Engineering MXene-, LDH-, and MOF-Based NO2 Sensors for Sustainable Environmental Monitoring: Mechanisms, Performance, and Practical Challenges

Nitrogen dioxide (NO2) is one of the hazardous air pollutants, and its sensitive detection requires sensors that combine low-temperature operation with high selectivity, recovery, humidity tolerance, and long-term stability. This review critically evaluates MXene-, layered double hydroxide (LDH)-, and metal–organic framework (MOF)-based materials for NO2 sensing applications. The effects of composition, morphology, porosity, defects, heterointerfaces, and conductivity are also correlated to sensing performance. MXenes provide conductive, termination-rich surfaces but are vulnerable to oxidation, restacking, humidity-dependent behavior, and incomplete recovery. On the other hand, LDHs provide compositionally tunable layered and porous structures; however, their limited conductivity necessitates the use of conductive supports or coupling with semiconductors. In contrast, MOFs and MOF-derived materials provide accessible pores, metal sites, and structurally controllable derivatives, but electrical and moisture stability remains material-dependent. Improved sensing performance is generally linked to oxygen vacancies, charge transfer, stronger adsorption, and heterojunction effects; however, these mechanisms are proposed rather than directly confirmed under actual sensing conditions. The review also discusses humidity control, theory–experiment alignment, and requirements for wireless deployment. Practical use may require standardized testing, long-term stability assessment, full device power analysis, and validation under realistic conditions.