Synthesis and Structural Characterization of Morphologically Distinct Nanoscale Hydroxyapatites

Nanoscale hydroxyapatites (HAPs) with different morphologies and crystal phases including highly crystalline nanorods, single-crystal nanowires, and nanosheets with the c or a(b) lattice planes were selectively developed and a semi amorphous form was synthesized and structurally characterized. Powder X-ray diffraction, electron microscopy, and multinuclear magic angle spinning (MAS) solid-state nuclear magnetic resonance (ssNMR) were used for structure determination of the bulk and surface species. Proton MAS ssNMR provides considerable insights into surface hydroxyl defect sites that are significantly more abundant in c-lattice plane-dominated nano sheets compared to the other HAP forms. Through a series of hydration and amino acid adsorption experiments, it is proposed that these hydroxyl defects are likely to be OH groups that bind to surface calcium sites and may be important in biomineralization processes. For a(b) lattice plane-preferred HAP samples, the H-1 MAS NMR reveals a low population of these environments with a spectrum dominated by structural hydroxyls and physisorbed water, while nanosheet morphologies with the a(b) lattice plane selectively developed reveal a considerably higher population of surface-associated defect hydroxyls. This work provides considerable new insights into the structure and surface chemistry of nanoscale HAP where surface hydroxyl environments show a strong dependence upon the morphologically exposed lattice planes.