Gold-dotted hydroxyapatite nanoparticles as multifunctional platforms for medical applications

A novel synthetic approach for coupling metallic gold nanoparticles (AuNPs) to the facets of hydroxyapatite crystalline nanoparticles (HapNPs) without an additional reducing agent was developed. Hexagonal prism-shaped hydroxyapatite nanoparticles (HapNPs) were first precipitated by a hydrothermal route assisted by citric acid, which left behind carboxylate species adsorbed on the HapNP facets. HapNP were then used as templates for gold precipitation. Upon exposure to gold ions, the carboxylate species on the HapNP prismatic facets were able to trigger the nucleation of metallic gold by reducing the gold ions in situ. The used conditions did not allow for the extensive growth of gold nuclei and hence gold dots with diameters not larger than 2.5 nm were formed on the HapNP facets. The obtained (Hap-AuNP) heteronanostructures combine Hap nanoprisms, a highly biocompatible material easily recognised by the human body, with metallic nanosized gold that imparts a plasmon resonance (SPR) effect to the synthesized nanostructures, which is of great potential for further exploration within the contexts of thermotherapy and imaging strategies. Human mesenchymal stem cells were exposed to a range of Hap-AuNP concentrations, but no cytotoxic effects from the heteronanostructures were observed. Furthermore, Hap-AuNP greatly enhanced Runx2 and ALP expressions. In addition, these results predict that the synthesized Hap-AuNP particles could hold potential interest for medical applications requiring nontoxic materials capable of bone-defect repair and furthermore exhibiting SPR properties, which can be potentially advantageous for therapeutic and diagnostic purposes.