Direct Optical Carbon Dioxide Sensing Based on a Polymeric Film Doped with a Selective Molecular Tweezer-Type Ionophore

A novel optical method for the determination of CO₂ concentration in aqueous and gaseous samples of plasticized PVC film is presented. The detection principle makes use of a direct molecular recognition of the carbonate ion by a molecular tweezer-type ionophore, which has previously been demonstrated to exhibit excellent carbonate selectivity. The carbonate ion is extracted together with hydrogen ions into a polymeric film that contains the anion exchanger tridodecylmethylammonium chloride, a lipophilic, electrically charged, and highly basic pH indicator, which is used for the readout in absorbance mode, in addition to the lipophilic carbonate ionophore. According to known bulk optode principles, such an optical sensor responds to the product of the carbonate ion activity and the square of hydrogen ion activity. This quantity is thermodynamically linked to the activity of carbon dioxide. This allows one to realize a direct carbon dioxide sensor that does not make use of the traditional Severinghaus sensing principle of measuring a pH change upon CO₂ equilibration across a membrane. A selectivity analysis shows that common ions such as chloride are sufficiently suppressed for direct <i>P</i>CO₂ measurements in freshwater samples at pH 8. Chloride interference, however, is too severe for direct seawater measurements at the same pH. This may be overcome by placing a gas-permeable membrane over the optode sensing film. This is conceptually confirmed by establishing that the sensor is equally useful for gas-phase <i>P</i>CO₂ measurements. As expected, humid air samples are required for proper sensor functioning, as dry CO₂ gas will not cause any signal change. The sensor showed acceptable response times and good reproducibility under both conditions