Physical Mechanisms of Crystal Growth Modification by Biomolecules

During the process of biomineralization, living organisms use macromolecules to direct the nucleation and growth of a variety of inorganic materials. Because biomineral structures exhibit complex topologies, hierarchical design, and unique materials properties, an understanding of the underlying mechanisms of biomolecular controls over mineral growth presents an opportunity to develop new strategies towards synthesis of novel materials for applications across a wide range of technologies. Herein the results from a combination of in situ atomic force microscopy (AFM) and molecular modeling (MM) studies to investigate the effect of specific interactions between carboxylate-rich biomolecules and atomic steps on calcium carbonate and calcium oxalate crystal surfaces during the growth are reviewed. The results show how the stereochemical relationships between additive and atomic step leads to modifications of crystal shape. In some cases, the inhibitory effects of strong binders are well-explained by a model of growth inhibition based on the classic Cabrera-Vermilyea theory, but updated to take into account the particular nature of biomolecular adsorption dynamics. The consequences include a positive feedback between peptide adsorption and step inhibition that results in bistable growth with rapid switching from fast to near-zero growth rates for very small changes in supersaturation. The phenomenon of biomolecule-induced growth acceleration is also reviewed and shown to be common to both the oxalate and carbonate systems. The source of acceleration is related to the activation barrier for solute attachment to steps. Finally, experimental and theoretical results are presented that suggest most biomineral phases can not be described by conventional models in which kink formation due to thermal fluctuations at step edges is rapid enough to ensure the availability of kinks. Instead, growth is kink-limited. As a consequence, biomolecule-step interactions cannot be interpreted with traditional thermodynamic models based on minimization of the Gibbs free energy. Instead these interactions follow a different mechanism determined by the kinetics of attachment and detachment. The general nature of these findings support the plausibility of their application to industrial systems.