## 3 Results
The detection limit of the sensing system, calculated as the acetone concentration that produced an analytical signal three times the standard deviation of the RTP intensity in pure air, was 9 ppm of acetone in air. This value is competitive with those obtained by other reported portable acetone sensing devices, as summarized in Table 1.  
The precision was evaluated in terms of repeatability (n = 4). The RSD values on the blank and samples were below 3% in all cases for the analysis of a gaseous sample containing 157 ppm of acetone. A linear response was found up to 1700 ppm (maximum concentration assayed; see Figure 8b). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sensing Device** & **Limit of Detection (ppm)** & **Working Principle (Analytical Signal)** & **Working Temperature** & **Reference** \\ \hline Mn:ZnS QDs (this work) & 9 & Quenching on the photoluminescence & Room temperature & This reference \\ \hline Si:WO\({}_{3}\) sensor & 0.02 & Chemo-resisist channel change & 350 \({}^{\circ}\)C & [25] \\ \hline Cataluminescence sensor & 5 & \begin{tabular}{c} Electron release in \\ the presence of an \\ analyte \\ \end{tabular} & 205 \({}^{\circ}\)C & [26] \\ \hline \begin{tabular}{c} YSZ-based \\ electrochemical sensor \\ \end{tabular} & 0.3 & \begin{tabular}{c} Electrochemical \\ interaction \\ \end{tabular} & 675 \({}^{\circ}\)C & [27] \\ \hline Si:WO\({}_{3}\) sensor & 0.02 & \
\begin{tabular}{c} Chemo-resisist \\ resistance change \\ \end{tabular} & 325–500 \({}^{\circ}\)C & [28] \\ \hline Ultrathin InN epilayer & 0.4 & Resistance change & 200 \({}^{\circ}\)C & [29] \\ \hline SnO\({}_{2}\) thin films & 2 & Resistance change & Room temperature & [30] \\ \hline Ni-doped ZnO nanorods & 100 &
\begin{tabular}{c} Photo-induced \\ electron release \\ \end{tabular} & Room temperature & [31] \\ \hline In\({}_{2}\)O\({}_{3}\) nanowires & 25 & Resistance change & 400 \({}^{\circ}\)C & [32] \\ \hline Ferroelectric WO\({}_{3}\) nanoparticles & 0.2 & Resistance change & 400 \({}^{\circ}\)C & [33] \\ \hline ZnO thin-films & 100 & Resistance change & 200 \({}^{\circ}\)C & [34] \\ \hline Sr-doped nanostructured & 500 & Resistance change & 275 \({}^{\circ}\)C & [35] \\ LaFeO\({}_{3}\) & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of the detection limits for acetone quantification obtained using the developed sensor with other reported portable acetone sensing devices. Figure 9: Quenching effect of acetone (three injections of 157 ppm acetone). The emission stability was evaluated both temporally and by studying the effect of variations on the humidity and temperature of the air stream on the phosphorescence emission under different conditions: dry air flux, humid air flux, hot air flux, and cold air flux. \"Dry air flux\" refers to the air stream coming directly from the compressed-air cylinders (purity \(>\) 99.999%), in which the H\({}_{2}\)O content is negligible (and is therefore considered to have a 0% relative humidity content). \"Humid air flux\" refers to air bubbled through a bubble gas humidifier three-quarters filled with water (which is widely assumed to result in a 100% relative humidity content due to the equilibrium present between water and gas in the top part of the bubbler). \"Cold air flux\" refers to room-temperature readings at 20 \({}^{\circ}\)C. \"Hot air flux\" refers to a system in which the tube used to circulate the air stream is heated using flexible heating blankets to ensure an air temperature of 50 \({}^{\circ}\)C.  
No significant changes in the emission intensity of the sensing material were observed between dry and humid flows; the difference in intensity was below 3%. However, a significant decrease in luminescence (around 10%) was registered in cold conditions. This could be attributed to the condensation of the impurities present in the gas carrier in the sensing phase or to a change in the optical path due to an increase in the sol-gel packing density at low temperatures.