Nanostructured Materials for Room-Temperature Gas Sensors

Enhancing sensitivity, selectivity, and intelligence of gas detection based on field-effect transistors: Principle, process, and materials

A sensor, serving as a transducer, produces a quantifiable output in response to a predetermined input stimulus, which may be of a chemical or physical nature. The field of gas detection has experienced a substantial surge in research activity, attributable to the diverse functionalities and enhanced accessibility of advanced active materials. In this work, recent advances in gas sensors, specifically those utilizing Field Effect Transistors (FETs), are summarized, including device configurations, response characteristics, sensor materials, and application domains. In pursuing high-performance artificial olfactory systems, the evolution of FET gas sensors necessitates their synchronization with material advancements. These materials should have large surface areas to enhance gas adsorption, efficient conversion of gas input to detectable signals, and strong mechanical qualities. The exploration of gas-sensitive materials has covered diverse categories, such as organic semiconductor polymers, conductive organic compounds and polymers, metal oxides, metal-organic frameworks, and low-dimensional materials. The application of gas sensing technology holds significant promise in domains such as industrial safety, environmental monitoring, and medical diagnostics. This comprehensive review thoroughly examines recent progress, identifies prevailing technical challenges, and outlines prospects for gas detection technology utilizing field effect transistors. The primary aim is to provide a valuable reference for driving the development of the next generation of gas-sensitive monitoring and detection systems characterized by improved sensitivity, selectivity, and intelligence.

Temperature dependent dual functional SnO2 sensor for Cl2 and NO2

Gas sensors play a crucial role in monitoring toxic and harmful gases. A large number of sensors or frequent replacement of different sensors is used to meet the purpose of testing different target gases, in order to meet multiple gas monitoring requirements of complex industrial and living environments. The increase in energy consumption and costs has forced the development of sensors capable of monitoring two or more gases. Herein, the SnO2 thin film assembled from nanoparticles (4-6 nm) was prepared in situ using hydrothermal method. The sensor prepared by the SnO2 thin film can achieve dual functional sensing of Cl2 and NO2 by adjusting the working temperature. At 50 °C, the response of the SnO2-10 thin film sensor to 5 ppm Cl2 was as high as 1386. When the working temperature was adjusted to 90 °C, the SnO2-10 thin film sensor had excellent sensing performance toward NO2, achieving the high response of 417 and the short recovery time of 15 s for 5 ppm NO2. Moreover, the SnO2-10 thin film sensor also demonstrated good reproducibility, reliable long-term stability, and excellent moisture resistance. This work will inspire the synthesis of multifunctional sensing materials and boost their sensing performance and sensing mechanism exploration.

Nitrogen Plasma-Driven Oxygen Vacancy Modulation in Tin Dioxide Nanosheets Enables Sub-Parts-per-Billion Nitrogen Dioxide Detection at Low Operating Temperature

Metal oxide semiconductor (MOS) materials have been widely used in gas sensing. However, they generally face challenges such as high operating temperatures and limited sensitivity/selectivity, which hinder their applications in areas like medical diagnosis based on human exhaled breath and ultralow concentration gas detection in harsh environments. Developing general strategies to enhance the sensing performance of MOS materials is both challenging and highly desired. Herein, we demonstrate nitrogen plasma-driven oxygen vacancy modulation in tin dioxide nanosheets (SnO2 NSs) that enables sub-parts-per-billion-level nitrogen dioxide (NO2) detection at low temperatures. SnO2 NSs, oriented predominantly along the (110) crystal facet, are synthesized using graphene oxide templates and treated with nitrogen plasma, which can generate abundant oxygen vacancies. The oxygen vacancy-rich SnO2 NSs exhibit exceptional NO2 sensing performance, with a theoretical detection limit of 0.154 ppb and a response that is 3.4 times higher than that of the untreated SnO2 NSs at 80 °C. Mechanism studies reveal that the improved sensitivity is attributed to the large surface area, favorable crystal orientation, and oxygen vacancies introduced by nitrogen plasma treatment. This work not only provides a promising strategy for modulating the oxygen vacancies in MOS materials, but also offers valuable insights for the development of high-performance MOS-based gas sensors.

Enhanced Gas Adsorption and Robust Multi-Interface Charge Transfer in Ternary Co3O4/ZnIn2S4/Pt Heterostructure Arrays for Efficient Triethylamine Detection

The resistive gas sensors based on semiconductor materials provide an effective strategy for the detection of harmful gases. However, the limitations of surface gas adsorption activity and interface charge transfer efficiency of semiconductor sensing materials, as well as the complex device fabrication process, pose significant challenges to the development of sensors. Here, a ternary Co3O4/ZnIn2S4/Pt heterostructure arrays gas sensor is designed, which consists of Co3O4 nanowire arrays grown in situ on an alumina flat substrate as backbones, ultrathin ZnIn2S4 nanosheets wrapped around the surface of Co3O4 nanowires, and highly dispersed Pt nanoparticles on the outermost layer. It enables superior sensing performance for the detection of the volatile organic compound triethylamine, which exhibits a significant response of ∼118.97 (Ra/Rg) toward 100 ppm of triethylamine at a relatively low working temperature of 200 °C, along with excellent response/recovery speed, selectivity, and enduring stability (over 3 months). Based on first-principles calculation and a series of spectroscopic characterization (including in situ spectroscopy), it is revealed that the heterostructure arrays exhibited enhanced adsorption activity for both oxygen and triethylamine molecules. Most importantly, the robust p-n heterointerface (Co3O4/ZnIn2S4) and semiconductor-metal heterointerface (Co3O4/Pt, ZnIn2S4/Pt) are formed in the ternary heterostructure, achieving efficient multi-interface charge transfer characteristics. In addition, thanks to the design of in situ 1D/2D/0D porous array structures, the ternary Co3O4/ZnIn2S4/Pt heterostructure arrays not only have large specific surface areas for gas reaction but also simplify device manufacturing. This research offers novel perspectives on boosting the gas sensing performance of semiconductor materials through the comprehensive design of ternary heterostructures with robust multi-interfaces.

All-aerosol-jet-printed Fe3+ modified bilayers polyaniline flexible room temperature sensor with enhanced ammonia sensing properties

The rapid advancement of human-machine interaction (HMI), the Internet of Things (IoTs), and artificial intelligence (AI) has imposed greater demands on the ambient temperature wearable performance of sensors. In this study, the Fe3+ modified bilayers polyaniline (PANI) flexible room temperature ammonia sensor is prepared by all-aerosol-jet-printed. The increased protonation degree of the PANI film produced by this method was elucidated through analysis of aerosol microdroplet evaporation behavior, while the improved ammonia sensing performance of the PANI/Fe3+ dendritic structure was explained using soft and hard acid-base theory. Gas sensing tests demonstrated that the PANI/Fe3+ sensor exhibited high sensitivity to ammonia (776 % at 55 ppm), a wide detection range (547 ppb-547 ppm), as well as excellent selectivity, flexibility, and cyclic stability. These results underscore its potential for application in ambient temperature wearable fields.

