Facile Synthesis of Pt-Functionalized Meso/Macroporous SnO2 Hollow Spheres through in Situ Templating with SiO2 for H2S Sensors

A Gas Sensor with Enhanced Sensing Properties towards Butyl Acetate: Vascular Bundle Structure Zn2 SnO4 Derived from Maize Straw

The development of butyl acetate sensors with high sensitivity and selectivity has been highly desirable for its harmful effects on human health. In this work, we developed a high-performance butyl acetate sensor based on vascular bundle structure Zn2 SnO4 nanomaterial derived from maize straw. The vascular bundle structure Zn2 SnO4 with higher specific surface area obtained by calcination to remove the maize straw template, plays the dual role of accelerating the diffusion of gas molecules and providing more active sites. Our research showed that the sensor had a response of 18 to 100 ppm butyl acetate at a working temperature of 250 °C, with a fast response recovery rate (18 s/25 s), which showed significant improvement compared to the Zn2 SnO4 sensor prepared without templates. The improved performance can be attributed to the cross-linked nanoparticle chains and gas collision mechanism of the sensor. These findings highlight the potential of our sensor for the detection of butyl acetate gas.

Engineering a Heterophase Interface by Tailoring the Pt Coverage Density on an Amorphous Ru Surface for Ultrasensitive H2S Detection

Amorphous/crystalline heterophase engineering is emerging as an attractive strategy to adjust the properties and functions of nanomaterials. Here, we reveal a heterophase interface role by precisely tailoring the crystalline Pt coverage density on an amorphous Ru surface (cPt/aRu) for ultrasensitive H2S detection. We found that when the atomic ratio of Pt/Ru increased from 10 to 50%, the loading modes of Pt changed from island coverage (1cPt/aRu) to cross-linkable coverage (3cPt/aRu) and further to dense coverage (5cPt/aRu). The differences in coverage models further regulate the chemical adsorption of H2S on Pt and the electronic transformation process on Ru, which can be proved by ex situ X-ray photoelectron spectroscopy experiments. Notably, a special cross-linkable coverage 3cPt/aRu on ZnO shows the best gas-sensitive performance, in which the operating temperature reduces from 240 to 160 °C compared with pristine ZnO and the selectivity coefficient for H2S gas improves from ∼1.2 to ∼4.6. This is mainly benefit from the maximized exposure of the amorphous/crystalline heterophase interface. Our work thus provides a new platform for future applications of amorphous/crystalline heterogeneous nanostructures in gas sensors and catalysis.

Nanotheranostics: application of nanosensors in diabetes management

Objectives

The objective of the present study is to discuss the use of nanomaterials like nanosensors for diagnosing Diabetes and highlight their applications in the treatment of Diabetes.

Methods

Diabetes mellitus (D.M.) is a group of metabolic diseases characterized by hyperglycemia. Orally administered antidiabetic drugs like glibenclamide, glipalamide, and metformin can partially lower blood sugar levels, but long-term use causes kidney and liver damage. Recent breakthroughs in nanotheranostics have emerged as a powerful tool for diabetes treatment and diagnosis.

Results

Nanotheranostics is a rapidly developing area that can revolutionize diabetes diagnosis and treatment by combining therapy and imaging in a single probe, allowing for pancreas-specific drug and insulin delivery. Nanotheranostic in Diabetes research has facilitated the development of improved glucose monitoring and insulin administration modalities, which promise to improve the quality of life for people with Diabetes drastically. Further, nanomaterials like nanocarriers and unique functional nanomaterials used as nano theranostics tools for treating Diabetes will also be highlighted.

Conclusion

The nanosensors discussed in this review article will encourage researchers to develop innovative nanomaterials with novel functionalities and properties for diabetes detection and treatment.

An Au/SnO-SnO2 nanosheet based composite used for rapid detection of hydrogen sulphide

In this work, a new type of H₂S sensor was fabricated by means of drop-coating of an Au/SnO-SnO₂ nanosheet material, which was prepared by a one-pot hydrothermal reaction, onto a gold electrode in an alumina ceramic tube with the formation of a thin nanocomposite film. The microstructure and morphology of the nanosheet composites were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A gas-sensitivity study presented good H₂S-sensing performance of such Au/SnO-SnO₂ nanosheet composites. At an optimal operating temperature of 240 °C and ambient temperature of 25 °C, the resulting sensor showed a good linear response to H₂S in a range of 1.0 to 100 ppm with a low detection limit of 0.7 ppm, and a very fast response-recovery time of 22 s for response and 63 s for recovery, respectively. The sensor was also unaffected by ambient humidity and had good reproducibility and selectivity. When being applied to the monitoring of H₂S in an atmospheric environment in a pig farm, the response signal to H₂S was only attenuated by 4.69% within 90 days, proving that the sensor had a long and stable service lifetime for continuous running and showing its important practical application prospects.

