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In this manuscript, a simple method combining atomic layer deposition and magnetron sputtering is developed to fabricate high-performance Pd/SnO2 film patterns applied for micro-electro-mechanical systems (MEMS) H2 sensing chips. SnO2 film is first accurately deposited in the central areas of MEMS micro hotplate arrays by a mask-assistant method, leading the patterns with wafer-level high consistency in thickness. The grain size and density of Pd nanoparticles modified on the surface of the SnO2 film are further regulated to obtain an optimized sensing performance. The resulting MEMS H2 sensing chips show a wide detection range from 0.5 to 500 ppm, high resolution, and good repeatability. Based on the experiments and density functional theory calculations, a sensing enhancement mechanism is also proposed: a certain amount of Pd nanoparticles modified on the SnO2 surface could bring stronger H2 adsorption followed by dissociation, diffusion, and reaction with surface adsorbed oxygen species. Obviously, the method provided here is quite simple and effective for the manufacturing of MEMS H2 sensing chips with high consistency and optimized performance, which may also find broad applications in other MEMS chip technologies.

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Emissions from mobile sources and stationary sources contribute to atmospheric pollution in China, and its components, which include ultrafine particles (UFPs), volatile organic compounds (VOCs), and other reactive gases, such as NH3 and NOx, are the most harmful to human health. China has released various regulations and standards to address pollution from mobile and stationary sources. Thus, it is urgent to develop online monitoring technology for atmospheric pollution source emissions. This study provides an overview of the main progress in mobile and stationary source monitoring technology in China and describes the comprehensive application of some typical instruments in vital areas in recent years. These instruments have been applied to monitor emissions from motor vehicles, ships, airports, the chemical industry, and electric power generation. Not only has the level of atmospheric environment monitoring technology and equipment been improving, but relevant regulations and standards have also been constantly updated. Meanwhile, the developed instruments can provide scientific assistance for the successful implementation of regulations. According to the potential problem areas in atmospheric pollution in China, some research hotspots and future trends of atmospheric online monitoring technology are summarized. Furthermore, more advanced atmospheric online monitoring technology will contribute to a comprehensive understanding of atmospheric pollution and improve environmental monitoring capacity.

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The supported metal catalysts on scaffolds usually reveal multiple active sites, resulting in the occurrence of side reaction and being detrimental to the achievement of highly consistent catalysis. Single atom catalysts (SACs), possessed with highly consistent single active sites, have great potentials for overcoming such issues. Herein, the authors used SACs to modulate kinetic process of gas sensitive reaction. The supported Pd SACs, established by a metal organic frameworks-templated approach, promoted greatly the detection capacity to hydrogen sulfide (H2 S) gas with a very high sensitivity and selectivity. Density functional theory calculations show that the supported Pd SACs not only increased the number of electrons transferring from H2 S molecules to Pd SACs, but strengthened surface affinity to H2 S. Moreover, the HS bonds of H2 S molecules absorbed on Pd atomic sites are more likely to be dehydrogenated directly into sulfur species. Significantly, quasi in situ XPS analysis confirmed the presence of sulfur species during H2 S detection process, which may be a major cause for such detection signal. Based on these results, a suitable sensing principle for H2 S gas driven by Pd SACs was put forward. This work will enrich catalytic electronics in chemiresistive gas sensing.

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Accurate detection of glucose is essential for the diagnosis of diabetes, wherein effective and sensitive biosensors for glucose detection are needed. Here, we fabricated a glucose sensor based on field-effect transistor (FET) with bimetallic nickel-copper metal-organic frameworks (Ni/Cu-MOFs) as its channel layers which were grown in-situ through a simple one-step hydrothermal method and modified with glucose oxidase (GOD) by using glutaraldehyde (GA) as linkers. Due to the synergistic effect of Ni ions and Cu ions in MOFs, the sensor (GOD-GA-Ni/Cu-MOFs-FET) showed good field effect performance and great responses to glucose through enzymatic reactions. It displayed a piecewise linear relationship in the wide range (1 µM-20 mM), and provided high sensitivity (26.05 µAcm-2mM-1) in the low concentration (1-100 µM) and a low detection limit (0.51 µM). The sensor also had these advantages of high specificity, excellent reproducibility, good short-term stability and fast response time. Especially, it is indicated that the Ni/Cu-MOFs-FETs with high performance have the potential to be available sensors, paving the way for the application of bimetallic MOFs in biosensing.