Smart Breath Sentinel: A NO2 Gas Sensor with ppt-Level Detection Lower Limit and High Signal-to-Noise Ratio Based on In(OH)3-α-Fe2O3-ZnO for an Application on Intelligent Upgrade of Ordinary Masks

Nitrogen dioxide (NO2), the main component of pollutants in atmospheric environments, causes and exacerbates respiratory diseases, especially during outdoor sports even at the 100 ppb level. Currently, environmental gas detection still faces challenges such as high detection limits, low SNR, and low sensitivity. A NO2 sensor based on In(OH)3-α-Fe2O3-ZnO was prepared using a hydrothermal method, featuring an ultralow detection limit of 82 ppt, an exceptionally high SNR of 574,000, and an ultrahigh sensitivity of 252.25 mV/ppm (100 ppb to 1 ppm). And this sensor exhibits excellent response, selectivity, and repeatability. These excellent sensing characteristics come from the adsorbed oxygen on the surface of the material with the formation of n-n heterojunctions. Additionally, a low-power, portable, and cost-effective environmental monitoring system (named Smart Breath Sentinel (SBS)) was designed to intelligently upgrade ordinary masks. SBS enables real-time wireless environmental gas monitoring with integrated humidity compensation to ensure accurate measurements in high-humidity environments. SBS has already been tested in multiple environments, and the test results have proven the feasibility of SBS. This sensor based on In(OH)3-α-Fe2O3-ZnO demonstrates significant potential for applications aimed at enhancing human safety.

Autonomous and Ultrasensitive Low-Power Metal Oxide Nanofiber Gas Sensor for Source Tracking and Localization

Current toxic gas detection methods in industrial and environmental settings are limited by their reliance on manual monitoring and stationary sensors. Here, we present an autonomous mobile gas sensing system offering real-time monitoring and precise gas source localization without the need for human intervention. Room-temperature gas sensors based on high specific surface area indium gallium zinc oxide nanofibers (IGZO NFs) are developed, which exhibit low power consumption (∼0.5 mW), exceptional sensitivity (∼1290% ppb-1), and a low detection limit of 20 ppb for toxic NO2. When integrated into an autonomous mobile platform and supported by adaptive biologically inspired algorithms, the system exhibits a source localization efficiency of ∼1.5 m min-1, offering a remote, scalable, and efficient solution for detecting and localizing toxic gas leaks.

Integrating Vacancies and Defect Levels in Heterojunctions to Synergistically Enhance the Performance of H2S Chemiresistors for Periodontitis Diagnosis

Exhaled breath is considered an important source of samples and a reservoir of biomarkers, especially for disease diagnosis. In this study, we developed an ultrasensitive point-of-care gas sensor for the analysis of hydrogen sulfide (H2S), which is a typical biomarker for periodontitis. A high-performance metal oxide semiconductor (MOS)-based chemiresistive H2S sensor was developed by integrating Fe-doped MoO3-x onto TiO2 nanotube arrays. The substitution of Fe atoms into MoO3-x not only induced oxygen vacancies, but also generated defect levels in the MoO3-x/TiO2 heterostructure, thus synergistically activating the gas sensing reaction at room temperature under ambient light. The resulting gas sensor exhibited ultrahigh sensitivity, fast response/recovery ability, and wide-range response to H2S concentrations up to 400 ppm. Furthermore, the sensing device maintained more than 95% of its original response at 70% relative humidity. With a subparts-per-billion limit of detection (the LOD for H2S was 0.34 ppb), the present sensor represents the most sensitive H2S chemiresistor reported to date for room-temperature, real-time monitoring of H2S concentration changes in the breath of healthy subjects, as well as for distinguishing breath samples of periodontitis patients and healthy individuals. This study utilizes the synergistic action of defects to provide an effective route for developing MOS-based ultrasensitive H2S sensors for periodontitis diagnosis.

Ultrasensitive Gas Sensor of Mixed-dimensional Heterostructures Combining Borophene and BC2N Quantum Dots: Enhanced Detection through Binary Cooperative Effects

The emergence of mixed-dimensional van der Waals heterostructures has inspired worldwide interests in recent years, opening up new avenues for potential nanotechnology applications. Herein, we proposed a mixed-dimensional heterostructure composed of borophene sheets and BC2N quantum dots. The gas sensing performance of the heterostructure was evaluated through a combination of theoretical calculations and experimental methods. Specifically, first-principles calculation results show that NO2 is the most strongly interacting molecule and induces the largest amount of charge transfer between the molecule and the heterostructure, suggesting exceptional sensitivity and selectivity of the heterostructure to NO2 gas. Following the theoretical insights, a borophene-BC2N heterostructure gas sensor was developed and its gas detection abilities were assessed with exposure to various gases at room temperature. Remarkably, this sensor displayed a sensitivity of 1170 % to 30 ppm NO2 and remain a high sensitivity of 108 % even to 0.2 ppm NO2. These results highlight borophene-BC2N heterostructure as a superior NO2 gas sensor, demonstrating enhanced sensing via BC2N quantum dots and integrated theoretical-experimental approaches.

Selectivity in Chemiresistive Gas Sensors: Strategies and Challenges

The demand for highly functional chemical gas sensors has surged due to the increasing awareness of human health to monitor metabolic disorders or noncommunicable diseases, safety measures against harmful greenhouse and/or explosive gases, and determination of food freshness. Over the years of dedicated research, several types of chemiresistive gas sensors have been realized with appreciable sensitivities toward various gases. However, critical issues such as poor selectivity and sluggish response/recovery speeds continue to impede their widespread commercialization. Specifically, the mechanisms behind the selective response of some chemiresistive materials toward specific gas analytes remain unclear. In this review, we discuss state-of-the-art strategies employed to attain gas-selective chemiresistive materials, with particular emphasis on materials design, surface modification or functionalization with catalysts, defect engineering, material structure control, and integration with physical/chemical gas filtration media. The nature of material surface-gas interactions and the supporting mechanisms are elucidated, opening opportunities for optimizing the materials design, fine-tuning the gas sensing performance, and guiding the selection of the most appropriate materials for the accurate detection of specific gases. This review concludes with recommendations for future research directions and potential opportunities for further selectivity improvements.

Unraveling the Synergy of Interfacial Engineering in In Situ Prepared NiO/NdNiO3 for ppb-Level SO2 Sensing: Mechanistic and First-Principles Insights

Interfacial engineering of semiconductor metal oxides offers a plethora of features to overcome the limitations of chemiresistive gas sensors, thereby increasing their practical viability. Herein, the SO2 sensing characteristics of NiO are modulated through the incorporation of NdNiO3, via a facile in situ synthesis of NiO/NdNiO3 nanostructures that significantly enhance the sensor performance. To this end, systematic control of the Nd/Ni molar ratio is employed during the synthesis of NiO/NdNiO3, enabling the regulation of structural properties and interfacial interactions. The optimized NiO/NdNiO3-based sensor demonstrates superior SO2 detection at 140 °C, outperforming pristine NiO, owing to tunable charge carrier dynamics at the heterointerface during gas adsorption. The sensor showcases an extensive dynamic response range from 450 ppb to 200 ppm and an impressive detection limit (320 ppb), along with remarkable selectivity and excellent stability. First-principles calculations reveal NiO and NdNiO3 play distinct roles in SO2 adsorption, with NiO functioning as the receptor, selectively interacting with SO2 through dissociated oxygen, and NdNiO3 serving as the transducer, facilitating signal conversion by inhibiting oxygen dissociation. Additionally, the designed portable, threshold-triggered sensor prototype, integrating the developed NiO/NdNiO3 sensor with enhanced SO2 detection, presents a promising avenue for applications in industrial and environmental monitoring.

