| Summary: | A reflection-mode carbon dioxide (CO2) fibre optic sensor (FOS) is developed based on the colorimetric change of pH indicator dye thymol blue. The CO2 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 CO2 range from 0 to 6% with a response time of 19 s and recovery time of 170 s for 6% of CO2. 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 CO2 for testing its applicability in healthcare. The CO2 measurement from breath sample using our FOS shows a good agreement (percentage error~3%) with a commercial CO2 datalogger (K-33 BLG, CO2 Meter, USA). Due to the limitation of response time, at present the FOS is limited to measuring variation of the CO2 from slow breath to breath. Keywords: Optical fibre CO2 sensor, Colorimetric change, pH indicator, Thymol blue, Sol-gel, End-tidal CO2
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