Two-dimensional black phosphorus/tin oxide heterojunctions for high-performance chemiresistive H2S sensing

Hydrogen sulfide (H₂S) emission from industrial fields and bacteria decomposing of sulfur-containing organic matter poses a significant impact on human health and atmospheric environment, thus necessitating the development of a H₂S sensor with high sensitivity and exclusive selectivity especially at a very low dose. Chemiresistive sensors based on traditional metal oxides were readily limited by the elevated operating temperature and severe cross-sensitivity. To overcome these obstacles, we prepared two dimensional (2D) tin oxide (SnO₂) nanosheets decorated with thin black phosphorus (BP) as the sensing layer of MEMS H₂S sensors. Compared with pure SnO₂ counterparts, BP-SnO₂ sensors demonstrated lower optimal working temperature (130 °C vs. 160 °C), higher response (8.1 vs. 4.6) and faster response/recovery speeds (39.8 s/47.4 s vs. 79 s/140 s) toward 5 ppm H₂S as well as larger sensitivity (1.3/ppm vs. 0.342/ppm). In addition, favorable repeatability, long-term stability, selectivity and humidity tolerance were exhibited. Thin BP not only served as an excellent conductivity platform within the composites, but enriched the adsorption sites by constructing p-n heterojunctions and introducing more oxygen vacancy, thus separately accelerating and strengthening the gas-solid interaction. This study showcased the application superiorities of BP nanosheets in the field of gas sensing, simultaneously providing a new strategy for trace H₂S sensing via the 2D heterojunctions.

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

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

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.

Porous Pd-Sn Alloy Nanotube-Based Chemiresistor for Highly Stable and Sensitive H2 Detection

While H₂ is indispensable as a green fuel source, it is highly flammable and explosive. Because it is difficult to detect due to its lack of odor and color, a solution for proper monitoring of H₂ leakage is essential to ensure safe handling. To this end, we have successfully fabricated hollow Pd-Sn alloy nanotubes (NTs) with a Brunauer-Emmett-Teller surface area of 223.0 m²/g through electrospinning and a subsequent etching method, which is the first demonstration of synthesizing Pd-based hollow alloy nanofibers with ultrafine grain sizes. We found that the alloying of Pd with Sn could effectively prevent degradation of the sensing performance upon the α-β phase transition during hydrogen detection. Besides, the highly porous structure with smaller nanograins offered more exposed active sites and higher gas accessibility to bulk materials. The resultant Pd-Sn NTs exhibited excellent sensitivity toward H₂ (0.00005-3%). Notably, the limit of detection of 0.0001% is an outstanding achievement on H₂ sensing among state-of-the-art H₂ sensors. Moreover, when exposed to a high concentration of H₂ (3%), Pd-Sn NTs showed excellent cycling stability with a standard deviation of 0.07% and a sensitivity of 9.27%. These obtained sensing results indicate that Pd-Sn NTs can be used as a highly sensitive and stable H₂ gas sensor at room temperature (25 °C).

Pt-Anchored CuCrO2 for Low-Temperature-Operating High-Performance H2S Chemiresistors

Recent advances in heterogeneous catalysts indicate that single atoms (SAs), anchored/stabilized on metal oxide nanostructures, exhibit not only high catalyst atom efficiency but also intriguing reactivity and selectivity. Herein, isolated Pt SA-anchored CuCrO₂ (CCO) has been designed by a glycine-nitrate solution combustion synthesis (SCS) route. The density of isolated Pt SAs achieves the highest value of ∼100 μm^-2 for the 1.39 wt % Pt-anchored CCO sample, which results in the drastically boosted H₂S response characteristics, including a high response of 1250 (35 times higher than that of pure CCO) at 10 ppm H₂S and a low operating temperature of 100 °C. Except for CH₄S, the responses of a 1.39 wt % Pt-anchored CCO chemiresistor to diverse vapors with concentrations of 50-100 ppm are less than 2, exhibiting excellent selectivity. Various ex situ characterizations indicate that the spillover catalytic effect of Pt SA sites, other than the conventional sulfuration-desulfuration mechanism, plays a dominant role in the outstanding H₂S response characteristics.

