Surface Modification of SnO 2 Gas Sensors with WO 3 for Temperature-Dependent Selective Detection of Ethanol and Acetone

Detection of volatile organic compounds (VOCs) has attracted considerable attention for indoor air quality and human breath analysis. Here, resistive-type gas sensors with porous films made of Au-loaded TiO2 nanotubes were developed for the detection of large-sized VOCs. Well-dispersed Au nanoparticles (10 to 20 nm) were photo-chemically deposited on TiO2 nanotubes (80 nm in diameter, 700 nm in length) prepared by a hydrothermal method. The device using the Au-loaded TiO2 nanotubes showed improved sensor responses to ethanol and toluene (50 ppm) in air at 500°C. This suggested that Au nanoparticles deposited on the surface of TiO2 nanotubules without aggregation. We also demonstrated that the device could be used for the detection of large-sized VOC molecule, i.e., 2,6-diisopropylphenol (propofol), an intravenously administered hypnotic drug for induction and maintenance of anesthesia. The Au-loaded TiO2 nanotubes would offer a way to continuously monitor a change in the concentration of VOCs in exhaled air for medical diagnostics.

The widespread application of SnO2-based semiconductor gas sensors stems from their exceptional sensitivity and stability in detecting various gases, including volatile organic compounds (VOCs). The fundamental mechanism involves the interaction between the semiconductor material's surface and oxidizing or reducing gases, resulting in a change in electrical resistance. Additionally, the receptor function stands out as a pivotal factor influencing the sensing performance of chemical sensors [1]. Modification of the surface of SnO2 with acidic or basic oxides has been shown to effectively alter its catalytic properties, thereby changing the reaction pathways during ethanol oxidation. This study focuses on an acidic oxide, WO3, which is a promising candidate receptor for improving gas selectivity due to its property of not adsorbing oxygen as well as its high sensitivity to VOC gases. Our previous results demonstrated that similar ethanol consumption rates are observed between neat SnO2 and WO3-loaded SnO2 within the range of 150 to 350℃ during catalytic combustion. Over neat SnO2, dehydrogenation is primarily observed, resulting in CH3CHO being the main product at 150℃, with peak production occurring at 250℃, gradually transitioning to CO2. Furthermore, CO2 generation is initiated at 250℃ and rapidly increases at higher temperatures, attributed to the high catalytic activity of SnO2. Conversely, loading WO3 onto the surface of SnO2 successfully modifies the reaction pathway of ethanol oxidation, with the primary reaction shifting from dehydrogenation to dehydration and selectively generating C2H4 at temperature exceeding 300℃. When materials synthesized by different compositing methods are applied in thick film sensors, the interaction between WO3 and SnO2 significantly influences their sensing properties, with the fine dispersion of WO3 playing a crucial role. Both WO3-loaded and mixed SnO2 exhibited higher responses compared to neat SnO2, with the loaded group showing the highest response to ethanol and acetone. Additionally, 3 mol% WO3-loaded SnO2 demonstrated selective detection properties, with the highest response to 20 ppm acetone at 250℃, which was 22.8 times higher than that of neat SnO2.Efforts have also been made to enhance the gas sensing properties of VOC gases by integrating thermal modulation techniques into miniaturized gas sensors, known as MEMS sensors. These sensors enable operation in a pulse-driven mode [2,3], where the microheater is capable of rapidly heating and cooling the sensor device. Such a driving mode offers selectivity due to the gas adsorption and combustion properties occurring during the heating and cooling phases. By utilizing the pulse-driven mode, it is suggested that during the low-temperature heat-off phase, VOC gases adsorb to the surface of particles, followed by their combustion during the high-temperature heat-on phase. This gas detection model facilitates enhancements in both sensitivity and selectivity, capitalizing on the gas adsorption, diffusion, and catalytic combustion properties on the particle surface. Such developments offer significant improvements in gas detection based on catalytic combustion properties. These insights are crucial for the development of high-performance volatile organic compound gas sensor materials.

