Advances in Noble Metal-Decorated Metal Oxide Nanomaterials for Chemiresistive Gas Sensors: Overview

Recent insights on MXene-based architectures for monitoring and sensing of gaseous pollutants: A review

Epidemiology and public health concerns have primarily relied on the accurate control of gas pollutants, requiring highly efficient gas sensor devices for detecting hazardous gases. Despite the dedication of many efforts in this era, the precise, continuous scrutiny of gases remains elusive for appropriate gas selectivity, prompt response and recovery time, proper repeatability, as well as low cost. Accordingly, nanostructured architectural sensing cues have received enormous attention toward versatile detection and sensing procedures. As a representational nanostructure, the MXene family has been widely introduced to tailor and augment sensor patterns by providing large surface area, tunable surface chemistry, superior electrical conductivity, chemical stability, compatibility with flexible substrates, and potential for multifunctionality. Additionally, they could be synthesized in various formations of film and layered designs, fibrous membranes, and gel-like structures, creating synergetic effects that can provide superior gas-sensing performance. Herein, the synthesis and benefits of MXene nanosheets as gas-sensitive materials, in tandem with the past-to-present progress of MXene-based gas sensors in the formation of films, fibrous, and gel-like configurations, are comprehensively reviewed. As an in-depth reference, the present overview could shed light on further advancing gas sensor architectures developed based on MXene structures.

Recent advances in biomaterials based near-infrared mild photothermal therapy for biomedical application: A review

Mild photothermal therapy (MPTT) generates heat therapeutic effect at the temperature below 45 °C under near-infrared (NIR) irradiation, which has the advantages of controllable treatment efficacy, lower hyperthermia temperatures, reduced dosage, and minimized damage to surrounding tissues. Despite significant progress has been achieved in MPTT, it remains primarily in the stage of basic and clinical research and has not yet seen widespread clinical adoption. Herein, a comprehensive overview of the recent NIR MPTT development was provided, aiming to emphasize the mechanism and obstacles, summarize the used photothermal agents, and introduce various biomedical applications such as anti-tumor, wound healing, and vascular disease treatment. The challenges of MPTT were proposed with potential solutions, and the future development direction in MPTT was outlooked to enhance the prospects for clinical translation.

Polyoxometalate Cluster-Guided Dynamic Nucleation and Hierarchical Growth of Branched WO3 Nanofibers with Ultrafine Pt Nanoparticles for Advanced Gas Sensing

In the food industry, 2,3-butanedione is a significant volatile organic compound valued for its unique aroma and flavor. Real-time detection of its concentration during food preparation is crucial for ensuring optimal taste and food safety. However, accurately detecting low concentrations of 2,3-butanedione requires highly sensitive sensing materials. Herein, we present a novel synthesis of branched WO3 nanofibers decorated with ultrafine Pt nanoparticles (Pt NPs-WO3 NFs), templated by polyoxometalate (POM) clusters, through a combination of electrospinning and thermal oxidation strategies for advanced gas sensing applications. This Pt NPs-WO3 NFs-based sensor exhibits impressive sensitivity (Ra/Rg = 2.25 vs 500 ppb), a low detection limit of 10 ppb, high selectivity, excellent repeatability, and stable performance over a period of 25 days. Using POM clusters as templates offers significant advantages over the traditional WCl6 salt in synthesizing WO3 NFs with smooth surfaces. Specifically, the POM clusters guide the dynamic nucleation and hierarchical growth of branched NFs, enhancing the concentration of oxygen vacancies and increasing the number of active adsorption sites. Furthermore, the uniform dispersion of ultrafine Pt NPs (≈ 4 nm) within the WO3 NFs further enhances the catalytic activation of 2,3-butanedione, significantly improving the gas sensing performance. This study introduces an efficient method to synthesize Pt NPs-WO3 NFs with potential for manufacturing advanced nanostructured sensing materials using POM clusters as templates, paving the way for high-performance gas sensing technologies.

Real-Time and Wireless Transmission of a Nitrogen-Doped Ti3C2Tx Wearable Gas Sensor for Efficient Detection of Food Spoilage and Ammonia Leakage

With the rising popularity of smart homes, there is an urgent need for devices that can perform real-time online detection of ammonia (NH3) concentrations for food quality measurement. In addition, timely warning is crucial to preventing individual deaths from NH3. However, few studies can realize continuous monitoring of NH3 with high stability and subsequent application validation. Herein, we report on an integrated device equipped with a nitrogen-doped Ti3C2Tx gas sensor that shows great potential in detecting food spoilage and NH3 leakage. The nitrogen doping results in the lattice misalignment of Ti3C2Tx, subsequently realizing effective barrier height modulation and enhanced charge transfer efficiency of nitrogen-doped Ti3C2Tx. Density functional theory calculations confirm the greatly enhanced adsorption of NH3 on nitrogen-doped Ti3C2Tx. Our work can inspire the design of efficient gas sensors for real-time and wireless detection of food spoilage and NH3 leakage.

Magnetic Field Assisted Enhanced Sensitivity of Nonferromagnetic Materials Boosting the Carrier Transfer: Mechanistic Studies

The performance of semiconductor sensors is determined by reaction kinetics, conductivity, and electron mobility, which are undoubtedly closely related to the electron motion behavior. Therefore, the effective regulation of electronic states is crucial for improving gas sensing properties. Previous methods of enhancing the gas-sensing performance have induced complex material modifications, and the extent of performance improvement is usually very limited. Further optimization of the gas sensing performance requires continuous efforts to advance new technologies. Toward this issue, a novel magnetic field-induced strategy is adopted to boost the carrier transfer efficiency of nonferromagnetic semiconductors. The gas sensing investigation results manifest that the applied magnetic field can effectively enhance the sensitivity and reduce the baseline resistance. The In2O3 NC-2 (In2O3 nanocubes) with an applied magnetic field have a greatly enhanced response of 161.4 toward 100 ppm formaldehyde, which is 2.5 times higher than that without magnetic field. The enhanced gas sensing properties can be mainly attributed to magnetization of reactive materials, which makes the orientation of electronic magnetic moments consistent, thus greatly contributing to reactivity. This work introduces a practical approach to effectively improve gas sensing performance without further morphology optimization, noble metal catalysis, structural modification, and material cladding. The results of this study provide new insights for designing novel gas sensors to improve the gas sensing performance.

Recent Advances in Structure Design and Application of Metal Halide Perovskite-Based Gas Sensor

Metal halide perovskites (MHPs) are emerging gas-sensing materials and have attracted considerable attention in gas sensors due to their unique bandgap structure and tunable optoelectronic properties. The past decade has witnessed significant developments in the gas-sensing field; however, their intrinsic structural instability and ambiguous gas-sensing mechanisms hamper their practical applications. Herein, we summarize the recent advances in MHP-based gas sensors. The physicochemical properties of MHPs are discussed at first. The structure design, including dimension design and engineering design, is overviewed as well as their fabrication methods, and we put forward our insights into the gas-sensing mechanism of MHPs. It is believed that enhanced understanding of gas-sensing mechanisms of MHPs are helpful for their application as gas-sensing materials, and structure design can enhance their stability, sensing sensitivity, and selectivity to target gases as gas sensors. Subsequently, the latest developments in MHP-based gas sensors are summarized according to their different application scenarios. Finally, we conclude with the current status and challenges in this field and propose future perspectives.