Effect of Polymeric Moisture Barriers on ZnO Nanofiber Gas Sensors Operating at Room Temperature

Relative humidity is a significant factor that impairs gas sensor performance. Some investigations have heated sensors to temperatures beyond 100 °C to eliminate relative humidity, negatively affecting the sensor stability and application range. To improve the sensitivity of gas sensors that operate at room temperatures, free-standing polymer films with moisture barrier qualities are applied to various sensor architectures. A selective permeable polymer membrane applied to the air interface of the sensors, with or without contact, is intended to lessen the effects of relative humidity on sensor sensitivity. In this study, ZnO structures were synthesized by using electrospinning methods. Selective permeable polystyrene/poly(ethylene glycol) (PS/PEG) polymer film membranes were produced on the synthesized nanofiber structures and applied to the sensor. In the membrane synthesis, the CO2 annealing process was applied to control the porosity O2 and moisture permeability. Gas sensor performance tests for NO2 and H2 gases were conducted for these synthesized nanostructures and membranes by using various characterization techniques and analyses. Gas-sensing measurements were performed in dry air and a relative humidity (RH) of 50%, employing different concentrations of NO2 and H2 gases. Different sensing parameters (response time, recovery time, sensitivity) were estimated at room temperature for samples, and the sensor sensitivity was 0.0152 at 50 ppm. Sensor response is enhanced approximately fivefold by samples with polymeric membrane measurements compared to without. Nanofibers exhibit 120 and 300 s response time and recovery time for NO2 gas, respectively. As a result, a new approach to the literature has been provided to reduce the effects of RH on the sensor, which is one of the biggest obstacles in the scope of gas sensors operating at room temperature. Therefore, this study's findings open a general approach for fabricating flexible devices for gas detection applications.

Recent advances in 2D MXene-based heterostructures for gas sensing: mechanisms and applications in environmental and biomedical fields

MXenes, a unique class of 2D transition metal carbides, have gained attention for gas sensing applications due to their distinctive properties. Since the synthesis of Ti3C2Tx MXene in 2011, significant progress has been made in using MXenes as chemiresistive sensors. Their layered structure, abundant surface groups, hydrophilicity, tunable conductivity, and excellent thermal properties make MXenes ideal for low-power, flexible, room temperature gas sensors, fostering scalable and reproducible applications in portable devices. This review evaluates the latest advancements in MXene-based gas sensors, beginning with an overview of the elemental compositions, structures, and typical fabrication process of MXenes. We subsequently examine their applications in gas sensing domains, evaluating the proposed mechanisms for detecting common volatile organic compounds such as acetone, formaldehyde, ethanol, ammonia, and nitrogen oxides. To set this apart from similar reviews, our focus centered on the mechanistic interactions between MXene sensing materials and analytes (particularly for chemiresistive gas sensors), leveraging the distinct functionalities of MXene chemistries, which can be finely tuned for specific applications. Ultimately, we examine the current limitations and prospective research avenues concerning the utilization of MXenes in environmental and biomedical applications.

Heteroatom decorated C2N monolayer for gas-sensing application: Insight from first-principles

Development of novel gas-sensing materials is essential to high-performance gas sensors for monitoring target gases in industrial production and environmental protection. Herein, we investigate two types of the heteroatom-decorated C2N monolayer, denoted as M@C2N (M = Mn and Ni) and B-C2N, for their gas-sensing functionality toward seven small gaseous molecules (H2, O2, N2, CO, CO2, NH3, and H2O). The key gas-sensing characteristics concerning chemiresistive (CR) and field-effect transistor (FET) gas sensing have been thoroughly explored. The results show that Mn@C2N and Ni@C2N can work as either CR or FET gas-sensing materials for detecting H2, O2, N2, CO, CO2, NH3, and H2O, whereas B-C2N can work as a disposable gas sensor for O2, H2O, and NH3. Mn@C2N and Ni@C2N are the most selective toward O2 and NH3, followed by CO and H2O in an oxygen- and ammonia-free environment, while B-C2N is the most selective toward H2O and NH3. More importantly, the adsorption strength of target molecule plays a critical role in gas-sensing mechanism as well as selectivity, recovery time, and sensitivity. This study offers theoretical perspectives on 2D hybrid carbon-based nanomaterials for efficient gas sensing.

Sensing of NO2, NH3, and C3H6O by graphene-Si Schottky diode at chosen voltage biases

Our research aims to propose a selective gas sensor based on a single graphene-silicon Schottky junction. By polarizing the sensor with selected voltage, we move between different regions of its gas sensitivity dependent on the lighting conditions. Our results show that low detection limits (36 ppb for NO2, 238 ppb for NH3, and 640 ppb for C3H6O) are obtained under UV irradiation at -0.4 V; however, a light source power supply is required. Then, the Schottky barrier height is primarily sensitive to gas adsorption. When the sensor operates in the dark, the characteristic region of graphene between the ohmic contact with the Ni/Au electrode and a Schottky contact with Si is responsible for observing the gating effect in the structure. It is visible as a bending in the I-V curve near 0.7 V, which shifts with the adsorption of gases due to potential induced by molecular dipoles. Observing and analyzing these two effects on DC characteristics makes our sensor a highly sensitive and selective platform of low-power consumption and low-cost methodology. Additionally, the applied Schottky junction between the graphene layer and the n-doped Si base makes the sensor more resistant to humidity adsorption during storage in ambient air than other graphene-based gas sensors (graphene back-gated FET).

Light-activated semiconductor gas sensors: pathways to improve sensitivity and reduce energy consumption

Resistive type gas sensors based on wide-bandgap semiconductor oxides are remaining one of the principal players in environmental air monitoring. The rapid development of technology and the desire to miniaturize electronics require the creation of devices with minimal energy consumption. A promising solution may be the use of photoactivation, which can initiate/accelerate physico-chemical processes at the solid-gas interface and realize detection of flammable and explosive gases at close to room temperature. This work examines the mechanism underlying the increased sensitivity to various gases under photoactivation. The review is intended to clarify the current situation in the field of light-activated gas sensors and set the vector for their further development in order to integrate with the latest technological projects.

Universal Gas-Sensitive Detection of Various Lithium-Ion Battery Electrolyte Leakages via Ag@Ag2O-Functionalized SnO2 Nanoflowers with Abundant Oxygen Vacancies

Lithium-ion batteries (LIBs) provide many benefits, but trace electrolyte leakage can cause serious safety risks such as thermal runaway. Although gas sensors offer a potential solution, the complexity of electrolyte solvents in LIBs makes it challenging to develop sensing materials capable of universally detecting multiple solvent molecules. Here, Ag@Ag2O-functionalized SnO2 nanoflowers were synthesized using a self-template pyrolysis strategy for the sensitive detection of both common solvent molecules and widely used electrolytes. These sensors, enhanced by abundant oxygen vacancies introduced by Ag@Ag2O functionalization, exhibit excellent sensitivity, particularly to dimethyl carbonate, with a response of 106-100 ppm, a low detection limit of 11.76 ppb, and rapid response/recovery times (28/55 s) at an operating temperature of 200 °C. The sensor performance was validated by density functional theory calculations, which corroborated the effectiveness of the sensing material. In simulated LIB leakage scenarios, such as puncture and electrolyte injection, the sensors demonstrated quick responses to various common electrolyte compositions, indicating their potential for practical applications. This study highlights an effective method for fabricating composite sensing materials and emphasizes the practical significance of our universal detection approach for practical monitoring of electrolyte leakage in energy storage devices.

