Length-Dependent Inversion of Ion Current Rectification in Conical Nanopores

Ion current rectification (ICR) in conical nanopores represents a fundamental electrokinetic phenomenon with broad applications in blue energy harvesting, single-molecule sensing, and neuromorphic computing. However, the influence of nanopore length on ICR remains poorly understood. Here, using size-modified Poisson-Nernst-Planck (SMPNP) simulations that incorporate finite-size effects, we demonstrate a length-dependent inversion of ICR in conical nanopores: as pore length increases, the preferential ion transport direction reverses from base-to-tip (reverse rectification, ICR > 1) to tip-to-base (forward rectification, ICR < 1). This inversion arises from the progressive localization of the electric potential drop at the tip region with increasing pore length, which inverts the ion enrichment/depletion states under different voltage biases. We further reveal that the critical length for this inversion is tunable via electrolyte concentration, surface charge density, tip radius, and cone angle. These findings establish nanopore length as a critical design parameter for nanofluidic diodes, providing a predictive framework for engineering rectification direction without modifying surface chemistry.