Recent Advances in Electrochemical Sensors for Detecting Toxic Gases: NO2, SO2 and H2S

Toxic gases, such as NOx, SOx, H2S and other S-containing gases, cause numerous harmful effects on human health even at very low gas concentrations. Reliable detection of various gases in low concentration is mandatory in the fields such as industrial plants, environmental monitoring, air quality assurance, automotive technologies and so on. In this paper, the recent advances in electrochemical sensors for toxic gas detections were reviewed and summarized with a focus on NO2, SO2 and H2S gas sensors. The recent progress of the detection of each of these toxic gases was categorized by the highly explored sensing materials over the past few decades. The important sensing performance parameters like sensitivity/response, response and recovery times at certain gas concentration and operating temperature for different sensor materials and structures have been summarized and tabulated to provide a thorough performance comparison. A novel metric, sensitivity per ppm/response time ratio has been calculated for each sensor in order to compare the overall sensing performance on the same reference. It is found that hybrid materials-based sensors exhibit the highest average ratio for NO2 gas sensing, whereas GaN and metal-oxide based sensors possess the highest ratio for SO2 and H2S gas sensing, respectively. Recently, significant research efforts have been made exploring new sensor materials, such as graphene and its derivatives, transition metal dichalcogenides (TMDs), GaN, metal-metal oxide nanostructures, solid electrolytes and organic materials to detect the above-mentioned toxic gases. In addition, the contemporary progress in SO2 gas sensors based on zeolite and paper and H2S gas sensors based on colorimetric and metal-organic framework (MOF) structures have also been reviewed. Finally, this work reviewed the recent first principle studies on the interaction between gas molecules and novel promising materials like arsenene, borophene, blue phosphorene, GeSe monolayer and germanene. The goal is to understand the surface interaction mechanism.


Introduction
Humans are exposed to various air toxins in the indoor and outdoor environment. Poor air quality is a well-known trigger for various health problems which can often result in life threatening and expensive emergency care. Therefore, precise toxic gas sensing will not only bring a major benefit to industries but also to day-to-day life for all people. Nitrogen dioxide (NO 2 ) is one of the common toxic air pollutants, which is mostly found as a mixture of nitrogen oxides (NO x ) with different ratios (x). NO 2 is a reddish-brown, irritant, toxic gas having a characteristic sharp and biting odor. The LC 50 (the lethal concentration for 50% of those exposed) for one hour of NO 2 exposure for humans has been estimated as 174 ppm. The major sources of NO 2 are from combustion of fuels such as certain coals and oil [1], biomass burning due to the extreme heat of lightning during thunderstorms [2], and nitrogen fixation by microorganisms due to agricultural fertilization [3]. The noteworthy impacts of NO 2 include: respiratory inflammation of the airways, decreased lung function due to long term exposure, increased risk of respiratory conditions [4,5], increased responsiveness to allergens, contribution to the formation of fine particulate matter (PM) and ground level ozone which have adverse health effects, and contribution to acid rain causing damage to vegetation, buildings and acidification of lakes and streams [6,7].
Sulphur dioxide (SO 2 ) is the most common air pollutant, mostly found as a mixture of sulfur oxides (SO x ). It is an invisible gas with a nasty, sharp smell. The maximum concentration for SO 2 exposures of 30 min to 1 h has been estimated as 50 to 100 ppm. The main sources of SO 2 include burning of fossil fuels (fuel oil, coal) in power stations, oil refineries, other large industrial plants, motor vehicles and domestic boilers [8,9]. It is also produced from natural sources like active volcanoes towards Cl2, NO and some common volatile organic compounds (VOCs). Results indicated that the Pd-SnO2-RGO hybrid is highly selective to NO2 gas ( Figure 1C). Preferred adsorption sites providing for NO2, high conductivity and the catalytic properties of Pd NPs are mainly responsible for the sensing performance improvement. However, no sulfur-containing gas was included in the test interference gases. Since Pd is known to interact strongly with S [68], the fabricated sensor should have been tested with S-containing gases to get the complete selectivity test picture. The same Wang et al. group [69] experimented with the introduction of oxygen vacancies (OV) into reduced graphene oxide nanosheets decorated with SnO2 nanoparticles (NPs). OVs enhance the adsorption of O2 molecules which in turn enhances the adsorption of NO2 molecules onto SnO2 NPs. Upon exposure to 1 ppm of NO2 gas, the SnO2-RGO-OVs-based sensor showed a response of 3.80 with reasonable response and recovery time. These NO2 sensing performances are better than those of other previously reported RGO-based sensors. In another study, Akbari et al. [70] decomposed methane in an arc discharge experiment to get carbonaceous materials (C-strands) between graphite electrodes. Upon NO2 exposure, the conductivity of the fabricated C strands was altered due to charge transfer between the carbon film and NO2 molecules. Previously, Zhang et al. [71] reported a rGO/Au nanocomposite-based NO2 sensor using a hydrothermal treatment. It provided good sensitivity with a quick response-recovery process at 50 °C.

Transition Metal Dichalcogenide (TMD)-based NO2 Sensors
Two-dimensional (2D) transition metal dichalcogenides (TMDs) possess semiconducting nature, high surface-to-volume ratio and atomically thin-layered structures which are useful properties required to be a convincing sensing material [72]. MoS2, WS2, ReS2, MoSe2, MoTe2, WSe2 and ReSe2 are very promising 2D TMDs for gas sensing purposes [73][74][75]. Agrawal et al. prepared in-plane and edge-enriched p-MoS2 flakes (mixed MoS2) to detect NO2 gas at room temperature [76]. A FE-SEM image of the mixed MoS2 flakes is shown in Figure 2A. The blackish region represents the in-plane MoS2 flakes and the white region represents the edge-enriched MoS2 flakes. Most likely, the edgeenriched MoS2 flakes are white due to their height from the substrate surface. Figure 2B displays a sensitivity vs NO2 concentration bar graph at RT and 125 °C. NO2 is an electron acceptor and it withdraws electrons from the MoS2 flakes, thus causing the resistance decrease of the mixed MoS2 flake-based sensor. The response and recovery time of the sensor were better at 125 °C than at RT. This happened because the adsorption energy of the NO2 gas molecule with the MoS2 flakes is very high at RT. The sensitivity of the sensor had been enhanced under UV light illumination as shown in Figure 2C. This improvement is attributed to the photoactivated desorption of adsorbed oxygen and creation of fresh active sites on the edges of MoS2 flakes. In another study, Kumar et al. [77] prepared a 1D MoS2 nanowire network which showed a detection limit of 4.6 ppb NO2 with good sensitivity. At the estimated optimum operating temperature (60 °C), response and recovery times were found In another study, Akbari et al. [70] decomposed methane in an arc discharge experiment to get carbonaceous materials (C-strands) between graphite electrodes. Upon NO 2 exposure, the conductivity of the fabricated C strands was altered due to charge transfer between the carbon film and NO 2 molecules. Previously, Zhang et al. [71] reported a rGO/Au nanocomposite-based NO 2 sensor using a hydrothermal treatment. It provided good sensitivity with a quick response-recovery process at 50 • C.

Transition Metal Dichalcogenide (TMD)-Based NO 2 Sensors
Two-dimensional (2D) transition metal dichalcogenides (TMDs) possess semiconducting nature, high surface-to-volume ratio and atomically thin-layered structures which are useful properties required to be a convincing sensing material [72]. MoS 2 , WS 2 , ReS 2 , MoSe 2 , MoTe 2 , WSe 2 and ReSe 2 are very promising 2D TMDs for gas sensing purposes [73][74][75]. Agrawal et al. prepared in-plane and edge-enriched p-MoS 2 flakes (mixed MoS 2 ) to detect NO 2 gas at room temperature [76]. A FE-SEM image of the mixed MoS 2 flakes is shown in Figure 2A. The blackish region represents the in-plane MoS 2 flakes and the white region represents the edge-enriched MoS 2 flakes. Most likely, the edge-enriched MoS 2 flakes are white due to their height from the substrate surface. Figure 2B displays a sensitivity vs. NO 2 concentration bar graph at RT and 125 • C. NO 2 is an electron acceptor and it withdraws electrons from the MoS 2 flakes, thus causing the resistance decrease of the mixed MoS 2 flake-based sensor. The response and recovery time of the sensor were better at 125 • C than at RT. This happened because the adsorption energy of the NO 2 gas molecule with the MoS 2 flakes is very high at RT. The sensitivity of the sensor had been enhanced under UV light illumination as shown in Figure 2C. This improvement is attributed to the photoactivated desorption of adsorbed oxygen and creation of fresh active sites on the edges of MoS 2 flakes. In another study, Kumar et al. [77] prepared a 1D MoS 2 nanowire network which showed a detection limit of 4.6 ppb NO 2 with good sensitivity. At the estimated optimum operating temperature (60 • C), response and recovery times were found as 16 s and 172 s, respectively, at 5 ppm NO 2 exposure. Previously, Choi et al. [78] introduced Nb atoms into 2D MoSe 2 host films. Figure 2D displays the low magnification planar annular dark-field scanning transmission electron microscopy (ADF-STEM) images and FFT patterns (inset) of MoSe 2 : Nb 1C, where 1C indicates one deposition cycle in the plasma-enhanced atomic layer deposition (PEALD) process. The polycrystal ring patterns in the image represent the presence of a few grains. Variably Nb-doped MoSe 2 sensor films were exposed to different NO 2 concentrations as shown in Figure 2E. The highest gas response was found for a MoSe 2 :Nb 1C device among the three tested devices because at low Nb dopant concentrations, MoSe 2 showed an improved NO 2 gas response due to its small grains and stabilized grain boundaries. At high Nb dopant concentrations, the NO 2 gas response was degraded due to the increase of gas-unresponsive metallic NbSe 2 regions, so an optimum Nb concentration is required for achieving a better gas response. The resistance of the MoSe 2 -based sensor gradually increased due to oxidation, whereas the Nb-doped MoSe 2 sensor showed very stable response ( Figure 2F). This means, introduction of Nb atoms onto 2D layered MoSe 2 promotes a stable gas response and the long-term stability of the sensor. Also, a significant enhancement in sensing response with quick response-recovery toward NO 2 was observed on WS 2 nanosheet functionalized with Ag NWs [79].  [78] introduced Nb atoms into 2D MoSe2 host films. Figure 2D displays the low magnification planar annular dark-field scanning transmission electron microscopy (ADF-STEM) images and FFT patterns (inset) of MoSe2: Nb 1C, where 1C indicates one deposition cycle in the plasma-enhanced atomic layer deposition (PEALD) process. The polycrystal ring patterns in the image represent the presence of a few grains.
Variably Nb-doped MoSe2 sensor films were exposed to different NO2 concentrations as shown in Figure 2E. The highest gas response was found for a MoSe2:Nb 1C device among the three tested devices because at low Nb dopant concentrations, MoSe2 showed an improved NO2 gas response due to its small grains and stabilized grain boundaries. At high Nb dopant concentrations, the NO2 gas response was degraded due to the increase of gas-unresponsive metallic NbSe2 regions, so an optimum Nb concentration is required for achieving a better gas response. The resistance of the MoSe2-based sensor gradually increased due to oxidation, whereas the Nb-doped MoSe2 sensor showed very stable response ( Figure 2F). This means, introduction of Nb atoms onto 2D layered MoSe2 promotes a stable gas response and the long-term stability of the sensor. Also, a significant enhancement in sensing response with quick response-recovery toward NO2 was observed on WS2 nanosheet functionalized with Ag NWs [79].

