Michael T. Klem

Biomimetic magnetic nanoparticles

Magnetic nanoparticles are of considerable interest because of their potential use in high-density memory devices, spintronics, and applications in diagnostic medicine. The conditions for synthesis of these materials are often complicated by their high reaction temperatures, costly reagents, and post-processing requirements. Practical applications of magnetic nanoparticles will require the development of alternate synthetic strategies that can overcome these impediments. Biomimetic approaches to materials chemistry have provided a new avenue for the synthesis and assembly of magnetic nanomaterials that has great potential for overcoming these obstacles.

Photochemical Mineralization of Europium, Titanium, and Iron Oxyhydroxide Nanoparticles in the Ferritin Protein Cage

The Fe storage protein ferritin was used as a size-constrained reaction vessel for the photoreduction and reoxidation of complexed Eu, Fe, and Ti precursors for the formation of oxyhydroxide nanoparticles. The resultant materials were characterized by dynamic light scattering, gel electrophoresis, UV-vis spectroscopy, and transmission electron microscopy. The photoreduction and reoxidation process is inspired by biological sequestration mechanisms observed in some marine siderophore systems.

Biomimetic synthesis of β-TiO2 inside a viral capsid

The Cowpea chlorotic mottle virus (CCMV) was used as a size-constrained reaction vessel for the synthesis of Ti(IV) oxyhydroxide and TiO2.

Magnetic properties of Co3O4 nanoparticles mineralized in Listeria innocua Dps

Temperature-dependent magnetic measurements are reported for 4.34 nm antiferromagnetic Co3O4 nanoparticles mineralized in the Listeria innocua Dps protein cage. ac measurements show a superparamagnetic blocking temperature of roughly 5.4 K and give an extracted anisotropy energy density of (7.6±0.4)×104J∕m3. The Neel temperature for the Co3O4 nanoparticles, determined with dc magnetometry, was determined to be roughly 15±2K.

Surface contribution to the anisotropy energy of spherical magnetite particles

The surface contribution to the magneto-crystalline anisotropy energy of spherical magnetite nanoparticles has been investigated. Magnetite particles of three sizes (3.5, 7, and 18nm diameter) were grown inside protein cages. Alternating current magnetic susceptibility measurements revealed the particles to be noninteracting, and allowed a determination of the average anisotropy energy for each sample. Surface atoms were found to increase the volume anisotropy energy density of the particles, and this effect increased sublinearly with particle curvature.

Modeling of the magnetic behavior of γ-Fe2O3 nanoparticles mineralized in ferritin

The temperature dependent initial magnetization of γ-Fe2O3 (maghemite) mineralized inside ferritin protein cages has been investigated with a vibrating sample magnetometer up to 8 T. The data are fit to different magnetic models to extract values of the magnetic moment of each cluster. It is found that the application of a simple Langevin model with a first and second order term in the susceptibility greatly enhances the quality of the fit to the data suggesting that the inclusion of crystalline anisotropy is important in extracting the magnetic moment of each core.

Biomimetic synthesis of photoactive α-Fe2O3 templated by the hyperthermophilic ferritin from Pyrococus furiosus

Utilizing a biomimetic approach toward materials synthesis a ferritin protein cage, from the hyperthermophilic archeaon Pyrococcus furiosus, was utilized to first tempate the formation of a largely amorphous ferrihydrite iron oxide and then bring about its transformation to hematite (α-Fe2O3). This was achieved under boiling aqueous conditions by refluxing the ferritin protein cage/ferrihydrite composite. The resultant material showed diffraction indicative of hematite, a visible band gap semiconductor, and photocurrents were measured under visible illumination. The resultant protein/mineral composite was also studied via dynamic light scattering, transmission electron microscopy, and size exclusion chromatography.

Electron magnetic resonance of iron oxide nanoparticles mineralized in protein cages

Magnetic and structural properties determined by electron magnetic resonance (EMR) spectroscopy are reported for maghemite (γ‐Fe2O3) nanoparticles formed through template-constrained mineralization within three protein cages with nominal diameters of 5, 8, and 24 nm. EMR spectra, obtained at 4.0, 9.2, 34.6, 94.9, and 130.0 GHz, and at ambient temperature, show dramatic frequency dependent effects in the line shapes, line-widths, and resonance-field shifts. Simulations of the spectra are used to obtain moment distribution parameters, which are consistent with size limitations imposed by the protein cages, but which reflect significant departures from bulk magnetization properties.

Site determination and magnetism of Mn doping in protein encapsulated iron oxide nanoparticles

Soft x-ray absorption spectroscopy, soft x-ray magnetic circular dichroism, and alternating current magnetic susceptibility were performed on 6.7 nm iron oxide nanoparticles doped with (5%–33%) Mn grown inside the horse-spleen ferritin protein cages and compared to similarly protein encapsulated pure Fe-oxide and Mn-oxide nanoparticles to determine the site of the Mn dopant and to quantify the magnetic behavior with varying Mn concentration. The Mn dopant is shown to substitute preferentially as Mn+2 and prefers the octahedral site in the defected spinel structure. The Mn multiplet structure for the nanoparticles is simpler than for the bulk standards, suggesting that the nanoparticle lattices are relaxed from the distortions present in the bulk. Addition of Mn is found to alter the host Fe-oxide lattice from a defected ferrimagnetic spinel structure similar to γ-Fe2O3 to a nonferromagnetic spinel structure with a local Fe environment similar to Fe3O4.

Site determination of Zn doping in protein encapsulated ZnxFe3−xO4 nanoparticles

The x-ray absorption spectra of the Fe and Zn L edges for 6.7nm Fe3O4 nanoparticles grown inside 12nm ferritin protein cages with 10%, 15%, 20%, and 33% zinc doping show that Zn is substitutional as Zn2+ within the iron oxide host structure. A Neel–Arrhenius plot of the blocking temperature in frequency dependent ac-susceptibility measurements shows that the particles are noninteracting and that the anisotropy energy barrier is reduced with Zn loading. X-ray magnetic circular dichroism of the Fe displays a linear decrease with Zn doping in sharp contrast to the initial increase present in the bulk system. The most plausible explanation for the decrease in moment is that Zn substitutes preferentially into the tetrahedral A site as a Zn2+ cation, generating a mixed spinel.