Modified Surfaces in Redox-Magnetohydrodynamics
Authors: Nash Christena K. , Jones Benjamin J. , Sahore Vishal , Fritsch Ingrid 
Published: February 2020

Redox-magnetohydrodynamics (MHD) has demonstrated unique control of fluid motion on a small scale. This includes a flat flow profile across macroscopic dimensions,1of importance in separations of mixtures and parallel, undistorted transport of samples for chemical analysis. Rotational flow is also possible, for tunable mixing and stirring under laminar flow conditions. These are features of great interest for lab-on-a-chip (LOAC) applications.
The MHD phenomenon is derived from the body force, FB
, which determines the direction and magnitude of the fluid flow. It is related to the ionic current density, j, in solution and the magnetic flux density, B, through the cross product: FB
= j x B. The manipulation of the j distribution within a miniaturized electrochemical cell is achieved by activating different individually-addressable microelectrodes having various geometries and locations on a chip and by setting the sign and magnitude of the electronic current. For a fixed magnetic field, as determined by a permanent NdFeB magnet placed beneath the miniaturized electrochemical cell, the fluid velocity is proportional to the magnitude of j,2 and thus, the flow is programmable.
Adding redox species to the solution that can be easily oxidized and reduced through electron transfer processes at the electrodes allows j to be produced without electrode dissolution and bubble formation from solvent electrolysis that were previously problematic.3,4 A low concentration of redox species, such as 5 mM Ru(NH3)6
3+/4+ in a buffer is enough to achieve sufficient speeds of 30 µm/s in small volumes.5 In addition, enzyme activity, important for enzyme-linked immunosorbant assays, is sustainable with this redox concentration. This composition has also been shown to be compatible with heart tissue.6 Nonetheless, it remains desirable to avoid the addition of chemical species to samples, which may have adverse chemical reactions and interfere with detection.
We have recently demonstrated that by modifying the surfaces of the electrodes with the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT),electropolymerized directly onto the electrodes can substitute for solution redox species. PEDOT produces currents that are 10 to 1000 times of those from solution redox species, and thus FB

,fluid velocities, and pressures increase dramatically, too. We are taking advantage of this discovery to explore the use of redox-MHD for a broader range of applications, such as in analytical separations and energy conversion.
We will present recent work on not only the modification of the electrode surfaces, but on the insulated ones as well, and their impact on redox-MHD microfluidics. We will discuss insights into new applications based on the results. The surfaces are derivatized with conducting polymers, non-conducting polymers, and enzymes. Characterization of these surfaces and their activity includes electrochemical impedance spectroscopy, chronoamperometry, and cyclic voltammetry. Visual evaluation is performed by optical and scanning electron microscopy.
The electrochemical cell consists of oppositely polarized, gold, parallel band microelectrodes on a chip that are as long as 2.5 cm. The electrolyte/buffer is mixed with microbeads and placed over the electrode chip. The solution is contained within a rectangular cutout of a poly(dimethylsiloxane) film that forms the sidewalls. The film’s thickness determines the height of the cell. The sidewalls are distant from the activated electrodes. A glass microscope slide, placed on top, serves as the lid. Fluid velocities are determined by tracking the microbeads by video microscopy.

Acknowledgments

Research was supported through the National Science Foundation (NSF) (CHE-0719097 and CBET-1336853) and the Arkansas Biosciences Institute.

References

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