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The development of advanced metal-oxide-semiconductor sensing technologies for the detection of Volatile Organic Compounds (VOCs) present in exhaled breath is of great importance for non-invasive, cheap and fast medical diagnostics. Our experimental studies investigate the effects of operating temperature selection and UV-light irradiation on improving the response of WO 3 nanowire sensors towards exhaled breath exposure. Herein, six WO 3 nanowire sensors (both pristine and doped with a range of metal nanoparticles such as Pt, Au, Au/Pt, Ni and Fe) were synthesised via Aerosol-Assisted Chemical Vapour Deposition (AACVD) and characterized by means of Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray (EDX-ray). Breath measurements were performed in the dark and under UV-light irradiation at various sensor operating temperatures. The results demonstrate that UV-light irradiation combined with the optimisation of the sensors’ operating temperature can greatly enhance the sensors’ responses towards breath exposure. [ABSTRACT FROM AUTHOR]

Sensing technology has been developed for detection of gases in some environmental, industrial, medical, and scientific applications. The main tasks of these works is to enhance performance of gas sensors taking into account their different applicability and scenarios of operation. This paper presents the descriptions, comparison and recent progress in some existing gas sensing technologies. Detailed introduction to optical sensing methods is presented. In a general way, other kinds of various sensors, such as catalytic, thermal conductivity, electrochemical, semiconductor and surface acoustic wave ones, are also presented. Furthermore, this paper focuses on performance of the optical method in detecting biomarkers in the exhaled air. There are discussed some examination results of the constructed devices. The devices operated on the basis of enhanced cavity and wavelength modulation spectroscopies. The experimental data used for analyzing applicability of these different sensing technologies in medical screening. Several suggestions related to future development are also discussed. [ABSTRACT FROM AUTHOR]

Highlights: [•] 1/f Noise can be used as a diagnostic tool of surge arrester varistor structures. [•] Noise is a sensitive tool for identification of different grain structures. [•] In diagnostics noise gives more distinctive results than DC characteristics. [•] Noise data can determine technology parameters of the produced varistors. [ABSTRACT FROM AUTHOR]

The chemoresistive sensor response of nanoporous NiO films prepared by advanced gas deposition was investigated by combined resistivity and in situ FTIR spectroscopy, with and without simultaneous light illumination, to detect NO 2 and acetone gases. The sensitivity towards NO 2 increased dramatically under UV irradiation employing 275 nm light. Improved sensitivity was observed at an elevated temperature of 150 °C. In situ FTIR measurements were performed to record the transient gas adsorption/desorption processes. The sustained sensitivity and repeatability for NO 2 sensing could be attributed to reversible surface-nitro and nitrate species formation, which are stable on the surface at relative humidity up to 40%. In contrast, acetone sensing results in irreversible decomposition and accumulation of reaction products on the NiO sensor surface, covering the surface and limiting gas sensing. Implications of the study for improved and sustained NiO gas sensor properties in gas mixtures are discussed. • NiO layer enhances its sensitivity to NO 2 by bandgap UV irradiation. • In situ FTIR spectra explain the gas sensing mechanisms of the NiO sensor. • Steady-state surface-nitro and nitrate formation leads to repeatable NO 2 sensing. • Acetone decomposition products cover the NiO surface and limit gas sensitivity. [ABSTRACT FROM AUTHOR]

