Oxidation at the Core–Ligand Interface of Au Lipoic Acid Nanoclusters That Enhances the Near-IR Luminescence

A new surface oxidation mechanism that enhances the luminescence of Au nanoclusters is discovered in Au nanoclusters synthesized with disulfide lipoic acid. The quantum efficiency increased from 1 to 2% up to 10% upon the sulfur oxidation at the Au–ligand interface. Relatively low quantum efficiency has been a bottleneck barrier to exploit the appealing near-infrared luminescence from molecular-like gold nanoclusters for broader and effective applications. Combined IR, XPS, and NMR analysis reveals that the outer sulfur atoms on ca. half of the lipoic acid ligands were partially oxidized accompanying the luminescence enhancement. Opposite to those from the widely studied monothiolate Au nanoclusters, the quantum efficiency increases at lower pH. The observation is explained by the electron density changes at the core–ligand interfaces under the ligand dielectric layer. Multivalent binding of the lipoic acid ligands on Au core drastically reduces the exchange or addition of monothiols into ligand monolayer. The improved resistance to excess thiols is significant because the stability of Au nanoclusters is a critical concern in their applications in thiol-rich physiological environment. The fundamental materials and chemistry insights suggest promising routes to further enhance the near-IR luminescence and chemical stability that are critical factors in biomedical and sensing applications.