Fabrication of a Micro-Electromechanical System-Based Acetone Gas Sensor Using CeO2 Nanodot-Decorated WO3 Nanowires

Development and integration of a hierarchical Pd/WO3 acetone-sensing device for real-time exhaled breath monitoring with disposable face mask

This study introduces an inventive acetone-sensing device seamlessly integrated into a disposable face mask, enabling real-time continuous breath monitoring. The sensor demonstrates exceptional sensitivity, registering a response of 8.22 at 1 ppm and an impressive sensor response of 57.33 at 100 ppm of acetone concentration. Particularly noteworthy is the remarkable lower limit of detection (LOD) of 0.076 ppm within the concentration range of 0.1-0.8 ppm, underscored by a robust R2 value of 0.994. To validate practicality, the Pd/WO3 sensor was fabricated onto cellulose paper and utilized for real-time breath analysis, yielding a substantial sensor response of 1.70 at 8 vol% (equivalent to a single exhale breath volume). The unique design incorporates a built-in disposable face mask, facilitating dependable and convenient real-time breath analysis. Additionally, this research explores the profound impact of introducing acetone and Pd atoms on the energy levels and dipole moments. The species elucidated through density functional theory (DFT) investigations encompassing WO3, WO3-acetone, Pd-WO3, and Pd-WO3-acetone species. This work presents an innovative and cost-effective approach for developing a portable, non-invasive, and highly sensitive acetone-sensing device, effectively integrated into a disposable face mask for real-time breath analysis. This pioneering technology holds immense potential for various applications in healthcare and beyond.

Flexible Paper-Based Room-Temperature Acetone Sensors with Ultrafast Regeneration

Paper-based lightweight, degradable, low-cost, and eco-friendly substrates are extensively used in wearable biosensor applications, albeit to a lesser extent in sensing acetone and other gas-phase analytes. Generally, rigid substrates with heaters have been employed to develop acetone sensors due to the high operating/recovery temperature (typically above 200 °C), limiting the use of papers as substrates in such sensing applications. In this work, we proposed fabricating the paper-based, room-temperature-operatable acetone sensor using ZnO-polyaniline-based acetone-sensing inks by a facile fabrication method. The fabricated paper-based electrodes showed good electrical conductivity (80 S/m) and mechanical stability (∼1000 bending cycles). The acetone sensors showed a sensitivity of 0.02/100 ppm and 0.6/10 μL with an ultrafast response (4 s) and recovery time (15 s) at room temperature. The sensors delivered a broad sensitivity over a physiological range of 260 to >1000 ppm with R² > 0.98 under atmospheric conditions. Further, the role of the surface, interfacial, microstructure, electrical, and electromechanical properties of the paper-based sensor devices has been correlated with the sensitivity and room-temperature recovery observed in our system. These versatile, green, flexible electronic devices would be ideal for low-cost, highly regenerative, room-/low-temperature-operable wearable sensor applications.

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

Highly sensitive gas sensors with remarkably low detection limits are attractive for diverse practical application fields including real-time environmental monitoring, exhaled breath diagnosis, and food freshness analysis. Among various chemiresistive sensing materials, noble metal-decorated semiconducting metal oxides (SMOs) have currently aroused extensive attention by virtue of the unique electronic and catalytic properties of noble metals. This review highlights the research progress on the designs and applications of different noble metal-decorated SMOs with diverse nanostructures (e.g., nanoparticles, nanowires, nanorods, nanosheets, nanoflowers, and microspheres) for high-performance gas sensors with higher response, faster response/recovery speed, lower operating temperature, and ultra-low detection limits. The key topics include Pt, Pd, Au, other noble metals (e.g., Ag, Ru, and Rh.), and bimetals-decorated SMOs containing ZnO, SnO2, WO3, other SMOs (e.g., In2O3, Fe2O3, and CuO), and heterostructured SMOs. In addition to conventional devices, the innovative applications like photo-assisted room temperature gas sensors and mechanically flexible smart wearable devices are also discussed. Moreover, the relevant mechanisms for the sensing performance improvement caused by noble metal decoration, including the electronic sensitization effect and the chemical sensitization effect, have also been summarized in detail. Finally, major challenges and future perspectives towards noble metal-decorated SMOs-based chemiresistive gas sensors are proposed.

Construction of Flower-like PtOx@ZnO/In2O3 Hollow Microspheres for Ultrasensitive and Rapid Trace Detection of Isopropanol

The design of a highly effective isopropanol gas sensor with high response and trace detection capability is extremely important for environmental surveillance and human health. Here, novel flower-like PtO_x@ZnO/In₂O₃ hollow microspheres were prepared by a three-step approach. The hollow structure was composed of an In₂O₃ shell inside and layered ZnO/In₂O₃ nanosheets outside with PtO_x nanoparticles (NPs) on the surface. Meanwhile, the gas sensing performances of the ZnO/In₂O₃ composite with different Zn/In ratios and PtO_x@ZnO/In₂O₃ composites were evaluated and compared systematically. The measurement results indicated that the ratio of Zn/In affected the sensing performance and the ZnIn₂ sensor presented a higher response, which was then modified with PtO_x NPs to further enhance its sensing property. The Pt@ZnIn₂ sensor exhibited outstanding isopropanol detection performance with ultrahigh response values under 22 and 95% relative humidity (RH). In addition, it also showed a rapid response/recovery speed, good linearity, and low theoretical limit of detection (LOD) regardless of being under a relatively dry or ultrahumid atmosphere. The enhancement of isopropanol sensing properties might be ascribed to the unique structure of PtO_x@ZnO/In₂O₃, heterojunctions between the components, and catalytic effect of Pt NPs.

