Progress of sensitive materials in chemiresistive sensors for detecting chemical warfare agent simulants: A review

Liu Yang; Molin Qin; Genwei Zhang; Jie Yang; Junchao Yang; Jiang Zhao

doi:10.1515/revac-2022-0052

Article Open Access

Progress of sensitive materials in chemiresistive sensors for detecting chemical warfare agent simulants: A review

Liu Yang , Molin Qin , Genwei Zhang , Jie Yang , Junchao Yang and Jiang Zhao

Published/Copyright: February 16, 2023

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From the journal Reviews in Analytical Chemistry Volume 42 Issue 1

Abstract

Chemical warfare agents (CWAs) are chemical substances intended for use in military operations to kill, injure, or incapacitate an enemy. It is very important to detect toxic CWAs at trace levels on site. Compared with traditional CWA analysis instrument methods, chemiresistive gas sensors present advantages of being small, fast, and inexpensive. Chemiresistive gas sensors are still an important research topic with the development of nanofabrication technology and new kinds of sensing materials, including carbon nanotubes, graphene, and black phosphorus (BP). Chemiresistive gas sensors are divided into three categories according to the type of sensitive materials: carbon- and BP-based materials, conductive polymers, and metal oxide semiconductors. A brief analysis was carried out on fabrication strategies using sensitive materials, including hydrogen bonding strategies, nanostructures, morphology, doping effects, composite materials, and other material application methods, and the sensitive materials and detection effects are summarized in this study. This review aims to provide guidance in the development of chemiresistive sensors for the detection of CWAs.

Graphical abstract

Keywords: chemical warfare agent simulants; chemiresistive sensor; sensing material; composite material; functional group modification; nanostructure

1 Introduction

Chemical warfare agents (CWAs) are chemical substances intended for use in military operations to kill, injure, or incapacitate an enemy [1]. These threatening agents are classified according to the mechanism of toxicity in humans into blister agents, nerve agents, blood agents, and choking agents. These agents still remain a threat. Compared with traditional CWA analysis instrument methods [2], such as ion mobility spectrometry, gas chromatography-mass spectrometry, Fourier transform-infrared spectrometry, and Raman spectrometry, chemiresistive gas sensors are smaller, faster, and inexpensive. However, the applicability of these sensors in actual situations is limited by their selectivity. With the development of new kinds of sensing material, including carbon nanotubes, graphene, black phosphorus (BP), and nanofabrication technology, the chemiresistive gas sensor research field is one of the research focuses. CWAs are dangerous substances, and CWA simulants are used for chemical sensor detection. Some CWAs and CWA simulant compounds are shown in Figure 1. CWA simulant compounds were included such as dimethyl methylphosphonate (DMMP), diisopropyl methylphosphonate (DIMP), diethyl methylphosphonate, diethyl ethylphosphonate (DEEP), diethyl chlorophosphate (DECP), diethyl cyanophosphonate (DCNP), diethyl chlorophosphate (DCP), diisopropyl fluorophosphate, 2-chloroethyl ethyl sulfide (CEES), 2-chloroethyl phenyl sulfide, and di(propylene glycol) monomethyl ether (DPGME).

Figure 1

Some CWAs and CWA simulant compounds.

Chemiresistive gas sensors use analyte gases in the environment for detection through a sensing layer that induces an electronic or resistance change and the collection of electrical or resistance signals. These sensors were introduced for the first time 50 years ago. Commercial sensors were invented in the 1960s, and chemiresistive gas sensors have been used in various fields worldwide. Chemiresistive gas detection methods are simple and effective, and sensing materials are crucial in chemiresistive gas sensors. Many kinds of sensing materials, including carbon-based materials, such as graphene and carbon nanotubes, and BP; conducting polymers such as polypyrrole (PPy), polyaniline (PANI), and poly(3,4-ethylenedioxythiophene) (PEDOT); semiconductor metal oxide and metal oxide hybrids, such as nontransition metal oxide materials (e.g., SnO₂); and transition metal oxide materials, such as ZnO and WO₃, have been applied to chemiresistive gas sensors. Some articles and reviews refer to the detection sensors for CWAs, such as Fennell et al. [3], Lee et al. [4], and Srivastava and Rai [5], but only a part of the materials and mechanisms involved. This study systematically introduces various types of sensing materials for chemiresistive sensors used to detect CWAs.

A satisfactory gas sensor should demonstrate stability, sensitivity, and selectivity or “3Ss.” The requirements are the same, and even more important, for chemiresistive sensor technology for potential detection of CWAs. The immediately dangerous to life or health value is also low and detection sensitivity is required because the toxicity of CWAs is high. In addition, the selectivity of the CWA sensors prevents not only the occurrence of false positives but also missed or incorrect detection of CWAs to avoid health damage or even death. Therefore, the continuous investigation and the optimized use of sensitive materials and preparation methods are necessary to obtain CWA sensors with ideal effect.

