UV illumination-enhanced ultrasensitive ammonia gas sensor based on (001)TiO2/MXene heterostructure for food spoilage detection

An electrochemical sensing potential of cobalt oxide nanoparticles towards citric acid integrated with computational approach in food and biological media

Although citric acid (CA) has antioxidant, antibacterial, and acidulating properties, chronic ingestion of CA can cause urolithiasis, hypocalcemia, and duodenal cancer, emphasizing the need for early detection. There are very few documented electrochemical-based sensing methods for CA detection due to the challenging behavior of electrode fouling caused by reactive oxidation products. In this study, a novel, non-enzymatic, and economical electrochemical sensor based on cobalt oxide nanoparticles (CoOxNPs) is successfully reported for detection CA. The CoOxNPs were synthesized through a simple thermal decomposition method and characterized by SEM, FT-IR, EDX, and XRD techniques. The proposed sensing platform was optimized by various parameters, including pH (7.0), time (15 min), and concentration of nanoparticles (100 mM) etc. In a linear range of 0.05-2500 μM, a low detection limit (LOD) of 0.13 μM was achieved. Theoretical calculations (ΔRT), confirmed hydrogen bonding and electrostatic interactions between CoOxNPs and CA. The detection method exhibited high selectivity in real media like food and biological samples, with good recovery values when compared favorably to the HPLC method. To facilitate effective on-site investigation, such a sensing platform can be assembled into a portable device.

The Application of Multi-Parameter Multi-Modal Technology Integrating Biological Sensors and Artificial Intelligence in the Rapid Detection of Food Contaminants

Against the backdrop of continuous socio-economic development, there is a growing concern among people about food quality and safety. Individuals are increasingly realizing the critical importance of healthy eating for bodily health; hence the continuous rise in demand for detecting food pollution. Simultaneously, the rapid expansion of global food trade has made people's pursuit of high-quality food more urgent. However, traditional methods of food analysis have certain limitations, mainly manifested in the high degree of reliance on personal subjective judgment for assessing food quality. In this context, the emergence of artificial intelligence and biosensors has provided new possibilities for the evaluation of food quality. This paper proposes a comprehensive approach that involves aggregating data relevant to food quality indices and developing corresponding evaluation models to highlight the effectiveness and comprehensiveness of artificial intelligence and biosensors in food quality evaluation. The potential prospects and challenges of this method in the field of food safety are comprehensively discussed, aiming to provide valuable references for future research and practice.

Enhanced oxygen anions generation on Bi2S3/Sb2S3 heterostructure by visible light for trace H2S detection at room temperature

Bismuth sulfide (Bi2S3) possesses unique properties that make it a promising material for effective hydrogen sulfide (H2S) detection at room temperature. However, when exposed to light, the oxygen anions (O2-(ads)) adsorbed on the surface of Bi2S3 can react with photoinduced holes, ultimately reducing the ability to respond to H2S. In this study, Bi2S3/Sb2S3 heterostructures were synthesized, producing photoinduced oxygen anions (O2-(hv)) under visible light conditions, resulting in enhanced H2S sensing capability. The Bi2S3/Sb2S3 heterostructure sensor exhibits a two-fold increase in sensing response to 500 ppb H2S under in door light conditions relative to its performance in darkness. Additionally, the sensing response of the Bi2S3/Sb2S3 sensor (Ra/Rg= 23.3) was approximately five times higher than pure Bi2S3. The improved sensing performance of the Bi2S3/Sb2S3 heterostructures is attributable to the synergistic influence of the heterostructure configuration and light modulation, which enhances the H2S sensing performance by facilitating rapid charge transfer and increasing active sites (O2-(hv)) when exposed to visible light.

Advancement of metal oxide nanomaterials on agri-food fronts

The application of metal oxide nanomaterials (MOx NMs) in the agrifood industry offers innovative solutions that can facilitate a paradigm shift in a sector that is currently facing challenges in meeting the growing requirements for food production, while safeguarding the environment from the impacts of current agriculture practices. This review comprehensively illustrates recent advancements and applications of MOx for sustainable practices in the food and agricultural industries and environmental preservation. Relevant published data point out that MOx NMs can be tailored for specific properties, enabling advanced design concepts with improved features for various applications in the agrifood industry. Applications include nano-agrochemical formulation, control of food quality through nanosensors, and smart food packaging. Furthermore, recent research suggests MOx's vital role in addressing environmental challenges by removing toxic elements from contaminated soil and water. This mitigates the environmental effects of widespread agrichemical use and creates a more favorable environment for plant growth. The review also discusses potential barriers, particularly regarding MOx toxicity and risk evaluation. Fundamental concerns about possible adverse effects on human health and the environment must be addressed to establish an appropriate regulatory framework for nano metal oxide-based food and agricultural products.

MXene Key Composites: A New Arena for Gas Sensors

With the development of science and technology, the scale of industrial production continues to grow, and the types and quantities of gas raw materials used in industrial production and produced during the production process are also constantly increasing. These gases include flammable and explosive gases, and even contain toxic gases. Therefore, it is very important and necessary for gas sensors to detect and monitor these gases quickly and accurately. In recent years, a new two-dimensional material called MXene has attracted widespread attention in various applications. Their abundant surface functional groups and sites, excellent current conductivity, tunable surface chemistry, and outstanding stability make them promising for gas sensor applications. Since the birth of MXene materials, researchers have utilized the efficient and convenient solution etching preparation, high flexibility, and easily functionalize MXene with other materials to prepare composites for gas sensing. This has opened a new chapter in high-performance gas sensing materials and provided a new approach for advanced sensor research. However, previous reviews on MXene-based composite materials in gas sensing only focused on the performance of gas sensing, without systematically explaining the gas sensing mechanisms generated by different gases, as well as summarizing and predicting the advantages and disadvantages of MXene-based composite materials. This article reviews the latest progress in the application of MXene-based composite materials in gas sensing. Firstly, a brief summary was given of the commonly used methods for preparing gas sensing device structures, followed by an introduction to the key attributes of MXene related to gas sensing performance. This article focuses on the performance of MXene-based composite materials used for gas sensing, such as MXene/graphene, MXene/Metal oxide, MXene/Transition metal sulfides (TMDs), MXene/Metal-organic framework (MOF), MXene/Polymer. It summarizes the advantages and disadvantages of MXene composite materials with different composites and discusses the possible gas sensing mechanisms of MXene-based composite materials for different gases. Finally, future directions and inroads of MXenes-based composites in gas sensing are presented and discussed.

Quantum-level investigation of air decomposed pollutants gas sensor (Pd-modified g-C3N4) influenced by micro-water content

In the electrical industry, there are many hazardous gases that pollute the environment and even jeopardize human health, so timely detection and effective control of these hazardous gases is of great significance. In this work, the gas-sensitive properties of Pd-modified g-C3N4 interface for each hazardous gas molecule were investigated from a microscopic viewpoint, taking the hazardous gases (CO, NOx) that may be generated in the power industry as the detection target. Then, the performance of Pd-modifiedg-C3N4 was evaluated for practical applications as a gas sensor material. Novelly, an unconventional means was designed to briefly predict the effect of humidity on the adsorption properties of this sensor material. The final results found that Pd-modified g-C3N4 is most suitable as a potential gas-sensitizing material for NO2 gas sensors, followed by CO. Interestingly, Pd-modified g-C3N4 is less suitable as a potential gas-sensitizing material for NO gas sensors, but has the potential to be used as a NO cleaner (adsorbent). Unconventional simulation explorations of humidity effects show that in practical applications Pd-modified g-C3N4 remains a promising material for gas sensing in specific humidity environments. This work reveals the origin of the excellent properties of Pd-modified g-C3N4 as a gas sensor material and provides new ideas for the detection and treatment of these three hazardous gases.

