Efficient metal ion sieving in rectifying subnanochannels enabled by metal-organic frameworks

Biological ion channels have remarkable ion selectivity, permeability and rectification properties, but it is challenging to develop artificial analogues. Here, we report a metal-organic framework-based subnanochannel (MOFSNC) with heterogeneous structure and surface chemistry to achieve these properties. The asymmetrically structured MOFSNC can rapidly conduct K⁺, Na⁺ and Li⁺ in the subnanometre-to-nanometre channel direction, with conductivities up to three orders of magnitude higher than those of Ca²⁺ and Mg²⁺, equivalent to a mono/divalent ion selectivity of 10³. Moreover, by varying the pH from 3 to 8 the ion selectivity can be tuned further by a factor of 10² to 10⁴. Theoretical simulations indicate that ion-carboxyl interactions substantially reduce the energy barrier for monovalent cations to pass through the MOFSNC, and thus lead to ultrahigh ion selectivity. These findings suggest ways to develop ion selective devices for efficient ion separation, energy reservation and power generation.