Molecular Dynamics of Ion Transport through Nanopores with Different Diameters for Applications in Nanofluidic Sensing

Abstract

Nanopores have wide applications in nanofluidic sensing, energy conversion, and material separation, and their performance is closely related to ion transport through nanopores. In this study, molecular dynamics (MD) simulations are performed to systematically investigate dynamic ionic behaviors inside nanopores of varying diameters (D) from 2 to 10 nm under applied voltages. From radial ion distributions, the increase of D can modulate the ion concentration inside electric double layers (EDLs), especially for D changing from 2 to 5 nm. With ion numbers inside nanopores, the electroneutrality breakdown presents a linear decrease with D. Through dividing ions into those in the EDLs and those in the pore center regions, contributing to the surface conductance and bulk conductance, quantitative analysis of ion flux demonstrates that the surface conductance is proportional to D for 2 ≤ D ≤ 6 nm and D2 for 6 < D ≤ 10 nm, while the bulk conductance is correlated to D2 for 2 ≤ D ≤ 10 nm. For ionic current, the cation current and anion current depend on D and D2, respectively. The conductivity of nanopores exhibits an increasing-decreasing trend with D, presenting a peak at D = 3 nm, where K+ ions have the shortest transit time through the pore. Based on the power spectral density of recorded current traces, the analysis of current noise inside nanopores reveals stronger normalized current noise at smaller diameters. Our results reveal detailed microscopic characteristics of ion transport under varying confinements, providing quantitative guidance for the structural design and performance optimization of solid-state nanopore sensors.