Indirect imaging of singlet oxygen generation from a single cell

Singlet oxygen (1O2) can be generated in a living cell upon focused laser irradiation of the intracellular photosensitizers. 1O2 lifetime in the living cells is shortened by the reactions with cellular molecules, and thus the 1O2 diffusion in a single cell has attracted much attention. In this study, 1O2 generation from the plasma membrane-targeted protoporphyrin IX (PpIX) and nuclear-targeted meso-Tetra (N-methyl-4-pyridyl) porphine tetra tosylate (TMPyP) in human nasopharyngeal carcinoma CNE2 cells was indirectly imaged by using a fluorescence probe Singlet Oxygen Sensor Green agent (SOSG), respectively. The confocal images indicate that the green fluorescence of SOSG in the vicinity of the PpIX-sensitized cells was dramatically enhanced with the increase of the irradiation time and intracellular PpIX, while there is no significant enhancement for the unsensitized and TMPyP-sensitized cells. The obtained results suggest that the 1O2 generated from the plasma membrane-targeted PpIX in the CNE2 cells can escape into the extracellular medium and react with the SOSG to produce SOSG endoperoxides (SOSG-EP). Moreover, the fluorescence enhancement of SOSG mainly depends on the subcellular localization and intracellular uptake of the photosensitizers. Depending on the site of 1O2 generation, 1O2 generated in the plasma membrane can escape from the cell interior into the extracellular environment, while the 1O2 generated in the nucleus cannot. Our findings indicate that SOSG holds great promise for the indirect imaging of the 1O2 that can escape from single intact living cells