Multi-gas Sensor Research Articles

UV-triggered and temperature-control carrier transport regulation of ZnO/Ti3C2Tx MXene heterostructures for dual selective HCHO and triethylamine detection

Food safety is crucial to the health and life of every individual. Triethylamine and formaldehyde, as key indicators for assessing the freshness of seafood, are increasingly important for food safety. Consequently, this study has developed a dual-gas sensor that is easy to operate and can rapidly detect both formaldehyde and triethylamine. The ZnO-MXene based gas sensors were prepared by a two-step electrostatic adsorption method. Under irradiation with 450 nm LED light, the sensors can detect formaldehyde gas at room temperature. By adjusting the temperature, it is possible to detect 100 ppm triethylamine in the dark at 160 ℃, with a response value of 28.2, which is 5.2 times that of the pure ZnO based sensors. Additionally, it exhibits good selectivity, excellent stability, and reproducibility. This paper also provides a detailed discussion of the sensing mechanism for detecting dual gases. It is significant for the development of dual-gas or multi-gas sensors and has potential applications in food safety testing and addressing complex changes in air environments.

Integrated-path multi-gas sensor using near-infrared diode lasers: An alternative to vehicle-driven point gas analyzer

Sensors for quantitative monitoring of greenhouse gases from high-emission facilities, such as wastewater treatment and industrial plants, are becoming essential to enforce regulatory compliance in climate action initiatives. A predominant sensing technique is the tracer-gas dispersion method (TDM) using closed-path gas analyzers typically installed in a vehicle to measure concentration as the gas plume is transected. Here, we validate for the first time the use of integrated-path gas sensors in TDM, measuring instantaneously the concentration along a predetermined laser beam path, thereby facilitating a stationary setup with orders of magnitude higher update rate and without the need for vehicles or road access. Our approach relies on a sensitive, integrated-path multi-gas sensor for real-time emission quantification using tunable diode laser absorption spectroscopy. An optical sensor head with spectral coverage from 1.52 μm to 1.65 μm is demonstrated, supporting multiplexed gas sensing in a modular fashion using low-power fiber-coupled diode lasers. The sensor measures real-time integrated-path concentrations of CO2 and CH4, and the tracer-gas C2H2 at 130 Hz with 0.5 ppm, 3 ppb, and 2 ppb sensitivity, respectively, at 1 s averaging. The integrated-path sensor is systematically benchmarked for absolute accuracy against a state-of-the-art point sensor, revealing an excellent match over 16 hours. Furthermore, the integrated-path sensor is deployed outdoors for emission quantification using TDM. A comparison to the gold-standard point sensor reveals identical results, demonstrating the integrated-path sensor as a potent substitute for vehicle-driven point sensors.

EC-QCL sensing system–incorporated adaptive baseline correction algorithm for simultaneous detection of multiple gas components

In this study, we presented a multi-gas sensor developed by employing an external cavity quantum cascade laser (EC-QCL)-integrated adaptive baseline correction algorithm for the simultaneous detection of ammonia (NH3), ozone (O3), and carbon dioxide (CO2). The EC-QCL sensing system was selected due to its high wavelength resolution and wide tuning range (1033–1068 cm−1), enabling the simultaneous measurement of multi-gases. The system complexity was effectively decreased and the superimposed noise was avoided by employing an adaptive baseline correction algorithm based on differential transformation and B-Spline approximation (ADTBA), which enabled the resolution of the uneven EC-QCL baseline signals. The performance validation of the ADTBA algorithm by comparing it with the conventional polynomial fitting for detecting the spectra of NH3, O3, and CO2 showed that the former outperformed the latter. The results showed a sensitivity enhancement factor of 3.7, indicating that the newly developed algorithm can generate a high-quality gas absorption spectrum. Allan deviation analysis of the system's responsiveness revealed an averaging time of 120 s resulting in sensitivities of 1.3, 26.7, and 37.0 ppm for NH3, O3, and CO2, respectively.

