Bright, NIR-Emitting Au23 from Au25: Characterization and Applications Including Biolabeling

A novel interfacial route has been developed for the synthesis of a bright-red-emitting new subnanocluster, Au, by the core etching of a widely explored and more stable cluster, Au(25)SG(18) (in which SG is glutethione thiolate). A slight modification of this procedure results in the formation of two other known Subnanoclusters. Au-22 and Au-33. Whereas Au-22 and Au lire water soluble and brightly fluorescent with quantum yields of 2.5 and 13%, respectively, Au-23 is organic soluble and less fluorescent. with a quantum yield of 0.1 %. Au-23 exhibits quenching of fluorescence selectively in the presence of Cu2+ ions and it can therefore be used as a metal-ion sensor. Aqueous- to organic-phase transfer of Au-23 has been carried out with fluorescence enhancement. Solvent dependency on the fluorescence of Au-23 before and after phase transfer has been studied extensively and the quantum yield of the cluster varies with the solvent used. The temperature response of Au-23 emission has been demonstrated. The inherent fluorescence of Au-23 was used for imaging human hepatoma cells by employing the avidin-biotin interaction.