The exceptionally large active surface-to-volume ratio of carbon nanotubes makes it an appealing candidate for gas sensing applications. Here, we studied the DC and low-frequency noise characteristics of a randomly oriented network of carbon nanotubes under NO 2 gas atmosphere at two different wavelengths of the UV light-emitting diodes. The UV irradiation allowed to sense lower concentrations of NO 2 (at least 1 ppm) compared to dark conditions. Our experimental studies confirmed that the flicker noise of resistance fluctuations under UV irradiation significantly enhanced the sensing characteristics of nanotube networks at low concentrations. We observed a dominating 1/ f -like noise component below 1 kHz. The sensitivity of nanotube networks was higher for shorter wavelength, whereas drift in the resistance was smaller for longer wavelength. The measurements under the NO 2 gas atmosphere revealed a remarkable reduction in DC resistance drift of the nanotube network between consecutive cycles of gas sensing. This phenomenon was explained via absorption-desorption of NO 2 gas molecules on nanotubes surface. Since small concentrations of NO 2 pose a threat to the ecosystem, these results might play a significant role in the development of sensitive nanotubes-based photo-activated gas sensors. [Display omitted] • Flicker noise can be effectively used for NO 2 sensing by carbon nanotube networks. • UV illumination enhances sensitivity to NO 2 gas in carbon nanotubes. • Response to NO 2 is more accelerated under UV illumination by shorter wavelengths. • Flicker noise in the carbon nanotube network has dependence on UV light wavelengths. [ABSTRACT FROM AUTHOR]

Non-invasive measurement methods offer great benefits in the field of medical diagnostics with molecular-specific techniques such as Raman spectroscopy which is increasingly being used for quantitative measurements of tissue biochemistry in vivo. However, some important challenges still remain for label-free optical spectroscopy to be incorporated into the clinical laboratory for routine testing. In particular, non-analyte-specific variations in tissue properties introduce significant variability of the spectra, thereby preventing reliable calibration. For measurements of blood analytes such as glucose, we propose to decrease the interference from individual tissue characteristics by exploiting the known dynamics of the blood-tissue matrix. We reason that by leveraging the natural blood pulse rhythm, the signals from the blood analytes can be enhanced while those from the static components can be effectively suppressed. Here, time-resolved measurements with subsequent pulse frequency estimation and phase-sensitive detection are proposed to recover the Raman spectra correlated with the dynamic changes at blood-pulse frequency. Pilot in vivo study results are presented to establish the benefits as well as outline the challenges of the proposed method in terms of instrumentation and signal processing. • Natural heart rhythm is used as reference source for phase-sensitive detection. • Method enhances dynamic pulse-related Raman spectrum blood. • Reduction of the influence of static tissues which cause individual variability. • Raman bands of blood components can be retrieved from non-invasive measurements. • Heart rhythm frequency can be estimated from noninvasive Raman measurements. [ABSTRACT FROM AUTHOR]

• UV light irradiation of doped WO 3 gas sensing layers reduced response time to formaldehyde. • The gas induced baseline shift was diminished under UV light irradiation. • UV light irradiation induces similar effect as the doping of WO 3 gas sensing layers with metal nanoparticles. • Energy induced by the UV light, accelerates the adsorption-desorption processes in WO 3 gas sensing layers. We report results of formaldehyde gas (CH 2 O) detection under dark conditions and UV light irradiation with pristine tungsten trioxide nanowires (WO 3 NWs) and metal nanoparticles decorated WO 3 NWs gas sensing layers. The resistive layers were deposited by one step aerosol assisted chemical vapor deposition (AACVD) on commercial alumina substrates with 10-pair interdigitated platinum electrodes. The elaborated gas sensors, based on pristine WO 3 and on WO 3 decorated with Au, Pt, Au/Pt, Ni and Fe nanoparticles, were investigated towards three concentrations of formaldehyde gas (5, 10 and 15 ppm) under dark conditions and under UV light irradiation at the wavelength of 394 nm. Two main effects were observed: firstly, under UV light irradiation the response time for CH 2 O desorption was significantly reduced with the exception of the nanomaterial with Fe NPs dopant; secondly, the gas induced baseline shift was reduced under UV light irradiation conditions. These results can be explained by the additional energy induced by the UV light, accelerating the adsorption-desorption processes. The results obtained confirmed that both the decoration of WO 3 NWs with selected metal nanoparticles as well as sensors operation under UV light irradiation are a practical and affordable way to enhance gas sensing towards formaldehyde detection, although both strategies applied together did not introduce an amplified synergetic effect. [ABSTRACT FROM AUTHOR]

