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

The generation of highly reactive singlet oxygen (O-1(2)) 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 O-1(2) can be efficiently produced through the direct photosensitization by Au-25(SR)(18)(-) clusters (H-SR = phenylethanethiol or captopril) without using conventional organic photosensitizers under visible/near-IR (532,650, and 808 nm) irradiation. O-1(2) was successfully detected by direct observation of the characteristic O-1(2) emission around 1276 nm as well as three different O-1(2)-selective probes. Water-soluble Au-25(captopril)(18)(-)clusters were explored for cytocompatibility and photodynamic activity toward cancer cells. In addition selective catalytic oxidation of organic sulfide to sulfoxide by O-1(2) was demonstrated on the photoexcited Au-25(SC2H4Ph)(18)(-)clusters. It is suggested that the optical gap of Au-25(SR)(18) clusters (similar to 1.3 eV) being larger than the energy of O-1(2)(0.97 eV) allows for the eficient energy transfer to O-3(2). In addition the long lifetime of the electronic excited states of Au-25(SR)(18) and the well-defined O-2 adsorption sites are the key factors that promote energy transfer from Au-25(SR)(18)(-) to molecular oxygen thus facilitating the formation of O-1(2). Finally neutral Au-25(SR)(18)(0) can also produce O-1(2) as efficiently as does the anionic Au-25(SR)(18)(-).