Jan 13 2016
Seashells, lobster claws, and chalk are all made of calcium carbonate crystals. While chalk is soft enough to draw on pathways, seashells and lobster claws are tough to break. This strength is due to the clumps of soft biological matter they contain.
When calcium carbonate crystallizes into hard shells, it incorporates soft bits of proteins to add strength. Research done in the lab shows how this might happen, and why it works. (Image courtesy of Wikipedia)
Research in Nature Communications discovered how the the soft clumps entered the crystals, and gave them this extraordinary strength. The outcome of the study revealed that chemical interactions with atoms caused these clumps to become incorporated into the crystals, an unexpected mechanism based on previous understanding.
This study offers an insight into how natural minerals, which are a composite of hard and soft components, are formed. Based on this study, material scientists can create new materials for a sustainable energy future.
"This work helps us to sort out how rather weak crystals can form composite materials with remarkable mechanical properties. It also provides us with ideas for trapping carbon dioxide in useful materials to deal with the excess greenhouse gases we're putting in the atmosphere, or for incorporating light-responsive nanoparticles into highly ordered crystalline matrices for solar energy applications."
Jim De Yoreo, Materials Scientist, Pacific Northwest National Laboratory’s Department of Energy
Calcium carbonate is a key material on the Earth, crystallizing into chalk, shells and rocks. Animals, from mollusks to humans, use calcium carbonate to make biominerals such as pearls, seashells, exoskeletons, or the tiny organs in ears that maintain balance. In order to convert the weak calcium carbonate into hard, durable materials, these biominerals include proteins or other organic matter in the crystalline matrix.
Scientists have been investigating the way organisms produce these biominerals in order to discover the basic geochemical principles of how they form, and also how to construct synthetic materials with unique properties in any preferred size or shape.
A material’s strength depends on how easily the underlying crystal matrix can be disrupted, the matrix is harder to break apart when the material is compressed. The proteins found trapped in the calcium carbonate crystals generate a strain or compressive force inside the crystal structure. This compressive strain is helpful in materials, it makes disrupting the underlying crystal structure harder and therefore adding strength. While scientists understand how a combination of forces, stress and strain make strong materials, they know less about creating these materials.
The leading explanation for how growing crystals incorporate proteins and other particles is by simple mechanics. When particles fall on the flat surface of calcium carbonate during crystallization, calcium carbonate units fasten over and around the particles, trapping them.
"The standard view is that the crystal front moves too fast for the inclusions to move out of the way, like a wave washing over a rock," said De Yoreo.
Knowing exactly where the strain occurs within the material is the drawback, De Yoreo and his colleagues new results provide an explanation. "We've found a completely different mechanism," he said.
With the aid of the atomic force microscopy (AFM) at the Molecular Foundry, a DOE Office of Science User Facility at Lawrence Berkeley National Laboratory, the team sought to identify how calcium carbonate incorporates proteins or other strength-building components. Using AFM, the tip of the microscope runs over the surface of a sample, much like a needle running over the grooves of a vinyl record, and this produces a three-dimensional image of the sample under the scope.
The team used a high concentration of calcium carbonate that naturally forms a crystalline mineral known as calcite. The calcite creates uneven surfaces during growth as it builds up in layers, much like steps and terraces on a mountainside. Or, imagine a staircase. A terrace is the flat landing at the bottom; the stair steps have vertical edges from which calcite grows out, eventually turning into terraces too.
The team then created spheres from organic molecules and added them into the mix. These spheres are referred to as micelles, and are molecules that roll up like roly-poly bugs based on the chemistry along their bodies. Inside of the spheres are the parts of their molecule that don't get along with the watery environment, while pointing outwards are the parts that work well chemically with both the surrounding water and calcite.
Using the microscope the team observed that the micelles do not randomly fall on the flat terraces, but stick to the edges of the steps.
"The step edge has chemistry that the terrace doesn't," said De Yoreo. "There are these extra dangling bonds that the micelles can interact with."
The edges cling to the micelles as the calcium carbonate steps close around them, one by one. The team observed the growing steps squeeze the micelles. As the steps close around the top of a micelle a cavity is created, this is followed by the entire structure vanishing below the growing crystal’s surface.
To prove that the micelles were truly embedded within the crystals, the researchers dissolved the crystal and studied it. Similar to rewinding a movie, the micelles began to appear as the crystal layers vanished.
Finally, the researchers used a mathematical simulation to recreate the process. It illustrated that any micelle, or any spherical inclusion, are compressed like springs when the steps close in around it. These compressed springs generate the strain in the crystal lattice found among the micelles. This strain produces the mechanical strength and is likely the key to the extra strength displayed in pearls, seashells and similar biominerals.
"The steps capture the micelles for a chemical reason, not a mechanical one, and the resulting compression of the micelles by the steps then leads to forces that explain where the strength comes from."
Jim De Yoreo, Materials Scientist, Pacific Northwest National Laboratory’s Department of Energy
This work was supported by the Department of Energy Office of Science, National Institutes of Health.