Influence of Phosphate Ions on the Stability of Amorphous Calcium Carbonate

Akhtar, Shahmima; Barralet, J.E.; Farrar, David; Wright, Adrian J.

doi:10.4028/www.scientific.net/kem.309-311.179

Cited by 7 publications

(14 citation statements)

References 11 publications

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“…9 Synthetic ACC is highly reactive and the reported preparative routes require meticulous control of reaction conditions, including temperature and pH, to obtain the unstable product. 10,11 Amorphous calcium orthophosphates (ACP) have also been widely studied, although their involvement in biomineralisation processes in mammals is still the subject of much controversy. While there is no irrefutable proof of the involvement of ACP as a precursor to hard tissue, increasing evidence is now being reported to support this supposition and the reader is directed to the literature for a more detailed discussion (see, for example, studies by Rey et al 12 or Nancollas et al 13 for further details).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2011

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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 (P 2 O 7 4À ); species themselves ubiquitous in vivo. Ambient temperature precipitation reactions were employed to synthesise amorphous Ca 2 P 2 O 7 .nH 2 O and Sr 2 P 2 O 7 .nH 2 O (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 A in both phases, with this local order found to resemble crystalline analogues. Further studies, including 1 H and 31 P 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 P 2 O 7 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 P 2 O 7 anions, leading to the hydrolysis of some P-O-P linkages and the formation of HPO 4 2À anions within the amorphous matrix. The latter anions then recombined into P 2 O 7 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. IntroductionA growing awareness of the existence and potential of amorphous inorganic materials has led to a rapid increase in their study in recent years. Often previously neglected or undetected as a consequence of their lack of long-range structural order and relative instability, amorphous inorganic materials are now more readily evident, partly through the continuing development of techniques sensitive to local order, such as magic angle spinning (MAS) solid state NMR and atomic pair distribution functions. Of particular interest has been the discovery of their prevalence in a diverse range of biological systems, 1-8 where it appears organisms exploit their unique properties in a number of processes.Amorphous calcium carbonate (ACC) is one of the most widely studied amorphous materials in nature, 1,2 with numerous studies on marine creatures, 9 where ACC has been shown to be a transient precursor to hard external shell formation. In effect, the highly unstable and soluble ACC is used as a reactive store of calcium and carbonate ions whose transformation in...

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Section: Introductionmentioning

confidence: 99%

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

et al. 2011

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“…A range of methods have been applied, 7,19,20 with insight into the formation of synthetic ACC being provided by Wegner et al who discussed the mechanism in terms of a liquid-liquid phase separation between calcium carbonate hydrated phases of high and low concentration, such that droplets are formed spontaneously through binodal decomposition of the supersaturated solution. [21][22][23] Typically conferring greater stability to the amorphous phase, additives including magnesium 7,[24][25][26] and phosphate 17,27 are also widely used in the synthesis of ACC. Providing a biomimetic approach to its formation, organic additives including poly(acrylic acid) [28][29][30] and poly-(aspartic acid), 31 dendrimers, 32 block copolymers 33,34 and phytic acid 35 are effective in stabilising ACC.…”

Section: Introductionmentioning

confidence: 99%

Synthesis-dependant structural variations in amorphous calcium carbonate

et al. 2007

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“…Sodium hydrogen carbonate (NaHCO 3 , VWR, 27778.293, 100.1%), 13 ) was synthesised by condensation of sodium hydrogen phosphate (Na 2 HPO 4 , Merck, 1.06586.2500, 99.0%) overnight at 400 °C, following the method of Mayen et al 45 The X-ray diffractogram and FTIR spectrum of the resulting powder are presented in Fig. S1.…”

Section: Reagent and Precursor Saltsmentioning

confidence: 99%

“…In view of increasing the signal to noise ratio of 13 C solidstate NMR measurements, we prepared ACC powders stabilised with 5 or 10% of pyrophosphate and enriched in 13 C (5PyACC13 and 10PyACC13). We followed the standard synthesis protocol reported above except that sodium hydrogen carbonate salt was replaced, at the same molar quantity, with a 13 C-enriched sodium carbonate (Na 2 13 CO 3 , Cambridge IsotopeLab, CLM-306-5, 99%); a lower quantity of sodium hydroxide was necessary to adjust the pH of the obtained anionic solution to 12.1. During vacuum filtration, the precipitate was protected by a parafilm and the obtained lyophilised powder was then stored in a sealed container into the freezer, to avoid or limit carbonate ion exchange with carbon dioxide from air.…”

Section: Synthesismentioning

confidence: 99%

“…12 Several synthesis processes can lead to ACC, the most mentioned being co-precipitation using calcium and carbonate salt precursors in solution. 13,14 The associated formation mechanisms have been described via classical and non-classical nucleation theories. The classical pathway for explaining preferential formation of ACC compared to crystalline CC is based either on the thermodynamically or kinetically-driven route.…”

Section: Introductionmentioning

confidence: 99%

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