First-Principles Predictions of MoS2-WS2 In-Plane Heterostructures for Sensing Dissolved Gas Species in Oil-Immersed Transformers

This work from first-principles insight uses a MoS2-WS2 in-plane heterostructure as a potential sensing material for detection of CO and C2H2, two typical dissolved gases in oil-immersed transformers, in order to evaluate the operation status. The adsorption performance of the MoS2-WS2 heterostructure upon two gas species is assessed via three adsorption sites and compared with isolated MoS2 and WS2. Results indicate that MoS2-WS2 performs with a much stronger binding force and charge-transfer for adsorptions of CO and C2H2 in comparison to the isolated counterpart, which gives rise to more obvious deformation in the electronic property of MoS2-WS2 as well as a much larger resistance-based sensing response. The recovery time of MoS2-WS2 for desorption of CO and C2H2 molecules is also appropriate to allow the reusability of such a sensor. The findings in this work uncover the admirable sensing potential of transition metal dichalcogenides (TMDs)-based heterostructures upon oil dissolved gases, which opens up a new way to explore novel 2D nanomaterials as resistive gas sensors for dissolved gas analysis in electrical oil-immersed transformers.

The Challenges and Opportunities for TiO2 Nanostructures in Gas Sensing

Chemiresistive gas sensors based on metal oxides have been widely applied in industrial monitoring, medical diagnosis, environmental pollutant detection, and food safety. To further enhance the gas sensing performance, researchers have worked to modify the structure and function of the material so that it can adapt to different gas types and environmental conditions. Among the numerous gas-sensitive materials, n-type TiO2 semiconductors are a focus of attention for their high stability, excellent biosafety, controllable carrier concentration, and low manufacturing cost. This Perspective first introduces the sensing mechanism of TiO2 nanostructures and composite TiO2-based nanomaterials and then analyzes the relationship between their gas-sensitive properties and their structure and composition, focusing also on technical issues such as doping, heterojunctions, and functional applications. The applications and challenges of TiO2-based nanostructured gas sensors in food safety, medical diagnosis, environmental detection, and other fields are also summarized in detail. Finally, in the context of their practical application challenges, future development technologies and new sensing concepts are explored, providing new ideas and directions for the development of multifunctional intelligent gas sensors in various application fields.

Nitrogen-Based Gas Molecule Adsorption on a ReSe2 Monolayer via Single-Atom Doping: A First-Principles Study

Two-dimensional materials have shown immense promise for gas-sensing applications due to their remarkable surface-to-volume ratios and tunable chemical properties. However, despite their potential, the utilization of ReSe2 as a gas-sensing material for nitrogen-containing molecules, including NO2, NO, and NH3, has remained unexplored. The choice of doping atoms in ReSe2 plays a pivotal role in enhancing the gas adsorption and gas-sensing capabilities. Herein, the adsorption properties of nitrogen-containing gas molecules on metal and non-metal single-atom (Au, Pt, Ni, P, and S)-doped ReSe2 monolayers have been evaluated systematically via ab initio calculations based on density functional theory. The findings strongly suggest that intrinsic ReSe2 has better selectivity toward NO2 than toward NO and NH3. Moreover, our results provide compelling evidence that all of the dopants, with the exception of S, significantly enhance both the adsorption strength and charge transfer between ReSe2 and the investigated molecules. Notably, P-decorated ReSe2 showed the highest adsorption energy for NO2 and NO (-1.93 and -1.52 eV, respectively) with charge transfer above 0.5e, while Ni-decorated ReSe2 exhibited the highest adsorption energy for NH3 (-0.76 eV). In addition, on the basis of transition theory, we found that only Au-ReSe2 and Ni-ReSe2 can serve as reusable chemiresisitve gas sensors for reliable detection of NO and NH3, respectively. Hence, our findings indicate that gas-sensing applications can be significantly improved by utilizing a single-atom-doped ReSe2 monolayer.

Adsorption Properties of Dissolved Gas Molecules in Transformer Oil on the ReSe2 Monolayer: A DFT Study

Based on density functional theory, the adsorption behavior of seven typical dissolved gas molecules (CO, CO2, H2, CH4, C2H2, C2H4, and C2H6) and H2O molecule on the ReSe2 monolayer was systematically investigated. The interactions between the ReSe2 monolayer and eight gas molecules were investigated by calculating the adsorption energies, charge transfer, density of states (DOS), and deformation charge density (DCD) for eight different adsorption systems. The gas sensitivity of the ReSe2 monolayer toward these gases was studied using frontier molecular orbital theory and work function analysis. The results demonstrate that compared to other gas molecules, the ReSe2 monolayer exhibits a stronger interaction with CO with an adsorption energy of -1.49 eV. It also displays excellent sensitivity and selectivity toward CO making it a promising candidate for CO gas sensing applications. We aspire that this research will offer theoretical guidance for the development of ReSe2-based gas sensors and contribute to state monitoring technology in oil-immersed power equipment.

Flexible Antibacterial Respiratory Monitoring Sensor Based on Controllable Au-Modified Surface of Highly {001} Preferred Anatase Titanium Dioxide Thin Film

Respiratory signals are critical clinical diagnostic criteria for respiratory diseases and health conditions, and respiratory sensors play a crucial role in achieving the desired respiratory monitoring effect. High sensitivity to a single factor can improve the reliability of respiratory monitoring, and maintaining the hygiene of the sensors is also important for daily health monitoring. Herein, we propose a flexible Au-modified anatase titanium dioxide resistive respiratory sensor, which can be mechanically compliantly attached to curved surfaces for respiratory monitoring in different modalities (i.e., respiratory intensity, frequency, and rate). The uniform and preferentially oriented anatase titanium dioxide films gained by the polymer-assisted deposition technique can be fabricated on flexible substrates through a liquid-assisted transferring process. The Au modification can enhance surface plasmon resonance to facilitate the photocatalytic activity of titanium dioxide, and the optimized distribution of Au on the surface of titanium dioxide film made the sensor have an excellent antibacterial effect. The uniquely designed encapsulation can effectively control the contact between the surface of titanium dioxide films and electrodes, allowing the flexible sensor to exhibit fast response time (0.71 s) and recovery time (1.06 s) to respiratory as well as insensitivity or low sensitivity to other factors (i.e., gas composition, humidity, temperature, stress, and strain). This work provided an effective strategy for flexible wearable respiratory sensors and has great potential in daily respiratory monitoring for health management and pandemic control.

