Angew.Chem.Int.Ed.http://doi.org/f2ppq8(2014)

Credit: © 2014 WILEY

Sharks have extremely healthy teeth, not only because they shed them often, but also because they are protected by a very hard layer of enameloid that guards against caries. This biomineral consists of bunches of single-crystal fluorapatite nanorods with a matrix of organic material and knowing its exact structure could explain its impressive properties — which might ultimately provide useful information for human dentistry. Direct imaging of biominerals with single-atom sensitivity, however, has proved difficult. This is because acquiring transmission electron microscopy images with high resolution requires a strong electron beam, which damages the biomineral and consequently it has so far only been possible to obtain nanoscale structural information.

Using a new aberration-corrected scanning transmission electron microscopy technique with a 'low-dose' electron beam, Zhongchang Wang from Tohoku University has led a team of researchers that now present an atomically resolved image of the complex fluorapatite structure of shark teeth. High-angle annular-dark-field images can resolve the heavier calcium atoms, but it is the structure of the light fluorine atoms that are most interesting. After all, it is the caries-reducing effects of fluorine that have saved many a human tooth. The team accomplishes this task with annular-bright-field images revealing the exact positions of the fluorine atoms in the fluorapatite.

The fluorine atoms are shown to be surrounded by a hexagon of calcium atoms. Adjacent to these hexagons reside equilateral triangles that feature pairs of closely spaced oxygen atoms at their tips and another calcium atom in the centre. Contour plots of this structure's charge density — obtained from density functional theory calculations — suggest that the calcium–fluorine bond has a highly covalent character. This bonding might be the secret to healthy teeth because it seems to be the strength-limiting factor. With this new low-dose electron microscopy method in the imaging toolkit, we can expect a wealth of atomic-scale information on various biominerals soon.