Sensing of volatile pollutants on reduced graphene oxide-polypyrrole composite: DFT investigation.

Air pollutants generated from volatile toxic chemicals pose significant public health concerns. Density functional theory (DFT) computations were used in this research to explore the efficiency and mechanism of harmful gas sensing over the reduced graphene oxide-polypyrrole (rGO-PPy) composite. Volatile molecule sensing was investigated for the NH3, H2CO, CH3OH, and C2H5OH gas molecules over three PPy orientations on the rGO substrate. Results showed that PPy orientation over rGO plays a crucial role in the sensing efficiency of the investigated gas molecules. The rGO-PPy composite, with PPy in a vertical orientation, demonstrated higher stability and enhanced sensing than other orientations. The results indicate that the strong hydrogen bonding of NH3 and CH3OH with both PPy and rGO significantly enhanced the sensing of these gas molecules on rGO by influencing the charge transfer with adsorption energy values of - 0.84 and - 0.92 eV, respectively. The lack of a direct hydrogen bonding with rGO and the weak hydrogen bonding with PPy caused a weak adsorption of H2CO and C2H5OH over rGO as indicated by the adsorption energy values of - 0.60 and - 0.78 eV, respectively. Selectivity analysis for the NH3 and C2H5OH gas molecules showed that NH3 can maintain hydrogen bonding with PPy in the presence of C2H5OH while C2H5OH cannot sustain this interaction. This study highlights the importance of the structural and electronic properties of the rGO-PPy composite in volatile pollutant sensing, providing insights for designing high-performance gas sensors.

Response Enhancement in High-Temperature H2S-Treated Metal Oxide Gas Sensors.

Metal oxide gas sensors (MOGS), crucial components in monitoring air quality and detecting hazardous gases, are well known for their poisoning effects when exposed to certain gas molecules, such as hydrogen sulfide. Surprisingly, our research reveals that high-temperature H2S treatment leads to an enhancement effect rather than response decay. This study investigates the time-decaying response enhancement, being attributed to the formation of metal sulfide and metal sulfate on the metal oxide's surface, enhancing the electronic sensitization. Such an enhancement effect is demonstrated for various gases, including CO, CH3CH2OH, CH4, HCHO, and NH3. Additionally, the impacts of H2S treatment on the response and recovery time are also observed. Surface compositional analysis are conducted with X-ray photoelectron spectroscopy. A proposed mechanism for the enhancement effect is elaborated, highlighting the role of electronic sensitization and the sulfide-sulfate component. This research offers valuable insights into the potential applications of metal oxide sensors in sulfide-presented harsh environments in gas sensing, encouraging future exploration of optimized sensor materials, operation temperature, and the development of hydrogen sulfide poisoning-resistant and higher sensitivity MOGS.

Defect-Engineered WO3-x Architectures Coupled with Random Forest Algorithm Enables Real-Time Seafood Quality Assessment.

Reliable and real-time monitoring of seafood decay is attracting growing interest for food safety and human health, while it is still a great challenge to accurately identify the released triethylamine (TEA) from the complex volatilome. Herein, defect-engineered WO3-x architectures are presented to design advanced TEA sensors for seafood quality assessment. Benefiting from abundant oxygen vacancies, the obtained WO2.91 sensor exhibits remarkable TEA-sensing performance in terms of higher response (1.9 times), faster response time (2.1 times), lower detection limit (3.2 times), and higher TEA/NH3 selectivity (2.8 times) compared with the air-annealed WO2.96 sensor. Furthermore, the definite WO2.91 sensor demonstrates long-term stability and anti-interference in complex gases, enabling the accurate recognition of TEA during halibut decay (0-48 h). Coupled with the random forest algorithm with 70 estimators, the WO2.91 sensor enables accurate prediction of halibut storage with an accuracy of 95%. This work not only provides deep insights into improving gas-sensing performance by defect engineering but also offers a rational solution for reliably assessing seafood quality.

Electronic Noses: From Gas-Sensitive Components and Practical Applications to Data Processing.

