|Abstract:||Amorphous calcium carbonate (ACC) is of high interest because of its critical role in biomineralization. ACC can be a precursor of crystalline calcium carbonate phases that are more energetically stable. ACC is not only stored in organisms for long time for the subsequent mineralization of new tissues, but it can also be the final carbonate phase in various organisms. The presence of organic additives has been shown to affect the mineralization pathway in solution by either inhibiting the aggregation of ACC or stabilizing it.
In this study, ACC was precipitated in solution within a poly (acrylic acid) (PAA) matrix. AFM imaging revealed that the PAA-ACC nanoparticles are composed of ACC nanograins with embedded PAA. The influence of organic additives (glucose, aspartic acid, glutamic acid) and Ca2+ on the mineralization and stabilization of PAA-ACC nanoparticles was investigated by measuring the size of the nanoparticles as a function of time using Dynamic Light Scattering (DLS); the adsorption of organic additives to PAA-ACC nanoparticles was investigated using Quartz Crystal Microbalance (QCM). QCM results demonstrate that glutamic and aspartic acid have a stronger interaction with PAA-ACC nanoparticles than glucose, probably because of their net negative charge, but the three of them interact with the nanoparticles, and hence, they can intervene in the stabilization of the nanoparticles. DLS results show that calcium ions can accelerate the aggregation of PAA-ACC nanoparticles by mitigating the electrostatic stabilization provided by PAA, while glucose, glutamic acid and aspartic acid can neutrally influence, inhibit and enhance the aggregation process, respectively. In absence of calcium ions, glucose can promote aggregation of PAA-ACC particles, while glutamic acid has no significant effect on stabilization and aspartic acid will lead to disaggregation. In the presence of calcium ions, synergistic effects between calcium and the organic additives are revealed. The additives also affect the mineralization pathway of ACC, as demonstrated by AFM imaging. With higher concentration of glucose, ACC nanograins become smother and larger, and fill the space within the nanoparticle more efficiently. With higher concentration of aspartic acid, ACC nanograins become smaller and less compact. The study of this biomimetic system will help us understand better how calcium carbonate constituents can be chemically and structurally controlled by the biomineralization process and to be applied to other inorganic systems in advanced materials applications such as additives-control crystallization.