Advanced Micro- and Nano-Gas Sensor Technology: A Review
2. Electrochemical Sensors
2.1. MOS Gas Sensors
2.1.1. Structure and Mechanism
- MOS sensors for detection of VOCs: A MOS gas sensor in temperature cycled operation can be used to detect formaldehyde, benzene and naphthalene in ppb and sub-ppb concentrations with a varying background of ethanol with concentrations of up to 2 ppm . It has been shown that selective detection of VOCs in the ppb range is possible even with an intensive background of various VOCs. However, sensitivity is reduced compared to the ideal laboratory case and in the absence of background VOCs. For ppb-level VOC detection, MOS gas sensors are designed with temperature cycled operation, which can achieve sufficient sensitivity and selectivity in combination with signal analysis based on pattern recognition. In the temperature cycled operation mode, the heater unit of the sensor is periodically set to different temperature steps and therefore, the MOS sensing layer goes through various states resulting in different level of interaction characteristics between the sensing layer and the target analyte at those specific temperature range [12,13]. Employing the temperature cycled MOS sensor in combination with the pattern recognition techniques can correlate the sensor response to the present analyte in a complex environment by providing an additional identifying parameter. Temperature cycled MOS sensors with detection capability of 100 ppb of formaldehyde and 20 ppb of naphthalene have been reported .
- MOS-based sensors with porous sensing layer: To enhance the interaction with the analytes and increase the reaction surface area, the porous structural MOS can be employed that offer high porosity, highly interconnected pore channels, and high surface area and active sites. These porous MOS sensors can be chemically synthesized via soft-templating method and nano-casting strategy , which enhances the device performance through facilitating the gas diffusion and improves the sensor’s sensitivity, response and recovery time, and selectivity. The tunability of this synthesis approach provides a potential to develop porous MOS sensors with various compositions, where pore size, film thickness, temperature, and humidity are the factors that affect sensing performance .
2.2. Organic-Based Chemiresistive Gas Sensors
2.2.1. Structure and Mechanism
- Capped nano-particle sensors for VOCs detection: Molecularly capped nano-particles have been intensively investigated as the chemiresistor sensing materials to detect various VOCs. Chemiresistor arrays have been fabricated with cross-linked Au nano-particle thin films with subtle structural differences. These chemiresistive sensors detect VOCs and breath biomarker under ambient conditions. The nano-particle composition and size are the key parameters in designing the sensor array with desired sensitivity, selectivity, and stability. This sensor configuration has been used for breath recognition of lung cancer patients. Nano-structured sensor arrays have shown high sensitivity and selectivity in detecting mixtures of VOCs with a LOD as low as 20 ppb .
- Polypyrrole sensors for detection of explosive gases: Chemiresistor gas sensor based on sulfonated dye-doped modified conducting polypyrrole (PPy) film has been designed and fabricated for highly sensitive detection of 2,4,6-trinitrotoluene that is one of the most commonly used explosives. The sensor uses electro synthesis of PPy on Au-IDEs in the presence of the sulfonated dyes and exhibits high sensitivity and good selectivity to TNT with no response to other related gases and with a LOD and linear dynamic range of about 0.2 ppb and 10–800 ppb, respectively .
3. CNT Gas Sensors
3.1. Structure and Mechanism
- Chemiresistive gas sorption CNT sensors: In these sensors exposing CNTs to the target gas results in charge transfer between the CNT and the gas. This phenomenon results in a change in conductivity of the CNT sensing material. The change in the device conductivity is correlated with the gas properties and concentration. In these sensors, recovery time can be improved by heating the sensing film . The sensing property of sorption-based CNT gas sensors can be modified by using different chemical functional groups such as oxygen on the surface of CNTs where they can lead to selective interaction to desired analytes such as hydrogen-containing molecules. However, this can decrease the accessibility of the analytes to the CNT surface, hence reducing the sensitivity . Common disadvantages of these sensors are long recovery time, irreversibility of CNT conductivity and decreased sensitivity for low gas energy levels .
