Tunable Ordered Nanostructured Phases by Co-assembly of Amphiphilic Polyoxometalates and Pluronic Block Copolymers

The assembly of polyoxometalate (POM) metal–oxygen clusters into ordered nanostructures is attracting a growing interest for catalytic and sensing applications. However, assembly of ordered nanostructured POMs from solution can be impaired by aggregation, and the structural diversity is poorly understood. Here, we present a time-resolved small-angle X-ray scattering (SAXS) study of the co-assembly in aqueous solutions of amphiphilic organo-functionalized Wells-Dawson-type POMs with a Pluronic block copolymer over a wide concentration range in levitating droplets. SAXS analysis revealed the formation and subsequent transformation with increasing concentration of large vesicles, a lamellar phase, a mixture of two cubic phases that evolved into one dominating cubic phase, and eventually a hexagonal phase formed at concentrations above 110 mM. The structural versatility of co-assembled amphiphilic POMs and Pluronic block copolymers was supported by dissipative particle dynamics simulations and cryo-TEM.

P olyoxometalates (POMs) are polynuclear metal-oxo clusters that can host a range of noble metal atoms to provide a well-defined coordination environment of singleatom sites. 1−3 The potential applications of single-atom catalysis of POMs have attracted significant research interest due to their versatility and potential for very high atomic utilization and minimal use of scarce and/or potentially toxic metals. 4−6 It is important to avoid agglomeration and the formation of dense and disordered structures that limit the accessibility of the single-atomic catalytic sites, but the preparation of ordered nanostructures with single-atom sites remains a challenge. 7,8 Assembly from solution is a powerful and extensively used approach to prepare periodic nanostructures from oligomeric, 9 polymeric, 10 and nanoparticle 11−13 building blocks. Surfactantencapsulated POMs (SEPOMs) have resulted in the formation of ordered structures in organic solvents 8,14−16 and have also facilitated recovery. 17,18 However, the stability of the physically bonded SEPOMs is poor, and the assembly of SEPOMs approach is limited to low concentrations and low POM content. 19 Grafting organic molecules to form covalently linked POMorganic hybrids avoids some of the issues related to the physically encapsulated SEPOMs and may improve compatibility and processability. Covalently linked POM-organic hybrids have been used to prepare periodic POM-containing nanostructures both in solid state 20,21 and in organic solvents. 19,22,23 However, studies of POM-containing nanostructures in aqueous solutions, which offer high single-atom accessibility compared to the reversed micelles in organic solvents, are sparse. 22,24 Here, we present a study on how periodic nanostructures containing single-dispersed organo-functionalized Wells−Dawson polyoxometalates can be prepared by co-assembly with a Pluronic block copolymer (P123) in aqueous solutions. Covalent grafting of two hydrophobic hydrocarbon chains resulted in amphiphilic Wells−Dawson polyoxometalates (K 6 [P 2 W 17 OSi 2 (C 12 H 25 ) 2 ], denoted as POM-2C 12 ) that could be dissolved in water without agglomeration or precipitation. The phase behavior and formation of ordered nanostructures by co-assembly of P123 and POM-2C 12 in aqueous solutions were investigated by time-resolved smallangle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) in levitating drops in combination with dissipative particle dynamics simulations and cryo-transmission electron microscopy (cryo-TEM). The use of shrinking levitating drops allowed a wide concentration range to be probed, revealing that the co-assembly strategy resulted in the formation of large vesicles, a lamellar phase, a cubic phase, and eventually a hexagonal phase with increasing concentration. The rich structural diversity and the demonstration of tunable manipulation of ordered POM-based nanostructures by coassembly could serve as a basis for the production, possibly by supercritical drying, 25 of structured POM-based materials.
Time-resolved SAXS/WAXS measurements (refer to Figure  1a for the experimental setup) of the aqueous solutions in the levitating droplet (Figure 1b) was used to follow in situ the evolution of the nanostructures in the shrinking aqueous droplet that contains POM-2C 12 and P123. The molecular structure of POM-2C 12 is illustrated in Figure S1; the molecular structure was confirmed by C−H correlated NMR; see Figure S2. The molar concentrations in the droplet were calculated according to the volume of the shrinking droplet, and we followed the co-assembly behavior up to the concentration of 115.0 mM (Figure 1c). The minor fluctuations of the concentration and volume are mainly related to the acoustic-field-induced fluctuations of the droplet shape.
