Inducing convection in solutions on a small scale: electrochemistry at microelectrodes embedded in permanent magnets

Magnetic field-induced convection in solutions containing electroactive species that can undergo reduction-oxidation (redox) chemistry has been demonstrated using a 125-μm-diameter platinum disk electrode embedded in Nd-Fe-B bonded magnets. Stirring near the electrode surface occurs through several convective forces: the Lorentz force (magnetohydrodynamics), the magnetic field gradient force, and the paramagnetic gradient force. Stirring was determined by comparing the mass transport-limited current before and after magnetization of the bonded material. Magnetic field effects were studied by performing cyclic voltammetry (CV) in a solution of nitrobenzene at different concentrations. The CV responses showed that with concentrations from 0.25 to 2.0 mol/L, the limiting current increased as large as 54% because of magnetic forces being parallel to and in the same direction as natural convection. Above 2.0 mol/L, the solution viscosity in the diffusion layer dominates, resulting in a decrease in current and, hence, less convection. Embedding microelectrodes in magnetic materials yields measurable enhancements of stirring of a small volume (∼1 μL), defined by the diffusion layer adjacent to the electrode, even at weak magnetic fields of ∼0.13 T.