Starch-Directed Synthesis of Worm-Shaped Silica Microtubes

Many strategies have been adopted to prepare silica materials with highly controlled structures, typically using sol-gel chemistry. Frequently, the alkoxysilanes used in sol-gel chemistry are based on monoalcohols, e.g., Si(OEt)₄. The structural control over silica synthesis achieved by these precursors is highly sensitive to pH and solvency. Alkoxysilanes derived from the sugar alcohol glycerol (diglycerylsilane) react more slowly and with much less sensitivity to pH. We report that, in the presence of cooled aqueous starch solutions, glyceroxysilanes undergo transesterification with the sugars on starch, leading to (hollow) microtubules resembling worms of about 400 nm in diameter. The tubes arise from the pre-assembly of starch bundles, which occurs only well below room temperature. It is straightforward to treat the first-formed starch/silica composite with the enzyme amylase to, in a programmed fashion, increasingly expose porosity, including the worm morphology, while washing away untethered silica and digested starch to leave an open, highly porous materials. Sintering at 600 °C completely removes the starch silane moieties.