Large-lateral-area SnO2 nanosheets with a loose structure for high-performance acetone sensor at the ppt level.

Gas sensors with high sensitivity and high selectivity are required in practical applications to distinguish between target molecules in the detection of volatile organic compounds, real-time security alerts, and clinical diagnostics. Semiconducting tin oxide (SnO2) is highly regarded as a gas-sensing material due to its exceptional responsiveness to changes in gaseous environments and outstanding chemical stability. Herein, we successfully synthesized a large-lateral-area SnO2 nanosheet with a loose structure as a gas sensing material by a one-step facile aqueous solution process without a surfactant or template. The SnO2 sensor exhibited a remarkable sensitivity (Ra/Rg = 1.33) at 40 ppt for acetone, with a theoretical limit of detection of 1.37 ppt, which is the lowest among metal oxide semiconductor-based gas sensors. The anti-interference ability of acetone was higher than those of pristine SnO2 and commercial sensors. These sensors also demonstrated perfect reproducibility and long-term stability of 100 days. The ultrasensitive response of the SnO2 nanosheets toward acetone was attributed to the specific loose large lateral area structure, small grain size, and metastable (101) crystal facets. Considering these advantages, SnO2 nanosheets with larger lateral area sensors have great potential for the detection and monitoring of acetone.

Facet-Controlled Synthesis of CeO2 Nanoparticles for High-Performance CeO2 Nanoparticle/SnO2 Nanosheet Hybrid Gas Sensors.

CeO2 nanocubes with metastable {100} facets and CeO2 nanooctahedrons with the most stable {111} facets are herein fabricated by controlling the morphology and facets of CeO2 nanoparticles. SnO2 nanosheet-based assembled films coated with these CeO2 nanocubes or CeO2 nanooctahedrons yield {100} CeO2 nanocubes/SnO2 nanosheets and {111} CeO2 nanooctahedron/SnO2 nanosheet hybrid gas sensors, respectively. The hybrid sensors with CeO2 nanoparticles exhibited enhanced sensing responses to numerous chemical species relative to a pristine SnO2 nanosheet gas sensor, including acetone, hydrogen, ethanol, ammonia, acetaldehyde, and allyl mercaptan. In particular, the responses of {100} CeO2 nanocubes/SnO2 nanosheets and {111} CeO2 nanooctahedron/SnO2 nanosheet gas sensors to acetone or allyl mercaptan were 6.8 and 10.3 times higher, respectively, than that of the pristine SnO2 nanosheet gas sensor. Furthermore, the sensor response to ammonia was 2.5 times higher than that of a commercial volatile organic compound (VOC) gas sensor (TGS2602, Figaro Engineering Inc.). The CeO2 nanocube-based sensor with exposed metastable {100} facets promotes the adsorption and oxidation of VOCs owing to the higher surface energy of the metastable {100} facets and therefore exhibits a higher sensing performance than the CeO2 nanooctahedron-based sensor with an exposed {111} facet. The developed sensors show excellent potential for the detection of gas markers in human breath and perspiration for disease diagnosis.

Nanosheet-type tin oxide gas sensor array for mental stress monitoring.

Mental stress management has become significantly important because excessive and sustained mental stress can damage human health. In recent years, various biomarkers associated with mental stress have been identified. One such biomarker is allyl mercaptan. A nanosheet-type tin oxide exhibited high gas selectivity for allyl mercaptan; thus, in this study, a sensor array comprising nanosheet-type tin oxide gas sensors was fabricated to detecting allyl mercaptan. Supervised learning algorithms were use to build gas classification models based on the principal component analysis of the sensor signal responses from the sensor array. The comprehensive data provided by the classification models can be used to forecast allyl mercaptan with high accuracy.

Highly Sensitive and Selective Gas Sensors Based on NiO/MnO2 @NiO Nanosheets to Detect Allyl Mercaptan Gas Released by Humans under Psychological Stress.

