Materials for Agricultural Gas Sensors

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This entry is adapted from the peer-reviewed paper 10.3390/s21103423

A sensing material employed as a gas sensor will react with multiple gases, and for this reason, multiple sensing materials are employed in a network of gas sensors known as an electronic nose (eNose) system. By recording the response of this network of gas sensors, a signature which relates to the target analyte is detected, mitigating the issue of selectivity.

carbon nano-tube (CNT) sensors chemiresistive gas sensors fibre-optic gas sensors multi-walled carbon nanotubes (MWCNTs) polymers sensing materials volatile organic compound (VOC)

1. Introduction

A significant proportion of agricultural fruits and vegetables originates from greenhouses, with increasing frequency and a steady rise in harvest land area for crops produced via greenhouse methods ^[1]. One main reason for the significance of greenhouse practices is that they offer the advantage of year-round production, enabling financial stability for the grower ^[2]. During the phases of plant growth and storage in agricultural greenhouse environments, there are various volatiles that can affect growth quality and prolong the shelf life of crops, including ethylene, carbon dioxide (CO₂) hydrogen sulfide (H₂S), ethanol and water vapor (humidity) ^[3]^[4]^[5].

Because of the key roles played by these small molecules, it is important to monitor them in plant growth environments such as greenhouses, as well as in storage and transport environments. Monitoring of such volatiles can be achieved with sensors that typically require the use of a sensing material. According to B. Eggins, there are three general classifications of sensors containing sensing materials: (1) chemical sensors where the analyte interacts with the sensing material via chemical or physical responses, (2) physical sensors which measure a physical change such as length, weight and temperature and (3) biosensors, which utilize a biosensing element to measure chemical substances ^[6]. The chemical sensing technologies (including electrochemical sensors) often work by transforming gas concentrations into an electrical signal such as current (amperometric sensors), potential (voltammetric), resistance (chemoresistive sensors) and frequency response (capacitive sensors, acoustic sensors and thermal magnetic) ^[7]^[8]^[9].

Some commonly reported classes of sensing materials are metal oxides ^[10]^[11], polymers (conducting and non-conducting) ^[12]^[13], and carbon nanotubes (including other allotropes of carbon such as graphene) ^[14]^[15]. There are also reports of multiple classes used simultaneously, like metal oxide/CNTs composites ^[16], polymer/graphene composites ^[17] and less commonly used materials like metal-organic frameworks (MOFs) ^[18] and ionic liquids ^[19]. This review discusses the above-mentioned types of sensing materials in terms of their method of application onto a sensor, physical properties once applied and mechanism of operation for detecting relevant agricultural analytes such as ethylene, CO₂, ethanol, H₂S and water vapor. It is observed that some of these sensing materials demonstrate sensitivity and selectivity to certain target analytes, while others are responsive to multiple analytes present in an agricultural greenhouse environment.

2. Sensing Materials for Target Analyte Detection

2.1. Ethylene Detection

Many metal oxide materials have been developed and tested for the detection of ethylene. Li, Jin et al. reported the use of porous zinc oxide nanosheets (ZnO NS) as an ethylene sensing material to determine fruit ripeness ^[20]. Another notable metal oxide; commercially available tin oxide, SnO₂ nanoparticles, reported by Agarwal and colleagues, show the capability of detecting ethylene at 20 ppm levels with CO₂, SO₂, NH₃, NO₂, and H₂S, NH as an interfering gas at concentrations ranging from 1000–3000 ppm at room temperature ^[11]. In regards to carbon allotropes, Swager et al. used single-walled carbon nanotubes (SWCNTs) functionalized with 4-pyridyl moieties as a sensing material in monitoring the senescence in red carnation via the detection of trace levels of ethylene gas ^[21]. In a different class of viable materials, the use of commercially available ionic liquids (ILs), such as 1-butyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide ([BMIM][NTf₂]), is reported by Zevenbergen et al.