Chemiresistive gas sensors perform detection by varying the electrical resistance in a semiconducting material upon its interaction with a target analyte. Although the transduction mechanism is simple, molecular-level processes, including gas diffusion, surface reactions, and charge carrier transfer, are extraordinarily complicated. The sensing mechanism strongly depends on the nature of the semiconducting material and the analyte as well as the adopted working conditions. Therefore, different sensitive materials can be used for chemical resistance sensors to detect CWA simulant targets, and various material applications and preparation strategies can be selected to achieve the ideal detection performance.

2 Carbon materials and BP-based sensors

Some studies have been conducted on CWA simulant detection using chemiresistive sensors with carbon-based materials, such as graphene and carbon nanotubes, and SWNTs are almost entirely composed of surface atoms and are expected to exhibit excellent sensitivity toward adsorbates. Graphene presents a unique two-dimensional structure, high specific surface area, and distinctive electrical properties, such as high mobility and low electrical noise. Carbon-based chemiresistive sensors show evident advantages over traditional metal oxide-based sensors, such as room temperature operation, high sensitivity, and ease of use. However, carbon-based materials, such as pristine graphene sensors, suffer from dangling bonds on their surface that limit the chemisorption of target molecules on the graphene surface. Material preparation methods, such as chemical modification, are usually required to enhance the sensing performance of carbon-based materials.

The application of carbon and BP materials for the detection of chemical resistance principle includes pristine graphene, graphene oxide, reduced graphene oxide (rGO), porous rGO, single-walled carbon nanotubes (SWCNTs), multiple-walled carbon nanotubes (MWCNs), BP materials, and their modification or doping hybrid composite materials.

The application method of these kinds of materials usually introduces sensitive groups that present an affinity with organophosphorus simulants, control the structure and morphology, and are used to prepare composites to increase sensitivity and selectivity of the sensor.

Pristine material sensors are characterized by their simple method and used for sensing CWA simulants through different preparation methods. Horrillo et al. [6] reported on chemiresistive sensors fabricated by SWCNTs. DMMP, DPGME, and dimethylacetamide (DMA) are CWA simulants of sarin, nitrogen mustard, and distilled mustard that were tested with a sensor. Mirica et al. [7] fabricated a gas sensor using carbon nanotubes and graphite on the surface of study. Saetia et al. [8] manufactured chemiresistive sensors with MWNT films on a porous electrospun fiber. The material is assembled layer by layer with MWNTs on individual electrospun fibers. Robinson et al. [9] fabricated rGO chemiresistive sensors for CWA simulant DMMP detection. Fang et al. [10] used a chemiresistive sensor-fabricated method by changing the material and the geometry of inkjet-printed interdigitated electrodes (IDEs). SWCNT-based sensing materials were deposited on a Kapton® substrate using a layer-by-layer chemical method. SWCNT-based Au-electrode sensors increased Schottky contacts between porous Au IDEs and semiconducting SWCNTs and facilitated an estimated fivefold increase in sensitivity to DEEP. Ammu et al. [11] demonstrated that chemiresistors made of thin films of SWCNT bundles on cellulosics (paper and cloth) can detect chlorine.

Liu et al. [12] proposed a BP gas sensor for the detection of DMMP. Sensitivity ranges of CNT, BP, and BP–CNT sensors in a concentration range of 1.5–30 mg·m⁻³ are 0.3–1.6%, 4.2–13.5%, and 1.8–4.8%, respectively. The detection performance and characteristics of sensors prepared using different materials are listed in Table 1.