Fabrication of one-dimensional nanostructures standing vertically on a substrate through layer-by-layer deposition

Layer-by-layer deposition has been widely used to prepare heterogeneous nanocomposites with controllable properties, where the controllability of nanocomposite properties can be further enhanced by combining the self-assembly of hard materials, such as nanocrystals, and soft materials, such as polymer matrices. However, the self-assembled structure of nanocrystals in a polymer matrix is limited by thermodynamics. Herein, we introduced the strong interaction between nanocrystals and a substrate to generate a thermodynamically unfavorable one-dimensional nanostructure standing vertically on the substrate. Two different shapes-"wire-like" and "steeple-like" or "antenna-like"-of one-dimensional nanostructures standing vertically on the substrate can be obtained using cube-like nanocrystals as building blocks and carefully controlling their size in each deposited layer. This low-cost and massively parallel scale synthesis method to generate one-dimensional nanostructures standing vertically on a substrate can be used to replace the expensive and time-consuming "lithography" synthesis method. This synthesis method also provides a simple way to design and fabricate one-dimensional nanostructures with desired properties standing vertically on a substrate by controlling nanocrystal properties in each deposited layer.

Performance of a Novel Electronic Nose for the Detection of Volatile Organic Compounds Relating to Starvation or Human Decomposition Post-Mass Disaster

There has been a recent increase in the frequency of mass disaster events. Following these events, the rapid location of victims is paramount. Currently, the most reliable search method is scent detection dogs, which use their sense of smell to locate victims accurately and efficiently. Despite their efficacy, they have limited working times, can give false positive responses, and involve high costs. Therefore, alternative methods for detecting volatile compounds are needed, such as using electronic noses (e-noses). An e-nose named the 'NOS.E' was developed and has been used successfully to detect VOCs released from human remains in an open-air environment. However, the system's full capabilities are currently unknown, and therefore, this work aimed to evaluate the NOS.E to determine the efficacy of detection and expected sensor response. This was achieved using analytical standards representative of known human ante-mortem and decomposition VOCs. Standards were air diluted in Tedlar gas sampling bags and sampled using the NOS.E. This study concluded that the e-nose could detect and differentiate a range of VOCs prevalent in ante-mortem and decomposition VOC profiles, with an average LOD of 7.9 ppm, across a range of different chemical classes. The NOS.E was then utilized in a simulated mass disaster scenario using donated human cadavers, where the system showed a significant difference between the known human donor and control samples from day 3 post-mortem. Overall, the NOS.E was advantageous: the system had low detection limits while offering portability, shorter sampling times, and lower costs than dogs and benchtop analytical instruments.

Anelosimuseximius bioinspired ZnO nano cobwebs for environmental remediation of drugs and endocrine disruptors from water

The pollution of wastewater with pharmaceuticals and endocrine-disrupting chemicals (EDCs) in populated areas poses a growing threat to humans and ecosystems. To address this serious problem, various one-dimensional (1D) hierarchical ZnO-based nanostructures inspired by Anelosimus eximius cobwebs were developed and successfully grown on a glass substrate through simple hydrothermal synthesis. The nanorods (nr) obtained during primary growth were chemically etched with KOH (ZnOnr-KOH), followed by the secondary growth of nano cobweb-like (ncw) structures using polyethyleneimine (ZnOnr/ncw). These structures were further decorated by the photoreduction of Ag nanoparticles (ZnOnr/ncw/Ag). The feasibility of ZnO-based 1D nanostructures to remove pollutants was demonstrated by degrading commonly prescribed pharmaceutical drugs (diclofenac and carbamazepine) in a miniature cuvette reactor. The photocatalytic activities for drug degradation generally decreased in the order ZnOnr/ncw/Ag > ZnOnr/ncw > ZnOnr-KOH. Additionally, the suitability of the samples for scaling up and practical application was demonstrated by photocatalytic degradation of the hormone estriol (E3) in a flow-through photoreactor. The photocatalytic degradation efficiency of E3 followed the same trend observed for drug degradation, with the complete elimination of the endocrine disruptor achieved by the best-performing ZnOnr/ncw/Ag within 4 h, due to optimized charge transfer and separation at the heterostructure interface.

MXenes for Wearable Physical Sensors toward Smart Healthcare

The gradual rise of personal healthcare awareness is accelerating the deployment of wearable sensors, whose ability of acquiring physiological vital signs depends on sensing materials. MXenes have distinct chemical and physical superiorities over other 2D nanomaterials for wearable sensors. This review presents a comprehensive summary of the latest advancements in MXenes-based materials for wearable physical sensors. It begins with an introduction to special structural features of MXenes for sensing performance, followed by an in-depth exploration of versatile functionalities. A detailed description of different sensing mechanisms is also included to illustrate the contribution of MXenes to the sensing performance and its improvement. In addition, the real-world applications of MXenes-based physical sensors for monitoring different physiological signs are included as well. The remaining challenges of MXenes-based materials for wearable physical sensors and their promising opportunities are finally narrated, in conjunction with a prospective for future development.

Single-Atom Au-Functionalized Mesoporous SnO2 Nanospheres for Ultrasensitive Detection of Listeria monocytogenes Biomarker at Low Temperatures

Semiconductor metal oxide gas sensors have been proven to be capable of detecting Listeria monocytogenes, one kind of foodborne bacteria, through monitoring the characteristic gaseous metabolic product 3-hydroxy-2-butanone. However, the detection still faces challenges because the sensors need to work at high temperatures and output limited gas sensing performance. The present study focuses on the design of single-atom Au-functionalized mesoporous SnO2 nanospheres for the sensitive detection of ppb-level 3-hydroxy-2-butanone at low temperatures (50 °C). The fabricated sensors exhibit high sensitivity (291.5 ppm-1), excellent selectivity, short response time (10 s), and ultralow detection limit (10 ppb). The gas sensors exhibit exceptional efficacy in distinguishing L. monocytogenes from other bacterial strains (e.g., Escherichia coli). Additionally, wireless detection of 3-hydroxy-2-butanone vapor is successfully achieved through microelectromechanical systems sensors, enabling real-time monitoring of the biomarker 3-hydroxy-2-butanone. The superior sensing performance is ascribed to the mesoporous framework with accessible active Au-O-Sn sites in the uniform sensing layer consisting of single-atom Au-modified mesoporous SnO2 nanospheres, and such a feature facilitates the gas diffusion, adsorption, and catalytic conversion of 3-hydroxy-2-butanone molecules in the sensing layer, resulting in excellent sensing signal output at relatively low temperature that is favorable for developing low-energy-consumption gas sensors.

In-Sensor Organic Electrochemical Transistor for the Multimode Neuromorphic Olfactory System

The olfactory system is one of the six basic sensory nervous systems. Developing artificial olfactory systems is challenging due to the complexity of chemical information decoding and memory. Conventional chemical sensors can convert chemical signals into electric signals to decode gas information but they lack memory functions. Additional storage and processing units would significantly increase the complexity and power consumption of the devices, especially for portable and wearable devices. Here, an olfactory-inspired in-sensor organic electrochemical transistor (OI-OECT) is proposed, with the integrated functions of chemical information decoding, tunable memory level, and selectivity of vapor sensing. The ion-gel electrolyte endows the OI-OECT with the function of tunable memory levels and a low operating voltage. Typical synaptic behaviors, including inhibitory postsynaptic current and paired-pulse facilitations, are successfully achieved. Importantly, the gas memory level can be effectively modulated by the gate voltages (0 and -1 V), which realized the transformation of volatile and nonvolatile memory. Furthermore, benefiting from the recognition of multiple gases and ability to detect cumulative damage caused by gases, the OI-OECT is demonstrated for early warning system targeting leakage detection of two gases (NH3 and H2S). This work achieves the integrated functions of chemical gas information decode, tunable gas memory level, and selectivity of gas in a single device, which provides a promising pathway for the development of future artificial olfactory systems.

