Abstract
Amorphous calcium carbonate (ACC) has been widely found in biomineralization, both as a transient precursor and a stable phase, but how organisms accurately control its formation and crystallization pathway remains unclear. Here, we aim to illuminate the role of biologically relevant additives on the phase behaviour of calcium carbonate solution by investigating their effects on the formation of ACC. Results show that divalent cations like magnesium (Mg2+) ions and negatively charged small organic molecules like aspartic acid (Asp) have little/no effect on ACC formation. However, the particle size of ACC is significantly reduced by poly(aspartic acid) (pAsp) with long chain-length, but no effect on the position of the phase boundary for ACC formation was observed. Phosphate (PO4 3-) ions are even more effective in reducing ACC particle size, and shift the phase boundary for ACC formation to lower concentrations. These phenomena can be explained by a cooperative ion-association process where the formation of ACC is only influenced by additives that are able to attract either Ca2+ ions or CO3 2- ions and, more importantly, introduce an additional long range interaction between the CaCO complexes and promote the phase separation process. The findings corroborate with our proposed model of ACC formation via spinodal decomposition and provide a more realistic representation of how biology can direct mineralization processes.
Highlights
According to the spinodal decomposition theory, the fastest growing wavelength of concentration fluctuation will determine PaperJournal of Materials Chemistry B the size of the domain and this characteristic length scale diverges at the spinodal line
Gower et al.[12] showed that negatively charged poly(aspartic acid) (pAsp) with an average molecular weight (Mw) of 8600 is able to induce the formation of a so-called polymer-induced liquid precursor (PILP) phase of calcium carbonate, which consists of shapeless droplets of large dimensions instead of the typical nanospheres
At a concentration of 5 mM, the average diameter of Amorphous calcium carbonate (ACC) spheres decreased from B200 nm for the pure system to B90 nm in the presence of only 0.3% of pAsp[200]
Summary
According to the spinodal decomposition theory, the fastest growing wavelength of concentration fluctuation will determine PaperJournal of Materials Chemistry B the size of the domain and this characteristic length scale diverges at the spinodal line. Results showed that the average diameter of ACC did not change even at a citrate/Ca2+ molar ratio up to 10%.30 This is surprising considering the fact that the stability constant for the complexation of Ca2+ and citrate (B7.0 Â 104 L molÀ1) is more than 40 times higher than that of CaCO03.31 Compared with citrate and Asp, another important characteristic of pAsp is that being a polymer it can locally concentrate the Ca2+ ions and the CaCO03 complexes through the chain.
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