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Current projects

The group’s research interests fall under the general category of materials chemistry, with particular emphasis on inorganic crystal growth in synthetic and biological systems. Research is focussed on controlling crystal growth to produce inorganic solids with defined sizes, morphologies, organisation, mechanical and indeed general physical properties, using natural systems as an inspiration. While synthetic methods typically employ high temperatures and pressures, or elaborate starting materials to produce advanced materials, nature clearly does not have these options available and operates under mild conditions to produce intricate structures, perfectly optimised to their function within an organism. We are currently performing research under a number of major themes:

  • Crystallization in Confinement
  • Using Microfluidic Devices to Study Crystallization Processes
  • Crystals with Composite structures
  • Applying Genetic Algorithms to Inorganic Materials Chemistry
  • Artificial Cells
  •  Evolving Nanomaterials
  • Crystallization Mechanisms

Biomineralization

Biomineralization describes the production of inorganic solids by organisms, and is extremely common in nature, with over 60 biominerals having been identified among all five of the animal kingdoms. Bones, teeth and seashells are common examples. Of known minerals, approximately 20 % are amorphous and 80 % crystalline, although the number of amorphous minerals may be an underestimate due to the problems of detecting amorphous materials in the presence of crystalline ones. Biominerals are typically characterised by unique morphologies, and display properties optimised for their function. For example, biominerals fulfilling structural roles typically possess remarkable mechanical properties which can rival those of engineering materials. The magnetite crystals providing magnetotactic bacteria with a magnetic dipole and enabling navigation in the earth’s magnetic field are always a single magnetic domain in size. Of note also is the ability of organisms to selectively precipitate minerals such as SrSO4 and BaSO4 that are significantly undersaturated in the surrounding environment of the organism.

Perhaps the most immediately striking aspect of biominerals is the range of truly exquisite and elaborate morphologies observed, which are frequently entirely disparate from their synthetic counterparts. Structurally, biominerals are typically composite materials, being intimately associated with organic macromolecules, and are often hierarchically organised on a scale from Angstroms to millimeters. The organic macromolecules are a vital component of the mineral, being involved in nucleation and growth control, and definition of mechanical properties. In consequence, although precipitated under ambient conditions, biominerals often exhibit superior mechanical properties, which is particularly important when they are being exploited for skeletal roles.

Although there is no single route by which nature achieves this goal, a number of common strategies are recognised which can be applied to produce minerals of specific size, morphology, structure and orientation. As organisms clearly cannot manipulate parameters such as temperature or pressure, as is commonly done synthetically to control crystal growth, the strategies they use rely on organic molecules to control mineralisation. These can be in the form of an insoluble organic matrix, which generates a unique environment in which crystallisation occurs and can influence nucleation processes. Soluble organic additives are also typically present during crystal growth, and can influence crystal texture and morphology. These processes can in turn provide inspiration for synthetic crystal growth experiments.

Synthetic Crystal Growth

A variety of strategies are being used to control the properties of crystals grown in synthetic systems, which have been developed using biological mechanisms as an inspiration. This work falls within a number of categories: (1) producing crystals with designer structures and properties, (2) amorphous precursor phases, (3) crystallization on surfaces and (4) crystallization in confined volumes. These bio-inspired methodologies are being used to produce crystals with complex morphologies, with composite structures in which particles of one phase are embedded within the matrix of another and showing superior mechanical properties. Crystallization in biological systems often proceeds via amorphous precursor phases, and study and exploitation of this mechanism is being carried out to gain greater control over the crystallization process and to use amorphous phases as novel synthetic materials.

While methods exist to modify the form of a crystal during growth, it is control at the earliest stages – at nucleation – which is essential for definition of features such as the polymorph, orientation, particle size and size distribution. we are investigating surfaces with well-defined topographies as a novel route to controlling crystal nucleation. Finally, as a significant current interest in the group, we are currently actively investigating the effect of confinement on crystallization processes. A key feature of biological systems is that crystallisation always occurs within compartments, which is likely to significantly affect the progress and outcome of reations occuring within. We are exploring a range of systems, including porous glasses, track etch membranes, droplet arrays and microfluidic systems to to study the effects of confinement of crystal nucleationa and growth from the nanometer to the micron scale.

