Mechanism of electrostatic gating at conical glass nanopore electrodes

The mechanism of molecule-based electrostatic gating of redox fluxes at conical glass nanopore (GNP) electrodes has been investigated using finite-element simulations. The results demonstrate that the fluxes of cationic redox molecules through the nanopore orifice can be reduced to negligibly small values when the surface charge of the nanopore is switched from a negative to a positive value. Electrostatic charge reversal can be affected by ionization of surface-bound moieties in response to environmental stimuli (e.g., photoionization or acid protonation), but only if the negative charge of the glass is included in the analysis. Numerical simulations of the responses of GNP electrodes are based on a simultaneous solution of the Poisson and Nernst-Planck equations and are in excellent agreement with our previously reported experimental results for electrostatic gating of the fluxes of Ru(NH 3) 6 (3+) and Fe(bpy) 3 (2+) at GNP electrodes with orifice radii between 15 and 100 nm. The gating mechanism is discussed in terms of three components: (1) migration of ionic redox species in the depletion layer adjacent to the electrode surface; (2) migrational transport along the charged pore walls; (3) electrostatic rejection of charged molecules at the pore orifice. The numerical results indicate that all three components are operative, but that ion migration along the pore walls is dominant.