Tailored order: the mesocrystalline nature of sea urchin teeth.

We investigated the pattern of crystal co-orientation at different length scales, together with variations in chemical composition and nanomechanical properties in the teeth of the modern sea urchin Paracentrotus lividus with electron backscatter diffraction (EBSD), electron probe microanalysis, energy-dispersive X-ray spectroscopy and nanoindentation testing. Modern sea urchin teeth are Mg-dominated calcite composite materials. They are distinctly harder than inorganically precipitated calcite. Some parts exceed even the hardness of dolomite. The teeth show a structuring of their mechanical properties that can be correlated to variations in major element chemical composition, such that their hardness is positively correlated to their magnesium contents. Mg/Ca ratio in Paracentrotus lividus varies between 10 and 26mol.%. Nanohardness of the tooth scatters between 3.5 and >8GPa compared to values of 3.0±0.2, 7.3±0.1 and 9.2±0.9GPa measured on the (104) planes of inorganic calcite, dolomite and magnesite, respectively. High-resolution EBSD shows that major structural units and subunits of the tooth of Paracentrotus lividus are tilted to each other by ∼3-5° and 1-2°, respectively. This indicates that the tooth is not a single crystal. With EBSD we can show that the tooth of the sea urchin Paracentrotus lividus is a hierarchically assembled biological mesocrystal with a mosaic texture. In comparison to the misorientation spread of 0.5° of calcite grown from solution, misorientation in the tooth varies between 2° and 4°. Thus, the self-sharpening feature of the tooth is enabled by a close interplay of its highly evolved micro- to nanostructure, structural unit size variations with a varying degree of crystal orientation, chemical structuring of the mineral component and a gradation of incorporated organic polymers.

Synthesis of well-ordered COF monolayers: surface growth of nanocrystalline precursors versus direct on-surface polycondensation.

Two different straightforward synthetic approaches are presented to fabricate long-range-ordered monolayers of a covalent organic framework (COF) on an inert, catalytically inactive graphite surface. Boronic acid condensation (dehydration) is employed as the polymerization reaction. In the first approach, the monomer is prepolymerized by a mere thermal treatment into nanocrystalline precursor COFs. The precursors are then deposited by drop-casting onto a graphite substrate and characterized by scanning tunneling microscopy (STM). While in the precursors monomers are already covalently interlinked into the final COF structure, the resulting domain size is still rather small. We show that a thermal treatment under reversible reaction conditions facilitates on-surface ripening associated with a striking increase of the domain size. Although this first approach allows studying different stages of the polymerization, the direct polymerization, that is, without the necessity of preceding reaction steps, is desirable. We demonstrate that even for a comparatively small diboronic acid monomer a direct thermally activated polymerization into extended COF monolayers is achievable.

Interdigitating biocalcite dendrites form a 3-D jigsaw structure in brachiopod shells.

We report a newly discovered dense microstructure of dendrite-like biocalcite that is formed by marine organisms. High spatial resolution electron backscatter diffraction (EBSD) was carried out under specific analytical conditions (15 and 10 kV) on the primary layer of the modern brachiopod Gryphus vitreus. The primary layer of modern brachiopods, previously termed nanocrystalline, is formed by an array of concave/convex calcite grains with interdigitated recesses and protrusions of abutting crystals without any cavities in or between the dendrites. The interface topology of this structure ranges from a few tens of nanometres to tens of micrometres, giving a nanoscale structure to the material fabric. The dendritic grains show a spread of crystallographic orientation of several degrees and can thus be referred to as mesocrystals. Individual dendritic mesocrystals reach sizes in one dimension larger than 20 µm. The preferred crystallographic orientation is similar in the primary and adjacent fibrous shell layers, even though these two layers show completely different crystal morphologies and grain boundary topologies. This observation indicates that two separate control mechanisms are active when the primary and the fibrous shell layers are formed. We propose a growth model for the interdigitated dendritic calcite grain structure based on a precursor of vesicles filled with amorphous calcium carbonate (ACC).