Metal and Metal-oxide Nanostructure-based NO2 Sensors
Metal oxides can be synthesized in various nanostructure morphologies like nanowires, nanoparticles, nanotubes, nanoflowers, nanocomposites and nanosheets for the enhancement of sensing performance [80][81][82]. Besides, porosity and permeable shell layers contribute to absolute electron depletion and gas diffusion that allow sensor devices to achieve high sensitivity toward gases [83]. Qiang et al. reported a NO2 gas sensor based on porous silicon (PS)/WO3 nanorods (NRs) functionalized with Pd NPs [84]. PS WO3 NRs were synthesized by electrochemical methods and thermal oxidation of W film, respectively. Pd NPs were deposited onto WO3 NRs, by the reduction

Metal and Metal-Oxide Nanostructure-Based NO 2 Sensors
Metal oxides can be synthesized in various nanostructure morphologies like nanowires, nanoparticles, nanotubes, nanoflowers, nanocomposites and nanosheets for the enhancement of sensing performance [80][81][82]. Besides, porosity and permeable shell layers contribute to absolute electron depletion and gas diffusion that allow sensor devices to achieve high sensitivity toward gases [83]. Qiang  functionalized with Pd NPs [84]. PS WO 3 NRs were synthesized by electrochemical methods and thermal oxidation of W film, respectively. Pd NPs were deposited onto WO 3 NRs, by the reduction of a Pd complex solution. Three different samples of PS/WO 3 NRs-Pd NPs were prepared by varying the amount of Pd NPs on the substrate. These are PS/WO 3 -Pd20, PS/WO 3 -Pd40 and PS/WO 3 -Pd60, where the order of the amount of Pd NPs is Pd60 > Pd40 > Pd20. A TEM image of PS/WO 3 -Pd60 displays the agglomeration of Pd NPs on WO 3 NRs ( Figure 3A). Gas concentration tests on the PS/WO 3 -Pd60 sensor revealed a ppb level detection capacity at RT with a faster response time ( Figure 3B). The catalytic activity of Pd NPs enhanced the NO 2 molecule adsorption and thereby enhanced the sensor response, so a PS/WO 3 -Pd60 sensor having the highest amount of Pd NPs showed the highest sensor response at room temperature ( Figure 3C). With a facile fabrication process and being compatible with the planar processes of the microelectronics industry, ultra-thin PdO films provided good sensing performances toward NO 2 [85], but they require a long recovery period (600-700 s) because of the lack of immediate interaction between NO 2 molecules and oxygen molecules adsorbed on sensor material surface. Also, ZnO nanostructured films obtained by a thermal evaporation method offered significantly enhanced response (622 at 100 ppm NO 2 ) with good response-recovery at 200 • C [86]. The microwave-synthesized NiO film has been found to operate using ultra-low power of 0.2 µW at room temperature. It achieved a response of 4991% to 3 ppm NO 2 along with fast response-recovery [87]. Moreover, a reasonable sensor response toward low concentration of NO 2 was exhibited by the multicomponent oxide CuBi 2 O 4 at 400 • C [88]. Recently, Hung et al. synthesized three sensors of ZnO (Z2, Z4 and Z6) and Zn 2 SnO 4 (ZS2, ZS4 and ZS6) NWs on microelectrode chips at 2, 4 and 6 cm from the thermal evaporation source, respectively [89]. It was found that the distance between the source and substrate strongly affected the gas response of the Zn 2 SnO 4 NW sensors. Figure 3D,E show FESEM images of the on-chip grown ZnO (Z2) NW and Zn 2 SnO 4 (ZS2) NW respectively.  Figure 3A). Gas concentration tests on the PS/WO3-Pd60 sensor revealed a ppb level detection capacity at RT with a faster response time ( Figure  3B). The catalytic activity of Pd NPs enhanced the NO2 molecule adsorption and thereby enhanced the sensor response, so a PS/WO3-Pd60 sensor having the highest amount of Pd NPs showed the highest sensor response at room temperature ( Figure 3C). With a facile fabrication process and being compatible with the planar processes of the microelectronics industry, ultra-thin PdO films provided good sensing performances toward NO2 [85], but they require a long recovery period (600-700 s) because of the lack of immediate interaction between NO2 molecules and oxygen molecules adsorbed on sensor material surface. Also, ZnO nanostructured films obtained by a thermal evaporation method offered significantly enhanced response (622 at 100 ppm NO2) with good response-recovery at 200 °C [86]. The microwave-synthesized NiO film has been found to operate using ultra-low power of 0.2 μW at room temperature. It achieved a response of 4991% to 3 ppm NO2 along with fast response-recovery [87]. Moreover, a reasonable sensor response toward low concentration of NO2 was exhibited by the multicomponent oxide CuBi2O4 at 400°C [88]. Recently, Hung et al. synthesized three sensors of ZnO (Z2, Z4 and Z6) and Zn2SnO4 (ZS2, ZS4 and ZS6) NWs on microelectrode chips at 2, 4 and 6 cm from the thermal evaporation source, respectively [89]. It was found that the distance between the source and substrate strongly affected the gas response of the Zn2SnO4 NW sensors. Figures 3D and 3E show FESEM images of the on-chip grown ZnO (Z2) NW and Zn2SnO4 (ZS2) NW respectively. The sensing performances of ZnO and Zn2SnO4 NW sensors to NO2 and other reducing gases are displayed in Figure 3F. Zn2SnO4 NW exhibited significantly better response towards NO2 gas in comparison to ZnO NW. Also, ZS2 showed higher response than ZS4 and ZS6, because placing the sensors far from the source resulted in several surface defects due to the lack of a Sn source. Responses The sensing performances of ZnO and Zn 2 SnO 4 NW sensors to NO 2 and other reducing gases are displayed in Figure 3F. Zn 2 SnO 4 NW exhibited significantly better response towards NO 2 gas in comparison to ZnO NW. Also, ZS2 showed higher response than ZS4 and ZS6, because placing the sensors far from the source resulted in several surface defects due to the lack of a Sn source. Responses for Zn 2 SnO 4 NW sensors with growth times of 15, 30 and 60 min are shown in Figure 3G. It is revealed that comparatively high or low density of NWs decreases the gas response.

GaN-Based NO 2 Sensors
Having a wide bandgap energy (3.4 eV), gallium nitride (GaN) is found to support higher peak internal electric fields than silicon or gallium arsenide (GaAs). This wide bandgap causes lower thermal electron-hole pair generation, hence allowing high working temperatures. GaN is less vulnerable to attack in caustic environments, and resistant to radiation because of the larger cohesion energies among its constituent atoms [90][91][92][93]. Bishop et al. proposed a double Schottky junction gas sensor based on BGaN/GaN [94]. Two devices were developed; first, 10 periods of 20 nm thick undoped GaN, and 20 nm thick BGaN formed the BGaN/GaN superlattice structure. Then a circular diode having 300 µm as diameter was made with a 200 µm spacing between two Pt contacts on the n-type GaN sample ( Figure 4A). When the sensors were exposed to 450 ppm NO 2 gas at different temperatures, BGaN/GaN SL sensor exhibited higher current change and sensitivity than GaN monolayer sensors ( Figure 4B). This enhancement is caused by two main reasons: firstly, BGaN has more interface traps than GaN, which creates more adsorption sites at the interface for gas molecules resulting a greater SBH change. Secondly, BGaN shows columnar growth thus a decrease in the volume-to-area ratio at the interface that provides more interface traps within a given area. It was found that at higher temperatures and concentrations, saturation of the signal change leads to a nonlinear response for the BGaN/GaN SL resulting into a decrease in the responsivity of the device ( Figure 4C). In another study, an AlGaN/GaN high electron mobility transistor (HEMT) with Pt functionalized gate demonstrated a high sensitivity of 38.5% toward 900 ppm NO 2 at high operating temperature of 600 • C [95]. The fabricated heterostructure sensor exhibited robustness under severe environmental conditions with a very quick response time of 1 s. When sensors are integrated in chips, low power sensor operation is required. Lim et al. [96] made SnO 2 sensitized AlGaN/GaN sensor operating at ultra-low power without using any heater. The fabricated sensor exhibited ppb level detection as well as fast response times. for Zn2SnO4 NW sensors with growth times of 15, 30 and 60 min are shown in Figure 3G. It is revealed that comparatively high or low density of NWs decreases the gas response.