Colorimetric and fluorometric bimodal amine chemosensor based on deprotonation-induced intramolecular charge transfer: Application to food spoilage detection

Amine derivatives are signature organic compounds generated from rotten protein food. Thus, sensitive detection of the presence of amines in protein foods can be a critical technique for monitoring their quality. In this study, we develop an organic chemosensor probe, 4-(2-(3-(dicyanomethylene)- 5,5-dimethylcyclohex-1-en-1-yl)vinyl)-N,N-diethylbenzenaminium chloride (DEAH), to effectively detect amines through discernible bimodal (colorimetric and fluorometric) changes. By exploiting the amine-triggered intramolecular charge transfer behavior, DEAH exhibits rapid color changes (<1 s) with an excellent detection limit (36.9 nM) and also fluorescence turn-on in response to amine gas. Moreover, it possesses detection capabilities in versatile conditions, including solutions, solids, and coated films, suggesting its practical applicability. In particular, DEAH shows dramatic color change from yellow to violet with exceptional color difference (△Eab) over 98, repeatable usability, and excellent selectivity to amines. Based on these compelling advantages, we successfully demonstrate real-time monitoring of amine gas generated from spoiled protein foods using DEAH-coated films.

Adsorption Behavior of Dissolved Gas Molecules in Transformer Oil on Rh Modified GeSe Monolayer

Dissolved gas analysis in transformer oil is useful for detecting early transformer failures. The research on gas sensors for monitoring dissolved gas in transformer oil has attracted wide attention from academia and industry. In this study, Rh-doped GeSe monolayers were used as gas sensing materials based on the density functional theory (DFT). The potential of the Rh-GeSe monolayer as a gas sensor was evaluated by calculating the geometric structure, adsorption distance (dsub/gas), binding energy (Eb), adsorption energy (Eads), transfer charge (ΔQ), the density of states (DOS), band structure, electron localization function (ELF), charge difference density (CDD), and sensitivity (S) of Rh-GeSe monolayer with eight gas molecules (SO2, C2H2, NO2, H2, CH4, CO2, H2S, and CO). The results show that the Rh-GeSe monolayer has a prominent response to SO2, C2H2, and NO2 gas molecules and has great potential to become an excellent gas sensor. This study provides a theoretical basis for the application of Rh-GeSe monolayer in the field of gas sensing and provides a new way for the development of other gas sensors.

Metal (Ni, Pd, and Pt)-Doped BS Monolayers as a Gas Sensor upon Vented Gases in Lithium-Ion Batteries: A First-Principles Study

Real-time monitoring of the vented gases emitted by the thermal runaway of lithium-ion batteries (LIBs) is of great significance to the normal use of LIBs. We study systematically the adsorption and sensing performances of pristine and metal-doped BS monolayers to five typical gases (CO, CO2, CH4, C2H2, and C2H4) emitted from LIBs employing the first-principles method. The adsorption structure and energetics, charge transfer, band structure, density of states, sensitivity, and recovery time are simulated and analyzed. Outstanding sensing properties are predicted for the Ni-, Pd-, and Pt-doped BS monolayers, although their recently synthesized pristine counterpart shows little sensing potential for those gases. The magnitude of the adsorption energy increases from 0.249 eV to 2.32 eV (Ni-BS), 1.954 eV(Pd-BS), and 2.994 eV (Pt-BS) for the CO gas after doping. Besides, significant variation of band gap is observed after gas adsorption in doped BS nanosheets, which leads to huge theoretical values of the sensitivity. The sensitivity for CO, CO2, CH4, C2H2, and C2H4 on Pt-BS may reach up to 5.87 × 105, 1.57 × 106, 1.81 × 105, 8.33 × 104, and 8.18 × 103, respectively. In addition, the calculated recovery times indicate that the doped BS monolayers have strong selectivity for the adsorption and detection of these five gases. The three metal-doped BS monolayers should have great potential for application in sensors monitoring the gases emitted from LIBs.

A Highly Sensitive Surface-Modified Porous Carbon Nanotube-Based Sensor for Ammonia Gas Detection

In this work, we compared the gas sensing behaviors of pristine and decorated multi-walled carbon nanotubes (MWCNTs) and examined the response behavior of bare and adorned MWCNTs in gas sensing. According to the data, the decorated response was 144%, which is higher than the bare CNT response of 117% in terms of the sensing response. The RF-sputtering method is used to decorate the carbon nanotubes by pure Indium (In) metal nanoparticles. Every measurement was performed in a temperature-controlled environment. Tests of the entire procedure were conducted at a 10 ppm concentration of ammonia gas. We have observed the quick reaction time (1-10 s) in pristine and (1-7 s) in decorated MWCNTs. The response was obtained 117% for the pristine and 144, 115, and 73% for the second (3 min decoration), third (6 min decoration), and fourth (9 min decoration) MWCNTs, respectively. The as-prepared pristine samples and all the decorated sensors had sensitivity values of 0.45, 0.50, 0.51, and 0.57 for time intervals of 0, 3, 6, and 9 min, respectively. It amounted to 45% for the pure and 50, 51, and 57% for the remaining as-prepared decorated sensors, respectively. Based on the measured sensor response graph, a recovery of between 80 and 85% was achieved. For a period of 10 days at a constant concentration, the stability was also assessed and we have analyzed the structural, electrical, and elemental composition of the prepared CNTs by FESEM, EDX, Raman spectroscopy, FTIR, and XRD.

Synergistic Integration of Machine Learning with Microstructure/Composition-Designed SnO2 and WO3 Breath Sensors

A high-performance semiconductor metal oxide gas sensing strategy is proposed for efficient sensor-based disease prediction by integrating a machine learning methodology with complementary sensor arrays composed of SnO2- and WO3-based sensors. The six sensors, including SnO2- and WO3-based sensors and neural network algorithms, were used to measure gas mixtures. The six constituent sensors were subjected to acetone and hydrogen environments to monitor the effect of diet and/or irritable bowel syndrome (IBS) under the interference of ethanol. The SnO2- and WO3-based sensors suffer from poor discrimination ability if sensors (a single sensor or multiple sensors) within the same group (SnO2- or WO3-based) are separately applied, even when deep learning is applied to enhance the sensing operation. However, hybrid integration is proven to be effective in discerning acetone from hydrogen even in a two-sensor configuration through the synergistic contribution of supervised learning, i.e., neural network approaches involving deep neural networks (DNNs) and convolutional neural networks (CNNs). DNN-based numeric data and CNN-based image data can be exploited for discriminating acetone and hydrogen, with the aim of predicting the status of an exercise-driven diet and IBS. The ramifications of the proposed hybrid sensor combinations and machine learning for the high-performance breath sensor domain are discussed.

Facile synthesis of Ag lattice doped mesoporous In2O3 nanocubes for high performance ethanol sensing

Ag lattice doped In2O3 with a mesoporous structure was synthesized through a combination of hydrothermal and calcination methods. The structural and morphological characteristics were assessed using XRD, SEM, TEM, TGA, BET, and XPS analyses. Gas sensing measurements revealed that the 7.0 mol% Ag-doped In2O3 sensor displayed a response of 420 towards 100 ppm ethanol at 140 °C, which was 19 times higher than that of the pure In2O3 gas sensor. Density functional theory calculations indicated that Ag-doped In2O3 exhibited enhanced adsorption performance, higher adsorption energy, and electron transfer, resulting in higher sensitivity to ethanol. These findings were also supported by the electronic band structure, work function, and DOS analyses. These results indicated that the Ag doped mesoporous In2O3 has high potential for the preparation of high-performance ethanol sensors in practical applications.