Metal-Organic Framework (MOF) Derivatives as Promising Chemiresistive Gas Sensing Materials: A Review

The emission of harmful gases has seriously exceeded relative standards with the rapid development of modern industry, which has shown various negative impacts on human health and the natural environment. Recently, metal-organic frameworks (MOFs)-based materials have been widely used as chemiresistive gas sensing materials for the sensitive detection and monitoring of harmful gases such as NO_x, H₂S, and many volatile organic compounds (VOCs). In particular, the derivatives of MOFs, which are usually semiconducting metal oxides and oxide-carbon composites, hold great potential to prompt the surface reactions with analytes and thus output amplified resistance changing signals of the chemiresistors, due to their high specific surface areas, versatile structural tunability, diversified surface architectures, as well as their superior selectivity. In this review, we introduce the recent progress in applying sophisticated MOFs-derived materials for chemiresistive gas sensors, with specific emphasis placed on the synthesis and structural regulation of the MOF derivatives, and the promoted surface reaction mechanisms between MOF derivatives and gas analytes. Furthermore, the practical application of MOF derivatives for chemiresistive sensing of NO₂, H₂S, and typical VOCs (e.g., acetone and ethanol) has been discussed in detail.

Engineering of ZnO/rGO towards NO2 Gas Detection: Ratio Modulated Sensing Type and Heterojunction Determined Response

Nanoscale heterostructured zinc oxide/reduced graphene oxide (ZnO/rGO) materials with p-n heterojunctions exhibit excellent low temperature NO₂ gas sensing performance, but their doping ratio modulated sensing properties remain poorly understood. Herein, ZnO nanoparticles were loaded with 0.1~4% rGO by a facile hydrothermal method and evaluated as NO₂ gas chemiresistor. We have the following key findings. First, ZnO/rGO manifests doping ratio-dependent sensing type switching. Increasing the rGO concentration changes the type of ZnO/rGO conductivity from n-type (<0.6% rGO) to mixed n/p -type (0.6~1.4% rGO) and finally to p-type (>1.4% rGO). Second, interestingly, different sensing regions exhibit different sensing characteristics. In the n-type NO₂ gas sensing region, all the sensors exhibit the maximum gas response at the optimum working temperature. Among them, the sensor that shows the maximum gas response exhibits a minimum optimum working temperature. In the mixed n/p-type region, the material displays abnormal reversal from n- to p-type sensing transitions as a function of the doping ratio, NO₂ concentration and working temperature. In the p-type gas sensing region, the response decreases with increasing rGO ratio and working temperature. Third, we derive a conduction path model that shows how the sensing type switches in ZnO/rGO. We also find that p-n heterojunction ratio (n_p-n/n_rGO) plays a key role in the optimal response condition. The model is supported by UV-vis experimental data. The approach presented in this work can be extended to other p-n heterostructures and the insights will benefit the design of more efficient chemiresistive gas sensors.

Formation of sub-100-nm suspended nanowires with various materials using thermally adjusted electrospun nanofibers as templates

The air suspension and location specification properties of nanowires are crucial factors for optimizing nanowires in electronic devices and suppressing undesirable interactions with substrates. Although various strategies have been proposed to fabricate suspended nanowires, placing a nanowire in desired microstructures without material constraints or high-temperature processes remains a challenge. In this study, suspended nanowires were formed using a thermally aggregated electrospun polymer as a template. An elaborately designed microstructure enables an electrospun fiber template to be formed at the desired location during thermal treatment. Moreover, the desired thickness of the nanowires is easily controlled with the electrospun fiber templates, resulting in the parallel formation of suspended nanowires that are less than 100 nm thick. Furthermore, this approach facilitates the formation of suspended nanowires with various materials. This is accomplished by evaporating various materials onto the electrospun fiber template and by removing the template. Palladium, copper, tungsten oxide (WO₃), and tin oxide nanowires are formed as examples to demonstrate the advantage of this approach in terms of nanowire material selection. Hydrogen (H₂) and nitrogen dioxide (NO₂) gas sensors comprising palladium and tungsten oxide, respectively, are demonstrated as exemplary devices of the proposed method.

Wearable chemical sensors based on 2D materials for healthcare applications

Chemical sensors worn on the body could make possible the continuous, noninvasive, and accurate monitoring of vital human signals, which is necessary for remote health monitoring and telemedicine. Attractive for creating high-performance, wearable chemical sensors are atomically thin materials with intriguing physical features, abundant chemistry, and high surface-to-volume ratios. These advantages allow for appropriate material-analyte interactions, resulting in a high level of sensitivity even at trace analyte concentrations. Previous review articles covered the material and device elements of 2D material-based wearable devices extensively. In contrast, little research has addressed the existing state, future outlook, and promise of 2D materials for wearable chemical sensors. We provide an overview of recent advances in 2D-material-based wearable chemical sensors to overcome this deficiency. The structure design, manufacturing techniques, and mechanisms of 2D material-based wearable chemical sensors will be evaluated, as well as their applicability in human health monitoring. Importantly, we present a thorough review of the current state of the art and the technological gaps that would enable the future design and nanomanufacturing of 2D materials and wearable chemical sensors. Finally, we explore the challenges and opportunities associated with designing and implementing 2D wearable chemical sensors.

