Highly sensitive and linear optical fiber carbon dioxide sensor based on sol-gel matrix doped with silica particles and HPTS

This paper presents a high-sensitive and linear fiber-optic carbon dioxide (CO₂) sensor based on a sol-gel matrix composed of n-octyltriethoxysilane (Octyl-triEOS)/tetraethylorthosilane (TEOS) doped with 1-hydroxy-3,6,8-pyrenetrisulfonic acid trisodium salt (HPTS, PTS-), silica particles and a tetraoctylammonium hydroxide (TOAOH) phase transfer agent (i.e. the base). The experimental results indicate that the relative fluorescence intensity of the ion pair form of HPTS dye decreases as the CO₂ gas phase concentration increases. The sensor has a sensitivity of approximately 26 and exhibits a uniquely linear response for CO₂ concentrations in the range of 0-100%. The sensitivity of the optical fiber CO₂ sensor is quantified in terms of the ratio I_N2 / I_{C O2}, where I_N2 and I_{C O2} represent the detected fluorescence intensities in pure nitrogen and pure carbon dioxide environments, respectively. The response time of the sensor is 9.8 s when switching from a pure N₂ atmosphere to a pure CO₂ atmosphere and 195.4 s when switching from CO₂ to N₂. The signal changes are fully reversible as the atmosphere is switched repeatedly between N₂ and CO₂. Overall, the experimental results demonstrate that the use of a sol-gel matrix doped with the ion pair form of HPTS and silica particles uniquely enhances the sensitivity and linearity of the proposed optic-fiber CO₂ sensor.