Enhanced stability and local structure in biologically relevant amorphous materials containing pyrophosphate

Abstract

There is increasing evidence that amorphous inorganic materials play a key role in biomineralisation in many organisms, however the inherent instability of synthetic analogues in the absence of the complex in vivo matrix limits their study and clinical exploitation. To address this, we report here an approach that enhances long-term stability to >1 year of biologically relevant amorphous metal phosphates, in the absence of any complex stabilisers, by utilising pyrophosphates (P₂O₇⁴⁻); species themselves ubiquitous in vivo. Ambient temperature precipitation reactions were employed to synthesise amorphous Ca₂P₂O₇.nH₂O and Sr₂P₂O₇.nH₂O (3.8 < n < 4.2) and their stability and structure were investigated. Pair distribution functions (PDF) derived from synchrotron X-ray data indicated a lack of structural order beyond ∼8 Å in both phases, with this local order found to resemble crystalline analogues. Further studies, including ¹H and ³¹P solid state NMR, suggest the unusually high stability of these purely inorganic amorphous phases is partly due to disorder in the P–O–P bond angles within the P₂O₇ units, which impede crystallization, and to water molecules, which are involved in H-bonds of various strengths within the structures and hamper the formation of an ordered network. In situ high temperature powder X-ray diffraction data indicated that the amorphous nature of both phases surprisingly persisted to ∼450 °C. Further NMR and TGA studies found that above ambient temperature some water molecules reacted with P₂O₇ anions, leading to the hydrolysis of some P–O–P linkages and the formation of HPO₄²⁻ anions within the amorphous matrix. The latter anions then recombined into P₂O₇ ions at higher temperatures prior to crystallization. Together, these findings provide important new materials with unexplored potential for enzyme-assisted resorption and establish factors crucial to isolate further stable amorphous inorganic materials.