# Competitive Detection of Volatile Compounds from Food Degradation by a Zinc Oxide Sensor
## 3 Results
### Sensing Experiments  
The sensor response was defined as the ratio of the reference resistance measured in dry air (R\({}_{0}\)) and the resistance when the sensor was exposed to the vapor mixtures (R). In all sensing experiments the sensor temperature was maintained at 523 K because this working condition resulted in the highest response in a previous study on the ZnO sensitivity to hexanal [8]. The first set of experiments was carried out by measuring the response of pure compounds to different concentrations in dry air, as reported in Table 2 and shown in Figure 2. The lowest concentration was set at 5 ppm (in dry air) because this is considered the threshold value for the perception of hexanal by the human nose and therefore is used as an indication of the onset of meat degradation processes [23; 24]. Comparing the data in Table 2, it is evident that ZnO is responsive to all three organic vapors, although in a different way. The sensor's response to hexanal, as already reported in a previous work in which the ZnO response to different hexanal concentrations was studied [8], is the highest up to C = 10 ppm. However, the response to 1-octen-3-ol shows the faster rise and exceeds all others at C = 50 ppm (Figure 2a). In contrast, the ZnO sensitivity for 1-pentanol is the lowest at all concentrations. The high sensitivity \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Chemical**} & \multicolumn{3}{c}{**Response (R\({}_{0}\)/R)**} \\ \cline{2-4}  **Compound** & **C = 5 ppm** & **C = 10 ppm** & **C = 50 ppm** \\ \hline Hexanal & 2.12 & 2.42 & 3.28 \\
1-pentanol & 1.30 & 1.68 & 2.60 \\
1-octen-3-ol & 1.50 & 2.10 & 4.10 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: ZnO sensor’s response to pure compounds (T = 523 K). Figure 1: Electron microscopy (SEM) image of the ZnO powder as prepared at 298 K and after the annealing treatment at 673 K (**a**), X-ray patterns of ZnO from 298 K to 673 K and (inset) the temperature shift at 20 = 31.7\({}^{\circ}\) (**b**). of ZnO to hexanal at 5 ppm and the growth rate of the signal with the concentration (in Figure 2b) indicate that the sensor's surface at C = 50 ppm of hexanal is close to the saturation of the adsorption centers so that the increase in the response is close to reaching an asymptotic value [22; 23; 24]. The response of the MOX sensor is the result of a complex interaction between the oxide surface and the adsorbed gas molecules so that the electrical properties of the ZnO layer are modified due to the effect of the redox reactions taking place with the adsorbed molecules and the oxygen ions on the layer surface [20; 21; 25; 26; 27; 28; 29; 30]. The organic substances considered in our tests are chemically different with different reagent groups, which makes the comparison of the sensor response more difficult: hexanal is an alkyl aldehyde, 1-octen-3-ol is an unsaturated secondary alcohol, and 1-pentanol is an aliphatic alcohol. In these cases, the mechanism of reaction involves the preferential interaction of hexanal with the ZnO surface, which occurs through the carbonyl group of the aldehyde [5; 7], while in the case of alcohols, the main interaction occurs through the hydroxyl group. Both aldehyde and alcohol compounds interact with oxygen ions on the sensor's surface by oxidation reactions of the respective functional groups. Another important property that can affect the interaction at the interface between a vapor and the sensor's surface is the volatility of the interacting substance. Table 3 shows the boiling temperatures and vapor pressures of the three pure compounds for a comparison with the observed sensor responses. Hexanal, as expected, is the chemical compound that showed the lowest boiling temperature and highest vapor pressure; however, the sensor showed a higher response to 1-octen-3-ol than 1-pentanol despite the higher Tb and lower vapor pressure shown by unsaturated alcohol (Figure 2a).