Search

Detection of volatile organic compounds (VOCs) is crucial in industrial production, environmental monitoring, and public safety. VOCs sensors need to be intrinsically safe, given the flammability and toxicity of common VOCs. Fiber optic sensors offer a passive and flexible solution for VOCs detection, attracting significant attention from researchers. In this study, ZIF-8, a subset of metal-organic frameworks, is applied to a side-polished silicon wafer, forming an open-cavity optical fiber Fabry-Pérot interferometer (FPI) with a fiber patch cable and a 3D-printed structural part. The sensing performance for prevalent VOCs, including methylbenzene, methanol, and ethanol, is experimentally explored, exhibiting sensitivities of 0.118 p.m./ppm, 0.177 p.m./ppm, and 0.412 p.m./ppm, respectively. Sensitivity differences are analyzed and demonstrated at the molecular level. The proposed technologies offer advantages such as easy fabrication, intrinsic safety, small size, and good selectivity, providing an alternative for VOCs detection in industrial production. [Display omitted] • A mass-produced and reusable fiber optic sensor for VOCs was prepared. • The coated ZIF-8 film enhances the selectivity of sensing for alcohol. • The sensing characteristics of the sensor for VOCs were experimentally verified. [ABSTRACT FROM AUTHOR]

Gas sensor constructed with metal oxide semiconductor (MOS) has attracted a growing concern owing to its feasibility for rapid detection of various harmful and pollutant gases. However, the sensors made of pure MOS usually suffer from the drawbacks of high working temperature, low sensitivity and poor selectivity. Herein, we propose a Co-doping strategy to upgrade the gas sensing performance of In 2 O 3. To expound it, Co-doped In 2 O 3 porous nanospheres (Co-In 2 O 3 PNSs) with the diameter of ∼120 nm was prepared from home-made In(OH) 3 NSs via a soaking-lyophilization-calcination method and their sensitive performance towards several volatile organic compounds (VOCs) was investigated in detail. Compared to pure In 2 O 3 sensor, the best Co-In 2 O 3 sensor (with optimized 0.5 mol% Co) exhibited outstanding improvements in triethylamine (TEA) sensing performance, especially including lower optimal working temperature (OWT: 220 °C vs 320 °C), higher response (R a /R g : 3500 vs 60 for 10 ppm TEA), and better selectivity (selectivity coefficient S TEA /S ethanol : 400 vs 1850). Besides, an interesting selectivity transformation from ethanol to formaldehyde was also observed after Co doping. These improvements of Co-In 2 O 3 PNSs were mainly attributed to their reduced crystallite size, increased oxygen vacancy concentration, narrowed bandgap, as well as upshifted Fermi level, whose mechanism was discussed. • Co-In 2 O 3 PNSs with boosted VOC sensing performances were synthesized. • The Co-In 2 O 3 sensor can easily detect ppb level TEA with high response. • Co doping caused a selectivity transformation from ethanol to formaldehyde. [ABSTRACT FROM AUTHOR]

Operating temperature for detection of a gas/ vapour is an important factor to control the selectivity of a chemi-resistive type sensor. By governing the temperature dependent adsorption/desorption process of a gas/vapor on the surface of a sensing prototype, its selectivity can be switched from one target gas to another, which is useful for field applications. In this vision, this study represents a high performance, low temperature, and versatile volatile organic compound (VOC) sensor fabricated by hydrothermally synthesized ZnO nanorod networks. The selectivity of the prepared sensor prototype has been switched from methanol (at low operating temperature i.e < 50 °C) to formaldehyde (at high operating temperature i.e > 50 °C) by varying the operating temperature of the sensor. The sensor can selectively detect methanol vapor (400 ppm) with a response value of 600 operating at room temperature (RT, 27 °C). On the other hand, the same prototype is selective toward formaldehyde (400 ppm) with an extremely high response of 12000 at an operating temperature of 100 °C. Moreover, high sensitivity, selectivity, repeatability, and low response time (4.2 s) towards methanol at room temperature fulfill the purpose of a reliable VOC sensor for practical applications. The mechanism of selectivity switching associated with the present sensor has been correlated with the activation energy values for methanol and formaldehyde. By virtue of temperature driven selectivity switching phenomena, the ZnO nanorod networks based present sensor prototype can be considered as a smart sensing element for a futuristic, versatile, low temperature, dual mode VOC sensor. [Display omitted] • ZnO nanorod networks were synthesized in low temperature hydrothermal method. • ZnO nanorod network sensor shows a unique selectivity switching in VOC detection. • Selectively detect methanol (R ∼ 600) at RT and formaldehyde (R ∼ 12000) at high temperature (≥ 50 °C). • Activation energy values has been correlated to explain the sensing mechanism behind selectivity switching. [ABSTRACT FROM AUTHOR]

