Chemically gated ion transport in two-dimensional heterogeneous membranes with ultrahigh metal ion selectivity.

Abstract

Membranes are of particular importance in maintaining physiological activities, with crucial functions including selective permeation of certain ions. Such ion selectivity is achieved via complex molecular machinery to distinguish ions depending on their local physical-chemical environments, which are triggered by a variety of stimuli. Reproduction of such machinery in solid state systems is desirable for applications such as smart filtration, chemical sensing, and energy conversion, but remains a challenging task. We report regulated ion transport in artificial two-dimensional heterogeneous membranes, where the assembled polyacrylic acid macromolecules on the surface of functionalized graphene oxide membranes behave as a molecular regulator. The membrane exhibits chemically gated ion permeation, with the permeability of bi-valent metal cations tunable in response to the concentration of certain signaling ions. The chemistry gating effect is further found bi-valent ion-specific since it negligibly influences the rapid flow of monovalent metal cations. Molecular dynamics simulations reveal its origin as the high energy barriers required for reconfiguring the anion shells formed around bi-valent cations upon their entry. That also explains the observed mono-/bi-valent ion permeability ratio of up to >10⁴. Our work allows fabricating membranes with implanted biomimetic functions of adaptive permeability and selectivity, governed by the choice of components and their responses to the chemical environments.