A Dynamic Range Preservation Readout Integrated Circuit for Multi-Gas Sensor Array Applications

This study introduces a readout integrated circuit (ROIC) tailored for multi-gas sensor arrays featuring a proposed baseline calibration scheme aimed at mitigating the issue of sensor baseline variation. Unlike previous approaches, the proposed scheme stores each sensor’s baseline value and dynamically updates the signal extraction range accordingly during ROIC operation. This adjustment allows for the optimal use of the ROIC’s dynamic range, enhancing sensor uniformity and accuracy without the need for complex additional circuitry or advanced post-processing algorithms. We fabricated a prototype ROIC using a 180 nm CMOS process, achieving a low power consumption of 0.43 mW and a conversion rate of 50 kSPS. The prototype boasts an integrated noise level of 9.9 μVRMS across a frequency range of 0.1 Hz to 5 kHz and a dynamic range of 142.6 dB, coupled with superior linearity, indicated by a maximum integral non-linearity (INL) of −75.71 dB. This design significantly reduces sensor offset scattering to within 1 LSB of the A/D reference scale. In this study, the efficacy of the proposed scheme was validated using Figaro TGS-2600. The ROIC targets a sensitivity range from 0.54 to 0.23 for gas concentrations ranging from 5 ppm to 20 ppm and a resolution of 39 Ω for sensor resistance range from 10 kΩ to 90 kΩ. The enhancements in performance make the proposed ROIC a promising solution for precise gas concentration detection in sensor applications.

Simultaneous atmospheric CH4, CO2 and O2 detection using fiber-optic switch (FOS)-based time-division multiplexed NIR laser long optical path absorption spectroscopy

A near-infrared (NIR) distributed feedback (DFB) laser sensor based on long optical path absorption spectroscopy technique was developed for simultaneous measurement of atmospheric methane (CH4), carbon dioxide (CO2) and oxygen (O2). A fiber-optic switch (FOS) in conjunction with the time-division multiplexing (TDM) technique was adopted to enable different lasers scan over the target gas absorption lines in turn. The long optical path wavelength modulation spectroscopy (WMS) technique with second harmonic detection as well as a laser power fluctuation correction method was used to improve the sensor’s precision and accuracy. For the selected gas absorption lines, located at 6046.945 cm-1, 6361.245 cm-1 and 7877.647 cm-1, the detection limit (1σ) of 0.034 parts per million (ppm) for CH4, 11.921 ppm for CO2 and 0.14% for O2 was achieved according to the concentration and noise level. Allan deviation analysis indicates that the 1-s measurement precision was 0.030 ppm for CH4, 11.518 ppm for CO2 and 0.14% for O2, which could be improved to 6.5E-4 ppm, 0.255 ppm and 0.005% at an optimal average time of 800 s. A two-day continuous measurement of the CH4, CO2, and O2 concentration from ambient air was carried out, which verifies the stability and robustness of the developed multi-gas sensor.

Environment-Adaptable Edge-Computing Gas-Sensor Device With Analog-Assisted Continual Learning Scheme

This paper presents a multi-gas sensor device whose structure is optimized for edge computing capability under internet of things (IoT) environments. Considering inherent sensor device characteristics susceptible to environmental factors like temperature and humidity, edge-computing capability for the on-site sensor calibration and pattern recognition (PR) is facilitated through a proposed analog-assisted continual learning scheme. An environment-adaptable continual learning (EACL) is proposed to combine multiple learning processes under different environments including chamber and on-site. Its computation burden is much relieved to be integrated into the edge device by adopting the analog-assisted structure, where a designed readout integrated circuit (ROIC) for automatic calibration normalizes gas-sensor data. For functional feasibility, an edge-computing IoT device prototype is manufactured with a fabricated ROIC and an in-house semiconductor-type sensor array, supporting wireless on-site monitoring platform interfaces. The environment-adaptable edge-computing capability is functionally verified through EACL-PR experiments on hazardous gases such as NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and CO under environmental factor variations. The average PR accuracy of 97% is achieved on several kinds of mixture gas patterns. The analog-assisted operation is verified to reduce the training cycles by 3 times while the EACL itself achieves 25% better efficiency.

Colorimetric Multigas Sensor Arrays and an Artificial Olfactory Platform for Volatile Organic Compounds.

