And Paper-Based Colorimetric Gas Detectors

Review The Past, Present and Future in Tube- and Paper-Based Colorimetric Gas Detectors

Koji Kawamura 1,*, Kazumasa Miyazawa 1 and Lloyd Kent 2

1 Komyo Rikagaku Kogyo K. K., 1-8-28 Shimonoge, Takatsu-Ku, Kanagawa, Kawasaki City 213-0006, Japan; [email protected] 2 Kitagawa America, 200 Wanaque Avenue, Suite 204, Pompton Lakes, NJ 07442, USA; [email protected] * Correspondence: [email protected]; Tel.: +81-(0)44-833-3127

Abstract: Colorimetric gas detectors have been widely applied in many fields such as environmental sciences, industrial hygiene, process control, forensic science and indoor air quality monitoring. They have a history of about 100 years and include devices such as gas detector tubes and paper-based gas detectors. The sensitivity and selectivity of the colorimetric gas detector are relatively high compared to other types of gas detectors such as semiconductor, catalytic combustion and electrochemical gas detectors. Detection of gas concentration can be performed by the naked eye in some colorimetric gas detectors. These methods do not require an electrical power source and are simple, so they are suitable for field operations. This review introduces the history and provides a general overview of the development in the research of colorimetric gas detectors. Recently, the sensitivity and selectivity of colorimetric gas detectors have improved. New materials such as enzymes or particles with a large surface area have been utilized to improve selectivity and sensitivity. Moreover, new gas detectors without toxic materials have been developed to reduce the environmental load. At present, there is a rapid development of IoT sensors in many industrial fields, which might extend the applications of Citation: Kawamura, K.; Miyazawa, colorimetric gas detectors in the near future. K.; Kent, L. The Past, Present and Future in Tube- and Paper-Based Keywords: colorimetric gas detector; gas detector tube; Kitagawa gas detector Colorimetric Gas Detectors. AppliedChem 2021, 1, 14–40. https:// doi.org/10.3390/appliedchem1010003 1. Introduction Academic Editor: Andrea L. Rizzo Gas detectors have been developed to protect human life, property and the environment. They are widely applicable and convenient for users who are not specialists in this field. Received: 8 July 2021 In World War I, animals such as dogs, birds and snails were used to alert people to Accepted: 4 August 2021 Published: 18 August 2021 the presence of toxic gases or oxygen deficiency [1] (p. 5) [2]. The operator confirmed the presence of toxic gases from the abnormal behavior of these animals. These were called “animal detectors”. However, some animals are not sensitive to toxic gases. Moreover, it is Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in not easy to confirm the information as to the presence of toxic gases from these animals. published maps and institutional affil- The development of simplified detectors for mustard agent was attempted with iations. selenious acid, iodine pentoxide or hydrogen sulfide based on the principle of chemical discoloration. However, none of these detectors were perfected before the end of World War I [1] (p. 4). A gas detector tube method was developed in the USA in 1918. Hoover et al. developed the first detector tube for carbon monoxide (CO). The gas detector tube method Copyright: © 2021 by the authors. consists of a manual aspirating sampling pump and a disposable gas detector tube. The Licensee MDPI, Basel, Switzerland. This article is an open access article color of a chemical reagent in a glass tube changes upon reaction with CO. That tube was distributed under the terms and only used for qualitative detection as quantitative measurement was not yet possible [3]. conditions of the Creative Commons Kitagawa developed a detector tube that was able to quantify hydrogen sulfide (H2S) gas Attribution (CC BY) license (https:// concentration in 1946 [4]. Today, many companies have expanded the product line of creativecommons.org/licenses/by/ detector tubes capable of detecting more than 300 different gases. 4.0/).

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A gas sensing paper and color intensity reader for the gas monitoring system using a color reaction was developed in 1940 [5]. The degree of color change of the gas detection paper was monitored as a function of the intensity of the reﬂected light from a lamp. A tape- type gas detection paper monitor has a continuous monitoring and alarm capability [6]. A gas detection paper method is a visual detector and does not need any other extra devices for its operation [7]. Some gas detection paper can be used as a “passive sampler”. The sampler is designed like a badge and clips to a worker’s body for personal sampling. It can also be placed in an appropriate location in the area to be sampled. After several hours, the sampler is simply detached from the worker’s body and transferred to a photometer for measurement of gas concentration [8]. Recently, biosensors for detecting various gases have been developed to improve the selectivity for target gases. Enzymes with high selectivity are used for gas detectors. The color of detection paper is changed by enzymatic reaction. Gas concentration can be read by the naked eye and can be measured by a photometer [9,10]. Colloidal silica gel possesses many superior properties because of its large surface area [11]. In particular, the reactivity of a sensing material coated with colloidal silica can be changed because of its high surface-to-volume ratio. Colloidal silica-decorated materials such as particles or paper are sensing materials with high sensitivity. As a recent trend, research on gas sensors using Internet of Things (IoT) and smartphone technology has been reported. In the future, the digitization of gas sensing will be promoted with the development of IoT [12,13]. These applications will have broad market prospects for gas sensing such as in industrial safety and process control. In this present review, the highlights of the history and new technology of tube- and paper-based colorimetric gas detectors are introduced. Colorimetric gas detectors are a simple, easy to operate and rapid technique without expensive devices or speciﬁed skilled operators for analytical chemistry. The application of these colorimetric gas detectors in collaboration with new materials and IoT technology is very bright.

2. Gas Detector Tube Method The gas detector tube method was developed by A.B. Lamb and C.R. Hoover in 1918. They developed a simplified CO detector for mines, industrial plants, etc. A mixture of pumice stone, fuming sulfuric acid and iodic anhydride was used as a chemical reagent in the glass tube. Both ends of the tube were hermetically sealed with flame. Before measurement, both ends of the detector tube were opened with an ampoule cutter. A rubber bulb was attached to the end of the tube to force gas through the tube (Figure1). The chemical reagent for the detection of CO was in the glass tube, where the presence of CO would cause the characteristic color change from white to brown. The color of the tube changed instantly at a concentration of 1% and changed within 15 s at a concentration of 0.01%. This tube was only used for qualitative detection [3]. Because the amount and flow rate of gas was not controlled, there was no stable relationship between discoloration and gas concentration. Therefore, quantitative measurement was not yet possible. Kitagawa developed a detector tube that can quantify H2S gas concentration from the length of the discolored layer in 1946 [4]. In order to control the process during fertilizer production, it was necessary to easily measure H2S, which is a catalytic poison. The amount and flow rate of gas through the tube were controlled in this method. The sample gas was sent into the detector tube at a definite velocity (1 min) and volume (100 mL) with a piston-type sampling pump and rubber tubing, as shown in Figure2a. AppliedChem 2021, 1, FOR PEER REVIEW 3

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Figure 1. Picture of the first gas detector tube: (H) gas detector tube of CO; (K) stopper; (F) detector tube fixing tube (M) granular active charcoal; (N) porous plug; (L) spring; (O) rubber bulb [3].

The chemical reagent was made of lead acetate and silica gel particle. This reagent was loaded into a glass tube with a 3 mm inner diameter. If H2S exists in the sample, the detector tube changes immediately from its white original color to dark brown from the gas inlet side and shows a distinct length of the discolored layer when gas sampling is finished. The length of the discolored layer is proportional to the concentration of H2S in Figure 1. Picture of the first gas detector Figuretube:the sample (H) 1. Picturegas gas. detector of The the tubemeasurement ﬁrst of gas CO; detector (K) stopper;of tube: a discol (H) (F) ored gasdetec- detector length tubeenables of CO; the (K) determination stopper; (F) detector of the tor tube fixing tube (M) granular active charcoaltubeconcentration ﬁxing; (N) tube porous (M)of H granularplug;2S with (L) activespring;the concentration charcoal; (O) rubber (N) bulb porouschart [3]. shown plug; (L) in spring;Figure (O) 2b. rubber bulb [3]. This method was confirmed with an accuracy of ±5%. The chemical reagent was made of lead acetate and silica gel particle. This reagent was loaded into a glass tube with a 3 mm inner diameter. If H2S exists in the sample, the detector tube changes immediately from its white original color to dark brown from the gas inlet side and shows a distinct length of the discolored layer when gas sampling is finished. The length of the discolored layer is proportional to the concentration of H2S in the sample gas. The measurement of a discolored length enables the determination of the concentration of H2S with the concentration chart shown in Figure 2b. This method was confirmed with an accuracy of ±5%.

(a) (b)

FigureFigure 2.2.Picture Picture ofof thethe gasgas detectordetector tubetube andand concentrationconcentration chart:chart: ((aa)) picturepicture ofof thethe gas gas detector detector tube tube for for simultaneous simultaneous detection of H2S and PH3 [14]; (b) first concentration chart for gas detector tube of H2S [4]. detection of H2S and PH3 [14]; (b) ﬁrst concentration chart for gas detector tube of H2S[4].

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Nowadays, the aspirating pump and direct reading tube method have been used for gas detector tubes, as shown in Figure 3. By pulling the pump handle, the pressure inside the pump is reduced, and the sample gas is aspirated into the pump through the detector tube. Sampling velocity is controlled by the pressure drop of the detector tube.

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Nowadays, the aspiratingNowadays, pump theand aspirating direct reading pump tube and method direct reading have been tube used method for have been used for gas detector tubes, asgas shown detector in Figure tubes, 3. as By shown pulling in Figurethe pump3. By handle, pulling the the pressure pump handle, inside the pressure inside the pump is reduced,the and pump the sample is reduced, gas is and aspirated the sample into gasthe ispump aspirated through into the the detector pump through the detector tube. Sampling velocitytube. is Samplingcontrolled velocity by the pressure is controlled drop by of thethe pressuredetector droptube. of the detector tube.

Figure 3. Picture of the pump and gas detector tube: (a) aspirating pump for gas detector tube; (b) sampling method for gas detector tube. Adapted with permission from ref. [15]. Copyright 2021 Japan Society for Analytical Chemistry.

The gas concentration scale is printed directly on the detector tube. This makes it convenient for taking gas measurements on-site because the tube is read just like a thermometer (Figure 4). Temperature can affect the measurement in several ways: (1) an increase or decrease in the sample volume due to temperature change, (2) a change in the quantity of gas absorbed by the detecting reagent and (3) a change in the rate in which the detecting reagent reacts with the gases. These effects can overlap each other, causing the length of the discolored layer in the detector tube to increase or decrease, or they can offset one another, thereby having no effect on the measurement result. Some gas detector tubes that Figurecan 3. Picture be affected of theFigure pumpby the 3.and temperaturePicture gas detector of the are pumptube: provided (a and) aspirating gas with detector pumpa temperature tube: for gas (a) detector aspirating correction tube; pump (tableb) for as gas detector tube; samplingpart method of the instructionfor gas(b) samplingdetector manual tube. method of Adapted the for detector gas with detector permission tube. tube. Adaptedfrom ref. with[15]. Copyright permission 2021 from ref. [15]. Copyright Japan SocietyThe for detecting Analytical2021 Japan reagentsChemistry. Society in gas for Analyticaldetector tubes Chemistry. are formulated to react uniquely with the gas to be measured. However, detector tubes can also show a similar reaction (color Thechange) gas concentrationwith anotherThe gasscale gas having is concentration printed similar direct prop scalely erties.on is the printed A detector coexisting directly tube. gas on This can the makeshave detector a itsimilar tube. This makes convenientor different for taking colorit convenient change.gas measurements It is for necessary taking on-site gasto be measurements becauseaware of thea color tube on-site change is read because different just the like from tube a that is read just like a thermometerstated in (Figure the instructionthermometer 4). manual (Figure of the4). detector tube. Temperature can affect the measurement in several ways: (1) an increase or decrease in the sample volume due to temperature change, (2) a change in the quantity of gas absorbed by the detecting reagent and (3) a change in the rate in which the detecting reagent reacts with the gases. These effects can overlap each other, causing the length of the discolored layer in the detector tube to increase or decrease, or they can offset one another, thereby having no effect on the measurement result. Some gas detector tubes that can be affected by the temperature are provided with a temperature correction table as part of the instruction manual of the detector tube. Figure 4. ExampleFigure of the 4. printedExample scale of on the the printed detector scale tube on and the discolored detector tube layer. and Adapted discolored with layer. Adapted with Thepermission detecting from reagents ref. [15]. inCopyright gas detector 2021 Ja tubespan Society are formulated for Analytical to Chemistry.react uniquely with permission from ref. [15]. Copyright 2021 Japan Society for Analytical Chemistry. the gas to be measured. However, detector tubes can also show a similar reaction (color change) withNowadays, another gasmoreTemperature having than 300 similar different can prop affect gaseserties. the measurement and A coexisting vapors such in gas several as can CO have ways:2, CO, a (1) Osimilar2, an O3 increase, HCN, or decrease in or differentHCl, NOx, color SO change.2the, H2 sampleO, It H is2O necessary2 volume, NH3, Cl due to2, H be to2 and aware temperature VOCs of a can color change, be changemeasured (2) adifferent change using inthefrom the gas quantitythat detector of gas absorbed statedtube in the method instruction [16].by the Thismanual detecting method of the reagentis detectorused in and vari tube. (3)ous a change fields such in the as rateindu instrial which hygiene the detecting [17,18], reagent reacts with the gases. These effects can overlap each other, causing the length of the discolored layer in the detector tube to increase or decrease, or they can offset one another, thereby having no effect on the measurement result. Some gas detector tubes that can be affected by the temperature are provided with a temperature correction table as part of the instruction manual of the detector tube. The detecting reagents in gas detector tubes are formulated to react uniquely with the gas to be measured. However, detector tubes can also show a similar reaction (color Figure 4. Example of thechange) printed with scale anotheron the detector gas having tube and similar discolored properties. layer. Adapted A coexisting with gas can have a similar permission from ref. [15].or differentCopyright color 2021 Ja change.pan Society It is for necessary Analytical to Chemistry. be aware of a color change different from that stated in the instruction manual of the detector tube. Nowadays, more thanNowadays, 300 different more gases than and 300 vapors different such gasesas CO and2, CO, vapors O2, O3, such HCN, as CO2, CO, O2,O3, HCl, NOx, SO2, H2O,HCN, H2O2, HCl,NH3, NOx,Cl2, H2 SO and2,H VOCs2O,H can2O be2, NHmeasured3, Cl2,H using2 and the VOCs gas detector can be measured using the tube method [16]. Thisgas method detector is tubeused methodin various [16 fields]. This such method as indu isstrial used hygiene in various [17,18], ﬁelds such as industrial