The Piezoresponse in WO3 Thin Films Due to N2-Filled Nanovoids Enrichment by Atom Probe Tomography

Tungsten trioxide (WO₃) is a versatile n-type semiconductor with outstanding chromogenic properties highly used to fabricate sensors and electrochromic devices. We present a comprehensive experimental study related to piezoresponse with piezoelectric coefficient d₃₃ = 35 pmV^-1 on WO₃ thin films ~200 nm deposited using RF-sputtering onto alumina (Al₂O₃) substrate with post-deposit annealing treatment of 400 °C in a 3% H₂/N₂-forming gas environment. X-ray diffraction (XRD) confirms a mixture of orthorhombic and tetragonal phases of WO₃ with domains with different polarization orientations and hysteresis behavior as observed by piezoresponse force microscopy (PFM). Furthermore, using atom probe tomography (APT), the microstructure reveals the formation of N₂-filled nanovoids that acts as strain centers producing a local deformation of the WO₃ lattice into a non-centrosymmetric structure, which is related to piezoresponse observations.

Ultrasonically-assisted synthesis of CeO2 within WS2 interlayers forming type II heterojunction for a VOC photocatalytic oxidation

Here, we investigate the band structure, density of states, photocatalytic activity, and heterojunction mechanism of WS₂ with CeO₂ (CeO₂@WS₂) as a photoactive heterostructure. In this heterostructure, CeO₂'s growth within WS₂ layers is achieved through ultrasonicating WS₂ and intercalating CeO₂'s precursor within the WS₂ interlayers, followed by hydrothermal treatment. Through a set of density functional calculations, we demonstrate that CeO₂ and WS₂ form an interface through a covalent bonding that can be highly stable. The electrochemical impedance spectroscopy (EIS) found that the CeO₂@WS₂ heterostructure exhibits a remarkably higher conductivity (22.23 mS cm^-2) compared to either WS₂ and CeO₂, assignable to the interface in CeO₂@WS₂. Furthermore, in a physically mixed CeO₂-WS₂ where the interaction between particles is noncovalent, the resistance was significantly higher (0.67 mS cm^-2), confirming that the heterostructure in the interface is covalently bonded. In addition, Mott-Schottky and the bandgap measurements through Tauc plots demonstrate that the heterojunction in CeO₂ and WS₂ is type II. Eventually, the CeO₂@WS₂ heterostructure indicated 446.7 µmol g ^-1 CO₂ generation from photocatalytic oxidation of a volatile organic compound (VOC), formic acid, compared to WS₂ and CeO₂ alone.

Tungsten-based Nanomaterials in the Biomedical Field: A Bibliometric Analysis of Research Progress and Prospects

Tungsten-based nanomaterials (TNMs) with diverse nanostructures and unique physicochemical properties have been widely applied in the biomedical field. Although various reviews have described the application of TNMs in specific biomedical fields, there are still no comprehensive studies that summarize and analyze research trends of the field as a whole. To identify and further promote the development of biomedical TNMs, a bibliometric analysis method is used to analyze all relevant literature on this topic. First, general bibliometric distributions of the dataset by year, country, institute, referenced source, and research hotspots are recognized. Next, a comprehensive review of the subjectively recognized research hotspots in various biomedical fields, including biological sensing, anticancer treatments, antibacterials, and toxicity evaluation, is provided. Finally, the prospects and challenges of TNMs are discussed to provide a new perspective for further promoting their development in biomedical research.

WS2 Nanorod as a Remarkable Acetone Sensor for Monitoring Work/Public Places

Here, we report the synthesis of the WS₂ nanorods (NRs) using an eco-friendly and facile hydrothermal method for an acetone-sensing application. This study explores the acetone gas-sensing characteristics of the WS₂ nanorod sensor for 5, 10, and 15 ppm concentrations at 25 °C, 50 °C, 75 °C, and 100 °C. The WS₂ nanorod sensor shows the highest sensitivity of 94.5% at 100 °C for the 15 ppm acetone concentration. The WS₂ nanorod sensor also reveals the outstanding selectivity of acetone compared to other gases, such as ammonia, ethanol, acetaldehyde, methanol, and xylene at 100 °C with a 15 ppm concentration. The estimated selectivity coefficient indicates that the selectivity of the WS₂ nanorod acetone sensor is 7.1, 4.5, 3.7, 2.9, and 2.0 times higher than xylene, acetaldehyde, ammonia, methanol, and ethanol, respectively. In addition, the WS₂ nanorod sensor also divulges remarkable stability of 98.5% during the 20 days of study. Therefore, it is concluded that the WS₂ nanorod can be an excellent nanomaterial for developing acetone sensors for monitoring work/public places.

