In Situ Detection of Calcium Phosphate Clusters in Solution and Wet Amorphous Phase by Synchrotron X-ray Absorption Near-Edge Spectroscopy at Calcium K-Edge

Calcium phosphate clusters are present in both aqueous solutions and an amorphous phase during crystallization. It is a challenging task to acquire the structural information on such clusters, owing to their small size, chemical lability, and inaccessibility to most detection techniques. Here, we demonstrate the feasibility of detecting such clusters in situ by synchrotron X-ray absorption near-edge spectroscopy at calcium K-edge, a technique that is sensitive to the short-range order in calcium coordination sphere. At the initial stage of crystallization, the most abundant clusters are detected to be Ca(eta(2)-PO43-)(2)L-2 (L = H2O or eta(1)-PO43-) in solution. More reactive clusters engage in the development of an amorphous phase via growing and fusing. The amorphous phase exhibits a dual character in its short-range order: Some of its clusters are similar to hydrated calcium ions, and some others to those in crystalline hydroxyapatite. When the amorphous dissolves, the detected unit is mainly Ca(eta(2)-PO43-)(H2O)(4) in the released solution clusters. While these findings provide a basis for a better understanding and rational control of calcium phosphate crystallization at molecular level, the experimental technique in assessing wet samples adopted in this work might be applicable to the crystallization studies of other materials as well.