Marine organisms, such as tropical corals and mollusks, are masters of constructing calcium carbonate (CaCO₃) microarchitectures. They can control the crystallization of CaCO₃ and choose between its two most stable polymorphs, aragonite and calcite, to build parts of their skeletons and shells. The mechanism behind the selective synthesis of these biominerals remains unclear.

A team of researchers from the University of Leeds, UK and the Institute of Materials Science of Barcelona (ICMAB), Spain has shown, for the first time known, that confinement in very small volumes promotes the formation of aragonite to such an extent that pure aragonite crystals can form at room temperature in nanoscale pores in the absence of organic additives. Their work, published in PNAS (Proceedings of the National Academy of Sciences of the United States), furthers the understanding of biomineralization processes and offers new tools to control polymorph formation with great potential for synthetic systems.  

The crystallization of aragonite in the laboratory, which is marginally less thermodynamically stable than calcite, typically requires a calcium carbonate solution at high temperatures or the presence of magnesium ions (Mg²⁺). Research on the mechanism behind the selective precipitation of aragonite in vivo has typically focused on the presence of soluble organic additives. These additives, mostly proteins and other synthetic or natural-occurring organic macromolecules, are usually studied in bulk solution.

For Fiona Meldrum, professor at the University of Leeds and head of the research team who conducted the study, one critical aspect of the process is often overlooked. “Biology is nanoscience, everything happens in small compartments. And yet, when we try to mimic that synthetically, typically we grow crystals in bulk solutions. So we were interested in seeing what the effect of confined volumes has on crystallization,” she says.

The results were unexpected, showing that confinement plays an important role in controlling the polymorph structure. The researchers were also able to change the shape and orientation of the crystals produced, yielding entirely different crystals to those formed under the same chemical conditions in bulk solutions.

To systematically study the role of confinement, the research team needed to limit the complexity of the biomineralization process. Their chosen model system was based on track-etched (TE) membranes, which are commercially available filtration membranes with a range of pore sizes from the microscale down to 10 nm. “These membranes offer a well-defined environment and give us the opportunity to systematically vary the confinement by varying the pore size,” Meldrum says.

The membranes separated two half U-tube arms, one of which was filled with a solution of CaCl₂ and the other with a solution of Na₂CO₃, while MgCl₂ and Na₂SO₄, respectively, were also added in some cases. The diffusion of the ions led to CaCO₃ precipitation within the membranes, with nanometer-sized pores of 1200, 800, 200, 50, and 25. The crystals formed were then isolated by dissolving the membrane, and analyzed by transmission electron microscopy (TEM), powder x-ray diffraction (XRD), and other techniques.

Powder XRD showed that the crystals formed within the 1200 nm pores were almost entirely calcite, in common with bulk solution. Aragonite was the only polymorph identified in 25 nm pores under additive-free conditions. In pores of sizes between 800 nm and 50 nm, aragonite production was promoted in the presence of both Mg²⁺ and SO₄²⁻.

Pupa Gilbert, professor of physics at the University of Wisconsin–Madison, says, “The Meldrum group has excelled, yet again, at demonstrating that the role of confinement goes well beyond generating calcite from amorphous calcium carbonate. They now show that reducing the diameter of the cylindrical pores linearly increases the amount of aragonite formed in them.” Gilbert points out that as the diameter approached 25 nm, the research group obtained pure aragonite in calcite-growth conditions. “This is a splendid result, elegant in its linear simplicity and hugely important for our quantitative understanding of biomineralization processes. The challenge ahead is discovering where, when, and how this role is harnessed for the evolutionary advantage of the biomineral-forming organism,” she says.

Meldrum says that the big challenge now is to discover and understand the origins of the effects of confinement in crystallization.

Read the abstract in PNAS.