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Stable Cellulose Nanofibril Microcapsules from Pickering Emulsion Templates

  • Hui Shi
    Hui Shi
    Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
    Centre for Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
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  • Kazi M. Zakir Hossain
    Kazi M. Zakir Hossain
    Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
    Centre for Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
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  • , 
  • Davide Califano
    Davide Califano
    Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
    Centre for Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
    More by Davide Califano
    Orcidhttps://orcid.org/0000-0002-9416-2021
  • , 
  • Ciaran Callaghan
    Ciaran Callaghan
    Centre for Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
    Department of Chemical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
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  • Ekanem E. Ekanem
    Ekanem E. Ekanem
    Department of Chemical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
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  • , 
  • Janet L. Scott
    Janet L. Scott
    Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
    Centre for Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
    More by Janet L. Scott
    Orcidhttps://orcid.org/0000-0001-8021-2860
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  • Davide Mattia
    Davide Mattia
    Department of Chemical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
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  • , and 
  • Karen J. Edler*
    Karen J. Edler
    Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
    Centre for Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
    *E-mail: [email protected]
    More by Karen J. Edler
    Orcidhttps://orcid.org/0000-0001-5822-0127
Cite this: Langmuir 2022, 38, 11, 3370–3379
Publication Date (Web):March 9, 2022
https://doi.org/10.1021/acs.langmuir.1c03025

Copyright © 2022 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Electrostatic attractions are essential in any complex formation between the nanofibrils of the opposite charge for a specific application, such as microcapsule production. Here, we used cationized cellulose nanofibril (CCNF)-stabilized Pickering emulsions (PEs) as templates, and the electrostatic interactions were induced by adding oxidized cellulose nanofibrils (OCNFs) at the oil–water interface to form microcapsules (MCs). The oppositely charged cellulose nanofibrils enhanced the solidity of interfaces, allowing the encapsulation of Nile red (NR) in sunflower oil droplets. Microcapsules exhibited a low and controlled release of NR at room temperature. Furthermore, membrane emulsification was employed to scale up the preparation of microcapsules with sunflower oil (SFO) encapsulated by CCNF/OCNF complex networks.

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Introduction

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Microcapsules (MCs) have an extensive range of applications as carriers and release vehicles for drugs, cosmetics, dyes, and many other bioactive/functional ingredients. (1,2) However, the vast majority of microcapsules are manufactured using nonbiodegradable polymers obtained from nonrenewable sources. (3,4) With growing awareness of pollution arising from microplastics and fossil fuels, polysaccharide-based microcapsules have attracted considerable attention as these are biodegradable, renewable, and biocompatible. (5,6) Compared with synthetic biopolymers or heavily modified biomolecules, the utilization of polysaccharides can reduce the burden on the environment, lower the cost, and increase the industrial use of these microcapsules. (7,8) However, the production of polysaccharide-based microcapsules is not a straightforward process. Many reported methods employ complex preparation methods, such as multiple deposition steps, cross-linking of biopolymers, (9,10) or decomposition of template cores after coating biopolymers. (11,12) Furthermore, most of the microcapsule production techniques reported in the literature are of low feasibility for scale-up industrial production. (13) In contrast, membrane emulsification (ME) is a well-controlled and highly engineered process that satisfies the essential requirements for scaling-up manufacturing volumes. (14,15) With further postprocessing, emulsions prepared via membrane emulsification can be turned into microcapsules for the delivery of actives. (16,17) Furthermore, a much lower shear force is generated in ME compared to other emulsification processes, which allows the encapsulation of sensitive materials without damage. (18)
Recently, microcapsules prepared by electrostatic interaction between polysaccharides have drawn more attention. (19−21) Among all of the polysaccharides used, cellulose is the most abundant organic polymer on earth. (22) It is obtained largely from plants where components of the biomass can be removed, leaving cellulose structures that can be further broken down mechanically or chemically to create particles that are nanometers in diameter but up to microns long depending on the preparation method. The high density of surface hydroxyl groups on the cellulose surface enables various chemical functionalization reactions such as oxidation (23,24) and cationization (25) to tweak their surface charge profiles. (26) Incorporating a specific amount of charge density on the cellulose surface is crucial to provide the necessary electrostatic repulsion forces to enable the proper dispersion of cellulose fibers in an aqueous medium. (27)
In this work, the electrostatic attraction of oppositely charged cellulose nanofibrils (CCNFs) was used in situ at the oil–water interface of an oil-in-water (O/W) emulsion. The formation of cellulose nanofibril complexes at the oil–water interface was found to stabilize emulsion droplets. The effect of introducing oppositely charged cellulose nanofibrils to the Pickering emulsion (PE) template was investigated via dye release studies under static (diffusion) and force field conditions (centrifugation and mechanical stirring) at room temperature. The dye release percentage from the most stable microcapsules was also investigated after conditioning at various pH environments (4.0, 5.0, 6.5, and 8.5). Membrane emulsification (ME) was applied to prepare microcapsules using bioderived cellulose particles and natural sunflower oil (SFO), proving the feasibility of scaling-up production of sustainable and biodegradable microcapsules.

Experimental Section

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Materials

α-Cellulose powder (C8002), glycidyl trimethylammonium chloride (GTMAC, ≥90%), 0.1 M AgNO3 aqueous solution (≥95%), calcofluor white stain (18909), Nile red (NR), sodium hydroxide pellets (≥98%), and hydrochloric acid (EMPLURA 32%) were purchased from Merck, U.K. and used as received. Pure sunflower oil made from 100% sunflower seeds was purchased from TESCO UK (pack size: 3 L). Ultrapure DI water (18.2 MΩ cm) was used for all dilutions and sample preparation. Oxidized cellulose nanofibrils (OCNFs) as a ca. 8 wt % solids paste in water with a degree of oxidation degree of 25% were prepared via 2,2,6,6-tetramethylpiperidinyloxy (TEMPO)-mediated oxidation as previously reported. (28,29)

Experiments

Preparation and Characterization of Modified Cellulose Nanofibrils

CCNF was produced by modifying α-cellulose with GTMAC following the semidry procedure using 3 mol equivalents of GTMAC relative to cellulose anhydroglucose units, as previously reported. (25,28) For the purification process after cationization, the paste was first washed with absolute ethanol (50%) followed by DI water and centrifugation at 8000 rpm for 5 min (three cycles for each solvent). After each cycle, after discarding the supernatant, the sediment was redispersed in fresh solvent for repeated washing. The prewashed CCNF dispersion was dialyzed against DI water for 5 days (medium replaced twice daily). The purified CCNF dispersion was freeze-dried, and the degree of substitution (DS) of cationic cellulose was determined to be 20% by conductometric titration of chloride ions (trimethylammonium chloride groups) with AgNO3 aqueous solution (see Figure S1). (25,28)
The OCNF was further purified via dialysis under ultrapure DI water to remove any salt and preservatives, as described elsewhere. (30) Briefly, approximately 20 g of OCNF slurry was dispersed in 100 mL of deionized water and stirred at room temperature for 30 min. After being acidified to pH 3 using 1 M aqueous HCl solution, the OCNF dispersion was dialyzed against DI water (cellulose dialysis tubing molecular weight cut-off (MWCO) 12,400) for 5 days (medium replaced twice daily). The dialyzed OCNF dispersion was homogenized at 6500 rpm for 30 min using an Ultra Turrax (IKA T25 digital) and adjusted to pH 7 using 0.1 M NaOH. After a second dialysis step for an additional 3 days (medium replaced twice daily), the purified OCNF dispersion was freeze-dried.

Cellulose Dispersion Preparation

The CCNF dispersions (0.05 and 0.1 wt %) were prepared by dispersing the required amount of freeze-dried CCNF in DI water using a sonication probe (Ultrasonic Processor, FB-505, power 500 W, using 1 s on/off pulses for a net time of 10 min at an amplitude of 30% in an ice bath for 20 mL of the dispersion). OCNF dispersions of varying concentrations (0.05, 0.2, and 0.5 wt %) were prepared using the same sonication protocol used for CCNF dispersions.
The morphology of the OCNF and CCNF were characterized using transmission electron microscopy (TEM, JEOL, JEM-2100 Plus) at an operating voltage of 200 kV. A dilute OCNF or CCNF suspension (0.01 wt %) was added onto a Cu grid (mesh size 300) and then negatively stained using uranyl acetate (from Merck, U.K.) (2 wt %) for enhanced contrast during TEM measurements.

Preparation of Microcapsules via the Pickering Emulsion Template

This study prepared microcapsules by encapsulating sunflower oil (SFO) using oppositely charged cellulose nanoparticles. SFO is a plant-sourced oil as well as edible, hence chosen in this study along with bioderived functionalized cellulose nanoparticles for sustainable microcapsules preparation. First, CCNF-stabilized Pickering emulsions (PEs) were prepared by blending 1 mL of sunflower oil (SFO) and CCNF dispersion (9 mL of 0.05 or 0.1 wt % dispersion) using an Ultra Turrax (IKA T25 digital) at 20 000 rpm for 1 min at room temperature (Figure 1). The first microcapsules (first MC) were prepared by replacing 1.5 mL of CCNF-stabilized PE with 1.5 mL of oppositely charged OCNF dispersion (0.05, 0.2, or 0.5 wt % dispersion added dropwise) while homogenizing at 8000 rpm for 3 min at room temperature. Then, the second and third MCs were subsequently prepared by successive replacement of 1.5 mL of previous microcapsules with an equal amount of OCNF dispersion (Figure 1). Typically, 1.5 mL of PE or MCs was replaced with the same amount of fresh OCNF dispersion at every successive stage of MCs preparation, to retain the final volume of the MC sample at 10 mL to maintain similar shear forces during the homogenization process.

Figure 1

Figure 1. Schematic of primary Pickering emulsion (PE) and microcapsule (MC) preparation protocol.

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Characterization

Microscopic Imaging

Optical images of the PE and MCs were taken after homogenization using an EVOS AMG (AMF 4300) microscope, and their stability was monitored visually over time (for any creaming/phase separation). The stability of the PE and MCs was further confirmed via confocal imaging (ZEISS; LSM50META), where the oil phase was dyed with NR and cellulose fibrils were stained with calcofluor white.

Surface Charge

The ζ-potential of the OCNF (0.05 wt %), CCNF (0.05 wt %), PE, and MC suspensions was measured using a Zetasizer (Malvern Zetasizer Nano ZSP, U.K.). PE/MC samples (100 μL) were diluted using DI water (900 μL) before the measurement. The samples (∼850 μL) were injected in the folded capillary electrode cell and equilibrated at 25 °C for 120 s before measuring as an average of 5 from 100 scans each. The ζ-potential was calculated using Henry’s law utilizing the Smoluchowski model loaded in the Zetasizer software (Malvern).

Quantification of Nile Red (NR) Release

For the dye release study, NR was dissolved in SFO at a weight fraction of 0.00275 wt %. The Pickering emulsions and various microcapsules (first, second, and third MCs) were prepared in a 50 mL centrifuge tube (Falcon) according to the method illustrated in Figure 1. Then, 5 mL of pure sunflower oil (NR-free) was added carefully on the top of the PE and MCs (10 mL) suspension along the tube wall. These were then stored at room temperature, allowing for any NR to diffuse from the PE or MC phase to the top oil layer. The absorbance of the upper oil layer was measured using a UV–visible spectrophotometer (Agilent 8453) at 520 nm after 1 and 7 days. The NR released was calculated from the calibration curve (Figure S2), and the percentage of dye release with respect to its initial concentration was reported.
Dye release studies were also conducted under different applied force fields, namely, centrifugation and mechanical stirring at room temperature. In the case of centrifugation, the abovementioned PE and MCs (10 mL of sample + 5 mL of dye-free fresh oil) were centrifuged at 8000 rpm for 10 min and then allowed to diffuse for 1 and 7 days before taking the absorbance of the upper oil layer at 520 nm. For mechanical stirring studies, the samples were stirred at 2000 rpm for 10 min using an overhead stirrer (propeller dimension 14.5 mm × 12 mm and Falcon tube internal diameter ∼27 mm) and then allowed to diffuse for 1 and 7 days. To separate the oil layer from the blend, a short centrifugation (8000 rpm for 2 min) was done after 1 and 7 days just before the absorbance measurement. The protocols used for all dye release studies (diffusion, centrifugation, and mechanical stirring) are shown in Figure S3.
In addition, a dye release study on the most stable MCs (third MCs) was done in different pH environments (4.0, 5.0, 6.5, and 8.5). The pH of the as-prepared MCs was ∼6.5 (without any adjustment); therefore, the lower and higher pH values were obtained using HCl (0.1 M) and NaOH (0.1 M) solution, respectively.
All dye release experiments were repeated in triplicate, and error bars report the standard deviation for each data point.

