Digital Simulation of the Measured Electrochemical Response of Reversible Redox Couples at Microelectrode Arrays: Consequences Arising from Closely Spaced Ultramicroelectrodes

Diffusion to arrays of closely spaced (1.2-0.2 μm) ultramicroelectrodes (50 μm x 2.3 μm) was studied by digital simulation and by examining the redox behavior of Ru(NH₃)₆³⁺ in H₂O. Cylindrical diffusion of solution species resulted in quasi-steady-state currents at the microband electrodes. Generation-collection experiments, analogous to rotating ring-disk collection experiments, resulted in larger generator currents than those observed at a single microelectrode due to the back diffusion of products to the neighboring microelectrode. A collection efficiency of 93% was observed for the reoxidation of Ru(NH₃)₆²⁺ generated at a central microelectrode 0.2 μm from two flanking collector microelectrodes. This experiment as well as generator-single collector electrode pairs was simulated at a two-dimensional rectangular expanding grid and yielded results in good agreement with the experiment. Predictions of the model that the collection efficiency principally depends on the gap size, rather than electrode width, were tested experimentally. The novel application of microelectrode arrays to the study of the follow-up reactions of electrogenerated Intermediates is demonstrated.