A Classic Near-Infrared Probe Indocyanine Green for Detecting Singlet Oxygen

The revelation of mechanisms of photodynamic therapy (PDT) at the cellular level as well as singlet oxygen ((1)O(2)) as a second messengers requires the quantification of intracellular (1)O(2). To detect singlet oxygen, directly measuring the phosphorescence emitted from (1)O(2) at 1270 nm is simple but limited for the low quantum yield and intrinsic efficiency of (1)O(2) emission. Another method is chemically trapping (1)O(2) and measuring fluorescence, absorption and Electron Spin Resonance (ESR). In this paper, we used indocyanine green (ICG), the only near-infrared (NIR) probe approved by the Food and Drug Administration (FDA), to detect (1)O(2) in vitro. Once it reacts with (1)O(2), ICG is decomposed and its UV absorption at 780 nm decreases with the laser irradiation. Our data demonstrated that ICG could be more sensitive and accurate than Singlet Oxygen Sensor Green reagent(®) (SOSG, a commercialized fluorescence probe) in vitro, moreover, ICG functioned with Eosin Y while SOSG failed. Thus, ICG would reasonably provide the possibility to sense (1)O(2) in vitro, with high sensitivity, selectivity and suitability to most photosensitizers.