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Electronic Materials Letters https://doi.org/10.1007/s13391-020-00254-9

REVIEW PAPER

Colorimetric Sensors for Toxic and Hazardous Gas Detection: A Review

Sung Hwan Cho1 · Jun Min Suh1 · Tae Hoon Eom1 · Taehoon Kim1 · Ho Won Jang1

Received: 5 October 2020 / Accepted: 19 October 2020 © The Korean Institute of Metals and Materials 2020

Abstract Since the industries are vastly rising, the threat of toxic and hazardous substance to human beings and demands of the accurate sensor is increasing. Colorimetric sensors that detect substances by measuring the absorbance or fluorescence spectra shift are one of the most emerging strategies these days. However, conventional colorimetric gas sensors are limited to specific application due to the limitation of detecting only liquid phase substances. For practical applications of the colorimetric sen-sors, it is necessary to detect low concentrations of toxic and hazardous substances in gaseous media. Besides, operation with low power consumption and excellent selectivity and sensitivity should be considered for Internet of Things (IoT) application. In this paper, various efforts on the investigation of several materials, including dyes, polymers, metal–organic complexes, and metal oxides as active sensor elements of the colorimetric gas sensors for IoT application are summarized. This paper also reviews various kinds of colorimetric gas sensor that exhibit great sensing properties to toxic and hazardous gases and introduce a brief overview of the challenges of colorimetric gas sensors as a candidate for future IoT gas sensor technology.

Graphic abstract

Keywords Gas sensor · Colorimetric · Internet of Things · Fluorescent · Low power consumption · Chemical warfare agents

1 Introduction

With the advent of the Internet of Things (IoT) that also called the Internet of Everything (IoE) is the network of computing device and machines and the ability to transfer data over the network, gas sensor technology has been inten-sively developed and researched during last decades [1, 2].

* Ho Won Jang hwjang@snu.ac.kr

1 Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 08826, Republic of Korea

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Before IoT era, "Classic" gas sensors based on gas chroma-tography (GC), mass spectrometry (MS) and ion mobility spectrometry (IMS) have been commercialized and preferred due to their stable and reliable operation despite their mas-sive power consumption, cost, and size [3, 4]. As the impor-tance of portable devices increases, IoT networks demand gas sensors with low power consumption, low power trans-missions, and accurate sensing performance. As a result, these days, "Classic" gas sensors are no longer proper plat-forms to apply to IoT [5–10].

As an alternative, various gas sensor principles have been suggested, including cantilever-based gas sensors, capacitive gas sensors, thermometric gas sensors, optical gas sensors, field-effect transistor gas sensors, chemoresistive gas sen-sors, solid-state electrochemical gas sensors, or colorimetric gas sensors to satisfy these requirements of gas sensors for IoT [11–17]. Among various types of gas sensors, colorimet-ric gas sensors have been considered as a suitable candidate for the IoT due to low power consumption and low limit of detection (LOD) [18]. In addition, their visualized sensor signals through color changes can provide the most intuitive information to IoT experiences [19]. These advantages make the colorimetric gas sensors as a front running candidate for IoT in various fields including hazardous and toxic material sensors that can pose a threat to human life.

The colorimetric sensors usually utilize various organic compounds for capturing target molecules, including dyes, fetal organic complexes, and polymers [17, 20, 21]. Although various functional materials have been investigated for col-orimetric sensors, most of them were limited to the detec-tion of ionic or molecular substances in solution and had difficulties in the detection of gaseous hazardous and toxic substances effectively. While hazardous substances in liquid media can easily be noticed by basic human senses, those in gaseous form in a low concentration are reluctant to be noticed easily by human body. Since a prolonged exposure to hazardous gas substances can result in severe damages in

human body, the gas sensors with intuitive indication of tar-get gas molecules with high sensing performance are highly desired in IoT. According to the literature survey, number of publication and citations regarding colorimetric sensors have been significantly increasing and indicate the significance of their development in IoT era (Fig. 1).

In this review, we will summarize various functional materials and strategies for the application to colorimetric sensors for the detection of gaseous toxic and hazardous materials. To the best of the author’s knowledge, there has been no systematic review of the colorimetric gas sensors that can provide the most intuitive gas detection experiences which can be recognized by human naked eyes or fluorescent changes. We strongly believe this review can provide a new perspective toward the development of colorimetric gas sen-sors to IoT application.

