Phosphorescent sensing of carbon dioxide based on secondary inner-filter quenching Original Research Article

A study of different strategies to prepare phosphorescence-based sensors for gaseous CO2 determination has been performed. It includes the characterization of different configurations tested, a discussion of the results obtained and possibilities for the future. The optical sensor for gaseous CO2 is based on changes in the phosphorescence intensity of the platinum octaethylporphyrin (PtOEP) complex trapped both on oxygen-insensitive poly(vinylidene chloride-co-vinyl chloride) (PVCD) membranes and PVCD microparticles, due to the displacement of the α-naphtholphthalein acid–base equilibrium with CO2 concentration. A secondary inner-filter mechanism was tested for the sensor and a full range linearized calibration was obtained by plotting (I100 − I0)/(I − I0) versus the inverse of the CO2 concentration, where I0 and I100 are the detected luminescence intensities from a membrane exposed to 100% nitrogen and 100% CO2, respectively, and I at a defined CO2 concentration. The different configurations tested included the use of membranes containing luminophore and pH-sensitive dye placed on two opposite sides of a transparent support to prevent the observed degradation of the PtOEP complex in the presence of the tetraoctylammonium hydroxide (TOAOH) phase transfer agent, which produced better results regarding stability and sensitivity. The CO2 gas sensor based on PtOEP homogeneous membranes presented better properties in terms of response time and sensitivity than that based on PtOEP microparticles. With a detection limit of 0.02%, the response time (10–90% maximum signal) is 9 s and the recovery time (90–10%) is 115 s. The lifetime of the membranes for CO2 sensing preserved in a 94% RH atmosphere and dark conditions is longer than at least 4 months.