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Abstract Metal oxide semiconductor (MOS) gas sensors possess extensive applications due to their high sensitivity, low cost, and simplicity. To boost their excellent sensing performance and meet the growing demand for applications, a series of strategies have been developed, such as the surface morphology engineering and function manipulation. Recently, the controlled morphology with exposed high-energy facets and the facet-dependent sensing properties have attracted much attention. Because of its abundant unsaturated active sites, the crystal planes with high surface energy usually serve as promising platform for gas sensing. After a lot of survey of literature, the authors provide a review of recent efforts on engineering crystal structures with exposed high-energy facets of MOS nanomaterials and their improved gas-sensitive performance, emphasis on six kinds of common gas-sensitive MOS including ZnO, SnO 2 , TiO 2 , α-Fe 2 O 3 , NiO and Cu 2 O. Also, the relationship between dangling bonds density and gas-sensing properties has been systematically discussed and used as one significant factor to evaluate superior sensing surface of MOS. According to the research and calculation, surface engineering by selectively exposing high-energy facets provides an effective way to obtain MOS gas-sensitive materials with superior performance. The understanding of the facet-dependent properties of MOS will assist in and guide the fabrication of more excellent gas sensors in the future. [ABSTRACT FROM AUTHOR]

In this paper, SnO 2 nanoparticles (NPs) decorated reduced graphene oxide hybrids with abundant vacancies (designated as SnO 2 -RGO-OVs) have been successfully prepared by a combined hydrothermal synthesis and chemical solution deposition method. It is found that high density SnO 2 NPs with the size of 3–5 nm are uniformly distributed on the surface of RGO nanosheets. Most importantly, SnO 2 -RGO-OVs hybrids exhibit excellent room-temperature NO 2 sensing properties with the low detection limit of 50 ppb. When SnO 2 -RGO-OVs-based sensor was exposed to 1 ppm NO 2 , the response is 3.80 and response time and recovery time are 14 s and 190 s, respectively. These sensing performances are superior to those of most reported room-temperature NO 2 sensors based on RGO-based materials and other materials. The excellent sensing performances of SnO 2 -RGO-OVs hybrids can be attributed to their specific structure, e.g., RGO that could facilitate transferring carriers during sensing progress, and abundant OVs that could facilitate adsorption of more NO 2 molecules onto SnO 2 NPs in SnO 2 -RGO-OVs hybrids. [ABSTRACT FROM AUTHOR]

Composites fabricated by mesoporous silica loading hydrophilic elements have been widely investigated as humidity sensitive materials. The physical mixing process for loading hydrophilic elements is difficult to guarantee the uniformity and structural stability of the composites. Furthermore, the poor dispersibility of the silica skeleton restricts its solution processability. Herein, solution processible organic-inorganic hybrid materials were synthesized via a thiol-ene click reaction between sulfhydryl functionalized microporous silica nanoparticles (MSNs) and hydrophilic alkenyl monomer. The optimal hybrid material could disperse in water stably with a concentration of 50 mg/mL owing to the small particle size (about 105 nm) and the hydrophilic characteristic. Humidity sensors were fabricated by solution process and the humidity sensing properties were investigated. The sensor exhibits high sensitivity (about three orders impedance modulus change), small hysteresis (∼2% RH) and rapid response (the response and recovery time are 5 and 40 s, respectively) in the humidity range of 11%–95% RH. [ABSTRACT FROM AUTHOR]

In this work, we have reported a novel NO 2 sensor using molybdenum disulfide nanoparticles (MoS 2 NPs) decorated RGO (MoS 2 -RGO) hybrids as sensing materials, where MoS 2 -RGO hybrids were prepared by a two-step wet-chemical method. Firstly, MoS 2 NPs prepared by modified liquid exfoliation method from bulky MoS 2 powder. Then, MoS 2 -RGO hybrids were obtained by self-assembly of MoS 2 NPs and GO nanosheets, followed by a facile hydrothermal treatment progress. The combined characterizations indicate that MoS 2 NPs with the size of 3–5 nm are uniformly dispersed on RGO nanosheets. Most importantly, the sensor based on MoS 2 -RGO hybrids could detect NO 2 at room temperature. To further improve sensing performances, especially response and recovery rate, sensing properties are further examined by increasing the operation temperature to 160 °C. It is notably seen that MoS 2 -RGO-based NO 2 sensor not only shows improved sensitivity to NO 2 compared to pure RGO-based sensor, but also exhibits fast response and recovery characteristics (response time and recovery time are 8 s and 20 s toward 3 ppm NO 2 ). This work paves a new way for application of MoS 2 and RGO in chemical sensors, providing an effective method for fabrication of NO 2 sensors with high sensitivity and fast response/recovery rate. [ABSTRACT FROM AUTHOR]

The sensing layer composited from the inorganic particles, organic materials, polymers could be regarded as an important and functional component used in electronic devices, such as chemical sensor. Furthermore, it is very important for these materials to own hierarchical and porous structures leading to a large effective specific surface area. Among these, hybrid materials with yolk double-shelled architecture can further realize the required surface chemical reactions. However, it is rarely reported in gas sensors. Herein, a novel SnO 2 -TiO 2 hollow nanostructure consisting of double shells was successfully designed and synthesized. Due to the special multi-shelled structure and abundant hetero-interface, the sensing layer based on SnO 2 -TiO 2 realizes a rapid response rate within 1.7 s to ethanol gas and alluring reproducibility (15 days), which are superior over those for compact structure with similar sensors. The results indicated that the SnO 2 -TiO 2 yolk double-shelled microsphere have the huge potential for designing high performance practical ethanol devices in environmental monitoring and drunk driving. [ABSTRACT FROM AUTHOR]