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fire preventionhygiene [19], forensics [17,18], science ﬁre prevention [20], atmospheric [19], forensics environment science [[21],20], atmosphericindoor air qual- environment [21], ity [22], odor measurementindoor air quality [23] and [22], science odor measurement education [24–26]. [23] and science education [24–26]. The detector tubeThe has detector been improved tube has been to me improvedasure ultralow to measure concentrations ultralow concentrationsof gas with of gas with an electric samplingan electric pump sampling (Figure 5a). pump The (Figuresensitivity5a). is The improved sensitivity by increasing is improved the byvol- increasing the ume of the samplevolume being of drawn the sample [22,27]. being The drawnformaldehyde [22,27]. (HCHO) The formaldehyde detector tube (HCHO) using detector tube hydroxylamineusing as a reaction hydroxylamine reagent can as a dete reactionct down reagent to 0.08 can ppm detect (World down Health to 0.08 Organ- ppm (World Health ization standardOrganization [28]) of HCHO standard gas for [28 preven]) of HCHOtion of gas sick for building prevention syndrome of sick building(SBS). The syndrome (SBS). sampling time Theis 30 sampling min and th timee sampling is 30 min volume and the is sampling 9 L. In addition, volume a is detector 9 L. In addition,tube with a detector tube improved sensitivitywith improved has been sensitivity developed has by been increasing developed theby surface increasing area theof the surface reagent area of the reagent carrier. A detectioncarrier. reagent A detection was prepared reagent wasusing prepared quartz using sand coated quartz with sand colloidal coated with silica colloidal silica to to increase the increasesurface area the surface as a carrier. area as The a carrier. detector The tube detector for acetic tube acid for aceticand formic acid and acid formic acid was was developeddeveloped by using this by using method. this That method. detector That tube detector can tubemeasure can measurea concentration a concentration as as low as low as 10 to 100010 μ tog/m 10003 (Figureµg/m 5b).3 (Figure The sampling5b). The samplingtime is 60 min time and is 60 the min sampling and the volume sampling volume is is 12 L [11]. 12 L [11].

Figure 5. ExampleFigure of the 5. electricExample samplin of theg pump electric and sampling detector pump tube: ( anda) picture detector of the tube: electric (a) picture sam- of the electric pling pump; (b) samplinggas detector pump; tube( bof) acetic gas detector and formic tube ofacid acetic for low and concentration formic acid for gas low detection. concentration gas detection.

A simplified detectionA simpliﬁed method detection based methodon thermal based desorption on thermal (TD) desorption coupled to (TD) a gas coupled to a gas detector tube (TD-GDdetector method) tube (TD-GD was developed method) was to improve developed the to sensitivity improve the of sensitivitythe gas detec- of the gas detector tor tube methodtube in 2000 method [29]. in The 2000 TD-GD [29]. The method TD-GD is based method on is the based absorption on the absorption of the VOCs of the VOCs in a in a charcoal tubecharcoal and then tube thermally and then thermallydesorbing desorbingthem. A large them. amount A large of amount gas is sampled of gas is sampled and and concentratedconcentrated in the charcoal in the tube charcoal with tubean electric with an sampling electric samplingpump. The pump. VOCs The are VOCs de- are desorbed sorbed with heatwith and heat are anddetected are detected with a gas with de atector gas detector tube (Figure tube (Figure6). The 6detector). The detector tube is tube is based based on the principleon the principle that toluene that toluenegas reac gasts with reacts iodine with pentoxide iodine pentoxide producing producing a brown- a brown-colored 3 colored product,product, iodine. iodine. This method This method is able isto able detect to detectdown downto 0.012 to 0.012ppm (50 ppmμg/m (50 3µ) g/mof ) of toluene toluene gas, a lowergas, a lowerconcentration concentration than the than WHO the WHO guideline guideline value value (0.07 (0.07ppm) ppm) for indoor for indoor air [28]. It air [28]. It can becan utilized be utilized in th ine control the control and andprevention prevention of SBS. of SBS. AppliedChem 2021, 1, FOR PEER REVIEW 6 AppliedChem 2021, 1 19

FigureFigure 6. 6. DiagramDiagram of of TD-GD TD-GD method. method. Total volatile organic compounds (TVOCs) that cause suspended particulate matter Total volatile organic compounds (TVOCs) that cause suspended particulate matter (SPM) and oxidants have been used as a pollution index of the atmospheric environ- (SPM) and oxidants have been used as a pollution index of the atmospheric environment ment [30]. The Japanese Air Pollution Control Act has an emission regulation for exhaust [30]. The Japanese Air Pollution Control Act has an emission regulation for exhaust TVOCs from factories. However, the composition ratio of VOCs in the exhausted gases TVOCs from factories. However, the composition ratio of VOCs in the exhausted gases always differs depending on the factory’s process of manufacturing. Because the sensitivity always differs depending on the factory’s process of manufacturing. Because the sensitiv- of simple methods such as gas detector tubes differs depending on the type of VOC, the ity of simple methods such as gas detector tubes differs depending on the type of VOC, simple gas detector cannot measure the absolute concentration (mg/m3, ppm) of mixed the simple gas detector cannot measure the absolute concentration (mg/m3, ppm) of mixed VOC gases in principle. VOC gases in principle. The catalytic combustion gas detector method (CC-GD method) was developed to detectThe TVOCs catalytic with combustion a gas detector gas detector tube [31 ].me TVOCsthod (CC-GD are oxidized method) by awas heated developed catalyst to at detect◦ TVOCs with a gas detector tube [31]. TVOCs are oxidized by a heated catalyst at 300 C to produce CO2 as shown in Equation (1). Generated CO2 is measured with a gas 300 °C to produce CO2 as shown in Equation (1). Generated CO2 is measured with a gas detector tube for CO2 (Figure7a,b). detector tube for CO2 (Figure 7a,b). Catalyst(Pt) at 300◦C Catalyst ℃() Pt at 300 AppliedChem 2021, 1, FOR PEER REVIEW Various VOCs + O2 → Carbon Dioxide7 → (1) CVarious VOCs + O7H8(Toluene) 2 Carbon Dioxide7CO2 100 ppm() 700 ppm (1) C78 H Toluene 7CO 2

100 ppm 700 ppm

Various VOCs are oxidized and produce an amount of CO2 in accordance with the carbon amount of each VOC. As an example, 100 ppm of toluene is oxidized to 700 ppm of CO2. A pH indicator is used as the reaction reagent in the CO2 detector tube. Sample gas is collected in a gasbag. Background CO2 in the bag is measured as a blank. The gasbag, catalytic combustor and gas detector tube are then connected as shown in Figure 7a. Sample gas in the gasbag is aspirated into the gas detector tube through the catalytic combustor. VOCs are decomposed into carbon dioxide by heated catalysis and then measured as CO2. The concentration of background CO2 in the blank is subtracted from the concentration of CO2 that passed through the catalytic combustor to determine the concentration of TVOCs. The concentration of TVOCs measured by the CC-GD method can be confirmed by a gas chromatography–flame ionization detection (GC-FID) method. The results are displayed in Table 1. The concentration of VOCs obtained by the CC-GD method was in good agreement with that of the GC-FID method. Figure 7. Figure(a) Diagram 7. (a) Diagram of the CC-GD of the method; CC-GD method; (b) diagram (b) diagram and picture andof picture the CC-GD of the method.CC-GD method. Adapted with permission from ref. [Adapted31]. Copyright with permission 2006 Kawamura from ref. and [31]. Honma. Copyright 2006 Kawamura and Honma.

Table 1. Measurement resultsVarious for VOCs CC-GD are method oxidized and GC-FID and produce method an [31]. amount of CO2 in accordance with the carbon amount of each VOC. As an example, 100 ppm of toluene is oxidized to 700 ppm of CC-GD GC-FID Bias of CC-GD for GC-FID VOC CO2. A pH indicator is used as the reaction reagent in the CO2 detector tube. Sample gas (ppmC) (ppmC) (%) 1 is collected in a gasbag. Background CO2 in the bag is measured as a blank. The gasbag, Toluene catalytic combustor2960 and gas detector 2945 tube are then connected 0.5 as shown in Figure7a. Sample Ethyl acetate 2445 2618 −1.4 Mixed VOC 1 2 2660 2440 9.0 3 Mixed VOC 2 420 428 −1.9 1 Bias % = measurement result of (CC-GD − GC-FID)/GC-FID × 100. 2 Toluene: 463 ppmC, methyl ethyl ketone: 542 ppmC, ethyl acetate: 755 ppmC, hexane: 680 ppmC. 3 Toluene: 419 ppmC, methyl ethyl ketone: 9 ppmC.

Toxic heavy metals such as mercury or chromium oxide (VI) (Cr(VI)) are used for gas detector tube reaction reagents as shown in Equations (2)–(4) [32–34]. Mercury chloride (HgCl2) is used for the detection of PH3, H2S, mercaptans and so on. Cr(VI) is used for the detection of VOCs as an oxidizer in detector tubes. The color of mercury and Cr(VI) in the detector tubes changes upon reaction with target gases. Figures 8 and 9 show a gas detector tube that contains mercury and Cr(VI) as an example. However, these metals are toxic and regulated in many countries to prevent environmental pollution [35]. → H22 S + HgCl HgS + 2 HCl 2 (2) White Black → PH32 + 3HgCl P() HgCl 3 + 3HCl (3) White Yellow

The reagent color of the detector tube changes with HCl and a pH indicator.

(a)

(b)

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gas in the gasbag is aspirated into the gas detector tube through the catalytic combustor. VOCs are decomposed into carbon dioxide by heated catalysis and then measured as CO 2. Figure 7.The (a) concentrationDiagram of the ofCC-GD background method;CO (b) 2diagramin the blank and picture is subtracted of the CC-GD from method. the concentration of AdaptedCO with2 that permission passed throughfrom ref. the[31]. catalytic Copyright combustor 2006 Kawamura to determine and Honma. the concentration of TVOCs. The concentration of TVOCs measured by the CC-GD method can be conﬁrmed Table 1.by Measurement a gas chromatography–ﬂame results for CC-GD method ionization and GC-FID detection method (GC-FID) [31]. method. The results are displayed in Table1. The concentration of VOCs obtained by the CC-GD method was in CC-GD GC-FID Bias of CC-GD for GC-FID goodVOC agreement with that of the GC-FID method. (ppmC) (ppmC) (%) 1 TableToluene 1. Measurement results2960 for CC-GD method 2945 and GC-FID method [31 0.5]. Ethyl acetate 2445 2618 −1.4 CC-GD GC-FID Bias of CC-GD for GC-FID Mixed VOC VOC1 2 2660 2440 9.0 (ppmC) (ppmC) (%) 1 Mixed VOC 2 3 420 428 −1.9 1 Bias % = measurementToluene result of (CC-GD 2960 − GC-FID)/GC-FID 2945 × 100. 2 Toluene: 463 ppmC, 0.5 methyl Ethyl acetate 2445 2618 −1.4 ethyl ketone: 542 ppmC, ethyl acetate: 755 ppmC, hexane: 680 ppmC. 3 Toluene: 419 ppmC, methyl Mixed VOC 1 2 2660 2440 9.0 ethyl ketone: 9 ppmC. Mixed VOC 2 3 420 428 −1.9 1 2 ToxicBias heavy % = measurement metals such result as of mercury (CC-GD − orGC-FID)/GC-FID chromium oxide× 100. (VI)Toluene: (Cr(VI)) 463 are ppmC, used methyl for ethylgas ketone: 542 ppmC, ethyl acetate: 755 ppmC, hexane: 680 ppmC. 3 Toluene: 419 ppmC, methyl ethyl ketone: 9 ppmC. detector tube reaction reagents as shown in Equations (2)–(4) [32–34]. Mercury chloride (HgCl2) is usedToxic for heavy the detection metals such of PH as3 mercury, H2S, mercaptans or chromium and oxideso on. (VI) Cr(VI) (Cr(VI)) is used are for used the for gas detectiondetector of VOCs tube as reactionan oxidizer reagents in detector as shown tubes. in The Equations color of (2)–(4) mercury [32 and–34]. Cr(VI) Mercury in the chloride detector(HgCl tubes2) changes is used forupon the reaction detection with of ta PHrget3,H gases.2S, mercaptans Figures 8 and and 9 soshow on. a Cr(VI) gas detec- is used for tor tubethe that detection contains of mercury VOCs as and an Cr(VI) oxidizer as inan detectorexample. tubes. However, The colorthese ofmetals mercury are toxic and Cr(VI) and regulatedin the detector in many tubes countries changes to prevent upon reaction environmental with target pollution gases. Figures[35]. 8 and9 show a gas detector tube that contains mercury→ and Cr(VI) as an example. However, these metals are H22 S + HgCl HgS + 2 HCl 2 toxic and regulated in many countries to prevent environmental pollution [35]. (2) White Black H S + HgCl → HgS + 2 HCl 2 2 2 (2) White→ Black PH32 + 3HgCl P() HgCl 3 + 3HCl (3) PHWhite+ 3HgCl Yellow→ P(HgCl) + 3HCl 3 2 3 (3) White Yellow The reagent color of the detector tube changes with HCl and a pH indicator.

