Studies Toward Lab-on-a-Chip Separations and Detection Using Redox Magnetohydrodynamic Microfluidics

Low cost, miniaturized, simple, and rapid isolation and detection of target chemicals and biological analytes from a complex mixture is of great interest to the point-of-care (POC) diagnostics community. On-chip redox- magnetohydrodynamics (MHD) microfluidics is being studied for complex chemical and biological mixture separations as a promising lab-on-a-chip application. Redox MHD is a particularly attractive pumping technique for this purpose because it does not rely on an external pump and, more importantly for separations, produces a flat flow profile within a small fluid volume.¹

In Redox-MHD, a force (F_MHD) acts on ions with ionic current density of j, perpendicular to the magnetic field (B). The resultant force acts on the surrounding fluid causing the fluid to flow according to right hand rule F_MHD = j x B. Fluid flow may also be programmed by activating different electrodes of a microfabricated chip resting on a magnet and both permanent magnets and electromagnets can be used, allowing for wide flexibility and tunability in application design.^2,3  Previously, studies used a mixture of redox species, K₃Fe(CN)₆ and K₄Fe (CN)₆, producing j without any bubble formation from solvent electrolysis and achieving very flat flow profiles.¹ More recently, redox species in solution were replaced with the conducting polymer poly (3, 4 ethylenedioxythiophene) (PEDOT) directly deposited on the electrodes. These, too, produced a flat flow profile and, in addition, produced higher fluid velocities since they generate about 10 to 1000 times more current.⁴

To apply these chips for separation purposes, we have modified a glass lid and chip surface between the pumping and detecting electrode with acrylic polymer (silicone acrylate and butyl acrylate) to serve as the stationary phase.⁵ Long wavelength (366 nm) UV polymerization of the acrylate monomer solution directed the polymerization between these electrodes. Here, we will report on our results addressing the effect of stationary and the mobile phase on fluid control, speed, and flow profile, as well as considerations for optimizing pumping parameters for efficient separation. We will also report on detection methods to quantify the separation of model analytes. One such method involves the use of fluorescence microscopy and single photon counting detection to directly follow the flow and separation of fluorescent analytes inside these microfluidic devices.⁶

Acknowledgments

Research was supported partially through the National Science Foundation (CBET-1336853) and the Arkansas Biosciences Institute, the major research component of the Arkansas Tobacco Settlement Proceeds Act of 2000.

References

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2. Qian, S.; Bau, H. H.  Mech. Res. Commun. 2009, 36, 10-21.

3. Weston, M. C.; Gerner, M. D.; Fritsch, I. Anal. Chem. 2010, 82, 3411-3418

4. Nash, C. K. Modified-Electrodes for Redox-Magnetohydrodynamic (MHD) Pumping for Microfluidic Applications. Ph.D. Dissertation, University of Arkansas, Fayetteville, AR, 2014

5. Stewart, R.; Kraak, J.C.; Poppe, H.; J. Chromatogr. A. 1994, 670, 25-38.

6. Gao, F.; Kreidermacher, A.; Fritsch, I.; Heyes, C.D.  Anal. Chem. 2013, 85, 4414-4422.