN:.
    Spherical biopolymer particles have been fabricated, applying coprecipitation with calcium carbonate, followed by cross-linking of the macromolecules and dissolution of the inorganic support. Particles made of roughly 80% horseradish peroxidase (HRP) as well as glucose oxidase (GOX) were prepared and enzyme activities were confirmed, applying the Amplex Red assay. The enzyme particles were reusable for at least six times, with a remaining activity of 30-50% from the initial one. When multiple coprecipitation steps and one or several cross-linking procedures were applied, multicompartment particles were obtained. Each of the resulting concentric compartments could be independently loaded with biomolecules. Three coupled enzymes, beta-glucosidase (beta-Glu), GOX, and HRP have been incorporated stepwise in such particles. Each of these enzymes was located in a separate compartment, in a desired sequence, and at a defined position. The distance between the enzyme containing compartments was also varied, including spacing compartments consisting of bovine serum albumin (BSA). When fluorogenic substrates for beta-Glu and HPR were used, the start and the end of the coupled enzyme reaction were visualized and recorded inside of individual particles, applying confocal laser scanning microscopy. A strong influence of the spacing on the reaction kinetics of the last enzyme was observed, suggesting an impaired diffusion of the intermediate products of the chain reaction through the spacing compartments made of BSA. The influence of the spacing between compartments containing different enzymes on the reaction kinetics was demonstrated on the microscopic scale within one microparticle, which to the best of our knowledge was not achieved until now.
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    Stoll, C.; Kudera, S.; Parak, W. J.; Lisdat, F. Quantum Dots on Gold: Electrodes For Photoswitchable Cytochrome c Electrochemistry. Small 2006, 2, 741– 743,  DOI: 10.1002/smll.200500441
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    Quantum dots on gold: electrodes for photoswitchable cytochrome c electrochemistry
    Stoll, Christian; Kudera, Stefan; Parak, Wolfgang J.; Lisdat, Fred
    Small (2006), 2 (6), 741-743CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A photoswitchable arrangement for direct electrochem. of the redox protein cytochrome c can result from the application of CdSe/ZnS quantum dots on Au electrodes. On/off responses can be realized through photoexcitation of the quantum dots. Electrode illumination resulted in stepwise heterogeneous electron transfer from the electrode via the semiconductor particles to the heme proteins (see figure).
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    Katz, E.; Zayats, M.; Willner, I.; Lisdat, F. Controlling the Direction of Photocurrents by Means of CdS Nanoparticles and Cytochrome c-Mediated Biocatalytic Cascades. Chem. Commun. 2006, 0, 1395– 1397,  DOI: 10.1039/b517332a
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    Controlling the direction of photocurrents by means of CdS nanoparticles and cytochrome c-mediated biocatalytic cascades
    Katz, Eugenii; Zayats, Maya; Willner, Itamar; Lisdat, Fred
    Chemical Communications (Cambridge, United Kingdom) (2006), (13), 1395-1397CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Cathodic or anodic photocurrents are generated by a monolayer of CdS nanoparticles in the presence of the oxidized or reduced states of cytochrome c, resp., and the photocurrents are amplified by enzyme-generated biocatalytic cascades mediated by cytochrome c.
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    Rajendran, V.; Lehnig, M.; Niemeyer, C. M. Photocatalytic Activity of Colloidal CdS Nanoparticles with Different Capping Ligands. J. Mater. Chem. 2009, 19, 6348– 6353,  DOI: 10.1039/b902187f
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    Photocatalytic activity of colloidal CdS nanoparticles with different capping ligands
    Rajendran, Vidyalakshmi; Lehnig, Manfred; Niemeyer, Christof M.
    Journal of Materials Chemistry (2009), 19 (35), 6348-6353CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    We report on the synthesis and characterization of cadmiumsulfide quantum dots (CdS-QDs) with three different types of capping ligands, mercaptoacetic acid (CdS-MAA), cysteamine, and 2-mercaptoethanesulfonic acid (CdS-CM), or tiopronin (CdS-TPN). All three types of QDs generate reactive oxygen species (ROS), in particular, superoxide- and hydroxyl radicals, upon irradn. with UV light. The ROS were analyzed using ESR (EPR) spectroscopy. Moreover, the hydroxyl radical generation from the three different QDs was monitored using a terephthalate-based fluorimetric assay. The three QDs were then used as photocatalyst to trigger the peroxidase activity of horseradish peroxidase.
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    Ipe, B. I.; Niemeyer, C. M. Nanohybrids Composed of Quantum Dots and Cytochrome P450 as Photocatalysts. Angew. Chem., Int. Ed. 2006, 45, 504– 507,  DOI: 10.1002/anie.200503084
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    Nanohybrids composed of quantum dots and cytochrome P450 as photocatalysts
    Ipe, Binil Itty; Niemeyer, Christof M.
    Angewandte Chemie, International Edition (2006), 45 (3), 504-507CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Synthesis with light: CdS quantum dots (QDs) generate superoxide and hydroxyl radicals upon UV irradn. in aq. soln. The radicals are used for activating P 450BSβ enzymes attached at the QD surface, effecting the catalytic transformation of myristic acid into α- and β- hydroxymyristic acid (see picture, R = (CH2)10CH3).
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    Yue, Z.; Lisdat, F.; Parak, W. J.; Hickey, S. G.; Tu, L. P.; Sabir, N.; Dorfs, D.; Bigall, N. C. Quantum-Dot-Based Photoelectrochemical Sensors for Chemical and Biological Detection. ACS Appl. Mater. Interfaces 2013, 5, 2800– 2814,  DOI: 10.1021/am3028662
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    Quantum-Dot-Based Photoelectrochemical Sensors for Chemical and Biological Detection
    Yue, Zhao; Lisdat, Fred; Parak, Wolfgang J.; Hickey, Stephen G.; Tu, Liping; Sabir, Nadeem; Dorfs, Dirk; Bigall, Nadja C.
    ACS Applied Materials & Interfaces (2013), 5 (8), 2800-2814CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    A review. Quantum-dot-based photoelectrochem. sensors are powerful alternatives for the detection of chems. and biochem. mols. compared to other sensor types, which is the primary reason as to why they have become a hot topic in nanotechnol.-related anal. methods. These sensors basically consist of QDs immobilized by a linking mol. (linker) to an electrode, so that upon their illumination, a photocurrent is generated which depends on the type and concn. of the resp. analyte in the immediate environment of the electrode. The present review provides an overview of recent developments in the fabrication methods and sensing concepts concerning direct and indirect interactions of the analyte with quantum dot modified electrodes. Furthermore, it describes in detail the broad range of different sensing applications of such quantum-dot-based photoelectrochem. sensors for inorg. and org. (small and macro-) mols. that have arisen in recent years. Finally, a no. of aspects concerning current challenges on the way to achieving real-life applications of QD-based photochem. sensing are addressed.
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    Friebe, V. M.; Frese, R. N. Photosynthetic Reaction Center-Based Biophotovoltaics. Curr. Opin. Electrochem. 2017, 5, 126– 134,  DOI: 10.1016/j.coelec.2017.08.001
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    Photosynthetic reaction center-based biophotovoltaics
    Friebe, Vincent M.; Frese, Raoul N.
    Current Opinion in Electrochemistry (2017), 5 (1), 126-134CODEN: COEUCY; ISSN:2451-9111. (Elsevier B.V.)
    A review. Photosynthesis is the principal solar energy conversion process on Earth, but is low in overall efficiency due to numerous steps necessary to fix carbon. Tapping into the high-efficiency components which drive photosynthesis, known as reaction centers, offers a route to efficiently capture solar energy. This may be achieved by direct integration of reaction centers onto electrode materials in order to ext. the high potential electrons and produce electricity or fuels. This review describes the various strategies and current benchmark configurations of Photosystem-I, Photosystem-II and bacterial reaction center-based biophotoelectrodes.
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    Stieger, K. R.; Ciornii, D.; Kölsch, A.; Hejazi, M.; Lokstein, H.; Feifel, S. C.; Zouni, A.; Lisdat, F. Engineering of Supramolecular Photoactive Protein Architectures: the Defined Co-Assembly of Photosystem I and Cytochrome c Using a Nanoscaled DNA-Matrix. Nanoscale 2016, 8, 10695– 10705,  DOI: 10.1039/C6NR00097E
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    Engineering of supramolecular photoactive protein architectures: the defined co-assembly of photosystem I and cytochrome c using a nanoscaled DNA-matrix
    Stieger, Kai R.; Ciornii, Dmitri; Koelsch, Adrian; Hejazi, Mahdi; Lokstein, Heiko; Feifel, Sven C.; Zouni, Athina; Lisdat, Fred
    Nanoscale (2016), 8 (20), 10695-10705CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    The engineering of renewable and sustainable protein-based light-to-energy converting systems is an emerging field of research. Here, we report on the development of supramol. light-harvesting electrodes, consisting of the redox protein cytochrome c working as a mol. scaffold as well as a conductive wiring network and photosystem I as a photo-functional matrix element. Both proteins form complexes in soln., which in turn can be adsorbed on thiol-modified gold electrodes through a self-assembly mechanism. To overcome the limited stability of self-grown assemblies, DNA, a natural polyelectrolyte, is used as a further building block for the construction of a photo-active 3D architecture. DNA acts as a structural matrix element holding larger protein amts. and thus remarkably improving the max. photocurrent and electrode stability. On investigating the photophys. properties, this system demonstrates that effective electron pathways have been created.
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    Carregal-Romero, S.; Rinklin, P.; Schulze, S.; Schäfer, M.; Ott, A.; Hühn, D.; Yu, X.; Wolfrum, B.; Weitzel, K. M.; Parak, W. J. Ion Transport through Polyelectrolyte Multilayers. Macromol. Rapid Commun. 2013, 34, 1820– 1826,  DOI: 10.1002/marc.201300571
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    Ion Transport Through Polyelectrolyte Multilayers
    Carregal-Romero, Susana; Rinklin, Philipp; Schulze, Susanne; Schaefer, Martin; Ott, Andrea; Huehn, Dominik; Yu, Xiang; Wolfrum, Bernhard; Weitzel, Karl-Michael; Parak, Wolfgang J.
    Macromolecular Rapid Communications (2013), 34 (23-24), 1820-1826CODEN: MRCOE3; ISSN:1022-1336. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Polyelectrolyte multilayer (PEM) films and capsules loaded with ion-sensitive fluorophores can be used as ion-sensors for many applications including measurements of intracellular ion concn. Previous studies have shown the influence of the PEM films/shells on the specific response of encapsulated ion-sensitive fluorophores. PEM shells are considered as semipermeable barriers between the environment and the encapsulated fluorophores. Parameters such as the time response of the encapsulated sensor can be affected by the porosity and charge of the PEM shell. In this study, the time response of an encapsulated pH-sensitive fluorophore towards pH changes in the surrounding environment is investigated. Furthermore, the conductance of PEM films for potassium ions is detd.
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    Wesp, V.; Hermann, M.; Schäfer, M.; Hühn, J.; Parak, W. J.; Weitzel, K. M. Bombardment Induced Ion Transport-Part IV: Ionic Conductivity of Ultra-Thin Polyelectrolyte Multilayer Films. Phys. Chem. Chem. Phys. 2016, 18, 4345– 4351,  DOI: 10.1039/C5CP04004C
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    Bombardment induced ion transport - part IV: ionic conductivity of ultra-thin polyelectrolyte multilayer films
    Wesp, Veronika; Hermann, Matthias; Schaefer, Martin; Huehn, Jonas; Parak, Wolfgang J.; Weitzel, Karl-Michael
    Physical Chemistry Chemical Physics (2016), 18 (6), 4345-4351CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The dependence of the ionic conductance of ultra-thin polyelectrolyte multilayer (PEM) films on the temp. and the no. of bilayers was studied by the recently developed low energy bombardment induced ion transport (BIIT) method. To this end multilayers of alternating poly(sodium 4-styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) layers were deposited on a metal electrode and subsequently bombarded by a low energy potassium ion beam. Ions are transported through the film according to the laws of electro-diffusion towards a grounded backside electrode. They are neutralized at the interface between the polymer film and the metal electrode. The detected neutralization current scales linearly with the acceleration potential of the ion beam indicating ohmic behavior for the (PAH/PSS)x multilayer, where x denotes the no. of bilayers. The conductance exhibits a nonmonotonic dependence on the no. of bilayers, x. For 2 ≤ x ≤ 8 the conductance increases nonlinearly with the no. of bilayers. For x ≥ 8 the conductance decreases with increasing no. of bilayers. The variation of the conductance is rationalized by a model accounting for the structure dependence of the cond. The thinnest sample for which the conductance was measured is the single bilayer reflecting properties dominated by the interface. The activation energy for the ion transport is 0.49 eV.
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    v. Klitzing, R. Internal Structure of Polyelectrolyte Multilayer Assemblies. Phys. Chem. Chem. Phys. 2006, 8, 5012– 5033,  DOI: 10.1039/b607760a
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    Internal structure of polyelectrolyte multilayer assemblies
    V Klitzing Regine
    Physical chemistry chemical physics : PCCP (2006), 8 (43), 5012-33 ISSN:1463-9076.
    The paper deals with the correlation between the internal structure and dynamics of polyelectrolyte multilayers on one hand and their functional properties on the other hand. It considers different concepts of multilayer formation like driving forces, adsorption kinetics, mode of growth and stability aspects. A further focus is the control of internal structure and dynamics which is of high impact with respect to the design of stimuli-responsive material.
