Tuning hardness in calcite by incorporation of amino acids

Abstract

(SPB) or y.y.kim@leeds.ac.uk (YYK). 2Structural biominerals are inorganic/organic composites that exhibit remarkable mechanical properties. However, the structure-property relationships of even the simplest building unitmineral single crystals containing embedded macromolecules -remain poorly understood. Here, by means of a model biomineral made from calcite single crystals containing glycine (0-7 mol%) or aspartic acid (0-4 mol%), we elucidate the origin of the superior hardness of biogenic calcite.We analyzed lattice distortions in these model crystals by using x-ray diffraction and molecular dynamics simulations, and by means of solid-state nuclear magnetic resonance show that the amino acids are incorporated as individual molecules. We also demonstrate that nanoindentation hardness increased with amino acid content, reaching values equivalent to their biogenic counterparts. A dislocation pinning model reveals that the enhanced hardness is determined by the force required to cut covalent bonds in the molecules.3 Biominerals such as bones, teeth and seashells are characterized by properties optimized for their functions. Despite being formed from brittle minerals and flexible polymers, nature demonstrates that it is possible to generate materials with strengths and toughnesses appropriate for structural applications 1 . At one level, the mechanical properties of these hierarchically structured materials are modelled as classical composites consisting of a mineral phase embedded in an organic matrix 2 . However, the single crystal mineral building blocks of biominerals are also composites 3 , containing both aggregates of biomacromolecules as large as 20 nm 4,5 and inorganic impurities 6,7 . While it should be entirely possible to employ this simple biogenic strategy in materials synthesis 8,9 , the strengthening and toughening mechanisms that result from these inclusions are still poorly understood 10,11 . This work addresses this challenge by analyzing hardening mechanisms in a simple model biomineral system: calcite single crystals containing known amounts of amino acids. We report synthetic calcite crystals with hardnesses equivalent to those of their biogenic counterparts, and offer a detailed explanation for the observed hardening.Since plastic deformation in single crystals occurs by the motion of dislocations, hardness is enhanced by features that inhibit dislocation motion. The mechanisms by which guest species may harden ionic single crystals generally fall into two categories. Second phase particles directly block dislocation motion, requiring a dislocation to either cut through (shear) a particle or bypass it by a diffusive process to keep going 12 . Solutes (point defects) do not directly block dislocation motion, but the stress fields of the dislocations interact with those associated with misfitting solutes, retarding dislocation motion 12 . Biominerals, notably calcite, often deform plastically by twinning 11 , but since twins grow by motion of "twinning dislocations" 13 , these concep...

3D visualization of additive occlusion and tunable full-spectrum fluorescence in calcite

“…All dyes showed an approximately linear increase in occlusion with the initial [Dye]/[Ca 2+ ], where a similar trend has been observed with the occlusion of aspartic acid and glycine in calcite (Supplementary Fig. 7)40. RED incorporated to the highest extent, followed by BLUE and GREEN; under conditions of [Ca 2+ ]=[CO 3 2− ]=5 mM and [Dye]/[Ca 2+ ]=0.004, RED, BLUE and GREEN are occluded to levels of ≈0.05, 0.02 and 0.002 mol%, respectively.…”

“…Anisotropic lattice distortions are observed, which provide insight into the orientation of the additives within the crystals. It is noted, however, that calcite is elastically anisotropic and more flexible along the c axis than the a axis40. Therefore, greater distortion along the c axis does not necessarily mean that the additive is located on planes perpendicular to the c axis.…”

Occlusion of magnetic nanoparticles within calcium carbonate single crystals under external magnetic field

“…With the incorporation of guest materials such as bio-micromolecules [1,2], cellular tissue networks [3], and organic molecules into the inorganic matrix [4][5][6][7], the biominerals generated often show enhanced mechanical and optical properties. Examples include sea urchin skeletons, mollusk shells, teeth, and bones, etc.…”

Effect of Nanoscale Confinement on the Crystallization of Potassium Ferrocyanide

“…1,2 Soluble additives can be particularly effective in defining crystal sizes, morphologies and polymorphs, 3 delivering for example, crystalline particles with complex hierarchical structures. [4][5][6][7][8] The encapsulation of additives within the crystal lattice can also change the textures, 9 mechanical properties, [10][11][12] and optical properties 13,14 of crystals. As an alternative to additive-directed crystallization, it is also becoming increasingly evident that the micro-environments in which crystals form can themselves define crystallization pathways and products.…”