The partial scattering invariant (Q In (t)) is a measure of the total scattering power of the sample 26−28 and was calculated through the expression Q In (t) = ∫ I(q)q 2 dq (q ranges between 0.00416 and 0.4472 Å −1 ) of the 1D reduced data as detailed in the calculation section in Supporting Information (SI). Q In (t) is a constant directly proportional to the mean-square average fluctuation in the scattering length density (Δρ) and the phase composition (ϕ(1 − ϕ), ϕ is the volume fraction of the nanostructures in the solution). The Δρ was kept constant for a specific solution system; thus, the apparent variation of Q In (t) can be used to probe the phase boundaries. 29 The timeresolved Q In (t) for each collected SAXS pattern, as a function of experimental time (t) is displayed in Figure 1d. The temporal changes of Q In (t), that is independent of concentration, provide important insight into the structural evolution of the aqueous solution, 30,31 though a large variation was spotted at the end of the measurement due to an instability of the shrinking droplet. The time-dependent co-assembly process was divided into five regions t 1 −t 5 that are related to different nanostructures that form with increasing concentration, Figure 1d.
The POM-2C 12 and P123 solution was studied at a starting molar concentration of 12.0 mM (ca. 5.02 wt %), which lies in the t 1 region and corresponds to the red SAXS patterns in Figure 1e. Dynamic light scattering (DLS) ( Figure S3) and cryo-TEM (Figure 2a) indicate that the dilute solution contains vesicles with sizes ranging between 235 and 265 nm. The SAXS pattern of the solution at 12.0 mM (Figure 2b) could be fitted to a random lamellar phase using SasView software (version 5.05), with a hydrophobic and hydrophilic slab at 35.2 ± 0.2 Å and 10.2 ± 3.5 Å, respectively. This indicates that the vesicles that formed in the diluted concentration range have a bilayer thickness of ca. 45.2 ± 3.5 Å, as illustrated in Figure S4. The zeta potential of the vesicles was determined to be −16.9 ± 3.2 eV, Figure S5, which is in between the zeta potential of the 12.0 mM POM-2C 12 aqueous solution (−80.0 ± 6.0 eV) and the zeta potential The SAXS pattern displayed several distinct peaks from 368 s, which related to a lamellar phase (Figure 1e and Figure 2c). This transformation corresponds to the t 2 region (Figure 1d) with an onset at a concentration of 28.7 mM. The correlation peak shifted to higher q with time, Figure S7, between 368 and 638 s, which indicates the formation of a more closely packed lamellar phase with increasing concentration.
At concentrations of 52.6 mM and above (from 638 s, related to the region t 3 in Figure 1d), the SAXS pattern corresponded to a mixed cubic phase, consisting of the Fm3̅ m space group and the Pm3̅ m space group, Figure 2c. The Bonnet ratio (the ratio of the lattice parameters of the two coexisting cubic phases, a Fm3m /a Pm3m ) 32−34 was calculated for the concentration at 52.6 mM, as listed in Table S2. As the droplet continued to shrink and the concentration thus increased, the SAXS analysis suggested that the nanostructures in the droplet transform to a single cubic phase with the Fm3̅ m space group from 74.3 mM (t 4 region in Figure 1d), followed by a final phase transformation into a hexagonal phase (t 5 region) toward the end of the measurement, at 111.2 mM (Figure 1d).
The large vesicles that formed in the dilute concentration range in the POM-2C 12 /P123 = 1:3 system can be attributed to the phase behavior of P123. 35 The transition from vesicles to lamellar sheets with increasing concentration (indicated by an arrow in Figure 1e and further illustrated in Figure S8) is caused by the interactions between the overcrowded vesicles. 36 The subsequent cubic phase existed at higher concentrations (to 106 mM) compared to a previously reported pure P123 system (to 93 mM at 20°C). 37 The difference might be due to the large size/high charge of the POM headgroups of POM-2C 12 in the system, which may delay the subsequent formation of the hexagonal phase.