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  146. 146
    Zhao, S.; Zhang, J.; Li, Z.; Zhang, P.; Li, Y.; Liu, G.; Wang, Y.; Yue, Z. Photoelectrochemical Determination of Hydrogen Peroxide Using a Gold Electrode Modified with Fluorescent Gold Nanoclusters and Graphene Oxide. Microchim. Acta 2017, 184, 677– 686,  DOI: 10.1007/s00604-016-2035-9
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    146
    Photoelectrochemical determination of hydrogen peroxide using a gold electrode modified with fluorescent gold nanoclusters and graphene oxide
    Zhao, Shuang; Zhang, Jun; Li, Zhengping; Zhang, Peixin; Li, Yunxiao; Liu, Guohua; Wang, Yong; Yue, Zhao
    Microchimica Acta (2017), 184 (3), 677-686CODEN: MIACAQ; ISSN:0026-3672. (Springer-Verlag GmbH)
    Nanocomposites consisting of gold nanoclusters and graphene oxide (AuNC/GO) were prepd. and investigated with respect to the design of new sensors for hydrogen peroxide (H2O2). The AuNC/GO hybrid nanomaterials were deposited on a gold electrode by the layer-by-layer assembly method, where they showed enhanced photoelec. and sensing properties. The presence of graphene oxide improves the photoinduced electron sepn. efficiency of the AuNCs, as well as the catalytic effect of AuNCs on the electroredn. of H2O2. Compared to an electrode modified with AuNCs only, the new electrodes display a more than ten-fold enhanced photocurrent at a working voltage of -500 mV (vs. Ag/AgCl), higher sensitivity for H2O2 (25.76 nA·mM-1), lower LOD (2 μM) and extended linear range (from 30 μM to 5 mM). The sensors were applied to the detn. of H2O2 extd. from living human umbilical vein endothelial cells stimulated by angiotensin II. [Figure not available: see fulltext.].
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  147. 147
    Zhao, S.; Li, Z.; Li, Y.; Yu, J.; Liu, G.; Liu, R.; Yue, Z. BSA-AuNCs Based Enhanced Photoelectrochemical Biosensors and Its Potential Use in Multichannel Detections. J. Photochem. Photobiol., A 2017, 342, 15– 24,  DOI: 10.1016/j.jphotochem.2017.03.034
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    147
    BSA-AuNCs based enhanced photoelectrochemical biosensors and its potential use in multichannel detections
    Zhao, Shuang; Li, Zhengping; Li, Yunxiao; Yu, Jun; Liu, Guohua; Liu, Ruping; Yue, Zhao
    Journal of Photochemistry and Photobiology, A: Chemistry (2017), 342 (), 15-24CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
    Herein we present the development of a bovine serum albumin (BSA)-coated fluorescent gold nanocluster (BSA-AuNC)/graphene oxide (GO) hybrid nanocomposite-based photoelectrochem. (PEC) biosensor, which can be used for multichannel detection of both hydrogen peroxide (H2O2) and dopamine (DA) within one chip at neg. and pos. potentials, resp. BSA-AuNC-based biosensors generally exhibit poor PEC sensing performance owing to their poor sepn. of photoexcited electron-hole pairs and low electrode coverage. Consequently, improved AuNC/GO hybrid nanocomposites and a layer-by-layer (LBL) method are introduced in this study. The enhancement was investigated by exploring the optimum concn. of GO (0.12 mg mL-1) and LBL no. (four layers) for the prepn. of the BSA-AuNC materials, revealing that GO plays different enhancement roles in the detection of H2O2 and DA. Compared to the results from other gold nanoclusters with different ligands, the photoelec. enhancement of BSA-AuNCs can be achieved for both neg. and pos. potentials simultaneously with the same optimum GO concn. and LBL no. Thus, the detection of H2O2 and DA was achieved by BSA-AuNC/GO multilayers with enhanced sensing properties and easier fabrication processes. The two enhancement methods acting together led to an improvement in the limits of detection (LODs) from 325 μM to 23 μM (H2O2) and from 7.45 μM to 1.5 μM (DA). Thus, our strategy shows great potential for the improvement of PEC sensing structure, and provides a method for multichannel detection.
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  148. 148
    Khalid, W.; El Helou, M.; Murböck, T.; Yue, Z.; Montenegro, J. M.; Schubert, K.; Göbel, G.; Lisdat, F.; Witte, G.; Parak, W. J. Immobilization of Quantum Dots via Conjugated Self-Assembled Monolayers and Their Application as a Light-Controlled Sensor for the Detection of Hydrogen Peroxide. ACS Nano 2011, 5, 9870– 9876,  DOI: 10.1021/nn2035582
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    148
    Immobilization of Quantum Dots via Conjugated Self-Assembled Monolayers and Their Application as a Light-Controlled Sensor for the Detection of Hydrogen Peroxide
    Khalid, Waqas; El Helou, Mira; Murboeck, Tobias; Yue, Zhao; Montenegro, Jose-Maria; Schubert, Kirsten; Goebel, Gero; Lisdat, Fred; Witte, Gregor; Parak, Wolfgang J.
    ACS Nano (2011), 5 (12), 9870-9876CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    A light-addressable gold electrode modified with CdS and FePt or with [email protected] nanoparticles via an interfacial dithiol linker layer is presented. XPS measurements reveal that trans-stilbenedithiol provides high-quality self-assembled monolayers compared to benzenedithiol and biphenyldithiol, in case they are formed at elevated temps. The CdS nanoparticles in good elec. contact with the electrode allow for current generation under illumination and appropriate polarization. FePt nanoparticles serve as catalytic sites for the redn. of hydrogen peroxide to water. Advantageously, both properties can be combined by the use of hybrid nanoparticles fixed on the electrode by the optimized stilbenedithiol layer. This allows a light-controlled anal. of different hydrogen peroxide concns.
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  149. 149
    Sabir, N.; Khan, N.; Völkner, J.; Widdascheck, F.; del Pino, P.; Witte, G.; Riedel, M.; Lisdat, F.; Konrad, M.; Parak, W. J. Photo-Electrochemical Bioanalysis of Guanosine Monophosphate Using Coupled Enzymatic Reactions at a CdS/ZnS Quantum Dot Electrode. Small 2015, 11, 5844– 5850,  DOI: 10.1002/smll.201501883
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    Photo-electrochemical Bioanalysis of Guanosine Monophosphate Using Coupled Enzymatic Reactions at a CdS/ZnS Quantum Dot Electrode
    Sabir, Nadeem; Khan, Nazimuddin; Voelkner, Johannes; Widdascheck, Felix; del Pino, Pablo; Witte, Gregor; Riedel, Marc; Lisdat, Fred; Konrad, Manfred; Parak, Wolfgang J.
    Small (2015), 11 (43), 5844-5850CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A photo-electrochem. sensor for the specific detection of guanosine monophosphate (GMP) is demonstrated, based on three enzymes combined in a coupled reaction assay. The first reaction involves the ATP-dependent conversion of GMP to guanosine diphosphate (GDP) by guanylate kinase, which warrants substrate specificity. The reaction products ADP and GDP are co-substrates for the enzymic conversion of phosphoenolpyruvate to pyruvate in a second reaction mediated by pyruvate kinase. Pyruvate in turn is the co-substrate for lactate dehydrogenase that generates lactate via oxidn. of NAD (reduced form) NADH to NAD+. This third enzymic reaction is electrochem. detected. For this purpose a CdS/ZnS quantum dot (QD) electrode is illuminated and the photocurrent response under fixed potential conditions is evaluated. The sequential enzyme reactions are first evaluated in soln. Subsequently, a sensor for GMP is constructed using polyelectrolytes for enzyme immobilization.
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    Tanne, J.; Schafer, D.; Khalid, W.; Parak, W. J.; Lisdat, F. Light-Controlled Bioelectrochemical Sensor Based on CdSe/ZnS Quantum Dots. Anal. Chem. 2011, 83, 7778– 7785,  DOI: 10.1021/ac201329u
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    150
    Light-Controlled Bioelectrochemical Sensor Based on CdSe/ZnS Quantum Dots
    Tanne, J.; Schaefer, D.; Khalid, W.; Parak, W. J.; Lisdat, F.
    Analytical Chemistry (Washington, DC, United States) (2011), 83 (20), 7778-7785CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
    This study reports on the oxygen sensitivity of quantum dot electrodes modified with CdSe/ZnS nanocrystals. The photocurrent behavior is analyzed for dependence on pH and applied potential by potentiostatic and potentiodynamic measurements. On the basis of the influence of the oxygen content in soln. on the photocurrent generation, the enzymic activity of glucose oxidase is evaluated in soln. To construct a photobioelectrochem. sensor which can be read out by illuminating the resp. electrode area, two different immobilization methods for the fixation of the biocatalyst have been investigated. Both covalent crosslinking and layer-by-layer deposition of GOD by the polyelectrolyte polyallylamine hydrochloride show that a sensor construction is possible. The sensing properties of this type of electrode are drastically influenced by the amt. and d. of the enzyme on top of the quantum dot layer, which can be advantageously adjusted by the layer-by-layer technique. By depositing four bilayers [GOD/PAH]4 on the CdSe/ZnS electrode, a fast-responding sensor for the concn. range of 0.1-5 mM glucose can be prepd. This study opens the door to multianalyte detection with a nonstructured sensing electrode, localized enzymes, and spatial read-out by light.
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    Meyer, R.; Sacca, B.; Niemeyer, C. M. Site-Directed, On-Surface Assembly of DNA Nanostructures. Angew. Chem. 2015, 127, 12207– 12211,  DOI: 10.1002/ange.201505553
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    Chen, X.; Venkatachalapathy, M.; Kamps, D.; Weigel, S.; Kumar, R.; Orlich, M.; Garrecht, R.; Hirtz, M.; Niemeyer, C. M.; Wu, Y. W.; Dehmelt, L. Molecular Activity Painting“: Switch-Like, Light-Controlled Perturbations inside Living Cells. Angew. Chem., Int. Ed. 2017, 56, 5916– 5920,  DOI: 10.1002/anie.201611432
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    152
    "Molecular-Activity Painting": Switch-like, Light-Controlled Perturbations inside Living Cells
    Chen, Xi; Venkatachalapathy, Muthukumaran; Kamps, Dominic; Weigel, Simone; Kumar, Ravi; Orlich, Michael; Garrecht, Ruben; Hirtz, Michael; Niemeyer, Christof M.; Wu, Yao-Wen; Dehmelt, Leif
    Angewandte Chemie, International Edition (2017), 56 (21), 5916-5920CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Acute subcellular protein targeting is a powerful tool to study biol. networks. However, signaling at the plasma membrane is highly dynamic, making it difficult to study in space and time. In particular, sustained local control of mol. function is challenging owing to the lateral diffusion of plasma membrane targeted mols. Herein we present "mol.-activity painting" (MAP), a novel technol. which combines photoactivatable chem. induced dimerization (pCID) with immobilized artificial receptors. The immobilization of artificial receptors by surface-immobilized antibodies blocks lateral diffusion, enabling rapid and stable "painting" of signaling mols. and their activity at the plasma membrane with micrometer precision. Using this method, we show that painting of the RhoA-myosin activator GEF-H1 induces patterned acto-myosin contraction inside living cells.
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    Salles, P.; Pinto, D.; Hantanasirisakul, K.; Maleski, K.; Shuck, C. E.; Gogotsi, Y. Electrochromic Effect in Titanium Carbide MXene Thin Films Produced by Dip-Coating. Adv. Funct. Mater. 2019, 29, 1809223,  DOI: 10.1002/adfm.201809223
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    Liu, S.; Kurth, D. G.; Möhwald, H.; Volkmer, D. A Thin-Film Electrochromic Device Based on a Polyoxometalate Cluster. Adv. Mater. 2002, 14, 225– 228,  DOI: 10.1002/1521-4095(20020205)14:3<225::AID-ADMA225>3.0.CO;2-F
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    154
    A thin-film electrochromic device based on a polyoxometalate cluster
    Liu, Shaoqin; Kurth, Dirk G.; Mohwald, Helmuth; Volkmer, Dirk
    Advanced Materials (Weinheim, Germany) (2002), 14 (3), 225-228CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH)
    Eu(III) deriv. of the Preyssler-type heteropolytungstate [Eu-(H2O)P5W30O110]12- was used as an active component for an electrochromic device. Such a device was constructed through layer-by-layer (LbL) deposition of Eu-POM, poly(allylamine hydrochloride) (PAH), and poly(styrenesulfonate) (PSS) on In Sn oxide (ITO)-coated glass slides. The multilayer cyclic voltammogram showed that the electrochem. property of Eu-POM is fully maintained in the LbL film. For poly(ethylenimine)/(PSS/PAH/Eu-POM/PAH)m multilayers (m ≤ 20), the peak currents increased linearly with the no. of layers, confirming that equal amts. of Eu-POM anions were adsorbed in each deposition cycle. The device operated reversibly with suitable response times, which were further reduced by optimizing the polyelectrolyte matrix. Sufficiently high optical contrast was achieved even for very thin films.
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    Liu, S.; Möhwald, H.; Volkmer, D.; Kurth, D. G. Polyoxometalate-Based Electro- and Photochromic Dual-Mode Devices. Langmuir 2006, 22, 1949– 1951,  DOI: 10.1021/la0523863
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    Polyoxometalate-Based Electro- and Photochromic Dual-Mode Devices
    Liu, Shaoqin; Moehwald, Helmuth; Volkmer, Dirk; Kurth, Dirk G.
