## 2 Materials and Methods
### S3 Instrumentation  
A new state-of-the-art nanowire gas sensor device Small Sensors System (S3) is presented for a quick and high-throughput quality and VOCs analysis of the Parmigiano Reggiano cheese samples considered in this work. The device has been completely designed and constructed at SENSOR Laboratory, Brescia, Italy. It is a miniaturized portable, compact, and automatic instrument. The system is designed in such a way that a greater amount of information can be gathered through a user-friendly control panel. It comprises an array of MOS gas sensors, flow sensors, temperature and humidity sensors, and actuators (valves, pumps), all embedded in a stainless steel cell. The sensing elements are Zinc and Tin oxide crystalline nanowires and thin films that have been fabricated and incorporated in the S3 sensor network. In particular, there are six metal oxide semiconductor gas sensors, three of them prepared with the nanowire technology [33] and the remaining three with the Rheotaxial Growth and Thermal Oxidation (RGTO) thin film technology [34]. Out of the three nanowires in the array, two of them are ZnO sensors with different operating temperatures and the third one is a SnO\({}_{2}\) sensor. Regarding RGTO sensors, the first one is constructed based on a blend of SnO\({}_{2}\) and MoO\({}_{3}\), the second one is a SnO\({}_{2}\) sensor catalyzed with Ag nanoparticles, and the third one consists of a blend of SnO\({}_{2}\) and WO\({}_{3}\). Each sensor is deposited on an Alumina substrate and doped with a Platinum heater on the sides. All the sensors in the array are unspecific. A photograph of the S3 device is shown in Figure 1, and a deep description of the sensors is explained in Table 2.  
The excellent characteristics of metal oxide nanowires in terms of sensitivity, selectivity, and stability toward different molecules, 1-dimensional (1-D) nanostructure, low power dissipation, and content production costs allowed their use for the manufacture of S3 device as a versatile gas detection equipment [35]. The instrument was also connected with an aut \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sensor Type** & **Sensor Composition** & **Morphology** & **Operating Temperature** (\({}^{\circ}\)**C**) \\ \hline SnO\({}_{2}\)–MoO\({}_{3}\) & Blend of SnO\({}_{2}\) and MoO\({}_{3}\) & RGTO & 245 \\ ZnO & ZnO & Nanowire & 280 \\ SnO\({}_{2}\) & SnO\({}_{2}\) & Nanowire & 375 \\ SnO\({}_{2}\)//Ag & SnO\({}_{2}\) catalyzed with Ag nanoparticles & RGTO & 400 \\ ZnO & ZnO & Nanowire & 500 \\ SnO\({}_{2}\)//WO\({}_{3}\) & Blend of SnO\({}_{2}\) and WO\({}_{3}\) & RGTO & 500 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Sensor array characteristics, composition, morphology, and operating temperature. Figure 1: Portable S3 sensor device developed at SENSOR Laboratory, Brescia, Italy. HT280T (HITA srl, Brescia, Italy), bearing a 40-loading-site carousel and a shaking oven to equilibrate the sample headspace in order to provide a high amount of replicate for obtaining a consistent dataset. The operation principle of S3 is based on the analysis of the head space (HS) of a given sample. This volatile fraction is formed inside a vial during the achievement of the equilibrium between the solid phase and the vapor phase. The creation of the HS depends on the test substance (vapor pressure) and the conditioning temperature of the sample, which plays a decisive role determining the concentration of volatile compounds. When compounds are extracted at the equilibrium point between the solid phase and the vapor phase the measurements are done in static HS, while if the extraction occurs continuously, it is called dynamic HS. In this case, the balance achieved between the solid phase and the vapor phase occurs during the measurement. The volatile fraction is then aspirated and transported to the sensor chamber where it will be analyzed.