Abstract:
Many organisms produce mineral structures. Growth of the minerals is manipulated to impart properties that are beneficial to the organism (such as mechanical strength of an exoskeleton). The highly controlled manipulation is often achieved by the use of an ordered organic framework. The organic layer in the nacre (iridescent layer) of molluscs, such as the paua, consists mainly of the polysaccharide chitin. This work relates to mimicking the organic framework, by changing the conformation and ionic environment of the polymer and elucidating its role in controlling calcium carbonate nucleation and growth. A soluble derivative of chitin, chitosan, was used to manipulate the growth of calcium carbonate. Specifically, calcium carbonate was grown with the polymer as an oligomer and a low molecular weight polymer in solution, and on/in a polymer film. Poly(acrylic acid) and poly(aspartic acid) were also used as crystal growth modifiers to mimic the role of acidic proteins in the shell.
Chitosan in solution assembles to form an upper surface at the air water interface which is preferential for nucleation and growth. The recipitation of chitosan leads to the formation of a concentration gradient in chitosan, and in the opposite direction a concentration gradient in Ca2+. The chitosan that remains in solution inhibits growth. The viscosity of the solution influences the morphology by limiting the diffusion of ions across the crystal surface. Chitosan oligomers remain in solution and show specific binding to growing faces, resulting in the formation of elongated crystals. PAA induces the formation of colloidal calcium carbonate crystals that aggregate producing layer clusters or strings of particles. PAsp induces multinucleation clusters with the oligomers and elongated crystals with the polymer solution.
Chitosan hydrogel films were also used as substrates for crystallisation. Doping ions into the polymer hydrogel matrix alters film structure. This also alters the morphology of the calcium carbonate crystals produced. In the case of doping Ca2+ and CO2- 3 ions into the polymer matrix, the first ion had a much greater effect, resulting in morphologies similar to only doping with that ion. PAA induces lateral aggregation of the primary particles produced by PAA on the films. PAsp produced rounded morphologies, controlled by PAsp after nucleation. This behaviour is explained in terms of a complex formed between the polymers, and the action of the free polymer in solution. These results support the idea that the chitin layer in nacre acts as a nucleation surface, and shows that the ions present within the film play a crucial role in determining morphology. Furthermore, the use of hydrogel films has potential in the synthesis of composite materials which mimic biominerals.