Flicker Noise in Resistive Gas Sensors-Measurement Setups and Applications for Enhanced Gas Sensing

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., MoS2, WS2, and ZrS3) 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.

Gas Sensing Properties of Pd-Decorated GeSe Monolayer toward Formaldehyde and Benzene Molecules: A First-Principles Study

In this study, the gas sensing properties of formaldehyde (HCHO) and benzene (C6H6) adsorbed on two-dimensional (2D) pristine GeSe and Pd-decorated GeSe (Pd-GeSe) monolayers are studied by using first-principles calculations. The adsorption energies, electronic properties, optical properties, sensitivity, and recovery time of the gas adsorption systems have been thoroughly investigated. It is found that the adsorption of C6H6 on two substrate surfaces and the adsorption of HCHO on pristine GeSe are examples of physical adsorption. However, after HCHO adsorption on the Pd-GeSe monolayer, the adsorption system exhibits an increased adsorption energy of -1.21 eV, which is more favorable compared with the other adsorption systems studied. Moreover, the electron localization function and charge transfer from Pd-GeSe to HCHO are significantly enhanced, indicating distinct chemical adsorption behavior. Furthermore, it demonstrates a larger band gap change rate of 18.8% and a significant enhancement of optical absorption upon the adsorption of HCHO on the Pd-GeSe monolayer. Additionally, the appropriate sensitivity and moderate recovery time for the adsorption of HCHO on the Pd-GeSe surface indicate that the Pd-GeSe monolayer possesses an outstanding sensing capability for HCHO gas.

Modification engineering of TiO2-based nanoheterojunction photocatalysts

Titanium dioxide (TiO2)-based photocatalysts have gained increasing attention for their versatile applications in organic degradation, hydrogen production, air purification, and CO2 reduction. Various TiO2-based heterojunction structures, including type I, type II, Schottky junction, Z-scheme, and S-scheme, have been extensively studied. The current research frontier is centered on the engineering modifications of TiO2-based nanoheterojunction photocatalysts, such as defect engineering, morphological engineering, crystal phase/facet engineering, and multijunction engineering. These modifications enhance carrier transport, separation, and light absorption, thereby improving the photocatalytic performance. Remarkably, this aspect has been less addressed in existing reviews. This review aims to fill this gap by focusing on the engineering modifications of TiO2-based nanoheterojunction photocatalysts. We delve into specific topics like oxygen vacancies, n-p homojunctions, and double defects. The review also systematically discusses the applications of multidimensional heterojunctions and examines carrier transport pathways in heterophase/facet junctions and their interactions with heterojunctions. A comprehensive summary of multijunction systems, including multi-Schottky junctions, semiconductor-based heterojunction-attached Schottky junctions, and multisemiconductor-based heterojunctions, is presented. Lastly, we outline future perspectives in this promising research field. This paper will assist researchers in constructing more efficient TiO2-based nanoheterojunction photocatalysts.

Surface crafting and entrapment of CsPbBr3 perovskite QDs in ZIF-8 for ammonia recognition

The substantial optical features of perovskite quantum dots (PQD) lead to rapid growth in the investigation of their surface and lattice doping for optoelectronic and biochemical sensor advancements. Herein, we have used the surface ligand crafting model of PQD by ammonia and its optimum response to recognise ammonia in the sensing cellulose paper. The PQD with acetyl amine and octanoic acid capped were synthesized and entrapped in zeolites imidazole framework to delay the instant quenching and envisaged response to ammonia with high sensitivity. The hybrid perovskite quantum dots and Zeolite imidazolate framework-8 (PQD@ZIF-8) materials were further immersed in cellulose paper for solid-state sensor fabrication for the detection of ammonia by naked-eye and a Xiaomi Note-5 mobile camera. The ammonia was measured with high sensitivity at ambient conditions, with a detection limit of 16 ppm and a linear detection range of 1 to 500 ppm. This research provides a new platform for designing sensor selectivity and sensitivity, which could be used to further develop fluorescent nanomaterials-based sensors for small molecule detection.

2D/2D Dy2O3 Nanosheet/MoO3 Nanoflake Heterostructures for Humidity-Independent and Sensitive Ammonia Detection

Chemiresistive ammonia gas (NH3) sensors have been playing a significant role in the fields of environmental protection, food safety monitoring, and air quality evaluation. Nevertheless, balancing the high sensitivity and humidity tolerance remains challenging. Herein, the two-dimensional (2D) heterostructures of molybdenum trioxide (MoO3) nanoflakes decorated with dysprosium oxide (Dy2O3) nanosheets (termed Dy2O3/MoO3) were synthesized via a facile probe-sonication method. With respect to pristine MoO3 counterparts, the optimal Dy2O3/MoO3 sensors possessed a 4.49-fold larger response at a lower temperature (30.52@328.2 °C vs 6.8@369.7 °C toward 10 ppm of NH3), shorter response/recovery times (11.6/2.9 s vs 26.9/43.4 s), 52.6-fold higher sensitivity (17.35/ppm vs 0.33/ppm), and a lower theoretical detection limit (1.02 vs 32.82 ppb). Besides the nice reversibility, wide detection range (0.45-100 ppm) and robust long-term stability, inspiringly, the Dy2O3/MoO3 sensors showed a nearly humidity-independent response. These impressive improvements in the NH3-sensing performance were attributed to numerous heterojunctions to strengthen the carrier concentration modulation and the compensation/protection effect of Dy2O3 to mitigate the humidity effect. Moreover, the Dy2O3/MoO3 sensors showed preliminary application potential in monitoring pork freshness. This work provides a universal methodology for constructing NH3 gas sensors with high sensitivity and good humidity resistance and probably extends the application scenarios of MoO3-based sensors in the Internet of Things in the future.

Studying the Adsorption of Gas Molecules and Defects on Modulating the Electronic Transport Characteristics of Monolayer Penta-BN2-Based Devices

Two-dimensional atomic layer materials, as an important part of the post-Moore era, have recently become an ideal choice for the preparation of high-efficiency, low-power, and miniaturized gas sensors. In this work, our study utilized density functional theory and the nonequilibrium Green's function method to investigate the electronic properties of the pentagonal BN2 (P-BN2) monolayer, as well as its gas-sensing properties for organic and inorganic gases. We also investigated how defects affect the quantum transport properties of the P-BN2-based device. Our findings demonstrate that the CO, H2S, NH3, SO2, C2H5OH, C3H6OH, CH3OH, and CH4 undergo physisorption on the P-BN2 monolayer, while NO, NO2, C2H2, C2H4, and HCHO undergo chemisorption. Then, we analyzed the impact of gas molecules chemisorbed on the P-BN2 monolayer on the electronic transport properties of the P-BN2-based gas sensor. When these five gas molecules are adsorbed, the current of the P-BN2-based gas sensor is greatly reduced. In addition, the effect of defects on the quantum transport properties of the P-BN2-based device is investigated. The results indicate that defects of N, B, and BN atoms lead to a decrease in the current of P-BN2-based nanodevices. Moreover, both the adsorption of gas molecules and the formation of vacancy defects leading to a decrease in device current can be revealed by the local device density of states near the zero-bias Fermi level, elucidating their microscopic mechanisms. Finally, gas molecules can also cause a decrease in the current of defect systems. These theoretical studies are of great significance for exploring two-dimensional atomic layer materials as high-efficiency gas sensors.