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Noble metals supported on metal oxides are promising materials for widely applying on gas sensors because of their enviable physical and chemical properties in enhancing the sensitivity and selectivity. Herein, pristine ZFO yolk-shell spheres composed of ultrathin nanosheets and ultrasmall nanoparticles decorated with nanosized Au particles with a diameter of 1-2 nm are fabricated using the method of solution-phase deposition-precipitation. As a result, the Au@ZFO yolk-shell sphere based sensor exhibits significantly sensing performances for chlorobenzene (CB). In comparison with pristine ZFO, the response (Rair/Rgas= 90.9) of a Au@ZFO based sensor with a low detection limit of 100 ppb increases 4-fold when exposed to 10 ppm chlorobezene at 150 °C. Excitingly, the sensing response for chlorobenzene is the highest among metal oxides semiconductor based sensors. Moreover, the sensors can be further applied in the field of chlorobenzene monitoring, owing to its outstanding selectivity. The results elaborated that the enhanced sensing mechanism is mainly attributed to the effects of electronic sensitization and chemical sensitization, which are induced by the Au nanoparticles on the surface of ZFO yolk-shell spheres. Density functional theory (DFT) calculations further illustrated that the existence of Au nanoparticles exhibits higher adsorption energy and net charge transfer for CB. In addition, the relationship between the sensing performances of pristine ZFO and Au@ZFO yolk-shell spheres for chlorobenzene and the factors of Au loading amount, operating temperature, and humidity was also fully investigated in this work.

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The low enhancement factor of semiconductor SERS substrates is a major obstacle for their practical application. Therefore, there is a need to explore the facile synthesis of new SERS substrates and reveal the SERS enhancement mechanism. Here, we develop a simple, facile and low-cost two-step method to synthesize copper sulfide based nanostructures with different Cu7.2S4 contents. The as-synthesized sample is composed of nanosheets with the CuS phase structure. With the increase of the annealing temperature to 300 °C, the CuS content gradually decreases and disappears, and the content of Cu7.2S4 and CuSO4 appears and gradually increases. At the annealing temperature of 350 °C, only CuSO4 exists. Compared with pure CuS or pure CuSO4, the detection limit of R6G molecules is the lowest for the composite sample with a higher content of Cu7.2S4, indicating that the introduction of non-stoichiometric Cu7.2S4 can improve the SERS performance and the higher content of Cu7.2S4 leads to a higher SERS activity. Furthermore, to investigate the SERS mechanism, the energy band structures and energy-level diagrams of different probe molecules over CuS, Cu7.2S4 and CuxS are studied by DFT calculations. Theoretical calculations indicate that the excellent SERS behavior depends on charge transfer resonance. Our work provides a general approach for the construction of excellent metal compound semiconductor SERS active substrates.

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Ni3(HITP)2, a novel and promising two-dimensional metal-organic framework (MOF) material, has been utilized in the areas of catalysis, sensing, and supercapacitors. It is very suitable for preparing field-effect transistor (FET) devices due to its good conductivity, porous structure, as well as easy film formation. Nevertheless, there is a challenge to transfer membrane materials undamaged to the substrates. Here, we reported a simple approach to fabricate the Ni-MOF-based FET with an in situ grown Ni3(HITP)2 membrane as the channel material of the FET. With this method, we obtained a large-area, dense, and uniform film composed of thin sheets, and the thickness and density of the MOF film were tunable through changing the reaction time. The as-prepared Ni-MOF-FET had a good mobility of 45.4 cm2 V-1 s-1 and on/off current ratio of 2.29 × 103. Moreover, this FET served as a liquid-gated device for the first time with bipolar behavior and good response to the gluconic acid at the range from 10-6 to 10-3 g/mL, verifying the potential of the Ni-MOF-FET as biosensors.

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The composites of polyaniline and TiO2 nanoparticles with different contents were prepared in the aqueous solution of phosphoric acid, in which the phosphoric acid was selected as the protonic acid to improve the conductivity of polyaniline. In the composites, the TiO2 nanoparticles with the size of about 20 nm were coated by a layer of polyaniline film with a thickness of about 5 nm. Then, the gas sensors were constructed by a liquid⁻gas interfacial self-assembly method. The gas-sensing properties of the composites-based gas sensors obviously improved after doping with TiO2 nanoparticles, and the sensor response of the composites increased several times to NH3 from 10 ppm to 50 ppm than that of pure polyaniline. Especially when the mass ratio of TiO2 to aniline monomer was 2, it exhibited the best gas response (about 11.2⁻50 ppm NH3), repeatability and good selectivity to NH3 at room temperature. The p⁻n junction structure consisting of the polyaniline and TiO2 nanoparticles played an important role in improving gas-sensing properties. This paper will provide a method to improve the gas-sensing properties of polyaniline and optimum doping proportion of TiO2 nanoparticles.

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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.

