Fiber-optic carbon dioxide sensor based on fluorinated xerogels doped with HPTS

This paper presents a high-performance fiber-optic carbon dioxide (CO2) sensor based on hybrid xerogels composed of alkyl and perfluoroalkyl ORMOSILs (organically modified silicates) doped with pH-sensitive fluorescent dye. The sensor film consists of 1-hydroxy-3,6,8-pyrenetrisulfonic acid trisodium salt (HPTS, PTS−), tetraoctylammonium cation (TOA+), and a tetraoctylammonium hydroxide (TOAOH) phase transfer agent (i.e. the base) immobilized within the hybrid xerogels. The experimental results indicate that the fluorescence intensity of the HPTS dye decreases as the CO2 gas phase concentration increases. The sensor has a response level of (I0 − I)/I ≈ 18, where I0 and I are the fluorescence intensities in 100% N2 and in 100% CO2, respectively, and exhibits a linear response to CO2 concentrations in the range 0–30%. The resolution is ±2.5% in this level and the limit of detection (LOD) is 0.03% CO2, calculated as three times the standard deviation (3σ). The response time of the sensor is 1.7 s when switching from a pure nitrogen atmosphere to a pure CO2 atmosphere and 38.5 s when switching from CO2 to nitrogen. The signal changes are fully reversible as the atmosphere is switched repeatedly between nitrogen and CO2. Overall, the experimental results demonstrate that the use of hybrid xerogels doped with HPTS enhances both the response level and the response speed of the proposed optic-fiber CO2 sensor.