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Ficure 3: Characteristics of a good sensor.

Figure 2 Ficure 3: Characteristics of a good sensor.

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Ficure 1: Bar chart of published manuscripts focusing on a metal oxide-based gas sensor. TABLE 1: Reports on the efficient gas sensing performance.
Ficure 1: Bar chart of published manuscripts focusing on a metal oxide-based gas sensor. TABLE 1: Reports on the efficient gas sensing performance.
TaBLeE 2: The numerical properties of chlorine (http://www .environmentalpollution.in/water-pollution/chlorine-numerical- properties-hydrolysis-and-role/1774). is often called brine. A solution of sodium chloride contains Na® (aq) and Cl (aq) ions and, from the dissociation of water, very low concentrations of H* (aq) and OH’ (aq) ions. During the electrolysis of the solution, Cl, and H, gases are produced (see the equations below):
TaBLeE 2: The numerical properties of chlorine (http://www .environmentalpollution.in/water-pollution/chlorine-numerical- properties-hydrolysis-and-role/1774). is often called brine. A solution of sodium chloride contains Na® (aq) and Cl (aq) ions and, from the dissociation of water, very low concentrations of H* (aq) and OH’ (aq) ions. During the electrolysis of the solution, Cl, and H, gases are produced (see the equations below):
Figure 5: Chlorine gas applications [67]. Figure 4: An ion exchange membrane cell [50].
Figure 5: Chlorine gas applications [67]. Figure 4: An ion exchange membrane cell [50].
can all affect the sensitivity and response of the sensing material. All of these criteria have the potential to improve the performance of sensing materials and provide a great potential for use in low- or high-concentration chlorine detection. ric reaction between free chlorine and specific or nonspecific reagents, with an oxidizing capacity. Additionally, through the action of oxidants, this enables electrochemical detec- tion. From literature, it is known that most of the reagents used contain toxic components and are pollutants. Among the most frequent reagents used is toluidine as it is highly sensitive and specific for chlorine ion determination. Nev- ertheless, it is also known to be a carcinogenic, toxic, and polluting reagent. Thus, Mesquita and Rangel chose o- dianisidine as the reagent to use as it is noncarcinogenic, has low toxicity, and has good sensitivity for chlorine ion determination. However, o-dianisidine is known to have ow selectivity. To overcome this, the researchers discovered that to avoid interference, they had to separate the free Cl, from the sample. This separation was based on the ability of the free Cl,, which is in the form of molecular Cl,, a gas at room temperature, to be isolated from the sample through a diffusion membrane. The sample thus must undergo acidification with HCl to ensure that all free chlo- rine is in molecular chlorine form. Then, the dissolved gas will diffuse from the sample through a hydrophobic mem- brane in a gas diffusion unit. The diffusion of chlorine gas is then converted to hypochlorite using hydroxide before reacting with o-dianisidine thus resulting in a colored prod- uct being measured [69].
can all affect the sensitivity and response of the sensing material. All of these criteria have the potential to improve the performance of sensing materials and provide a great potential for use in low- or high-concentration chlorine detection. ric reaction between free chlorine and specific or nonspecific reagents, with an oxidizing capacity. Additionally, through the action of oxidants, this enables electrochemical detec- tion. From literature, it is known that most of the reagents used contain toxic components and are pollutants. Among the most frequent reagents used is toluidine as it is highly sensitive and specific for chlorine ion determination. Nev- ertheless, it is also known to be a carcinogenic, toxic, and polluting reagent. Thus, Mesquita and Rangel chose o- dianisidine as the reagent to use as it is noncarcinogenic, has low toxicity, and has good sensitivity for chlorine ion determination. However, o-dianisidine is known to have ow selectivity. To overcome this, the researchers discovered that to avoid interference, they had to separate the free Cl, from the sample. This separation was based on the ability of the free Cl,, which is in the form of molecular Cl,, a gas at room temperature, to be isolated from the sample through a diffusion membrane. The sample thus must undergo acidification with HCl to ensure that all free chlo- rine is in molecular chlorine form. Then, the dissolved gas will diffuse from the sample through a hydrophobic mem- brane in a gas diffusion unit. The diffusion of chlorine gas is then converted to hypochlorite using hydroxide before reacting with o-dianisidine thus resulting in a colored prod- uct being measured [69].
Figure 7: (a) Chlorine gas hydrolyses in water to hypochlorous acid. (b) The ratio of hypochlorous acid to hypochlorite ion varies dependent on the pH in aqueous solution. (c) Oxidation reaction of SA with free chlorine and (d) oxidation reaction of DPD with free chlorine [70].
Figure 7: (a) Chlorine gas hydrolyses in water to hypochlorous acid. (b) The ratio of hypochlorous acid to hypochlorite ion varies dependent on the pH in aqueous solution. (c) Oxidation reaction of SA with free chlorine and (d) oxidation reaction of DPD with free chlorine [70].
Figure 8: Mechanism of WO, sensing with different oxygen vacancy concentrations [81].
