Nanoscale ferrimagnetic particles have a diverse range of uses from directed cancer therapy and drug delivery systems to magnetic recording media and transducers. Such applications require the production of monodisperse nanoparticles with well-controlled size, composition and magnetic properties. To fabricate these materials purely using synthetic methods is costly in both environmental and economical terms. However, metal-reducing microorganisms offer an untapped energy source for producing these materials. Specifically the Fe(III)-reducing bacteria Geobacter sulfurreducens has been used to synthesize magnetic iron oxide nanoparticles containing varying amounts of additional transition metals, altering the magnetic properties of the particulates. The element and crystallographic site specific nature of X-ray magnetic circular dichroism (XMCD) has been an invaluable tool for assessing the site occupancy of the different cations in the nanoparticles. In conjunction with electron microscopy and magnetometry techniques, soft X-ray spectroscopy has been employed to show that this method of biosynthesis results in high yields of crystalline nanoparticles with a narrow size distribution and magnetic properties equal to the best chemically synthesized materials. The successful production of nanoparticulate ferrites achieved in this study at high yields could open up the way for the scaled-up industrial manufacture of nanoparticles using environmentally benign methodologies.
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