Membrane Emulsification

A micropore LDC-1 dispersion cell (Micropore Technologies Ltd.), a 72–2685 digital-control DC power supply (Tenma 72-2685 Digital-Control power supply, 30 V, 3 A), a membrane with 30 μm pores (Micropore Technologies Ltd.), and a syringe pump were used for the membrane emulsification process. Around 45 mL of 0.1 wt % CCNF dispersion was used as the continuous phase, and stirring was applied by adjusting the power supply to 6 V (∼530 rpm). Then, 3.75 mL of sunflower oil stained with NR was injected at a speed of 0.1 mL/min from a 20 mL syringe through the membrane as the dispersed phase. The produced oil droplets were stabilized by CCNF in the continuous phase forming the primary Pickering emulsion (PE). After injecting all of the dispersed phase, 20 mL of 0.5 wt % oppositely charged OCNF aqueous dispersion was injected at 0.1 mL/min into the CCNF-stabilized primary emulsion while stirring at 6 V (∼530 rpm) to prepare the microcapsules. The microcapsules were then collected, and optical images were taken just after preparation and after a week of storage.

Results and Discussion

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Morphology of OCNF and CCNF

TEM micrographs, presented in Figure 2a,b, revealed fibrillar particles of OCNF and CCNF. Both materials have been previously characterized with a length (L) of 160 ± 60 nm and a diameter (D) of 7 ± 2 nm for OCNF (statistical image analysis of TEM micrographs from averaging 175 measurements) (29) and L = 105 ± 35 nm and D = 7 ± 2 nm for CCNF (measurements of over 100 particles). (31)

Figure 2

Figure 2. TEM micrographs of (a) OCNF and (b) CCNF dispersion in DI water at 0.01 wt %. Uranyl acetate (2%) was used for contrast imaging.

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Interactions of CCNF-Stabilized Pickering Emulsion with OCNF at the Oil–Water Interface

The primary oil-in-water (O/W) emulsion stabilized with CCNF was used as a template for the adsorption of the second layer of OCNF. A CCNF dispersion of 0.05 wt % was used to generate O/W primary PE, which remained stable after 1 week of storage at room temperature. This is attributed to the fact that hydrophobic CCNF (contact angle of water on the glass slide coated with CCNF = 95 ± 3°, see Figure S4) is stable at the O/W interface. An OCNF dispersion of 0.05 wt % was added stepwise to the CCNF-stabilized PE following the protocol in Figure 1. Results show that there are free oil droplets in the first MC sample after the first addition of OCNF compared to the PE (Figure 3a,b). This suggests that the stabilizing ability of CCNF/OCNF complexes at the oil–water interface is lower than that of the CCNF alone. Upon adding more OCNF, droplet coalescence was hindered, producing fewer free oil droplets (second MCs in Figure 3c). In the third MC sample, droplets were stable with no oil released (Figure 3d). As seen from the confocal images of the third MC sample (Figure 3e,f), most cellulose gathers at the surface of the droplets, preventing coalescence of the oil droplets; they retain their spherical shapes, showing the significance of the CCNF/OCNF interaction.

Figure 3

Figure 3. Optical microscope images of the PE and MCs prepared by replacing 1.5 mL of CCNF (0.05 wt %)-stabilized PE with an equal amount of 0.05 wt % OCNF dispersion: (a) control CCNF (0.05 wt %)-stabilized PE before adding any OCNF dispersion, (b) first MCs, (c) second MCs, and (d) third MCs. Images were taken just after preparation and (e, f) confocal microscope images of third MCs after standing at room temperature for 1 week (sunflower oil dyed with NR showing in red and cellulose stained with calcofluor white stain showing in blue).

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The first and second MCs were still positively charged, suggesting an insufficient amount of the OCNF interacted with the CCNF at the oil–water interface (Table 1). Hence, coalescence was observed in the optical micrographs for these samples. However, in the case of the third MCs, charge inversion was observed (Table 1), and droplets were found to be stable as observed via the confocal laser microscopic image (Figure 3e,f). The attributions “stable” and “unstable” in Table 1 are based on the visual observation of the MC samples after storage at room temperature for 1 week: unstable (visible free oil phase) or stable (no free oil phase), as shown in Figure S5.
Table 1. Summary of the Interaction between CCNF (0.05 wt %)-Stabilized Emulsion with OCNF
CCNF stock (wt %)OCNF stock (wt %)samples (MCs were prepared according to the scheme presented in Figure 1 using CCNF and OCNF stock dispersions)ζ-potential (mV)CCNF/OCNF mass ratio in MCs (see Table S1)phenomenon
0.05-CCNF dispersion+42 (±2) stable
-0.05OCNF dispersion–60 (±4) stable
0.05-PE+38 (±2) stable
0.05first MCs+35 (±1)5.10unstable
 second MCs+21 (±2)2.34unstable
 third MCs–42 (±2)1.43stable
A higher concentration of CCNF (0.1 wt %) dispersion was also utilized with SFO to prepare another primary PE. At this concentration, the CCNF (0.1 wt %)-stabilized primary emulsion was more stable against significant coalescence (Figure 4a). OCNF dispersions of 0.05 and 0.5 wt % were used to interact with the CCNF (0.1 wt %)-stabilized emulsions to form microcapsules, according to the protocol in Figure 1. The interaction of OCNF at lower concentration (0.05 wt %) with the primary PE at different stages (first, second, third MC samples) resulted in the coalescence of droplets and release of free oil, as observed via microscopic images in Figure 4b. This reveals that the transformation of CCNF to OCNF/CCNF complexes at the interface negatively influences emulsion stability. Moreover, the ζ-potential decreased from +37 to +12 mV (Table 2). Although an insufficient amount of OCNF was added to cause a reversal of the surface charge of droplets, the decrease of ζ-potential with increasing OCNF confirms the interfacial electrostatic interaction.

Figure 4

Figure 4. Optical microscope images of microcapsule samples prepared using CCNF (0.1 wt %)-stabilized emulsion as a template with the various concentrations of OCNF dispersion: (a) MCs prepared using 0.05 wt % OCNF dispersion (inset shows the control CCNF (0.1 wt %)-stabilized PE). (b) MCs prepared using 0.2 wt % OCNF dispersion. (c) MCs prepared using 0.5 wt % OCNF dispersion; optical images were taken just after preparation. (d) Confocal microscope images of MC samples (c) after standing at room temperature for 1 week (SFO dyed with NR showing red and cellulose stained with calcofluor white stain showing blue).

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Table 2. Summary of the Interaction between the Various Concentrations of OCNF with CCNF (0.1 wt %)-Stabilized PE
CCNF stock (wt %)OCNF stock (wt %)samples (MCs were prepared according to the scheme presented in Figure 1 using CCNF and OCNF stock dispersions)ζ-potential (mV)CCNF/OCNF mass ratio in MCs (see Table S1)phenomenon
0.1−PE+40 (±1) stable
0.05first MCs+37 (±2)10.20unstable
 second MCs+33 (±3)4.68unstable
 third MCs+12 (±1)2.87unstable
0.10.2first MCs+31 (±2)2.55unstable
second MCs–22 (±1)1.17stable
third MCs–45 (± 4)0.71stable
0.10.5first MCs–45 (±2)1.02stable
second MCs–56 (±2)0.47stable
third MCs–60 (±4)0.29stable
To prevent coalescence, the concentration of the added OCNF was increased to 0.2 wt %. The ζ-potential values of the first and third MC samples in this set were reversed from +31 to −45 mV (see Table 2), reflecting the adsorption of OCNF particles at the CCNF interface. Using an even higher OCNF concentration (0.5 wt %), the ζ-potential values of all microcapsules (first, second, third MC samples) reversed from positively charged to negatively charged with stability unchanged (Table 2). The second and third MC samples achieved ζ-potential values similar to the pure OCNF in DI water (−60 mV), which might be due to the complete coverage of CCNF-stabilized oil droplets and the surrounding particles by the oppositely charged OCNF. Thus, the surface charge of the droplets can be reversed in a more controlled way by adjusting the concentrations of CCNF and OCNF. Once coalescence is prevented, a further dilution or washing process does not affect the stability, as confirmed via confocal imaging (Figure 4d). This implies that the adsorption of OCNF/CCNF complexes at the interface is irreversible. Further addition of oppositely charged cellulose particles with respect to the particle’s charge of the outer surface, for example by adding another layer of CCNF, would increase the shell thickness, potentially forming stronger MCs. However, this would include additional preparation steps, making the process more complex.
MCs prepared using the same materials but added in the opposite order, an OCNF-stabilized PE template followed by adsorption of CCNF, revealed good dispersion for the control OCNF-stabilized PE (Figure S6); however, after 1 week of storage at room temperature, the primary OCNF-stabilized PE and all of the MCs showed clear phase separation. This destabilization may be due to the high surface charge of OCNF (−60 ± 4 mV). Since the repulsion between OCNF is strong, they are easily removed from the interface, and hence a low surface coverage induces coalescence of the oil droplets. Furthermore, cellulose aggregates were observed in the water phase (freed from the oil–water interface), as shown in the confocal image (Figure S6). This might be due to the hydrophilic nature of the OCNF (contact angle of water on the glass slide coated with OCNF = 41 ± 2°, see Figure S4), which are poorly attached to the oil–water interface of the emulsion.

Dye Release from Microcapsules

Here, the most stable formulation, i.e., the CCNF (0.1 wt %)-stabilized PE and the associated MCs prepared using 0.5 wt % OCNF dispersion (see Table 2), was used for dye release studies using the lipophilic stain NR as an active substance in the oil phase. The release of NR from these microcapsules can be attributed to two distinct mechanisms: first, the release could occur during coalescence of the droplets due to poor encapsulation caused by weak CCNF/OCNF interactions at the oil–water interface. Second, in droplets with stable and solid encapsulation (CCNF (0.1 wt %)-stabilized emulsion/OCNF complex), the diffusion of dye from the capsule core through the cellulose shell could also induce dye release. Figure 5a shows the dye release percentage variation via diffusion for the CCNF (0.1 wt %)-stabilized PE and the stable first, second, and third MCs kept at room temperature for 7 days. After 1 day, the PE released ∼3.5 wt % dye and the first and second MCs did not show any significant reduction in dye release compared to the PE. However, the third MCs showed a noticeable decrease in dye release percentage (∼2.4%) compared to the PE and first and second MCs. After 7 days of storage at room temperature, the primary PE released ∼8.7% dye, whereas the first, second, and third MCs released ∼6.2, 5.2, and 4.6% dye, respectively. This suggests that with the increasing concentration of OCNF, the interaction of OCNF/CCNF at the CCNF-stabilized O/W interface increased, leading to better encapsulation.

