Regulation of Silica Nanotube Diameters: Sol−Gel Transcription Using Solvent-Sensitive Morphological Change of Peptidic Lipid Nanotubes as Templates

A secondary ammonium hydrochloride of the peptidic lipid, in which the l -prolyl- l -prolyl- l -proline fragment is coupled with an l -glutamate derivative carrying two long alkyl chains, self-assembles in water to form nanotube structures and acts as a template to transcribe them to silica nanotubes through sol−gel reaction. When the peptidic lipids self-assemble in mixtures of ethanol and water, the resultant self-assembled structures showed notable morphological changes from tubular structures with different diameters to spherical ones. Using the obtained self-assembled structures as templates in different ethanol/water mixtures, we carried out the sol−gel reaction of tetraethoxysilane (TEOS). The obtained silica structures gave tubular structures with different diameters of 80, 50, and 30 nm and hollow spheres, just corresponding to the morphological change in the self-assembled templates. We thus controlled the diameters and dimensions of the silica nanotubes, which depend on the ethanol volume fractions. In addition, we have also performed in situ control of silica tubular morphologies by the addition of ethanol in a silica/lipid xerogel state, producing a unique tube-in-tube structure.