A novel organic-inorganic hybrid material with the core-shell framework was prepared by modifying the poly(vinylsulfonic acid, sodium salt) (PSS) on vinyl functionalized silica nanoparticles (VSNs) via the free radical polymerization. SNs with different contents of PSS were prepared and the structures were carefully characterized. The hybrid material could enhance the durability of PSS under high humidity environment. Meanwhile, the SNs skeleton provides interstitial channels in the PSS@SNs films, which are available for water molecules transport. The PSS coated SNs (PSS@SNs) films were formed by drop-coating and their sensing properties were investigated. The impedance of the optimal humidity sensor changed more than four orders of magnitude over the relative humidity range of 33–95%, with good linearity under semi-logarithm scale (R 2 = 0.950) and a small humidity hysteresis of 2% RH. The obtained sensor also showed rapid response to humidity change (the response and recovery time were 2 and 65 s, respectively). Finally, the resultant sensor was explored for respiratory rate monitoring. [ABSTRACT FROM AUTHOR]

The sensing layer in electronic device, such as chemical sensor, is an important and functional component consisting of polymer, organic materials, inorganic particles, and other components. These materials are generally hierarchical and porous with a large effective specific surface area to achieve the required surface chemical reactions. Despite the great progress, the synthesis of hollow core-shell nanostructure, especially ternary transition metal with core-in-hollow-shell nanostructure, is rarely reported. Herein, a novel NiFe 2 O 4 core-in-hollow-shell nanosphere with a porous inner and outer surface was prepared by a hydrothermal and co-precipitation method and the acetone sensing properties were reported. The NiFe 2 O 4 sensing layer exhibited high response to 100 ppm acetone gas (10.6) and achieved a rapid response rate and recovery process (within 1 s/7 s) at 280 °C, which are superior over those for nanoparticles of NiFe 2 O 4 and those reported for similar devices. In addition, after 20 days, the response of sensor based on NiFe 2 O 4 decreased ∼4%, indicating an alluring long-term stability. This is attributed to the highly porous but robust core-shell architecture, large utilization ratio of sensing layer and excellent catalytic activity. [ABSTRACT FROM AUTHOR]

The high sensing performance of chemical sensing sensors depends greatly on rational design of nanostructure and smart heterogenization of tailorable nanomaterials. Hybrid materials (HMs) as active sensing layers, are generally used for sensing devices. Here, the heteroassembly of n–type α–Fe 2 O 3 nanoparticles on a p–type LaFeO 3 matrix into a pore-rich hollow nanostructure is reported. The fabrication process uses a hydrothermal and co–precipitation synthesis method and begins from carbon nanospheres as sacrificial templates. The hetero-interfacial area and surface structure can be successfully tailored by changing the composition ratio of α–Fe 2 O 3 to LaFeO 3 . Consequently, as sensing materials in gas sensors, the α–Fe 2 O 3 /LaFeO 3 hetero-materials have a fast response/recovery time of 1 s/5 s after 5 cycles and are able to retain 92.4% of this sensitivity after 15 days, i.e., 10.1, to 100 ppm ethanol gas, which is closely related with the strong hetero–interfacial contact and unique hollow structures. This work offers a new insight into the rational design of chemical sensing devices from the synergetic effect of p–type ternary transition metal oxides as well as n–type nanostructures. [ABSTRACT FROM AUTHOR]

A physical mixing is usually needed in order to load lithium chloride (LiCl) in a matrix. A method of in situ loading LiCl in the cross-linked polymer framework by click reaction is demonstrated in this work. Pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) and divinylbenzene (DVB) were taken as the monomers to construct stable cross-linked polymer matrix, while different contents of LiCl were introduced in the polymerization process for preparing humidity sensitive composites. The cross-linked composites with uniform distribution of LiCl were obtained. Humidity sensors based on the composites were fabricated and their sensing properties were systematically investigated. The optimum mass ratio of LiCl was determined to be 8 wt%, the corresponding sensor owns a variation of impedance modulus beyond two orders of magnitudes from 11% to 95% relative humidity (RH), a maximum humidity hysteresis of ∼1.5% RH, and a short response and recovery time of 3.5 and 63 s, respectively. The humidity sensing mechanism of the obtained sensor was explored by the complex impedance spectroscopy and the equivalent circuit. [ABSTRACT FROM AUTHOR]

Mesoporous silicas loading hydrophilic materials composites were widely used in humidity sensors during the last decade. However, the physical mixing process used for loading the hydrophilic materials in the channel of mesoporous silicas is hard to guarantee their uniform dispersion, and there is no strong force to bind the host and guest materials together. To solve above problems, novel vinyl functionalized mesoporous silicas (SBA-15-vinyl) were chemically modified with hydrophilic sodium p -styrenesulfonate (SSS) with free radical polymerization. The chemical and porous structures of the obtained hybrid materials (SBA-15-PSSx) were characterized by fourier transform infrared spectroscopy (FT-IR), elemental analyses (EA), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and N 2 adsorption-desorption. The humidity sensing properties of the sensors based on SBA-15-vinyl and SBA-15-PSS indicate the introduction of PSS could effectively enhance the sensing properties of SBA-15-vinyl. The impedance of the optimized sensor changed by more than three orders of magnitude over the relative humidity (RH) range of 11%–95%, with rapid response to RH change. [ABSTRACT FROM AUTHOR]

Water-resistance and defined structure are important characteristics for the polymers used in humidity sensors. Here, cross-linked organic-inorganic hybrid polymers based on mercaptopropyl polyhedral oligomeric silsesquioxane (MPOSS), 1,4-divinylbenzene (DVB) and methacrylatoethyl trimethyl ammonium chloride (DMC) with defined structures have been synthesized via click chemistry. The structures of the resultant polymers were characterized by Fourier transform infrared spectra (FT-IR). Humidity sensors based on the cross-linked polymers were fabricated and their humidity sensing properties were systematically investigated. The impedance modules of the optimized sensor change about three orders of magnitude over the relative humidity range from 11% to 95%. The obtained sensor also shows small humidity hysteresis and rapid response to humidity change. [ABSTRACT FROM AUTHOR]