Figure 5

Figure 5. Variation of the NR release from the primary CCNF (0.1 wt %)-stabilized PE and the MCs prepared using 0.5 wt % OCNF dispersion: (a) via diffusion and (b) via centrifugation (8000 rpm for 10 min). Dye release percentage of the third MCs in various pH environments: (c) via diffusion and (d) via centrifugation (8000 rpm for 10 min).

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As the third MC samples were found to be more stable, only these MCs were considered in further studies under varying pH environments. With decreasing pH, after 1 day of storage at room temperature, the dye release percentage of the third MCs decreased marginally, from ∼2.4% (at pH 6.5) to ∼2.2% (at pH 4.0); however, after 7 days, the dye release percentage reduced from ∼4.6% (at pH 6.5) to ∼3.6% (at pH 4.0), as can be seen in Figure 5c. A further solidification of the outer shell of these MCs was achieved by lowering the pH further, here attributed the proton-induced interfibrillar interactions of OCNF networks. (30) On the contrary, at higher pH (8.5), dye release increased to ∼6.4% (after 1 day) and ∼7.5% (after 7 days). This is tentatively attributed to the presence of Na+ ions (used to increase the pH), which screened the external negative charge on the microcapsules, causing significant aggregation, hence disrupting the OCNF/CCNF complex shell networks at the O/W interface.
The dye release of third MCs was also characterized under two distinct force fields, centrifugation and mechanical stirring. After centrifugation at 8000 rpm for 10 min at room temperature, the most stable third MCs showed 30 and 43% lower dye release than the primary PE after 1 and 7 days, respectively (Figure 5b). The dye release obtained via centrifugation was higher compared to the diffusion case. However, in the case of the third MCs, no remarkable increase in dye release was observed after 7 days compared to the first day, suggesting that after an initial release caused by the centrifugation, there was no further release due to diffusion through the cellulose shell. In addition, by lowering the pH, the third MCs showed that the release profile did not change after an additional 7 days of storage at room temperature (Figure 5d). Higher pH (8.5), on the other hand, led to further dye release after centrifugation and after extra 7 days of storage at room temperature, which further confirms the lack of stability of these MCs at higher pH.
The robustness of the third MCs was further characterized by blending the MCs with fresh oil using an overhead stirrer at 2000 rpm for 10 min and then allowing the suspension to settle at room temperature. After 1 day of settling, the MCs and the free oil phases had not separated; as such, a short centrifugation (8000 rpm for 2 min) was used to separate the oil layer for the absorbance measurement (as shown in Figure S3). After 1 day, the dye release of these mechanically stirred third MCs was ∼5.8, 6.4, 7.4, and 10.5% at pH 4.0, 5.0, 6.5, and 8.5, respectively (Figure 6a). However, after 7 days of storage, the mechanically stirred MCs showed a higher release compared to day 1 for all pH environments investigated. The mechanical stirring process might have weakened some of the shell networks even in lower pH environments, causing the weaker capsules to coalesce and thus increasing the dye released over the storage period.

Figure 6

Figure 6. (a) Dye release from the third MCs obtained after stirring using an overhead mechanical stirrer (2000 rpm for 10 min) in various pH environments. (b–d) Optical micrographs show that the MCs are stable after diffusion, centrifugation, and mechanical stirring processes at control pH (∼6.5) after 7 days of storage at room temperature.

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The third MCs showed a similar increasing trend of dye release with increasing pH environment for both the static (diffusion) and applied force field conditions (centrifugation and mechanical stirring) after 7 days of storage at room temperature. Although the mechanical stirring showed a higher release profile than the centrifugation and diffusion, the release percentage was still below 9% after 7 days at control pH (∼6.5). This suggests that, under these conditions, the third MCs are highly stable, as also evidenced from their optical micrographs (Figure 6b–d). This low dye release profile demonstrates that the MCs can withstand conditions used in a formulation, where the MC would be incorporated using mechanical stirring into commercial products. The full release of oil-soluble active ingredients (such as micronutrients or drugs) from these microcapsules could then be triggered via enzymatic degradation of the cellulose particles, e.g., if they are formulated for agricultural (32) or pharmaceutical applications. (33)

Microcapsules Prepared Using Membrane Emulsification

The CCNF (0.1 wt %)-stabilized primary PE was also prepared using membrane emulsification to provide proof-of-concept for scaling up the microcapsule production. As NR was dissolved in sunflower oil and encapsulated by the CCNF/OCNF complex layer, the continuous phase for the membrane emulsification was the CCNF aqueous dispersion (0.1 wt %), and NR dissolved in sunflower oil was used as the dispersed phase, which was injected through a 30 μm pore membrane into the continuous phase. The oil droplets formed at the membrane permeate side were stabilized by the CCNF in the continuous phase, forming the primary PE. At the low rotation speed used (∼530 rpm), larger droplets (190 ± 35 μm, measured using ImageJ) were obtained, compared to the batch process (which was formed via high shear generated using an Ultra Turrax homogenizer), as expected (Figure 7a). After slowly adding the OCNF dispersion to the oppositely charged primary emulsion (maintaining the same OCNF/CCNF ratio as that in the third MCs prepared earlier), electrostatic interactions at the O/W interface led to the formation of microcapsules with an average diameter of 220 ± 48 μm (Figure 7b). No significant change in the size of the droplets was observed for the microcapsules even after storage at room temperature for 1 week (220 ± 60 μm) and at 50 °C (224 ± 65 μm) compared to the primary PE droplets (Figure 7c,d).

Figure 7

Figure 7. Micrographs of (a) control CCNF (0.1 wt %)-stabilized Pickering emulsion prepared upon adding sunflower oil (dispersed phase) to 0.1 wt % CCNF dispersion (continuous phase) via membrane emulsification; (b) microcapsules prepared by the addition of the OCNF dispersion to the CCNF (0.1 wt %)-stabilized primary PE, (c) microcapsules after storage for 7 days at room temperature, and (d) microcapsules after storage for 7 days at 50 °C.

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Conclusions

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This study utilized the electrostatic attraction of various concentrations of oppositely charged cellulose nanofibrils at the oil–water interface to form microcapsules. The most stable microcapsules were produced from the cationized cellulose nanofibrils (CCNF∼0.1 wt %)-stabilized Pickering emulsions after the addition of negatively charged oxidized cellulose nanofibril (OCNF ∼0.5 wt %) dispersions. Complex CCNF/OCNF networks formed at the oil–water interface via the charge inversion of the primary Pickering emulsion, which played a vital role in controlling the stability of the microcapsules against coalescence. The stability of these microcapsules was further enhanced by increasing the amount of OCNF to allow more interactions with the CCNF that anchored at the oil–water interface. NR dye release from the oil phase through the CCNF/OCNF complex shell networks via diffusion and centrifugation processes revealed that dye release decreased with increasing concentration of OCNF at the oil–water interface. In addition, mechanical stirring showed a higher dye release profile compared to the centrifugation and diffusion measurements; however, the release percentage was still below 9% (at pH 6.5) after 1 week of storage at room temperature. Dye release from these microcapsules was reduced by decreasing the pH of microcapsule suspensions due to proton-induced interfibrillar attractions between the OCNF present at the oil–water interface. At the higher pH (∼8.5), the stability of the microcapsules reduced. The microcapsules were also successfully produced via membrane emulsification as a proof-of-concept for potential scale-up, showing promise for the large-scale manufacturing of stable microcapsules produced using renewable cellulose and their use in the controlled release of oil-based ingredients.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.1c03025.

  • Conductometric analysis of the degree of substitution of CCNF; calibration curve for absorbance of NR in sunflower oil; photographs showing the stages in the dye diffusion and release protocols used in this study; photographs of stable and unstable microcapsules; optical microscope images of microcapsule samples; images of the contact angle of water on glass slides coated with OCNF or CCNF; and summary of the OCNF and CCNF concentrations in the samples (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
  • Authors
    • Hui Shi - Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.;  Centre for Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
    • Kazi M. Zakir Hossain - Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.;  Centre for Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
    • Davide Califano - Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.;  Centre for Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath BA2 7AY, U.K.;  Orcidhttps://orcid.org/0000-0002-9416-2021
    • Ciaran Callaghan - Centre for Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath BA2 7AY, U.K.;  Department of Chemical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
    • Ekanem E. Ekanem - Department of Chemical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
    • Janet L. Scott - Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.;  Centre for Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath BA2 7AY, U.K.;  Orcidhttps://orcid.org/0000-0001-8021-2860
    • Davide Mattia - Department of Chemical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors would like to thank the EPSRC for funding this project (Grant EP/P027490/1). The authors thank Diana Lednitzky and Philip Fletcher of the Material and Chemical Characterisation Facility (MC2) at the University of Bath for helping with TEM imaging. Data supporting this work is freely accessible in the Bath research data archive system at DOI: 10.15125/BATH-01110.

Note Added After ASAP Publication

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This paper was originally published ASAP on March 9, 2022. A correction was made in the OCNF stock column of Table 1, and the paper was reposted on March 9, 2022.