In this work, MoS 2 flakes were printed on ceramic substrates and investigated toward 1-10 ppm of nitrogen dioxide (NO 2 ), 2-12 ppm of ammonia (NH 3 ), and 2-12 ppm acetone (C 3 H 6 O) under UV light (275 nm). The structure of overlapping MoS 2 flakes and UV light assistance affected high responsivity to NO 2 when DC resistance was monitored, and superior sensitivity to NH 3 was obtained from the low-frequency noise spectra. MoS 2 exhibited response and recovery times in hundreds of seconds and stability throughout the experiments conducted within a few months. MoS 2 sensor exhibited a resistance drift during the detection of a specific relaxation time. Subtracting the baseline burden with exponential drift exposed the direction of changes induced by oxidizing and reducing gases and reduced DL to 80 ppb, 130 ppb, and 360 ppb for NO 2 , NH 3 , and C 3 H 6 O, respectively. The fluctuation-enhanced sensing (FES) revealed that the adsorption of NO 2 on MoS 2 decreases the noise intensity, whereas adsorbed NH 3 increases the fluctuations of current flowing through the sensor, and these changes are proportional to the concentration of gases. The noise responses for NO 2 and NH 3 were opposite and higher than DC resistance responses with subtracted baseline (an increase of 50% for 10 ppm of NO 2 and an increase of more than 600% for 12 ppm of NH 3 ), showing that FES is a highly sensitive tool to detect and distinguish between these two gases. This way, we introduce a simple and low-cost method of gas sensor fabrication using ink-printed MoS 2 and the possibility of enhancing its sensitivity through data processing and the FES method., (© 2024. The Author(s).)

We discuss the implementation challenges of gas sensing systems based on low-frequency noise measurements on chemoresistive sensors. Resistance fluctuations in various gas sensing materials, in a frequency range typically up to a few kHz, can enhance gas sensing by considering its intensity and the slope of power spectral density. The issues of low-frequency noise measurements in resistive gas sensors, specifically in two-dimensional materials exhibiting gas-sensing properties, are considered. We present measurement setups and noise-processing methods for gas detection. The chemoresistive sensors show various DC resistances requiring different flicker noise measurement approaches. Separate noise measurement setups are used for resistances up to a few hundred kΩ and for resistances with much higher values. Noise measurements in highly resistive materials (e.g., MoS 2 , WS 2 , and ZrS 3 ) are prone to external interferences but can be modulated using temperature or light irradiation for enhanced sensing. Therefore, such materials are of considerable interest for gas sensing.

We investigated the noise and photoresponse characteristics of various optical transparencies of nanotube networks to identify an optimal randomly oriented network of carbon nanotube (CNT)-based devices for UV-assisted gas sensing applications. Our investigation reveals that all of the studied devices demonstrate negative photoconductivity upon exposure to UV light. Our studies confirm the effect of UV irradiation on the electrical properties of CNT networks and the increased photoresponse with decreasing UV light wavelength. We also extend our analysis to explore the low-frequency noise properties of different nanotube network transparencies. Our findings indicate that devices with higher nanotube network transparencies exhibit lower noise levels. We conduct additional measurements of noise and resistance in an ethanol and acetone gas environment, demonstrating the high sensitivity of higher-transparent (lower-density) nanotube networks. Overall, our results indicate that lower-density nanotube networks hold significant promise as a viable choice for UV-assisted gas sensing applications.

The gas sensing properties of graphene back-gated field-effect transistor (GFET) sensors toward acetonitrile, tetrahydrofuran, and chloroform vapors were investigated with the focus on unfolding possible gas detection mechanisms. The FET configuration of the sensor device enabled gate voltage tuning for enhanced measurements of changes in DC electrical characteristics. Electrical measurements were combined with a fluctuation-enhanced sensing methodology and intermittent UV irradiation. Distinctly different features in 1/ f noise spectra for the organic gases measured under UV irradiation and in the dark were observed. The most intense response observed for tetrahydrofuran prompted the decomposition of the DC characteristic, revealing the photoconductive and photogating effect occurring in the graphene channel with the dominance of the latter. Our observations shed light on understanding surface processes at the interface between graphene and volatile organic compounds for graphene-based sensors in ambient conditions that yield enhanced sensitivity and selectivity.