No abstract available

[en] We present magnetization isotherms and 9 GHz ferromagnetic resonance (FMR) spectra recorded on powders containing 6 nm-diameter nanocrystals, prepared by a high-temperature organometallic route. Upon lowering the temperature from 400 K towards the blocking temperature, T_b∼50 K, deviations from Langevin isotherms and shifts of the FMR field occur, which for the first time are consistently explained by introducing a uniaxial anisotropy which, for both CoPt₃ and FePt₂, strongly decay with the 6th power of the particle moments μ_p(T)

[en] Liquid suspensions of magnetic nanoparticles play a crucial role in current biomedical research and applications. While their magnetic characteristics are of importance for their utilization, no measurement standard exists and an uncertainty budget for static magnetisations measurements of MNP is lacking. Here we present the structure of the uncertainty budget for static magnetisation measurements by stratifying the measurement process and data analysis into seven levels of rising complexity. The main uncertainty contributions of each level are stated and briefly described. This paves the way towards a reliable quantitative uncertainty budget that could be used for interlaboratory comparisons as well as for the development of a measurement standard. (paper)

No abstract available

[en] The aggregation behaviour of magnetic nanoparticles (MNP) is a decisive factor for their application in medicine and biotechnology. We extended the moment superposition model developed earlier for describing the Neel relaxation of an ensemble of immobilized particles with a given size distribution by including the Brownian relaxation mechanism. The resulting cluster moment superposition model is used to characterize the aggregation of magnetic nanoparticles in various suspensions in terms of mean cluster size, aggregate fraction, and size dispersion. We found that in stable ferrofluids 50%-80% of larger magnetic nanoparticles are organized in dimers and trimers. The scaling of the relaxation curves with respect to MNP concentration is found to be a sensitive indicator of the tendency of a MNP suspension to form large aggregates, which may limit the biocompatibility of the preparation. Scaling violation was observed in aged water based ferrofluids, and may originate from damaged MNP shells. In biological media such as foetal calf serum, bovine serum albumin, and human serum we observed an aggregation behaviour which reaches a maximum at a specific MNP concentration. We relate this to agglutination of the particles by macromolecular bridges between the nanoparticle shells. Analysis of the scaling behaviour helps to identify the bridging component of the suspension medium that causes agglutination

[en] New therapies against cancer based on magnetic nanoparticles (MNPs) require a quantitative spatially resolved imaging of MNPs inside a body. In magnetorelaxometry (MRX), a distribution of nanoparticles can be quantified non-invasively by measuring its relaxation after removal of an external magnetizing field. Conventionally, in MRX the sample is exposed to a homogeneous magnetizing field resulting in a quantitative reconstruction with rather poor spatial resolution. Theoretical work suggests an improvement of spatial resolution may be achieved by a sequential application of inhomogeneous fields magnetizing only parts of a sample. Here, we experimentally demonstrate the feasibility of this approach by reconstructing a nanoparticle distribution inside a compact three-dimensional volume phantom made of 54 gypsum cubes (1 cm"3 cube"−"1), of which 12 gypsum cubes were filled with MNPs. Using 48 small excitation coils surrounding the phantom, a sequence of MRX signals was obtained where only those MNPs near an individual coil contribute. By combined evaluation of these 48 MRX measurements, the positions and content of the 12 MNP-filled cubes could be determined accurately with a deviation below 4%, while by conventional homogeneous MRX only the MNP content was reconstructable with a deviation of about 9%. The results demonstrate the improvement of quantitative MRX imaging by using sequential activation of multiple magnetizing fields. (paper)

