Direct Sol-Gel Replication without Catalyst in an Aqueous Gel System: From a Lipid Nanotube with a Single Bilayer Wall to a Uniform Silica Hollow Cylinder with an Ultrathin Wall

A combination of transmission electron microscopy (TEM) and atomic force microscopy (AFM) revealed that a secondary ammonium hydrochloride of a peptidic lipid, in which an 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 consisting of a single bilayer wall. Mixing an aqueous dispersion containing the lipid nanotubes with tetraethoxysilane (TEOS) led to slow gelation due to sol-gel condensation in the absence of solution catalyst. TEM analysis of the aqueous gel phase, coupled with electron energy-loss spectroscopy (EELS), revealed the presence of a high population of hybrid nanotube architectures with a well-defined organic\inorganic interface. Lyophilization and subsequent calcination of the aqueous gels produced silica nanotubes with uniform walls 8-nm thick. The weakly acidic and mildly catalytic lipid headgroup is responsible for the characteristic formation of the silica nanotubes. The minimal amount of positive charges on the surfaces of the lipid nanotube also contributes to this mechanism. Thus, in an aqueous gel system, the morphology of the lipid nanotube consisting of a single bilayer wall was replicated directly into a silica nanotube with an ultrathin wall.