Recent Progress on Flexible Room-Temperature Gas Sensors Based on Metal Oxide Semiconductor

With the rapid development of the Internet of Things, there is a great demand for portable gas sensors. Metal oxide semiconductors (MOS) are one of the most traditional and well-studied gas sensing materials and have been widely used to prepare various commercial gas sensors. However, it is limited by high operating temperature. The current research works are directed towards fabricating high-performance flexible room-temperature (FRT) gas sensors, which are effective in simplifying the structure of MOS-based sensors, reducing power consumption, and expanding the application of portable devices. This article presents the recent research progress of MOS-based FRT gas sensors in terms of sensing mechanism, performance, flexibility characteristics, and applications. This review comprehensively summarizes and discusses five types of MOS-based FRT gas sensors, including pristine MOS, noble metal nanoparticles modified MOS, organic polymers modified MOS, carbon-based materials (carbon nanotubes and graphene derivatives) modified MOS, and two-dimensional transition metal dichalcogenides materials modified MOS. The effect of light-illuminated to improve gas sensing performance is further discussed. Furthermore, the applications and future perspectives of FRT gas sensors are also discussed.

A Dual Sensing Platform for Human Exhaled Breath Enabled by Fe-MIL-101-NH2 Metal-Organic Frameworks and its Derived Co/Ni/Fe Trimetallic Oxides

Limited by the insufficient active sites and the interference from breath humidity, designing reliable gas sensing materials with high activity and moisture resistance remains a challenge to analyze human exhaled breath for the translational application of medical diagnostics. Herein, the dual sensing and cooperative diagnosis is achieved by utilizing metal-organic frameworks (MOFs) and its derivative. The Fe-MIL-101-NH₂ serves as the quartz crystal microbalance humidity sensing layer, which exhibits high selectivity and rapid response time (16 s/15 s) to water vapor. Then, the Co²⁺ and Ni²⁺ cations are further co-doped into Fe-MIL-101-NH₂ host to obtain the derived Co/Ni/Fe trimetallic  oxides (CoNiFe-MOS-n). The chemiresistive CoNiFe-MOS-n sensor displays the high sensitivity (560) and good selectivity to acetone, together with a lower original resistance compared with Fe₂ O₃ and NiFe₂ O₄ . Moreover, as a proof-of-concept application, synergistic integration of Fe-MIL-101-NH₂ and derived CoNiFe-MOS-n is carried out. The Fe-MIL-101-NH₂ is applied as moisture sorbent materials, which realize a sensitivity compensation of CoNiFe-MOS-n sensors for the detection of acetone (biomarker gas of diabetes). The findings provide an insight for effective utilization of MOFs and the derived materials to achieve a trace gas detection in exhaled breath analysis.

Oxygen Vacancy Injection on (111) CeO2 Nanocrystal Facets for Efficient H2O2 Detection

Facet and defect engineering have achieved great success in improving the catalytic performance of CeO₂, but the inconsistent reports on the synergistic effect of facet and oxygen vacancy and the lack of investigation on the heavily doped oxygen vacancy keeps it an attractive subject. Inspired by this, CeO₂ nanocrystals with selectively exposed crystalline facets (octahedron, cube, sphere, rod) and abundant oxygen vacancies have been synthesized to investigate the synergistic effect of facet and heavily doped oxygen vacancy. The contrasting electrochemical behavior displayed by diverse reduced CeO₂ nanocrystals verifies that oxygen vacancy acts distinctly on different facets. The thermodynamically most stable CeO₂ octahedron enclosed by heavily doped (111) facets surprisingly exhibited the optimum non-enzymatic H₂O₂ sensing performance, with a high sensitivity (128.83 µA mM^-1 cm^-2), a broad linear range (20 µM~13.61 mM), and a low detection limit (1.63 µM). Meanwhile, the sensor presented satisfying selectivity, repeatability, stability, as well as its feasibility in medical disinfectants. Furthermore, the synergistic effect of facet and oxygen vacancy was clarified by the inclined distribution states of oxygen vacancy and the electronic transmission property. This work enlightens prospective research on the synergistic effect of alternative crystal surface engineering strategies.

Heteronanostructural metal oxide-based gas microsensors

The development of high-performance, portable and miniaturized gas sensors has aroused increasing interest in the fields of environmental monitoring, security, medical diagnosis, and agriculture. Among different detection tools, metal oxide semiconductor (MOS)-based chemiresistive gas sensors are the most popular choice in commercial applications and have the advantages of high stability, low cost, and high sensitivity. One of the most important ways to further enhance the sensor performance is to construct MOS-based nanoscale heterojunctions (heteronanostructural MOSs) from MOS nanomaterials. However, the sensing mechanism of heteronanostructural MOS-based sensors is different from that of single MOS-based gas sensors in that it is fairly complex. The performance of the sensors is influenced by various parameters, including the physical and chemical properties of the sensing materials (e.g., grain size, density of defects, and oxygen vacancies of materials), working temperatures, and device structures. This review introduces several concepts in the design of high-performance gas sensors by analyzing the sensing mechanism of heteronanostructural MOS-based sensors. In addition, the influence of the geometric device structure determined by the interconnection between the sensing materials and the working electrodes is discussed. To systematically investigate the sensing behavior of the sensor, the general sensing mechanism of three typical types of geometric device structures based on different heteronanostructural materials are introduced and discussed in this review. This review will provide guidelines for readers studying the sensing mechanism of gas sensors and designing high-performance gas sensors in the future.

