My 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 and mechanical 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.

Although there is no single route by which nature achieves this goal, a number of common strategies are recognised. Key is the use of organic macromolecules. 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.

We are currently investigating a variety of strategies which mimic some of the processes employed in biology. A broad range of projects are carried out, varying from using optical techniques such as Brewster angle microscopy for studying crystallisation, to using templates to control crystal morphologies, to the organisation of crystals on surfaces. Many of these experiments also enable us to better understand natural crystal growth phenomena.

• Crystal growth within confined volumes
• Control of single crystal morphologies
• Crystal engineering using block-copolymers
• Formation of macroporous inorganic solids using a templating approach

Crystal growth is typically heterogeneous and occurs in association with surfaces. We are interested in producing well-characterised surfaces and studying how crystal nucleation and growth is affected by the surface. Surfaces of controlled topography are being formed by adsorption of nanoparticles.

• Brewster angle microscopy study of crystal growth under Langmuir monolayers
• Surfaces of controlled roughness for microbial and crystal growth

The work employs a wide range of physical techniques including microscopy methods such as Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Brewster Angle Microscopy (BAM), as well as techniques such as X-ray diffraction and IR spectroscopy.

Current Projects

A number of current projects are described below:

Crystal growth in constrained volumes

Crystal growth within constrained volumes is being investigated. Crystals of defined morphology have been produced on precipitation of calcium carbonate within the pores of Track-etch membranes. Under conditions where the particles formed via an amorphous calcium carbonate (ACC) precursor phase, perfect rod-shaped particles were obtained. There is increasing evidence that many calcium carbonate biominerals contain both calcite and ACC, and that some form on transformation of ACC into calcite over time. The ACC therefore acts as a transient precursor to the more stable calcite phase. Our results suggest that transformation of ACC within a constrained volume may provide a mechanism for morphology control.

CaCO₃ particles produced in membrane pores at 4-6 ^oC (a) after 24 hrs, (b) after 15 mins, showing ACC particles

Morphological Control of Single Crystals

One of the most remarkable aspects of many biominerals is the range of unusual morphologies, which are typically quite unmatched by their synthetic counterparts. This is perfectly exemplified by sea urchin skeletal elements, each of which displays a complex fenestrated structure, and yet is a single crystal of calcite. We are currently investigating the premise that such morphological control can be achieved by growing crystals within a suitable template, under controlled growth conditions. Experiments have utilised sea urchin templates, from which a polymer replica is produced. The polymer replica exhibits an identical structure to the original plates. Growth of calcium carbonate within these membranes produces either polycrystalline or single crystal particles, depending on solution concentrations, demonstrating that single crystals with complex form is within the grasp of synthetic chemistry.

(a) Polymer template and (b) Single crystal of calcite with complex porous structure

Brewster Angle Microscope Study of Crystal Growth under Langmuir Monolayers

We are currently investigating CaCO₃ crystallisation under a series of Langmuir monolayers of long-chain fatty acids, using Brewster Angle Microscopy (BAM). To fully understand the role of the monolayer in controlling crystal growth it is necessary to actually view the monolayer itself. This can be achieved using BAM. Our results demonstrate that the induction time, the crystal polymorph and calcite crystal morphology is affected by the monolayer chain length. This can be attributed to slower CO₂ diffusion through thicker monolayers, and changes in the monolayer structure as a function of the fatty acid chain length. In addition, our results show that CaCO₃ crystals preferentially nucleate at the boundaries of phase domains and under condensed domians.

[Work supported by the Royal Society, and London University Central Funds]

Before crystal growth	After 30 minutes

Novel Macroporous Materials by Biomineral Templating

Profiting from existing materials with unusual forms, the skeletal plates of sea urchins are being used as templates to produce macro-porous organic and inorganic solids. Among other materials, we have produced gold replicas of the macroporous sea urchin plates, and have recently extended the technique to produce templated polymer membranes. We are currently investigating techniques to template a wide range of other materials, such as electroless deposition of metals. [EPSRC Grant GR/N65585/01].

(a) Templated Gold	(b) Templated polymer membrane

Investigation of the Microstructure of Sea Urchin Plates

The plates forming sea urchin tests exhibit a unique fenestrated structure. However, despite their morphological complexity, each plate is actually a single crystal of high-Mg calcite. We are currently using X-ray tomography to study the structure of the plates, to determine how the sea urchin manipulates calcium carbonate growth to produce a structure of apparent cubic symmetry, when calcite possesses a triclinic crystal structure.