A highly sensitive and flexible gas sensor that can detect a wide range of chemicals is crucial for wearable applications. However, conventional single resistance-based flexible sensors face challenges in maintaining chemical sensitivity under mechanical stress and can be affected by interfering gases. This study presents a versatile approach for fabricating a micropyramidal flexible ion gel sensor, which accomplishes sub-ppm sensitivity (<80 ppb) at room temperature and discrimination capability between various analytes, including toluene, isobutylene, ammonia, ethanol, and humidity. The discrimination accuracy of our flexible sensor is as high as 95.86%, enhanced by using machine learning-based algorithms. Moreover, its sensing capability remains stable with only a 2.09% change from the flat state to a 6.5 mm bending radius, further amplifying its universal usage for wearable chemical sensing. Therefore, we envision that a micropyramidal flexible ion gel sensor platform assisted by machine learning-based algorithms will provide a new strategy toward next-generation wearable sensing technology.

The present investigation deals with the development of ethanol-vapour-sensing materials coated with the semiconducting oxide TiO2. Thick films of anatase TiO2 were deposited using the sol-gel dip-coating technique on alumina substrates by conventional alkoxide sol and modified sol added with Degussa P-25 as the sensing medium. It was shown that crystallised TiO2 anatase was obtained at the annealing temperature of 500°C. The fabricated TiO2 sensors exhibited highest sensitivity at the sensing temperature of 350 °C. Sensitivity towards the ethanol vapour was further increased with UV light effect. The enhancement of the sensitivity of the modified catalytic pellet can be explained by the crystallite of anatase TiO2 and the effect of the photocatalytic of TiO2. The high sensitivity of the TiO2 film deposited with modified sol revealed that the modified sol could be a new alternative in the development of a TiO2 ethanol sensor. [ABSTRACT FROM AUTHOR]

Abstract: The response of the electrode potential to several volatile organic compounds was examined for a concentration cell-type oxygen sensor with a Pt|8YSZ|Pt (ref. 21% oxygen) structure. The contamination of air with ppm levels of volatile organic compounds resulted a decrease in the oxide activity at the sensing electrode with a Pt layer, i.e., a decrease in the half-cell potential. The decrease of the potential at the Pt sensing electrode was modified with an over-coating of the Pt electrode with a second material. The formation of a dense Au layer on the Pt electrode was effectively decreased the half-cell potential with the VOCs. On the other hand, the over-coating of the Pt electrode with materials having a high catalytic activity of volatile organic compounds oxidation depressed the sensitivity to the volatile organic compounds. [Copyright &y& Elsevier]

The characteristic and effectiveness of pure and modified tin dioxide catalytic pellets as volatile organic compound sensors are discussed. The pure and La-doped SnO2 thick film sensor pellets exhibited highest sensitivity to ethanol, methanol and acetone vapour at sensing temperature 300°C. The highest sensitivity towards 500 ppm of ethanol, acetone and methanol was achieved by 5 at.% of La-doped SnO2 with values of ~57, ~55 and ~59, respectively. The enhancement of the sensitivity of La-doped SnO2 can be explained by the increase of surface areas; decrease of crystallites sizes and the basicity of the lanthanum dopants. The response times for the developed sensors were about 13-15 s for both pure SnO2 and La-doped SnO2 illustrated a fast response sensor that could be used as VOC sensors. [ABSTRACT FROM AUTHOR]

