Editorial: Sensors for Air Quality Monitoring, Indoor and Outdoor

Sensors for air quality monitoring have quickly gained popularity due to increased concerns related to air pollution and spread of air contaminants both indoors and outdoors. The protracted experiences of lockdowns, self-quarantines, use of facemasks during the coronavirus pandemic (COVID-19) put the air quality issue in the spotlight worldwide, and have made people more environmentally and health aware. The increasing availability and applicability of sensors for air quality monitoring offers the possibility to design and prototype customized low-cost sensor systems (LCSS) and multisensor platforms easier than ever before, not only for research or industrial purposes, but also for personal exposure assessment, complementary network measurements, educational training, and student projects (Lenartz et al.; Höfner et al.). Sensor-based intelligent systems have entered our daily life and found an increased use in health, environmental, and safety-related applications, thanks to the remarkable improvements on sensing materials and device performance. Significant efforts have been done to achieve increased sensitivity, selectivity, long-term stability, reproducibility, as well as decreased response time and operation temperature (Saruhan et al.; Domènch-Gil et al.). Moreover, proper calibration, rigorous data analysis, evaluation and validation methods are key factors of substantial improvement of sensor performance and enhanced reliability of concentration readings (Lenartz et al.). Advances in materials research have played a crucial role for decades for the development of highperformance gas sensors. Metal oxide (MOx) semiconductor materials have been widely used since the 1950s to fabricate chemoresistive gas sensors due to their excellent sensing properties such as high sensitivity and long-term stability, possibility to control their properties by synthesis methods, ease of manufacture, cost effectiveness, and large-scale production potential compared to other types of gas sensors (Saruhan et al.; Domènch-Gil et al.). Among all, particular focus has been given to nanostructured SnO2 and TiO2 as outstandingMOx sensing materials. The review article by Saruhan et al. reports the most important achievements related to SnO2 and TiO2 over the past 2 decades. The effects of physical and chemical material characteristics, such as crystal structure, morphology, size, surface modification, bulk doping on the gas sensor properties are also described. It has been proven that MOx are suitable for detecting a large variety of gases and that the sensing properties can be tuned by, e.g., bulk-doping, heterojunctions or surface functionalization. However, MOx-based sensors usually suffer from poor selectivity due to their high sensitivity to several gas species, which is an issue that needs further research to be overcome. An important factor to consider when fabricating micro or nanostructured MOx-based chemoresistive gas sensors is the amount and distribution of the structures that shape the sensing material, as they determine the gas sensing performance. The material characteristics can, therefore, enhance the sensitivity, selectivity, and working temperature for each relevant gas. In Edited and reviewed by: Elisabetta Comini, University of Brescia, Italy