Optical sensor for carbon dioxide combining colorimetric change of a pH indicator and a reference luminescent dye

A new optical CO(2) sensor based on the luminescence intensity change of the europium(III) complex tris(thenoyltrifluoroacetonato) europium(III) dihydrate ([Eu(tta)(3)]) caused by the absorption change of various pH indicators-thymol blue, phenol red, or cresol red-with CO(2) was developed and its CO(2) sensing properties were investigated. For all the CO(2) sensors using pH indicators the observed luminescence intensity from [Eu(tta)(3)] at 613 nm increased with increasing CO(2) concentration. The linear calibration method based on the plot of (I(100)-I(0))/(I-I(0)) versus the inverse of CO(2) concentration was suggested, where I(0) and I(100) were luminescence intensities at 613 nm of the CO(2) sensor film in 100% nitrogen and 100% gaseous CO(2). In all cases the plots showed good linearity and the correlation factors of the plots, r(2), were 0.991 for thymol blue, 0.990 for phenol red, and 0.998 for cresol red. The slopes of the plots (A/B) for thymol blue, phenol red, and cresol red were 2.2, 5.2, and 9.0%, respectively. The response times of the CO(2) sensor film were 4.0 s for thymol blue, 4.4 s for phenol red, and 8.8 s for cresol red for switching from nitrogen to CO(2), and the recovery times of films were 36 s for thymol blue, 39.2 s for phenol red, and 56.6 s for cresol red for switching from CO(2) to nitrogen. The signal changes were fully reversible and hysteresis was not observed during the measurements. The highly sensitive CO(2) sensor was developed using thymol blue as an indicator for the CO(2)-sensing probe.