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.

Left: Transmission electron microscopy (TEM) images of the Fe(III)-reducing bacterium, Geobacter sulfurreducens. Right: TEM images of magnetite nanoparticles produced by the bacteria. Note the difference in the scale bars.

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 L_2,3 and Co L_2,3 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 (T_d) and octahedral (O_h) 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., T_d or O_h), 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 L_2,3 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 O_h sites.

Top left: Crystal structure of cobalt ferrite (CoFe₂O₄). Bottom left: Magnetic hysteresis loops measured at T = 5 K for biogenic magnetite (red) and cobalt ferrite (blue). A wider hysteresis loop indicates a higher resistance to demagnetization. Center: Experimental (black) and best-fit (red) Fe L_2,3 XMCD spectra for biogenic magnetite (top), cobalt-ferrite with 6 atom % Co (middle) and 23 atom % Co (bottom). Right: The decomposition of the XMCD spectra into the Fe²⁺ O_h (purple), Fe³⁺ T_d (blue), and Fe³⁺ O_h (green) components. The color-coded numbers indicate the proportions of the components in the XMCD spectrum.

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).

ALSNews Vol. 302