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Figure 8. Picture of a PH3 detector tube that contains mercury chloride: (a) before gas sampling; (b) after gas sampling. Adapted with permission(b) from ref. [36]. Copyright 2019 Kawamura, Sae- gusa and Miyazawa.

6+→ 3+ VOCs + CrFigure 8. Picture + Hof a PH3 24detector SO tube that contains Cr + Oxidation product mercury chloride: (a) before gas sampling; (4) (b) after gas Yellow or orange P sampling. Adapted with permission fromale blue or brown ref. [36]. Copyright 2019 Kawamura, Saegusa and Miyazawa. The color change depends on the design of the chemical reagent. (a)

(b)

FigureFigure 9. Picture 9. Picture of an ethanol of an ethanol detector detector tube that tube contains that contains Cr(VI): Cr(VI): (a) before (a) before gas sampling; gas sampling; (b) after (b) after gas sampling. gas sampling.

Recently, detector tubes that do not use toxic heavy metals have been developed to reduce the environmental load. In particular, mercury and its compounds are regulated in many countries to prevent mercury poisoning. More than 140 countries agreed in the Minamata Convention on Mercury by the UNEP to prevent the emission of mercury [37]. Therefore, detector tubes using gold chloride have been developed, but the price of gold is high and it is not preferred as a reagent material for the product. Then, gas detector tubes for PH3 and H2S detection made without mercury and gold were developed. Silver was chosen as an indicator for PH3 and H2S detection because silver is a well-described reagent and good for environmental load reduction [36,38]. Figure 10 shows a PH3 gas detector tube with silver sulfate as an example. The measurement range of the detector tube is 10–700 ppm. The sampling time is one minute and the sampling volume is 100 mL. The chemical reaction is shown in Equation (5).

Figure 10. Picture of a PH3 detector tube without mercury (uses silver sulfate): (a) before gas sampling; (b) after gas sampling. Adapted with permission from ref. [36]. Copyright 2019 Kawa- mura, Saegusa and Miyazawa.

→ 2PH 3243424 + 3Ag SO 2Ag PO + 3H SO (5) White Brown Gas detector tubes for VOC detection without Cr(VI) have been developed [15]. Po- tassium manganate (VII) (KMnO4) was chosen as an indicator because it is a well-described VOC oxidizer and its environmental impact potential is relatively low. The white discolored layer on the purple detector tube with KMnO4 increased as vapor concentrations of alcohols (ethanol, methanol and 2-propanol) increased in a range of 0–300 ppm. Figure 11 shows a gas detector tube for alcohols as an example. The sampling time is three minutes and the sampling volume is 100 mL. This method is based on the principle that alcohols react with KMnO4 to produce a colorless product (Equation (6)).

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Figure 8. Picture of a PH3 detector tube that contains mercury chloride: (a) before gas sampling; (b) after gas sampling. Adapted with permission from ref. [36]. Copyright 2019 Kawamura, Sae- gusa and Miyazawa.

6+→ 3+ VOCs + Cr + H24 SO Cr + Oxidation product (4)

AppliedChem 2021, 1 Yellow or orange Pale blue or brown 21 The color change depends on the design of the chemical reagent. (a) The reagent color of the detector tube changes with HCl and a pH indicator.

VOCs + Cr6++H SO(b) → Cr3++Oxidation product 2 4 (4) Yellow or orange Pale blue or brown

The color change depends on the design of the chemical reagent. Figure 9. PictureRecently, of an ethanol detector detector tubes tube that that do notcontains use toxicCr(VI): heavy (a) before metals gas havesampling; been (b developed) after to gas samplingreduce. the environmental load. In particular, mercury and its compounds are regulated in many countries to prevent mercury poisoning. More than 140 countries agreed in the Recently,Minamata detector Convention tubes that on Mercurydo not use by toxic the UNEP heavy to metals prevent have the been emission developed of mercury to [37]. reduce Therefore,the environmental detector tubesload. usingIn particul goldar, chloride mercury have and been its developed,compounds but are the regulated price of gold is in manyhigh countries and it isto notprevent preferred mercury as a reagentpoisoning. material More forthan the 140 product. countries agreed in the Minamata ConventionThen, gas detector on Mercury tubes by for the PH UNEP3 and to H 2preventS detection the madeemission without of mercury mercury [37]. and gold Therefore,were detector developed. tubes Silverusing wasgold chosenchloride as have an indicator been developed, for PH3 butand the H2 Sprice detection of gold because is high silverand it isis anot well-described preferred as a reagent reagent and material good forfor environmentalthe product. load reduction [36,38]. Then, gasFigure detector 10 shows tubes for a PH PH3 3gas and detector H2S detection tube withmade silver without sulfate mercury as an and example. gold The were developed.measurement Silver range was ofchosen the detector as an indicator tube is 10–700 for PH ppm.3 and TheH2S samplingdetection timebecause is one sil- minute ver is a andwell-described the sampling reagent volume and is 100good mL. for Theenvironmental chemical reaction load reduction is shown [36,38]. in Equation (5). Figure 10 shows a PH3 gas detector tube with silver sulfate as an example. The meas- 2PH + 3Ag SO → 2Ag PO + 3H SO urement range of the detector tube3 is 10–7002 4 ppm. The sampling3 4 time is2 one4 minute and (5) the sampling volume is 100 mL. The chemicalWhite reaction is shown Brown in Equation (5).

Figure 10.Figure Picture 10. ofPicture a PH of3 detector a PH3 detector tube without tube withoutmercury mercury (uses silver (uses sulfate): silver sulfate): (a) before (a) gas before gas sampling;sampling; (b) after ( bgas) after sampling. gas sampling. Adapted Adapted with permissi with permissionon from fromref. [36]. ref. [36Copyright]. Copyright 2019 2019 Kawa- Kawamura, mura, Saegusa and Miyazawa. Saegusa and Miyazawa.

Gas detector tubes for VOC→ detection without Cr(VI) have been developed [15]. AppliedChem 2021, 1, FOR PEER REVIEW 2PH 3243424 + 3Ag SO 2Ag PO + 3H SO 9 Potassium manganate (VII) (KMnO4) was chosen as an indicator because it(5) is a well- White described VOC oxidizer and its Brown environmental impact potential is relatively low. The white Gasdiscolored detector layertubes onfor the VOC purple detection detector withou tube witht Cr(VI) KMnO have4 increased been developed as vapor [15]. concentrations Po- Mercury and Cr(IV) continue to be used in some gas detector tubes for the detection tassiumof manganate alcohols (ethanol, (VII) (KMnO methanol4) was and 2-propanol)chosen as an increased indicator in abecause range of it 0–300 is a well-de- ppm. Figure 11 scribedofshows VOC certain oxidizer a gas other detector and gases its tube suchenvironmental for as alcoholsHCN or impa asVOCs anct example. potentialeven today. The is relatively samplingMoreover, low. time lead The is oxide three white has minutes also beenand theused sampling as an oxidizer volume in isdetector 100 mL. tubes. This methodNovel principles is based onwithout the principle requiring that these alcohols toxic discolored layer on the purple detector tube with KMnO4 increased as vapor concentrations ofmaterialsreact alcohols with for(ethanol, KMnO gas detector4 tomethanol produce tubes and a should colorless 2-propanol) be productdeveloped increased (Equation to reduce in a (6)). range the environmental of 0–300 ppm. load. Figure 11 shows a gas detector tube for alcohols as an example. The sampling time is three minutes and the sampling volume is 100 mL. This( amethod) is based on the principle that alcohols react with KMnO4 to produce a colorless product (Equation (6)).

(b)

Figure 11. Picture of the alcohol detector tube without Cr(VI) (KMnO4): (a) before gas sampling; (b) Figure 11. Picture of the alcohol detector tube without Cr(VI) (KMnO4): (a) before gas sampling; after gas sampling. (b) after gas sampling.

Mercury and Cr(IV) continue to be→ used+ in some gas detector tubes for the detection Alcohols + KMnO42 K + MnO + Oxidation product of certain other gases such as HCN or VOCs even today. Moreover, lead oxide has also(6) Purple White

Possible oxidation products: carboxylic acid. Kubota et al. developed the inorganic qualitative gas detector using five types of discoloration reagents together in a single tube for one-time measurement (Figure 12) [39,40]. The color of the reagents changes upon reaction with various inorganic gases. Inorganic gas can be qualitatively detected based on the combination of color changes in the tube (Figure 13). Organic gases also can be detected with chemical reagents for organic gases using a similar principle [40].

(a) (b)

Most gas detector tubes are made with a glass tube filled with a chemical reagent particle. The length of the tube depends on the manufacturer; however, it is often about 80–120 mm. Small detector tube systems have been required, and the splintering of detector tube glass might cause an injury. Therefore, a gas detector tube in a small chip was developed. The chip (50 mm × 10.5 mm × 5 mm) contains 10 measurement capillaries filled with a chemical reagent for gas detection (Figure 14a) [41,42]. When the chip (Figure 14b) is inserted into the analyzer, all information required for the measurement is transferred to the handy size analyzer by means of a bar code on the chip. Gas concentration is measured from the discoloration time of the tube with the photometer in the analyzer. The measurement results are interpreted and corrected under the operating temperature specification automatically. This system can be used to measure 35 different gases and vapors.

AppliedChem 2021, 1, FOR PEER REVIEW 9

Mercury and Cr(IV) continue to be used in some gas detector tubes for the detection of certain other gases such as HCN or VOCs even today. Moreover, lead oxide has also been used as an oxidizer in detector tubes. Novel principles without requiring these toxic materials for gas detector tubes should be developed to reduce the environmental load.

(a)

(b) AppliedChem 2021, 1 22

Figure 11. Picture of the alcohol detector tube without Cr(VI) (KMnO4): (a) before gas sampling; (b) afterbeen gas used sampling. as an oxidizer in detector tubes. Novel principles without requiring these toxic materials for gas detector tubes should be developed to reduce the environmental load. → + AlcoholsAlcohols ++ KMnOKMnO42→ KK + MnO++MnO + + Oxidation productOxidation product 4 2 (6)(6) PurpPurplele White White

AppliedChem 2021, 1, FOR PEER REVIEW PossiblePossible oxidation oxidation products: products: carboxylic carboxylic acid. acid. 10

KubotaKubota et et al. al. developed developed the the inorganic inorganic qualit qualitativeative gas gas detector detector using using five ﬁve types types of of dis- dis- colorationcoloration reagents reagents together together in in a asingle single tube tube for for one-time one-time measurement measurement (Figure(Figure 12) 12 [39,40].)[39,40]. TheThe color color of of the the reagents reagents changes changes upon upon reacti reactionon with with various various inorganic inorganic gases. gases. Inorganic Inorganic gasSECTION CHEMICAL REACTION PRINCIPLES gas can can be be qualitatively qualitatively detected detected based based on on the the combination combination of of color color changes changes in in the the tube tube (FigureA By reacting with Phosphoric acid, PH indica(Figure 13). 13). Organic Organic gases gases also also can can be be detect detecteded with withtor is discoloured. chemical chemical reagents reagents for for organic organic gases gases usingusing a asimilar similar principle principle [40]. [40]. 2NH33 ＋ H PO42 → (NH 44 ) HPO B By reacting with an Alkaline, PH indicator is discoloured. (a) (b) SO2232 ＋ 2NaOH → Na SO＋ H O C By reacting with oo -Toluidine, Nitro- -Toluidine(Dyestuff) is liberated. (7) D By reacting with Lead Acetate(II), Lead sulphide is produced.

H2323 S ＋ Pd(CH COO) → PdS＋ 2CH COOH E Potasium disulphide palladate (II) is reduced and Palladium is liberated.

CO ＋ K23223 Pd(SO ) K (SO )2PdCO FigureFigure 12. 12. InorganicInorganic gas gas qualitative qualitative detector detector tube: tube: (a ()a picture) picture of of the the detector detector tube; tube; (b ()b original) original color. color. K232 (SO ) PdCO → CO2＋＋ SO 2 K 23 SO AdaptedAdapted with with permission permission from from ref. ref. [40] [40.]. Copyright Copyright 2021 2021 Komyo Komyo Rikagaku Rikagaku Kogyo Kogyo K.K. K.K.

