Hidemoto Nakagawa

A fiber-optic evanescent-wave hydrogen gas sensor using palladium-supported tungsten oxide

A new optical-fiber hydrogen sensor has been developed. The sensor utilizes the absorption change of the evanescent field in the clad region. The platinum- or palladium-supported tungsten oxide was used as sensing media. Two different approaches were adopted for the fiber fabrication. One used Pd/WO3 containing silicone resin as the clad. The other utilized the sol–gel process to form a thin Pd/WO3 clad. In the presence of hydrogen, strong evanescent-wave absorption was observed as a result of the formation of tungsten bronze. The sensor sensitively and immediately responded to hydrogen. It was found that the characteristics of the sensor were easily controlled by the amount of catalysts. The sensor developed in this study has potential to measure the spatial distribution along the fiber line, unlike the traditional hydrogen sensors that measure the concentration of a certain spatial point.

Re-activation of an amperometric organophosphate pesticide biosensor by 2-pyridinealdoxime methochloride

Abstract Inhibitions and re-activation characteristics of a biosensor by the organophosphate pesticides were investigated. The amperometric enzyme sensor co-immobilized on a platinum electrode by crosslinking was fabricated. Inhibition of the biosensor by dimethyl 2,2-dichlorovynyl phosphate (Dichlorvos, DDVP) as a model organophosphate pesticide was tested. The relative inhibition parameter (RI) was proportional to the concentration over a wide range. It was found that a nearly reversible inhibition process was observed if the inhibition is weak. However, irreversibility was appeared when the strongly inhibition took place. In this case, only 2-pyridinealdoxime methochloride (PAM) successfully regenerated the enzyme activity. PAM treatment guaranteed almost stable recovery rate and sensitivity. The results clarified that proposed re-activation procedures could realize inexpensive and reliable continuous monitoring of organophosphate pesticides.

An automated car ventilation system

Abstract A prototype of an automated car ventilation system that inhibits the intake of exhaust gas was constructed to explore suitable choices and application methods of gas sensors. The requirements for the gas sensors were discussed and the sensing characteristics of candidate sensors were investigated. The system adopted three metal-oxide semiconductor-type gas sensors. A CO/HC (hydrocarbon) gas sensor and an NO 2 sensor were utilized for the detection of exhaust fume in outside-air, and an odor sensor for the monitoring of inside-air. The sensing characteristics of these sensors, including humidity effects were investigated. It has been found that the humidity interference is not negligible, and computer algorithm to cope with the interference was presented. The prototype system satisfactorily shut out exhaust-containing air when the pollution level was high. The data collected on highways indicated that the wind efficiently reduces pollution level.

The interference elimination for gas sensor by catalyst filters

Abstract The method to eliminate alcohol interference for CO gas sensor using oxide semiconductor is explored. Some oxidizing agents and reducing agents were tested as the filtering materials. Nafion was found to be the most effective. This property was improved by the treatment to form the Nafion into complete H-type before measurement. The elimination capability is correlated with the acidities of the filtering materials.

A new ozone sensor for an ozone generator

Abstract A stable ozone sensor for quantitative analysis, targeted to be a monitor for an ozone generator, is proposed. The operation principle of the sensor is similar to that of a catalytic combustion sensor. The proposed sensor uses two thermal devices of physically and chemically identical characteristics. No catalytic materials such as noble metals are utilized in the present sensor. Unlike a catalytic combustion sensor, which measures catalytic oxidation heat, the proposed ozone sensor measures thermal decomposition heat. The first device, a detecting device, is operated well above the thermal decomposition temperature of ozone, whereas the second device, a reference device is operated near room temperature to avoid thermal decomposition. The detecting device detects the ozone by temperature rise of ozone decomposition origin, and the reference device compensates ambient temperature drift. The reference signal is amplified with appropriate gain and is subtracted from the signal of the detecting device to form temperature-compensated output signal. The proposed sensor exhibited excellent sensing characteristics with good reproducibility and long-term stability. Effect of ambient temperature drift was suppressed almost completely. Other sensing characteristics, such as effect of source gas composition, humidity, and pressure is also discussed.

A novel method of temperature compensation for a stable combustion-type gas sensor

Abstract A novel application method of combustion-type gas sensor mainly for thermally decomposable gas determination and a scheme to compensate the temperature dependence is suggested. In this method, a pair of heat detecting devices which have same physical and chemical property are utilized. These devices are driven with different currents. One for gas detection is operated at above the thermal decomposition temperature with larger current, and the other for compensation, reference device, at below the corresponding temperature. As ambient temperature varies, the resistances of a detecting device and a reference one vary, as well as their terminal voltages. Unlike a bridge configurated devices, the reference device exhibits smaller temperature dependence. Amplification of the reference signal with appropriate gain and subtracting of the amplified reference signal and the detecting device signal could compensate the ambient temperature variations. The method is successfully applied to ozone detection with excellent stability. A prudent design of the sensor chamber demonstrates good immunity against gas flow rate variation. But change in thermal conductivity of the detecting gas is not compensated with the present method.

Nitrate-sensitive corroding metal electrode

Abstract The electrode potential of Devardas alloy in phosphate solutions buffered at pH = 12 containing different amounts of nitrate ions has been measured. A proportional relation between the electrode potential and the concentration of nitrate ions [NO3−] is observed when [NO3−] is less than 40 mg 1−1. When [NO3−] is larger than 200 mg 1−1, log[NO3−] is linearly related to the electrode potential, with a slope of about 120 mV decade−1 up to [NO3−]=10 000 mg 1−1. The presence of chloride sulfate and perchlorate ions has almost no effect on the above relations. The sensitivity and selectivity of the above metal electrode exceed those of nitrate-ion-selective electrodes. Since the electrical impedance of the corroding metal/solution interface is much lower than that of other methods, the potential measurement should be easier.

Application of bistable nature of metal surface in aqueous solution to an oxygen sensor

A new oxygen sensor is proposed. It utilizes the electrochemically bistable nature of an iron surface in neutral buffer solutions. Iron placed in neutral solutions takes two discrete and stable states. The electrode potential is about -0.75 V (always versus SCE) in one state and about -0.25 V in the other. Dissolved-oxygen concentrations, as well as pH, govern the surface states. When the oxygen concentration and pH are low, the electrode stays at the former potential. The electrode potential indicates whether the oxygen concentration is above or below a threshold value. The exact dissolved-oxygen concentration can be determined from the transient time of an externally polarized electrode.

High-Performance Current-Voltage Measurement System for Scanning Tunneling Spectroscopy of Deposited Molecules

A tunnel current-voltage measurement system adopting analog integration of the current to improve the signal-to-noise ratio and high resolution (16-bit) analog-to-digital conversion of the integrated current was developed for scanning tunneling spectroscopy of deposited organic molecules. First-derivative images of good quality were successfully obtained with atomic-scale resolution for the first time.