Johan van Lierop

Hierarchical self-assembly and optical disassembly for controlled switching of magnetoferritin nanoparticle magnetism

Protein cages such as ferritin and viral capsids are interesting building blocks for nanotechnology due to their monodisperse structure and ability to encapsulate various functional moieties. Here we show that recombinant ferritin protein cages encapsulating Fe(3)O(4)-γ-Fe(2)O(3) iron oxide (magnetoferritin) nanoparticles and photodegradable Newkome-type dendrons self-assemble into micrometer-sized complexes with a face-centered-cubic (fcc) superstructure and a lattice constant of 13.1 nm. The magnetic properties of the magnetoferritin particles are affected directly by the hierarchical organization. Magnetoferritin nanoparticles dispersed in water exhibit typical magnetism of single domain noninteracting nanoparticles; however, the same nanoparticles organized into fcc superstructures show clearly the effects of the altered magnetostatic (e.g., dipole-dipole) interactions by exhibiting, for example, different hysteresis of the field-dependent magnetization. The magnetoferritin-dendron assemblies can be efficiently disassembled by a short optical stimulus resulting in release of free magnetoferritin particles. After the triggered release the nanomagnetic properties of the pristine magnetoferritin nanoparticles are regained.

Magnetic field enhanced convective diffusion of iron oxide nanoparticles in an osmotically disrupted cell culture model of the blood-brain barrier.

Purpose The present study examines the use of an external magnetic field in combination with the disruption of tight junctions to enhance the permeability of iron oxide nanoparticles (IONPs) across an in vitro model of the blood–brain barrier (BBB). The feasibility of such an approach, termed magnetic field enhanced convective diffusion (MFECD), along with the effect of IONP surface charge on permeability, was examined. Methods The effect of magnetic field on the permeability of positively (aminosilane-coated [AmS]-IONPs) and negatively (N-(trimethoxysilylpropyl)ethylenediaminetriacetate [EDT]-IONPs) charged IONPs was evaluated in confluent monolayers of mouse brain endothelial cells under normal and osmotically disrupted conditions. Results Neither IONP formulation was permeable across an intact cell monolayer. However, when tight junctions were disrupted using D-mannitol, flux of EDT-IONPs across the bEnd.3 monolayers was 28%, increasing to 44% when a magnetic field was present. In contrast, the permeability of AmS-IONPs after osmotic disruption was less than 5%. The cellular uptake profile of both IONPs was not altered by the presence of mannitol. Conclusions MFECD improved the permeability of EDT-IONPs through the paracellular route. The MFECD approach favors negatively charged IONPs that have low affinity for the brain endothelial cells and high colloidal stability. This suggests that MFECD may improve IONP-based drug delivery to the brain.

Spontaneously Formed Interfacial Metal Silicates and Their Effect on the Magnetism of Superparamagnetic FeCo/SiO2 Core/Shell Nanoparticles

The integration of superparamagnetic core/shell nanoparticles into devices and other nanoscale technological applications requires a detailed understanding of how the intimate contact between core and shell nanophases affects the magnetism. We report how, for single-domain FeCo nanoparticles, an FeCo phase unique to the nanoscale with silica shells of increasing thicknesses spontaneously formed interfacial metal silicates between the core and shell (such as Fe2SiO4 and Co2SiO4) and altered the overall magnetism of the nanomaterial significantly. The influence of this previously overlooked phenomenon on magnetic properties is reported. Evidence of these metal silicate interfacial layers was observed by X-ray absorption spectroscopy (XAS) collected over the L3,2 absorption edges of Fe and X-ray photoelectron spectra (XPS) collected over the 2p transitions of Fe and Co. Through the correlation of magnetometry and XPS data, the evolution of nanoparticle magnetic anisotropy is shown to increase with the metal silicate.

Differential internalization of brick shaped iron oxide nanoparticles by endothelial cells

Nanoparticles targeting endothelial cells to treat diseases such as cancer, oxidative stress, and inflammation have traditionally relied on ligand-receptor based delivery. The present studies examined the influence of nanoparticle shape in regulating preferential uptake of nanoparticles in endothelial cells. Spherical and brick shaped iron oxide nanoparticles (IONPs) were synthesized with identical negatively charged surface coating. The nanobricks showed a significantly greater uptake profile in endothelial cells compared to nanospheres. Application of an external magnetic field significantly enhanced the uptake of nanobricks but not nanospheres. Transmission electron microscopy revealed differential internalization of nanobricks in endothelial cells compared to epithelial cells. Given the reduced uptake of nanobricks in endothelial cells treated with caveolin inhibitors, the increased expression of caveolin-1 in endothelial cells compared to epithelial cells, and the ability of IONP nanobricks to interfere with caveolae-mediated endocytosis process, a caveolae-mediated pathway is proposed as the mechanism for differential internalization of nanobricks in endothelial cells.

Rational selection of superparamagnetic iron oxide/silica nanoparticles to create nanocomposite inductors

The detailed characterization of core/shell iron-oxide/silica nanoparticles reveals how these superparamagnetic systems are actually composed of a Fe3O4 inner core, γ-Fe2O3, outer core, iron orthosilicate interphase layer, and exterior silica shell. The performance of a superparamagnetic inductor device is then reported for the nanocomposite formed from the binder-free consolidation of these core/shell nanoparticles.