Multishell SnO2 Hollow Microspheres Loaded with Bimetal PdPt Nanoparticles for Ultrasensitive and Rapid Formaldehyde MEMS Sensors

Low-cost and real-time formaldehyde (HCHO) monitoring is of great importance due to its volatility, extreme toxicity, and ready accessibility. In this work, a low-cost and integrated microelectromechanical system (MEMS) HCHO sensor is developed based on SnO₂ multishell hollow microspheres loaded with a bimetallic PdPt (PdPt/SnO₂-M) sensitizer. The MEMS sensor exhibits a high sensitivity to HCHO ((R_a/R_g - 1) % = 83.7 @ 1 ppm), ultralow detection limit of 50 ppb, and ultrashort response/recovery time (5.0/7.0 s @ 1 ppm). These excellent HCHO sensing properties are attributed to its unique multishell hollow structure with a large and accessible surface, abundant interfaces, suitable mesoporous structure, and synergistic catalytic effects of bimetal PdPt. The well-defined multishell hollow structure also shows fascinating capacities as good hosts for noble metal loading. Therefore, PdPt bimetallic nanoparticles can be employed to construct a synergistic sensitizer with a high content and good dispersity on this multishell hollow structure, further exhibiting a reduced working temperature and ultrasensitive detection of HCHO. This PdPt/SnO₂-M-based MEMS sensor presents a unique and highly sensitive means to detect HCHO, establishing its great promise for potential application in environmental monitoring.

Highly Selective Ethyl Mercaptan Sensing Using a MoSe2/SnO2 Composite at Room Temperature

Volatile organic sulfur compounds (VOSCs) serve not only as biomarkers for dental diseases such as halitosis but also as a tracer for monitoring air quality. Room-temperature selective detection and superior sensitivity against VOSCs at a sub-ppm level has remained a challenging task. Here, we propose a heterostructure-based design using a MoSe2/SnO2 composite for achieving sensitive and selective detection of ethyl mercaptan at room temperature. The composite was synthesized via a facile two-step method. A composite-based device has shown detection down to 1 ppm of ethyl mercaptan over a wider range of relative humidity (40-90%). Notably, the composite has shown adsorption selectivity toward ethyl mercaptan compared to hydrogen sulfide and other reducing or oxidizing analytes. Moreover, a density functional theory (DFT) study has been performed to understand the adsorption selectivity, charge transfer, and modification in the electronic properties after molecule adsorption on the host surface. Simulations predicted the lowest negative adsorption energy for ethyl mercaptan, implying the chemisorption (-142.029 kJ mol-1) process of adsorption. The device thus-obtained has also shown a stable response even at an extreme relative humidity level of 90%. The obtained results and superior signal-to-noise ratio indicate that a MoSe2/SnO2-based sensor may be a promising candidate for highly selective and sensitive detection of ethyl mercaptan even below 1 ppm.

Mesoporous Ti0.5Cr0.5N for trace H2S detection with excellent long-term stability

Efficient, accurate and reliable detection and monitoring of H₂S is of significance in a wide range of areas: industrial production, medical diagnosis, environmental monitoring, and health screening. However the rapid corrosion of commercial platinum-on-carbon (Pt/C) sensing electrodes in the presence of H₂S presents a fundamental challenge for fuel cell gas sensors. Herein we report a solution to the issue through the design of a sensing electrode, which is based on Pt supported on mesoporous titanium chromium nitrides (Pt/Ti_0.5Cr_0.5N). Its desirable characteristics are due to its high electrochemical stability and strong metal-support interactions. The Pt/Ti_0.5Cr_0.5N-based sensors exhibit a much smaller attenuation (1.3%) in response to H₂S than Pt/C-sensor (40%), after 2 months sensing test. Furthermore, the Pt/Ti_0.5Cr_0.5N-based sensors exhibit negligible cross response to other interfering gases compared with hydrogen sulfide. Results of density functional theory calculation also verify the excellent long-term stability and selectivity of the gas sensor. Our work hence points to a new sensing electrode system that offers a combination of high performance and stability for fuel-cell gas sensors.

