Volatile Organic Compounds Sensor Research Articles

Abaxial leaf surface-mounted multimodal wearable sensor for continuous plant physiology monitoring

Wearable plant sensors hold tremendous potential for smart agriculture. We report a lower leaf surface-attached multimodal wearable sensor for continuous monitoring of plant physiology by tracking both biochemical and biophysical signals of the plant and its microenvironment. Sensors for detecting volatile organic compounds (VOCs), temperature, and humidity are integrated into a single platform. The abaxial leaf attachment position is selected on the basis of the stomata density to improve the sensor signal strength. This versatile platform enables various stress monitoring applications, ranging from tracking plant water loss to early detection of plant pathogens. A machine learning model was also developed to analyze multichannel sensor data for quantitative detection of tomato spotted wilt virus as early as 4 days after inoculation. The model also evaluates different sensor combinations for early disease detection and predicts that minimally three sensors are required including the VOC sensors.

Selective Sensing of Volatile Organic Compounds Using an Electrostatically Formed Nanowire Sensor Based on Automatic Machine Learning.

With the development of Internet of Things technology, various sensors are under intense development. Electrostatically formed nanowire (EFN) gas sensors are multigate Si sensors based on CMOS technology and have the unique advantages of ultralow power consumption and very large-scale integration (VLSI) compatibility for mass production. In order to achieve selectivity, machine learning is required to accurately identify the detected gas. In this work, we introduce automatic learning technology, by which the common algorithms are sorted and applied to the EFN gas sensor. The advantages and disadvantages of the top four tree-based model algorithms are discussed, and the unilateral training models are ensembled to further improve the accuracy of the algorithm. The analyses of two groups of experiments show that the CatBoost algorithm has the highest evaluation index. In addition, the feature importance of the classification is analyzed from the physical meaning of electrostatically formed nanowire dimensions, paving the way for model fusion and mechanism exploration.

High performance of p-p heterojunction LaFeO3/YFeO3 planar electrode sensor for volatile organic compounds under multi-wavelength light illumination

The challenge encountered by the sensor in practical application is that the optimal working temperature is high, which will cause certain dangers in the work. Herein, we fabricated a p-p heterojunction composite LaFeO3/YFeO3 planar electrode sensor for gas detection. The planar electrode sensor adopts the method of direct heating, which reduces the optimal working temperature of the sensor compared with the traditional side-heated alumina ceramic tube sensor. The optimal operating temperature of the LaFeO3/YFeO3 (0.8:0.2) planar electrode sensor is 110 °C. In addition, we also performed multi-wavelength (470, 420, and 370 nm) light illumination experiments on the LaFeO3/YFeO3 sensor to improve gas response. Under 370 nm light illumination, the response values of the LaFeO3/YFeO3 (0.8:0.2) sensor to 30 ppm methanol, acetone, and ethanol were 166.82, 199.58, and 292.67, which were 1.19, 1.33, and 1.36 times than those in the dark. The lowest detection limits of the LaFeO3/YFeO3 (0.8:0.2) sensor under 370 nm light illumination for methanol, acetone, and ethanol were 0.3, 0.8, and 0.2 ppm, respectively. Therefore, the LaFeO3/YFeO3 (0.8:0.2) planar electrode sensor has the advantages of small size, low optimal operating temperature, low power consumption, high response value to volatile organic compounds gases, and high-cost performance for use in different gas environments.

Wearable electrochemical gas sensor for methanol leakage detection

To ensure worker and environmental safety in daily work, it is an urgent need in industrial safety field to construct wearable sensing devices for volatile organic compounds (VOCs) leakage detection in chemical production. In this work, methanol gas was selected as the research object, and the wearable gas sensor has been constructed on a flexible substrate for systematically investigating the sensing process and performance, as well as practicality of leak detection. Electrochemical measurements demonstrated the constructed sensor possessed obvious electrocatalytic capability at 0.6 V, good linearity in the detection range of 20–100 ppm, 200–3000 ppm and 4000–30,000 ppm, and low limit of detection (3.3 ppm). This methanol gas sensor also exhibited high reproducibility, lifetime, and stable sensitivity according to the continuous repeated and long-time stability measurement results. High gas-sensitive selectivity to methanol under multiple interference gases, such as acetone, toluene, xylene, methylene chloride, acetonitrile had been demonstrated, and the response change under selectivity test was less than 16%. Significantly, it also displayed detectability in actual methanol gasoline sample, and the response time was less than 16 s. Moreover, experimental results also have proved higher content of platinum (20%) with better theoretical CO-tolerance capacity and sensing sensitivity. When maintaining a platinum content of 20%, the introduction of support materials with high specific surface area and conductivity in place of activated carbon will ensure that all sites are catalytically active, further improve the sensing sensitivity and be extended to the design of other VOCs sensors.