Energy-Level Alignment at TiO2@NH2-MIL-125 Interface for High-Performance Gas Sensing

Metal oxide (MO)-based chemiresistive sensors have great potential in environmental monitoring, security protection, and disease diagnosis. However, the thermally activated sensing mechanism in pristine MOs leads to high working temperature and poor selectivity, which are the main challenges impeding practical applications. Precise modulation of the band structure at the heterojunction interfaces of MOs offers the opportunity to unlock unique electrical and optical properties, enabling us to overcome these challenges. Metal-organic frameworks (MOFs) with tunable structures are promising materials for aligning the energy levels at the heterojunctions of MOs. Herein, we report the energy-level structural engineering of MO@MOF heterojunctions to optimize chemiresistive sensing performance. The interface was flexibly modulated from a straddling gap to a staggered gap by -NH2 functionalization of TiO2@(NH2)x-MIL-125, varying x from 0 to 1 and 2, respectively. TiO2@(NH2)x-MIL-125 combines the advantages of MOs and MOFs to synergistically improve gas-sensing properties. As a result, TiO2@NH2-MIL-125 is the first light-activated material to detect NO2 at 1 ppb with a response time of < 0.3 min at room temperature. It also exhibited excellent selectivity and long-term stability. Our study underscores the potential of energy band engineering in creating high-performance sensors, offering a strategy to overcome current material limits.

Ti3C2Tx MXene Functionalized via Boron Doped MoS2 Quantum Dots: A Synergy of Heterojunctions and Doping Effect Enabling Ultrasensitive SO2 Detection at Room Temperature

The design of mixed-dimensional heterostructures has emerged to be a new frontier of research as it induces exciting physical/chemical properties that extend beyond the fundamental properties of single dimensional systems. Therefore, rational design of heterostructured materials with novel surface chemistry and tailored interfacial properties appears to be very promising for the devices such as the gas sensors. Here, a highly sensitive gas sensor device is constructed by employing heterostructures of boron doped molybdenum disulfide quantum dots (B-MoS2 Qdots) assembled into the matrix of Ti3C2Tx MXene. Functionalization of MXene surface with B-MoS2 Qdots as a result of strong electrostatic attraction leads to improved charge migration behavior, active site exposure and abundant specific surface area. As a result, the Ti3C2Tx/B-MoS2 sensor device shows ultra-high response (28,998.3% @ 3 ppm), ultra-fast response rate (23.1% s-1), sub-ppm (10 ppb lowest) detection of sulfur dioxide (SO2) gas and excellent reversibility at room temperature. Density functional theory-based calculations indicate that enhanced SO2 sensing performance results from synergy of the 2D-0D heterostructure formation and preferential adsorption of SO2, induced by doped boron (B) heteroatoms in Qdots. Finally, a portable and wireless SO2 monitoring system is demonstrated for real-time detection of SO2 leakage and quantification under certain circumstances.

Roadmap for Borophene Gas Sensors

Borophene, a two-dimensional allotrope of boron, has emerged as a promising material for gas sensing because of its exceptional electronic properties and high surface reactivity. This review comprehensively overviews borophene synthesis methods, properties, and sensing applications. However, it is crucial to acknowledge the substantial gap between the abundance of theoretical literature and the limited experimental studies. While theoretical investigations have elucidated the stability and remarkable sensing capabilities of various borophene polymorphs across different gases, significant experimental challenges have hindered the translation of these theoretical predictions into practical devices. Consequently, this review carefully studies these challenges and shortcomings that are jeopardizing the practical implementation of borophene in real-world settings. Specifically, four key issues are thoroughly studied, such as superficial borophene oxidation upon exposure to the air, interference from relative humidity on gas molecule detection, lack of selectivity, and synthesis scalability. Finally, novel strategies are proposed to overcome these bottlenecks. By adopting these approaches, borophene can pave the way to drive the advancement of the next generation of sensing devices.

Pt-Decorated ZnO Nanorods for Light-Assisted Ethanol Sensing and In Situ Analysis of the Sensing Mechanism under Light Irradiation

ZnO nanorods have attracted much attention owing to their outstanding properties for chemical gas sensors. Although they show greater sensing properties than conventional nanoparticulate ZnO, high operation temperature (>250-350 °C) is required for them to work even if precious metals are deposited on them to sensitize their sensing properties. Light irradiation is one solution for overcoming the high operation temperature and the gas selectivity because it assists the oxidation activity on the surface that affects the sensor response. In this work, the sensing properties of Pt/ZnO nanorods and ZnO nanorods are examined under light irradiation, and the relationship between their sensing properties and surface reaction (ethanol oxidation) is elucidated. Pt/ZnO nanorods show selective sensor responses to ethanol (conditions: 150 °C, 50 ppm ethanol; sensor response, 843; response time, 4.0 min; recovery time, 22 min). In situ spectroscopic observations reveal that the largest amount of oxidation intermediates (acetate species) and oxidation products (CO2 and acetaldehyde) is confirmed during light irradiation. The oxidation reaction of ethanol is facilitated by the deposition of Pt and light irradiation. Thus, the operation temperature of ZnO nanorods decreases, and the selectivity to ethanol is enhanced.

The Cross-Sensitivity of Chemiresistive Gas Sensors: Nature, Methods, and Peculiarities: A Systematic Review

The evaluation of selectivity/cross-sensitivity is one of the most important tests for gas sensor development, particularly that based on chemiresistive technology. It is known that chemiresistive gas sensors suffer from low selectivity when they provide sensitivity to several analytes. Typically, selectivity testing involves independently assessing a sensor's response to a specific gas. However, there is a growing need to evaluate performance with interfering gases or gas mixtures since gas sensors are always exposed to gas mixtures in practice. Despite the great importance of selectivity characterization, currently, there are no standard methods of selectivity tests at conditions when target gas coexists with interfering gas compounds, which mimics real conditions. We outlined the four main methods researchers use to evaluate the cross-sensitivity of gas sensors. It highlights key aspects of selectivity test performance, assessment methodologies, and procedure features, attempting to classify them by their distinct characteristics. This review covers the essentials of gas properties, adsorption and desorption processes, and gas molecule interactions. Finally, we tried to address the lack of standardized protocols for evaluating chemiresistive gas sensors' cross-sensitivity to interfering gases and guide researchers.

Room Temperature NO2-Sensing Properties of N-Doped ZnO Nanoparticles Activated by UV-Vis Light

Zinc oxide nanoparticles (ZnO NPs) with varying levels of nitrogen (N) doping were synthesized using a straightforward sol-gel approach. The morphology and microstructure of the N-doped ZnO NPs were examined through techniques such as SEM, XRD, photoluminescence, and Raman spectroscopy. The characterization revealed visible changes in the morphology and microstructure resulting from the incorporation of nitrogen into the ZnO lattice. These N-doped ZnO NPs were used in the fabrication of conductometric gas sensors designed to operate at room temperature (RT) for detecting low concentrations of NO2 in the air, under LED UV-Vis irradiation (λ = 400 nm). The influence of nitrogen doping on sensor performance was systematically studied. The findings indicate that N-doping effectively enhances ZnO-based sensors' ability to detect NO2 at RT, achieving a notable response (S = R/R0) of approximately 18 when exposed to 5 ppm of NO2. These improvements in gas-sensing capabilities are attributed to the reduction in particle size and the narrowing of the optical band gap.

