Revisiting Physicochemical and Biological Properties of Zn²⁺ - Enriched Hydroxyapatite

This nanotechnology-oriented study provides insights into the nanoscale structural and compositional modulation of hydroxyapatite. This study investigated the effect of zinc ions (Zn²⁺) content (0-1.8 mol%) in nanocrystalline hydroxyapatite on its physicochemical and biological properties, focusing on biomedical applications.

A series of zinc-enriched nanocrystalline hydroxyapatites was synthesized via aqueous precipitation. Their ultrastructure and crystallinity were characterized by transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD), including unit cell analysis. Chemical composition-specifically OH^-, HPO₄ ^2-, and CO₃ ^2- groups-was examined using Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, and solid-state nuclear magnetic resonance (ssNMR). Zn²⁺ content and release over seven weeks were quantified via flame atomic absorption spectrometry (F-AAS). Cytotoxicity was evaluated using MTT and NRU assays.

Increasing Zn²⁺ concentration led to reduced crystal size and crystallinity. Zinc ions were incorporated both into the crystalline core and the hydrated surface layer of hydroxyapatite. At concentrations ≥1.0 mol%, an amorphous zinc phosphate phase appeared. Higher Zn²⁺ levels also correlated with decreased hydroxyl groups and carbonate impurities, accompanied by increased water content and acidic phosphate groups. Zinc ion release remained minimal across all samples, independent of the initial zinc concentration. Cytotoxicity assays revealed that samples containing 0-0.6 mol% Zn²⁺ were non-toxic, while those with 1.0 mol% and 1.8 mol% Zn²⁺ exhibited cytotoxic effects.

Zn-doped hydroxyapatite containing up to 0.6 mol% Zn²⁺ exhibits enhanced structural stability and cytocompatibility, establishing 0.6 mol% as the optimal threshold for biomedical applications.