## 2 Materials and Methods
### S3 Analysis  
The innovative Small Sensors System S3 device used in the present work has been completely designed and constructed at SENSOR Laboratory (University of Brescia, Italy) in collaboration with NASYS S.r.l., a spin-off of the University of Brescia. The tool comprises a metal oxide (MOX) gas sensors array, flow sensors, temperature and humidity sensors, fluidodinamic system, electronic control system. In particular, the sensors used in this study are 8 MOX gas sensors. Three of them are nanowires of MOX, as presented in References [37; 38]. Two of them are tin oxides nanowires sensors, both grown by means of the Vapor Liquid Solid technique [39], using a gold catalyst on the alumina substrate and functionalizing one of them with gold clusters; the third sensor has an active layer of copper oxide nanowires. The working temperature is 350 \({}^{\circ}\)C, 350 \({}^{\circ}\)C and 400 \({}^{\circ}\)C, respectively. The other three sensors are prepared with Rheotaaxial Growth and Thermal Oxidation (RGTO) thin film technology; one is tin oxide functionalized with gold clusters (working at 400 \({}^{\circ}\)C), while the other two are pure tin oxide (working at 300 \({}^{\circ}\)C and at 400 \({}^{\circ}\)C, respectively). The last two are commercial MOX sensors produced by Figaro Engineering Inc. (Osaka, Japan). In particular, they are the TGS2611 and TGS2602, which are sensitive to natural gases and odorous gases like ammonia, respectively, according to the datasheet of the company. Commercial sensors have been mounted on our e-nose in order to evaluate the performances of nanowire sensors. Details of S3 sensors made at SENSOR Laboratory are summarized in Table 1. Response to 5 ppm of ethanol, selectivity (response ethanol/response carbon monoxide) and limit of detection (LOD) of ethanol are also included. The MOX nanowires are gas sensors with a high sensitivity to a broad range of chemicals; they exhibit physical properties that are significantly different from their polycrystalline counterpart. The nanowires have a high degree of crystallinity, atomically-sharp terminations and an extraordinary length-to-width ratio, resulting in enhanced sensing capability as well as long-term material stability for prolonged operation. In addition, the three-dimensional network formed by the nanowiresincreases the adsorption surface and the catalytic activity, improving the response and decreasing the instrument threshold [40]. SnO\({}_{2}\) and CuO nanowires structures obtained from SEM are shown in Figure 1A,B, respectively. S3 analyzes the head space (HS), i.e., the volatile fraction of the samples formed when the equilibrium of the solid-liquid phase and the vapor phase of all volatile compounds is reached. The creation of the HS depends on the test substance (vapor pressure) and the conditioning temperature of the sample. The compounds are extracted at the equilibrium point between the solid phase and the vapor in a dynamic head space. This characteristic allows for a non-destructive samples analysis. In this case, the sensor base line is obtained from the air of the surrounding environment; no gas cylinder of chromatographic air is required (an essential feature that makes it a portable instrument). The environmental air was filtered using a small metal cylinder (21.5 cm in length, 5 cm in diameter) filled with activated carbons. The volatile fraction is then aspirated and transported to the sensor chamber to be analyzed. In order to avoid any influence of the surrounding environment to the sensor response, the chamber has been thermostated and isolated. To prevent the absorption of volatile substances that could be released during subsequent analysis, the chamber and the connection between the elements' tires are made using steel. The air is flown into the sensor chamber using a pump through a needle valve. This is used to adjust the total airflow, which is measured by a flowmeter downstream from the pump.