    Langmuir (2006), 22 (5), 1949-1951CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Mol. or supramol. systems capable of electro- and photostimulated color changes are still rare. The authors present a device design based on an electrostatic complex of a nanoscopic polyoxometalate cluster and a polyelectrolyte. The coating reversibly changes color from transparent to blue by photo- and/or electroinduced stimulation. The choice of the components results in perfect transparency over the entire visible range, a large optical response, reversible operation, and excellent stability.
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  156. 156
    Sergeeva, Y. N.; Huang, T.; Felix, O.; Jung, L.; Tropel, P.; Viville, S.; Decher, G. What is Really Driving Cell-Surface Interactions? Layer-by-Layer Assembled Films May Help To Answer Questions Concerning Cell Attachment and Response to Biomaterials. Biointerphases 2016, 11, 019009,  DOI: 10.1116/1.4943046
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    156
    What is really driving cell-surface interactions? Layer-by-layer assembled films may help to answer questions concerning cell attachment and response to biomaterials
    Sergeeva, Yulia N.; Huang, Tongtong; Felix, Olivier; Jung, Laura; Tropel, Philippe; Viville, Stephane; Decher, Gero
    Biointerphases (2016), 11 (1), 019009/1-019009/14CODEN: BJIOBN; ISSN:1559-4106. (American Institute of Physics)
    Layer-by-layer (LbL) assembled multicomponent films offer the opportunity to control and to fine-tune cell attachment and behavior on solid surfaces [Layer-by-Layer Films for Biomedical Applications, edited by Picart et al. (Wiley, Weinheim, 2014) and El-Khouri et al., "Multifunctional layer-by-layer architectures for biol. applications," in Functional Polymeric Ultrathin Films, edited by Advincula and Knoll (Wiley, Weinheim, 2011), Vol. 1]. At the same time, these films allow for quite detailed physicochem. characterization of static and dynamic surface properties that are typically not available in classic cell culture. In this report, the authors investigate cell adhesion and cytocompatibility of compositionally and morphol. similar thin films composed of oppositely charged synthetic or natural polyelectrolytes in which different phys. parameters such as surface charge or water content are varied through chem. compn. and deposition conditions. Human adult dermal fibroblasts were chosen as a model because of the need for chem. defined matrix in the field of primary cell amplification. The growth and the stability of the multilayer films in the incubation media were studied dissipation- enhanced quartz crystal micobalance (QCM-D) and ellipsometry. The QCM-D signals obsd. during the film deposition were analyzed qual. to est. the viscoelastic properties of the films. The authors used contact angle measurements with water to study the contribution of the chem. functionalities to wetting behavior of the films. Most importantly, they also studied the interaction of the films with serum components. Our results underline that cell adhesion is a highly complex process which is not only governed by the functionality of a surface but also by its morphol., its affinity for serum components, and also by changes of surface properties brought about by adsorbing mols. Of the many LbL-films tested, poly(4-styrenesulfonate)/ poly(allyl amine) multilayers were best suited for our fibroblast cultures, which opens a way to avoid gelatin based and similar substrates whose exact chem. compn. is unknown. (c) 2016 American Institute of Physics.
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  157. 157
    Muzzio, N. E.; Pasquale, M. A.; Rios, X.; Azzaroni, O.; Llop, J.; Moya, S. E. Adsorption and Exchangeability of Fibronectin and Serum Albumin Protein Corona on Annealed Polyelectrolyte Multilayers and Their Consequences on Cell Adhesion. Adv. Mater. Interfaces 2019, 6, 1900008,  DOI: 10.1002/admi.201900008
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    El-Khouri, R. J.; Szamocki, R.; Sergeeva, Y.; Felix, O.; Decher, G. Multifunctional Layer-by-Layer Architectures for Biological Applications. In Functional Polymer Films; Knoll, W., Advincula, R. C., Eds.; Wiley-VCH: Weinheim, Germany, 2011; Vol. 2, pp 11– 58.
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    Jan, E.; Pereira, F. N.; Turner, D. L.; Kotov, N. A. In Situ Gene Transfection and Neuronal Programming on Electroconductive Nanocomposite to Reduce Inflammatory Response. J. Mater. Chem. 2011, 21, 1109– 1114,  DOI: 10.1039/C0JM01895C
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    159
    In situ gene transfection and neuronal programming on electroconductive nanocomposite to reduce inflammatory response
    Jan, Edward; Pereira, Felipe N.; Turner, David L.; Kotov, Nicholas A.
    Journal of Materials Chemistry (2011), 21 (4), 1109-1114CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    Inflammatory reactions, such as encapsulation of implanted electrodes by scar tissues and gradual degrdn. of neurons, are the key problems for neural tissue interfacing. These problems must be resolved for treatments of debilitating conditions to be effective. One strategy to mitigate them is to engineer neural electrodes with the ability to control cell response via in situ gene transfection. Taking advantage of layer-by-layer (LBL) assembled carbon nanotube (CNT) composites, purposeful engineering of electrostimulating implants with these functionalities becomes realistic. LBL assembled CNT composites are conductive and can incorporate plasmid DNA capable of altering the response/functionality of surrounding cells. Successful expression of Lyn-citrine plasmid DNA was achieved in attached neurons. The transfection efficiency was found to be remarkably higher than conventional soln.-mediated techniques. Most importantly, by using plasmid expression vectors for neural basic helix-loop-helix proteins, neurons were generated from multipotent P19 embryonal carcinoma cells adhering to the CNT multilayers. This study illustrates the possibility of fabricating an electrostimulating implant capable of recruiting and programming resident stem cells in the nervous system to provide a substantially improved level of tissue-device integration.
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    Crouzier, T.; Boudou, T.; Picart, C. Polysaccharide-Based Polyelectrolyte Multilayers. Curr. Opin. Colloid Interface Sci. 2010, 15, 417– 426,  DOI: 10.1016/j.cocis.2010.05.007
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    Polysaccharide-based polyelectrolyte multilayers
    Crouzier, Thomas; Boudou, Thomas; Picart, Catherine
    Current Opinion in Colloid & Interface Science (2010), 15 (6), 417-426CODEN: COCSFL; ISSN:1359-0294. (Elsevier B.V.)
    A review. In recent years, the layer-by-layer technique has grown in various fields. One of the emerging trends of bio-applications is the use of polysaccharides as main film components, which stems from their intrinsic phys., chem. and biol. properties. These allow the simple formation, by self-assembly, of new kinds of mimics of extracellular matrixes from plant and animal tissues. These assemblies, which possess specific properties arising from their hydration and internal compn., can indeed contain addnl. functionalities obtained by chem. modification of the biopolymers or film post-processing. They can be molded into different forms (films, membranes, and capsules).
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    Romero, G.; Ochoteco, O.; Sanz, D. J.; Estrela-Lopis, I.; Donath, E.; Moya, S. E. Poly(lactide-co-glycolide) Nanoparticles, Layer-by-Layer Engineered for the Sustainable Delivery of AntiTNF-α. Macromol. Biosci. 2013, 13, 903– 912,  DOI: 10.1002/mabi.201200478
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    Poly(lactide-co-glycolide) nanoparticles, layer by layer engineered for the sustainable delivery of antiTNF-α
    Romero, Gabriela; Ochoteco, Olaia; Sanz, David J.; Estrela-Lopis, Irina; Donath, Edwin; Moya, Sergio E.
    Macromolecular Bioscience (2013), 13 (7), 903-912CODEN: MBAIBU; ISSN:1616-5187. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A strategy of encapsulation of the antiTNF-α antibody on top of poly(lactide-co-glycolide) nanoparticles (PLGA NPs) is presented on the basis of the complexation of antiTNF-α with alginate (Alg) and subsequent assembly layer by layer with poly(L-lysine) (PLL). The assembly of the antiTNF-α/Alg complex with PLL and its stability in PBS and lysozymes are monitored on a planar support using a quartz crystal microbalance with dissipation. The assembly of the antiTNF-α/Alg complex on PLGA NPs is followed by zeta potential measurements. AntiTNF-α release from the PLGA NPs is measured in PBS at 37 and 60° and in the HepG2 cell line following NP uptake, using the Q-ADA kit detection kit. The release follows first-order kinetics with an initial burst. Intracellular release of antiTNF-α is confirmed by confocal Raman microscopy.
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    Naves, A. F.; Motay, M.; Mérindol, R.; Davi, C. P.; Felix, O.; Catalani, L. H.; Decher, G. Layer-by-Layer Assembled Growth Factor Reservoirs for Steering the Response of 3T3-cells. Colloids Surf., B 2016, 139, 79– 86,  DOI: 10.1016/j.colsurfb.2015.11.019
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    Layer-by-Layer assembled growth factor reservoirs for steering the response of 3T3-cells
    Naves, Alliny F.; Motay, Marvin; Merindol, Remi; Davi, Christiane P.; Felix, Olivier; Catalani, Luiz H.; Decher, Gero
    Colloids and Surfaces, B: Biointerfaces (2016), 139 (), 79-86CODEN: CSBBEQ; ISSN:0927-7765. (Elsevier B.V.)
    Layer-by-Layer (LbL) assemblies of heparin (Hep) and chitosan (Chi) were prepd. for use as reservoirs for acidic and basic fibroblast growth factors (aFGFs and bFGFs, resp.). The effects of the architecture and compn. of the reservoirs on the viability and proliferation of NIH-3T3 fibroblast cells were studied under starvation conditions. The reservoir stability was monitored by ellipsometry. The aFGF and bFGF loadings were detd. using a dissipation-enhanced quartz crystal microbalance (QCM-D). Stability and release assays were performed in a phosphate buffer at physiol. conditions. The results demonstrated that the amt. of aFGF and bFGF loaded into and released from LbL reservoirs composed of 3 and 6 layer pairs could be controlled. Cell culture assays in low serum culture medium (LSCM) demonstrated that incorporating very small amts. of aFGF and bFGF into the (Hep/Chi)n multilayers significantly improved the proliferation of the NIH-3T3 fibroblasts. The cells did not proliferate on (Hep/Chi)n assemblies prepd. in the absence of FGF under identical conditions. The LbL reservoirs were highly effective for the long-term storage (up to 9 mo) of aFGF and bFGF. This work demonstrates the potential of LbL reservoirs for use as biomaterial coatings.
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    Muzzio, N. E.; Gregurec, D.; Diamanti, E.; Irigoyen, J.; Pasquale, M. A.; Azzaroni, O.; Moya, S. E. Thermal Annealing of Polyelectrolyte Multilayers: An Effective Approach for the Enhancement of Cell Adhesion. Adv. Mater. Interfaces 2017, 4, 1600126,  DOI: 10.1002/admi.201600126
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  164. 164
    Skirtach, A. G.; Volodkin, D. V.; Möhwald, H. Bio-Interfaces-Interaction of PLL/HA Thick Films with Nanoparticles and Microcapsules. ChemPhysChem 2010, 11, 822– 829,  DOI: 10.1002/cphc.200900676
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    Bio-interfaces-Interaction of PLL/HA Thick Films with Nanoparticles and Microcapsules
    Skirtach, Andre G.; Volodkin, Dmitry V.; Moehwald, Helmuth
    ChemPhysChem (2010), 11 (4), 822-829CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The interaction of biocompatible, exponentially grown films composed of poly-L-lysine (PLL) and hyaluronic acid (HA) polymers with gold nanoparticles and microcapsules is studied. Both aggregated and non-aggregated nanoparticle states are achieved; desorption of PLL accounts for aggregation of nanoparticles. The presence of aggregates of gold nanoparticles on films enables remote activation by near-IR irradn. due to local, nanometer confined heating. Thermally shrunk microcapsules, which are remarkably monodisperse upon prepn. but gain polydispersity after months of storage, are also adsorbed onto films. PLL polymers desorbed from films interact with microcapsules introducing a charge imbalance which leads to an increase of the microcapsule size, thus films amplify this effect. Multifunctional, biocompatible, thick gel films with remote activation and release capabilities are targeted for cell cultures in biol. and tissue engineering in medicine.
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  165. 165
    Kohler, D.; Madaboosi, N.; Delcea, M.; Schmidt, S.; De Geest, B. G.; Volodkin, D. V.; Möhwald, H.; Skirtach, A. G. Patchiness of Embedded Particles and Film Stiffness Control through Concentration of Gold Nanoparticles. Adv. Mater. 2012, 24, 1095– 1100,  DOI: 10.1002/adma.201103958
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    165
    Patchiness of Embedded Particles and Film Stiffness Control Through Concentration of Gold Nanoparticles
    Kohler, D.; Madaboosi, N.; Delcea, M.; Schmidt, S.; De Geest, B. G.; Volodkin, D. V.; Moehwald, H.; Skirtach, A. G.
    Advanced Materials (Weinheim, Germany) (2012), 24 (8), 1095-1100CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We present here (a) an entirely biocompatible method for controlling the patch size of particles, and (b) results of modifying biocompatible exponentially grown films with the purpose of increasing film stiffness. This is accomplished by adjusting the concn. of gold nanoparticles on the surface of hyaluronic acid (HA)/poly-(L-Lysine) (PLL) (further denoted as (HA/PLL)n, films, where n describes the no. of deposited bilayers), micrometer thick, gellike films in which microparticles protrude and from which they are later on extd. Com. available silica (SiO2) microparticles are used as a model system due to their good monodispersity and ease in chem. surface modification. They are deposited onto AuNP modified (HA/PLL)12 films and their protrusion into HA/PLL films is thereby controlled via the film stiffness achieved by AuNP deposition on HA/PLL films.
