In this Special Issue dedicated to the State of the art of Sensors in Italy, we report the recent achievements obtained at the SENSOR Lab in Brescia. To properly frame these results, the paper starts with a short introduction on the working principle of metal oxide chemiresistors and reports the strategies followed at SENSOR to optimize the structure of sensitive layer, from thin film to nanowire technology. The gas-sensing performances of these technologies are compared choosing the detection of chemical warfare agents (CWAs) as target application. Finally the results obtained integrating these devices in artificial olfactory systems (AOSs) are shown focusing on three paradigmatic applications of food-quality control, namely detection of bacteria-contamination in beverages, identification of fraud in extra virgin olive oil and identification of errors in a tomato processing line.

2.?Working PrincipleConductometric gas sensors, also named chemiresistors, transduce the presence in the atmosphere of a given chemical compound through a variation of their electrical resistance. They are based on semiconducting metal oxides, whose electrical properties are modulated by red-ox interactions with adsorbing gaseous molecules.In particular, active species, such as O?, O2?, O2?, OH?, have been identified as the active centers responsible for the above red-ox reactions [2]. Such species cover the oxide surface with their relative population depending on the oxide temperature and atmospheric composition AV-951 [3].

In the typical temperature range of metal oxide chemiresistors (200�C500 ��C), O2? ions are the most abundant at low temperature (below 300�C350 ��C), while a higher temperature favors the dissociation of molecular oxygen leading to atomic oxygen ions O?.From an electrical point of view, when a semiconducting oxide is exposed to air, the adsorption of water and/or oxygen from the atmosphere modifies the band structure of the material at the surface with respect to the bulk. Chemisorption, involving the transfer of Cilengitide electrons between the conduction of the semiconductor and the adsorbed atom/molecule, is that particular form of adsorption responsible for building up the population of active ions at the oxide surface and the consequent band bending [4]. In particular, chemisorption of oxygen creates acceptor surface states that withdraw electrons from the outermost layer of the semiconductor, thus inducing a surface up-ward bending of the oxide band structure.