Generation of Singlet Oxygen by Photoexcited Au25(SR)18 Clusters

The generation of highly reactive singlet oxygen (1O2) is of major importance for a variety of applications such as photodynamic therapy (PDT) for cancer treatment, water treatment, catalytic oxidation, and others. Herein, we demonstrate that 1O2 can be efficiently produced through the direct photosensitization by Au25(SR)18 clusters (H−SR = phenylethanethiol or captopril) without using conventional organic photosensitizers under visible/near-IR (532, 650, and 808 nm) irradiation. 1O2 was successfully detected by direct observation of the characteristic 1O2 emission around 1276 nm as well as three different 1O2-selective probes. Water-soluble Au25(captopril)18 clusters were explored for cytocompatibility and photodynamic activity toward cancer cells. In addition, selective catalytic oxidation of organic sulfide to sulfoxide by 1O2 was demonstrated on the photoexcited Au25(SC2H4Ph)18 clusters. It is suggested that the optical gap of Au25(SR)18 clusters (∼1.3 eV) being larger than the energy of 1O2 (0.97 eV) allows for the efficient energy transfer to 3O2. In addition, the long lifetime of the electronic excited states of Au25(SR)18 and the well-defined O2 adsorption sites are the key factors that promote energy transfer from Au25(SR)18 to molecular oxygen, thus facilitating the formation of 1O2. Finally, neutral Au25(SR)180 can also produce 1O2 as efficiently as does the anionic Au25(SR)18.