Resistive Gas Sensors Based on 2D TMDs and MXenes

ConspectusGas sensors are used in various applications to sense toxic gases, mainly for enhanced safety. Resistive sensors are particularly popular owing to their ability to detect trace amounts of gases, high stability, fast response times, and affordability. Semiconducting metal oxides are commonly employed in the fabrication of resistive gas sensors. However, these sensors often require high working temperatures, bringing about increased energy consumption and reduced selectivity. Furthermore, they do not have enough flexibility, and their performance is significantly decreased under bending, stretching, or twisting. To address these challenges, alternative materials capable of operating at lower temperatures with high flexibility are needed. Two-dimensional (2D) materials such as MXenes and transition-metal dichalcogenides (TMDs) offer high surface area and conductivity owing to their unique 2D structure, making them promising candidates for realization of resistive gas sensors. Nevertheless, their sensing performance in pristine form is typically weak and unacceptable, particularly in terms of response, selectivity, and recovery time (trec). To overcome these drawbacks, several strategies can be employed to enhance their sensing properties. Noble-metal decoration such as (Au, Pt, Pd, Rh, Ag) is a highly promising method, in which the catalytic effects of noble metals as well as formation of potential barriers with MXenes or TMDs eventually contribute to boosted response. Additionally, bimetallic noble metals such as Pt-Pd and Au/Pd with their synergistic properties can further improve sensor performance. Ion implantation is another feasible approach, involving doping of sensing materials with the desired concentration of dopants through control over the energy and dosage of the irradiation ions as well as creation of structural defects such as oxygen vacancies through high-energy ion-beam irradiation, contributing to enhanced sensing capabilities. The formation of core-shell structures is also effective, creating numerous interfaces between core and shell materials that optimize the sensing characteristics. However, the shell thickness needs to be carefully optimized to achieve the best sensing output. To reduce energy consumption, sensors can operate in a self-heating condition where an external voltage is applied to the electrodes, significantly lowering the power requirements. This enables sensors to function in energy-constrained environments, such as remote or low-energy areas. An important advantage of 2D MXenes and TMDs is their high mechanical flexibility. Unlike semiconducting metal oxides that lack mechanical flexibility, MXenes and TMDs can maintain their sensing performance even when integrated onto flexible substrates and subjected to bending, tilting, or stretching. This flexibility makes them ideal for fabricating flexible and portable gas sensors that rigid sensors cannot achieve.

Rapid and sensitive determination of legacy and emerging per- and poly-fluoroalkyl substances with solid-phase microextraction probe coupled with mass spectrometry

This study was designed to develop a rapid and sensitive method for quantifying legacy and emerging per- and polyfluoroalkyl substances (PFASs) in environmental samples with solid-phase microextraction (SPME) coupled with mass spectrometry (MS). An innovative SPME probe was fabricated via in situ polymerization, and the probe coating was optimized with response surface methodology to maximize the fluorine-fluorine interactions and electrostatic properties and ensure high selectivity for the target PFASs with enrichment factors of 48-491. The coupled SPME and MS provided a rapid and sensitive method for analyses of PFASs, with excellent linearity (r ≥ 0.9962) over the concentration range 0.001-1 μg/L and remarkably low detection limits of 0.1-13.0 ng/L. This method was used to analyze trace PFASs in tap water, river water, and wastewater samples and proved to be a simple and efficient analytical method for selective enrichment and detection of contaminants in the environment.

Real-Time Tunable Gas Sensing Platform Based on SnO2 Nanoparticles Activated by Blue Micro-Light-Emitting Diodes

Micro-light-emitting diodes (μLEDs) have gained significant interest as an activation source for gas sensors owing to their advantages, including room temperature operation and low power consumption. However, despite these benefits, challenges still exist such as a limited range of detectable gases and slow response. In this study, we present a blue μLED-integrated light-activated gas sensor array based on SnO2 nanoparticles (NPs) that exhibit excellent sensitivity, tunable selectivity, and rapid detection with micro-watt level power consumption. The optimal power for μLED is observed at the highest gas response, supported by finite-difference time-domain simulation. Additionally, we first report the visible light-activated selective detection of reducing gases using noble metal-decorated SnO2 NPs. The noble metals induce catalytic interaction with reducing gases, clearly distinguishing NH3, H2, and C2H5OH. Real-time gas monitoring based on a fully hardware-implemented light-activated sensing array was demonstrated, opening up new avenues for advancements in light-activated electronic nose technologies.

Cobalt-Doped Ceria Sensitizer Effects on Metal Oxide Nanofibers: Heightened Surface Reactivity for High-Performing Chemiresistive Sensors

Chemiresistive gas sensors based on semiconducting metal oxides typically rely on noble metal catalysts to enhance their sensitivity and selectivity. However, noble metal catalysts have several drawbacks for practical utilization, including their high cost, their propensity for spontaneous agglomeration, and poisoning effects with certain types of gases. As such, in the interest of commercializing the chemiresistive gas sensor technology, we propose an alternative design for a noble-metal-free sensing material through the case study of Co-doped ceria (Co-CeO2) catalysts embedded in a SnO2 matrix. In this investigation, we utilized electrospinning and subsequent calcination to prepare Co-CeO2 catalyst nanoparticles integrated with SnO2 nanofibers (NFs) with uniform particle distribution and particle size regulation down to the sub-2 nm regime. The resulting Co-CeO2@SnO2 NFs exhibited superior gas sensing characteristics toward isoprene (C5H8) gas, a significant biomarker for monitoring the onset of various diseases through breath diagnostics. In particular, we identified that the Co-CeO2 catalysts, owing to the transition metal doping, facilitated the spillover of chemisorbed oxygen species to the SnO2 sensing body. This resulting in the sensor having a 27.4-fold higher response toward 5 ppm of C5H8 (compared to pristine SnO2), exceptionally high selectivity, and a low detection limit of 100 ppb. The sensor also exhibited high stability for prolonged response-recovery cycles, attesting to the strong anchoring of Co-CeO2 catalysts in the SnO2 matrix. Based on our findings, the transition metal-doped metal oxide catalysts, such as Co-CeO2, demonstrate strong potential to completely replace noble metal catalysts, thereby advancing the development of the commercially viable chemiresistive gas sensors free from noble metals, capable of detecting target gases at sub-ppm levels.

Sensitive detection of H2S based on Ce doped ZnCo2O4 hollow microspheres at low working temperature

In order to develop a highly efficient H2S gas sensor at low working temperature, in this work, a kind of novel Ce-doped ZnCo2O4 hollow microspheres (Ce/ZnCo2O4 HMSs) were successfully synthesized using a template-free one-pot method, showing a sensitive response toward H2S. The microstructure and morphology of the material were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The gas-sensing performance of the composite was investigated, showing that the ZnCo2O4 doped with 6 mol% Ce had the highest response to 20 ppm H2S at a low operating temperature of 160 °C with a response value of 67.42, which was about 2 times higher than that of original ZnCo2O4. The prepared Ce/ZnCo2O4 HMS sensor in response to H2S exhibited a linear range of 0.1-200 ppm with a low detection limit of 0.1 ppm under the conditions of ambient humidity of 45% and ambient temperature of 20 °C. Meanwhile, it also possessed good selectivity, repeatability and reproducibility. The response value of the sensor decreased by 5.32% after 7 months of continuous monitoring of H2S in an atmospheric environment of a pig farm, indicating that the sensor had a long-term stability and continuous service life with important application prospects.

Machine Learning Driven Atom-Thin Materials for Fragrance Sensing

Fragrance plays a crucial role in the daily lives. Its importance spans various sectors, from therapeutic purposes to personal care, making the understanding and accurate identification of fragrances essential. To fully harness the potential of fragrances, efficient and precise fragrance sensing and identification are necessary. However, current fragrance sensors face several limitations, particularly in detecting and differentiating complex scent profiles with high accuracy. To address these challenges, the use of atom-thin materials in fragrance sensors has emerged as a groundbreaking approach. These atom-thin sensors, characterized by their enhanced sensitivity and selectivity, offer significant improvements over traditional sensing technology. Moreover, the integration of Machine Learning (ML) into fragrance sensing has opened new opportunities in the field. ML algorithms applied to fragrance sensing facilitate advancements in four key domains: accurate fragrance identification, precise discrimination between different fragrances, improved detection thresholds for subtle scents, and prediction of fragrance properties. This comprehensive review delves into the synergistic use of atom-thin materials and ML in fragrance sensing, providing an in-depth analysis of how these technologies are revolutionizing the field, offering insights into their current applications and future potential in enhancing the understanding and utilization of fragrances.

