We present a scalable thermolysis and high temperature oxidation procedure for synthesizing monodisperse magnetite nanoparticles with saturation magnetization of up to 80 emu g −1 (412 kA m −1 ), 92% ...of bulk magnetite. Diameters in the 15–30 nm size range are produced from iron oleate via the thermolysis method at 324 °C and varying oleic acid ratios for size control (6.7–7.6 equivalents per Fe). The influence of the iron oleate synthesis procedure on the quality of resulting nanoparticles is examined and the structure of the iron oleate is proposed to have a triironoxonium core Fe 3 O + based on magnetic susceptibility measurements. The thermolysis method is shown to initially give wüstite nanoparticles, which are oxidized in situ at 318 °C using 1% oxygen in argon to form highly magnetic magnetite nanoparticles. The use of 1% oxygen offers broad application as a safe and efficient reagent for the high temperature oxidation of nanoparticles. Special consideration to the reproducibility of nanoparticle diameter and monodispersity has uncovered critical factors. Additionally, the reduction of Fe( iii ) to Fe( ii ) is shown to occur during the heat up stage of thermolysis, beginning at less than 180 °C and being complete by 320 °C. Evidence for the reduction occurring by the oxidative decarboxylation of oleic acid is presented. Decomposition of the remaining oleic acid is shown to occur by a ketonization reaction producing oleone. The nucleation event and growth of particles is examined by TEM. Comparison of the solvents 1-octadecene and octadecane are presented demonstrating the effect on the reduction of Fe( iii ) during heat up, the large difference in particle size, and effects on the oxidation rate of iron oxide nanoparticles. Determination of Fe( ii ) content in magnetic iron oxide nanoparticles by titration is presented.
We describe the microfabrication and magnetic behavior of a composite/hybrid, two-dimensional, magnetostatically interacting array of nanomagnets of Fe and exchange-biased bilayer Fe/IrMn ...heterostructures. Such an interacting array of nanomagnets, forming an artificial spin ice lattice but with a hybrid structure, has not been demonstrated before. These devices are fabricated of epitaxially grown Fe/IrMn thin films by a two-stage electron beam lithography process involving metal mask transfer and controlled ion milling. Following the epitaxial deposition of Fe/IrMn bilayer films, the first step involves electron beam lithography fabrication of nanomagnet arrays, followed by selective removal of exchange-bias by etching away IrMn layer at specific nanomagnet elements by ion milling. The technique described provides a way to apply a site-specific magnetic field at the nanometer length scale, utilizing the phenomenon of exchange-bias, as demonstrated here for an array with local fields applied at twice the period of the artificial spin ice lattice. This technology can also be readily extended to different spintronic devices requiring spatial distribution of exchange-bias fields.
Magnetic particle imaging (MPI) is an imaging modality that detects the response of a distribution of magnetic nanoparticle tracers to alternating magnetic fields. There has recently been exploration ...into multi-contrast MPI, in which the signal from different tracer materials or environments is separately reconstructed, resulting in multi-channel images that could enable temperature or viscosity quantification. In this work, we apply a multi-contrast reconstruction technique to discriminate between nanoparticle tracers of different core sizes. Three nanoparticle types with core diameters of 21.9 nm, 25.3 nm and 27.7 nm were each imaged at 21 different locations within the scanner field of view. Multi-channel images were reconstructed for each sample and location, with each channel corresponding to one of the three core sizes. For each image, signal weight vectors were calculated, which were then used to classify each image by core size. With a block averaging length of 10 000, the median signal-to-noise ratio was 40 or higher for all three sample types, and a correct prediction rate of 96.7% was achieved, indicating that core size can effectively be predicted using signal weight vector classification with close to 100% accuracy while retaining high MPI image quality. The discrimination of the core size was reliable even when multiple samples of different core sizes were placed in the measuring field.
Purpose:
Magnetic particle imaging (MPI), using magnetite nanoparticles (MNPs) as tracer material, shows great promise as a platform for fast tomographic imaging. To date, the magnetic properties of ...MNPs used in imaging have not been optimized. As nanoparticle magnetism shows strong size dependence, the authors explore how varying MNP size impacts imaging performance in order to determine optimal MNP characteristics for MPI at any driving field frequency
f
0
.
Methods:
Monodisperse MNPs of varying size were synthesized and their magnetic properties characterized. Their MPI response was measured experimentally using a custom-built MPI transceiver designed to detect the third harmonic of MNP magnetization. The driving field amplitude
H
0
=
6
mT
μ
0
−
1
and frequency
f
0
=
250
kHz
were chosen to be suitable for imaging small animals. Experimental results were interpreted using a model of dynamic MNP magnetization that is based on the Langevin theory of superparamagnetism and accounts for sample size distribution and size-dependent magnetic relaxation.
Results:
The experimental results show a clear variation in the MPI signal intensity as a function of MNP diameter that is in agreement with simulated results. A maximum in the plot of MPI signal vs MNP size indicates there is a particular size that is optimal for the chosen
f
0
.
Conclusions:
The authors observed that MNPs 15 nm in diameter generate maximum signal amplitude in MPI experiments at 250 kHz. The authors expect the physical basis for this result, the change in magnetic relaxation with MNP size, will impact MPI under other experimental conditions.