GaN-based NO2 Sensors
Having a wide bandgap energy (3.4 eV), gallium nitride (GaN) is found to support higher peak internal electric fields than silicon or gallium arsenide (GaAs). This wide bandgap causes lower thermal electron-hole pair generation, hence allowing high working temperatures. GaN is less vulnerable to attack in caustic environments, and resistant to radiation because of the larger cohesion energies among its constituent atoms [90][91][92][93]. Bishop et al. proposed a double Schottky junction gas sensor based on BGaN/GaN [94]. Two devices were developed; first, 10 periods of 20 nm thick undoped GaN, and 20 nm thick BGaN formed the BGaN/GaN superlattice structure. Then a circular diode having 300 μm as diameter was made with a 200 μm spacing between two Pt contacts on the n-type GaN sample ( Figure 4A). When the sensors were exposed to 450 ppm NO2 gas at different temperatures, BGaN/GaN SL sensor exhibited higher current change and sensitivity than GaN monolayer sensors ( Figure 4B). This enhancement is caused by two main reasons: firstly, BGaN has more interface traps than GaN, which creates more adsorption sites at the interface for gas molecules resulting a greater SBH change. Secondly, BGaN shows columnar growth thus a decrease in the volume-to-area ratio at the interface that provides more interface traps within a given area. It was found that at higher temperatures and concentrations, saturation of the signal change leads to a nonlinear response for the BGaN/GaN SL resulting into a decrease in the responsivity of the device ( Figure 4C). In another study, an AlGaN/GaN high electron mobility transistor (HEMT) with Pt functionalized gate demonstrated a high sensitivity of 38.5% toward 900 ppm NO2 at high operating temperature of 600 °C [95]. The fabricated heterostructure sensor exhibited robustness under severe environmental conditions with a very quick response time of 1 s. When sensors are integrated in chips, low power sensor operation is required. Lim et al. [96] made SnO2 sensitized AlGaN/GaN sensor operating at ultra-low power without using any heater. The fabricated sensor exhibited ppb level detection as well as fast response times.

Organic Materials-based NO2 Sensors
Conducting and semiconducting organic films are promising gas sensing materials due to their excellent ability of tuning the chemical and physical properties on exposure to gas molecules. Also, recognition groups can be integrated covalently on organic sensing materials in order to get high selectivity and response [97]. Organic field effect transistors (OFETs) and thin film transistors (TFTs) are two major forms of organic material-based sensors. Kumar et al. [98] synthesized an OFET to detect NO2 gas using gate bias as control unit. The active layer of the OFET was the polymer poly [N-90-heptadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole] (PCDTBT). The electron removal of NO2 molecule from the p-type conducting polymer PCDTBT led to an increase of conductivity. The typical transfer and output characteristics of OFET sensor are shown in Figures  5A and 5B, respectively. From the attained transfer and output characteristics, the mobility (μsat) and

Organic Materials-Based NO 2 Sensors
Conducting and semiconducting organic films are promising gas sensing materials due to their excellent ability of tuning the chemical and physical properties on exposure to gas molecules. Also, recognition groups can be integrated covalently on organic sensing materials in order to get high selectivity and response [97]. Organic field effect transistors (OFETs) and thin film transistors (TFTs) are two major forms of organic material-based sensors. Kumar et al. [98] synthesized an OFET to detect NO 2 gas using gate bias as control unit. The active layer of the OFET was the polymer poly [N-90-heptadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole] (PCDTBT). The electron removal of NO 2 molecule from the p-type conducting polymer PCDTBT led to an increase of conductivity. The typical transfer and output characteristics of OFET sensor are shown in Figure 5A,B, respectively. From the attained transfer and output characteristics, the mobility (µ sat ) and threshold voltage (V th ) were obtained as 1.13 × 10 −4 cm 2 V −1 s −1 and −9 V. From the gas concentration test, as shown in Figure 5C, the response increases linearly up to 10 ppm of exposure and then the increasing trend drops at higher concentrations. This happens because most of the active adsorption sites of the active PCDTBT layer get populated by NO 2 molecules. The response and recovery time of the sensor at 1 ppm of NO 2 exposure were obtained as shown in the inset of Figure 5C. The selectivity of the sensor upon exposure 10 ppm of different toxic gases was studied. Figure 5D displays that the OFET sensor exhibits the highest selectivity towards NO 2 gas. Although the H 2 S gas response was moderate, the recovery was incomplete. In another study, a 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) film-based NO 2 sensor attained a sensitivity above 1000%/ppm along with quick response-recovery [99]. It was predicted that the high sensing performance is attributable to the effective charge transport on the top of low original carrier concentration. Huang et al. [100] fabricated TFTs using copper phthalocyanine (CuPc) for NO 2 gas detection. The gate dielectric used here is a UV-ozone (UVO)-treated polymer. Figure 5E shows sensitivities of the TFT biased at V D = V G = −40 V toward different NO 2 concentrations and UVO treatment times (t UVO ). It is seen that the sensitivity enhances significantly for sensors with longer t UVO at all NO 2 gas concentrations because of UVO-derived hydroxylated species on the dielectric surface. Gas selectivity tests revealed that without UVO treatment of the dielectric, the sensors are not at all selective to NO 2 gas. However, at t UVO = 360 s, the sensitivity increased from 10% to almost 600% at a concentration of 20 ppm NO 2 which is six times more sensitive than all other test gases ( Figure 5F). threshold voltage (Vth) were obtained as 1.13 × 10 −4 cm 2 V -1 s -1 and −9 V. From the gas concentration test, as shown in Figure 5C, the response increases linearly up to 10 ppm of exposure and then the increasing trend drops at higher concentrations. This happens because most of the active adsorption sites of the active PCDTBT layer get populated by NO2 molecules. The response and recovery time of the sensor at 1 ppm of NO2 exposure were obtained as shown in the inset of Figure 5C. The selectivity of the sensor upon exposure 10 ppm of different toxic gases was studied. Figure 5D displays that the OFET sensor exhibits the highest selectivity towards NO2 gas. Although the H2S gas response was moderate, the recovery was incomplete. In another study, a 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) film-based NO2 sensor attained a sensitivity above 1000%/ppm along with quick response-recovery [99]. It was predicted that the high sensing performance is attributable to the effective charge transport on the top of low original carrier concentration. Huang et al. [100] fabricated TFTs using copper phthalocyanine (CuPc) for NO2 gas detection. The gate dielectric used here is a UV-ozone (UVO)-treated polymer. Figure 5E shows sensitivities of the TFT biased at VD = VG = −40 V toward different NO2 concentrations and UVO treatment times (tUVO). It is seen that the sensitivity enhances significantly for sensors with longer tUVO at all NO2 gas concentrations because of UVO-derived hydroxylated species on the dielectric surface. Gas selectivity tests revealed that without UVO treatment of the dielectric, the sensors are not at all selective to NO2 gas. However, at tUVO = 360 s, the sensitivity increased from 10% to almost 600% at a concentration of 20 ppm NO2 which is six times more sensitive than all other test gases ( Figure 5F).

Hybrid Materials-based NO2 Sensors
In most cases hybrid materials combine and exhibit the useful characteristics of their constituent materials to promote high sensing performances. For instances, both MoS2 and RGO show good conductivity changes upon adsorption of NO2 molecules, and thus a combination of MoS2 and RGO results in high performance NO2 gas sensors [101]. Recently, Wang et al. synthesized a MoS2 nanoparticles-incorporated RGO hybrid material for NO2 detection by a two-step wet-chemical

Hybrid Materials-Based NO 2 Sensors
In most cases hybrid materials combine and exhibit the useful characteristics of their constituent materials to promote high sensing performances. For instances, both MoS 2 and RGO show good conductivity changes upon adsorption of NO 2 molecules, and thus a combination of MoS 2 and RGO results in high performance NO 2 gas sensors [101]. Recently, Wang et al. synthesized a MoS 2 nanoparticles-incorporated RGO hybrid material for NO 2 detection by a two-step wet-chemical method [102]. In the first step, from powdered MoS 2 NPs were prepared by a modified liquid exfoliation method. Next, self-assembly of MoS 2 NPs and GO nanosheets, and hydrothermal treatment provided MoS 2 -RGO hybrid nanosheets. A high magnification SEM image of MoS 2 -RGO hybrids, shown in Figure 6A, reveals the presence of NPs on the RGO surface. It was found that the response time and recovery time decrease with increasing operating temperature while the sensor responses to NO 2 remain almost unchanged. The optimum operating temperature was obtained as 160 • C. A response-recovery curve to 3 ppm NO 2 gas at 160 • C is illustrated in Figure 6B. When the fabricated MoS 2 -RGO based sensor was exposed to NO 2 gas concentrations ranging from 100 ppb to 3 ppm, the response followed an increasing trend due to the increased amount of NO 2 molecules absorbed ( Figure 6C). Wang et al. [103] synthesized a hybrid sensing material made of ZnO and poly(3-hexylthiophene) for the detection of NO 2 at room temperature. The fabricated nanosheet-nanorod structured bilayer film sensor showed a sensitivity of 180% at 50 ppm of gas exposure.
The sensing performance metrics like sensitivity/response, response and recovery times at certain gas concentration and operating temperatures, and sensitivity per ppm/response time ratio for different NO 2 sensor materials and structures are summarized in Table 1. It provides a brief comparative performances outline among different NO 2 sensor reported in recent years. method [102]. In the first step, from powdered MoS2 NPs were prepared by a modified liquid exfoliation method. Next, self-assembly of MoS2 NPs and GO nanosheets, and hydrothermal treatment provided MoS2-RGO hybrid nanosheets. A high magnification SEM image of MoS2-RGO hybrids, shown in Figure 6A, reveals the presence of NPs on the RGO surface. It was found that the response time and recovery time decrease with increasing operating temperature while the sensor responses to NO2 remain almost unchanged. The optimum operating temperature was obtained as 160 °C. A response-recovery curve to 3 ppm NO2 gas at 160 °C is illustrated in Figure 6B. When the fabricated MoS2-RGO based sensor was exposed to NO2 gas concentrations ranging from 100 ppb to 3 ppm, the response followed an increasing trend due to the increased amount of NO2 molecules absorbed ( Figure 6C). Wang et al. [103] synthesized a hybrid sensing material made of ZnO and poly(3-hexylthiophene) for the detection of NO2 at room temperature. The fabricated nanosheetnanorod structured bilayer film sensor showed a sensitivity of 180% at 50 ppm of gas exposure. The sensing performance metrics like sensitivity/response, response and recovery times at certain gas concentration and operating temperatures, and sensitivity per ppm/response time ratio for different NO2 sensor materials and structures are summarized in Table 1. It provides a brief comparative performances outline among different NO2 sensor reported in recent years.

