Preparation of calcium phosphate nanoparticles hybridized with europium(III) complex for novel luminescent organic-inorganic systems

We have synthesized novel inorganic–organic hybrid systems based on the interactions between calcium phosphate (CP) and tris(2,2,6,6-tetramethyl-3,5-heptanedionato)europium (III) (EuTH) complex to clarify the states of the EuTH molecules. We achieved the nucleation and crystal growth of electronically localized of EuTH molecules using two different synthetic processes (based on the timing of the addition EuTH before and after the CP nucleation stage). The coordination environment of the Eu³⁺ ion inside/on the hybrids was clearly changed, having the higher spatial symmetry states in the hybrid. The aspect ratios of the hybrid crystallites were 2.5–6.0, suggesting that the EuTH molecules selectively interacted with the a–plane of the CP nuclei to induce preferential crystal growth along with the uncovered c–plane. The segregated EuTH molecules on CP surfaces suppressed Ca²⁺ dissolution from the hybrids in phosphate buffer saline. In the excitation spectra of the hybrids, the ⁷F₀ → ⁵L₆ transition appeared as a result of the interaction between the EuTH and phosphate ions, suggesting that the phosphate ions affected the coordination environment. In the luminescence spectra of the hybrids, the luminescence intensity ratio of the magnetic dipole transition (⁵D₀ → ⁷F₁) to electric dipole transition (⁵D₀ → ⁷F₂) were higher than those of EuTH and the other Eu³⁺-doped inorganic systems. Thus, the higher spatial symmetry states of the Eu³⁺ ion in the hybrids resulted in greater luminescence intensities and quantum efficiencies, which would be affected by the phosphorus anti-quenching. In the future, CP hybrids with various organic complexes will be prepared, and these could be applied in the biomedical fields such as in bio-imaging and drug delivery systems.