FigureFigure 13. 13. DetectionDetection chart chart of of the the inorganic inorganic gas gas qualitat qualitativeive detector detector tube. tube. Adapted Adapted with with permission permission fromfrom ref. ref. [40]. [40 ].Copyright Copyright 2021 2021 Komyo Komyo Rikagaku Rikagaku Kogyo Kogyo K.K. K.K. Most gas detector tubes are made with a glass tube ﬁlled with a chemical reagent particle. The length of the tube depends on the manufacturer; however, it is often about 80–120 mm. Small detector tube systems have been required, and the splintering of detector tube glass might cause an injury. Therefore, a gas detector tube in a small chip was developed. The chip (50 mm × 10.5 mm × 5 mm) contains 10 measurement capillaries ﬁlled with a chemical reagent for gas detection (Figure 14a) [41,42]. When the chip (Figure 14b) is inserted into the analyzer, all information required for the measurement is transferred to the handy size analyzer by means of a bar code on the chip. Gas concentration is

(a) (b) Figure 14. Picture of the gas detector tube in the chip system: (a) gas detector (photometer and air pump); (b) gas detector tube in small chip (10 tubes in the chip).

AppliedChem 2021, 1, FOR PEER REVIEW 10

SECTION CHEMICAL REACTION PRINCIPLES A By reacting with Phosphoric acid, PH indicator is discoloured.

2NH33 ＋ H PO42 → (NH 44 ) HPO B By reacting with an Alkaline, PH indicator is discoloured.

SO2232 ＋ 2NaOH → Na SO＋ H O C By reacting with oo -Toluidine, Nitro- -Toluidine(Dyestuff) is liberated. (7) D By reacting with Lead Acetate(II), Lead sulphide is produced.

H2323 S ＋ Pd(CH COO) → PdS＋ 2CH COOH E Potasium disulphide palladate (II) is reduced and Palladium is liberated.

CO ＋ K23223 Pd(SO ) K (SO )2PdCO

K232 (SO ) PdCO → CO2＋＋ SO 2 K 23 SO

AppliedChem 2021, 1 23

measured from the discoloration time of the tube with the photometer in the analyzer. The measurement results are interpreted and corrected under the operating temperature speciﬁcation automatically. This system can be used to measure 35 different gases and vapors.

SECTION CHEMICAL REACTION PRINCIPLES A By reacting with Phosphoric acid, PH indicator is discoloured. 2NH3 + H3PO4 → (NH4)2HPO4 B By reacting with an Alkaline, PH indicator is discoloured. SO2 + 2NaOH → Na2SO3 + H2O C By reacting with o−Toluidine, Nitro−o−Toluidine(Dyestuff) is liberated. (7) D By reacting with Lead Acetate(II), Lead sulphide is produced. H2S + Pd(CH3COO)2 → PdS + 2CH3COOH E Potasium disulphide palladate (II) is reduced and Palladium is liberated. CO + K Pd(SO ) K (SO ) PdCO Figure 13. Detection2 chart3 2 of the2 inorganic3 2 gas qualitative detector tube. Adapted with permission K (SO ) PdCO → CO + SO + K SO from ref.2 [40].3 Copyright2 2021 Komyo2 Rikagaku2 2 Kogyo3 K.K.

(a) (b)

FigureFigure 14. Picture 14. Picture of the of gas the detector gas detector tube tubein the in chip the chipsystem: system: (a) gas (a) detector gas detector (photometer (photometer and air and pump); air pump); (b) gas (b) detector gas detector tubetube in small in small chip chip(10 tubes (10 tubes in the in chip). the chip).

3. Gas Sensing Paper and Color Intensity Reader for a Gas Monitoring System Gas detector tubes have a great advantage in terms of selectivity and rapidity for practical analysis compared to other types of gas detectors such as semiconductor, combustible and electrochemical gas sensors because they use a selective chemical detection principle [23]. However, detector tubes are spot testers and not adequate for continuous detection. The tape monitor method was developed for continuous gas detection method using a colorimetric principle that has gas selectivity [5,43,44]. The gases in the sample react with the chemical reagent in the tape to produce the chemical reaction product, and hence the color of the tape changes much like it does in a detector tube. The sample gas is drawn through the sampling chamber at a constant flow rate (e.g., 400 mL/min) using an air pump in the detector. The degree of color change is recorded by measuring the relative light reflected off of the tape. The length of tape exposed to the sample is renewed by moving the tape every measurement interval (Figure 15). Measurement interval depends on the gas detector design (e.g., 5 min, 30 min). Nakano et al. developed the HCHO gas detector using the tape monitor method [45,46]. The dried tape contained silica gel and was impregnated with a processing solution containing hydroxylamine sulfate. Methyl Yellow (pH indicator) was used as the detection reagent. When an air sample including HCHO was exposed to the cellulose tape, the color of the tape changed from yellow to red. The degree of color change was proportional to the concentration of HCHO, and it was recorded by measuring the AppliedChem 2021, 1 intensity of reflected light at 555 nm. The tape could detect down to 0.08 ppm of HCHO 24 (WHO standards for indoor air quality control [28]) with a sampling time of 30 min.

FigureFigure 15. 15.SchematicSchematic of the of thetape tape monitor monitor method. method. Adapte Adaptedd with with permission permission from from ref. ref.[8] [(p.8] (p.16). 16). CopyrightCopyright 2004 2004 Kawamura. Kawamura.

TheNakano chemical et al.reaction developed of this the meth HCHOod is gas shown detector in Equation using the (8). tape monitor method [45,46]. The dried tape contained silica gel and was impregnated with a processing solution 2HCHO+ () NH OH・ H SO → 2H C=NOH + H SO +H O containing hydroxylamine22422 sulfate. Methyl Yellow (pH indicator) 242was used as the detection(8) reagent. When an air sample including HCHO was exposed to the cellulose tape, the color ofThe the tapeMethyl changed Yellow from pH yellow indicator to red. on The the degreetape changed of color changecolor by was reacting proportional with the to the sulfuricconcentration acid that ofwas HCHO, produced and by it was the reacti recordedon of by hydroxylamine measuring the sulfate intensity reagent of reﬂected with the light HCHO.at 555 The nm. response The tape of could the tape detect was down interpreted to 0.08 ppmusing of the HCHO following (WHO equation: standards for indoor air quality control [28]) with a sampling time of 30 min. Response ()() % of voltage drop = VV−× / V 100 (9) The chemical reaction of this method01 is shown 0 in Equation (8).

where VV01 n is the voltage before gas sampling and lig is the voltage after gas samp 2HCHO + (NH2OH)2·H2SO4 → 2H2C = NOH + H2SO4 +H2O (8) The tape used above does not respond to CO, NOx, CO2 and VOCs such as toluene and xylene.The HCl Methyl and Yellowaldehydes pH give indicator a response; on the however, tape changed the interference color by reacting of these withgases the is notsulfuric a serious acid problem that was for produced monitoring by the ambient reaction air. of hydroxylamine sulfate reagent with the HCHO. The response of the tape was interpreted using the following equation:

Response (% of voltage drop) = (V0 − V1) / V0 × 100 (9)

V V where 0 is the voltage before gas sampling and 1 is the voltage after gas sampling. AppliedChem 2021, 1, FOR PEER REVIEW 12 The tape used above does not respond to CO, NOx, CO2 and VOCs such as toluene and xylene. HCl and aldehydes give a response; however, the interference of these gases is not a serious problem for monitoring ambient air.

Various tapes for gases suchVarious as HCN tapes [47], for Cl2 gases [6], NH such3[48], as HCl HCN [49], [47 H],2S Cl [50],2 [6 ],silane NH3 [48], HCl [49], H2S[50], [51], PH3 and AsH3 [52] silanehave been [51], PHdeveloped.3 and AsH This3 [52 ]method have been is simple, developed. specific This and method is simple, specific and recommended for continuousrecommended gas detection for in continuous the field. gas detection in the field. A circle detection paper tabletA circle and the detection measurement paper tabletof the relative and the light measurement reflected has of the relative light reflected been developed as spot samplehas been and developed detection asmethod. spot sample However, and detectionthis method method. is not However, this method is adequate for continuous detection,not adequate although for continuous it is suitable detection, for portable although gas it isdetection suitable for portable gas detection applications due to its smallapplications size. Figure due 16 shows to its small an example size. Figure of a detection 16 shows paper an example tablet. of a detection paper tablet.

Figure 16. Example of detectionFigure paper 16. tablet.Example Adapted of detectionwith permission paper tablet.from ref. Adapted [8] (p. 27). with Copy- permission from ref. [8] (p. 27). right 2004 Kawamura. Copyright 2004 Kawamura.

The detection paper on the tablet is exposed to sample gas using an air pump in the photometer. The degree of color change ias recorded by measuring the reflected light with a photodiode and LED (Figure 17).

Detection paper tablet

Figure 17. Schematic of photometer and detection paper tablet.

A portable HCHO gas detector was developed using a reagent “KD-XA01”(4-amino- 4-phenylbut-2-en-2-on) in a dried state on the detection paper tablet. HCHO reacted with KD-XA01, and the color of the detection paper changed to yellow. The degree of the color change was recorded by measuring the relative intensity of reflected light at 400 nm with a gas detection device called an FP-30 (photometer and sampling pump). The flow rate of the sample gas was 250 mL/min [53–55]. The reaction of HCHO and KD-XA01 is presented in Figure 18. The KD-XA01-based method showed no significant response towards various VOCs or aldehydes other than HCHO, assuring high reliability. The limit of detection was 0.05 ppm after a 5-min test.

AppliedChem 2021, 1, FOR PEER REVIEW 12

Various tapes for gases such as HCN [47], Cl2 [6], NH3[48], HCl [49], H2S [50], silane [51], PH3 and AsH3 [52] have been developed. This method is simple, specific and recommended for continuous gas detection in the field. A circle detection paper tablet and the measurement of the relative light reflected has been developed as spot sample and detection method. However, this method is not adequate for continuous detection, although it is suitable for portable gas detection applications due to its small size. Figure 16 shows an example of a detection paper tablet.

AppliedChem 2021Figure, 1 16. Example of detection paper tablet. Adapted with permission from ref. [8] (p. 27). Copy- 25 right 2004 Kawamura.

The detection paper on the tablet is exposed to sample gas using an air pump in the photometer. The degreeThe of detection color change paper ias on thereco tabletrded isby exposed measuring to sample the reflected gas using light an air with pump in the photometer. The degree of color change ias recorded by measuring the reﬂected light with a photodiode and LED (Figure 17). a photodiode and LED (Figure 17).

Detection paper tablet

Figure 17. Schematic of photometer and detection paper tablet. Figure 17. Schematic of photometer and detection paper tablet. A portable HCHO gas detector was developed using a reagent “KD-XA01”(4-amino- A portable HCHO4-phenylbut-2-en-2-on) gas detector was in develo a driedped state using on the a detection reagent “KD-XA01”(4-amino- paper tablet. HCHO reacted with 4-phenylbut-2-en-2-on)KD-XA01, in a and dried the colorstate ofon the the detection detection paper paper changed tablet. to HCHO yellow. reacted The degree with of the color KD-XA01, and thechange color wasof the recorded detection by measuring paper changed the relative to yellow. intensity The of reflecteddegree of light the at color 400 nm with a change was recordedgas detection by measuring device called the relative an FP-30 intensity (photometer of reflected and sampling light pump). at 400 Thenm flowwith rate of the a gas detection devicesample called gas was an 250FP-30 mL/min (photo [53meter–55]. Theand reactionsampling of HCHOpump). and The KD-XA01 flow rate is presentedof in Figure 18. The KD-XA01-based method showed no significant response towards various AppliedChem 2021, 1, FOR PEERthe REVIEW sample gas was 250 mL/min [53–55]. The reaction of HCHO and KD-XA01 is presented 13 VOCs or aldehydes other than HCHO, assuring high reliability. The limit of detection was in Figure 18. The0.05 KD-XA01-based ppm after a 5-min method test. showed no significant response towards various VOCs or aldehydes other than HCHO, assuring high reliability. The limit of detection was 0.05 ppm after a 5-min test.

Figure 18. TheFigure reaction 18. Theof HCHO reaction and of HCHO KD-XA01. and KD-XA01.

A NO2 detector was also developed using the FP-30 [56]. N-1-Naphthylethylenediamine 2 A NOdihydrochloride detector onwas the detectionalso developed paper reacted using with NO the2 to giveFP-30 a color [56]. change N to-1- yellow. NaphthylethylenediamineThe degree of color dihydrochloride change of paper on from the whitedetection to yellow paper was reacted monitored with as NO a function2 to give a color ofchange the intensity to yellow. of the The reflected degree light of color (λ = 475change nm) of anpaper LED. from The white limit of to the yellow detection was monitoredwas as 0.01 a function ppm when of thethe sampling intensity time of the was reflected 30 min, andlight the (λflow = 475 rate nm) of of sample an LED. gas was The limit of the250 detection mL/min. Thiswas device0.01 ppm is useful when for the the sampling detection time of NO was2 in 30 a min, room and using the an flow exhaust room discharge type heater. rate of sample gas was 250 mL/min. This device is useful for the detection of NO2 in a Another type of HCHO gas detector for SBS was developed with a significantly short room using an exhaust room discharge type heater. sampling time. 4-Amino hydrazine-5-mercapto-1,2,4-triazole (AHMT) reagent was chosen Anotheras type an indicator of HCHO for gas HCHO detector detection for becauseSBS was AHMT developed is a well-described with a significantly reagent with short a high sampling time.sensitivity 4-Amino and selectivity hydrazine-5-mercapto-1,2,4-triazole towards HCHO [57]. A disposable (AHMT) circular glassreagent paper onwas a chip chosen as an indicator for HCHO detection because AHMT is a well-described reagent with a high sensitivity and selectivity towards HCHO [57]. A disposable circular glass paper on a chip and hanging drop kit were developed for simple and routine use. The glass paper was impregnated with AHMT and detected HCHO concentrations in air at less than 0.08 ppm in 3 min. The limit of detection was 0.04 ppm. Before detection, 50 μL of AHMT solution and 50 μL of KOH solution (1:1) were dropped on the glass paper. Then, the reagents on the glass paper were exposed to HCHO gas in a photometer (Figure 19a). An air pump was installed in this photometer. The degree of the color change was recorded by measuring the relative intensity of reflected light at 550 nm. The flow rate of sample gas was 200 mL/min. HCHO reacted with AHMT and KOH, and the color of the reagent solutions changed to purple in 3 min (Figure 19b). This AHMT-based method exhibits high sensitivity and a short measurement time since the liquid reagent and HCHO gas react on the glass paper. In Figure 20, the AHMT- based method was compared with a conventionally used hydroxylamine-based one. The AHMT-based method is more sensitive and has a shorter sampling time than the hydroxylamine-based method.

