Schematic diagram of the fabrication of the urea sensor based on the MWCNT composite; (e) Optical image of the fabricated urea sensor. Graph (b) shows the response of the sensor to different gases such as acetone, methanol, ethanol, propanol, NO2, CO2 and CO.
Introduction
Overview
The excessive presence of environmental pollutants has negative effects on the environment and human health. Integration of nanomaterials with sensors has opened up a large scope of development towards miniaturized devices for various sensing applications.
Classification of sensors
- Chemiresistor sensor
- Chemicapacitor sensor
- Field effect transistor (FET) sensor
The specificity of the chemiresistor sensor for a particular analyte can be achieved by including appropriate functional groups on the surface of the nanomaterial. Various functional groups can be incorporated on the surface of the nanomaterial layer to improve the sensing performance of the chemicapacitor sensor.
Classification of sensing nanomaterials
- Carbon nanomaterials
- Metal based nanomaterials
Several metal oxide nanomaterials such as zinc oxide (ZnO), copper oxide (CuO), titanium dioxide (TiO2), tin oxide (SnO2), iron oxide (Fe2O3), indium oxide (In2O3), tungsten trioxide (WO3) and cobalt oxide (Co3O4) have been reported for various sensing applications [ 186-197]. In this regard, several transition metal dichalcogenides, including molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), molybdenum ditelluride (MoTe2), tungsten disulfide (WS2), tungsten diselenide (WSe2), and tungsten ditelluride (MoTe2) have been reported for various sensing applications [209–214].
Knowledge gap
Molybdenum disulfide (MoS2) is a typical transition metal dichalcogenide with n-type semiconducting behavior and exhibits excellent properties such as high mobility, high surface to mass ratio, tunable band gap, non-toxicity, excellent mechanical strength and so on [221-225]. Recently, hybrids of metal oxide and transition metal dichalcogenides as sensing materials have attracted the academics around the world for various applications, including electronic devices, energy storage, biosensing, gas and chemical sensing, and so on [226–234].
Objective of the Thesis
Organization of the Thesis
The sensing performance of the sensor is examined for different concentrations of urea and a possible sensing mechanism is proposed. The sensing performance of the MWCNT-PDDA composite is explored for CO gas and a possible sensing mechanism is proposed.
Fabrication of a carbon nanotube based non-enzymatic
Overview
However, excessive use of urea in agricultural fields leads to an indiscriminate increase in residual nitrate in the soil and causes soil pollution. Currently, the presence of urea derivatives on products for daily use is a major concern worldwide [22, 23]. The detection and monitoring of urea is of great importance in clinical analysis, food processing technology and dairy industries.
In addition, most of these techniques use urease as an enzyme, which catalyzes the hydrolysis of urea to ammonia (NH3) and carbon dioxide (CO2), but stability and storage of enzyme-based sensors are always a major concern. Various non-enzymatic sensors based on materials such as tin oxide (SnO2), nickel cobalt oxide (NiCo2O4) nanoneedles, nickel/cobalt oxide decorated graphene and polypyrrole/platinum electrode have been used for the detection of urea concentrations [35-38].
Experimental section
- Sensor fabrication
- Characterizations
The MWCNT surface was functionalized by oxidizing the MWCNTs, followed by attaching a thiol group to the MWCNT surface. The stability and reproducibility of the urea sensor were investigated for the feasibility of its commercial potential. Here, 2 mg of MWCNTs were dispersed in 8 ml of a mixed acid solution and treated with ultrasound for 3 hours.
The manufacturing process steps of the urea sensor are illustrated in Figure 2.1 (a)–(d), and the optical image of the urea sensor is shown in Figure 2.1 (e). The surface morphology of the materials was recorded using field emission scanning electron microscope (FESEM) (JEOL, JSM-7610F).
Results and discussion
- Material’s characteristics
- Sensing mechanism
- Sensing performance
- Milk quality assessment
The HRTEM image of S-AuNP is illustrated in Figure 2.3 (b), and the calculated interplanar distance is ~0.21 nm. In the present case, we use this concept for the development of the urea sensor. The calculated limit of detection (LOD) of the urea sensor was found to be 0.48 mg/dL.
