Combined Effect of Citrate and Fluoride Ions on Hydroxyapatite Nanoparticles

Citrate and fluoride ions are two constituents of dental enamel hydroxyapatite (HA) nanocrystals. Their individual effect on HA crystallization has already been studied, and it was proven that both citrate and fluoride ions regulate HA crystal growth. However, the combined effect of citrate and fluoride ions on HA nanocrystals has never been reported so far. In this work, we have prepared citrate-fluoride-HA (citrate-FHA) nanoparticles in mild conditions, and we have studied the evolution of morphology and composition upon maturation. We have proven that even in the presence of citrate, fluoride ions are incorporated in the apatitic structure (replacing hydroxyl ions) and accelerate the crystallization process. Interestingly, citrate-FHA nanoparticles exhibit a flattened hexagonal rod-like morphology in contrast to the needle-like platelet morphology of citrate-HA. The density of citrate ions bound on the citrate-FHA surface is higher than that on citrate-HA. Moreover, the relative amount of unidentate citrate-Ca2+ adducts versus the ionic-like ones is higher for citrate-FHA than for citrate-HA. Our results provide a deeper understanding of the combined effect of citrate and fluoride ions on HA nanocrystals that can be used for the design of advanced biomaterials with tailored features, for a better comprehension of the enamel biomineralization process, and for the synthesis of enamel-like nanocrystals.