Semiconducting two-dimensional (2D) transition-metal dichalcogenides (TMDCs) are considered promising sensing materials due to the high surface-to-volume ratio and active sensing sites. However, the reported strategies for 2D TMDCs toward sensing of volatile organic compounds (VOCs) present with some drawbacks. These include high operation temperatures, low gas response, and complex fabrication, limiting the development of room-temperature gas sensors. In this study, 2D MoS2 nanoflakes were prepared by liquid-phase exfoliation, and their surface was functionalized with Au nanoparticles (NPs) through a facile solution mixing method. MoS2 decorated with Au NPs with an average size of 10 nm was used as a material platform for an electrochemical sensor to detect a wide variety of VOCs at room temperature. Through dynamic sensing tests, the enhancement of gas-sensing performance in terms of response and selectivity, especially in detecting oxygen-based VOCs (acetone, ethanol, and 2-propanol), was demonstrated. After Au functionalization, the response of the gas sensor to acetone improved by 131% (changing from 13.7% for pristine MoS2 to 31.6% for MoS2-Au(0.5)). Sensing tests under various relative humidity values (10-80%), bending or long-term conditions, indicated the sound robustness and flexibility of the sensor. Density functional theory simulations suggested that the adsorption energy of VOC molecules on MoS2-Au is significantly higher than that on pristine MoS2, contributing to the gas-sensing enhancement; a VOC-sensing mechanism for Au-decorated MoS2 nanoflakes was proposed for the first time for the highly sensitive and selective detection of oxygen-based VOCs.

The emergence of MXene, a group of two-dimensional materials, has shown its potential to be used in diverse fields especially in energy storage since 2011. Being inspired by the intriguing properties of MXene used for energy storage, such as the high electrical conductivity and the ability to accommodate ions in the interlayer space with controllable manner, the possibility to accommodate the volatile organic compounds (VOCs) in the interlayer space and to release the entrapped VOCs with controllable approaches have drawn our attention for the potential application on the controlled release of odors. Additionally, due to the abundant termination groups on the surface of MXene, it could also be the promising candidate for the detection of VOCs. The surface functional groups on MXene could not only serve as the active sites for the interaction between MXene and VOC molecules but also provide the potential anchoring points for the surface functionalization, endowing diverse surface properties by the additional termination groups. In this thesis, Ti3C2 MXene is selected to be the main material for the studies of VOC controlled release and detection due to its superior electrical conductivity, versatile surface functional groups and better thermal stability among the MXene materials. With the polar functional group on the pristine Ti3C2 MXene surface, Ti3C2 MXene is first applied to perform the controlled release of polar aroma VOC. The surface of the Ti3C2 MXene is also functionalized with non-polar groups to improve the adsorption of the non-polar VOCs. With the improved adsorption of non-polar VOCs by surface functionalized Ti3C2 MXene, the modified MXene is further applied for the detection of the non-polar VOC. The controlled release of the aroma VOC is monitored by the ex-situ and in-situ X-ray diffraction, which show the significant change of the interlayer distance during the adsorption and desorption of the VOCs. The adsorption mechanism is also proved by the X-ray photoelectron microscopy. The modified Ti3C2 MXene also shows the improve sensing specificity towards the non-polar VOC. These findings offer the insight of Ti3C2 MXene as a potential material used for the controlled release of aroma VOCs and the detection of VOCs.

For field-like environmentalgas monitoring and noninvasive illnessdiagnostics, effective sensing materials with exceptional sensingcapabilities of sensitive, quick detection of volatile organic compounds(VOCs) are required. Carbon-based nanomaterials (CNMs), like CNTs,graphene, carbon dots (Cdots), and others, have recently drawn a lotof interest for their future application as an elevated-performancesensor for the detection of VOCs. CNMs have a greater potential fordeveloping selective sensors that target VOCs due to their tunablechemical and surface properties. Additionally, the mechanical versatilityof CNMs enables the development of novel gas sensors and places themahead of other sensing materials for wearable applications. An overviewof the latest advancements in the study of CNM-based sensors is givenin this comprehensive organized review.

