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The sliver (Ag) modified zinc oxide (ZnO) nanorods were successfully obtained with a simplified and environmentally friendly solvothermal method. Materials characterization indicated that the metallic Ag was located on the outside of ZnO nanorods after annealing. In comparison with ZnO nanorods, Ag modified ZnO (Ag–ZnO) nanorods exhibited a considerably enhanced response to C2H2. The response of the 3 at% Ag–ZnO based sensor operating at 175 °C is 539 (Ra/Rg), which is the highest value among all the sensors in detecting 100 ppm C2H2. The Ag–ZnO based sensors exhibited fast response speed, lower operation temperature and higher selectivity.

In this study, long-term ordered mesoporous In2O3 was prepared by replicating the structure of SBA-15. Samples prepared with Au-doped and Au-loaded mesoporous In2O3 were aimed at studying the influence of the modification method (doping and loading) on acetone sensing properties, and the corresponding sensing mechanism was discussed in detail. The structural and chemical characterizations of the samples were carried out by XRD, BET, TEM and XPS. The best sensing performance was achieved in the In2O3 Au-doped (IO-Au-D) sample; the response reached 19.01 to 100 ppm of acetone at 250 °C. The effects of the polarity and bond dissociation energy of acetone benefited the improved selectivity among volatile organic compounds (VOCs). The excellent gas sensing performance not only depended on the ordered mesoporous structure of the In2O3 matrix, but could also be attributed to the “electronic sensitization” and “chemical sensitization” of Au. Furthermore, uniformly dispersed Au doping depressed the lattice growth and enlarged the depletion region, playing a great role in enhancing the sensing performance. However, the limited surface decorated region of the Au-loaded sample suppressed the enhancement of the acetone sensing performance. This work significantly reveals that the sensing performance can be controlled by different methods of introducing additives.

In this work, the mixed-potential-type gas sensor based on NASICON solid electrolyte and MMnO3 (M: Gd, Sm, La) sensing electrode (SE) was developed and fabricated aiming at the low-concentration triethylamine (TEA) detection. The research point of the fabricated sensors mainly concentrated on the effect of different A-site elements in MMnO3-SE (M: Gd, Sm, La) on TEA sensing characteristics. The result reveals that the highest response value (−217.5 mV) towards 50 ppm TEA was observed for the device utilizing SmMnO3-SE. The highest electrochemical catalytic activity of the fabricated sensors attached with SmMnO3-SE was further authenticated by the polarization curve measurement. Moreover, the response value displays a segmentally linear relationship with TEA concentrations and the logarithm of TEA concentrations, and the slopes are −49 mV/ppm and −105 mV/decade at the ranges of 0.05–0.5 ppm and 1–50 ppm TEA, respectively. The device could also produce an acceptable response value (−5.6 mV) towards 50 ppb TEA. Moreover, it also displays good repeatability and selectivity, along with steady response signals towards 5 ppm and 50 ppm TEA for 15 days and 1 ppm TEA at different humidity levels.

Here, the porous SnO2/Zn2SnO4 sensing electrode material prepared by facile hydrothermal method was applied to fabricate the mixed-potential-type gas sensor based on yttrium-stabilized zirconia (8 mol% Y2O3-doped ZrO2) solid electrolyte plane substrate for effectively detecting acetylene (C2H2) at 700 °C. In terms of the sensing characteristics of the C2H2 gas sensor, the response value toward 100 ppm C2H2 was -82.3 mV, and the detection limit of C2H2 was lowered to 500 ppb. The ΔV of the sensing device varied piecewise linearly with the logarithm of C2H2 concentration gradient range of 0.5–2 and 5–1000 ppm, with sensitivities of -12 and −56 mV/decade, respectively. In addition, the sensor exhibited good humidity stability, long stability, and selectivity to C2H2 at 700 °C. More interestingly, after high-temperature measurement (700 °C) for 20 consecutive days, the sensor continued to present good response transients, sensitivity, and reproducibility to C2H2. Furthermore, the sensing mechanism of the mixed-potential-type sensor was verified by measuring the polarization curves and complex impedance curves.

