Silica-based materials have found many applications in various fields. Alkoxysilanes have been most widely used as precursors. Fine structural control of silica-based materials has become increasingly important for tuning their properties and for developing new functions. In this perspective, utilization of alkoxysilyl groups has been reviewed from the viewpoint of designing siloxane-based nanomaterials. Alkoxy groups have generally been used only as eliminating groups in the sol−gel processing; however, recent research has shown that they are useful for molecular assembly, for generating pores, for linking nanobuiliding blocks, and for selective synthesis of new oligosiloxane compounds.
The design of siloxane-based nanoparticles is important for many applications. Here we show a novel approach to form core-shell silica nanoparticles of a few nanometers in size through the principle of "dispersion of ordered mesostructures into single nanocomponents". Self-assembled siloxane-organic hybrids derived from amphiphilic alkyl-oligosiloxanes were postsynthetically dispersed in organic solvent to yield uniform nanoparticles consisting of dense lipophilic shells and hydrophilic siloxane cores. In situ encapsulation of fluorescent dyes into the nanoparticles demonstrated their ability to function as nanocarriers.
Alkoxychlorosilanes are scientifically and industrially important toward preparing silicone and silica as well as preparation of siloxane-based nanomaterials by stepwise reactions of Si-OR (R=alkyl) and Si-Cl groups. Intermolecular exchange of alkoxy and chloro groups between alkoxysilanes and chlorosilanes (functional group exchange reaction) provides an efficient and environmentally benign route to alkoxychlorosilanes. BiCl as a Lewis acid catalyst can promote the functional group exchange reactions more efficiently than conventional acid catalysts. Higher reactivity has been observed for chlorosilanes with smaller numbers of Si-CH groups and for alkoxysilanes with larger numbers of Si-CH groups. The reaction mechanism is proposed and selective syntheses of alkoxychlorosilanes are demonstrated. These findings also enable us to synthesize an organotrialkoxysilane with four different substituents.
Mesostructured silica having mesopore surface functionalized with poly(ethylene oxide) 20 -bpoly(propylene oxide) 70 -b-poly(ethylene oxide) 20 (EO 20 PO 70 EO 20 , P123) micelles was synthesized by using triethoxysilyl-terminated P123 (TES-P123) with dual functions of templating and surface anchoring. The amount of anchored P123 was controlled by mixing TES-P123 and conventional P123 as a cotemplate and the subsequent removal of the cotemplate by extraction. All the samples show the presence of ordered mesopores after the extraction. Unexpectedly, the d-spacing and the pore size increased after the extraction when the ratio of TES-P123/(TES-P123 + P123) was over 75%, which is explained by an osmotic force caused by anchored P123 swollen with THF during the extraction. To evaluate the stability of anchored P123, ibuprofen was simultaneously incorporated into mesopores during the formation of mesostructured silica. When TES-P123 was exclusively used, the percentage of retained P123 after the extraction of IBU was higher than that found for the case of conventional P123. This approach will open a new way to stabilize assembled amphiphiles in regularly ordered siloxane frameworks.
The addition of 2,2,2-trifluoroethanol (TFE) induces both the transition from β-sheet to α-helix structure of peptides and disassembly of wormlike micelles of peptide amphiphiles. The hierarchical structural changes were utilized for the preparation of silica nanotubes from silica-micelle complexes with avoiding structural deterioration that is normally caused by thermal treatment. This method is advantageous for the preparation of silica nanotubes because of reusability of peptide templates, possible replication of the surface of β-sheet structure, and very mild conditions.
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