Graphene Oxide Membrane/Conical Nanoporous Polyimide Composites for Regulating Ion Transport

The ionic rectification effect, an important phenomenon existing in the solid-state nanopore system, has attracted much attention due to its ability to significantly regulate ion transport in the nanopores. However, current research mainly focuses on the single nanopore systems with low ionic currents, and thus further practical applications are limited. In this work, the conical nanoporous polyimide (CNPI) membrane was prepared by ion irradiation and asymmetric chemical etching to increase the ionic current, and the graphene oxide membrane (GOM) was spin-coated on it to improve the corresponding ionic flux and rectification effect after that. It was found that the ionic rectification ratio (0.05 M KCl at 3 V) increased from 17 (CNPI membrane) to 63 (GOM/CNPI composite) with the ionic current in microamperes. The enhanced rectification effect was ascribed to the increased geometry asymmetry and charge asymmetry, which further increased the difference of ionic conductance at opposite bias. Moreover, the corresponding COMSOL simulation revealed that the enhanced ion depletion and enrichment appeared in the nanochannel, especially at the tip side of conical nanopores after the GOM was spin-coated on the CNPI membrane. Finally, it was also confirmed that the GOM/CNPI composite can exhibit a good rectification effect and chemical durability in a wide acidic condition (pH 1 to 6), which extends the practical applications of the conical nanopore in a nanofluidic system.