2 Mechanism of Colorimetric Gas Sensing

2.1 Ring Opening and Closing Reactions

The first primary possible mechanism of colorimetric gas sensing is ring opening and closing reaction of the sens-ing materials as shown in Fig. 2a. The first suggestion of colorimetric gas sensors can probably be dated back to 1992 when Arnold G. Fogg et al. reported p-Aminophenol sensors as indophenol blue dye [22]. Although this study introduced not vapor but solution phase detections, Arnold G. Fogg et al. revealed the role of dyes and organic com-pounds as colorimetric gas sensing materials. These dyes and organic compounds consisted of aromatic structures that have many conjugated pi-system. In a conjugated pi sys-tem, electrons can capture specific photons when resonating along with a specific distance of p-orbitals. In other words, the wavelength of photon captured is affected by the length of the number of conjugated the pi-system. For example,

Fig. 1 Results from a Web of Science literature survey for articles about a number of publications and b citations from 2012 to August, 2020. Web of Science Core Collection Search: keywords of colorimetric gas sensor

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the number of conjugated pi-systems and lengthening or extending a conjugated system can tend to shift the absorp-tion spectra of materials. Dyes and other organic compounds that have been used as colorimetric gas sensors usually show ring opening or closing reactions that could lead to change in the number of conjugated pi bondings. These changes can make us use dyes as colorimetric gas sensor materials.

2.2 Change of Functional Groups

Functional groups are specific moieties within organic compounds. Functional groups can be divided into several groups because of their similar chemical reactions [23]. They give characteristic features to organic compounds and increase reactivity. For example, organic compounds that have hydroxyl groups are easy to be oxidized. So it is essen-tial to fabricate functionalized compounds to detect the gas. After these functional groups react with the target gas, the functional groups change to the other functional groups as shown in Fig. 2b. As a result, the color of organic com-pounds has been changed.

2.3 Ligand Exchange

Transition metal complexes are usually colored due to elec-tronic transitions in the energy level of d orbitals that are easily affected by ligands around the transition metal ions. When coordination complexes are formed by transition met-als, the d-orbitals of the transition metal (inside) interact with the electron cloud of the ligands (outside) in such a

manner that the d-orbitals have non-degeneracy of electrons, as shown in Fig. 2c. If ligands are modified or substituted by surroundings, the state of electrons could be changed and absorb the different wavelengths of lights [24]. This phe-nomenon is called Spin Crossover (SCO). Low transition complexes that have d4 and d7 orbital states usually show spin crossover [25]. Spin crossover properties can be applied to colorimetric gas sensing. The reaction between gas and ligands connected to center transition metal ion induces ligand substitution or change. As a result, spin crossover occurs, and we can detect the gas.

2.4 Phase Transition

Many inorganic compounds can react in the ambient air and lead to color change because of products as shown in Fig. 2d. For example, iron is oxidized in the air and produces iron oxide. This phase transition also triggers color change, gray to brown. Materials or structure that has selectivity to gas need to stable in ambient air to adapt optical proper-ties. Using catalysis that reacts with specific gas and induces phase transition is an effective way to solve the problems.

3 Colorimetric Gas Sensors by Target Gases

There are various kinds of gases we can detect by colori-metric gas sensors, which can be divided into four major groups, Chemical Warfare Agents (CWAs), Volatile Organic

Fig. 2 Schematic illustration of 4 major colorimetric gas sensor mechanisms: a ring opening and closing reaction, b functional group change, c ligand exchange and d phase transition

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Compounds (VOCs), carbon monoxide, hydrogen sulfide, and other gases.

3.1 Chemical Warfare Agents (CWAs)

CWAs are toxic chemicals or its precursor that can lead to lethal influence on people, animals, or other living things through its chemical action and effect with very low con-centration [17]. CWAs consist of 5 main categories; nerve, asphyxiants, blistering, toxic industrial, and blood agents [26]. The first historically recorded use of war during WWI. The Franch military used tear gas to control the crowd. Since then because of their toxic effects, other agents were consid-ered for military use [27].

3.1.1 Phosgene

Phosgene is a kind of asphyxiants that can cause severe dam-age to the lung and respiratory system of humans, which does not cause immediate damage even at the high concen-trations. Still, after a few hours later, phosgene gas leads to severe symptoms, such as pulmonary edema and respiratory failure [28]. But it is widely used in the chemical industry for the production of various polymers or compounds. As the result, the employees are easily exposed to phosgene gases, thus there a strong need to develop accurate and pre-cise phosgene sensors to protect human from the threat of the danger [29, 30].