References

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    Piacentini, E.; Poerio, T.; Bazzarelli, F.; Giorno, L. Microencapsulation by Membrane Emulsification of Biophenols Recovered from Olive Mill Wastewaters. Membranes 2016, 6 (2), 25  DOI: 10.3390/membranes6020025
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    Microencapsulation by membrane emulsification of biophenols recovered from olive mill wastewaters
    Piacentini, Emma; Poerio, Teresa; Bazzarelli, Fabio; Giorno, Lidietta
    Membranes (Basel, Switzerland) (2016), 6 (2), 25/1-25/11CODEN: MBSEB6; ISSN:2077-0375. (MDPI AG)
    Biophenols are highly prized for their free radical scavenging and antioxidant activities. Olive mill wastewaters (OMWWs) are rich in biophenols. For this reason, there is a growing interest in the recovery and valorization of these compds. Applications for the encapsulation have increased in the food industry as well as the pharmaceutical and cosmetic fields, among others. Advancements in micro-fabrication methods are needed to design new functional particles with target properties in terms of size, size distribution, and functional activity. This paper describes the use of the membrane emulsification method for the fine-tuning of microparticle prodn. with biofunctional activity. In particular, in this pioneering work, membrane emulsification has been used as an advanced method for biophenols encapsulation. Catechol has been used as a biophenol model, while a biophenols mixt. recovered from OMWWs were used as a real matrix. Water-in-oil emulsions with droplet sizes approx. 2.3 times the membrane pore diam., a distribution span of 0.33, and high encapsulation efficiency (98% ± 1% and 92% ± 3%, for catechol and biophenols, resp.) were produced. The release of biophenols was also investigated.
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    Akamatsu, K.; Chen, W.; Suzuki, Y.; Ito, T.; Nakao, A.; Sugawara, T.; Kikuchi, R.; Nakao, S.-i. Preparation of Monodisperse Chitosan Microcapsules with Hollow Structures Using the SPG Membrane Emulsification Technique. Langmuir 2010, 26, 14854– 14860,  DOI: 10.1021/la101967u
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    Preparation of Monodisperse Chitosan Microcapsules with Hollow Structures Using the SPG Membrane Emulsification Technique
    Akamatsu, Kazuki; Chen, Wei; Suzuki, Yukimitsu; Ito, Taichi; Nakao, Aiko; Sugawara, Takashi; Kikuchi, Ryuji; Nakao, Shin-ichi
    Langmuir (2010), 26 (18), 14854-14860CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    We describe herein successful prepns. of monodisperse chitosan microcapsules with hollow structures using the SPG membrane emulsification technique. Two prepn. procedures were examd. in this study. In the first method, monodisperse calcium alginate microspheres were prepd. and then coated with unmodified chitosan. Subsequently, tripolyphosphate treatment was conducted to phys. crosslink chitosan and solubilize the alginate core at the same time. In the second method, photo-crosslinkable chitosan was coated onto the monodisperse calcium alginate microspheres, followed by UV irradn. to chem. crosslink the chitosan shell and tripolyphosphate treatment to solubilize the core. For both methods, it was detd. that the av. diams. of the chitosan microcapsules depended on those of the calcium alginate microparticles and that the microcapsules have hollow structures. In addn., the first phys. crosslinking method using tripolyphosphate was found to be preferable to obtain the hollow structure, compared with the second method using chem. crosslinking by UV irradn. This was because of the difference in the resistance to permeation of the solubilized alginate through the chitosan shell layers.
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    Charcosset, C.; Limayem, I.; Fessi, H. The membrane emulsification process─a review. J. Chem. Technol. Biotechnol. 2004, 79 (3), 209– 218,  DOI: 10.1002/jctb.969
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    The membrane emulsification process - a review
    Charcosset, C.; Limayem, I.; Fessi, H.
    Journal of Chemical Technology and Biotechnology (2004), 79 (3), 209-218CODEN: JCTBED; ISSN:0268-2575. (John Wiley & Sons Ltd.)
    A review of the membrane emulsification process including principles of membrane emulsification, effect of process parameters and industrial applications. Membrane emulsification has received increasing attention over the last 10 yr, with potential applications in many fields. In the membrane emulsification process, a liq. phase is pressed through the membrane pores to form droplets at the permeate side of a membrane; the droplets are then carried away by a continuous phase flowing across the membrane surface. Under specific conditions, monodispersed emulsions can be produced using this technique. Small-scale applications such as drug delivery systems, food emulsions, and the prodn. of monodispersed microspheres are also included. Compared with conventional techniques for emulsification, membrane processes offer advantages such as control of av. droplet diam. by av. membrane pore size and lower energy input.
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    Jia, Y.; Feng, X.; Li, J. Polysaccharides-Based Microcapsules. In Supramolecular Chemistry of Biomimetic Systems; Li, J., Ed.; Springer: Singapore, 2017; Vol. 63, pp 63– 84.
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    Mohanta, V.; Madras, G.; Patil, S. Layer-by-Layer Assembled Thin Films and Microcapsules of Nanocrystalline Cellulose for Hydrophobic Drug Delivery. ACS Appl. Mater. Interfaces 2014, 6, 20093– 20101,  DOI: 10.1021/am505681e
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    Layer-by-Layer Assembled Thin Films and Microcapsules of Nanocrystalline Cellulose for Hydrophobic Drug Delivery
    Mohanta, Vaishakhi; Madras, Giridhar; Patil, Satish
    ACS Applied Materials & Interfaces (2014), 6 (22), 20093-20101CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    A layer-by-layer (LbL) approach has been employed for the fabrication of multilayer thin films and microcapsules having nanofibrous morphol. using nanocryst. cellulose (NCC) as one of the components of the assembly. The applicability of these nanoassemblies as drug delivery carriers has been explored by the loading of an anticancer drug, doxorubicin hydrochloride, and a water-insol. drug, curcumin. Doxorubicin hydrochloride, having a good water soly., is postloaded in the assembly. In the case of curcumin, which is very hydrophobic and has limited soly. in water, a stable dispersion is prepd. via noncovalent interaction with NCC prior to incorporation in the LbL assembly. The interaction of various other lipophilic drugs with NCC was analyzed theor. by mol. docking in consideration of NCC as a general carrier for hydrophobic drugs.
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    Feng, X.; Du, C.; Li, J. Molecular Assembly of Polysaccharide-Based Microcapsules and Their Biomedical Applications. Chem. Rec. 2016, 16 (4), 1991– 2004,  DOI: 10.1002/tcr.201600051
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    Molecular Assembly of Polysaccharide-Based Microcapsules and Their Biomedical Applications
    Feng, Xiyun; Du, Cuiling; Li, Junbai
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    A review. Advanced multifunctional microcapsules have revealed great potential in biomedical applications owing to their tunable size, shape, surface properties, and stimuli responsiveness. Polysaccharides are one of the most acceptable biomaterials for biomedical applications because of their outstanding virtues such as biocompatibility, biodegradability, and low toxicity. Many efforts have been devoted to investigating novel mol. design and efficient building blocks for polysaccharide-based microcapsules. In this Personal Account, we first summarize the common features of polysaccharides and the main principles of the design and fabrication of polysaccharide-based microcapsules, and further discuss their applications in biomedical areas and perspectives for future research.
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    Klemm, D.; Heublein, B.; Fink, H.-P.; Bohn, A. Cellulose: Fascinating Biopolymer and Sustainable Raw Material. Angew. Chem., Int. Ed. 2005, 44 (22), 3358– 3393,  DOI: 10.1002/anie.200460587
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    Cellulose: Fascinating biopolymer and sustainable raw material
    Klemm, Dieter; Heublein, Brigitte; Fink, Hans-Peter; Bohn, Andreas
    Angewandte Chemie, International Edition (2005), 44 (22), 3358-3393CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. As the most important skeletal component in plants, the polysaccharide cellulose is an almost inexhaustible polymeric raw material with fascinating structure and properties. Formed by the repeated connection of D-glucose building blocks, the highly functionalized, linear stiff-chain homopolymer is characterized by its hydrophilicity, chirality, biodegradability, broad chem. modifying capacity, and its formation of versatile semicryst. fiber morphologies. In view of the considerable increase in interdisciplinary cellulose research and product development over the past decade worldwide, this paper assembles the current knowledge in the structure and chem. of cellulose, and in the development of innovative cellulose esters and ethers for coatings, films, membranes, building materials, drilling techniques, pharmaceuticals, and foodstuffs. New frontiers, including environmentally friendly cellulose fiber technologies, bacterial cellulose biomaterials, and in-vitro syntheses of cellulose are highlighted together with future aims, strategies, and perspectives of cellulose research and its applications.
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    Bragd, P. L.; van Bekkum, H.; Besemer, A. C. TEMPO-Mediated Oxidation of Polysaccharides: Survey of Methods and Applications. Top. Catal. 2004, 27, 49– 66,  DOI: 10.1023/b:toca.0000013540.69309.46
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    TEMPO-Mediated Oxidation of Polysaccharides: Survey of Methods and Applications
    Bragd, P. L.; van Bekkum, H.; Besemer, A. C.
    Topics in Catalysis (2004), 27 (1-4), 49-66CODEN: TOCAFI; ISSN:1022-5528. (Kluwer Academic/Plenum Publishers)
    A review dealing with TEMPO as a catalyst in oxidn. of alc. functions in polysaccharides. Synthesis of TEMPO and derivs. and the mechanism of the oxidative cycle in which TEMPO is involved in oxidn. of alcs. are discussed. Results of oxidn. of various polysaccharides with respect to yield, and introduction of the functional groups (aldehyde and/or carboxylate) are presented. Most of the primary oxidants are not ideal, as they produce large amts. of salts, e.g., sodium chloride from sodium hypochlorite. Results and perspectives are given to change the salt-based oxidative systems for much cleaner oxygen or hydrogen peroxide/enzyme-based TEMPO systems. Moreover, several immobilized TEMPO systems have been developed.
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    Saito, T.; Kimura, S.; Nishiyama, Y.; Isogai, A. Cellulose Nanofibers Prepared by TEMPO-Mediated Oxidation of Native Cellulose. Biomacromolecules 2007, 8, 2485– 2491,  DOI: 10.1021/bm0703970
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    24
    Cellulose Nanofibers Prepared by TEMPO-Mediated Oxidation of Native Cellulose
    Saito, Tsuguyuki; Kimura, Satoshi; Nishiyama, Yoshiharu; Isogai, Akira
    Biomacromolecules (2007), 8 (8), 2485-2491CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
    Never-dried and once-dried hardwood celluloses were oxidized by a 2,2,6,6-tetra-Me piperidine-1-oxyl radical (TEMPO)-mediated system, and highly cryst. and individualized cellulose nanofibers, dispersed in water, were prepd. by mech. treatment of the oxidized cellulose/water slurries. When carboxylate contents formed from the primary hydroxyl groups of the celluloses reached approx. 1.5 mmol/g, the oxidized cellulose/water slurries were mostly converted to transparent and highly viscous dispersions by mech. treatment. Transmission electron microscopic observation showed that the dispersions consisted of individualized cellulose nanofibers 3-4 nm in width and a few microns in length. No intrinsic differences between never-dried and once-dried celluloses were found for prepg. the dispersion, as long as carboxylate contents in the TEMPO-oxidized celluloses reached approx. 1.5 mmol/g. Changes in viscosity of the dispersions during the mech. treatment corresponded with those in the dispersed states of the cellulose nanofibers in water.
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    Courtenay, J. C.; Ramalhete, S. M.; Skuze, W. J.; Soni, R.; Khimyak, Y. Z.; Edler, K. J.; Scott, J. L. Unravelling cationic cellulose nanofibril hydrogel structure: NMR spectroscopy and small angle neutron scattering analyses. Soft Matter 2018, 14, 255– 263,  DOI: 10.1039/C7SM02113E
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    Unravelling cationic cellulose nanofibril hydrogel structure: NMR spectroscopy and small angle neutron scattering analysis
    Courtenay, James C.; Ramalhete, Susana M.; Skuze, William J.; Soni, Rhea; Khimyak, Yaroslav Z.; Edler, Karen J.; Scott, Janet L.
    Soft Matter (2018), 14 (2), 255-263CODEN: SMOABF; ISSN:1744-6848. (Royal Society of Chemistry)
    Stiff, elastic, viscous shear thinning aq. gels are formed upon dispersion of low wt. percent concns. of cationically modified cellulose nanofibrils (CCNF) in water. CCNF hydrogels produced from cellulose modified with glycidyltrimethylammonium chloride, with degree of substitution (DS) in the range 10.6(3)-23.0(9)%, were characterised using NMR spectroscopy, rheol. and small angle neutron scattering (SANS) to probe the fundamental form and dimensions of the CCNF and to reveal interfibrillar interactions leading to gelation. As DS increased CCNF became more rigid as evidenced by longer Kuhn lengths, 18-30 nm, derived from fitting of SANS data to an elliptical cross-section, cylinder model. Furthermore, apparent changes in CCNF cross-section dimensions suggested an "unravelling" of initially twisted fibrils into more flattened ribbon-like forms. Increases in elastic modulus (7.9-62.5 Pa) were detected with increased DS and 1H soln.-state NMR T1 relaxation times of the introduced surface -N+(CH3)3 groups were found to be longer in hydrogels with lower DS, reflecting the greater flexibility of the low DS CCNF. This is the first time that such correlation between DS and fibrillar form and stiffness has been reported for these potentially useful rheol. modifiers derived from renewable cellulose.
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    Eyley, S.; Thielemans, W. Surface modification of cellulose nanocrystals. Nanoscale 2014, 6, 7764– 7779,  DOI: 10.1039/C4NR01756K
