Rectification of ionic current in nanopores functionalized with bipolar polyelectrolyte brushes

In this study, control of ion transport is demonstrated by surface modification of bipolar polyelectrolyte (PE) brushes on a nanopore wall. Three types of nanopore are considered: (i) Only the inner wall of a nanopore is modified and its outer walls are charge-free (type I). (ii) Only the outer walls of a nanopore are modified (type II) and its inner wall is charge-free. (iii) Both the inner and the outer walls of a nanopore are modified (type III). Results reveal that for the levels of salt concentration considered all these three types of nanopore show apparent current rectification. The rectification ability depends highly on both the structure and the space charge densities of the bipolar PE brush layers. For example, if the charge densities of this layer are low, the rectification ability of the types of nanopore ranks as type III > type II > type I; otherwise, it ranks as type III > type I > type II if they are sufficiently high. In addition, if the charge densities of those layers are symmetric, the rectification ability increases with increasing charge density for all three types of nanopore. For nanopores having asymmetric charge densities in their PE brush layers, similar charge density-dependent rectification behavior occurs only to type II nanopore. It is interesting to observe in the other two types of nanopore, the increase in the charge densities results in a decrease in the rectification effect. This is unexpected because previous studies suggested that the nanopore rectification depends positively on its charge density. (C) 2017 Elsevier B.V. All rights reserved.