Photodynamic therapy (PDT) is a relatively new method for cancer treatment, where tumor cells are destroyed by light-induced, local production of a reactive oxygen species (ROS) such as singlet oxygen (1O2) and superoxide(O2�-) via photosensitizers. It is considered that singlet oxygen is the primary cytotoxic agent responsible for PDT. The development of singlet oxygen photosensitizers to produce highly reactive oxygen species is a key step for effective PDT. In recent years, an increasing number of researchers have considered the possibility of nanomaterials as singlet oxygen photosensitizers such as semiconductor quantum dots (QDs), fullerene C60, metal nanoparticles, and QDs conjugated with aromatic photosensitizers. Unfortunately, most photosensitizers have some major drawbacks, including poor water solubility, toxicity, instability, and ineffective excitation wavelengths for the tissue penetration.Recently, significant progress has been made in the synthesis of atomically precise thiolate-protected gold nanoclusters (denoted as Aun(SR)m , where SR refers to thiolate ligand), such as Au25(SR)18, Au38(SR)24, Au102(SR)44, and Au144(SR)60. The photoluminescent properties of Aun(SR)m render such nanoclusters as promising imaging and sensing agents. However, till now there have been no reports on the exploitation of Aun(SR)m nanoclusters as potential photosensitizers toward PDT. Herein we first demonstrate that singlet oxygen can be produced through the direct photosensitization by Au25(SR)18- nanoclusters (SR=phenylethanethiol or captopril) without the presence of any organic photosensitizers. The experimental evidence includes (1) two sensitive probes for singlet oxygen, namely, diaminobenzidine and diphenylisobenzofuran, (2) quenching of singlet oxygen production by histidine as the efficient scavenger for singlet oxygen, and (3) enhancement of singlet oxygen production in D2O. It is also suggested that the singlet oxygen from Au25(SR)18-nanoclusters is in competition with the �quenching� of the singlet oxygen by simultaneously produced superoxide.

Generation of singlet oxygen by atomically precise Au25 (SR)18� nanoclusters

246th National Meeting (2013)

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