Analyzing volatile organic compounds in exhaled breath is a noninvasive method that has potential for disease assessment. Herein, acetone sensors based on porous In2O3 flowers are synthesized through a facile solvothermal method. Structural characterizations demonstrate that the three-dimensional In2O3 flower-like structure comprises porous ultrathin two-dimensional nanosheets. The structural porosity results in a large specific surface area and fast gas diffusion, which can improve the gas sensing performance. The crystalline structure of the prepared hierarchical flower-like In2O3 is a mixed phase of rhombohedral and cubic phases according to the XRD patterns. The sensing properties of the cubic-rhombohedral In2O3 (bcc-rh-In2O3) are also investigated. Compared with commercial-In2O3-based sensor, the bcc-rh-In2O3 sensor exhibits 2.9 times higher response (Ra/Rg = 12) and shorter response time (ca. 2 s) toward 50 ppm acetone. The bcc-rh-In2O3 sensor has a low detection limit of 10 ppb in atmosphere and can easily distinguish acetone from ethanol. The extraordinary properties of the bcc-rh-In2O3 sensor are mainly attributed to the n-n heterojunction between rh-In2O3 and bcc-In2O3.

Nitrogen dioxide (NO2) sensing is important in terms of the agriculture development and healthcare monitoring. However, relatively poor recovery kinetics at a low temperature is still a problem for NO2 sensing and remains to be solved. In this work, we combine the excellent sensing properties and gas adsorption capacity of two- dimensional (2D) tin disulfide (SnS2) hexagonal nanoflakes with the beneficial electrical properties of graphene to build high-performance NO2 sensors by a simple and low-cost method. We found that the sensors based on the quasi-2D rGO-SnS2 hybrids exhibited exclusive selectivity towards NO2. The presence of rGO in hybrids decreased optimal working temperature effectively. Sensors based on 0.8% rGO-SnS2 (G-SS) show almost an order of magnitude increase in response (32) compared to pristine SnS2 (3.5). Especially, the response/recovery time is 50/48 s, respectively, demonstrating the favorable response/recovery kinetics of sensors. Such significant features can be attributed to the synergistic effect of the quasi-2D G-SS hybrid and the local heterojunctions in the interface between SnS2 and rGO.

In this work, we designed a multi-signal readout platform for sensitive monitoring carbamate pesticide based on gold nanoclusters-anchored MnO2 (AuNCs-MnO2) nanocomposite. The fluorescence of AuNCs was quenched by MnO2 via Forster resonance energy transfer effect. In the presence of acetylcholinesterase and choline oxidase, the substrate acetylcholine (ACh) is catalyzed to generate H2O2 that can specially induce the decomposition of MnO2, coupling with change of color and recovery of fluorescence. Carbamate pesticide as an inhibitor of acetylcholinesterase prevent the generation of H2O2, further block the breakage of MnO2, which caused the fluorescence switch. Such color and fluorescence respond can be utilized for highly sensitive carbamate pesticide identification and quantification. Under the optimum conditions, the dual-output assay possessed good sensitivity for rapid detection of carbaryl (model analyte) with a detection limit of 0.125 μg L−1. The fluorometric/colorimetric bimodal platform showed good stability and anti-interference ability comparing to that of others enzyme-based sensor.

Spinel-type oxides have attracted a broad interest in sensing materials research owing to their high catalytic activity and flexibly tunable chemical properties. Here, we report the synthesis of superfine porous NiFe2O4 microspheres by one-step solvothermal approach, in order to fabricate ultra-sensitive acetone sensors for real-time monitoring, relying on the high catalytic activity of NiFe2O4 and effective mass transfer property of porous microspheres structure. The porous NiFe2O4 sensors displayed high selectivity to acetone against other interfering gases, giving a high sensitivity (27.4), fast response time (2 s) towards 100 ppm acetone and low detection limit (200 ppb) at 250 °C. This paper proposed a general approach for fabricating highly sensitive gas sensor based on spinel-type oxides in monitoring volatile organic pollutants.

