Ion-pairing design of covalent organic framework membranes for separation and controlled release of pharmaceuticals

Intrinsically charged covalent organic frameworks (COFs) afford specific ionic nanochannels for mass transport, and thus become promising platforms to design membranes with unique selectivity. However, internal electrostatic repulsion and large-pore frameworks cause significant barriers that greatly limit membrane performances. Herein, we report a novel strategy to synchronously crystallize and upgrade cationic COF membranes by ion-pairing design. Ion-paired guest molecules are involved in electric-driven interfacial crystallization to offset the charges of the ionic monomers and host frameworks. This host-guest neutralization promotes the crystallization of compact COF membranes. Lateral dimension and charge nature of guest molecules fundamentally affect the ion-pairing efficiency. Tight encapsulation of the large-sized electronegative molecules effectually narrows the molecular sieving channels, yielding a significant elevation in membrane selectivity. The membrane rejection of organic ions with a size larger than 1.2 nm thus can be improved from below 50 % to above 85 % with a water permeance of ∼10 L m⁻² h⁻¹ bar⁻¹. Prominently, our membranes demonstrate efficient recovery and pH-dependent release of bioactive pharmaceuticals, with a release rate that is 12 times higher in an acidic solution compared to a neutral environment. This work provides an ion-pairing strategy to regulate COF membranes for pharmaceutical industries and beyond.