Hydrogen-Bonded Organic Framework Nanosheets with Rich Free Oxygen Atoms for NO₂ Sensing

Hydrogen-bonded organic frameworks (HOFs) are under fast development in broad applications but have not been well explored for chemiresistive gas sensing yet primarily due to insufficient active sites. Herein, a new porphyrin-based HOF-199 is constructed by OH···O hydrogen bonds featuring layered networks and rich free oxygen (O) atoms, which is further exfoliated into few-layer nonosheets with more dangling O sites through an ultrasound-assisted liquid exfoliation method (namely L-HOF-199). Benefiting from rich electron-donor sites, L-HOF-199 demonstrates exceptional NO₂ sensing properties under ambient conditions, achieving a remarkable 3.25-fold improvement in sensitivity (152% toward 5 ppm of NO₂) and a faster response speed (52 s), relative to HOF-199. This work provides a promising platform for the rational design of advanced gas sensors via functional HOF chemistry.