Recent advances in MXene, MOF, and COF based NH3 gas sensors for environmental monitoring: sensing mechanisms, analytical performance, and future perspectives.

The increasing use of ammonia (NH3) in agriculture, refrigeration, chemical processing, and energy applications has increased concerns over its toxicity and corrosiveness. This suggest the need for sensitive NH3 monitoring under practical conditions. This review critically evaluates recent advances in MXene, metal-organic framework (MOF), and covalent organic framework (COF) based NH3 sensors. MXenes are promising for room-temperature sensing because of their high conductivity and efficient charge transport, but oxidation, restacking, and humidity interference continue to hinder stable and reproducible performance. In contrast, MOFs provide molecular-sieving capability, high porosity, and Lewis-acidic adsorption sites, but their poor conductivity and limited stability necessitate integrating it with semiconductors, polymers, or carbon-based materials. Similarly, COFs exhibit tunable pore structures, ordered transport pathways, and good chemical stability. Despite these advantages, their application in NH3 sensing remains relatively limited. The analytical performance of MXene, MOF, and COF based NH3 sensors has been critically discussed with the underlying sensing mechanisms. The key challenges, future perspectives, and emerging research directions are also discussed. Unlike earlier reviews that discuss these materials separately, this review compares MXene, MOF, and COF based NH3 sensors using the same performance criteria, providing a clearer view of their strengths and remaining limitations. We believe that this review will provide a useful overview for researchers developing MXene, MOF, and COF based NH3 sensors.