The Power Spectrum of Ionic Nanopore Currents: The Role of Ion Correlations

TL;DR

This study combines theoretical calculations and simulations to analyze the power spectrum of ionic currents through nanopores, revealing the influence of ion correlations and surface charge on current fluctuations.

Contribution

It introduces a combined mean-field and Langevin dynamics approach to accurately predict power spectra and identifies ion-ion correlations as key to low-frequency behavior.

Findings

Mean-field theory accurately predicts power spectral density dependence on pore radius and electric field.

Ion-ion correlations cause a low-frequency power law in the power spectrum.

Surface charge density does not affect the frequency dependence of the power spectrum.

Abstract

We calculate the power spectrum of electric-field-driven ion transport through cylindrical nanometer-scale pores using both linearized mean-field theory and Langevin dynamics simulations. With the atom-sized cutoff radius as the only fitting parameter, the linearized mean-field theory accurately captures the dependence of the simulated power spectral density on the pore radius and the applied electric field. Remarkably, the linearized mean-field theory predicts a plateau in the power spectral density at low frequency ${\omega}$, which is confirmed by the Langevin dynamics simulations at low ion concentration. At high ion concentration, however, the power spectral density follows a power law that is reminiscent of the $1/{\omega}^{\alpha}$ dependence found experimentally at low frequency. Based on simulations with and without ion-ion interactions, we attribute the low-frequency power law dependence to ion-ion correlations. Finally, we show that the surface charge density has no effect on the frequency dependence of the power spectrum.