Singlet Oxygen Generation Using Thiolated Gold Nanoclusters under Photo- and Ultrasonic Excitation: Size and Ligand Effect

Ultrasonic irradiation of liquids can induce catalytic activity in semiconductor nanoparticles (sonocatalysis/sonosensi-tization) similar to light-induced photocatalysis/photosensitization. However, due to the complexity of the acoustic cavitation processes involved in sonocatalysis/sonosensitization, an ideal nanoparticle design has not been identified for them. Herein, the size-and ligand-dependent ultrasonic activation of thiolate gold nanoclusters (Au NCs) and their photosensitizing and sonosensi-tizing abilities for singlet oxygen (1O2) generation were investigated. The difference between Au NC-based photosensitiza-tion and sonosensitization was also elucidated, along with a mechanism for the latter. For Au25 NC-based sonosensitization and photosensitization, the ligand effect on the 1O2-generation efficiency was in the order of glutathione < captopril < 4-mercaptobenzoic acid. The competing 1O2 production and quenching reactions using sono-/photo-excited Au25 NCs determined the net 1O2 production. The size effects on the 1O2-generation efficiency were in the order of Au144 >> Au25 > plasmonic Au nanoparticle for sonosensitization, as opposed to the case of photosensitization: Au25 >> Au144 similar to plasmonic Au nanoparticle. The 1O2 generation via ultrasonically excited Au144 NCs correlated with high-energy ultrasonic cavitation depending on the ultrasonication power and frequency. Therefore, high-energy ultrasonic cavitation-mediated Au144 NC-based sonosensitizers could be effectively used in the production of 1O2 for various chemical and biomedical applications.