Carbon Material-Based SO 2 Sensors
Aligned carbon nanotubes possess high surface-to-volume ratios which promote efficient physical and chemical adsorption of target gases [114]. Recently Zouaghi et al. have initiated research on vertically aligned carbon nanotube (VACNT)-based gas sensors interrogated by THz radiation [115]. They synthesized VACNT on SiO 2 coated, boron-doped Si substrate by a water-assisted chemical vapor deposition method. Figure 7A shows a SEM image of vertically aligned CNT indicating a layer thickness of 95µm. The transmission spectrum upon SO 2 gas exposure is illustrated in Figure 7B. The denser rotational spectrum of SO 2 is attributed to the bent structure of SO 2 molecule. The highest relative transmittance was obtained around 0.2 THz. When SO 2 gas was flowed abruptly into a Si/SiO 2 /VACNT sensor, the maximum of transmitted electric field amplitude decreased to a steady value with fast response time of 2-3 min ( Figure 7C). However, the recovery time was too long (>70 min). It has been predicted that the slow recovery was caused from the high sticking coefficient of SO 2 gas to steel walls in the system. In a previous research, cholesteric-nematic mixture intercalated with CNT walls had been prepared and physical adsorption between the CNT and SO 2 molecules was observed [116]. This adsorption phenomenon altered the CNT conductivity that in turn resulted into sensing signal for SO 2 . Zhang et al. synthesized TiO 2 /graphene film using layer-by-layer self-assembly technique for room temperature SO 2 detection [117]. Excellent contacts between TiO 2 and rGO are achieved from the wrapping of rGO flakes on TiO 2 nanosphere surface or bridge-connection between TiO 2 balls as shown in SEM image ( Figure 7D). The sensor was exposed to 1, 50, 250, 1000 ppb SO 2 gas to study the response-recovery behavior plotted in Figure 7E. It was observed that with increasing gas concentration, the sensor response kept increasing but the response-recovery time became longer. It has been predicted that the large interspace is responsible for the increase of response and recovery time. The TiO 2 /rGO film sensor showed much higher sensitivity to 1 ppm SO 2 gas at room temperature than other target gases such as-CH 4 , C 2 H 2 , H 2 , CO, NO 2 ( Figure 7F). So, the synthesized sensor was selective enough to SO 2 gas.

Carbon Material-based SO2 Sensors
Aligned carbon nanotubes possess high surface-to-volume ratios which promote efficient physical and chemical adsorption of target gases [114]. Recently Zouaghi et al. have initiated research on vertically aligned carbon nanotube (VACNT)-based gas sensors interrogated by THz radiation [115]. They synthesized VACNT on SiO2 coated, boron-doped Si substrate by a water-assisted chemical vapor deposition method. Figure 7A shows a SEM image of vertically aligned CNT indicating a layer thickness of 95μm. The transmission spectrum upon SO2 gas exposure is illustrated in Figure 7B. The denser rotational spectrum of SO2 is attributed to the bent structure of SO2 molecule. The highest relative transmittance was obtained around 0.2 THz. When SO2 gas was flowed abruptly into a Si/SiO2/VACNT sensor, the maximum of transmitted electric field amplitude decreased to a steady value with fast response time of 2-3 min ( Figure 7C). However, the recovery time was too long (>70 min). It has been predicted that the slow recovery was caused from the high sticking coefficient of SO2 gas to steel walls in the system. In a previous research, cholesteric-nematic mixture intercalated with CNT walls had been prepared and physical adsorption between the CNT and SO2 molecules was observed [116]. This adsorption phenomenon altered the CNT conductivity that in turn resulted into sensing signal for SO2. Zhang et al. synthesized TiO2/graphene film using layer-bylayer self-assembly technique for room temperature SO2 detection [117]. Excellent contacts between TiO2 and rGO are achieved from the wrapping of rGO flakes on TiO2 nanosphere surface or bridgeconnection between TiO2 balls as shown in SEM image ( Figure 7D). The sensor was exposed to 1, 50, 250, 1000 ppb SO2 gas to study the response-recovery behavior plotted in Figure 7E. It was observed that with increasing gas concentration, the sensor response kept increasing but the response-recovery time became longer. It has been predicted that the large interspace is responsible for the increase of response and recovery time. The TiO2/rGO film sensor showed much higher sensitivity to 1 ppm SO2 gas at room temperature than other target gases such as-CH4, C2H2, H2, CO, NO2 ( Figure 7F). So, the synthesized sensor was selective enough to SO2 gas.

Metal and Metal-Oxide Nanostructures-Based SO 2 Sensors
Many attempts had been made for SO 2 gas detection using various semi-conducting metal oxides, such as-CeO 2 , WO 3 , V 2 O 5 -TiO 2 , MoO 3 -SnO 2 and NiO [118]. However, due to instability in the highly reducing atmospheres, these sensors can only operate at low temperature (<500 • C) [119]. Liu et al. fabricated ZnO nanosheets decorated with Ru/Al 2 O 3 catalyst and integrated them with a microsensor to detect SO 2 gas [120]. Inkjet printing technology was used to load the sensor. AFM image in Figure 8A reveals the uniformity of the prepared ZnO 2D nanosheet and the thickness is indicated as about 1.5 nm. Different concentrations of SO 2 gas had been exposed to Ru/Al 2 O 3 /ZnO sensor and the corresponding resistance responses are shown in Figure 8B. It is seen that resistance notably decreased at SO 2 exposure and percent sensor response increased linearly with SO 2 concentration. At 25 ppm of SO 2 , the obtained response and recovery times were about 1 min and 6 min, respectively. The SO 2 selectivity test is displayed in Figure 8C, where the fabricated sensor responded negligibly to the test gases CO, CH 3 OH, C 2 H 5 OH, acetone, CO 2 , NO and HCHO in comparison to SO 2 gas. From on-line mass spectrometry experiments, it was found that the catalyst Ru/Al 2 O 3 dissociates SO 2 molecules into easily detectable SO• species. Being captured by ZnO nanosheet, these species contribute to the sensor output signal. In another study, Ciftyürek et al. prepared and then evaluated molybdenum and tungsten binary and ternary oxide thick films for gas sulfur species sensing [121]. It was found that hydrothermally synthesized nano-SrMoO 4 exhibited the highest sensor response among those fabricated oxide films. The SrMoO 4 -based sensors were able to operate at very high temperature (>600 • C) while maintaining their sensing performances, and thus can be useful in gas monitoring at industries. SnO 2 thin film had been prepared by Tyagi et al. [122] using sputtering technique. Then, the film was functionalized with various metal oxide catalyst such as-PdO, CuO, NiO, MgO, V 2 O 5 to make SO 2 gas sensor. The uniform distribution of NiO nanoclusters on the surface of SnO 2 film is noticed in the SEM image ( Figure 8D). 500 ppm of SO 2 gas was exposed to different metal-oxides deposited on SnO 2 sensors to study the response at various operating temperatures ( Figure 8E). NiO/SnO 2 structure showed the highest response (∼56) at 180 • C due to two main reasons. Firstly, the spill-over effect from NiO nanoclusters toward SO 2 molecules. Secondly, increase of adsorbed oxygen species sites at the porous and rough surface of SnO 2 film [123]. The response and recovery time of NiO/SnO 2 sensor were estimated as 80 s and 70 s respectively towards 500 ppm of SO 2 gas at 180 • C as shown in Figure 8F. Also, the sensor exhibited good reproducibility and selectivity under SO 2 exposure. In another study, it had been reported that BiFeO 3 is highly selective to SO 2 against carbon monoxide and butane [124]. Also, it was found that BiFeO 3 synthesized by a sonochemical method provides better sensing performances than when prepared by conventional methods.

GaN-based SO2 Sensors
AlGaN/GaN heterostructure semiconductors facilitate low power consumption, miniaturization and excellent sensing performances [125]. Also, AlGaN/GaN-based sensors can operate in chemically harsh environments, at high temperatures and under radiation fluxes due to having thermally and chemically stable structures [126]. Triet et al. synthesized Al0.27Ga0.73N/GaN-based Schottky diode sensors for SO2 gas detection [127]. Vertical zinc oxide nanorods (ZnO NRs) and a RGO nanosheet hybrid was formed on a AlGaN/GaN/sapphire heterostructure where the RGO and AlGaN surface made a Schottky contact with each other. From the FE-SEM image in Figure 9A, it is observed that neighboring ZnO NRs are attached to each other by RGO. During the gas exposure, the Schottky barrier between RGO and AlGaN layers changes. As a result, thermionic emission carrier transport is altered which in turn modifies the reverse saturation current. In the case of detecting SO2 ( Figure  9B), the resistance response increased with increasing gas concentration because SO2 molecules are electron withdrawers. The non-linearity of the response with gas concentration is attributed to incomplete recovery of the sensing material RGO-ZnO NRs ( Figure 9C). Here, SO2 gas molecules react with interaction sites resulting into slow diffusion of gas molecules within the RGO multilayer structure [128].

GaN-Based SO 2 Sensors
AlGaN/GaN heterostructure semiconductors facilitate low power consumption, miniaturization and excellent sensing performances [125]. Also, AlGaN/GaN-based sensors can operate in chemically harsh environments, at high temperatures and under radiation fluxes due to having thermally and chemically stable structures [126]. Triet et al. synthesized Al 0.27 Ga 0.73 N/GaN-based Schottky diode sensors for SO 2 gas detection [127]. Vertical zinc oxide nanorods (ZnO NRs) and a RGO nanosheet hybrid was formed on a AlGaN/GaN/sapphire heterostructure where the RGO and AlGaN surface made a Schottky contact with each other. From the FE-SEM image in Figure 9A, it is observed that neighboring ZnO NRs are attached to each other by RGO. During the gas exposure, the Schottky barrier between RGO and AlGaN layers changes. As a result, thermionic emission carrier transport is altered which in turn modifies the reverse saturation current. In the case of detecting SO 2 ( Figure 9B), the resistance response increased with increasing gas concentration because SO 2 molecules are electron withdrawers. The non-linearity of the response with gas concentration is attributed to incomplete recovery of the sensing material RGO-ZnO NRs ( Figure 9C). Here, SO 2 gas molecules react with interaction sites resulting into slow diffusion of gas molecules within the RGO multilayer structure [128].

