Regulation Of Poly(Aspartic Acid) On The Stabilization And Crystallization Of Amorphous Calcium Carbonate

Biomineralization is the process by which organisms guide the formation of delicate and complex biominerals in an efficient and accurate manner with the participation of organic matter.The use of amorphous phase as the precursor of mature biominerals is a common strategy in biomineralization.Amorphous calcium carbonate（ACC）is widely recognised as a sub-stable precursor to the formation of crystalline biominerals with unique morphology and superior properties,and can have a significant impact on the crystallisation pathway of calcium carbonate.In biomimetic synthesis,modulation of ACC has often been achieved through the addition of soluble additives,with polyaspartic acid（p Asp）being one of the most commonly used organic molecules in in vitro experiments.However,the crystallization mechanism of ACC remains elusive and the mechanism by which p Asp regulates ACC is poorly understood,making it difficult to subtly control the crystallization pathway of ACC.This paper therefore systematically investigates the regulation of ACC stability and crystallization by p Asp in two dimensions:concentration and the chain length.The details of the study are as follows.Firstly,exploring the role of p Asp with the chain length of 10（p Asp-10）in the ACC crystal process.p Asp-10 is effective in improving the stability of ACC in solution and can finely control the crystallization conversion process of ACC.The crystallisation process from ACC to calcite at different concentrations of p Asp-10 is quite different,（1）ACC forms a typical rhombic calcite through a dissolution recrystallisation mechanism.（2）ACC and its small particles form agglomerates that transform into calcite by localised dissolution recrystallisation.（3）ACC transforms into spherulite nanoparticles through a pseudocrystalline transformation mechanism,followed by the formation of micron-sized agglomerates of vaterite,which form calcite through a localised dissolution recrystallisation mechanism.Secondly,the effect of additive concentration on the crystallisation pathways of ACC was further investigated systematically using p Asp with the chain length of 30（p Asp-30）as an additive.The results show that the multiple crystallisation pathways of ACC depend specifically on the concentration of p Asp-30.At low concentrations of p Asp-30,ACC is transformed into a mixture of calcite and vaterite by the typical dissolution recrystallisation mechanism.At moderate concentrations of p Asp-30,ACC is first converted to pure vaterite by a pseudocrystalline transformation mechanism,followed by the aggregation of these vaterite nanoparticles and their conversion to pure calcite by a localised dissolution recrystallisation mechanism.At high concentrations of p Asp-30,ACC is also first converted to pure vaterite,but the aggregation of vaterite nanoparticles is inhibited,eventually forming a mixture of calcite and vaterite.Thus,fine control of polymorph selection between pure calcite and pure vaterite can be achieved by simply varying the concentration of p Asp-30.Finally,the effects of different chain lengths of p Asp on ACC stability,crystal shape selection and crystallization processes were systematically investigated using Asp monomer,p Asp-10,p Asp-30,p Asp-50 and p Asp-100 as additives,respectively.The results show that the stability and crystallization of ACC depend on the chain length of p Asp.The longer the chain length of p Asp,the stronger the binding capacity to free Ca²⁺ions and calcite crystals in solution,and the more pronounced the inhibition effect on calcite.p Asp-100 also significantly retarded the crystallisation process.With increasing chain length,in addition to the classical nucleation and growth processes of solution ions,significant pseudocrystal transformation and particle attachment mechanisms were observed.By integrating and comparing the results under both dimensions,we judged that the effect of p Asp on the stability and crystallisation of ACC is the result of a synergistic effect of concentration and chain length.This study has important implications for understanding the mechanisms behind additive control of amorphous phase crystallisation and for understanding biomolecules in biomineralisation.