Inducing Convection in Solutions on a Small Scale: Electrochemistry at Microelectrodes Embedded in Permanent Magnets

Abstract

Magnetic field-induced convection in solutions containing electroactive species that can undergo reduction-oxidation (redox) chemistry has been demonstrated using a 125-m-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 L), defined by the diffusion layer adjacent to the electrode, even at weak magnetic fields of 0 13 T.

Modeling RedOx-based magnetohydrodynamics in three-dimensional microfluidic channels

“…The introduction of RedOx species into the liquid is a potential solution to the problems associated with the DC MHD microfluidics. [20][21][22][23][24][25][26] RedOx-based DC MHD has several benefits. For example, the electric potential applied across the electrodes can be very low ͑several mV to ϳ1 V͒, which eliminates the bubble generation problem.…”

Scanning Electrochemical Microscopy with Forced Convection Introduced by High-Precision Stirring

“…Various techniques of hydrodynamic voltammetry exploiting the increased mass transport are known. Typical examples are different kinds of rotating electrodes, − a variety of flow systems like channels, wall- and microjets , or tubes, as well as convective systems based on sonication, microwave radiation, and even magnetic fields , have been applied.…”

Validation of Electrochemical Sensor for Determination of Manganese in Drinking Water

“…Thus, microscale electrochemical assays are carried out as acquiescent, with diffusion often limiting the reaction rate. 44,45 Indeed, this can be seen in our earlier work, where an increase in deposition time always resulted in current (or charge) saturation. 23,33 In biotechnology and medical diagnostics, where surface assays such as enzyme-linked immunosorbent assay (ELISA) and immunofluorescence have become ubiquitous, sample volumes have decreased to ∼200 μL in 96-well plates and the prevalent approach to generating advection is using an orbital shaker.…”

“…Magnetic stirrers, however, cannot be used in miniaturized assays aimed at point-of-care applications where sample volumes are small, often <100 μL. Thus, microscale electrochemical assays are carried out as acquiescent, with diffusion often limiting the reaction rate. , Indeed, this can be seen in our earlier work, where an increase in deposition time always resulted in current (or charge) saturation. , In biotechnology and medical diagnostics, where surface assays such as enzyme-linked immunosorbent assay (ELISA) and immunofluorescence have become ubiquitous, sample volumes have decreased to ∼200 μL in 96-well plates and the prevalent approach to generating advection is using an orbital shaker . While shakers improve mass transport in miniature samples, and are widely accepted due to their ease of use, they are not suited for point-of-care applications due to size and power requirements.…”