ELECTRIC-FIELD MEDIATED TRANSPORT IN NANOMETER DIAMETER CHANNELS

Nanoporous polycarbonate (PCTE) nuclear-track-etched membranes were us ed to effect electric field modulation of the mass transport of cation ic, anionic, and neutral species in aqueous buffer. The permeability r esponse to electric fields depended on the molecular charge and electr olyte concentration; Solute and solvent fluxes through nanopores under an electrical potential result from a balance of diffusion, electroos mosis, and ion migration The Debye length, kappa(-1), associated with the electrical double layer within the pores relative to the pore diam eter, 2a, plays a critical role in determining electrokinetic transpor t 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 t he supporting electrolyte concentration, kappa a can be tuned such tha t 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 bu ffer cations, dominates when kappa a < 1. In this case the pores are e ssentially permselective, and anion electromigration is virtually elim inated. When kappa a greater than or equal to 1, the sign of the appli ed potential can be used to select for anion vs cation/neutral molecul e transport.