Biomimetic synthesis of ribbon-like hydroxyapatite employing poly(l-arginine)

Binding of a polypeptide on colloidal particles can affect the dissolution of mineral, initiate crystal nucleation and change the growth kinetics of the precipitated crystal. In this study the synthesis of hydroxyapatite nanocrystals was performed in the presence of poly(L-arginine), PA. Aqueous solutions of Ca(2+), phosphate, and PA were employed at calcium:PA molar ratios ranging from 2:1 to 1:2 and the resulting suspensions were hydrothermally treated at 80 °C for 16 h, or at 130 °C for 6h. The resulting nanomaterials were characterized by XRD, FTIR, TEM, SEM, and TGA. It was found that the presence of PA promotes HAP formation and affects its crystal size and morphology possibly through a rather specific interaction between the homopeptide that is positively charged and also that adopts a β-sheet conformation and the negatively charged c-plane of the growing HAP crystal. In all cases, hexagonal HAP crystals with thin ribbon-like morphology were obtained. Increase of the PA ratio and of the hydrothermal temperature leads to more homogeneous and narrower size distributions with crystallites having widths ranging between 5 to 50 nm and lengths ranging from 50 to 450 nm.