Electric field mediated transport in nanometer diameter channels

Nanoporous polycarbonate (PCTE) nuclear-track-etched membranes were used to effect electric field modulation of the mass transport of cationic, anionic, and neutral species in aqueous buffer. The permeability response to electric fields depended on the molecular charge and electrolyte concentration; Solute and solvent fluxes through nanopores under an electrical potential result from a balance of diffusion, electroosmosis, and ion migration The Debye length, kappa(-1), associated with the electrical double layer within the pores relative to the pore diameter, 2a, plays a critical role in determining electrokinetic transport behavior. The channel walls adsorb anions preferentially to produce a largely immobile negative charge density, leaving a large and mobile cation population to mediate transport in the channel. By adjusting the supporting electrolyte concentration, kappa a can be tuned such that the electrical double layer is either small in relation to the pore (kappa a greater than or equal to 1) or more diffuse and spanning the pore (kappa a < 1). Electroosmotic transport, mediated primarily by buffer cations, dominates when kappa a < 1. In this case the pores are essentially permselective, and anion electromigration is virtually eliminated. When kappa a greater than or equal to 1, the sign of the applied potential can be used to select for anion vs cation/neutral molecule transport.