Metal Oxide Semiconductor Nanostructure Gas Sensors with Different Morphologies

There is an increasing need for the development of low-cost and highly sensitive gas sensors for environmental, commercial, and industrial applications in various areas, such as hazardous gas monitoring, safety, and emission control in combustion processes. Considering this, resistive-based gas sensors using metal oxide semiconductors (MOSs) have gained special attention owing to their high sensing performance, high stability, and low cost of synthesis and fabrication. The relatively low final costs of these gas sensors allow their commercialization; consequently, they are widely used and available at low prices. This review focuses on the important MOSs with different morphologies, including quantum dots, nanowires, nanofibers, nanotubes, hierarchical nanostructures, and other structures for the fabrication of resistive gas sensors.

Humidity-Tolerant Chemiresistive Gas Sensors Based on Hydrophobic CeO2/SnO2 Heterostructure Films

The accelerated evolution of the Internet of Things has brought new challenges to the gas sensors, which are required to work persistently under harsh conditions, like high humidity. However, currently, it is quite challenging to solve the hindrance of the trade-off between gas-sensing performance and anti-humidity ability of the chemiresistive gas sensors. Herein, hydrophobic inorganic CeO₂/SnO₂ heterostructure films were prepared by depositing the CeO₂ layers with a thickness of a few nanometers onto the SnO₂ film via a magnetron sputtering method. The sensors based on the CeO₂/SnO₂ heterostructure films demonstrated excellent gas-sensing performance toward trimethylamine (TEA) with high response, wide detection range (0.04-500 ppm), low record detection limit (0.04 ppm), ideal reproducibility, and long-term stability, while concurrently possessing promising anti-humidity ability. A portable, wireless TEA-sensing system containing the CeO₂/SnO₂ sensor was constructed to realize the real-time monitoring of trace concentration of the volatiles released from a fish. This work provides a novel strategy to prepare advanced chemiresistive gas sensors for humidity-independent detection of harmful gases and vapors and will accelerate their commercialization process in the field of food safety and public health.

Superior acetone sensor based on hetero-interface of SnSe2/SnO2 quasi core shell nanoparticles for previewing diabetes

To improve gas sensing performance of SnO₂ sensor, a heterostructure constructed by SnO₂ and SnSe₂ is designed and synthesized via hydrothermal method and post thermal oxidation treatment. The obtained SnSe₂/SnO₂ composite nanoparticles demonstrate a special core-shell structure with SnO₂ nanograins distributed in the shell and mixed SnSe₂ and SnO₂ nanograins in the core. Owning to the promoted charge transfer effect invited by SnSe₂, the sensor based on SnSe₂/SnO₂ composite nanoparticles exhibit expressively enhanced acetone sensing performance compared to the pristine SnO₂ sensor. At the working temperature of 300 °C, the SnSe₂/SnO₂ composite sensor with optimized composition exhibits superior sensing property towards acetone, including high response (10.77-100 ppm), low theoretical limit of detection (0.354 ppm), high selectivity and good reproducibility. Moreover, the sensor shows a satisfactory sensing performance in trace acetone gas detection under high humidity condition (relative humidity: 70-90%), making it a promising candidate to constructing exhaled breath sensors for acetone detection.

Research Progress on Coating of Sensitive Materials for Micro-Hotplate Gas Sensor

Micro-hotplate gas sensors are widely used in air quality monitoring, identification of hazardous chemicals, human health monitoring, and other fields due to their advantages of small size, low power consumption, excellent consistency, and fast response speed. The micro-hotplate gas sensor comprises a micro-hotplate and a gas-sensitive material layer. The micro-hotplate is responsible for providing temperature conditions for the sensor to work. The gas-sensitive material layer is responsible for the redox reaction with the gas molecules to be measured, causing the resistance value to change. The gas-sensitive material film with high stability, fantastic adhesion, and amazing uniformity is prepared on the surface of the micro-hotplate to realize the reliable assembly of the gas-sensitive material and the micro-hotplate, which can improve the response speed, response value, and selectivity. This paper first introduces the classification and structural characteristics of micro-hotplates. Then the assembly process and characteristics of various gas-sensing materials and micro-hotplates are summarized. Finally, the assembly method of the gas-sensing material and the micro-hotplate prospects.

Metal–Organic-Framework-Derived Ball-Flower-like Porous Co3O4/Fe2O3 Heterostructure with Enhanced Visible-Light-Driven Photocatalytic Activity

A porous ball-flower-like Co₃O₄/Fe₂O₃ heterostructural photocatalyst was synthesized via a facile metal–organic-framework-templated method, and showed an excellent degradation performance in the model molecule rhodamine B under visible light irradiation. This enhanced photocatalytic activity can be attributed to abundant photo-generated holes and hydroxyl radicals, and the combined effects involving a porous structure, strong visible-light absorption, and improved interfacial charge separation. It is notable that the ecotoxicity of the treated reaction solution was also evaluated, confirming that an as-synthesized Co₃O₄/Fe₂O₃ catalyst could afford the sunlight-driven long-term recyclable degradation of dye-contaminated wastewater into non-toxic and colorless wastewater.

Nickel Oxide-Carbon Soot-Cellulose Acetate Nanocomposite for the Detection of Mesitylene Vapour: Investigating the Sensing Mechanism Using an LCR Meter Coupled to an FTIR Spectrometer

Nanocomposite sensors were prepared using carbon soot (CNPs), nickel oxide nanoparticles (NiO-NPs), and cellulose acetate (CA), which was used to detect and study the sensing mechanism of mesitylene vapour at room temperature. Synthesised materials were characterised using high-resolution transmission electron microscopy (HR-TEM), powder x-ray diffraction (PXRD), Raman spectroscopy, and nitrogen sorption at 77 K. Various sensors were prepared using individual nanomaterials (NiO-NPs, CNPs, and CA), binary combinations of the nanomaterials (CNPs-NiO, CNPs-CA, and NiO-CA), and ternary composites (NiO-CNPs-CA). Among all of the prepared and tested sensors, the ternary nanocomposites (NiO-CNPs-CA) were found to be the most sensitive for the detection of mesitylene, with acceptable response recovery times. Fourier-transform infrared (FTIR) spectroscopy coupled with an LCR meter revealed that the mesitylene decomposes into carbon dioxide.

