## 4 Results
### Sensor Characterizations with Interface Electronics
The repeatability of the sensor's characteristics was assessed at 90 \({}^{\circ}\)C for 5 ppm over five cycles, with nearly identical frequency changes, response times, and consistency observed across all cycles. The sensor's selectivity to gases similar to those found in spacecraft was evaluated, with the concentrations of the tested gases remaining within the spacecraft's allowable range. As shown in Figure 10, at 90 \({}^{\circ}\)C and a 2 ppm NO\({}_{2}\) concentration, the sensor exhibited a significant frequency shift of approximately 2208 Hz, while only minimal changes were observed for C\({}_{2}\)H\({}_{6}\), CO\({}_{2}\), and H\({}_{2}\) gases. This suggests that the developed sensor exhibits high selectivity for NO\({}_{2}\), which can be attributed to the highly reactive sensing material and its optimized operating temperature. For comparison of the sensor properties in terms of the internal volume of the sensing material, the sensor with a graphene/MoS\({}_{2}\) film as the sensing material was also characterized (Figure 11). The entire measurement process was carried out in the same manner as for the previously mentioned sensor. At room temperature, the sensor's signal-to-noise ratio (SNR) is too low to enable measurements. Using a microheater, the temperature was set to 90 \({}^{\circ}\)C, and a 5 ppm NO\({}_{2}\) concentration was applied. A significant frequency shift of 500 Hz was observed, along with a fast response time of 10 s and a recovery time of 14 s. The sensitivity, evaluated from the calibration plot for NO\({}_{2}\) concentrations in the range of 2-10 ppm, was \(-\)161.3 Hz/ppm (Figure 11a). Compared to the initially measured sensor with graphene/MoS\({}_{2}\) flakes, the frequency shift and response time of the graphene/MoS\({}_{2}\) film sensor were slightly worse. This degradation can be attributed to several factors, including the MoS\({}_{2}\) film, which covers the entire sensing surface and limits the carrier changes in the graphene beneath it. Additionally, the increased total mass of the sensing material results in smaller percentage changes in mass due to NO\({}_{2}\) gas adsorption. The minimum detection limit (MDL) was approximately 300 ppb, based on the concentration of NO\({}_{2}\) that produced a signal three times the standard deviation of the blank (without NO\({}_{2}\)). The repeatability characteristics were measured over five cycles at a concentration of 5 ppm. Almost identical responsiveness was observed in all cycles, confirming stable repeatability. The evaluated sensitivity was 161 Hz/ppm at 90 \({}^{\circ}\)C, which is lower than that of the sensor with graphene/MoS\({}_{2}\) flakes as the sensing material (Figure 11d). A selectivity test was conducted using gases that coexist inside spacecraft. The developed sensor exhibited a significant frequency change of 139 Hz for 2 ppm NO\({}_{2}\), while CO\({}_{2}\) and \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sensing Material** & **Center Freq. (MHz)** & **Res. (kHz/ppm)** & **Res./Rec. Time (s)** & **Temp.** & **Ref.** \\ \hline TiO\({}_{2}\) NP & 135.8 & 8.5/100 & 310/182 & RT & [34] \\ g-C\({}_{3}\)N\({}_{4}\)@TiO\({}_{2}\) NP & 135.7 & 19.7/100 & 143/114 & RT & [34] \\ Ag/rGO-ppy & 246.99 & 13.2/100 & 68/190 & RT & [35] \\ GO-PEDOT:PSS & 135.78 & 5.7/100 & 35/10 & RT & [36] \\ ZnO nano belt & 111 & 3.5/10 & 90/140 & 160 \({}^{\circ}\)C & [37] \\ Graphene/MoS\({}_{2}\) film & 222 & 0.14/2 & 10/14 & 90 \({}^{\circ}\)C & This work \\ Graphene/MoS\({}_{2}\) flakes & 222 & 2.1/2 & 2.5/8 & 90 \({}^{\circ}\)C & This work \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of response/recovery time and sensitivity in SAW-based NO\({}_{2}\) sensors from previous studies. Table 3 presents a comparison of the characteristics between the two types of sensors we developed and the existing SAW sensors reported in the literature. The reported SAW gas sensors suffer from long response and recovery times, ranging from tens to hundreds of seconds.