2.2. Carbon Dioxide Detection

A wide variety of metal oxide sensing materials have been reported for their use in CO₂ detection. One such sensing material under this class is reported by Karthik et al., who developed a Zinc oxide (ZnO) sensing material, synthesized by the thermal decomposition of precursors such as zinc acetate and zinc nitrate ^[22]. Another notable metal oxide for use in CO₂ detection is cerium oxide (CeO₂) nanospheres ^[23]. Karthik et al. coated a g-C₃N₄ nanosheet with TiO₂, forming a hybrid 2D sensing material for the purposes of CO₂ detection ^[24].

Baltrusaitis et al. reported a material under the polymer class; methylated poly(ethylene) imine (mPEI) for CO₂ detection, synthesized by previously reported work ^[13]. This polymer is also sensitive to sulfur dioxide (SO₂) detection, to which the material shows a lower sensory response. Among the viable ionic liquid sensing materials is 1-ethyl-3-methylimidazolium bis (trifluoromethyl-sulfonyl)-imide (EMIM[NTf₂]), which was investigated by Bhide et al. ^[25]. Wei et al. reported a rather exotic sensing material, which was a functionalized pillar[5]arene/bipyridine salt for the detection of CO₂ at a detection limit of 2.2 ppm ^[26].

2.3. Hydrogen Sulfide Detection

Under the metal oxide material class, Li et al. used indium oxide (In₂O₃) nano-cubes for sensing H₂S at room 25 °C and 100 °C ^[27]. Synthesis of this material involved a cetyltrimethyl ammonium bromide (CTAB)-assisted solvothermal and subsequent calcination process. This material has a very impressive 5 ppb limit of detection and interestingly, selectivity between NO₂ (also able to detect) and H $_{2}$ S sensing can be tuned using temperature (25 °C versus 100 °C). A viable material under the polymer material class—polyaniline/metal chloride nanofiber composites as sensing materials for H₂S detection—was reported by Virji et al. ^[28]. Among the carbon allotrope material class, Asad et al. reported single-walled carbon nanotubes (SWCNTs) modified with copper nanoparticles for H₂S detection ^[29].

2.4. Ethanol Detection

Among the metal oxide class, palladium/titanium oxide (Pd/TiO₂) nanorod arrays and tin sulfide (SnS) nanoflakes were reported by Dutta ^[30] and Afsar ^[31]. The Pd/TiO₂ nanorod arrays are also sensitive to 2-propanol and able to detect down to 1 ppm of theses alcohol vapors. Within the polymer material class, Yoon et al. used poly(styrene-co-allyl alcohol) (PSAA) as a sensing material (other materials also shown) in a wireless sensor to detect ethanol, which proved to be cross sensitive to acetone and ethylene ^[32]. Among ionic liquids, Xu et al. reported a viable material for the detection of ethanol are alkyl-imidazolium halide ^[33], which can be synthesized using simple solution synthetic methods, or can be procured commercially.

2.5. Humidity Detection

Within the metal oxide material glass, Zhang et al. reported a graphene oxide/polymer composite for humidity detection ^[10]. Shifting focus to the polymer sensing materials, Zhao et al. reported MWCNTs functionalized with poly-L-lysin (PLL) to be a viable sensing material for humidity detection ^[34]. Qi et al. demonstrated a material under the carbon allotrope material class known as chitosan-wrapped multi-walled carbon nanotubes (MWCNTs-CS) for detecting humidity ^[15]. Duan et al. reported Halloysite nanotubes as a sensing material for humidity which has a dynamic range of 0–91.5% RH ^[35]. For use in fibre-optic sensing applications, a wide range of sensing materials is available for humidity detection. This group of gas sensors employs several techniques, including fibre grating, evanescent wave monitoring, interferometric approach and absorption measurements, as well as hybrid sensors ^[36]^[37]^[38]^[39].