Artificial olfaction, also known as an electronic nose, is a gas identification device that replicates the human olfactory organ. This system integrates sensor arrays to detect gases, data acquisition for signal processing, and data analysis for precise identification, enabling it to assess gases both qualitatively and quantitatively in complex settings. This article provides a brief overview of the research progress in electronic nose technology, which is divided into three main elements, focusing on gas-sensitive materials, electronic nose applications, and data analysis methods. Furthermore, the review explores both traditional MOS materials and the newer porous materials like MOFs for gas sensors, summarizing the applications of electronic noses across diverse fields including disease diagnosis, environmental monitoring, food safety, and agricultural production. Additionally, it covers electronic nose pattern recognition and signal drift suppression algorithms. Ultimately, the summary identifies challenges faced by current systems and offers innovative solutions for future advancements. Overall, this endeavor forges a solid foundation and establishes a conceptual framework for ongoing research in the field.

Compact 3D-Printed Unit for Separation of Simple Gas Mixtures Combined with Chemiresistive Sensors.

Inexpensive chemiresistive sensors are often insufficiently selective as they are sensitive to multiple components of the gas mixture at the same time. One solution would be to insert a device in front of the sensor that separates the measured gas mixture and possibly isolates the unwanted components. This study focused on the fabrication and characterization of a compact unit, which was fabricated by 3D printing, for the separation and detection of simple gas mixtures. The capillary, the basic part of the compact unit, was 4.689 m long and had a diameter of 0.7 mm. The compact unit also contained a mixing chamber on the inlet side and a measuring chamber with a MiCS-6814 sensor on the outlet side. Mixtures of ethanol and water at different concentrations were chosen for characterization. The measured calibration curve was found to have a reliability of R2 = 0.9941. The study further addressed the elements of environmental friendliness of the materials used and their sustainability.

Preparation of Pt and bamboo charcoal co-modified TiO2 for formaldehyde sensing at room temperature.

Anatase TiO2 has evolved into one of the most attractive materials for gas sensing owing to its strong oxidation activity and excellent sensing properties. In this study, we prepared Pt and bamboo charcoal co-modified nano-TiO2 using a one-pot hydrothermal process and applied it to detect formaldehyde. The successful incorporation of the precious metal Pt and bamboo charcoal onto TiO2 was confirmed by scanning electron microscope, transmission electron microscopy, energy dispersive spectrometer, X-ray diffraction and X-ray photoelectron spectroscopy. Detailed analysis revealed a homogeneous distribution of Pt nanoparticles and bamboo charcoal on the TiO2 surface, which significantly improved the surface area and facilitated gas adsorption. These modifiers significantly enhanced the response of TiO2 to formaldehyde, for instance, the response signal increased fourfold, while the response time decreased from 91 to 68 s. The sample with 0.5@Pt and 0.5@C bamboo charcoal performed the best, showcasing the synergistic effect of metal nanoparticles and carbonaceous materials on gas-sensing properties. Our work highlighted the potential of using biomass-derived carbon to enhance the detection of formaldehyde and demonstrated the importance of material characteristics in designing effective gas sensors.

Ambipolar Heterojunction Sensors: Another Way to Detect Polluting Gases.

Gas sensors based on ambipolar materials offer significant advantages in reducing the size of the analytical system and enhancing its efficiency. Here, bilayer heterojunction devices are constructed using different octafluorinated phthalocyanine complexes, with Zn and Co as metal centers, combined with a lutetium bisphthalocyanine complex (LuPc2). Stable p-type behavior is observed for the ZnF8Pc/LuPc2 device under both electron-donating (NH3) and -oxidizing (NO2 and O3) gaseous species, while the CoF8Pc/LuPc2 device exhibits n-type behavior under reducing gases and p-type behavior under oxidizing gases. The nature of majority of the charge carriers of Co-based devices varies depending on the nature of target gases, displaying an ambipolar behavior. Both heterojunction devices demonstrate stable and observable response toward all three toxic gases in the sub-ppm range. Remarkably, the Co-based device is highly sensitive toward ammonia with a limit of detection (LOD) of 200 ppb, whereas the Zn-based device demonstrates exceptional sensitivity toward oxidizing gases, with excellent LOD values of 4.9 and 0.75 ppb toward NO2 and O3, respectively, which makes it one of the most effective organic heterojunction sensors reported so far for oxidizing gases.

