Surface Functionalization of Layered Molybdenum Disulfide for the Selective Detection of Volatile Organic Compounds at Room Temperature.

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.

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)

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.

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.

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

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.

Carbon nanotubes-coated multi-transducing sensors for VOCs detection

Various metal oxide and conducting polymer-based sensors have been used in electronic nose systems for the detection of volatile organic compounds (VOCs). Some of the major constrains of these sensor materials are their sensitivity and selectivity. Recently, much research has been towards the development of biosensors that mimic biological olfactory mechanism. The sensing materials based on biological olfactory system can be well adapted to detect low concentrations of VOCs as it exhibits high selectivity/specificity, fast response time, high sensitivity, simplicity in fabrication, and are not toxic resulting from their biocompatibility and rapid biodegradability. A better understanding of human olfactory system can aid in the development of an artificial olfactory sensing system. The application domain of such sensors/systems could be potentially expanded for rapid and accurate detection of VOCs in environment and other biological media. Therefore, this paper provides a brief overview of the biology, associated mechanisms, and the functional principles of the olfactory sensing system in humans. Moreover, it summarizes the recent research on the development of an olfactory sensor utilizing olfactory receptors, odorant binding proteins and olfactory epithelium for the detection of different VOCs, and research needs in this area.

Chemiresistive gas sensors are essential for detecting harmful gases such as NO2, CO, H2S, and volatile organic compounds, as exposure to these gases poses severe risks to human health, including fatal outcomes. Prior to recent advancements and research in two-dimensional (2D) transitional metal dichalcogenides (TMDs), metal-oxide-based gas sensors were predominantly used.[1] However, their high operating temperatures and reliance on ionized oxygen present challenges for the development of low-power and miniaturized devices.As a promising alternative material, TMDs have garnered significant attention in the field of sensors for their high surface-to-volume ratio, enabling the sensitive detection of aforementioned gas species even at room temperature.[2] Despite its immense potential, intrinsic defects or vacancies on the TMD basal plane—where strong chemisorption of gas molecules occurs—hinder the recovery rate of TMD based gas sensors, compromising their reliability and long-term performance.[3] In this work, we use Atomic Layer Deposition (ALD) to selectively deposit and passivate defects and grain boundaries of WS2 and MoS2, resulting in a significant enhancement of the device’s recovery rate and longevity. We evaluate the gas-sensing properties of pristine and Al2O3-passivated MoS2 and WS2 when exposed to NH3 and NO2 gases at ppb levels. SEM analysis confirms selective nucleation and growth of Al2O3 on the defects and grain boundaries.As a result, the recovery rate of passivated sensors increased significantly from 74% (pristine sensors) to 96%. Additionally, Al₂O₃-passivated sensors demonstrated superior responsiveness to NO₂ gas compared to their pristine counterparts. The effects of ALD-deposited Al₂O₃ on the 2D TMD surface were characterized using Raman and photoelectron spectroscopy, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and field-emission scanning electron microscopy (FE-SEM).Our findings establish that passivation is a pivotal strategy for significantly enhancing the recovery performance and long-term reliability of 2D TMD-based gas sensors.[1] M. Swan et al., J. Sens. Actuator Networks 1 (2012) 217–253.[2] Lee, K et al., Adv. Mater. 25, 6699-6702 (2013).[3] H. Li et al., Small 8, 63–67 (2012). Figure 1

In this work, Hollow-core Optical Fibers (HOF) functionalized with Single Walled Carbon NanoTubes (SWCNTs) are proposed for volatile organic compounds (VOCs) detection. The sensing probe is composed by a piece of HOF with a termination coated and partially filled by SWCNTs. The infiltration of the SWCNTs inside the HOF holes has been accomplished by means of the Langmuir-Blodgett technique. Reflectance and far field transmission characteristics have been carried out within the HOF bandwidth. Finally the sensing capability of the proposed sensors has been investigated by exposure in a proper designed test chamber to traces of toluene. The experimental results obtained demonstrate the success of the SWCNTs partial filling within the HOF holes and the sensor capability to perform VOCs detection with a good sensitivity and fast response times.

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 United States Environmental Protection Agency (USEPA) Airborne Spectral Photometric Environmental Collection Technology (APSECT) program is the nation’s only airborne emergency response program with remote chemical and radiological detection capability. The goal of the ASPECT program is to provide real-time or near-real-time hazard information to first responders so they can respond appropriately to the specific hazard present. The instrumentation in the ASPECT aircraft includes a passive Fourier transform infrared (FTIR) spectrometer that is used for the detection of many volatile organic compounds (VOCs) that can be hazardous to the environment and human health. The high data acquisition rate required for the aircraft-based measurement and the need to provide timely information to first responders dictate that the data interpretation must be automated. This is accomplished through the application of numerical pattern recognition methods to the acquired data. The effectiveness of detection is based upon our ability to develop effective classification models that are specific to each analyte of interest. In the field, there is constantly changing background information in each scan. Unlike in traditional laboratory-based spectroscopy, we do not have the luxury of taking representative background spectra to use to remove background information from field-collected spectra. For this reason, analysis is performed directly on the FTIR interferograms, rather than on spectra. Selecting a segment from an optimized location offset from the interferogram centerburst is a strategy employed here to remove background information. This thesis work is focused on the improvement of ground resolution in the FTIR application. Because scans are taken as the aircraft flies, each interferogram point is representative of a slightly different spot on the ground. The ASPECT program has traditionally v implemented classifiers based upon using one segment from one side of each interferogram. Not only does this discard potentially useful information, but it also holds the assumption that what was viewed by the spectrometer at that time was representative of the area viewed throughout the entire scan. This research explores the possibility of using more than one segment from a single interferogram to improve the ground resolution of passive FTIR data. The work here focused on three analytes: methanol, ethanol, and ammonia. Laboratory data of these three analytes collected to mimic field conditions were used to complete studies of classification error of segments of lengths 25, 50, 75, 100, and 150 points spanning the entire length of each interferogram. This gave us a profile of where information valuable for classification resided in the length of the interferogram for each analyte. Exploration of how the signal-to-noise ratio affects classification error was also explored using these data. The research performed here shows the feasibility of improving ground resolution of FTIR data by using multiple interferogram segments. This methodology, once fully developed, could provide more useful information to first responders in their response to and mitigation of hazardous VOC releases.

Monitoring of volatile organic compounds (VOCs) is important in many fields. MXene, a class of two-dimensional materials, exhibits significant potential for high-sensitivity VOCs detection owing to its high specific surface area and abundant functional groups. However, MXene layers tend to restack or aggregate in practical applications due to van der Waals forces and hydrogen bonding, which leads to a decrease of surface area compared to theoretical performance. One-dimensional (1D) carbon nanotube (CNT)/two-dimensional (2D) MXene mixed-dimensional heterostructures have been proven to be effective in improving catalytic efficiency or battery performance. Therefore, in this study, CNT and MXene (Nb2CTx) are constructed as hybrids, which effectively reduced the restack of MXene layers, thereby increasing the surface area available for VOCs binding. The sensitivity of the hybrids to ethanol and acetone is 3.9 and 5.6 times higher than that of pristine Nb2CTx, respectively. In addition, the sensor based on Nb2CTx/CNT hybrids shows a higher response to a variety of VOCs and exhibits good stability. It is an effective method for VOCs detection to take advantage of the high specific surface area of low-dimensional materials by constructing mixed-dimensional heterostructures.