Dye-based colorimetric gas sensors have been researched a lot because of their color [21, 31, 32]. Hu et al. reported 4-chloro-7-nitrobenzo[c]-[ 1,2,5] oxadiazole (NBD) based o-phenylenediamine (OPD) group colorimetric gas sensing materials that can detect phosgene rapidly and precisely [21]. However, NBD-OPD phosgene gas sensors produce HCl as by-products that can lead to secondary pollution. Hu et al. reported new ring structure materials to overcome this shortage. Hu et al. synthesized sensor-1, a benzimida-zole-fused rhodamine dye, which could detect phosgene gas effectively, with solution of RB-OPD in dry THF and aluminum hydride [31]. Hu et  al. propose the possible mechanism of the reaction between sensor-1 and phosgene in Fig. 3a. According to 1H NMR spectrum of the product, products produced after phosgene gas sensing showed that it possesses the structure represented by the ring opened N-shloroformyl -benzimadazle 2. Senor-1, based polyeth-ylene oxide (PEO Mw = 600,000) nanofiber format was fabricated by electrospinning method that could produce a uniform size of nanofibers (Fig. 3b). The nanofiber platform based sensor-1 showed color and fluorescence change upon exposure to phosgene gas. As shown in Fig. 3c, sensor-1 based test paper is white colored and green colored in color and fluorescent each. After phosgene gas reactions, there are color change to dark pink and emit pink fluorescence.

Wang et al. also reported phosgene colorimetric gas sen-sors. The named gas sensing materials as Phos-3 synthe-sized with 2′-amionmethy-3,3′-dimethylpyrrole as an active site [33]. And the possible sensing mechanism of Phos-3

Fig. 3 a Reaction mechanism of the gas sensor-1 with triphosgene. b SEM images of sensor-1 before and after exposure to phosgene. c Visual and fluorescent response of sensor-1 to phosgene (0.8  mg/L

phosgene gas). Reproduced with permission from [27]. Copyright © 2018, American Chemical Society

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is in Fig. 4a. They fabricated Phos-3 embedded nanofiber by electrospinning method using Phos-3 and poly ethylene oxide (Mw = 1,000,000) in CH3CN to make phosgene col-orimetric gas sensor test strips. And the method was carried out on the filter paper. The test trips emitted yellow fluo-rescence under daylight and 365 nm light. After exposure to various concentrations of phosgene gases (0 to 50 ppm), the color of test strip change to colorless and cyan gradually each in Fig. 4b. The fluorescence intensity of the test strips at 495 nm also changed gradually by the concentration of phosgene in Fig. 4c, and it showed good linearity to concen-tration in Fig. 4d.

3.1.2 Sarin Gas

Nerve agents have similar structures to organophosphates that help to deactivate the enzyme. These similar struc-tures lead to a reaction between the nicotinic receptor and nerve agents. Thus nerve agents block the biologi-cal response of enzymes that catalyzed the breakdown of acetylcholine [34, 35]. Nerve agents are regulated and restricted by laws all over the world [36]. However, sarin gas that is one of the nerve agents, had been used several

times. In 1988, sarin gas was used to control Kurds. And terrorist attacked civilian people by this deadly chemical weapons on Tokyo’s subway in 1995 [13]. Several materi-als and devices have been researched and developed, such as GC/MS, ion mobility spectrometers, and flame photo-metric detectors to protect people from the threat of Sarin gas [38–40]. But these devices and systems have critical limitations such as, slow to counter the attack, expensive to use, and hard to use. Colorimetric nerve gas sensors could be a strong way to solve the limitations because of low-cost and easy to read.

Ordronneau et  al. reported organophosphorus verve agents (OPs) colorimetric gas sensors based on ligand reac-tions of metal complexes synthesized by mixing bipyridine ligand and iron chloride tetrahydrate [41, 42]. They propose a mechanism of the reaction between ligands and OPs in Fig. 5a. During the reaction, the cis geometry structure is changed to trans structure and the state of the nitrogen atom changed to ammonium form. The iron complex solution was dropped on the paper and dried in ambient air for 30 min to make colorimetric Sarin gas sensor platforms. After the papers were exposed to vapor phase diphenyl-chlorophos-phate (DPCP), it turned in to colorless in Fig. 5b.