Room Temperature Chemiresistive Gas Sensors Based on 2D MXenes

Owing to their large surface area, two-dimensional (2D) semiconducting nanomaterials have been extensively studied for gas-sensing applications in recent years. In particular, the possibility of operating at room temperature (RT) is desirable for 2D gas sensors because it significantly reduces the power consumption of the sensing device. Furthermore, RT gas sensors are among the first choices for the development of flexible and wearable devices. In this review, we focus on the 2D MXenes used for the realization of RT gas sensors. Hence, pristine, doped, decorated, and composites of MXenes with other semiconductors for gas sensing are discussed. Two-dimensional MXene nanomaterials are discussed, with greater emphasis on the sensing mechanism. MXenes with the ability to work at RT have great potential for practical applications such as flexible and/or wearable gas sensors.

Applications of Gas Sensing in Food Quality Detection: A Review

Food products often face the risk of spoilage during processing, storage, and transportation, necessitating the use of rapid and effective technologies for quality assessment. In recent years, gas sensors have gained prominence for their ability to swiftly and sensitively detect gases, making them valuable tools for food quality evaluation. The various gas sensor types, such as metal oxide (MOX), metal oxide semiconductor (MOS) gas sensors, surface acoustic wave (SAW) sensors, colorimetric sensors, and electrochemical sensors, each offer distinct advantages. They hold significant potential for practical applications in food quality monitoring. This review comprehensively covers the progress in gas sensor technology for food quality assessment, outlining their advantages, features, and principles. It also summarizes their applications in detecting volatile gases during the deterioration of aquatic products, meat products, fruit, and vegetables over the past decade. Furthermore, the integration of data analytics and artificial intelligence into gas sensor arrays is discussed, enhancing their adaptability and reliability in diverse food environments and improving food quality assessment efficiency. In conclusion, this paper addresses the multifaceted challenges faced by rapid gas sensor-based food quality detection technologies and suggests potential interdisciplinary solutions and directions.

Adsorption of NO2, SO2, H2S, and NH3 on Os-Doped WSe2 Monolayers: A First-Principles Study

In this study, DFT calculations are used to analyze the adsorption of industrial waste gases (NO2, SO2, H2S, and NH3) on WSe2 monolayers. The adsorption energy, energy band, density of states, charge transfer, and recovery time of the adsorption structures between the target gas molecules and the Os-doped WSe2 are studied. Compared with pure WSe2 monolayer, Os surface bonding doping WSe2 (Os-modified WSe2) and Os doping with Se vacancy of WSe2 (Os-embedded WSe2) exhibit improved gas molecule adsorption ability. Among them, the adsorption energy of the Os-modified WSe2 monolayer on NO2, SO2, H2S, and NH3 is greater than that of the WSe2 monolayer. At the same time, it is proved that the Os-embedded WSe2 can be used as a gas sensor for H2S and NH3 gas molecules at a high temperature.

Room-temperature chemiresistive ammonia sensors based on 2D MXenes and their hybrids: recent developments and future prospects

Detection of ammonia (NH3) gas at room temperature is essential in a variety of sectors, including pollution monitoring, commercial safety and medical services, etc. Two-dimensional (2D) materials have emerged as fascinating candidates for gas-sensing applications due to their distinct properties. MXenes, a type of 2D transition metal carbides/nitrides/carbonotrides, have drawn the interest of researchers due to their high conductivity, large surface area, and changing surface chemistry. The review begins by describing the NH3 gas-detecting methods of 2D materials and then concentrates on MXene-based sensors, emphasising the benefits that MXenes provide in this context. The study also explains the prime factors involved in evaluating sensor performance, which include sensor response, sensitivity, selectivity, stability, charge transfer values, adsorption energy and response/recovery times. Subsequently, the review covers two main categories: pristine/intercalated MXenes and MXene-based hybrid materials. The review investigates the approaches for improving the sensing characteristics of pristine and intercalated MXenes by introducing MXene hybrids like MXene-metal oxide hybrids, MXene-transition metal dichalcogenides hybrid, MXene-other 2D materials hybrid, MXene-polymers and other hybrids and other MXene-derived materials. In summary, this review offers a thorough overview of current advancements and potential applications for room-temperature ammonia sensors based on 2D MXenes and their hybrids. In order to pave the way for future improvements in MXene-based gas-sensing technology for room temperature ammonia detection, the study concludes by outlining potential future scope and conclusions.

Synthesis of TiO2-(B) Nanobelts for Acetone Sensing

Titanium dioxide nanobelts were prepared via the alkali-hydrothermal method for application in chemical gas sensing. The formation process of TiO2-(B) nanobelts and their sensing properties were investigated in detail. FE-SEM was used to study the surface of the obtained structures. The TEM and XRD analyses show that the prepared TiO2 nanobelts are in the monoclinic phase. Furthermore, TEM shows the formation of porous-like morphology due to crystal defects in the TiO2-(B) nanobelts. The gas-sensing performance of the structure toward various concentrations of hydrogen, ethanol, acetone, nitrogen dioxide, and methane gases was studied at a temperature range between 100 and 500 °C. The fabricated sensor shows a high response toward acetone at a relatively low working temperature (150 °C), which is important for the development of low-power-consumption functional devices. Moreover, the obtained results indicate that monoclinic TiO2-B is a promising material for applications in chemo-resistive gas detectors.

Adsorption of CH4, CO, and H2S on a MoTe2 Monolayer Doped with Metal Atoms (Au and Ru): An Ab Initio Study

Detecting toxic gases, such as CH4, CO, and H2S, in everyday life holds great significance. This research article focuses on investigating the adsorption characteristics of CH4, CO, and H2S on MoTe2 and MoTe2 doped with Au and Ru using the density functional theory. The study examines various aspects, including adsorption energy, charge transfer, density of states, and charge density difference of the adsorption configuration. The findings demonstrate that the adsorption properties of Ru-doped MoTe2 exhibit a significant enhancement for all three gases, with CO displaying the highest adsorption performance. Through comparative analysis, it is evident that the adsorption affinity between MoTe2-Ru and the three gases is robust, thus indicating improved gas detection capabilities.

Pt-Doped HfS2 Monolayer as a Novel Sensor and Scavenger for Dissolved Gases (H2, CO2, CH4, and C2H2) in Transformer Oil: A Density Functional Theory Study

Detecting the types and concentrations of dissolved gases in insulating oil by resistivity-type sensors is an extremely effective means for diagnosing faults in an oil-immersed transformer. However, further breakthroughs and innovations are needed in gas-sensitive materials for preparing high-performance resistivity-type sensors. In this investigation, the application possibility of using Pt-doped HfS2 (Pt-HfS2) as gas-sensitive materials for the detection of dissolved H2, CO2, CH4, and C2H2 in oil has been verified by analyzing the adsorption energy (Ead), differential charge density (DCD), density of states (DOS), frontier molecular orbital, and desorption time based on density functional theory (DFT). The outcomes suggest that the band gap of HfS2 is obviously narrowed after doping Pt at the position of the bridge between the S and Hf atoms, resulting in a significant increase in the conductivity of HfS2. The low adsorption energy implies that there is only weak physical adsorption between Pt-HfS2 and CO2 (-0.783 eV). In contrast, the highly hybridized atomic orbitals of Pt with H2, CH4, and C2H2 indicate that strong chemical adsorption reactions occur. Two-dimensional Pt-HfS2 as a gas sensor has a great monitoring performance for CH4 at 298 K (room temperature). This research serves as theoretical guidelines for probing the application potential of Pt-HfS2 in fault diagnosis and predictive maintenance of an oil-immersed transformer.

