Kei Toda

Recent progress in applications of graphene oxide for gas sensing: A review.

This paper is a review of the recent progress on gas sensors using graphene oxide (GO). GO is not a new material but its unique features have recently been of interest for gas sensing applications, and not just as an intermediate for reduced graphene oxide (RGO). Graphene and RGO have been well known gas-sensing materials, but GO is also an attractive sensing material that has been well studied these last few years. The functional groups on GO nanosheets play important roles in adsorbing gas molecules, and the electric or optical properties of GO materials change with exposure to certain gases. Addition of metal nanoparticles and metal oxide nanocomposites is an effective way to make GO materials selective and sensitive to analyte gases. In this paper, several applications of GO based sensors are summarized for detection of water vapor, NO2, H2, NH3, H2S, and organic vapors. Also binding energies of gas molecules onto graphene and the oxygenous functional groups are summarized, and problems and possible solutions are discussed for the GO-based gas sensors.

Micro gas analyzers for environmental and medical applications

In this review, novel microsystems and microdevices to measure gaseous species for environmental analysis and medical diagnostics are described. Miniaturization of analyzers makes field measurements affordable. As well, high sensitivity and good time resolution can be achieved by miniaturization. Some such devices have already been successfully applied to real environmental analyses. Mobile monitoring is available with the use of micro gas analyzers to investigate the natural environment, air pollution and to detect nerve or explosive gases released accidentally or through terrorist activities. Miniature devices are also attractive for medical analyses. Gases produced from the human body reflect gases contained in the blood and certain metabolic conditions. Noninvasive monitoring using miniature devices is available in hospitals and in a patients home. Many investigations have been conducted using wet and dry chemistry methods for both applications. Instruments employing wet chemistries, which comprise liquid droplets, liquid film, miniature diffusion scrubbers, and microfluidic devices have been studied. Among the instruments using dry methods, miniature samplers, portable gas chromatographs, and microfabricated gas chromatographs have all been investigated. These instruments are expected to usher in a new era of environmental monitoring and will find uses in many medical applications.

A Capacitance Sensor for Water: Trace Moisture Measurement in Gases and Organic Solvents

The determination of water in various matrices is one of the most important analytical measurements. We report on a high-resolution capacitance-based moisture sensor utilizing a thin film of a perfluorosulfonate ionomer (PFSI)-H(3)PO(4) composite in a flow-through configuration, for both gas and liquid samples. Incorporation of H(3)PO(4) into a PFSI sensing film improved the limit of detection (LOD) (signal-to-noise ratio, S/N = 3) by a factor of 16 in the gas phase to 0.075% relative humidity (RH) (dew point = -56 °C). The response time was dependent on the sensing film thickness and composition and was as low as ∼60 ms. The temperature dependence of the sensor response, and its relative selectivity over alcohol and various other solvents, are reported. Measurement of water in organic solvents was carried out in two different ways. In one procedure, the sample was vaporized and swept into the detector (e.g., in a gas chromatograph (GC) without a column); it permitted a throughput of 80 samples/h. This is well-suited for higher (%) levels of water. In the other method, a flow injection analysis system integrated to a tubular dialysis membrane pervaporizer (PV-FIA) was used; the LOD for water in ethanol was 0.019% (w/w). We demonstrated the temporal course of drying of ethanol by Drierite; the PV-FIA results showed excellent correspondence (r(2) > 0.99) with results from GC-thermal conductivity detection. The system can measure trace water in many types of organic solvents; no reagent consumption is involved.

Simple temperature compensation of thermal air-flow sensor

Abstract The thermal air-flow sensor, which is heated to a temperature with a constant degree of difference (Δt) from that of the gas, provides power consumption according to the flow rates. The flow signal output of the sensor decreases depending on the rise in ambient temperature. This effect is easily compensated by inserting two identical compensation resistors (Rcom) in the Wheatstone bridge circuit without Δt change. This compensation is accomplished based on the influential change of ambient temperature on the sensor power consumption in proportion to the ratio of Rcom to the bridge resistor.

Sulfurized limonite as material for fast decomposition of organic compounds by heterogeneous Fenton reaction.

Rapid decomposition of wastewater contaminants using sulfurized limonite (S-limonite) was investigated. Limonite is used for desulfurization of biogases, and S-limonite is obtained from desulfurization plants as solid waste. In this work, the profitable use of S-limonite in water treatment was examined. The divalent Fe in S-limonite was expected to produce OH radicals, as Fe(2+) ions and limonite thermally treated with H2 do. Methylene blue was used for batch-wise monitoring of the decomposition performance. The decomposition rate was fast and the methylene blue solution color disappeared in only 10s when a small amount of H2O2 was added (1mM in the sample solution) in the presence of S-limonite. The OH radicals were formed by a heterogeneous reaction on the S-limonite surface and Fenton reaction with dissolved Fe(2+). The decomposition of pentachlorophenol was also examined; it was successfully decomposed in batch-wise tests. The surfaces of limonite before sulfurization, S-limonite, and S-limonite after use for water treatment were performed using scanning electron microscopy and X-ray photoelectron spectroscopy. The results show that S-limonite reverted to limonite after being used for water treatment.

NEW APPLICATIONS OF CHEMILUMINESCENCE FOR SELECTIVE GAS ANALYSIS

Chemiluminescence (CL) is a powerful analytical tool for trace gas measurements. In this mini-review, we present reactions and spectra of ozone-induced CL for various compounds and techniques for detecting such CL. Next discussed are CL applications where real gas samples were successfully measured. Chemiluminescence monitoring has been used as universal nitrogen and sulfur detectors for gas chromatography and capillary electrophoresis. Chemiluminescence detection can be used as the basis of compact, affordable, and sensitive analyzers for real-sample analysis. Isoprene and sulfur compounds in breath and atmospheric samples have been successfully measured by coupling with a small collection system. Short-term (5 min) sorbent collection enhances the CL signal and considerably reduces interference. For sulfur gas analysis, methyl mercaptan and dimethyl sulfide can be separated on the same column that is used for collection. Waterborne arsenic is measured by automated arsine generation and CL reaction of arsine and ozone. In addition to gas-phase CL, more recent efforts towards the determination of gases by CL generated at solid/gas and liquid/gas interfaces are also discussed.