Table 1

Sensitive materials, simulant, and detection effects on chemiresistive sensor

Sensitive materials	CWA simulant	Temp.	Conc.	Response	LOD	Response/recovery time	Ref.
SWNT networks	DMMP	RT	10 ppm	0.8–2.1%	1 ppm	0.15 min	[23]
HFIPP–SWNT	DMMP	RT	200 ppb	1%	50 ppb	—	[16]
SWNT–6FBPA	DMMP*	RT	20 ppm	5.1%	0.5 ppm	—	[17]
SWNT–TFQ	DMMP	RT	20 ppt	1.7%	0.02 ppb	Tres = 2 min	[18]
SWCNTs	DMMP	RT	—	—	0.01 ppm	—	[6]
SWCNTs	DPGMA	RT	—	—	0.1 ppm	—	[6]
SWCNTs	DMA	RT	—	—	50 ppm	—	[6]
Nanocarbon	DMMP*	RT	—	—	12 ppm	—	[7]
TF-MWNT/ES	DMMP	RT	—	—	5 ppm	—	[8]
SWCNT–HFiP-1	DMMP*	RT	24 ppm	16%	480 ppb	300 s/90 s	[20]
MSP–SWCNTs	DECP	RT	—	—	0.1 ppm	Tres = 5 s	[24]
SWCNT	DEEP	RT	2 ppm	5.2%	2 ppm	63 min	[10]
rGO	DMMP	RT	—		5 ppb	—	[9]
rGO	CEES	RT	—	—	0.5 ppb	—	[9]
rGO	HCN				70 ppb	—	[9]
PPD–CRG	DMMP	RT	20 ppm	8%	5 ppm	1,080 s/360 s	[15]
rGO-B/rGO-H	DMMP*	RT	—	—	10 ppm	—	[14]
TFQ–graphene	DMMP	RT	105 ppm	6%	4 ppm	—	[21]
Triphenylene-G	DMMP*	RT	1.3 ppm	10%	1.3 ppm	50 s	[22]
r(GO/rGO)₁₂	DMMP	RT	1–50 ppm	2.21–8.95	1 ppm	4 min/3 min	[13]
BP–CNT	DMMP	RT	1.5 mg·m⁻³	1.8%	—	—	[12]
3D–HFIP–CNFs	DMMP	RT	1 ppm	25.5 S^g	—	2.4 s/4.7 s	[19]
CNT	Cl₂*	RT	—	—	500 ppb	—	[11]
CB/polymer	DIMP	RT	—	—	0.049 mg·m⁻³	—	[36]
CB/polymer	DMMP	RT	—	—	0.0474 mg·m⁻³		[36]
Polymer ribbons	DMMP*	RT	—	—	2.2 ppm	—	[37]
Shape PANI	DMMP*		500 ppb	3%	5 ppb	1 s/40 s	[29]
CA–Ppy–NT	DMMP*	RT	—	—	0.5 ppb	—	[26]
PEDOT–HPNTs	DMMP	RT	—	—	10 ppt	1 s/3–25 s	[3]
FeOOH–PPy	DMMP	RT	10 ppb	7%	0.1 ppb	1.5 s/8.5 s	[34]
PPY–SnO₂	DMMP*	RT	0.05 ppb	0.5%	0.05 ppb	15 s/30 s	[37]
PPy–rGO	DMMP	RT	100 ppm	12.9%	5 ppm	43 s/75 s	[38]
PANI-G	DMMP*	RT	—	—	3 ppb	2 s/35 s	[40]
PEDOT/SWCNT	DMMP*	RT	11 ppm	0.95%	2.7 ppm	60 s/140 s	[3]
CoPc–HFIP–GQD	DMMP*	RT	20 ppm	8.4%	500 ppb	600 s/640 s	[28]
RR-P3HT	DMMP*	RT	6 ppm	8%	0.4 ppm	—	[33]
PAN–ACFs	DMMP	RT	10 ppm	5.1%	—	—	[41]
RR-P3HT–Al₂O₃	DMMP*	RT	1.1 ppm	0.5%	1.1 ppm	50 min/50 min	[32]
PANI–SWNT	DMMP	RT	10 ppm	27.1%	1 ppm	5.5 s	[39]
CoPc–Pd	DMMP	RT	—	—	60 ppb	—	[30]
Au–O–SnO₂	DMMP*	320	680 ppb	1.67	4.8 ppb	26 s/32 s	[64]
WO₃/WS₂	CEES*	240	5.7 ppm	81%	0.1 ppm	20 s/55 s	[43]
WSe₂	DMMP*	RT	10 ppm	8.91%	122 ppb	100 s/1,260 s	[53]
Al–ZnO	DMMP*	300	—	—	2 ppm	11 s/27 s	[65]
Al–ZnO	CEES*	500	—	—	10 ppm	—	[67]
SnO₂	DMMP	350	0.5 ppm	60%	—	—	[50]
SnO₂ (Ni/Mo/Sb)	DMMP	350	0.6 ppm	2.33	—	10 min/90 min	[63]
SnO₂	DMMP	400	0.7 ppm	0.1	—	—	[55]
SnO₂	ACN	500	0.5 ppm	0.4	0.5 ppm	1–2 min/1–2 min	[56]
SnO₂–CNF	DMMP	RT	1 ppm	0.036	—	0.02 min/1 min	[34]
SnO₂	Sarin	400	125 ppb	0.5	6 ppb	—	[57]
Pt-ZnO	DMMP	300	2 ppm	250	—	0.25 min/0.75 min	[68]
Al–ZnO	DMMP	350	5 ppm	4,347	100 ppb	0.03 min/1.6 min	[66]
CuO–ZnO	DMMP	350	10 ppm	626	—	Tres 0.4 min	[44]
Mn₃O₄	DMMP	200	5 ppm	0.55	0.04 ppm	Tres 0.83 min	[58]
SnO₂/MnO₂	DMMP	250	5 ppm	0.1	2.3 ppb	11 min/26 min	[45]
WO₃/CuO	DMMP	RT	10 ppm	18.3	—	0.15 min/0.26 min	[46]
CdSnO₃	DMMP	350	4 ppm	2	—	—	[69]
Fe₂O₃ complex	DMMP	RT	1 ppm	0.12	1 ppb	2 min/5 min	[51]
Co₃O₄	DMMP	150	0.5 ppm	2	—	0.22 min/0.20 min	[47]
rGO/WO₃-HFIP	DMMP	150	10 ppm	15.4	0.1 ppm	0.38 min/0.41 min	[48]
HFIP–Co₃O₄/CuO	DMMP	90	0.5 ppm	1.5	0.5 ppm	0.16 min/0.14 min	[49]
SnO₂	ACN	300	2 ppm	0.2	—	8.5 min/11.7 min	[60]
MnO₂	ACN	200	25 ppm	0.24	0.05 ppm	2.0 min/6.0 min	[61]
Cr0.8Fe0.2NbO₄	ACN	300	200 ppm	0.75	—	—	[32]
SnO₂–NiO	DPGME	350	0.05 ppm	14%	—	0.3 min/6.7 min	[71]
Mn₃O₄–Au	DPGME	200	5 ppm	6	0.6 ppb	9 min/28 min	[52]
SnO₂	CEES	250	10 ppm	179	—	0.75 min/10 min	[72]
Al–ZnO–QD	CEES	450	20 ppm	5,392	0.5 ppm	0.05 min/6.8 min	[74]
Al–ZnO	CEES	500	20 ppm	953	—	0.03 min/2.1 min	[73]
Ru–CdSnO₃	CEES	350	4 ppm	61	—	0.08 min/2.1 min	[59]
Pt–CdSnO₃	CEES	250	4 ppm	58	—	0.5 min/5 min	[69]

