Direct visualization of calcium oxalate monohydrate crystallization and dissolution with atomic force microscopy and the role of polymeric additives

The growth and dissolution of calcium oxalate monohydrate (COM) were investigated by real-time in situ atomic force microscopy (AFM). The (100) surfaces of COM crystals were sufficiently rough that direct AFM imaging of terrace growth and step motion was not feasible. In undersaturated aqueous solutions, however, COM crystals dissolved, developing elongated hexagonal pits oriented along the [001] direction and having perimeters defined by {010} and {021} planes, which mimics the habit of the macroscopic crystals. Increasing the concentration of calcium oxalate to supersaturated levels reversed etching, resulting in gradual filling of the pits, which is tantamount to crystal growth. The confinement of growth within the pits permitted observation of crystal growth events, in real time and at the microscopic level, which could not be deduced by inspection of the rough surfaces of the COM crystals. This approach allowed determination of growth and dissolution rates along specific crystallographic directions, as well as the influence of solute concentration and additives on those rates. The polymeric additives tested were effective inhibitors of both COM growth and dissolution (pit filling and etching, respectively). Small amounts of poly(aspartate) (polyD) altered the aspect ratio of the (100) pits during growth and dissolution when compared to the behavior observed in the absence of polymer. The observed effects were consistent with preferential binding of the polymer to the {001} or {021} apical planes relative to the {010} planes. In contrast, poly(glutamate) (polyE), which was approximately 16 times less effective than polyD with respect to suppressing growth or dissolution, altered the aspect ratio in a manner consistent with preferred binding of polyE at the {010} surface. These observations provide a basis for understanding, and potentially regulating, calcium oxalate crystallization in important biomineralization processes, such as kidney stone formation.