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  166. 166
    Merindol, R.; Diabang, S.; Felix, O.; Roland, T.; Gauthier, C.; Decher, G. Bio-Inspired Multiproperty Materials: Strong, Self-Healing, and Transparent Artificial Wood Nanostructures. ACS Nano 2015, 9, 1127– 1136,  DOI: 10.1021/nn504334u
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    166
    Bio-Inspired Multiproperty Materials: Strong, Self-Healing, and Transparent Artificial Wood Nanostructures
    Merindol, Remi; Diabang, Seydina; Felix, Olivier; Roland, Thierry; Gauthier, Christian; Decher, Gero
    ACS Nano (2015), 9 (2), 1127-1136CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Nanocomposite films possessing multiple interesting properties (mech. strength, optical transparency, self-healing, and partial biodegradability) were discussed. We used Layer-by-Layer assembly to prep. micron thick wood-inspired films from anionic nanofibrillated cellulose and cationic poly(vinyl amine). The film growth was carried out at different pH values to obtain films of different chem. compn., whereby, and as expected, higher pH values led to a higher polycation content and also to 6 times higher film growth increments (from 9 to 55 nm per layer pair). In the pH range from 8 to 11, micron thick and optically transparent LbL films are obtained by automated dipping when dried regularly in a stream of air. Films with a size of 10 cm2 or more can be peeled from flat surfaces; they show tensile strengths up to about 250 MPa and Young's moduli up to about 18 GPa as controlled by the polycation/polyanion ratio of the film. Expts. at different humidities revealed the plasticizing effect of water in the films and allowed reversible switching of their mech. properties. Whereas dry films are strong and brittle (Young's modulus: 16 GPa, strain at break: 1.7%), wet films were soft and ductile (Young's modulus: 0.1 GPa, strain at break: 49%). Wet film surfaces even amalgamate upon contact to yield mech. stable junctions. We attribute the switchability of the mech. properties and the propensity for self-repair to changes in the polycation mobility that are brought about by the plasticizing effect of water.
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  167. 167
    Khademhosseini, A.; Suh, Y. K.; Yang, M. J.; Eng, G.; Yeh, J.; Levenberg, S.; Langer, R. Layer-by-Layer Deposition of Hyaluronic Acid and Poly-l-lysine for Patterned Cell Co-Cultures. Biomaterials 2004, 25, 3583– 3592,  DOI: 10.1016/j.biomaterials.2003.10.033
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    167
    Layer-by-layer deposition of hyaluronic acid and poly-L-lysine for patterned cell co-cultures
    Khademhosseini, Ali; Suh, Kahp Y.; Yang, Jen M.; Eng, George; Yeh, Judy; Levenberg, Shulamit; Langer, Robert
    Biomaterials (2004), 25 (17), 3583-3592CODEN: BIMADU; ISSN:0142-9612. (Elsevier Science Ltd.)
    A novel method for patterning cellular co-cultures that uses the layer-by-layer deposition of ionic biopolymers is described. Non-biofouling hyaluronic acid (HA) micropatterns were used to immobilize cells and proteins to glass substrates. Subsequent ionic adsorption of poly-L-lysine (PLL) to HA patterns was used to switch the HA surfaces from cell repulsive to adherent thereby facilitating the adhesion of a second cell type. The utility of this approach to pattern co-cultures of hepatocytes or embryonic stem cells with fibroblasts was demonstrated. In addn., the versatility of this approach to generate patterned co-cultures irresp. of the primary cell seeding and relative adhesion of the seeded cells was demonstrated. Thus, the proposed method may be a useful tool for fabricating controlled cellular co-cultures for cell-cell interaction studies and tissue engineering applications.
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    Fukuda, J.; Khademhosseini, A.; Yeh, J.; Eng, G.; Cheng, J.; Farokhzad, O. C.; Langer, R. Micropatterned Cell Co-Cultures Using Layer-by-Layer Deposition of Extracellular Matrix Components. Biomaterials 2006, 27, 1479– 1486,  DOI: 10.1016/j.biomaterials.2005.09.015
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    Micropatterned cell co-cultures using layer-by-layer deposition of extracellular matrix components
    Fukuda, Junji; Khademhosseini, Ali; Yeh, Judy; Eng, George; Cheng, Jianjun; Farokhzad, Omid C.; Langer, Robert
    Biomaterials (2006), 27 (8), 1479-1486CODEN: BIMADU; ISSN:0142-9612. (Elsevier Ltd.)
    Micropatterned cellular co-cultures were fabricated using 3 major extracellular matrix components: hyaluronic acid (HA), fibronectin (FN) and collagen. To fabricate co-cultures with these components, HA was micropatterned on a glass substrate by capillary force lithog., and the regions of exposed glass were coated with FN to generate cell adhesive islands. Once the first cell type was immobilized on the adhesive islands, the subsequent electrostatic adsorption of collagen to HA patterns switched the non-adherent HA surfaces to adherent, thereby facilitating the adhesion of a second cell type. This technique utilized native extracellular matrix components and therefore affords high biol. affinity and no cytotoxicity. This biocompatible co-culture system could potentially provide a new tool to study cell behavior such as cell-cell communication and cell-matrix interactions, as well as tissue-engineering applications.
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    Tamada, Y.; Ikada, Y. Effect of Preadsorbed Proteins on Cell Adhesion to Polymer Surfaces. J. Colloid Interface Sci. 1993, 155, 334– 339,  DOI: 10.1006/jcis.1993.1044
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    Effect of preadsorbed proteins on cell adhesion to polymer surfaces
    Tamada, Yasushi; Ikada, Yoshito
    Journal of Colloid and Interface Science (1993), 155 (2), 334-9CODEN: JCISA5; ISSN:0021-9797.
    Adsorption of three different proteins and adhesion of cells onto various substrates in the presence of serum proteins were studied. Both the maximal protein adsorption and the maximal cell adhesion were obsd. on surfaces with water contact angle around 70°. Preadsorption of serum albumin prevented cell adhesion to all the substrates, whereas preadsorbed fibronectin (FN) enhanced cell adhesion to all the substrates, independent of their water wettability, except for poly(vinyl alc.) and acrylamide-grafted films. Competitive adsorption of FN from mixed proteins, ranging from 0.03 to 0.07 μg/cm2, markedly influenced cell adhesion in the presence of serum. These results suggest that the effect of the water wettability of surfaces on cell adhesion in the presence of serum should occur through protein layers adsorbed directly to the substrate surfaces.
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    Yeom, B.; Sain, T.; Lacevic, N.; Bukharina, D.; Cha, S. H.; Waas, A. M.; Arruda, E. M.; Kotov, N. A. Abiotic Tooth Enamel. Nature 2017, 543, 95– 98,  DOI: 10.1038/nature21410
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    Abiotic tooth enamel
    Yeom, Bongjun; Sain, Trisha; Lacevic, Naida; Bukharina, Daria; Cha, Sang-Ho; Waas, Anthony M.; Arruda, Ellen M.; Kotov, Nicholas A.
    Nature (London, United Kingdom) (2017), 543 (7643), 95-98CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Tooth enamel comprises parallel microscale and nanoscale ceramic columns or prisms interlaced with a soft protein matrix. This structural motif is unusually consistent across all species from all geol. eras. Such invariability-esp. when juxtaposed with the diversity of other tissues-suggests the existence of a functional basis. Here we performed ex vivo replication of enamel-inspired columnar nanocomposites by sequential growth of zinc oxide nanowire carpets followed by layer-by-layer deposition of a polymeric matrix around these. We show that the mech. properties of these nanocomposites, including hardness, are comparable to those of enamel despite the nanocomposites having a smaller hard-phase content. Our abiotic enamels have viscoelastic figures of merit (VFOM) and wt.-adjusted VFOM that are similar to, or higher than, those of natural tooth enamels-we achieve values that exceed the traditional materials limits of 0.6 and 0.8, resp. VFOM values describe resistance to vibrational damage, and our columnar composites demonstrate that light-wt. materials of unusually high resistance to structural damage from shocks, environmental vibrations and oscillatory stress can be made using biomimetic design. The previously inaccessible combinations of high stiffness, damping and light wt. that we achieve in these layer-by-layer composites are attributed to efficient energy dissipation in the interfacial portion of the org. phase. The in vivo contribution of this interfacial portion to macroscale deformations along the tooth's normal is maximized when the architecture is columnar, suggesting an evolutionary advantage of the columnar motif in the enamel of living species. We expect our findings to apply to all columnar composites and to lead to the development of high-performance load-bearing materials.
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    Vendamme, R.; Onoue, S. Y.; Nakao, A.; Kunitake, T. Robust Free-Standing Nanomembranes of Organic/Inorganic Interpenetrating Networks. Nat. Mater. 2006, 5, 494– 501,  DOI: 10.1038/nmat1655
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    Robust free-standing nanomembranes of organic/inorganic interpenetrating networks
    Vendamme, Richard; Onoue, Shin-Ya; Nakao, Aiko; Kunitake, Toyoki
    Nature Materials (2006), 5 (6), 494-501CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Hybrid sol-gel materials have been a subject of intensive research during the past decades because these nanocomposites combine the versatility of org. polymers with the superior phys. properties of glass. Here, we report the synthesis, by spin coating, of hybrid interpenetrating networks in the form of free-standing nanomembrane (around 35-nm thick) with unprecedented macroscopic size and characteristics. The quasi-2D interpenetration of the org. and inorg. networks brings to these materials a unique combination of properties that are not usually compatible within the same film: macroscopic robustness and homogeneity, nanoscale thickness, mech. strength, high flexibility and optical transparency. Interestingly, such free-standing nanofilms of macroscopic size can seal large openings, are strong enough to hold amts. of liq. 70,000 times heavier than their own wt., and are flexible enough to reversibly pass through holes 30,000 times smaller than their own size.
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    Ariga, K.; Yamauchi, Y.; Rydzek, G.; Ji, Q.; Yonamine, Y.; Wu, K. C. W.; Hill, J. P. Layer-by-Layer Nanoarchitectonics: Invention, Innovation, and Evolution. Chem. Lett. 2014, 43, 36– 68,  DOI: 10.1246/cl.130987
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    Layer-by-layer nanoarchitectonics: invention, innovation, and evolution
    Ariga, Katsuhiko; Yamauchi, Yusuke; Rydzek, Gaulthier; Ji, Qingmin; Yonamine, Yusuke; Wu, Kevin C.-W.; Hill, Jonathan P.
    Chemistry Letters (2014), 43 (1), 36-68CODEN: CMLTAG; ISSN:0366-7022. (Chemical Society of Japan)
    A review. Materials fabrication with nanoscale structural precision based on bottom-up-type self-assembly has become more important in various current disciplines in chem. including materials chem., org. chem., phys. chem., anal. chem., biochem., colloid and surface chem., and supramol. chem. Although the design of new materials based on nanoscale self-assembly is anticipated as a key concept, prepg. complete three-dimensional structures at nanoscale precision remains a difficult target using current technologies. Rather, dimension-reduced approaches such as layering of two-dimensional nanostructures into precisely controlled lamellar nanomaterials are currently achievable. In particular, layer-by-layer (LbL) assembly is known as a highly versatile method for fabrication of controlled layered structures from various kinds of component materials using very simple, inexpensive, and rapid procedures. Therefore, fabrication of multilayer films through the LbL deposition process has attracted growing interest from various research communities. The high versatility and flexibility of LbL assembly is continuously creating new concepts, new materials, new procedures, and new applications. In this highlight review, we focus on nanoarchitectonics by LbL assembly. After an initial introduction on the invention and a brief history of the LbL assembly technique, innovations and the evolution of the technique are described based mainly on recent examples, which are categorized into two sections: (i) developments in methodol. (tech., material, and phenomenol. aspects with expansion of concept) and (ii) progress in applications (phys., chem./biochem., and biomedical applications).
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    Tang, Z.; Wang, Y.; Podsiadlo, P.; Kotov, N. A. Biomedical Applications of Layer-by-Layer Assembly: From Biomimetics to Tissue Engineering. Adv. Mater. 2006, 18, 3203– 3224,  DOI: 10.1002/adma.200600113
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    Biomedical applications of layer-by-layer assembly: from biomimetics to tissue engineering
    Tang, Zhiyong; Wang, Ying; Podsiadlo, Paul; Kotov, Nicholas A.
    Advanced Materials (Weinheim, Germany) (2006), 18 (24), 3203-3224CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. The design of advanced, nanostructured materials at the mol. level is of tremendous interest for the scientific and engineering communities because of the broad application of these materials in the biomedical field. Among the available techniques, the layer-by-layer assembly method introduced by Decher and co-workers in 1992 has attracted extensive attention because it possesses extraordinary advantages for biomedical applications: ease of prepn., versatility, capability of incorporating high loadings of different types of biomols. in the films, fine control over the materials' structure, and robustness of the products under ambient and physiol. conditions. In this context, a systematic review of current research on biomedical applications of layer-by-layer assembly is presented. The structure and bioactivity of biomols. in thin films fabricated by layer-by-layer assembly are introduced. The applications of layer-by-layer assembly in biomimetics, biosensors, drug delivery, protein and cell adhesion, mediation of cellular functions, and implantable materials are addressed. Future developments in the field of biomedical applications of layer-by-layer assembly are also discussed.
