A fiber-optic sensor has been developed to measure and monitor the partial pressure of carbon dioxide in blood via fluorescence or phosphorescence changes in sensing dyes. The pCO2optrode uses a nanoliter size droplet of a concentrated fluorescein derivative in a buffered sodium ascorbate solution as a pH indicator. The pH of the droplet equilibrates with the volatile constituents of blood across an air bubble trapped at the tip of the capillary surrounding the fiber. Ascorbic acid is used to reduce photobleaching, which is further minimized by working on the right portion of the concentration quenching curve. Signal levels of 300 000 counts per second are obtained at 0.5 μW using the 488-nm line of an argon ion laser. A response range of 25 percent was obtained when the sensor was evaluated from 1.97 to 7.40 percent CO2in air at controlled humidity and temperature.