Enhanced Interface Charge Carrier Transport of SnO2/CeO2 via Oxygen Vacancy Synergized Heterojunction for Triethylamine Sensing Property

Efficient charge carrier transport characteristics are critical to achieving the excellent performance of metal-oxide semiconductor gas sensors. Herein, SnO2/CeO2 heterojunction layered nanosheets with abundant oxygen vacancies were successfully synthesized through a simple solvothermal assisted high-temperature calcination method. The synergistic effect of oxygen vacancies and heterojunctions promoting the charge carrier transport properties at the SnO2/CeO2 interface for the enhanced sensing properties of triethylamine (TEA) was highlighted. As a result, the optimized SnO2/CeO2 exhibits improved gas sensing performance at 173 °C to 50 ppm of TEA. These include high response (205), excellent selectivity, low detection limit, and good long-term stability. This enhanced gas sensing property of SnO2/CeO2 is mainly attributed to the fact that the heterojunction and oxygen vacancies act as dual active sites synergistically inducing electron transfer, thereby effectively modulating the transport properties of the interfacial charge carriers, and thus facilitate the surface reactions efficiently. In this work, the dual-engineering strategy of synergistic interaction of heterojunction and oxygen vacancies can provide new perspectives for the design of advanced gas sensing materials.

Tandem Electric-Fields Prolong Energetic Hot Electrons Lifetime for Ultra-Fast and Stable NO2 Detection

Prolonging energetic hot electrons lifetimes and surface activity in the reactive site can overcome the slow kinetics and unfavorable thermodynamics of photo-activated gas sensors. However, bulk and surface recombination limit the simultaneous optimization of both kinetics and thermodynamics. Here tandem electric fields are deployed at (111)/(100)Au-CeO2 to ensure a sufficient driving force for carrier transfer and elucidate the mechanism of the relationship between charge transport and gas-sensing performance. The asymmetric structure of the (111)/(100)CeO2 facet junction provides interior electric fields, which facilitates electron transfer from the (100)face to the (111)face. This separation of reduction and oxidation reaction sites across different crystal faces helps inhibit surface recombination. The increased electron concentration at the (111)face intensifies the interface electric field, which promotes electron transfer to the Au site. The local electric field generated by the surface plasmon resonance effect promotes the generation of high-energy energy hot-electrons, which maintains charge concentration in the interface field by injecting into (111)/(100)CeO2, thereby provide thermodynamic contributions and inhibit bulk recombination. The tandem electric fields enable the (111)/(100)Au-CeO2 to rapidly detect 5 ppm of NO2 at room temperature with stability maintained within 20 s.

Redispersing Ir Nanoparticles via a Carbon-Assisted Pyrolysis Process to Break the Activity-Stability Trade-Off of H2 Sensors

Oxide semiconductor-supported metal nanoparticles often suffer from a high-temperature gas sensing process, resulting in agglomeration and coalescence, which significantly decrease their surface activity and stability. Here, we develop an in situ pyrolysis strategy to redisperse commercial Ir particles (∼15.6 nm) into monodisperse Ir species (∼5.4 nm) on ZnO supports, exhibiting excellent sintering-resistant properties and H2 sensing. We find that large-size Ir nanoparticles can undergo an unexpected splitting decomposition process and spontaneously migrate along the encapsulated carbon layer surface during high-temperature pyrolysis of ZIF-8. This resultant monodisperse status can be integrally reserved, accompanying further oxidation sintering. The final Irred/ZnO-450-based sensor exhibits outstanding stability, H2 response (10-2000 ppm), fast response/recovery capability (7/9.7 s@100 ppm), and good moisture resistance. In situ Raman and ex situ XPS further experimentally verify that highly dispersive Ir species can promote the electron transfer process during the gas sensing process. Our strategy thus provides important insights into the design of agglomeration-resistant gas sensing materials for highly effective H2 detection.

A Strategy for Multigas Identification Using Multielectrical Parameters Extracted from a Single Carbon-Based Field-Effect Transistor Sensor

Given the widespread utilization of gas sensors across various industries, the detection of diverse and complex target gases presents a significant challenge in designing sensors with multigas detection capability. Although constructing a sensor array with widely used chemiresistive gas sensors is one solution, it is difficult for a single chemiresistive gas sensor to simultaneously detect different gases, as it can only detect a single target gas. The intrinsic reason for this bottleneck is that chemiresistive gas sensors rely entirely on the resistivity as the unique parameter to evaluate the diverse gas sensing properties of sensors, such as sensitivity, selectivity, etc. Herein, a field-effect transistor (FET) with abundant electrical parameters is employed to prepare a gas sensor for the detection of a variety of gases. Semiconducting carbon nanotubes (CNTs) are selected as the channel material, which is modified by Pd nanoparticles to enhance the gas sensing properties of the sensors. By extracting various electrical parameters such as transconductance, threshold voltage, etc. from the transfer characteristic curves of FET, a correlation between multielectrical parameters and various gas detection information is established for subsequent data analysis. Through the utilization of the principal component analysis algorithm, the identification of six gases can be finally achieved by relying solely on a single carbon-based FET-type gas sensor. We hope our work can solve the bottleneck of multigas identification by a single sensor in principle and is expected to reduce the system complexity and cost caused by the design of sensor arrays, offering a valuable guidance for multigas identification technology.

Bimetallic PtAu-Decorated SnO2 Nanospheres Exhibiting Enhanced Gas Sensitivity for Ppb-Level Acetone Detection

Acetone is a biomarker found in the expired air of patients suffering from diabetes. Therefore, early and accurate detection of its concentration in the breath of such patients is extremely important. We prepared Tin(IV) oxide (SnO2) nanospheres via hydrothermal treatment and then decorated them with bimetallic PtAu nanoparticles (NPs) employing the approach of in situ reduction. The topology, elemental composition, as well as crystal structure of the prepared materials were studied via field emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The findings revealed that bimetallic PtAu-decorated SnO2 nanospheres (PtAu/SnO2) were effectively synthesized as well as PtAu NPs evenly deposited onto the surface of the SnO2 nanospheres. Pure SnO2 nanospheres and PtAu/SnO2 sensors were prepared, and their acetone gas sensitivity was explored. The findings demonstrated that in comparison to pristine SnO2 nanosphere sensors, the sensors based on PtAu/SnO2 displayed superior sensitivity to acetone of 0.166-100 ppm at 300 °C, providing a low theoretical limit of detection equal to 158 ppm. Moreover, the PtAu/SnO2 sensors showed excellent gas response (Ra/Rg = 492.3 to 100 ppm), along with fast response and recovery (14 s/13 s to 10 ppm), good linearity of correlation, excellent repeatability, long-term stability, and satisfactory selectivity at 300 °C. This improved gas sensitivity was because of the electron sensitization of the Pt NPs, the chemical sensitization of the Au NPs, as well as the synergistic effects of bimetallic PtAu. The PtAu/SnO2 sensors have considerable potential for the early diagnosis and screening of diabetes.