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    Sinani, V. A.; Koktysh, D. S.; Yun, B. G.; Matts, R. L.; Pappas, T. C.; Motamedi, M.; Thomas, S. N.; Kotov, N. A. Collagen Coating Promotes Biocompatibility of Semiconductor Nanoparticles in Stratified LbL Films. Nano Lett. 2003, 3, 1177– 1182,  DOI: 10.1021/nl0255045
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    Collagen Coating Promotes Biocompatibility of Semiconductor Nanoparticles in Stratified LBL Films
    Sinani, Vladimir A.; Koktysh, Dmitry S.; Yun, Bo-Geon; Matts, Robert L.; Pappas, Todd C.; Motamedi, Massoud; Thomas, Stephanie N.; Kotov, Nicholas A.
    Nano Letters (2003), 3 (9), 1177-1182CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Nanostructured thin films fabricated from semiconductor nanoparticles (NPs) are of great interest for biomedical applications, but NP materials based on heavy metals can be cytotoxic. In this work, the prepn. of semiconductor NPs followed the protocol of layer-by-layer (LBL) assembly, which alleviates this problem. Collagen/poly(acrylic acid) bilayers were added to CdTe/polycation LBL films to produce porous collagen bilayers. Such stratified multilayer systems showed successful cell attachment and survival while native NP films were strongly cytotoxic.
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    Zhang, Y.; Sun, J. Multilevel and Multicomponent Layer-by-Layer Assembly for the Fabrication of Nanofibrillar Films. ACS Nano 2015, 9, 7124– 7132,  DOI: 10.1021/acsnano.5b01832
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    Multilevel and Multicomponent Layer-by-Layer Assembly for the Fabrication of Nanofibrillar Films
    Zhang, Yuanyuan; Sun, Junqi
    ACS Nano (2015), 9 (7), 7124-7132CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    In this study, we demonstrate multilevel and multicomponent layer-by-layer (LbL) assembly as a convenient and generally applicable method for the fabrication of nanofibrillar films by exploiting the dynamic nature of polymeric complexes. The alternate deposition of poly(allylamine hydrochloride)-methyl red (PAH-MR) complexes with poly(acrylic acid) (PAA) produces nanofibrillar PAH-MR/PAA films, which involves the disassembly of PAH-MR complexes, the subsequent assembly of PAH with PAA, and the PAA-induced assembly of MR mols. into MR nanofibrils via a π-π stacking interaction. The aq. soln. of weak polyelectrolyte PAA with a low soln. pH plays an important role in fabricating nanofibrillar PAH-MR/PAA films because proton transfer from acidic PAA to MR mols. induces the formation of MR nanofibrils. The generality of the multilevel and multicomponent LbL assembly is verified by alternate assembly of complexes of 1-pyrenylbutyric acid (PYA) and PAH with PAA to fabricate PAH-PYA/PAA films with organized nanofibrillar structures. Unlike the traditional static LbL assembly, the multilevel and multicomponent LbL assembly is dynamic and more flexible and powerful in controlling the interfacial assembly process and in fabricating composite films with sophisticated structures. These characteristics of multilevel and multicomponent LbL assembly will enrich the functionalities of the LbL-assembled films.
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    Volodkin, D.; von Klitzing, R.; Möhwald, H. Polyelectrolyte Multilayers: Towards Single Cell Studies. Polymers 2014, 6, 1502– 1527,  DOI: 10.3390/polym6051502
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    Polyelectrolyte multilayers: towards single cell studies
    Volodkin, Dmitry; von Klitzing, Regine; Moehwald, Helmuth
    Polymers (Basel, Switzerland) (2014), 6 (5), 1502-1527, 26 pp.CODEN: POLYCK; ISSN:2073-4360. (MDPI AG)
    A review. Single cell anal. (SCA) is nowadays recognized as one of the key tools for diagnostics and fundamental cell biol. studies. The Layer-by-layer (LbL) polyelectrolyte assembly is a rather new but powerful technique to produce multilayers. It allows to model the extracellular matrix in terms of its chem. and phys. properties. Utilization of the multilayers for SCA may open new avenues in SCA because of the triple role of the multilayer film: (i) high capacity for various biomols.; (ii) natural mimics of signal mol. diffusion to a cell and (iii) cell patterning opportunities. Besides, light-triggered release from multilayer films offers a way to deliver biomols. with high spatio-temporal resoln. Here we review recent works showing strong potential to use multilayers for SCA and address accordingly the following issues: biomol. loading, cell patterning, and light-triggered release.
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    Johnston, A. P. R.; Cortez, C.; Angelatos, A. S.; Caruso, F. Layer-by-Layer Engineered Capsules and Their Applications. Curr. Opin. Colloid Interface Sci. 2006, 11, 203– 209,  DOI: 10.1016/j.cocis.2006.05.001
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    Layer-by-layer engineered capsules and their applications
    Johnston, Angus P. R.; Cortez, Christina; Angelatos, Alexandra S.; Caruso, Frank
    Current Opinion in Colloid & Interface Science (2006), 11 (4), 203-209CODEN: COCSFL; ISSN:1359-0294. (Elsevier B.V.)
    A review. Over the last 15 years, development of the layer-by-layer (LbL) technique for prepg. thin films has greatly improved control over the phys. and chem. properties of thin films. When applied to colloidal particles, the LbL technol. permits the formation of hollow capsules with engineered features, including size, shape, compn. and function. In this review, the authors outline the potential applications of LbL assembled micro- and nanometer-sized capsules. These lie principally in the areas of drug delivery, sensing, catalysis and microreactors. The authors focus on the potential use of LbL capsules as drug delivery vehicles and, specifically, on methods for encapsulating and releasing mols. from capsules as well as on targeting capsules to cells.
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    Sukhorukov, G. B.; Donath, E.; Lichtenfeld, H.; Knippel, E.; Knippel, M.; Budde, A.; Möhwald, H. Layer-by-Layer Self Assembly of Polyelectrolytes on Colloidal Particles. Colloids Surf., A 1998, 137, 253– 266,  DOI: 10.1016/S0927-7757(98)00213-1
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    Layer-by-layer self assembly of polyelectrolytes on colloidal particles
    Sukhorukov, Gleb B.; Donath, Edwin; Lichtenfeld, Heinz; Knippel, Eberhard; Knippel, Monika; Budde, Axel; Mohwald, Helmuth
    Colloids and Surfaces, A: Physicochemical and Engineering Aspects (1998), 137 (1-3), 253-266CODEN: CPEAEH; ISSN:0927-7757. (Elsevier Science B.V.)
    Polyelectrolytes have been consecutively adsorbed onto charged polystyrene latex particles, forming stable ultrathin multilayer films. Film growth has been followed by electrophoresis, dynamic light scattering, single particle light scattering and fluorescence intensity measurements. Desorption of bound polyelectrolytes has been followed by HPLC. A low percentage of particle aggregates were found if a centrifugation-based technique was applied, whereas a larger no. of particle aggregates were obsd. if polyelectrolytes were consecutively added at satg. concns. and if centrifugation was avoided. Single particle light scattering allowed for adsorption layer thickness detn. If a layer refractive index of 1.47 was assumed the thickness of a poly(allylamine hydrochloride)/poly(styrene sulfonate) layer pair formed in 0.5 M NaCl was found to be 2-3 nm.
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    Kotov, N. A.; Liu, Y. F.; Wang, S. P.; Cumming, C.; Eghtedari, M.; Vargas, G.; Motamedi, M.; Nichols, J.; Cortiella, J. Inverted Colloidal Crystals as Three-Dimensional Cell Scaffolds. Langmuir 2004, 20, 7887– 7892,  DOI: 10.1021/la049958o
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    Inverted colloidal crystals as three-dimensional cell scaffolds
    Kotov, Nicholas A.; Liu, Yuanfang; Wang, Shaopeng; Cumming, Colin; Eghtedari, Mohammad; Vargas, Gracie; Motamedi, Massoud; Nichols, Joan; Cortiella, Joaquin
    Langmuir (2004), 20 (19), 7887-7892CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    A new type of three-dimensional scaffold with inverted colloidal crystal geometry for the investigation of topol. effects in cell cultures is introduced in this publication. The scaffolds are made by infiltration of the hexagonal crystal lattice of polystyrene spheres with sol-gel formulation and subsequent annealing. It possesses a relatively high degree of order among existing cell scaffolds and affords tight control over the scaffold porosity and tissue organization. The prepd. scaffolds can be a convenient system for the investigation of cell-cell and cell-matrix interactions. Their biocompatibility is demonstrated for human hepatocellular carcinoma HEP G2 and human bone marrow HS-5 cell cultures. A preliminary effect of the scaffold topol. on cell proliferation is obsd. HEP G2 hepatocytes form a large no. of 10-15 cell colonies on scaffolds made from 75-μm spheres, while their no. diminishes for scaffolds from 10- and 160-μm spheres. Under similar conditions, HS-5 forms smaller colonies consisting of three to four cells in 90-μm cavities.
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  180. 180
    Nichols, J. E.; Cortiella, J. Q.; Lee, J.; Niles, J. A.; Cuddihy, M.; Wang, S. P.; Bielitzki, J.; Cantu, A.; Mlcak, R.; Valdivia, E.; Yancy, R.; McClure, M. L.; Kotov, N. A. In Vitro Analog of Human Bone Marrow from 3D Scaffolds with Biomimetic Inverted Colloidal Crystal Geometry. Biomaterials 2009, 30, 1071– 1079,  DOI: 10.1016/j.biomaterials.2008.10.041
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    180
    In vitro analog of human bone marrow from 3D scaffolds with biomimetic inverted colloidal crystal geometry
    Nichols, Joan E.; Cortiella, Joaquin; Lee, Jungwoo; Niles, Jean A.; Cuddihy, Meghan; Wang, Shaopeng; Bielitzki, Joseph; Cantu, Andrea; Mlcak, Ron; Valdivia, Esther; Yancy, Ryan; McClure, Matthew L.; Kotov, Nicholas A.
    Biomaterials (2009), 30 (6), 1071-1079CODEN: BIMADU; ISSN:0142-9612. (Elsevier Ltd.)
    In vitro replicas of bone marrow can potentially provide a continuous source of blood cells for transplantation and serve as a lab. model to examine human immune system dysfunctions and drug toxicol. Here we report the development of an in vitro artificial bone marrow based on a 3D scaffold with inverted colloidal crystal (ICC) geometry mimicking the structural topol. of actual bone marrow matrix. To facilitate adhesion of cells, scaffolds were coated with a layer of transparent nanocomposite. After seeding with hematopoietic stem cells (HSCs), ICC scaffolds were capable of supporting expansion of CD34+ HSCs with B-lymphocyte differentiation. Three-dimensional organization was shown to be crit. for prodn. of B cells and antigen-specific antibodies. Functionality of bone marrow constructs was confirmed by implantation of matrixes contg. human CD34+ cells onto the backs of severe combined immunodeficiency (SCID) mice with subsequent generation of human immune cells.
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  181. 181
    Kozai, T. D. Y.; Langhals, N. B.; Patel, P. R.; Deng, X.; Zhang, H.; Smith, K. L.; Lahann, J.; Kotov, N. A.; Kipke, D. R. Ultrasmall Implantable Composite Microelectrodes with Bioactive Surfaces for Chronic Neural Interfaces. Nat. Mater. 2012, 11, 1065– 1073,  DOI: 10.1038/nmat3468
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    181
    Ultrasmall implantable composite microelectrodes with bioactive surfaces for chronic neural interfaces
    Kozai, Takashi D. Yoshida; Langhals, Nicholas B.; Patel, Paras R.; Deng, Xiaopei; Zhang, Huanan; Smith, Karen L.; Lahann, Joerg; Kotov, Nicholas A.; Kipke, Daryl R.
    Nature Materials (2012), 11 (12), 1065-1073CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Implantable neural microelectrodes that can record extracellular biopotentials from small, targeted groups of neurons are crit. for neuroscience research and emerging clin. applications including brain-controlled prosthetic devices. The crucial material-dependent problem is developing microelectrodes that record neural activity from the same neurons for years with high fidelity and reliability. Here, we report the development of an integrated composite electrode consisting of a carbon-fiber core, a poly(p-xylylene)-based thin-film coating that acts as a dielec. barrier and that is functionalized to control intrinsic biol. processes, and a poly(thiophene)-based recording pad. The resulting implants are an order of magnitude smaller than traditional recording electrodes, and more mech. compliant with brain tissue. They were found to elicit much reduced chronic reactive tissue responses and enabled single-neuron recording in acute and early chronic expts. in rats. This technol., taking advantage of new composites, makes possible highly selective and stealthy neural interface devices towards realizing long-lasting implants.
