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 and conductor/insulator materials. The resulting charging current can overwhelm the current from redox chemistry. The device studied here features a 70-µm×100-µm 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 unexpected, nonlinear dependence of current on scan rate, which can be explained with two empirical approaches. The first employs an equivalent circuit, 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 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 and Tefzel.