Effective Thermal Diffusion of Eu(III) and F Ions into Hydroxyapatite Nanoparticles by Citric Acid Coordinative Mediation

We synthesized citric acid (Cit)-coordinated Eu 3+ and F – codoped hydroxyapatite (Cit/Eu:HA-F) nanoparticles (NPs) and investigated their ion diffusion processes based on photoluminescence (PL) properties at the near-surfaces with thermal treatment. At 85 °C, the Cit coordination layer effectively suppressed the nonradiative deactivation of HA NP surfaces in the PL process, and the quantum yield (η int ) of Eu 3+ ions reached a maximum value of ca. 33%, indicating the highest PL efficiency in the HA system. At 250 °C, Cit was decomposed into aconitic acid with a high optical absorption coefficient and reducing ability, so that the η int of the Eu ions in the NPs were minimized and the partial Eu 3+ ions were resultantly reduced to Eu 2+ ions. At around 350 °C, the Eu ions were partially diffused from Ca(I) into Ca(II) sites. At 550 °C, the proportion of Eu ions in Ca(II) sites dramatically reached up to ca. 31%, and the F – ions simultaneously diffused into the HA structures by substituting for the OH sites. Therefore, it was elucidated that the Cit molecules coordinated to the Eu and F – codoped HA surfaces achieved higher η int and promoted the diffusion of various ions into the HA structures with thermal treatment and resultantly controlled the existence ratio of the Eu 3+ and Eu 2+ ions at the near surfaces, which could be expected to control the PL color by the Cit coordinative mediation technique and realize the creation of cell-labeling luminescent nanomaterials in biomedical fields.