Coordination State Control of Citric Acid Molecules on Europium(III) Ion-Doped Hydroxyapatite Nanoparticles for Highly Efficient Photoluminescence toward Biomedical Applications

We successfully controlled the coordination states of citric acid (Cit) molecules on europium (Eu3+) ion-doped hydroxyapatite (Eu:HA) nanoparticles (NPs) by changing the feed Ca/P molar ratio at the nucleation stage and precisely investigated the effect of Cit states on the photoluminescence (PL) properties of the Cit-coordinated Eu:HA (Cit/Eu:HA) NPs. The increase in the molar ratio of (Ca + Eu) to P significantly increased the coordination amount of Cit on the Eu:HA NP surfaces. Transmission electron microscopy (TEM) observation demonstrated that with the increase in the coordination amount of Cit, the primary Cit/Eu:HA NPs with low crystallinity spherified and the aggregation among the NPs partially occurred, suggesting bond formation between the −COO– of Cit and the Ca2+ (or Eu3+, K+) of Eu:HA. The Fourier transform infrared spectra demonstrated that the COO– of Cit is initially substituted with HPO42– on the Cit/Eu:HA NP surfaces, followed by electrostatic adsorption of Cit on the cations (Ca2+, Eu3+, and K+) of the NP surfaces, resulting in the increase of the Cit coordination amount. When the coordination occupancy of Cit molecules exceeded 100%, the molecules would form bonds with the cations such as −COO––Ca2+ (or Eu3+ or K+)–COO– on the surfaces in the bimolecular coordination state, which was observed as an amorphous surface layer by TEM. In the cation-rich surface environment during NP formation, the existence of the excess Ca2+, Eu3+, and K+ ions would be enhanced on the NP surfaces while interacting with COO– in addition to the inclusion state inside the Eu:HA structures. Moreover, the surface Cit layer containing Eu3+ formed on the NP surfaces contributed to the effective increase in the PL intensity area and the internal quantum efficiency for biomedical applications.