Fig. 4 a Schematic mechanism of gas sensing. b Images test paper with Phos-3 based nanofiber gas sensors upon exposure to phos-gene from 0 to 50 ppm. c Fluorescence spectra of the Phos-3 based nanofiber gas sensor. d The plot of fluorescence intensity at 495 nm

as a function of phosgene concentrations λex = 365  nm. Reproduced with permission from [29]. Copyright © 2019 Royal Society of Chemistry

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Heo et  al. also reported high selective OPs sensors based on organic materials. Rhodamine 6G based sensors (RDS) showed excellent selectivity to DCP in DMF through fluorescence spectroscopy in Fig. 6b [32]. They synthe-sized RDS by 3′,6′-Bis(ethylamino)-2-(2-hydroxyethyl)-2′,7′-dimethylspiro[isoindoline-1,9′-xanthen]-3-one (1 g, 2.18 mmol) that was dissolved in anhydrous tetrahydrofuran. After exposure to phosgene gas, RDS react with DCP and produce phosphorylated intermediate 1, then P=O bond-ing formed by the cyclization reaction of RDS and DCP in Fig. 6a. RDS and polyurethane monomer in acetone /dimeth-ylacetamide and the mixture was loaded into a syringe and electrospun to the paper to fabricate RDS that could detect the gas phase DCP. The paper was white and blue colored before DCP exposure, after exposure, it turned to pink and yellow under daylight and UV light each in Fig. 6c.

3.2 Volatile Organic Compounds (VOCs)

Organic compounds, in general, can be defined as com-pounds that contain at least one carbon-hydrogen bond. Vol-atile organic compounds (VOCs) refer to organic compounds that exist as a gas at room temperature or as a liquid or solid with high vapor pressure. Since the industrial revolution, many industries and many activities have emitted pollut-ants, including toxic gases and VOCs [43]. But these days, interest in indoor air quality is increasing due to its high indoor concentrations because of VOCs that occur in every-day life like cooking or painting and serious effect on human health such as cancer, neurobehavioral effects, immune sys-tem disorder, and adverse effects on the kidney [44–46]. As a result, control and detect the indoor concentration of

VOCs is indispensable. Many chemoresistive VOCs sen-sors have been researched, but they have excellent response only at high temperature that need high energy consump-tion [47–49]. Colorimetric gas sensors are good alternatives to solving the above problems. It has excellent sensitivity at room temperature, fast response time, and low power consumption.

3.2.1 Formaldehyde

Formaldehyde is one of the most popular but fatal VOCs. Formaldehyde is produced 46 billion pounds a year and is used in paint used in the house. But it leads to bronchitis, pneumonia, and other severe diseases [50]. The emission of formaldehyde is indispensable in indoor because of paints and other construction materials [51]. As a result, all day formaldehyde sensors are necessary to control indoor concentrations.

Wang et  al. demonstrated 4-amino-3-penten-2-one (fluoral-p) functionalized electrospun polyacrylonitrile (PAN/fluoral-p) based selective sensor strip. It makes people detect formaldehyde with naked eyes by a color change from white to yellow. Wang and their group made PAN nanofibers via the electrospinning method and dipped the fabricated PAN nanofibers into 1 wt% fluoral-p solution that prepared by dissolving a mixed solution consisting of methanol, glycerin, and phosphate buffer solution for 1 min. Figure 7a shows the reaction mechanism of fluoral-p and formalde-hyde. Blue marked carbon in fluoral-p attacked the carbon of formaldehyde and generated conjugated intermediate 1. This intermediate 1 and another fluoral-p do Michael addition

Fig. 5 a Ligand reaction mechanism of metal coordina-tion complex. b Colorimteric response of filter paper with before (left) and after (right) exposure to gaseous of DPCP. Reproduced with permission from [37]. Copyright © 2013 Royal Society of Chemistry

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Fig. 6 a Reaction mechanism of RDS-based colorimetric gas sen-sor to Diethyl chlorophosphate. b Fluorescence spectra (λex. 510 nm) changes of RDS with DCP. c Colorimetric response to DCP under

day light (above) and UV light (below) Reproduced with permission from [28]. Copyright © 2019 Elsevier Ltd. All rights reserved

Fig. 7 a Sensing mechanism of reaction between Fluoral-p and for-maldehyde. b Reflectance spectra and c color change upon exposure to different concentration of formaldehyde (0 to 6  ppm). d Reflec-

tance spectra of various VOCs, e color change upon exposure to dif-ferent VOCs. Reproduced with permission from [45]. Copyright © 2013 Royal Society of Chemistry

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reaction to form product 3 and then formed a cycle structure through deamination and cyclization [52–54].

Figure 7b shows reflectance spectra of colorimetric strip upon exposure to different formaldehyde concentrations (0 to 6 ppm). Reflectance decrease as the concentration of for-maldehyde increases at 417 nm. Figure 7c shows visual color change of gas sensor platforms to formaldehyde gas that can be readily identified by naked eye, even under 80 ppb that is the WHO indoor air mandatory standard, easy to detect the color change. Figure 7d, e shows the selectivity of gas sen-sors to other VOCs, reflectance and color changes appears in only to formaldehyde.

