Hiroaki Suzuki

Micromachined liquid-junction Ag/AgCl reference electrode

Abstract A liquid-junction Ag/AgCl reference electrode was fabricated using microfabrication techniques. It employed a novel thin-film Ag/AgCl structure which functioned normally even in a solution saturated with KCl. The container for the internal electrolyte solution and the liquid junction were formed by anisotropically etching a silicon substrate. A pin-hole junction and a combination of the pin-hole and a cellulose-acetate-plug junctions were tested. Although the electrode with the former junction showed constant drift of the potential to the positive side, the electrode with the latter junction could maintain a potential level within ±1 mV for several h. No dependence on pH and KCl concentration was observed for the latter type, which was also applied to pH measurement in combination with a commercial pH glass electrode.

Fabrication of oxygen electrode arrays and their incorporation into sensors for measuring biochemical oxygen demand

Abstract Miniature Clark-type oxygen electrode arrays were fabricated using thin film technology. Each oxygen electrode comprised a silver cathode and a Ag/AgCl anode. The use of a dry electrolyte containing polyvinylpyrrolidone (PVP) permitted the use of solid state fabrication processes. The electrolyte was left dry until it was used, when the water essential for oxygen reduction was introduced through a gas-permeable membrane and absorbed by the PVP. The miniature oxygen electrode arrays showed good characteristics for monitoring dissolved oxygen concentrations and could be mass produced with assured quality. The yeast Trichosporon cutaneum was immobilized onto the cathode of an oxygen electrode using a photo-crosslinkable resin and the response of the sensor was the difference between the output of a yeast-immobilized electrode and that of a bare oxygen electrode. Measurements using the dual-type sensor for biochemical oxygen demand (BOD) were markedly simplified as air-saturated samples were not required, so air pumps and stirrers can be omitted from the sensing system. The BOD sensor was used for the analysis of real samples and the results agreed well with those determined using the conventional 5-day BOD determination method. This is a significant step toward the development of a portable BOD measuring system incorporating disposable electrodes and containing no mechanical parts.

An integrated three-electrode system with a micromachined liquid-junction Ag/AgCl reference electrode

A micromachined liquid-junction Ag/AgCl reference electrode was employed to fabricate an integrated three-electrode system. The reference electrode features a durable thin-film Ag/AgCl element whose entire surface was coated with a hydrophobic membrane and AgCl layer grown from its periphery. On the other side of the chip were formed a platinum working electrode and a platinum counter electrode. The cleanliness of the thin-film platinum working electrode was satisfactory to conduct electrochemical analyses including cyclic voltammetry. No substantial difference was observed in the cyclic voltammograms between the integrated system and the macroscopic three-electrode system in terms of their peak current and half-wave potential. The integrated electrode system was also applied to fabricate one-chip bio/chemical sensors. The miniature glucose sensor showed a distinct response against step changes of glucose concentration. The one-chip hydrogen electrode was used to conduct potentiometric titration. A good coincidence was again confirmed between the integrated system and the macroscopic system.

An integrated module for sensing pO2, pCO2, and pH

Abstract An integrated sensing module for monitoring pO 2 , pCO 2 , and pH was fabricated. Electrode patterns were formed on a glass substrate and the containers for corresponding electrolyte solutions were formed by anisotropically etching a silicon substrate. The Clark-type oxygen electrode and the Severinghaus-type pCO 2 electrode were used for sensing pO 2 and pCO 2 , respectively. An iridium oxide indicator electrode was used for the sensing element of the pCO 2 electrode and the pH electrode. The pH electrode was a combination electrode with an on-chip liquid-junction reference electrode. Distinct responses were observed for each of these electrodes. The linear relationships between the output current of the oxygen electrode and pO 2 , and between the potential of the carbon dioxide electrode and logarithm of pCO 2 were confirmed. The output potential of the pH electrode was compared using the on-chip liquid-junction reference electrode and a macroscopic commercial reference electrode. No substantial difference was observed. Simultaneous operation of the three electrodes was confirmed.

Miniature Clark-type oxygen electrode with a three-electrode configuration

Abstract A Clark-type oxygen electrode with three electrodes has been fabricated using semiconductor techniques and its characteristics compared with those of double oxygen electrodes with a similar structure. The 90% response time is 40 s, the residual current at zero oxygen concentration is less than 7%, and the linearity is good. The oxygen electrode can be used for 25 h at −0.6 V. These characteristics are affected by electrochemical crosstalk between the electrodes and are improved by varying the electrode configuration.

Development of a miniature clark-type oxygen electrode using semiconductor techniques and its improvement for practical applications

Abstract A disposable minature Clark-type oxygen electrode has been fabricated using semiconductor techniques. The oxygen electrode consists of: (a) anisotropically etched U- or V-grooves on a (100) silicon substrate as the electrolyte container; (b) a calcium alginate gell containing a 0.1 M KCl electrolyte; and (c) a directly formed gas-permeable membrane. A two-electrode configuration is used with a gold cathode and a gold anode. A novel fabrication process has been developed to allow miniaturization and mass production. Several important factors which influence the oxygen electrode characteristics have been examined. A 90% response time of 10 s is attained. A linear relationship is observed for a wide range of oxygen concentrations at 27°C. 34% of the oxygen electrodes work normally after sterlization in an autoclave. The oxygen electrode can be operated continuously for more than 40 h.

Disposable oxygen electrodes fabricated by semiconductor techniques and their application to biosensors

Abstract We fabricated a miniature Clark-type oxygen electrode using semiconductor techniques. The oxygen electrode features V-grooves to contain a 0.1 M KCl electrolyte impregnated in a gel, and a directly formed gas-permeable membrane over the gel. A fast response and a good linearity from zero oxygen concentration to saturation was obtained using the Ag/AgCl anode. The oxygen electrode was used in a disposable miniature CO 2 sensor. Also, an l -lysine sensor was developed that functions by immobilizing l -lysine decarboxylase on the sensitized area of the CO 2 sensor.