The d spacing decreased gradually with increasing concentration (Figure 2d). The most pronounced decrease of the d spacing occurred within the lamellar region where the distance between two adjacent bilayers experienced a decrease from ca. 296 Å to ca. 220 Å as the concentration increased from 28.7 mM to 45.9 mM. Between 47.0 and 65.0 mM, the d spacing remained almost unchanged at the beginning and then decreased slightly. The analysis of the d spacing plotted in Figure 2d that was calculated according to the first primary peak of the Fm3̅ m space group was complicated by the existence of two cubic phases. The estimated d spacing decreased as only a single cubic phase with the Fm3̅ m space group was formed (t 4 ).
The time-resolved WAXS patterns of the POM-2C 12 /P123 system ( Figure S9) showed that the crystal structure of the phosphotungstate headgroup remained unchanged throughout the measurement. This indicates that the POM headgroup is unaffected by the concentration increase and the X-ray beam. The dried beads that were collected from the levitator after the SAXS measurements of the POM-2C 12 /P123 (1:3) system have an ellipsoid shape with a cross-sectional size of 646 μm × 721 μm ( Figure S10a). The rough surface of the dry bead ( Figure S10b) is probably related to the drying-induced shrinkage. The EDS analyses ( Figure S11) showed the existence of P, W, Si, C, K and O elements in the dry bead, which are the main components of the two molecules we have studied.
The SAXS measurements confirm that the dilute POM-2C 12 solution (12.0 mM) contains ellipsoid micelles ( Figure S12) similar to previous work where the POM-containing micelles were used as the template to encapsulate POMs in porous carriers. 38 Interestingly, no ordered structures were observed at higher concentrations (between ca. 18.7 mM and 94.7 mM) in POM-2C 12 solutions, as illustrated in Figure S12a. The scattering invariant ( Figure S12b) displayed a continuous increase, which corroborated that no phase transition occurred within the investigated concentration range. We speculate that the high charge (refer to the zeta potential in Figure S5) and the large size of the hydrophilic headgroup 39 of the POM-2C 12 molecules impeded the organization of molecules even at high concentrations. Drying of the POM-2C 12 solution on a glass slide, Figure S13, resulted in large rod-like deposits with a size of ca. 1.5 μm, similar to what has been found for a modified lacunary polyoxometalate cluster ([PW 11 O 39 ]). 40 We have also investigated the structural evolution of POM-2C 12 -rich mixed solutions with a mixing ratio between POM-2C 12 Figure 3b, corroborated the elliptical morphology. The formation of ellipsoids was shown in the POM-2C 12 /P123 = 3:1 system at 12.0 mM while the POM-2C 12 /P123 = 1:3 system was dominated by vesicles at similar concentrations, which could be related to a higher headgroup area due to a higher charge density for the system that contains more POM-2C 12 . 36 This leads to a lower value of the packing parameter of the POM-2C 12 /P123 = 3:1 mixture that may reach under 1/3, a value range that predicts the formation of ellipsoids or spheres. 42 Time-resolved SAXS measurements of the shrinking droplets of the POM-2C 12 -rich mixed solution, Figure 3c, showed one sharp peak and one broad peak (indicated by the arrows) at the end, which is related to a wormlike structure. No additional ordered structures or phase transitions could be detected up to the highest investigated concentration of 102.9 mM ( Figure S14).
Mixing POM-2C 12 with small nonionic molecules, e.g. octaethylene glycol monododecyl ether (C 12 EO 8 ), resulted in the formation of disordered micelles at a concentration of 80.0 mM (see the brown curve in Figure S15). Increasing the concentration to 133.0 mM, resulted in the formation of an ordered lamellar phase (see the green curve in Figure S15).
These comparative studies clearly show that co-assembly of the covalently organo-functionalized POM clusters with the amphiphilic P123 block copolymer is responsible for the rich phase behavior in aqueous solutions.
The morphologies of the nanostructures formed in POM-2C 12 /P123 systems with mixing ratios of both 1:3 and 3:1 were investigated by dissipative particle dynamics (DPD) simulations at total molar concentrations where different phases were observed experimentally. The molecular structures and coarse-grained models are shown in Figure 4a.