- Gas ionization CNT sensors: High aspect ratio of CNTs provides an ideal geometry for creation of an electrical field by applying voltage. In ionization CNT gas sensors, CNTs are used for both anode and cathode electrodes to create electric field . Analytes is ionized to be in plasma state by the accelerated electrons from the electrode. The ionization energy and the current though the plasma can be measured for identification of gas properties and concentration. This mechanism is useful for detection of gases with low sorption energy. Common gas ionization sensors are bulky with high energy consumption level; however, the use of CNTs can reduce the size and ionization energy of gas significantly due to the easier ionization enabled by CNT’s sharp tip structure and low work function [30,31].
- Capacitive-based and resonant frequency CNT sensors: CNTs can be used as the sensing element of capacitive-based gas sensors. In this structure, one plate of the capacitor is silicon and the other plate is made of is CNT coated silicon. By applying a voltage across the capacitor, CNT creates a high electric field which results in polarization of the gas molecules, and therefore, a change in the capacitance. The shift in the sensor’s capacitance is due to the dielectric constant change of CNT which is correlated with the target VOC concentration. This dielectric change of CNT sensor can also be used in a resonance frequency sensor configuration which measures the frequency shift associated with the gas properties and concentrations .
- Arc discharge technique: Since the synthesis temperature is above 1700 C, arc discharge technique has been reported to create less defects in the CNTs. In this method, fabrication is processed in helium, hydrogen, or methane-filled chamber containing graphite electrodes as shown in Figure 4. By applying the voltage, the electrode evaporates in gas and form CNTs on the other electrode [33,34].
- Pulsed laser ablation: PLA method has been used to produce SWCNTs of high quality and purity. The procedure is very similar to the arc discharge technique except that the energy is provided by a laser source. The laser is introduced onto the graphite layer which contains cobalt or other catalysts. A schematic diagram of PLA method is shown in Figure 5.
- Chemical vapor deposition: CVD has been proposed as an alternative to conventional CNT fabrication techniques due to its controllable process as well as high purity of its product.There are different CVD methods that have been used for CNT synthesis fabrication, such as catalyst CVD (CCVD), plasma enhanced CVD (PECVD) shown in Figure 6, radiofrequency CVD (RF-CVD), micro-wave plasma CVD (MPECVD) and water and oxygen assisted CVD. These days, CCVD and PECVD techniques are standard methods to synthesize CNTs, since they provide purer CNTs while using low temperature processes.
- Gas sorption CNT sensors for NH, NO and organic compounds detection: Monolayer CNTs have been used for detection of NH and NO in mass adsorption principle. Limit of 44 ppb and 262 ppb have been reported for detection of NO and nitrotoluene, respectively . Long recovery time up to 10 h is reported due to the very strong molecular bonds between some analytes and carbon, which is improved to 10 min by applying ultra-violet (UV) radiation to break bonds . In addition, CNTs have been used in sensors employing field effect transistors (FET) to improve selectivity between NO, CO, CO, O and H [35,36].
- Gas sorption CNT sensors for CH and HO detection: CNT sensors have difficulty in detecting gases such as CO, water vapor and bimolecular gas since they are limited to detect molecules with high bonding energy and ability to transfer charges to the nano-tubes. To overcome this disadvantage, various doping of CNTs have been proposed. In mass adsorption gas sensors, CNTs doped with nitrogen and boron are reported to detect CH, HO and NO in a low concentration at room temperature. Boron doped CNTs have good sensitivity for ethylene (CH), while nitrogen doped CNTs are sensitive to NO and CO .
4. Acoustic Gas Sensors
4.1. QCM Gas Sensors
4.1.1. Structure and Mechanism
- Calixarene coated QCM for VOCs detection: QCM sensors with calixarene coating or calixarene derivatives have been employed for detection of analytes such as alcohols, halocarbons, esters, ethers, chemical warfare agents, and toxic gases. Calixarene modified QCM sensors exhibit strong sensing ability to methylene chloride emissions with a LOD of about 54 ppm. The cyclic structures, hydrogen-bonding capabilities, and highly organized properties of the calixarene derivatives play a key role in VOC detection. However, QCM sensing properties are affected due to the random arrangement of calixarene derivatives’ molecules on the surface of the crystal .
