Ionic Current Saturation Enabled by Cation Gating Effect in Metal-Organic-Framework Membranes.

Ion transport through nanoporous two-dimensional (2D) membranes is predicted to be tunable by controlling the charging status of the membranes' planar surfaces, the behavior of which though remains to be assessed experimentally. Here we investigate ion transport through intrinsically porous membranes made of 2D metal-organic-framework layers. In the presence of certain cations, we observe a linear-to-nonlinear transition of the ionic current in response to the applied electric field, the behavior of which is analogous to the cation gating effect in the biological ion channels. Specifically, the ionic currents saturate at transmembrane voltages exceeding a few hundreds of millivolts, depending on the concentration of the gating cations. This is attributed to the binding of cations at the membranes' surfaces, tuning the charging states there and affecting the entry/exit process of translocating ions. Our work also provides 2D membranes as candidates for building nanofluidic devices with tunable transport properties.

Polyamide Membranes with Tunable Surface Charge Induced by Dipole-Dipole Interaction for Selective Ion Separation.

Nanofiltration (NF) has the potential to achieve precise ion-ion separation at the subnanometer scale, which is necessary for resource recovery and a circular water economy. Fabricating NF membranes for selective ion separation is highly desirable but represents a substantial technical challenge. Dipole-dipole interaction is a mechanism of intermolecular attractions between polar molecules with a dipole moment due to uneven charge distribution, but such an interaction has not been leveraged to tune membrane structure and selectivity. Herein, we propose a novel strategy to achieve tunable surface charge of polyamide membrane by introducing polar solvent with a large dipole moment during interfacial polymerization, in which the dipole-dipole interaction with acyl chloride groups of trimesoyl chloride (TMC) can successfully intervene in the amidation reaction to alter the density of surface carboxyl groups in the polyamide selective layer. As a result, the prepared positively charged (PEI-TMC)-NH2 and negatively charged (PEI-TMC)-COOH composite membranes, which show similarly high water permeance, demonstrate highly selective separations of cations and anions in engineering applications, respectively. Our findings, for the first time, confirm that solvent-induced dipole-dipole interactions are able to alter the charge type and density of polyamide membranes and achieve tunable surface charge for selective and efficient ion separation.