Temp. – working temperature (°C); CB – carbon black; CA – carboxylic acid; NT – nanotube; Fe₂O₃ complex – Fe₂O₃/graphite/CaSO₄; Conc. – concentration; LOD – limit of detection; Pd – palladium.

*Representatives also tested for interferents or other simulants.

Controlling the structure and morphology of materials is important in the preparation of sensitive materials. Wang and Yang [13] fabricated a sensor with 12 layers of porous graphene (r(GO/rGO)₁₂. The resistance change value was threefold higher than that obtained with the sensor with one layer of graphene. Leyla and Soltani [14] established a gas sensor array using rGO. Three kinds of rGO-based sensors were compared on the basis of DMMP detection. The results showed that sodium borohydride and ascorbic acid lead to rGOs, which are considerably more sensitive to DMMP than that reduced by hydrazine hydrate. Hu et al. [15] tested a p-phenylenediamine (PPD) rGO gas sensor for DMMP detection. PPD can be efficiently dispersed in organic solvents and benefit the formation of conductive circuits between electrode arrays through drop drying method. The response of DMMP by PPD-fabricated rGO sensors exhibits 5.7 times than hydrazine-fabricated rGO sensors.

The method of introducing a sensitive group that presents affinity with organophosphorus CWA simulants is important for the preparation of chemiresistive sensors. Kong et al. [16] developed the method of chemiresistive sensors with N-4-hexafluoro isopropanolphenyl-1-pyrenebutyramide (HFIPP)-decorated SWNT to ensure that a strong hydrogen bond can be formed between HFIPP and DMMP. Wang et al. [17] fabricated chemiresistive sensors with hexafluorobisphenol covalently functionalized SWNT. Wei et al. [18] investigated the SWNT-TFQ (tetrafluorohydroquinone) network used for a chemiresistive sensor. The sensor can detect DMMP at a concentration of 20 ppt. Heavy hole doping of SWNTs with TFQ can not only achieve selective interaction of DMMP with functionalized SWCNTs but also contribute to the high sensitivity of sensors. Alali et al. [19] constructed a 3D carbon nanostructure via in situ growth of carbon nanofibers (CNFs) on electrospun CNFs (ECNFs) and then activated it with hexafluoroisopropanol (HFIP) to detect DMMP via H-bonding. The fabric structure of 3D CNFs/ECNFs-HFIP provides a fast charge carrier pathway and accelerates the response (2.4 s) and recovery (4.7 s) times. Kumar et al. [20] developed the method of gas sensor using 4-(hexafluoro-2-hydroxy isopropyl) aniline-functionalized SWCNT for DMMP detection. Lee et al. [21] synthesized TFQ-functionalized graphene for DMMP sensing.

Kim et al. [22] investigated graphene chemiresistors modified with functionalized triphenylene for DMMP detection. The response of the functionalized graphene sensor to DMMP is two orders of magnitude higher than that of other gases. High sensitivity of the functionalized graphene sensor is attributed to the strong hydrogen bonding between DMMP and N-substituted triphenylene, and the hole-doping effect caused by triphenylene that increases the binding affinity to the electron-donating DMMP.

Fabricated complex materials are important in the preparation of chemiresistive sensors. Kim et al. [23] explore SWNT networks used in chemiresistive sensors. Random networks of SWNTs were fabricated by drop-casting an SWNT-containing solution onto a surface-oxidized Si substrate. Pd-contacting SWNT network sensors exhibited higher response and excellent recovery than those of Au-contacting SWNT network sensors at the same DMMP concentration because of the stronger interactions between SWNTs and palladium (Pd) surface atoms.

