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Studies in nanoscale magnetism: Core/shell magnetic nano-architectures in biology and materials science

Final Activity Report Summary - COMAGMAT (Studies in nanoscale magnetism: Core/shell magnetic nano-architectures in biology and materials science)

This project was interdisciplinary combining experimental and theoretical components in the study of nanomagnetism with applications in biology and bio-medicine. State of the art experimental and theoretical ltechniques were used for the characterisation of a variety of magnetic nanoparticle assemblies. These core/shell magnetic nanoarchitectures were either of biological origin, related to the iron storage protein ferritin, or synthesised using a variety of micro-emulsion and co-precipitation methods producing biocompatible nanoparticles for medical applications. Four laboratories in various Universities and research centres on three continents participated in this collaborative effort to advance our knowledge of magnetism at the nanoscale and push the frontier of applications for societal benefit.

Ferritins are iron storage proteins distributed throughout the plant and animal kingdoms. Mammalian ferritins oxidise and accumulate iron as a ferrihydrite mineral within a shell-like protein cavity. Iron deposition utilises O2 and/or H2O2 as oxidants for Fe2+, where oxidation can occur either at protein ferroxidase centers or directly on the surface of the growing mineral core. Our studies determined that similar cores are produced in all instances, composed of a crystalline centre and a more amorphous surface shell, suggesting that the structure of the biomineral is thermodynamically, not kinetically controlled. In another study, using apoferritin nanotemplating the biomimetic synthesis of nanophase iron-phosphate, -arsenate, -vanadate and -molybdate was undertaken. The oxoanion containing cores were more amorphous than that of ferrihydrite. The lack of crystallinity in the oxoanion containing nanophases was attributed to the faster kinetics of core formation observed in the presence of oxo-anions.

Reverse microemulsion techniques were combined with nano-templating strategies for the synthesis of higher core/shell nano-architectures. Homogeneous Fe2O3-core/solid- silica-shell nanocomposite particles with well-controlled shell thickness at the nm-scale were synthesised and magnetically characterised. They were then used as templates for fabrication of Fe2O3-core/solid-silica-shell/mesoporous-silica-shell nanoarchitectures. The resulting nanoparticle assemblies exhibited unprecedented monodispersity and homogeneity presenting ideal experimental systems for the study of magnetic interparticle interactions. Dipole-dipole interactions were studied via magnetisation and Mössbauer measurements and Monte Carlo calculation techniques. The results indicate that dipole-dipole interactions increase in strength sharply at interparticle distances less than 20 nm while at distances greater than 80 nm the particles are magnetically isolated, with the strength of the interaction reduced essentially to zero.

These magnetic nanoparticles have important biotechnological applications. For instance, the mesoporous silica coat can be used for the adsorption of DNA fragments or pharmaceuticals making them useful agents in targeted gene and drug delivery. In addition, bifunctional magnetic nanoparticles with fluorescent quantum-dots anchored on their surface, specifically Fe2O3-CdSe, were synthesised and their magnetic and optical properties characterised. Furthermore, surface plasmon resonance phenomena in noble metal nanoparticles were also utilised in the production of Fe3O4-Ag heterodimers, which were successfully used for cell biolabelling and magnetic manipulation. Water based ferrofluids of Fe2O3 were also prepared by the co-precipitation method and stabilised with a 20-nm corona of gummic acid. They were magnetically characterised and their specific absorption rate was measured by calorimetric methods at a frequency of 150 kHz and a field of 105 G. Preliminary hyperthermia studies on induced brain tumours in mice indicated that these ferrofluids are effective hyperthermia agents.