Figure 8: Mechanism of WO, sensing with different oxygen vacancy concentrations [81].
be modified to respond to specific analytes, with their response being based on inherent properties such as electri- cal conductivity and rate of electron transfer, they require only simple fabrication which then leads to the ability to miniaturize these sensors [3], [86-90]. From these character- istics, Sivakamasundari et al. developed chlorine sensors using poly(norborene)s bound to cyanuric acid as a polymer film and attached it onto a glass substrate for use as a sensor membrane. The film was then used to measure absorbance intensity during interaction between cyanuric acid and free chlorine [91]. Semenistaya et al. implemented silver (Ag) nanoparticles in polyacrylonitrile (PAN) using infrared (IR) pyrolysis to detect chlorine at room temperature. PA was chosen based on several advantages including solubility in polar solvents, ability to form a thin film sensor, and change of electrophysical properties from being dielectric to semimetal under IR annealing. Response, sensitivity based on choice of the deposition method, stability, recovery, and effect on air humidity were studied to examine the perfor- mance of the Ag-PAN-based chlorine sensor. The sensor showed an increase in conductivity upon exposure of Cl, gas, as Cl, is a gas oxidizer and acceptor of electron, decreas- ing the hole concentration in sensing material. The reaction mechanism of chlorine detection pointing to the sensing material is a p-type semiconductor characteristic [92]. Sultan et al. fabricated a toxic chlorine sensor using polypyrrole (PPy) as the sensing film. The polymer was functionalized with silicon carbide nanocomposites (SiC) and dodecylbenzene sulphonic acid (DBSA) via in situ poly- merization in order to enhance its sensing properties. The PPy-based sensor was found to have the highest sensitivity compared to PPy/SiC and PPy/DBS. However, these mate- rials had poor reproducibility performance. All the sensing measurements were performed at room temperature, and the high mobility of charge carriers from all the polymer composites contributed to the good performance of the sen- sor [93]. Conducting polymer also could be applied in the determination of free chlorine. For instance, paper-based chemiresistive free chlorine sensors were developed by Qin et al. using poly(3,4-ethylenedioxythiophene) : poly(styrene- sulfonate) (PEDOT: PSS). The main advantage of this free chlorine sensor was that the fabrication could be conducted at room temperature by untrained personnel and without any special equipment or machines needed. It was found
be modified to respond to specific analytes, with their response being based on inherent properties such as electri- cal conductivity and rate of electron transfer, they require only simple fabrication which then leads to the ability to miniaturize these sensors [3], [86-90]. From these character- istics, Sivakamasundari et al. developed chlorine sensors using poly(norborene)s bound to cyanuric acid as a polymer film and attached it onto a glass substrate for use as a sensor membrane. The film was then used to measure absorbance intensity during interaction between cyanuric acid and free chlorine [91]. Semenistaya et al. implemented silver (Ag) nanoparticles in polyacrylonitrile (PAN) using infrared (IR) pyrolysis to detect chlorine at room temperature. PA was chosen based on several advantages including solubility in polar solvents, ability to form a thin film sensor, and change of electrophysical properties from being dielectric to semimetal under IR annealing. Response, sensitivity based on choice of the deposition method, stability, recovery, and effect on air humidity were studied to examine the perfor- mance of the Ag-PAN-based chlorine sensor. The sensor showed an increase in conductivity upon exposure of Cl, gas, as Cl, is a gas oxidizer and acceptor of electron, decreas- ing the hole concentration in sensing material. The reaction mechanism of chlorine detection pointing to the sensing material is a p-type semiconductor characteristic [92]. Sultan et al. fabricated a toxic chlorine sensor using polypyrrole (PPy) as the sensing film. The polymer was functionalized with silicon carbide nanocomposites (SiC) and dodecylbenzene sulphonic acid (DBSA) via in situ poly- merization in order to enhance its sensing properties. The PPy-based sensor was found to have the highest sensitivity compared to PPy/SiC and PPy/DBS. However, these mate- rials had poor reproducibility performance. All the sensing measurements were performed at room temperature, and the high mobility of charge carriers from all the polymer composites contributed to the good performance of the sen- sor [93]. Conducting polymer also could be applied in the determination of free chlorine. For instance, paper-based chemiresistive free chlorine sensors were developed by Qin et al. using poly(3,4-ethylenedioxythiophene) : poly(styrene- sulfonate) (PEDOT: PSS). The main advantage of this free chlorine sensor was that the fabrication could be conducted at room temperature by untrained personnel and without any special equipment or machines needed. It was found
Ficure 9: Semiconductor interfaces into three different types of heterojunctions [85].
Ficure 9: Semiconductor interfaces into three different types of heterojunctions [85].
Figure 10: Typical conducting polymers used as sensing materials.
Figure 10: Typical conducting polymers used as sensing materials.
Figure 11: Fibre optic sensor system basic components [99].
Figure 11: Fibre optic sensor system basic components [99].
Figure 12: Basic principles of the chemiluminescence mechanism.
Figure 12: Basic principles of the chemiluminescence mechanism.
