Zinc incorporation into hydroxylapatite

Area of Science:

• Materials Science
• Solid-State Chemistry
• Biomineralization

Background:

• Hydroxylapatite is a key biomaterial with applications in bone regeneration and drug delivery.
• Understanding trace element incorporation, such as zinc (Zn), is crucial for tailoring material properties.
• The precise local structure of incorporated Zn in hydroxylapatite has not been fully elucidated.

Purpose of the Study:

• To determine the preferred crystallographic site and coordination geometry of Zn within the hydroxylapatite lattice.
• To investigate the local coordination environment of Zn using complementary theoretical and experimental techniques.

Main Methods:

• Density Functional Theory (DFT) modeling to predict favored Zn substitution sites and coordination.
• X-ray Absorption Spectroscopy (XAS), including X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS), to probe the local atomic structure around Zn.

Main Results:

• DFT calculations indicate Zn preferentially occupies the Ca2 site over the Ca1 site, favoring tetrahedral coordination.
• XANES spectroscopy reveals a single dominant coordination environment for incorporated Zn, with no evidence of secondary Zn phases.
• EXAFS fitting confirms tetrahedral coordination of Zn, with characteristic distances to neighboring P and Ca atoms.

Conclusions:

• Zn substitution in hydroxylapatite predominantly occurs at the Ca2 site with tetrahedral coordination.
• The experimental findings from XAS are in excellent agreement with the theoretical predictions from DFT.
• This study provides a detailed understanding of Zn's local structure in hydroxylapatite, relevant for biomaterial design.