Crystal Growth in Confined Volumes

A fundamental characteristic of biological systems is that their organisation and function are based on compartmentalisation. Indeed, there is increasing realisation that biological and chemical reactions can be dramatically affected by confinement, stimulating a rapidly growing interest in this effect. Biomineralization processes are no exception to this, and it is well-recognised that a key step in the biological control of mineralisation is the initial construction of a “privileged environment” within the organism in which mineralisation occurs. Within these localised microenvironments organisms actively select the mineral phase, and determine the morphology, orientation and location of the biomineral product through control of precursor ions and phases, and via interaction with soluble organic macromolecules and insoluble organic matrices. However, despite the fact that crystallisation invariably occurs within such confined volumes, experiments modelling biomineralisation are invariably carried out in bulk solution.

As a major research effort, we are investigating the effects of confinement on crystallisation over length scales ranging from the micrometer to the nanometer level. A range of experimental systems are being used including controlled-pore glasses, track-etch membranes, droplet arrays, crossed cylinders and droplets created within microfluidic devices. These experiments have demonstrated that confinement can have a significant effect on crystallisation even at remarkably large length scales, enabling control over features including polymorph, orientation, single crystal/ polycrystalline structure and morphology. The mechanisms underlying such control are currently being investigated. Therefore, while precipitating minerals within localised environments is fundamental to biologically-controlled biomineralisation processes, it can be suggested that confinement – and specifically the large ratio of mineral/ organic surface area to mineral volume – also provides organisms with an additional mechanism of control over mineral formation.

Using Microfluidic Devices to Study Crystallization Processes

Microfluidic devices offer some unique features which make them ideally suited to studying crystallization processes. One of the reasons that understanding of crystal nucleation and growth mechanisms is so poor is that these processes are very difficult to study experimentally. Precipitation often occurs very rapidly, is affected by impurities (that can be almost impossible to eliminate), mixing and convection, which leads to problems with reproducibility. In the case of biomineralization, crystals are often generated in the presence of mixtures of soluble additives such that different additives can be utilized at different points in the reaction.

Our group has been developing expertise in microfluidics in order to use these platforms to study crystallization processes over the past 5 years. Microfluidic devices offer many advantages over bulk solution for studying crystallization, for example by providing highly reproducible reaction conditions, a precisely-defined flow rate such that different locations along the flow-channels correspond to specific time-points in the reaction, and reactions can be analysed at very early times. Despite this, they are still to be widely used in crystallization studies, where existing work has focussed on their ability to support the growth of large protein crystals.

We are using microfluidic devices to study crystallization both on-chip (using Raman microscopy and optical microscopy) and off-chip by transferring crystals to TEM grids (where this is achieved by integrating the TEM grid into the device design). Our results show that the droplets formed in segmented-flow microfluidic devices support extremely reproducible crystallization, such that we can selectively produce either calcite of vaterite according to the reaction conditions. We have also developed devices including a “Crystal Hotel” to study crystallization within well-defined environments, and under non-equilibrium conditions.

Crystals with Composite Structures

Advances in technology demand an ever-increasing degree of control over material structure, properties and function. Even so, the range of properties that can be obtained from monolithic materials remains relatively limited. In contrast, the creation of composite materials, in which two dissimilar materials are combined, opens the door to the fabrication of “new” materials with many potential applications. We are investigating and exploiting a simple biomimetic approach to forming crystals with composite structures. Biological crystals are almost without exception composite materials, where organic additives are occluded within inorganic crystals.

In this work we use biology as an inspiration for the production of new materials, by encapsulating “impurity species” within crystals using a simple one-pot method. These range from 500 nm polymer particles, to 20 nm copolymer micelles, and also inorganic nanoparticles whose incorporation leads to the formation of inorganic/ inorganic heterostructures. The beauty of this approach is that the particles are used as simple growth additives and that their incorporation changes the properties of the host crystals. Occlusion of block copolymer micelles within calcite single crystals leads to the formation of an “artificial biomineral”, which, like calcite biominerals, is harder than synthetic calcite. Similarly, occlusion of gold or magnetite particles within calcite single crystals endows the host crystal with new colour or magnetic properties.

