Bipolar electrode focusing: tuning the electric field gradient.

Bipolar electrode (BPE) focusing is a developing technique for enrichment and separation of charged analytes in a microfluidic channel. The technique employs a bipolar electrode that initiates faradaic processes that subsequently lead to formation of an ion depletion zone. The electric field gradient resulting from this depletion zone focuses ions on the basis of their individual electrophoretic mobilities. The nature of the gradient is of primary importance to the performance of the technique. Here, we report dynamic measurements of the electric field gradient showing that it is stable over time and that its axial position in the microchannel is directly correlated to the location of an enriched tracer band. The position of the gradient can be tuned with pressure-driven flow. We also show that a steeper electric field gradient decreases the breadth of the enriched tracer band and therefore enhances the enrichment process. The slope of the gradient can be tuned by altering the buffer concentration: higher concentrations result in a steeper gradient. Coating the channel with the neutral block co-polymer Pluronic also results in enhanced enrichment.