Toward On-Chip Processes using Redox- Magnetohydrodynamic Microfluidics without Channels Vishal Sahore, Christena K. Nash, Melissa C. Weston, Matthew D. Gerner, and Ingrid Fritsch University of Arkansas Department of Chemistry and Biochemistry Fayetteville, AR 72701 Controlling localized motion of small volumes of fluid with redox-magnetohydrodynamics (redox-MHD) will be discussed, with an emphasis on performing multiple steps for chemical analysis processes on a chip. Redox-MHD has the unique capability to do this without the need for traditional channels that physically confine and direct fluid flow on the small scale. The phenomenon of MHD is well-known in fields distant from electrochemistry. This includes plasma physics and metallurgy, where a highly conductive fluid (ionized gases and liquid metals, respectively) moving in a magnetic field produces a current, which then further affects the movement and shape of the fluid. The MHD body force, F B , which determines the direction and magnitude of the fluid flow, is best described through the right-hand rule as the cross product of the charge or ion flux, j, with the magnetic flux density, B: F B = j × B. Liquid solutions (aqueous and non-aqueous), which are much more resistive than ionized gases and liquid metals, exhibit subtler MHD effects because of the lower magnitude of j. The addition of a chemical (redox) species to an electrolyte solution that can be easily oxidized and reduced through electron transfer processes at electrodes allows current to pass without electrode dissolution and bubble formation from solvent electrolysis, unlike in early attempts to use MHD as a microfluidic pump. 1, 2 This redox-MHD approach has now been explored for microfluidics on a chip. 3-5 Redox-MHD on a chip containing individually- addressable microband electrodes in the presence of a permanent magnet placed beneath the chip has demonstrated several interesting microfluidic capabilities. 4 Localized flow can be started, stopped, and easily reversed simply by turning on, off, and reversing the polarity of the electrodes, respectively. The fluid can be pumped in separate or larger single loops depending on the distance, size, and current at the electrodes. Oppositely polarized electrodes reinforce the flow, increasing the speed for the same given current. In addition, a flat flow profile is achievable, which is of great interest in performing efficient chemical separations. The fluid speed for a given cell geometry is proportional to the applied current, allowing fine-tuning of flow simply by tuning the electrode current. 6 A concern about redox-MHD had been the presence of high concentrations of redox species. Originally, such high concentrations were thought necessary for the solution to exhibit MHD in low B-fields generated by small permanent magnets that would be of interested in hand-held devices. This is because evidence of MHD convection was often followed by measuring a change in current under applied potential conditions, 7-10 thus requiring high currents from large redox concentrations. However, with the ability to monitor fluid flow by tracking microbeads with video microscopy, redox concentrations as low as 5 mM can be shown to be enough to achieve sufficient speeds of 30 μm/s in the small volumes on a chip. 11 This concentration does not significantly affect the enzymatic reaction of alkaline phosphatase (AP) with the substrate p-aminophenyl phosphate (PAPP). This is important because AP is an enzyme label used in enzyme-linked immunosorbant assays (ELISAs), suggesting that redox-MHD should be compatible with immunoassay analysis. The low redox concentration allows for not only pumping, but also simultaneous electrochemical detection of the species p- aminophenol that is enzymatically generated from the PAPP. This has been performed under circumstances where a fluid segment or plug was directed to the electrochemical detector with redox-MHD pumping alone, without channel sidewalls. These outcomes suggest that redox-MHD could be used to perform the microfluidics in carrying out the multiple steps of an ELISA on a chip and without the need for channels. A chip without channels would be easier to fabricate and could be reprogrammed for different applications, having different microfluidic needs. In our presentation, we will include new studies that take these concepts to the next level. Extremely straight fluid flow will be shown between microband electrodes across much longer stretches, > 2 cm. Fluid manipulation using other electrode geometries, including concentric disk and ring electrodes, will illustrate different approaches to stir and trap small segments of solution. We will also provide updates on new strategies to insure compatibility of the pumping method with the application. We will discuss how these results can lead to the design of chips based on the right-hand rule to achieve complete analysis processes. It is hoped that the high level of complexity of this topic is well-suited for initiating the kind of intense scientific discussions that W. Ves Childs would have enjoyed. We present our research to honor his memory and with thanks to him for stimulating intellectual excitement on many other topics in the past. ACKNOWLEDGMENTS Research was supported through the National Science Foundation (NSF) (CHE-0719097) and the Arkansas Biosciences Institute. REFERENCES (1) Jang, J.; Lee, S. S. Sensors and Actuators A 2000, 80, 84-89. (2) Lemoff, A. V.; Lee, A. P. Sensors and Actuators B 2000, 63, 178-185. (3) Aguilar, Z. P.; Arumugam, P. U.; Fritsch, I. J. Electroanal. Chem. 2006, 591, 201-209. (4) Anderson, E. C.; Weston, M. C.; Fritsch, I. Anal. Chem. 2010, 82, 2643-2651. (5) Arumugam, P. U.; Fakunle, E. S.; Anderson, E. C.; Evans, S. R.; King, K. G.; Aguilar, Z. P.; Carter, C. S.; Fritsch, I. J. Electrochem. Soc. 2006, E185- E194. (6) Weston, M. C.; Fritsch, I. Sens. Actuat. B 2010, submitted. (7) Ragsdale, S. R.; Lee, J.; Gao, X.; White, H. S. J. Phys. Chem. 1996, 100, 5913-5922. (8) Ragsdale, S. R.; Lee, J.; White, H. S. Anal. Chem. 1997, 69, 2070-2076. (9) Leventis, N.; Chen, M.; Gao, X.; Canalas, M.; Zhang, P. J. Phys. Chem. B 1998, 102, 3512-3522. (10) Leventis, N.; Gao, X. J. Phys. Chem. B 1999, 103, 5832-5840. (11) Weston, M. C.; Nash, C. K.; Fritsch, I. Anal. Chem. 2010, 82, 7068-7072. Abstract #1659, 219th ECS Meeting, © 2011 The Electrochemical Society ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.187.191.249 Downloaded on 2014-11-17 to IP View publication stats View publication stats