The sensors were exposed to urea solutions (60 mg/dL) and the response of the sensor is shown in Figure 2.8 (b). The stability of the sensors was evaluated over two months and the response is shown in Figure 2.8(c).
Conclusions
Development of a chemiresistor with surface modified
Overview
The advantages of a nanomaterial chemiresistive sensor over conventional sensing techniques have been discussed in detail in the previous chapters. In this regard, various nanoengineered materials such as nanotubes, nanowires, nanoribbons, nanobands have been used to fabricate chemiresistive sensors for chemical and biosensing applications [1-6]. The prominent features of carbon nanotube (CNT) based sensors and different surface modification/functionalization approaches have been discussed in detail in the previous chapters.
In recent decades, various techniques have been used, such as electrochemical, thermoelectric, optical, potentiometric and resistive sensors for CO gas detection [22-25]. Of recent times, various nanocomposites based on CNT such as PANI-MWCNT, ZnO/MWCNT, Pt/MWCNTs, Pt-doped SWCNT have been used as sensing material for CO detection [33-36].
Experimental section
- Materials
- Preparation of MWCNT-PDDA composite
- Sensor fabrication
- Sensing mechanism
IDEs were patterned on glass substrates using a shadow mask, and Ag electrodes were thermally evaporated (Hind High Vacuum, Auto 500) onto glass substrates. The sample substrates were treated with UV-ozone for 10 minutes for better adhesion of the MWCNT-PDDA composite to the substrates. Typically, 10 µL of the MWCNT-PDDA composite solution was drop-cast onto the patterned substrate and dried at room temperature.
The remediation mechanism for CO sensing is based on the adsorption (physisorption) phenomenon of the CO molecule on the MWCNT-PDDA composite. Due to this charge transfer to the composite, the overall resistance of the sensor decreases.
Results and discussion
- Characterizations
- Response of CO sensor
- Adsorption-desorption kinetics
The transient response of the MWCNT-PDDA composite sensor was measured for various concentrations of CO gas ranging from 1 to 20 ppm. The response and recovery time of the sensor for 1 ppm CO gas is shown in Figure 3.5(c). The variations in response and recovery time of the MWCNT-PDDA composite sensor when exposed to different CO gas concentrations are shown in Figure 3.5(d).
The response of the sensor is measured by exposing the MWCNT-PDDA composite to various concentrations of CO gas and 20 ppm). The effect of humidity on the base resistance of the sensors is shown in Figure 3.7 (b).
Conclusions
A carbon monoxide sensor model 91 was also studied to understand the sensing mechanism of the CO gas sensor.
Synthesis of MoS 2 -CuO nanocomposite for room
Experimental section
- Materials
- Preparation of materials
- Characterization
- Sensor fabrication and measurement
The precipitates were thoroughly washed several times with deionized water and absolute ethanol until the pH of the solution became neutral. All electrical characterizations of the acetone sensor were performed with a Keithley 4200-SCS semiconductor parameter analyzer at room temperature. Next, MoS2-CuO nanocomposite was deposited on the channel area of the sensor by drop casting and dried at 60℃ for 1 hour.
The manufacturing process steps of the acetone sensor are illustrated in Figure 4.1(a)–(d), and the optical image of a fabricated acetone sensor is shown in Figure 4.1(e). The electrical response of the sensor was measured by exposing the sensor to target gas concentrations.
Results and discussion
- Morphological and structural characteristics
- Gas sensing performance
- Acetone sensing mechanism
Some important information about the synthesized CuO nanoparticle and MoS2-CuO nanocomposite can be obtained from the Raman spectra shown in Figure 4.4 (a). The shifts in the peaks can also be considered as confirmations of the formation of the nanocomposite (MoS2-CuO). The MoS2-CuO nanocomposite-based sensor was exposed to a wide range of acetone concentrations from 0.5 ppm to 10 ppm at room temperature, and the transient response of the acetone sensor is shown in Figure 4.6 (a).
