Biomimetic Mineralization Of Calcite In Vitro-An Atomic Force Microscopy Study

An intensive understanding of biomineralization and its mechanisms not only directs thesynthesis of hybrid materials with both hierarchical structure and functionality, but alsoprovides new approaches for the resolution of pathological mineralization in human body.Calcite is widely distributed in nature and has significant implications in biology andgeochemistry. Therefore it is selected as an object of study so as to examine the effects oforganic additives on crystal growth and dissolution using AFM. The study aims at revealingthe interfacial interaction mechanisms at molecular level and providing new understandingsfor the synthesis of biomaterials.The calcite (104) surface dissolves in the form of step retreat and etch pit nucleation indeionized water. The rhombohedral etch pits are monolayer or multilayer deep and arecomprised of monomolecular steps. These etch pits coalesce when they are expanding in fourcrystallographic directions. For the disparity of crystal defects, the formed pits can be dividedinto nucleation pits and inverted pyramid pits. The (104) face that has grown for a whileresults in higher pit density than the inherent (104) face when exposed to deionized water. InCaCO₃ supersaturated solution, crystal growth occurs via step advancement, surfacenucleation and spiral growth which are closely related to supersaturation state and surfacedefects. Step velocity mainly depends on supersaturation and solution flow rate. Crystalgrowth is only surface interaction controlled when flow rate is higher than 0.5 ml/min.In the presence of carboxylic acids and amino acids, the etch pit shape is dependent onadditive concentration, solution pH and fluid flow. Under the same condition, the developedpit shape highly depends on the conformation of additives, especially the distances betweenfunctional groups. Pit shape becomes irrespective of additive concentration when the additiveconcentration exceeds 50 mmol/L. Fluid flow affects the mass transfer at the solid/fluidinterface, which results in the change of etch pit shape. The effect of pH results from the variation of calcite surface net charge and the protonation state of organic acids. Gly, L-Asp,L-Glu, L-Lys, malonate and succinate have modifications on pit shape while 6-aminohexanoic acid, acetic acid, oxalic acid and glutaric acid have no effect on pit shape.AFM results showed that both the ammonium and carboxylate groups are active in surfaceinteractions and are able to bind with calcium atoms or H-bond with carbonate oxygens.Geometrical matching plays a crucial role in solid/fluid recognition, while stereochemicalcorrespondence, molecular chirality and electrostatic attraction play a secondary role.