Ionic Strength Gated Redox Current Rectification by Ferrocene Grafted in Silica Nanochannels.

Abstract

In this work, a silica nanochannel membrane (SNM) consisting of a high density of vertically aligned channels on an indium tin oxide (ITO) electrode surface was used as a stencil to confine the grafting of ferrocene (Fc) via electrochemical reduction of diazonium cations. The grafting selectively occurs on the underlying ITO surface (namely, the bottom of silica nanochannels) instead of on the nanochannel walls. Thus, the obtained electrode, designated as Fc@SNM/ITO, is composed of an array of silica nanochannels with redox activity on the bottom. Immobilized Fc molecules are able to rectify the electron transfer between the underlying ITO electrode and soluble redox species. In other words, they can promote the electron transfer in one direction while suppressing that in the opposite direction. Anodic and cathodic redox rectification was observed for Fe(CN)64- and IrCl62-, respectively, with the magnitude of current rectification directly proportional to the concentration of redox species. Moreover, because of strong permselectivity of silica nanochannels arising from their ultrasmall size (2-3 nm in diameter) and negatively charged surface (due to deprotonation of surface silanol groups), the access of Fe(CN)64- and IrCl62- to nanochannels strongly depends on the ionic strength of electrolyte solution. A higher ionic strength favors the access of anionic redox species, thus leading to a larger redox current rectification. The redox current rectification phenomenon not only proves the permselective nature of silica nanochannels but also holds potential for the development of electrochemical current rectification-based sensors.