3. Deposition Methods

One of the challenges in sensor development for agricultural monitoring is to apply the developed sensing material to the active area of a sensor; this is often referred to as material deposition. In addition, since there are different mechanisms of operation for gas sensors due to their different structures, including capacitive and piezoelectric-based, QCM, chemiresistive and fibre-optic gas sensors, particular deposition techniques should be utilized to improve sensors’ performance in static and dynamic operations. Therefore, the sensor’s mechanism of operation, along with desired sensing material thickness and active area, which agree to the optimum sensor’s response point, can define the potential deposition technique. The most common deposition method observed using all the sensing materials mentioned in previous sections, is drop-coating. This technique is mostly used in chemiresistive gas sensors; obtaining a layer of few nanometers sensing material is not required ^[23]^[24]. Other common methods that are also suitable and utilized to deposit the aforementioned sensing materials include spin-coating, dip-coating, spraying, electro-spinning, and inkjet printing, as shown in Table 1. Gas sensors such as capacitive-based structures, which have a thick layer of sensing material, can have a negative impact on their operation benefit from the inkjet printing technique ^[40].

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Figure 1. Schematic illustration of deposition methods: (a) drop-coating, (b) dip-coating, (c) electro-spinning and (d) spraying.

Table 1. Material deposition methods, sensing technologies, sensor performance parameters and operating temperatures with various sensing materials and target analytes in gas phase.

Sensing Material	Target Analyte	Sensing Technology	Deposition Method	Material Thickness	Dynamic Range & Limit of Detection	Recovery Time	Operating Temperature	Long-Term Stability	Sensitivity (Output/Input)	Refs.