MOFTextile: Metal‐organic frameworks nanosheets incorporated cotton textile for selective vapochromic sensing and capture of pyridine

Metal–organic frameworks (MOFs) improved several trends and are promising for industrial applications. However, current synthesis processes offer powder form, rendering their applications difficult. A simple solvothermal method offered an in situ growth of copper‐based MOFs, for example, CuBDC (BDC: benzene‐1,4‐dicarboxylic acid) into a cotton textile; the material was denoted as CuBDC@Textile. CuBDCTextile was used as a solid sensor and adsorbent for volatile organic compounds (VOCs). It exhibited good vapochromic properties that enabled a colorimetric detection of pyridine (Py) via naked eyes with high selectivity and good sensitivity. Adsorption of pyridine via pervaporation using CuBDC@Textile was recorded. CuBDCTextile is a flexible textile with a high adsorption capacity (137.9 mg g−1) toward pyridine. It offered dual functional: sensor probe and adsorbent. The synthesis of CuBDC@Textile and their excellent performance as a sensor and adsorbent are promising for further investigation of the “MOFs on textile materials” topic.

Textural landscapes of VOC-sensitive chiral liquid crystal-based materials

Liquid crystal-based materials, in which liquid crystal molecules are confined and ordered in compartments, are dynamic materials yielding a variety of optical textures that can be tuned as a response to physical and chemical stimuli. While nematic and smectic-based gel materials have been reported as dynamic optical sensors to report volatile organic compounds (VOCs), chiral systems are less explored despite having the potential to yield extremely rich optical landscapes. Here, we report for the first time the confinement of chiral liquid crystal formulations by an interface formed by ionic liquid molecules. The resultant self-assembled ionic liquid/liquid crystal droplets are simultaneously immobilized on a gelatin matrix. The droplets feature a rich variety of unique topological states. We explored, by means of polarizing optical microscopy, the various droplet optical textures and categorized them with regard to their relative chirality parameter. We further investigated their optical response in the presence of gas analytes and discussed their potential utilization as dynamic liquid crystal-based optical VOC sensors. The newly generated soft materials with semi-selective VOC sensing capabilities can be further utilized in arrays of liquid crystal-based gas sensors for the analysis of complex gas samples using artificial olfaction approaches.

Continuous monitoring of volatile organic compounds through sensorization. Automatic sampling during pollution/odour/nuisance episodic events

Volatile organic compounds (VOCs) are a highly diverse class of chemical contaminants and between 50 and 300 of them may be found in ambient air. In urbanized areas, VOCs are emitted from industrial activities, as well as from vehicle-related and combustion sources. VOCs outdoors can be detected in a broad range of concentrations, usually varying seasonally. The presence of VOCs at relatively high concentrations has been related to poor air quality, discomfort and odorous nuisances. Additionally, they can have negative health effects to the human organism. Hence, in locations where recurrent sporadic situations of high VOCs levels take place, episodic samples' evaluation is necessary instead of 24 h or longer sampling period's evaluations. The use of commercially available metal oxide semiconductor gas sensors for a continuous monitoring of VOCs concentrations in outdoor air is an interesting and innovative technology. Additionally, the use of these sensors for the activation of a VOCs sampler when episodic events of nuisance/odorous annoyance occur was successfully evaluated. The sensor activation is induced by higher VOCs concentrations from a wide number of VOC chemical families. Two sensor stations, developed at our laboratory and provided with sampling pumps, were located in the municipality of Santa Margarida i els Monjos (Catalunya, Spain) in January 2021. The stations started recording data continuously from two different types of VOCs sensors, temperature, relative humidity and pressure in 1.5-min periods. Automatic VOCs sampling was conducted, using multi-sorbent bed tubes, during the months of June–July when the sensors electronic values exceeded a set point value. Samples were analysed through TD-GC/MS. TVOC concentrations in episode samples ranged between 78-669 and 12–159 μg m−3 in Site 1 and Site 2, respectively. Although TVOC concentrations were not high in all cases, relevant concentrations of chloroform were observed, especially in Site 1, with concentrations ranging from 19 to 159 μg m−3.

