Ligand-capped gold nanoparticles were synthesized by capping monothiol derivatives of 2, 2'-dipyridyl onto the surface of Au nanoparticles (Au-BT). The average size of the metal core is around 4 nm, with a shell of~ 340 bipyridine ligands around the Au nanoparticle. The high local concentration of the chelating ligands (~ 5 M) around the Au nanoparticle makes these particles excellent ion sponges, and their complexation with EuIII/TbIII ions yields ...