On Rectification of Ionic Current in Nanopores.

Rectification of ionic current, a frequently observed phenomenon with asymmetric nanopores varying in geometry and/or surface charge, has been utilized for studies of microfluidic circuits, nanopore sensors, and energy conversion devices. However, the physics behind the rectification phenomenon deserves further analysis, and the involved processes need renewed organization; however, the origin is known, and numerous simulations based on the Poisson-Nernst-Planck formalism provide details of the observation. Here, we present an analytical model by identifying the causal chain connecting the key physical factors and processes leading to rectification: the charge present on the pore sidewalls causing the selectivity of ion fluxes through the pore, the selectivity inducing enrichment-depletion of ions around the pore, and the established ion concentration gradient rendering the electric field redistribution in the pore. Our analytical model that considers nanopore geometry, surface charge density, and electrolyte concentration calculates the ionic current and corresponding rectification factor at given bias voltages. The model is validated by numerical simulations, and the model results agree well with experimental data. It is, therefore, a useful tool not only for gaining physical insights into ionic current rectification but also for providing practical guidelines in designing nanopore- and nanopipette-based ion sensors for a range of applications.