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Bipolar electrode focusing in microchannels

A thin floating electrode attached locally to the wall of a straight microchannel results in a redistribution of local field strength after application of an external electric field. Together with bulk convection based on (cathodic) electroosmotic flow, an extended field gradient is formed in the anodic microchannel segment. It imparts a spatially dependent electrophoretic force on charged analytes and, in combination with the bulk convection, results in electric field gradient focusing at analyte-specific positions. Analyte concentration in the enriched zone approaches a maximum value which is independent of its concentration in the supplying reservoirs.

Figure 1: (A) Optical fluorescence micrograph of a microchannel with embedded Au electrode. The micrograph shows the concentration distribution of BODIPY disulfonate in 1 mM Tris–HCl buffer at pH 8.1 after applying a potential bias of 30 V for 240 s (top). Schematic illustration of the proposed mechanism of tracer accumulation in the microchannel with a bipolar electrode (middle). Maximum concentration factor in the enriched zones experimentally measured for different initial concentrations of BODIPY disulfonate in 1 mM Tris–HCl buffer at pH 8.1 with an applied potential bias of 30 V as a function of time (bottom). (B) Simulated profiles of the local axial electric field (top) and tracer concentration profiles (bottom) for t = 50, 100, 150, and 200 s. Applied field strength is 5 kV/m, initial tracer concentration is 5 µM. Profiles represent the distributions of field strength along the geometrical axis of the channel.