## 3 Capacitive Sensing with Electronic Detection
### Response of the Electronic Detection System to TNT and RDX Vapours  
The measurements were performed in the controlled laboratory environment using a gas generator as described in Section 4.1, as well as in a real environment. The sampling gas was pumped to the sensor using miniature piezoelectric pumps with a flow rate of 15 mL/min. For laboratory experiments, the gas input was switched between dry N\({}_{2}\) gas and the N\({}_{2}\) contaminated with target molecules in equal proportion. At room temperature, the vapour pressure of the TNT is 6 \(\times 10^{-4}\) Pa, which means that the density of target TNT molecules relative to the N\({}_{2}\) molecules is at the vapour pressure in a range of \(X_{{}_{\mathrm{r}arg}}\) = 10\({}^{-9}\)[32]. Figure 14 shows the response of one channel to the gas switched between pure N\({}_{2}\) and N\({}_{2}\) contaminated with TNT in equal proportions. From the difference between the two readings, \(\Delta N_{{}_{\mathrm{TNT,CE}}}\) = 4000, the standard deviation \(\sigma_{{}_{\mathrm{CE}}}\) = 30 and the bandwidth \(BW_{{}_{\mathrm{CE}}}\) = 14 Hz and taking into account the fact that the signal must be at least three times as large as the background noise (CE stands for **C**apacitive sensing with **E**lectronic detection), one can estimate the normalized sensitivity of the sensor and measurement system for TNT target molecules using Equation (5):  
\[S_{{}_{\mathrm{TNT,CE}}}\cong\frac{0.5\cdot X_{{}_{\mathrm{r}arg}}\cdot\sigma _{{}_{\mathrm{CE}}}\cdot 3}{\Delta N_{{}_{\mathrm{TNT,CE}}}\sqrt{BW_{{}_{ \mathrm{CE}}}}}=\frac{3\cdot 10^{-12}}{\  
Figure 15 shows the concentration dependence of the response of CE measurement system, using the comb sensor that was surface-functionalized with APS molecules. Panel (a) shows the time dependence of CE-APS response to the gas, switched between pure N\({}_{2}\) and N\({}_{2}\) with added TNT molecules in controlled concentration. During the measurement, the concentration of TNT molecules was increased in steps of 20% of the vapour pressure of TNT. The amplitude of the response of CE-APS measurement system to various TNT concentrations is shown in Figure 15b.  
Figure 14: Measured response of a CE (Comb capacitive sensor with Electronic detection) surface-functionalized with APS molecules, to the gas, switched between pure N\({}_{2}\) and N\({}_{2}\) contaminated with 50% vapour pressure of TNT at room temperature. The normalized sensitivity of the sensor and the measurement system to the vapour of TNT is approximately \(3\times 10^{-12}\). The RESPONSE is dimensionless and represents the number at the output of the DSP that is proportional to the relative change of the capacitance \(\delta C_{x}\big{/}C_{y}\).  
Figure 15: Response of a CE measurement system using comb sensors, surface-functionalized with APS (CE-APS), to various concentrations of TNT vapours in the carrier gas N\({}_{2}\). **(a)** Time-dependence of the CE-APS response to the gas, switched between pure N\({}_{2}\) and N\({}_{2}\) with added TNT vapours of different concentrations. The concentration of TNT was first increased in steps of 20% and then decreased back. Black symbols represent raw data, red symbols are 2000 point averaged data. **(b)** Signal of the CE-APS measurement system as a function of peak concentration of TNT in N\({}_{2}\).