Hydroxyapatite nucleation and growth modulated by amino acid-capped gold nanoparticles: An in vitro study

Surface modification of biomaterials significantly influences implantable materials' surface reactivity to induce an apatite-like layer formation. This study observed the bioactivity and surface reactivity of modified bio-materials (calcium-deficient hydroxyapatite [CDHA] on amino acid-capped AuNPs [AA-AuNPs]) through im-mersion in simulated body fluid (SBF). AuNPs capped with eight different amino acids (AA)-both charged and polar uncharged AAs-showed the ability to induce the biomimetic hydroxyapatite formation. The presence of a hydroxyapatite peak in the X-ray powder diffraction (XRD) pattern indicated hydroxyapatite formation after all samples' immersion in SBF. Moreover, longer immersion time in SBF contributed to the nucleation and growth of hydroxyapatite shown by the changing of the crystal structure including lattice parameters, crystallinity index, and crystallite size. The XRD results confirmed that surface modification using AA-AuNPs significantly affected the orientation of the hydroxyapatite crystal, where AAs served as a binding site for Ca2+ and PO43- ions. Furthermore, grayscale analysis based on scanning electron microscope images showed that charged AAs exhibited a role in hydroxyapatite nucleation and growth while polar uncharged AAs only contributed to hy-droxyapatite growth. The calculated Ca/P ratio also confirmed the formation of hydroxyapatite from CDHA. XRD and scanning electron microscopy/energy-dispersive X-ray spectroscopy results signify the bioactivity of CDHA-modulated AA-AuNPs in hydroxyapatite formation for new bone tissue regeneration.