- PANI-ES coated QCM for vapor detection: QCM can be fabricated based on dip coated polyaniline emeraldine (PANI-ES) salt thin films. In these devices, three different acids thin films such as hydrochloric acid (HCl), dodecylbenzene sulfonic acid (DBSA) and 1,5-naphtalene disulfonic acids (1,5-NDSA) are doped on the AT-cut 10 MHz QCM electrode. These sensors exhibit a frequency shift linear to both the vapor concentrations in part per million (ppm) and the sensing film thickness in nano-meter (nm). The frequency changes in these sensors are mainly due to the electrostatic interactions between the dopant agents within PANI-ES films and the vapor molecules. PANI-DBSA films show a highly sensitive of ~7 Hz/ppm and selectivity to para-xylene over toluene and benzene with a LOD of 3 ppm. They exhibit a relatively short recovery time of less than 3 min and an acceptable sensitivity in the presence of humidity interference .
- Ultrasensitive PPy-BPB and -4BP-based QCM for gas detection: Polypyrrole-bromophenol blue (PPy-BPB) nano-structure-based QCM has been developed for the detection of very small trace amounts of nitro-explosive vapors. This ultrasensitive and selective QCM sensor uses PPy-BPB compound in nano-sphere and nano-rod forms on the gold electrode. At room temperature the sensors are stable, reversible, and exhibit fast response time. PPy modified QCM sensors can be doped with various bromine containing anion dopants to improve the sensor performance. The enhanced sensitivity of this sensor towards nitro-explosives is related to the non-covalent interaction of halogen-nitro synthons between the bromine atoms and nitro-explosive groups as electron deficient acceptors as well as the partial charge transfer interaction between the nitro-explosive groups and electron rich polymer film .
4.2. SAW Device
4.2.1. Structure and Mechanism
- Polymer-based SAW for biomarker and VOCs detection: SAW gas sensors can employ various polymers as their sensing element that can react to different analytes such as biomarkers associated with lung cancer. However, many of these polymers respond to the presence of more than one analyte. The number of chosen sensing materials and their properties are designed according to the type of biomarkers that need to be identified. Pattern recognition and neural network techniques are employed to discriminate various chemical analytes by analyzing signal obtained from these sensors with different sensing material .
- Palladium- and CuPc-based SAW for hydrogen detection: Palladium has been used as the sensing material on a SAW sensor to detect hydrogen. Absorbing and desorbing hydrogen molecules result in a change in density and electrical conductivity of the sensing material. Copper phthalocyanine (CuPc) has also been used in SAW system for hydrogen detection. It has been shown that CuPc layer alone is not sensitive enough to hydrogen, which required high operation temperature of more than 70 C. This operating temperature can be lowered by using CuPc or Pd thin film as sensing layer down to room temperature. In this design, the change in the sensor’s output is mainly due to the change in the electrical conductivity rather than the mass change of the sensing layer .
4.3.1. Structure and Mechanism
- CMUT sensors for dimethyl methylphosphonate (DMMP) detection: Employing a very thin layer of DKAP polymer, CMUT sensors have been reported to detect DMMP, a simulant for sarin gas, with a good selectivity and a sensitivity of 48.8 zg/Hz/m . In addition, polyisobutylene (PIB) coated CMUT sensor is also reported to detect DMMP with a sensitivity of 130 zg/Hz/m  with a minimum LOD for DMMP of 16.8 pptv .
- CMUT sensors for carbon dioxide detection: CMUT sensors employing different materials such as polyimide, amine-bearing functional groups and quinidine can be fabricated as a highly sensitive CO detector. A CO sensitivity of 1.06 ppm/Hz at 50 MHz and a resolution of 4.9 ppm in the ambient temperature has been reported with consideration of other influencing parameters such cross sensitivity with water vapor, sensor repeatability and regeneration .