Multiarray Gas Sensors Using Ternary Combined Ti3C2Tx MXene-Based Nanocomposites

This paper reports chemiresistive multiarray gas sensors through the synthesized ternary nanocomposites, using a one-pot method to integrate two-dimensional MXene (Ti3C2Tx) with Ti-doped WO3 (Ti-WO3/Ti3C2Tx) and Ti3C2Tx with Pd-doped SnO2 (Pd-SnO2/Ti3C2Tx). The gas sensors based on Ti-WO3/Ti3C2Tx and Pd-SnO2/Ti3C2Tx exhibit exceptional sensitivity, particularly in detecting 70% at 1 ppm acetone and 91.1% at 1 ppm of H2S. Notably, our sensors demonstrate a remarkable sensing performance in the low-ppb range for acetone and H2S. Specifically, the Ti-WO3/Ti3C2Tx sensor demonstrates a detection limit of 0.035 ppb for acetone, and the Pd-SnO2/Ti3C2Tx sensor shows 0.116 ppb for H2S. Simultaneous measurements with Ti-WO3/Ti3C2Tx- and Pd-SnO2/Ti3C2Tx-based sensors enable the evaluation of both the concentration and type of unknown target gases, such as acetone or H2S. Furthermore, density functional theory calculations are performed to clarify the role of Ti and Pd doping in enhancing the performance of Ti-WO3/Ti3C2Tx and Pd-SnO2/Ti3C2Tx nanocomposites. Theoretical simulations contribute to a deeper understanding of the doping effects, providing essential insights into the mechanisms underlying the enhanced gas response of the gas sensors. Overall, this work provides valuable insights into the gas-sensing mechanisms and introduces a novel approach for high-performance multiarray gas sensing.

CuO nanostructure-decorated InGaN nanorods for selective H2S gas detection

Establishing a heterostructure is one of the adequate strategies for enhancing device performance and has been explored in sensing, and energy applications. In this study, we constructed a heterostructure through a two-step process involving hydrothermal synthesis of CuO nanostructures and subsequent spin coating on MBE-grown InGaN NRs. We found that the CuO content on the InGaN NRs has a great impact on carrier injection at the heterojunction and thus the H2S gas sensing performance. Popcorn CuO/InGaN NR shows excellent gas sensing performance towards different concentrations of H2S at room temperature. The highest response is up to 35.54% to a H2S concentration of 100 ppm. Even more significantly, this response is further enhanced significantly (123.70%) under 365 nm UV light. In contrast, this composite structure exhibits negligibly low responses to 100 ppm of NO2, H2, CO, and NH3. The heterostructure band model associated with a surface reaction model is manifested to elucidate the sensing mechanism.

Comparison of Sub-Ppm Instrument Response Suggests Higher Detection Limits Could Be Used to Quantify Methane Emissions from Oil and Gas Infrastructure

Quantifying and controlling fugitive methane emissions from oil and gas facilities remains essential for addressing climate goals, but the costs associated with monitoring millions of production sites remain prohibitively expensive. Current thinking, supported by measurement and simple dispersion modelling, assumes single-digit parts-per-million instrumentation is required. To investigate instrument response, the inlets of three trace-methane (sub-ppm) analyzers were collocated on a facility designed to release gas of known composition at known flow rates between 0.4 and 5.2 kg CH4 h-1 from simulated oil and gas infrastructure. Methane mixing ratios were measured by each instrument at 1 Hertz resolution over nine hours. While mixing ratios reported by a cavity ring-down spectrometer (CRDS)-based instrument were on average 10.0 ppm (range 1.8 to 83 ppm), a mid-infrared laser absorption spectroscopy (MIRA)-based instrument reported short-lived mixing ratios far larger than expected (range 1.8 to 779 ppm) with a similar nine-hour average to the CRDS (10.1 ppm). We suggest the peaks detected by the MIRA are likely caused by a micrometeorological phenomenon, where vortex shedding has resulted in heterogeneous methane plumes which only the MIRA can observe. Further analysis suggests an instrument like the MIRA (an optical-cavity-based instrument with cavity size ≤10 cm3 measuring at ≥2 Hz with air flow rates in the order of ≤0.3 slpm at distances of ≤20 m from the source) but with a higher detection limit (25 ppm) could detect enough of the high-concentration events to generate representative 20 min-average methane mixing ratios. Even though development of a lower-cost, high-precision, high-accuracy instrument with a 25 ppm detection threshold remains a significant problem, this has implications for the use of instrumentation with higher detection thresholds, resulting in the reduction in cost to measure methane emissions and providing a mechanism for the widespread deployment of effective leak detection and repair programs for all oil and gas infrastructure.

Wireless Gas Sensor Based on the Mesoporous ZnO-SnO2 Heterostructure Enables Ultrasensitive and Rapid Detection of 3-Methylbutyraldehyde

Achieving ultrasensitive and rapid detection of 3-methylbutyraldehyde is crucial for monitoring chemical intermediate leakage in pharmaceutical and chemical industries as well as diagnosing ventilator-associated pneumonia by monitoring exhaled gas. However, developing a sensitive and rapid method for detecting 3-methylbutyraldehyde poses challenges. Herein, a wireless chemiresistive gas sensor based on a mesoporous ZnO-SnO2 heterostructure is fabricated to enable the ultrasensitive and rapid detection of 3-methylbutyraldehyde for the first time. The mesoporous ZnO-SnO2 heterostructure exhibits a uniform spherical shape (∼79 nm in diameter), a high specific surface area (54.8 m2 g-1), a small crystal size (∼4 nm), and a large pore size (6.7 nm). The gas sensor demonstrates high response (18.98@20 ppm), short response/recovery times (13/13 s), and a low detection limit (0.48 ppm) toward 3-methylbutyraldehyde. Furthermore, a real-time monitoring system is developed utilizing microelectromechanical systems gas sensors. The modification of amorphous ZnO on the mesoporous SnO2 pore wall can effectively increase the chemisorbed oxygen content and the thickness of the electron depletion layer at the gas-solid interface, which facilitates the interface redox reaction and enhances the sensing performance. This work presents an initial example of semiconductor metal oxide gas sensors for efficient detection of 3-methylbutyraldehyde that holds great potential for ensuring safety during chemical production and disease diagnosis.

Ultrathin Boundary-Less SnO2 Films with Surface-Activated Two-Dimensional Nanograins Enable Fast and Sensitive Hydrogen Gas Sensing

Fast and reliable semiconductor hydrogen sensors are crucially important for the large-scale utilization of hydrogen energy. One major challenge that hinders their practical application is the elevated temperature required, arising from undesirable surface passivation and grain-boundary-dominated electron transportation in the conventional nanocrystalline sensing layers. To address this long-standing issue, in the present work, we report a class of highly reactive and boundary-less ultrathin SnO2 films, which are fabricated by the topochemical transformation of 2D SnO transferred from liquid Sn-Bi droplets. The ultrathin SnO2 films are purposely made to consist of well-crystallized quasi-2D nanograins with in-plane grain sizes going beyond 30 nm, whereby the hydroxyl adsorption and grain boundary side-effects are effectively suppressed, giving rise to an activated (101)-dominating dangling-bond surface and a surface-controlled electrical transportation with an exceptional electron mobility of 209 cm2 V-1 s-1. Our work provides a new cost-effective strategy to disruptively improve the gas reception and transduction of SnO2. The proposed chemiresistive sensors exhibit fast, sensitive, and selective hydrogen sensing performance at a much-reduced working temperature of 60 °C. The remarkable sensing performance as well as the simple and scalable fabrication process of the ultrathin SnO2 films render the thus-developed sensors attractive for long awaited practical applications in hydrogen-related industries.

