Evidence for Boundary Layer Effects Influencing the Sensitivity of Microencapsulated O₂ Fluorescence Indicator Molecules

Abstract

As Kautsky and Hirsch (1935) and later also Vaughan and Weber (1970) have shown fluorescence quenching by molecular oxygen is often a diffusion-controlled collisional process. Thus, it is not surprising that the oxygen solubility coefficient, and the oxygen diffusion coefficient D of various solvents are of great importance with regard to the O₂ sensitivity of the indicator molecules. Fig. 1 shows this effect on different calibration curves for the indicator pyrenebutyric acid dissolved in alkaline water and dioctylphthalate and embedded in a silicone membrane. Calibration has been performed by means of the so-called “Po₂ optode”, a schematic drawing of which is shown in Fig. 2 (Lübbers and Opitz, 1975a, b; Opitz and Lübbers, 1976). This optical device incorporates a thin indicator film (1–5 um) between a radiation permeable quartz window and a membrane which separates the fluorescence probes from the medium to be measured (i.e., different partial oxygen pressures in N₂-O₂ gas mixtures).