Analytical evaluation of polymeric CNTs/CuO nanocomposite electrode for the room temperature detection of volatile organic compounds (VOCs)

Detection of volatile organic compounds (VOCs) in human urine has potential application value in screening for disease and toxin exposure. However, the current technologies are too slow to detect the concentration of VOCs in fresh urine. In this study, we developed a novel ultrasonic nebulization extraction proton transfer reaction mass spectrometry (UNE-PTR-MS) technology. The urinary VOCs can be rapidly extracted to gaseous VOCs using the UNE system and then delivered using a carrier gas to the PTR-MS instrument for rapid detection. The carrier gas flow and sample size were optimized to 100 mL/min and 100 μL, respectively. The limits of detection (LODs) and response time of the UNE-PTR-MS were evaluated by detecting three VOCs that are common in human urine: methanol, acetaldehyde, and acetone. The LODs determined for methanol (4.47 μg/L), acetaldehyde (1.98 μg/L), and acetone (3.47 μg/L) are 2-3 orders of magnitude lower than the mean concentrations of that in healthy human urine. The response time of the UNE-PTR-MS is 34 s and only 0.66 mL of urine is required for a full scan. The repeatability of this UNE-PTR-MS was evaluated, and the relative standard deviations of 5 independent determinations were between 4.62% and 5.21%. Lastly, the UNE-PTR-MS was applied for detection of methanol, acetaldehyde, and acetone in real human urine to test matrix effects, yielding relative recoveries of between 88.39% and 94.54%. These results indicate the UNE-PTR-MS can be used for the rapid detection of VOCs in a drop of urine and has practical potential for diagnosing disease or toxin exposure.

Metal-organic frameworks (MOFs): A review of volatile organic compounds (VOCs) detection.

Volatile organic compounds (VOCs) serve as critical biomarkers in exhaled breath for early-stage cancer patients, and their rapid, trace-level detection holds marked implications for cancer screening. Surface-enhanced Raman scattering (SERS) technology demonstrates strong potential for trace VOC gas detection due to its ultra-high sensitivity and immunity to water interference. However, while surface plasmon resonance (SPR)-free semiconductor substrates offer superior spectral stability and selectivity, their sensitivity toward VOC detection remains suboptimal. This study introduces a novel semiconductor-based SERS substrate composed of copper single atoms anchored on UiO-66 (Cu1/UiO-66), achieving a record-low detection limit of 10 parts per billion for VOC gases with a rapid 2-min response time, thereby elevating the gas-sensing performance of SPR-free substrates to unprecedented levels. The exceptional SERS activity originates from the highly delocalized electron properties of single-atomic copper, which effectively facilitates single-atom charge transfer processes. Concurrently, the incorporation of copper single atoms modulates the band structure of UiO-66, substantially enhancing the coupling resonance between the substrate and target molecules. In simulated breath tests mimicking lung cancer patients’ exhalations, Cu1/UiO-66 exhibits remarkable VOC recognition capability and robust anti-interference performance. This work pioneers a new paradigm for ultra-sensitive, rapid detection of trace VOCs in exhaled breath, holding substantial promise for early cancer diagnostics and clinical translation.

High-performance virtual sensors array based on a single-chip FBAR for volatile organic compounds (VOCs) detection and classification

We have reported the sensing characteristics of an yttria-stabilized zirconia (YSZ)-based gas sensor using NiO sensing-electrode (SE), aiming at selective detection of indoor volatile organic compounds (VOCs) at ppb levels. However, the detection of toluene (a typical VOCs) is strongly affected by the presence of ethanol which is a common interfering gas in indoor environments. To overcome this issue, both an SnO2 catalyst layer and an Al2O3 gas-diffusion barrier layer were applied on the surface and ends of the NiO-SE, respectively. The barrier layer can facilitate effective ethanol oxidation through the SnO2 catalyst layer by hindering direct penetration of ethanol into the NiO-SE layer. As a result, the fabricated sensor was found to be capable of detecting toluene in several tens ppb level, with low interference from high ethanol concentrations. In addition, the sensor showed selective responses toward aromatic VOCs rather than aldehyde. This behavior was confirmed to be due to the facilitated catalytic oxidation of aldehyde in the SnO2 layer. Furthermore, the present sensor exhibited stable sensitivity and selectivity toward 50 ppb toluene after the 13 days operation examined.

