PH-modulated ion transport and amplified redox response of Keggin-type polyoxometalates through vertically-oriented mesoporous silica nanochannels

Abstract

Cyclic voltammetry has been applied to characterize mass transport of a Keggin-type polyoxometalates ([SiW12O40]4-) through vertically-aligned mesoporous silica thin films in aqueous media. The results indicate a drastic influence of pH and the ionic strength which may be rationalized in terms of electrostatic interactions and confinement effects. Diffusion of such bulky anionic electroactive species is considerably restricted at pH values higher than the point of zero charge of the silica for which the walls of the nanochannels are negatively charged. On the other hand, the positive charge density generated in more acidic solutions favors the ingress of [SiW12O40]4- species through the mesochannels, enhancing considerably the electrochemical response, except at high ionic strength due to competition for the binding sites. The accumulation of POM anions into the silica films can be even promoted by functionalization of the silica surface with propylammonium groups. In summary, pH tuning induced gating of the ionic flux through the silica nanopores, which can be somewhat modulated by the ionic strength by varying the thickness of the electrical double layer generated at the silica walls surface.