Room temperature ionic liquids as optical sensor matrix materials for gaseous and dissolved CO2

A new optical CO2 sensor based on the spectrophotometric signal changes of the bromothymol blue/tetraoctylammonium (BTB−/TOA+) ion pair in room temperature ionic liquids (RTILs) has been proposed. Ionic liquids, also known as green chemistry reagents, have been used for the first time as a matrix material in the optical CO2 sensor design. In the first stage of the study, determination of the acidity constant (pKa) of the modified BTB has been performed in the employed ionic liquids. In the second stage, response of the sensor composition to gaseous and dissolved CO2 has been evaluated. It should be noted that the solubility of CO2 in water miscible ionic liquids is about 10–20 times as that observed in the conventional solvents, polymer matrices or in water and enhance the response of the sensing agent. The detection limits were 1.4% for gaseous, and 10−6 M [HCO3−] for dissolved CO2. CO2 can be completely extruded from the ionic liquid by heating, under vacuum or sonification. The regenerated ionic liquid has been used for CO2 sensing without any loss of efficiency.