An emission based optical CO2 sensor fabricated on grating-like TiO2 substrates using HPTS

TiO2 is a widely used material in various applications, including photo-catalysis, sensing, and dye-sensitized solar cells. In this work, we presented a stable and sensitive CO2 sensor working on principle of emission signals of the HPTS dye immobilized on TiO2 substrates whose effective refractive indices in the parallel and perpendicular grating orientations evidenced earlier. The HPTS exhibited 8.4-fold enhanced emission signal on the offered substrates with respect to the previously reported intensities recorded for the polymeric supports. The emission peaks of the HPTS exhibited CO2 induced intensity quenching when excited at 466 nm. Calibration sensitivity of the offered composites has been tested exposing the sensor materials to varying concentrations of the CO2 between 0.0 and 100.0 % pCO2 and correlating the measured fluorescence intensity with the corresponding CO2 levels. The dye revealed enhanced CO2 induced response exhibiting the I0/100 values of 11.9, 13.3, 11.2 and 6.5. We also recorded bi-exponential excited state lifetimes for the HPTS on four different test materials both in the absence and presence of the CO2. The emission based variations were followed as the analytical signal due to higher CO2-induced relative signal changes than that of the lifetime based variations. Herein we used the ion pair form of the HPTS in a protective chemical microenvironment and obtained considerable long term stability extending to16 mounts which can be attributed to the presence of the 1-butyl-3-methylimidazolium tetrafluoroborate as an internal buffering system in the sensing composition as well as the inert and robust structure of the substrate. The extremely high optical response, advantage of excitation with blue LEDs and long-term stability observed on grating-like TiO2 surfaces make the proposed system a promising design for the quantification of CO2 for further applications