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Salt-Dependent Ion Current Rectification in Conical Nanopores: Impact of Salt Concentration and Cone Angle

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journal contribution
posted on 02.11.2017, 00:00 by Jyh-Ping Hsu, Tsai-Wei Lin, Chih-Yuan Lin, Shiojenn Tseng
Inspired by biological ion channels, many artificial nanopores are fabricated for regulating ion transport based on functionality of discriminating ion types. It is expected that the transport of ions in these nanopores is influenced by their physicochemical nature such as their geometry and the conditions of the liquid phase such as the salt concentration. Taking account of possible presence of electroosmotic flow, these influences are investigated comprehensively in this work. Three types of salt are considered: KCl, NaCl, and LiCl. Several interesting behaviors are observed, and the associated mechanisms are discussed in detail. For example, if the bulk salt concentration is low and the cone angle is sufficiently small, then the rectification ratio of the salts examined ranks as LiCl > NaCl > KCl. However, this trend could be reversed at a larger applied voltage bias if the bulk salt concentration is sufficiently high or the cone angle is sufficiently large. This can be attributed to the fact that the degree of ion enrichment arising from the applied voltage bias is quite different for the types of salt examined. In addition, the behavior of ion selectivity as the applied voltage bias varies depends largely on the level of bulk salt concentration. In particular, if the bulk salt concentration is sufficiently low, then the selectivity decreases monotonically with increasing applied voltage bias, and if it is sufficiently high and the cone angle is sufficiently large, then the selectivity shows both a local minimum and a local maximum.