Uniform Co 3 O 4 nanocubes have been prepared via an effective one-pot hydrothermal method. Sphere-shape Co 3 O 4 nanomaterials can be synthesized by varying the concentration of Co(NO 3 ) 2 ·6H 2 O and NaOH without the utilization of surfactants. Co 3 O 4 nanocubes and Co 3 O 4 nanospheres show regular geometry structure with diameters of 250 nm and 300 nm, respectively. Gas sensors based on different Co 3 O 4 nanostructured are fabricated and used for investigating acetone sensing performances. The response of Co 3 O 4 nanocubes based sensor to 500 ppm acetone is 4.88 at 240 °C, which is higher than that of Co 3 O 4 nanospheres. In addition, sensors based on Co 3 O 4 nanocubes not only exhibit a high response but also have a rapid response time (2 s) and recovery process (5 s) to acetone. The enhanced sensing performances to acetone can be attributed to the effective catalytic activity of Co 2+ at special low-index crystal facets. This finding demonstrates the shape-related synthesis of nanomaterials can be an effective way of designing high performance gas sensor, and also provides a new insight into gas-sensing mechanism. [ABSTRACT FROM AUTHOR]

Mesoporous silica based composites are widely used for humidity sensors. But a physical mixing is needed to load humidity sensitive materials in the mesoporous silica materials, so the uniform dispersion of humidity active materials could not be guaranteed. A chemically modification method has been used to ensure the uniform dispersion and stable structure of the humidity active units in this work. Novel organic-inorganic hybrid materials have been prepared via modifying the sulfhydryl functionalized mesoporous silica SBA-15 (SBA-15-propyl-SH) with hydrophilic unit methacrylatoethyl trimethyl ammonium chloride (DMC) via a thiol-ene click reaction. Mesoporous silica skeletons could ensure the stability at high humidities and hydrophilic groups modified on the skeletons behave as humidity sensitive parts. The content of DMC on the silica could be tuned by the feed ratios. The obtained hybrid materials keep the mesoporous characteristics of silica, which is beneficial for the transport of water molecules in adsorption and desorption processes. The humidity sensing properties of the sensors based on SBA-15-propyl-SH and SBA-15-propyl-S-DMCx were investigated in details. The impedance modulus of the optimized sensor changes by more than three orders of magnitude over the relative humidity (RH) range of 11% to 95%, with prompt response to RH change (11 s and 60 s for adsorption and desorption processes, respectively). [ABSTRACT FROM AUTHOR]

Recently, the intelligent, smart gas sensing devices have shown great prospects in applications of gas component recognition and concentration detection, which largely depends on the precise structures and compositions of sensing layers. Various synthetic strategies are promising for the scalable production and performance optimization of the sensing materials. In this review, different gas sensing materials with controlled preparation such as wet-chemical synthesis, electrospinning method, vapor-phase synthesis and exfoliation routes are summarized. Then, the representative design strategies and gas sensing performances in gas sensors are also discussed. Finally, the recent progress of some gas sensing devices is presented, with the emphasis on their device structure, circuit design, and applications. It is believed that this review will provide some guidelines for designing novel gas sensing materials with controllable synthetic methods and sensing behaviors for advanced gas sensors. • The various synthetic strategies were summarized for a rational design of advanced gas sensing materials. • The gas sensing performance and optimize strategy were well provided for the sensing materials. • Future challenges were suggested, including the trade-off between materials and performances, and sensing mechanism study. [ABSTRACT FROM AUTHOR]

Abstract: N-type Fe2O3 nanobelts and P-type LaFeO3 nanobelts were prepared by electrospinning. The structure and micro-morphology of the materials were characterized by X-ray diffraction (XRD) and scanning of electron microscopy (SEM). The gas sensing properties of the materials were investigated. The results show that the optimum operating temperature of the gas sensors fabricated from Fe2O3 nanobelts is 285°C, whereas that from LaFeO3 nanobelts is 170°C. Under optimum operating temperatures at 500ppm ethanol, the response of the gas sensors based on these two materials is 4.9 and 8.9, respectively. The response of LaFeO3-based gas sensors behaves linearly with the ethanol concentration at 10–200ppm. Sensitivities to different gases were examined, and the results show that LaFeO3 nanobelts exhibit good selectivity to ethanol, making them promising candidates as practical detectors of ethanol. [Copyright &y& Elsevier]

Abstract: Barium titanate (BaTiO3) nanofiber was synthesized by electrospinning and by using the calcinations technique. Its morphologies were characterized by scanning electron microscopy, which showed that its average diameter was 80–200nm, and its length was over 0.1mm. An impedance-type humidity sensor was then fabricated based on this BaTiO3 nanofiber. It was unexpected to find that such sensor exhibited fast response and recovery, which were 5 and 4s between 11% and 95% relative humidity, respectively. The long stability and humidity hysteresis properties of this sensor were also tested, and they were found to be optimum. These excellent sensing characteristics prove that BaTiO3 nanofiber is applicable for high-performance humidity sensors. [Copyright &y& Elsevier]

Abstract: Hollow ZnSnO3 nanocubes with peculiar cage- and skeleton-like architectures are successfully synthesized by a simple hydrothermal process at 180°C for 12h. These ZnSnO3 nanostructures exhibit almost uniform cubic structures with side length of 200–400nm. The gas sensor based on these ZnSnO3 nanostructures exhibits high response and quick response-recovery to ethanol and HCHO, which also shows superior sensitive performance compared with the results of sensor based on solid ZnSnO3 nanocubes. [Copyright &y& Elsevier]

Abstract: High preferential c-axis oriented ZnO nanorods aligned on SiO2/Si substrates with interdigital electrodes have been successfully fabricated through aqueous solution methods. The transparent ZnO buffer layer covered on both the conducting electrodes and the insulating SiO2 spacer regions acts as lattice-matched template for the assembly of c-axis oriented ZnO nanorod arrays, and the subsequent growth of ZnO nanorods has been completed via a low temperature hydrothermal process. Detailed structural characterizations reveal that the as-synthesized nanorods are single crystalline, with a hexagonal phase, and with growth along the [001] direction. The gas-sensing properties of ZnO nanorod thin films are studied. It is found that the ZnO nanorod thin film sensor exhibits excellent sensing properties towards acetone at 300°C with the response time shorter than 5s. [Copyright &y& Elsevier]

