An investigation into nanohybrid states of europium (III) complex with hydroxyapatite nanocrystals

We synthesized the novel inorganic-organic nanohybrid systems in hydroxyapatite (HA) nanocrystal growth processes under the presence of tris(2,2,6,6-tetramethyl-3,5-heptanedionato)europium (III) (EuTH) complex for the first time. The nanohybrid states were successfully controlled by the EuTH concentration to Ca ions in HA. The morphology of the HA was observed by a transmission electron microscope and the nanocrystalline aspect ratios were estimated to be 2.5–4.8. The segregated EuTH molecules on the HA nanocrystals covered over the a-plane in HA, leading to the effective crystal growth along with the uncovered c-plane. The luminescence properties of the nanohybrids were different from that of the EuTH itself in the solution and solid states, indicating the unique Eu³⁺ ion states on the HA nanocrystals because of the different ligand field symmetry. Moreover, the relative luminescence intensity ratio based on the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions in the nanohybrid became lower with increasing the EuTH concentration, suggesting that the hybridization lowered the coordination environment symmetry of Eu³⁺ ion. The luminescence intensity and quantum efficiency of the nanohybrids were higher than that of the EuTH itself. Therefore, the interaction at EuTH–HA interface as well as between EuTH–EuTH molecules significantly changed by the inorganic-organic hybridization under different EuTH concentrations. In future, the present hybridization technique can be applied for preparing the nanohybrids with various metallic complexes.