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The DPD simulations for the POM-2C 12 /P123 system with a mixing ratio of 1:3 at a concentration of 12.0 mM (Figure 4b and Figure S16a) show large, spherical assembles that correspond well to the large vesicles that were observed experimentally at this concentration. The DPD simulations at increasing concentration (28.7 mM) also corroborate the experimental observation and indicate the formation of lamellar phase structures. The DPD simulations for the mixture with a concentration of 74.3 mM show a face-centered structure, where the SAXS study indicated that a cubic phase with the Fm3̅ m space group is formed. At the highest simulated concentration (111.2 mM), the DPD simulation reproduce a hexagonal packing structure.
The morphologies of the nanostructures formed in the POM-2C 12 /P123 system with a mixing ratio of 3:1 (Figure 4c) are difficult to define quantitatively, but the DPD simulations appear to reproduce elliptical nanostructures at 12.0 mM, which corroborate with both the SAXS (Figure 3a) and cryo-TEM (Figure 3b) analyses. Simulations of solutions at a higher concentration (28.7, 52.6, and 74.3 mM) (Figure 4c, Figure  S16b, Figure S17) indicated the formation of wormlike nanostructures.
Overall, the simulated structures follow the general trends that were inferred from the time-resolved SAXS data, but more detailed analyses of, e.g., the region where two cubic space groups were found to coexist probably require that larger systems are used for simulations. The simulations clearly illustrate that the covalent organo-functionalization is able to minimize aggregation and promote the formation of ordered POM-containing nanostructures in aqueous solutions.
We have investigated the nanostructures formed by coassembly of POM-2C 12 and P123 in levitating aqueous droplets by time-resolved SAXS combined with dissipative particle dynamic simulations and cryo-TEM. The timeresolved SAXS measurements were performed on evaporating and thus shrinking levitating droplets that enabled studies of the structural evolution for concentrations ranging between 12.0 mM and 133.0 mM. The mixtures of POM-2C 12 and P123 with a 1:3 ratio displayed a rich phase behavior with the formation of large vesicles at the lowest concentration (12.0 mM) that transformed into a lamellar phase, a mixture of two cubic phases that evolved into one dominating cubic phase, and eventually the formation of a hexagonal phase at the highest investigated concentration. The high electron density of POMs promoted the determination of nanostructures formed by the covalent-modified amphiphilic polyoxometalates in the shrinking levitating droplet and ensured a sufficient contrast of the cryo-TEM images.

■ METHODS
Small/wide angle X-ray scattering experiments were carried out at the ID02 beamline, 43 European Synchrotron Radiation Facility (ESRF), Grenoble, France, using a standard X-ray energy of 12.3 keV. 2D SAXS patterns were collected using an Eiger 2-4 M detector using a sample-to-detector distance at 1.5, 3, or 5 m depending on the pretest of the size of the colloid in the solution. 2D WAXS patterns were collected using a Rayonix LX 170HS CCD camera at a sample to detector distance of 13 cm. The online data reduction pipeline is based on Python Fast Azimuthal Integration (PyFAI). 44 The 2D SAXS/WAXS images were background-corrected and converted to 1D data by azimuthal integration of the 2D signal, and the obtained 1D data are visualized via SAXSutilities2 software, 45 where q is defined by q = 4π/λ sin(θ/2) with the wavelength (λ) and the scattering angle θ. We used a customized setup to measure the self-assembled nanostructures while the concentration increased. An acoustic levitator, with a microscope camera in place to follow the status of the levitating droplet, was placed in positions as schematically depicted in Figure 1a. At the beginning of the measurement, a droplet of the aqueous solution (total surfactant molar concentration, 12.0 mM) was injected into the acoustic levitator (model 13K11, tec5, Oberursel, Germany) using microliter syringes (Hamilton Company, USA). The size of the droplet in the acoustic levitator was followed by the microscope camera. Videos that capture the shrinking process of the droplet were converted to image sequences by VirtualDub software. The size of the shrinking droplet with time was analyzed using ImageJ software and calibrated using the height scan at the beginning of each time-resolved SAXS measurement. For absolute intensity calibration and solvent background subtraction, scattering of the empty levitator chamber and levitator chamber with a water droplet were also recorded. Each experiment was repeated 2−3 times.