Ionic Covalent Organic Framework Membranes for Rapid Moisture-Driven Actuation and Sensing.

The development of smart materials capable of rapid and reversible responses to ambient humidity is essential for next-generation sensors, monitoring systems, and adaptive devices. Covalent organic frameworks (COFs), with their tunable porosity and designable architectures, represent a promising class of materials for such stimuli-responsive systems, yet practical implementation remains limited. In this study, we report a self-standing ionic COF membrane, synthesized by integrating hydrogen-bonding ionic functionalities into the framework backbone. This rational design endows the membrane with exceptional moisture-driven actuation and sensing behavior. The unique hydrogen bonding interactions within the framework facilitate rapid water uptake and release, enabling a rapid response time of 1 s. The membrane demonstrates excellent mechanical flexibility, high water sorption capacity, and robust cycling durability. Results from DFT calculations and MD simulations revealed that water molecules could strongly adsorb onto the membrane via hydrogen bonding to modulate its micropore structure and facilitate its responsive behavior. Furthermore, its responsive behavior to subtle humidity changes makes it suitable for applications such as human-interfacing soft actuators, smart switches, and soil moisture sensors. This study highlights the utility of ionic COF membranes as a versatile platform for creating next-generation intelligent materials.