Nanoscale iron(III) oxyhydroxy aggregates formed in the presence of functional water-soluble polymers: models for iron(III) biomineralisation processes.

Superparamagnetic clusters of iron(III) oxyhydroxide in the form of poorly crystalline ferrihydrite (formally, 5Fe2O3 x 9H2O) have been synthesised in the presence of the polymers polyvinyl alcohol (PVA), polyacrylic acid (PAA) and alginic acid. The solutions have been characterised by viscosity studies and the resultant arrays isolated from these solutions have been imaged under an electron microscope and their magnetic properties determined by 57Fe-Mössbauer spectroscopic studies at 293 and 77 K and magnetisation measurements at 293 and 5 K. The magnetic data show that the iron(III) oxyhydroxy particles are superparamagnetic. All preparations show hysteretic behaviour with coercive fields being approximately half or less than half of that of ferrihydrite (3.4 kOe) and values of magnetic moment per iron particle less than that of ferrihydrite. Nanoscale aggregates (2-4 nm) are formed in the presence of PVA and PAA while, with alginic acid, extended branch-like structures are observed, their formation being facilitated by the comparatively rigid polysaccharide chain, a process related to iron biomineralisation in diverse biological systems.