It is observed that the response of the sensor increases with an increase in acetone gas concentration. The inset graph of Figure 4.6 (b) demonstrates the response of the sensor for low acetone concentrations in the range of 0.5–2 ppm.
Conclusions
Development of a paper based enzymatic chemiresistor
Overview
A charge transfer during the adsorption of the molecules to be observed in the functionalized CNTs causes a significant change in the electrical properties of the CNTs, which is translated for chemical and biosensing applications. Consumption of alcoholic beverages can increase the concentration of ethanol in exhaled air, which is an intoxicated state of health. A low-cost, portable and user-friendly device with fast response time is expected to specifically detect ethanol from human breath or air in the presence of other VOCs or gases.
The major challenge in the development of such sensors has been the significantly low concentrations of ethanol in human breath or air [32]. The interferences of other volatile organic materials were also tested to prove the selectivity and sensitivity of the sensor towards ethanol in the presence of a gas-vapor mixture.
Experimental section
- Materials
- Preparation of materials
- Sensor fabrication
Image (a) shows sensor fabrication steps, namely electrode deposition and drop-casting of MWCNT-PDDA-ADH composite as sensor material. Image (b) shows the top view of the fabricated sensor and image (c) shows the sensor connection with a source meter for measurement purposes. Typically, 10 µL of MWCNT-PDDA-ADH solution was deposited between two aluminum electrodes by drop casting.
The sensor was connected to a source meter (Model 2614B, Keithley, U.S.A.), as shown in Figure 5.1(c) and the initial resistance was measured without the sample solution. The variation in the resistance of the sensors was studied at different concentrations of ethanol solutions.
Results and discussion
- Sensor characteristics
- Sensor performance
- Surface potential measurements
- Breath analyzer
The limit of detection (LOD) of the sensor could be calculated using the equation. Surface potential (SP) measurement of the sensor was performed using Kelvin Probe Force Microscopy (KPFM) (Bruker, Innova series, USA) to understand the surface charge distribution on the channel before and after the alcohol vapor exposure. Alternatively, the average CPD of the surface after the exposure was found to be nearly - 52.4 mV.
These values also justified the variation of the surface potential after exposure to alcohol vapors. The position of the sample holder and the demonstration of the circuit after alcohol vapor exposure to the sensor are shown in Figure 5.7(b).
Conclusions
Summary
The sensing performance of the MWCNT-PDDA composite was determined for different concentrations of CO gas. The MoS2-CuO nanocomposite sensor was exposed to different concentrations of acetone gas at room temperature, and the response of the sensor was systematically investigated. The selectivity of the MoS2-CuO nanocomposite sensor was studied by exposing the sensor to various interfering gases and volatile organic compounds.
The sensing performance of the paper-based sensor was measured by exposing the sensor to various ethanol concentrations in the gas vapor. The interferences of other volatile organic compounds were also investigated to prove the selectivity and sensitivity of the sensor towards ethanol in the presence of a gas-vapor mixture.
Salient outcomes of the thesis
In addition, the variation of the resistance during the interaction between the sensor and ethanol was also characterized by measuring the surface potential of the channel material using atomic force microscopy. Furthermore, the sensor was integrated with an electronic circuit to develop a low-cost, portable, and user-friendly point-of-care (POC) ethanol detection in human breath with fast response and recovery times.
Future scope
The surface of MWCNTs can be customized with other suitable functional groups for the detection of various environmental pollutants such as NO, NO2, H2S, CO2, SO2 and other volatile organic compound gases. Metal-based nanomaterial heterojunction can be used to detect various volatile organic compounds apart from acetone detection with MoS2-CuO nanocomposite, as discussed. The disposable paper-based sensor can also be used for other biosensor applications by including suitable enzymes on the surface of the sensor material.
Growth of carbon-dotted ZnO nanorods on a graphite-coated paper substrate to fabricate a flexible and self-powered Schottky diode for UV sensing. Mandal, International Conference on Advances in Sustainable Research for Energy and Environmental Management (ASREEM-2021), SVNIT Surat, India, 2021.
