Transient effects on microchannel electrokinetic filtering with an ion-permselective membrane.

The electrokinetics and hydrodynamics in a hybrid microfluidic/nanofluidic pore network configuration and its effect on the concentration enrichment of charged analytes are described. A hydrogel microplug, photopolymerized in a microfluidic channel, with negative surface charge serves as a nanoporous membrane and dictates the electrokinetic behavior within the adjoining microchannel compartments. The nanoporous hydrogel with a mean pore size on the order of the electrical double layer thickness imparts ion-permselectivity (cation-selectivity) to the migration of ionic species which, under the influence of an applied electrical field, drives concentration polarization in bulk solution near the interfaces between the two microchannel compartments and the hydrogel-based nanopores. The concentration enrichment efficiency for charged analytes depends on this concentration polarization, which strongly affects the distribution of local electrical field strength. In addition, electroosmotic flow in the device plays a critical role in determining the location of the analyte enrichment zone. A theoretical model and simulations are presented to explain the interplay of concentration polarization and electroosmotic flow with respect to the observed concentration enrichment of negatively charged analytes at the cathodic hydrogel plug-microchannel solution interface.