Ultralarge-pore FDU-12 (ULP-FDU-12) silicas with face-centered cubic structures (Fm3m symmetry) of spherical mesopores were synthesized at low initial temperature (∼14 °C) using commercially available PEO-PPO-PEO triblock copolymer Pluronic F127 as a micellar template and xylene as a micelle expander. Xylene was selected on the basis of its predicted higher swelling ability for the Pluronic surfactant micelles in comparison to 1,3,5-trimethylbenzene that was used previously to obtain large-pore FDU-12. The optimization of the synthesis conditions afforded as-synthesized ULP-FDU-12 materials with unit-cell parameters up to 56 nm, which is comparable to the highest reported values for Fm3m structures templated by custom-made surfactants. Calcined silicas were obtained with unit-cell parameters up to 53 nm and pore diameters up to ∼36 nm (for N(2) adsorption at 77 K, the capillary condensation relative pressure was up to 0.938). The preferred silica source was tetraethylorthosilicate, but tetramethylorthosilicate was also found suitable. The pore diameter was dependent on the unit-cell size of the as-synthesized material, but was further tuned by adjusting the time and temperature of the treatment in the HCl solution. If the synthesis was performed at low temperature only, highly ordered closed-pore silicas were obtained at calcination temperatures as low as 450 °C. On the other hand, the hydrothermal treatments, including the acid treatment at 130 °C, afforded silicas with large pore entrance sizes. The present synthesis constitutes a major advancement in the synthesis of ordered silicas with very large open and closed spherical mesopores.
Creating secondary nanostructures from fundamental building blocks with simultaneous high loading capacity and well-controlled size/uniformity, is highly desired for nanoscale synergism and integration of functional units. Here a novel strategy is reported for hydrophobic quantum dots (QDs) assembley with porous templates, to form pitaya-type fluorescent silica colloids with densely packed and intact QDs throughout the silica matrix. The mercapto-terminated dendritic silica spheres with highly accessible centralradial pores and metal-affinity interior surface, are adopted as a powerful absorbent host for direct immobilization of QDs from organic phase with high loading capacity. The alkylsilane mediated silica encapsulation prevents QDs' optical degradation induced by ligand exchange and favors the homogeneous silica shell formation. These multiple QD embedded silica spheres exhibit good compatibility for different colored QDs with well-preserved fluorescence, high colloidal/optical stability, and versatile surface functionality. It is demonstrated that after integration with a lateral flow strip platform, these silica colloids provide an ultrasensitive, specific, and robust immunoassay for C-reaction protein in clinical samples as promising fluorescent reporters.
A surfactant/swelling-agent pair suitable for templating a variety of well-defined large-pore nanoporous silicas was identified. The pair includes a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), PEO-PPO-PEO, block copolymer surfactant (Pluronic F127, EO106PO70EO106) with a large fraction of long hydrophilic PEO blocks and a swelling agent (toluene) that strongly solubilizes in micelles of the PEO-PPO-PEO surfactant family. Such a combination affords micellar templates for both spherical and cylindrical mesopores with potential to hinder cross-linking of micelle-templated nanostructures due to stabilization of nanoparticles by long PEO chains. Under low-temperature conditions (11–12 °C), the Pluronic F127/toluene pair affords ultralarge-pore FDU-12 (ULP-FDU-12) silica with face-centered cubic structure of spherical mesopores and related hollow nanospheres, as well as large-pore SBA-15 (LP-SBA-15) with two-dimensional hexagonal structure of cylindrical mesopores and related silica nanotubes. ULP-FDU-12 reaches the unit-cell parameter of 69 nm, which is very large. LP-SBA-15 has a unit-cell parameter up to 26 nm and pore diameter up to ∼20 nm and is exceptionally well ordered. The hollow nanospheres and nanotubes are attainable through lowering of the silica-precursor/surfactant ratio. The materials templated by spherical micelles form when the surfactant/swelling-agent solution is kept under stirring for extended periods of time before the addition of the silica precursor. The sizes of entrances to the hollow nanospheres can be continuously tuned by adjusting the hydrothermal treatment temperature. The ordered mesoporous silicas can be converted from open-pore to closed-pore materials through the thermally induced pore closing. The diversity in morphology, pore size, and pore connectivity makes the proposed surfactant/swelling-agent templating system unprecedented in the large mesopore domain.
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