Ion-Permselective pH-Switchable Mesoporous Silica Thin Layers

Abstract

Organized mesoporous layers of SiO2 with narrow pore size distribution centered at ca. 7 nm, which were uniformly functionalized with readily protonated amino groups, exhibit excellent permselective membrane properties. Their selectivity and permeability can be reversibly controlled by changing pH. The flux of hexacyanoferrate Fe(CN)63- anions, hexaamino-ruthenium Ru(NH3)63+ cations, and neutral ferrocene methanol molecules through the amino functionalized silica layers was monitored by cyclic voltammetry at different solution pH values. Depending on their charges, the flux of ions can be completely blocked or enhanced, while the permeation of uncharged species is affected adversely only in a limited extent by the change in pH. The high membrane porosity ensures fast penetration rates of the analytes and much faster response to the changes in pH compared to those of typical compact organic polymer layers. As the amino functionalized mesoporous silica layers enable the efficient separation of differently charged species from complex mixtures, they are attractive for applications in biology and medicine, where such mixtures are often encountered, e.g., for the separation of blood, urine, or sera components.