## 4 Discussion  
Bromocresol green was successfully embedded into a sol-gel matrix and then deposited onto a filter paper base to build a vapochromic sensor. The deposition was confirmed by both the visible green color of the sensor and the surface image of the sensor obtained via scanning electron microscopy. Some aspects of the sol-gel formulation were optimized, including the amount of base catalyst and indicator dye added. In addition to other factors, such as catalyst concentration and solution temperature, gelation time for the sol-gel solution has previously been observed to be dependent on the time of hydrolysis for the tetraethyl orthosilicate (TEOS) alkyl chains [35]. However, in the case of the two-catalyst process implemented here, it was the amount of base catalyst that had a more immediate and visible role in the polycondensation of the sol-gel material. Contrary to the initial hypothesis, gelation time did not decrease with the increase in the base catalyst. Based on the results in Table 1, both an excess and deficiency of the base catalyst caused a longer gelation time in this formulation. Similarly, an excess of pH indicator dye loaded into the sensor (0.12% \(w/v\) and 0.08% \(w/v\)) caused a decrease in reaction time, whereas an insufficient amount (0.02% \(w/v\)) did not produce a sensor with an indistinguishable color change. During this sensor development, ammonium hydroxide (NH4 OH) was identified as a potential target chemical that would generate a basic vapor in its surrounding environment and trigger a colorimetric response in the sensor. In the headspace of the target chemical, the sensor will change color from green to blue (Figure 2). Depending on the NH4 OH concentration, this reaction can take place between 2 s to 132 s on average. Once exposed to NH4 OH, the paper sensor can be returned to its original state by placing it in the oven at 50 \({}^{\circ}\)C to evaporate any lingering chemicals. It cannot be assumed that other ammonium-containing compounds will also generate a sensor response. Initial testing in the headspace of ammonium nitrate (NH4 NO3 or AN) did not trigger a positive response since AN does not create a basic environment; it exists in an equilibrium with ammonia and nitric acid [36]. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Conditions**} & \multirow{2}{*}{**Sensor**} & \multicolumn{2}{c}{**Headspace Analysis**} \\ \cline{3-4}  & & **2E1H Contamination** & **2E1H Contamination** \\  & & **Detected in Jar B** & **Detected in Kit** \\ \hline
25 \({}^{\circ}\)C (47.5\% RH) & No change & n.d. & n.d. \\ \hline
50 \({}^{\circ}\)C (9.22\% RH) & 24 h & 48 h & 24 h \\ \hline
70 \({}^{\circ}\)C (3.17\% RH) & 24 h & 24 h & 24 h \\ \hline Control (25 \({}^{\circ}\)C) & No change & n.d. & n.d. \\ \hline n.d.—not detected. & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Results for sensor performance indicating the time that contamination was detected and the sensor changes color. This is indicative of a nonbasic environment in the headspace and, thus, would not allow the sensor to react and change color. Other compounds with similar alkaline properties to ammonium hydroxide may also work as a target chemical for the sensor's application. The reported working range for the bromocresol green pH indicator is pH 3.8-5.4 [34]; however, the BCG sensor was found to cease color change between 10 ppm and 4 ppm dilutions of the target chemical, corresponding to a limit of detection at a pH range between pH 7.53 \(\pm\) 0.19 and 7.89 \(\pm\) 0.06 in the headspace of the NH\({}_{4}\)OH dilutions. Low standard deviations throughout the analyses performed on the sensors indicated good reproducibility for the developed sensor. The decrease in sensitivity in the vapor phase compared to aqueous BCG solutions might be due to a limited contact between the indicator molecules and the air moisture as opposed to the direct contact that occurs when they are submerged or mixed into a solution.