Redox-Magnetohydrodynamic Microfluidics Without Channels and Compatible with Electrochemical Detection Under Immunoassay Conditions

A unique capability of redox-magnetohydrodynamics (redox-MHD) for handling liquids on a small scale was demonstrated. A 1.2 mu L 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(NH3)(6)Cl-2 and 2.5 mM Ru(NH3)(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 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 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 similar to 30 mu m/s was generated in a reinforcing flow configuration between oppositely polarized sets of pumping electrodes with similar to 2.1 mu A.