The amperometric sensor is an electrochemical device in which the current flowing through the system is related to the concentration of the gaseous species [120]. Basically, amperometry consists of a two-electrode configuration (Figure 16). However, since there are limits to the concentra- tions of the reactant gas, a three-electrode scheme was devel- oped (Figure 17). In the three-electrode configuration, the current at the sensing electrode can be measured with a constant potential condition which generates a genuine ther- modynamic potential for all reactions without the involve- ment of a reference electrode. This is commonly known as “constant-potential amperometry.” When exposed to a vapour or gas that consists of an electroactive analyte, the amperometric gas sensor generates a current due to the diffusion of the analyte into electrochemical cell. Initially, the analyte will diffuse to the working electrode surface then to the working electrode surface and there on will participate in an electrochemical reaction that either accepts or pro- duces electrons. The current produced as a result of the target gas at the sensing or working electrode is measured as the sensor signal which can then be quantified at either a fixed or variable electrode potential [125]. Selectivity to certain gaseous analytes is an important consideration for sensing applications. Thus, amperometric sensors used for detecting various gases can be managed by changing the type of electrolyte used. a ee Se Oe eT Se, | a a ee, ee: eet |
The amperometric sensor is an electrochemical device in which the current flowing through the system is related to the concentration of the gaseous species [120]. Basically, amperometry consists of a two-electrode configuration (Figure 16). However, since there are limits to the concentra- tions of the reactant gas, a three-electrode scheme was devel- oped (Figure 17). In the three-electrode configuration, the current at the sensing electrode can be measured with a constant potential condition which generates a genuine ther- modynamic potential for all reactions without the involve- ment of a reference electrode. This is commonly known as “constant-potential amperometry.” When exposed to a vapour or gas that consists of an electroactive analyte, the amperometric gas sensor generates a current due to the diffusion of the analyte into electrochemical cell. Initially, the analyte will diffuse to the working electrode surface then to the working electrode surface and there on will participate in an electrochemical reaction that either accepts or pro- duces electrons. The current produced as a result of the target gas at the sensing or working electrode is measured as the sensor signal which can then be quantified at either a fixed or variable electrode potential [125]. Selectivity to certain gaseous analytes is an important consideration for sensing applications. Thus, amperometric sensors used for detecting various gases can be managed by changing the type of electrolyte used. a ee Se Oe eT Se, | a a ee, ee: eet |
Figure 13: Schematic diagram of a type II potentiometric sensor (-) Ag, O,, |Ag,SO,|Pt, SO,, and O, (+) [122]. TABLE 4: Solid-state electrochemical sensor using metal chloride and Ag*-(B ' 6") alumina.
Figure 13: Schematic diagram of a type II potentiometric sensor (-) Ag, O,, |Ag,SO,|Pt, SO,, and O, (+) [122]. TABLE 4: Solid-state electrochemical sensor using metal chloride and Ag*-(B ' 6") alumina.
FicureE 14: The schematic structure of type III arrangement. (—) Na,ZrO; + ZrO,, Au| Na+ |Au, and NaNO, (+) [122].
FicureE 14: The schematic structure of type III arrangement. (—) Na,ZrO; + ZrO,, Au| Na+ |Au, and NaNO, (+) [122].
FicurE 15: Response and recovery of sensor towards 10 ppm Cl, gas and repeatability of the Cl, sensor [123].
FicurE 15: Response and recovery of sensor towards 10 ppm Cl, gas and repeatability of the Cl, sensor [123].
Ficure 16: Amperometric sensor with a two-electrode configuration [120].
Ficure 16: Amperometric sensor with a two-electrode configuration [120].
Ficure 17: Amperometric gas sensor with a three-electrode configuration [120].
Ficure 17: Amperometric gas sensor with a three-electrode configuration [120].
Figure 18: Metal phthalocyanine structure [130, 131].
Figure 18: Metal phthalocyanine structure [130, 131].
FrcurE 20: Chemical structure of the ZnPc(OBu), molecule [134]. Figure 19: (a) Sensor response towards 1 ppm of Cl,, NO,, NO, ethanol, hydrogen sulphide, NH,, and CO. (b) Sensor response towards lppm Cl, in different temperatures. (c) Sensor response in different Cl, gas concentrations at 150°C. (d) Variation in the response of sensor with Cl, concentration (experimental curve (dot lines) and fitting curve (solid lines)).
FrcurE 20: Chemical structure of the ZnPc(OBu), molecule [134]. Figure 19: (a) Sensor response towards 1 ppm of Cl,, NO,, NO, ethanol, hydrogen sulphide, NH,, and CO. (b) Sensor response towards lppm Cl, in different temperatures. (c) Sensor response in different Cl, gas concentrations at 150°C. (d) Variation in the response of sensor with Cl, concentration (experimental curve (dot lines) and fitting curve (solid lines)).
FicurE 21: Mechanism of detection of free chlorine by inkjet-printed silver electrodes and signal plot obtained from the third step of linear sweep voltammetry [127].
FicurE 21: Mechanism of detection of free chlorine by inkjet-printed silver electrodes and signal plot obtained from the third step of linear sweep voltammetry [127].
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