Due to the ultrathin structure, functional groups and excellent physical-chemical properties, Two-dimensional (2D) MXenes shows great potential for biomedical applications. Here, we prepared TiO2 inverse opal photonic crystals (IOPCs)/Ti3C2 quantum dots (QDs) composite film through the self-assembly method. Because of the widen absorption band of Ti3C2 QDs, TiO2 IOPCs/Ti3C2 composite film could have photocurrent effect from 280 to 900 nm, and the internal power conversion efficiency could reach to 26% at 350 nm. Owing to favourable energy levels, Ti3C2 QDs were formed the Type-II Heterojunction with TiO2, thus enabling directional transport charge for efficient charge separation. The Type-II Heterojunction photoelectrochemical (PEC) sensor consisting of TiO2/Ti3C2/Nafion electrode exhibited high stability, sensitivity, and selectivity for the detection of glutathione in buffered solution and cell extracts. Thus, the TiO2/Ti3C2/Nafion composite film provides a promising green electrode for sensitive PEC detection, which could realize some potential applications for disease early precaution and diagnose.

The development of room temperature NH3 sensors with flexible substrate can fulfill the future demands of high NH3 sensing performance under the premise of lower energy consumption and safety circumstance in wearable electronics field. The polyaniline and a serious of WO3 hollow spheres@ polyaniline hybrid were successfully synthesized via a combined route of simple hydrothermal method and in-situ chemical oxidative polymerization. The room temperature NH3 sensors were fabricated by assembling WO3 hollow spheres@ polyaniline hybrid on flexible polyethylene terephthalate (PET) films. The NH3 sensing properties of the fabricated devices demonstrated that the device utilizing 10 mol% WO3 hollow spheres@ polyaniline (PAWHs 10) hybrid exhibited the best sensing performance to 100 ppm NH3 (response value at 25) at 20 °C. Moreover, the PAWHs10 hybrid sensor also achieved excellent NH3 selectivity, ppb-level detection limit (1.67–500 ppb NH3), and good response/recovery rates (136 s/130 s) at 20 °C. The enhanced sensing performance of incorporation of polyaniline and WO3 hollow spheres was mainly attributed to the hollow structure of WO3 and the formation of p-n heterojunction between PANI and WO3 hollow spheres.

The preparation of size-controlled materials has shown remarkable potential with regard to their reinforced sensing performance. Herein, we report a facile solvothermal synthesis of dispersed WO3 nanoparticles with porous nanostructure caused by particle assembly. Various characterizations of the as-obtained samples were analyzed. Furthermore, the gas sensing properties of WO3 samples on volatile organic compounds were investigated. The results reveal that the gas sensors based on synthesized WO3 samples with suitable hexamethylenetetramine (HMT) amount and sintering treatment exhibit the highest response (132.0) towards 100 ppm toluene at the optimal working temperature of 225 °C with fast response/recovery time (2 s/6 s), which is attributed to the porous structure of the sensing film and the high resistance caused by well crystallization and grain boundary among the nanoparticles.

Herein, ZnO nanoflowers without any doping and modification were synthesized by hydrothermal method. The room temperature nitrogen dioxide sensor with high sensitivity, selectivity and long-term stability was prepared by combining ZnO nanoflowers with interdigital electrodes. The average diameter of the ZnO nanoflowers was about 0.9–1 μm. In addition, the gas sensing properties of the nitrogen dioxide gas sensors at room temperature were investigated. Compared with other ZnO sensors, the sensor based on the ZnO nanoflowers revealed outstanding sensing response to NO2, giving a response of 0.05–6 ppm. The sensor exhibited a lower detection limit of 0.05 ppm and had an ultra-high response to 1 ppm NO2 at room temperature. The response was only decreased by 16% of the initial value after 40 days of measurement. The Schottky barrier could be the key factor for the excellent NO2 sensing properties, which was formed between the Au electrodes and ZnO nanoflowers.

In this work, we developed a high-response mixed potential type yttria stabilized zirconia (YSZ) based planar gas sensing device utilizing CoTiO3 sensing electrode (SE) prepared via a facile sol-gel method to sensitively detect NO2 at an operating temperature of 650 °C. The present gas sensor coupled with 1200 ℃-sintered CoTiO3-SE exhibited high responses to NO2 in the concentration range of 500 ppb-200 ppm, as the value of the response was 130 mV to 100 ppm NO2. A linear relationship was observed between the ΔV of the sensor and the logarithmic scale of the NO2 concentration, which the sensitivity was 60 mV/decade. In addition, the sensor also displayed good reproducibility, the maximum response change of 5.4% and 16% in different coexisting gases and 98% high relative humidity, and response decrease of 6.9%–100 ppm NO2 after high temperature test for 20 days. Moreover, the sensing mechanism of sensor, which was based on mixed potential model, was confirmed using polarization curve measurements in air and different concentrations of NO2.

