By weaving tiny bits of protein into the lattices of growing crystals, nature long ago figured out how to convert calcium carbonate, a brittle, fragile material, to a tough biomineralized form of calcite, the carbonate-based material from which seashells and some animal claws are made. Just recently, scientists figured out how nature does it.
The new study shows that the mechanism controlling calcite crystallization is directed by surface chemistry, not simple physical processes, as had been commonly assumed. The findings could lead to new strategies for synthesizing tough composites, CO2-storage materials, and nanoparticle-doped crystalline matrices for solar energy applications.
To probe the biomineralization process, De Yoreo and colleagues exposed a freshly cleaved calcite crystal to a concentrated water-based solution of calcium carbonate in an atomic force microscopy liquid sample cell. They spiked the solution with a synthetic diblock copolymer that can form micelles, tiny spherical aggregates that serve as stand-ins for the small bits of protein that naturally make their way into calcite.
The team observed the crystal growing as calcium carbonate from solution added one uneven layer after another to the calcite surface, forming something similar to a jagged staircase, which is typical for calcite. Surprisingly, however, the researchers found that the micelles did not end up distributed randomly across the relatively large terraces, as has been predicted previously.
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Instead, the aggregates bonded selectively to step edges, which are crystal sites rich in so-called dangling bonds. Speaking during a session sponsored by the Division of Environmental Chemistry, De Yoreo explained that these chemically active features arise from the presence of undercoordinated atoms.
The AFM results and computational analysis showed that after bonding to the step edges, micelles become compressed like springs as the crystal continues to grow around them. Eventually, the micelles are trapped inside cavities within the crystal. Just as the crystal exerts compressive forces on the micelles, the micelles push back on the lattice, which enhances its mechanical properties, De Yoreo’s team found (Nat. Commun. DOI: 10.1038/ncomms10187).
Summarizing the findings, De Yoreo noted that “the steps capture the micelles for a chemical reason, not a mechanical one, and the resulting compression of the micelles leads to forces that explain where calcite’s strength comes from.”
“By taking full advantage of AFM’s capabilities for in situ analysis to observe interactions of micelles with mineral surfaces, De Yoreo and colleagues have given us a new understanding of additive-directed mineralization,” said Bucknell University’s Molly M. McGuire, a scanning probe microscopy specialist. She added, “This is a wonderful example of why the snapshots we get from structural mineral analyses alone cannot give us a complete picture of processes at the mineral-water interface.”
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Correction: This story was updated on March 17, 2016, to clarify that the material being described is not just calcite but a biomineralized version of calcite.