Ultrasensitive Detection of Dimethylamine Gas for Early Diagnosis of Parkinson's Disease Using CeO2-Coated Ti3C2Tx MXene/Carbon Nanofibers

Parkinson's disease is a prevalent neurological disorder, with dimethylamine (DMA) recognized as a crucial breath biomarker, particularly at the parts per billion (ppb) level. Detecting DMA gas at this level, especially at room temperature and high humidity, remains a formidable challenge. This study presents an ultrasensitive chemiresistor DMA gas sensor, leveraging the CeO2-coated Ti3C2Tx MXene/carbon nanofiber (CeO2/MXene/C NFs) heterostructure to enhance dimethylamine sensing. The high conductivity of MXene, combined with C-Ti-O bonds and a sp2 hybridized hexagonal carbon structure, increases surface active sites. The presence of Ce3+ promotes the formation of surface-active oxygen species, while the MXene-CeO2 heterojunction broadens the electron depletion layer. Theoretical calculations reveal that the highest adsorption energy for DMA gas is at the Ce top site, explaining the sensor's satisfactory sensitivity, rapid response and recovery process, low detection limit (5 ppb), and high selectivity at room temperature. The Ce3+/Ce4+ dynamic self-refresh mechanism, involving surface hydroxyl elimination, enhances the sensor's performance under high-humid conditions. Clinical breath tests demonstrate the sensor's ability to distinguish between healthy individuals and Parkinson's disease patients, paving the way for developing next-generation sensors for early diagnosis of neurological disorders.

Transparent TiO2/MoO3 Heterojunction-Based Photovoltaic Self-Powered Triethylamine Gas Sensor with IoT-Enabled Smartphone Interface

Conventional gas sensors encounter a significant obstacle in terms of power consumption, making them unsuitable for integration with the next generation of smartphones, wireless platforms, and the Internet of Things (IoT). Energy-efficient gas sensors, particularly self-powered gas sensors, can effectively tackle this problem. The researchers are making significant strides in advancing photovoltaic self-powered gas sensors by employing diverse materials and their compositions. Unfortunately, several of these sensors seem complex in fabrication and mainly target oxidizing species detection. To address these issues, we have successfully employed a transparent, cost-efficient solution processed bilayer TiO2/MoO3 heterojunction-based photovoltaic self-powered gas sensor with superior VOC sensing capabilities, marking a significant milestone in this field. The scanning Kelvin probe (SKP) measurement reveals the remarkable change in contact potential difference (-23 mV/kPa) of the TiO2/MoO3 bilayered film after UV light exposure in a triethylamine (TEA) atmosphere, indicating the highest reactivity between TEA molecules and TiO2/MoO3. Under photovoltaic mode, the sensor further demonstrates exceptional sensitivity (∼2.35 × 10-3 ppm-1) to TEA compared to other studied VOCs, with an admirable limit of detection (22 ppm) and signal-to-noise ratio (1540). Additionally, the sensor shows the ability to recognize TEA and estimate its composition in a binary mixture of VOCs from a similar class. The strongest affinity of TiO2/MoO3 toward the TEA molecule, the lowest covalent bond energy, and the highest electron-donating nature of TEA may be mainly attributed to the highest adsorption between TiO2/MoO3 and TEA. We further demonstrate the practical applicability of the TEA sensor with a prototype device connected to a smartphone via the IoT, enabling continuous surveillance of TEA.

First-principles investigations of As-doped tetragonal boron nitride nanosheets for toxic gas sensing applications

Pristine and arsenic-doped tetragonal boron nitride nanosheets (BNNS and As-BNNS) have been reported as potential candidates for toxic gas sensing applications. We have investigated the adsorption behavior of BNNS and As-BNNS for CO2, H2S, and SO3 gas molecules using first-principles density functional theory (DFT). Both BNNS and As-BNNS possess negative cohesive energies of -8.47 and -8.22 eV, respectively, which indicates that both sheets are energetically stable. Successful adsorption is inferred from the negative adsorption energy and structural deformation in the vicinity of the adsorbent and adsorbate. As-doping results in a significant increase in adsorption energies from -0.094, -0.175, and -0.462 eV to -2.748, -2.637, and 3.057 eV for CO2, H2S and SO3 gases, respectively. Due to gas adsorption, the electronic bandgap in As-BNNS varies by approximately 32% compared to a maximum of 24% in BNNS. A notable fluctuation in the energy gap and electrical conductivity is seen, with ambient temperature being the point of maximal sensitivity. For SO3, the maximum charge transfer during adsorption in BNNS and As-BNNS is determined to be 0.08|e| and 0.25|e|, respectively. Due to the interaction with gases, all structures exhibit an extremely high absorption coefficient on the order of 104 cm-1 with minimal peak shifting. Additionally, doping an As atom on BNNS' surface remarkably improved its ability to sense CO2, H2S, and SO3 gasses.

Sensitivity Analysis of an Optical Interferometric Surface Stress Ethanol Gas Sensor with a Freestanding Nanosheet

Ethanol (EtOH) gas detection has garnered considerable attention owing to its wide range of applications in industries such as food, pharmaceuticals, medical diagnostics, and fuel management. The development of highly sensitive EtOH-gas sensors has become a focus of research. This study proposes an optical interferometric surface stress sensor for detecting EtOH gas. The sensor incorporates a 100 nm-thick freestanding membrane of Parylene C and gas-sensitive polymethylmethacrylate (PMMA) fabricated within a microcavity on a Si substrate. The results showed that reducing the thickness of the freestanding Parylene C membrane is essential for achieving higher sensitivity. Previously, a 100-nm-thick membrane transfer onto microcavities was achieved using a surfactant-assisted release technique. However, polymerization inhibition caused by the surfactant presented challenges in forming ultrathin membranes of several tens of nanometers. In this study, we employed a surfactant-free release technique using a hydrophilic natural oxide layer to successfully form a 14-nm-thick freestanding Parylene C membrane. In contrast, the optimum thickness of the gas-adsorbed PMMA membrane was approximately 295 nm. Moreover, we demonstrated that this thinner membrane improved EtOH gas detection sensitivity by a factor of eight compared with our previously reported sensor. Thus, this study advances the field of nanoscale materials and sensor technology.

Recent Advances in Engineering of 2D Layered Metal Chalcogenides for Resistive-Type Gas Sensor

2D nanomaterials have triggered widespread attention in sensing applications. Especially for 2D layered metal chalcogenides (LMCs), the unique semiconducting properties and high surface area endow them with great potential for gas sensors. The assembly of 2D LMCs with guest species is an effective functionalization method to produce the synergistic effects of hybridization for greatly enhancing the gas-sensing properties. This review starts with the synthetic techniques, sensing properties, and principles, and then comprehensively compiles the advanced achievements of the pristine 2D LMCs gas sensors. Key advances in the development of the functionalization of 2D LMCs for enhancing gas-sensing properties are categorized according to the spatial architectures. It is systematically discussed in three aspects: surface, lattice, and interlayer, to comprehend the benefits of the functionalized 2D LMCs from surface chemical effect, electronic properties, and structure features. The challenges and outlooks for developing high-performance 2D LMCs-based gas sensors are also proposed.

Iron-based electrode material composites for electrochemical sensor application in the environment: A review

This review explores the expanding role of electrochemical sensors across diverse domains such as environmental monitoring, medical diagnostics, and food quality assurance. In recent years, iron-based electrocatalysts have emerged as promising candidates for enhancing sensor performance. Notable for their non-toxicity, abundance, catalytic activity, and cost-effectiveness, these materials offer significant advantages. However, further investigation is needed to fully understand how iron-based materials' physical, chemical, and electrical properties influence their catalytic performance in sensor applications. It explores the overview of electrochemical sensor technology, examines the impact of iron-based materials and their characteristics on catalytic activity, and investigates various iron-based materials, their advantages, functionalization, and modification techniques. Additionally, the review investigates the application of iron-based electrode material composites in electrochemical sensors for real sample detections. Ultimately, continued research and development in this area promise to unlock new avenues for using iron-based electrode materials in sensor applications.

