Ion Current Rectification at Nanopores in Glass Membranes

TLDR

The study investigates the origin of ion current rectification in conical glass nanopores in KCl solutions via finite‑element simulations. The authors modeled ion transport by solving coupled Nernst‑Planck, Poisson, and Navier‑Stokes equations for K⁺ and Cl⁻, incorporating fixed surface charge on the glass walls to capture electric fields and spatially varying conductivity. The simulations show that rectification arises from voltage‑dependent conductivity near the pore mouth, peaks at intermediate bulk concentrations due to competing screening and ion‑number effects, and is only modestly influenced by electroosmotic flow.

Abstract

The origin of ion current rectification observed at conical-shaped nanopores in glass membranes immersed in KCl solutions has been investigated using finite-element simulations. The ion concentrations and fluxes (due to diffusion, migration, and electroosmotic convection) were determined by the simultaneous solution of the Nernst-Planck, Poisson, and Navier-Stokes equations for the two-ion (K+ and Cl-) system. Fixed surface charge on both the internal and external glass surfaces that define the pore structure was included to account for electric fields and nonuniform ion conductivity within the nanopores and electric fields in the external solution near the pore mouth. We demonstrate that previous observations of ion current rectification in conical-shaped glass nanopores are a consequence of the voltage-dependent solution conductivity in the vicinity of the pore mouth, both inside and outside of the pore. The simulations also demonstrate that current rectification is maximized at intermediate bulk ion concentrations, a combination of (i) the electrical screening of surface charge at high concentrations and (ii) a fixed number of charge-carrying ions in the pore at lower concentration, which are physical conditions where the voltage dependence of the conductivity disappears. In addition, we have quantitatively shown that electroosmotic flow gives rise to a significant but small contribution to current rectification.