title: Ion current rectification, limiting and overlimiting conductances in nanopores.
abstract: Previous reports on Poisson-Nernst-Planck (PNP) simulations of solid-state nanopores have focused on steady state behaviour under simplified boundary conditions. These are Neumann boundary conditions for the voltage at the pore walls, and in some cases also Donnan equilibrium boundary conditions for concentrations and voltages at both entrances of the nanopore. In this paper, we report time-dependent and steady state PNP simulations under less restrictive boundary conditions, including Neumann boundary conditions applied throughout the membrane relatively far away from the nanopore. We simulated ion currents through cylindrical and conical nanopores with several surface charge configurations, studying the spatial and temporal dependence of the currents contributed by each ion species. This revealed that, due to slow co-diffusion of oppositely charged ions, steady state is generally not reached in simulations or in practice. Furthermore, it is shown that ion concentration polarization is responsible for the observed limiting conductances and ion current rectification in nanopores with asymmetric surface charges or shapes. Hence, after more than a decade of collective research attempting to understand the nature of ion current rectification in solid-state nanopores, a relatively intuitive model is retrieved. Moreover, we measured and simulated current-voltage characteristics of rectifying silicon nitride nanopores presenting overlimiting conductances. The similarity between measurement and simulation shows that overlimiting conductances can result from the increased conductance of the electric double-layer at the membrane surface at the depletion side due to voltage-induced polarization charges. The MATLAB source code of the simulation software is available via the website http://micr.vub.ac.be.
authors.affiliation: Research group of Microbiology, Vrije Universiteit Brussel, Brussels, Belgium; IMEC, Leuven, Belgium.
authors.forename: Liesbeth
authors.initials: L
authors.lastname: van Oeffelen
authors.affiliation: IMEC, Leuven, Belgium.
authors.forename: Willem
authors.initials: W
authors.lastname: Van Roy
authors.affiliation: Electron Microscopy for Materials Science (EMAT), Department of Physics, University of Antwerp, Antwerp, Belgium; Institute of Mechanics, Materials and Civil Engineering, Université catholique de Louvain, Louvain-la-Neuve, Belgium.
authors.forename: Hosni
authors.initials: H
authors.lastname: Idrissi
authors.affiliation: Research group of Microbiology, Vrije Universiteit Brussel, Brussels, Belgium.
authors.forename: Daniel
authors.initials: D
authors.lastname: Charlier
authors.affiliation: IMEC, Leuven, Belgium; Department of Physics, Katholieke Universiteit Leuven, Leuven, Belgium.
authors.forename: Liesbet
authors.initials: L
authors.lastname: Lagae
authors.affiliation: IMEC, Leuven, Belgium; Department of Physics, Katholieke Universiteit Leuven, Leuven, Belgium.
authors.forename: Gustaaf
authors.initials: G
authors.lastname: Borghs
mesh_terms: D003198:Computer Simulation; D017136:Ion Transport / Q000502:physiology; D058608:Nanopores