Ion and Molecular Sieving with Ultrathin Polydopamine Nanomembranes

In contrast to biological cell membranes, it is still a major challenge for synthetic membranes to efficiently separate ions and small molecules, due to their similar sizes in the sub-nanometer range. Inspired by biological ion channels with their unique channel wall chemistry that facilitates ion sieving by ion-channel interactions, we report here the first free-standing, ultrathin (10-17 nm) nanomembranes composed entirely of polydopamine (PDA) for efficient ion and molecular sieving via an easily scalable electropolymerization strategy. These nanomembranes feature well-defined nanochannels, as well as abundant amine and catechol groups, which provide a favourable chemical environment for ion-channel electrostatic and hydrogen bond interactions, emulating the analogous function of biological ion channels. They exhibit remarkable selectivity for monovalent ions over multivalent ions and larger species with K+/Mg2+ of ≈4.2, K+/[Fe(CN)6]3- of ≈10.2, and K+/Rhodamine B base of ≈261.9 in a pressure-driven process, as well as cyclic reversible pH-responsive gating properties. Infrared spectra reveal hydrogen bond formation between hydrated multivalent ions and PDA, which prevents transport of multivalent ions and facilitates the high selectivity. We propose chemically rich, free-standing and pH-responsive PDA nanomembranes with specific interaction sites as customizable high-performance sieves for a wide range of challenging separation requirements.