Tailoring ion-channel affinity for enhanced COF membrane mono/divalent cation selectivity

Precise regulation of the transport properties of monovalent and divalent cations is essential for applications in biosensors, membrane separation, nanofluidic devices, and energy conversion systems. For artificial ion channels, the cooperative interaction and release of target ions with affinity sites are significant for enhancing ion recognition and selectivity. Here, we constructed a series of ion channels by embedding crown ether molecules (12C4, 15C5, 18C6) into a covalent organic framework (COF) membrane via in situ assembly. Experiments have demonstrated that the presence of crown ethers results in reduced water permeability and induces forced dehydration of ions, with their interaction depending on the hydration state of ions. Molecular simulations further reveal that monovalent cations experience moderate binding and lower dehydration barriers within the 12C4-confined nanochannels, enabling rapid ion release, whereas divalent cations are kinetically trapped by excessively strong coordination. The interaction between 12C4 molecules and monovalent cations is optimal, facilitating dehydration within the membrane and expediting their rapid release, achieving an ideal equilibrium between thermodynamic adsorption and kinetic transmission of monovalent ions and enhancing their efficient transport while obstructing the passage of divalent ions. The 12C4@COF membrane showed the highest K⁺/Mg²⁺ selectivity (232) under concentration dialysis and retained strong performance (K⁺/Mg²⁺~100, K⁺ permeance ~612 mmol·m^-2·h^-1) under electrodialysis. This work demonstrates a generalizable strategy for tuning ion transport via molecular-level binding affinity modulation, providing a robust and generalizable strategy for high-performance mono/divalent cation separation.