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  182. 182
    Kotov, N. A.; Winter, J. O.; Clements, I. P.; Jan, E.; Timko, B. P.; Campidelli, S.; Pathak, S.; Mazzatenta, A.; Lieber, C. M.; Prato, M.; Bellamkonda, R. V.; Silva, G. A.; Kam, N. W. S.; Patolsky, F.; Ballerini, L. Nanomaterials for Neural Interfaces. Adv. Mater. 2009, 21, 3970– 4004,  DOI: 10.1002/adma.200801984
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    182
    Nanomaterials for Neural Interfaces
    Kotov, Nicholas A.; Winter, Jessica O.; Clements, Isaac P.; Jan, Edward; Timko, Brian P.; Campidelli, Stephane; Pathak, Smita; Mazzatenta, Andrea; Lieber, Charles M.; Prato, Maurizio; Bellamkonda, Ravi V.; Silva, Gabriel A.; Kam, Nadine Wong Shi; Patolsky, Fernando; Ballerini, Laura
    Advanced Materials (Weinheim, Germany) (2009), 21 (40), 3970-4004CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    This review focuses on the application of nanomaterials for neural interfacing. The junction between nanotechnol. and neural tissues can be particularly worthy of scientific attention for several reasons: (i) Neural cells are electroactive, and the electronic properties of nanostructures can be tailored to match the charge transport requirements of elec. cellular interfacing. (ii) The unique mech. and chem. properties of nanomaterials are crit. for integration with neural tissue as long-term implants. (iii) Solns. to many crit. problems in neural biol./medicine are limited by the availability of specialized materials. (iv) Neuronal stimulation is needed for a variety of common and severe health problems. This confluence of need, accumulated expertise, and potential impact on the well-being of people suggests the potential of nanomaterials to revolutionize the field of neural interfacing. In this review, we begin with foundational topics, such as the current status of neural electrode (NE) technol., the key challenges facing the practical utilization of NEs, and the potential advantages of nanostructures as components of chronic implants. After that the detailed account of toxicol. and biocompatibility of nanomaterials in respect to neural tissues is given. Next, we cover a variety of specific applications of nanoengineered devices, including drug delivery, imaging, topog. patterning, electrode design, nanoscale transistors for high-resoln. neural interfacing, and photoactivated interfaces. We also critically evaluate the specific properties of particular nanomaterials-including nanoparticles, nanowires, and carbon nanotubes-that can be taken advantage of in neuroprosthetic devices. The most promising future areas of research and practical device engineering are discussed as a conclusion to the review.
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  183. 183
    Montjoy, D. G.; Bahng, J. H.; Eskafi, A.; Hou, H.; Kotov, N. A. Omnidispersible Hedgehog Particles with Multilayer Coatings for Multiplexed Biosensing. J. Am. Chem. Soc. 2018, 140, 7835– 7845,  DOI: 10.1021/jacs.8b02666
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    183
    Omnidispersible Hedgehog Particles with Multilayer Coatings for Multiplexed Biosensing
    Montjoy, Douglas G.; Bahng, Joong Hwan; Eskafi, Aydin; Hou, Harrison; Kotov, Nicholas A.
    Journal of the American Chemical Society (2018), 140 (25), 7835-7845CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Hedgehog particles (HPs) replicating the spiky geometry of pollen grains revealed surprisingly high dispersion stability regardless of whether their hydrophobicity/hydrophilicity matches that of the media or not. This property designated as omnidispersibility, is attributed to the drastic redn. of van der Waals interactions between particles coated with stiff nanoscale spikes compared to particles of the same dimensions with smooth surfaces. One may hypothesize but it remains unknown, however, whether HPs modified with polymers or nanoparticles (NPs) would retain this property. Such modifications will expand the functionalities of HPs making possible their use as omnidispersible carriers. HPs carrying dense conformal coatings made by layer-by-layer (LBL) assembly maintain dispersion stability in environments of extreme polarity and ionic strength. HPs, surface-modified by multilayers of polymers and gold NPs are capable of surface enhanced Raman scattering (SERS) and overcome the limited colloidal stability of other SERS probes. The agglomeration resilience of HPs leads to greater than one order of magnitude increase of SERS intensity compared to colloids with smooth surfaces and enables simultaneous detection of several targets in complex media with high ionic strength. Omnidispersible optically active colloids open the door for rapid multiplexed SERS- anal. in biol. fluids and other applications.
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  184. 184
    Lvov, Y. M.; Lu, Z.; Schenkman, J. B.; Zu, X.; Rusling, J. F. Direct Electrochemistry of Myoglobin and Cytochrome P450cam in Alternate Layer-by-Layer Films with DNA and Other Polyions. J. Am. Chem. Soc. 1998, 120, 4073– 4080,  DOI: 10.1021/ja9737984
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    184
    Direct Electrochemistry of Myoglobin and Cytochrome p450cam in Alternate Polyion Layer-by-Layer Films with DNA and Poly(styrenesulfonate)
    Lvov, Yuri M.; Lu, Zhongqing; Schenkman, John B.; Zu, Xiaolin; Rusling, James F.
    Journal of the American Chemical Society (1998), 120 (17), 4073-4080CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Alternate layer-by-layer polyion adsorption onto gold electrodes coated with chemisorbed mercaptopropanesulfonic acid gave stable, electroactive multilayer films contg. the proteins myoglobin and cytochrome P 450cam. Direct, reversible, electron exchange between gold electrodes and proteins involved heme FeIII/FeII redox couples. With oxygen in soln., electrons were also transferred to the FeII-O2 complexes of these proteins, a key step for oxidative enzyme catalysis. Film assembly for Mb was done by sequential adsorption with poly(styrenesulfonate) (PSS) or DNA. Cyt P 450cam was assembled with layers of PSS. Quartz crystal microbalance and voltammetric studies on the same films allowed quantitation of electroactive and nonelectroactive protein. At pH 5.5, the first protein monolayer in all films was fully electroactive. A second monolayer added 30-40% redox activity, but addnl. protein layers did not communicate with the electrode. Using various film construction strategies, Mb monolayers were also placed at distances from the electrodes of 0.5, 1.8, and 3.8 nm. Full electroactivity was found at 0.5 nm, and about 70-80% electroactivity at 1.8 and 3.8 nm. Results suggest the possibility of enhanced electron transport by partial intermixing of protein and nonprotein layers. Polyion films contg. Mb and cyt P 450cam were active for enzyme-like catalysis of styrene epoxidn. in aerobic solns.
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  185. 185
    Tong, W.; Song, X.; Gao, C. Layer-by-Layer Assembly of Microcapsules and Their Biomedical Applications. Chem. Soc. Rev. 2012, 41, 6103– 6124,  DOI: 10.1039/c2cs35088b
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    185
    Layer-by-layer assembly of microcapsules and their biomedical applications
    Tong, Weijun; Song, Xiaoxue; Gao, Changyou
    Chemical Society Reviews (2012), 41 (18), 6103-6124CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Nanoengineered multifunctional capsules with tailored structures and properties are of particular interest due to their multifunctions and potential applications as new colloidal structures in diverse fields. Among the available fabrication methods, the layer-by-layer (LbL) assembly of multilayer films onto colloidal particles followed by selective template removal has attracted extensive attention due to its advantages of precise control over the size, shape, compn., wall thickness and functions of the obtained capsules. The past decade has witnessed a rapid increase of research concerning the new fabrication strategies, functionalization and applications of this kind of capsules, particularly in the biomedical fields such as drug delivery, biosensors and bioreactors. In this crit. review, the very recent progress of the multilayer capsules is summarized. First, the advances in assembly of capsules by the LbL technique are introduced with focus on tailoring the properties of hydrogen-bonded multilayer capsules by crosslinking, and fabrication of capsules based on covalent bonding and bio-specific interactions. Then the fabrication strategies which can speed up capsule fabrication are reviewed. In the following sections, the multi-compartmental capsules and the capsules that can transform their shape under stimulus are presented. Finally, the biomedical applications of multilayer capsules with particular emphasis on drug carriers, biosensors and bioreactors are described (306 refs.).
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  186. 186
    Yashchenok, A. M.; Bratashov, D. N.; Gorin, D. A.; Lomova, M. V.; Pavlov, A. M.; Sapelkin, A. V.; Shim, B. S.; Khomutov, G. B.; Kotov, N. A.; Sukhorukov, G. B.; Möhwald, H.; Skirtach, A. G. Carbon Nanotubes on Polymeric Microcapsules: Free-Standing Structures and Point-Wise Laser Openings. Adv. Funct. Mater. 2010, 20, 3136– 3142,  DOI: 10.1002/adfm.201000846
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    186
    Carbon Nanotubes on Polymeric Microcapsules: free-Standing Structures and Point-Wise Laser Openings
    Yashchenok, Alexey M.; Bratashov, Daniil N.; Gorin, Dmitry A.; Lomova, Maria V.; Pavlov, Anton M.; Sapelkin, Andrei V.; Shim, Bong Sup; Khomutov, Gennady B.; Kotov, Nicholas A.; Sukhorukov, Gleb B.; Moehwald, Helmuth; Skirtach, Andre G.
    Advanced Functional Materials (2010), 20 (18), 3136-3142CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Single-wall carbon nanotubes modified by anionic polyelectrolyte mols. are embedded into the shells of microcapsules. Carbon nanotubes serve as rigid rods in a softer polymeric capsule, which forms a free-standing shell upon treatment with glutaraldehyde and subsequent drying. The embedded carbon nanotubes exhibit a broad absorption in the UV-near-IR part of the spectrum, and that allows point-wise activation and opening of the microcapsules by laser. Raman signal anal. shows changes of carbon-nanotube-specific lines after high-power laser irradn., which is characteristic of the formation of disordered carbonlike structures. These polyelectrolyte/carbon nanotube composite capsules represent a novel light-addressable type of microcontainers.
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  187. 187
    Shyu, T. C.; Damasceno, P. F.; Dodd, P. M.; Lamoureux, A.; Xu, L.; Shlian, M.; Shtein, M.; Glotzer, S. C.; Kotov, N. A. A Kirigami Approach to Engineering Elasticity in Nanocomposites Through Patterned Defects. Nat. Mater. 2015, 14, 785– 789,  DOI: 10.1038/nmat4327
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    187
    A kirigami approach to engineering elasticity in nanocomposites through patterned defects
    Shyu, Terry C.; Damasceno, Pablo F.; Dodd, Paul M.; Lamoureux, Aaron; Xu, Lizhi; Shlian, Matthew; Shtein, Max; Glotzer, Sharon C.; Kotov, Nicholas A.
    Nature Materials (2015), 14 (8), 785-789CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Efforts to impart elasticity and multifunctionality in nanocomposites focus mainly on integrating polymeric and nanoscale components. Yet owing to the stochastic emergence and distribution of strain-concg. defects and to the stiffening of nanoscale components at high strains, such composites often possess unpredictable strain-property relationships. Here, by taking inspiration from kirigami-the Japanese art of paper cutting-we show that a network of notches made in rigid nanocomposite and other composite sheets by top-down patterning techniques prevents unpredictable local failure and increases the ultimate strain of the sheets from 4 to 370%. We also show that the sheets' tensile behavior can be accurately predicted through finite-element modeling. Moreover, in marked contrast to other stretchable conductors, the elec. conductance of the stretchable kirigami sheets is maintained over the entire strain regime, and we demonstrate their use to tune plasma-discharge phenomena. The unique properties of kirigami nanocomposites as plasma electrodes open up a wide range of novel technol. solns. for stretchable electronics and optoelectronic devices, among other application possibilities.
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  188. 188
    Kim, Y.; Yeom, B.; Arteaga, O.; Yoo, S. J.; Lee, S. G.; Kim, J. G.; Kotov, N. A. Reconfigurable Chiroptical Nanocomposites with Chirality Transfer From the Macro- to the Nanoscale. Nat. Mater. 2016, 15, 461– 468,  DOI: 10.1038/nmat4525
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    188
    Reconfigurable chiroptical nanocomposites with chirality transfer from the macro- to the nanoscale
    Kim, Yoonseob; Yeom, Bongjun; Arteaga, Oriol; Yoo, Seung Jo; Lee, Sang-Gil; Kim, Jin-Gyu; Kotov, Nicholas A.
    Nature Materials (2016), 15 (4), 461-468CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Nanostructures with chiral geometries exhibit strong polarization rotation. However, achieving reversible modulation of chirality and polarization rotation in device-friendly solid-state films is difficult for rigid materials. Here, we describe nanocomposites, made by conformally coating twisted elastic substrates with films assembled layer-by-layer from plasmonic nanocolloids, whose nanoscale geometry and rotatory optical activity can be reversibly reconfigured and cyclically modulated by macroscale stretching, with up to tenfold concomitant increases in ellipticity. We show that the chiroptical activity at 660 nm of gold nanoparticle composites is assocd. with circular extinction from linear effects. The polarization rotation at 550 nm originates from the chirality of nanoparticle chains with an S-like shape that exhibit a non-planar buckled geometry, with the handedness of the substrate's macroscale twist detg. the handedness of the S-like chains. Chiroptical effects at the nexus of mechanics, excitonics and plasmonics open new operational principles for optical and optoelectronic devices from nanoparticles, carbon nanotubes and other nanoscale components.
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  189. 189
    Muzzio, N. E.; Pasquale, M. A.; Diamanti, E.; Gregurec, D.; Moro, M. M.; Azzaroni, O.; Moya, S. E. Enhanced Antiadhesive Properties of Chitosan/Hyaluronic Acid Polyelectrolyte Multilayers Driven by Thermal Annealing: Low Adherence for Mammalian Cells and Selective Decrease in Adhesion for Gram-Positive Bacteria. Mater. Sci. Eng., C 2017, 80, 677– 687,  DOI: 10.1016/j.msec.2017.07.016
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    189
    Enhanced antiadhesive properties of chitosan/hyaluronic acid polyelectrolyte multilayers driven by thermal annealing: Low adherence for mammalian cells and selective decrease in adhesion for Gram-positive bacteria
    Muzzio, Nicolas E.; Pasquale, Miguel A.; Diamanti, Eleftheria; Gregurec, Danijela; Moro, Marta Martinez; Azzaroni, Omar; Moya, Sergio E.
