Abstract
The spherical cage-like protein ferritin was reconstituted with varying numbers of iron atoms per protein shell (ranging from approximately 20 to 1100) at temperatures of both 25 and 50 °C to produce iron(III) oxyhydroxide (ferrihydrite) particles with different average particle sizes and degrees of crystallinity. After characterization of the structural properties of the resulting iron-oxyhydroxide nanoparticles with transmission electron microscopy and Mössbauer spectroscopy, magnetic viscosity measurements were made in zero applied magnetic field and the resulting data were used to calculate the apparent magnetic-moment-weighted energy barrier distributions for the samples. The distributions measured were typically comprised of both a lognormal distribution and an exponential decay of barrier frequency with increasing barrier height. Evidence that the lognormal component of this distribution arises from the distribution of particle volumes and moments within the ensemble is strongly supported by the increase in the mode of the energy barrier distribution with increasing particle size. The exponentially decaying distribution has a relatively higher contribution to the overall distribution for the more crystalline reconstituted ferritin samples suggesting that it may be associated predominantly with uncompensated spins at particle surfaces.
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