Recent Advances in Artificial Anion Channels and Their Selectivity

Nature performs critical physiological functions using a series of structurally and functionally diverse membrane proteins embedded in cell membranes, in which native ion protein channels modify the electrical potential inside and outside the cell membrane through charged ion movements. Consequently, the cell responds to external stimuli, playing an essential role in various life activities, such as nerve excitation conduction, neurotransmitter release, muscle movement, and control of cell differentiation. Supramolecular artificial channels, which mimic native protein channels in structure and function, adopt unimolecular or self-assembled structures, such as crown ethers, cyclodextrins, cucurbiturils, column arenes, cyclic peptide nanotubes, and metal-organic artificial channels, in channel construction strategies. Owing to the various driving forces involved, artificial synthetic ion channels can be divided into artificial cation and anion channels in terms of ion selectivity. Cation selectivity usually originates from ion coordination, whereas anion selectivity is related to hydrogen bonding, ion pairing, and anion-dipole interactions. Several studies have been conducted on artificial cation channels, and several reviews have summarized them in detail; however, the research on anions is still in the initial stages, and related reviews have rarely been reported. Hence, this article primarily focuses on the recent research on anion channels.