Beyond steric selectivity of ions using angstrom-scale capillaries

Ionic flow through angstrom-scale channels facilitates selectivity beyond steric effects between ions of same charge and hydrated diameter. Here, with two-dimensional channels, the authors show that ion position in the channel influences selectivity. Ion-selective channels play a key role in physiological processes and are used in many technologies. Although biological channels can efficiently separate same-charge ions with similar hydration shells, it remains a challenge to mimic such exquisite selectivity using artificial solid-state channels. Although there are several nanoporous membranes that show high selectivity with respect to certain ions, the underlying mechanisms are based on the hydrated ion size and/or charge. There is a need to rationalize the design of artificial channels to make them capable of selecting between similar-sized same-charge ions, which, in turn, requires an understanding of why and how such selectivity can occur. Here we study angstrom-scale artificial channels made by van der Waals assembly, which are comparable in size with typical ions and carry little residual charge on the channel walls. This allows us to exclude the first-order effects of steric- and Coulomb-based exclusion. We show that the studied two-dimensional angstrom-scale capillaries can distinguish between same-charge ions with similar hydrated diameters. The selectivity is attributed to different positions occupied by ions within the layered structure of nanoconfined water, which depend on the ion-core size and differ for anions and cations. The revealed mechanism points at the possibilities of ion separation beyond simple steric sieving.

Automated summary

Ionic flow through angstrom-scale channels can selectively separate ions of the same charge and hydrated size beyond steric and Coulomb effects. Two-dimensional channels have been shown to be influenced by the position of ions, leading to selectivity. Understanding and designing artificial channels that can mimic the exquisite selectivity of biological channels is a challenge, but necessary for various applications. This study investigates Å-scale artificial channels made by van der Waals assembly and demonstrates their ability to distinguish between same-charge ions based on their positions within nanoconfined water.