Solid Electrolyte-based SO2 Sensors
Different solid electrolytes such as-NASICON [129], YSZ [130] and alkali metal sulfates [131] have been exploited during the past decades to fabricate high performance SO2 sensors. Among all the solid electrolytes, NASICON is widely used in the mixed-potential sensors due to its high ionic conductivity. Ma et al. [132] reported a mixed-potential gas sensor using NASICON and orthoferrite (La0.5Sm0.5FeO3) as sensing electrode. The SEM image of powdered La0.5Sm0.5FeO3 having a perovskite crystal structure reveals the uniformity of size and porosity ( Figure 10A). La 3+ doping level had been varied to study the variation of sensing performances. The highest response (−86.5 mV) was obtained for sensor with La0.5Sm0.5FeO3 as sensing electrode to 1 ppm SO2 ( Figure 10B). The response order was found as ΔV(La0.5) > ΔV(La0.4) > ΔV(La0.6) > ΔV(La0.8) > ΔV (La0.2). The porous structure and electrocatalytic property are possibly responsible for the variation of responses. Responses were recorded at different operating temperatures. The equity between the amount of adhering gas and the activation energy demand indicated 275 °C as the optimum operating temperature with the highest response. The prepared mixed-potential sensor was exposed to other test gases such as-NO2, Cl2, NH3, CO, NO, acetone, H2, CH4, ethanol and methanol for a gas selectivity test. The sensor remained selective enough to detect SO2 gas even in very low amounts as illustrated in Figure 10C. In another study, a zirconia-based solid state electrochemical SO2 sensor had been demonstrated with MnNb2O6 as sensing electrode [133]. Under very high operating temperature (700 °C), the sensor attained good sensitivity along with rapid and stable response-recovery of gas molecules.

Solid Electrolyte-Based SO 2 Sensors
Different solid electrolytes such as-NASICON [129], YSZ [130] and alkali metal sulfates [131] have been exploited during the past decades to fabricate high performance SO 2 sensors. Among all the solid electrolytes, NASICON is widely used in the mixed-potential sensors due to its high ionic conductivity. Ma et al. [132] reported a mixed-potential gas sensor using NASICON and orthoferrite (La 0.5 Sm 0.5 FeO 3 ) as sensing electrode. The SEM image of powdered La 0.5 Sm 0.5 FeO 3 having a perovskite crystal structure reveals the uniformity of size and porosity ( Figure 10A). La 3+ doping level had been varied to study the variation of sensing performances. The highest response (−86.5 mV) was obtained for sensor with La 0.5 Sm 0.5 FeO 3 as sensing electrode to 1 ppm SO 2 ( Figure 10B). The response order was found as ∆V(La 0.5 ) > ∆V(La 0.4 ) > ∆V(La 0.6 ) > ∆V(La 0.8 ) > ∆V (La 0.2 ). The porous structure and electrocatalytic property are possibly responsible for the variation of responses. Responses were recorded at different operating temperatures. The equity between the amount of adhering gas and the activation energy demand indicated 275 • C as the optimum operating temperature with the highest response. The prepared mixed-potential sensor was exposed to other test gases such as-NO 2 , Cl 2 , NH 3 , CO, NO, acetone, H 2 , CH 4 , ethanol and methanol for a gas selectivity test. The sensor remained selective enough to detect SO 2 gas even in very low amounts as illustrated in Figure 10C. In another study, a zirconia-based solid state electrochemical SO 2 sensor had been demonstrated with MnNb 2 O 6 as sensing electrode [133]. Under very high operating temperature (700 • C), the sensor attained good sensitivity along with rapid and stable response-recovery of gas molecules.

Zeolite-based SO2 Sensors
Zeolites are aluminosilicates possessing immensely porous crystal structure, high specific surface area, high chemical and thermal stability, good adsorption properties, alterable chemical composition, presence of mobile ions, ability to undergo ion-exchange process and variable hydrophobic or hydrophilic features [134,135]. These characteristics make zeolites very attractive for gas detection. Choeichom et al. studied the effects of zeolite type, cation type and Si/Al ratio within various zeolites when exposed to SO2 gas [136]. During the exposure to 4200 ppm SO2, pristine zeolites exhibited the different sensor responses plotted in Figure 10D. It was found that the relative response of each pristine zeolite type showed the following decreasing order: ZSM-5 > beta > 13X > Y > 4A > ferrierite > mordenite > 5A > 3A. The three key factors contributing to the variation of these zeolite responses are pore size, cation type and Si/Al ratio. It was observed that the relative response increases with increasing zeolite pore size, however, decreases with a too large pore size. Among the monovalent cation zeolites focused here, the NH4 + zeolite response was the highest because of formation of hydrogen bonds with more than one SO2 molecule. With decreasing of Si/Al ratio, the responses kept increasing. The combined effect of the above discussed factors contributed to NH4 + ZSM-5 (23) achieving the highest relative response toward SO2 with 23 as Si/Al ratio and medium pore size. Recovery and repeatability assessments were performed by flowing 4200 ppm SO2 for four cycles as illustrated in Figure 10E. The sensor conductivity returned to its initial value after SO2 was removed and again produced the same response to SO2 in the subsequent cycles. These results indicate the complete recovery and strong repeatability of the zeolite sensor. The sensor responses of various ion-exchanged ZSM-5 (23) towards 4200 ppm SO2 had been investigated as well ( Figure 10F). It was found that Al 3+ ZSM-5 (23) provides the highest relative response due to two key factors: firstly, the magnitude of the ion-dipole attraction increases with the increasing ionic charge. Al 3+ having higher ionic charge than Mg 2+ and Na + , promotes a higher degree of interaction with SO2 molecules which in turn results in a higher sensor response. Secondly, the higher electronegativity of

Zeolite-Based SO 2 Sensors
Zeolites are aluminosilicates possessing immensely porous crystal structure, high specific surface area, high chemical and thermal stability, good adsorption properties, alterable chemical composition, presence of mobile ions, ability to undergo ion-exchange process and variable hydrophobic or hydrophilic features [134,135]. These characteristics make zeolites very attractive for gas detection. Choeichom et al. studied the effects of zeolite type, cation type and Si/Al ratio within various zeolites when exposed to SO 2 gas [136]. During the exposure to 4200 ppm SO 2 , pristine zeolites exhibited the different sensor responses plotted in Figure 10D. It was found that the relative response of each pristine zeolite type showed the following decreasing order: ZSM-5 > beta > 13X > Y > 4A > ferrierite > mordenite > 5A > 3A. The three key factors contributing to the variation of these zeolite responses are pore size, cation type and Si/Al ratio. It was observed that the relative response increases with increasing zeolite pore size, however, decreases with a too large pore size. Among the monovalent cation zeolites focused here, the NH 4 + zeolite response was the highest because of formation of hydrogen bonds with more than one SO 2 molecule. With decreasing of Si/Al ratio, the responses kept increasing. The combined effect of the above discussed factors contributed to NH 4 + ZSM-5 (23) achieving the highest relative response toward SO 2 with 23 as Si/Al ratio and medium pore size. Recovery and repeatability assessments were performed by flowing 4200 ppm SO 2 for four cycles as illustrated in Figure 10E. The sensor conductivity returned to its initial value after SO 2 was removed and again produced the same response to SO 2 in the subsequent cycles. These results indicate the complete recovery and strong repeatability of the zeolite sensor. The sensor responses of various ion-exchanged ZSM-5 (23) towards 4200 ppm SO 2 had been investigated as well ( Figure 10F). It was found that Al 3+ ZSM-5 (23) provides the highest relative response due to two key factors: firstly, the magnitude of the ion-dipole attraction increases with the increasing ionic charge. Al 3+ having higher ionic charge than Mg 2+ and Na + , promotes a higher degree of interaction with SO 2 molecules which in turn results in a higher sensor response. Secondly, the higher electronegativity of Al 3+ ZSM-5 (23) governs the stronger cation-dipole interaction with SO 2 and thus facilitates a higher sensor response. Recently, a zinc-doped zeolitic imidazolate framework (ZIF-67) attached with CNT has been reported as SO 2 sensing material [137]. It provided notable sensing performances at room temperature due to its porous polyhedral structure of metal particles with numerous interlinked CNTs. Previously, conductive polymer/zeolite composite based SO 2 detection had been studied [138]. It was observed that PEDOT-PSS/KY zeolite composite achieved the highest sensor response having gas adsorption-desorption dependence on the cation types of Y zeolite.

Paper-Based SO 2 Sensors
Sensing materials incorporated onto paper offer color transition sensing with the eyes, whereby measurement systems and electric circuits are not needed [139]. Paper-based analytical devices (PADs) provide the advantages of ease of production, low cost, flexibility, efficient sample collection, and easy disposability [140]. Li et al. coupled headspace sampling (HS) with PAD in order to detect SO 2 through surface-enhanced Raman scattering (SERS) [141]. Hybrids consisting of 4-mercapto-pyridine (Mpy)-modified gold nanorods (GNRs) and reduced graphene oxide (rGO) were prepared. Then along with anhydrous methanol and starch iodine complex, the rGO/MPy-GNRs hybrids were immobilized upon cellulose-based filter papers using a vacuum filtration method. This process promotes the formation of a dense blue colored film on the filter paper as shown in the SEM images ( Figure 11A-C). Uniform cellulose fibers of 12.5 µm width adopt wrinkle-like structures because of the attachment with rGO ( Figure 11B). On exposing the fabricated rGO/MPy-GNRs/SIC paper to SO 2 , the blue color faded within minutes as illustrated in Figure 11C. It was found that the intermolecular charge-transfer complex between starch and iodine produces a broad band at 600 nm as indicated by curve d of Figure 11D. Al 3+ ZSM-5 (23) governs the stronger cation-dipole interaction with SO2 and thus facilitates a higher sensor response. Recently, a zinc-doped zeolitic imidazolate framework (ZIF-67) attached with CNT has been reported as SO2 sensing material [137]. It provided notable sensing performances at room temperature due to its porous polyhedral structure of metal particles with numerous interlinked CNTs. Previously, conductive polymer/zeolite composite based SO2 detection had been studied [138]. It was observed that PEDOT-PSS/KY zeolite composite achieved the highest sensor response having gas adsorption-desorption dependence on the cation types of Y zeolite.