Applications, challenges and prospects of bionic nose in rapid perception of volatile organic compounds of food

Bionic nose, a technology that mimics the human olfactory system, has been widely used to assess food quality due to their high sensitivity, low cost, portability and simplicity. This review briefly describes that bionic noses with multiple transduction mechanisms are developed based on gas molecules' physical properties: electrical conductivity, visible optical absorption, and mass sensing. To enhance their superior sensing performance and meet the growing demand for applications, a range of strategies have been developed, such as peripheral substitutions, molecular backbones, and ligand metals that can finely tune the properties of sensitive materials. In addition, challenges and prospects coexist are covered. Cross-selective receptors of bionic nose will help and guide the selection of the best array for a particular application scenario. It provides an odour-based monitoring tool for rapid, reliable and online assessment of food safety and quality.

In-situ construction of direct Z-scheme NiO/Bi2MoO6 heterostructure arrays with enhanced room temperature ether sensing properties under visible light irradiation

Light irradiation has emerged as a promising strategy to promote room temperature sensing of resistive-type semiconductor gas sensors recently. However, high recombination rate of photo-generated carriers and poor visible light response of conventional semiconductor sensing materials have greatly limited the further performance improvement. It is urgent to develop gas sensing materials with high photo-generated carrier separation efficiency and excellent visible light response. Herein, a novel direct Z-scheme NiO/Bi₂MoO₆ heterostructure arrays were designed and in-situ constructed on alumina flat substrate to form thin film sensors, which realized excellent room temperature gas response towards ether under irradiation of visible light for the first time, together with excellent stability and selectivity. Based on density functional theory calculation and experimental characterization, it was demonstrated that the construction of Z-scheme heterostructure could greatly promote the separation of photo-generated carriers and adsorption of ether. Moreover, the excellent visible light response characteristics of NiO/Bi₂MoO₆ could improve the utilization of visible light. In addition, the in-situ construction of array structure could avoid a series of problems caused by the conventional thick film devices. The work not only provides a promising guideline for Z-scheme heterostructure arrays in promoting the room temperature sensing performance of semiconductors gas sensors under visible light irradiation, but also clarifies the gas sensing mechanism of Z-scheme heterostructure at the atomic and electronic level.

Recent Progress in Screening of Mycotoxins in Foods and Other Commodities Using MXenes-Based Nanomaterials

Mycotoxin pollution in agricultural food products endangers animal and human health during the supply chains, therefore the development of accurate and rapid techniques for the determination of mycotoxins is of great importance for food safety guarantee. MXenes-based nanoprobes have attracted enormous attention as a complementary analysis and promising alternative strategies to conventional diagnostic methods, because of their fascinating features, like high electrical conductivity, various surface functional groups, high surface area, superb thermal resistance, good hydrophilicity, and environmentally-friendlier characteristics. In this study, we outline the state-of-the-art research on MXenes-based probes in detecting various mycotoxins like aflatoxin, ochratoxin, deoxynivalenol, zearalenone, and other toxins as a most commonly founded mycotoxin in the agri-food supply chain. First, we present the diverse synthesis approaches and exceptional characteristics of MXenes. Afterward, based on the detecting mechanism, we divide the biosensing utilizations of MXenes into two subcategories: electrochemical, and optical biosensors. Then their performance in effective sensing of mycotoxins is comprehensively deliberated. Finally, present challenges and prospective opportunities for MXenes are debated.

An antibacterial and intelligent cellulose-based label self-assembled via electrovalent bonds for a multi-range sensing of food freshness

Intelligent labels provide customers with food freshness information. However, the existing label response is limited and can only detect a single kind of food. Here, an intelligent cellulose-based label with highly antibacterial activity for a multi-range sensing freshness was developed to overcome the limitation. Cellulose fibers were modified using oxalic acid to graft -COO^- followed by binding chitosan quaternary ammonium salt (CQAS), the remaining charges of which attached methylene red and bromothymol blue to form response fibers and to further self-assemble into the intelligent label. CQAS electrostatically gathered the dispersed fibers, resulting in an increase in TS and EB of 282 % and 16.2 %, respectively. After that, the rest positive charges fixed the anionic dyes to broaden pH response range of 3-9 effectively. More significantly, the intelligent label exhibited highly antimicrobial activity, killing 100 % of staphylococcus aureus. The rapid acid-base response revealed the potential for practical application in which the label color from green to orange represented the milk or spinach from fresh to close to spoiled, and from green to yellow, and to light green indicated the pork fresh, acceptable, and close to spoiled. This study paves a way for the preparation of intelligent labels in large-scale and promote the commercial application to improve food safety.

IoT-Enabled Electronic Nose System for Beef Quality Monitoring and Spoilage Detection

Food spoilage is a major concern in the food industry, especially for highly perishable foods such as beef. In this paper, we present a versatile Internet of Things (IoT)-enabled electronic nose system to monitor food quality by evaluating the concentrations of volatile organic compounds (VOCs). The IoT system consists mainly of an electronic nose, temperature/humidity sensors, and an ESP32-S3 microcontroller to send the sensors' data to the server. The electronic nose consists of a carbon dioxide gas sensor, an ammonia gas sensor, and an ethylene gas sensor. This paper's primary focus is to use the system for identifying beef spoilage. Hence, the system performance was examined on four beef samples stored at different temperatures: two at 4 °C and two at 21 °C. Microbial population quantifications of aerobic bacteria, Lactic Acid Bacteria (LAB), and Pseudomonas spp., in addition to pH measurements, were conducted to evaluate the beef quality during a period of 7 days to identify the VOCs concentrations that are associated with raw beef spoilage. The spoilage concentrations that were identified using the carbon dioxide, ammonia, and ethylene sensors were 552 ppm-4751 ppm, 6 ppm-8 ppm, and 18.4 ppm-21.1 ppm, respectively, as determined using a 500 mL gas sensing chamber. Statistical analysis was conducted to correlate the bacterial growth with the VOCs production, where it was found that aerobic bacteria and Pseudomonas spp. are responsible for most of the VOCs production in raw beef.

Highly Efficient NO2 Sensors Based on Al-ZnOHF under UV Assistance

Zinc hydroxyfluoride (ZnOHF) is a newly found resistive semiconductor used as a gas-sensing material with excellent selectivity to NO₂ because of its unique energy band structure. In this paper, Al³⁺ doping and UV radiation were used to further improve the gas-sensing performance of ZnOHF. The optimized 0.5 at.% Al-ZnOHF sample exhibits improved sensitivity to 10 ppm NO₂ at a lower temperature (100 °C) under UV assistance, as well as a short response/recovery time (35 s/96 s). The gas-sensing mechanism demonstrates that Al³⁺ doping increases electron concentration and promotes electron transfer of the nanorods by reducing the bandgap of ZnOHF, and the photogenerated electrons and holes with high activity under UV irradiation provide new reaction routes in the gas adsorption and desorption process, effectively promoting the gas-sensing process. The synergistic effect of Al³⁺ and UV radiation contribute to the enhanced performance of Al-ZnOHF.

