Enlarged Pore Size in Mesoporous Silica Films Templated by Pluronic F127: Use of Poloxamer Mixtures and Increased Template/SiO₂ Ratios in Materials Synthesized by Evaporation-Induced Self-Assembly

Although evaporation-induced self-assembly (EISA) has proven to be a convenient method for synthesizing nanoporous silica films (and particles), accessing material structures with pore sizes larger than ca. 10 nm remains experimentally inconvenient. The use of pore swelling agents (SAs), commonly used during the hydrothermal synthesis of mesoporous silicas, results in little or no pore size expansion due to evaporation or phase separation. Moreover, diblock copolymer templates can yield large pores but are quite expensive and generally require the addition of strong organic cosolvents. Here, we hypothesized that pores templated by the Pluronic triblock polymer F127 could be successfully enlarged, without phase separation, by using a chemically similar, nonvolatile, secondary Pluronic polymer (P103) as the SA. We find pore size increased up to 15 nm for a spherical pore morphology, with a phase transition to a multilamellar vesicle (MLV)-based nanostructure occurring as the P103/F127 ratio is further increased. This MLV phase produces even larger pore sizes due to the collapse of concentric silica shells upon template removal. Remarkably, F127 alone exhibits expansion of pore size (up to ca. 16 nm) as the template/silica ratio is increased. We find appearance of the MLV phase is due to geometric packing considerations, with expansion of F127 micelle size being a result of favorable intermolecular interactions driven by the large poly­(ethylene oxide) content of F127. Other Pluronic polymers with this feature also exhibit variable pore size based on the template/silica ratio, enabling the synthesis of mesoporous films with 3D pore connectivity and truly variable pore size of ca. 4.5 to almost 20 nm

XRD spectra of RR2, RR3, RR4 (rutile rod) samples.

<p>XRD spectra of RR2, RR3, RR4 (rutile rod) samples.</p

Measured properties of the TiO₂ samples.

a<p>primary particle dimensions are determined by measuring 100 randomly-chosen particles on TEM images.</p>b<p>specific surface area is calculated based on the particle dimensions, assuming rutile rods as cylinders and anatase spheres as perfect spheres. The density of TiO2 was 4.23×10⁶ g/m³.</p>c<p>CCC of RR3 with NOM appeared to be below the lowest tested electrolyte concentration, thus no CCC is reported here.</p

Electrophoretic moblity of anatase spheres (a) and rutile rods (b) as a function of pH.

<p>A general trend of PZC shift toward a lower pH can be observed for both anatase spheres and rutile rods.</p

Representative TEM images of rutile rods and anatase spheroids.

<p>Representative TEM images of rutile rods and anatase spheroids.</p

Correlation between specific surface area and CCC for rutile rods.

<p>Correlation between specific surface area and CCC for rutile rods.</p

Anatase sphere CCC-particle size correlation and the theoretical prediction of the energy barrier.

<p>(a). Correlation between nanoparticle diameter and CCC for anatase sphere (AS) TiO₂; (b). Predicted energy barrier contour map of TiO₂ anatase nanospheres. The color bar denotes the energy barrier (unit kT).</p

The attachment efficiency as a function of NaCl concentration for AS3 (anatase sphere).

<p>A reaction-limited cluster aggregation regime (RLCA, left region) and a diffusion-limited cluster aggregation regime (DLCA, right region) can be observed.</p

Influence of Silica Matrix Composition and Functional Component Additives on the Bioactivity and Viability of Encapsulated Living Cells

The remarkable impact encapsulation matrix chemistry can have on the bioactivity and viability of integrated living cells is reported. Two silica chemistries (aqueous silicate and alkoxysilane), and a functional component additive (glycerol), are employed to generate three distinct silica matrices. These matrices are used to encapsulate living <i>E. coli</i> cells engineered with a synthetic riboswitch for cell-based biosensing. Following encapsulation, membrane integrity, reproductive capability, and riboswitch-based protein expression levels and rates are measured over a 5 week period. Striking differences in <i>E. coli</i> bioactivity, viability, and biosensing performance are observed for cells encapsulated within the different matrices. <i>E. coli</i> cells encapsulated for 35 days in aqueous silicate-based (AqS) matrices showed relatively low membrane integrity, but high reproductive capability in comparison to cells encapsulated in glycerol containing sodium silicate-based (AqS + g) and alkoxysilane-based (PGS) gels. Further, cells in sodium silicate-based matrices showed increasing fluorescence output over time, resulting in a 1.8-fold higher fluorescence level, and a faster expression rate, over cells free in solution. This unusual and unique combination of biological properties demonstrates that careful design of the encapsulation matrix chemistry can improve functionality of the biocomposite material, and result in new and unexpected physiological states

Revealing the Interfacial Self-Assembly Pathway of Large-Scale, Highly-Ordered, Nanoparticle/Polymer Monolayer Arrays at an Air/Water Interface

The pathway of interfacial self-assembly of large-scale, highly ordered 2D nanoparticle/polymer monolayer or bilayer arrays from a toluene solution at an air/water interface was investigated using grazing-incidence small-angle scattering at a synchrotron source. Interfacial-assembly of the ordered nanoparticle/polymer array was found to occur through two stages: formation of an incipient randomly close-packed interfacial monolayer followed by compression of the monolayer to form a close-packed lattice driven by solvent evaporation from the polymer. Because the nanoparticles are hydrophobic, they localize exclusively to the polymer–air interface during self-assembly, creating a through thickness asymmetric film as confirmed by X-ray reflectivity. The interfacial self-assembly approach can be extended to form binary NP/polymer arrays. It is anticipated that by understanding the interfacial self-assembly pathway, this simple evaporative procedure could be conducted as a continuous process amenable to large area nanoparticle-based manufacturing needed for emerging energy technologies