# Innovative Sensor Approach to Follow _Campylobacter jejuni_ Development
## Materials and Methods
### S3 Analysis  
The S3 device used in the present work has been completely designed and constructed at SENSOR Laboratory (University of Brescia, Brescia, Italy) in collaboration with NASYS S.r.l., a spin-off of the University of Brescia, Brescia, Italy. It has been described in other works [25; 26; 27; 28]. Briefly, the tool comprises three parts: Pneumatic components that transfer VOCs from the headspace of samples to the sensing chamber; electronic boards that manage the acquisition and transmission of the data from the device to the dedicated Web-App and allow the synchronization between S3 and the auto-sampler, and a sensing chamber, that can host from five to ten different metal oxide (MOX) gas sensors and is thermostatically isolated in order to avoid any influence of the surrounding environment. To function properly, sensors need a reference value that has been obtained by filtering the ambient air with a small metal cylinder (21.5 cm in length, 5 cm in diameter) filled with activated carbons.  
Eight MOX gas sensors were used. Three of them are MOX nanowire [29; 30]. Two of them are tin oxide nanowires sensors, both grown by means of the Vapor Liquid Solid technique [31], 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 Rheotaxial Growth and Thermal Oxidation (RGTO) thin film technology [32]; 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 autosampler headspace system HT2010H (HTA s.r.l., Brescia, Italy) was coupled with S3. It supports a 42-loading-sites carousel and a shaking oven to equilibrate the sample headspace. 40 vials were placed in a randomized mode into the carousel. Among these vials, 5 were control samples analyzed at times 2.5 h, 5.5 h, 8.5 h, 11.5 h and 14.5 h. Each vial was incubated at 40 \({}^{\circ}\)C for 10 min in the autosampler oven. The sample headspace was then extracted from the vial in the dynamic headspace path and released into the carried flow (100 sccm). The analysis timeline can be divided into three different steps for a duration of 30 min per sample divided as follows: 5 min to analyze samples, 5 min to clean the tube that carries VOCs from sample headspace to sensing chamber and 20 min to restore the baseline. Thanks to the processor integrated in the S3 instrument, the frequency at which the equipment works is equal to 1 Hz.