Influence of the Pd Oxidation State in PdNi Thin Films on Surface Acoustic Wave Hydrogen Sensing Performance

PdNi alloy thin films demonstrate exceptional hydrogen sensing performance and exhibit significant potential for application in surface acoustic wave (SAW) hydrogen sensors. However, the long-term stability of SAW H2 sensors utilizing PdNi films as catalysts experiences a substantial decrease during operation. In this paper, X-ray photoelectron spectroscopy (XPS) is employed to investigate the failure mechanisms of PdNi thin films under operational conditions. The XPS analysis reveals that the formation of PdO species on PdNi thin films plays a crucial role in the failure of hydrogen sensing. Additionally, density functional theory (DFT) calculations indicate that hydrogen atoms encounter a diffusion energy barrier during the penetration process from the PdNiOx surface to the subsurface region. The identification of PdNi film failure mechanisms through XPS and DFT offers valuable insights into the development of gas sensors with enhanced long-term stability. Guided by these mechanisms, we propose a method to restore the hydrogen sensing response time and magnitude to a certain extent by reducing the partially oxidized surface of the PdNi alloy under a hydrogen atmosphere at 70 °C, thereby restoring Pd to its metallic state with zero valence.

Mechanism of Interfacial Molecular Interactions Reveals the Intrinsic Factors for the Highly Enhanced Sensing Performance of Ag-Loaded Co3O4

The noble metal-loaded strategy can effectively improve the gas-sensing performances of metal oxide sensors. However, the gas-solid interfacial interactions between noble metal-loaded sensing materials and gaseous species remain unclear, posing a significant challenge in correlating the physical and chemical processes during gas sensing. In this study, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and in situ Raman spectroscopy were conducted to collaboratively investigate the interfacial interactions involved in the ethanol gas-sensing processes over Co3O4 and Ag-loaded Co3O4 sensors. In situ DRIFTS revealed differences in the compositions and quantities of sensing reaction products, as well as in the adsorption-desorption interactions of surface species, among Co3O4 and Ag-loaded Co3O4 materials. In parallel, in situ Raman spectroscopy demonstrated that the ethanol atmosphere can modulate the electron scattering of Ag-loaded Co3O4 materials but not of raw Co3O4. In situ experimental results revealed the intrinsic reason for the highly enhanced sensing performances of the Ag-loaded Co3O4 sensors toward ethanol gas, including a decreased optimal working temperature (from 250 to 150 °C), an improved gas response level (from 24 to 257), and accelerated gas recovery dynamics. This work provides an effective platform to investigate the interfacial interactions of sensing processes at the molecular level and further advances the development of high-performance gas sensors.

Regulating electron transfer and orbital interaction within metalloporphyrin-MOFs for highly sensitive NO2 sensing

The understanding of electron transfer pathways and orbital interactions between analytes and adsorption sites in gas-sensitive studies, especially at the atomic level, is currently limited. Herein, we have designed eight isoreticular catechol-metalloporphyrin scaffolds, FeTCP-M and InTCP-M (TCP = 5,10,15,20-tetrakis-catechol-porphyrin, M = Fe, Co, Ni and Zn) with adjustable charge transfer schemes in the coordination microenvironment and precise tuning of orbital interactions between analytes and adsorption sites, which can be used as models for exploring the influence of these factors on gas sensing. Our experimental findings indicate that the sensitivity and selectivity can be modulated using the type of metals in the metal-catechol chains (which regulate the electron transfer routes) and the metalloporphyrin rings (which fine-tune the orbital interactions between analytes and adsorption sites). Among the isostructures, InTCP-Co demonstrates the highest response and selectivity to NO2 under visible light irradiation, which could be attributed to the more favorable transfer pathway of charge carriers in the coordination microenvironment under visible light illumination, as well as the better electron spin state compatibility, higher orbital overlap and orbital symmetry matching between the N-2s2pz hybrid orbital of NO2 and the Co-3dz2 orbital of InTCP-Co.

Wearable Sensors for Breath Monitoring Based on Water-Based Hexagonal Boron Nitride Inks Made with Supramolecular Functionalization

Wearable humidity sensors are attracting strong attention as they allow for real-time and continuous monitoring of important physiological information by enabling activity tracking as well as air quality assessment. Amongst 2Dimensional (2D) materials, graphene oxide (GO) is very attractive for humidity sensing due to its tuneable surface chemistry, high surface area, processability in water, and easy integration onto flexible substrates. However, strong hysteresis, low sensitivity, and cross-sensitivity issues limit the use of GO in practical applications, where continuous monitoring is preferred. Herein, a wearable and wireless impedance-based humidity sensor made with pyrene-functionalized hexagonal boron nitride (h-BN) nanosheets is demonstrated. The device shows enhanced sensitivity towards relative humidity (RH) (>1010 Ohms/%RH in the range from 5% to 100% RH), fast response (0.1 ms), no appreciable hysteresis, and no cross-sensitivity with temperature in the range of 25-60 °C. The h-BN-based sensor is able to monitor the whole breathing cycle process of exhaling and inhaling, hence enabling to record in real-time the subtlest changes of respiratory signals associated with different daily activities as well as various symptoms of flu, without requiring any direct contact with the individual.

Emerging single-atom catalysts in the detection and purification of contaminated gases

Single atom catalysts (SACs) show exceptional molecular adsorption and electron transfer capabilities owing to their remarkable atomic efficiency and tunable electronic structure, thereby providing promising solutions for diverse important processes including photocatalysis, electrocatalysis, thermal catalysis, etc. Consequently, SACs hold great potential in the detection and degradation of pollutants present in contaminated gases. Over the past few years, SACs have made remarkable achievements in the field of contaminated gas detection and purification. In this review, we first provide a concise introduction to the significance and urgency of gas detection and pollutant purification, followed by a comprehensive overview of the structural feature identification methods for SACs. Subsequently, we systematically summarize the three key properties of SACs for detecting contaminated gases and discuss the research progress made in utilizing SACs to purify polluted gases. Finally, we analyze the enhancement mechanism and advantages of SACs in polluted gas detection and purification, and propose strategies to address challenges and expedite the development of SACs in polluted gas detection and purification.

Development of Pd/In2O3 hybrid nanoclusters to optimize ethanol vapor sensing

In this study, we successfully synthesize palladium-decorated indium trioxide (Pd/In2O3) hybrid nanoclusters (NCs) using an advanced dual-target cluster beam deposition (CBD) method, a significant stride in developing high-performance ethanol sensors. The prepared Pd/In2O3 hybrid NCs exhibit exceptional sensitivity, stability, and selectivity to low concentrations of ethanol vapor, with a maximum response value of 101.2 at an optimal operating temperature of 260 °C for 6 at% Pd loading. The dynamic response of the Pd/In2O3-based sensor shows an increase in response with increasing ethanol vapor concentrations within the range of 50 to 1000 ppm. The limit of detection is as low as 24 ppb. The sensor exhibits a high sensitivity of 28.24 ppm-1/2, with response and recovery times of 2.7 and 4.4 seconds, respectively, for 100 ppm ethanol vapor. Additionally, the sensor demonstrates excellent repeatability and stability, with only a minor decrease in response observed over 30 days and notable selectivity for ethanol compared to other common volatile organic compounds. The study highlights the potential of Pd/In2O3 NCs as promising materials for ethanol gas sensors, leveraging the unique capabilities of CBD for controlled synthesis and the catalytic properties of Pd for enhanced gas-sensing performance.

