ABSTRACT
In photobiology, a photosensitizer (PS) is a light-absorbing molecule that mediates a reaction in a cell, tissue, or whole organism in response to light, without undergoing any permanent change itself. Since the PS is normally not consumed during this photodynamic reaction, it can mediate many reaction cycles that can increase the damage to a substrate (the nal target of the process). Dierent types of photosensitization have been distinguished in the literature based on the intramolecular interaction of the PS aer being excited. e so-called type II photochemical pathway stands for a direct energy transfer from the PS triplet state to molecular oxygen, forming singlet oxygen (1O2), the lowest electronically excited state of oxygen. Besides this pathway, electron-transfer reactions there might occur in the substrate or oxygen, followed by chemical reactions of the radicals that are produced. Today there is common agreement (Baier et al. 2005; Snyder et al. 2005;
15.1 Introduction 293
15.2 How the diffusion of 1O2 affects its detection 296
15.2.1 Diffusion range of 1O2 in cells 296
15.2.2 How 1O2 diffusion limits the quantitative accuracy of 1O2 imaging 297
15.2.3 Influence of 1O2 diffusion on the luminescence intensity 298
15.2.4 Influence of 1O2 diffusion on the luminescence kinetics 299
15.3 Interaction of 1O2 with cellular quenchers 302
15.3.1 1O2 in living cells: The majority is deactivated by cellular quenchers 302
15.3.2 How measurement of 1O2 changes the object under investigation 303
15.3.3 Applying changes in 1O2 kinetics as sensor for cell vitality 305
15.4 Detection limits in living cells 305
15.4.1 Direct observation in vital single cells 305
15.4.2 Fluorescent probes in vital cells 306
15.4.3 Direct observation disregarding cell vitality 306
15.4.4 1O2 luminescence kinetics in cell ensembles 307
15.4.5 Estimating 1O2 in cells based on PS fluorescence and triplet kinetics 308
15.5 Conclusions 308
References 309
Redmond and Kochevar 2006; Juzeniene et al. 2007; Jiménez-Banzo et al. 2008b; Cavalcante et al. 2009; Price et al. 2009; Hackbarth et al. 2010; Wang et al. 2010; Jarvi et al. 2011, 2012; Kanofsky 2011; Shen et al. 2011) that 1O2 is the key agent in photosensitization in biological environments. e term “PS activity,” as used in this chapter, points toward the amount of 1O2 generated as well as its reactions with the biological environment.