Dipicolinate Sensitization of Europium Luminescence in Dispersible 5% Eu:LaF3 Nanoparticles

An earlier synthetic procedure was adapted to produce water-dispersible, citrate-stabilized LaF3 nanoparticles of 3-4 nm diameter, which exhibit a high degree of crystallinity (tysonite structure). The samples, as isolated from synthesis, consist of nanoparticles with a monolayer coverage of citrate capping ligands (with no excess, unbound citrate) and ammonium ions which balance charge. In aqueous dispersions, dipicolinate (DPA) displaces citrate on the surface of 5%Eu-doped LaF3 nanoparticles and strongly sensitizes Eu3+(D-5(0)) emission. At low concentrations, the DPA does not adversely affect the dispersibility or the structural integrity of the nanoparticles and binds to the particle surface, selectively sensitizing Eu3+ surface and near-surface sites. The DPA, therefore, serves both as a sensitizer of Eu3+ luminescence and as a selective probe of the surface. The dipicolinate strongly sensitizes Eu3+(D-5(0)) emission, increasing the 614 nm emission intensity by a factor of 100 with less than 1% citrate replacement. Addition of an LaF3 shell (thickness approximate to 4 angstrom) over the 5%Eu:LaF3 core (3-4 nm) results in dispersible nanoparticles which exhibit 4-fold reduction in dipicolinate sensitization. The DPA probe gives strong evidence that a true core-shell structure is formed and that DPA sensitization can penetrate to near-surface sites to which the DPA is not directly bound.