Highly luminescent water-soluble lanthanide nanoparticles through surface coating sensitization

Abstract

Water-soluble La_0.95Eu_0.05F₃·AEP (AEP = aminoethylphosphate) nanoparticles (NPs) were shown to react with 6-carboxy-5′-methyl-2,2′-bipyridine (bipyCOO⁻) in water, resulting in an increase of more than two orders of magnitude of the europium centred emission upon excitation at 305 nm. Nanoparticles coated by the bipyCOO⁻ ligands were prepared by surface ligand exchange in water, isolated and characterized by infrared spectroscopy, transmission electron microscopy, energy dispersion X-ray analysis and dynamic light scattering. These analyses compared to those of the genuine NPs sample confirmed the partial replacement of AEP by bipyCOO⁻, with retention of the original morphology of the particles. The photo-physical properties of the isolated La_0.95Eu_0.05F₃·bipyCOO⁻ NPs were measured in water by absorption, steady-state and time-resolved luminescence spectroscopies and revealed that the large increase of the luminescence of the NPs is due to a surface generated antenna effect where coated bipyCOO⁻ moieties efficiently absorb light and transfer the energy to the emitting europium atoms of the NPs.