# Internet of Food (IoF), Tailor-Made Metal Oxide Gas Sensors to Support Tea Supply Chain
## 2 Materials and Methods
### S3 Analysis Conditions  
The chosen sensors for the application were installed on the S3 device, an acronym that stands for small sensor system. The device that was designed and built by Nasys S.r.l. (www.nasys.it, accessed on 21 June 2021) is an innovative spin-off born in the University of Brescia, where tailor-made sensors were produced. S3 is composed of three essential parts: (A) sensors chamber, (B) fluid dynamic circuit for the distribution of volatile compounds, and (C) electronics control system. 1. The sensors are housed inside a steel chamber isolated from the external environment, except for an inlet and an outlet path for the passage of volatile compounds. In addition to the MOX sensors, a temperature, humidity sensor, and a flow sensor are also allocated as necessary to take into account the number of variables during the analysis. The dimensions of the chamber are 11 \(\times\) 6.5 \(\times\) 1.3 cm. On the 11 and 6.5 cm sides, 6 and 5 positions have been obtained, respectively: 10 positions were available for the use of metal oxide (MOX) sensors and one for the temperature and humidity sensor. The list of MOX used in this study is represented in Table 2, where the technology of production, RGTO (rheotaxial growth and thermal oxidation) or nanowire, the sensing material (SnO\({}_{2}\) or CuO), and the working temperature are indicated. 2. The fluid dynamic circuit consists of a pump (Knf, model: NMP05B), polyurethane pipes, a solenoid valve, and a metal cylinder containing activated carbon for filtering possible interfering odors present in the environment. The pump flow is regulated by a needle valve placed at the chamber inlet; the flow range for tea analysis was set to 100 sccm. 3. The electronic boards make it possible to acquire the resistances of the sensors, the correct heating of the sensors themselves to their operating temperature, and the sending of data to the Web App dedicated to the S3 device through an internet connection. In addition, it allows communication and synchronization with an autosampler. This is an autosampler of the company HTA S.r.l. (model HT2010H) which allows one to prepare batches of 42 samples per measurement session. Tea samples were conditioned for 5 min at 40 \({}^{\circ}\)C with 1 min in a shaking mode in order to equilibrate the headspace (HS). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Material** & **Kind** & **Working Temperature** \\ \hline SnO\({}_{2}\) + Au & RGTO & 400 \({}^{\circ}\)C \\ SnO\({}_{2}\) & RGTO & 400 \({}^{\circ}\)C \\ CuO & Nanowire & 350 \({}^{\circ}\)C \\ SnO\({}_{2}\) + Au & Nanowire & 350 \({}^{\circ}\)C \\ SnO\({}_{2}\) & Nanowire & 350 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Sensor array composition of S3 device and sensors characteristics. The RGTO technique requires two phases of deposition: the first step is the metallic thin film by DC magnetron sputtering from a metallic target to a substrate at higher temperatures than the melting point of the metal; and the second step is the thermal oxidation period in order to produce a metal oxide coating with stable stoichiometry [25]. The surface of the thin film is rough, and this is desirable since it has a high surface-to-volume ratio and reactivity to the gaseous species [26]. In addition, the presence of this very rough surface morphology, also known as'spongy agglomerates', gives rise to a highly specific area required for high-sensitivity gas sensors [27]. Nanowires display extraordinary crystalline quality and a very high length-to-width ratio, resulting in improved sensitivity and long-term material stability for extended operation [28; 29]. The fabrication method consists of the evaporation of the powder (metal oxide) at high temperatures in a controlled atmosphere at pressures of less than a hundred mbar (50-200 mbar) and the subsequent mass transfer of the vapor (50-100 sccm) to substrates held at lower temperatures in relation to the evaporation source area. This growth technique is called a vapor-liquid-solid (VLS) mechanism.