Herein, we develop colorimetric multigas sensor arrays assembling chemo-reactive fluorescent patch arrays and 10 × 10 indium gallium zinc oxide phototransistor arrays and apply them to an artificial olfactory platform to recognize five different volatile organic compounds (VOCs). Porous nanofibers, coupled with two organic emitters and emitting fluorescence, rapidly respond to gas-phased VOCs and offer unique fluorescent patterns associated with particular gas conditions, including gas kinds, concentrations, and exposure times by forming patch arrays with different fluorophore component ratios. These VOC-induced fluorescent patterns could be quantified and amplified by indium gallium zinc oxide (IGZO) phototransistor arrays functioning as a signal-generating component, resulting in gas-fingerprint patterns regarding electrical signals. Thus, the pattern library associated with VOCs and their concentration enables us to determine each airborne analyte as the artificial olfactory platform. Therefore, this system could achieve rapid, early quantitative recognition of hazardous gases and be applied as a preventative, portable, and wearable multigas identifier in various fields.

The equivalence ratio concept in fire safety science

The equivalence ratio is a well-established metric used to evaluate a fuel–air mixture in combustion. Since its inception, many have used this metric to evaluate the reactive properties of gas mixtures and their impact on combustion. Some have expanded beyond this concept using the compartment (i.e., global), plume, upper layer, and local equivalence ratios as criteria for predicting species yields in fire experiments, particularly carbon monoxide generation. In this work, an overview of the equivalence ratio concept in fire safety science is discussed, focusing on research conducted in multi-scale fire experiments. Gas analysis techniques used to measure the equivalence ratio, such as the phi meter, gas chromatography, or multi-gas sensor systems, are examined. The notable takeaways from works examining the equivalence ratio concept across various fire experiments suggest opportunities to address challenges in fire safety sciences.

Resistive Multi-Gas Sensor for Simultaneously Measuring the Oxygen Stoichiometry (λ) and the NOx Concentration in Exhausts: Engine Tests under Dynamic Conditions.

Due to increasingly stringent limits for NOx emissions, there is now more interest than ever in cost-effective, precise, and durable exhaust gas sensor technology for combustion processes. This study presents a novel multi-gas sensor with resistive sensing principles for the determination of oxygen stoichiometry and NOx concentration in the exhaust gas of a diesel engine (OM 651). A screen-printed porous KMnO4/La-Al2O3 film is used as the NOx sensitive film, while a dense ceramic BFAT (BaFe0.74Ta0.25Al0.01O3-δ) film prepared by the PAD method is used for λ-measurement in real exhaust gas. The latter is also used to correct the O2 cross-sensitivity of the NOx sensitive film. This study presents results under dynamic conditions during an NEDC (new European driving cycle) based on a prior characterization of the sensor films in an isolated sensor chamber with static engine operation. The low-cost sensor is analyzed in a wide operation field and its potential for real exhaust gas applications is evaluated. The results are promising and, all in all, comparable with established, but usually more expensive, exhaust gas sensors.

A near-infrared multi-gas sensor based on IWTD-CEEMDAN and WOA-BiLSTM for detection of CH4 and NH3 leaked in industrial production

A multi-gas trace sensor based on tunable diode laser absorption spectroscopy (TDLAS) with two distributed feedback (DFB) lasers is designed, which was applied to measure methane (CH4) and ammonia (NH3) leaked in industrial production. The multi-frequency synchronous modulation strategy was used to drive two lasers simultaneously, where the lasers have a central wavelength of 1653.7 nm and 1512.2 nm, corresponding to the target absorption lines of CH4 and NH3 respectively. A miniaturized multi-pass gas cell (MPGC) is used as the gas absorption cell. Improved wavelet threshold denoising algorithm based on complete ensemble empirical mode decomposition with the adaptive noise (IWTD-CEEMDAN) is applied to improve the SNR of the spectral signals, and the signal-to-noise ratio (SNR) could be enhanced by approximately 26.34 dB. The concentration of CH4 and NH3 is inverted by whale optimization algorithm–bidirectional long short-term memory (WOA-BiLSTM) with a root-mean-square error (RMSE) of 0.691 ppb and 0.111 ppb respectively. The linearity, stability and response time were verified and the determination coefficients (R2) were 0.9994 and 0.9992 for CH4 and NH3. The response time of the sensor is 4.8 s, which satisfies the real-time detection demands and has the RMSE of 43.24 ppb and 11.83 ppb respectively. The limit of detection (LoD) for CH4 is 6.69 ppb with 212 s integration time, and the LoD for NH3 is 3.78 ppb with 36 s integration time, which is obtained by Allan-Werle deviation. The measurement results of CH4 and NH3 concentrations in atmosphere indicate that the sensors provide a reliable method for simultaneous real-time detection of multiple leaking gases.

