UC Santa Barbara

Biological organisms are able to selectively synthesize and stabilize different polymorphs of calcium carbonate, including metastable vaterite and amorphous calcium carbonate (ACC), in a process known as biomineralization. Stabilization is accomplished by introducing other ions, such as magnesium or silicate, into the calcium carbonate material to stabilize the formation of less energetically favorable polymorphs, and also via interactions of biomolecules, such as proteins or saccharides, with the particle surface to slow the kinetics of the crystallization processes. In this work, the effects of two different saccharide surface additives, glucose and sucrose, on the crystallization process of amorphous calcium carbonate were investigated. The crystallization process was characterized using solid-state nuclear magnetic resonance (NMR) spectroscopy, wide-angle x-ray diffractometry (XRD), transmission and scanning electron microscopy (TEM and SEM), and nitrogen adsorption porosimetry. Both the glucose and sucrose adsorbates delayed the onset and progression of crystallization of amorphous calcium carbonate, though the respective products of the crystallization process differed. Adsorbed glucose surface favored the formation of the thermodynamically stable calcite polymorph, while adsorbed sucrose favored the formation of the metastable vaterite polymorph. The observed differences likely arose from the fact that glucose is a reducing saccharide that interacted with the surface of the amorphous calcium carbonate particles primarily via electrostatic interactions, while sucrose, a non-reducing saccharide, interacted with the particle surfaces via hydrogen bonding in addition to electrostatic interactions. Furthermore, initial studies of adsorbed maltose (reducing saccharide) and trehalose (non-reducing saccharide) revealed that these surface additives exhibited the same inhibition of crystallization.