Conversion of biogenic aragonite into hydroxyapatite scaffolds in boiling solutions

Abstract

The interaction of Sepia officinalis cuttlefish bone, made of aragonitic CaCO₃, with (NH₄)₂HPO₄ boiling solutions produces porous carbonate-bearing hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) scaffolds with extremely large specific surface area. Percentages of aragonite into hydroxyapatite conversion close to 90% are reached after 10 hours for all the conditions explored, though higher efficiency of the transformation process is achieved by increasing the concentration of the (NH₄)₂HPO₄ solution. The conversion has a pseudomorphic character since it occurs with the accurate preservation of the external shape as well as the primary macro- and microtextural features of the biomineral. Moreover, the mineral replacement transformation is accompanied by a significant increase in surface area, which most likely relates to newly formed porosity. We proposed that this transformation occurs through an interface coupled dissolution–crystallization mechanism, which is continuously promoted by the release of CO₂ to the atmosphere. The generation of newly formed porosity would be the consequence of the preservation of the biomineral's external shape, which requires the balance of both molar volume and solubility changes involved in the mineral transformation.