Abstract: SnO2 nanofibers with and without block copolymer P123 (EO20PO70EO20) in the precursor are synthesized via a simple electrospinning method. The Brunauer–Emmett–Teller surface areas of the nanofibers with and without P123 are 113.261 and 18.025m2 g−1, respectively. Gas sensing properties of these two samples are investigated by exposing the corresponding sensors to NH3, C2H5OH, and CH3COCH3 at 280, 300, and 340°C, respectively. Comparing with nanofibers without P123, the SnO2 nanofibers with P123 hold enhanced response value, higher saturated-detection-concentration, and lower minimum-detection-limit. The response is 59, 75, and 65 for nanofibers with P123 to 500ppm NH3, C2H5OH, and CH3COCH3, respectively, and is 28, 50, and 43 for nanofibers without P123. The minimum-detection-limit is 5, 1, and 1ppm for nanofibers with P123 to detect NH3, C2H5OH, and CH3COCH3, respectively, and is 50, 20, and 10ppm for nanofibers without P123. The improved gas sensing properties are based on their coarse and loose morphology and higher Brunauer–Emmett–Teller surface area brought by adding P123. [Copyright &y& Elsevier]

Abstract: The large-scale flowerlike ZnO nanostructures, consisting of many aggregative nanorods with the diameter of about 60nm, were prepared by a simple and facile hydrothermal route. The toluene sensing properties of the pure ZnO and TiO2-doped ZnO nanostructures were tested at different operating temperatures from 160 to 390°C and toluene concentrations ranging from 1 to 3000ppm. The results indicate that the response of the ZnO nanostructures towards toluene was enhanced significantly by TiO2 doping, which was deposited onto the ZnO products by evaporation. It is found that the TiO2-doped ZnO sensor exhibits remarkably enhanced response of 17.1 at the optimal operating temperature of 290°C to 100ppm toluene, which has been improved up from 7.4 at 390°C from pure ZnO nanostructures. [Copyright &y& Elsevier]

Abstract: In/Pd-doped SnO2 is synthesized via a sol–gel method and coated on a silicon substrate with Pt electrodes to fabricate a microstructure sensor. The sensor shows high response to CO with very low cross response to common interference gases at an operating temperature of 140°C. Especially, the sensor can detect CO down to 1ppm (the response value is about 3), and the response time and recovery time are about 15 and 20s, respectively. These results make our sensor a good candidate in practical CO sensors. [Copyright &y& Elsevier]

Abstract: KCl-doped ZnO nanofibers are synthesized via an electrospinning method and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The humidity sensing properties of these nanofibers are investigated by screen-printing them on a ceramic substrate with a pair of Ag–Pd interdigitated electrodes. The experimental results show that the 5.7wt% KCl-doped ZnO nanofibers hold super-rapid response and recovery, high response value, good reproducibility, linearity, selectivity, and stability at 100Hz. Especially, the response time and recovery time is only about 2 and 1s, respectively. These results demonstrate the potential application of KCl-doped ZnO nanofibers for fabricating high performance humidity sensors. [Copyright &y& Elsevier]

Abstract: A simple method for the large-scale synthesis of SnO2 nanofibers has been demonstrated through an electrospinning method. The as-synthesized fibers are characterized by scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, X-ray diffraction, and energy dispersive X-ray analysis. The sensor fabricated from these fibers exhibits high response to toluene at 350°C with good selectivity. The response time and recovery time are about 1 and 5s, respectively. The linear dependence of the response value on the toluene concentration is observed in the range of 10–300ppm. The potential application of SnO2 nanofibers for fabricating high performance toluene sensors at industry level has been demonstrated. [Copyright &y& Elsevier]

Abstract: Cu–Zn alloy nanoparticles in mass production are synthesized through a wire electrical explosion (WEE) method. The atoms on the surface of the as-synthesized nanoparticles are oxidized to form the Cu–Zn/CuO–ZnO nanoparticles (CZ/CZNs). The humidity sensing properties of CZ/CZNs and KCl-doped Cu–Zn/CuO–ZnO nanoparticles (KCZ/CZNs) are investigated by screen-printing them on ceramic substrates with Ag–Pd interdigitated electrodes, respectively. The measurement results show that KCZ/CZNs hold the improved humidity sensing properties with the impedance varying about four orders of magnitude in the range of 11–95% relative humidity (RH), while the impedance of CZ/CZNs changes only about two orders of magnitude. The response and recovery time of KCZ/CZNs are about 40 and 50s, respectively. And the maximum hysteresis is around 4% RH. Our results demonstrate the potential application of KCZ/CZNs for fabricating high performance humidity sensors. [Copyright &y& Elsevier]

Abstract: Fe-doped mesoporous silica SBA-15 has been studied as a humidity sensing material. Comparing with pure SBA-15, Fe-doped SBA-15 shows improved humidity sensing properties within the relative humidity (RH) range of 11–95%. The impedance of our product changes by more than four orders of magnitude over the whole humidity range with the response time and recovery time are about 20 and 50s, respectively. High selectivity and good stability are also observed based on our product. These results make Fe-doped SBA-15 a good candidate in fabricating humidity sensor. [Copyright &y& Elsevier]

Abstract: Dumbbell-like ZnO microcrystals have been obtained through a facile solution method. The structure, morphology and optical properties of the as-prepared ZnO microcrystals have been characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy and photoluminescence. The as-prepared ZnO microcrystals exhibit excellent sensing properties against acetone at an operating temperature of 300°C. The response and recovery times are found to be 1.5 and 3s, respectively. Moreover, the sensor holds the successful discrimination between acetone and ethanol, which makes our product a good candidate in fabricating highly selective sensors in practice. [Copyright &y& Elsevier]