The growth behavior of brain-like SnO2 microspheres under a solvothermal reaction with tetrahydrofuran as a solvent and their gas sensitivity

In this paper, the growth behavior of brain-like SnO2 microspheres synthesized by a tetrahydrofuran (THF) solvothermal method was studied. Unlike water or ethanol as the solvent, THF is a medium polar and aprotic solvent. Compared with other common polar solvents, the THF has no strong irregular effects on the growth process of SnO2. In addition, the viscosity of THF also helps the SnO2 to form a regular microstructure. The growth behavior of the brain-like SnO2 microspheres is controlled by changing the synthesis temperature of the reaction. The SEM and TEM results reveal that the SnO2 forms particles first (125 °C/3 h), and then these nanoparticles connect to each other forming nanowires and microspheres (diameter ≈ 1-2 μm) at 135 °C for 3 h; finally the microspheres further aggregate to form double or multi-sphere structures at 180 °C for 3 h. In this paper, the brain-like SnO2 microspheres obtained at 125 °C for 3 h were selected as sensitive materials to test their gas sensing performance at different operating temperature (50 °C and 350 °C). The H2S was tested at 50 °C which is the lowest operating temperature for the sensor. The combustible gas (H2/CH4/CO) was measured at 350 °C which is the highest temperature for the sensor. They all have extremely high sensitivity, but only H2S has excellent selectivity.

Surface Activity-Tuned Metal Oxide Chemiresistor: Toward Direct and Quantitative Halitosis Diagnosis

Continuous monitoring of hydrogen sulfide (H₂S) in human breath for early stage diagnosis of halitosis is of great significance for prevention of dental diseases. However, fabrication of a highly selective and sensitive H₂S gas sensor material still remains a challenge, and direct analysis of real breath samples has not been properly attempted, to the best of our knowledge. To address the issue, herein, we introduce facile cofunctionalization of WO₃ nanofibers with alkaline metal (Na) and noble metal (Pt) catalysts via the simple addition of sodium chloride (NaCl) and Pt nanoparticles (NPs), followed by electrospinning process. The Na-doping and Pt NPs decoration in WO₃ grains induces the partial evolution of the Na₂W₄O₁₃ phase, causing the buildup of Pt/Na₂W₄O₁₃/WO₃ multi-interface heterojunctions that selectively interacts with sulfur-containing species. As a result, we achieved the highest-ranked sensing performances, that is, response (R_air/R_gas) = 780 @ 1 ppm and selectivity (R_H2S/R_EtOH) = 277 against 1 ppm ethanol, among the chemiresistor-based H₂S sensors, owing to the synergistic chemical and electronic sensitization effects of the Pt NP/Na compound cocatalysts. The as-prepared sensing layer was proven to be practically effective for direct, and quantitative halitosis analysis based on the correlation (accuracy = 86.3%) between the H₂S concentration measured using the direct breath signals obtained by our test device (80 cases) and gas chromatography. This study offers possibilities for direct, highly reliable and rapid detection of H₂S in real human breath without the need of any collection or filtering equipment.

MOF-Derived Porous Hollow Co3O4@ZnO Cages for High-Performance MEMS Trimethylamine Sensors

Trimethylamine (TMA) sensors based on metal oxide semiconductors (MOS) have drawn great attention for real-time seafood quality evaluation. However, poor selectivity and baseline drift limit the practical applications of MOS TMA sensors. Engineering core@shell heterojunction structures with accumulation and depletion layers formed at the interface is regarded as an appealing way for enhanced gas sensing performances. Herein, we design porous hollow Co₃O₄@ZnO cages via a facile ZIF-67@ZIF-8-derived approach for TMA sensors. These sensors demonstrate great TMA resistive sensing performance (linear response at moderate TMA concentrations (<33 ppm)), and a high sensitivity of ∼41 is observed when exposed to 33 ppm TMA, with a response/recovery time of only 3/2 s. This superior performance benefits from the Co₃O₄@ZnO porous hollow structure with maximum heterojunctions and high surface area. Furthermore, great capacitive TMA sensing with linear sensitivity over the full testing concentration range (0.33-66 ppm) and better baseline stability were investigated. A possible capacitive sensing mechanism of TMA polarization was proposed. For practical usage, a portable sensing prototype based on the Co₃O₄@ZnO sensor was fabricated, and its satisfactory sensing behavior further confirms the potential for real-time TMA detection.

