Science Concentrates: ACS meeting news MATERIALS
How nature grows one tough biomineral Study points to chemistry’s role in trapping organics and forming calcite nism 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 and CO2 storage materials. Materials scientist James J. De Yoreo of Pacific Northwest National Laboratory described his team’s study at the American Chemical Society national Compressed Micelle meeting in San Diego last week. micelle To probe the biomineralization process, De Yoreo and colleagues exposed a freshly cleaved calcite crystal to a concentrated water-based solution Stage 1 Stage 2 of calcium carbonate in an atomic force microscopy liquid samCavity Buried ple cell. They spiked the solution cavity 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. Stage 3 Stage 4 The team observed the An AFM study shows that micelles attach selectively crystal growing as calcium to certain calcite crystal features and compress carbonate from solution added (stage 2) as the crystal grows and forms a cavity one uneven layer after another around the micelle, finally swallowing it (stage 3-4). to the calcite surface, forming
NAT. COMMUN.
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 the 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 mecha-
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. Instead, the aggregates bonded selectively to step edges, which are rich in “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 (Nat. Commun. 2016, DOI: 10.1038/ncomms10187). “By taking full advantage of AFM’s capabilities for in situ analysis, De Yoreo and colleagues have given us a new understanding of additive-directed mineralization,” said Bucknell University’s Molly M. McGuire. 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.”—MITCH JACOBY
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C&EN | CEN.ACS.ORG | MARCH 21, 2016
