Red-emitting Au7 nanoclusters with fluorescence sensitivity to Fe2+ ions

Luminescent gold nanoclusters have recently emerged as an important class of sensing and imaging materials in optical applications. Here we report a red-emitting gold nanocluster with a precise molecular formula of Au-7(DHLA)(2)Cl-2, synthesized via the size-focusing strategy using a bidentate ligand of dihydrolipoic acid (DHLA). This Au nanocluster is water-soluble and emits fluorescence at 683 nm with a quantum yield of 3.6% in water. The intense fluorescence of Au-7(DHLA)(2)Cl-2 originates from the aggregation of Au(I)-DHLA/Cl motifs on the Au(0) core in a rigid core/shell-like structure. Moreover, we find that trace levels of Fe2+ ions can quench selectively and sensitively the fluorescence of the Au-7(DHLA)(2)Cl-2 nanocluster, making it a potential fluorescent sensor for Fe2+ with a limit of detection of 3.8 mM (0.2 ppm). A dissociation-induced fluorescence quenching mechanism is also proposed to describe the fluorescence response of the nanocluster to Fe2+ ions.