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    Surface modification of cellulose nanocrystals
    Eyley, Samuel; Thielemans, Wim
    Nanoscale (2014), 6 (14), 7764-7779CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    A review. Chem. modification of cellulose nanocrystals is an increasingly popular topic in the literature. This review analyses the type of cellulose nanocrystal modification reactions that have been published in the literature thus far and looks at the steps that have been taken towards analyzing the products of the nanocrystal modifications. The main categories of reactions carried out on cellulose nanocrystals were oxidns., esterifications, amidations, carbamations and etherifications. More recently nucleophilic substitutions had been used to introduce more complex functionality to cellulose nanocrystals. Multi-step modifications were also considered. This review emphasizes quantification of modification at the nanocrystal surface in terms of degree of substitution and the validity of conclusions drawn from different anal. techniques in this area. The mechanisms of the modification reactions were presented and considered with respect to the effect on the outcome of the reactions. While great strides had been made in the quality of anal. data published in the field of cellulose nanocrystal modification, there was still vast scope for improvement, both in data quality and the quality of anal. of data. Given the difficulty of surface anal., cross-checking of results from different anal. techniques was fundamental for the development of reliable cellulose nanocrystal modification techniques.
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    Han, J.; Zhou, C.; Wu, Y.; Liu, F.; Wu, Q. Self-Assembling Behavior of Cellulose Nanoparticles during Freeze-Drying: Effect of Suspension Concentration, Particle Size, Crystal Structure, and Surface Charge. Biomacromolecules 2013, 14, 1529– 1540,  DOI: 10.1021/bm4001734
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    27
    Self-Assembling Behavior of Cellulose Nanoparticles during Freeze-Drying: Effect of Suspension Concentration, Particle Size, Crystal Structure, and Surface Charge
    Han, Jingquan; Zhou, Chengjun; Wu, Yiqiang; Liu, Fangyang; Wu, Qinglin
    Biomacromolecules (2013), 14 (5), 1529-1540CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
    Cellulose nanocrystals and cellulose nanofibers with I and II cryst. allomorphs (designated as CNC I, CNC II, CNF I, and CNF II) were isolated from bleached wood fibers by alk. pretreatment and acid hydrolysis. The effects of concn., particle size, surface charge, and crystal structure on the lyophilization-induced self-assembly of cellulose particles in aq. suspensions were studied. Within the concn. range of 0.5 to 1.0 wt. %, cellulose particles self-organized into lamellar structured foam composed of aligned membrane layers with widths between 0.5 and 3 μm. At 0.05 wt. %, CNC I, CNF I, CNC II, and CNF II self-assembled into oriented ultrafine fibers with mean diams. of 0.57, 1.02, 1.50, and 1.00 μm, resp. The size of self-assembled fibers became larger when more hydroxyl groups and fewer sulfates (weaker electrostatic repulsion) were on cellulose surfaces. Possible formation mechanism was inferred from ice growth and interaction between cellulose nanoparticles in liq.-cryst. suspensions.
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    Courtenay, J. C.; Johns, M. A.; Galembeck, F.; Deneke, C.; Lanzoni, E. M.; Costa, C. A.; Scott, J. L.; Sharma, R. I. Surface modified cellulose scaffolds for tissue engineering. Cellulose 2017, 24, 253– 267,  DOI: 10.1007/s10570-016-1111-y
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    Surface modified cellulose scaffolds for tissue engineering
    Courtenay, James C.; Johns, Marcus A.; Galembeck, Fernando; Deneke, Christoph; Lanzoni, Evandro M.; Costa, Carlos A.; Scott, Janet L.; Sharma, Ram I.
    Cellulose (Dordrecht, Netherlands) (2017), 24 (1), 253-267CODEN: CELLE8; ISSN:0969-0239. (Springer)
    We report the ability of cellulose to support cells without the use of matrix ligands on the surface of the material, thus creating a two-component system for tissue engineering of cells and materials. Sheets of bacterial cellulose, grown from a culture medium contg. Acetobacter organism were chem. modified with glycidyltrimethylammonium chloride or by oxidn. with sodium hypochlorite in the presence of sodium bromide and 2,2,6,6-tetramethylpipiridine 1-oxyl radical to introduce a pos., or neg., charge, resp. This modification process did not degrade the mech. properties of the bulk material, but grafting of a pos. charged moiety to the cellulose surface (cationic cellulose) increased cell attachment by 70% compared to unmodified cellulose, while neg. charged, oxidized cellulose films (anionic cellulose), showed low levels of cell attachment comparable to those seen for unmodified cellulose. Only a minimal level of cationic surface derivitization (ca 3% degree of substitution) was required for increased cell attachment and no mediating proteins were required. Cell adhesion studies exhibited the same trends as the attachment studies, while the mean cell area and aspect ratio was highest on the cationic surfaces. Overall, we demonstrated the utility of pos. charged bacterial cellulose in tissue engineering in the absence of proteins for cell attachment.
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    Schmitt, J.; Calabrese, V.; da Silva, M. A.; Lindhoud, S.; Alfredsson, V.; Scott, J. L.; Edler, K. J. TEMPO-oxidised cellulose nanofibrils; probing the mechanisms of gelation via small angle X-ray scattering. Phys. Chem. Chem. Phys. 2018, 20, 16012– 16020,  DOI: 10.1039/C8CP00355F
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    29
    TEMPO-oxidised cellulose nanofibrils; probing the mechanisms of gelation via small angle X-ray scattering
    Schmitt, Julien; Calabrese, Vincenzo; da Silva, Marcelo A.; Lindhoud, Saskia; Alfredsson, Viveka; Scott, Janet L.; Edler, Karen J.
    Physical Chemistry Chemical Physics (2018), 20 (23), 16012-16020CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The structure of dispersions of TEMPO-oxidised cellulose nanofibrils (OCNF), at various concns., in water and in NaCl aq. solns., was probed using small angle X-ray scattering (SAXS). OCNF are modelled as rod-like particles with an elliptical cross-section of 10 nm and a length greater than 100 nm. As OCNF concn. increases above 1.5 wt%, repulsive interactions between fibrils are evidenced, modelled by the interaction parameter νRPA > 0. This corresponds to gel-like behavior, where G' > G'' and the storage modulus, G', shows weak frequency dependence. Hydrogels can also be formed at OCNF concn. of 1 wt% in 0.1 M NaCl(aq). SAXS patterns shows an increase of the intensity at low angle that is modelled by attractive interactions (νRPA < 0) between OCNF, arising from the screening of the surface charge of the fibrils. Results are supported by ζ potential and cryo-TEM measurements.
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  30. 30
    Hossain, K. M. Z.; Calabrese, V.; da Silva, M. A.; Bryant, S. J.; Schmitt, J.; Ahn-Jarvis, J. H.; Warren, F. J.; Khimyak, Y. Z.; Scott, J. L.; Edler, K. J. Monovalent Salt and pH-Induced Gelation of Oxidised Cellulose Nanofibrils and Starch Networks: Combining Rheology and Small-Angle X-ray Scattering. Polymers 2021, 13, 951  DOI: 10.3390/polym13060951
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    30
    Monovalent salt and pH-induced gelation of oxidized cellulose nanofibrils and starch networks: combining rheology and small-angle X-ray scattering
    Hossain, Kazi M. Zakir; Calabrese, Vincenzo; da Silva, Marcelo A.; Bryant, Saffron J.; Schmitt, Julien; Ahn-Jarvis, Jennifer H.; Warren, Frederick J.; Khimyak, Yaroslav Z.; Scott, Janet L.; Edler, Karen J.
    Polymers (Basel, Switzerland) (2021), 13 (6), 951CODEN: POLYCK; ISSN:2073-4360. (MDPI AG)
    Water quality parameters such as salt content and various pH environments can alter the stability of gels as well as their rheol. properties. Here, we investigated the effect of various concns. of NaCl and different pH environments on the rheol. properties of TEMPO-oxidized cellulose nanofibril (OCNF) and starch-based hydrogels. Addn. of NaCl caused an increased stiffness of the OCNF:starch (1:1 wt.%) blend gels, where salt played an important role in reducing the repulsive OCNF fibrillar interactions. The rheol. properties of these hydrogels were unchanged at pH 5.0 to 9.0. However, at lower pH (4.0), the stiffness and viscosity of the OCNF and OCNF:starch gels appeared to increase due to proton-induced fibrillar interactions. In contrast, at higher pH (11.5), syneresis was obsd. due to the formation of denser and aggregated gel networks. Interactions as well as aggregation behavior of these hydrogels were explored via ζ-potential measurements. Furthermore, the nanostructure of the OCNF gels was probed using small-angle X-ray scattering (SAXS), where the SAXS patterns showed an increase of slope in the low-q region with increasing salt concn. arising from aggregation due to the screening of the surface charge of the fibrils.
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    Courtenay, J. C.; Jin, Y.; Schmitt, J.; Hossain, K. M. Z.; Mahmoudi, N.; Edler, K. J.; Scott, J. L. Salt-Responsive Pickering Emulsions Stabilized by Functionalized Cellulose Nanofibrils. Langmuir 2021, 37, 6864– 6873,  DOI: 10.1021/acs.langmuir.0c03306
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    31
    Salt-Responsive Pickering Emulsions Stabilized by Functionalized Cellulose Nanofibrils
    Courtenay, James C.; Jin, Yun; Schmitt, Julien; Hossain, Kazi M. Zakir; Mahmoudi, Najet; Edler, Karen J.; Scott, Janet L.
    Langmuir (2021), 37 (23), 6864-6873CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Oil-in-water emulsions have been stabilized by functionalized cellulose nanofibrils bearing either a neg. (oxidized cellulose nanofibrils, OCNF) or a pos. (cationic cellulose nanofibrils, CCNF) surface charge. The size of the droplets was measured by laser diffraction, while the structure of the shell of the Pickering emulsion droplets was probed using small-angle neutron scattering (SANS), confocal laser scanning microscopy (CLSM), SEM, and rheol. measurements. Both OCNF- and CCNF-stabilized emulsions present a very thick shell (>100 nm) comprised of densely packed CNF. OCNF-stabilized emulsions proved to be salt responsive, influencing the droplet aggregation and ultimately the gel properties of the emulsions, while CCNF emulsions, on the other hand, showed very little salt-dependent behavior.
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  32. 32
    López-Mondéjar, R.; Zühlke, D.; Becher, D.; Riedel, K.; Baldrian, P. Cellulose and hemicellulose decomposition by forest soil bacteria proceeds by the action of structurally variable enzymatic systems. Sci. Rep. 2016, 6, 25279  DOI: 10.1038/srep25279
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    32
    Cellulose and hemicellulose decomposition by forest soil bacteria proceeds by the action of structurally variable enzymatic systems
    Lopez-Mondejar, Ruben; Zuhlke, Daniela; Becher, Dorte; Riedel, Katharina; Baldrian, Petr
    Scientific Reports (2016), 6 (), 25279CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    Evidence shows that bacteria contribute actively to the decompn. of cellulose and hemicellulose in forest soil; however, their role in this process is still unclear. Here we performed the screening and identification of bacteria showing potential cellulolytic activity from litter and org. soil of a temperate oak forest. The genomes of 3 cellulolytic isolates previously described as abundant in this ecosystem were sequenced and their proteomes were characterized during the growth on plant biomass and on microcryst. cellulose. Pedobacter and Mucilaginibacter showed complex enzymic systems contg. highly diverse carbohydrate-active enzymes for the degrdn. of cellulose and hemicellulose, which were functionally redundant for endoglucanases, β-glucosidases, endoxylanases, β-xylosidases, mannosidases, and carbohydrate-binding modules. Luteibacter did not express any glycosyl hydrolases traditionally recognized as cellulases. Instead, cellulose decompn. was likely performed by an expressed GH23 family protein contg. a cellulose-binding domain. Interestingly, the presence of plant lignocellulose as well as cryst. cellulose both trigger the prodn. of a wide set of hydrolytic proteins including cellulases, hemicellulases, and other glycosyl hydrolases. Our findings highlight the extensive and unexplored structural diversity of enzymic systems in cellulolytic soil bacteria and indicate the roles of multiple abundant bacterial taxa in the decompn. of cellulose and other plant polysaccharides.
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  33. 33
    Lakhundi, S.; Siddiqui, R.; Khan, N. A. Cellulose degradation: a therapeutic strategy in the improved treatment of Acanthamoeba infections. Parasites Vectors 2015, 8, 23  DOI: 10.1186/s13071-015-0642-7
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    33
    Cellulose degradation: a therapeutic strategy in the improved treatment of Acanthamoeba infections
    Lakhundi Sahreena; Siddiqui Ruqaiyyah; Khan Naveed Ahmed
    Parasites & vectors (2015), 8 (), 23 ISSN:.
    Acanthamoeba is an opportunistic free-living amoeba that can cause blinding keratitis and fatal brain infection. Early diagnosis, followed by aggressive treatment is a pre-requisite in the successful treatment but even then the prognosis remains poor. A major drawback during the course of treatment is the ability of the amoeba to enclose itself within a shell (a process known as encystment), making it resistant to chemotherapeutic agents. As the cyst wall is partly made of cellulose, thus cellulose degradation offers a potential therapeutic strategy in the effective targeting of trophozoite encased within the cyst walls. Here, we present a comprehensive report on the structure of cellulose and cellulases, as well as known cellulose degradation mechanisms with an eye to target the Acanthamoeba cyst wall. The disruption of the cyst wall will make amoeba (concealed within) susceptible to chemotherapeutic agents, and at the very least inhibition of the excystment process will impede infection recurrence, as we bring these promising drug targets into focus so that they can be explored to their fullest.
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  1. Wei Zhang, Hua Cheng, Rui Pan, Ji Hua Yang, Yi Gong, Zhengya Gan, Rui Hu, Jianjun Ding, Lin Chen, Xian Zhang, Xingyou Tian. Phase Change Microcapsules with a Polystyrene/Boron Nitride Nanosheet Hybrid Shell for Enhanced Thermal Management of Electronics. Langmuir 2022, 38 (51) , 16055-16066. https://doi.org/10.1021/acs.langmuir.2c02660
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  • Abstract