Effects of UAS Rotor Wash on Air Quality Measurements.

Laboratory and field tests examined the potential for unmanned aircraft system (UAS) rotor wash effects on gas and particle measurements from a biomass combustion source. Tests compared simultaneous placement of two sets of CO and CO2 gas sensors and PM2.5 instruments on a UAS body and on a vertical or horizontal extension arm beyond the rotors. For 1 Hz temporal concentration comparisons, correlations of body versus arm placement for the PM2.5 particle sensors yielded R2 = 0.85 and for both gas sensor pairs exceeded R2 of 0.90. Increasing the timestep to 10 s average concentrations throughout the burns improved the R2 value for the PM2.5 to 0.95 from 0.85. Finally, comparison of whole-test average concentrations further increased the correlations between body- and arm-mounted sensors, exceeding R2 of 0.98 for both gases and particle measurements. Evaluation of PM2.5 emission factors with single factor ANOVA analyses showed no significant differences between the values derived from the arm, either vertical or horizontal, and those from the body. These results suggest that rotor wash effects on body- and arm-mounted sensors are minimal in scenarios where short duration, time-averaged concentrations are used to calculate emission factors and whole-area flux values.

2D Amino-Functionalized Black Phosphorus: A New Approach to Improve Hydrogen Gas Detection Performance.

In recent years, hydrogen has gained attention as a potential solution to replace fossil fuels, thus reducing greenhouse gas emissions. The development of ever improving hydrogen sensors is a topic that is constantly under study due to concerns about the inherent risk of leaks of this gas and potential explosions. In this work, a new, long-term, stable phosphorene-based sensor was developed for hydrogen detection. A simple functionalization of phosphorene using urea was employed to synthesize an air-stable material, subsequently used to prepare films for gas sensing applications, via the drop casting method. The material was deeply characterized by different techniques (scanning electron microscopy, X-ray diffraction, X-ray photoelectron, and Raman spectroscopy), and the stability of the material in a noninert atmosphere was evaluated. The phosphorene-based sensor exhibited high sensitivity (up to 700 ppm) and selectivity toward hydrogen at room temperature, as well as long-term stability over five months under ambient conditions. To gain further insight into the gas sensing mechanism over the surface, we employed a dedicated apparatus, namely operando diffuse reflectance infrared Fourier transform, by exposing the chemoresistive sensor to hydrogen gas under dry air conditions.

A Prototype of Graphene E-Nose for Exhaled Breath Detection and Label-Free Diagnosis of Helicobacter Pylori Infection.

Helicobacter pylori (HP), a common microanaerobic bacteria that lives in the human mouth and stomach, is reported to infect ≈50% of the global population. The current diagnostic methods for HP are either invasive, time-consuming, or harmful. Therefore, a noninvasive and label-free HP diagnostic method needs to be developed urgently. Herein, reduced graphene oxide (rGO) is composited with different metal-based materials to construct a graphene-based electronic nose (e-nose), which exhibits excellent sensitivity and cross-reactive response to several gases in exhaled breath (EB). Principal component analysis (PCA) shows that four typical types of gases in EB can be well discriminated. Additionally, the potential of the e-nose in label-free detection of HP infection is demonstrated through the measurement and analysis of EB samples. Furthermore, a prototype of an e-nose device is designed and constructed for automatic EB detection and HP diagnosis. The accuracy of the prototype machine integrated with the graphene-based e-nose can reach 92% and 91% in the training and validation sets, respectively. These results demonstrate that the highly sensitive graphene-based e-nose has great potential for the label-free diagnosis of HP and may become a novel tool for non-invasive disease screening and diagnosis.

Balancing Pd-H Interactions: Thiolate-Protected Palladium Nanoclusters for Robust and Rapid Hydrogen Gas Sensing.

