Electrohydrodynamic Origin of Direction-Dependent Conductive and Resistive Events in Nanopipettes.

Understanding ion transport in nanopores is critical for optimizing nanopore-based sensing performance. Here, we report conductive events (CEs) during DNA translocation through conical nanopipettes under high and symmetric electrolyte conditions (1 M KCl), where conventional membrane nanopore systems generally exhibit resistive signals. Notably, these conductive signals show strong directional dependence, occurring only when λ-DNA translocates from the nanopipette interior to the external reservoir. To elucidate the origin and temporal characteristics of this behavior, we combine experiments with three-dimensional transient simulations based on the finite element method. The results reveal that conductive pulses arise from the coupled effects of ion enrichment and electrophoretic ion transport, governed by the interplay of applied bias, DNA surface charge, and electric double-layer effects along the nanopipette walls. Furthermore, we systematically investigate the roles of electrolyte concentration, DNA surface charge density, and nanopipette surface charge. Increased ionic strength and higher DNA charge enhance ion enrichment and current amplitude, while reduced nanopipette surface charge prolongs translocation duration. These findings provide mechanistic insight into ionic current modulation and offer guidance for optimizing nanopore-based biomolecular sensing.