Arrays of Functionalized Graphene Chemiresistors for Selective Sensing of Volatile Organic Compounds

Real-time detection of volatile organic compounds (VOCs) is being spotlighted for its importance in environmental monitoring and disease detection from the breath. However, the selective detection of VOCs in a small form factor gas sensor remains a challenge. Here, we present the VOC sensing performance of phthalocyanine (Pc)-functionalized graphene chemiresistors supported by the theoretical prediction of Pc’s VOC interactions. Pc functionalization, with and without metalation, was studied parametrically with varying deposition parameters to explore impacts on the gas sensing response toward VOCs as a function of substrate coverage. Sensing data were collected to establish fingerprint signatures for a variety of VOCs. Improvement in selectivity is shown by successfully differentiating the VOCs from each other when combined into a gas sensor array tested with acetone, ethanol, formaldehyde, and toluene.

Nanofibers as optical sensors for model VOCs dispersed in aqueous phase

AbstractNanofibers mats, prepared from poly(vinyl chloride) (PVC) containing dispersed dye Nile red (NR) were applied in a proof of concept study as optical sensors for Volatile Organic Compounds (VOCs) dispersed in an aqueous phase. Benefiting from the solubility of the dye, and in some cases, also of the polymer in model solvents belonging to the group of VOCs, an increase of emission was observed for increasing solvent concentration in the sample. The optical signal formation was observed regardless if only the dye or both dye and PVC were soluble in the tested solvent. In both cases, high sensitivity emission increases for increasing VOCs present in the aqueous phase were observed within the range of concentration of model analytes: from 200 ppm of m‐xylene or from 300 ppm of styrene, to up to ca 1500 ppm. The obtained higher detection limit was lower compared to films of PVC containing the dye due to the lower availability of the material to be dissolved by analyte – solvent. The large surface area of nanofibers was useful in the detection, leading to higher signal changes compared to films.

Regulatory Significance of Plastic Manufacturing Air Pollution Discharged into Terrestrial Environments and Real-Time Sensing Challenges.

Cured-in-place-pipe (CIPP) is an onsite plastic manufacturing technology used in the U.S. and has not been evaluated for regulatory compliance with federal air pollution laws. The practice involves the discharge of manufacturing waste into the environment. The study goal was to estimate the magnitude of volatile organic compounds (VOCs) discharged into the atmosphere for styrene and nonstyrene composite manufacture and examine low-cost air monitoring sensor reliability. Time-resolved emission analysis revealed that VOC emission was not only isolated to the thermal curing period but also occurred before and after curing. In addition to the styrene monomer, other gas-phase hazardous air pollutants regulated under the Clean Air Act were also emitted. Based on typical CIPP installations, 0.9 to 16.6 U.S. tons of emitted VOCs were estimated for styrene CIPPs, and 0.09 to 1.6 U.S. tons of emitted VOCs were estimated for nonstyrene CIPPs. Because the number and size of CIPPs manufactured in a single community can vary, the total air pollution burden will significantly differ across communities. Low-cost VOC sensors commonly utilized near CIPP manufacturing activities did not accurately quantify styrene and should not be relied upon for that purpose. Up to several thousand-fold detection differences were observed. Regulatory evaluation of CIPP air pollution and PID sensor reliability assessments are recommended.

Unique Photoactivated Time-Resolved Response in 2D GeS for Selective Detection of Volatile Organic Compounds.

Volatile organic compounds (VOCs) sensors have a broad range of applications including healthcare, process control, and air quality analysis. There are a variety of techniques for detecting VOCs such as optical, acoustic, electrochemical, and chemiresistive sensors. However, existing commercial VOC detectors have drawbacks such as high cost, large size, or lack of selectivity. Herein, a new sensing mechanism is demonstrated based on surface interactions between VOC and UV-excited 2D germanium sulfide (GeS), which provides an effective solution to distinguish VOCs. The GeS sensor shows a unique time-resolved electrical response to different VOC species, facilitating identification and qualitative measurement of VOCs. Moreover, machine learning is utilized to distinguish VOC species from their dynamic response via visualization with high accuracy. The proposed approach demonstrates the potential of 2D GeS as a promising candidate for selective miniature VOCs sensors in critical applications such as non-invasive diagnosis of diseases and health monitoring.

Recent Advances in Sensing Materials Targeting Clinical Volatile Organic Compound (VOC) Biomarkers: A Review.