[Collaboration with Professor Stephen Hyde, Department of Applied Mathematics, Australian National University, Canberra and Dr Christoph Rau and Dr Timm Weitkamp, ESRF, Grenoble.]

Structure of echinoid skeletal plate	Tomograph of echinoid plate

Calcification in the Coccolithophorid Emiliania huxleyi

Coccosphere

Coccolithophorids are algae that mineralise to produce calcium carbonate scales (termed coccoliths) as crystalline calcite. The morphology of the crystals are quite unique and are entirely distinct from their synthetic counterparts, suggesting that the crystals produced fulfil a specific biological function. As algae, the coccolithophorids photosynthesise, and it has been demonstrated that the processes of calcification and photosynthesis are co-dependent. The relationship between the photosynthesis and calcification processes is being investigated to determine the correlation between them and ultimately to gain insight into why coccolithophorids calcify to produce crystals of such distinct morphologies. [Collaboration with Mrs Brenda Thake, Department of Biological Sciences, QMUL]

Crystal Engineering using Block-Copolymers

The influence of a series of water-soluble, block copolymers on the crystallisation of minerals such as BaSO₄ and CaCO₃ is being investigated. The structure of the copolymers is readily controlled to generate blocks with selected lengths and functional groups. We have demonstrated that these polymer additives are active in controlling the nucleation and morphologies of growing crystals.

[Collaboration with Professor Steve Armes, School of Chemistry, Physics and Environmental Science, University of Sussex, Brighton]

BaSO4 crystals grown: (a) without, and (b) with block copolymers

Arrays of Metal Nanoparticles as Potential Microcapacitor Devices

2.8 nm gold nanoparticles have been self-assembled on gold, and immobilised by chemisorption, using decanedithiol during as a linking molecule. The electronic properties of the particles are then being probed by Scanning tunnelling microscopy (STM). At low substrate - tip voltages, rastering of the tip sweeps the nanoparticles from the raster area (but not at high tip voltage). It is envisaged that this phenomenon could be used to separate nanoparticles of differing size. [Collaboration with Professor Guy Wilson, Department of Physics, Queen Mary, Dr Mike Watkinson, Department of Chemistry, Queen Mary]

Thiol-coated Au nanoparticles

Selected Publications

1. Fendler J.H., Meldrum F.C. "Colloid Chemistry Approach to Nanostructured Materials" (1995) Adv. Mater. 7(7), 607-631.
2. Meldrum F.C., Flath J., Knoll W. "Formation of Patterned PbS and ZnS Films on Self-Assembled Monolayers" (1999) Thin Solid Films 348, 188-195.
3. Meldrum F.C., Flath J., Knoll W. "Chemical Deposition of PbS on a Series of w-Functionalised Self-Assembled Monolayers” (1999) J. Mater. Chem. 9, 711-723.
4. Meldrum F.C., Seshadri R. “Porous Gold Structures through Templating by Echinoid Skeletal Plates” (2000) Chem. Commun. 29-30.
5. Seshadri R., Meldrum F.C., “Templating Approaches to Materials with Ordered, Macroporous Structures” (2000) Adv. Mater. 12, 1149-1151.
6. Rolandi M., Meldrum F.C., Scott K. and Wilson E.G. “Manipulation and Immobilisation of Alkane coated Gold Nanocrystals using Scanning Tunnelling Microscopy” (2001) J. Appl. Phys. 89(3), 1588-1595.
7. Gautam U.K., Rajamathi M., Meldrum F.C., Morgan P., Seshadri R. “A Solvochemical Route to Capped CdSe Nanoparticles” (2001) Chem. Comm. 629-630.
8. Meldrum F.C., Hyde S.T. “Morphological Influence of Magnesium and Organic Additives on the Precipitation of Calcite” (2001) J. Crystal Growth. 231(4), 544-558.
9. Loste E., Meldrum F.C. “Control of calcium carbonate morphology by transformation of an amorphous precursor in a constrained volume” (2001) Chem. Comm. 10, 901-902.
10. Robinson K.L., Weaver J.V.M, Armes S.P., Diaz-Marti E., Meldrum F.C., “Synthesis of controlled-structure sulfate-based copolymers via atom transfer radical polymerisation and their use as crystal habit modifiers for BaSO4” (2002) J. Mater. Chem. 12, 890-896.
11. Park R.J., Meldrum F.C., “Synthesis of Single Crystals of Calcite with Complex Morphologies” (2002), Adv. Mater. 14, 1167-1169.