Chitosan-Gated Polycarbonate Nanochannels for Tunable Water and Cation Transport

Biological membranes regulate transport by responding to pH changes and ion binding. Emulating this logic, chitosan is introduced into cylindrical nanochannels of multiporous polycarbonate ion-track-etched membranes via covalent immobilization or electrostatic adsorption. Chitosan is confined within the nanochannels, leaving outer-surface liquid-solid interactions unchanged. The channels remain open when dry, while upon hydration the polymer swells and modulates water and ion transport. Chitosan shifts the membrane from microfiltration to ultrafiltration via swelling-driven steric hindrance. Consistent with reduced water flux, the flux of tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)₃ ^{2 +}) decreases by ∼75% and ∼92% at pH 7.5 and pH 2.5, respectively, compared to pristine nanochannels. The stronger limitation at pH 2.5 arises from combined steric hindrance and electrostatic exclusion by protonated chitosan. Salts provide an independent stimulus: at neutral pH, they promote electrostatic screening, polymer-chain reorganization, and chitosan deswelling, thereby increasing water flux, whereas under acidic conditions, the addition of KCl or CaCl₂ mainly affects cation transport through electrostatic screening, while chitosan remains highly swollen. This biomimetic platform enables pH- and ion-triggered, bidirectional control of both water flux and cation transport under different pH and ionic conditions and thereby advances nanoporous materials for smart separations, selective ion handling, and adaptive flow regulation.