Micromachined severinghaus-type carbon dioxide electrode.

The Severinghaus-type pCO(2) electrode was miniaturized and batch-fabricated using semiconductor and micromachining techniques. Anodically grown iridium oxide film (AIROF) was employed as the pH sensing element to detect a local pH change caused by the infusion of CO(2). The AIROF showed a super-Nernstian response with a slope of approximately -80 mV/pH at 25 °C. A novel thin-film Ag/AgCl structure was also used. It features a hydrophobic membrane which covers the entire silver layer and the AgCl layer grown from the periphery of the silver pattern. The open-circuit potential of the Ag/AgCl element drifted to the negative side at -0.1 to -0.2 mV h(-1). A microcavity in which the electrolyte solution was filled was anisotropically etched in a silicon substrate, and a silicone rubber gas-permeable membrane was formed on the sensitive area. The miniature pCO(2) electrode showed a distinct response to the variation in concentration of dissolved CO(2). The inherent characteristics of the Severinghaus electrode were confirmed in terms of its response and calibration curve. The selectivity of the electrode was satisfactory in view of its application to clinical analysis.