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 (w gap) 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 (N E = 5), w = 4.3 μm, w gap = 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 w gap decreases and diffusional flux outpaces reaction rates. Simulations aid investigations of various array designs, achievable through conventional photolithography, by decreasing w and w gap and increasing N E to fit within the electroactive window. A coplanar array, N E = 58, w = w gap = 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 = w gap = 4 μm, and projected dopamine lower limits of quantitation of 139 nM, 171 nM at generator and collector electrodes, respectively.