A 4+3 <i>β</i> ‐Ketoenamine‐Linked Covalent Organic Framework Membrane for High‐Performance Film‐Based Fluorescent NO ₂ Sensor

Abstract Developing compact and structurally simple sensors for reliable and rapid monitoring of nitrogen dioxide (NO 2 ) remains a challenge. In this study, we synthesized a novel β ‐ketoenamine‐linked 4 + 3 covalent organic framework (COF) membrane with unique topology structure using tetrakis(4‐aminophenyl)ethene (ETTA) and 1,3,5‐triformylphloroglucinol (TP) as monomers through liquid‐liquid interfacial polymerization. The sensitive NO 2 response of the ETTA‐TP COF membrane enables the creation of the first and high‐performance NO 2 film‐based fluorescent sensor, achieving fastest response/recovery time (1.5 s/2.0 s) and a high selectivity (over 16 potential interferents). This sensor realizes a low detection limit of 0.1 ppm and a broad detection range from 0.1 to 50 ppm, while maintaining stable performance over 5000 continuous tests. Furthermore, it demonstrates on‐site, real‐time monitoring of NO 2 emissions from automotive exhaust and waste incineration. The sensing mechanism studies reveal that the carbonyl groups of β ‐ketoenamine structure can bind NO 2 via electrostatic interactions and undergoes an energy‐level‐matching photoinduced electron transfer process under photoexcitation. The responses of other carbonyl‐containing fluorescent molecules and COF materials to NO 2 corroborate the generality of this mechanism. This study offers valuable insights into the development of oxidizing gas sensors characterized by fast response time, high sensitivity, and robust in situ online monitoring capabilities.