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.