RESUMEN
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.
RESUMEN
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.
RESUMEN
A unique capability of redox-magnetohydrodynamics (redox-MHD) for handling liquids on a small scale was demonstrated. A 1.2 muL 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 (PAP(R)) in the presence of 2.5 mM Ru(NH(3))(6)Cl(2) and 2.5 mM Ru(NH(3))(6) Cl(3), 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 PAP(R) 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 x 15.5 mm long x 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 approximately 30 mum/s was generated in a reinforcing flow configuration between oppositely polarized sets of pumping electrodes with approximately 2.1 muA.