Microbially mediated synthesis of metal-substituted magnetic nanoparticles

Synthetic or substituted powder magnetites are generally prepared by solid phase reaction or by wet chemical methods. The objective of this study is to examine biologically facilitated production of copious amounts of extracellular metal-substituted magnetite nanoparticles. The microbially mediated synthesis of metal-substituted magnetites by Tberoanaerobacter ethanolicus (TOR-39, C1) isolated from a deep subsurface environment was investigated under anaerobic conditions. The thermophilic iron-reducing bacterium has the ability to reduce various metals in aqueous media and forms magnetite nanocrystals. No exogeneous electron carriers (i.e., anthraquinone disulfonate) or reducing agents (i.e., cysteine) were added to the anaerobic medium. The iron-reducing bacteria formed metal-substituted magnetites using iron oxide plus metals (e.g, Co, Cr, Mn, Ni. Pd, Zn, etc) under conditions of relatively low temperature (<70 °C), ambient pressure, and pH values near neutral to slightly basic (pH=6.5-9). Precise biological control over activation and regulation of the biosolid-state processes can produce magnetite particles of well-defined nanometer size and crystallographic morphology, including the doping of the metals into the magnetite (Fe_3-yX_yO₄) structure (where X=Co, Cr, Mn, Ni, Pd, Zn etc). The microbially produced Co-substituted magnetite showed greater magnetism (AH>2200 Gauss) than the magnetism (AH∼1800 Gauss) exhibited by magnetite without metal substitution. Microbially facilitated synthesis of metal-substituted magnetites at near ambient temperatures may expand the possible use of specialized ferromagnetic nanoparticles.