Novel Dye-Solvent Solutions for the Simultaneous Detection of Oxygen and Carbon Dioxide

Abstract

The effects of the compositions of N,N-dimethyl-p-toluidine/N,N-dimethylformamide (DMF/DMF) solvent mixtures, the types of bases, the initial base concentrations, and the water content on the performance of alkaline fluorescein (FL)-DMT/DMF (dye-solvent) solutions in determining oxygen (O 2 ) and carbon dioxide (CO 2 ) have been investigated. Increased [O 2 ] causes the absorbance of dye-solvent solutions at 400 nm to increase because of a contact charge transfer existing between DMT and O 2 molecules, and increased [CO 2 ] produces a nonlinear decrease in absorbance at 520 nm as the color of FL changes from its orange dianion (FL 2- ) to the colorless, neutral, lactonic forms. The sensitivity to O 2 can be enhanced by increasing [DMT] in the DMT/DMF solvent mixture. A linear equation (i.e., log(A o - A)/A = 2β log [CO 2 ] + log(α 2 /K), where A o and A are the absorbances of dye-solvent solutions with nitrogen and CO 2 standards passing through, respectively, α and β are constants, and K is the dissociation constant for FL) is derived to relate the change of absorbance and applied [CO 2 ]. The sensitivity of dye-solvent solutions to O 2 is independent of the types of bases, but the sensitivity to CO 2 is not. Increased base concentration causes a change in the sensitivity to CO 2 but has no effect on O 2 . The higher water concentration in dye-solvent solutions has two effects. First, dye-solvent solutions are more sensitive to CO 2 . Second, there is a hypsochromic shift of FL 2- ions in DMT/DMF solvent mixture. A fiber-optic detecting system based on a solution of 10 μM FL and 336 μM tetrabutylammonium hydroxide in 1 :1 (v/v) DMT/DMF has been developed for the determination of O 2 and CO 2 . Their responses are reversible and independent. This solution can be used for future development of a single fiber-optic-based O 2 /CO 2 sensor.