BMIM-NTf₂ $_{2}$	Ethylene	Amperometric	Drop-coating	63 µm	760 ppb–10 ppm	-	22 °C	-	51 pA/ppm	^[19]
Porous ZnO NS	Ethylene	Chemiresistive	Dip-coating	10 nm	5–2000 ppm	20 s	350–500 °C	30 days	0.6 µA/ppm	^[20]
LaFeO₃	Ethylene	Chemiresistive	Screen printing	37–38.3 µm	25–5000 ppm	~1 s	20–200 °C	-	0.4Ω/ppm	^[41]^[42]
SWCNTs	Ethylene	Chemiresistive	-	1 µL	0.5–50 ppm	-	4 °C	16 days	1.2%R/ppm	^[21]
SnO₂ nanoparticles	Ethylene	Chemicapacitive	Dip-coating/Sputtering	1300 nm	20–100 ppm	~10 s	22 °C	-	0.0531 pF/ppm	^[11]
PtTiO₂	Ethylene	Magnetoelastic	Dip-Coating	31–155 nm	0.5–50 ppm	-	19 °C	-	8.5 Hz/ppm	^[43]
ZnO	CO₂ $_{2}$	Chemiresistive	Spray pyrolysis	8.3 nm	50–1000 ppm	100 s	300 °C	-	800 Ω/ppm $Ω$	^[22]
PEDOT PSS/graphene	CO₂ $_{2}$ $_{2}$	Chemiresistive	Calibrated spreader	10 µm	4.7–4500 ppm	-	35–65 °C	-	0.004–0.0047%R/%RH	^[44]
TIO₂ coated g-C₃N₄ NS	CO₂ $_{2}$	Chemiresistive	Drop-coating	30 nm	100–2500 ppm	35 s	22 °C	60 days	406 µΩ/ppm $μ$	^[24]
CeO₂	CO₂ $_{2}$	Chemiresistive	Drop-coating	170–210 nm diam.	150–2400 ppm	~1 s	100–250 °C	-	4.88 kΩ/ppm	^[23]
EMIM[NTF₂]	CO₂ $_{2}$	Chemicapacitive	Dip-coating	<1 µm	50,000–1,000,000 ppm	38.5 s	Room temperature	-	29 pF/ppm	^[25]
HPTS	CO₂ $_{2}$	Fibre-Optic	Dip-coating	>1 µm	300–300,000 ppm	50–100 s	22 °C	-	0.00055 a.u./ppm	^[45]
mPEI	CO₂ $_{2}$	Resonator	Spin coating	-	0.011%	-	-	-	8 Hz/ppm	^[13]
CuO,Fe₂O₃ $_{2}$	H₂S	Amperometric	-	-	10ppm	-	−15 °C–65 °C	-	700 µA/ppm	^[46]
CNTs/SnO₂/CuO	H₂S	Chemiresistive	Spin-coating	>6 nm	10–80 ppm	10 min	25 °C	-	4.41Ω/ppm	^[16]
SnO₂ nanofibres	H₂S	Chemiresistive	Electro-spinning	150 nm diam.	0.1–1 ppm	230 s	200–350 °C	-	970kΩ/ppm	^[47]
Zn₂SnO₄ NS	H₂S	Chemiresistive	Dip-coating	100 nm	5–1000 ppb	1300 s	133–170 °C	60 days	1.08MΩ/ppb	^[48]
In₂O₃	H₂S	Chemiresistive	Dip-coating	100 um	5 ppb	5 min	25–100 °C	30 days	13.02 kΩ/ppm	^[27]
WO₃, PPy	H₂S	Chemiresistive	-	50–100 nm	200 ppm	>1 day	90 °C	-	490 µV/ppm	^[49]
SWCNTs	H₂S	Chemiresistive	Spin-coating	1–2 nm diam.	5 ppm–150 ppm	10–15 s	20 °C	-	0.47%R/ppm	^[29]
ZnO Nanowires	Ethanol	Chemiresistive	Spin-coating	25 nm diam.	1–200 ppm	120 s	300 °C	-	644Ω/ppm	^[50]
SnS	Ethanol	Chemiresistive	-	-	10 ppm	9 s	200 °C	6 weeks	0.27–13.5%R/ppm	^[31]
Pd/TiO₂	Ethanol	Chemicapacitive	Nanorod growth	710–750 nm	1–100 ppm	2.4–3.8 s	100 °C	-	7.5%C/ppm	^[30]
SiO2/Si NW	Ethanol	MGFET	vapor-liquid-sold growth	16 nm diam.	26–2000 ppm	4 min	60 °C	-	16–40 pA/ppm	^[51]^[52]
PSAA	Ethanol	Resonator	Drop-coating	19.9 nm	13.3 ppm	20 min	24 °C	-	1.5 Hz/ppm	^[53]
CuO particles	Water Vapor	Chemiresistive	Drop-coating	140 µm	33–90%RH	-	22 °C	-	0.5–30kΩ/%RH	^[54]
WS₂ NS	Water Vapor	Chemiresistive	Drop-coating	6 nm	8–85%RH	30–140 s	-	several weeks	580 MΩ/%RH $M$	^[55]
MWCNTs-CS	Water Vapor	Chemiresistive	-	-	11–95%RH	-	Room temperature	-	2.4 mΩ/%RH	^[34]
MWCNTs-PLL	Water Vapor	Chemiresistive	Drop-coating	-	0–91.5%RH	-	Room temperature	-	3.78kΩ/%RH	^[34]
MoS₂/ND	Water Vapor	Chemicapacitive	-	-	11–97%RH	-	Room temperature	-	6.5 nF/%RH	^[56]