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    Figure 1

    Figure 1. Schematic of primary Pickering emulsion (PE) and microcapsule (MC) preparation protocol.

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    Figure 2

    Figure 2. TEM micrographs of (a) OCNF and (b) CCNF dispersion in DI water at 0.01 wt %. Uranyl acetate (2%) was used for contrast imaging.

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    Figure 3

    Figure 3. Optical microscope images of the PE and MCs prepared by replacing 1.5 mL of CCNF (0.05 wt %)-stabilized PE with an equal amount of 0.05 wt % OCNF dispersion: (a) control CCNF (0.05 wt %)-stabilized PE before adding any OCNF dispersion, (b) first MCs, (c) second MCs, and (d) third MCs. Images were taken just after preparation and (e, f) confocal microscope images of third MCs after standing at room temperature for 1 week (sunflower oil dyed with NR showing in red and cellulose stained with calcofluor white stain showing in blue).

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    Figure 4

    Figure 4. Optical microscope images of microcapsule samples prepared using CCNF (0.1 wt %)-stabilized emulsion as a template with the various concentrations of OCNF dispersion: (a) MCs prepared using 0.05 wt % OCNF dispersion (inset shows the control CCNF (0.1 wt %)-stabilized PE). (b) MCs prepared using 0.2 wt % OCNF dispersion. (c) MCs prepared using 0.5 wt % OCNF dispersion; optical images were taken just after preparation. (d) Confocal microscope images of MC samples (c) after standing at room temperature for 1 week (SFO dyed with NR showing red and cellulose stained with calcofluor white stain showing blue).

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    Figure 5

    Figure 5. Variation of the NR release from the primary CCNF (0.1 wt %)-stabilized PE and the MCs prepared using 0.5 wt % OCNF dispersion: (a) via diffusion and (b) via centrifugation (8000 rpm for 10 min). Dye release percentage of the third MCs in various pH environments: (c) via diffusion and (d) via centrifugation (8000 rpm for 10 min).

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    Figure 6. (a) Dye release from the third MCs obtained after stirring using an overhead mechanical stirrer (2000 rpm for 10 min) in various pH environments. (b–d) Optical micrographs show that the MCs are stable after diffusion, centrifugation, and mechanical stirring processes at control pH (∼6.5) after 7 days of storage at room temperature.

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    Figure 7. Micrographs of (a) control CCNF (0.1 wt %)-stabilized Pickering emulsion prepared upon adding sunflower oil (dispersed phase) to 0.1 wt % CCNF dispersion (continuous phase) via membrane emulsification; (b) microcapsules prepared by the addition of the OCNF dispersion to the CCNF (0.1 wt %)-stabilized primary PE, (c) microcapsules after storage for 7 days at room temperature, and (d) microcapsules after storage for 7 days at 50 °C.

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  • References

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    This article references 33 other publications.

