Investigation of the Surface Elution Mechanism of Citric Acid-Coordinated Hydroxyapatite Nanoparticles in Biological Solutions

We clarified the surface elution behavior of citric acid (Cit) ions from Cit-coordinated europium(III) ion (Eu³⁺)-doped hydroxyapatite (CEH) nanoparticles in three different biological solutions (i.e., phosphate buffer (PB), carbonate buffer (CB), and simulated body fluid (SBF)). Specifically, the elution behavior of Cit from the CEH nanoparticles in SBF was clarified as compared with the cases in PB and CB. Based on the elution results of Cit from the nanoparticles, two-stepwise (i.e., initially dramatic and subsequentially gradual) elution behavior during the immersion was observed, which was mainly driven by the substitutional coordination of HPO₄^2-and CO₃^2-to the Cit ions on the nanoparticle surfaces. The Cit elution behavior by the immersion was effectively affected by the anions in the biological solutions. The substitutional coordination of HPO₄^2-or CO₃^2-to the Cit ions was the dominant driving force for the Cit ion elution chelating with Eu³⁺and Ca²⁺ions, which progressed significantly at the immersion time of 0-3 h in SBF. Subsequently, the complex reactions based on the substitutional coordination would become the equilibrium state at the immersion time of 3-12 h in SBF, suggesting the control of nanoparticle surface states by optimizing the Cit elution behavior. Therefore, we believe that the nanoparticles will be the base nanobiomaterials toward the application for drug delivery system carriers containing both photoluminescence and drug loading functions.