Confinement generates single-crystal aragonite rods at room temperature
Proceedings of the National Academy of Sciences of the United States of America
The topic of calcite and aragonite polymorphism attracts enormous interest from fields including biomineralization and paleogeochemistry. While aragonite is only slightly less thermodynamically stable than calcite under ambient conditions, it typically only forms as a minor product in additive-free solutions at room temperature. However, aragonite is an abundant biomineral, and certain organisms can selectively generate calcite and aragonite. This fascinating behavior has been the focus of
... n the focus of decades of research, where this has been driven by a search for specific organic macromolecules that can generate these polymorphs. However, despite these efforts, we still have a poor understanding of how organisms achieve such selectivity. In this work, we consider an alternative possibility and explore whether the confined volumes in which all biomineralization occurs could also influence polymorph. Calcium carbonate was precipitated within the cylindrical pores of track-etched membranes, where these enabled us to systematically investigate the relationship between the membrane pore diameter and polymorph formation. Aragonite was obtained in increasing quantities as the pore size was reduced, such that oriented single crystals of aragonite were the sole product from additive-free solutions in 25-nm pores and significant quantities of aragonite formed in pores as large as 200 nm in the presence of low concentrations of magnesium and sulfate ions. This effect can be attributed to the effect of the pore size on the ion distribution, which becomes of increasing importance in small pores. These intriguing results suggest that organisms may exploit confinement effects to gain control over crystal polymorph. calcium carbonate | biomineralization | bioinspired | biomimetic B iominerals provide a wonderful demonstration of the extent to which crystallization processes may be controlled (1). However, while many of the general strategies that organisms use to control biomineralization are known (2), the mechanisms used to achieve control over polymorph remain unclear. From the outset of the field of biomineralization, researchers have isolated proteins entrapped within calcite and aragonite biominerals, with the expectation that these would enable polymorph control. However, despite a few isolated reports (3), it appears that the mechanism is not so simple. There are also few examples of synthetic organic additives which induce aragonite precipitation at room temperature in the absence of magnesium ions (4-6). Nevertheless, there is a general strategy that reproducibly generates aragonite in the presence of organic additives: insoluble organic matrices containing soluble additives. Aragonite has been precipitated within cross-linked collagen films in the presence of poly(aspartic acid) and poly(glutamic acid) (7), and within reacetylated chitosan thin films (8) and poly(vinyl alcohol) matrices in the presence of poly(acrylic acid) (PAA) (9, 10). A more elaborate scaffold mimicking the organic matrix in which nacre forms was also created from β-chitin, silk fibroin, and macromolecules extracted from the aragonitic or calcitic layers of a mollusk; aragonite or calcite precipitated according to whether the macromolecules had been extracted from the aragonite or calcite biomineral, respectively (11). Finally, the acidic matrix protein Pif promoted aragonite precipitation between a chitin membrane and glass slide (12) . Common to all of the above systems is that the crystals form in defined microenvironments rather than in bulk solution, which is a feature that is intrinsic to all biomineralization processes. However, their complexity makes it difficult to investigate the factors that give rise to aragonite formation, and the role played by confinement. The work described here employs a simple system-crystallization within the cylindrical pores of track-etched (TE) membranes-to systematically investigate how confinement influences calcium carbonate polymorph. Our data show that aragonite forms in increasing quantities as the pore size decreases, and that low concentrations of magnesium and sulfate ions support the formation of pure aragonite in larger pore sizes than under additive-free conditions. Magnesium and sulfate ions are significant components of the seawater in which many biomineralizing organisms live and promote aragonite formation (13-15). It would hence be surprising if these ions do not contribute to aragonite formation in vivo (16) . That significant quantities of aragonite are formed in pores as large as 200 nm when magnesium and sulfate ions are present is also of direct relevance to calcium carbonate biomineralization, where organisms such as mollusks and coccoliths can generate nanosized CaCO 3 ; pteropods form beautiful shells comprising curved aragonite nanofibers 50-500 nm thick (17), while Significance Calcium carbonate is a widespread compound whose two common crystalline forms, calcite and aragonite, are important biominerals. Although aragonite is only marginally less stable than calcite under ambient conditions, it usually only crystallizes from solution at high temperatures or in the presence of magnesium ions. However, organisms readily form both calcite and aragonite biominerals, a capacity usually attributed to the action of specific organic macromolecules. By investigating calcium carbonate precipitation in submicron pores we here show that aragonite is promoted in confinement and that pure aragonite crystallizes in nanoscale pores in the absence of any additives. This is of great significance to biomineralization processes, which invariably occur in small volumes, and suggests that organisms may exploit confinement effects to control polymorph.