Resistive, Temperature-Independent Metal Oxide Gas Sensor for Detecting the Oxygen Stoichiometry (Air-Fuel Ratio) of Lean Engine Exhaust Gases.

This study presents a resistive sensor concept based on Barium Iron Tantalate (BFT) to measure the oxygen stoichiometry in exhaust gases of combustion processes. The BFT sensor film was deposited on the substrate by the Powder Aerosol Deposition (PAD) method. In initial laboratory experiments, the sensitivity to pO2 in the gas phase was analyzed. The results agree with the defect chemical model of BFT materials that suggests the formation of holes h• by filling oxygen vacancies VO•• in the lattice at higher oxygen partial pressures pO2. The sensor signal was found to be sufficiently accurate and to have low time constants with changing oxygen stoichiometry. Further investigations on reproducibility and cross-sensitivities to typical exhaust gas species (CO2, H2O, CO, NO, …) confirmed a robust sensor signal that was hardly affected by other gas components. The sensor concept was also tested in real engine exhausts for the first time. The experimental data showed that the air-fuel ratio can be monitored by measuring the resistance of the sensor element, including partial and full-load operation modes. Furthermore, no signs of inactivation or aging during the test cycles were observed for the sensor film. Overall, a promising first data set was obtained in engine exhausts and therefore the BFT system is a possible cost-effective alternative concept to existing commercial sensors in the future. Moreover, the integration of other sensitive films for multi-gas sensor purposes might be an attractive field for future studies.

2-D Be2C as Multitime Reversible Environmental Oxygen- and Nitrogen-Containing Gases Sensor Material Under First Principle Investigations

A combined density functional theory and nonequilibrium Green’s function (DFT-NEGF) approach has been conducted to examine the potential of monolayer Be2C nanosheet as a promising sensor for various environmentally toxic/nontoxic nitrogen- and oxygen-containing multigases (NCGs and OCGs). A total of eight gases comprising four OCGs (O2, CO, CO2, and H2O) and four NCGs (NO, NO2, N2O, and NH3) are investigated on the surface of monolayer Be2C to identify its gas sensing potential. The nontoxic O2, CO2, and H2O are also included in the study considering their obvious presence in the environment. Our investigation suggests that at room temperature, larger adsorption energy of NO, NH3, and O2 gas molecules on the surface of Be2C reflects a more significant recovery time as 1016, 105, and 1012 s, respectively. The positive higher adsorption energy of NO2 gas molecules as 0.91 eV adsorbed on the Be2C nanosheet at room temperature results in a very short recovery time of 10−29 s. The adsorption energy of the NO2 gas molecule is positive, making it unsuitable for gas sensors. In all cases, notable variation is observed in electronic and adsorption properties, charge transfer, recovery time, work function, and transport characteristics of OCGs and NCGs on Be2C surface. OCGs and NCGs show electron acceptor nature during interaction with Be2C nanosheet. The electronic transport of Be2C sheet shows its highly selective and sensitive nature toward OCGs and NCGs. Overall, monolayer Be2C reveals as an excellent multitime reversible multigas sensor material for four gases, viz. CO, CO2, H2O, and N2O with their significantly low recovery time, whereas it appears to be one-time disposable sensor material for O2, NO, and NH3 gases with higher adsorption energy and relatively long recovery time.

Calibrating and Graduating of Multi-Sensor Gas Analyzer

Introduction. The article is devoted to the calibration and graduation of the multi-sensor gas analyzer developed at the V.M. Glushkov Institute of Cybernetics of National Academy of Sciences of Ukraine. The modern trend in the development of medical equipment, in particular, containing built-in measuring devices, consists in its miniaturization, intellectualization, the use of information technologies, and the introduction of wireless and network means of data transmission. The purpose of the article is to design, create electronics and software for recording the concentration of three gases (oxygen, carbon dioxide and carbon monoxide), test and calibrate the device. Results. Three prototypes of sensor modules for oxygen, carbon dioxide and carbon monoxide have been created. The structure, scheme of connection of sensor modules, and the principle of operation of the gas analyzer was described. Testing and optimization of these prototypes were performed. Auxiliary pneumatic equipment for checking the performance of devices, their calibration and graduation was manufactured. Measurement technique was developed and test measurements of the specified gases were performed. The calibration technique was developed and the device was calibrated using two test gas samples such as ethanol vapor and air-gas mixture of carbon dioxide. In addition, a special software has been developed, which allow observe a registered input signal from sensor modules and store it to the file. Conclusions. As a result of the work, prototype of the device was created, its testing, calibration and graduation were carried out at the State Enterprise "Ukrmetrteststandart", 4 certificates of metrological attestation were obtained. The gas analyzer can be used for screening of healthy persons based on study of exhalation to predict the risk of diseases. Keywords: multi-sensor gas analyzer, oxygen, carbon dioxide, carbon monoxide, calibration, graduation.