Figure 19. Photograph of sensor for HCHO gas detection: (a) photometer; Adapted with permission from ref. [57]. Copyright 2005 Elsevier. (b) glass paper with AHMT/KOH solution after HCHO gas exposure. Adapted with permission from ref. [8] (p. 40). Copyright 2004 Kawamura.

“This article was published in Publication title, Vol number, Author(s), Title of article, Page Nos, Copyright Elsevier (or appropriate Society name) (Year).”

AppliedChem 2021, 1, FOR PEER REVIEW 13

Figure 18. The reaction of HCHO and KD-XA01.

A NO2 detector was also developed using the FP-30 [56]. N-1- Naphthylethylenediamine dihydrochloride on the detection paper reacted with NO2 to give a color change to yellow. The degree of color change of paper from white to yellow was monitored as a function of the intensity of the reflected light (λ = 475 nm) of an LED. The limit of the detection was 0.01 ppm when the sampling time was 30 min, and the flow rate of sample gas was 250 mL/min. This device is useful for the detection of NO2 in a room using an exhaust room discharge type heater. Another type of HCHO gas detector for SBS was developed with a significantly short sampling time. 4-Amino hydrazine-5-mercapto-1,2,4-triazole (AHMT) reagent was chosen as an indicator for HCHO detection because AHMT is a well-described reagent with a high sensitivity and selectivity towards HCHO [57]. A disposable circular glass paper on a chip and hanging drop kit were developed for simple and routine use. The glass paper was impregnated with AHMT and detected HCHO concentrations in air at less than 0.08 ppm in 3 min. The limit of detection was 0.04 ppm. Before detection, 50 μL of AHMT solution and 50 μL of KOH solution (1:1) were AppliedChem 2021, 1 dropped on the glass paper. Then, the reagents on the glass paper were exposed26 to HCHO gas in a photometer (Figure 19a). An air pump was installed in this photometer. The degree of the color change was recorded by measuring the relative intensity of reflected lightand hanging at 550 nm. drop The kit were flow developed rate of sample for simple gas and was routine 200 mL/min. use. The glassHCHO paper reacted was with AHMT impregnated with AHMT and detected HCHO concentrations in air at less than 0.08 ppm andin 3 min.KOH, The and limit the of detection color of was the 0.04reagent ppm. solutions changed to purple in 3 min (Figure 19b). BeforeThis AHMT-based detection, 50 µL method of AHMT exhibits solution andhigh 50 sensitivityµL of KOH and solution a short (1:1) weremeasurement time sincedropped the on liquid the glass reagent paper. Then,and HCHO the reagents gas onreact the glasson the paper glass were paper. exposed In to Figure HCHO 20, the AHMT- basedgas in a method photometer was (Figure compared 19a). An with air pump a conventi was installedonally in this used photometer. hydroxylamine-based The degree one. The of the color change was recorded by measuring the relative intensity of reﬂected light at AHMT-based550 nm. The ﬂow method rate of sample is more gas was sensitive 200 mL/min. and HCHO has a reacted shorter with sampling AHMT and time than the hydroxylamine-basedKOH, and the color of the reagentmethod. solutions changed to purple in 3 min (Figure 19b).

Figure 19. 19.Photograph Photograph of sensor of sensor for HCHO for HCHO gas detection: gas detection: (a) photometer; (a) photometer; Adapted with permissionAdapted with permission fromfrom ref. ref. [57 [57].]. Copyright Copyright 2005 2005 Elsevier. Elsevier. (b) glass (b paper) glass with paper AHMT/KOH with AHMT/KOH solution after HCHOsolution gas after HCHO gas exposure. Adapted Adapted with with permission permission from ref. from [8] (p. ref. 40). [8] Copyright (p. 40). 2004Copyright Kawamura. 2004 Kawamura.

AppliedChem 2021, 1, FOR PEER REVIEW This AHMT-based method exhibits high sensitivity and a short measurement time14 “sinceThis the article liquid was reagent published and HCHO in Publication gas react on title, the glassVol number, paper. In FigureAuthor(s), 20, the Title AHMT- of article, Page Nos, Copyrightbased method Elsevier was compared(or appropriate with aSociety conventionally name) (Year).” used hydroxylamine-based one. The AHMT-based method is more sensitive and has a shorter sampling time than the hydroxylamine-basedThe reaction of HCHO method. and AHMT is presented in Figure 21.

FigureFigure 20.20.Comparison Comparison of of AHMT- AHMT- and and hydroxylamine-based hydroxylamine-based methods methods (n = 5) (n with = 5) calibration with calibration graphs forgraphs AHMT-based for AHMT-based method method after (a )after 5 min (a) and 5 min (b )and 3 min (b) and3 min (c )and hydroxylamine-based (c) hydroxylamine-based method method after after 30 min. Hydroxylamine is a dry-based method. The corresponding coefficient variation value 30 min. Hydroxylamine is a dry-based method. The corresponding coefﬁcient variation value is is shown over each position. Adapted with permission from ref. [57]. Copyright 2005 Elsevier. shown over each position. Adapted with permission from ref. [57]. Copyright 2005 Elsevier.

The reaction of HCHO and AHMT is presented in Figure 21.

Figure 21. The reaction of HCHO and AHMT.

Moreover, the AHMT-based method was not affected by the presence of various aldehydes other than HCHO, VOCs, acidic or alkaline gases. A toluene gas detector was also developed with the photometer in Figure 19a [58]. This detector is based on the principle that toluene reacts with iodine pentoxide (I2O5) to produce a brown-colored product, iodine (Figure 22). I2O5 solution (50 μL) was pipetted onto the glass paper filter, which was subsequently placed into the photometer. Immediately after, the reagent solution was exposed to toluene gas for a duration of 30 min at a flow rate of 480 mL/min. The product is detected by measuring the intensity of reflected light at 460 nm. The degree of color change is proportional to the concentration of toluene gas. This detector is able to detect down to 0.05 ppm toluene, a lower concentration than the WHO guideline value (0.07 ppm) for indoor air. The chemical reaction of this method is shown in Equation (10).

AppliedChem 2021, 1, FOR PEER REVIEW 14

The reaction of HCHO and AHMT is presented in Figure 21.

Figure 20. Comparison of AHMT- and hydroxylamine-based methods (n = 5) with calibration

AppliedChem 2021, 1 graphs for AHMT-based method after (a) 5 min and (b) 3 min and (c) hydroxylamine-based27 method Figure 20. Comparisonafter 30 min. of AHMT- Hydroxylamine and hydroxylamine-based is a dry-based method methods. The corresponding(n = 5) with calibrationcoefficient variation value graphs for AHMT-basedis shown methodover each after position. (a) 5 min Adapted and (b )with 3 min perm andission (c) hydroxylamine-based from ref. [57]. Copyright method 2005 Elsevier. after 30 min. Hydroxylamine is a dry-based method. The corresponding coefficient variation value is shown over each position. Adapted with permission from ref. [57]. Copyright 2005 Elsevier.

Figure 21. The reaction of HCHOFigure 21. and The AHMT. reaction of HCHO and AHMT.

Moreover,Figure the21. TheMoreover, AHMT-based reaction ofthe HCHO methodAHMT-based and was AHMT. not method affected was by not the affected presence by of the various presence of various aldehydes otheraldehydes than HCHO, other VOCs, than HCHO, acidic or VOCs, alkaline acidic gases. or alkaline gases. AMoreover, toluene gas the detectorA AHMT-based toluene was gas also detector method developed waswas with also not the developedaffected photometer by wi theth in Figurethepresence photometer 19a of [58 various]. This in Figure 19a [58]. detectoraldehydes is based other on than the HCHO, principle VOCs, that toluene acidic reactsor alkaline with iodinegases. pentoxide (I2O5) to produce This detector is based on the principle that toluene reacts with iodine pentoxide (I2O5) to a brown-coloredA toluene gas product, detector iodine was (Figurealso developed 22). I2O5 wisolutionth the photometer (50 µL) was in pipetted Figure onto19a [58]. the produce a brown-colored product, iodine (Figure 22). I2O5 solution (50 μL) was pipetted glass paper filter, which was subsequently placed into the photometer. Immediately2 after,5 This detector isonto based the on glass the principle paper filter,that toluene which reactswas withsubsequently iodine pentoxide placed (IintoO ) theto photometer. the reagent solution was exposed to toluene gas for a duration2 5 of 30 min at a flow rate produce a brown-coloredImmediately prod after,uct, the iodine reagent (Figure solution 22). IwasO solutionexposed (50to tolueneμL) was gas pipetted for a duration of 30 of 480 mL/min. The product is detected by measuring the intensity of reflected light at onto the glassmin paper at a flowfilter, rate which of 480 was mL/min. subsequently The produc placedt is detected into the by photometer.measuring the intensity of 460 nm. The degree of color change is proportional to the concentration of toluene gas. Immediately after,reflected the reagentlight at 460solution nm. Thewas degree exposed of tocolor toluene change gas is fo proportionalr a duration to of the 30 concentration This detector is able to detect down to 0.05 ppm toluene, a lower concentration than the min at a flow rateof toluene of 480 gas.mL/min. This detectorThe produc is ablet is detectedto detect bydown measuring to 0.05 ppmthe intensity toluene, ofa lower concen- WHO guideline value (0.07 ppm) for indoor air. The chemical reaction of this method is reflected light trationat 460 nm.than The the degreeWHO guidelineof color change value (0.07is proportional ppm) for indoor to the air.concentration The chemical reaction of shown in Equation (10). of toluene gas.this This method detector is shownis able toin deteEquationct down (10). to 0.05 ppm toluene, a lower concentration than the WHO guideline value (0.07 ppm) for indoor air. The chemical reaction of + → + this method is shownC in6H Equation5CH3 (10).I2O5 I2 Oxidation products (10)

Possible oxidation products: C6H5CHO, C6H5COOH. The measurement results using a photometer in this section are interpreted and corrected under the operating temperature range speciﬁcation automatically.

→ C65 H CH 3 + I 25 O I 2 + Oxidation products (10)

4. Passive Sampling Detection Paper Method Generally, a passive sampling detection paper method is not sensitive compared to an active sampling method that uses an air pump. Passive sampling is better for qualitative detection and not quantitative measurement. However, the detection paper method is extremely easy to use, does not need electronic devices and is maintenance- free. The detector paper in the chemical agent analyzer kit “M10” was used for the detection of most chemical warfare agents except for nerve agents in 1945 [1] (p. 11). A test strip capable of detecting low concentrations of chlorine gas in air was developed for use in detecting chlorine gas leaks in 1946. This strip was used for industrial hygiene [59]. Orthotoluidine was used as a detecting reagent for chlorine detection. It can AppliedChem 2021, 1 detect chlorine gas at less than 4 ppm and preferably less than 2 ppm, and it retains its28 sensitivity for relatively long periods of time, on the order of months, when properly stored. Early methods for the detection of phosgene utilized absorption into a solution that Early methods for the detection of phosgene utilized absorption into a solution that changes color (4-(4′-nitrobenzyl)-pyridine) and stabilizes the color (N-phenyl benzene). changes color (4-(40-nitrobenzyl)-pyridine) and stabilizes the color (N-phenyl benzene). The absorbance was then read on a spectrophotometerspectrophotometer [[60].60]. Badges that change color uponupon exposure exposure to phosgene are commercially available for industrial hygiene. The badge forfor phosgene phosgene determination determination was was developed developed with with a a paper paper strip strip impregnated with with 4-(4 4-(4′0-- nitrobenzyl)-pyridinenitrobenzyl)-pyridine and N-phenyl benzylamin benzylamine.e. Wernaer Wernaer et al. developed a badge that hashas three three indicators for for rapid recognition of th thee dose of reactive gas. The surface of each window is is covered covered with with a a film ﬁlm with with different different permeability. permeability. Each Each of of the the three three indicators indicators is designedis designed to discolor to discolor with with low low (<5ppm (<5ppm min min−1), medium−1), medium (50–80 (50–80 ppm ppmmin−1 min) and−1 high) and (100– high 150(100–150 ppm min ppm−1 min) concentrations−1) concentrations of gases. of gases.Since checking Since checking the discoloration the discoloration of the of three the threewin- dowswindows is easier is easier than thanreading reading the density the density from froma color a colorstandard standard chart, chart,the amount the amount of phos- of genephosgene can be can checked be checked immediately immediately [61]. [61]. AA test stripstrip forfor HCHOHCHO was was developed developed for for indoor indoor air air quality quality control control with with KD-XA01 KD-XA01 [56]. [56].The colorThe color of the oftest the striptest strip changed changed to yellow to yellow upon upon exposing exposing it to it HCHO to HCHO gas gas (Figure (Figure 23). 23).Most Most of the of passivethe passive sampling sampling methods methods such such as the as test the strip test needstrip severalneed several hours tohours detect to detectgas concentrations gas concentrations of less thanof less 1 ppm than due 1 ppm to their due low to their sensitivity. low sensitivity. However, However, low sensitivity low sensitivityhas advantages has advantages for long-term for long-term measurements, measurements, such as in such the as detection in the detection of threshold of thresh- limit oldvalues—time-weighted limit values—time-weighted average (TLVs-TWA)average (TLVs-TWA) of a typical of worka typical shift work of 8 hshift [62 ].of 8 h [62].

