COMMENTARY Aragonite formation in confinements: A step toward understanding polymorph control COMMENTARY Yifei Xua,b,c and Nico A. J. M. Sommerdijka,b,c,1 Calcium carbonate (CaCO3) is one of the most com- unclear but were generally attributed to the interac- mon minerals on Earth; it not only forms rocks like tion between the acidic functional groups of the limestone or marble but is also a main component of biomacromolecules and the mineral components. Ad- biominerals such as pearls, the nacre of seashells, and ditionally, it was reported that a macromolecular sea-urchin skeletons (1). Despite many years of re- hydrogel-like 3D network is formed before the miner- search, the polymorphism of CaCO3 is still far from alization of nacre (11), which may play a role in the being understood. CaCO3 has three anhydrous crys- crystallization process by confining the crystallization talline forms: calcite, aragonite, and vaterite, with a to defined small volumes. Nonetheless, confinement decreasing thermodynamic stability under aqueous has never been directly correlated with aragonite for- ambient conditions (calcite > aragonite > vaterite) mation in biominerals, although its capability of con- (2). While vaterite is rare in nature, calcite and arago- trolling crystal orientation and polymorphism has nite are both frequently found in rocks or biominerals been shown in many recent reports (12–15). (1). A well-known example is the aragonite structure of Now, Zeng et al. (5) explore for the first time the nacre (3), where the organization of the crystals leads impact of confinement on aragonite formation. By to extraordinary mechanical performance. However, precipitating CaCO3 within the cylindrical pores of in synthetic systems, crystallization experiments only track-etched membranes (Fig. 1), they investigated generate a small fraction of aragonite compared with the relationship between pore size and the polymor- calcite at ambient conditions and in the absence of phism of CaCO3. Strikingly, a high level of aragonite additives (4). So, how is the formation of aragonite formation was detected within these nanosized con- + facilitated in nature, especially in biominerals? In finements. Using the same concentrations of Mg2 2− PNAS, Zeng et al. (5) shed light on this matter by and SO4 as additives, the aragonite proportion in showing that aragonite formation is dramatically pro- bulk solution was only 7%, while this value increased moted within confinements. to 69% in 200-nm pores and reached 100% in 50-nm In recent decades, great efforts have been spent pores. When even smaller pores were used (25 nm), toward understanding the strategies exploited by pure aragonite crystals were obtained even in the ab- organisms to regulate aragonite formation, and sev- sence of any additives. More than that, the aragonite eral key factors have been identified. Up to now, the crystallized within these nanopores was mainly single + effect of Mg2 additive is the most well established. crystal rods, highly oriented along the c-axis. + Mg2 is abundant in seawater and is expected to be The authors subsequently examined several possi- present during the formation of many marine biomin- ble origins for the preferred formation of aragonite + + erals (6, 7). At high Mg2 :Ca2 ratios, aragonite forms within these confinements. Confinement is known to as the major crystalline form instead of calcite at room increase the incubation time for crystallization nucle- temperature. This was recently explained by Sun et al. ation, inhibiting the formation of thermodynamically + (8), who show that Mg2 can significantly increase the stable phases (e.g., calcite) in favor of metastable surface energy of calcite and raise its nucleation bar- phases (13). This is, however, not likely the reason for rier, while aragonite is much less affected. Meanwhile, aragonite formation here, since aragonite is seldom insoluble organic matrices and soluble acidic macro- seen as a precursor to calcite. Computations demon- molecules extracted from aragonite-forming tissues strated that the reaction was also not affected by a also favor aragonite formation to different extents variation of diffusion rates. Hence, the only reasonable (9, 10). Detailed mechanisms of the effects remain cause for the promotion of aragonite formation seems aLaboratory of Materials and Interface Chemistry, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands; bCenter for Multiscale Electron Microscopy, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands; and cInstitute for Complex Molecular Systems, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands Author contributions: Y.X. and N.A.J.M.S. wrote the paper. The authors declare no conflict of interest. Published under the PNAS license. See companion article on page 7670 in issue 30 of volume 115. 