Richard Crooks and colleagues, researchers at the University of Texas, Austin developed a way to locally concentrate and move analytes using internal bipolar electrodes (bypassing the need for an outside driver of fluid flow). Electrodes printed on the bottom of the microfluidic channels create controllable gates which balance the convective and electrokinetic forces acting on charged sample molecules. A single DC power supply and controller box is needed to open/close these gates to deliver analytes to different regions of the chip.
Crooks and his group have extensively investigated bipolar electrochemistry theory and in this paper demonstrate the use of bipolar electrodes to separate, enrich, and transport bands of analytes in microfluidic channels. Electric potentials applied across a channel induce an electric field within the buffer whilst conductive substrates present on the floor of the microchannel also adopt a potential between their two poles. H+ ions within the buffer are partially neutralized by electrogenerated OH– and thus regions of ion depletion appear. These depletion zones attract charged analytes from the solution to maintain the charge gradient induced by the electric field (the speed of electromigration of analytes to the depletion zone is proportional to the electric field). Bipolar electrodes on the channel bottom contain their own local field and so analytes concentrate in these areas, leading to enrichment near the electrodes.
In this work, Crooks and his team demonstrate separation and enrichment of two common fluorescent dyes: BODIPY2- and MPTS3-. The two dye bands are then directed to two separate reservoirs. In previous papers, the group focused on optimizing enrichment and achieved enrichment rates of up to 0.57 BODIPY2 1, . The current extension and integration of online separation and enrichment achieves comparable rates of enrichment, 0.11 and 0.31 fold/second for BODIPY and MPTS, respectively, while also enabling control over separating analytes of different electromobility (μep) and transporting these bands to designated areas of the device.
To create the devices presented, the group used conventional photolithography techniques to pattern gold bipolar passive electrodes (BPEs) on glass and bonded PDMS channels on top of the regions. This method can be easily multiplexed as additional BPEs can be activated to guide separated and enriched analytes to different areas of the chip.
1 R. K. Anand, E. Sheridan, D. Hlushkou, U. Tallarek and R. M. Crooks, Lab on a Chip, 2011, 11, 518-527.
Electrochemically-gated delivery of analyte bands in microfluidic devices using bipolar electrodes
Karen Scida, Eoin Sheridan and Richard M. Crooks, Lab Chip, 2013, 13, 2292-2299.