Many organisms use amorphous calcium carbonate (ACC) and control its stability by various additives and water; however, the underlying mechanisms are yet unclear. Here, the effect of water and inorganic additives commonly found in biology on the dynamics of the structure of ACC during crystallization and on the energetics of this process is studied. Total X‐ray scattering and pair distribution function analysis show that the short‐ and medium‐range order of all studied ACC samples are similar; however, the use of in situ methodologies allow the observation of small structural modifications that are otherwise easily overlooked. Isothermal calorimetric coupled with microgravimetric measurements show that the presence of Mg2+ and of PO4 3− in ACC retards the crystallization whereas increased water content accelerates the transformation. The enthalpy of ACC with respect to calcite appears, however, independent of the additive concentration but decreases with water content. Surprisingly, the enthalpic contribution of water is compensated for by an equal and opposite entropic term leading to a net independence of ACC thermodynamic stability on its hydration level. Together, these results point toward a kinetic stabilization effect of inorganic additives and water, and may contribute to the understanding of the biological control of mineral stability.

Biomineralization processes in living organisms result in the formation of skeletal elements with complex ultrastructures. Although the formation pathways in sea urchin larvae are known, the interrelation between calcite, amorphous calcium carbonate (ACC), and intra-crystalline organics in adult sea urchin biominerals is less clear. Here, we study this interplay in the spines and test 2 plates of the Paracentrotus lividus sea urchins whose skeletal elements have optimized functionproperties relationships. Thermogravimetric analysis coupled with differential scanning calorimetry or mass spectrometry measurements, nuclear magnetic resonance technique and high-resolution powder X-ray diffraction show that pristine spines and test plates are composed of Mg-rich calcite and comprise about 10 wt. % of anhydrous ACC, 1.2 to 1.6 wt. % of organics, and less than 0.2 wt. % of water. Anhydrous ACC originates from incomplete crystallization of a precursor ACC phase during biomineralization and is associated with intra-crystalline organics at the molecular level. Molecular interactions at organic/inorganic interfaces cause significant calcite lattice distortions of the tensile type. The latter are amplified during ACC crystallization and finally disappear after heat-assisted destruction of organic molecules. Converting the measured lattice distortions (strains) into internal stress components, we follow stress evolution upon annealing and find that complete crystallization of ACC leads to the isotropy of residual stresses in all investigated skeletal parts. These results allow us to speculate that organic macromolecules are preferentially attached to different crystallographic planes in the pristine test and spine samples.

The mechanisms by which organisms control the stability of amorphous calcium carbonate (ACC) are yet not fully understood. Previous studies have shown that the intrinsic properties of ACC and its environment are critical in determining ACC stability. Here, the question, what is the effect of bulk incorporation versus surface adsorption of additives on the stability of synthetic ACC, is addressed. Using a wide range of in situ characterization techniques, it is shown that surface adsorption of poly(Aspartic acid) (pAsp) has a much larger stabilization effect than bulk incorporation of pAsp and only 1.5% pAsp could dramatically increase the crystallization temperature from 141 to 350 °C. On the contrary, surface adsorption of PO43− ions and OH− ions does not effectively stabilize ACC. However, bulk incorporation of these ions could significantly improve the ACC stability. It is concluded that the stabilization mechanism of pAsp is entirely different from that of PO43− and OH− ions: while pAsp is effectively inhibiting calcite nucleation at the surface of ACC particle, the latter acts to modify the ion mobility and delay crystal propagation. Thus, new insights on controlling the stability and crystallization processes of metastable amorphous materials are provided.

A one-step method is reported to synthesize hybrid gold nanoparticles (AuNPs) by reduction of HAuCl4 in acetic solution in the presence of collagen (Col), dicarboxylic acid-terminated polyethylene glycol (PEG), and cetyltetrammonium bromide (CTAB) mixed with hydoxyapatite (HAP) as surfactants. Such formation process of AuNPs was shown to be responsible for purple stains naturally formed on Egyptianizing archaeological gilded ivories from 8th BC Syria. The understanding of this formation mechanism, which most likely involves a step with hybrid AuNPs, allows the establishing of an authenticity marker of ancient gold-plated ivories.