Paper-based SO2 Sensors
Sensing materials incorporated onto paper offer color transition sensing with the eyes, whereby measurement systems and electric circuits are not needed [139]. Paper-based analytical devices (PADs) provide the advantages of ease of production, low cost, flexibility, efficient sample collection, and easy disposability [140]. Li et al. coupled headspace sampling (HS) with PAD in order to detect SO2 through surface-enhanced Raman scattering (SERS) [141]. Hybrids consisting of 4-mercaptopyridine (Mpy)-modified gold nanorods (GNRs) and reduced graphene oxide (rGO) were prepared. Then along with anhydrous methanol and starch iodine complex, the rGO/MPy-GNRs hybrids were immobilized upon cellulose-based filter papers using a vacuum filtration method. This process promotes the formation of a dense blue colored film on the filter paper as shown in the SEM images ( Figures 11A-C). Uniform cellulose fibers of 12.5 μm width adopt wrinkle-like structures because of the attachment with rGO ( Figure 11B). On exposing the fabricated rGO/MPy-GNRs/SIC paper to SO2, the blue color faded within minutes as illustrated in Figure 11C. It was found that the intermolecular charge-transfer complex between starch and iodine produces a broad band at 600 nm as indicated by curve d of Figure 11D.  The IR spectrum of rGO/MPy-GNRs/SIC is displayed in Figure 11E indicating the modification in the response after SO 2 exposure. Along with the distinct and typical peaks for MPy, SO 2 adsorption introduces a new peak having increased intensity in the SERS spectra of rGO/MPy-GNRs/SIC as shown by curve e in Figure 11F. This additional peak occurs because SO 2 possibly affects the bending vibration of pyridine, and the characteristic peaks of SO 2 -pyridine complex are reflected in the bands. Recently, an amino-functionalized luminescent MOF material (MOF-5-NH 2 ) was incorporated onto test paper for portable SO 2 sensing [142]. It was seen that the prepared luminescent paper got lightened upon SO 2 gas exposure with high selectivity. Also, it detected as low as 0.05 ppm SO 2 having reusability advantages. In another research work, a microfluidic paper-based integrated detection system had been reported to monitor SO 2 concentrations using RGB color analysis software [143]. The sensing performance metrics like sensitivity/response, response and recovery times at certain gas concentration and operating temperatures, and sensitivity per ppm/response time ratio for different SO 2 sensor materials and structures have been summarized in Table 2. It provides a brief comparative performances outline among different SO 2 sensor reported in recent years.

Carbon Material-Based H 2 S Sensors
In recent years, many research efforts have been made for H 2 S detection using graphene, reduced graphene oxide and carbon nanofibers. Ovsianytskyi et al. [152] proposed a graphene-based H 2 S gas sensor functionalized with Ag nanoparticles (Ag NPs) and charged impurities. Graphene was grown by the CVD technique, and then Ag NPs and impurities were incorporated on the graphene by a wet chemical method. The SEM image of graphene after immersing into AgNO 3 /Fe(NO 3 ) 3 solution reveals the presence of large number of nanoparticles (10-100 nm) uniformly distributed on its surface ( Figure 12A). The comparative responses obtained on exposing 500 ppb of H 2 S gas for 400 s to pristine graphene, graphene doped with Fe(NO 3 ) 3 solution, graphene doped with AgNO 3 solution, and graphene doped with a mixed AgNO 3 /Fe(NO 3 ) 3 solution are displayed in Figure 12B. Graphene doped with the mixed solution exhibited the highest response. Since Ag is less electronegative than graphene, adsorption of H 2 S occurs because of its interaction with the adsorbed oxygen species on Ag mostly. Then, electrons released from dissociation of H 2 S are accumulated in graphene. This phenomenon causes a decrease in graphene hole concentration, and thus resistance of Ag-doped graphene increases.
The relationship between gas concentrations and corresponding relative responses of the synthesized sensor is quite linear, as plotted in Figure 12C. Also, the sensor was strongly selective to H 2 S gas against CH 4

Carbon Material-based H2S Sensors
In recent years, many research efforts have been made for H2S detection using graphene, reduced graphene oxide and carbon nanofibers. Ovsianytskyi et al. [152] proposed a graphene-based H2S gas sensor functionalized with Ag nanoparticles (Ag NPs) and charged impurities. Graphene was grown by the CVD technique, and then Ag NPs and impurities were incorporated on the graphene by a wet chemical method. The SEM image of graphene after immersing into AgNO3/Fe(NO3)3 solution reveals the presence of large number of nanoparticles (10-100 nm) uniformly distributed on its surface ( Figure 12A). The comparative responses obtained on exposing 500 ppb of H2S gas for 400 s to pristine graphene, graphene doped with Fe(NO3)3 solution, graphene doped with AgNO3 solution, and graphene doped with a mixed AgNO3/Fe(NO3)3 solution are displayed in Figure 12B. Graphene doped with the mixed solution exhibited the highest response. Since Ag is less electronegative than graphene, adsorption of H2S occurs because of its interaction with the adsorbed oxygen species on Ag mostly. Then, electrons released from dissociation of H2S are accumulated in graphene. This phenomenon causes a decrease in graphene hole concentration, and thus resistance of Ag-doped graphene increases.
The relationship between gas concentrations and corresponding relative responses of the synthesized sensor is quite linear, as plotted in Figure 12C. Also, the sensor was strongly selective to H2S gas against CH4, CO2, N2, and O2 gases. Similarly, Chu et al. [153] obtained a sensitivity of 34.31% toward 100 ppm H2S with tin oxide-modified reduced graphene oxide (SnO2-rGO) at 125 °C. In another study, Zhang et al. [154] developed a stable sensor using ZnO-carbon nanofibers (30.34 wt% carbon) that exhibited good H2S sensing performances. It was found that the protection of carbon provides high stability of ZnO and oxygen vacancies to allow improved sensor responses.

GaN-Based H 2 S Sensors
Several AlGaN/GaN-based gas sensors have been reported, including NO, NO 2 , NH 3 , Cl 2 , CO, CO 2 and CH 4 [156][157][158]. However, H 2 S sensing using wide bandgap semiconductors like GaN have not been explored yet that much. In order to sense H 2 S gas even at very low amounts, Sokolovskij et al. [155] synthesized AlGaN/GaN HEMT-based sensor with platinum as gate. The top view optical micrograph of the synthesized device shown in Figure 12D reveals the gate dimensions, gate-source and gate-drain spacing. For high temperature operations, each device was wire bonded to ceramic substrates. The variation of drain current was observed under different H 2 S concentrations and gate bias voltages ( Figure 12E).
Because of the increasing baseline current with increasing gate bias, variation of the drain current was highly influenced. Also, the fabricated HEMT sensor operates in a wide range of biasing conditions without degrading the sensing performances and thus shows an excellent stability. It was found that when gate bias approaches pinch-off state, it minimizes power consumption and thus enables the sensor to operate at high response mode. The sensitivity of AlGaN/GaN HEMT sensor clearly increases with higher temperatures as plotted in Figure 12F. The rise and fall time were estimated 219 s and 507 s, respectively, at 250 • C. At lower temperatures, rise and fall times have gone higher. Further, Zhang et al. [159] pre-treated the Pt-gated AlGaN/GaN HEMT sensor with H 2 pulses in dry air ambient at 250 • C. This treatment facilitated the enlargement of the H 2 S detection range up to 90 ppm.

Nanostructured Metal Oxide-Based Sensors
It was found that metal oxides such as-SnO 2 , WO 3 , ZnO, and α-Fe 2 O 3 based sensors exhibited superior sensing performances toward H 2 S due to their stable nanostructures [160][161][162]. Zhang et al. [163] reported a α-Fe 2 O 3 nanosheet-based H 2 S gas sensor using a solvothermal method. Figure 13A shows the SEM image of a sample obtained at reaction temperature of 160 • C denoted as S 160 . It was observed that at low temperatures, the morphology of the samples is not uniform. Since both α-Fe 2 O 3 and Fe 3 O 4 exist in the nanostructure, uniform morphology can't be obtained under the reaction temperature of 160 • C. It was seen that sensor response of α-Fe 2 O 3 to H 2 S decreases with the increasing working temperature. However, the recovery time is too long at low temperature, so taking sensor response, response time and recovery time into account, 135 • C was estimated as the optimum working temperature. Figure 13B displays the response of the prepared sensor to different concentrations of H 2 S ranging from 1 to 50 ppm at 135 • C. The response and recovery time were estimated to be less than 10 s and 45 s, respectively, indicating very a rapid response in comparison to other H 2 S gas sensors. The changes in the electric resistances were found negligible for the sensor to 50 ppm acetone, ethanol, methanol and H 2 gases at 135 • C. On the contrary, the sensor response was very large to H 2 S under same conditions, thus reflecting excellent selectivity of the α-Fe 2 O 3 nanosheet-based H 2 S sensor ( Figure 13C).
In another study, Li et al. [164] developed ZnO/CuO nanotube arrays to sense H 2 S at low-working temperatures. It was observed that the nanotube structures promoted the diffusion and adsorption of gas with many active sites between H 2 S molecules and adsorbed oxygen molecules. Thus, they contributed to achieve good sensitivity along with fast response time. It was found that porous In 2 O 3 nanoparticles provide large surface areas and pore volumes which create numerous active sites to produce active oxygen species [165]. These sites facilitate a significant improvement in H 2 S gas sensing with 1 ppb of detection limit. Also, a dense array of intrinsic ZnO NWs has been reported for H 2 S detection by exploiting a sulfuration-desulfuration reaction mechanism [166]. In another work, Eom et al. [167] fabricated Cu x(x=1,2) O:SnO 2 thin films to detect H 2 S gas at room temperature. Enhanced sensitivity with rapid response-recovery was obtained due to enhanced adsorption sites arising from abounding domains of p-n heterojunctions on the Cu x O:SnO 2 film surfaces. Besides, a Cu-doped BaSrTiO 3 -based H 2 S sensor was reported [168]. Herein along with gas-surface interaction, the role of pre-adsorbed oxygen species and surface dipolar hydroxyl groups has been investigated as well. temperature. Enhanced sensitivity with rapid response-recovery was obtained due to enhanced adsorption sites arising from abounding domains of p-n heterojunctions on the CuxO:SnO2 film surfaces. Besides, a Cu-doped BaSrTiO3-based H2S sensor was reported [168]. Herein along with gassurface interaction, the role of pre-adsorbed oxygen species and surface dipolar hydroxyl groups has been investigated as well.