Ishihara et al. [24] fabricated SWCNTs wrapped with metallosupramolecular polymer sensors for CWA gas detection. Semiconducting SWCNT sensors show significantly enhanced sensitivity with DECP compared with metallic SWCNT (M-SWCNT) sensors, but responses of M-SWCNT sensors were smaller but less susceptible to interfering signals. Mechanisms at the interface between tubes can exert a significant influence on electronic properties of the overall network for devices with a network of CNTs. Small changes in the distance between two CNTs can remarkably influence the contact resistance because the probability of charge tunneling decreases exponentially with distance. Intertube conduction pathways can be modulated either by partitioning analytes into interstitial spaces between tubes or swelling the supporting matrix/wrapper. Alternatively, an analyte can trigger the disassembly of the molecular/polymer wrapping of CNTs (Figure 2). Ganji et al. [25] manufactured an aluminum nitride graphene-based sensor for DMMP adsorption and detection.

(1) S ⁎ ( % ) = R a − R g R a × 100

(2) s g = R a R g

Figure 2

3 Conducting polymer material-based sensors

Conducting polymer-based sensing materials is also used in chemiresistive sensors. Conducting polymers are suitable as sensing materials for different gases, with their earliest application dating back to the 1980s. Conducting polymers and their hybrids have been utilized in chemiresistive CWA simulant gas sensors in recent years. The morphology of new conducting polymers and their hybrids play an important role in sensors. Nanostructures provide a large specific surface area or surface-to-volume ratio to improve the diffusion rate of gas molecules.

Three methods, namely, preparation and control of the structure and morphology of polymers, functionalized group modification of conductive polymers, and formation of composite materials with carbons or semiconductors, have been used for the preparation of chemical poison simulants from conductive polymers and the application of materials in recent years.

The functionalized group of conductive polymers is a material application method that mainly forms hydrogen bonds with DMMP after polymer modification. Kwon et al. [26] adopted hydroxylated PEDOT (HEDOT) nanotubes as the sensing material for DMMP detection. PEDOT is a P-type semiconductor material, and the hydroxylation modification increases the sensitivity. The results showed the formation of hydrogen bonds between nerve agent stimulant molecules and HEDOT. Kwon et al. [27] explored the effect of the degree of polymer substitution using different functional groups on detection in 2016. Fennell et al. [3] adopted a chemiresistive CWA sensor using SWCNTs wrapped with HFIP-PEDOT derivatives. Jiang et al. [28] proposed the use of two kinds of cobalt phthalocyanine (CoPc) derivatives containing HFIP and hexafluorobisphenol A (6FBPA) substituents as sensing materials. Graphene quantum dots (GQDs) were anchored to CoPc derivatives via π–π bonding to form hybrid materials (Figure 3).

Figure 3

Sensing mechanism. Computed 3D graphics showing the interaction of P3CA with (a) DMMP and (b) sarin, where the white dotted line indicates a hydrogen bond between the corresponding atoms. (c) Scheme of describing the effect of hydrogen bonding on the oxidation state of PPy [27]. Copyright © 2017; Springer Nature.

Methods for the preparation or control of morphology are a class of material applications. Cho et al. [29] manufactured shape-controlled PANI-based chemiresistors for DMMP detection. PANI nanomaterials of three different shapes were synthesized via chemical oxidation polymerization. Powroźnik et al. [30] assessed the sensing mechanisms in sensors on the basis of phthalocyanine–palladium structures. Sensors based on the monolayer of CuPc did not respond to DMMP. Sensors based on bilayer structures exhibited a maximum sensitivity of 60 pbb to DMMP at room temperature but showed no sensitivity at 85°C. Physisorption is likely the dominant sensing mechanism. Phulgirkar [31] used oligoaniline as the sensing material for a chemiresistive sensor. Tetraaniline, octaaniline, hexadecaaniline, and PANI were utilized to fabricate sensing thin films. The inkjet-printing method provided improved thickness control and low variability between different sensors. Powroźnik et al. [32] fabricated regioregular poly(3-hexylthiophene) polymer (RR-P3HT) as a resistive sensor for the detection of DMMP. Powroźnik et al. [33] examined photoconductive polymer sensing properties of light-activated regioregular RR-P3HT in DMMP detection.

The integration of conducting polymers and carbons or semiconductors into composite materials is another method for preparing sensitive layers of CWA simulant sensors. Lee et al. [34] created a multi-dimensional FeOOH nanoneedle-decorated hybrid PPy-based sensor for DMMP detection. FeOOH surfaces adsorbed the DMMP via charge interaction and hydrogen bonding through the −OH group of FeOOH and the phosphate. Hopkins and Lewis [35] investigated an organic polymer/carbon black composite material as the sensing material for a chemiresistive sensor. DMMP and DIMP were detected using a chemiresistive sensor. Benz and Patel [36] fabricated a chemiresistive gas sensor with polymer composite ribbons. DMMP and different interferents were detected with a polymer composite ribbon sensor. Jun et al. [37] utilized one-dimensional tube-in-tube PPy/tin oxide as the sensing material for a CWA chemiresistive sensor. The PPy/SnO₂ material with tube-in-tube structure was established via mixed-solvent single-nozzle electrospinning and vapor deposition polymerization.