1To whom correspondence should be addressed. Email: [email protected]. Published online August 3, 2018. www.pnas.org/cgi/doi/10.1073/pnas.1811696115 PNAS | August 21, 2018 | vol. 115 | no. 34 | 8469–8471 Downloaded by guest on September 27, 2021 Fig. 1. Scheme of CaCO3 crystallization within the cylindrical nanopores of track-etched membranes. While crystals formed in the bulk solution are mainly calcite, aragonite single crystals oriented in the c-axis are formed within the nanopores. to be the influence of the pore surface on crystal nucleation. In- ∼500-nm-thick aragonite layers (17). As this dimension is still a bit deed, aragonite formation was further promoted when smaller above the largest effective pore size (200 nm) reported by Zeng pores were used and larger pore surfaces were generated. Zeng et al. (5), it is likely that in addition to confinement also the in- et al. (5) suggest that the pore surface may modulate ion activity teraction between the mineral and biomacromolecular matrix (9– and thus facilitate aragonite formation. Unfortunately, experimen- 11) plays a role in the preferred aragonite formation in nacre. tal confirmation was hindered by the difficulty in directly measur- A similar fundamental question remains on how confinement is ing the ionic profiles within the nanopores. correlated to the selection of the aragonite polymorph. Zeng et al. By showing that pure aragonite single crystals can be synthe- (5) show that the two main volume effects of confinement, that is, sized at ambient conditions by using only nanosized confinements, inhibiting nucleation and limiting diffusion rate, are both irrelevant the work of Zeng et al. (5) provides a route for CaCO3 polymorph to the preferred aragonite formation, and the main role of con- control that may possibly also be applied by biomineralizing organ- finement appears to be enhancing the surface effect. Indeed, isms. In particular, it points to the importance of surface effects in when track-etched membranes from different manufacturers were controlling aragonite formation. This also aligns with the computa- used, different CaCO3 polymorphs formed within pores of the + tional work of Sun et al. (8) showing that the presence of Mg2 same size (15). This was attributed to possible differences in the favors aragonite formation by modulating the surface energy of density or conformation of the chemical species lining the mem- the crystals. Nevertheless, the details of the mechanism by which brane pores, as only minor differences of surface roughness and surface effects facilitate aragonite growth for now remain unknown. no differences in composition were detected for these mem- The surface interactions may modulate the CaCO3 polymorph branes. Hence, maybe not only aragonite but also other poly- through tuning the distribution of ions, as proposed by the authors; morphs of CaCO3 can be selected by nanosized confinements alternatively, the effect may be due to a lower interfacial energy with the appropriate surface chemistry. This could certainly be a between the pore surface and aragonite, compared with calcite. strategy employed in biomineralization but could also potentially Another intriguing question is how the confinement promotes the provide a new window for controlling polymorphism in the syn- formation of oriented single crystals, as was also reported for other thesis of crystalline materials. In conclusion, the results of Zeng mineral systems (14–16). Clearly, future investigations of crystalliza- et al. (5) are of great significance for the understanding of poly- tion within porous membranes are likely to give us more important morph control, which has seen some recent interesting advances insights into the control of crystal orientation and polymorphism (18, 19) but is still in its infancy despite its importance, in particular with possible relevance for both synthetic and biological systems. in the preparation of pharmaceuticals. Meanwhile, to what extent organisms indeed deploy nano- sized confinements to promote aragonite formation is a question Acknowledgments that needs further discussion. One interesting system is nacre, This work was supported by a TOP-PUNT grant from the Netherlands Organi- where lamellar organic matrices were found between adjacent zation of Scientific Research. 1 Lowenstam HA, Weiner S (1989) On Biomineralization (Oxford Univ Press, New York). 2 Plummer LN, Busenberg E (1982) The solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90 C, and an evaluation of the aqueous model for the system CaCO3-CO2-H2O. Geochim Cosmochim Acta 46:1011–1040. 3 Jackson A, Vincent JF, Turner R (1988) The mechanical design of nacre. Proc R Soc Lond B Biol Sci 234:415–440. 4 Ogino T, Suzuki T, Sawada K (1987) The formation and transformation mechanism of calcium carbonate in water. Geochim Cosmochim Acta 51:2757–2767. 5 Zeng M, et al. (2018) Confinement generates single-crystal aragonite rods at room temperature. 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