Stability of Fluorescence Emission from Gold Nanoclusters and Photocatalytic Redox on Surface^†

The fluorescent emission and the photocatalytic activity of gold nanoclusters (AuNCs) are important properties of AuNCs. Here, we synthesized water-soluble, captopril (Capt) and glutathione (GSH) covered Au₂₅(Capt)₁₈ and Au₁₅(GSH)₁₃ nanoclusters and investigated their fluorescence stabilities which are correlated with the photo-redox of their surface. It was observed that Au₂₅(Capt)₁₈ exhibited a decay in its two-photon fluorescence emission induced by surface oxidation upon irradiation of femtosecond near-infrared laser (810 nm, 300 mW) in the presence of oxygen (O₂). On the other hand, the two-photon fluorescence from Au₁₅(GSH)₁₃ was stable upon the same excitation. The different stabilities of the fluorescence of the two AuNCs were attributed to their different band gaps. This interpretation was confirmed by one-photon excitation with photon energy beyond the band gaps of the two AuNCs. Using experiments with the addition of several agents, the photocatalytic redox activities of gold atoms on the surface of Au₂₅(Capt)₁₈ were understood and tuned.