Fabrication of high-fluorescence composite nanoparticles of gold nanoclusters and calcium phosphate for macrophage imaging

Gold nanoclusters (AuNCs) have great potential as a dual-modal fluorescence/X-ray computed tomography imaging agent. Herein, we fabricated composite nanoparticles of glutathione (GSH)-capped AuNCs and calcium phosphate (CaP), Au–CaP nanoparticles, via coprecipitation in highly supersaturated CaP solutions containing various concentrations of GSH-capped AuNCs. The resulting nanoparticles were nearly spherical in shape and immobilized high amounts of GSH-capped AuNCs dispersed within the CaP matrix. The Au–CaP nanoparticles decreased in size (from ∼250 to ∼50 nm in hydrodynamic diameter) and increased in Au content (Au/Ca elemental ratio) with an increase in initial Au concentration of the supersaturated CaP solution. The Au–CaP nanoparticles showed good dispersibility in water owing to the relatively large negative zeta potentials (from −18 to −15 mV). Additionally, the Au–CaP nanoparticles showed much higher (from 33- to 140-fold) fluorescence intensities (per one mole of Au atom) than free GSH-capped AuNCs. The Au–CaP nanoparticles fluorescently labeled macrophage cells (RAW264.7) more efficiently than free GSH-capped AuNCs without exhibiting significant cytotoxicity. These results suggest that the Au–CaP nanoparticles have potential as an imaging agent for macrophage cells.