Thomas Rheinländer

Magnetic fractionation of magnetic fluids

The properties of magnetic nanoparticles in magnetic fluids often exhibit a broad distribution, so in many applications only a small number of particles contribute to the desired magnetic effect. In order to optimize magnetic fluids for applications, preference is given to methods that separate the nanoparticles on the basis of their magnetic properties. Therefore, a magnetic method has been developed for the fractionation of magnetic fluids into two or more fractions. A common magnetic fluid was fractionated by this method. Magnetic and nonmagnetic properties of the fractions obtained and the original sample were measured. In addition to measurement of their magnetization curves they were also investigated by magnetic resonance and magnetorelaxometry, two biomedical applications of magnetic nanoparticles. The influence of the ion concentration of the washing solutions on the magnetic fractionation was additionally tested. The magnetic fractions have distinctly better magnetic properties than the original samples and are therefore especially suited for applications. Furthermore, the results indicate that the magnetic method fractionates the particles in accordance with their magnetic moment and that it has good recovery as well as reproducibility. Finally, magnetic fractionation is compared with other fractionation techniques.

Comparison of size-selective techniques for the fractionation of magnetic fluids

Abstract Nanoparticles in magnetic fluids commonly display a broad distribution of magnetic and nonmagnetic parameters such as particle size. Nevertheless, magnetic fluids are used in many fields of application like material separation and biomedicine. Thereby, it is often the case that only a small number of particles contribute to the desired magnetic effect. Two fractionation methods, which separate according to the particle size, were tested in order to optimize magnetic fluids for applications. On the one hand, flow field-flow fractionation was used with an online multi-angle laser light-scattering detector that measures the particle size independently. For comparison, on the other hand, well-known size-exclusion chromatography was performed. The fractions obtained by both methods were magnetically characterized by magnetorelaxometry, a biomedical application of magnetic nanoparticles. The fractionations yielded are similar, independent of the method used. In this respect, flow field-flow fractionation has several advantages over size-exclusion chromatography in analytical use. Thus, flow field-flow fractionation requires neither the addition of electrolytes nor column materials. The fractions obtained display distinctly different magnetic properties compared to the original sample and are therefore especially suited for applications like magnetorelaxometry.

Magnetic nanoparticle relaxation measured by a low-Tc SQUID system

A low-Tc SQUID system was developed for measuring magnetic relaxation of polymer-coated magnetic nanoparticles (MNPs) in a liquid carrier (e.g. water). The system consists of two low-Tc SQUIDs which are electronically combined to form an axial gradiometer using high-bandwidth directly coupled FLL electronics. The system is operated in a magnetically shielded room. The magnetic relaxation of the investigated MNPs in a liquid carrier is dominated by Brownian motion. In a solid phase, when the MNPs are immobilized, the magnetization of the sample decays via the Neel mechanism. A similar situation occurs when the mobility of the MNPs is reduced by a biochemical binding reaction. This effect is used for identifying biological reactions for purposes of medical diagnostics, e.g. immunoassays. By investigating the magnetic relaxation of dried samples, quantities as small as 1 nmol Fe of -Fe2O3 were detected. In the first agglomeration assay the binding reaction of the biochemical model biotin-avidin complexes can be clearly identified down to concentrations of <1 µg avidin in a volume of 150 µl of human blood.

Novel physicochemical characterization of magnetic fluids

Magnetic fluids are widely used in technical and biomedical applications. Depending on the application, different properties of the magnetic fluids are of interest. In order to choose magnetic fluids for a specific application, a comprehensive characterization is needed. In this work various aqueous magnetic fluids were systematically investigated with regard to their physicochemical properties. Besides magnetic properties the chemical composition, zeta potential, core and particle size of the samples were determined. Among techniques like magnetization curve measurements or photon correlation spectroscopy novel methods like acoustic and electroacoustic spectroscopy were used, which allow size and zeta potential measurements especially in highly concentrated dispersions. Comparisons of the results obtained by the different methods show good agreements. Furthermore, the magnetic relaxation was investigated, which is of significance for a novel biomedical application. The complex requirements of magnetic fluids for this application are considered in more detail.