FIBEROPTIC SENSING OF CARBON-DIOXIDE BASED ON EXCITED-STATE PROTON-TRANSFER TO A LUMINESCENT RUTHENIUM(II) COMPLEX

A pH transduction chemistry, based on luminescence quenching by proton transfer to the excited tris[ 2-(2-pyrazinyl)thiazole]ruthenium(II) cation, is presented. Absorption (460 nm) and emission (652 nm, 320-ns lifetime) features of the complex ion are discussed as a function of pH and nature and concentration of buffer. In this way, a pK(a1) (-1.9) has been determined, and the pK(a)* evaluated (ca. 10(6)-fold basicity increase). Proton-transfer rates in the range 1.3 X 10(8) to 5 X 10(9) M-1 sa-1 have been measured, depending on the strength of the Bronsted acid. Since the working principle does not rely on ground-state acid/base equilibrium of the sensitive dye, both emission intensity and lifetime can be used to monitor changes in the concentration of acidic species. The effect of temperature and dissolved gases (argon, air, carbon dioxide) on the luminescence lifetime is discussed as well. The transduction chemistry has been applied to the construction of a fiber-optic sensor for carbon dioxide (up to 50 % concentration in the gas phase with a response time of about 1 min). Its best performance has been found when the luminescent dye is electrostatically immobilized onto a functionalized sepharose gel swollen in pH 7.2 0.1 M hydrogen phthalate buffer and separated from the analyte phase by a gas-permeable silicone membrane.