A reflection-mode fibre-optic sensor for breath carbon dioxide measurement in healthcare

Abstract A reflection-mode carbon dioxide (CO 2 ) fibre optic sensor (FOS) is developed based on the colorimetric change of pH indicator dye thymol blue. The CO 2 sensing film comprises thymol blue and tetramethylammonium hydroxide in the form of an ion pair doped in an organically modified silica film. A dip coating process was conducted to coat the sensing film on the tip of optical fibre with a film thickness approximately 2.7 ± 0.1 μm. The observed reflection signal at a wavelength of 608 nm exhibits a reversible change for the concentration of CO 2 range from 0 to 6% with a response time of 19 s and recovery time of 170 s for 6% of CO 2 . An optimal concentration of thymol blue in the preparation of coating solution that provides a highest sensitivity was found to be 2.9 mg/ml. Storing the FOS in a pure nitrogen atmosphere provided a stable film with a shelf-life of over 15 days as compared to a gradual degradation in film performance and complete loss of sensitivity after 10 days with storage in ambient air. The FOS response is affected by relative humidity (RH) and a humidity filter was demonstrated to reduce humidity levels in breath from 90% to 40–50% to reduce this effect. The developed FOS was used to measure the end-tidal breath CO 2 for testing its applicability in healthcare. The CO 2 measurement from breath sample using our FOS shows a good agreement (percentage error~3%) with a commercial CO 2 datalogger (K-33 BLG, CO 2 Meter, USA). Due to the limitation of response time, at present the FOS is limited to measuring variation of the CO 2 from slow breath to breath.