Rectification behavior of a conical nanopore subject to extra simultaneously applied concentration and thermal gradients

Abstract

The transport of ions in biological cells can be efficiently regulated by a uniformly distributed salt concentration and temperature in their ion channels. Attempting to understand the interactions between these two factors, we consider the ionic transport of a conical nanopore functionalized with bipolar polyelectrolyte (PE) layers subject to applied electric field, concentration gradient (∇C), and thermal gradient (∇T). The electrokinetic behavior of the system considered is primarily governed by ∇C which contributes to diffusion flow. In contrast, the contribution of ∇T is only auxiliary. Through varying the directions of ∇C and ∇T (four possible combinations), we show that the accumulation and depletion of ions in the nanopore are mainly influenced by the shielding effect of the charged region of PE layers. If a small negative voltage bias is applied, an anion-selective nanopore switches to a cation-selective one, and this phenomenon is pronounced when a higher concentration is placed on the nanopore tip side. However, that phenomenon is temperature independent. When ∇C is applied, the ionic current rectification factor Rf shows a maximum value as a higher concentration is placed on the nanopore tip side, while it monotonically decreases as a higher concentration is placed on the other side. As for applying ∇T, the rectification capability of the nanopore is only slightly affected. The ion selectivity S under opposite polarities of voltage bias shows unique phenomena: If V=+1V, S mainly depends on temperature, which is almost uniform in the nanopore. In contrast, S is dominated by concentration when V=−1V, which arises from the enhancement or offset of diffusion and migration fluxes.