Yang et al. [38] developed a PPy-decorated rGO hybrid (PPy-rGO) through redox reactions between pyrrole and graphene oxide (GO) during the hydrothermal treatment process in DMMP sensing. The enhanced DMMP sensing performance of PPy-rGO hybrids is caused by the following. On the one hand, the formation of hydrogen bonds between PPy-rGO hybrids and DMMP molecules regulates the adsorption/desorption of DMMP. On the other hand, increasing the Brunauer Emmett Teller surface area through the introduction of PPy into the rGO matrix facilitates the DMMP diffusion among the sensing materials. Yoo et al. [39] developed a composite sensor composed of SWCNTs and PANI for DMMP detection. The DMMP-sensing mechanism is shown in Figure 4.

Figure 4

Illustration of DMMP-sensing mechanism in pure SWCNT network sensors (top) and SWCNT–PANI composite sensors (bottom). The top panel inset shows a magnified view of two crossed SWCNTs and the corresponding energy barrier. The bottom panel lower inset magnifies the SWCNT–PANI interface and corresponding energy barrier; the right inset shows the distortion of the PANI matrix around an adsorbed DMMP molecule [39]. Copyright © 2014; Elsevier.

Yu et al. [40] fabricated conductive paper with PANI nanofiber and graphene sheet as the sensor for DMMP detection. The mechanism for detecting DMMP molecules using PANI molecules forms an H-bond with DMMP. Kang et al. [41] examined the influence of micropore structures of polyaniline (PAN)-based activated carbon fibers (ACF) on DMMP detection. PAN-based activated carbon fiber electrode is an N-type semiconductor. The sensitivity of the fabricated DMMP gas sensor increased from 1.7% to 5.1% as the micropore volume increased. Wiederoder et al. [42] fabricated a gas sensor array using a polymer–graphene nanoplate. An array with 12 sensors was established via coating with a different polymer–graphene nanoplate. Five kinds of CWA simulants and eight kinds of common background interferents were detected by the array. Discrimination accuracy for all analytes was 99%.

4 Metal oxide semiconductor (MOS) material-based sensors

MOSs are a kind of sensing material for chemiresistive sensors. MOS sensors have been used in a wide range of products, especially in industrial and safety products, for more than 30 years. These materials usually operate at high temperatures when used in detecting gaseous species. The two main types of semiconducting devices are based on SnO₂ and ZnO. The two groups of devices can be differentiated according to thickness and size (e.g., thick film, thin film, and microsize).

Different doped materials are usually introduced to enhance the sensitivity and stability. New material application methods, including the construction of heterojunctions between different oxide materials, have been proposed in recent years. Different methods, such as forming sensitive materials modified by hydrogen bond functional groups, forming complexes with carbons and polymers, controlling the structure and morphology, and doping materials, have been applied to improve the detection effect on CWA simulants.

Different MOS materials can be used in the construction of heterojunctions for the preparation of CWA simulant sensors. Fan et al. [43] proposed a highly selective gas sensor based on the WO₃/WS₂ van der Waals heterojunction for the detection of 2-CEES. A novel WO₃/WS₂ van der Waals heterostructured material is controllably constructed by simply annealing WS₂ nanosheets in air. The sensor exhibits higher selectivity to 2-CEES compared with ammonia, common VOC gases (acetone and ethanol), and other toxic gases. Yoo et al. [44] fabricated high-surface area CuO nanoparticles (NPs) on micron-scale ZnO(CuO/ZnO) “flowers” for DMMP gas sensing. The CuO NP/ZnO heterojunction structure provides an extension of the depletion layer and increases the resistance (R _a) in air, thereby reducing the depletion layer and resistance (R _g) when exposed to reducing DMMP gas. Quasi-1D MnO₂ nanocomposite sensing materials (A/MnO₂ with A = CuO, SnO₂) were developed by Bigiani et al. [45]. The sensing mechanism is the formation of built-in p–n and n–n junctions or CuO/MnO₂ and SnO₂/MnO₂ systems. Alali et al. [46] fabricated n–p heterojunction composite WO₃/CuO nanofibers as the sensing material. The 1D-nanocomposite structure and n–p heterojunction between CuO and WO₃ crystals provided sufficient oxygen vacancies to absorb and desorb oxygen ions and achieve a high response. Composite WO₃/CuO NFs showed a remarkable response to 0.5 ppm of DMMP at room temperature (Figure 5).

Figure 5

Schematic diagram of the reaction of the composite WO₃/CuO NFs with oxygen and DMMP molecules in the air (a) and DMMP atmosphere (b), respectively. Illustration diagram of the energy band gap in each case is shown in the bottom figure. The abbreviation means that Ф _eff is the potential junction barrier on the surface of crystals, E _c is the lower level of the conduction band gap, E _f is the Fermi level, Ф _w is the work function, and E _v is the higher level of the conduction band gap [46]. Copyright © 2019; Elsevier.

