Nacre surface transformation to hydroxyapatite in a phosphate buffer solution

Abstract

Nacre, also known as mother-of-pearl, constitutes the inner layer of mollusc shells. Nacre is a natural composite material consisting mostly of calcium carbonate in the aragonite crystal form and some organic matter. Previous studies have shown that geological aragonite, coral and nacre can convert hydrothermally to hydroxyapatite (HAP) in phosphate solution by a solid-state topotactic ion-exchange reaction. This conversion typically occurs within the range of 140–260°C, although higher temperatures are possible. In this work, we have found that nacre can transform to HAP in a phosphate buffer solution at room temperature via a surface reaction. The morphology of the nacre-transformed HAP surface was investigated by scanning electron microscopy (SEM). The HAP surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). A layer covered with packed particles was found in contrast to the tablet structure typical of nacre surfaces. XPS and SIMS indicated that the mineral phase of the nacre surface had converted from an aragonite phase to an HAP phase. Fourier transform infrared spectroscopy (FTIR) showed that phosphate (PO 4) bands appeared after nacre was soaked in a phosphate buffer and the intensity of the PO 4 bands increased with exposure time. The FTIR was consistent with XPS and SIMS results. We suggest that this surface reaction occurs by a dissolution–precipitation mechanism. Calcium ions are released from the nacre surface, react with phosphate ions in the buffer solution, and then precipitate as HAP on the nacre surface.