Abstract: Pure and Sm2O3-doped SnO2 are prepared through a sol–gel method and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The sensor based on 6wt% Sm2O3-doped SnO2 displays superior response at an operating temperature of 180°C, and the response magnitude to 1000ppm C2H2 can reach 63.8, which is 16.8 times larger than that of pure SnO2. This sensor also shows high sensitivity under various humidity conditions. These results make our product be a good candidate in fabricating C2H2 sensors. [Copyright &y& Elsevier]

Abstract: The humidity sensitive characteristics of a sensor fabricated from flower-like ZnO nanorods by screen-printing on a ceramic substrate with Ag–Pd interdigital electrodes have been investigated. The sensor shows high humidity sensitivity, rapid response and recovery, small hysteresis, and good stability. It is found that the impedance of the sensor decreases by about five orders of magnitude with increasing relative humidity (RH) from 11 to 95%. The response and recovery time of the sensor is about 5 and 10s, respectively. These results indicate that the flower-like ZnO nanorods can be used in fabricating high-performance humidity sensors. [Copyright &y& Elsevier]

Abstract: Polypyrrole (PPY) materials were prepared by using chemical polymerization at room temperature for 96h. A resistive-type humidity sensor was fabricated on a ceramic substrate with Ag–Pd interdigital electrodes. The humidity-sensing mechanism of the polypyrrole materials was discussed in terms of capacitance, direct current, complex impedance and dielectric loss properties. The discussion on the sensing mechanism indicated that not only electrons or ions but also dipoles contributed to the conduction in the whole relative humidity range. [Copyright &y& Elsevier]

Abstract: P-SBA-15 and A-SBA-15 and LiCl-A-SBA-15 (X) were prepared and characterized by IR and XRD. These different characteristic results suggest that these materials have the similar mesoporous structure. The dopant of LiCl improves the pure A-SBA-15 sorption ability to moisture. A resistive-type humidity sensor is fabricated on a ceramic substrate with Ag-Pd interdigital electrodes. The testing results on humidity sensing properties show that the material with 15wt.% LiCl as a dopant has the best sensing properties to humidity. The discussion on the sensing principle suggests that the protons are the domination carrier in a low humidity range and ions in a high humidity range. The complex impedance plots support this discussion result. [Copyright &y& Elsevier]

• An ultrasensitive electrochemical immunosensor was developed for detection of AFP. • Electrodeposited Au NPs was used as platform to achieve signal amplification. • MoS 2 @Cu 2 O-Au showed superb tri-amplification current response. • The new immunosensor showed excellent sensitivity, stability and selectivity. Recently, alpha fetoprotein (AFP), a tumour marker to identify adult primary liver cancer, has attracted increasingly widespread attention. In this work, an ultrasensitive sandwich-type electrochemical biosensor was fabricated for AFP detection. Electrodeposited gold nanoparticles (D-Au NPs) modified on glassy carbon electrodes (GCE) were used as antibody carriers and sensing platforms. Furthermore, a molybdenum disulfide @ cuprous oxide hybrid (MoS 2 @Cu 2 O) was prepared to combine gold nanoparticles (Au NPs) as a strategy for achieving signal amplification. The experimental results revealed that molybdenum disulfide @ cuprous oxide-gold nanoparticles (MoS 2 @Cu 2 O-Au) showed superb tri-amplification electrochemical signals when catalyzing hydrogen peroxide (H 2 O 2) reduction. This method was demonstrated as a novel tool for the highly reliable separation and quantification of AFP. Under the most suitable experimental conditions, the proposed method exhibits a linear range from 0.1 pg/mL to 50 ng/mL and a low detection limit of 0.037 pg/mL (S/N = 3). The ultrasensitive electrochemical immunosensor has been tested for human serum, and satisfactory recoveries were obtained, indicating that the proposed method is promising for practical applications in the clinical diagnosis of AFP detection. [ABSTRACT FROM AUTHOR]

• A sandwich-type immunosensor for detection of PCT was designed. • CeO 2 -CuO-Au as platform can promote electron transfer. • Au@Ag-Th as signal labels can further effectually amplify the current signal. • The immunosensor presents prominent performance for PCT detection. Procalcitonin (PCT) is a kind of protein exists in the human blood. If the human bodies are infected with bacteria, fungi or parasites, the value of PCT in blood will be significantly raised. Thereby, we report an electrochemical immunosensor for measurement of PCT using gold nanoparticle (Au NP) decorated CeO 2 -CuO (CeO 2 -CuO-Au) as the platform and thionine modified Au@Ag heterojunction nanorods (Au@Ag-Th) as signal labels. Au NP decorated CeO 2 -CuO exhibits high surface area, high activity and good biocompatibility, which can immobilize more primary antibodies and promote electron transfer. The Au@Ag-Th provides good active sites to link secondary antibodies, further effectually amplify the current signal. This immunosensor exhibits a well-pleasing response with a limit detection of 0.17 pg mL−1 and linear detection between 0.5 pg mL−1 and 50 ng mL−1. Meanwhile, the high sensitivity, excellent specificity and acceptable human serum samples testing results of this method lead to potential applications in diagnosis of bacterial infections. [ABSTRACT FROM AUTHOR]

