Redox-magnetohydrodynamic microfluidics using multiple electrode pairs and sweep-step potential waveforms

by Haas, Elaine M.; Fritsch, Ingrid

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Microfluidics is integral for automating lab-on-a-chip (LOAC) devices, esp. if it is possible to program fluid flow. Redox-MHDs (MHD) offers unique fluid control for LOAC devices. The MHD array setup induces fluid flow while eliminating the need for channels, thus reducing the complexity of the detection system. This project addresses a complication assocd. with redox-MHD: the need for redox species. In order to minimize interference with samples and detection, a strategy to immobilize these species to the electrodes was investigated. Hexamine ruthenium chloride was tethered to the electrodes using the polymer Nafion. Characterization with cyclic voltammetry showed that the pos.-charged redox species loaded into the neg.-charged Nafion polymer (through electrostatic interactions). Because of the finite no. of redox species at modified electrodes, they deplete quickly under electrolysis conditions. A method of redox replenishment was investigated with redox species in soln. for preliminary studies (0.1 M potassium ferricyanide, 0.1 M potassium ferrocyanide in 0.1 M potassium chloride). A chip was used with two arrays of individually-addressable microelectrodes to serve as anodes and cathodes. With a multipotentiostat, different pairs of anodes and cathodes were activated with stepped potential (0 V to 0.3 V at the working electrode). At 0.220 s, one set underwent a sweep reversing its potential (in one expt., a fast sweep over 0.220 s was used, and in another case a slow sweep over 0.660 s was used). The sweeps replenished the redox species to their original state. This sequence was performed for 6 cycles. Fluid speed was twice as great with both sets during the potential step than when one set was swept back. The results are encouraging for redox species recovery. In the future, another instrument will allow programming of multiple potential waveforms out of phase, thus producing a continuous fluid flow without interference from redox species.