Micro gas analyzer measurement of nitric oxide in breath by direct wet scrubbing and fluorescence detection

A novel method is proposed to measure NO in breath. Breath NO is a useful diagnostic measure for asthma patients. Due to the low water solubility of NO, existing wet chemical NO measurements are conducted on NO(2) after removal of pre-existing NO(2) and conversion of NO to NO(2). In contrast, this study utilizes direct measurement of NO by wet chemistry. Gaseous NO was collected into an aqueous phase by a honeycomb-patterned microchannel scrubber and reacted with diaminofluorescein-2 (DAF-2). Fluorescence of the product was measured using a miniature detector, comprising a blue light-emitting diode (LED) and a photodiode. The response intensity was found to dramatically increase following addition of NO(2) into the absorbing solution or air sample. By optimizing the conditions, the sensitivity obtained was sufficient to measure parts per billion by volume levels of NO continuously. The system was applied to real analysis of NO in breath, and the effect of coexisting compounds was investigated. The proposed system could successfully measure breath NO.

Electrochemical enzyme immunoassay using immobilized antibody on gold film with monitoring of surface plasmon resonance signal

Sandwich immunoassay was conducted on a thin gold film set in a surface plasmon resonance (SPR) cell. Monochronal antibody (anti-IgG) was immobilized onto the gold film via 4,4′-dithiodibutyric acid (DDA) and avidin–biotin bonding. Next, IgG sample and alkaline phosphatase-conjugated anti-IgG (ALP anti-IgG) were introduced into the cell successively. Finally, p-aminophenyl phosphate (PAPP) was injected as an enzyme substrate, and the produced p-aminophenol (PAP) was electrochemically measured. Flow did not need to be stopped for incubation for the enzyme reaction, because of the thinness of the cell. In these processes, all the antigen–antibody reactions took place on the gold film. Therefore, the immobilization was performed quickly, and each process could be confirmed by SPR signal. This system had the advantage that the middle of the complicated process could be monitored. For example, the amount of antibody immobilized, which affected on the final electrochemical signal, could be confirmed in the course of immobilization. It was also convenient to investigate process conditions, such as removal of used antigens and labeled antibodies. Good correlation was obtained between the electrochemical current and the SPR signals due to the adsorption of IgG and ALP anti-IgG, and the sensitivity of the electrochemical measurement was much higher than the SPR’s.

A wall-jet ring disk electrode fabricated within a thin-layered micromachined cell

Abstract The wall-jet ring disk electrode (WJRDE) has been used with a thin layer cell in a flow system. Usually, the cavity for the solution flow and reactions on the ring disk electrode (RDE) is formed with an elastomer spacer. Accordingly, the thickness is more than several tenths of a micrometer. If a very thin-layered WJRDE (TLWJRDE) is fabricated, the collection efficiency N, which indicates the rate at which the product on the disk is captured onto the ring electrode, is expected to increase. Therefore, a TLWJRDE was prepared in a very thin cavity and was fabricated by micromachining. Ditches and the cavity were formed on a glass plate by photolithography and chemical etching, and this chip was pasted onto another glass substrate prepared with the RDE on it. The sample solution was introduced from the center of the device, and was spread over the electrodes. After this, it went out from the periphery of the device. The TLWJRDE was evaluated using the Fe(CN)64−/3− redox system, and its characteristics are discussed. When the flow rate was decreased, N increased to nearly 100%. This tendency was stronger, especially when the flow-through cell was thinner. Compared to other RDEs reported previously, a very thin cavity of micrometer order can be fabricated, and large N values were obtained in the TLWJRDE system. The TLWJRDE can be manufactured by micromachining and is expected to be used as a disposable detector.

Formaldehyde content of atmospheric aerosol.

Formaldehyde (HCHO) is a highly soluble polar molecule with a large sticking coefficient and thus likely exists in both gaseous and particulate forms. Few studies, however, address particulate HCHO (HCHO(p)). Some report that HCHO(p) concentrations (obtained only with long duration sampling) are very low. The lack of data partly reflects the difficulty of specifically measuring HCHO(p). Long duration filter sampling may not produce meaningful results for a variety of reasons. In this work, gaseous HCHO (HCHO(g)) and (HCHO(p)) were, respectively, collected with a parallel plate wet denuder (PPWD) followed by a mist chamber/hydrophilic filter particle collector (PC). The PPWD quantitatively removed HCHO(g) and the PC then collected the transmitted aerosol. The collected HCHO from either device was alternately analyzed by Hantzsch reaction-based continuous flow fluorometry. Each gas and particle phase measurement took 5 min each, with a 10 min cycle. The limits of detection were 0.048 and 0.0033 μg m(-3), respectively, for HCHO(g) and HCHO(p). The instrument was deployed in three separate campaigns in a forest station in western Japan in March, May, and July of 2013. Based on 1296 data pairs, HCHO(p), was on the average, 5% of the total HCHO. Strong diurnal patterns were observed, with the HCHO(p) fraction peaking in the morning. The relative humidity dependence of the partition strongly suggests that it is driven by the liquid water content of the aerosol phase. However, HCHO(p) was 100× greater than that expected from Henrys law. We propose that the low water activity in the highly saline droplets lead to HCHO oligomerization.