Crystallisation and Directed Assembly
Our research areas include:
- Crystal engineering
- Supramolecular assembly
- Nanoreactor templating
- Carbon nanostructure assembly
- Functional materials
- Biomolecular self-assembly
- Polymer synthesis and self-assembly
We’re interested in directing self-assembly to generate functional molecular crystals and coordination polymers.
One set of projects involves switchable materials based on spin-crossover metal complexes, which reversibly change their magnetic moment, colour and conductivity in response to a physical stimulus like a change in temperature or laser irradiation.
These switching events can be gradual or abrupt, and sometimes exhibit thermal hysteresis. These properties are controlled by the way each switching site interacts with its nearest neighbours in the crystal, and we are leading efforts to understand these phenomena.
Coordination polymers, in contrast, are crystalline metal-ligand systems with infinite structures such as chains, helices, grids and 3D lattice which find application in areas including molecular separations, catalysis and gas storage.
Our ultimate goal is to be able to build these types of molecular material (by design) from the bottom-up, using either simple bridging ligands or bowl-shaped cavitand-ligands. The latter can create materials with different types of pore space to make hierarchical materials.
The same principles of self-assembly can also be used to produce discrete molecules with three-dimensional topologies and cage structures. One active project focuses on unusually large metal/organic molecular cages with dimensions in the nm range. These hollow molecules can bind smaller guest molecules in their internal cavity.
The structures of the cages, or their constituent cavitands, can be systematically modified by appropriate substitution of the walls of the cavities, allowing us to vary the hydrophobicity and steric properties of their internal cavities.
Specific host-guest interactions can thus be designed into the cage structures, enabling them to be used as nano-scale reaction vessels and in a wide range of other applications. Functionality such as phosphorescence, catalytic centres, or light-triggered structure-switching can be introduced to the cages.
In a new project, the same principles of self-assembly are being applied using redox-active components, to produce new molecular architectures exhibiting strong optical absorptions in the infra-red that are switchable by guest binding.
We also exploit the porous nanomaterials including carbon nanostructures, such as nanotubes and hollow fibres, and also zeolites to direct the assembly of molecular complex and nanoparticulate catalysts to create hybrid nanomaterials for application in sustainable heterogeneous catalysis, nanoelectronics and sensors. This work combines fullerene based synthesis, coordination chemistry, supramolecular assembly and nanomaterial fabrication.
We are also using nano-sized test tubes to study the interactions, self-assembly and reactions of molecules and nanostructures at the atomic level using electron microscopy and electrochemistry with the principle aim of learning fundamental information about the chemistry of transition metal/carbon nanotube based nanomaterials.
This work is underpinned by a detailed understanding of the structure of catalytic materials at the atomic level. We use a wide range of solid state analytical techniques including aberration corrected high resolution electron microscopies (AC-HRTEM and STEM), X-ray absorption and diffraction and gas adsorption to elucidate the exact structure of our catalytic materials. In addition, it is possible to 'watch' the formation of heterogeneous catalysts from metallic precursors and the evolution and behaviour of the catalyst during chemical reactions in real time using a combination of in situ AC-HRTEM and near edge X ray absorption fine structure experiments to give atomic level structural information and reveal mechanistic details of how nanomaterials are formed and function.
Carbon nanostructure assembly
Carbon nanostructures, including one-dimensional nanotubes and two-dimensional graphene sheets, provide unique building blocks for the construction of structured, macroscopic materials, such as aerogels and membranes, with great potential for integration into macro-scale technologies. Control over the microstructure and surface chemistry of the nanostructure assemblies is key to create novel materials with enhanced or completely new properties.
We explore soft and hard templating strategies to fabricate monolithic nanocarbon assemblies with well-defined internal morphologies and pore structures over several length scales. Manipulation of nanocarbon cross-linking and surface chemistry is studied to create mechanically robust and durable materials with targeted, specific chemical functionalities. We also investigate approaches to combine nanocarbon assemblies with functional molecules and nanoparticles to create highly-structured, multifunctional hybrid materials for specific applications in heterogeneous catalysis, environmental remediation, separation, sensing, and energy research.
Self-assembly of low molecular weight organogelators into fibrils can form gels: when the gelators are flat, aromatic (discotic) molecules (eg triphenylene derivatives) these gels may be capable of conducting charge. One project currently looks at the applications of such pi-gels in organic electronics.