High-Sensitive MEMS Hydrogen Sulfide Sensor made from PdRh Bimetal Hollow Nanoframe Decorated Metal Oxides and Sensitization Mechanism Study

Here we report the fabrication of a high performance metal oxide semiconductor (MOS) sensor for the detection of hydrogen sulfide (H₂S) using PdRh bimetal hollow nanocube (HC) with Rh-rich hollow frame and Pd-rich core frame as sensitizing materials. PdRh bimetal HC with the edge-length about 10 nm was prepared by chemical etching PdRh bimetal solid nanocube (SC) in HNO₃ aqueous solution. The results of gas-sensing tests indicate that the response value order of the MEMS gas sensors based on MOSs (including ZnO, MoO₃ and SnO₂) is as follows: R_PdRh HC/MOS > R_PdRh SC/MOS > R_MOS. First, in the system of ZnO, gas sensor modified by PdRh (PdRh SC/ZnO and PdRh HC/ZnO) possess enhanced H₂S sensing performance with a better response and excellent low-concentration detection capability (down to 15 ppb) comparing to pure ZnO. The improved H₂S sensing performance could be attributed to the good conductivity of Rh-rich frame, the high catalytic activity of PdRh bimetal and formation of Schottky barrier-type junctions and defect. Second, PdRh HC/ZnO sensor shows better response (185-1 ppm of H₂S) compared to PdRh SC/ZnO sensor (108-1 ppm of H₂S), which is due to the higher specific surface area of PdRh HC/ZnO and good gas diffusion of the hollow structure. This work indicate that the sensitization characteristics of PdRh bimetal HC will provide new paradigms for the future development of the high performance sensor.

Novel Two-Dimensional WO3/Bi2W2O9 Nanocomposites for Rapid H2S Detection at Low Temperatures

Compared with single-component metal oxides, multicomponent metal oxides show good gas sensing performance in the field of gas sensing, but they still need to be further improved in terms of rapid response. In this paper, a two-dimensional flaky WO₃/Bi₂W₂O₉ composite material with a thickness of about 32.3 nm was synthesized by a simple solvothermal method. The composite has good sensing performance and selectivity toward H₂S. When the operating temperature is as low as 92 °C, the response to 100 ppm H₂S reaches 84.18, and the response time is 2 s, which is extremely fast due to the open system of the two-dimensional nanosheet. A combination of gas chromatography-mass spectrometry (GC-MS) and X-ray photoelectron spectroscopy (XPS) is used to analyze the changes of H₂S and the surface chemistry of WO₃/Bi₂W₂O₉ composite materials; the sensing mechanism of H₂S was studied by a Kelvin probe and UV diffuse reflection. Compared with the pure phase WO₃ and Bi₂W₂O₉, good gas sensing properties of the WO₃/Bi₂W₂O₉ composite may be due to its unique heterostructure. This is the first application of WO₃/Bi₂W₂O₉ in the field of gas sensing and is of great significance for the rapid detection of H₂S at low temperatures for multicomponent metal oxides.

Single-Atom Pt Stabilized on One-Dimensional Nanostructure Support via Carbon Nitride/SnO2 Heterojunction Trapping

Catalysis with single-atom catalysts (SACs) exhibits outstanding reactivity and selectivity. However, fabrication of supports for the single atoms with structural versatility remains a challenge to be overcome, for further steps toward catalytic activity augmentation. Here, we demonstrate an effective synthetic approach for a Pt SAC stabilized on a controllable one-dimensional (1D) metal oxide nano-heterostructure support, by trapping the single atoms at heterojunctions of a carbon nitride/SnO₂ heterostructure. With the ultrahigh specific surface area (54.29 m² g^-1) of the nanostructure, we obtained maximized catalytic active sites, as well as further catalytic enhancement achieved with the heterojunction between carbon nitride and SnO₂. X-ray absorption fine structure analysis and HAADF-STEM analysis reveal a homogeneous atomic dispersion of Pt species between carbon nitride and SnO₂ nanograins. This Pt SAC system with the 1D nano-heterostructure support exhibits high sensitivity and selectivity toward detection of formaldehyde gas among state-of-the-art gas sensors. Further ex situ TEM analysis confirms excellent thermal stability and sinter resistance of the heterojunction-immobilized Pt single atoms.

Low-Temperature Carbon Dioxide Gas Sensor Based on Yolk-Shell Ceria Nanospheres

Monitoring carbon dioxide (CO₂) levels is extremely important in a wide range of applications. Although metal oxide-based chemoresistive sensors have emerged as a promising approach for CO₂ detection, the development of efficient CO₂ sensors at low temperature remains a challenge. Herein, we report a low-temperature hollow nanostructured CeO₂-based sensor for CO₂ detection. We monitor the changes in the electrical resistance after CO₂ pulses in a relative humidity of 70% and show the high performance of the sensor at 100 °C. The yolk-shell nanospheres have not only 2 times higher sensitivity but also significantly increased stability and reversibility, faster response times, and greater CO₂ adsorption capacity than commercial ceria nanoparticles. The improvements in the CO₂ sensing performance are attributed to hollow and porous structure of the yolk-shell nanoparticles, allowing for enhanced gas diffusion and high specific surface area. We present an easy strategy to enhance the electrical and sensing properties of metal oxides at a low operating temperature that is desirable for practical applications of CO₂ sensors.

