Evolution of Amorphous Calcium Phosphate to Hydroxyapatite Probed by Gold Nanoparticles

Amorphous calcium phosphate (ACP) is an important intermediate phase during the biomineralization of apatite. But the detailed mechanism of transformation from ACP to crystallized hydroxyapatite (HAP) is still unclear. Gold nanoparticles are used as ex-situ probes to monitor the detailed evolution process of ACP in vitro. The rearrangement of gold nanoparticles during the evolution of ACP indicates the aggregation and surface-mediated crystallization subprocesses. The former process can be observed by the bottleneck-like connection and the distribution of gold nanoparticles inside the microspheres. After aggregation, surface-mediated crystallization dominated. The HAP nanoneedles first formed at the ACP-solution interface and extended outward radially. In this process, the ACP microsphere provides a template and nutrient for the growth and assembly of HAP nanoneedles, which is consistent with the well-known modified Kirkendall process. Neither the dissolution-recrystallization process nor the internal rearrangement mechanism is applicable for the explanation of such phenomena. Accordingly, the aggregation and Kirkendall process coupled with surface crystallization are proposed for an alternative mechanism for the evolution of ACP into HAP probed by gold nanoparticles. Although biomineralization is a complicated process in the presence of numerous biomolecules, our conceptual mechanism, revealed by the gold probe, may be also applied to understand the phase transformation in living organisms. This study shows that the involvement of gold nanoparticles can provide a new strategy to investigate the detailed evolution process of amorphous materials.