Linkage Engineering of Semiconductive Covalent‐Organic Frameworks toward Room‐Temperature Ppb‐Level Selective Ammonia Sensing

Rational design of molecular architectures is crucial for developing advanced materials such as covalent-organic frameworks (COFs) with excellent sensing performance. In this work, two isostructural COFs (β-keto-AnCOF and imine-AnCOF) with the same conjugated linkers but distinct linkages are constructed. Although both COFs have porous structure and semiconductor behavior conferred by the identical conjugated backbones, β-keto-AnCOF with ─C═O side groups exhibits superior room-temperature ammonia (NH₃) sensing performance than imine-AnCOF and even the state-of-the-art dynamic and commercial NH₃ sensors, i.e., high sensitivity up to 18.94% ppm^-1, ultralow experimental detection limit of 1 ppb, outstanding selectivity, and remarkable response stability and reproducibility after 180 days. In situ spectroscopy and theoretical calculation reveal that the additional charge transfer between NH₃ and ─C═O sites in β-keto-AnCOF effectively increases the distance between Fermi level and the valence band, enabling highly-sensitive NH₃ detection at ppb levels. This work provides novel molecular architectures for next-generation high-performance sensors.