Morphological Changes, Antibacterial Activity, and Cytotoxicity Characterization of Hydrothermally Synthesized Metal Ions-Incorporated Nanoapatites for Biomedical Application

The objective of this study was to prepare hydroxyapatite (HA) with potential antibacterial activity against gram-negative and gram-positive bacteria by incorporating different atomic ratios of Cu(2+) (0.1–1.0%), Mg(2+) (1.0–7.0%), and Zn(2+) (1.0–7.0%) to theoretically replace Ca(2+) ions during the hydrothermal synthesis of grown precipitated HA nanorods. This study highlights the role of comparing different metal ions on synthetic nanoapatite in regulating the antibacterial properties and toxicity. The comparisons between infrared spectra and between diffractograms have confirmed that metal ions do not affect the formation of HA phases. The results show that after doped Cu(2+), Mg(2+), and Zn(2+) ions replace Ca(2+), the ionic radius is almost the same, but significantly smaller than that of the original Ca(2+) ions, and the substitution effect causes the lattice distance to change, resulting in crystal structure distortion and reducing crystallinity. The reduction in the length of the nanopatites after the incorporation of Cu(2+), Mg(2+), and Zn(2+) ions confirmed that the metal ions were mainly substituted during the growth of the rod-shape nanoapatite Ca(2+) distributed along the longitudinal site. The antibacterial results show that nanoapatite containing Cu(2+) (0.1%), Mg(2+) (3%), and Zn(2+) (5–7%) has obvious and higher antibacterial activity against gram-positive bacteria Staphylococcus aureus within 2 days. The antibacterial effect against the gram-negative bacillus Escherichia coli is not as pronounced as against Staphylococcus aureus. The antibacterial effect of Cu(2+) substituted Ca(2+) with an atomic ratio of 0.1~1.0% is even better than that of Mg(2+)- and Zn(2+)- doped with 1~7% groups. In terms of cytotoxicity, nanoapatites with Cu(2+) (~0.2%) exhibit cytotoxicity, whereas Mg(2+)- (1–5%) and Zn(2+)- (~1%) doped nanoapatites are biocompatible at low concentrations but become cytotoxic as ionic concentration increases. The results show that the hydrothermally synthesized nanoapatite combined with Cu(2+) (0.2%), Mg(2+) (3%), and Zn(2+) (3%) exhibits low toxicity and high antibacterial activity, which provides a good prospect for bypassing antibiotics for future biomedical applications.