The production of reactive oxygen species (ROS), such as hydroxyl radicals, by ultrasonic activation of semiconductor nanoparticles (NPs), including TiO2, has excellent potential for use in sonodynamic therapy and for the sonocatalytic degradation of pollutants. However, TiO2 NPs have limitations including low yields of generated ROS that result from fast electron–hole recombination. In this study, we first investigated the sonocatalytic activity of TiO2-supported Au nanoclusters (NCs) (Au NCs/TiO2) by monitoring the production of hydroxyl radicals (•OH) under ultrasonication conditions. The deposition of Au144 NCs on TiO2 NPs was found to enhance sonocatalytic activity for •OH production by approximately a factor of 2. Electron–hole recombination in ultrasonically excited TiO2 NPs is suppressed by Au144 NCs acting as an electron trap; this charge separation resulted in enhanced •OH production. In contrast, the deposition of Au25 NCs on TiO2 NPs resulted in lower sonocatalytic activity due to less charge separation, which highlights the effectiveness of combining Au144 NCs with TiO2 NPs for enhancing sonocatalytic activity. The sonocatalytic action that forms electron–hole pairs on the Au144/TiO2 catalyst is due to both heat and sonoluminescence from the implosive collapse of cavitation bubbles. Consequently, the ultrasonically excited Au144 (3 wt. %)/TiO2 catalyst exhibited higher catalytic activity for the production of •OH because of less light shadowing effect, in contrast to the lower catalytic activity when irradiated with only external light.
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 ( 1 O 2 ) generation were investigated. The difference between Au NC-based photosensitization and sonosensitization was also elucidated, along with a mechanism for the latter. For Au 25 NC-based sonosensitization and photosensitization, the ligand effect on the 1 O 2 -generation efficiency was in the order of glutathione < captopril < 4-mercaptobenzoic acid. The competing 1 O 2 production and quenching reactions using sono-/photo-excited Au 25 NCs determined the net 1 O 2 production. The size effects on the 1 O 2 -generation efficiency were in the order of Au 144 ≫ Au 25 > plasmonic Au nanoparticle for sonosensitization, as opposed to the case of photosensitization: Au 25 ≫ Au 144 ∼ plasmonic Au nanoparticle. The 1 O 2 generation via ultrasonically excited Au 144 NCs correlated with high-energy ultrasonic cavitation depending on the ultrasonication power and frequency. Therefore, high-energy ultrasonic cavitation-mediated Au 144 NC-based sonosensitizers could be effectively used in the production of 1 O 2 for various chemical and biomedical applications.
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