NiO nanosheets are synthesized in situ on gas sensor chips using a facile solvothermal method. These NiO nanosheets are then used as gas sensors to analyze allyl mercaptan (AM) gas, an exhaled biomarker of psychological stress. Additionally, MnO2 nanosheets are synthesized onto the surfaces of the NiO nanosheets to enhance the gas-sensing performance. The gas-sensing response of the NiO nanosheet sensor is higher than that of the MnO2 @NiO nanosheet sensor. The response value can reach 56.69, when the NiO nanosheet sensor detects 40 ppm AM gas. Interestingly, a faster response time (115 s) is obtained when the MnO2 @NiO nanosheet sensor is exposed to 40 ppm of AM gas. Moreover, the selectivity toward AM gas is about 17-37 times greater than those toward confounders. The mechanism of gas sensing and the factors contributing to the enhance gas response of the NiO and MnO2 @NiO nanosheets are discussed. The products of AM gas oxidized by the gas sensor are identified by gas chromatography-mass spectrometry (GC/MS). AM gas detection is an unprecedented application for semiconductor metal oxides. From a broader perspective, the developed sensors represent a new platform for the identification and monitoring of gases released by humans under psychological stress, which is increasing in modern life.

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

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

Facile synthesis of ZnO nanobullets by solution plasma without chemical additives.

ZnO nano-bullets were synthesized using solution plasma from only Zn electrode in water without any chemical agents. In this sustainable synthesis system, the rapid quenching reaction at the interface between the plasma/liquid phases facilitates the fast formation of nano-sized materials. The coil-to-pin type electrode geometry, which overcomes the discharge interruption owing to the electrode gap broadening of the typical pin-to-pin type enables the synthesis of numerous nanomaterials through a stable discharge for 1 h. The as-prepared samples exhibited a high crystalline ZnO structure without post calcination, and the length and width were 71.8 and 29.1 nm, respectively. The main exposed facet of ZnO nano-bullets was the (100) crystal facet, but interestingly, the (101) facet was confirmed at the inclined surfaces in the edges. The (101) crystal facet has an asymmetric Zn and O atom arrangement, and it could result in a focused electron density area with relatively high reactivity. Therefore, ZnO nano-bullets are promising materials for applications in advanced technologies.

Catalyst-free Highly Sensitive SnO2 Nanosheet Gas Sensors for Parts per Billion-Level Detection of Acetone.

The development of a facile gas sensor for the ppb-level detection of acetone is required for realizing health diagnosis systems that utilize human breath. Controlling the crystal facet of a nanomaterial is an effective strategy to fabricate a high-response gas sensor without a novel metal catalyst. Herein, we successfully synthesized a SnO2 nanosheet structure, with mainly exposed (101) crystal facets, using a SnF2 aqueous solution at 90 °C. The SnO2 nanosheets obtained after various synthesis durations (2, 6, and 24 h) were investigated. The sample synthesized for 6 h (NS-6) exhibited a 10-fold higher response (Ra/Rg = 10.4) for 1 ppm of acetone compared to the other samples, where Ra and Rg are the electrical resistances under air and the target gas. Furthermore, NS-6 detected up to 200 ppb of acetone (response = 3). In this study, we attributed the high response (of low concentrations of acetone) to the (101) crystal facet, which is the main reaction surface. The (101) crystal facet allows the facile formation of a depletion layer due to the highly reactive Sn2+. Additionally, the acetone adsorption energy of the (101) crystal facet is relatively lower than that of other crystal facets. Owing to these factors, our pristine SnO2 nanosheet gas sensor exhibited significantly high sensitivity to ppb levels of acetone.

Effect of Crystal Defect on Gas Sensing Properties of Co3O4 Nanoparticles.

Crystal growth-controlled Co3O4 nanoparticles were prepared to examine gas sensing properties. A cube-like, an irregular shaped, and three kinds of raspberry-type structures were observed by morphology analysis. The raspberry-type structures have an expanded lattice volume with a large oxygen deficiency area, and the cube-like structure has a contracted lattice volume as compared to the irregular shaped structure. The raspberry-type structures exhibited a higher sensor signal response than the others. A relationship between sensor properties and crystal defect was investigated, and it was revealed that the gas selectivity to a high dipole moment value of a reducing gas molecule increased with increasing oxygen deficiency area of the Co3O4 nanoparticle. It was considered that the oxygen deficiency area acted as an important reaction site, which can be attributed to the selective reaction of the Co3O4 nanoparticle with gas molecules.

Catalytic Liquid-Phase Oxidation of Phenolic Compounds Using Ceria-Zirconia Based Catalysts.

