Atomically Precise Au 22 (Lys-Cys-Lys) 16 Gold Nanocluster as Type I Photosensitizer: ROS Generation Mechanism

The exceptional response of the gold nanocluster Au 22 (Lys-Cys-Lys) 16 , capable of generating reactive oxygen species (ROS) via electron transfer, was studied from both computational modeling and experimental characterization. An electron density perspective on remarkable features of gold nanoclusters and their photophysical applications provides a comprehensive study of the mechanism the gold nanocluster Au 22 (Lys-Cys-Lys) 16 undergoes as a type I photosensitizer. By means of molecular dynamics simulations, density functional theory for ground state and excited states, and linear response - time dependent density functional theory calculations, the optical properties of the nanocluster and its interactions in aqueous media were studied. Analysis of frontier molecular orbitals of Au 22 (Lys-Cys-Lys) 16 revealed that this nanocluster can transfer an electron from the gold core to a dioxygen molecule in the vicinity, generating ROS by following a spin-flip transition. The results suggest a strong connection between redistribution of electron density in the excited state and the ability of Au 22 (Lys-Cys-Lys) 16 to act as an electron donor rather than as a more commonly studied energy transfer mechanism for gold nanoclusters, mainly reported as photosensitizers type II. This study also probes the underlying mechanism of fluorescence quenching in aqueous media providing a comprehensive description of the experimental detection of ROS at different pH.