Ionic Current Rectification in a Conical Nanopore: Influence of Salt

Abstract

Because it is capable of mimicking the ion channels of living organisms, the ionic transport in an artificial nanopore has drawn the attention of many researchers, recently. A considerable amount of efforts has also been made on relevant analyses due to the fast advances in nanotechnology and wide potential applications. As the linear size of a channel is down to the order of Debye length (or thickness of electric double layer), the overlapping of electric double layers yields distinctive electrokinetic phenomena. These include, for example, ion concentration polarization, ion selectivity, and ion current rectification. Ion current rectification is the phenomenon that a nanopore shows a diode-like current-voltage behavior, or the preference for ionic current in a certain direction when an electric potential is applied. For charged nanopores, ion current rectification might arise from several reasons, such as asymmetric pore geometry, imposed salt gradient, and bipolar surface properties. In the present study, the influence of electroosmotic flow on the behavior of the ion current rectification in a conical nanopore connecting two large identical reservoirs is investigated. In particular, the effect of the type of salt is examined by considering LiCl, NaCl, and KCl. We show that neglecting electroosmotic flow is capable of influencing ion current rectification both quantitatively and qualitatively. If electroosmotic flow is neglected, the rectification factor at each level of the applied electric potential bias across the two reservoirs for the case of LiCl (KCl) is always the largest (smallest). However, if electroosmotic flow is taking into account, the relative magnitude of the rectification factors for various salt depends upon the level of the applied electric potential bias. This behavior is consistent with the experimental observation in the literature, and can be explained by the degree of ion deplection/enrichment in a nanopore.