    Materials Science & Engineering, C: Materials for Biological Applications (2017), 80 (), 677-687CODEN: MSCEEE; ISSN:0928-4931. (Elsevier B.V.)
    The development of antifouling coatings with restricted cell and bacteria adherence is fundamental for many biomedical applications. A strategy for the fabrication of antifouling coatings based on the layer-by-layer assembly and thermal annealing is presented. Polyelectrolyte multilayers (PEMs) assembled from chitosan and hyaluronic acid were thermally annealed in an oven at 37°C for 72 h. The effect of annealing on the PEM properties and topog. was studied by at. force microscopy, ζ-potential, CD and contact angle measurements. Cell adherence on PEMs before and after annealing was evaluated by measuring the cell spreading area and aspect ratio for the A549 epithelial, BHK kidney fibroblast, C2C12 myoblast and MC-3T3-E1 osteoblast cell lines. Chitosan/hyaluronic acid PEMs show a low cell adherence that decreases with the thermal annealing, as obsd. from the redn. in the av. cell spreading area and more rounded cell morphol. The adhesion of S. aureus (Gram-pos.) and E. coli (Gram-neg.) bacteria strains was quantified by optical microscopy, counting the no. of colony-forming units and measuring the light scattering of bacteria suspension after detachment from the PEM surface. A 20% decrease in bacteria adhesion was selectively obsd. in the S. aureus strain after annealing. The changes in mammalian cell and bacteria adhesion correlate with the changes in topog. of the chitosan/hyaluronic PEMs from a rough fibrillar 3D structure to a smoother and planar surface after thermal annealing.
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  190. 190
    Dubas, S. T.; Kumlangdudsana, P.; Potiyaraj, P. Layer-by-Layer Deposition of Antimicrobial Silver Nanoparticles on Textile Fibers. Colloids Surf., A 2006, 289, 105– 109,  DOI: 10.1016/j.colsurfa.2006.04.012
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    190
    Layer-by-layer deposition of antimicrobial silver nanoparticles on textile fibers
    Dubas, Stephan T.; Kumlangdudsana, Panittamat; Potiyaraj, Pranut
    Colloids and Surfaces, A: Physicochemical and Engineering Aspects (2006), 289 (1-3), 105-109CODEN: CPEAEH; ISSN:0927-7757. (Elsevier B.V.)
    Antimicrobial silver nanoparticles were immobilized on nylon and silk fibers by following the layer-by-layer deposition method. The sequential dipping of nylon or silk fibers in dil. solns. of poly(diallyldimethylammonium chloride) (PDADMAC) and silver nanoparticles capped with poly(methacrylic acid) (PMA) led to the formation of a colored thin film possessing antimicrobial properties. The layer-by-layer deposition was monitored by measuring the K/S value, which is the ratio between the sorption coeff. (K) and the scattering (S) of the coated fibers, with a reflectance spectrophotometer. The K/S values for both silk and nylon fibers were found to increase as a function of the no. of deposited layers. Although the film growth was obsd. on both fibers, the K/S value of the nylon fiber was significantly lower than silk fibers. SEM studies of both fibers confirmed that the layer-by-layer coating on the nylon fibers was not as uniform as on the silk fibers. Antimicrobial tests against Staphylococcus aureus bacteria were performed and antimicrobial activity was demonstrated for both coated fibers. The deposition of 20 PDADMAC/PMAcapAg layers onto the fibers resulted in 80% bacteria redn. for the silk fiber and 50% for the nylon fiber. Although the film growth was more efficient on the silk fibers, these results suggest that this technique could be used in the design of new synthetic or natural tech. fibers where antimicrobial properties are required.
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  191. 191
    Moskowitz, J. S.; Blaisse, M. R.; Samuel, R. E.; Hsu, H. P.; Harris, M. B.; Martin, S. D.; Lee, J. C.; Spector, M.; Hammond, P. T. The Effectiveness of the Controlled Release of Gentamicin from Polyelectrolyte Multilayers in the Treatment of Staphylococcus aureus Infection in a Rabbit Bone Model. Biomaterials 2010, 31, 6019– 6030,  DOI: 10.1016/j.biomaterials.2010.04.011
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    191
    The effectiveness of the controlled release of gentamicin from polyelectrolyte multilayers in the treatment of Staphylococcus aureus infection in a rabbit bone model
    Moskowitz, Joshua S.; Blaisse, Michael R.; Samuel, Raymond E.; Hsu, Hu-Ping; Harris, Mitchel B.; Martin, Scott D.; Lee, Jean C.; Spector, Myron; Hammond, Paula T.
    Biomaterials (2010), 31 (23), 6019-6030CODEN: BIMADU; ISSN:0142-9612. (Elsevier Ltd.)
    While the infection rate of orthopedic implants is low, the required treatment, which can involve six weeks of antibiotic therapy and two addnl. surgical operations, is life threatening and expensive, and thus motivates the development of a one-stage re-implantation procedure. Polyelectrolyte multilayers incorporating gentamicin were fabricated using the layer-by-layer deposition process for use as a device coating to address an existing bone infection in a direct implant exchange operation. The films eluted about 70% of their payload in vitro during the first three days and subsequently continued to release drug for more than four addnl. weeks, reaching a total av. release of over 550 μg/cm2. The coatings were demonstrated to be bactericidal against Staphylococcus aureus, and degrdn. products were generally nontoxic towards MC3T3-E1 murine preosteoblasts. Film-coated titanium implants were compared to uncoated implants in an in vivo S. aureus bone infection model. After a direct exchange procedure, the antimicrobial-coated devices yielded bone homogenates with a significantly lower degree of infection than uncoated devices at both day four (p < 0.004) and day seven (p < 0.03). This study has demonstrated that a self-assembled ultrathin film coating is capable of effectively treating an exptl. bone infection in vivo and lays the foundation for development of a multi-therapeutic film for optimized, synergistic treatment of pain, infection, and osteomyelitis.
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  192. 192
    Shah, N. J.; Macdonald, M. L.; Beben, M. Y.; Padera, R. F.; Samuel, R. E.; Hammond, P. T. Tunable Dual Growth Factor Delivery from Polyelectrolyte Multilayer Films. Biomaterials 2011, 32, 6183– 6193,  DOI: 10.1016/j.biomaterials.2011.04.036
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    192
    Tunable dual growth factor delivery from polyelectrolyte multilayer films
    Shah, Nisarg J.; MacDonald, Mara L.; Beben, Yvette M.; Padera, Robert F.; Samuel, Raymond E.; Hammond, Paula T.
    Biomaterials (2011), 32 (26), 6183-6193CODEN: BIMADU; ISSN:0142-9612. (Elsevier Ltd.)
    A promising strategy to accelerate joint implant integration and reduce recovery time and failure rates is to deliver a combination of certain growth factors to the integration site. There is a need to control the quantity of growth factors delivered at different times during the healing process to maximize efficacy. Polyelectrolyte multilayer (PEM) films, built using the layer-by-layer (LbL) technique, are attractive for releasing controlled amts. of potent growth factors over a sustained period. Here, we present PEM films that sequester physiol. amts. of osteogenic rhBMP-2 (recombinant human bone morphogenetic protein - 2) and angiogenic rhVEGF165 (recombinant human vascular endothelial growth factor) in different ratios in a degradable [poly(β-amino ester)/polyanion/growth factor/polyanion] LbL tetralayer repeat architecture where the biol. load scaled linearly with the no. of tetralayers. No burst release of either growth factor was obsd. as the films degraded. The release of rhBMP-2 was sustained over a period of 2 wk, while rhVEGF165 eluted from the film over the first 8 days. Both growth factors retained their efficacy, as quantified with relevant in vitro assays. rhBMP-2 initiated a dose dependent differentiation cascade in MC3T3-E1S4 pre-osteoblasts while rhVEGF165 upregulated HUVEC proliferation, and accelerated closure of a scratch in HUVEC cell cultures in a dose dependent manner. In vivo, the mineral d. of ectopic bone formed de novo by rhBMP-2/rhVEGF165 PEM films was approx. 33% higher than when only rhBMP-2 was introduced, with a higher trabecular thickness, which would indicate a decrease in the risk of osteoporotic fracture. Bone formed throughout the scaffold when both growth factors were released, which suggests more complete remodeling due to an increased local vascular network. This study demonstrates a promising approach to delivering precise doses of multiple growth factors for a variety of implant applications where control over spatial and temporal release profile of the biol. is desired.
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  193. 193
    Hammond, P. T. Building Biomedical Materials Layer-by-Layer. Mater. Today 2012, 15, 196– 206,  DOI: 10.1016/S1369-7021(12)70090-1
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    Building biomedical materials layer-by-layer
    Hammond, Paula T.
    Materials Today (Oxford, United Kingdom) (2012), 15 (5), 196-206CODEN: MTOUAN; ISSN:1369-7021. (Elsevier Ltd.)
    A review. In this materials perspective, the promise of water based layer-by-layer (LbL) assembly as a means of generating drug-releasing surfaces for biomedical applications, from small mol. therapeutics to biol. drugs and nucleic acids, is examd. Specific advantages of the use of LbL assembly vs. traditional polymeric blend encapsulation are discussed. Examples are provided to present potential new directions. Translational opportunities are discussed to examine the impact and potential for true biomedical translation using rapid assembly methods, and applications are discussed with high need and medical return.
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  194. 194
    Macdonald, M. L.; Samuel, R. E.; Shah, N. J.; Padera, R. F.; Beben, Y. M.; Hammond, P. T. Tissue Integration of Growth Factor-Eluting Layer-by-Layer Polyelectrolyte Multilayer Coated Implants. Biomaterials 2011, 32, 1446– 1453,  DOI: 10.1016/j.biomaterials.2010.10.052
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    194
    Tissue integration of growth factor-eluting layer-by-layer polyelectrolyte multilayer coated implants
    MacDonald, Mara L.; Samuel, Raymond E.; Shah, Nisarg J.; Padera, Robert F.; Beben, Yvette M.; Hammond, Paula T.
    Biomaterials (2011), 32 (5), 1446-1453CODEN: BIMADU; ISSN:0142-9612. (Elsevier Ltd.)
    Drug eluting coatings that can direct the host tissue response to implanted medical devices have the potential to ameliorate both the medical and financial burden of complications from implantation. However, because many drugs useful in this arena are biol. in nature, a paucity of delivery strategies for biologics, including growth factors, currently limits the control that can be exerted on the implantation environment. Layer-by-Layer (LbL) polyelectrolyte multilayer films are highly attractive as ultrathin biol. reservoirs, due to the capability to conformally coat difficult geometries, the use of aq. processing likely to preserve fragile protein function, and the tunability of incorporation and release profiles. Herein, we describe the first LbL films capable of microgram-scale release of the biol. Bone Morphogenetic Protein 2 (BMP-2), which is capable of directing the host tissue response to create bone from native progenitor cells. Ten micrograms of BMP-2 are released over a period of two weeks in vitro; less than 1% is released in the first 3 h (compared with com. collagen matrixes which can release up to 60% of BMP-2, too quickly to induce differentiation). BMP-2 released from LbL films retains its ability to induce bone differentiation in MC3T3 E1S4 pre-osteoblasts, as measured by induction of alk. phosphatase and stains for calcium (via Alizarin Red) and calcium matrix (via Von Kossa). In vivo, BMP-2 film coated scaffolds were compared with film coated scaffolds lacking BMP-2. BMP-2 coatings implanted i.m. were able to initiate host progenitor cells to differentiate into bone, which matured and expanded from four to 9 wk as measured by MicroCT and histol. Such LbL films represent new steps towards controlling and tuning host response to implanted medical devices, which may ultimately increase the success of implanted devices, provide alternative new approaches toward bone wound healing, and lay the foundation for development of a multi-therapeutic release coating.
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    Joseph, N.; Ahmadiannamini, P.; Hoogenboom, R.; Vankelecom, I. F. J. Layer-by-Layer Preparation of Polyelectrolyte Multilayer Membranes for Separation. Polym. Chem. 2014, 5, 1817– 1831,  DOI: 10.1039/C3PY01262J
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    Layer-by-layer preparation of polyelectrolyte multilayer membranes for separation
    Joseph, Nithya; Ahmadiannamini, Pejman; Hoogenboom, Richard; Vankelecom, Ivo. F. J.
    Polymer Chemistry (2014), 5 (6), 1817-1831CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
    A review. Polymer membranes provide a highly promising platform for the development of an efficient and sustainable technique for sepn. Ideally such membranes combine a high flux with a high selectivity requiring thin defect-free membranes. The layer by layer (LBL) assembly technique has proven to be a versatile and simple method for the fabrication of very thin polyelectrolyte multilayers making it highly suitable for the prepn. of sepn. membranes. Recent developments in this field related to membrane prepn. and their applications in sepn. processes are presented and discussed in this review. An overview of the different fabrication techniques of such membranes will be first provided. In addn., the formation mechanism and the parameters that can be varied to tune the properties of the membranes will be discussed. Finally, the potential applications of these membranes in different sepn. areas such as pervaporation, nanofiltration, solvent resistant nanofiltration, reverse osmosis, gas sepn. and forward osmosis will be addressed.
