Abstract
The revelation of mechanisms of photodynamic therapy (PDT) at the cellular level as well as singlet oxygen (1O2) as a second messengers requires the quantification of intracellular 1O2. To detect singlet oxygen, directly measuring the phosphorescence emitted from 1O2 at 1270 nm is simple but limited for the low quantum yield and intrinsic efficiency of 1O2 emission. Another method is chemically trapping 1O2 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 1O2 in vitro. Once it reacts with 1O2, 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 1O2 in vitro, with high sensitivity, selectivity and suitability to most photosensitizers.
Highlights
Singlet oxygen (1O2), a highly reactive oxygen species (ROS), plays a crucial role in photodynamic therapy (PDT) by causing oxidative damage to proteins, DNA and lipids [1,2,3]
chlorin e6 (Ce6) had no influence on absorption of indocyanine green (ICG) at 780 nm (Figure 1c)
The solution was exposed to laser irradiation, the absorption of ICG at 780 nm significantly reduced after each instance of laser irradiation (Figure 1d)
Summary
Singlet oxygen (1O2), a highly reactive oxygen species (ROS), plays a crucial role in photodynamic therapy (PDT) by causing oxidative damage to proteins, DNA and lipids [1,2,3]. Fluorescence probes for 1O2 have drawn much attention, including DPAX or DMAX [11], ATTA-Eu3+ [12] and Singlet Oxygen Sensor Green reagent® (SOSG) They use an anthracene moiety to trap 1O2 that quenches the fluorescence of the fluorophore through an electron transfer reaction. Upon reaction with 1O2, the immediate product SOSG endoperoxide (SOSG-EP) exhibits green fluorescence, with excitation and emission peak at 504 and 525 nm, respectively [13] These probes are convenient, highly sensitive and widely used, especially SOSG, which has been broadly used in recent studies [14,15,16,17]. Once irradiated under 660 nm laser light, Ce6 can produce 1O2, which decomposes ICG through the break of alkene (double bond in a polymethine chain) hypothetically (Supplementary Material, Figure S1) [21] This probe is safe, sensitive and broadly suited for most photosensitizers. To better evaluate its detecting ability, we chose the commercialized 1O2 probe SOSG as a reference probe
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