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.