A Comprehensive Review of Biomarker Sensors for a Breathalyzer Platform

Detecting volatile organic compounds (VOCs) is increasingly recognized as a pivotal tool in non-invasive disease diagnostics. VOCs are metabolic byproducts, mostly found in human breath, urine, feces, and sweat, whose profiles may shift significantly due to pathological conditions. This paper presents a thorough review of the latest advancements in sensor technologies for VOC detection, with a focus on their healthcare applications. It begins by introducing VOC detection principles, followed by a review of the rapidly evolving technologies in this area. Special emphasis is given to functionalized molecularly imprinted polymer-based biochemical sensors for detecting breath biomarkers, owing to their exceptional selectivity. The discussion examines SWaP-C considerations alongside the respective advantages and disadvantages of VOC sensing technologies. The paper also tackles the principal challenges facing the field and concludes by outlining the current status and proposing directions for future research.

Nanostructured NiS2-based flexible smart sensors for human respiration monitoring

The growing demand for wearable healthcare devices has led to an urgent need for cost-effective, wireless and portable breath monitoring systems. However, it is essential to explore novel nanomaterials that combine state-of-the-art flexible sensors with high performance and sensing capabilities along with scalability and industrially acceptable processing. In this study, we demonstrate a highly efficient NiS2-based flexible capacitive sensor fabricated via a solution-processible route using a novel single-source precursor [Ni{S2P(OPr)2}2]. The developed sensor could precisely detect the human respiration rate and exhibit rapid responsiveness, exceptional sensitivity and selectivity at ambient temperatures, with an ultra-fast response and recovery. The device effectively differentiates the exhaled breath patterns including slow, fast, oral and nasal breath, as well as post-exercise breath rates. Moreover, the sensor shows outstanding bending stability, repeatability, reliable and robust sensing performance and is capable of contactless sensing. The sensor was further employed with a user-friendly wireless interface to facilitate smartphone-enabled real-time breath monitoring systems. This work opens up numerous avenues for cost-effective, sustainable and versatile sensors with potential applications for Internet of Things-based flexible and wearable electronics.This article is part of the theme issue 'Celebrating the 15th anniversary of the Royal Society Newton International Fellowship'.

High-Entropy Sulfide (FeCoNiCrMn)S2 for Room Temperature NO2 Sensors

Metal sulfides have been extensively studied in the field of chemical sensors working at room temperature (RT). However, compared to metal oxides, metal sulfides often suffer from incomplete recovery and unsatisfactory selectivity. In this work, we for the first time report the high-entropy sulfide (HES) (FeCoNiCrMn)S2, prepared by the solvothermal method, is a promising candidate for utilization as the sensitive layer in gas sensors. Unlike traditional single-metal sulfides, this HES (FeCoNiCrMn)S2 exhibits reversible recovery for NO2 at RT as well as excellent selectivity. To unveil the sensing mechanism, we simulated the adsorption and charge transfer between (FeCoNiCrMn)S2 and gas molecules by the density functional theory (DFT) calculation, revealing the reason (FeCoNiCrMn)S2 is uniquely selective to NO2. This work explores the application potential of HES (FeCoNiCrMn)S2 as an RT sensor and enriches the material selection for NO2 sensors.

Single stranded 1D-helical Cu coordination polymer for ultra-sensitive ammonia sensing at room temperature

With the increasing demand for ammonia applications, there is a significant focus on improving NH3 detection performance at room temperature. In this study, we introduce a groundbreaking NH3 gas sensor based on Cu(I)-based coordination polymers, featuring semiconducting, single stranded 1D-helical nanowires constructed from Cu-Cl and N-methylthiourea (MTCP). The MTCP demonstrates an exceptional response to NH3 gas (>900% at 100 ppm) and superior selectivity at room temperature compared to current materials. The interaction mechanism between NH3 and the MTCP sensor is elucidated through a combination of empirical results and computational calculations, leveraging a crystal-determined structure. This reveals the formation of NH3-Cu and NH3-H3C complexes, indicative of a thermodynamically favorable reaction. Additionally, Ag-doped MTCP exhibits higher selectivity and a response over two times greater than the original MTCP, establishing it as a prominent NH3 detection system at room temperature.

Notable synthesis, properties and chemical gas sensing trends on molybdenum disulphides and diselenides two-dimensional nanostructures: A critical review

Evaluation of synthesis methods, notable properties, and chemical gas sensing properties of molybdenum disulphides and diselenides two-dimensional nanosheets is unfold. This is motivated by the fact that the two dichalcogenides have good sensitivity and selectivity to different harmful gases at ambient temperatures. Synthesis methods explored include exceptional top-down and bottom-up approaches, which consider physical and chemical compositional inceptions. Mechanical exfoliation in both molybdenum disulphides and diselenides nanosheets demonstrate good crystalline purity with structural alterations under varying stacking conditions. These chalcogenides exhibit low energy band gaps of ±1.80 eV for MoS2 and ±1.60 eV for MoSe2, which reduces with introduction of impurities. Thus, upon doping with other metal elements, a transformation from either n-type or p-type semiconductors is normally observed, leading to tuneable electronic properties. Thus, different gases such as methane, ethanol, toluene, ammonia, nitrogen oxide have been systematically detected using molybdenum disulphide and diselenide based thin films as sensing platforms. This review highlights structural, electronic and morphological characteristics of the two dichalcogenides which influences the sensitivity and selectivity ability for a couple of gases at ambient temperatures. The strategies for enhancing the selectivity by introducing defects, impurities and interfacing with other composites expanding the choice of these gases wider is also discussed in details. The review also provides overviews of challenges and limitations that open new research avenues to further enriching both chalcogenides as flexible, stable and cost effective state-of-the-art chemical gas sensors.

UV-assisted fluctuation-enhanced gas sensing by ink-printed MoS2 devices

In this work, MoS2 flakes were printed on ceramic substrates and investigated toward 1-10 ppm of nitrogen dioxide (NO2), 2-12 ppm of ammonia (NH3), and 2-12 ppm acetone (C3H6O) under UV light (275 nm). The structure of overlapping MoS2 flakes and UV light assistance affected high responsivity to NO2 when DC resistance was monitored, and superior sensitivity to NH3 was obtained from the low-frequency noise spectra. MoS2 exhibited response and recovery times in hundreds of seconds and stability throughout the experiments conducted within a few months. MoS2 sensor exhibited a resistance drift during the detection of a specific relaxation time. Subtracting the baseline burden with exponential drift exposed the direction of changes induced by oxidizing and reducing gases and reduced DL to 80 ppb, 130 ppb, and 360 ppb for NO2, NH3, and C3H6O, respectively. The fluctuation-enhanced sensing (FES) revealed that the adsorption of NO2 on MoS2 decreases the noise intensity, whereas adsorbed NH3 increases the fluctuations of current flowing through the sensor, and these changes are proportional to the concentration of gases. The noise responses for NO2 and NH3 were opposite and higher than DC resistance responses with subtracted baseline (an increase of 50% for 10 ppm of NO2 and an increase of more than 600% for 12 ppm of NH3), showing that FES is a highly sensitive tool to detect and distinguish between these two gases. This way, we introduce a simple and low-cost method of gas sensor fabrication using ink-printed MoS2 and the possibility of enhancing its sensitivity through data processing and the FES method.