Song et al. reported another type of formaldehyde col-orimetric gas sensors. They produced gas sensors by Cel-lulose nanocrystals (CNCs) with the help of ionic-liquid (1-butyl-3-methylimidazlium) without the use of any other chemical pigments. CNCs have been considered attractive materials because of their physical and chemical properties, such as forming colored structures because of their liquid crystalline arrangements [55–57]. Song et al. demonstrated the ionic-liquid assisted method of CNCs that is faster and more controllable than the previous method [58–60]. They dipped silicon substrate vertically into the BmimCl/CNC solution prepared by mixing 1 mL of the CNC solution and 0.8 mg of BmimCl. Then pulled the substrate from the solu-tion at a rate of 3–9 μm/min to deposit the CNCs in the glovebox to reduce noises and impurities. At each pulling speed, different colored CNCs films were fabricated because of different thicknesses. Figure 8a shows different colored CNCs films according to pulling speeds. Hydroxyl groups of films are changed into amine groups by surface modi-fication. Their amine groups react with formaldehyde gas selectively, and the reaction formula is R–COH (target gas

molecule) + R′–NH2 (sensor surface) → R–C=N–R′ + H2O. Figure 8b shows the color change of 3 sensors to different aldehyde gas, the most left column of each figure shows the control corresponding to 0 ppm aldehyde gas. Figure 8c shows selectivity of 3 differently colored CNCs film gas sen-sors to other VOCs.

3.2.2 Alcohol

Guo et al. reported iron(II) neutral complex based alco-hols. They prepared crystalline [Fe(trz-tet)2(H2O)4]_2H2O according to a literature procedure [61]. The crystalline sam-ple was grinded to fine powder and insert the powder into a Qwik Handi-press and press it to make small size particles. Figure 9a shows the color change of the sensors. After expo-sure to methanol and ethanol at room temperature, the sen-sors begin to turn colorless to pink, but this pink color can be changed into colorless by water vapor in Fig. 9a. The sens-ing mechanism is based on a subsequent spin state change of centered iron ion. The spin state changes low spin state to high spin state by exposure to alcohol. Figure 9b shows electronic diagrams of the low spin state and high spin state for Fe(II) ion [62].

3.2.3 Other VOCs

There are so many kinds of VOCs in the air and almost VOCs lead to public safety concerns. As a result, many researchers have been focusing on sensor array that could monitor various VOCs by one sensor platform, and many materials have been reported include organic compounds and metal complexes [63–66]. Among them, polydiacety-lenes (PDA) are promising because of their unique optical

Fig. 8 a Schematic illustration of fabricating CNC-based color film on solid substrate and gas reaction mechanism between color film and aldehyde gas. b CNC-based gas sensor color change to different con-centration of formaldehyde. c Colorimetric response of CNC-based

gas sensor band upon to exposure to 5 different VOCs. Reproduced with permission from [49]. Copyright © 2018, American Chemical Society

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properties. PDA has conjugated structures that can lead to color changes from blue to red under various stimuli, and modulation of headgroups can lead to different chromatic properties [67–69].

Dolai et al. present a new way to produce PDA based gas sensors with aerogel [70]. This method was an effective and easy way to improve sensitivity and decrease response

time. Aerogel was synthesized by mixing tetraethylorthosili-cate (TEOS), ethanol, distilled water, and hydrochloric acid. PDA-aerogel was fabricated by drop casting of PDA solu-tions to aerogel. Figure 10a shows schematic steps of fabri-cating PDA-aerogel and color change of PDA-aerogel sen-sors. Figure 10b shows the change of fluorescence emission change to 5 different VOCs i.e., benzene, acetone, toluene,

Fig. 9 a Process of reaction and recovery of iron(II)-based colorimetric sensor upon exposure to different VOCs. b Change of Fe(II) spin state after exposure to VOCs to 5 different VOCs. Reproduced with permission from [57]. Copyright © 2018 Royal Society of Chemistry

Fig. 10 a Schematic steps of fabricating PDA-aerogel and response to different VOCs (concentrations 1000 ppm). b Fluorescence spectra change of PDA-aerogel upon exposure to different VOCs and change of confocal fluorescence microscopy images (excitation 488  nm) of PDA–aerogel upon exposure to different VOCs. Scale bar corre-

sponds to 10  μm. c Three different types of monomer of PDA and color change of PDA-aerogel gas sensor to different 1000 ppm VOCs. Reproduced with permission from [65]. Copyright © 2017, American Chemical Society

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2-propanol, and pentane. Figure 10c shows three monomers that have different headgroups and its colorimetric response to 6 different VOCs i.e., benzene, acetone, toluene, 2-pro-panol, pentane, and diethyl ether. Monomer 3 that has phenyl moieties shows the best sensitivity to different VOCs.