Synthesis and applications of MXene-based composites: a review

Recently, there has been considerable interest in a new family of transition metal carbides, carbonitrides, and nitrides referred to as MXenes (Ti₃C₂T_x) due to the variety of their elemental compositions and surface terminations that exhibit many fascinating physical and chemical properties. As a result of their easy formability, MXenes may be combined with other materials, such as polymers, oxides, and carbon nanotubes, which can be used to tune their properties for various applications. As is widely known, MXenes and MXene-based composites have gained considerable prominence as electrode materials in the energy storage field. In addition to their high conductivity, reducibility, and biocompatibility, they have also demonstrated outstanding potential for applications related to the environment, including electro/photocatalytic water splitting, photocatalytic carbon dioxide reduction, water purification, and sensors. This review discusses MXene-based composite used in anode materials, while the electrochemical performance of MXene-based anodes for Li-based batteries (LiBs) is discussed in addition to key findings, operating processes, and factors influencing electrochemical performance.

Highly Sensitive Chemiresistive H2S Detection at Subzero Temperature over the Sb-Doped SnO2@g-C3N4 Heterojunctions under UV Illumination

NASA has detected H₂S in the persistently shadowed region of the lunar South Pole through NIR and UV/vis spectroscopy remotely, but in situ detection is generally considered to be more accurate and convincing. However, subzero temperatures in space drastically reduce chemisorbed oxygen ions for gas sensing reactions, making gas sensing at subzero temperature something that has rarely been attempted. Herein, we report an in situ semiconductor H₂S gas sensor assisted by UV illumination at subzero temperature. We constructed a g-C₃N₄ network to wrap the porous Sb doped SnO₂ microspheres to form type II heterojunctions, which facilitate the separation and transport of photoinduced charge carriers under UV irradiation. This UV-driven technique affords the gas sensor a fast response time of 14 s and a response value of 20.1 toward 2 ppm H₂S at -20 °C, realizing the sensitive response of the semiconductor gas sensor at subzero temperature for the first time. Both the experimental observations and theoretical calculation results provide evidence that UV irradiation and the formation of type II heterojunctions together promote the performance at subzero temperature. This work fills the gap of semiconductor gas sensors working at subzero temperature and suggests a feasible method for deep space gas detection.

Enhanced ammonia sensing response based on Pt-decorated Ti3C2Tx/TiO2composite at room temperature

Herein, we report a Pt-decorated Ti₃C₂T_x/TiO₂gas sensor for the enhanced NH₃sensing response at room temperature. Firstly, the TiO₂nanosheets (NSs) arein situgrown onto the two-dimensional (2D) Ti₃C₂T_xby hydrothermal treatment. Similar to Ti₃C₂T_xsensor, the Ti₃C₂T_x/TiO₂sensor has a positive resistance variation upon exposure to NH₃, but with slight enhancement in response. However, after the loading of Pt nanoparticles (NPs), the Pt-Ti₃C₂T_x/TiO₂sensor shows a negative response with significantly improved NH₃sensing performance. The shift in response direction indicates that the dominant sensing mechanism has changed under the sensitization effect of Pt NPs. At room temperature, the response of Pt-Ti₃C₂T_x/TiO₂gas sensor to 100 ppm NH₃is about 45.5%, which is 13.8- and 10.8- times higher than those of Ti₃C₂T_xand Ti₃C₂T_x/TiO₂gas sensors, respectively. The experimental detection limit of the Pt-Ti₃C₂T_x/TiO₂gas sensor to detect NH₃is 10 ppm, and the corresponding response is 10.0%. In addition, the Pt-Ti₃C₂T_x/TiO₂gas sensor shows the fast response/recovery speed (23/34 s to 100 ppm NH₃), high selectivity and good stability. Considering both the response value and the response direction, the corresponding gas-sensing mechanism is also deeply discussed. This work is expected to shed a new light on the development of noble metals decorated MXene-metal oxide gas sensors.

Metal-oxide-semiconductor resistive gas sensors for fish freshness detection

Fish are prone to spoilage and deterioration during processing, storage, or transportation. Therefore, there is a need for rapid and efficient techniques to detect and evaluate fish freshness during different periods or conditions. Gas sensors are increasingly important in the qualitative and quantitative evaluation of high-protein foods, including fish. Among them, metal-oxide-semiconductor resistive (MOSR) sensors with advantages such as low cost, small size, easy integration, and high sensitivity have been extensively studied in the past few years, which gradually show promising practical application prospects. Herein, we take the detection, classification, and assessment of fish freshness as the actual demand, and summarize the physical and chemical changes of fish during the spoilage process, the volatile marker gases released, and their production mechanisms. Then, we introduce the advantages, performance parameters, and working principles of gas sensors, and summarize the MOSR gas sensors aimed at detecting different kinds of volatile marker gases of fish spoiling in the last 5 years. After that, this paper reviews the research and application progress of MOSR gas sensor arrays and electronic nose technology for various odor indicators and fish freshness detection. Finally, this review points out the multifaceted challenges (sampling system, sensing module, and pattern recognition technology) faced by the rapid detection technology of fish freshness based on metal oxide gas sensors, and the potential solutions and development directions are proposed from the view of multidisciplinary intersection.

Novel ammonia-responsive carboxymethyl cellulose/Co-MOF multifunctional films for real-time visual monitoring of seafood freshness

Nowadays, ammonia-responsive biopolymer-based intelligent active films are of great interest for their huge potential in maintaining and monitoring the freshness of seafood. However, it is still a challenge to create biopolymer-based intelligent active films with favorable color stability, antibacterial and visual freshness indication functions. Herein, cobalt-based metal-organic framework (Co-MOF) nanosheets with ammonia-sensitive and antibacterial functions were successfully synthesized and then embedded into carboxymethyl cellulose (CMC) matrix to develop high performance and multifunctional CMC-based intelligent active films. The influence of Co-MOF addition on the structure, physical and functional characters of CMC film was comprehensively studied. The results showed that the Co-MOF nanofillers were homogeneously embedded within the CMC matrix, bringing about remarkable promotion on tensile strength (from 45.3 to 62.2 MPa), toughness (from 0.7 to 2.3 MJ/m³), water barrier and UV-blocking performance of CMC film. Notably, the obtained CMC/Co-MOF nanocomposite films also presented excellent long-term color stability, antibacterial activity (with the bacteriostatic efficiency of 99.6 % and 99.3 % against Escherichia coli and Staphylococcus aureus), and ammonia-sensitive discoloration performance. Finally, the CMC/Co-MOF nanocomposite films were successfully applied for real-time visual monitoring of shrimp freshness. The above results demonstrate that the CMC/Co-MOF nanocomposite films possess huge potential applications in intelligent active packaging.

