A comprehensive survey of NO2 gas sensors: Functional materials, recent advances, present challenges, and future directions

Nitrogen dioxide (NO₂), a major atmospheric pollutant, poses significant risks to human health even at low concentrations and exerts similarly harmful effects on plants and terrestrial ecosystems. Therefore, the real-time, accurate and reliable monitoring of NO₂ concentrations in the atmosphere is very important. NO₂ gas sensors translate the concentration of NO₂ at any given time into analysable signals. So, different materials/technologies have been explored to develop NO₂ sensors. However, there is an increasing global awareness about the possible connections between technological advances, waste generation, and environmental pollution. Moreover, research and technological advancements have recently tilted towards the utilisation of artificial intelligence to develop smart devices. Therefore, this paper reviews the different functional materials in NO₂ sensors, including their advantages and limitations. Likewise, the techniques/technologies for the fabrication of NO₂ gas sensors are discussed. Moreover, recent developments that support sustainability, circularity, or end-of-life composability are presented. Furthermore, the contemporary trend of research towards fast and seamless integration of new generation NO₂ gas sensors with AI and the use of IoT for remote data collection is included in this review. Finally, future perspectives on the manufacture of smart, durable, mechanical, and environmentally robust NO₂ gas sensors are discussed.