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Mineralization of CaCO³ and SiO² within functionalized hydrogel networks emulating natural matrices

Subject Area Biological and Biomimetic Chemistry
Term from 2001 to 2006
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 5313096
 
Final Report Year 2013

Final Report Abstract

Biomineral structures invariably contain organic substances in the form of network matrices and/or unconnected macromolecules. These organic substances certainly play a role in biomineralization processes. In many cases, gelatinous organic network matrices are formed prior to the onset of mineralization. Mineral precipitation takes place inside the network from the supersaturated pore fluid. The precipitate that forms can be influenced through functional (mostly anionic) molecular groups bound to the network matrix and also through polyanionic molecules dissolved in the fluid from which precipitation takes place. Anionic functional groups may interact with cations that are either dissolved or a constituent of a crystal surface. As an approach to understanding the role of organic matrices in biomineralization, CaCO3 crystal growth experiments were conducted in gelatinous matrices with a focus on polyacrylamide (pAAm) hydrogel as a model system. Surprisingly, these hydrogel matrices change the pattern of calcite crystal growth from normal rhombohedral habits to aligned cluster-like aggregates with octahedral morphology and incorporated organic matrix. This growth pattern develops after the growing crystal initially had displaced the organic network and compresses it at the crystal surface, forcing the crystal to grow around the organic matrix and incorporate it. The calcite clusters grow by aggregation of nanoparticles at the crystal surface, which then coalesce in a crystallographically aligned way. These observed crystal growth phenomena may also play a role in biomineralization. Taking the next step towards biomimetics, the pAAm-matrix was functionalized with anionic i) sulfonate groups, ii) carboxylate groups. While in sulfonate-functionalized pAAm hydrogel the morphology of the calcite clusters changed without changing the precipitation pattern and cluster structure, carboxylate functionalization caused an initial precipitation of vaterite particles as a transient phase, which was consumed and replaced by larger spherical calcite particles. Addition of polyanionic poly-L-aspartate molecules to the pore fluid in pAAm-hydrogel resulted in the precipitation of spherical vaterite particles as the stable phase. Additional helical vaterite structures were observed to protrude from the spherical particles. Also the effect of cations on gel-grown precipitates was tested. While lithium ions only changed the morphology of the calcite precipitates by stabilizing {00.1} faces, strontium ions precipitated as strontianite on calcite surfaces. Magnesium ions alone favor the precipitation of the CaCO3 polymorph aragonite as spherical particles, while a combination of magnesium ions and poly-L-aspartate in sulfonate-functionalized pAAm hydrogel stabilizes amorphous CaCO3. To further investigate the influence of poly-L-aspartate on calcite growth, experiments were conducted that could be observed in-situ with an atomic force microscope (AFM). The polyanionic molecules were shown to adsorb on the calcite surface and to induce subsequently a nanocluster-like pattern of crystal growth, reminiscent to that in pAAm hydrogel. As a control of the success of our biomimetic approaches, we investigated a number of natural biominerals for their micro- and nanostructures. Biomineral structures, such as sea urchin skeletons, sponge spicules and octocoral sclerites, have been found to grow by aggregation of small particles that coalesce to form solid crystals with complex morphologies. We found striking similarities between natural biominerals and precipitates grown in the laboratory under bio-inspired conditions. Organic matrices of different types obviously override normal crystal growth patterns, such as step advancement on calcite faces, and induce nano-granular or nano-clustered crystal growth structures instead. We suggest that unusual macroscopic crystal structures and the precipitation and stabilization of metastable mineral phases, which both are common features in biomineralization as well as in our experimental systems, is causally connected to the micro- to nano-cluster precipitation pattern observed. This clustered precipitation may be a basic principle in biomineralization.

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