Modulating the bandgap of Cr-intercalated bilayer graphene via combining the 18-electron rule and the 2D superatomic-molecule theory

Bandgap engineering of graphene is of great significance for its potential applications in electronic devices. Herein, we used a sandwich compound Cr(C6H6)2 as the building block to construct Cr-intercalated bilayer graphene (BLG), namely a C12Cr monolayer. Chemical bonding analysis reveals that strong d-π interaction ensures π electrons of the graphene layers and d orbitals of the Cr atoms localized in C6CrC6 units to achieve the favored 18-electron rule, thus leading to a bandgap of 0.24 eV. Subsequently, a C48Cr monolayer with lower proportion of Cr is further designed using Cr(C54H18)2 as building units, where a newly developed two-dimensional (2D) superatomic-molecule theory is introduced to rationalize its electronic structure. The C48Cr monolayer not only satisfies the 18-electron rule, but also localizes extra π electrons to form two layers of 2D superatomic crystals composed of 2D superatoms (◊O and ◊N), resulting in a wider bandgap of 0.74 eV. This work opens an effective avenue to modulate the bandgap of BLG via combining the 18-electron rule and the 2D superatomic-molecule theory.

Emerging Low Detection Limit of Optically Activated Gas Sensors Based on 2D and Hybrid Nanostructures

Gas sensing is essential for detecting and measuring gas concentrations across various environments, with applications in environmental monitoring, industrial safety, and healthcare. The integration of two-dimensional (2D) materials, organic materials, and metal oxides has significantly advanced gas sensor technology, enhancing its sensitivity, selectivity, and response times at room temperature. This review examines the progress in optically activated gas sensors, with emphasis on 2D materials, metal oxides, and organic materials, due to limited studies on their use in optically activated gas sensors, in contrast to other traditional gas-sensing technologies. We detail the unique properties of these materials and their impact on improving the figures of merit (FoMs) of gas sensors. Transition metal dichalcogenides (TMDCs), with their high surface-to-volume ratio and tunable band gap, show exceptional performance in gas detection, especially when activated by UV light. Graphene-based sensors also demonstrate high sensitivity and low detection limits, making them suitable for various applications. Although organic materials and hybrid structures, such as metal-organic frameworks (MoFs) and conducting polymers, face challenges related to stability and sensitivity at room temperature, they hold potential for future advancements. Optically activated gas sensors incorporating metal oxides benefit from photoactive nanomaterials and UV irradiation, further enhancing their performance. This review highlights the potential of the advanced materials in developing the next generation of gas sensors, addressing current research gaps and paving the way for future innovations.

Monolayer AlC3: Mechanical, Thermoelectric, and Promising Sensing Properties for Environmentally Toxic/Nontoxic Oxygen-Containing Gases

This work investigated the possibility of a monolayer AlC3 nanosheet as an optimistic sensor for ecologically poisonous/harmless oxygen-containing gases (OCGs), which includes CO, CO2, and H2O, implementing nonequilibrium Green's function (NEGF) and first-principles computations. The sensing properties of a pure AlC3 monolayer were explored in this study, including their mechanical and thermoelectric properties. The pristine AlC3 has a maximum mobility in the armchair direction for the hole carrier of 62797.44 cm2 V-1 s-1. At room temperature, the value of electrical conductivity (σ), thermal conductivity (κ), figure of merit (ZT), and Seebeck coefficient (S) are 2.07 × 1018 S m-1, 3.84 × 1013 W m-1 K-1 s-1, 0.64, and -199.35 × 10-6 V K-1, respectively. On the surface of the AlC3 nanosheet, we found variability in adsorption energies, electronic parameters, charge transfer, recovery time, work function, and I/V transport properties for these OCGs. When gas molecules cooperate with the surface of the AlC3 nanosheet, all of the OCGs exhibit electron acceptor behavior. The OCGs (CO, CO2, and H2O) are noticed to have relatively short recovery times, though, because of their low adsorption energies. Overall, due to the remarkably short recovery time of OCGs, monolayer AlC3 proves to be an outstanding multigas reversible sensor material designed for CO, CO2, and H2O. As a result, AlC3 seems to be an excellent reversible sensor for CO, CO2, and H2O. We found remarkable variations in transport (I/V) properties and sensitivity for OCGs adsorbed on the AlC3 monolayer, which illustrates its applicability for real-world applications. The current study successfully demonstrates that the AlC3 monolayer may be utilized to create a gas sensor with outstanding sensing performance and also thermoelectric efficiency.

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.

Development of a Screening Platform for Optimizing Chemical Nanosensor Materials

Chemical sensors, relying on changes in the electrical conductance of a gas-sensitive material due to the surrounding gas, typically react with multiple target gases and the resulting response is not specific for a certain analyte species. The purpose of this study was the development of a multi-sensor platform for systematic screening of gas-sensitive nanomaterials. We have developed a specific Si-based platform chip, which integrates a total of 16 sensor structures. Along with a newly developed measurement setup, this multi-sensor platform enables simultaneous performance characterization of up to 16 different sensor materials in parallel in an automated gas measurement setup. In this study, we chose the well-established ultrathin SnO2 films as base material. In order to screen the sensor performance towards type and areal density of nanoparticles on the SnO2 films, the films are functionalized by ESJET printing Au-, NiPt-, and Pd-nanoparticle solutions with five different concentrations. The functionalized sensors have been tested toward the target gases: carbon monoxide and a specific hydrogen carbon gas mixture of acetylene, ethane, ethne, and propene. The measurements have been performed in three different humidity conditions (25%, 50% and 75% r.h.). We have found that all investigated types of NPs (except Pd) increase the responses of the sensors towards CO and HCmix and reach a maximum for an NP type specific concentration.

A Review of Gas Sensors for CO2 Based on Copper Oxides and Their Derivatives

Buildings worldwide are becoming more thermally insulated, and air circulation is being reduced to a minimum. As a result, measuring indoor air quality is important to prevent harmful concentrations of various gases that can lead to safety risks and health problems. To measure such gases, it is necessary to produce low-cost and low-power-consuming sensors. Researchers have been focusing on semiconducting metal oxide (SMOx) gas sensors that can be combined with intelligent technologies such as smart homes, smart phones or smart watches to enable gas sensing anywhere and at any time. As a type of SMOx, p-type gas sensors are promising candidates and have attracted more interest in recent years due to their excellent electrical properties and stability. This review paper gives a short overview of the main development of sensors based on copper oxides and their composites, highlighting their potential for detecting CO2 and the factors influencing their performance.

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.

Chemiresistive Gas Sensing using Graphene-Metal Oxide Hybrids

Chemiresistive sensing lies in its ability to provide fast, accurate, and reliable detection of various gases in a cost-effective and non-invasive manner. In this context, graphene-functionalized metal oxides play crucial role in hydrogen gas sensing. However, a cost-effective, defect-free, and large production schemes of graphene-based sensors are required for industrial applications. This review focuses on graphene-functionalized metal oxide nanostructures designed for gaseous molecules detection, mainly hydrogen gas sensing applications. For the convenience of the reader and to understand the role of graphene-metal oxide hybrids (GMOH) in gas sensing activities, a brief overview of the properties and synthesis routes of graphene and GMOH have been reported in this paper. Metal oxides play an essential role in the GMOH construct for hydrogen gas sensing. Therefore, various metal oxides-decorated GMOH constructs are detailed in this review as gas sensing platforms, particularly for hydrogen detection. Finally, specific directions for future research works and challenges ahead in designing highly selective and sensitive hydrogen gas sensors have been highlighted. As illustrated in this review, understanding of the metal oxides-decorated GMOH constructs is expected to guide ones in developing emerging hybrid nanomaterials that are suitable for hydrogen gas sensing applications.

