Mechanism-to-Deployment Engineering of NO2 Gas Sensors: Materials, Interfaces, Transducers, and Environmental Validation.

NO2 sensing is shifting from the optimization of individual receptor materials toward integrated systems in which surface chemistry, interfacial charge transfer, transducer architecture, operating environment, and regeneration jointly determine performance. This review argues that deployment readiness is set not by peak response but by the coupled performance of five layers: (i) receptor and interface chemistry, (ii) transducer and device architecture, (iii) gas delivery and environmental conditions, (iv) regeneration and aging, and (v) calibration, uncertainty, and system integration. Within this mechanism-to-deployment framework, we examine how oxygen adsorption, depletion and accumulation layers, heterojunction and Schottky barriers, defects, catalytic sensitization, and percolation govern the electrical signal across metal oxides, carbon materials, transition-metal dichalcogenides, MXenes, porous and MOF-derived architectures, and organic semiconductors and how MEMS microheaters, FET/TFT/MOSFET transducers, flexible platforms, optical and electrical regeneration, and AI-assisted arrays read it out. Despite this progress, translation remains limited by environmental interference, incomplete recovery, transport-dependent response, aging, and device-to-device variability so that record responses seldom survive realistic operation. Reliable NO2 monitoring therefore requires application-specific validation of the complete measurement cycle-exposure, readout, recovery, environmental perturbation, calibration, and long-term operation-rather than isolated sensitivity metrics.