5. Optical Gas Sensors
5.1. Fiber-Optic Gas Sensors
5.1.1. Structure and Mechanism
- Cholesteric liquid crystal film coated fiber-optic sensors for VOCs detection: A cholesteric liquid crystal film (CLCF) coated side polished fiber (SPF) has been used for VOC sensing where an increase in VOC concentration on CLCF results in an increase in the pitch of the resultant light. This creates a blue shift of the resonant dips which can be correlated with the exposed VOCs. The sensitivities of the CLCFC-SPF have been reported to be 7.08 nm.L/mmol, 3.46 nm.L/mmol and 0.52 nm.L/mmol for tetrahydrofuran, acetone, and methanol gas, respectively, where the sensitivity of CLCFC-SPF increases with the molar mass of the VOCs .
- ZnO nano-particle-based fiber-optic gas sensor: ZnO nano-particles coated fiber-optic sensors have shown concentration selectivity dependency for acetone, ammonia, and ethanol. The ZnO nano-particles show good sensitivity towards ammonia at low concentrations up to 150 ppm and acetone at high concentrations above 150 ppm due to the enhanced catalytic reactivity of acetone at high concentrations .
- ZnO thin film-based fiber-optic gas sensors for carbon monoxide (CO) detection: Room temperature operating CO gas sensor using ZnO sensing film has been developed. The sensor operates based on the surface plasmon resonance (SPR) mechanism. This sensor has been reported to have a high sensitivity of 0.091/ppm and a fast response time of about 1 s towards a wide CO concentration range of 0.5–100 ppm at room temperature. These sensors are shown to selective towards CO, with a negligible interference with other gases such as NH, CO, NO, LPG and H. Therefore, ZnO thin film-based fiber-optic sensors have been shown as potential candidates for commercial applications of CO detection .
5.2. Photonic Crystal Gas Sensors
5.2.1. Structure and Mechanism
- Mesoporous Si-based photonic crystal gas sensors for VOC detection: Si-based photonic crystal have been researched to identify various VOCs by three differently sized patterns on one substrate. The patterns are etched on Si wafer by electrochemical anodization for mesopores with different sizes. The mesopores of 8 nm in diameter are produced in multilayer with 178, 229, and 300 nm of vertical spacings on Si substrate, which allows 430, 580, and 740 nm light to reflect. The analytes introduced onto this multi-layer-patterned PhC increases the effective refractive index and shifts the allowed wavelength of reflected light. Sensing performance of the device was tested with methanol, ethanol, and isopropanol in nitrogen carrier gas, which showed LOD in ppm range. In this method, analytes can be identified by measuring the wavelength shift gradient in time of each VOCs .
- Silica nano-sphere-based photonic crystal gas sensor: Self-assembled silica nano-spheres have been investigated to form photonic crystal for detection of water, ethanol and carbon disulfide (). The silica photonic crystal can be synthesized from silica colloid by drying the silica colloid on the substrate, followed by annealing at 600 C for sintering. This nano-structured silica can be coated with HKUST-1 to increase the interaction to analytes. The size of each silica nano-sphere is approximately 300 nm in diameter and arranged in face-centered cubic structure. Near infrared light can be introduced on  direction of silica nano-structure for gas-sensing test in the presence of analytes. Water, ethanol and tested for sensing performance have showed the response time at a few seconds with the estimated detection limit at 2.6 ppm for water, 0.3 ppm for ethanol and 0.5 ppm for .
Conflicts of Interest
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Nazemi, H.; Joseph, A.; Park, J.; Emadi, A. Advanced Micro- and Nano-Gas Sensor Technology: A Review. Sensors 2019, 19, 1285. https://doi.org/10.3390/s19061285
Nazemi H, Joseph A, Park J, Emadi A. Advanced Micro- and Nano-Gas Sensor Technology: A Review. Sensors. 2019; 19(6):1285. https://doi.org/10.3390/s19061285Chicago/Turabian Style
Nazemi, Haleh, Aashish Joseph, Jaewoo Park, and Arezoo Emadi. 2019. "Advanced Micro- and Nano-Gas Sensor Technology: A Review" Sensors 19, no. 6: 1285. https://doi.org/10.3390/s19061285