Generating Materials with Complex, Life-like Morphologies

This project will couple experiments and mathematical modelling to explore the self-assembly of inorganic microstructures with complex, life-like morphologies. Living systems provide a unique inspiration for the design and construction of new materials. With complex, three-dimensional morphologies and hierarchical structures, biominerals such as bones, teeth and seashells exhibit properties unparalleled by their synthetic counterparts. A key feature of these biological structures is that they often form by assembly-based mechanisms under far-from equilibrium conditions.

In exciting new results we have recently shown that the commercially-available polyelectrolytes can direct the formation of inorganic microstructures with remarkable morphologies including spirals, cones and twisted tapes in aqueous solution. This is achieved using a simple one-pot method. Notably, we have produced comparable structures for a range of compounds including calcium carbonate and strontium sulfate. Inorganic structures that have morphologies reminiscent of living, biological materials have been termed "biomorphs". However, to date, biomorphs have only been generated in the presence of silicate ions - and have been restricted to metal carbonates. Our methodology is therefore far more general.

This system will now be investigated in depth to explore the influence of the reaction conditions, including the inorganic compound, organic additives, solution concentrations and reaction times on the product morphologies. Experimental work will generate "morphology maps" that relate the experimental conditions to the structures formed, and explore the range of morphologies that can be formed by this self-assembly route. A wide range of insoluble inorganic compounds and organic polymers will be screened, as well as solution conditions. The morphologies and structures of the product crystals will be characterised using optical microscopy, scanning electron microscopy (SEM), Raman spectroscopy and X-ray diffraction. In this way we will be able to build an understanding of how the crystal chemistry and solution conditions dictate the structures formed.

The mechanisms by which these structures form is of significant interest and will be determined by characterising the evolving structures using electron microscopy and analytical techniques. We will use cryo-TEM to investigate the early stages of formation of these structures, where this technique allows us to preserve and thus characterise the structures as they are in the solution. Cryo-electron tomography will be used to image the internal structures of these hierarchical structures in 3D. We will also explore liquid cell (LC-TEM) to actually watch the process of formation of these structures in real time in solution with nanometre resolution. This would represent the first time anyone has been able to study in situ the formation of such structures.

In parallel with the experimental studies we will also develop mathematical models that can rationalise the formation of these microstructures by modelling the interdependence of concentration profiles, diffusion rates, reaction rates and curvature of the structures. The formation of silica/ metal carbonate biomorphs has been proposed to derive from an autocatalytic co-precipitation cycle, that occurs due to alternating local changes in pH that occur when metal carbonate, and then silica forms. We have no silicate in our system, and can form these structures from compounds whose formation does not involve a pH change. This suggests a much more general underlying mechanism.

This project will ultimately deliver a framework that will allow us to predict and control the synthesis of morphologically-complex microstructures by design, where inorganic materials with complex forms are important in applications including next-generation optical metamaterials.