AppliedChem 2021, 1, FOR PEER REVIEW 16 FigureFigure 23. PhotographPhotograph of of test test strips strips with with KD-XA01 KD-XA01 after after exposure exposure to to 0.1 0.1 ppm ppm HCHO HCHO gas gas for for 8 8 h. h. Adapted with permission from ref. [8] (p. 126). Copyright 2004 Kawamura. Adapted with permission from ref. [8] (p. 126). Copyright 2004 Kawamura.

The test strip methods can be prone to analystanalyst error, asas itit isis notnot easyeasy toto readread thethe measured value from the paper or interpret interpret a a color color change. change. Kawamura Kawamura et et al. al. developed developed a amethod method to to obtain obtain quantified quantiﬁed results results using using pa passivessive detection detection paper and a photometerphotometer asas shown inin FigureFigure 1919aa [[8]8] (p.(p. 59–83).59–83). The methodmethod waswas basedbased onon thethe principleprinciple thatthat HCHOHCHO reactsreacts withwith KD-XA01KD-XA01 toto produceproduce aa yellowyellow colorcolor (Figure(Figure 2424a).a).

Figure 24.24.Photograph Photograph of of sensors sensors for KD-XA01-basedfor KD-XA01-base methodd method and HCHO and HCHO gas detector gas detector paper method: paper (method:a) detector (a) paper detector containing paper KD-XA01containing before KD-XA01 and after before reaction and withafter HCHOreaction gas; with (b) absorptionHCHO gas; spec- (b) traabsorption of 124 mM spectra of KD-XA01 of 124 mM before of andKD-XA01 after reaction before and with after 1000 reaction mg/L of with HCHO 1000 at roommg/L temperature.of HCHO at room temperature. Adapted with permission from ref. [8] (pp. 59–83). Copyright 2004 Kawamura. Adapted with permission from ref. [8] (pp. 59–83). Copyright 2004 Kawamura. The discoloration is recorded by measuring the intensity of reflected light at 460 nm using a photodiode after 8 h exposure of the paper to atmospheric air. The calibration graph of HCHO was linear between 0.04 and 0.57 ppm HCHO after 8 h sampling. Thus, this method can be used to detect HCHO at the TLV-TWA exposure value of 0.1 ppm set by ACGIH in the work environment for 8 h. The passive detection paper sampler was designed like a badge and clips to a worker’s collar for personal sampling (Figure 25). It could also be placed in an appropriate location in the area to be sampled. After 8 h, the sampler was simply detached from the worker’s collar, and the paper was transferred to a photometer for detection of HCHO gas.

Figure 25. A passive sampling badge shown attached to a worker’s collar for personal sampling and clipped in an appropriate location for use in a workplace. Adapted with permission from ref. [8] (pp. 59–83). Copyright 2004 Kawamura.

Maruo et al. also developed a HCHO detector with a colorimetric passive detection element in a portable photometer for indoor air monitoring [63]. The rutidine derivative that was formed as a yellow product of the reaction between β-diketone and HCHO was stable in the porous glass block (sensor element, 8 mm × 8 mm × 1 mm). The detection limit was 5 ppb per hour, and it was estimated it took about 1 h to detect a HCHO concentration of 94%. The sensor device was small and easy to use and it successfully carried out hourly HCHO monitoring. The sensitivity of many detection papers is low. However, fast-response paper-based visual color change film for efficient ammonia detection was developed [7]. Inexpensive and disposable rapid detection film was made using perovskite halide CH3NH3PbI3 (MAPI) to detect the presence of NH3 by color change, where the black-colored MAPI film (on the paper) changes to yellow color in the presence of a very low concentration of NH3 gas (Figure 26). The mechanism by which the color changes is based on the complete

AppliedChem 2021, 1, FOR PEER REVIEW 16

The test strip methods can be prone to analyst error, as it is not easy to read the measured value from the paper or interpret a color change. Kawamura et al. developed a method to obtain quantified results using passive detection paper and a photometer as shown in Figure 19a [8] (p. 59–83). The method was based on the principle that HCHO reacts with KD-XA01 to produce a yellow color (Figure 24a).

Figure 24. Photograph of sensors for KD-XA01-based method and HCHO gas detector paper method: (a) detector paper containing KD-XA01 before and after reaction with HCHO gas; (b) AppliedChem 2021, 1 absorption spectra of 124 mM of KD-XA01 before and after reaction with 1000 mg/L of HCHO29 at room temperature. Adapted with permission from ref. [8] (pp. 59–83). Copyright 2004 Kawamura.

The discoloration is recorded by measuring the intensity of reflected light at 460 nm usingThe a photodiode discoloration after is recorded 8 h exposure by measuring of the paper the to intensity atmospheric of reﬂected air. light at 460 nm usingThe a photodiode calibration after graph 8 h of exposure HCHO ofwas the linear paper between to atmospheric 0.04 and air. 0.57 ppm HCHO after 8 h sampling.The calibration Thus, graph this method of HCHO can wasbe used linear tobetween detect HCHO 0.04 and at the 0.57 TLV-TWA ppm HCHO exposure after 8value h sampling. of 0.1 ppm Thus, set this by methodACGIH in can the be work used environment to detect HCHO for 8 at h. the TLV-TWA exposure valueThe of 0.1 passive ppm set detection by ACGIH paper in the sampler work environment was designed for 8like h. a badge and clips to a worker’sThe passive collar for detection personal paper sampling sampler (Figure was designed 25). It could like also a badge be placed and clips in an to appropriate a worker’s collarlocation for personalin the area sampling to be sampled. (Figure 25After). It could8 h, the also sampler be placed was in simply an appropriate detached locationfrom the inworker’s the area collar, to be sampled. and the paper After 8was h, the transferr samplered wasto a simplyphotometer detached for detection from the worker’sof HCHO collar,gas. and the paper was transferred to a photometer for detection of HCHO gas.

FigureFigure 25. 25.A A passive passive sampling sampling badge badge shown shown attached attached to to a a worker’s worker’s collar collar for for personal personal sampling sampling and and clippedclipped in in an an appropriate appropriate location location for for use use in in a a workplace. workplace. Adapted Adapted with with permission permission from from ref. ref. [8 [8]] (pp. 59–83). Copyright 2004 Kawamura. (pp. 59–83). Copyright 2004 Kawamura.

MaruoMaruo et et al. al. also also developed developed a a HCHO HCHO detector detector with with a a colorimetric colorimetric passive passive detection detection elementelement in in a a portable portable photometer photometer for for indoor indoor air air monitoring monitoring [63 [63].]. The The rutidine rutidine derivative derivative thatthat was was formed formed as as a a yellow yellow product product of of the the reaction reaction between betweenβ β-diketone-diketone and and HCHO HCHO was was stablestable in in the the porous porous glass glass block block (sensor (sensor element, element, 8 mm 8 ×mm8 mm × 8× mm1 mm). × 1 mm). The detection The detection limit waslimit 5 ppbwas per 5 ppb hour, per and hour, it was and estimated it was itestima took aboutted it 1took h to detectabout a1 HCHOh to detect concentration a HCHO ofconcentration 94%. The sensor of 94%. device The was sensor small device and easy was to small use and and it successfullyeasy to use carriedand it successfully out hourly HCHOcarried monitoring. out hourly HCHO monitoring. TheThe sensitivity sensitivity of of many many detection detection papers papers is is low. low. However, However, fast-response fast-response paper-based paper-based AppliedChem 2021, 1, FOR PEER REVIEW 17 visualvisual color color change change film film for for efficient efficient ammonia ammonia detection detection was was developed developed [7 [7].]. Inexpensive Inexpensive andand disposabledisposable rapid rapid detection detection film film was was made made using using perovskite perovskite halide halide CH CH3NH3NH3PbI3PbI3 3 (MAPI)(MAPI) to to detectdetect thethe presence of of NH NH33 byby color color change, change, where where the the black-colored black-colored MAPI MAPI film degradation filmof(on MAPI (onthe thepaper) to paper)PbI changes2, a changessolid to with yellow to yellowdistinct color color yellow in the in thepresencecolor, presence upon of a exposure ofvery a very low low toconcentration NH concentration3. of NH of 3 The color changeNHgas3 togas(Figure yellow-colored (Figure 26). 26 The). The mechanismPbI mechanism2 is a structural by by which which phase th thee transition color color changes changes that occurs is is based due onon to the the complete complete the interactiondegradation of MAPI with of MAPI NH3. to PbI2, a solid with distinct yellow color, upon exposure to NH3. AccordingThe to color the experimental change to yellow-colored observations PbI, the2 isfilm a structural could detect phase gas transition concentrations that occurs due to of 10 ppm withthe a interaction visual color of change MAPI with within NH a3 response. time of around 12 s (Figure 27).

(a) (b)

Figure 26. ColorFigure change 26. responseColor change of the response detection of film: the detection (a) the original film: (a black) the originalcolor of blackthe unexposed color of the unexposed film; (b) the yellow color of the film in presence of NH3 gas [7]. film; (b) the yellow color of the film in presence of NH3 gas [7].

According to the experimental observations, the ﬁlm could detect gas concentrations of 10 ppm with a visual color change within a response time of around 12 s (Figure 27).

Figure 27. The dependence of color change response time of the MAPI film on different gas concentrations [7].

The response time of the film is very fast compared to reported NH3 sensors using other materials. The film works at room temperature and shows a rapid response that is faster than the response of electrical sensors available commercially for the same gas (Table 2).

Table 2. Different ammonia sensors based on other materials and their response time to detect the ammonia gas concentration [7].

NH3 Gas Concentration (in NH3Sensors Response Time (in s) ppm) rGo 1 108 10 PANI 2 800 50 SnO2 3000 10 CNT–TiO2 600 Not mentioned 1 Reduced graphene oxide. 2 Polyaniline.

They used paper with a high porosity as a substrate. As a result, the morphologies are different, and in their case, the nanorod-like structure is responsible for the fast response towards NH3 (Figure 28). The MAPI film was easy to fabricate via a wet chemistry route, and being a visual color change detection film, it does not need any other extra equipment for its operation. This film has a long shelf-life (180 days at room temperature) and almost constant performance for detection.

AppliedChem 2021, 1, FOR PEER REVIEW 17

degradation of MAPI to PbI2, a solid with distinct yellow color, upon exposure to NH3. The color change to yellow-colored PbI2 is a structural phase transition that occurs due to the interaction of MAPI with NH3. According to the experimental observations, the film could detect gas concentrations of 10 ppm with a visual color change within a response time of around 12 s (Figure 27).

(a) (b) AppliedChem 2021, 1 30 Figure 26. Color change response of the detection film: (a) the original black color of the unexposed film; (b) the yellow color of the film in presence of NH3 gas [7].

Figure 27. The dependence Figureof color 27. changeThe dependence response oftime color of changethe MAPI response film timeon different of the MAPI gas ﬁlm on different gas concen- concentrations [7]. trations [7].

The response time of the filmThe is response very fast time compared of the ﬁlm to reported is very fast NH compared3 sensors using to reported NH3 sensors using other materials. The film worksother materials.at room temp Theerature ﬁlm works and shows at room a rapid temperature response and that shows is a rapid response that faster than the response ofis fasterelectrical than se thensors response available of electricalcommercially sensors for available the same commercially gas for the same gas (Table 2). (Table2).

Table 2. Different ammonia sensorsTable 2. basedDifferent on other ammonia materials sensors and basedtheir response on other materialstime to detect and theirthe response time to detect the ammonia gas concentration [7].ammonia gas concentration [7].

NH3Sensors ResponseNH3 Gas Time Concentration (in s) NH (in3 Gas Concentration (in ppm) NH3Sensors Response Time (in s) rGo 1 108ppm) 10 rGo 1 PANI 2 108 800 10 50 PANI 2 SnO2 800 3000 50 10 CNT–TiO 600 Not mentioned SnO2 30002 10 1 Reduced graphene oxide. 2 Polyaniline. CNT–TiO2 600 Not mentioned 1 Reduced graphene oxide. 2 Polyaniline. They used paper with a high porosity as a substrate. As a result, the morphologies are They used paper withdifferent, a high porosity and in their as a case,substrate. the nanorod-like As a result, structurethe morphologies is responsible for the fast response are different, and in theirtowards case, the NH nanorod-3 (Figurelike 28). structure The MAPI is film responsible was easy for to fabricatethe fast via a wet chemistry route, and being a visual color change detection film, it does not need any other extra equipment AppliedChemresponse 2021, 1, FOR towards PEER REVIEW NH3 (Figure 28). The MAPI film was easy to fabricate via a wet 18 chemistry route, and beingfor a visual its operation. color chan Thisge filmdetection has a film, long it shelf-life does not (180 need days any at other room temperature) and almost extra equipment for its operation.constant performance This film has for a detection. long shelf-life (180 days at room temperature) and almost constant performance for detection.