Self-Adaptive Gas Sensor System Based on Operating Conditions Using Data Prediction

Through the improvement of nanomaterial technologies, a gas sensor was developed for detecting ppm or ppb levels of gas. Our SnO₂ nanosheet gas sensor can detect 50 ppb of acetone without the requirement of a novel metal catalyst by exposing the (101) facet containing the Sn²⁺ state. Despite the high performance, the fluctuation of the gas response value based on operating conditions, even at the same concentration, is a critical problem in gas sensors. Thus, the alarm criteria of the sensor are typically determined by a safety factor. However, this method is not suitable for application in ultrasensitive sensors that require distinguishing minute differences in extremely low concentrations for medical examination or odor analysis. Therefore, we suggest a self-adaptive system that is based on operating conditions in collaboration with the data prediction model. The sensor system is based on a predictive model obtained by the response surface methodology. When the system detects a change in conditions, the alarm criteria are changed appropriately through the calculated values from the predictive model. To prepare a database for an effective predictive model, the gas responses of the SnO₂ nanosheet sensor were measured with 20 treatments with 3 independent variables, namely, the temperature, flow rate, and concentration. Our prediction model achieved its best performance on training data with R² = 0.9299 and less than 5% error in the prediction of unseen data.

Plasmonic Oxygen Defects in MO3- x (M = W or Mo) Nanomaterials: Synthesis, Modifications, and Biomedical Applications

Nanomedicine is a promising technology with many advantages and provides exciting opportunities for cancer diagnosis and therapy. During recent years, the newly developed oxygen-deficiency transition metal oxides MO_{3- x} (M = W or Mo) have received significant attention due to the unique optical properties, such as strong localized surface plasmon resonance (LSPR) , tunable and broad near-IR absorption, high photothermal conversion efficiency, and large X-ray attenuation coefficient. This review presents an overview of recent advances in the development of MO_{3- x} nanomaterials for biomedical applications. First, the fundamentals of the LSPR effect are introduced. Then, the preparation and modification methods of MO_{3- x} nanomaterials are summarized. In addition, the biological effects of MO_{3- x} nanomaterials are highlighted and their applications in the biomedical field are outlined. This includes imaging modalities, cancer treatment, and antibacterial capability. Finally, the prospects and challenges of MO_{3- x} and MO_{3- x} -based nanomaterial for fundamental studies and clinical applications are also discussed.

Low-operating temperature and remarkably responsive methanol sensors using Pt-decorated hierarchical ZnO structure

Highly responsive methanol sensors working at low temperatures are developed using hierarchical ZnO nanorods decorated by Pt nanoparticles. The sensing materials are fabricated following a 3-step process: electrospinning of ZnO nanofibers, hydrothermal growth of hierarchical ZnO nanorods on the nanofibers and UV-assisted deposition of Pt nanoparticles. The morphology, structure and properties of the materials are examined by field-effect scanning electron microscopy, transmission electron microscope, x-ray diffraction, x-ray photoelectron spectroscopy, UV-Vis absorption spectroscopy, and electrical measurements. The methanol sensing performance is investigated at different working temperatures in the range of 110 °C-260 °C. It is observed that the surface modification of the ZnO hierarchical nanorods by Pt nanoparticles results in a remarkable enhancement of the sensing response toward methanol, which can reach approximately 19 500 times higher than that of the unmodified ZnO nanorods-based sensor. In addition, this modification enables lower working temperatures with an optimum range of 140 °C-200 °C. Based on the achieved results, a methanol sensing mechanism of the Pt/ZnO structure is proposed.

Nanowire-based sensor electronics for chemical and biological applications

Detection and recognition of chemical and biological species via sensor electronics are important not only for various sensing applications but also for fundamental scientific understanding. In the past two decades, sensor devices using one-dimensional (1D) nanowires have emerged as promising and powerful platforms for electrical detection of chemical species and biologically relevant molecules due to their superior sensing performance, long-term stability, and ultra-low power consumption. This paper presents a comprehensive overview of the recent progress and achievements in 1D nanowire synthesis, working principles of nanowire-based sensors, and the applications of nanowire-based sensor electronics in chemical and biological analytes detection and recognition. In addition, some critical issues that hinder the practical applications of 1D nanowire-based sensor electronics, including device reproducibility and selectivity, stability, and power consumption, will be highlighted. Finally, challenges, perspectives, and opportunities for developing advanced and innovative nanowire-based sensor electronics in chemical and biological applications are featured.

