Free-standing silica colloidal nanoporous membranes.

We prepared robust free-standing 200 microm-thick colloidal membranes (nanofrits) with a relatively large area and no mechanical defects by sintering silica colloidal films. The silica spheres used to prepare the nanofrits were 338, 300, or 251 nm in diameter, leading to 25, 22.5, and 19 nm nanopore sizes, respectively. The room-temperature diffusional flux through these membranes is of the order of 3.6 x 10(-10) mol s(-1) cm(-2) for a Fe(bpy)32+ ion in acetonitrile test solution in the absence of applied pressure and is in good agreement with the calculated diffusional flux for colloidal crystals of the same thickness. To evaluate the feasibility of nanofrit surface modification, we treated them with 3-aminopropyltriethoxysilane after rehydroxylation. We found, by measuring the surface coverage for dansyl amide on the surface, that the number of the amines on the nanofrit surface is lower as compared to that observed for colloidal films not treated with heat. As a result, the selectivity for the transport of Fe(bpy)3(2+) through the aminated nanofrits in the presence of acid is lower than the selectivity observed for amine-modified colloidal films.