FigureFigure 28. FESEM 28. FESEM Images: Images: (a) bare (a) paper; bare paper; (b) MAPI (b) MAPI thin film thin grown ﬁlm grown on paper on paper [7]. [7]. 5. Colorimetric Sensor Arrays 5. Colorimetric Sensor Arrays The development of optoelectronic nose sensors using colorimetric array chips has The development of optoelectronic nose sensors using colorimetric array chips has also been reported. Since the judgment is made using many reagents (for instance, Fe2+ 2+ also beenredox, reported. several types Since of the pH judgment indicators, is Schiff made test, using nitro-sensitive, many reagents solvatochromic, (for instance, metal–dye Fe redox,chromogens), several types the of gaspH indicators, selectivity canSchiff be test, improved nitro-sensitive, for toxic solvatochromic, and explosive gases.VOCsmetal– dye chromogens), the gas selectivity can be improved for toxic and explosive gases.VOCs such as ammonium nitrate, 2,4-dinitrotoluene and H2O2 can be qualitatively determined from the patterning of the sensor array (Figure 29). It has also been reported that discoloration can be determined by reading with a charge-coupled device (CCD) camera [64,65]. Sensors have also been reported that can identify the ppm levels of vapors of 20 toxic industrial gases, including PH3, HCN and fluorine, using a colorimetric 36- component sensor array [66,67]. In the future, the use of these sensors will improve the performance of even commercially available gas detectors.

Figure 29. The optoelectronic nose: (A) the linear array of colorimetric sensors and disposable cartridge, cartridge side view (7.9 × 2.8 × 1.0 cm3); (B) cartridge front view; (C) handheld reader/analyzer (12.8 × 9.5 × 4.0 cm3) based on a color contact line imager [64].

6. Colorimetric Gas Detector with Biomaterial The biosensor method is useful to improve the selectivity of the detector because biomaterials such as enzymes and antibodies have high substrate specificity. Mitsubayashi et al. developed bio-sniffers for gaseous chemicals (trimethylamine, methyl mercaptan, HCHO, ethanol, etc.) with metabolizing enzymes as chemical recognition proteins. The sniff devices monitor the concentration change of the gas-phase chemical with high selectivity because of enzyme specificity. An optical approach (Sniff- cam) allows one to visualize the spatiotemporal concentration change of chemical vapor such as body odor or wine ethanol. These gas sensors possessed high selectivity for

AppliedChem 2021, 1, FOR PEER REVIEW 18

Figure 28. FESEM Images: (a) bare paper; (b) MAPI thin film grown on paper [7].

5. Colorimetric Sensor Arrays The development of optoelectronic nose sensors using colorimetric array chips has also been reported. Since the judgment is made using many reagents (for instance, Fe2+ redox, several types of pH indicators, Schiff test, nitro-sensitive, solvatochromic, metal– AppliedChem 2021, 1 31 dye chromogens), the gas selectivity can be improved for toxic and explosive gases.VOCs such as ammonium nitrate, 2,4-dinitrotoluene and H2O2 can be qualitatively determined from the patterning of the sensor array (Figure 29). It has also been reported that suchdiscoloration as ammonium can be nitrate, determined 2,4-dinitrotoluene by reading with and a H charge-coupled2O2 can be qualitatively device (CCD) determined camera from[64,65]. the Sensors patterning have of also the been sensor reported array (Figure that can 29 identify). It has alsothe ppm been levels reported of vapors that discol- of 20 orationtoxic industrial can be determined gases, including by reading PH with3, HCN a charge-coupled and fluorine, device using (CCD) a colorimetric camera [64 ,6536-]. Sensorscomponent have sensor also beenarrayreported [66,67]. that can identify the ppm levels of vapors of 20 toxic industrialIn the gases, future, including the use PH of3 ,these HCN andsensors ﬂuorine, will usingimprove a colorimetric the performance 36-component of even sensorcommercially array [66 available,67]. gas detectors.

FigureFigure 29. TheThe optoelectronic optoelectronic nose: nose: (A) (theA) linear the linear array array of colorimetr of colorimetricic sensors sensors and disposable and dispos- car- abletridge, cartridge, cartridge cartridge side view side (7.9 view × 2.8 (7.9 × 1.0× cm2.83);× (B1.0) cartridge cm3); (B )front cartridge view; front(C) handheld view; (C )reader/ana- handheld reader/analyzerlyzer (12.8 × 9.5 × (12.8 4.0 cm× 39.5) based× 4.0 on cm a3 color) based contact on a color line imager contact [64]. line imager [64].

6. ColorimetricIn the future, Gas the Detector use of these with sensors Biomaterial will improve the performance of even commer- ciallyThe available biosensor gas detectors.method is useful to improve the selectivity of the detector because biomaterials such as enzymes and antibodies have high substrate specificity. 6. Colorimetric Gas Detector with Biomaterial Mitsubayashi et al. developed bio-sniffers for gaseous chemicals (trimethylamine, methylThe mercaptan, biosensor method HCHO, is ethanol, useful to etc.) improve with the metabolizing selectivity ofenzymes the detector as chemical because biomaterialsrecognition proteins. such as enzymes The sniff and devices antibodies monito haver the highconcentration substrate change speciﬁcity. of the gas-phase chemicalMitsubayashi with high et selectivity al. developed because bio-sniffers of enzyme for specificity. gaseous chemicalsAn optical (trimethylamine, approach (Sniff- methylcam) allows mercaptan, one to HCHO,visualize ethanol, the spatiotempor etc.) with metabolizingal concentration enzymes change as of chemical chemical recogni- vapor tion proteins. The sniff devices monitor the concentration change of the gas-phase chemical such as body odor or wine ethanol. These gas sensors possessed high selectivity for with high selectivity because of enzyme speciﬁcity. An optical approach (Sniff-cam) allows one to visualize the spatiotemporal concentration change of chemical vapor such as body odor or wine ethanol. These gas sensors possessed high selectivity for analyte vapor in the presence of other gas-phase chemicals and gave a negligible response to these other chemicals [9,68]. Enzymatic reaction of these detectors was as follows:

Monoamine oxidases + + (11) R − CH2−NH3 + H2O → O2R − CHO − NH4 + H2O2

A ruthenium organic complex was immobilized on the tip of an oxygen-sensitive optical fiber. The oxygen concentration can be quantified by the quenching phenomenon of the fluorescence reaction (excited light wavelength: 470 nm, fluorescence wavelength: 600 nm) due to the formation of an electrified transfer complex with oxygen molecules.

HCHO dehydrogenase (FALDH) + + (12) HCHO + NAD +H2O → HCOOH + NADH + H

In the presence of HCHO, FALDH uses oxidized nicotinamide adenine dinucleotide (NAD+) as an electron acceptor to produce formic acid and reduced NADH. Since NADH has fluorescence characteristics (ex.: 340 nm, fl.: 491 nm), HCHO could be detected by fluorescence measurement using UV-LED.

Alcohol oxidases (13) R − CH2OH + O2 → R − CHO + H2O2 AppliedChem 2021, 1 32

Using a mesh in which alcohol oxidase and horseradish peroxidase were immobilized and an electron-multiplying CCD camera was used, luminol luminescence associated with the enzyme reaction was imaged. The enzyme mesh was moistened with a luminol solution and reacted with ethanol gas in a dark room. Ethanol could be detected by photographing the luminescence by the enzyme with the electron-multiplying CCD. Yanagisawa et al. developed a HCHO gas detector with HCHO dehydrogenase using the principle of a passive emission colorimetric sensor (PECS) to measure the emission rates of HCHO from various surfaces in residential houses [10]. PECS is a very small device (diameter: 23 mm, thickness: 3.2 mm). PECS consists of a PET body and a test paper, which turns red by enzyme reaction (HCHO dehydrogenase, NAD+, NADPH oxidase diaphorase, 4-iodonitrotetrazolium violet) in the presence of HCHO. At the beginning of the measurement, one drop of pure water is put into the PECS. The PECS is placed on indoor materials, and 30 min later the coloration can be measured by visual observation or absorption photometry. Gas detector tubes using cholinesterase have been developed for the detection of nerve agents such as phosphate esters. Acetic acid is produced by the reaction of choline oxidase and acetylcholine. A pH indicator, cholinesterase, acetylcholine and a capsule containing water are loaded into the gas detector tube. The capsule is broken during measurement to allow the enzyme to react with acetylcholine. In the absence of a nerve agent that inhibits cholinesterase, the acetic acid produced reacts with the pH indicator and the detector tube is discolored. In the presence of nerve agents, the enzymatic activity of cholinesterase is inhibited, acetic acid is not produced and the detector tube is not discolored [69]. A kit for sensing phosphoric ester has also been developed (Figure 30). This sensing method uses dried cholinesterase or acetylcholinesterase, choline oxidase, peroxidase and water. The dry enzyme pad is brought into contact with the sample gas, and then water is added to the dry enzyme pad. After that, when the enzyme pad and the drying reagent pad are brought into contact with each other, the enzyme reacts with acetylcholine (substrate), and choline oxidase generates hydrogen peroxide. The color of the pad changes from white to blue by enzymatic reaction [70]. In the presence of nerve agents, the enzymatic activity AppliedChem 2021, 1, FOR PEER REVIEW 20 of acetylcholinesterase or cholinesterase is inhibited, H2O2 is not produced and the reagent pad is not discolored.

Figure 30. Picture of apparatusFigure for 30. sensingPicture phosphoric of apparatus ester. for Adapted sensing phosphoric with permission ester. Adaptedfrom ref. with permission from ref. [70]. [70]. Copyright 2002 Kawamura.Copyright 2002 Kawamura.

The enzymatic reaction ofThe discolor enzymatication reaction is as shown of discoloration in Figure 31. is as shown in Figure 31.

7. Colorimetric Gas Detector and ICT Technology Recently, smartphone colorimetric gas detectors have been developed. Rachel et al. proposed a thread sensor with a fabrication method for the stable entrapment of optically responsive dyes on a thread substrate. It is proposed to develop a detection system that can be integrated into clothing [71]. The dyes 5,10,15,20-tetraphenyl-21H,23H-porphine manganese(III) chloride (MnTPP), methyl red (MR) and bromothymol blue (BTB) were used for the detection of NH3 and HCl. Their optical approach utilizes a smartphone to extract and track changes in the RGB signal of the acquired images of the thread to detect the presence of an analyte. These threads could detect 50–1000 ppm of NH3 and HCl gases (Figure 32). The threads were shown to be stable over time, even with agitation in a centrifuge. This is attributed to the dual-step fabrication process that entraps the dye with polydimethylsiloxane (PDMS) coating in a stable manner. The facile fabrication of colorimetric gas-sensing washable threads is ideal for the next generation of smart textiles and intelligent clothing.

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Figure 30. Picture of apparatus for sensing phosphoric ester. Adapted with permission from ref. [70]. Copyright 2002 Kawamura. AppliedChem 2021, 1 33 The enzymatic reaction of discoloration is as shown in Figure 31.

Figure 31. Enzymatic reaction of the apparatus for se sensingnsing phosphoric ester. Adapted with per- permis- 0 0 missionsion from from ref. ref. [70]. [70]. Copyright Copyright 2002 2002 Kawamura. Kawamura. TMBZ: TMBZ: 3,3 ,5,53,3′,5,5-Tetramethylbenzidine.′-Tetramethylbenzidine. 7. Colorimetric Gas Detector and ICT Technology 7. Colorimetric Gas Detector and ICT Technology Recently, smartphone colorimetric gas detectors have been developed. Rachel et al. Recently, smartphone colorimetric gas detectors have been developed. Rachel et al. proposed a thread sensor with a fabrication method for the stable entrapment of optically proposedresponsive a dyesthread on sensor a thread with substrate. a fabrication It is me proposedthod for to the develop stable aentrapment detection systemof optically that responsivecan be integrated dyes on into a thread clothing substrate. [71]. The It dyesis proposed 5,10,15,20-tetraphenyl-21H,23H-porphine to develop a detection system that canmanganese(III) be integrated chloride into clothing (MnTPP), [71]. methyl The dyes red (MR)5,10,15,20-tetraphenyl-21H,23H-porphine and bromothymol blue (BTB) were manganese(III) chloride (MnTPP), methyl red (MR) and bromothymol blue (BTB) were used for the detection of NH3 and HCl. Their optical approach utilizes a smartphone to usedextract for and the track detection changes of NH in the3 and RGB HCl. signal Their of theoptical acquired approach images utilizes of the a thread smartphone to detect to extract and track changes in the RGB signal of the acquired images of the thread to detect the presence of an analyte. These threads could detect 50–1000 ppm of NH3 and HCl gases the(Figure presence 32). of an analyte. These threads could detect 50–1000 ppm of NH3 and HCl gases (FigureThe 32). threads were shown to be stable over time, even with agitation in a centrifuge. ThisThe is attributed threads were to the shown dual-step to be fabricationstable over time, process even that with entraps agitation the dyein a centrifuge. with poly- Thisdimethylsiloxane is attributed (PDMS) to the coating dual-step in a stablefabric manner.ation process The facile that fabrication entraps ofthe colorimetric dye with polydimethylsiloxanegas-sensing washable threads(PDMS) is coating ideal for in the a nextstable generation manner. of The smart facile textiles fabrication and intelli- of colorimetricgent clothing. gas-sensing washable threads is ideal for the next generation of smart textiles and intelligentColorimetric clothing. gas indicators for toxic gas scrubbers have been used in many factories [72,73]. The color of the indicator changes upon reaction with the toxic gas that has passed through the scrubber. Monitoring of the relative depletion of the scrubbing medium in a scrubber can be carried out with endpoint detection using a colorimetric indicator and reader (Figure 33).

