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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. 

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 × 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 (N_E= 5),w= 4.3μm,w_gap= 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 asw_gapdecreases and diffusional flux outpaces reaction rates. Simulations aid investigations of various array designs, achievable through conventional photolithography, by decreasingwandw_gapand increasingN_Eto fit within the electroactive window. A coplanar array,N_E= 58,w=w_gap= 0.6μm,h= 0.150μm andl= 100μm, yielded ferricyanide sensitivities of 0.266, 0.259 nA·μM⁻¹, enhancements of 8 × and 9 × overw=w_gap= 4μm, and projected dopamine lower limits of quantitation of 139 nM, 171 nM at generator and collector electrodes, respectively. 

Single particle electrochemical oxidation of polyvinylpyrrolidone-capped silver nanoparticles at a microdisk electrode is investigated as a function of particle shape (spheres, cubes, and plates) in potassium nitrate and potassium hydroxide solutions. In potassium nitrate, extreme anodic potentials (≥1500 mV vs Ag/AgCl (3 M KCl)) are necessary to achieve oxidation, while lower anodic potentials are required in potassium hydroxide (≥900 mV vs Ag/AgCl (saturated KCl)). Upon oxidation, silver oxide is formed, readily catalyzing water oxidation, producing a spike-step current response. The spike duration for each particle is used to probe effects of particle shape on the oxidation mechanism, and is substantially shorter in nitrate solution at the large overpotentials than in hydroxide solution. The integration of current spikes indicates oxidation to a mixed-valence complex. In both electrolytes, the rate of silver oxidation strongly depends on silver content of the nanoparticles, rather than the shape-dependent variable–surface area. The step height, which reflects rate of water oxidation, also tracks the silver content more so than shape. The reactivity of less-protected citrate-capped particles toward silver oxidation is also compared with that of the polymer-capped particles under these anodic conditions in the nitrate and hydroxide solutions. 

Magnetohydrodynamics (MHD) is a unique approach for pumping fluids on a microscale and is highly suitable for enabling multiple functions for chemical analysis on a chip. An ionic current, j , is established in the fluid between selectively-activated electrodes in the presence of a magnetic field, B , that is perpendicular to the current, to generate a force, F B , orthogonal to j and B , through the right hand rule. F B is a body force that propels the liquid in the same direction through momentum transfer. We use microelectrodes, which are patterned into different, individually-addressable geometries on chips. Those electrodes are modified with poly(3,4-ethylenedioxythiophene), PEDOT, a conducting polymer, that converts the applied electronic current in the external circuit to ionic current in the fluid [1]. A small NdFeB permanent magnet is placed under the chip to provide B . By strategic activation of the electrodes, fluid flow can be programmable. For example, we previously demonstrated that MHD can start, stop, reverse, adjust speed, and alter profiles of the fluid flow. We have also shown recently that MHD fluid flow can be diverted in a contactless way by magnetic field gradients when paramagnetic species are present [2]. In our presentation, we will discuss how MHD can control the paths of individual microvolumes of different fluids for mixing, sampling, and injection. We will describe the conditions that lead to and the resulting flow profiles that result from adjacent counter flows, transverse paths, and different solvent compositions. Acknowledgements: We are grateful for financial support from the National Science Foundation (CMI-1808286) and Arkansas Bioscience Institute, the major research component of the Arkansas Tobacco Settlement Proceeds Act of 2000. References [1] Khan, F. Z.; Fritsch, I. “Chip-Scale Electrodeposition and Analysis of Poly(3,4-ethylenedioxythiophene) (PEDOT) Films for Enhanced and Sustained Microfluidics Using DC-Redox-Magnetohydrodynamics”, Journal of The Electrochemical Society 2019 , 166 (13), H615-H627. [2] Hähnel, V.; Khan, F. Z.; Mutschke, G.; Cierpka, C.; Uhlemann, M.; Fritsch, I. “Combining magnetic forces for contactless manipulation of fluids in microelectrode-microfluidic systems:, Scientific Reports 2019 , 9:5103. 

Electrodeposited conductive copolymer films with predictable relative properties (quantities of functional groups for further modification and capacitance) are of interest in sensors, organic electronic materials and energy applications. Potentiodynamic copolymerization of films in aqueous solutions of two different thiophene derivatives, (2,3-dihydrothieno[3,4-b]dioxin-2-yl)methanol (1) and 4-((2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)-methoxy)-4-oxobutanoic acid (2), containing 0.02 M total monomer (0, 25, 34, 50, 66, 75, 100 mol%2), 0.05 M sodium dodecyl sulfate, and 0.1 M LiClO₄, on gold microelectrodes in an array was investigated. Decreasing monomer deposited (m)from 0 to 100 mol%2is attributed to a decreasing pH that inhibits electropolymerization. Molar ratios of1and2in the films, determined by micro-attenuated total reflectance Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, tracks closely with the ratio in the deposition solutions. Capacitances measured from cyclic voltammetry in aqueous buffer and electron transfer of ferrocyanide at the films are unaffected by copolymer composition, except for the 100 mol%2case. Ratios of reverse-to-forward faradaic peak currents suggest that films with high content of1expand in the anodic form and contract in the cathodic form and vice versa for films with high content of2, where anions and cations dominate counterion transport from solution, respectively.