Calibration of low-cost particulate matter sensors for coal dust monitoring

Mining-induced coal dust causes various respiratory diseases to mine workers mainly coal workers' pneumoconiosis (CWP). Currently available underground monitors are expensive and bulky. These disadvantages limit them for regulatory sample monitoring purposes. Moreover, personal exposure levels for most miners remain unknown, risking them to potential overexposures. Low-cost light scattering particulate matter (PM) sensors offer a potential solution to this problem with the capability to characterize PM concentration with high spatio-temporal resolution. However, these sensors require precise calibration before they can be deployed in mining environments. No previous study has promulgated a standard protocol to assess these sensors for coal dust monitoring. The goal of this study was to calibrate Plantower PMS5003 sensors for coal dust monitoring using linear regression models. Two other commercially available PM sensors, the Airtrek and Gaslab CM-505 multi-gas sensors, were also evaluated and calibrated. They were evaluated for factors including linearity, precision, limit of detection, upper concentration limits, and the influence of temperature and relative humidity in a laboratory wind tunnel. The PMS5003 sensors were observed to be accurate below 3.0 mg/m3 concentration levels with R-squared values of 0.70 to 0.90 which was the best among the sensors under with an acceptable precision below 1.5 mg/m3. Moreover, this study shows that temperature and relative humidity have minimal influence on the efficacy of low-cost PM sensors' ability to monitor coal dust. This investigation reveals the feasibility of low-cost sensors for real-time personal coal dust monitoring in underground coal mines if a robust calibration model is applied.

High-performance and self-calibrating multi-gas sensor interface to trace multiple gas species with sub-ppm level

Developing a better understanding of gas-metal oxide interactions as well as improving the output performance with the aging of the gas sensing array represents a critical challenge. In this work, we have designed a surface-functionalized metal-oxide sensors array containing six units for detecting NO2, SO2, H2S, and CO hazardous gases. The responses of the sensor array to four target gases are enhanced by various catalytic effects on the surface, and they show distinctive responses to each gas. For improved selectivity, a pattern recognition algorithm is implemented under an edge computing-based environment to relieve the overload of a monitoring server. Moreover, a self-calibration method for long-term stability is designed to indirectly calibrate gas responses with aging. With its edge-computing device prototype, which includes a sensor array and its readout integrated circuit, four target gases are distinguished by an artificial neural network algorithm with 97% accuracy. The present work describes the effective platforms for emerging next-generation gas sensors with improved communication and monitoring systems.

Compact Non-Dispersive Infrared Multi-Gas Sensing Platform for Large Scale Deployment with Sub-ppm Resolution

We report on a novel, cost-effective non-dispersive infrared (NDIR) multi-gas sensor aimed at environmental air pollution monitoring. The rugged design of the K96 sensor core combines highest compactness and low-power consumption with our unique multi-channel cell design, featuring the detection of up to three different gases simultaneously, including CO2, CH4, N2O, and H2O. Our sensing platform allows the selection of the target gases as well as the concentration ranges, thus providing highly customizable gas sensor systems targeting application-specific gas monitoring settings. The sensor core comes with an implemented calibration model, and can address in real time any cross-sensitivity between the NDIR gas-sensing channels. We provide an immensely versatile sensing system while ensuring high sensing stability combined with high precision (&lt;0.1 ppm for both CO2 and N2O, &lt;0.5 ppm for CH4). The K96 multi-gas sensor core offers a resilient sensor solution for the increasing demand of compact monitoring systems in the field of environmental monitoring at reasonable costs for medium-to-high volumes.