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    Miller, M. D.; Bruening, M. L. Controlling the Nanofiltration Properties of Multilayer Polyelectrolyte Membranes through Variation of Film Composition. Langmuir 2004, 20, 11545– 11551,  DOI: 10.1021/la0479859
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    196
    Controlling the nanofiltration properties of multilayer polyelectrolyte membranes through variation of film composition
    Miller, Matthew D.; Bruening, Merlin L.
    Langmuir (2004), 20 (26), 11545-11551CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    We report the use of a variety of polyelectrolyte multilayers (PEMs) as selective skins in composite membranes for nanofiltration (NF) and diffusion dialysis. Deposition of PEMs occurs through simple alternating adsorption of polycations and polyanions, and sepns. can be optimized by varying the constituent polyelectrolytes as well as deposition conditions. In general, the use of polycations and polyanions with lower charge densities allows sepn. of larger analytes. Depending on the polyelectrolytes employed, PEM membranes can remove salt from sugar solns., sep. proteins, or allow size-selective passage of specific sugars. Addnl., because of the minimal thickness of PEMs, NF pure water fluxes through these membranes typically range from 1.5 to 3 m3/(m2 day) at 4.8 bar. Specifically, to sep. sugars, we employed poly(styrene sulfonate) (PSS)/poly(diallyldimethylammonium chloride) (PDADMAC) films, which allow 42% passage of glucose along with a 98% rejection of raffinose and a pure water flux of 2.4 m3/(m2 day). PSS/PDADMAC membranes are also capable of sepg. NaCl and sucrose (selectivity of ∼10), while high-flux chitosan/hyaluronic acid membranes [pure water flux of 5 m3/(m2 day) at 4.8 bar] may prove useful in protein sepns.
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  197. 197
    Krasemann, L.; Tieke, B. Composite Membranes with Ultrathin Separation Layer Prepared by Self-Assembly of Polyelectrolytes. Mater. Sci. Eng., C 1999, 8–9, 513– 518,  DOI: 10.1016/S0928-4931(99)00030-2
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    Composite membranes with ultrathin separation layer prepared by self-assembly of polyelectrolytes
    Krasemann, Lutz; Tieke, Bernd
    Materials Science & Engineering, C: Biomimetic and Supramolecular Systems (1999), C8-C9 (), 513-518CODEN: MSCEEE; ISSN:0928-4931. (Elsevier Science B.V.)
    New composite membranes with ultrathin self-assembled polyelectrolyte sepg. layer and their use in gas, liq. and ion sepn. are described. Composite membranes were prepd. by alternating electrostatic adsorption of polycations and polyanions on a porous PAN/PET supporting membrane (a polyethyleneterephthalate fleece coated with a thin layer of polyacrylonitrile). As the polycations poly(allylamine hydrochloride) (PAH), poly(ethylenimine) (PEI), poly(diallyldimethyl-ammonium chloride) (PDADMAC), poly(4-vinylpyridine) (P4VP) and chitosan (CHI) were used, the polyanion was always poly(styrene sulfonate sodium salt) (PSS). Studying CO2 and N2 permeation rates, selectivities for CO2 (α = 1.5) were only found with P4VP/PSS sepn. layer. Studying ethanol-water pervaporation, sepn. factors α' up to 80 were found, when the sepn. layer was made of PAH/PSS and annealed at T≥60°. For non-annealed samples, the highest α' value was 20. It was found for a sepg. membrane of PEI/PSS. Various effects of prepn. and operation conditions on flux and sepn. factor are discussed. Studying permeation of NaCl and MgCl2 in aq. soln., a selective transport of the monovalent Na+ was found, if the sepn. layer was PAH/PSS. The permeation rate for Na+ was 15.1 times higher than for Mg2+. Our study indicates that composite membranes with self-assembled polyelectrolyte sepn. layer are useful in materials sepn., although individual polyions may exhibit a very different sepn. capability.
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  198. 198
    van Ackern, F.; Krasemann, L.; Tieke, B. Ultrathin Membranes for Gas Separation and Pervaporation Prepared upon Electrostatic Self-Assembly of Polyelectrolytes. Thin Solid Films 1998, 327–329, 762– 766,  DOI: 10.1016/S0040-6090(98)00782-2
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    Ultrathin membranes for gas separation and pervaporation prepared upon electrostatic self-assembly of polyelectrolytes
    van Ackern, Frank; Krasemann, Lutz; Tieke, Bernd
    Thin Solid Films (1998), 327-329 (), 762-766CODEN: THSFAP; ISSN:0040-6090. (Elsevier Science S.A.)
    Composite membranes with ultrathin sepn. layer were prepd. by alternating electrostatic adsorption of cationic and anionic compds. (polyelectrolytes and bolaamphiphiles) at porous supporting membranes such as Celgard 2400, PAN/PET (a polyethyleneterephthalate fleece coated with a thin layer of polyacrylonitrile) and Isopore (an etched ion-track polycarbonate membrane). As the ionic compds. poly(allylamine hydrochloride) (PAH), poly(styrenesulfonate sodium salt) (PSS), 1,4-diketo-3,6-diphenylpyrrolo-[3,4-c]-pyrrole-4,4'-disulfonic acid and 10,12-docosadiyne-1,22-disulfate disodium salt were used. The composite membranes were studied on their gas permeation and pervaporation properties. The adsorption of 20 layer pairs of PAH/PSS at PAN/PET membranes was sufficient to reduce the argon flow to 7% of the initial value, while 60 bilayers reduced the flow to 0.01%. Gas flow rates of oxygen, nitrogen and argon were nearly identical, while the flow rate of carbon dioxide was higher by a factor of up to 2.4. Toluene/heptane pervaporation through composite membranes of 60 bilayers of PAH/PSS on PAN/PET gave a toluene enrichment from 20% in the feed to 24% in the permeate, while for ethanol/water permeation a water enrichment from 3% in the feed up to 50% in the permeate was found.
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  199. 199
    White, N.; Misovich, M.; Alemayehu, E.; Yaroshchuk, A.; Bruening, M. L. Highly Selective Separations of Multivalent and Monovalent Cations in Electrodialysis through Nafion Membranes Coated with Polyelectrolyte Multilayers. Polymer 2016, 103, 478– 485,  DOI: 10.1016/j.polymer.2015.12.019
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    Highly selective separations of multivalent and monovalent cations in electrodialysis through Nafion membranes coated with polyelectrolyte multilayers
    White, Nicholas; Misovich, Maria; Alemayehu, Elena; Yaroshchuk, Andriy; Bruening, Merlin L.
    Polymer (2016), 103 (), 478-485CODEN: POLMAG; ISSN:0032-3861. (Elsevier Ltd.)
    Nafion membranes coated with 5.5-bilayer poly(4-styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) films show Li+/Co2+ and K+/La3+ selectivities >1000 in electrodialysis (ED). The corresponding selectivities of bare Nafion are <2. Even with adsorption of only 3 polyelectrolyte layers, Nafion membranes exhibit a Li+/Co2+ selectivity >23. However, the resistance to monovalent-ion passage does not decrease significantly with fewer polyelectrolyte layers. At overlimiting currents, H2O splitting near the polyelectrolyte/Nafion interface reduces current efficiency because protons and hydroxide ions carry current in the membrane. Also, hydroxides from H2O splitting form insol. metal hydroxides to foul the membrane. With 0.1 M source-phase salt concns., transference nos. for monovalent cations approach unity and selectivities are >5000 because the diffusion-limited K+ or Li+ currents exceed the applied current. However, ED selectivities gradually decline with time. Thus, future research should aim to increase membrane stability and limiting currents to fully exploit the remarkable selectivity of these membranes.
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  200. 200
    Krasemann, L.; Tieke, B. Selective Ion Transport Across Self-Assembled Alternating Multilayers of Cationic and Anionic Polyelectrolytes. Langmuir 2000, 16, 287– 290,  DOI: 10.1021/la991240z
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    200
    Selective Ion Transport across Self-Assembled Alternating Multilayers of Cationic and Anionic Polyelectrolytes
    Krasemann, Lutz; Tieke, Bernd
    Langmuir (2000), 16 (2), 287-290CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Ultrathin membranes consisting of an alternating sequence of cationic and anionic polyelectrolytes were prepd. by means of electrostatic layer-by-layer adsorption and investigated on their permeability for NaCl, Na2SO4, and MgCl2 in aq. soln. The multi-bipolar structure of the polyelectrolyte membranes favors the sepn. of mono- and divalent ions by Donnan exclusion of the divalent ions. Various effects on the rate of ion permeation and the selectivity were investigated. Addn. of salt to the polyelectrolyte solns. used for membrane prepn. led to improved ion sepn., while an increase of the pH had the opposite effect. Use of polyelectrolytes with high charge d. also improved the ion sepn. Esp. good results were obtained if membranes contg. polyallylamine (PAH) as the cationic polyelectrolyte were used. For 60 layer pairs of PAH/polystyrenesulfonate, for example, a sepn. factor α for Na+/Mg2+ up to 112.5 and for Cl-/SO42- up to 45.0 was found. The origins of the various effects are discussed in terms of different charge d. and concn. of excess charges in the polyelectrolyte membrane.
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    Liu, G.; Dotzauer, M. D.; Bruening, M. L. Ion-Exchange Membranes Prepared Using Layer-by-Layer Polyelectrolyte Deposition. J. Membr. Sci. 2010, 354, 198– 205,  DOI: 10.1016/j.memsci.2010.02.047
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    Ion-exchange membranes prepared using layer-by-layer polyelectrolyte deposition
    Liu, Guanqing; Dotzauer, David M.; Bruening, Merlin L.
    Journal of Membrane Science (2010), 354 (1-2), 198-205CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
    Layer-by-layer polyelectrolyte adsorption in porous polymeric membranes provides a simple way to create ion-exchange sites without greatly decreasing hydraulic permeability (<20% redn. in permeability). At 80% breakthrough, membranes coated with 3-bilayer poly(styrene sulfonate) (PSS)/polyethyleneimine (PEI) films bind 37 ± 6 mg of neg. charged Au colloids per mL of membrane vol. The binding capacity of membranes coated with 1-bilayer films decreases in the order PSS/PEI > PSS/poly(diallyldimethyl ammonium chloride) > PSS/poly(allylamine hydrochloride). Films terminated with a polyanion present cation-exchange sites that bind lysozyme, and the lysozyme-binding capacities of (PSS/PEI)3/PSS films increase with the ionic strength of the soln. from which the last PSS layer is deposited. Charge screening during deposition of the terminal PSS layer gives rise to a larger no. of ion-exchange sites and lysozyme binding capacities as high as 16 mg/mL of membrane. At 10% breakthrough, a stack of 3 membranes binds 3 times as much lysozyme as a single membrane, showing that stacking is an effective way to increase capacity.
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  202. 202
    de Grooth, J.; Oborný, R.; Potreck, J.; Nijmeijer, K.; de Vos, W. M. The Role of Ionic Strength and Odd-Even Effects on the Properties of Polyelectrolyte Multilayer Nanofiltration Membranes. J. Membr. Sci. 2015, 475, 311– 319,  DOI: 10.1016/j.memsci.2014.10.044
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    The role of ionic strength and odd-even effects on the properties of polyelectrolyte multilayer nanofiltration membranes
    de Grooth, Joris; Oborny, Radek; Potreck, Jens; Nijmeijer, Kitty; de Vos, Wiebe M.
    Journal of Membrane Science (2015), 475 (), 311-319CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
    The modification of membranes by polyelectrolytes via the Layer-by-Layer technique is an attractive method to obtain nanofiltration membranes. We prep. such membranes by alternatively coating a polycation (poly(diallyldimethylammonium chloride) (PDADAMAC)) and a polyanion (poly(styrene sulfonate) (PSS)) on a porous support. Depending on the coating conditions, hollow fiber membranes with rejections of up to 71% NaCl and 96% Na2SO4 are obtained. Moreover, we demonstrate that the final membrane properties can be easily controlled by variation of the ionic strength of the coating soln., the no. of layers and the choice of terminating polyelectrolyte layer. Coating at higher salt concns. results in thicker multilayers that are more open to permeation. Furthermore, we show that by taking the effect of the terminating layer (the so called "odd-even" effect) into account, information on the structure of the multilayers on the membrane is obtained. Depending on the coating conditions and no. of layers, two different regimes can be distinguished. Thinner layers show a pore-dominated regime, where the multilayer is coated on the inside of the pores. Thicker layers show a layer-dominated regime, in which case the multilayer is predominately coated on top of the pores. This conclusion is supported by our ion rejection measurements: for thin layers the rejections are primarily based on size exclusion, whereas for thick layers the ion rejections are based on Donnan exclusion.
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  203. 203
    Menne, D.; Üzüm, C.; Koppelmann, A.; Wong, J. E.; Foeken, C. V.; Borre, F.; Dähne, L.; Laakso, T.; Pihlajamäki, A.; Wessling, M. Regenerable Polymer/Ceramic Hybrid Nanofiltration Membrane Based on Polyelectrolyte Assembly by Layer-by-Layer Technique. J. Membr. Sci. 2016, 520, 924– 932,  DOI: 10.1016/j.memsci.2016.08.048
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    203
    Regenerable polymer/ceramic hybrid nanofiltration membrane based on polyelectrolyte assembly by layer-by-layer technique
    Menne, Daniel; Uezuem, Cagri; Koppelmann, Arne; Wong, John Erik; van Foeken, Chiel; Borre, Fokko; Daehne, Lars; Laakso, Timo; Pihlajamaeki, Arto; Wessling, Matthias