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CuO monolayer colloidal particle films with controllable thickness and homogeneous microstructure were prepared by self-assembly and subsequent calcination based on Cu2O colloidal particles. Large-scale CuO monolayer colloidal particle films have the particle size of 300-500 nm, and CuO colloidal particles are hollow. It was found that such a structure exhibits excellent room-temperature H2S-gas-sensing properties. It not only has high sensing response and excellent selectivity, but also has a low limit of detection of 100 ppb. The sensors exhibit different sensitive characteristics at low and high concentrations of H2S. At low concentration (100-500 ppb), the sensor can be recovered with the increase of gas response, although it takes a longer recovery time at room temperature. At medium concentration (1-100 ppm), although the gas response still increases, the sensor is irreversible at room temperature. When the concentration continues to increase (>100 ppm), the sensor is irreversible at room temperature, and the gas response first increases and then decreases. Two reaction mechanisms are proposed to explain the above-mentioned sensing behavior. More importantly, quasi in situ X-ray photoelectron spectra confirm the existence of CuS. The CuO sensor with room-temperature response and superselectivity will find potential applications in industry, environment, or intelligent electronics.

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A convenient and flexible route is presented to fabricate gold noble metal nanoparticles wrapped with a controllable ultrathin carbon layer (Au@C) in one step based on laser ablation of the noble metal targets in toluene-ethanol mixed solutions. The obtained metal nanoparticles were <20 nm in size after ablation, and the thickness of the wrapped ultrathin carbon layer was 2 nm in a typical reaction. The size of the inner noble metal nanoparticles could be controlled by adjusting the power of laser ablation, and the thickness of the ultrathin carbon layer can be controlled from 0.6 to 2 nm by laser ablation in different components of organic solution. Then the resultant Au@C core/shell nanoparticles were modified on the surface of In2O3 films through a sol-gel technique, and the hydrogen sulfide (H2S) gas-sensing characteristics of the products were examined. Compared to pure and Au-modified In2O3, the Au@C-modified In2O3 materials exhibited a revertible and reproducible performance with good sensitivity and very low response times (few seconds) for H2S gas with a concentrations of 1 to 5 ppm at room temperature. Evidence proved that the ultrathin carbon layer played an important role in the improved H2S sensor performance. Other noble metals wrapped by the homogeneous carbon shell, such as Ag@C, could also be prepared with this method.

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Styrene, a chronic toxic gas, is of great harm to human health. It is urgently required to develop a portable, efficient, and inexpensive method to detect this toxic gas. Chemiresistive gas sensor based on semiconductor metal-oxides is considered as one of the best candidate suited to above features, while its sensitivity and selectivity are not enough high for the applications. Herein, the ultrafine Pt NPs embellished SnO2/α-Fe2O3 hollow nanoheterojunctions were achieved by in-situ reduction and subsequent calcination treatment. Particularly, such yielding products exhibited excellent styrene sensing performances with a detection limit of 50 ppb and extremely fast response/recovery time (3/15 s, respectively). More importantly, this SnO2/α-Fe2O3/Pt sensing platform revealed improved styrene selectivity against other malodorous gases. Additionally, the significant enhancement for styrene sensing response was also obtained compared to other two sensors (pristine SnO2 and SnO2/α-Fe2O3, respectively). Further studies demonstrated that such enhanced performances possibly be owing to the "catalytic sensitization" effect driven by Pt NPs and "electronic sensitization" effect triggered through the formation of Schottky junction as well as n-n nanoheterojunction. Based on these sensing features, it is probably great promising in the detection of styrene gas in the future.

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The structure and thickness of the chemiresistive thin films can significantly affect their gas sensing performances for the heating-typed sensors. Under light irradiation, however, their influences are still to be addressed. In present paper, the multilayered ZnO porous thin films with different (three types) micro/nanostructures and controllable thickness are fabricated via layer by layer construction of the self-assembled colloidal-layers. The structural and thickness effects of such films on the gas sensing performances to NO2 under ultraviolet (UV) illumination are experimentally studied. It has been found that under UV irradiation, the responses of the ZnO porous thin films to NO2 increase upto the maxima with the rising film thickness. Further increasing the thickness would lead to the insignificantly or gradually decreasing responses. The film thicknesses corresponding to the maximal responses are associated with the porous structures and the porosity of the thin films. The films with the higher porosity would lead to the higher maximal responses and the larger corresponding film-thicknesses, or vice versa. Such thickness and porous-structure dependences of the responses are attributed to the ever-decaying light intensity (and hence ever-decreasing photo-generated carrier concentration) in the films along the depth from the films' surface. This study is of importance in design and development of the light illuminating-typed gas sensing devices with high performances.

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Based on hydrogen bonding, the highly uniform polyaniline (PANI) nanotubes were synthesized by self-assembly method using citric acid (CA) as the dopant and the structure-directing agent by optimizing the molar ratio of CA to aniline monomer (Ani). Synthesis conditions like reaction temperature and mechanical stirring were considered to explore the effects of hydrogen bonding on the morphologies. The effects of CA on the final morphology of the products were also investigated. The as-synthesized CA doped polyaniline (PANI) nanomaterials were further deposited on the plate electrodes for the test of gas sensing performance to ammonia (NH3). The sensitivity to various concentrations of NH3, the repeatability, and the stability of the sensors were also tested and analyzed. As a result, it was found that the PANI nanomaterial synthesized at the CA/Ani molar ratio of 0.5 has highly uniform tubular morphology and shows the best sensing performance to NH3. It makes the PANI nanotubes a promising material for high performance gas sensing to NH3.

