Enhancement of Redox Cycling Currents at Interdigitated Electrodes with Elevated Fingers

Abstract

An ideal interdigitated array (IDA) electrode is composed of a pair of comb-type finger electrodes flush with an insulating wall. However, the fingers that compose an actual IDA are elevated from the gap plane by their thickness. The side wall of the elevated part can work as an active electrode, and increases the current for redox cycling reactions, as in a thin layer cell. Here, the elevated fingers are deliberately fabricated to increase the redox cycling currents. Our fabrication technique involves combining the carbonization of thick polyimide films with photolithography on quartz substrates. One type of fabricated electrode has an elevated height ranging from 0.01 to 4.5 μm for a 5 μm gap. The other type has a height of 0.05 μm for gaps ranging from 0.25 to 2 μm. The theoretical current is evaluated by solving numerically the Laplace equation for the redox concentration by using the finite element method. An approximate equation for diffusion controlled currents is obtained based on the concept that the current is the sum of the current at the flat IDA and that at the area of the electrode side wall. Voltammograms of ferrocene derivatives demonstrate the validity of the approximate equation for the enhancement of the current.