Oxygen Vacancy-Rich Bimetallic Au@Pt Core-Shell Nanosphere-Functionalized Electrospun ZnFe2O4 Nanofibers for Chemiresistive Breath Acetone Detection

Sensitive and selective acetone detection is of great significance in the fields of environmental protection, industrial production, and individual health monitoring from exhaled breath. To achieve this goal, bimetallic Au@Pt core-shell nanospheres (BNSs) functionalized-electrospun ZnFe2O4 nanofibers (ZFO NFs) are prepared in this work. Compared to pure NFs-650 analogue, the ZFO NFs/BNSs-2 sensor exhibits a stronger mean response (3.32 vs 1.84), quicker response/recovery speeds (33 s/28 s vs 54 s/42 s), and lower operating temperature (188 vs 273 °C) toward 0.5 ppm acetone. Note that an experimental detection limit of 30 ppb is achieved, which ranks among the best cases reported thus far. Besides the demonstrated excellent repeatability, humidity-enhanced response, and long-term stability, the selectivity toward acetone is remarkably improved after BNSs functionalization. Through material characterizations and DFT calculations, all these improvements could be attributed to the boosted oxygen vacancies and abundant Schottky junctions between ZFO NFs and BNSs, and the synergistic catalytic effect of BNSs. This work offers an alternative strategy to realize selective subppm acetone under high-humidity conditions catering for the future requirements of noninvasive breath diabetes diagnosis in the field of individual healthcare.

Advances in Synthesis and Applications of Single-Atom Catalysts for Metal Oxide-Based Gas Sensors

As a stable, low-cost, environment-friendly, and gas-sensitive material, semiconductor metal oxides have been widely used for gas sensing. In the past few years, single-atom catalysts (SACs) have gained increasing attention in the field of gas sensing with the advantages of maximized atomic utilization and unique electronic and chemical properties and have successfully been applied to enhance the detection sensitivity and selectivity of metal oxide gas sensors. However, the application of SACs in gas sensors is still in its infancy. Herein, we critically review the recent advances and current status of single-atom catalysts in metal oxide gas sensors, providing some suggestions for the development of this field. The synthesis methods and characterization techniques of SAC-modified metal oxides are summarized. The interactions between SACs and metal oxides are crucial for the stable loading of single-atom catalysts and for improving gas-sensitive performance. Then, the current application progress of various SACs (Au, Pt, Cu, Ni, etc.) in metal oxide gas sensors is introduced. Finally, the challenges and perspectives of SACs in metal oxide gas sensors are presented.

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.

Functional Optical Fiber Sensors Detecting Imperceptible Physical/Chemical Changes for Smart Batteries

The battery technology progress has been a contradictory process in which performance improvement and hidden risks coexist. Now the battery is still a "black box", thus requiring a deep understanding of its internal state. The battery should "sense its internal physical/chemical conditions", which puts strict requirements on embedded sensing parts. This paper summarizes the application of advanced optical fiber sensors in lithium-ion batteries and energy storage technologies that may be mass deployed, focuses on the insights of advanced optical fiber sensors into the processes of one-dimensional nano-micro-level battery material structural phase transition, electrolyte degradation, electrode-electrolyte interface dynamics to three-dimensional macro-safety evolution. The paper contributes to understanding how to use optical fiber sensors to achieve "real" and "embedded" monitoring. Through the inherent advantages of the advanced optical fiber sensor, it helps clarify the battery internal state and reaction mechanism, aiding in the establishment of more detailed models. These advancements can promote the development of smart batteries, with significant importance lying in essentially promoting the improvement of system consistency. Furthermore, with the help of smart batteries in the future, the importance of consistency can be weakened or even eliminated. The application of advanced optical fiber sensors helps comprehensively improve the battery quality, reliability, and life.

2D nanomaterials for realization of flexible and wearable gas sensors: A review

Gas sensors are extensively employed for monitoring and detection of hazardous gases and vapors. Many of them are produced on rigid substrates, but flexible and wearable gas sensors are needed for intriguing usage including the internet of things (IoT) and medical devices. The materials with the greatest potential for the fabrication of flexible and wearable gas sensing devices are two-dimensional (2D) semiconducting nanomaterials, which consist of graphene and its substitutes, transition metal dichalcogenides, and MXenes. These types of materials have good mechanical flexibility, high charge carrier mobility, a large area of surface, an abundance of defects and dangling bonds, and, in certain instances adequate transparency and ease of synthesis. In this review, we have addressed the different 2D nonmaterial properties for gas sensing in the context of fabrication of flexible/wearable gas sensors. We have discussed the sensing performance of flexible/wearable gas sensors in various forms such as pristine, composite and noble metal decorated. We believe that content of this review paper is greatly useful for the researchers working in the research area of fabrication of flexible/wearable gas sensors.

Ultrasensitive and Exclusive Chemiresistors with a ZIF-67-Derived Oxide Cage/Nanofiber Co3O4/In2O3 Heterostructure for Acetone Detection

Gas sensors for acetone detection have received considerable attention because acetone has a significant influence on both the environment and human health, e.g., it is flammable and toxic and may be related to blood glucose levels. However, achieving high sensitivity and selectivity at low concentrations is still a great challenge to date. Here, we report a unique chemiresistive gas sensor for acetone detection, which is composed of In2O3 nanofibers loaded with a porous Co-based zeolitic imidazolate framework (ZIF-67)-derived Co3O4 cage prepared by simple electrospinning and solvothermal methods. The ZIF-67-derived oxide cage/nanofiber Co3O4/In2O3 heterostructure has abundant reversible active adsorption/reaction sites and a type-I heterojunction, resulting in an ultrasensitive response of 954-50 ppm acetone at 300 °C. In addition, it demonstrates a low detection limit of 18.8 ppb, a fast response time of 4 s, good selectivity and repeatability, acceptable humidity interference, and long-term stability. With such excellent sensing performance to acetone, our chemiresistive gas sensor could be potentially applied for environmental monitoring and early diagnosis of diabetes.

Adjusting surface coverage of Pt nanocatalyst decoration for selectivity control in CMOS-integrated SnO2 thin film gas sensors

Smart gas-sensor devices are of crucial importance for emerging consumer electronics and Internet-of-Things (IoT) applications, in particular for indoor and outdoor air quality monitoring (e.g., CO2 levels) or for detecting pollutants harmful for human health. Chemoresistive nanosensors based on metal-oxide semiconductors are among the most promising technologies due to their high sensitivity and suitability for scalable low-cost fabrication of miniaturised devices. However, poor selectivity between different target analytes restrains this technology from broader applicability. This is commonly addressed by chemical functionalisation of the sensor surface via catalytic nanoparticles. Yet, while the latter led to significant advances in gas selectivity, nanocatalyst decoration with precise size and coverage control remains challenging. Here, we present CMOS-integrated gas sensors based on tin oxide (SnO2) films deposited by spray pyrolysis technology, which were functionalised with platinum (Pt) nanocatalysts. We deposited size-selected Pt nanoparticles (narrow size distribution around 3 nm) by magnetron-sputtering inert-gas condensation, a technique which enables straightforward surface coverage control. The resulting impact on SnO2 sensor properties for CO and volatile organic compound (VOC) detection via functionalisation was investigated. We identified an upper threshold for nanoparticle deposition time above which increased surface coverage did not result in further CO or VOC sensitivity enhancement. Most importantly, we demonstrate a method to adjust the selectivity between these target gases by simply adjusting the Pt nanoparticle deposition time. Using a simple computational model for nanocatalyst coverage resulting from random gas-phase deposition, we support our findings and discuss the effects of nanoparticle coalescence as well as inter-particle distances on sensor functionalisation.

Chemresistive Detection of NO2 of ppb Level in Humid Air at 350 K Using Azo-Spaced Polycroconamide

Organic molecules are of great interest for gas sensing applications. However, achieving high-performance gas sensors with high sensitivity, fast response, low consumption, and workability in humid conditions is still challenging. Herein, we report the rational design and synthesis of an ion-in-conjugation polymer, PADC (poly-4,4'-azodianiline-croconamide), obtained by the condensation of croconic acid with 4-4'diaminoazobenzene for gas sensing under humid conditions. The as-fabricated PADC-based gas sensor exhibits ultrahigh sensitivity (802.7 ppm-1 at 1 ppm), subppb detection limit, and high selectivity under humid air with an 80% humidity effect at a temperature down to 350 K. PADC shows good planarity, excellent thermostability, and a narrow band gap of 1.2 eV because of azobenzene fragments spacing previously repulsed biphenyl rings. Compared to previous humidity immunity works, PADC-based sensors realized humidity immunity at a relatively lower temperature, resulting in lower energy consumption.