2D/2D Bi2Se3/SnSe2 heterostructure with rapid NO2 gas detection

Heterostructure engineering is crucial for enhancing gas sensing performance. However, achieving rapid response for room-temperature NO2 sensing through rational heterostructure design remains a challenge. In this study, a Bi2Se3/SnSe2 2D/2D heterostructure was synthesized by hydrothermal method for the rapid detection of NO2 at room temperature. By combining Bi2Se3 nanosheets with SnSe2 nanosheets, the Bi2Se3/SnSe2 sensor demonstrated and the lowest detection limit for NO2 a short response time (15 s) to 10 ppm NO2 at room temperature, reaches 25 ppb. Furthermore the sensor demonstrates significantly larger response to NO2 than to other interfering gases, including 10 ppm NO2, H2S, NH3, CH4, CO, and SO2,demonstrating its outstanding selectivity. And we discuss the mechanism of related performance enhancement.

Elevating nanomaterial optical sensor arrays through the integration of advanced machine learning techniques for enhancing visual inspection of food quality and safety

Food quality and safety problems caused by inefficient control in the food chain have significant implications for human health, social stability, and economic progress and optical sensor arrays (OSAs) can effectively address these challenges. This review aims to summarize the recent applications of nanomaterials-based OSA for food quality and safety visual monitoring, including colourimetric sensor array (CSA) and fluorescent sensor array (FSA). First, the fundamental properties of various advanced nanomaterials, mainly including metal nanoparticles (MNPs) and nanoclusters (MNCs), quantum dots (QDs), upconversion nanoparticles (UCNPs), and others, were described. Besides, the diverse machine learning (ML) and deep learning (DL) methods of high-dimensional data obtained from the responses between different sensing elements and analytes were presented. Moreover, the recent and representative applications in pesticide residues, heavy metal ions, bacterial contamination, antioxidants, flavor matters, and food freshness detection were comprehensively summarized. Finally, the challenges and future perspectives for nanomaterials-based OSAs are discussed. It is believed that with the advancements in artificial intelligence (AI) techniques and integrated technology, nanomaterials-based OSAs are expected to be an intelligent, effective, and rapid tool for food quality assessment and safety control.

Constructing Adsorption Site-Enhanced Vo-BiOCl/rGO Heterostructures for Efficient Response to NO2/NH3 Gases at Room Temperature

Real-time detection of harmful gases at room temperature has become a serious problem in public health and environmental monitoring. Two-dimensional materials with semiconductor properties BiOCl is a promising gas-sensitive material due to its large specific surface area and adjustable band gap as well as outstanding safety characteristics. However, limited by the weak gas adsorption sites and sluggish charge-transfer ability, the performance of BiOCl could not be fully exploited. Oxygen vacancy (Vo) engineering can introduce lattice defects, thereby significantly increasing the local charge density and enhancing the adsorption of gases, which is an effective strategy to enhance the gas-sensing performance. In this work, we composite BiOCl with a vacancy (Vo-BiOCl) and reduced graphene oxide (rGO) to construct a Vo-BiOCl/rGO heterostructure with enhanced gas adsorption sites. Experimental and theoretical calculations show that Vo can enhance the adsorption of gases and the introduction of rGO forms a high-quality heterostructure with BiOCl, which can effectively reduce the band gap of BiOCl and promote electron transfer, thereby improving the sensitivity of the sensor. Benefiting from above, Vo-BiOCl/rGO achieves the ability to detect low concentrations of NO2/NH3 at room temperature, with high sensitivity (55% at 1 ppm of NO2 and -28% at 1 ppm of NH3), fast response time (40 s at 1 ppm of NO2 and 2 s at 1 ppm of NH3), good stability (over 150 days), and fully recoverable gas sensitivity.

Construction of semiconductor nanocomposites for room-temperature gas sensors

Gas sensors are essential for ensuring public safety and improving quality of life. Room-temperature gas sensors are notable for their potential economic benefits and low energy consumption, and their expected integration with wearable electronics, making them a focal point of contemporary research. Advances in nanomaterials and low-dimensional semiconductors have significantly contributed to the enhancement of room-temperature gas sensors. These advancements have focused on improving sensitivity, selectivity, and response/recovery times, with nanocomposites offering distinct advantages. The discussion here focuses on the use of semiconductor nanocomposites for gas sensing at room temperature, and provides a review of the latest synthesis techniques for these materials. This involves the precise adjustment of chemical compositions, microstructures, and morphologies. In addition, the design principles and potential functional mechanisms are examined. This is crucial for deepening the understanding and enhancing the operational capabilities of sensors. We also highlight the challenges faced in scaling up the production of nanocomposite materials. Looking ahead, semiconductor nanocomposites are expected to drive innovation in gas sensor technology due to their carefully crafted design and construction, paving the way for their extensive use in various sectors.

Hierarchically porous ZnO derived from zeolitic imidazolate frameworks for high-sensitive MEMS NO2 sensor

Three-dimensional (3D) porous metal oxide nanomaterials with controllable morphology and well-defined pore size have attracted extensive attention in the field of gas sensing. Herein, hierarchically porous ZnO-450 was obtained simply by annealing Zeolitic Imidazolate Frameworks (ZIF-90) microcrystals at an optimal temperature of 450 °C, and the effect of annealing temperature on the formation of porous nanostructure was discussed. Then the as-obtained ZnO-450 was employed as sensing materials to construct a Micro-Electro-Mechanical System (MEMS) gas sensor for detecting NO2. The MEMS sensor based on ZnO-450 displays the excellent gas-sensing performances at a lower working temperature (190 °C), such as high response value (242.18% @ 10 ppm), fast response/recovery time (9/26 s) and ultralow limit of detection (35 ppb). The ZnO-450 sensor shows better sensing performance for NO2 detection than ZnO-based composites materials or commercial ZnO nanoparticles (NPs), which are attributed to its unique hierarchically structures with high porosity and larger surface area. This ZIFs driven strategy can be expected to pave a new pathway for the design of high-performance NO2 sensors.

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.

Nanotransistor-based gas sensing with record-high sensitivity enabled by electron trapping effect in nanoparticles

Highly sensitive, low-power, and chip-scale H2 gas sensors are of great interest to both academia and industry. Field-effect transistors (FETs) functionalized with Pd nanoparticles (PdNPs) have recently emerged as promising candidates for such H2 sensors. However, their sensitivity is limited by weak capacitive coupling between PdNPs and the FET channel. Herein we report a nanoscale FET gas sensor, where electrons can tunnel between the channel and PdNPs and thus equilibrate them. Gas reaction with PdNPs perturbs the equilibrium, and therefore triggers electron transfer between the channel and PdNPs via trapping or de-trapping with the PdNPs to form a new balance. This direct communication between the gas reaction and the channel enables the most efficient signal transduction. Record-high responses to 1-1000 ppm H2 at room temperature with detection limit in the low ppb regime and ultra-low power consumption of ~ 300 nW are demonstrated. The same mechanism could potentially be used for ultrasensitive detection of other gases. Our results present a supersensitive FET gas sensor based on electron trapping effect in nanoparticles.