Singlet Oxygen, the lowest electronically excited state of molecular oxygen, is highly reactive and involved in many chemical and biological processes. It is one major mediator during photosensitization, which has been used by mankind since ancient times, even though the mechanisms behind it were understood only about half a century ago.

The combination of high reactivity and very long natural lifetime allows for direct optical detection of singlet oxygen and its interactions using its characteristic phosphorescence at around 1270 nm. Since this emission is very weak, optical detection was technically very challenging for a long time. Therefore, even today, most laboratories only exploit the high reactivity to observe the interaction with sensor molecules, rather than singlet oxygen emission itself. However, in recent years highly sensitive optical detection was developed, the authors being major contributors.

This book is dedicated to the detection of singlet oxygen, discussing possibilities, pitfalls and limits of the various methods with a special focus on time-resolved phosphorescence and the kinetics of singlet oxygen generation and decay including involved and related processes, discussing investigated systems with various complexity from solutions over in vitro to in vivo.

The long-standing paradigm that singlet oxygen phosphorescence is a benchmark for detection systems rather than an option for process observation is still ubiquitous and this book hopes to contribute in overcoming this still prevailing bias.

Steffen Hackbarth received his Ph.D. degree in experimental physics from HU Berlin in 2000. Ever since, he has been working in the field of time-resolved spectroscopy in the time range ps to ms with a special focus at the triplet processes of photosensitizers. He is head of the singlet oxygen lab and as such, recently became a faculty member of the mathematical and natural sciences faculty. Since 2011 he is leader of the advanced practical courses for the physics students at HU Berlin. His research focuses on molecular photobiophysics, especially on fundamental research in the field of photosensitization, passive drug delivery, and diffusion processes during Photodynamic Therapy. Technical developments focus on singlet oxygen luminescence detection in vivo and other heterogeneous environments at highest sensitivity toward real-time supervision of individualized medical tumor treatments. In 2012, he was awarded (together with his colleague, Jan Schlothauer) the innovation award of the SPIE Europe for the development of a compact time-resolved table-top singlet oxygen luminescence detection system, which since then was improved to in vivo capability.

Michael Pfitzner received his M.SC in physics from HU Berlin in 2013. Since then, he was working in the field of time-resolved spectroscopy with a special focus at singlet oxygen spectroscopy. He will finish his Ph.D. this year, with a focus at pushing in vivo singlet oxygen measurements toward a medical application. His special interest lies within the detection of these very weak NIR signals as well as data evaluation. In 2017, he began expanding his studies toward the combination of time-resolved pointwise measurements with steadystate camera-based detection methods at the Fujian Normal University (Fuzhou, China).

Jakob Pohl studied biophysics at the Humboldt-Universität zu Berlin and received his Master's degree in 2013. During a three-year stipendium from Deutsche Bundesstiftung Umwelt, he developed protocols for a reproducible cultivation of phototrophic biofilms under conditions of photodynamic inactivation. Those are, among others, the major topics of his Ph.D. thesis which he completed in 2019. Since his Bachelor's thesis in 2009, he has been working in the field of photodynamic therapy and photodynamic inactivation, studying the effects of photosensitization on human cancer cells and microorganisms. Since 2016 he has worked as a research associate at the Humboldt-Universität zu Berlin in the group of photobiophysics to develop protocols for qualitative analysis of PDI, photophysical characterization of antimicrobial surfaces, and the inactivation of algal biofilms.