Eaidkong et al. reported VOCs sensor array fabricated by 8 different kinds of PDA in Fig. 11a. The sensor array was fabricated by dropping DA monomer solution on filter paper and drying at room temperature. Then the DA monomer was irradiated with UV light for 1 min to polymerize the monomers. Figure 11b shows tested results of the sensors for 18 different VOCs, i.e., pentane, hexane, cyclohexane, toluene, o-xylene, benzene, diethyl ether, dichloromethane, 2-propanol, THF, chloroform, ethanol, ethyl acetate, ace-tone, methanol, acetonitrile, DMF, and DMSO. The con-ventional image-processing program was used to convert the scanned images into the RGB values to evaluate response to several VOCs. Figure 11c shows PCA score plot of RGB color changes of 8 different gas sensors. 18 distinct clusters correspond to 18 different VOCs that are observed. These results show the ability to distinguish of paper-based PDA gas sensor array.

3.3 Hydrogen Sulfide

Hydrogen sulfide is a colorless and odorous compound [71]. At low concentrations (15 to 50 ppm), prolonged exposure of hydrogen sulfide can irritate the mucous membranes and cause headaches, dizziness, and nausea. And high concen-trations (200 ppm ~), sulfide become toxic because it can cause suffocation, coma or loss of consciousness [72–74]. But hydrogen sulfide readily produced in several categories of industry, even a third of petroleum workers experienc-ing some symptoms from sulfide exposure and 8% having become unconscious [75, 76]. To prevent workers from the danger of this toxic gas, a gas detection system such as gas sensors are essential all over the industry.

Lead(II) acetate is one of the most widely used materi-als to detect hydrogen sulfide because lead(II) acetate based dye changes its color white to dark brown after exposure to hydrogen sulfide. Lead(II) acetate embedded paper-based colorimetric hydrogen sulfide sensors are already available in the market. Still, usage of commercial sensors are limited due to the low detection limit between 5 and 10 ppm [77]. Cha et al. demonstrated sub-parts-per-million

Fig. 11 a Different monomer structure of 8 different polydiacetylene. b Colorimetric response of paper-based PDA sensor array to various VOCs. c PCA score plot of RGB changes. Reproduced with permission from [66]. Copyright © 2012 Royal Society of Chemistry

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hydrogen sulfide colorimetric sensors based on lead(II) ace-tate embedded nanofibers. As shown in Fig. 12, Lead(II) acetate embedded nanofibers have large reactions sites and turn dark blown after exposure of hydrogen sulfide. As the concentration of gas decreased, the degree of color change also decreased and has selectivity to other gases.

Lead(II) acetate hydrogen sulfide sensors are conven-tional sensors, but they have several drawbacks, such as the use of toxic metal and low sensitivity [78, 79]. Kuban et al. reported H2S gas in an alkaline solution has been able to detect at a sub ppm concentrations utilizing UV–Vis spec-troscopy [80]. While this method offers an attractive way to fabricate H2S sensor platform. As a result, Rosalina et al. demonstrated Bismuth based H2S colorimetric gas sen-sors [77]. It has high sensitivity and low limit of detection, however, react with CO2 in the air. Zhang et al. reported copper(II) pyridylazonaphthol (Cu-PAN) complex based gas sensors with high sensitivity and stability in the air. In comparison, these platforms have reactions only to liquid

phase hydrogen sulfide, not to gaseous hydrogen sulphide [81]. Carpenter et al. demonstrated the rapid, quantitative, and sensitive Cu-PAN based gas sensor platforms to gaseous hydrogen sulfide [82]. The gas sensors were prepared via dipping porous substrate in the Cu-PAN solution made by a mixture of ethanol and Cl-Cu-PAN about 5 s. As shown in Fig. 13a. hydrogen sulfide gas is captured by hydroxide solutions and Cu-PAN complex reacts with S2− ions and turns into azo-dye H-PAN [83]. Pink colored Cu-PAN is changed after exposure to 5 ppm H2S to a yellow colored substrate in Fig. 13b.

