Sugar-modified silanes: precursors for silica monoliths

Abstract

Sugarsilanes, alkoxysilanes derived from sugars and sugar alcohols including glycerol, sorbitol, maltose and dextran, were hydrolyzed to prepare monolithic, mesoporous silicas. Unlike conventional alkoxysilanes such as tetramethylorthosilicate (TMOS) and tetraethylorthosilicate (TEOS), the sol–gel hydrolysis and cure rates of sugarsilanes were very sensitive to ionic strength, but not to pH: comparable rates of gelation were observed for any specific compound at constant ionic strength over a pH range of about 5.5–11. Reduced levels of shrinkage when compared to TEOS (65% for diglycerylsilane (DGS)-derived silica; 50% for monosorbitylsilane (MSS)-derived silica) were also observed provided that the residual sugars were not washed or pyrolyzed from the silica monolith. Pore sizes in the dried silica monoliths (∼2–3 nm diameter) were marginally increased by the addition of non-functional polyethylene oxide (PEO) (mesopore sizes: no PEO, 3.1 nm; 4 wt% PEO MW 2000, 10000, 3.3 and 3.5 nm, respectively): the protein Human Serum Albumin did not act as a porogen. PEO terminated with Si(OEt)₃ groups (TES-PEO), however, was very efficient at increasing mesopore size (TES-PEO MW 200 and 10000, led to pores of average diameter 3.7 and 6.1 nm, respectively). The addition of a multivalent metal such as Mg²⁺ to the sol increased the pore sizes of glycerol silane-derived silica, but led to decreased sizes in silica prepared from TEOS. These changes in cure chemistry and final properties are attributed to a distortion of the silica cure equilibrium by the multidentate sugar ligands.