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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 × 100μm. These arrays can function inμL-volumes for chemical analysis (e.g., in-vivo dopamine detection using probes). Experiments were conducted with an array of five electrodes (NE= 5),w= 4.3μm,wgap= 3.7μm,h= 0.150μm, andl= 99.2μm. 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 aswgapdecreases and diffusional flux outpaces reaction rates. Simulations aid investigations of various array designs, achievable through conventional photolithography, by decreasingwandwgapand increasingNEto fit within the electroactive window. A coplanar array,NE= 58,w=wgap= 0.6μm,h= 0.150μm andl= 100μm, yielded ferricyanide sensitivities of 0.266, 0.259 nA·μM−1, enhancements of 8 × and 9 × overw=wgap= 4μm, and projected dopamine lower limits of quantitation of 139 nM, 171 nM at generator and collector electrodes, respectively.
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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 70μm × 100μm 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.
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- Award ID(s):
- 1808286
- PAR ID:
- 10519727
- Publisher / Repository:
- The Electrochemical Society
- Date Published:
- Journal Name:
- Journal of The Electrochemical Society
- Volume:
- 171
- Issue:
- 6
- ISSN:
- 0013-4651
- Format(s):
- Medium: X Size: Article No. 066511
- Size(s):
- Article No. 066511
- Sponsoring Org:
- National Science Foundation
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