[en] Magnetorelaxometry (MRX) is a well-known measurement technique which allows the retrieval of magnetic nanoparticle (MNP) characteristics such as size distribution and clustering behavior. This technique also enables the non-invasive reconstruction of the spatial MNP distribution by solving an inverse problem, referred to as MRX imaging . Although MRX allows the imaging of a broad range of MNP types, little research has been done on imaging different MNP types simultaneously. Biomedical applications can benefit significantly from a measurement technique that allows the separation of the resulting measurement signal into its components originating from different MNP types. In this paper, we present a theoretical procedure and experimental validation to show the feasibility of MRX imaging in reconstructing multiple MNP types simultaneously. Because each particle type has its own characteristic MRX signal, it is possible to take this a priori information into account while solving the inverse problem. This way each particle type’s signal can be separated and its spatial distribution reconstructed. By assigning a unique color code and intensity to each particle type’s signal, an image can be obtained in which each spatial distribution is depicted in the resulting color and with the intensity measuring the amount of particles of that type, hence the name multi-color MNP imaging. The theoretical procedure is validated by reconstructing six phantoms, with different spatial arrangements of multiple MNP types, using MRX imaging. It is observed that MRX imaging easily allows up to four particle types to be separated simultaneously, meaning their quantitative spatial distributions can be obtained. (paper)

[en] Suspensions of magnetic nanoparticles have received increasing interest in the biomedical field. While these ferrofluids are already used for magnetic resonance imaging, emerging research on cancer treatment focuses, for example, on employing the particles as drug carriers, or using them in magnetic hyperthermia to destroy diseased cells by heating of the particles. To enable safe and effective applications, an understanding of the flow behaviour of the ferrofluids is essential. Regarding the applications mentioned above, in which flow phenomena play an important role, viscosity under the influence of an external magnetic field is of special interest. In this respect, the magnetoviscous effect (MVE) leading to an increasing viscosity if an external magnetic field of a certain strength is applied, is well-known for singlecore ferrofluids used in the engineering context. In the biomedical context, multicore ferrofluids are preferred in order to avoid remanence magnetization and to enable a deposition of the particles by the organism without complications. This study focuses on a comparison of the MVE for three ferrofluids whose composition is identical except in relation to their hydrodynamic diameter and core composition—one of the fluids contains singlecore particles, while the other two feature multicore particles. This enables confident conclusions about the influence of those parameters on flow behaviour under the influence of a magnetic field. The strong effects found for two of the fluids should be taken into account, both in future investigations and in the potential use of such ferrofluids, as well as in manufacturing, in relation to the optimization of flow behaviour. (paper)

[en] In magnetic heating treatments, intratumorally injected superparamagnetic iron oxide nanoparticles (MNP) exposed to an externally applied alternating magnetic field generate heat, specifically at the tumor region. This inactivates cancer cells with minimal side effects to the normal tissue. Therefore, the quantity of MNP needs to be thoroughly controlled to govern adequate heat production. Here, we demonstrate the capability of magnetorelaxometry (MRX) for the non-invasive quantification and localization of MNP accumulation in small animal models. The results of our MRX measurements using a multichannel vector magnetometer system with 304 SQUIDs (superconductive quantum interference device) on three mice hosting different carcinoma models (9L/lacZ and MD-AMB-435) are presented. The position and magnitude of the magnetic moment are reconstructed from measured spatial magnetic field distributions by a magnetic dipole model fit applying a Levenberg-Marquadt algorithm. Therewith, the center of gravity and the total amount of MNP accumulation in the mice are determined. Additionally, for a fourth mouse the distribution of MNP over individual organs and the tumor is analyzed by single-channel SQUID measurements, obtaining a sensitive spatial quantification. This study shows that magnetorelaxometry is well suited to monitor MNP accumulation before cancer therapy, with magnetic heating being an important precondition for treatment success.

[en] In magnetic drug targeting a chemotherapeutic agent is bound to coated magnetic nanoparticles, which are administered to the blood vessel system and subsequently focused by an external applied magnetic field. The optimization of intra-arterial magnetic drug targeting (MDT) requires detailed knowledge about the biodistribution of particles in the artery and the respective surrounding after the application. Here, we demonstrate the potential of magnetorelaxometry for quantifying the distribution of magnetic nanoparticles in the artery. To this end, we present a magnetorelaxometry investigation of a MDT study in an artery model. In particular, the absolute magnetic nanoparticle accumulation along the artery as well as the uptake profile along the region around the MDT-magnet position was quantified. (note)