Ion-Beam Bombarded SiO2 Layer Effects on the Microstructure and Magnetism in FePt/SiO2 Bilayers

We have shown that the structural and magnetic properties of FePt thin films were affected strongly by capped SiO2 layers prepared by ion-beam bombardment followed by post-annealing. Compared to the single fcc FePt phase in the as-deposited FePt/SiO2 bilayer (0% O2/Ar), annealing at 550 °C produced an ordered L 10 FePt phase with enhanced coercivity (~14 kOe). Increasing the %O2/Ar during deposition of the top SiO2 layer resulted in smaller ordered FePt grains separated by grain boundaries of SiO2. We find that the (001) diffraction peak is broadened considerably with larger SiO2 deposition %O2/Ar and annealing, likely due to the induced strain. Our results indicate that FePt/SiO2 films deposited with lower %O2/Ar, the oxygen atoms created by the ion-beam bombardment act effectively to inhibit the FePt grain growth, whereas the excess oxygen atoms present during film deposition with higher %O2/Ar may induce a local strain on the FePt crystallites by occupying the interstitial sites in the FePt lattice.

Magnetic and Magnetotransport Properties of Exchange-Biased NiFe/NiO Bilayers

The magnetism of a series of NiFe (20 nm)/NiO (10 nm) bilayers with different Ni oxides has been studied. Ni oxides were made using ratios of O2 to Ar ions ranging from 7 to 33% during ion-beam deposition. Transmission electron microscopy (TEM) has shown that with 7% O2/Ar used during bottom layer deposition, the film produced consisted of fcc Ni (a=3.52 A) and rock-salt NiO (a=4.21 A) phases. A bottom film layer prepared with 33% O2/Ar consisted of a pure NiO phase with an expanded lattice constant (a=4.32 A). A strong temperature dependence of the coercivity (Hc) and a 20 kOe field-cooled loop shift exchange bias field (Hex) were observed below 100 K. In addition, Hex increased with increasing O2/Ar ratio for the bottom layer antiferromagnetic (AF) component. At 10 K, a NiFe/NiO (33% O2/Ar) bilayer exhibited the higest Hc (110 Oe) and Hex (-60 Oe). This indicated that the more expanded the NiO lattice, the stronger the exchange coupling with the NiFe. The magnetotransport studies have shown that these NiFe/NiO bilayers exhibit anisotropic magnetoresistance (AMR) behavior. In addition, the total MR ratio of these NiFe/NiO bilayers increases with increasing O2/Ar ratio owing to strong anisotropic scattering at the NiFe/NiO interface.

Incommensurate crystal supercell and polarization flop observed in the magnetoelectric ilmenite MnTiO3

In the last few years the magnetoelectric behavior of MnTiO3 has been observed even though its been studied for many decades. We use neutron scattering on two separately grown single crystals and two powder samples to show the presence of a supercell that breaks R (3) over bar symmetry. We also present the temperature and field dependence of the dielectric constant and pyroelectric current and show evidence of nonzero off-diagonal magnetoelectric tensor elements (forbidden by R (3) over bar symmetry) followed by a polarization flop accompanying the spin flop transition at mu H-0(SF) = 6.5T. Mossbauer spectroscopy on MnTiO3 gently doped with Fe-57 was used to help shed light on the impact of the supercell on the observed behavior. Moreover, the full supercell structure could not be solved at this time due to a lack of visible reflections, the full scope of the results presented here suggest that the role of local spin-lattice coupling in the magnetoelectric properties of MnTiO3 is likely more important than previously thought.

Effect of ion-beam bombardment on microstructural and magnetic properties of Ni80Fe20/α-Fe2O3 thin films

Ion-beam bombardment has been established as an effective way to tune the microstructure and thus modify the magnetic anisotropy of thin film materials, leading to certain remarkable magnetic properties. In this work, we investigated a Ni80Fe20/α-Fe2O3 bilayer deposited with a dual ion-beam deposition technique. Low-energy argon ion-beam bombardment during the α-Fe2O3 deposition led to a decline of crystallinity and interfacial roughness of the bilayer, whereas the grain size distribution remained essentially unchanged. At low temperature, the coercivity exhibited a pronounced decrease after the bombardment, indicating that the effective uniaxial anisotropy in the ferromagnetic layer was dramatically reduced. Such reduction in uniaxial anisotropy was likely attributed to the irreversible transition in the α-Fe2O3 grains caused by the ion-beam bombardment, which subsequently modified the anisotropy in the Ni80Fe20 layer. The bombarded bilayer also exhibited a larger ΔMFC–ZFC compared to the un-bombarded bilayer, which indicated a stronger exchange coupling between the ferromagnetic layer and the antiferromagnetic layer.

Correlating Uncompensated Antiferromagnetic Moments and Exchange Coupling Interactions in Interface Ion-Beam Bombarded Co90Fe10/CoFe-Oxide Bilayers

The coercivity and exchange bias field of ferro-/antiferromagnetic Co90Fe10/CoFe-oxide bilayers were studied as function of the surface morphology of the bottom CoFe-oxide layer. The CoFe-oxide surface structure was varied systematically by low energy (0–70 V) Argon ion-beam bombardment before subsequent deposition of the Co90Fe10 layer. Transmission electron microscopy results showed that the bilayer consisted of hcp Co90Fe10 and rock-salt CoFe-oxide. At low temperatures, enhanced coercivities and exchange bias fields with increasing ion-beam bombardment energy were observed, which are attributed to defects and uncompensated moments created near the CoFe-oxide surface in increasing amounts with larger ion-beam bombardment energies. Magnetometry results also showed an increasing divergence of the low field temperature dependent magnetization [ΔM(T)] between field-cooling and zero-field-cooling processes, and an increasing blocking temperature with increasing ion-beam bombardment energy.