A unique capability of redox-magnetohydrodynamics (redox-MHD) for handling liquids on a small scale was demonstrated. A 1.2-μL solution plug was pumped from an injection site to a detector without the need for a channel to direct the flow. The redox pumping species did not interfere with enzymatic activity in a solution compatible with enzyme-linked immunoassays. Alkaline phosphatase (AP), a common enzyme label, converted p-aminophenyl phosphate (PAPP) to p-aminophenol (PAPR) in the presence of 2.5 mM Ru(NH3)6Cl2 and 2.5 mM Ru(NH3)6 Cl3, in 0.1 M Tris buffer (pH=9). A solution plug containing PAPP (no AP) was pumped through the surrounding solution containing AP (no PAPP), and the enzymatically-generated PAPR was easily detected and distinguishable electrochemically from the pumping species with square wave voltammetry down to 0.1 mM concentrations. The test device consisted of a silicon chip containing individually-addressable microband electrodes, placed on a 0.5-T NdFeB permanent magnet with the field oriented perpendicular to the chip. A 8.0-mm wide × 15.5-mm long × 1.5-mm high volume of solution was contained by a poly(dimethylsiloxane) gasket and capped with a glass slide. A steady-state fluid velocity of ~30 μm/s was generated in a reinforcing flow configuration between oppositely polarized sets of pumping electrodes with ~2.1 μA.

A new approach using electrodes modified with poly(3,4-ethylenedioxythiophene) (PEDOT) was implemented to perform redox-magnetohydrodynamics (MHD) microfluidics and eliminate the need to add redox species to solution, thus removing interferences with detection, sample, and reagents for lab-on-a-chip applications. This accomplishment not only retains the unique properties of redox-MHD pumping (i.e., programmable fluid speeds and flow patterns without the need for side walls, horizontal flat flow profiles, looping flow, no electrode corrosion, and no bubble formation), but also achieves a wider sustainable voltage range and currents that can be as much as 7+ times higher (and therefore correspondingly higher velocities) than in past studies involving unmodified electrodes and redox species in solution. PEDOT, a conducting polymer that has been shown to exhibit low cytotoxicity, was electropolymerized on microband gold electrodes (25 mm long ×103 μm wide). A cell (325 μL) with distant side walls was formed by placing a 620 μm thick poly(dimethylsiloxane), PDMS, gasket with an opening of 3.2 cm × 1.5 cm on the chip, and a glass slide lid prevented evaporation. A 0.37 T magnet under the chip generated a magnetic field perpendicular to the chip surface. The cell was filled with 0.095 M NaCl electrolyte containing 10 μm polystyrene beads to visualize and quantify fluid flow using optical video microscopy. Fluid speeds of 590 μm s(-1) were observed immediately after applying a potential step. A linear relationship between applied electronic current and fluid velocity was shown. Vertical flow profiles under applied current conditions were curved, with a weak parabolic fit.

There is a need for a microfluidic pumping technique that is simple to fabricate, yet robust, compatible with a variety of solvents, and which has easily controlled fluid flow. Redox-magnetohydrodynamics (MHD) offers these advantages. However, the presence of high concentrations of redox species, important for inducing sufficient convection at low magnetic fields for hand-held devices, can limit the use of redox-MHD pumping for analytical applications. A new method for redox-MHD pumping is investigated that takes advantage of the large amplitude of the transient portion of the faradaic current response that occurs upon stepping the potential sufficiently past the standard electrode potential, E°, of the pumping redox species at an electrode. This approach increases the velocity of the fluid for a given redox concentration. An electronic switch was implemented between the potentiostat and electrochemical cell to alternately turn on and off different electrodes along the length of the flow path to maximize this transient electronic current and, as a result, the flow speed. Velocities were determined by tracking microbeads in a solution containing electroactive potassium ferrocyanide and potassium ferricyanide, and supporting electrolyte, potassium chloride, in the presence of a magnetic field. Fluid velocities with slight pulsation were obtained with the switch that were 70% faster than the smooth velocities without the switch. This indicates that redox species concentrations can be lowered by a similar amount to achieve a given speed, thereby diminishing interference of the redox species with detection of the analyte in applications of redox-MHD microfluidics for chemical analysis.

A p aradigm shift in microfluidic pumping using redox-magnetohydrodynamics (MHD) that preserves its advantages and resolves problems that previously slowed its application in analytical chemistry (for pumping small volumes and sustaining fluid flow) is demonstrated herein. Miniaturization of chemical analysis for labon-a-chip (LOAC) devices offers portability and automation with less power, reagent and waste volumes, and analysis time. A crucial feature is the programmable manipulation of fluid within the device. MHD microfluidics Fig. 1. (a) Photograph of the microelectrode array chip showing electrode features and dimensions. Expanded images show unmodified gold and PEDOT-modified gold band electrodes as viewed under a microscope. During the AC MHD experiments, electrode set 1 (working), and sets 2 and 3 (combined auxiliary/quasi-reference) were active. Set 1 was oppositely biased from sets 2 and 3. (b) Fluid flow profile based on bead speeds in the 5600 µm gap between PEDOT-modified microband electrodes. Error bars represent ± one standard deviation. (c) Particle image velocimetry (PIV) data from which points in (b) were obtained. (d) Vectors involved in AC-MHD for synchronized electrical and magnetic fields to maintain flow in a single direction.can provide continuous pumping without channels or moving parts and can stop or reverse fluid flow without the need for valves by switching off or changing the sign of the ionic current, respectively. 1 The magnetohydrodynamic force, F B (N•m −3 ), and therefore fluid flow, is generated by the ionic current density, j (C•s −1 •m −2 ), when perpendicular to a magnetic field, B (T), and follows the right-hand rule according to the cross-product relationship, F B = j × B. [2][3][4][5] In previous studies, we have shown that electrodes modified with poly(3,4-ethylenedioxythiophene) (PEDOT) are capable of high currents while limiting the interaction with the sample, thus improving compatibility. However, use of PEDOTmodified electrodes limits redox-MHD pumping to short times because it cannot sustain the current once charge transfer in the film is completed, unlike diffusionlimited redox species in solution. The device used in this study takes advantage of the highly reversible nature of PEDOT to sustain redox-MHD over indefinitely long periods by recycling PEDOT in real time.This study used an array chip with microband and microdisk-ring electrodes (Fig. 1) and a solution confined over them with a gasket and lid. Polystyrene latex beads (10 µm) added to the electrolyte solution allowed visualization of fluid movement using video microscopy. The chip was placed on an electromagnet with a magnetic field (0.033 T RMS) perpendicular to the chip. The electrodes were modified with a conducting polymer, PEDOT, instead of redox species added to solution, to generate an ionic current from the electrochemical reaction at the films, avoiding bubble formation and electrode corrosion.Synchronized sinusoidal potential waveforms applied to PEDOTmodified electrodes (producing an AC curre...

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