# Ammonia Gas Sensors: Comparison of Solid-State and Optical Methods
## 2 Solid-State Ammonia Sensors
The sensors are chemo-resistive and, therefore, can be used in wearable or handheld portable applications. Moreover, their resistance can be monitored by utilizing triboelectric charging in self-powered wearable applications. The second group of solid-state ammonia sensors consists of conducting polymer sensors. They represent important class of functional organic materials for next-generation sensors. Their features of high surface area, small dimensions, and unique properties have been used for various sensor constructions. Many remarkable examples have been reported over the past decade. The enhanced sensitivity of conducting polymer nanomaterials toward various chemical/biological species and external stimuli made them ideal candidates for incorporation into the design of the sensors. However, the selectivity and stability can be further improved. Advances in nanotechnology allow the fabrication of various conducting polymer nanomaterials [29]. Among the conducting polymers, polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), and poly (butyl acrylate) (PBuA) or poly (vinylidene fluoride) (PVDF) are the most frequently used in the ammonia sensor as the active layer [30; 31; 32]. Gas sensors with conducting polymer are based on amperometric, conductometric, colorimetric, gravimetric, and potentiometric measuring techniques. However, their selectivity and stability should be also improved. Some recently reported solid-state ammonia sensors are summarized in Table 1.  
Colorimetric sensors utilize optical measurements of the sample (porous matrix) interacting with the ambient gas. Therefore, we present these gas sensors in the group of solid-state sensors. They are used for the measurement of NH\({}_{3}\) in blood, urine, and wastewater. Colorimetric gas sensors are based on the change in color of a chemochromic reagent incorporated in a porous matrix such as porphyrin-based or pH indicator-based films. To detect this change in color, three basic components are needed: a light source, the chemochromic substance, and the light sensor. [PERSON] et al. [43] has developed a solid-state, portable, and automated device capable to measure total ammonia amount in liquids, including the biological samples (e.g., urine). The idea of operation of the colorimetric sensor is shown in Figure 2. A horizontal gas flow channel passing through the sensing chamber, a red LED light source, and four photodiodes (a sensing-reference pair and a sensing-reference backup pair) were applied. The target gas is exposed to the sensor which then exhibits a color change proportional to the NH\({}_{3}\) concentration. The photodiodes convert the color change to electronic signals. Such a sensor is of \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sensor** & **Detection** & **Response** & **Recovery Time** & **Operation** & **Reference** \\
**Material** & **Limit** & **Time** & **Recovery Time** & **Temperature** & **Reference** \\ \hline \hline \multicolumn{6}{c}{**Metal Oxide**} \\ \hline SnO\({}_{2}\)/In\({}_{2}\)O\({}_{3}\) & 100 ppb & \(\sim\)0.1 min & 10 s & RT & [33] \\ SnO\({}_{2}\)/Pd/RGO & 2 ppm & 7 min & 50 min & RT & [34] \\ SnO & 5 ppm & \(<\)2 min & 30 s & RT & [35] \\ TiO\({}_{2}\)/GO/PANI & 100 ppm & \(\sim\)0.5 min & 17 s & RT & [36] \\ SnO\({}_{2}\) (type & & & & \\ GGS10331) & 5 ppm & \(<\)1 min & \(\sim\)few minutes & 300–500 \({}^{\circ}\)C & [37] \\ \hline \hline \multicolumn{6}{c}{**Conducting Polymer**} \\ \hline PANI/SWNT & 50 ppb & \(\sim\)few minutes & \(\sim\)few hours & RT & [38] \\ PANI-TiO\({}_{2}\)-gold & 10 ppm & - & - & RT & [39] \\ PANI/TiO\({}_{2}\) & 25 ppb & \(<\)1.5 min & - & RT & [40] \\ PANI/Cu & 1 ppm & \(\sim\)0.1 min & 160 s & RT & [41] \\ PANI/graphene & 1 ppm & \(<\)1 min & 23 s & RT & [42] \\ \hline \hline \multicolumn{6}{c}{Where: GO—graphene oxide, RGO—reduced graphene oxide, RT—room temperature, PANI—polyaniline, PPs—polypyrrole, SWNT—single-walled carbon nanotubes.} \\ \end{tabular}
\end{table} 
Table 1: Parameters of some solid-state ammonia sensors.