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      A review. Bioactive compds. possess plenty of health benefits, but they are chem. unstable and susceptible to oxidative degrdn. The application of pure bioactive compds. is also very limited in food and drug formulations due to their fast release, low soly., and poor bioavailability. Encapsulation can preserve the bioactive compds. from environmental stresses, improve physicochem. functionalities, and enhance their health-promoting and anti-disease activities. Micro and nano-encapsulation based techniques and systems have great importance in food and pharmaceutical industries. This review highlights the recent advances in micro and nano-encapsulation of bioactive compds. We comprehensively discussed the importance of encapsulation, the application of biopolymer-based carrier agents and lipid-based transporters with their functionalities, suitability of encapsulation techniques in micro and nano-encapsulation, as well as different forms of improved and novel micro and nano-encapsulate systems. Both micro and nano-encapsulation have an extensive application, but nano-encapsulation can be a promising approach for encapsulation purposes. Maltodextrin in combination with gums or other polysaccharides or proteins can offer an advantageous formulation for the encapsulation of bioactive compds. by using encapsulation techniques. Electro-spinning and electro-spraying are promising technologies in micro and nano-encapsulation, while solid lipid nanoparticles and nanostructure lipid carriers are exposing themselves as the promising and new generation of lipid nano-carriers for bioactive compds. Moreover, phytosome, nano-hydrogel, and nano-fiber are also efficient and novel nano-vehicles for bioactive compds. Further studies are required for the improvement of existing encapsulate systems and exploring their application in food and gastrointestinal systems for industrial application.
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      Microfluidics and Nanofluidics (2012), 13 (1), 151-178CODEN: MNIAAR; ISSN:1613-4982. (Springer)
      This review provides an overview of major microengineering emulsification techniques for prodn. of monodispersed droplets. The main emphasis has been put on membrane emulsification using Shirasu Porous Glass and microsieve membrane, microchannel emulsification using grooved-type and straight-through microchannel plates, microfluidic junctions and flow focusing microfluidic devices. Microfabrication methods for prodn. of planar and 3D poly(dimethylsiloxane) devices, glass capillary microfluidic devices and single-crystal silicon microchannel array devices have been described including soft lithog., glass capillary pulling and microforging, hot embossing, anisotropic wet etching and deep reactive ion etching. In addn., fabrication methods for SPG and microseive membranes have been outlined, such as spinodal decompn., reactive ion etching and UV LIGA (Lithog., Electroplating, and Molding) process. The most widespread application of micromachined emulsification devices is in the synthesis of monodispersed particles and vesicles, such as polymeric particles, microgels, solid lipid particles, Janus particles, and functional vesicles (liposomes, polymersomes and colloidosomes). Glass capillary microfluidic devices are very suitable for prodn. of core/shell drops of controllable shell thickness and multiple emulsions contg. a controlled no. of inner droplets and/or inner droplets of two or more distinct phases. Microchannel emulsification is a very promising technique for prodn. of monodispersed droplets with droplet throughputs of up to 100 l h-1.
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    16. 16
      Piacentini, E.; Poerio, T.; Bazzarelli, F.; Giorno, L. Microencapsulation by Membrane Emulsification of Biophenols Recovered from Olive Mill Wastewaters. Membranes 2016, 6 (2), 25  DOI: 10.3390/membranes6020025
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      Microencapsulation by membrane emulsification of biophenols recovered from olive mill wastewaters
      Piacentini, Emma; Poerio, Teresa; Bazzarelli, Fabio; Giorno, Lidietta
      Membranes (Basel, Switzerland) (2016), 6 (2), 25/1-25/11CODEN: MBSEB6; ISSN:2077-0375. (MDPI AG)
      Biophenols are highly prized for their free radical scavenging and antioxidant activities. Olive mill wastewaters (OMWWs) are rich in biophenols. For this reason, there is a growing interest in the recovery and valorization of these compds. Applications for the encapsulation have increased in the food industry as well as the pharmaceutical and cosmetic fields, among others. Advancements in micro-fabrication methods are needed to design new functional particles with target properties in terms of size, size distribution, and functional activity. This paper describes the use of the membrane emulsification method for the fine-tuning of microparticle prodn. with biofunctional activity. In particular, in this pioneering work, membrane emulsification has been used as an advanced method for biophenols encapsulation. Catechol has been used as a biophenol model, while a biophenols mixt. recovered from OMWWs were used as a real matrix. Water-in-oil emulsions with droplet sizes approx. 2.3 times the membrane pore diam., a distribution span of 0.33, and high encapsulation efficiency (98% ± 1% and 92% ± 3%, for catechol and biophenols, resp.) were produced. The release of biophenols was also investigated.
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    17. 17
      Akamatsu, K.; Chen, W.; Suzuki, Y.; Ito, T.; Nakao, A.; Sugawara, T.; Kikuchi, R.; Nakao, S.-i. Preparation of Monodisperse Chitosan Microcapsules with Hollow Structures Using the SPG Membrane Emulsification Technique. Langmuir 2010, 26, 14854– 14860,  DOI: 10.1021/la101967u
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      Preparation of Monodisperse Chitosan Microcapsules with Hollow Structures Using the SPG Membrane Emulsification Technique
      Akamatsu, Kazuki; Chen, Wei; Suzuki, Yukimitsu; Ito, Taichi; Nakao, Aiko; Sugawara, Takashi; Kikuchi, Ryuji; Nakao, Shin-ichi
      Langmuir (2010), 26 (18), 14854-14860CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      We describe herein successful prepns. of monodisperse chitosan microcapsules with hollow structures using the SPG membrane emulsification technique. Two prepn. procedures were examd. in this study. In the first method, monodisperse calcium alginate microspheres were prepd. and then coated with unmodified chitosan. Subsequently, tripolyphosphate treatment was conducted to phys. crosslink chitosan and solubilize the alginate core at the same time. In the second method, photo-crosslinkable chitosan was coated onto the monodisperse calcium alginate microspheres, followed by UV irradn. to chem. crosslink the chitosan shell and tripolyphosphate treatment to solubilize the core. For both methods, it was detd. that the av. diams. of the chitosan microcapsules depended on those of the calcium alginate microparticles and that the microcapsules have hollow structures. In addn., the first phys. crosslinking method using tripolyphosphate was found to be preferable to obtain the hollow structure, compared with the second method using chem. crosslinking by UV irradn. This was because of the difference in the resistance to permeation of the solubilized alginate through the chitosan shell layers.
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      Charcosset, C.; Limayem, I.; Fessi, H. The membrane emulsification process─a review. J. Chem. Technol. Biotechnol. 2004, 79 (3), 209– 218,  DOI: 10.1002/jctb.969
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      The membrane emulsification process - a review
      Charcosset, C.; Limayem, I.; Fessi, H.
      Journal of Chemical Technology and Biotechnology (2004), 79 (3), 209-218CODEN: JCTBED; ISSN:0268-2575. (John Wiley & Sons Ltd.)
      A review of the membrane emulsification process including principles of membrane emulsification, effect of process parameters and industrial applications. Membrane emulsification has received increasing attention over the last 10 yr, with potential applications in many fields. In the membrane emulsification process, a liq. phase is pressed through the membrane pores to form droplets at the permeate side of a membrane; the droplets are then carried away by a continuous phase flowing across the membrane surface. Under specific conditions, monodispersed emulsions can be produced using this technique. Small-scale applications such as drug delivery systems, food emulsions, and the prodn. of monodispersed microspheres are also included. Compared with conventional techniques for emulsification, membrane processes offer advantages such as control of av. droplet diam. by av. membrane pore size and lower energy input.
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      Jia, Y.; Feng, X.; Li, J. Polysaccharides-Based Microcapsules. In Supramolecular Chemistry of Biomimetic Systems; Li, J., Ed.; Springer: Singapore, 2017; Vol. 63, pp 63– 84.
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      Mohanta, V.; Madras, G.; Patil, S. Layer-by-Layer Assembled Thin Films and Microcapsules of Nanocrystalline Cellulose for Hydrophobic Drug Delivery. ACS Appl. Mater. Interfaces 2014, 6, 20093– 20101,  DOI: 10.1021/am505681e
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      Layer-by-Layer Assembled Thin Films and Microcapsules of Nanocrystalline Cellulose for Hydrophobic Drug Delivery
      Mohanta, Vaishakhi; Madras, Giridhar; Patil, Satish
      ACS Applied Materials & Interfaces (2014), 6 (22), 20093-20101CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      A layer-by-layer (LbL) approach has been employed for the fabrication of multilayer thin films and microcapsules having nanofibrous morphol. using nanocryst. cellulose (NCC) as one of the components of the assembly. The applicability of these nanoassemblies as drug delivery carriers has been explored by the loading of an anticancer drug, doxorubicin hydrochloride, and a water-insol. drug, curcumin. Doxorubicin hydrochloride, having a good water soly., is postloaded in the assembly. In the case of curcumin, which is very hydrophobic and has limited soly. in water, a stable dispersion is prepd. via noncovalent interaction with NCC prior to incorporation in the LbL assembly. The interaction of various other lipophilic drugs with NCC was analyzed theor. by mol. docking in consideration of NCC as a general carrier for hydrophobic drugs.
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      Feng, X.; Du, C.; Li, J. Molecular Assembly of Polysaccharide-Based Microcapsules and Their Biomedical Applications. Chem. Rec. 2016, 16 (4), 1991– 2004,  DOI: 10.1002/tcr.201600051
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      Molecular Assembly of Polysaccharide-Based Microcapsules and Their Biomedical Applications
      Feng, Xiyun; Du, Cuiling; Li, Junbai
      Chemical Record (2016), 16 (4), 1991-2004CODEN: CRHEAK; ISSN:1528-0691. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Advanced multifunctional microcapsules have revealed great potential in biomedical applications owing to their tunable size, shape, surface properties, and stimuli responsiveness. Polysaccharides are one of the most acceptable biomaterials for biomedical applications because of their outstanding virtues such as biocompatibility, biodegradability, and low toxicity. Many efforts have been devoted to investigating novel mol. design and efficient building blocks for polysaccharide-based microcapsules. In this Personal Account, we first summarize the common features of polysaccharides and the main principles of the design and fabrication of polysaccharide-based microcapsules, and further discuss their applications in biomedical areas and perspectives for future research.
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      Klemm, D.; Heublein, B.; Fink, H.-P.; Bohn, A. Cellulose: Fascinating Biopolymer and Sustainable Raw Material. Angew. Chem., Int. Ed. 2005, 44 (22), 3358– 3393,  DOI: 10.1002/anie.200460587
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      Cellulose: Fascinating biopolymer and sustainable raw material
      Klemm, Dieter; Heublein, Brigitte; Fink, Hans-Peter; Bohn, Andreas
      Angewandte Chemie, International Edition (2005), 44 (22), 3358-3393CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. As the most important skeletal component in plants, the polysaccharide cellulose is an almost inexhaustible polymeric raw material with fascinating structure and properties. Formed by the repeated connection of D-glucose building blocks, the highly functionalized, linear stiff-chain homopolymer is characterized by its hydrophilicity, chirality, biodegradability, broad chem. modifying capacity, and its formation of versatile semicryst. fiber morphologies. In view of the considerable increase in interdisciplinary cellulose research and product development over the past decade worldwide, this paper assembles the current knowledge in the structure and chem. of cellulose, and in the development of innovative cellulose esters and ethers for coatings, films, membranes, building materials, drilling techniques, pharmaceuticals, and foodstuffs. New frontiers, including environmentally friendly cellulose fiber technologies, bacterial cellulose biomaterials, and in-vitro syntheses of cellulose are highlighted together with future aims, strategies, and perspectives of cellulose research and its applications.
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      Bragd, P. L.; van Bekkum, H.; Besemer, A. C. TEMPO-Mediated Oxidation of Polysaccharides: Survey of Methods and Applications. Top. Catal. 2004, 27, 49– 66,  DOI: 10.1023/b:toca.0000013540.69309.46
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      TEMPO-Mediated Oxidation of Polysaccharides: Survey of Methods and Applications
      Bragd, P. L.; van Bekkum, H.; Besemer, A. C.
      Topics in Catalysis (2004), 27 (1-4), 49-66CODEN: TOCAFI; ISSN:1022-5528. (Kluwer Academic/Plenum Publishers)
      A review dealing with TEMPO as a catalyst in oxidn. of alc. functions in polysaccharides. Synthesis of TEMPO and derivs. and the mechanism of the oxidative cycle in which TEMPO is involved in oxidn. of alcs. are discussed. Results of oxidn. of various polysaccharides with respect to yield, and introduction of the functional groups (aldehyde and/or carboxylate) are presented. Most of the primary oxidants are not ideal, as they produce large amts. of salts, e.g., sodium chloride from sodium hypochlorite. Results and perspectives are given to change the salt-based oxidative systems for much cleaner oxygen or hydrogen peroxide/enzyme-based TEMPO systems. Moreover, several immobilized TEMPO systems have been developed.
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      Saito, T.; Kimura, S.; Nishiyama, Y.; Isogai, A. Cellulose Nanofibers Prepared by TEMPO-Mediated Oxidation of Native Cellulose. Biomacromolecules 2007, 8, 2485– 2491,  DOI: 10.1021/bm0703970
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      24
      Cellulose Nanofibers Prepared by TEMPO-Mediated Oxidation of Native Cellulose
      Saito, Tsuguyuki; Kimura, Satoshi; Nishiyama, Yoshiharu; Isogai, Akira
      Biomacromolecules (2007), 8 (8), 2485-2491CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
      Never-dried and once-dried hardwood celluloses were oxidized by a 2,2,6,6-tetra-Me piperidine-1-oxyl radical (TEMPO)-mediated system, and highly cryst. and individualized cellulose nanofibers, dispersed in water, were prepd. by mech. treatment of the oxidized cellulose/water slurries. When carboxylate contents formed from the primary hydroxyl groups of the celluloses reached approx. 1.5 mmol/g, the oxidized cellulose/water slurries were mostly converted to transparent and highly viscous dispersions by mech. treatment. Transmission electron microscopic observation showed that the dispersions consisted of individualized cellulose nanofibers 3-4 nm in width and a few microns in length. No intrinsic differences between never-dried and once-dried celluloses were found for prepg. the dispersion, as long as carboxylate contents in the TEMPO-oxidized celluloses reached approx. 1.5 mmol/g. Changes in viscosity of the dispersions during the mech. treatment corresponded with those in the dispersed states of the cellulose nanofibers in water.
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    25. 25
      Courtenay, J. C.; Ramalhete, S. M.; Skuze, W. J.; Soni, R.; Khimyak, Y. Z.; Edler, K. J.; Scott, J. L. Unravelling cationic cellulose nanofibril hydrogel structure: NMR spectroscopy and small angle neutron scattering analyses. Soft Matter 2018, 14, 255– 263,  DOI: 10.1039/C7SM02113E
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      Unravelling cationic cellulose nanofibril hydrogel structure: NMR spectroscopy and small angle neutron scattering analysis
      Courtenay, James C.; Ramalhete, Susana M.; Skuze, William J.; Soni, Rhea; Khimyak, Yaroslav Z.; Edler, Karen J.; Scott, Janet L.
