Metal oxide nanomaterial-based sensors for monitoring environmental NO2 and its impact on the plant ecosystem: a review

The substantial increase in the emission of air pollutants due to the rapid industrial growth, combustion of fossil fuels, pesticides, and insecticides used in the agriculture sector in the last few decades urgently requires sincere remedial efforts from the global scientific community. The elevated concentration of nitrogen dioxide (NO2) makes it one of the most common and catastrophic air pollutants. NO2 acts as a precursor for ozone (O3) and particulate matter (PM) and participates in the phenomena of acidification and eutrophication in plant ecosystems. Therefore, worldwide research is ongoing towards designing an accurate, reliable, and environment-friendly NO2 gas detection technology for the monitoring of NO2 concentration in the earth's environment. Nanostructured metal oxide semiconductors (NMOSs) have been found to be very consistent and precise for NO2 sensing due to their outstanding structural and morphological properties. The first part of this review presents an overview of the effects of NO2 pollution on the plant ecosystem. The second part briefly describes the various other available techniques for NO2 detection. Further, this article presents a comprehensive review on the progress of a wide variety of NMOS-based NO2 gas sensors. NMOS materials with diverse morphologies such as nanorods, nanoparticles, nanowires, nano flowers, and nanosheets, fabricated into NO2 gas sensors, are discussed extensively. Moreover, their fabrication methods and performance in terms of sensitivity/selectivity, operating temperature, response/recovery time, and detection limit, along with their gas sensing mechanism, are explained in the review in order to establish a reliable NO2 gas detection technology.