Magnetic bacterial protein Mms6 controls morphology, crystallinity and magnetism of cobalt-doped magnetite nanoparticles in vitro

Magnetic nanoparticles (MNPs) are in high demand within biomedical and nanotechnological industries. Size, shape, material and crystal quality directly affect the particle's properties, namely their magnetic characteristics, and must be tuned and controlled to meet the specification of the application. A key challenge is to refine synthetic methods to tailor the MNP properties with precision, but using cheap, high-yield, industrially robust and environmentally friendly methods. In this study we compare simple high-yield precipitation methods of producing cobalt-doped magnetite MNPs. We explore the variation of magnetic coercivity and saturation with increasing Co-doping from 0-15% in magnetite MNPs, which increases coercivity from 5-62 mT, but decreases saturation from 91-28 emu g^-1. An optimum of 6% was further investigated as this produced the greatest increase in coercivity to 34 mT with a relatively small reduction in saturation magnetisation to 79 emu g ^-1. The methods compared are refined with the addition of the recombinant biomineralisation protein Mms6 from a magnetic bacterium, as this has been shown to help control magnetite MNP morphology and grainsize distribution in vitro. Similar control is seen here over our Co-doped magnetite synthesis. Mms6 increases the size and decreases the size distribution of room temperature co-precipitated particles from 11.7 nm to 31.7 nm. The affinity tagged protein his₆Mms6 also controls the size (23.2 nm) but less effectively than Mms6. Therefore the Mms6 mediated Co-doped MNP particles are found to be single domain and thus give very clear, square magnetic hysteresis with a coercivity of 48 mT at 10 K. Hysteresis of the smaller particles (Co-doped MNP with no protein and with his-tagged protein) clearly shows both superparamagnetic and single-domain magnetic behaviours. Powder X-ray diffraction shows that both the addition of Mms6 and cobalt increases the crystal quality of the MNP. Thus Mms6 protein mediated room temperature co-precipitation offers an environmentally friendly, industrially robust route towards tailored, uniform, single-domain, high-quality Co-doped magnetite MNPs.