Investigation of superparamagnetic Fe3O4 nanoparticles by fluxgate magnetorelaxometry for use in magnetic relaxation immunoassays

We have investigated the magnetorelaxation (MRX) of various magnetite $\left({\mathrm{Fe}}_3{\mathrm{O}}_4\right)$ nanoparticle samples with organic shells with regard to their suitability for the realization of a magnetic relaxation immunoassay. The MRX of magnetic nanoparticles (MNP), immobilized by freeze-drying, was measured for magnetization fields between 0.2 and $2.0\mathrm{mT}$ and for magnetization times between 0.2 and $20\mathrm{s}$ using a differential fluxgate setup. The experimental data were analyzed on the basis of the magnetic-moment superposition model, providing information on magnetic properties, such as anisotropy constant and saturation magnetization, on the dynamics of the magnetization and relaxation process, as well as on the size distribution of MNP cores.