Interfacially Fabricated Covalent Organic Framework Membranes for Film‐Based Fluorescence Humidity Sensors and Moisture Driven Actuators

Abstract Rapid, on‐site measurement of ppm‐level humidity in real time remains a challenge. In this work, we fabricated a few micrometer thick, β ‐ketoenamine‐linked covalent organic framework (COF) membrane via interfacially confined condensation of 1,3,5‐tris‐(4‐aminophenyl)triazine (TTA) with 1,3,5‐tri‐formylphloroglucinol (TP). Based on the super‐sensitive and reversible response of the COF membrane to water vapor, we developed a high‐performance film‐based fluorescence humidity sensor, depicting unprecedented detection limit of 0.005 ppm, fast response/recovery (2.2 s/2.0 s), and a detection range from 0.005 to 100 ppm. Remarkably, more than 7,000‐time continuous tests showed no observable change in the performance of the sensor. The applicability of the sensor was verified by on‐site and real‐time monitoring of humidity in a glovebox. The superior performance of the sensor was ascribed to the highly porous structure and unique affinity of the COF membrane to water molecules as they enable fast mass transfer and efficient utilization of the water binding sites. Moreover, based on the remarkable moisture driven deformation of the COF membrane and its composition with the known polyimide films, some conceptual actuators were created. This study brings new ideas to the design of ultra‐sensitive film‐based fluorescent sensors (FFSs) and high‐performance actuators.