Functional group modification methods similar to those used in carbon and conductive polymer materials have also been applied in MOS materials in recent years. For example, HFIP functional groups are introduced into the material to form hydrogen bonds with DMMP to improve the detection effect. Alali et al. [47] deposited micro–nano-octahedra Co₃O₄ functionalized with HFIP on a layer of rGO as double-layer sensing materials. The Co₃O₄ micro-nano-octahedra was synthesized via direct growth from electrospun fiber templates calcined in ambient air. The rGO/Co₃O₄-HFIP sensing materials presented high sensing selectivity toward DMMP due to the hydrogen bonding between DMMP molecules and Co₃O₄-HFIP. The functionalization with HFIP was grafted on tungsten trioxide (WO₃) hollow nanofibers (HNFs) for detecting DMMP by the same group [48]. rGO nanosheets were applied as the base layer to avoid grain boundary poisoning and enhance the charge mobility. Alali et al. [49] successfully extended functionalization with HFIP in 2020. Hexafluoroisopropanol (HFIP) groups were grafted on composite Co₃O₄/CuO nanotubes as a hybrid sensing material for detecting DMMP. Hybrid HFIP-Co₃O₄/CuO NTs showed high response and excellent selectivity to 0.5 ppm of DMMP under light irradiation. The enhanced DMMP detection is attributed to the photoactivation of p–p heterojunction Co₃O₄/CuO NTs and HFIP functionalization (Figure 6).

Figure 6

The method of forming a composite of MOS with carbon and conductive polymers can be applied in the preparation of sensitive layers. Lee et al. [50] fabricated ultrafine metal-oxide-decorated hybrid CNFs for DMMP gas sensing. These ultrafine hybrid CNFs were applied to a DMMP chemical sensor at room temperature and achieved excellent sensitivity because metal oxide nanonodules of hybrid CNFs increase the surface area and affinity to DMMP vapor. Alizadeh and Jahani [51] used a nanocomposite of nano-sized a-Fe₂O₃/graphite/CaSO₄ as the DMMP sensor and demonstrated that the presence of CaSO₄ in the nanocomposite is crucial for observing the sensing characteristic of nano-sized a-Fe₂O₃. Sulfated a-Fe₂O₃ acts as a p-type semiconductor in the nanocomposite despite being an n-type semiconductor. Bigiani et al. [52] fabricated Mn₃O₄-based nanocomposites containing noble metal particles for the detection of DPGME (simulant of vesicant nitrogen mustard). The selective sensing of DPGME was activated at the oxide−NP interface via dual anchoring to both oxide and metal components. The Schottky junction formed on the Au/Mn₃O₄ interface increases the sensing effect.

Controlling the structure and morphology is another application method of MOS materials. Nanowires, radial heterojunctions, nanotubes, nanorods, nanorings, and layered structures can be prepared with the continuous development of nanofabrication technology, nanowires, and nanosheets (Figure 7).

Figure 7

Structure and morphology of different nanomaterials.

Lia et al. [53] fabricated a room-temperature chemiresistive gas sensor based on two-dimensional few-layered tungsten diselenide (WSe2) nanosheets for DMMP sensing in 2021. The enhanced sensing performance of WSe2 nanosheets can be ascribed to the increase in specific surface area that provides additional active adsorption sites for DMMP molecules, thereby facilitating the charge transfer process between DMMP molecules and WSe2 nanosheets. Sberveglieri et al. [54] prepared tin oxide nanowires as functional materials for DMMP detection. The gas sensor based on tin oxide nanowires prepared via gas–liquid–solid vapor liquid solid method exhibits high sensitivity to DMMP. Comini et al. [55] used SnO₂-based nanowire networks as sensing materials to detect DMMP. Te conduction in bundles of nanowires is dominated by the space charge region formed in the contact area between different nanowires. Dai et al. [56] synthesized nanoporous n-type SnO₂ sensory films via a template-transferring method to create a sensor for sarin sensing. The SnO₂ sensor demonstrated an abnormal sensing response with increased resistance to sarin gas only at 300°C while preserving the reducing gas response with declined resistance at other operating temperatures for other gases. Maccato et al. [57] fabricated Mn₃O₄ nanosystems on Al₂O₃ substrates as the sensing layer. The structure/morphology was characterized as a function of the used growth atmosphere. Functional Mn₃O₄ nanosystems exhibit high surface and peculiar nano-organization. Kanan et al. [58] compared the response of the sensor prepared from porous WO₃ powder with that of nonporous WO₃ powder. Detection selectivity between methanol and DMMP is obtained because the size-dependent access of a gas molecule in the interior pore structure of WO₃ leads to a size-dependent magnitude change in the conductivity of the sensor. Patil et al. [59] fabricated CdSnO₃ thin sensing films using ultrasonic spray pyrolysis technique. The nanostructured perovskite-type thin films reported in the present article showed a satisfactory response to the CEES simulant. The mechanism may facilitate the adsorption of additional atmospheric oxygen species. Ponzoni et al. [60] prepared SnO₂ nanowires and WO₃ films as sensitive materials for sensors via rheological growth thermal oxidation to nanowire technology. The response of SnO₂ nanowires to the cyanide simulant acetonitrile (ACN) is satisfactory. Barreca et al. [61] established β-MnO₂ nanosystems as the sensor-sensitive layer. Fabricated sensors can efficiently detect ACN with optimal responses at moderate temperatures (≤200°C) because of the high oxygen vacancy content and fluorine concentration. Murthy et al. [62] fabricated Cr_0.8Fe_0.2NbO₄ thick film arrays to determine ACN, H₂, NH₃, acetone, alcohol, cyclohexane, and petroleum gas at different operating temperatures. Compounds were distinguished using principal component analysis. Lee et al. [63] investigated different pore-size SnO₂-based gas sensors. Tin oxide with large pore size shows higher sensor response to DPGME and DMMP compared with tin oxide with small pore size.