Humidity is a physical quantity that evaluates the concentration of water vapor in the air. The detection of humidity has received extensive attention in various fields such as industry, agriculture, meteorology, and national defense. Among the many reported humidity detection methods, humidity sensors stand out due to their advantages of portability, easy fabrication, low cost, high sensitivity, and fast response. It is worth noting that the humidity detection limit is becoming more and more stringent in some special industries, and the dynamic range of humidity measurement can be extended from low relative humidity to ppm or ppb or even ppt level. However, the development of high-performance low humidity sensors still faces great challenges due to the very small amount of water molecules that the sensor can adsorb and the limitation of some harsh environments. So far, the literature on low humidity sensors is scarce, and there is no review on its research progress. In this case, a comprehensive and in-depth understanding of the research progress of low humidity sensors is of great significance, and it may provide new insights to promote the development of low humidity sensors. Therefore, this paper reviewed the research progress of low humidity sensors for nearly three decades. Firstly, the necessity of low humidity detection, trace moisture generators and various low humidity measurement methods were introduced. Then, the research status of different types of low humidity sensors was highlighted, including capacitance, resistance, QCM, MCL, SAW, optical fiber, fluorescence and other types. [ABSTRACT FROM AUTHOR]

Humidity is a physical quantity that evaluates the concentration of water vapor in the air. The detection of humidity has received extensive attention in various fields such as industry, agriculture, meteorology, and national defense. Among the many reported humidity detection methods, humidity sensors stand out due to their advantages of portability, easy fabrication, low cost, high sensitivity, and fast response. It is worth noting that the humidity detection limit is becoming more and more stringent in some special industries, and the dynamic range of humidity measurement can be extended from low relative humidity to ppm or ppb or even ppt level. However, the development of high-performance low humidity sensors still faces great challenges due to the very small amount of water molecules that the sensor can adsorb and the limitation of some harsh environments. So far, the literature on low humidity sensors is scarce, and there is no review on its research progress. In this case, a comprehensive and in-depth understanding of the research progress of low humidity sensors is of great significance, and it may provide new insights to promote the development of low humidity sensors. Therefore, this paper reviewed the research progress of low humidity sensors for nearly three decades. Firstly, the necessity of low humidity detection, trace moisture generators and various low humidity measurement methods were introduced. Then, the research status of different types of low humidity sensors was highlighted, including capacitance, resistance, QCM, MCL, SAW, optical fiber, fluorescence and other types. [ABSTRACT FROM AUTHOR]

CO 2 is one of the main components of air, accounting for 0.03–0.04% (300–400 ppm) of the total volume of the atmosphere. CO 2 gas is chemically stable and is an inert gas, which makes the means of detecting it extremely limited. To overcome these challenges, in this paper, nanoscale M-MOF-74 (M=Mg, Ni, Zn, Co) was synthesized by a facile method in a two-hour reaction at room temperature, which combined with a quartz crystal microbalance (QCM) sensor-enabled the detection of CO 2 gas at room temperature. The CO 2 sensing performance of M-MOF-74 coated QCM sensors at room temperature was systematically investigated. The experimental results showed that the Mg-MOF-74 sensor responded to 2000 ppm CO 2 at room temperature with 66, and at the same time, had good linearity, high response/recovery (75 s/50 s), and good selectivity to CO 2. The electrical parameters and quality factor of the QCM were further verified by impedance analysis. In addition, the potential CO 2 adsorption mechanism may be mainly related to the open metal sites in the Mg-MOF-74 material and the synergistic effect of Mg-O bonds. This work confirms that the M-MOF-74 material provides a new avenue for QCM CO 2 sensing, and the sensor is expected to be a potential candidate for CO 2 detection. [Display omitted] • The nano-sized MOF-74 were successfully synthesized by a facile method. • The effect of open metal sites on the gas-sensitive properties was investigated. • The Mg-MOF-74 gas sensor has good sensing performance for CO 2 at room temperature. • The exceptional uptake of CO 2 by Mg-O bonds enhances the sensing performance. [ABSTRACT FROM AUTHOR]

Abstract: Photoluminescent (PL) sensing is a common method for the detection of trace nitroaromatic explosive 2,4,6-trinitrotoluene (TNT). In this work, a red emissive phosphorescent iridium(III) complex was synthesized and used for TNT sensing. The obtained iridium(III) complex is highly emissive, and the PL response of the complex in solution toward TNT was researched by examining the steady state emission intensity to the concentration of TNT. Efficient PL quenching was obtained when TNT was added to the detection solution. The long excited-state lifetime of obtained phosphorescent complex is beneficial for interaction with TNT molecules. In addition, suitable matching of the lowest unoccupied molecular orbital (LUMO) levels of the iridium(III) complex with TNT also benefits the electron transfer from the complex to TNT molecules and enhances the PL quenching. As a small molecular material, the obtained phosphorescent complex shows comparable PL quenching efficiency to TNT with reported conjugated polymers. [Copyright &y& Elsevier]

Abstract: A novel porous polymer was synthesized by Friedel-Crafts alkylation polymerization, and the structure and characteristic of the polymer was determined by Fourier transform infrared spectroscopy (FTIR), N2 adsorption/desorption analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The obtained polymer was used to prepare humidity sensitive composites by loading LiCl. Compared with the pure polymer, LiCl-loaded composites showed improved humidity sensing properties and the 4wt% LiCl/polymer sensor showed the best sensing properties. The impedance of the optimum sensor changed near four orders of magnitude over the whole humidity range, with a good linearity, little hysteresis, good stability and short response and recovery times (2 and 7s, respectively). The complex impedance plots were used to research conductive mechanism. [Copyright &y& Elsevier]

Abstract: Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3 (BNT–BKT) powder is synthesized by a metal-organic decomposition method and characterized by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). A humidity sensor, which is consisted of five pairs of Ag–Pd interdigitated electrodes and an Al2O3 ceramic substrate, is fabricated by spin-coating the BNT–BKT powder on the substrate. Good humidity sensing properties such as high response value, short response and recovery times, and small hysteresis are observed in the sensing measurement. The impedance changes more than four orders of magnitude within the whole humidity range from 11% to 95% relative humidity (RH) at 100Hz. The response time and recovery time are about 20 and 60s, respectively. The maximum hysteresis is around 4% RH. The results indicate that BNT–BKT powder is of potential applications for fabricating high performance humidity sensors. [Copyright &y& Elsevier]