All Fiber-Optic Multigas (NH₃, NO₂, and CO) Sensor Based on MoWS₂ Coated Fiber

In this paper, we fabricate and demonstrate Molybdenum Tungsten Disulfide (MoWS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) composite based all-optical gas sensor for environmental monitoring applications. This proposed sensor is fabricated by coating the synthesized MoWS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanosheets on a side polished fiber (SPF) and analyzed the sensing performance of three important hazardous gases such as ammonia (NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ), Nitrogen dioxide (NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) and Carbon monoxide (CO). The MoWS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanosheets are synthesized using the hydrothermal exfoliation technique. This novel nanocomposite offers broadband absorption and desorption of gas molecules due to more edge sites, which in turn enhances the sensing properties for various gases. Thus, the MoWS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> coated SPF sensor utilized for sensing three hazardous gases such as NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and CO. Besides, the gas sensor is operated at normal humidity (60–75%) and room temperature for varying gas concentration from 0 to 500 ppm. The sensor shows high sensitivity of 86, 72, and 48% for various gases such as NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and CO respectively. Furthermore, the results exhibit rapid response and fast recovery time, which are measured as 21 s and 60 s for NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> gas, whereas 42 s and 71 s for NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas and 93 s and 56 s for CO gas. The selectivity test for the sensor is conducted with methane and formaldehyde along with NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , CO, and NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gases in the same experimental condition. Further, the results confirm that the sensor possesses a high degree of selectivity for NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> gas among the other gases. Hence, the proposed sensor is eminently suitable to detect hazardous gases in high traffic and industrial fields.

Multi-laser sensor for simultaneous multi-gas measurements using off-axis cavity-enhanced absorption spectroscopy with an opposite two-way configuration.

An opposite two-way off-axis cavity-enhanced absorption spectroscopy-based multi-gas sensor is reported. More than two lasers can be employed in the sensor for simultaneous detection of different gas species. An approximately two times improvement in magnitude of the 2f signal and the signal-to-noise ratio is achieved because the concave spherical mirrors outside each end of the cavity and the narrow bandpass filters before the detectors can act as re-injection mirrors to re-inject the light into the cavity in the scheme. The performance of the sensor is demonstrated by simultaneous measurement of CO2 and CH4 in the atmosphere. This Letter highlights a new, to the best of our knowledge, strategy for simultaneous multi-gas measurement in a single integrated cavity by employing as many as four lasers.

Improvement of the Sensitivity and Selectivity of Gas Molecules of Graphene-base Gas Sensor by Applying Carbon Nanotubes

Chemical biosensors using graphene as a sensing element have been developed for a new generation of highly-sensitive miniature exhaled multi-gas sensors for early detection of various diseases. In this study, carbon nanotubes (CNTs) were synthesized directly on graphene to increase the surface area of the molecular adsorption in order to improve the sensitivity and selectivity of the graphene-based gas sensor. The synthesis of graphene was performed in a low-pressure chemical vapor deposition apparatus on a very pure copper catalyst substrate. The grown graphene was transferred onto a Si/SiO2 substrate. A CNT layer was also formed directly on the graphene sensor element using a thermochemical vapor deposition method. Stable ohmic contact was observed between the synthesized CNTs and the base graphene layer. It was also confirmed that the sensitivity of the graphene sensor with the CNT layer to gas molecules was improved by about 4 times compared with the conventional graphene sensor.

The Multi-Gas Sensor for Remote UAV and UGV Missions-Development and Tests.

In this article, we present a versatile gas detector that can operate on an unmanned aerial vehicle (UAV) or unmanned ground vehicle (UGV). The device has six electrochemical modules, which can be selected to measure specific gases, according to the mission requirements. The gas intake is realized by a miniaturized vacuum pump, which provides immediate gas distribution to the sensors and improves a fast response. The measurement data are sent wirelessly to the operator’s computer, which continuously stores results and presents them in real time. The 2 m tubing allows measurements to be taken in places that are not directly accessible to the UGV or the UAV. While UAVs significantly enhanced the versatility of sensing applications, point gas detection is challenging due to the downwash effect and gas dilution produced by the rotors. In our work, we demonstrated the method of downwash effect reduction at aerial point gas measurements by applying a long-distance probe, which was kept between the UAV and the examined object. Moreover, we developed a safety connection protecting the UAV and sensor in case of accidental jamming of the tubing inside the examined cavity. The methods presented provide an effective gas metering strategy using UAVs.