Chapter 26: Singlet Oxygen in Heterogeneous Systems

In heterogeneous environments any measurement of singlet-oxygen (¹O₂) luminescence becomes exceedingly difficult, the more so the analysis of its kinetics. Since the generation of ¹O₂ in such systems occurs at locations with different microenvironments it should be termed as an inhomogeneous process. Even more important, the diffusion of ¹O₂ in areas of different radiative rate constants and different solubility of oxygen has to be taken into account. Therefore, due to the extremely short ¹O₂ lifetimes in biological systems, the heterogeneity strongly influences the ¹O₂ luminescence kinetics. This chapter is dedicated to the discussion of ¹O₂ diffusion processes in heterogeneous systems and their consequences for the ¹O₂ luminescence kinetics. Starting with liposomes and continuing with cells in suspension, we introduce and discuss diffusion models, describing the ¹O₂ luminescence kinetics. These models demonstrate the influence of the localization of the photosensitizer on the ¹O₂ luminescence kinetics and intensity. The examples show that ¹O₂ luminescence kinetics may help to determine the exact localization of the photosensitizer. This knowledge is the pathway to a quantitative analysis of ¹O₂ luminescence kinetics in complex systems like cells away from the need of single-cell measurements.