We have investigated the crystal structure of nanosized iron-oxide by X-ray diffraction (XRD), extended X-ray absorption fine structure measurements at the iron K-edge as well as by transmission electron microscopy (TEM). Iron-oxide nanoparticles were produced by thermal treatment of horse spleen ferritin molecules. The structure of these particles was compared to α-Fe2O3 and γ-Fe2O3 nanopowder references. The thermal treatment of a submonolayer film of ferritin molecules results in pure γ-Fe2O3 nanoparticles, while for films above a certain thickness α-Fe2O3 and γ-Fe2O3 coexist, exhibiting two different crystallite sizes. TEM shows a characteristic particle diameter of ~7 nm for γ-Fe2O3 resulting from thermal treatment of monolayers, consistent with the crystallite size of the γ-phase as obtained from XRD measurements on multi-layered samples. XRD shows the α-Fe2O3 phase to be characterized by a crystallite size of ~34 nm.
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To produce nanosized iron oxide particles we made use of the ability of ferritin to self-assemble and construct a core, which shows a structure similar to that of the mineral ferrihydrite (5Fe2O3·9H2O). Ferritin is the major cellular iron-storage protein. It is able to store iron as hydrated iron oxide in the internal cavity which is composed of 24 polypeptide subunits. The inner and outer diameters of the protein shell are about 8 and 12.5 nm, respectively. Hydrophilic and hydrophobic channels penetrate the protein shell and provide the means by which iron ions can be accumulated within or removed from the molecules. (Cornell and Schwertmann 2003; Massover 1993; Mann et al. 1989).
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We acknowledge the ANKA Angstroemquelle Karlsruhe for the provision of beamtime and we would like to thank S. Mangold and S. Doyle for their valuable assistance using beamline ANKA-XAS and ANKA-PDIFF. We thank M. Klemm and P.S. Riseborough for fruitful discussion. This work was supported by the Deutsche Forschungsgemeinschaft in context of SFB 484 and PROALMEX Mexico-Germany binational collaboration project.
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Krispin, M., Ullrich, A. & Horn, S. Crystal structure of iron-oxide nanoparticles synthesized from ferritin. J Nanopart Res 14, 669 (2012). https://doi.org/10.1007/s11051-011-0669-4
- Iron-oxide nanoparticles
- Crystal structure
- X-ray diffraction
- Transmission electron microscopy
- Extended X-ray absorption fine structure