The transition toward hydrogen gas (H2) as an eco-friendly and renewable energy source necessitates advanced safety technologies, particularly robust sensors for H2 leak detection and concentration monitoring. Although palladium (Pd)-based materials are preferred for their strong H2 affinity, intense palladium-hydrogen (Pd-H) interactions lead to phase transitions to palladium hydride (PdHx), compromising sensors' durability and detection speeds after multiple uses. In response, this study introduces a high-performance H2 sensor designed from thiolate-protected Pd nanoclusters (Pd8SR16), which leverages the synergistic effect between the metal and protective ligands to form an intermediate palladium-hydrogen-sulfur (Pd-H-S) state during H2 adsorption. Striking a balance, it preserves Pd-H binding affinity while preventing excessive interaction, thus lowering the energy required for H2 desorption. The dynamic adsorption-dissociation-recombination-desorption process is efficiently and highly reversible with Pd8SR16, ensuring robust and rapid H2 sensing at parts per million (ppm). The Pd8SR16-based sensor demonstrates exceptional stability (50 cycles; 0.11% standard deviation in response), prompt response/recovery (t90 = 0.95 s/6 s), low limit of detection (LoD, 1 ppm), and ambient temperature operability, ranking it among the most sensitive Pd-based H2 sensors. Furthermore, a multifunctional prototype demonstrates the practicality of real-world gas sensing using ligand-protected metal nanoclusters.

Fit-for-purpose WWTP unmanned aerial systems: A game changer towards an integrated and sustainable management strategy.

In the ongoing Anthropocene era, air quality monitoring constitutes a primary axis of European and international policies for all sectors, including Waste Water Treatment Plants (WWTPs). Unmanned Aerial Systems (UASs) with proper sensing equipment provide an edge technology for air quality and odor monitoring. In addition, Unmanned Aerial Vehicle (UAV) photogrammetry has been used in civil engineering, environmental (water) quality assessment and lately for industrial facilities monitoring. This study constitutes a systematic review of the late advances and limitations of germane equipment and implementations. Despite their unassailable flexibility and efficiency, the employment of the aforementioned technologies in WWTP remote monitoring is yet sparse, partial, and concerns only particular aspects. The main finding of the review was the lack of a tailored UAS for WWTP monitoring in the literature. Therefore, to fill in this gap, we propose a fit-for-purpose remote monitoring system consisting of a UAS with a platform that would integrate all the required sensors for air quality (i.e., emissions of H2S, NH3, NOx, SO2, CH4, CO, CO2, VOCs, and PM) and odor monitoring, multispectral and thermal cameras for photogrammetric structural health monitoring (SHM) and wastewater/effluent properties (e.g., color, temperature, etc.) of a WWTP. It constitutes a novel, supreme and integrated approach to improve the sustainable management of WWTPs. Specifically, the developments that a fit-for-purpose WWTP UAS would launch, are fostering the decision-making of managers, administrations, and policymakers, both in operational conditions and in case of failures, accidents or natural disasters. Furthermore, it would significantly reduce the operational expenditure of a WWTP, ensuring personnel and population health standards, and local area sustainability.

Research Progress on Ammonia Sensors Based on Ti3C2Tx MXene at Room Temperature: A Review.

Ammonia (NH3) potentially harms human health, the ecosystem, industrial and agricultural production, and other fields. Therefore, the detection of NH3 has broad prospects and important significance. Ti3C2Tx is a common MXene material that is great for detecting NH3 at room temperature because it has a two-dimensional layered structure, a large specific surface area, is easy to functionalize on the surface, is sensitive to gases at room temperature, and is very selective for NH3. This review provides a detailed description of the preparation process as well as recent advances in the development of gas-sensing materials based on Ti3C2Tx MXene for room-temperature NH3 detection. It also analyzes the advantages and disadvantages of various preparation and synthesis methods for Ti3C2Tx MXene's performance. Since the gas-sensitive performance of pure Ti3C2Tx MXene regarding NH3 can be further improved, this review discusses additional composite materials, including metal oxides, conductive polymers, and two-dimensional materials that can be used to improve the sensitivity of pure Ti3C2Tx MXene to NH3. Furthermore, the present state of research on the NH3 sensitivity mechanism of Ti3C2Tx MXene-based sensors is summarized in this study. Finally, this paper analyzes the challenges and future prospects of Ti3C2Tx MXene-based gas-sensitive materials for room-temperature NH3 detection.

Finite Element Simulation Model of Metallic Thermal Conductivity Detectors for Compact Air Pollution Monitoring Devices.

