Functionalized gold nanoparticles as phosphorescent nanomaterials and sensors

Abstract

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 Eu^III/Tb^III ions yields phosphorescent nanomaterials. Absorption spectral studies confirm a 1:3 complexation between Eu^III/Tb^III ions and bipyridines, functionalized on the surface of Au nanoparticles. The red-emitting Au-BT:Eu^III complex exhibits a long lifetime of 0.36 ms with six line-like emission peaks, whereas the green-emitting Au-BT:Tb^III complex exhibits a lifetime of 0.7 ms with four line-like emission peaks. These phosphorescent nanomaterials, designed by linking BT:Eu^III complexes to Au nanoparticles, were further utilized as sensors for metal cations. A dramatic decrease in the luminescence was observed upon addition of alkaline earth metal ions (Ca²⁺, Mg²⁺) and transition metal ions (Cu²⁺, Zn²⁺, Ni²⁺), resulting from an isomorphous substitution of Eu^III ions, whereas the luminescence intensity was not influenced by the addition of Na⁺ and K⁺ ions. Direct interaction of bipyridine-capped Au nanoparticles with Cu²⁺ ions brings the nanohybrid systems closer, leading to the formation of three-dimensional superstructures. Strong interparticle plasmon interactions were observed in these closely spaced Au nanoparticles.