# Competitive Detection of Volatile Compounds from Food Degradation by a Zinc Oxide Sensor
## 3 Results
### Sensing Experiments
For a more complete characterization of the ZnO sensor, the transient responses corresponding to the data shown in Table 2 are shown in Figure 2b for 2 concentrations: 5 and 50 ppm, respectively. The electrical resistance of the ZnO layer decreases with time when exposed to the organic vapors (n-type behavior) as a consequence of the redox reactions  
Figure 2: (**a**) Comparison of the sensor’s response to pure compounds according to data from Table 2 and (**b**) the transient response of the ZnO sensor to pure volatiles at 5 (**top**) and 50 ppm (**bottom**). with the oxygen adsorbed on the sensor's surface and the consequent electron release into the conduction band of the semiconductor layer [7; 20; 31]. The transient response to hexanal compared to other volatiles was the fastest in the detection (R decrease), reaching the lowest resistance in seconds, and in regeneration in dry air (R increase) at all concentrations (Figure 2b). For the two alcohols, 1-pentanol and 1-ocen-3-ol, the transient response was significantly slower than the hexanal response for both the detection and regeneration (Figure 2b). According to these results, although the value of the R\({}_{0}\)/R ratio is the reference parameter used for evaluating the sensor's sensitivity, the response as a function of time can be important for discriminating among competing species, such as in the examples shown. During the degradation process, however, the alcohol vapors are released simultaneously with hexanal so the sensor's response cannot be considered a simple addition of the responses to pure compounds. To study the sensor's response in the presence of more than one volatile in the environment, a series of experiments were planned using binary and ternary mixtures. The results are shown in Table 4, where the composition of the vapor phases was determined as previously explained in the experimental paragraph. According to [2; 3; 9; 23; 32], the beginning of food degradation is mainly correlated to the production of hexanal, which is predominant among the other organic compounds released and, indeed, is used as an indicator of the freshness of food and shelf-life determination. In our tests, this condition was simulated by binary mixtures with a concentration of 98/2 (w%). However, for a more complete characterization, the sensor was also tested with mixtures 75/25 (w%), with a higher concentration of the \"competing\" components (Table 4). Under the experimental conditions adopted, the vapors were generated by the liquid phase at T = 258 K and any reaction between the components was inhibited. The vapors were, then, conveyed directly to the sensor's surface, the temperature of which was maintained at 523 K. The sensing mechanism is mainly based on the oxidation reactions that are dominant on the sensor's surface, where, even in this case, due to the experimental conditions adopted, reactions between gas components are unlikely [33]. Indeed, the trend of the sensor's response and the stability of its working temperature confirmed the absence of side reactions due to the formation of intermediate compounds (such as acetal and hemiacetal) in the gas phase [33]. However, with respect to pure substances, different signals were registered when the binary and ternary mixtures were considered. Therefore, due to the complexity of the phenomena involved, which cannot only be described by a superposition of the signals from pure components, studying the sensor's response to binary and ternary mixtures was of fundamental importance. The sensor's response to hexanal/1-pentanol vapors compared to pure components is shown in Figure 3b. According to the data shown in Table 4, the sensor's behavior is strongly influenced by the presence