Mesoporous Metal Oxide-based Sensors
Generally mesoporous materials contain pores with diameters of 2-50 nm. Mesoporous metaloxides offer efficient gas detection because they have large surface areas, open porosity, small pore sizes, and the ability to coat the surface of the mesoporous structure with one or more compounds. Quang et al. [169] reported a mesoporous Co3O4 nanochains-based H2S sensor. At first cobalt carbonate hydroxide (Co (CO3)0.5(OH)·11H2O) nanowires were synthesized using a hydrothermal route. Then heat treatment was applied in air at 600 °C for 5 h to form rough-surfaced mesoporous Co3O4 nanochains as shown by the TEM image in Figure 13D. From the gas responses vs working temperatures analysis, the optimum working temperature was obtained as 300 °C. At lower operating temperatures than the optimum, Co3O4 nanochains displayed sluggish chemical activity causing weak responses. Moreover, at higher working temperatures, adsorbed H2S molecules start escaping from the Co3O4 nanochain surface because of increased activation. As a result, the sensor response starts decreasing. The fabricated sensor exhibited quick responses and recovery to 1-100 ppm H2S at 300 °C. At 100 ppm H2S, response and recovery times were estimated as 46 s and 24 s, respectively, as illustrated in Figure 13E. The nanochain structure provides a high specific surface area, narrow pore size and rich mesopores which make the fabricated sensor more suitable for H2S sensing than other toxic gases. The comparative responses of Co3O4 nanochains toward H2S and other target gases are plotted in Figure 13F indicate strong selectivity for H2S. Previously, Stanoiu et al. [170] prepared a mesoporous SnO2-CuWO4-based cost-effective H2S sensor having high sensitivity at low working temperature. In addition, a short temperature trigger of 500 °C was applied to enhance the recovery operation of the fabricated sensor.

Mesoporous Metal Oxide-Based Sensors
Generally mesoporous materials contain pores with diameters of 2-50 nm. Mesoporous metaloxides offer efficient gas detection because they have large surface areas, open porosity, small pore sizes, and the ability to coat the surface of the mesoporous structure with one or more compounds. Quang et al. [169] reported a mesoporous Co 3 O 4 nanochains-based H 2 S sensor. At first cobalt carbonate hydroxide (Co (CO 3 ) 0.5 (OH)·11H 2 O) nanowires were synthesized using a hydrothermal route. Then heat treatment was applied in air at 600 • C for 5 h to form rough-surfaced mesoporous Co 3 O 4 nanochains as shown by the TEM image in Figure 13D. From the gas responses vs. working temperatures analysis, the optimum working temperature was obtained as 300 • C. At lower operating temperatures than the optimum, Co 3 O 4 nanochains displayed sluggish chemical activity causing weak responses. Moreover, at higher working temperatures, adsorbed H 2 S molecules start escaping from the Co 3 O 4 nanochain surface because of increased activation. As a result, the sensor response starts decreasing. The fabricated sensor exhibited quick responses and recovery to 1-100 ppm H 2 S at 300 • C. At 100 ppm H 2 S, response and recovery times were estimated as 46 s and 24 s, respectively, as illustrated in Figure 13E. The nanochain structure provides a high specific surface area, narrow pore size and rich mesopores which make the fabricated sensor more suitable for H 2 S sensing than other toxic gases. The comparative responses of Co 3 O 4 nanochains toward H 2 S and other target gases are plotted in Figure 13F indicate strong selectivity for H 2 S. Previously, Stanoiu et al. [170] prepared a mesoporous SnO 2 -CuWO 4 -based cost-effective H 2 S sensor having high sensitivity at low working temperature. In addition, a short temperature trigger of 500 • C was applied to enhance the recovery operation of the fabricated sensor.

Metal Oxide Microsphere-Based Sensors
Typically, microspheres are small spherical particles having diameters in the micrometer range. Hu et al. [171] reported CuFe 2 O 4 nanoparticles-decorated CuO microspheres-based H 2 S gas sensors. The synthesized CuO/CuFe 2 O 4 heterostructures provided a porous and rough surface due to the arbitrary deposition of nanoparticles as displayed by the FE-SEM image in Figure 14A. When the temperature is low, the response becomes low because of the weak chemical interaction between the gas molecules and adsorbed oxygen species. At higher working temperatures, the mentioned chemical interaction is strong and the responses keep increasing, but gas molecule diffusion becomes slower than the surface interaction causing a decrease of the response again, as illustrated in Figure 14B. The optimal operating temperature was estimated as 240 • C. The dependence of responses of the sensor on H 2 S concentrations is plotted in Figure 14C which exhibits a gradual increasing trend. The response and recovery time of the fabricated CuO/CuFe 2 O 4 sensor were obtained as 31 s and 40 s, respectively, at the optimal operating temperature (240 • C) with good reproducibility and selectivity toward H 2 S gas. In another study, Li et al. [172] prepared SiO 2 @TiO 2 microspheres and then formed Cd 2+ -doped TiO 2 shell-modified ITO electrodes for H 2 S detection. Exploiting the mismatch of energy band levels between TiO 2 shells and induced CdS nanoparticles, this device provided good sensing performances.

MOF-Based H 2 S Sensors
Metal organic frameworks (MOFs) offer highly selective and sensitive detection of H 2 S because of possessing chemical stability, custom tuning of porosity and functionalities, and various pre-or post-synthetic modifications to the structural framework [173]. Guo et al. [174] synthesized a MOF material named as Zr(TBAPy) 5 (TCPP) using a solvothermal method, where Zr is the metal center, and 1,3,6,8-tetra(4-carboxylphenyl) pyrene (TBAPy) and tetrakis(4-carboxyphenyl) porphyrin (TCPP) act as double linkers. The prepared Zr(TBAPy) 5 (TCPP) exhibited well-shaped shuttle structures with a particle size of about 100 nm as seen from the transmission electron microscope (TEM) image ( Figure 14D). However, Zr-MOF NU-1000 (synthesized for comparison) exhibited an irregular structure indicating the structural effect of TCPP on the synthesized materials. The FTIR spectrum of Zr(TBAPy) 5 (TCPP) is displayed in Figure 14E. There is a clear shift in the N-H and C=N peak on the addition of S 2− due to the attachment between S and N in the materials. Fluorescence enhancement of the fabricated Zr(TBAPy) 5 (TCPP) sensor provides a linear trend with the increase of S 2− concentration as plotted in Figure 14F. The interference effects of other anions such as-SO 4 2− , CNS − , COOH − , Br − , I − , IO3 − , F − , HSO 3 − , Cl − and NO 3 − was investigated and the results confirm that Zr(TBAPy) 5 (TCPP) is highly selective for S 2− sensing. In another study, Dong et al. [175] developed a ZIF-67-derived porous dodecahedra Co 3 O 4 sensor showing enhanced linear trend with H 2 S concentration. The significant improvement in the overall sensing performances is attributed to a high specific surface and the exposed {110} lattice planes of the fabricated structure.
provided good sensing performances.

MOF-based H2S Sensors
Metal organic frameworks (MOFs) offer highly selective and sensitive detection of H2S because of possessing chemical stability, custom tuning of porosity and functionalities, and various pre-or postsynthetic modifications to the structural framework [173]. Guo et al. [174] synthesized a MOF material named as Zr(TBAPy)5(TCPP) using a solvothermal method, where Zr is the metal center,

Organic Materials-Based H 2 S Sensors
In recent years, several attempts have been made for H 2 S sensing using organic semiconducting films and polymers. Different types of interactions like crosslinking, doping, grafting and scissioning between electrons and organic materials take place in subject to energy as well as the dose of incident electron beam [176] which help to attain high selectivity and sensitivity to target gas. Chaudhary et al. reported a polyaniline-silver (PANI-Ag) nanocomposite film-based H 2 S sensor [177]. After protonation of aniline monomers, photopolymerization of aniline on a bi-axially oriented polyethylene terephthalate (BOPET) sheet was performed. The prepared PANI-Ag films were irradiated by a 10 MeV electron beam. As the dose was increased, the nanofiber diameter increased and a 30 kGy dose promoted an interconnected microstructure with larger sized Ag particles. At a very high dose of 100 kGy, Ag clusters submerged inside the polymer matrix with denser structure. The bright spots observed in SEM image as shown in Figure 15A reveal that Ag nanoparticles are incorporated in the PANI matrix. After EB irradiation, Ohmic nature was retained as seen from the linear I-V relationship displayed in Figure 15B. The electrical conductivity kept increasing with the dose to achieve the highest value at 30 kGy and then started going down. The percent sensor responses under different H 2 S concentrations and irradiation doses are plotted in Figure 15C. Since lower irradiation doses cause higher conductivity changes, the corresponding sensing responses become lower upon H 2 S exposure. On the contrary, higher doses cause lower electrical conductivity due to crosslinking-induced structural defects, so, the corresponding sensing responses to H 2 S become larger. Abu-Hani et al. [178] engineered the conductivity of chitosan (CS) film to obtain a highly sensitive and selective sensor toward H 2 S gas. Glycerol ionic liquid (IL) had been incorporated to tune the conductivity of the CS film. It was able to operate at lower temperature and provided rapid response-recovery with low-power consumption. In another study, Cu 2+ -doped SnO 2 nanograin/polypyrrole nanospheres-based H 2 S gas detection was reported [179]. The enhanced sensing performance of the fabricated organic-inorganic nanohybrids is mainly attributed to improved surface potential barrier by surface defects tailoring, and numerous reaction sites to accelerate gas diffusion and adsorption.

