Quantitative analysis of the steady-state currents of reversible redox species at a microdisk array electrode embedded in a surface electrode

A new type of microdisk array electrode was developed. The microdisk array electrode was embedded in a plane surface electrode, and separated by a thin insulating step. The electrode was fabricated by conventional photolithography and dry-etching techniques, and a step height of 0.3 μm was realized. This electrode shows larger steady-state limiting currents of the redox species than those of an ordinary microdisk array electrode, because a steep concentration gradient between the microdisk and the surface electrode is established. The concentration profile and the limiting current of the electrodes were analysed by solving the boundary problem of a three-dimensional Laplace equation for diffusion using the toroidal coordinate. The equi-concentration surfaces of the new electrode are oblate hemispheroidal-like, but their edges are pinned at the edge of the disk electrode and have a compact diffusion zone unlike those of an ordinary microdisk array electrode. Our theory, which takes into account the step height effect using one empirical constant, could explain the dependence of the experimental limiting current on our disk electrode.