Application of Titanium Carbide MXenes in Chemiresistive Gas Sensors

The titanium carbide MXenes currently attract an extreme amount of interest from the material science community due to their promising functional properties arising from the two-dimensionality of these layered structures. In particular, the interaction between MXene and gaseous molecules, even at the physisorption level, yields a substantial shift in electrical parameters, which makes it possible to design gas sensors working at RT as a prerequisite to low-powered detection units. Herein, we consider to review such sensors, primarily based on Ti₃C₂T_x and Ti₂CT_x crystals as the most studied ones to date, delivering a chemiresistive type of signal. We analyze the ways reported in the literature to modify these 2D nanomaterials for (i) detecting various analyte gases, (ii) improving stability and sensitivity, (iii) reducing response/recovery times, and (iv) advancing a sensitivity to atmospheric humidity. The most powerful approach based on designing hetero-layers of MXenes with other crystals is discussed with regard to employing semiconductor metal oxides and chalcogenides, noble metal nanoparticles, carbon materials (graphene and nanotubes), and polymeric components. The current concepts on the detection mechanisms of MXenes and their hetero-composites are considered, and the background reasons for improving gas-sensing functionality in the hetero-composite when compared with pristine MXenes are classified. We formulate state-of-the-art advances and challenges in the field while proposing some possible solutions, in particular via employing a multisensor array paradigm.

Spectroscopic determination of the role of vanadyl oxygen in room temperature NH3 sensing by V2O5 nanoparticles

In the present study, a multi-modal approach consisting of in-situ photoluminescence, Raman, and UV-Vis absorption spectroscopic studies is carried out along with chemiresistive sensing to unveil the mechanism of NH₃ gas sensing by V₂O₅ nanoparticles in ambient air. V₂O₅ nanoparticles with an average size of 49 nm show a superior sensor response of 17 ± 1.5 % towards 1 ppm of NH₃ gas with a response and recovery time of 96 s and 45 s, respectively. The photoluminescence and UV-Vis absorption studies in the presence of NH₃ reveal electron doping to a new energy level at 1.84 eV, resulting in conduction band filling and increase in the optical band gap. The intensity of the photoluminescence spectrum shows an increase in the presence of NH₃ gas as a result of this electron doping. The sensor response from the optical sensing carried out by in-situ photoluminescence study is 43 % for 40 ppm of NH₃ gas. The vanadyl oxygen site is the most active in the sensing process, as evident by a selective enhancement in the intensity of V-O (vanadyl) bond vibration. This study gives an experimental evidence for the changes in optical and electronic properties of V₂O₅ on the adsorption of NH₃ gas molecules.

Eddy Current Measurement of Chemiresistive Sensing Transients in Graphene-hBN Heterostructures

The development of graphene-based electronic and gas sensing devices has motivated considerable research interest in the properties of graphene-hBN heterostructures. Eddy current measurements of the sheet conductance of graphene-hBN heterostructures show a relatively low conductance, as compared to results previously reported in the literature, all of which were obtained using contact-based measurement methods. Chemiresistive measurements of the graphene-hBN heterostructure response to oxygen adsorption, including hysteric effects under transient multicycle loading, show that the incremental sheet conductance responses of graphene and graphene-hBN sensors differ in sign. A transient, nonlinear, history dependent constitutive model of graphene-hBN response to oxygen adsorption distinguishes stochastic variations in material properties from deterministic variations in sensor performance. The deterministic variations are due to sensing process hysteresis, a phenomenon of central interest in the development of graphene-based sensor systems.

Recent advances in determination applications of emerging films based on nanomaterials

Sensitive and facile detection of analytes is crucial in various fields such as agriculture production, food safety, clinical diagnosis and therapy, and environmental monitoring. However, the synergy of complicated sample pretreatment and detection is an urgent challenge. By integrating the inherent porosity, processability and flexibility of films and the diversified merits of nanomaterials, nanomaterial-based films have evolved as preferred candidates to meet the above challenge. Recent years have witnessed the flourishment of films-based detection technologies due to their unique porous structures and integrated physical/chemical merits, which favors the separation/collection and detection of analytes in a rapid, efficient and facile way. In particular, films based on nanomaterials consisting of 0D metal-organic framework particles, 1D nanofibers and carbon nanotubes, and 2D graphene and analogs have drawn increasing attention due to incorporating new properties from nanomaterials. This paper summarizes the progress of the fabrication of emerging films based on nanomaterials and their detection applications in recent five years, focusing on typical electrochemical and optical methods. Some new interesting applications, such as point-of-care testing, wearable devices and detection chips, are proposed and emphasized. This review will provide insights into the integration and processability of films based on nanomaterials, thus stimulate further contributions towards films based on nanomaterials for high-performance analytical-chemistry-related applications.

In situ prepared composite of polypyrrole and multi-walled carbon nanotubes grafted with sodium polystyrenesulfonate as ammonia gas sensor with wide detection range

NH3 gas sensors with good sensing performance including wide detection range at room temperature are highly desirable for a large variety of applications. In this work, multi-walled carbon nanotubes grafted with sodium polystyrenesulfonate (PSSNa-MWCNTs) are prepared via a controlled radical polymerization and show good dispersibility in water. The composite of polypyrrole with PSSNa-MWCNTs (PPy/PSSNa-MWCNT) is prepared by in situ vapor phase polymerization of pyrrole to fabricate NH3 gas sensors. Effects of the content of PSSNa-MWCNTs, the concentration of the oxidant, polymerization time and temperature on the gas sensing properties of the composite are investigated at room temperature. It is revealed that the composite shows much higher response magnitude than the single components. Under optimal conditions, PPy/PSSNa-MWCNT exhibits very wide detection range from 5 to 2000 ppm, and good sensing linearity over 5–20 ppm and 20–100 ppm, respectively. Moreover, the electrical responses of the composite towards NH3 gas are fast (response and recovery time to 1000 ppm NH3 gas are 16.7 s and 143.6 s, respectively), reproducible and highly selective. The interactions between PPy and MWCNTs promote the charge transfer in the composite, leading to good sensing performance and exhibiting a synergetic effect.

Room-Temperature Detection of Perfluoroisobutyronitrile with SnO2/Ti3C2Tx Gas Sensors

Ti₃C₂T_x MXene is an emerging two-dimensional transition-metal carbide/nitride with excellent properties of large specific surface and high carrier mobility for room-temperature gas sensing. However, achieving high sensitivity and long-term stability of pristine Ti₃C₂T_x-based gas sensors remains challenging. SnO₂ is a typical semiconductor metal oxide with high reaction activity and stable chemical properties ideal for a dopant that can comprehensively improve sensing performance. Ti₃C₂T_x and SnO₂ are investigated for the first time in this study as functional materials for hybridization and room-temperature detection of the gas insulating medium fluorinated nitrile (C₄F₇N) with microtoxicity. A Ti₃C₂T_x-SnO₂ nanocomposite sensor exhibits superior sensitivity, high selectivity, strong anti-interference ability, and excellent long-term stability. The enhanced sensing mechanism is ascribed to the synergistic effect between SnO₂ and Ti₃C₂T_x and the strong adsorption ability of SnO₂ to C₄F₇N similar to bait for fish. We also established an actual leakage scene and demonstrated the feasibility of the Ti₃C₂T_x-SnO₂ sensor to provide distribution rules with high sensing efficiency for actual engineering applications. The results of this work can expand the gas sensing application of Ti₃C₂T_x MXene and provide a reference for maintaining C₄F₇N-based eco-friendly gas-insulated equipment.