Abstract: A compact optical fibre sensor coated with a fluorosiloxane polymer has been shown to be suitable for monitoring of some different classes of organic volatile compounds (VOCs), namely some chlorinated hydrocarbons, aromatic hydrocarbons, aliphatic hydrocarbons, acetate and alcohols. The sensing component consists of an optical fibre, with the top end surface coated with poly[methyl(3,3,3-trifluoropropyl)siloxane] by dip-coating technique. Variations of the light power guided through the fibre are detected as the organic vapour is sorbed in the thin polymeric film. The experimental set-up is further constituted by an optical source to generate the interrogating signal and a photodiode to measure the intensity modulated signal. In this work some operational conditions such as, temperature of cure of the polymeric material, injection cell temperature, carrier gas flow rate, laser working wavelengths and frequencies were studied in order to achieve higher sensitivity and accuracy concerning sensor system performance. High detection capability for volatile organic compounds, good sensibility, reversibility, reproducibility and linearity were analytical features checked for this sensor system. However, the main advantage of the developed sensor is its very short analytical time (response time and desorption time), found to be less than 20s. Compounds well known as potentially dangerous for human health and environment, such as toluene and benzene showed changes on the reflected optical power up to 26.0 and 4.0dB, respectively. [Copyright &y& Elsevier]

Recently virtual sensor arrays (VSAs) have been developed to improve the selectivity of volatile organic compound (VOC) sensors. However, most reported VSAs rely on detecting single property change of the sensing material after their exposure to VOCs, thus resulting in a loss of much valuable information. In this work, we propose a VSA with the high dimensionality of outputs based on a quartz crystal microbalance (QCM) and a sensing layer of MXene. Changes in both mechanical and electrical properties of the MXene film are utilized in the detection of the VOCs. We take the changes of parameters of the Butterworth-van Dyke model for the QCM-based sensor operated at multiple harmonics as the responses of the VSA to various VOCs. The dimensionality of the VSA's responses has been expanded to four independent outputs, and the responses to the VOCs have shown good linearity in multidimensional space. The response and recovery times are 16 and 54 s, respectively. Based on machine learning algorithms, the proposed VSA accurately identifies different VOCs and mixtures, as well as quantifies the targeted VOC in complex backgrounds (with an accuracy of 90.6%). Moreover, we demonstrate the capacity of the VSA to identify "patients with diabetic ketosis" from volunteers with an accuracy of 95%, based on the detection of their exhaled breath. The QCM-based VSA shows great potential for detecting VOC biomarkers in human breath for disease diagnosis.

Identifying the defect type which closely correlates with gas/metal oxide semiconductor (MOS) interfacial charge exchange is of utmost importance for rationally designing high performance MOS gas sensors, which are essential for the emerging sensing electronics for assessing the personal health and environment pollution. Herein, we report the sensing performance of p-type NiO toward volatile organic compounds (VOCs) molecules could be drastically improved by Sc dopant. At an optimal doping ratio of 7.4 at.%, the sensor response to 100 ppm acetone was boosted from 8.2 (pure NiO) to 109.4, with a low detection limit of 10 ppb. Comprehensive defect characterizations indicate surface unsaturated oxygen vacancy (V O) defects act as active sites that facilitate the interfacial charge exchange at elevated temperatures. Our work highlights that abundant V O defects induced by aliovalent Sc dopant play an important role in boosting the sensitivity of p-type NiO toward VOCs, and provide valuable guidance in designing high performance p-type MOS sensors. The surface unsaturated oxygen vacancy defects, induced by Sc aliovalent dopant, play a vital role in boosting the electrical response of p-type NiO sensor toward VOC molecules. Image 1 • Sc-doped NiO nanoflowers have been synthesized via a facile wet chemical method. • Sc aliovalent dopant could substantially boost the responses of p-type NiO sensor toward VOC molecules. • Other than Ni3+, surface unsaturated oxygen vacancy defects play an important role in facilitating VOC/NiO interfacial charge exchange. [ABSTRACT FROM AUTHOR]