In general, volatile organic compounds (VOCs) have a high vapor pressure at room temperature (RT). It has been reported that all humans generate unique VOC profiles in their exhaled breath which can be utilized as biomarkers to diagnose disease conditions. The VOCs available in exhaled human breath are the products of metabolic activity in the body and, therefore, any changes in its control level can be utilized to diagnose specific diseases. More than 1000 VOCs have been identified in exhaled human breath along with the respiratory droplets which provide rich information on overall health conditions. This provides great potential as a biomarker for a disease that can be sampled non-invasively from exhaled breath with breath biopsy. However, it is still a great challenge to develop a quick responsive, highly selective, and sensitive VOC-sensing system. The VOC sensors are usually coated with various sensing materials to achieve target-specific detection and real-time monitoring of the VOC molecules in the exhaled breath. These VOC-sensing materials have been the subject of huge interest and extensive research has been done in developing various sensing tools based on electrochemical, chemoresistive, and optical methods. The target-sensitive material with excellent sensing performance and capturing of the VOC molecules can be achieved by optimizing the materials, methods, and its thickness. This review paper extensively provides a detailed literature survey on various non-biological VOC-sensing materials including metal oxides, polymers, composites, and other novel materials. Furthermore, this review provides the associated limitations of each material and a summary table comparing the performance of various sensing materials to give a better insight to the readers.

Recent Advances in Luminescent Metal-Organic Frameworks for Detection of Gas and Volatile Organic Molecules

Gas molecules and volatile compounds in the air affect product purity and health. Because of increasing demand for better quality of life and health in industrial and general environments, timely detection of abnormal gas releases and poisonous compound volatilization is essential. Luminescence metal-organic frameworks (LMOFs) are materials with tunable pore sizes, defined structures, and luminescence properties, which have been widely reported as sensor materials for detecting various compounds. Here, luminescence mechanisms of LMOFs are reviewed and interactions between the host and guest molecules are explained at the molecular level. On the basis of these relationships, the state-of-the-art LMOF-based sensors for various common gas molecules (including H2O, O2, COx, and NOx) and volatile organic compounds (VOCs) are further described in depth. This review may benefit the further development of LMOFs and accelerate their practical application in various sensor combinations.

Guest-adaptive molecular sensing in a dynamic 3D covalent organic framework

Molecular recognition is an attractive approach to designing sensitive and selective sensors for volatile organic compounds (VOCs). Although organic macrocycles and cages have been well-developed for recognising organics by their adaptive pockets in liquids, porous solids for gas detection require a deliberate design balancing adaptability and robustness. Here we report a dynamic 3D covalent organic framework (dynaCOF) constructed from an environmentally sensitive fluorophore that can undergo concerted and adaptive structural transitions upon adsorption of gas and vapours. The COF is capable of rapid and reliable detection of various VOCs, even for non-polar hydrocarbon gas under humid conditions. The adaptive guest inclusion amplifies the host-guest interactions and facilitates the differentiation of organic vapours by their polarity and sizes/shapes, and the covalently linked 3D interwoven networks ensure the robustness and coherency of the materials. The present result paves the way for multiplex fluorescence sensing of various VOCs with molecular-specific responses.

Fabrication and application of free-standing fiber based on blue phase liquid crystal.

The application of blue phase liquid crystals (BPLCs) in optical control devices has been widely studied due to their fast response characteristics. However, the fabrication of free-standing BPLC fiber with emerging functionalities is challenging. Here, we demonstrate a free-standing fiber based on BPLC with excellent stability, flexibility, and multifunction. The multi-mode fiber (MMF) end face is etched by the etching agent of buffered oxide etch (BOE), which can be fixedly connected with the free-standing BPLC fiber after polymerization in order to overcome the problems of optical signals transmission and reception. Three types of free-standing BPLC fiber-based devices, including Volatile Organic Compound (VOC) vapor sensors, vector position sensors, and color fibers, are fabricated and investigated, to the best of our knowledge, for the first time. The free-standing BPLC fiber as a multifunctional material will provide broad application prospects in VOC sensors without power supply, smart fabrics, flexible displays, decorations fields with no dyes, as well as vector displacement sensors for robotic arm positioning.

A Peripherally Located Air Recirculation Device Containing an Activated Carbon Filter Reduces VOC Levels in a Simulated Operating Room.