Many H2S colorimetric gas sensors have been reported. The disadvantages of gas sensors are that they mainly detect S2− not H2S [84–86]. Das et al. reported a new chemodo-simeter DNPS (dansyl-naphthalimide conjugated sulfona-mide), which can detect H2S in a liquid and gaseous phase. DNPS was synthesized by adding 5-nitro-1H,3H-benzo[de] isochromene-1,3-dione to 5-(dimethylamino) naphtha-lene -1-sulfonohydrazide [87]. Das and their group proper

Fig. 12 a Schematic illustrations of Pb(AC)2 nanofiber color change upon exposure to H2S gas. b Visual color changes of Pb(AC)2 nanofiber gas sensor to different concentration of H2S (100  ppb to

5  ppm). c RGB sum variations of Pb(AC)2 nanofiber gas sensor to various gases. Reproduced with permission from [74]. Copyright © 2018, American Chemical Society

Fig. 13 a Possible sensing mechanism of Cu-PAN complex to H2S gas. b Color changes upon exposure to 5 ppm hydrogen sulfide for 10 s. Reproduced with permission from [79]. Copyright © 2017 Elsevier B.V. All rights reserved

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sensing mechanism. As shown in Fig. 14a, H2S or HS− ion attack the center of DNPS molecule following SN reaction and produce three kinds of products. This reaction leads to the color change of DNPS colorless to purple. Figure 14b shows absorbance spectra change of gas senor upon to expo-sure various concentrations of liquid phase H2S (0 to 3 μM). Figure 14c shows the gas sensor platform that was made by dip stick method to DNPS solution reactions to gas phase H2S.

3.4 Other Toxic and Hazardous Gases

3.4.1 Hydrazine

Hydrazine is one of the most explosive and toxic com-pounds. It is easy to absorb via inhalation and ingestion of polluted water and can cause skin irritation, burning to the eyes, shortness of breath, and edema [88, 89]. Despite these side effects, hydrazine is widely used in industrial fields. It is a precursor to pesticides, pharmaceuticals, and a compo-nent of rocket fuels. Because of its wide usage, monitoring technology of hydrazine has been researched [90–93].

Wang et al. reported renewable test strip hydrazine colori-metric gas sensors [94]. Test strips gas sensors were dipped in EPH solutions in 30 min and dried in air. Figure 15a

shows fluorescence color change of EPH embedded test trip to different hydrazine concentrations (0 to 50%). Figure 15b shows renewable properties of EPH embedded test strip with a response time of 5 min and mechanism of sensing reaction. Hydrazine reacts with the ester group of EPH and produces HMBT. Acetic anhydride reacts with HMBT and produces EPH back. EPH embedded test strip sensor could detect hydrazine fast and could be easily renewable by treating with acetic anhydride.

3.4.2 Hydrogen Peroxide

H2O2 could cause significant damages to cellular constitu-ents and case serer cardiovascular disease and cancer. But H2O2 is easy to be produced around everyday life, for exam-ple, textile and paper bleaching, fuel cell, and clinical appli-cations [95–99].

Xu et al. demonstrated paper-based H2O2 consisted of Ti(VI) oxo complexes. The sensor was fabricated by drop casting ammonium titanyl oxalate onto a paper. Figure 16a shows the reaction mechanism of gas sensing between H2O2 and the sensor. Colorless Ti(IV) oxo complex reacts with H2O2 and produces bright yellow colored Ti(IV)-peroxide bond. This color change only occurred to H2O2, not to other gases. Ti(IV) peroxide colorimetric sensor for solutions has

Fig. 14 a Possible sensing mechanism of DNPS to H2S gas. b Absorbance spectra of DNPS (1 μM) and c Color changes of DNPS based paper sensor to H2S upon exposure to different concentration

(0 to 3 equiv.). Reproduced with permission from [84]. Copyright © 2019 Elsevier B.V. All rights reserved

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Fig. 15 a Fluorescence color changes of the EPH based test strips upon exposure to dif-ferent concentrations of N2H4 gas under UV lamp irradiation (365 nm). b The fast fluores-cence color response and recov-ery of EPH based test strips to N2H4 and possible mechanism of N2H4 detection of gas sensor. Reproduced with permission from [91]. Copyright © 2018 Royal Society of Chemistry

Fig. 16 a Possible reaction mechanism of Ti(IV) oxo complexs to hydrogen peroxides. b Colorimetric sensing of Ti(IV) oxo complexs based paper towel gas sensor to 35 wt% H2O2. Reproduced with permission from [96]. Copyright © 2011, American Chemical Society

Fig. 17 a Schematic mechanism of gas sensing. b Transmittance spectra change of Pd-WO3 gas sensor to 1% H2. c Photographic images of flexible Pd-WO3 sen-sor before(left) and after(right) upon exposure to 1% H2. Reproduced with permission from [103]. Copyright © 2016 Elsevier B.V. All rights reserved

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been reported already, but Wu et al. reported a gaseous H2O2 sensor [100, 101]. Figure 16b shows the color change of paper-based sensor to vapor phase H2O2.

