Ultrasonic Activation of Water-Soluble Au25(SR)18 Nanoclusters for Singlet Oxygen Production

The production of reactive oxygen species (ROS) by ultrasonic activation of nanomaterials has great potential in sonodynamic therapy and sonocatalysis. Herein, we first demonstrate that water-soluble Au-25(Captril)(18) nanoclusters (NCs) can work as a sonosensitizer to produce singlet oxygen (O-1(2)) under ultrasound irradiation. The ultrasonically mediated O-1(2) production was also confirmed in other thiolated Au NCs, including those protected by glutathione or 6-aza-2-thiothymine (ATT). There are two possible pathways to ultrasonically excite the thiolated Au NCs via microbubble cavitation: (i) sonoluminescence from water and (ii) ROS production (hydroxyl radical) by pyrolysis cleavage of water. We suggest that the sonoluminescence from water could excite the Au NCs to produce O-1(2) under ultrasonication, when there is a spectral overlap between the sonoluminescence and the absorption of Au NCs to allow energy transfer. The ultrasonic excitation of ATT-Au NCs was confirmed by the observation of sonoluminescence. The Au(I)-SR shell of thiolated Au NCs is responsible for generating the triplet state of thiolated Au NCs to produce O-1(2) under ultrasonication. As thiolated Au NCs also have photosensitizing capability, it would be used as ROS-based nanomedicine for a sonodynamic and/or photodynamic therapy.