CO₂-responsive graphene oxide nanofiltration membranes for switchable rejection to cations and anions

Inspired by biological membranes with charge-gated channels that allow the transport of water and ions but restrict the undesired substances, various regulated nanofiltration (NF) membranes containing charge-gated channels have been developed for water purification. However, most of these membranes are mono-charged (positively or negatively) that show high rejection only to the corresponding co-ions but low rejection to the counter-ions, which limits their wider applications. Herein, we report NF membranes with gas-tunable charge-gated channels and high rejection to different salts on demand. The membranes are prepared by assembling a CO₂-responsive polymer, poly(N,N-dimethylaminoethyl methacrylate), on the surface of graphene oxide (GO). As a result of CO₂-induced reversal of the charge sign, the membranes exhibit “self-regulation” of their surface charges. Upon bubbling CO₂ into water, the membranes are positively charged, which shows high rejection to MgCl₂. After bubbling Ar to remove CO₂, the membranes gradually transform to negatively charged, and high rejection to Na₂SO₄ is obtained. This study demonstrates the gas-tunable charge-gated ion transport behavior of the CO₂-responsive NF membranes, and opens up new avenues for the design of on-demand NF membranes.