Molecular imprinting is a range of techniques for creating polymeric materials containing selective binding sites, which can be used in chromatographic separations, sensors or as catalysts. Functional monomers are polymerised around a template molecule, with complementary functional group interactions. After removal of the template, binding sites remain with appropriate shape and arrangement of functional groups to rebind the template or a close analogue. Current projects seek to improve the imprinting process by better understanding of the monomer-template interactions during the polymerization process, and at exploiting imprinted materials in environmental analysis, such as the detection of pesticides in water samples.
Crystallisation is a hugely important subject which lies at the heart of a vast array of natural phenomena and technological processes, including weathering and frost heave, scaling phenomena, the formation of ice in the atmosphere and applications such as the fabrication of nano-materials, mineral-based biomaterials, pharmaceuticals and food-stuffs.
We have particular interests in biomineralization – the formation of structures such as bones, teeth and seashells. Biominerals are characterized by many remarkable features, such as complex morphologies and superior mechanical properties, and yet are formed in water under ambient conditions. Using the strategies by which nature forms biominerals as an inspiration, we are developing new methods of controlling synthetic crystallisation processes. In this way we can, for example, generate crystals with composite structures whose hardnesses are superior even to biominerals, we can gain control over polymorph, and we can form single crystals with sponge-like morphologies.
This will enable us to form new materials, or indeed to prevent unwanted crystallisation events such as the formation of kidney stones, or scaling, and is thus highly important to fundamental research and technology across many disciplines.
Molecular self-assembly is an integral component of the formation and function of biological systems.
We’re interested in many aspects of these processes, where a principal goal is to build an improved understanding of biological systems, and then apply this new knowledge to the development of novel, functional nanostructured materials and devices. Current interests include bottom-up synthetic biology where new materials not found in nature are constructed form biomolecular and synthetic components, including artificial cells that encapsulate (bio)chemical within compartmentalised architectures that mimic the organisation and function of natural cells.
The research carried out covers many topics, including the biophysical properties of lipid membranes, the applications of membrane-based materials, and nanotoxicology – where we are developing high throughput toxicity sensors. We are collaborating with microfluidic engineers to develop animal-on-chip platforms to replace animal testing of nanomaterials and toxicants.
In collaboration with local SMEs we are engaged in an intense activity to commercialise these devices.
Looking to nanomedicine, we’re interested in topics such as targeted drug delivery, while biologically-inspired self-assembly is being used as a route to engineer structures such as nanotapes, ribbons and fibres, which have found application in regenerative dentistry.
Finally, theoretical, computational methods are being used to investigate the phase behaviour and transitions of complex systems of biomolecules, with the expectation that this will enable us to bridge our understanding of colloidal systems, polymers, and proteins. Such an approach is being used to study the aggregation of proteins into amyloid fibrils.
In this context, we have innovated methods of directing biomembrane assembly through the use of applied electric field. These methods use simulation techniques in concert with detailed experimental methods.
Polymer synthesis and self-assembly
Macromolecules that are capable of self-assembly to form discrete nanostructures may be exploited for a range of uses, including acting as vehicles for the controlled delivery of therapeutic/commodity compounds, providing scaffolds for tissue regeneration and providing a platform for catalyst immobilisation.
Current research focuses on utilising a range of controlled polymerisation techniques to create both biodegradable and hydrolytically stable polymers that are suited to an explicit application.
In particular, focus is afforded to the synthesis of poly(amino acid)s for use within a biomedical context. Such polymers are biodegradable, non-cytotoxic and are capable of forming a secondary structure, which drives their self-assembly to yield nanoparticles for use as drug delivery vehicles. Poly(hydroxy acid)s are closely related to poly(amino acid)s in that they are derived from amino acids, but are a class of polyester rather than polyamide. They are extremely promising macromolecules as they offer the biodegradability associated with polyesters, but crucially also provide functional groups that may be exploited for (bio)molecular conjugation. Current research involves the creation of poly(hydroxy acid)s that are derived from amino acids and so possess a varied range of functional groups. Such polymers are able to self-assemble in aqueous solution to form nanoparticles, before undergoing degradation to release a chemotherapeutic cargo in response to an acidic environment, such as that presented by tumour tissue.
Finally, we have a firm interest in employing biopolymers, including alginate, chitosan and cellulose, for the targeted removal of undesirable compounds, such as dye molecules from wastewater and rogue dye molecules from within the laundry wash cycle. Forming polymeric nanoparticulates from cost-effective renewable polymers offers a vast surface area to enable the effective adsorption and removal of the targeted molecule.