Stabilization and crystallization mechanism of amorphous calcium carbonate

Amorphous phases hold great promise in diverse applications and are widely used by organisms as precursors to produce biominerals with complex morphologies and excellent properties. However, the stabilization and crystallization mechanisms of amorphous phases are not fully understood, especially in the presence of additives. Here, using amorphous calcium carbonate (ACC) as the model system, we systematically investigate the crystallization pathways of amorphous phases in the presence of poly(Aspartic acid) (pAsp) with various chain lengths. Results show that pure ACC transforms into a mixture of calcite and vaterite via the typical dissolution-recrystallization mechanism and 3 % of Asp monomer exhibits negligible effect. However, pAsp with a chain length of only 10 strongly inhibits the aggregation-induced formation of vaterite spheres while slightly delaying the growth of calcite via classical ion-by-ion attachment, thus kinetically favoring the formation of calcite. Moreover, the inhibition effect of calcite growth from solution ions becomes more prominent with the increase of pAsp chain length or concentration, which significantly improves the stability of the amorphous phase and leads to crystallization of spherical or elongated calcite via the nonclassical particle attachment mechanism after pseudomorphic transformation of ACC into vaterite nanoparticles. These results allow us to reach a more comprehensive understanding of the stabilization and crystallization mechanism of ACC in the presence of additives and provide guidelines for controlling the polymorph selection and morphology of crystals during the crystallization of amorphous precursors.