Abstract: Hierarchical SnO2 microspheres were synthesized by a hydrothermal method at 140°C using stannic chloride hydrate and sodium hydroxide as starting materials. The individual hierarchical SnO2 microsphere ranged from 700 to 900nm in diameter. After these microspheres were heated at 600°C for 2h, the spheres were cross-linked into clusters by short SnO2 nanorods as revealed by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Most importantly, SnO2 hierarchical microsphere sensor exhibits excellent selectivity and fast response to ethanol. Response and recovery times were 0.6s and 11s when the sensor was exposed to 50ppm ethanol at an operating temperature of 300°C. Thus, hierarchical structures play a significant role in the field of gas sensing. [Copyright &y& Elsevier]

Abstract: An ordered mesoporous In2O3 material with crystalline walls has been synthesized through the nanocasting method. It was investigated as an impedance-type humidity sensor for the detection of water vapor. A nanostructured In2O3 matrix has been obtained by the hard template route from the KIT-6 silica template. The crystalline In2O3 belonged to the Ia3d space group, and its structure was characterized by X-ray diffraction (XRD), N2 adsorption–adsorption, and transmission electron microscopy (TEM). The sensor based on mesoporous In2O3 showed excellent performance in terms of humidity changes and favorable stability. Through the analysis of its semiconductor characteristics, Kelvin equation, and complex impedance, we found that the 3D mesoporous structure contributes greatly to the improvement of humidity-sensitive properties. A possible mechanism was established to explain the excellent performance of the mesoporous In2O3-made humidity-sensing device. The evaluation of its electrical characterization and the establishment of its sensing mechanism shows that mesoporous In2O3 is a good candidate for developing humidity sensors. It has potential applications in the chemical-sensing field. [Copyright &y& Elsevier]

Abstract: As a novel humidity sensing material, Bi0.5K0.5TiO3 (BKT) powder was synthesized by a chemical solution method, and the humidity sensor was composed of BKT powder, Ag–Pd interdigitated electrodes and Al2O3 ceramic substrate. The humidity sensing properties were measured by using LCR analyzer. The experimental results show that the impedance changes about four orders of magnitude within the whole humidity range from 11% to 95% relative humidity (RH). At the frequency of 100Hz, the response time and recovery time are about 12 and 25s, and the maximum hysteresis is around 3% RH. They indicate that BKT powder has good humidity sensing properties, and it is of the potential applications for fabricating high performance humidity sensors. [Copyright &y& Elsevier]

Abstract: Pure and KCl-doped TiO2 nanofibers with different crystallographic structures are synthesized by electrospinning and calcination techniques, and their humidity sensing properties are investigated. KCl-doped TiO2 nanofibers with mixed anatase and rutile structures show the highest sensing performance. The impedance of this sample linearly decreases by more than four orders of magnitude with increasing relative humidity from 11% to 95% on a semilogarithmic scale. Both the response time and recovery time are about 3s. Excellent selectivity, prominent stability, and good reproducibility are also observed based on this sample. The high humidity sensing performance is explained in terms of the ionic contribution of KCl doping and dissociating promotion of mixed crystallographic structure. [Copyright &y& Elsevier]

Abstract: A fast, simple and reliable humidity sensor based on ion-doped SnO2 nanofibers has been developed by screen-printing on ceramic substrates with Ag–Pd interdigital electrodes in this research. The optimized results show that the impedance of the sensor decreases by more than five orders of magnitude with increasing relative humidity (RH) from 11% to 95%, the response and recovery time of the sensor are about 5 and 6s, respectively. Compared to the previous electrical conductivity-based SnO2 humidity sensors, the proposed sensor exhibits higher humidity sensitivity, more rapid response and recovery, smaller hysteresis, and better linearity and stability. [Copyright &y& Elsevier]

Abstract: Nanocrystalline LaCo0.3Fe0.7O3 was prepared by a citrate method. A resistive-type humidity sensor built on a ceramic substrate with Ag–Pd interdigitated electrodes was fabricated. The humidity sensing mechanism of the ceramic materials was assessed by analyses of DC and AC test results. Electrons and ions contributing to the conduction were confirmed by DC analysis. Electrons, H+, and K+ ions were each found to dominate at different humidity ranges for the humidity sensor based on LaCo0.3Fe0.7O3 materials. The dielectric loss property was studied in detail and the humidity sensing mechanism was discussed. The results indicated that not only proton or ion but also molecular polarization contributes to the conduction in 11–95% RH. [Copyright &y& Elsevier]

Abstract: This paper reports the study of humidity sensitive properties of pure and K+-doped LaCo0.3Fe0.7O3 thick film sensors. The humidity sensitive thick films were prepared by screen printing on ceramic substrates with interdigitated electrodes. It was found that pure LaCo0.3Fe0.7O3 showed sensitivity only in the relative humidity (RH) above 54%. The addition of alkali ion K+ was beneficial for improving the humidity sensitive properties of the sample. The best results were obtained for 2wt.% K2CO3-doped sample, which exhibited excellent linearity in the whole humidity range of 11–95%. A resistance variation of about four orders of magnitude, narrow hysteresis loop, and rapid response were obtained. The effects of dopant content on humidity sensitivity were also discussed. [Copyright &y& Elsevier]

Low humidity detection has been a great challenge in the field of humidity sensing. In recent years, polyelectrolytes have been proven to be promising materials for highly sensitive humidity sensors. In this work, cross-linked poly(ionic liquid)s (PILs) were prepared for low humidity detection. The strong hydrophilicity of PILs contributes to humidity sensing while the cross-linked structure guarantees the stability of the sensing film. The optimized sensor shows high response (48.0) in 5–35% RH, with short response/recovery time (1 s/10 s) and a small humidity hysteresis (0.2% RH). The possible sensing mechanism of the sensors was discussed. • Cross-linked poly(ionic liquid)s were used as low humidity sensitive materials. • The optimized sensor shows high response in 5–35% RH. • The optimized sensor has short recovery time and small humidity hysteresis. • The introduction of glycerol could reduce the impedance of the sensor. [ABSTRACT FROM AUTHOR]

