Modeling Charging Current Dynamics at Microelectrodes and their Interfaces with Electrolyte and Insulators with a Focus on Microfabricated Gold Microband Electrodes on an SU-8 Substrate

The suitability of electrochemical methods for quantitative measurements at microdevices is influenced by the relatively large electrode-insulator interface-to-electrode area ratio, greatly impacting charging dynamics due to interactions among electrolyte, conductor material, and insulator layers. The resulting charging current can overwhelm the faradaic current from redox chemistry. The device studied here features a 70 μ m × 100 μ m electroactive window, hosts gold coplanar microband electrodes, and is insulated by SU-8, which serves as both overlayer and substrate. The overlayer defines the electroactive length and isolates the leads of the electrodes from the sample solution. Cyclic voltammetry in 0.10 M KCl yields an unexpected, nonlinear dependence of current on scan rate, which can be explained with two empirical approaches. The first employs an equivalent circuit model, involving leakage resistance and double-layer capacitance in parallel, to address both background processes and electrode imperfections as a function of scan rate. The second associates the enhanced current to a changing-chargeable area resulting from interface irregularities. Prior publications on alternative conductor-insulator materials are benchmarked in this study. The comparison of the materials shows that the charging dynamics for devices made with SU-8 lead to more favorable electrochemical performance than for those constructed with glass, epoxy, and silicon nitride, and under certain circumstances, polyimide.