Oxygen Vacancy Defects and a Field Effect-Mediated ZnO/WO2.92 Heterojunction for Enhanced Corrosion Resistance

The heterojunction constructed by tungsten oxide and zinc oxide materials can improve the problem of easy deactivation of electrons, which is a new and effective strategy for realizing anticorrosion. Here, the ZnO/WO_2.92 heterojunction modified by oxygen vacancies (OVs) serving as the photoelectric conversion center was not consumed to provide continuous light-induced protection for steel, and the impedance value was increased by 185.35% compared to that of epoxy resin after 72 h of corrosion. The enhanced anticorrosion activity was due to OV modification leading to oxygen adsorption and electron capture, which inhibited the cathodic corrosion reaction and effectively hindered electron transport. Additionally, the localized surface plasmon resonance effect produced by OVs improved light utilization efficiency and increased electron density, which enabled numerous photoelectrons to gather on the surface of the iron substrate to reduce the corrosion rate of metals. Besides, the cascade effect of the ZnO/WO_2.92 heterojunction promoted the transfer of e^-/h⁺ to form an electric field that allowed the directional flow of electrons to inhibit the anode dissolution process. Thus, exploring the corrosion reaction involving OVs and heterojunction structures was of great significance to the development of nonsacrificial and efficient anticorrosion materials.

Ultrasensitive ppb-level trimethylamine gas sensor based on p-n heterojunction of Co3O4/WO3

Trace poisonous and harmful gases in the air have been harming and affecting people's health for a long time. At present, effective and accurate detection of ppb-level harmful gas is still a bottleneck to be overcome. Herein, we report a ppb-level triethylamine (TEA) gas sensor based on p-n heterojunction of Co₃O₄/WO₃, which is prepared with ZIF-67 as the precursor and provides Co₃O₄deposited tungsten oxide flower-like structure. Due to the introduction of Co₃O₄and the 3D flower-like structure of WO₃, the Co₃O₄/WO₃-2 gas sensor shows excellent gas sensing performance (1101 for 10 ppm at 240 °C), superb selectivity, good long-term stability and linear response for TEA concentration. Moreover, the experimental results indicate that the Co₃O₄/WO₃-2 gas sensor also possesses a good response to 50 ppb TEA, in fact, the theoretical limit of detection is 0.6 ppb. Co₃O₄not only improves the efficiency of electron separation/transport, but also accelerates the oxidation rate of TEA. This method of synthesizing p-n heterojunction with ZIF as the precursor provides a new idea and method for the preparation of low detection limit gas sensors.

Synthesis and Characterization of Thin Films Based on Azobenzene Derivative Anchored to CeO2 Nanoparticle for Photoswitching Applications

Azobenzene has attracted substantial attention as a photoswitchable molecule since its applications range from energy and data storage to biomedical applications. This work reports a new type of thin-film based on azobenzene derivative anchored to cerium oxide nanoparticles CeO₂ NPs for photoswitching applications. The trans-cis isomerization and reverse isomerization occur by UV-light exposure and thermal relaxation process, respectively. The photoisomerization and reverse isomerization kinetics for CeO₂ NPs-MR thin films are studied, investigated, and analyzed using UV-Vis absorbance spectra, FTIR spectroscopy, XRD, and scanning electron microscopy (SEM), in addition to differential scanning calorimetry (DSC) measurement to study the energy storage capacity. The results found that anchoring azobenzene to CeO₂ NPs is successful in multisource storage of solar energy applications.

Socio-economic demands and challenges for non-invasive disease diagnosis through a portable breathalyzer by the incorporation of 2D nanosheets and SMO nanocomposites

Breath analysis for non-invasive clinical diagnostics and treatment progression has penetrated the research community owing to the technological developments in novel sensing nanomaterials. The trace level selective detection of volatile organic compounds (VOCs) in breath facilitates the study of physiological disorder and real-time health monitoring. This review focuses on advancements in chemiresistive gas sensor technology for biomarker detection associated with different diseases. Emphasis is placed on selective biomarker detection by semiconducting metal oxide (SMO) nanostructures, 2-dimensional nanomaterials (2DMs) and nanocomposites through various optimization strategies and sensing mechanisms. Their synergistic properties for incorporation in a portable breathalyzer have been elucidated. Furthermore, the socio-economic demands of a breathalyzer in terms of recent establishment of startups globally and challenges of a breathalyzer are critically reviewed. This initiative is aimed at highlighting the challenges and scope for improvement to realize a high performance chemiresistive gas sensor for non-invasive disease diagnosis.

Breath analysis for non-invasive clinical diagnostics and treatment progression has penetrated the research community owing to the technological developments in novel sensing nanomaterials.

Ex-situ XPS analysis of yolk-shell Sb2O3/WO3 for ultra-fast acetone resistive sensor

The preparation of fast, highly responsive and reliable gas sensing devices for the detection of acetone gas is considered to be a key challenge for the development of accurate disease diagnosis systems through exhaled respiratory gases. In the paper, yolk shell Sb₂O₃/WO₃ is synthesized and its gas sensing performance was studied by static test system. Special, the maximum response value of 1:1 Sb₂O₃/WO₃ yolk-shell (WO₃-1 YSL) sensor to 100 ppm acetone can reach as high as 50.0 at 200 ℃. And it also exhibits excellent response/recover time (4 s/5 s), low detection limit (2 ppm) and superior selectivity towards acetone. More importantly, in mixed selective gas test, the sensor shows high selectivity towards acetone. And the mechanism is analyzed by ex-situ XPS. The excellent gas-sensing performance can be attributed to unique yolk-shell structure, which facilitates the rapid transport of charge carriers from the surface to the bulk and provides more active sites for gas adsorption and desorption; the heterojunction between of Sb₂O₃ and WO₃, which promotes oxygen pre-adsorption on the surface and increasing the interfacial potential; the increased oxygen vacancies which allowing more chemisorbed oxygen to form.