Adding doped materials to enhance sensitivity and stability is crucial in the preparation of semiconductor-sensitive layers. Yang et al. [64] designed Au NP-decorated oxygen vacancy-enriched SnO₂ hybrids (Au–O—SnO₂) as sensing materials in 2022. The enhanced DMMP sensing performance of Au–O–SnO₂ hybrids can be mainly ascribed to the synergistic effect on many surface-active sites induced by oxygen vacancies and the chemical and electronic sensitization of Au NPs. Bagul et al. [65] used Al-doped nanocrystalline ZnO thin films as the sensing layer. The high DMMP response of Al-doped thin films may be due to surface defects, high adsorption capability for oxygen species, the presence of Al as an activator, and the nanocrystalline nature of ZnO, which is the host material. Yoo et al. [66] explored the hydrothermal preparation of Al-doped ZnO nanomaterials for DMMP sensing. DMMP showed the fastest response among the gases. The increased O2 vacancies and surface reactions with small nanocrystals in Al-doped ZnO NP sensors provide enhanced gas sensing performance for DMMP detection. Yoo et al. [67] also examined Al-doped ZnO NPs as sensors for sensitive materials and demonstrated that Al is more effective in detecting 2-CEES than Cu, Co, or Mn. Patil et al. [68] prepared thick films with nanocrystalline ZnO powder using ultrasonic atomization technique. Pt presents some applications as a catalyst for DMMP decomposition. Smaller nanocrystals are better than larger nanocrystals in terms of sensing performance. Patil et al. [69] utilized nanostructured perovskite CdSnO₃ and Pt-CdSnO₃ thin films as the sensing layer for DMMP sensing. The group also [70] investigated a ruthenium-loaded nanostructured CdSnO₃ thin film sensor for 2-CEES detection. Hwang et al. [71] prepared a new SnO₂-based thick film gas sensor based on NiO. NiO improved the sensor response and response time in detecting ppb-level DPGME. Aliha et al. [72] explored SnO₂-based semiconductor thick film gas sensors with added ZnO, CuO, and Sm₂O₃. The results showed that the presence of dopants causes an increase in surface area and a decrease in particle size of sensing materials. Yoo et al. [73] assessed the effect of doping on sensing properties of ZnO NPs for the detection of 2-CEES. The sensing response of Al-doped ZnO NPs was observed to show the maximum value due to the enhanced conductivity and concentration of oxygen vacancy after Al doping. Lee et al. [74] prepared a hydrothermally synthesized Al-doped ZnO quantum dot sensor for the detection of 2-CEES combined with micro-gas chromatography on packed chromatographic columns to achieve selective detection. The improved detection performance of the Al-doped sensor can be attributed to the increased specific surface area obtained by reducing the particle size, thereby increasing the chemical reaction between oxygen and free radicals.

5 Conclusions

Chemiresistive gas sensors still require further investigation with the development of new sensing materials and nanotechnology. The literature on chemiresistive sensors for the detection of chemical poison simulants in the past 10 years is reviewed in this work from the aspect of material application. Application methods of sensing materials for the detection of CWA simulants are analyzed, and application methods of hydrogen bond functional group modification strategies, nanostructures and morphology, hole doping effects, and formation of composite materials are introduced. The detection effect on chemical sensors for CWA simulants is examined, and the development of related chemiresistive sensor materials is discussed. Except for traditional doping materials, n-P, P-P heterojunction materials, semiconductor materials grafted with sensitive groups, and new composite materials are potential development hotspots of sensing materials. The chemiresistive sensor must be validated the interfering analytes in real environment for selectivity to avoid false responded and missed detection. New sensing materials will promote the further development of chemiresistive sensors in the field of detecting CWAs.

Funding information: The authors state no funding involved.
Author contributions: Liu Yang: writing – review and editing, Molin Qin: writing – original draft, Genwei Zhang: writing – original draft; Jie Yang: resources; Junchao Yang: resources; Jiang Zha: writing – review and editing.
Conflict of interest: The authors state no conflict of interest.

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Received: 2022-09-15

Revised: 2022-11-15

Accepted: 2022-11-25

Published Online: 2023-02-16

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/revac-2022-0052

Keywords for this article

chemical warfare agent simulants; chemiresistive sensor; sensing material; composite material; functional group modification; nanostructure

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