Catalytic liquid-phase oxidation using a catalyst and oxygen gas (Catalytic wet air oxidation, CWAO) is one of the most promising technology to remove hazardous organic compounds in wastewater. Up to now, various heterogeneous catalysts have been reported for phenolic compounds decomposition. The CeO2-ZrO2 based catalysts have been recently studied, because CeO2-ZrO2 works as a promoter which supplies active oxygen species from inside the lattice to the active sites. Since it is difficult to dissolve oxygen gas into water, the use of the promoter is effective for realizing the high catalytic activity at moderate conditions. Also, CeO2-ZrO2 shows high resistance for the metal leaching during the catalytic reaction in the liquid-phase. This article reviews the studies of the catalytic liquid-phase oxidation of phenolic compounds using CeO2-ZrO2 based catalysts.

Fabrication and H2-Sensing Properties of SnO2 Nanosheet Gas Sensors.

Vertically formed and well-defined SnO2 nanosheets are easy to fabricate, involving only a single process that is performed under moderate conditions. In this study, two different sizes of a SnO2 nanosheet were concurrently formed on a Pt interdigitated electrode chip, with interconnections between the two. As the SnO2 nanosheets were grown over time, the interconnections became stronger. The ability of the fabricated SnO2 nanosheets to sense H2 gas was evaluated in terms of the variation in their resistance. The resistance of a SnO2 nanosheet decreased with the introduction of H2 gas and returned to its initial level after the H2 gas was replaced with air. Also, the response-recovery behaviors were improved as a result of the growth of the SnO2 nanosheets owing to the presence of many reaction sites and strong interconnections, which may provide multipassages for the electron transfer channel, leading to the acceleration of the reaction between the H2 gas and SnO2 nanosheets.

Catalytic liquid-phase oxidation of acetaldehyde to acetic acid over a Pt/CeO2-ZrO2-SnO2/γ-alumina catalyst.

Pt/CeO2-ZrO2-SnO2/γ-Al2O3 catalysts were prepared by co-precipitation and wet impregnation methods for catalytic oxidation of acetaldehyde to acetic acid in water. In the present catalysts, Pt and CeO2-ZrO2-SnO2 were successfully dispersed on the γ-Al2O3 support. Dependences of platinum content and reaction time on the selective oxidation of acetaldehyde to acetic acid were investigated to optimize the reaction conditions for obtaining both high acetaldehyde conversion and highest selectivity to acetic acid. Among the catalysts, a Pt(6.4wt.%)/Ce0.68Zr0.17Sn0.15O2.0(16wt.%)/γ-Al2O3 catalyst showed the highest acetaldehyde oxidation activity. On this catalyst, acetaldehyde was completely oxidized after the reaction at 0°C for 8hr, and the selectivity to acetic acid reached to 95% and higher after the reaction for 4hr and longer.

Functional characterization of OsRacB GTPase--a potentially negative regulator of basal disease resistance in rice.

The rice genome contains at least seven expressed Rop small GTPase genes. Of these Rops, OsRac1 is the only characterized gene that has been implicated in disease resistance as a positive regulator. To our interest in finding a negative ROP regulator of disease resistance in rice, we applied a "phylogeny of function" approach to rice Rops, and identified OsRacB based on its close genetic orthologous relationship with the barley HvRacB gene, a known negative regulator of disease resistance. To determine the function of OsRacB, we isolated the OsRacB cDNA and conducted gene expression and transgenic studies. OsRacB, a single copy gene in the genome of rice, shared 98% identity with HvRacB at the amino acid level. Its mRNA was strongly expressed in leaf sheath (LS) and in panicles, but was very weakly expressed in young and mature leaves. The basal mRNA level of OsRacB in LS of two-week-old seedlings was strongly down-regulated upon wounding by cut and treatment with jasmonic acid. A dramatic down-regulation in the OsRacB transcripts was also found in plants inoculated with the blast pathogen, Magnaporthe grisea. Interestingly, transgenic rice plants over-expressing OsRacB showed increased symptom development in response to rice blast pathogens. Additionally, fluorescence microscopy of green fluorescent protein (GFP):OsRacB-transformed onion cells and Arabidopsis protoplasts revealed OsRacB association with plasma membrane (PM), suggesting that PM localization is required for proper function of OsRacB. Based on these results, we suggest that OsRacB functions as a potential regulator for a basal disease resistance pathway in rice.