Applying Genetic Algorithms to Inorganic Materials Chemistry

Biology uses a wide range of organic molecules to control mineralisation processes. By comparison, synthetic experiments tend to investigate organic additives individually, possibly due to the potential complexity of the problem. We are investigating the use of genetic algorithms to guide the combinatorial selection of additives which generate inorganic materials with desired properties. This approach is inspired by the diversification strategies observed in natural evolution. Briefly, the “fitness” of every individual in the initial generation of crystals is evaluated, and the “genomes” (here the additive compositions) of the fittest members are recorded. A new generation of additives is then created using this information, by recombining and possibly randomly mutating these winning genomes. The performances of the second generation additives are evaluated, and the fittest candidate solutions are then used in the next iteration of the algorithm.

Artificial Cells

Fluid-fluid interfaces are fundamental to biology and technology, defining cell membranes and droplet boundaries in water/oil emulsions. However, while biological membranes exhibit vast chemical and compositional diversities, interface compositions in industrial dispersions remain relatively unexplored. We have developed a novel screening approach that enables fluid-fluid interfaces to be rapidly tailored for specific applications, and have exploited this to generate artificial cells protected by optimised mineral coatings. Using a droplet-based microfluidic screening platform to sample diverse combinatorial oil/surfactant chemistries, genetic algorithms are being employed to rapidly optimise droplet stability. Selecting in turn for oil-water interfaces that support mineralization, we have succeeded in generating mineralised droplets that show considerable stability both on- and off-chip.

These droplets are then in turn being used as a new class of mineral-protected artificial cells. In addition to offering mechanical support, the mineralized shells are biocompatible and support in vitro protein expression in their interiors.

Evolving Nanomaterials

Through the process of evolution, biological systems have gained exquisite control over inorganic materials synthesis, generating structures such as bones and teeth, whose properties are perfectly optimised for their function. While this is clearly achieved by many routes, all are united by one common feature; nature controls the formation of inorganic solids using organic molecules. The structure of these biomolecules is acquired by evolutionary selection and differential replication such that the fittest biomolecules survive to produce materials with target properties. These properties are ‘tuned’ to match environmental pressures, yielding biominerals with controlled crystal size, shape, orientation and polymorph.

We are currently addressing this challenge and are using directed-evolution techniques to synthesise inorganic materials with target properties. Very large biomolecule libraries are being used to generate genetically-encoded quantum dot and magnetic nanoparticles, where evolutionary selection is being conducted within micro-droplets generated in microfluidic devices. Droplets containing “winning” nanoparticles with target photoluminescent or magnetic properties – together with their partner protein and encoding DNA – are isolated using fluorescence activated cell sorting (FACS) or magnetic separation.

Crystallization Mechanisms

The recognition that crystallisation often proceeds by non-classical mechanisms – that is by assembly-based processes, or via amorphous phases rather than via ion-by-ion growth has led to significant interest in crystallization mechanisms.

It is now well-accepted that calcification in biology frequently proceeds via an amorphous calcium carbonate (ACC) precursor phase, rather than by direct precipitation of crystalline calcium carbonate. The mechanisms by which amorphous intermediates transform into crystalline materials, however, are poorly understood. We are investigating the crystallization of ACC, a key intermediary in synthetic, biological and environmental systems. By investigating this process in detail, we have shown that ACC can dehydrate prior to crystallizing, both in solution and in air, while thermal analyses and solid-state NMR measurements reveal that its water is present in distinct environments. Loss of the final water fraction – comprising less than 15% of the total – then triggers crystallization. The high activation energy of this step suggests that it occurs by partial dissolution/ recrystallization, mediated by surface water, and the majority of the particle then crystallizes by a solid state transformation.

We are also addressing the structure and formation of mesocrystals, which are described as 3D crystals that form by oriented assembly, and which exhibit nanoparticle substructures. Rigorous structural analysis of calcite/polymer crystals described as mesocrystals has demonstrated that the data used to assign mesocrystal structure are frequently misinterpreted and that these calcite/polymer crystals do not have nanoparticle substructures. In particular, line-broadening in powder XRD spectra is due to lattice strain only, precluding the existence of a nanoparticle substructure. We are therefore actively re-evaluating the existing literature on mesocrystals!