Regulating Surface Charge of Mof Membranes for Enhanced Ion Selectivity

Abstract

Replicating the biological nanopore channel with regulated transport behavior is the ultimate goal for the artificial ion channel, which is primarily attributed to the exquisite charged residues inside the sub-nanoscale pores. However, there remains a deficiency in effective strategies and in-depth understanding of the correlation regarding how surface charge in the channel affects ion transport activities. Herein, inspired by the charge-based separation existing in nature, we propose a charge-modified strategy by anchoring the cations at charge-lock sites in the channel wall. In this work, we employ water-stable MOF-303, which contains the pyrazole ligand with a double N-based site as the charge-lock sites for anchoring Fe3+. The channel anchored with Fe3+ exhibits a high positive charge, thereby strongly repulsing the high charge valence of Mg2+. The strongly positively charged membrane exhibits an outstanding selectivity for monovalent and divalent cations (i.e., ~110.2 for K+/Mg2+). DFT theoretical calculations show that, compared with original MOF-303, the difference in entry energy (∆E) between monovalent ions and divalent ions through strongly positively charged Fe3+@MOF-303 is larger. It is expected that this study will provide a novel method for the creation of charge-tunable MOF membranes with superior performance for separation fields.

Keywords: Metal-organic frameworks (MOF), Charge regulation, Electrostatic repulsion, Ions sieving