Hierarchical highly ordered SnO2 nanobowl branched ZnO nanowires for ultrasensitive and selective hydrogen sulfide gas sensing

Highly sensitive and selective hydrogen sulfide (H₂S) sensors based on hierarchical highly ordered SnO₂ nanobowl branched ZnO nanowires (NWs) were synthesized via a sequential process combining hard template processing, atomic-layer deposition, and hydrothermal processing. The hierarchical sensing materials were prepared in situ on microelectromechanical systems, which are expected to achieve high-performance gas sensors with superior sensitivity, long-term stability and repeatability, as well as low power consumption. Specifically, the hierarchical nanobowl SnO₂@ZnO NW sensor displayed a high sensitivity of 6.24, a fast response and recovery speed (i.e., 14 s and 39 s, respectively), and an excellent selectivity when detecting 1 ppm H₂S at 250 °C, whose rate of resistance change (i.e., 5.24) is 2.6 times higher than that of the pristine SnO₂ nanobowl sensor. The improved sensing performance could be attributed to the increased specific surface area, the formation of heterojunctions and homojunctions, as well as the additional reaction between ZnO and H₂S, which were confirmed by electrochemical characterization and band alignment analysis. Moreover, the well-structured hierarchical sensors maintained stable performance after a month, suggesting excellent stability and repeatability. In summary, such well-designed hierarchical highly ordered nanobowl SnO₂@ZnO NW gas sensors demonstrate favorable potential for enhanced sensitive and selective H₂S detection with long-term stability and repeatability.

Controlled Synthesis of Pt Doped SnO2 Mesoporous Hollow Nanospheres for Highly Selective and Rapidly Detection of 3-Hydroxy-2-Butanone Biomarker

Listeria monocytogenes (L. monocytogenes) has been recognized as one of the extremely hazardous and potentially life-threatening food-borne pathogens, its real-time monitoring is of great importance to human health. Herein, a simple and effective method based on platinum sensitized tin dioxide semiconductor gas sensors has been proposed for selective and rapid detection of L. monocytogenes. Pt doped SnO2 nanospheres with particular mesoporous hollow structure have been synthesized successfully through a robust and template-free approach and used for the detection of 3-hydroxy-2-butanone biomarker of L. monocytogenes. The steady crystal structure, unique micromorphology, good monodispersit, and large specific surface area of the obtained materials have been confirmed by X-ray diffraction (XRD), Raman spectroscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), Brunauer-Emmett-Teller (BET), and Photoluminescence spectra (PL). Pt doped SnO2 mesoporous hollow nanosphere sensors reach the maximum response of 3-hydroxy-2-butanone at 250°C. Remarkably, sensors based on SnO2 mesoporous hollow nanospheres with 0.16 wt% Pt dopant exhibit excellent sensitivity (Rair/Rgas = 48.69) and short response/recovery time (11/20 s, respectively) to 10 ppm 3-hydroxy-2-butanone at the optimum working temperature. Moreover, 0.16 wt% Pt doped SnO2 gas sensors also present particularly low limit of detection (LOD = 0.5 ppm), superb long-term stability and prominent selectivity to 3-hydroxy-2-butanone. Such a gas sensor with high sensing performance foresees its tremendous application prospects for accurate and efficient detection of foodborne pathogens for the food security and public health.

In Situ Growth of NiO@SnO2 Hierarchical Nanostructures for High Performance H2S Sensing

Heterostructured metal oxides with large specific surface area are crucial for constructing gas sensors with high performance. However, using slurry-coating and screen-printing methods to fabricate gas sensors cannot result in high uniformity and reproducibility of the sensors. Here, NiO nanowalls decorated by SnO₂ nanoneedles (NiO@SnO₂) were in situ grown on ceramic microchips via a chemical bath deposition method to detect H₂S instead of print-coating and slurry-coating methods. The morphologies and compositions of the NiO@SnO₂ hierarchical nanostructures (HNSs) were well tuned by varying the growth time of the NiO@SnO₂ HNSs to optimize the sensing performance. The response of the NiO@SnO₂ HNSs (2 h) to 1 ppm of H₂S was over 23-fold higher than that of the pure NiO nanowalls and 17-fold higher than that of the pure SnO₂ nanosheets. This dramatic enhancement is attributed to the large surface area of the NiO@SnO₂ HNSs and the p-n heterojunction at the heterointerface of SnO₂ and NiO. The variation in the depletion layers (W_SnO2 and W_NiO) at the heterointerface of SnO₂ and NiO greatly depends on the properties of the target gases (e.g., electron-withdrawing property (NO₂) or electron-donating property (H₂S)).