Magnetic particle imaging (MPI) is a powerful new research and diagnostic imaging platform that is designed to image the amount and location of superparamagnetic nanoparticles in biological tissue. ...Here, we present mathematical modeling results that show how MPI sensitivity and spatial resolution both depend on the size of the nanoparticle core and its other physical properties, and how imaging performance can be effectively optimized through rational core design. Modeling is performed using the properties of magnetite cores, since these are readily produced with a controllable size that facilitates quantitative imaging. Results show that very low detection thresholds (of a few nanograms Fe
3O
4) and sub-millimeter spatial resolution are possible with MPI.
Abstract Magnetic Particle Imaging (MPI) is a new biomedical imaging modality that produces real-time, high-resolution tomographic images of superparamagnetic iron oxide (SPIO) nanoparticle tracer ...distributions. In this study, we synthesized monodisperse tracers for enhanced MPI performance and investigated both, their blood clearance time using a 25 kHz magnetic particle spectrometer (MPS), and biodistribution using a combination of quantitative T2-weighted MRI and tissue histology. In vitro and in vivo MPI performance of our magnetic nanoparticle tracers (MNTs), subject to biological constraints, were compared to commercially available SPIOs (Resovist). Monodisperse MNTs showed a 2-fold greater signal per unit mass, and 20% better spatial resolution. In vitro evaluation of tracers showed that MPI performance of our MNTs is preserved in blood, serum-rich cell-culture medium and gel; thus independent of changes in hydrodynamic volume and fluid viscosity – a critical prerequisite for in vivo MPI. In a rodent model, our MNTs circulated for 15 min – 3× longer than Resovist – and supported our in vitro evaluation that MPI signal is preserved in the physiological environment. Furthermore, MRI and histology analysis showed that MNTs distribute in the reticuloendothelial system (RES) in a manner similar to clinically approved SPIO agents. MNTs demonstrating long-circulation times and optimized MPI performance show potential as angiography tracers and blood-pool agents for the emerging MPI imaging modality.
We describe an artificial spin ice (ASI) composed of exchange biased heterostructured nanomagnetic elements with unidirectional anisotropy and compare it with a conventional ASI constituted by ...ferromagnets with uniaxial anisotropy (Ising spins). The introduction of a local exchange bias field, aligned along one of the sublattices of the square ASI, lifts the spin-reversal symmetry of the vertices. By varying the lattice constant of the square array, we control the ratio of exchange bias (EB) to dipolar field (HEB / Hdip) and tune the ground state from an antiferromagnetic to a ferromagnetic configuration with an effective magnetic moment. The geometric frustration of dipolar interactions is moderated by a nonfrustrated local field, leading to a mesoscopic system with specific metastable states observed during the demagnetization process.
We report the successful synthesis of Co−Au core−shell nanoparticles by reducing an organo-gold compound onto cobalt seeds with a weak reducer in a nonpolar solvent. The core−shell morphology was ...unequivocally confirmed by complementary structural, magnetic, and optical property measurements. High-resolution transmission electron microscopy and Z-contrast imaging shows a gold shell composed of multiple grains. Electron-energy loss spectroscopy confirms the chemically distinct characteristics of the core (Co) and shell (Au). Temperature-dependent magnetic property measurements confirm that the particles are superparamagnetic with a blocking temperature, T B ∼ 55 K, consistent with a magnetic diameter ∼6 nm. The UV−visible absorption spectra of these nanoparticles show a red shift (relative to pure gold nanoparticles) in agreement with a Au-shell morphology.
Magnetic particle imaging (MPI) shows promise for medical imaging, particularly in angiography of patients with chronic kidney disease. As the first biomedical imaging technique that truly depends on ...nanoscale materials properties, MPI requires highly optimized magnetic nanoparticle tracers to generate quality images. Until now, researchers have relied on tracers optimized for MRI T2*-weighted imaging that are sub-optimal for MPI. Here, we describe new tracers tailored to MPI's unique physics, synthesized using an organic-phase process and functionalized to ensure biocompatibility and adequate in vivo circulation time. Tailored tracers showed up to 3 × greater signal-to-noise ratio and better spatial resolution than existing commercial tracers in MPI images of phantoms.
Gastrointestinal bleeding, as a potentially life-threatening condition, is typically diagnosed by radiation-based imaging modalities like computed tomography or more invasively catheter-based ...angiography. Endoscopy enables examination of the upper gastrointestinal tract and the colon but not of the entire small bowel. Magnetic Particle Imaging (MPI) enables non-invasive, volumetric imaging without ionizing radiation. The aim of this study was to evaluate the feasibility of detecting gastrointestinal bleeding by single- and multi-contrast MPI using human-sized organs. A 3D-printed small bowel phantom and porcine small bowel specimens were prepared with a defect within the bowel wall as the source of a bleeding. For multi-contrast MPI, the bowel lumen was filled with an intestinal tracer representing an orally administered tracer. MPI was performed to evaluate the fluid exchange between the vascular compartment of the bowel wall and the lumen while a blood pool tracer was applied. Leakage of the blood pool tracer was observed to the bowel lumen. Multi-contrast MPI enabled co-registration of both tracers at the same location within the bowel lumen indicating gastrointestinal bleeding. Single- and multi-contrast MPI are feasible to visualize gastrointestinal bleeding. Therefore, MPI might emerge as a useful tool for radiation-free detection of bleeding within the entire gastrointestinal tract.
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