Air pollution has been associated with several health problems. Detecting and measuring the concentration of harmful pollutants present in complex air mixtures has been a long-standing challenge, due to the intrinsic difficulty of distinguishing among these substances from interferent species and environmental conditions, both indoor and outdoor. Despite all efforts devoted by the scientific and industrial communities to tackling this challenge, the availability of suitable device technologies able to selectively discriminate these pollutants present in the air at minute, yet dangerous, concentrations and provide a quantitative measure of their concentrations is still an unmet need. Thermal conductivity detectors (TCDs) show promising characteristics that make them ideal gas sensing tools capable of recognising different gas analytes based on their physical fingerprint characteristics at the molecular level, such as their density, thermal conductivity, dynamic viscosity, and others. In this paper, the operation of TCD gas sensors is presented and explored using a finite element simulation of Joule heating in a sensing electrode placed in a gas volume. The results obtained show that the temperature, and hence, the resistance of the individual suspended microbridge sensor device, depends on the surrounding gas and its thermal conductivity, while the sensitivity and power consumption depend on the properties of the constitutive metal. Moreover, the electrode resistance is proven to be linearly dependent on the applied voltage.

A Gas Sensors Detection System for Real-Time Monitoring of Changes in Volatile Organic Compounds during Oolong Tea Processing.

The oxidation step in Oolong tea processing significantly influences its final flavor and aroma. In this study, a gas sensors detection system based on 13 metal oxide semiconductors with strong stability and sensitivity to the aroma during the Oolong tea oxidation production is proposed. The gas sensors detection system consists of a gas path, a signal acquisition module, and a signal processing module. The characteristic response signals of the sensor exhibit rapid release of volatile organic compounds (VOCs) such as aldehydes, alcohols, and olefins during oxidative production. Furthermore, principal component analysis (PCA) is used to extract the features of the collected signals. Then, three classical recognition models and two convolutional neural network (CNN) deep learning models were established, including linear discriminant analysis (LDA), k-nearest neighbors (KNN), back-propagation neural network (BP-ANN), LeNet5, and AlexNet. The results indicate that the BP-ANN model achieved optimal recognition performance with a 3-4-1 topology at pc = 3 with accuracy rates for the calibration and prediction of 94.16% and 94.11%, respectively. Therefore, the proposed gas sensors detection system can effectively differentiate between the distinct stages of the Oolong tea oxidation process. This work can improve the stability of Oolong tea products and facilitate the automation of the oxidation process. The detection system is capable of long-term online real-time monitoring of the processing process.

Recent Progress in the Applications of Langmuir-Blodgett Film Technology.

Langmuir-Blodgett (LB) film technology is an advanced technique for the preparation of ordered molecular ultra-thin films at the molecular level, which transfers a single layer of film from the air/water interface to a solid substrate for the controlled assembly of molecules. LB technology has continually evolved over the past century, revealing its potential applications across diverse fields. In this study, the latest research progress of LB film technology is reviewed, with emphasis on its latest applications in gas sensors, electrochemical devices, and bionic films. Additionally, this review evaluates the strengths and weaknesses of LB technology in the application processes and discusses the promising prospects for future application of LB technology.

Ultrathin WO3 Nanosheets/Pd with Strong Metal-Support Interactions for Highly Sensitive and Selective Detection of Mustard-Gas Simulants.

Exposure to mustard gas can cause damage or death to human beings, depending on the concentration and duration. Thus, developing high-performance mustard-gas sensors is highly needed for early warning. Herein, ultrathin WO3 nanosheet-supported Pd nanoparticles hybrids (WO3 NSs/Pd) are prepared as chemiresistive sulfur mustard simulant (e.g., 2-chloroethyl ethyl sulfide, 2-CEES) gas sensors. As a result, the optimal WO3 NSs/Pd-2 (2 wt % of Pd)-based sensor exhibits a high response of 8.5 and a rapid response/recovery time of 9/92 s toward 700 ppb 2-CEES at 260 °C. The detection limit could be as low as 15 ppb with a response of 1.4. Moreover, WO3 NSs/Pd-2 shows good repeatability, 30-day operating stability, and good selectivity. In WO3 NSs/Pd-2, ultrathin WO3 NSs are rich in oxygen vacancies, offer more sites to adsorb oxygen species, and make their size close to or even within the thickness of the so-called electron depletion layer, thus inducing a large resistance change (response). Moreover, strong metal-support interactions (SMSIs) between WO3 NSs and Pd nanoparticles enhance the catalytic redox reaction performance, thereby achieving a superior sensing performance toward 2-CEES. These findings in this work provide a new approach to optimize the sensing performance of a chemiresistive sensor by constructing SMSIs in ultrathin metal oxides.