Solid Electrolytes-based H2S Sensors
The mixed potential type sensor requires a solid electrolyte having the ability to transfer oxygen ions between reference electrode and sensing electrode. Hao et al. [180] prepared a mixed-potential H2S gas sensor using YSZ as solid electrolyte and La2NiO4 as sensing electrode. The microstructure of the sensing material La2NiO4 was varied with three equivalents of citric acid and total metal irons which were 0.5:1 (LNO-0.5), 1:1 (LNO-1) and 1:2 (LNO-2), respectively. The SEM image of LNO-1 sensing material (La2NiO4 powders) reveals the porous structure and it has the largest pore size among the three samples as displayed in Figure 15D. Also, it was found that LNO-1 possesses the highest BET surface area and pore volume. On increasing the H2S exposure concentration, the electrode potential difference exhibited linear changes with the logarithm of H2S concentration at 500 °C as plotted in Figure 15E. The sensitivity of the sensor to H2S was changed to -10 mV/decade from -69 mV/decade. The recovery time was improved by applying a temperature pulse of 700 °C. For 500 ppb of H2S exposure, it decreased from 20 min to 150 s. The fabricated sensor was proved to be highly selective toward H2S compared to other target gases as observed from the responses toward 1 and 2 ppm of every test gas ( Figure 15F). Moreover, it was found quite stable in long term performance with lower detection limit. In another study, Yang et al. [181] used Nafion as a proton exchange membrane to demonstrate H2S sensing with the help of a sensing electrode. The electrode was made of Pt-Rh nanoparticles loaded on carbon fibers. The sensitivity was obtained 0.191 μA/ppm from the linear plot between sensor current changes and corresponding H2S concentrations. The fabricated sensor was highly selective toward H2S at room temperature with a fast recovery time (16 s) under 50 ppm of gas exposure. Recently, a promising TMD material, WS2 has been utilized to detect H2S with high sensitivity and selectivity [182]. It was observed that oxygen doping in the sulfur sites of the WS2 lattice promotes enhanced sensing performances towards H2S. Earlier, the adsorption properties of WS2 had been analyzed toward various target gas molecules along with its Fermi level pinning [183]. The sensing performance metrics like sensitivity/response, response and recovery times at certain gas concentration and operating temperatures, and sensitivity per ppm/response time ratio for different H2S sensor materials and structures have been summarized in Table 3. It provides a brief comparative performances outline among different H2S sensors reported in recent years.

Solid Electrolytes-Based H 2 S Sensors
The mixed potential type sensor requires a solid electrolyte having the ability to transfer oxygen ions between reference electrode and sensing electrode. Hao et al. [180] prepared a mixed-potential H 2 S gas sensor using YSZ as solid electrolyte and La 2 NiO 4 as sensing electrode. The microstructure of the sensing material La 2 NiO 4 was varied with three equivalents of citric acid and total metal irons which were 0.5:1 (LNO-0.5), 1:1 (LNO-1) and 1:2 (LNO-2), respectively. The SEM image of LNO-1 sensing material (La 2 NiO 4 powders) reveals the porous structure and it has the largest pore size among the three samples as displayed in Figure 15D. Also, it was found that LNO-1 possesses the highest BET surface area and pore volume. On increasing the H 2 S exposure concentration, the electrode potential difference exhibited linear changes with the logarithm of H 2 S concentration at 500 • C as plotted in Figure 15E. The sensitivity of the sensor to H 2 S was changed to −10 mV/decade from −69 mV/decade. The recovery time was improved by applying a temperature pulse of 700 • C. For 500 ppb of H 2 S exposure, it decreased from 20 min to 150 s. The fabricated sensor was proved to be highly selective toward H 2 S compared to other target gases as observed from the responses toward 1 and 2 ppm of every test gas ( Figure 15F). Moreover, it was found quite stable in long term performance with lower detection limit. In another study, Yang et al. [181] used Nafion as a proton exchange membrane to demonstrate H 2 S sensing with the help of a sensing electrode. The electrode was made of Pt-Rh nanoparticles loaded on carbon fibers. The sensitivity was obtained 0.191 µA/ppm from the linear plot between sensor current changes and corresponding H 2 S concentrations. The fabricated sensor was highly selective toward H 2 S at room temperature with a fast recovery time (16 s) under 50 ppm of gas exposure. Recently, a promising TMD material, WS 2 has been utilized to detect H 2 S with high sensitivity and selectivity [182]. It was observed that oxygen doping in the sulfur sites of the WS 2 lattice promotes enhanced sensing performances towards H 2 S. Earlier, the adsorption properties of WS 2 had been analyzed toward various target gas molecules along with its Fermi level pinning [183]. The sensing performance metrics like sensitivity/response, response and recovery times at certain gas concentration and operating temperatures, and sensitivity per ppm/response time ratio for different H 2 S sensor materials and structures have been summarized in Table 3. It provides a brief comparative performances outline among different H 2 S sensors reported in recent years.  [196] Multi-tube arrays RT 5 1.45 14 30 0.02 NiO [197] Porous A novel metric, sensitivity per ppm/response time ratio has been calculated for each sensor in order to compare the overall sensing performance on the same reference. The higher value of the calculated ratio indicates the better overall sensor performance. Average ratios have been obtained by taking the recently reported gas sensors into account for the highly focused sensing materials as illustrated in Figure 16. It is found that hybrid materials-based sensors exhibit the highest average ratio for NO 2 gas sensing, whereas GaN and Metal-oxide based sensors possess the highest ratio for SO 2 and H 2 S gas sensing respectively.
Sensors 2018, 18, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sensors A novel metric, sensitivity per ppm/response time ratio has been calculated for each sensor in order to compare the overall sensing performance on the same reference. The higher value of the calculated ratio indicates the better overall sensor performance. Average ratios have been obtained by taking the recently reported gas sensors into account for the highly focused sensing materials as illustrated in Figure 16. It is found that hybrid materials-based sensors exhibit the highest average ratio for NO2 gas sensing, whereas GaN and Metal-oxide based sensors possess the highest ratio for SO2 and H2S gas sensing respectively.

Recent Density-Functional Theory (DFT) Study of Gas Molecule-Sensor Interaction
Numerous efforts have been made to investigate the adsorption properties of various sensing materials toward different toxic gases including NO 2, SO 2 and H 2 S by first-principle method calculations using density functional theory (DFT) in recent years as shown in Table 4.

Calibration of Toxic Gas Sensors
In order to check sensor precision, toxic gas sensors must be calibrated at regular intervals. The sensor producers typically suggest a time interval between calibrations. Single toxic gas detectors are normally calibrated with a defined toxic gas depending on the gas type whereas multi-gas detectors are calibrated with their own specific calibration gas mixtures. There are mainly two steps in the gas sensor calibration. Firstly, a reference zero reading must be established using pure nitrogen or pure synthetic air. Secondly, the sensor operating range must be calibrated using a standard gas mixture. The ideal practice is to apply a mixture of the target gas balanced in the natural air as the calibration gas. Premixed calibration gas, permeation devices, cross calibration, gas mixing, Gaussian processes are some of the practical methods of calibrating the gas sensors [213][214][215].

Toxic Gas Sensors in Internet of Things (IoT) Applications
The Internet of Things is a network of physical objects that utilizes sensors and application programming interfaces (APIs) to collect and exchange data over the internet. IoT network requires ultra-low power, low cost, long lifetime, integrable into electronic circuits, and mini-sized gas sensors for remote air quality monitoring and enhanced automated system [216]. Electrochemical gas sensors can provide these characteristics required by IoT platforms, thus become suitable candidate for the IoT applications such as creating smart environment, smart home, smart parking system and so on [217,218]. Toxic gas sensors were incorporated into a multi-purpose field surveillance robot which uses multiple IoT cloud servers [219]. High performance gas sensors are utilized in IoT-based vehicle emission monitoring systems [220]. Besides, wireless sensor networks have been employed for toxic gas boundary area detection in large-scale petrochemical plants [221]. However, the gas sensing performances are strongly affected by miniaturization of sensor in terms of length and width between the electrodes, number of electrodes, sensing area etc. [222]. Extensive studies on sensing properties of miniaturized gas sensors can further facilitate the implementation of toxic gas sensors in IoT platforms.

Future Perspectives and Conclusions
Toxic gas sensors play an important role in many aspects of technology, industry, or daily life. In recent years, researchers have exploited the fundamental properties of various gas sensing materials to achieve high performance toxic gas sensors. Particularly, excellent improvements have been attained in terms of sensitivity, selectivity, limit of detection, miniaturization and portability for NO 2 , SO 2 and H 2 S gas sensors using novel combination of nanomaterials exhibiting various morphologies. However, the toxic gas sensors reported so far have limitations in some of the important performance metrics, such as-response and recovery times, stability, operating temperature, reproducibility, fabrication cost, reliability etc. These limitations can be overcome by further exploiting the hybrid and heterostructure, exploring more in surface functionalization, and adopting novel, efficient and cost-effective fabrication technique. This work reviews and categorizes the recent progress in electrochemical detection of NO 2 , SO 2 and H 2 S gases based on various highly explored sensing materials over the past few decades. Moreover, the sensing performance parameters like sensitivity/ response, response and recovery times at certain gas exposure concentration and operating temperature for various sensor materials and structures have been tabulated which provide a brief comparative performances outline to the reader. This study will give an overview on the research trend of the above-mentioned toxic gas sensors to the current and future researchers.