Development of room-temperature gas sensors with excellent sensing performances for detection of dimethyl methylphosphonate (DMMP, a simulant of organophosphate) has aroused considerable attention. However, the weak adsorption ability of sensing materials toward DMMP molecules causes poor sensing performances. In this work, a multisite strategy for enhancing sensitivity was proposed by introduction of both N-doped carbon nanoparticles (N-CNPs) and copper ions onto the surface of rGO (designated as GNC). Impressively, optimal GNC-3 exhibits the response value of 10.2% toward 100 ppm DMMP, which is 5.3 times higher than that of pristine rGO. This mechanism for multisite-assisted enhanced DMMP sensing performances was investigated by combined characterizations of Fourier-transform infrared spectra, UV-Vis spectra, photoluminescence spectra and Mott-Schottky curves. Besides the construction of multisite, improvement of hydrogen bonding interactions between DMMP molecules and N-CNPs in hybrids assisted by copper ions, and decrease of conduction band potential by formation of ternary hybrids also contribute to enhance the sensitivity of DMMP sensors. These findings not only provided new insights into designing high-performance room-temperature gas sensors, but also promoted the fundamental research on sensing mechanism for room-temperature gas sensors. • A multisite strategy for enhancing sensitivity was proposed by constructing three kinds of DMMP adsorption active sites. • GNC hybrids demonstrate 5.3-fold improvement in response to 100 ppm DMMP compared to pristine rGO hybrids. • This mechanism for multisite-assisted enhanced DMMP sensing performances was confirmed by the spectroscopy. [ABSTRACT FROM AUTHOR]

Mpox (formerly referred as Monkeypox) outbreak has been declared a Public Health Emergency of International Concern. However, traditional polymerase chain reaction (PCR) diagnostic technology is not ideal for on-site applications. To conduct the sample-to-result Mpox viral particles detection outside the laboratories, we developed an easy-to-operate palm-size pouch, termed Mpox At-home Self-Test and point-of-caRe Pouch (MASTR Pouch). In this MASTR Pouch, the fast and accurate visualization was achieved by incorporating recombinase polymerase amplification (RPA) with clustered regularly interspaced short palindromic repeat (CRISPR)/Cas12a system. From viral particle lysis to naked eye readout, MASTR Pouch required only four simple steps to accomplish the analysis process within 35 min. Fifty-three Mpox pseudo-viral particles in exudate (10.6 particles/μL) were able to be detected. To verify the practicability, 104 mock Mpox clinical exudate specimens were tested. The clinical sensitivities were determined to be 91.7%− 95.8%. There was no false-positive result, validating the 100% clinical specificity. MASTR Pouch approaches the WHO's ASSURD criteria for point-of-care diagnostic, which will be beneficial for mitigating Mpox's global spread. The versatility potential of MASTR Pouch could further revolutionize infection diagnosis. [Display omitted] • A palm-size tool pouch was developed for Mpox on-site detection, termed MASTR Pouch. • A complete sample-to-result analysis can be accomplished in 35 min. • A 3D-printed mini automatic readout device was fabricated to simplify operations. • For sensitive and specific detection, RPA and CRISPR technologies were incorporated. • Clinical sensitivity was determined to be 91.7–95.8%, with 100% clinical specificity. [ABSTRACT FROM AUTHOR]

• Surface loading with oxidized platinum clusters drastically changes WO 3 gas sensors. • Experimental proof of Fermi level pinning mechanism. • The homogeneous dispersion of the surface loading controls the strength of the effect. In order to increase the stability of semiconducting metal oxides and to modify their gas sensing behavior, noble metal surface additives are often used. In literature, there are numerous papers that report the drastic effect of platinum loading on the sensing characteristics of WO 3. Here a thorough characterization of two different platinum loaded WO 3 samples was coupled with operando diffuse reflectance infrared Fourier transform spectroscopy. Based on the results, it was possible to identify the mechanism which is responsible for the changed sensor behavior. It was found that in this case a Fermi level pinning mechanism dominates. In addition, by comparing the two samples prepared with different impregnation methods, it could be shown that the dispersion on the surface of the material determines how strong the effect of the loading is. This work complements recent findings in which the Fermi level pinning mechanism was identified for Rh 2 O 3 loaded samples (SnO 2 , In 2 O 3 and WO 3) and Pt-loadings on SnO 2. The results help in understanding reports already found in literature and provides useful inputs for the intentional tuning of sensor characteristics. [ABSTRACT FROM AUTHOR]

• SnO 2 samples with controllable oxygen vacancy concentration have been developed by organometallic chemistry-assisted strategy. • The SnO 2 samples with high oxygen vacancy concentration exhibit excellent sensing performances for detection of acetone. • This work paves a new avenue for fabricating metal oxides with controllable oxygen vacancies for various applications. Oxygen vacancies play a critical role in the sensing performances for metal oxides-based chemiresistor-type sensors. Developing facile and effective strategies for fabricating metal oxides with controllable oxygen vacancies is in great demand but remains paramount challenge. Herein, a novel organometallic chemistry-assisted method is reported to prepare SnO 2 samples containing controllable surface oxygen vacancies. This new approach combines modification of SnO 2 with dimethyltin dichloride, and subsequently thermal treatment in air atmosphere. Taking commercial SnO 2 as an example, it is found that the optimized SnO 2 sample possesses highest surface oxygen vacancy concentration of 37.13%. Owing to increased oxygen vacancy concentration, the resulting SnO 2 sample (SnO 2 -Sn-0.5) presents excellent sensing performances for detection of acetone, including high response, fast response and recovery rate, good selectivity, compared with pristine SnO 2 with low oxygen vacancy concentration (28.66%). This work paves a new avenue for fabricating metal oxides with controllable oxygen vacancies for various applications of sensing, energy storage, and so on. [ABSTRACT FROM AUTHOR]