Research progress on the catalytic and thermal decomposition of ammonium dinitramide (ADN)

Ammonium dinitramide (NH4N(NO3)2, ADN) is regarded as a promising oxidizer due to its low signature and high specific impulse. Generally, ADN undergoes exothermic decomposition above 140 °C accompanied by the byproduct of ammonium nitrate (AN). The inevitable endothermic decomposition of AN decreases the overall heat release, and so there is a need to develop efficient catalysts to guide ADN decomposition along desired pathways with a lower decomposition temperature and higher heat release. A suitable catalyst should be able to withstand the harsh conditions in a thruster to achieve a stable thrust force, which poses a huge obstacle for manufacturing a stable and active catalyst. This review gives a comprehensive summary of the thermal and catalytic decomposition pathways of ADN for the first time, which is expected to deepen the understanding of its reaction mechanism and provide useful guidance for designing prospective catalysts toward efficient ADN decomposition.

Preparation of Pt/WO3@ZnO hollow spheres for low-temperature and high-efficiency detection of triethylamine

In this work, hollow spherical Pt-loaded WO3/ZnO heterostructured composites were prepared by a chemical liquid phase synthesis method. The morphology, crystal structure and components of the composites were characterized by SEM, TEM, XRD, XPS, etc. The sensing performance for various gases was also tested. Compared with the pristine WO3 (S = 44@225 °C, 50 ppm) gas sensor, the gas sensor that is functionalized with 1 wt% Pt and 0.5 mmol ZnO (1Pt/WZ-2) has a high response of 842-50 ppm at a relatively low temperature of 100 °C for TEA, with a quick response/recovery time of 34/120 s, a lower detection limit of 50 ppb, and good selectivity and moisture resistance. This study provides a highly efficient synthesis method of composite materials for TEA gas detection and the sensitivity mechanism is also discussed in detail.

Design of a SnO2/Zeolite Gas Sensor to Enhance Formaldehyde Sensing Properties: From the Strategy of the Band Gap-Tunable Zeolite

ZSM-5 zeolite is usually used in gas sensors as an auxiliary material to improve the gas-sensitive properties of other semiconductor materials, such as its molecular sieve properties and surface adsorption properties. Here, the gas-sensitive mechanism analysis of SnO2/zeolite gas sensors is studied for the first time based on the perspective of zeolite as a band gap-tunable semiconductor that was reported recently. The gas-sensing mechanism of the zeolite/semiconductor has been modeled based on the surface charge theory, and the work function of the ZSM-5 zeolite has been revealed for the first time. A heterostructure of Ag and ZSM-5 was designed and compounded to tune the band gap of the ZSM-5 zeolite by the ammonia pool effect method. The band gap width of the zeolite decreases from 4.51 to 3.61 eV. A series of characterization techniques were used to analyze the distribution and morphology of silver nanoparticles in zeolites and the variation of the ZSM-5 band gap. Then, SnO2/Ag@ZSM-5 sensors were fabricated, and the gas-sensing performances were measured. The gas-sensing results show that the SnO2/Ag@ZSM-5 sensor has an improved response to formaldehyde in particular compared to the SnO2 sensor. The response value of the SnO2/Ag@ZSM-5 sensor to 70 ppm formaldehyde reached 29.4, which is a 528% improvement compared to the SnO2 sensor. Additionally, the selectivity was greatly enhanced. This study provides a strategy for designing and developing higher-performance gas sensors.

Metal Oxide Nanowire-Based Sensor Array for Hydrogen Detection

Accurate hydrogen leakage detection is a major requirement for the safe and widespread integration of this fuel in modern energy production devices, such as fuel cells. Quasi-1D nanowires of seven different metal oxides (CuO, WO3, Nb-added WO3, SnO2, ZnO, α-Bi2O3, NiO) were integrated into a conductometric sensor array to evaluate the hydrogen-sensing performances in the presence of interfering gaseous compounds, namely carbon monoxide, nitrogen dioxide, methane, acetone, and ethanol, at different operating temperatures (200-400 °C). Principal component analysis (PCA) was applied to data extracted from the array, demonstrating the ability to discriminate hydrogen over other interferent compounds. Moreover, a reduced array formed by only five sensors is proposed. This compact array may be easily implementable into artificial olfaction systems used in real hydrogen detection applications.

Synthesis of Mesoporous Lanthanum-Doped SnO2 Spheres for Sensitive and Selective Detection of the Glutaraldehyde Disinfectant

Glutaraldehyde disinfectant has been widely applied in aquaculture, farming, and medical treatment. Excessive concentrations of glutaraldehyde in the environment can lead to serious health hazards. Therefore, it is extremely important to develop high-performance glutaraldehyde sensors with low cost, high sensitivity, rapid response, fabulous selectivity, and low limit of detection. Herein, mesoporous lanthanum (La) doped SnO2 spheres with high specific surface area (52-59 m2 g-1), uniform mesopores (with a pore size concentrated at 5.7 nm), and highly crystalline frameworks are designed to fabricate highly sensitive gas sensors toward gaseous glutaraldehyde. The mesoporous lanthanum-doped SnO2 spheres exhibit excellent glutaraldehyde-sensing performance, including high response (13.5@10 ppm), rapid response time (28 s), and extremely low detection limit of 0.16 ppm. The excellent sensing performance is ascribed to the high specific surface area, high contents of chemisorbed oxygen species, and lanthanum doping. DFT calculations suggest that lanthanum doping in the SnO2 lattice can effectively improve the adsorption energy toward glutaraldehyde compared to pure SnO2 materials. Moreover, the fabricated gas sensors can effectively detect commercial glutaraldehyde disinfectants, indicating a potential application in aquaculture, farming, and medical treatment.

Impedance-Based Detection of NO2 Using Ni-MOF-74: Influence of Competitive Gas Adsorption

Chemically robust, low-power sensors are needed for the direct electrical detection of toxic gases. Metal-organic frameworks (MOFs) offer exceptional chemical and structural tunability to meet this challenge, though further understanding is needed regarding how coadsorbed gases influence or interfere with the electrical response. To probe the influence of competitive gases on trace NO2 detection in a simulated flue gas stream, a combined structure-property study integrating synchrotron powder diffraction and pair distribution function analyses was undertaken, to elucidate how structural changes associated with gas binding inside Ni-MOF-74 pores correlate with the electrical response from Ni-MOF-74-based sensors. Data were evaluated for 16 gas combinations of N2, NO2, SO2, CO2, and H2O at 50 °C. Fourier difference maps from a rigid-body Rietveld analysis showed that additional electron density localized around the Ni-MOF-74 lattice correlated with large decreases in Ni-MOF-74 film resistance of up to a factor of 6 × 103, observed only when NO2 was present. These changes in resistance were significantly amplified by the presence of competing gases, except for CO2. Without NO2, H2O rapidly (<120 s) produced small (1-3×) decreases in resistance, though this effect could be differentiated from the slower adsorption of NO2 by the evaluation of the MOF's capacitance. Furthermore, samples exposed to H2O displayed a significant shift in lattice parameters toward a larger lattice and more diffuse charge density in the MOF pore. Evaluating the Ni-MOF-74 impedance in real time, NO2 adsorption was associated with two electrically distinct processes, the faster of which was inhibited by competitive adsorption of CO2. Together, this work points to the unique interaction of NO2 and other specific gases (e.g., H2O, SO2) with the MOF's surface, leading to orders of magnitude decrease in MOF resistance and enhanced NO2 detection. Understanding and leveraging these coadsorbed gases will further improve the gas detection properties of MOF materials.