3.4.3 Hydrogen

As hydrogen rapidly emerges as eco-friendly alternative energy, an accurate and low power consumption hydrogen sensors are needed [102, 103]. Especially tungsten trioxide has been researched widely because of its superior optical properties [103, 104]. WO3 changes color from transparent to blue upon exposure to H2. This color change is due to the formation of tungsten oxide bronze [105]. Lee et al. reported WO3-Pd film H2 flexible gas sensors. They fabricated WO3 film by sputtering and Pd was deposited on the WO3 film by an e-beam evaporation system. Figure 17c shows the color change of WO3-Pd flexible gas sensors. After expo-sure 1% H2, the color of the film changes into transparent to dark blue. Figure 17b shows transmittance spectra change of WO3-Pd film to 1% H2. Figure 17a shows a schematic mechanism of gas sensing. H2 molecules absorb on the Pd and chemical bonding between H atoms are broken and these H atoms diffuse to the WO3 lattice, react to form an interme-diate W–O–H state that produces H2O and oxygen vacancy. Then oxygen vacancy induces increasing the number of W5+ site increase generating coloration [106].

4 Challenges

The colorimetric gas sensors have been so far highly effec-tive in sensitive and selective detection of toxic and hazardous gases [107]. However, colorimetric gas sensors are still facing a huge challenge of their poor reusability and slow response time that are one of the most important features toward ideal gas sensors for IoT application. The poor reusability and slow response time of the colorimetric gas sensors are widely known to originate from their distinctive sensing mechanism. They detect target gas molecules by measuring the absorbance or fluorescence change of the reactant and the product during chemical reactions. In other words, to reuse the colorimetric gas sensor, a reverse reaction of the gas capturing reaction should be available and occur. Meanwhile, it is difficult for the reverse reaction to occur naturally, without adding new substances or heating, because their reactions are based on chemical binding reaction, not physical adsorption/desorp-tion processes [107–109]. Collectively, the development of functional materials that have great sensitivity and selectiv-ity to target gas and can be reused by facilitating the reverse

reactions in the ambient air is significant challenges of future colorimetric gas sensor research.

Another issue of the colorimetric sensor is it is difficult to detect the color change with the naked eye at the low con-centrations of toxic and hazardous gases that could be fatal to human beings. The naked eye is hard to detect any subtle changes compared to the induction instrument, so we have to make a more dramatic color change to the target gases at low concentrations of gases. Surface Plasmon Resonance (SPR) is an effective way to induce dramatic color change make the detection of gases easier [110]. SPR can be made by vari-ous kinds of metal nanoparticles like gold and silver. By these properties, it is possible to detect a low concentration of haz-ardous gases by colorimetric gas sensors with naked eyes.

5 Conclusions

In this article, we reviewed various types of colorimetric sensors to gaseous toxic and hazardous materials. Organic dyes-based sensors show dynamic absorbance and fluores-cence spectra changes to CWAs, VOCs, hydrogen sulfide, and other various gases. Functionalized polymer-based gas sensors are promising to develop sensor array for VOCs due to showing dynamic response to various types of VOCs Metal organic complex can be used to detect explo-sive gas sensor like hydrogen peroxide. Last, metal oxide with metal catalysis-based gas sensors, which have excel-lent selectivity and a few reusabilities, have tremendous potential if it can detect various kinds of gases. The results described both organic and inorganic materials are pos-sible to detect danger at high speed through sight, one of the five senses, without the power consumption and make us safe from the threat. And if reusability is improved by research, it is expected to be used in real life with IoT.

Acknowledgements This research was supported by the Future Mate-rial Discovery Program (2016M3D1A1027666), the Basic Research Laboratory Program (2018R1A4A1022647), and the National Research Foundation of Korea (2019M3E6A110381812, 2017R1A2B3009135, 2016M3A7B4910, 2020M2D8A206983011).

Author contributions SHC and HWJ conceived of the outline of the manuscript and wrote the manuscript. JMS, THE and TK contributed to finding the references.

Compliance with Ethical Standards

Conflict of interest The authors declare no conflict of interest.

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