In Situ and 2D and 3D in Silico Redox Cycling Studies for Design Optimization of Coplanar Arrays of Microband Electrodes in a 70 µm × 100 µm Electroactive Footprint

Abstract

Optimization of redox-cycling currents was performed by adjusting the height (sidewalls, h), width (w) ,and length (l) of band electrodes and their spacing (wgap) in coplanar arrays restricted to a small-electroactive window of 70 × 100 µm. These arrays can function in µL-volumes for chemical analysis (e.g., in vivo dopamine detection using probes). Experiments were conducted with an array of five electrodes (NE = 5), w = 4.3 µm, wgap = 3.7 µm, h = 0.150 µm, and l = 99.2 µm. Reasons for disparities between currents from experiments and approximate equations were determined by high-density mesh simulations and were found to arise from sluggish heterogeneous electron transfer kinetics and diffusion at electrode ends, edges, and heights. Ferricyanide, with its moderately slow kinetics, exhibits redox-cycling currents that fall below predictions by the equations as wgap decreases and diffusional flux outpaces reaction rates. Simulations aid investigations of various array designs, achievable through conventional photolithography, by decreasing w and wgap and increasing NE to fit within the electroactive window. A coplanar array, NE = 58, w = wgap = 0.6 µm, h = 0.150 µm and l = 100 µm, yielded ferricyanide sensitivities of 0.266, 0.259 nA·µM−1, enhancements of 8× and 9× over w = wgap = 4 µm, and projected dopamine limits of quantification of 139 nM, 171 nM at generator and collector electrodes, respectively