|Harnessing the Bacterial Power of Nanomagnets|
Nanometer-size magnets have wide-ranging uses, from directed cancer therapy and drug delivery systems to magnetic recording media and transducers. Such applications require the production of nanoparticles with well-controlled size and tunable magnetic properties. The synthesis of such nanomagnets, however, often requires elevated temperatures and toxic solvents, resulting in high environmental and energy costs. Metal-reducing microorganisms offer an untapped resource to produce these materials in an environmentally benign way. At the ALS, researchers from the University of Manchester have shown that Fe(III)-reducing bacteria can be used to synthesize magnetic iron oxide nanoparticles with high yields, narrow size distribution, and magnetic properties equal to the best chemically synthesized materials.
A relatively unexplored resource for magnetic nanomaterial production is a type of subsurface microorganism capable of producing large quantities of nanoscale magnetite (Fe3O4) at ambient temperatures. Metal-reducing bacteria live in soils deficient in oxygen and conserve energy for growth through the oxidation of hydrogen or organic electron donors, coupled to the reduction of oxidized metals such as Fe(III)-bearing minerals. This can result in the formation of magnetite via the extracellular reduction of amorphous Fe(III)-oxyhydroxides, releasing soluble Fe(II) and completely recrystallizing the amorphous mineral into a new phase.
The Manchester team developed a method for producing large quantities of highly crystalline magnetite and cobalt ferrite (CoFe2O4) nanoparticles using the Fe(III)-reducing bacterium, Geobacter sulfurreducens. In particular, they demonstrated that cobalt ferrite nanoparticles with the high coercivity (i.e., resistance to demagnetization) important for applications can be manufactured through this biotechnological route. Three samples containing increasing amounts of Co in the biogenic magnetite structure were analyzed. X-ray diffraction and transmission electron microscopy showed that the material is nanocrystalline. Moreover, the coercivity of the samples increases with increasing Co content, so that it can be tuned for specific applications.
The cation distribution in the ferrite nanoparticles was investigated using x ray absorption (XA) and x-ray magnetic circular dichroism (XMCD) at the Fe L2,3 and Co L2,3 edges, measured at ALS Beamline 4.0.2. An XMCD spectrum is obtained as the difference between two XA spectra measured in opposite external magnetic fields. Magnetite has an inverse spinel crystal structure, which contains tetrahedral (Td) and octahedral (Oh) sites accommodating Fe2+ and Fe3+ cations. Each specific cation in the spinel structure generates a unique XMCD signature determined by its valence state (number of d electrons), site symmetry (i.e., Td or Oh), and moment direction, which can be computed using atomic multiplet calculations. By fitting a weighted sum of these calculated spectra to the measured XMCD spectra, the site occupations of the Fe cations can be obtained.
The biogenic materials show a striking change with increasing Co amount, namely a decrease in intensity of the leading negative peak in the Fe L3 edge, which implies that Co is predominantly replacing Fe2+ cations in octahedral sites. Similarly, the site occupancy and oxidation state of the Co can be directly assessed by examining the Co L2,3 XA and XMCD spectra. The close similarity with the spectra for synthetically produced CoFe2O4 thin films confirmed that the bacteria were able to suitably accommodate Co in the ferrite structure with the Co2+ residing primarily on Oh sites.
The XMCD measurements indicate a dramatic enhancement in the magnetic properties of biogenically produced nanoparticles when large quantities of Co are introduced into the spinel structure, a major advance over previous biomineralization studies. Inclusion of other transition metals into the spinel structure by Fe(III)-reducing bacteria to tailor the magnetic properties of nanoferrites could lead to a suite of materials required for different technological uses. The successful production of highly ordered crystalline nanoparticulate ferrites demonstrates the potential for scaled-up industrial manufacture of nanoparticles using environmentally benign and energy-efficient methodologies.
Research conducted by V.S. Coker, N.D. Telling, R.A.D. Pattrick, C.I. Pearce, J.R. Lloyd, F. Tuna, and R.E.P. Winpenny (University of Manchester, UK); G. van der Laan (Diamond Light Source, UK); and E. Arenholz (ALS).
Research funding: UK Engineering and Physical Sciences Research Council and UK Biotechnology and Biological Sciences Research Council. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.
Publication about this research: V.S. Coker, N.D. Telling, G. van der Laan, R.A.D. Pattrick, C.I. Pearce, E. Arenholz, F. Tuna, R. Winpenny, and J.R. Lloyd, "Harnessing the extracellular bacterial production of nanoscale cobalt ferrite with exploitable magnetic properties," ACS Nano3, 1922 (2009).