Unraveling Mass and Electron Transfer Kinetics at Silica Nanochannel Membrane Modified Electrodes by Scanning Electrochemical Microscopy.

An in-depth understanding of kinetic processes convoluting mass and charge transfer at nanoporous membrane modified electrodes is crucial for developing high-performance electrochemical sensors. In this work, we propose a theoretical model to unravel mass (km) and electron transfer rate (kf) from the apparent electrochemical rate constant (kapp) at silica nanoporous membrane (SNM) modified indium tin oxide (ITO) electrodes (designated as SNM/ITO for simplicity). Using scanning electrochemical microscopy (SECM), the kapp of charged redox species was first determined at the SNM/ITO in the absence and presence of surfactant micelles inside SNM. On the basis of the theory, in the presence of micelles inside SNM, km equals zero for all charged probes (Ru(NH3)62+, Ru(CN)63-, and FcMeOH+), thus the SNM behaves as an insulating barrier and the overall electrode reactivity is dominated by the permeability of SNM. After excluding micelles from SNM, the km of Ru(CN)63-/4- is strongly dependent on the KCl concentration in the solution, decreasing from 0.23/0.15 mm s-1 to almost zero upon decreasing the KCl concentration from 1.0 to 0.01 M. In contrast, km increases from 1.33 to 2.4 mm s-1 for Ru(NH3)62+ and from 0.18 to 0.33 mm s-1 for FcMeOH+, which are comparable to the electron transfer rate at the underlying ITO electrode surface (0.8 and 0.35 mm s-1). In these cases, both mass and electron transfer processes are important in determining the overall redox activity of SNM/ITO electrodes. The methodology reported in this work can provide a quantitative way of unraveling these processes and their respective contributions.