Methyl‐Tailored Covalent Organic Framework for Highly Sensitive Dual‐Mode Detection of Nitrogen Dioxide

ABSTRACT Development of dual‐mode sensors that synergize rapid visual detection with precise electrical quantification remains a significant challenge for reliable nitrogen dioxide (NO 2 ) monitoring. Herein, we report a vinylene‐linked covalent organic framework (COF), TMP‐TB‐COF, engineered as a high‐performance dual‐mode (colorimetric/chemiresistive) sensing platform for NO 2 . The conjugated and electron‐rich framework, featuring well‐ordered micropores, exhibits strong and specific affinity for electron‐accepting NO 2 molecules. This interaction triggers instantaneous color change from orange to wine‐red and concurrent fluorescence quenching, enabling visual qualification. Simultaneously, it induces a pronounced and reversible modulation in electrical resistance, allowing for ultrasensitive quantitative detection. The chemiresistive sensor demonstrates a high responsivity of 0.54% ppm −1 , a low detection limit of 7.8 ppb, excellent selectivity, and fast response/recovery (∼10 s). Mechanistic calculations reveal that the imine nitrogen atom adjacent to a methyl group serves as the key binding site, where a favorable orbital interaction facilitates selective charge transfer, underpinning the dual response. Leveraging its mechanical flexibility and stable performance, the COF film is integrated into a wireless, portable device, successfully demonstrating real‐time, spatial NO 2 monitoring and alerting via a smartphone. This work provides a novel paradigm for creating intelligent, reliable gas sensors by leveraging the multi‐signal output capability of tailor‐made COFs.