Roman Kötitz

Determination of the binding reaction between avidin and biotin by relaxation measurements of magnetic nanoparticles

The binding between avidin and biotinylated magnetic iron oxide nanoparticles can be monitored by means of magnetic relaxation measurements. The resultant signal, caused by particle aggregates, has Brownian and Neel components. The time constant and amplitude of Brownian relaxation were determined by a fit algorithm which also yielded Neel amplitudes proportional to the avidin content of the samples.

Time domain study of Brownian and Néel relaxation in ferrofluids

Abstract Relaxation of six ferrofluids differing in particle size was studied using a dc SQUID magnetometer. We observed Neel relaxation with nonlinear ln( t ) behaviour in freeze-dried samples and viscosity-dependent Brownian relaxation in glycerol-based dried samples. Investigations of the basic magnetic parameters, complex susceptibility and remanence complement the time-dependent measurements.

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.

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.

Superconducting quantum interference device-based magnetic nanoparticle relaxation measurement as a novel tool for the binding specific detection of biological binding reactions (abstract)

Biological binding reactions play a key role in biology and medicine. Their detection is usually achieved by labeling one of the reaction components with radioisotopes, enzymes, or fluorescence dyes. In particular assays, using the specificity of the reaction between antibodies and antigens are of outstanding importance. Most of these assays are hampered by the drawback that the label generates a signal that is not influenced by the binding reaction. Therefore, separation procedures between bound and unbound reaction components are mostly inevitable. Here, we present the use of superconducting quantum interference device (SQUID)-based magnetic nanoparticle relaxation measurement as a novel tool for the quantitative determination of biological binding reactions, where magnetic nanoparticles are used as labels to antibodies. The rotational diffusion of the label is hindered by the binding of the antibody to the antigen, which is adsorbed to the sample tube wall. As a result, the observed relaxation of its m...

LTS SQUID gradiometer system for in vivo magnetorelaxometry

A system for spatially resolved magnetorelaxometry in spatially expanded objects (e.g. for in vivo investigations) in a disturbed environment is presented. The system is based on an LTS SQUID gradiometer with a field gradient sensitivity of 30 fT cm-1 Hz-1/2 and an intrinsic balance of some 10-5. A Helmholtz coil of 80 cm diameter allows the magnetization of the samples with fields up to 7 mT. A nonmagnetic x-y stage (driven by step motors) covers a scanning area of 40 cm × 25 cm. We present spatially resolved measurements of the Neel relaxation of samples (dried ferrofluid) and compare them with model calculations. The data acquisition and monitoring as well as the filtering and fitting procedure are discussed.

Detection of Biodegradation of Ferrofluid by Relaxation Measurements

Ferrofluids are colloidal solutions of ferro- or ferrimagnetic particles of typically 10 nm size, coated with a surfactant and dispersed in a carrier liquid. Due to their size these particles are single domain and magnetized to saturation. Ferrofluids are superparamagnetic i.e. gain a very high magnetization in the presence of a magnetizing field. Thus the high sensitivity of SQUID sensors allows the detection of low concentrations of these particles. Fig.l gives an overview of the components and some relevant parameters of ferrofluids.