      Soft Matter (2018), 14 (2), 255-263CODEN: SMOABF; ISSN:1744-6848. (Royal Society of Chemistry)
      Stiff, elastic, viscous shear thinning aq. gels are formed upon dispersion of low wt. percent concns. of cationically modified cellulose nanofibrils (CCNF) in water. CCNF hydrogels produced from cellulose modified with glycidyltrimethylammonium chloride, with degree of substitution (DS) in the range 10.6(3)-23.0(9)%, were characterised using NMR spectroscopy, rheol. and small angle neutron scattering (SANS) to probe the fundamental form and dimensions of the CCNF and to reveal interfibrillar interactions leading to gelation. As DS increased CCNF became more rigid as evidenced by longer Kuhn lengths, 18-30 nm, derived from fitting of SANS data to an elliptical cross-section, cylinder model. Furthermore, apparent changes in CCNF cross-section dimensions suggested an "unravelling" of initially twisted fibrils into more flattened ribbon-like forms. Increases in elastic modulus (7.9-62.5 Pa) were detected with increased DS and 1H soln.-state NMR T1 relaxation times of the introduced surface -N+(CH3)3 groups were found to be longer in hydrogels with lower DS, reflecting the greater flexibility of the low DS CCNF. This is the first time that such correlation between DS and fibrillar form and stiffness has been reported for these potentially useful rheol. modifiers derived from renewable cellulose.
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      Eyley, S.; Thielemans, W. Surface modification of cellulose nanocrystals. Nanoscale 2014, 6, 7764– 7779,  DOI: 10.1039/C4NR01756K
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      Surface modification of cellulose nanocrystals
      Eyley, Samuel; Thielemans, Wim
      Nanoscale (2014), 6 (14), 7764-7779CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      A review. Chem. modification of cellulose nanocrystals is an increasingly popular topic in the literature. This review analyses the type of cellulose nanocrystal modification reactions that have been published in the literature thus far and looks at the steps that have been taken towards analyzing the products of the nanocrystal modifications. The main categories of reactions carried out on cellulose nanocrystals were oxidns., esterifications, amidations, carbamations and etherifications. More recently nucleophilic substitutions had been used to introduce more complex functionality to cellulose nanocrystals. Multi-step modifications were also considered. This review emphasizes quantification of modification at the nanocrystal surface in terms of degree of substitution and the validity of conclusions drawn from different anal. techniques in this area. The mechanisms of the modification reactions were presented and considered with respect to the effect on the outcome of the reactions. While great strides had been made in the quality of anal. data published in the field of cellulose nanocrystal modification, there was still vast scope for improvement, both in data quality and the quality of anal. of data. Given the difficulty of surface anal., cross-checking of results from different anal. techniques was fundamental for the development of reliable cellulose nanocrystal modification techniques.
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    27. 27
      Han, J.; Zhou, C.; Wu, Y.; Liu, F.; Wu, Q. Self-Assembling Behavior of Cellulose Nanoparticles during Freeze-Drying: Effect of Suspension Concentration, Particle Size, Crystal Structure, and Surface Charge. Biomacromolecules 2013, 14, 1529– 1540,  DOI: 10.1021/bm4001734
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      27
      Self-Assembling Behavior of Cellulose Nanoparticles during Freeze-Drying: Effect of Suspension Concentration, Particle Size, Crystal Structure, and Surface Charge
      Han, Jingquan; Zhou, Chengjun; Wu, Yiqiang; Liu, Fangyang; Wu, Qinglin
      Biomacromolecules (2013), 14 (5), 1529-1540CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
      Cellulose nanocrystals and cellulose nanofibers with I and II cryst. allomorphs (designated as CNC I, CNC II, CNF I, and CNF II) were isolated from bleached wood fibers by alk. pretreatment and acid hydrolysis. The effects of concn., particle size, surface charge, and crystal structure on the lyophilization-induced self-assembly of cellulose particles in aq. suspensions were studied. Within the concn. range of 0.5 to 1.0 wt. %, cellulose particles self-organized into lamellar structured foam composed of aligned membrane layers with widths between 0.5 and 3 μm. At 0.05 wt. %, CNC I, CNF I, CNC II, and CNF II self-assembled into oriented ultrafine fibers with mean diams. of 0.57, 1.02, 1.50, and 1.00 μm, resp. The size of self-assembled fibers became larger when more hydroxyl groups and fewer sulfates (weaker electrostatic repulsion) were on cellulose surfaces. Possible formation mechanism was inferred from ice growth and interaction between cellulose nanoparticles in liq.-cryst. suspensions.
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    28. 28
      Courtenay, J. C.; Johns, M. A.; Galembeck, F.; Deneke, C.; Lanzoni, E. M.; Costa, C. A.; Scott, J. L.; Sharma, R. I. Surface modified cellulose scaffolds for tissue engineering. Cellulose 2017, 24, 253– 267,  DOI: 10.1007/s10570-016-1111-y
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      Surface modified cellulose scaffolds for tissue engineering
      Courtenay, James C.; Johns, Marcus A.; Galembeck, Fernando; Deneke, Christoph; Lanzoni, Evandro M.; Costa, Carlos A.; Scott, Janet L.; Sharma, Ram I.
      Cellulose (Dordrecht, Netherlands) (2017), 24 (1), 253-267CODEN: CELLE8; ISSN:0969-0239. (Springer)
      We report the ability of cellulose to support cells without the use of matrix ligands on the surface of the material, thus creating a two-component system for tissue engineering of cells and materials. Sheets of bacterial cellulose, grown from a culture medium contg. Acetobacter organism were chem. modified with glycidyltrimethylammonium chloride or by oxidn. with sodium hypochlorite in the presence of sodium bromide and 2,2,6,6-tetramethylpipiridine 1-oxyl radical to introduce a pos., or neg., charge, resp. This modification process did not degrade the mech. properties of the bulk material, but grafting of a pos. charged moiety to the cellulose surface (cationic cellulose) increased cell attachment by 70% compared to unmodified cellulose, while neg. charged, oxidized cellulose films (anionic cellulose), showed low levels of cell attachment comparable to those seen for unmodified cellulose. Only a minimal level of cationic surface derivitization (ca 3% degree of substitution) was required for increased cell attachment and no mediating proteins were required. Cell adhesion studies exhibited the same trends as the attachment studies, while the mean cell area and aspect ratio was highest on the cationic surfaces. Overall, we demonstrated the utility of pos. charged bacterial cellulose in tissue engineering in the absence of proteins for cell attachment.
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      Schmitt, J.; Calabrese, V.; da Silva, M. A.; Lindhoud, S.; Alfredsson, V.; Scott, J. L.; Edler, K. J. TEMPO-oxidised cellulose nanofibrils; probing the mechanisms of gelation via small angle X-ray scattering. Phys. Chem. Chem. Phys. 2018, 20, 16012– 16020,  DOI: 10.1039/C8CP00355F
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      29
      TEMPO-oxidised cellulose nanofibrils; probing the mechanisms of gelation via small angle X-ray scattering
      Schmitt, Julien; Calabrese, Vincenzo; da Silva, Marcelo A.; Lindhoud, Saskia; Alfredsson, Viveka; Scott, Janet L.; Edler, Karen J.
      Physical Chemistry Chemical Physics (2018), 20 (23), 16012-16020CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The structure of dispersions of TEMPO-oxidised cellulose nanofibrils (OCNF), at various concns., in water and in NaCl aq. solns., was probed using small angle X-ray scattering (SAXS). OCNF are modelled as rod-like particles with an elliptical cross-section of 10 nm and a length greater than 100 nm. As OCNF concn. increases above 1.5 wt%, repulsive interactions between fibrils are evidenced, modelled by the interaction parameter νRPA > 0. This corresponds to gel-like behavior, where G' > G'' and the storage modulus, G', shows weak frequency dependence. Hydrogels can also be formed at OCNF concn. of 1 wt% in 0.1 M NaCl(aq). SAXS patterns shows an increase of the intensity at low angle that is modelled by attractive interactions (νRPA < 0) between OCNF, arising from the screening of the surface charge of the fibrils. Results are supported by ζ potential and cryo-TEM measurements.
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    30. 30
      Hossain, K. M. Z.; Calabrese, V.; da Silva, M. A.; Bryant, S. J.; Schmitt, J.; Ahn-Jarvis, J. H.; Warren, F. J.; Khimyak, Y. Z.; Scott, J. L.; Edler, K. J. Monovalent Salt and pH-Induced Gelation of Oxidised Cellulose Nanofibrils and Starch Networks: Combining Rheology and Small-Angle X-ray Scattering. Polymers 2021, 13, 951  DOI: 10.3390/polym13060951
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      30
      Monovalent salt and pH-induced gelation of oxidized cellulose nanofibrils and starch networks: combining rheology and small-angle X-ray scattering
      Hossain, Kazi M. Zakir; Calabrese, Vincenzo; da Silva, Marcelo A.; Bryant, Saffron J.; Schmitt, Julien; Ahn-Jarvis, Jennifer H.; Warren, Frederick J.; Khimyak, Yaroslav Z.; Scott, Janet L.; Edler, Karen J.
      Polymers (Basel, Switzerland) (2021), 13 (6), 951CODEN: POLYCK; ISSN:2073-4360. (MDPI AG)
      Water quality parameters such as salt content and various pH environments can alter the stability of gels as well as their rheol. properties. Here, we investigated the effect of various concns. of NaCl and different pH environments on the rheol. properties of TEMPO-oxidized cellulose nanofibril (OCNF) and starch-based hydrogels. Addn. of NaCl caused an increased stiffness of the OCNF:starch (1:1 wt.%) blend gels, where salt played an important role in reducing the repulsive OCNF fibrillar interactions. The rheol. properties of these hydrogels were unchanged at pH 5.0 to 9.0. However, at lower pH (4.0), the stiffness and viscosity of the OCNF and OCNF:starch gels appeared to increase due to proton-induced fibrillar interactions. In contrast, at higher pH (11.5), syneresis was obsd. due to the formation of denser and aggregated gel networks. Interactions as well as aggregation behavior of these hydrogels were explored via ζ-potential measurements. Furthermore, the nanostructure of the OCNF gels was probed using small-angle X-ray scattering (SAXS), where the SAXS patterns showed an increase of slope in the low-q region with increasing salt concn. arising from aggregation due to the screening of the surface charge of the fibrils.
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      Courtenay, J. C.; Jin, Y.; Schmitt, J.; Hossain, K. M. Z.; Mahmoudi, N.; Edler, K. J.; Scott, J. L. Salt-Responsive Pickering Emulsions Stabilized by Functionalized Cellulose Nanofibrils. Langmuir 2021, 37, 6864– 6873,  DOI: 10.1021/acs.langmuir.0c03306
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      31
      Salt-Responsive Pickering Emulsions Stabilized by Functionalized Cellulose Nanofibrils
      Courtenay, James C.; Jin, Yun; Schmitt, Julien; Hossain, Kazi M. Zakir; Mahmoudi, Najet; Edler, Karen J.; Scott, Janet L.
      Langmuir (2021), 37 (23), 6864-6873CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Oil-in-water emulsions have been stabilized by functionalized cellulose nanofibrils bearing either a neg. (oxidized cellulose nanofibrils, OCNF) or a pos. (cationic cellulose nanofibrils, CCNF) surface charge. The size of the droplets was measured by laser diffraction, while the structure of the shell of the Pickering emulsion droplets was probed using small-angle neutron scattering (SANS), confocal laser scanning microscopy (CLSM), SEM, and rheol. measurements. Both OCNF- and CCNF-stabilized emulsions present a very thick shell (>100 nm) comprised of densely packed CNF. OCNF-stabilized emulsions proved to be salt responsive, influencing the droplet aggregation and ultimately the gel properties of the emulsions, while CCNF emulsions, on the other hand, showed very little salt-dependent behavior.
      >> More from SciFinder ®
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    32. 32
      López-Mondéjar, R.; Zühlke, D.; Becher, D.; Riedel, K.; Baldrian, P. Cellulose and hemicellulose decomposition by forest soil bacteria proceeds by the action of structurally variable enzymatic systems. Sci. Rep. 2016, 6, 25279  DOI: 10.1038/srep25279
      [Crossref], [PubMed], [CAS], Google Scholar
      32
      Cellulose and hemicellulose decomposition by forest soil bacteria proceeds by the action of structurally variable enzymatic systems
      Lopez-Mondejar, Ruben; Zuhlke, Daniela; Becher, Dorte; Riedel, Katharina; Baldrian, Petr
      Scientific Reports (2016), 6 (), 25279CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      Evidence shows that bacteria contribute actively to the decompn. of cellulose and hemicellulose in forest soil; however, their role in this process is still unclear. Here we performed the screening and identification of bacteria showing potential cellulolytic activity from litter and org. soil of a temperate oak forest. The genomes of 3 cellulolytic isolates previously described as abundant in this ecosystem were sequenced and their proteomes were characterized during the growth on plant biomass and on microcryst. cellulose. Pedobacter and Mucilaginibacter showed complex enzymic systems contg. highly diverse carbohydrate-active enzymes for the degrdn. of cellulose and hemicellulose, which were functionally redundant for endoglucanases, β-glucosidases, endoxylanases, β-xylosidases, mannosidases, and carbohydrate-binding modules. Luteibacter did not express any glycosyl hydrolases traditionally recognized as cellulases. Instead, cellulose decompn. was likely performed by an expressed GH23 family protein contg. a cellulose-binding domain. Interestingly, the presence of plant lignocellulose as well as cryst. cellulose both trigger the prodn. of a wide set of hydrolytic proteins including cellulases, hemicellulases, and other glycosyl hydrolases. Our findings highlight the extensive and unexplored structural diversity of enzymic systems in cellulolytic soil bacteria and indicate the roles of multiple abundant bacterial taxa in the decompn. of cellulose and other plant polysaccharides.
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    33. 33
      Lakhundi, S.; Siddiqui, R.; Khan, N. A. Cellulose degradation: a therapeutic strategy in the improved treatment of Acanthamoeba infections. Parasites Vectors 2015, 8, 23  DOI: 10.1186/s13071-015-0642-7
      [Crossref], [PubMed], [CAS], Google Scholar
      33
      Cellulose degradation: a therapeutic strategy in the improved treatment of Acanthamoeba infections
      Lakhundi Sahreena; Siddiqui Ruqaiyyah; Khan Naveed Ahmed
      Parasites & vectors (2015), 8 (), 23 ISSN:.
      Acanthamoeba is an opportunistic free-living amoeba that can cause blinding keratitis and fatal brain infection. Early diagnosis, followed by aggressive treatment is a pre-requisite in the successful treatment but even then the prognosis remains poor. A major drawback during the course of treatment is the ability of the amoeba to enclose itself within a shell (a process known as encystment), making it resistant to chemotherapeutic agents. As the cyst wall is partly made of cellulose, thus cellulose degradation offers a potential therapeutic strategy in the effective targeting of trophozoite encased within the cyst walls. Here, we present a comprehensive report on the structure of cellulose and cellulases, as well as known cellulose degradation mechanisms with an eye to target the Acanthamoeba cyst wall. The disruption of the cyst wall will make amoeba (concealed within) susceptible to chemotherapeutic agents, and at the very least inhibition of the excystment process will impede infection recurrence, as we bring these promising drug targets into focus so that they can be explored to their fullest.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC2MvmtFyrtg%253D%253D&md5=50b4928ce9784d74154c7d83e8031ec8

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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.1c03025.

    • Conductometric analysis of the degree of substitution of CCNF; calibration curve for absorbance of NR in sunflower oil; photographs showing the stages in the dye diffusion and release protocols used in this study; photographs of stable and unstable microcapsules; optical microscope images of microcapsule samples; images of the contact angle of water on glass slides coated with OCNF or CCNF; and summary of the OCNF and CCNF concentrations in the samples (PDF)


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