The polycrystalline perovskite SmMO3 (M = Al, Cr, Co) sensing electrode was synthesized by the sol-gel method, and the NASICON-based sensors attached with SmMO3 were designed and fabricated for triethylamine (TEA) detection. This study investigated the effects of different M3+ ions in SmMO3 and various calcination temperatures of the sensing material on TEA sensing properties. The results reveal that compared with the other devices, the sensor attached with SmCrO3 sintered at 800 °C displays a higher response value (−108.2 mV) towards 100 ppm TEA, and that the sensor even detects 2 ppm TEA with an acceptable value (−6 mV). The sensor utilizing SmCrO3 (800 °C) also exhibits pretty swift response and recovery times of 2 and 22 s, respectively, toward 20 ppm TEA. A linear relationship exists between the response values and the logarithm of TEA concentrations, and the sensitivity is −64 mV/decade. At the same time, good repeatability, selectivity, and long-term stability of the sensor are also observed. Furthermore, the sensing mechanism related to the mixed potential model is demonstrated by the polarization curve measurements.

Highly selective oxidizing gas sensors are of great importance for environmental pollution monitoring. In this work, a hybrid material containing Zn2SnO4 nanoparticles (NPs) and immobilized reduced graphene oxide (Zn2SnO4-RGO) was developed as a high performance gas sensing material for the detection of ppb-levels of oxidizing gases (NO2 and O3). The structural, morphological and compositional properties of the Zn2SnO4-RGO hybrids were systematically characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), which demonstrated the successful anchoring of Zn2SnO4 NPs on RGO nanosheets. The obtained Zn2SnO4-RGO hybrids exhibited outstanding sensing performance for detecting oxidizing gases (NO2 and O3) with very low cross sensitivities to reducing gases, such as C2H5OH, CH3COCH3 and CO. The Zn2SnO4-RGO based sensors exhibited high response values of up to 3.50 for 500 ppb NO2, which is higher than that for detection of 500 ppb O3 (1.78) at 30 °C under 50% relative humidity (RH). Moreover, the NO2 sensing performances of Zn2SnO4-RGO-based sensors were investigated under various RH. In all cases, the sensors based on RGO and Zn2SnO4-RGO hybrids presented p-type behavior. The sensors based on Zn2SnO4-RGO hybrids also exhibited high response values of up to 3.62 for 1 ppm NO2 at 50 °C in 80% RH, which is much higher than that of pure RGO (1.31). The excellent sensing performances are mainly ascribed to the synergetic effect of Zn2SnO4 NPs and RGO. Furthermore, the surface reaction between Zn2SnO4-RGO hybrids and NO2 can be concluded from Operando diffuse reflectance infrared Fourier transformed spectroscopy (Operando DRIFT).

Xylene is a harmful and hazardous volatile organic compound (VOC) indoors, thus selective and sensitive detection for subppm-level xylene is crucial, however a remained challenge. In this work, p-NiO/p-NiCr2O4 nanocomposites were successfully synthesized through a simple hydrothermal route and used as sensing materials. In the comparative gas sensing test, the sensor fabricated from NiO/NiCr2O4 (Cr/Ni = 25 at%) nanocomposite exhibited the highest response (66.2–100 ppm) to xylene, which was 37.2 times higher than that of the pure NiO sensor. Moreover, the NiO/NiCr2O4 nanocomposite gas sensor showed not only superior xylene selectivity with low cross-responses to interfering gases such as ethanol (Sxylene/Sethanol = 11.8) and acetone (Sxylene/Sacetone = 10.2) but also ppb-level detection limit (1.2–50 ppb xylene) at 225 °C. The synergistic catalytic effect between NiO and NiCr2O4, optimized structural parameters and marked resistive variation due to the formation of nanoscale p-p heterojunctions were regarded as the main reasons for the ultrasensitive and selective xylene detection.

