Mineralization Pathways of Amorphous Calcium Phosphate in the Presence of Fluoride

Abstract

Amorphous calcium phosphate (ACP) has been widely reported as a metastable precursor during the mineralization of calcium phosphates in bone and enamel. Although the influence of fluoride on the crystallization of hydroxyapatite (HAP) has been extensively investigated, the mineralization pathways of ACP in the presence of fluoride are not fully understood. Here, using a combination of in situ monitoring and ex situ characterizations, we show that fluoride exhibits little effect on the formation and particle size of ACP nanospheres from a supersaturated calcium phosphate solution. However, the stability of ACP increases with increasing concentration of free fluoride ions in solution. We show that the aggregation of ACP nanospheres into larger spheres is an important step during ACP crystallization, which drives the nucleation of plate-like nanocrystals on the surface of ACP nanospheres and induces subsequent crystallization of ACP via a dissolutionrecrystallization pathway. In the presence of fluoride, the aggregation of ACP nanospheres and the nucleation of crystalline phases are retarded, thus stabilizing the amorphous phase. In addition, fluoride promotes the formation rod-like crystals on the surface of ACP nanospheres, which grow both outward from solution ions and inward from ACP nanospheres. These results significantly improve our understanding of the mineralization pathways of ACP and explain the inhibitory effect of fluoride during ACP crystallization.

Characterization and In Vitro Behavior of PEO Coated Mg Modified with Antibacterial Ag(I) and Cu(II) Complexes

“…No other crystalline phases were found. Considering the composition of the solution used to obtain the PEO2 specimens, the absence of a Mg 3 (PO 4 ) 2 crystalline phase in the PEO2 coating may be attributed to the presence of fluoride ion, known for its inhibitory effect in the phosphate's crystallization process [49] …”

“…Considering the composition of the solution used to obtain the PEO2 specimens, the absence of a Mg 3 (PO 4 ) 2 crystalline phase in the PEO2 coating may be attributed to the presence of fluoride ion, known for its inhibitory effect in the phosphate's crystallization process. [49] Analysis of the literature [20,50,51] indicates that formation of a certain phase state (amorphous or crystalline) highly depends on the process parameters like frequency, current density, temperature etc. Based on energy dispersive X-ray analysis and X-ray photoelectron spectroscopy data (will be discussed later), it is concluded that calcium-and fluoride-containing compounds are present in the coating composition but form no crystalline phase in the conditions described.…”

Time-Dependent Effect of Additives on Crystallization of Amorphous Calcium Phosphate: Faster Nucleation after Dilution

“…However, NaF and MgCl 2 increased the lifetime of ACP (Figures d and S2c). , The acceleration effect was present at lower concentrations of l -Asp (<75 mM) and l -Arg (<100 mM) (Figures f and S3e,f). Amino acids with uncharged side chains had a weaker effect on the ACP lifetime.…”

“…When F − was added after ACP formation, the surface of ACP adsorbed F − (Figure 6i−k), and its crystallization was inhibited (Figure 2d), further demon- strating that the crystallization of ACP is a surface-mediated process. 33 The surface-mediated dissolution reprecipitation process is the primary pathway for the crystallization of ACP in solution, 47,48 and we propose that additives that have weak interactions with ACP do not alter this process. The effects of the other additives on the ACP particle concentration (Figure S3c) and size (Figure S2j,k,p) of ACP particles were similar to those of H 2 O, which further strengthens our proposition.…”

“…32 At the final pH platform, the mineral showed sharp diffraction peaks at 2θ = 25.9°and 2θ = 31.7°(Figure 1b), and the two absorption peaks of ACP split into four peaks that were at 560, 600, 1110, and 1030 cm −1 (Figure S1d), indicating the mineral is mainly HAp at the final pH platform, which was nanoneedle or nanosheet (Figure 1c). 33 Based on the sharp peaks observed at 2θ = 11.6°, 2θ = 20.8°, and 2θ = 29.2°(Figure 1b), DCPD micron plates (Figure 1d) formed at the middle pH platform. 34 This demonstrated that the middle pH platform marks the formation and transformation of DCPD when mineralization occurs in a near-neutral solution, although DCPD may disappear at higher pH.…”