Emission-based optical carbon dioxide sensing with HPTS in green chemistry reagents: room-temperature ionic liquids

We describe the characterization of a new optical CO2 sensor based on the change in the fluorescence signal intensity of 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) in green chemistry reagents-room-temperature ionic liquids (RTILs). As far as we are aware, this is the first time RTILs, 1-methyl-3-butylimidazolium tetrafluoroborate (RTIL-I) and 1-methyl-3-butylimidazolium bromide (RTIL-II), have been used as matrix materials with HPTS in an optical CO2 sensor. It should be noted that the solubility of CO2 in water-miscible ionic liquids is approximately 10 to 20 times that in conventional solvents, polymer matrices, or water. The response of the sensor to gaseous and dissolved CO2 has been evaluated. The luminescence intensity of HPTS at 519 and 521 nm decreased with the increasing concentrations of CO2 by 90 and 75% in RTIL-I and RTIL-II, respectively. The response times of the sensing reagents were in the range 1-2 min for switching from nitrogen to CO2, and 7-10 min for switching from CO2 to nitrogen. The signal changes were fully reversible and no significant hysteresis was observed during the measurements. The stability of HPTS in RTILs was excellent and when stored in the ambient air of the laboratory there was no significant drift in signal intensity after 7 months. Our stability tests are still in progress.