Gas sensors provide versatile opportunities for detecting volatile organic compounds (VOCs) such as acetone, methanol, ethanol, propanol, isoprene, and aldehydes in exhaled breath (EB) associated with COVID-19 respiratory infections. These VOCs provide valuable information about metabolic markers linked with COVID-19. They have opened opportunities to develop sensors for COVID-19 screening based on breath analysis. These sensors have the potential to provide the rapid detection of viruses in healthcare settings. RT-PCR, as a conventionally adopted diagnostic method, has a detection limit around 10–100 RNA copies/mL, with an accuracy of around 95%. Gas sensors have demonstrated VOC detection limits at the ppm level in COVID-19 EB and have displayed a sensitivity and specificity of 98.2% and 74.3%, respectively. Multiple gas sensors combined with machine learning algorithms have the potential to enhance the specificity of VOC detection. In addition to having an accuracy similar to that of the PCR method, the VOC-based diagnosis of COVID-19 offers unique advantages in terms of non-invasive and rapid detection. This review provides an overview of state-of-the-art gas sensors developed for COVID-19 detection. Despite there being significant developments in this field, there are certain challenges that still need to be addressed—these include the impact of environmental factors, the specificity of detection, the sensing range, and precision limitations, leading to accuracy issues. Despite these existing challenges, the integration of gas sensors with machine learning methods can enhance the accuracy of the detection of COVID-19. Future research directions are proposed to validate and standardize the application of gas sensors for COVID-19 in clinical settings.

Although the development of gas sensing materials for room temperature operation is of great importance for environmental management, computational studies usually overlook the effectiveness of the adsorbent under realistic conditions. The FePS3 monolayer has recently been synthesized and recognized as a novel two-dimensional (2D) material with favorable electrical properties, but its potential for gas-sensing applications remains unexplored. In this study, first-principles calculations are employed to systematically investigate the feasibility of pristine and CuO-modified FePS3 monolayers for the detection of volatile organic compounds (VOCs). Relative energy is employed to evaluate the adsorption effectiveness of VOCs at different temperatures. The results demonstrate that VOCs are weakly physisorbed on the pristine FePS3 monolayer, resulting in poor adsorption stability. The introduction of CuO strengthens the adsorbate-substrate interactions by enhancing orbital hybridization and charge transfer between VOCs and the FePS3 monolayer. At room temperature, the CuO-modified FePS3 monolayer can stably capture VOCs and achieve repeated detection of CHCl3, CH3Cl, and CH2O with short recovery times. Moreover, the CuO-modified FePS3 monolayer exhibits high sensitivity and excellent selectivity toward VOCs, enabling its use as a high-performance resistance-type gas sensor. Our work reveals the potential of the FePS3 monolayer for gas sensing applications and promotes the design of gas sensors based on 2D metal phosphorus chalcogenides.

The presence of some volatile organic compounds (VOC) such as: ketone, alcohols, affect agriculture, blood and health. Some of these VOC can cause respiratory diseases such as asthma, irritate eyes and causes allergies. In this paper we presented the design and implementation of an unmanned mobile device for detection of VOC leaks. The proposed system has an emitter and a receiver. The transmitter has two systems embedded in a two 8-bit microcontroller, one for the control and decision making of a robot, which is equipped with proximity and line following sensors. The second embedded system is used for VOC monitoring, through commercial gas sensors, which have a detection range of 10 to 1000 parts per million (ppm.) In addition, in this embedded system the monitoring of the battery lifetime and the distance traveled by the robot is carried out, the information is processed and sent through a bidirectional radiofrequency antenna (Nrf24L01). The receiver has a system embedded in a microcontroller and a bidirectional Nrf24L01, the data is received and sent to a personal computer through the serial port. The data is processed, stored and presented with the help of a graphical user interface. This shows the status of the two robot motors, the battery charge, the operating time and the distance traveled. Also, the status of the gas sensors is observed, if one of them detects the presence of a gas, it is stored in the register and activates an alarm to complete the detection of one.

Direct characterization and quantification of volatile organic compounds by piezoelectric module chips sensor