Metal–Oxide Nanowire Molecular Sensors and Their Promises

During the past two decades, one–dimensional (1D) metal–oxide nanowire (NW)-based molecular sensors have been witnessed as promising candidates to electrically detect volatile organic compounds (VOCs) due to their high surface to volume ratio, single crystallinity, and well-defined crystal orientations. Furthermore, these unique physical/chemical features allow the integrated sensor electronics to work with a long-term stability, ultra-low power consumption, and miniature device size, which promote the fast development of “trillion sensor electronics” for Internet of things (IoT) applications. This review gives a comprehensive overview of the recent studies and achievements in 1D metal–oxide nanowire synthesis, sensor device fabrication, sensing material functionalization, and sensing mechanisms. In addition, some critical issues that impede the practical application of the 1D metal–oxide nanowire-based sensor electronics, including selectivity, long-term stability, and low power consumption, will be highlighted. Finally, we give a prospective account of the remaining issues toward the laboratory-to-market transformation of the 1D nanostructure-based sensor electronics.

Modulating Ni/Ce Ratio in NiyCe100-yOx Electrocatalysts for Enhanced Water Oxidation

Oxygen evolution reaction (OER) is the key reaction for water splitting, which is used for hydrogen production. Oxygen vacancy engineering is an effective method to tune the OER performance, but the direct relationship between the concentration of oxygen vacancy and OER activity is not well understood. Herein, a series of Ni_yCe_100−yO_x with different concentration of oxygen vacancies were successfully synthesized. The larger concentration of oxygen vacancies in Ni₇₅Ce₂₅O_x and Ni₅₀Ce₅₀O_x result in their lower Tafel slopes, small mass-transfer resistance, and larger electrochemical surface areas of the catalysts, which account for the higher OER activities for these two catalysts. Moreover, with a fixed current density of 10 mA/cm², the potential remains stable at 1.57 V for more than 100 h, indicating the long-term stability of the Ni₇₅Ce₂₅O_x catalyst.

Recent advances in energy-saving chemiresistive gas sensors: A review

With the tremendous advances in technology, gas-sensing devices are being popularly used in many distinct areas, including indoor environments, industries, aviation, and detectors for various toxic domestic gases and vapors. Even though the most popular type of gas sensor, namely, resistive-based gas sensors, have many advantages over other types of gas sensors, their high working temperatures lead to high energy consumption, thereby limiting their practical applications, especially in mobile and portable devices. As possible ways to deal with the high-power consumption of resistance-based sensors, different strategies such as self-heating, MEMS technology, and room-temperature operation using especial morphologies, have been introduced in recent years. In this review, we discuss different types of energy-saving chemisresitive gas sensors including self-heated gas sensors, MEMS based gas sensors, room temperature operated flexible/wearable sensor and their application in the fields of environmental monitoring. At the end, the review will be concluded by providing a summary, challenges, recent trends, and future perspectives.

Ultrasensitive and low detection limit of acetone gas sensor based on ZnO/SnO2 thick films

In this study, we synthesized ZnO/SnO₂ hybrid sensing nanostructures by a sol-gel method. The structures, composition and morphologies of the synthesized products were thoroughly studied by X-ray diffraction (XRD), field-emission electron scanning microscopy (FESEM) and transmission electron microscopy (TEM). After the gas sensing test, we found that the sensing performance of the ZnO/SnO₂ composite is improved obviously compared with that of single components ZnO and SnO₂. The response to 0.5 ppm acetone reaches 3.36, almost twice that of pure ZnO and SnO₂. Meanwhile, the detection limit can be reduced to the ppb level. The enhanced acetone sensing performance was mainly attributed to the formation of n-n heterojunctions and the synergistic effect of ZnO and SnO₂.

Self-Assembled SnO2/SnSe2 Heterostructures: A Suitable Platform for Ultrasensitive NO2 and H2 Sensing

By means of experiments and theory, the gas-sensing properties of tin diselenide (SnSe₂) were elucidated. We discover that, while the stoichiometric single crystal is chemically inert even in air, the nonstoichiometric sample assumes a subnanometric SnO₂ surface oxide layer once exposed to ambient atmosphere. The presence of Se vacancies induces the formation of a metastable SeO₂-like layer, which is finally transformed into a SnO₂ skin. Remarkably, the self-assembled SnO₂/SnSe_2-x heterostructure is particularly efficient in gas sensing, whereas the stoichiometric SnSe₂ sample does not show sensing properties. Congruently with the theoretical model, direct sensing tests carried out on SnO₂/SnSe_2-x at an operational temperature of 150 °C provided sensitivities of (1.06 ± 0.03) and (0.43 ± 0.02) [ppm]^-1 for NO₂ and H₂, respectively, in dry air. The corresponding calculated limits of detection are (0.36 ± 0.01) and (3.6 ± 0.1) ppm for NO₂ and H₂, respectively. No detectable changes in gas-sensing performances are observed in a time period extended above six months. Our results pave the way for a novel generation of ambient-stable gas sensor based on self-assembled heterostructures formed taking advantage on the natural interaction of substoichiometric van der Waals semiconductors with air.