**3.3 Optical device sensors**

*Gas Sensors*

For maximizing the renewable energy recovery, Xue and coworkers designed a flexible hierarchical PANI/CNT nanocomposite film-based electronic gas sensor for a real-time monitoring of NH3 in anaerobic digestion from 200 ppb to 50 ppm at room temperature [136]. The sensor exhibited a fast response/recovery time with excellent selectivity to NH3 compared to other VOCs, such as methanol, ethanol, acetone, dichloromethane, isopropyl alcohol, ethylene glycol, and pyridine due to the high surface area of nanocomposite films. An in situ synthesis of SnO2-rGO)- PANI (SGP) nanocomposite via surfactant-free precursor at low temperature was investigated for enhanced performance of NH3 gas sensor [137]. From XPS, the well-defined p-n hetero junction existed in the hybridized SGP nanocomposite dramatically enhanced the sensing activity, selectivity, and chemical stability, in comparison of pure SnO2 and SnO2-rGO hybrid. In addition, Ye et al. reported the rGO/Poly (3-hexylthiophene) (rGO/P3HT) composite film prepared by spray process for constructing the resistive NH3 sensor [138]. The composite film sensor exhibited better sensing properties and reversibility than pure rGO, as a result of π-π interaction between rGO and P3HT. Moreover, Sharma and coworkers demonstrated the synthesis of Al-SnO2-PANI, MWCNT-PANI, and MWCNT-PEDOTpolystyrene sulfonic acid (PSS) nanofibers via electrospinning technique for H2 and NH3 gas sensing application [139, 140]. On exposure to 1000 ppm of H2 gas, the Al-SnO2-PANI nanofiber composite showed fast response/recovery at 48°C [139]. MWCNT-PEDOT-PSS was found to be more effective than MWCNT-PANI composite in terms of sensitivity and repeatability for NH3 gas [140]. However, the sensor presented a major challenge of complete recovery of chemisorbed NH3 from CNT; the research group proposed a trial experiment for sensor complete recovery within

a short time (20 min) using a combination of heat and DC electric field.

Besides, various metals and/or metal oxides were also introduced to further enhance the response/recovery kinetics of the sensing materials. Chemiresistor gas sensing behavior of NH3 based on nanostructured PPY/SnO2 [141], PPY/ZnO [142–144], PPY/Zn2SnO4 [145], PPY/Ag-TiO2 [146], PPY/silicon nanowires (PPY/ SNWs) [147], PANI/SnO2 [148], PANI/ZnO [149], PANI/In2O3 [150], PANI/TiO2 [151], PANI/flower-like WO3 [152], PANI/SnO2/rGO [153], PANI-TiO2-Au [154], and Ag-AgCl/PPY [155] has recently been studied so far. The CP/metal oxide nanocomposite thin films exhibited an outstanding response time of 2 S for NH3 at very low concentration of 50 ppb in air with respect to methanol and ethanol vapors [156]. Thin films of Cu/PANI have also been examined as a sensor toward different gases, such as NH3, CO, CO2, NO, and CH4 at room temperature [157]. Incorporation of Cu nanoparticles improved the response and the recovery times, in addition to its excellent selectivity toward NH3 due to doping and dedoping processes of PANI. Composite of Pd-PANI-rGO [158] has been recently synthesized to fabricate a highly sensitive and selective chemiresistive H2 gas sensor. In addition to high surface area of the PANI-GO composite, the fast spillover effect and hydrogen dissociation over Pd significantly enhanced the sensing performance. Other studies by Xu and coworkers employing films of SnO2-ZnO/PANI [159] and SnO2/ PANI [160] hybrids as NO2 gas sensors prepared by the solvothermal hot-press (SHP) process were demonstrated. The later sensors exhibited much high affinity and selectivity to a low concentration of NO2 gas at 140°C caused by the formation of p-n junction. For porous SnO2-ZnO/PANI, a high selective sensor responded to a low NO2 concentration at 180°C, due to the porous nature of SnO2 and high ZnO content (20 wt.%). Mane et al. investigated chemiresistive NO2 gas sensors based on DBSA-doped PPY/WO3 and CSA-doped PPY/NiO nanocomposites at room temperature [161, 162]. The sensor can successfully detect NO2 gas at a concentration as low as 5 ppm. The enhanced gas sensing properties would be assigned to the formation of random nano p-n junctions distributed over the polymer surface film

**134**

The gas sensors based on optical transductions are described as change in absorbance and luminescence as a result of gas analytes, interaction with a sensitive material [164]. For signal generation, optical parameters such as refractive index and reflectivity have been used. Optical gas sensors have been recently utilized for multi-analyte array-based gas sensing, due to low cost, miniaturized optoelectronic light sources, and efficient detectors [164]. Based on the signal generated due to intrinsic properties of sensing material, optical sensors are classified as absorption and luminescence.