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It is important and necessary to effectively detect the chemical warfare agents, such as highly toxic never agent sarin. However, based on the surface-enhanced Raman scattering (SERS) effect, detection of nerve agent simulant dimethyl methylphosphonate (DMMP) which is weakly interacted with SERS-active substrate has been the most challenge for the routine SERS detection method. To overcome this challenge, we put forward a thin water film confined SERS strategy. Under the space-confinement of water film, Raman measurements are carried out in the water evaporation process. The subsequent water evaporation induces concentrating of the DMMP molecules, which are thus successfully restricted within the strong electromagnetic field enhanced area above the SERS substrates, leading to the enhancement of their Raman signals. This study provides a new way to achieve the efficient SERS-based detection of the target molecules weakly interacted with the metal substrates.

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Resistance-type metal-oxide semiconductor gas sensors with high sensitivity and low detection limit have been explored for practical applications. They require both sensing films with high sensitivity to target gases and an appropriate structure of the electrode-equipped substrate to support the sensing films, which is still challenging. In this paper, a new gas sensor of metal-oxide porous array films on a micro-gap electrode pair is designed and implemented by taking ZnO as a model material. First, a micro-gap electrode pair was constructed by sputtering deposition on a filament template, which was used as the sensor's supporting substrate. Then, the sensing film, made up of ZnO porous periodic arrays, was in situ synthesized onto the supporting substrate by a solution-dipping colloidal lithography strategy. The results demonstrated the validity of the strategy, and the as-designed sensor shows a small device-resistance, an enhanced sensing performance with high resolution and an ultralow detection limit. This work provides an alternative method to promote the practical application of resistance-type gas sensors.

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Nano-structured gas sensing materials, in particular nanoparticles, nanotubes, and nanowires, enable high sensitivity at a ppb level for gas sensors. For practical applications, it is highly desirable to be able to manufacture such gas sensors in batch and at low cost. We present here a strategy of in-situ wafer-level fabrication of the high-performance micro/nano gas sensing chips by naturally integrating microhotplatform (MHP) with nanopore array (NPA). By introducing colloidal crystal template, a wafer-level ordered homogenous SnO2 NPA is synthesized in-situ on a 4-inch MHP wafer, able to produce thousands of gas sensing units in one batch. The integration of micromachining process and nanofabrication process endues micro/nano gas sensing chips at low cost, high throughput, and with high sensitivity (down to ~20 ppb), fast response time (down to ~1 s), and low power consumption (down to ~30 mW). The proposed strategy of integrating MHP with NPA represents a versatile approach for in-situ wafer-level fabrication of high-performance micro/nano gas sensors for real industrial applications.

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A reversible janus gas redox transition was discovered in the trace Sarin sensing process using an ethanol-aged nanoporous SnO2 micro-chemiresistor, which was reliable to realize the identification of Sarin from other gases and even its simulant. This sensor also endows a rather low detection threshold of 6 ppb to Sarin.

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Silver nanoplate hierarchical turreted ordered arrays were fabricated through an electro-deposition method on ordered acuate silicon nanocone templates. Such arrays can be used as SERS substrates for trace analyses of streptomycin sulphate, and exhibit high activity and stability. This work is of importance in practical applications based on the SERS effect of noble metal micro/nano-structured arrays.

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We develop a facile and effective strategy to prepare monodispersed Au spherical nanoparticles by two steps. Large-scale monocrystalline Au nanooctahedra with uniform size were synthesized by a polyol-route and subsequently Au nanoparticles were transformed from octahedron to spherical shape in a liquid under ambient atmosphere by non-focused laser irradiation in very short time. High monodispersed, ultra-smooth gold nanospheres can be obtained by simply optimizing the laser fluence and irradiation time. Photothermal melting-evaporation model was employed to get a better understanding of the morphology transformation for the system of nanosecond pulsed-laser excitation. These Au nanoparticles were fabricated into periodic monolayer arrays by self-assembly utilizing their high monodispersity and perfect spherical shape. Importantly, such Au nanospheres arrays demonstrated very good SERS enhancement related to their periodic structure due to existence of many SERS hot spots between neighboring Au nanospheres caused by the electromagnetic coupling in an array. These gold nanospheres and their self-assembled arrays possess distinct physical and chemical properties. It will make them as an excellent and promising candidate for applying in sensing and spectroscopic enhancement, catalysis, energy, and biology.