Electrosurgery procedures produce airborne contaminants including volatile organic compounds (VOCs). The effectiveness of commercial grade activated carbon at removing toluene, a typical VOC, from the air in an enclosed simulated operating room (OR) when interfaced with an air recirculation device was tested. The concentration of toluene in the air was measured using gas sensitive semiconductor VOC sensors. When the air recirculation device containing activated carbon was turned on, the concentration of toluene in the air decayed exponentially. When the device was off, the toluene concentration reduced much more slowly. After 130 min, a VOC sensor placed near the air recirculation device showed VOC reductions of approximately 30% when the device is on and less than 1% when the device is off. Changing the activated carbon filter after 22 h of constant use showed an abrupt increase in the rate of toluene removal.

Optimization of light-analyte interaction in Si3N4/polymer hybrid waveguide for sensitive sensing of pyridine vapor with ppb-level detection limit

High-sensitivity and miniature volatile organic compound (VOC) sensor is considerably significant for environment monitoring. Herein we performed the theoretical and experimental optimization of waveguide sensor to realize the sensitive detection of volatile toxic pyridine vapor with ppb-level detection limit. The miniature sensor is based on the CMOS-compatible silicon nitride (Si3N4) Mach-Zehnder interferometer (MZI) waveguide with dipolar polycarbonate as sensitive cladding material. Theoretical optimization by varying the waveguide width and the thickness of sensitive cladding is carried out to tune the group refractive index and evanescent field. Experimentally, two MZIs with distinct waveguide widths and sensitive cladding thickness are fabricated to compare their performance in pyridine vapor sensing. The sensing results show that the thickness of sensitive layer plays a key role in sensing sensitivity and response time. The final obtained pyridine sensing sensitivity can reach 63 pm/ppm and the detection limit can be as low as 476 ppb. Our demonstration provide a simple and efficient strategy for sensitive sensing of hazardous chemical vapors, which may find application in biochemical sensing.

Flower-like NiO for the detection of 3-methyl-1-butanol, a microbial volatile organic compound

Volatile organic compounds (VOCs) can indicate the quality or freshness of food. Thus, the development of VOC sensors has a tremendous technological impact. Herein, we report the VOC sensing performance of porous flower-like NiO to 3-methyl-1-butanol (3M1B), a microbial VOC that can be used as a food spoilage marker. Our results show that the NiO-based sensor has high sensitivity and selectivity to 3M1B compared to the other VOCs, including acetone, ethanol, butanone, 2-nonanone, and m-xylene. Moreover, we show that NiO exhibits a high response to 3M1B with fast response times under humidity conditions, being a promising material for real applications.

Humidity effect on real-time response of tetranitro-oxacalix[4]arene-based surface plasmon resonance (SPR) acetone sensor at room temperature

We report the effect of humidity on the acetone sensitivity of nano-sized thin films obtained by spin coating the oxacalix[4]arenes(1,3) molecules on suitable substrates in this work. The surface morphologies and volatile organic compound (VOC) sensor properties of spin-coated thin films were examined using atomic force microscopy (AFM) and surface plasmon resonance (SPR), respectively. It was determined that the sensitivities calculated from the measurements performed using a mixture of acetone and water vapor were 14% and 11% incorrect for the 1 and 3 thin film sensors, respectively, compared to those performed with dry air and acetone mixture.

Rational Design of Peptide Biorecognition Elements on Carbon Nanotubes for Sensing Volatile Organic Compounds

AbstractCarbon nanotube (CNT) chemiresistors have emerged as miniaturized platforms for wearable volatile organic compound (VOC) sensors. As a promising biorecognition element (BRE), a short peptide can functionalize CNT to be sensitive and selective to target VOCs. However, unveiling the VOC‐optimized peptide‐CNT pair for gas‐phase sensing remains unclear. Here, a novel multimodal molecular toolset for designing, building, and probing suitable BRE‐CNT sensors using machine learning, molecular dynamics, and near‐edge X‐ray absorption fine structure spectroscopy is presented. This computational and experimental suite predicts the peptide conformation on the CNT surface and probes how the peptide–CNT interfaces affect the VOC sensing. Then, peptide‐functionalized CNT chemiresistors are tested against various VOCs to confirm the efficacy of the toolkit. The results show that the vertically oriented peptide on the CNT surface hinders VOC access to the peptide–CNT interface, resulting in a significantly lower sensor signal than the CNT chemiresistor with the horizontally oriented peptide. The interactive computational and experimental results strongly indicate that a peptide conformation plays an important role in VOC sensing sensitivity.