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/sonosensitization) 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 sonosensitizing abilities for singlet oxygen (¹O₂) generation were investigated. The difference between Au NC-based photosensitization and sonosensitization was also elucidated, along with a mechanism for the latter. For Au₂₅NC-based sonosensitization and photosensitization, the ligand effect on the ¹O₂-generation efficiency was in the order of glutathione < captopril < 4-mercaptobenzoic acid. The competing ¹O₂production and quenching reactions using sono-/photo-excited Au₂₅NCs determined the net ¹O₂production. 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∼plasmonic Au nanoparticle. The ¹O₂generation via ultrasonically excited Au₁₄₄NCs correlated with high-energy ultrasonic cavitation depending on the ultrasonication power and frequency. Therefore, high-energy ultrasonic cavitation-mediated Au₁₄₄NC-based sonosensitizers could be effectively used in the production of ¹O₂for various chemical and biomedical applications.