A facile hydrothermal process is used to prepare hybrid graphene-MoO3 particles to be used as hole transport layers (HTLs) in organic solar cells (OSCs). The OSCs with active layer donor/acceptor combinations of Poly[N-9″-hepta-decanyl-2,7-carbazolealt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and fullerene derivative [6,6]-phenyl-C71-butyric acid methylester (PC71BM) exhibit an enhanced power conversion efficiency (PCE) of 7.07%, an increase by 19% with the hybrid HTL compared to those devices with only MoO3 HTLs. Through investigating the optical and electrical properties of the devices, we found that the superior PCE originates from an enhanced hole transport property resulting from the extraction capabilities of G-MoO3. Comparing with thermal evaporated MoO3, the G-MoO3 exhibits a higher optical transmittance, improved electrical conductivity and enhanced hole mobility. Moreover, the work function of the hybrid G-MoO3 was close to the highest occupied molecular orbital (HOMO) level of PCDTBT, which reduced the energy barrier for the carriers and was suited for hole transport.

In this study, Sr2MMoO6 (M: Fe, Mg and Ni) were prepared successfully by sol-gel method and used as sensing electrode materials for CeO2-based mixed potential type acetone sensors. Results indicated that the sensor attached with Sr2FeMoO6 sensing electrode exhibited the best sensing characteristic to acetone among all the other sensors in this study, which showed the maximum response value of −147 mV to 100 ppm acetone at 590 °C and the typical 90% response time was about 13 s. The sensitivity of the sensor using Sr2FeMoO6-SE was the largest towards acetone at the range of 5–200 ppm, and the slope of which was −100 mV/decade. Moreover, the sensor could also detect 0.5 ppm acetone with an acceptable value. In addition, the device also exhibited excellent repeatability and humidity stability, good selectivity and stability during 25 days. Besides, the sensing mechanism of the mixed potential type acetone sensor was explained clearly in this study.

There is an urgent demand for on-site detection of biomarker, particularly in clinical diagnosis and therapeutics applications. Herein, we designed a convenient and sensitive fluorescence nanosensor for point-of-care determination of trypsin (TRY). The fluorescence intensity of gold nanoclusters (AuNCs) can be quenched by cytochrome c (Cyt c) via electron transfer mechanism. TRY specifically catalyzes the hydrolysis of Cyt c to produce small peptide fragments, inducing the significant fluorescence recovery. As a result, the AuNCs-based system processed a sensitive and selective response to TRY with the range 0.2–100 μg mL−1, accompanying a detection limit of 0.08 μg mL−1. Significantly, the sensing assay can be used to construct test strips for rapid and visual recognition of TRY. Combining with smartphone and ImageJ software, we further developed an image processing algorithm for quantitative detection of TRY with highly promising, which validated the potential point-of-care application.

In this work, hierarchucal porous SnO2/Zn2SnO4 nanospheres were succesfully prepared via a facile one-step hydrothermal method with subsequent calcination process. Scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were employed in order to investigate the structural and morphological properties of the as-prepared composites. The results showed that the SnO2/Zn2SnO4 composites were cpmposed of many porous nanospheres with a uniform diameter of about 500 nm. Moreover, the as-prepared products were used as sensing material for the fabrication of gas sensor. The sensing performance of the sensor was systematically evaluated, and the sensor exhibited excellent ethanol-sensing property. The optimum operating temperature was 250 °C with a reponse of 30.5 toward 100 ppm ethanol. Also, the sensor showed good selectivity, stability and a low detection limit of 0.5 ppm (response 1.4). The good sensing performance of SnO2/Zn2SnO4 nanospheres can be attibuted to the porous structure as well as the heterojunction formed between SnO2 and ZnSn2O4.

In this work, a novel branched heterostructural composite composed of nanorods ZnO backbone and SnO2 branches was prepared via a facile one-step hydrothermal method. The morphology, structure and component of the SnO2/ZnO composite was characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and elemental mapping analysis. The evolution process of the SnO2/ZnO composite was observed by SEM that the SnO2 branches gradually grow on ZnO backbones. The composite with novel heterostructure was applied as the sensing material for the fabrication of gas sensor, and their gas sensing properties were tested for response to various gases. Compared to pure ZnO gas sensors the branched SnO2/ZnO gas sensor exhibited enhanced gas sensing properties toward ethanol, giving a response of 18.1–100 ppm.