Hysteresis Charges in the Dynamic Enrichment and Depletion of Ions in Single Conical Nanopores

Inhomogeneous distribution of electrolyte ions at solid-solution interfaces at the nanometer scale is known to contribute to several emerging transport phenomena discovered from various nanopores or nanochannels. The topic has attracted extensive research effort, aiming to achieve better energy harvesting, storage, conversion, desalination and separation, sample concentration, stochastic sensing, and so on. In this report, dynamic ion concentration polarization inside charged conical nanopores, one of the two characteristics to describe the electrical double layer (EDL) structure (charge distribution and potential profile), is quantified. The hysteresis in the ion transport or redistribution in single nanopores is determined from the pinched current-potential loops by time-dependent electrokinetic measurements. The extent of polarization in ion concentrations over different timescales, or residual charges during the transition from one ion distribution/EDL structure to another, is quantitated from the area enclosed in the hysteresis current loops. The ratio of the residual charges during the high/ low conductivity states is correlated with the current ratio at the threshold potentials (rectification factor), which, in turn, serves as a convenient parameter to estimate the charge transport hysteresis. By adjusting the solution pH and ionic strength, and under different potential sweeping rates, the dynamics of ion enrichment or depletion is elucidated, respectively, and successfully described with a simple capacitive charging model. Taken together with our previous reports of the memristive ion transport in conical nanopores/nanopipettes and the physical meaning of the unique non-zero cross-point potential, a comprehensive view of the charge redistribution and EDL dynamics at nanoscale interfaces is offered.