PdPt Bimetal-Functionalized SnO2 Nanosheets: Controllable Synthesis and its Dual Selectivity for Detection of Carbon Monoxide and Methane

Bimetallic nanoparticles (NPs) usually exhibit some novel properties due to the synergistic effects of the two distinct metals, which is expected to play an important role in the field of gas sensing. PdPt bimetal NPs with Pd-rich shell and Pt-rich core were successfully synthesized and used to modify SnO₂ nanosheets. The 1P-PdPt/SnO₂-A sensor obtained by self-assemblies of PdPt NPs exhibited temperature-dependent dual selectivity to CO at 100 °C and CH₄ at 320 °C. Furthermore, the sensor possessed good long term stability and antihumidity interference. The activation energy of adsorption for CO and CH₄ were estimated by the temperature-dependent response process modeled using Langmuir adsorption kinetics, which proved that the lower activation energy of adsorption corresponded to better sensing performance. The gas-sensing mechanism based on the diffusion depth of the tested gas in the sensing layer was discussed. The dramatically improved sensing performance could be ascribed to the high catalytic activity of PdPt bimetal, the electron sensitization of PdO, and Schottky barrier-type junctions at the interface between SnO₂ and PdPt NPs. Our present results demonstrate that bimetal NPs with special structure and components can significantly improve the gas-sensing performance of metal oxide semiconductor and the obtained sensor has great potential in monitoring coal mine gas.

Gas sensors using ordered macroporous oxide nanostructures

Detection and monitoring of harmful and toxic gases have gained increased interest in relation to worldwide environmental issues. Semiconducting metal oxide gas sensors have been considered promising for the facile remote detection of gases and vapors over the past decades. However, their sensing performance is still a challenge to meet the demands for practical applications where excellent sensitivity, selectivity, stability, and response/recovery rate are imperative. Therefore, sensing materials with novel architectures and fabrication processes have been pursued with a flurry of research activity. In particular, the preparation of ordered macroporous metal oxide nanostructures is regarded as an intriguing candidate wherein ordered aperture sizes in the range from 50 nm to 1.5 μm can increase the chemical diffusion rate and considerably strengthen the performance stability and repeatability. This review highlights the recent advances in the fabrication of ordered macroporous nanostructures with different dimensions and compositions, discusses the sensing behavior evolution governed by structural layouts, hierarchy, doping, and heterojunctions, as well as considering their general principles and future prospects. This would provide a clear scale for others to tune the sensing performance of porous materials in terms of specific components and structural designs.

Pd-Catalyzed Reaction-Producing Intermediate S on a Pd/In2O3 Surface: A Key To Achieve the Enhanced CS2-Sensing Performances

Although chemiresistive gas sensors, based on metal-oxide semiconductors, have exhibited particular promise for the monitoring of air pollution, they are often limited because of poor selectivity. In that case, to overcome this issue, according to the essence of the gas-sensing process, the method of reforming the surface reaction path on the surface of the sensing materials was used. Here, we report that Pd nanoparticles supported over the In₂O₃ composites, featured with a yolk-shell structure, enable the trace detection of carbon disulfide (CS₂) gas molecules, which are immensely dangerous to humans and animals. Moreover, the prominent enhancement of the gas response and the ultraselective CS₂-sensing characteristic were acquired in comparison with pristine In₂O₃ sensors. Significantly, density functional theory calculations revealed that the Pd supported on In₂O₃ greatly facilitates the adsorption capacity to CS₂, and the intermediate S, produced by Pd-catalyzed desulfurization reaction, on the Pd/In₂O₃ surface during the sensing process is a key to achieving a high CS₂ gas response as well as ultraselectivity, which is well in agreement with the X-ray photoelectron spectroscopy analysis results. On the basis of these results, a new sensing mechanism model for the CS₂-sensing process was put forward.