SPEEK	Water Vapor	Impedance-based	Drop-coating	20 µm	11–95%RH	130 s	22 °C	30 days	12-120MΩ/%RH	^[57]
TiO₂ Nanowires	Water Vapor	Impedance-based	Dip-coating	40–50 nm	12–97%RH	<2 min	17–35 °C	250 days	144kΩ/%RH	^[58]
Silica/di-ureasil FBG	Water Vapor	Fibre-Optic	Dip-coating	450–591 µm	5–95%RH	-	5–40 °C	1 year	1.25–7.14 pm/%RH	^[59]
PI	Water Vapor	Fibre-Optic	Dip-coating	450–591 µm	5-95%RH	-	−15–20 °C	-	1.85–2.25 pm/%RH	^[60]
Al₂O₃⁺/PSS^- nano-film	Water Vapor	Fibre-Optic	ESA	84nm	22–39%RH	-	24.5 °C	-	1.43 nm/%RH	^[61]
SiO₂	Water Vapor	Fibre-Optic	ESA	300 nm	20–80%RH	150ms	10–40 °C	-	67.33–451.78 pm/%RH	^[62]
CaCl₂	Water Vapor	Fibre-Optic	-	3 µm	55–95%RH	-	30 °C	-	1.36 nm/%RH	^[63]
CoCl₂	Water Vapor	Fibre-Optic	Drop-coating	10 µm	50–95%RH	~40 s	25 °C	-	67–200 pm/%RH	^[64]
HEC/PVDF	Water Vapor	Fibre-Optic	Dip-impregnation	-	40–90%RH	-	28 °C	-	0.196 dB/%RH	^[65]
PAA Nanowires	Water Vapor	Fibre-Optic	Electrospinning	-	30–95%RH	210 ms	25 °C	-	0.01 dB/%RH	^[66]
ZnO Nanorods	Water Vapor	Fibre-Optic	Dip-coating	2.5 µm	10–95%RH	-	25 °C	-	0.0007–0.0057%P/%RH	^[67]
PVA	Water Vapor	Fibre-Optic	Dip-coating	8 µm	20–95%RH	500 ms	20–100 °C	7 days	25–980 pm/%RH	^[68]^[69]^[70]^[71]
PEO	Water Vapor	Fibre-Optic	Dip-coating	-	85–90%RH	~1 s	22 °C	-	1.17 dB/%RH	^[72]
Silica/methylene blue	Water Vapor	Fibre-Optic	Dip-coating	-	1.1-4.1%RH	<30 s	18 °C	-	0.0087 a.u./%RH	^[73]
Ag-Polyaniline	Water Vapor	Fibre-Optic	Dip-coating	15–30 nm diam.	5-95%RH	90s	25–30 °C	-	10–29 mV/%RH	^[74]
PGA/poly-lysine	Water Vapor	Fibre-Optic	Soaked in polymer	1 µm	50–92.9%RH	5.8 s	-	-	0.01 dBm/%RH	^[75]
ZnO	Water Vapor	Fibre-Optic	Dip/Spin-coating	70–80 nm diam.	5–50%RH	35 s	22 °C	-	0.45%dB/%RH	^[76]
Co/Polyaniline	Water Vapor	Fibre-Optic	Dip-coating	10.4 µm	20–92%RH	1 min	30 °C	-	0.024–3.406 mV/%RH	^[77]
Gelatin	Water Vapor	Fibre-Optic	Dip-coating	80 nm	9–94%RH	~50 s	22 °C	-	0.167 dBm/%RH	^[78]
Chitosan	Water Vapor	Fibre-Optic	Dip-coating	-	20–80%RH	-	25 °C	-	81 pm/%RH	^[79]

4. Conclusions

As new sensing materials and technologies continue to be developed for use in greenhouse environments, it will be essential to demonstrate their operation in representative environments that explore long-term stability and cross-sensitivity under realistic conditions. The rapid advances in sensing materials, morphology, and structure, as well as transduction mechanisms are expected to address current limitations in performance and are expected to enable miniaturized, low-power sensors capable of achieving wireless, distributed sensor networks for the continuous monitoring of agriculture environments. Further experimentation on the listed sensing materials should be implemented, recording the sensitivity of each material to their respective analyte over a long period of time to validate the usefulness of each material for greenhouse applications. Furthermore, the material’s solubility in water and sensitivity to elevated RH can help determine where the sensor ought to be located within the greenhouse.

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