Ultrafast and Parts-per-Billion-Level MEMS Gas Sensors by Hetero-Interface Engineering of 2D/2D Cu-TCPP@ZnIn2S4 with Enriched Surface Sulfur Vacancies.

The primary challenge for resonant-gravimetric gas sensors is the synchronous improvement of the sensitivity and response time, which is restricted by low adsorption capacity and slow mass transfer in the sensing process and remains a great challenge. In this study, a novel 2D/2D Cu-TCPP@ZnIn2S4 composite is successfully constructed, in which Cu-TCPP MOF is used as a core substrate for the growth of 2D ultrathin ZnIn2S4 nanosheets with well-defined {0001} crystalline facets. The Cu-TCPP@ZnIn2S4 sensor exhibited high sensitivity (1.5 Hz@50 and 2.3 Hz@100 ppb), limit of detection (LOD: 50 ppb), and ultrafast (9 s @500 ppb) detection of triethylamine (TEA), which is the lowest LOD and the fastest sensor among the reported TEA sensors at room temperature, tackling the bottleneck for the ultrafast detection of the resonant-gravimetric sensor. These above results provide an innovative and easily achievable pathway for the synthesis of heterogeneous structure sensing materials.

A portable gas sensor based on In2O3@CuO P-N heterojunction connected via Wi-Fi to a smartphone for real-time carbon monoxide determination.

This research presents a compact portable electronic gas sensor that can be monitored through a smartphone application. The smart sensor utilizes three state-of-the-art sensors. The sensors integrate an ESP8266 microcontroller within the same device. This facilitates their integration with the electronics and enhances their performance. Herein, primarily focuses on utilizing the sensor to detect carbon monoxide. This article outlines the fabrication process of a gas sensor utilizing a P-N heterojunction, eliminating the need for a binder. The sensor consists of CuO/copper foam nanowires and hierarchical In2O3. In order to verify the system's functionality, it underwent testing with various levels of CO concentrations (10-900 ppm), including particular tests designed to examine the device's performance in different humidity and temperature circumstances. A mobile application for the provision of monitoring services has been developed at last. To process the information obtained from the gas sensor, an algorithm has been constructed, trained, and integrated into a smartphone for this purpose. This research demonstrated that a smartphone-coupled gas sensor is a viable system for real-time monitoring and the detection of CO gas.

Novel Chemoresistive Sensors for Indoor CO2 Monitoring: Validation in an Operational Environment.

Health and safety considerations of indoor occupants in enclosed spaces are crucial for building management which involves the strict control and monitoring of carbon dioxide levels to maintain acceptable air quality standards. For this study, we developed a wireless, noninvasive, and portable platform for the continuous monitoring of carbon dioxide concentration in enclosed environments, i.e., academic rooms. The system aimed to monitor and detect carbon dioxide using novel low-cost metal oxide-based chemoresistive sensors, achieving sensing performance comparable to those of commercially available detectors based on optical working principle, e.g., nondispersive infrared sensors. In particular, a predictive study of carbon dioxide levels was performed by exploiting random forest and curve fitting algorithms on chemoresistive sensor data collected in an academic room, then comparing the results with lab-based measurements. The performance of the models was evaluated with real environment conditions during 7 weeks. The field measurements were conducted to validate and support the development of the system for real-time monitoring and alerting in the presence of relevant concentrations (above 1,000 ppm). Therefore, the study highlighted that the curve fitting model obtained was able to recognize with an F1-score of 0.77 the presence of poor air quality, defined as concentration above 1,000 ppm of carbon dioxide as reported by the Occupational Safety and Health Administration.