Facile Synthesis of Hierarchical Tin Oxide Nanoflowers with Ultra-High Methanol Gas Sensing at Low Working Temperature

In this work, the hierarchical tin oxide nanoflowers have been successfully synthesized via a simple hydrothermal method followed by calcination. The as-obtained samples were investigated as a kind of gas sensing material candidate for methanol. A series of examinations has been performed to explore the structure, morphology, element composition, and gas sensing performance of as-synthesized product. The hierarchical tin oxide nanoflowers exhibit sensitivity to 100 ppm methanol and the response is 58, which is ascribed to the hierarchical structure. The response and recovery time are 4 s and 8 s, respectively. Moreover, the as-prepared sensor has a low working temperature of 200 °C which is lower than that for other gas sensors of such type has been reported elsewhere. The excellent sensitivity of the sensor is caused by its complex phase mixture of SnO, SnO2, Sn2O3, and Sn6O4 revealed by XRD analysis. The proposed hierarchical tin oxide nanoflowers gas sensing material is promising for development of methanol gas sensor. The as-obtained hierarchical tin oxide nanoflower (HTONF) gas sensor shows excellent gas-sensing performance at low working temperature (200 °C) and high annealing temperature (400 °C).

Ultra-efficient room-temperature H2S gas sensor based on NiCo2O4/r-GO nanocomposites

NiCo₂O₄/r-GO nanocomposites were synthesized successfully; the sensor based on these nanocomposites exhibited a fast response and high selectivity towards H₂S at room temperature.

An effective H2S sensor based on SnO2 nanowires decorated with NiO nanoparticles by electron beam evaporation

The highly toxic hydrogen sulphide (H₂S) present in air can cause negative effects on human health. Thus, monitoring of this gas is vital in gas leak alarms and security. Efforts have been devoted to the fabrication and enhancement of the H₂S-sensing performance of gas sensors. Herein, we used electron beam evaporation to decorate nickel oxide (NiO) nanoparticles on the surface of tin oxide (SnO₂) nanowires to enhance their H₂S gas-sensing performance. The synthesised NiO–SnO₂ materials were characterised by field-emission scanning electron microscopy, transmission electron microscopy and energy dispersive spectroscopy analysis. H₂S gas-sensing characteristics were measured at various concentrations (1–10 ppm) at 200–350 °C. The results show that with effective decoration of NiO nanoparticles, the H₂S gas-sensing characteristics of SnO₂ nanowires are significantly enhanced by one or two orders compared with those of the bare material. The sensors showed an effective response to low-level concentrations of H₂S in the range of 1–10 ppm, suitable for application in monitoring of H₂S in biogas and in industrial controls. We also clarified the sensing mechanism of the sensor based on band structure and sulphurisation process.

NiO nanoparticles decorated on the surface of the on-chip grown SnO₂ nanowires exhibited excellent response to highly toxic hydrogen sulphide (H₂S) in air.

A highly sensitive gas sensor employing biomorphic SnO2 with multi-level tubes/pores structure: bio-templated from waste of flax

Metal oxide gas sensors with porous structures are widely used in numerous applications ranging from health monitoring and medical detection to safety; in this study, we report a highly sensitive SnO₂ gas sensor with a multi-level tube/pore structure prepared via biomimetic technology using flax waste as a bio-template and a simple wet chemical process combined with subsequent annealing. Indeed, MLTPS not only maintained and improved the excellence of porous structure gas sensing materials with abundant active sites and large surface-to-volume ratios, but also overcame the deficiency of the lack of gas diffusion channels in porous gas sensing materials. Thus, this novel multi-level tube/pore SnO₂ gas sensor exhibited significantly enhanced sensing performance, e.g. an ultra-low response concentration (250 ppb), a high response (87.9), a fast response (9.2 s), a low operating temperature (130 °C) and good stability, for formaldehyde. On the basis of these results, via the reuse of agricultural waste, this study provides a new concept for the low-cost synthesis of environmentally friendly and effective multi-level tube/pore gas sensor materials.

Metal oxides gas sensors are widely used in numerous applications from health, medical detection to safety. By bio-templating from waste of flax, this paper reports a highly sensitive SnO₂ gas sensor with multi-level tubes/pores structure.

Understanding the noble metal modifying effect on In2O3 nanowires: highly sensitive and selective gas sensors for potential early screening of multiple diseases

Sensor arrays consisting of In₂O₃ NWs loaded with different NMNPs can accurately distinguish different trace VOC biomarkers in simulated exhaled breath.