Figure 34 shows the color detection of the PH3 indicator for a scrubber using a smartphone camera (RGB information). Copper nitrate was used as the indicator reagent to detect the PH3. The color of the indicator changes from blue to grey upon reaction with PH3. Discoloration of the indicator provided RGB color information using the smartphone camera. Optical detection offers advantages over other sensing techniques, as scanners and smartphones can image and analyze the color difference with a color sensor. In particular, the utilization of a smartphone is useful in terms of promoting the Internet of Things (IoT) in the ﬁeld of gas detection. This is because the data can easily be sent to the internet through the smartphone. Recently,many researchers have tried to develop gas detectors with IoT technology [12,13]. Kawamura et al. developed an O2 monitoring system using a galvanic oxygen sensor and gateway (Silent system SB-1) for IoT sensing. Oxygen sensors are connected with LAN cable to the gateway (Figure 35a). Monitoring results are sent to the data server through the Internet gateway. They utilized open-source software and platforms (Amazon Web AppliedChem 2021, 1 34

AppliedChem 2021AppliedChem, 1, FOR 2021 PEER, 1, FOR REVIEW PEER REVIEW 21 21

Service, AWS and ZABBIX) for the data server and monitoring software to reduce the cost of the monitoring system (Figure 35b). However, this is not a colorimetric gas detector.

Figure 32. Gas sensing demonstration—optical images. Optical images of BTB, MR, and MnTPP devices for different concentrations of (a) NH3 and (b) HCl; corresponding RGB color information extracted from the optical images for (c) NH3 and (d) HCl [71].

Colorimetric gas indicators for toxic gas scrubbers have been used in many factories [72,73]. The color of the indicator changes upon reaction with the toxic gas that has passed Figure 32. Gas sensing demonstration—optical images. Optical images of BTB, MR, and MnTPP devices for different Figure 32. Gas sensing demonstration—opticalthrough the scrubber. images. Monitoring Optical im ofages the relati of BTB,ve depletion MR, and of MnTPPthe scrubbing devices medium for different in a concentrations of (a) NH3 andscrubber (b) HCl; can corresponding be carried RGBout colorwith informationendpoint detection extracted from using the a optical colorimetric images forindicator (c) NH3 and concentrations of (a) NH3 and (b) HCl; corresponding RGB color information extracted from the optical images for (c) NH3 and (d) HCl [71]. and (d) HCl [71]. reader (Figure 33).

Colorimetric gas indicators for toxic gas scrubbers have been used in many factories [72,73]. The color of the indicator changes upon reaction with the toxic gas that has passed through the scrubber. Monitoring of the relative depletion of the scrubbing medium in a scrubber can be carried out with endpoint detection using a colorimetric indicator and reader (Figure 33).

FigureFigure 33. 33. DiagramDiagram of of colorimetric colorimetric indicator indicator for for scrubber. scrubber.

Figure 34 shows the color detection of the PH3 indicator for a scrubber using a smartphone camera (RGB information). Copper nitrate was used as the indicator reagent to detect the PH3. The color of the indicator changes from blue to grey upon reaction with PH3. Discoloration of the indicator provided RGB color information using the smartphone camera.

Figure 33. Diagram of colorimetric indicator for scrubber.

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Figure 34. Color detection of PH3 indicator for scrubber by smartphone camera. Figure 34. Color detection of PH3 indicator for scrubber by smartphone camera.

Optical detection offers advantages(a) over other sensing techniques, as scanners and smartphones can image and analyze the color difference with a color sensor. In particular, the utilization of a smartphone is useful in terms of promoting the Internet of Things (IoT) in the field of gas detection. This is because the data can easily be sent to the internet through the smartphone. Recently, many researchers have tried to develop gas detectors with IoT technology [12,13]. Kawamura et al. developed an O2 monitoring system using a galvanic oxygen sensor and gateway (Silent system SB-1) for IoT sensing. Oxygen sensors are connected with LAN cable to the gateway (Figure 35a). Monitoring results are sent to the data server through the Internet gateway. They(b) utilized open-source software and platforms (Amazon Web Service, AWS and ZABBIX) for the data server and monitoring software to reduce the cost of the monitoring system (Figure 35b). However, this is not a colorimetric gas detector. João et al. proposed an IoT system with a multigas smart sensor for various industrial fields (Figure 36) [74]. The devices used in these studies utilize semiconductor, electrochemical and optical sensors, not colorimetric sensors. The development of colorimetric gas detectors for IoT has been delayed. This is because many colorimetric gas detectors utilize chemical reactions (usually irreversible reactions), so the sensor part is often disposable and not suitable for continuous measurement [74,75].

Figure Figure35. Oxygen 35. Oxygen monitoring monitoring with smartphone, with smartphone, AWS and AWS Zabbix and Zabbixthrough through the internet: the internet: (a) oxygen (a) oxygen monitor;monitor; (b) monitoring (b) monitoring result of result oxygen of oxygen with AWS with an AWSd ZABBIX. and ZABBIX. Adapted Adapted with permission with permission from from ref. [13].ref. Copyright [13]. Copyright 2002 Kawamura. 2002 Kawamura.

João et al. proposed an IoT system with a multigas smart sensor for various industrial ﬁelds (Figure 36)[74]. The devices used in these studies utilize semiconductor, electrochemical and optical sensors, not colorimetric sensors. The development of colorimetric gas detectors for IoT has been delayed. This is because many colorimetric gas detectors utilize chemical reactions (usually irreversible reactions), so the sensor part is often disposable and not suitable for continuous measurement [74,75].

Figure 36. Illustration of Internet of Things (IoT) verticals and market opportunities for multigas smart sensors: smart homes, agriculture, smart cities, industry, healthcare and smart grid [12].

However, many types of gas sensors are needed to promote IoT. The gas detectors of the tape monitor method are suitable for continuous measurement. Chenwen et al. developed a gradient-based colorimetric array sensor (GCS) for O3 with indigo carmine, citric acid and silica gel pieces (13 mm × 30 mm). The sensing reagents were printed on the substrate as straight lines, each with a length of 8 mm and a width of 0.3 mm. The lateral transport of analytes across a colorimetric sensor surface creates a color gradient that shifts along the transport direction over time. GCS tracks the gradient shift and converts it into analyte concentration. Differing from conventional colorimetric sensors that expose the whole sensing surface to analytes, GCS has a greatly extended lifetime.

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(a)

(b)

Figure 35. Oxygen monitoring with smartphone, AWS and Zabbix through the internet: (a) oxygen AppliedChem 2021, 1 monitor; (b) monitoring result of oxygen with AWS and ZABBIX. Adapted with permission from 36 ref. [13]. Copyright 2002 Kawamura.

FigureFigure 36. 36.IllustrationIllustration of Internet of Internet of Things of Things (IoT) (IoT) verticals verticals and and market market opportunities opportunities for formultigas multigas smartsmart sensors: sensors: smart smart homes, homes, agriculture, agriculture, smart smart cities, cities, industry, industry, healthcare healthcare and and smart smart grid grid [12]. [12 ].

However,However, many many types types of gas of gassensors sensors are needed are needed to promote to promote IoT. The IoT. gas The detectors gas detectors of theof tape the tapemonitor monitor method method are suitable are suitable for forcontinuous continuous measurement. measurement. Chenwen Chenwen et al. et al. developeddeveloped a gradient-based a gradient-based colorimetric colorimetric array array sensor sensor (GCS) (GCS) for forO3 Owith3 with indigo indigo carmine, carmine, citriccitric acid acid and and silica silica gel gel pieces pieces (13 (13 mm mm ×× 3030 mm). mm). The The sensing sensing reagents reagents were printedprinted onon the thesubstrate substrate as as straight straight lines, lines, each each with with a lengtha length of of 8 mm 8 mm and and a width a width of 0.3 of mm.0.3 mm. The The lateral lateraltransport transport of analytes of analytes across across a colorimetric a colorimetric sensor sensor surface surface creates creates a color a gradient color gradient that shifts thatalong shifts the along transport the transport direction overdirection time. over GCS time. tracks GCS the gradienttracks the shift gradient and converts shift and it into convertsanalyte it concentration.into analyte concentration. Differing from Differing conventional from conventional colorimetric sensorscolorimetric that exposesensors the thatwhole expose sensing the whole surface sensing to analytes, surface GCS to an hasalytes, a greatly GCS extendedhas a greatly lifetime. extended The sensorlifetime. was able to continuously monitor the O3 concentration in the atmosphere for 24 h [75]. These devices will be used as gas detectors in IoT in the future.

8. Comparison of Various Gas Detector Technologies The sensitivity and selectivity of the colorimetric gas detectors are relatively high compared to other types of gas detectors such as semiconductor, combustible, electrochemical and photon ionization (Table3).

Table 3. Comparison of various gas detector technologies.

Catalytic Semiconductor Combustible Electrochemical Photoionization Colorimetric [76–80] [76,81,82] [76,83,84] [85–87] [88–90] × 4 o o Sensitivity O (sub-ppm) (sub-%) (ppm) (ppb) (ppb) Selectivity × × 4 4 o Continuous monitoring/ OOOO 4 1 data transfer Oxygen requirement Required Required Required Not required Not required during measurement × × 4 Number of measurable gases For reducing o o For combustible gas (less than 100) (more than 300) (more than 300) for commercial products gas/oxidizing gas 1 Continuous gas detector with a colorimetric method such as the tape monitor method. o : very good, O: good, 4: not so good, ×: not good.

Gas detectors with semiconductor sensors can measure reducing/oxidizing gases because the conductivity of semiconductor materials changes in accordance with changes in gas concentration. This is caused by the adsorption and desorption of oxygen and the reaction between surface oxygen and gases. Gas detectors with combustible gas sensors can only measure combustible gases because the reaction principle is combustion. Moreover, AppliedChem 2021, 1 37

since the detection is based on combustion, the gas selectivity is low. The sensitivity of the combustible sensor is low, and it is not suitable for low concentration measurement. Gas detectors with electrochemical sensors can have a certain degree of gas selectivity by adjusting the electrical potential. However, there are fewer than 100 types of gas that can be measured using electrochemical sensors. They cannot measure nearly as many types of gases as colorimetric gas detectors can measure. Moreover, oxygen is required for gas measurements on semiconductor, combustible and electrochemical gas sensors. Most colorimetric gas detectors do not require oxygen. A gas detector with a photon ionization detector can have a certain degree of gas selectivity by changing the UV lamp type (voltage). There are three types of commonly used lamp voltages (9.8, 10.6 and 11.7 eV), but they are not sufﬁcient to improve selectivity by much. There are more than 300 types of gases that can be measured with photon ionization. The colorimetric gas detector is highly sensitive, and some products can measure the concentration down to part-per-billion levels. The gas selectivity depends on the chemical reagents used. It is possible to improve gas selectivity by using chemical reagents with high selectivity. In principle, more than 30 types of reagent reaction principles are used. This is signiﬁcantly more than other types of gas detectors and can easily be applied if new reagents become available in the future. This feature is an advantage when developing a new gas detector. In addition, colorimetric gas detectors currently commercially available can measure more than 300 types of gases [80–90]. The disadvantage of colorimetric gas detectors is that the detection component is often a disposable product, so many products are not suitable for continuous measurement or data transfer. However, tape monitor products [43–52] and continuously usable colorimetric gas sensors [75] are capable of continuous measurement and data transfer and do not have such drawbacks. Moreover, single-use disposable products have an impact on the environment. In the future, we should develop a colorimetric gas detector that can be recycled and has a low environmental impact.

9. Conclusions A review of various colorimetric gas detectors was presented. The history of devices such as detector tubes, tape monitor methods, photometer-type gas detectors and detector paper was discussed, as was the current situation and the direction to be taken in the future. In the future, to reduce the environmental load, products that do not use harmful substances such as heavy metals (Cr, mercury) will be desired. In order to improve the sensitivity, it is effective to use a material with a large surface area, and further research is needed. Gas detectors should be promoted in IoT. Some colorimetric gas detectors, such as tape monitoring, can perform continuous measurements. It is necessary to promote the IoT of these products and further promote their utilization in the ubiquitous sensor network. The colorimetric gas detector is highly sensitive and selective and has a large number of measurable gas species. No other gas detector with such characteristics can be found. More research utilizing these advantages of the colorimetric gas detector should be conducted in order to improve the ﬁeld of gas detection.

Author Contributions: Conceptualization, K.K. and K.M.; investigation, resources, writing—original draft preparation, K.K. and L.K.; writing—review and editing, L.K.; visualization, K.K.; supervision, K.M. and L.K. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest. AppliedChem 2021, 1 38

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