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.

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柠檬酸根和氟离子对羟基磷灰石纳米粒子的联合作用

柠檬酸根离子和氟离子是牙釉质羟基磷灰石（HA）纳米晶体的两个组成部分。已经研究了它们对HA结晶的单独影响，并且证明柠檬酸根离子和氟离子都可以调节HA晶体的生长。然而，到目前为止，尚未报道柠檬酸根离子和氟离子对HA纳米晶体的联合作用。在这项工作中，我们在温和的条件下制备了柠檬酸-氟化物-HA（citrate-FHA）纳米颗粒，并且研究了成熟时的形态和组成演变。我们已经证明，即使在存在柠檬酸盐的情况下，氟离子也被并入脂肪族结构中（取代羟基离子）并加速了结晶过程。有趣的是 与柠檬酸盐-HA的针状血小板形态相反，柠檬酸盐-FHA纳米粒子表现出扁平的六角棒状形态。结合在柠檬酸盐-FHA表面上的柠檬酸盐离子的密度高于在柠檬酸盐-HA上的密度。此外，柠檬酸根的相对含量柠檬酸盐-FHA的²⁺加合物相对于离子类化合物的加合物高于柠檬酸盐-HA。我们的结果提供了对柠檬酸根和氟离子对HA纳米晶体的联合作用的更深刻理解，可用于设计具有定制功能的先进生物材料，以更好地理解牙釉质的生物矿化过程，以及用于合成牙釉质纳米晶体。