Breath-by-breath measurement of carbon dioxide using a plastic film optical sensor

An optical sensor has been developed for breath-by-breath gaseous CO2 analysis. The detector is based on a general formulation described in previous work where a phase-transfer agent, tetraoctyl ammonium hydroxide, is used to incorporate a hydrophilic pH-sensitive dye into a hydrophobic plastic film to create an effectively solid-state colorimetric sensor. In this work the formulation has been modified to yield a sensor which is capable of responding to clinically important levels of CO2 (0.1-5%) in less than 200 ms. This is comparable with the response of commercially available capnometers based on infrared sensing of CO2, which are currently widely used for clinical analysis. These films have good stability under the hot humid conditions involved in respiratory gas monitoring (37 degrees C gas temperature, 100% humidity) and, when stored in a freezer, show no deterioration in performance over an 80 day period.