Nanoparticle Composition of a Ferrofluid and Its Effects on the Magnetic Properties

Abstract

Experiments were carried out on a water-based ferrofluid (gamma-Fe2O3 with carboxydextran shell) using photon correlation spectroscopy (PCS), atomic force microscopy, and magnetic nanoparticle relaxation measurements. The experiments were designed with the aim to relate the Néel signals that are in theory generated by large single core particles with nanoscopic properties, that is, particle size, particle size distribution, shell properties, and aggregation. For this purpose, the ferrofluid was fractionated by magnetic fractionation and size exclusion chromatography. Nanoparticles adsorbed onto positively charged substrates form a two-dimensional monolayer. Their mean core diameters are in the range of 6 to about 20 nm, and particles above 10 nm are mostly aggregates. The hydrodynamic particle diameters are between 13 and 80 nm. The core diameter of the smallest fraction is confirmed by X-ray reflectometry; the surface coverage is controlled by bulk diffusion. Comparison with the hydrodynamic radius yields a shell thickness of 3.8 nm. Considering the shell thickness to be constant for all particles, it was possible to calculate the apparent particle diameter in the original ferrofluid from the PCS signals of all fractions. As expected, the small cores yielded no Néel relaxation signals in freeze-dried samples; however, the fractions containing mostly aggregates yielded Néel relaxation signals.

Enhancement of AC-losses of magnetic nanoparticles for heating applications

“…The concentration data are in fair accordance with data derived from iron analysis by atom absorption spectroscopy which are given in Ref. [9]. Experimental coercivity data are extremely small in comparison to the anisotropy field of ellipsoidal single domain maghemite particles.…”

“…Three different fluid fractions were prepared from the original fluid by a magnetic separation technique described in detail elsewhere [9]. Shortly, a magnetic column (MACS Cell Separation Column LS, Miltenyi Biotec) positioned between the poles of an electromagnet was used and after stepwise reducing the applied coil current the respective fluid fractions were washed out of the magnetic column.…”

“…Particles of the original ferrofluid and different fractions were also investigated by atomic force microscopy (AFM). The method of investigation as well as results thereof are described in detail elsewhere [9]. The mean particle diameter derived from susceptibility spectra is considerably larger than data extracted from AFM images.…”

Synthesis of iron oxide nanoparticles using chitosan and starch templates

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Development of a liquid phase immunoassay by time-dependent measurements of the transient magneto-optical birefringence using functionalized magnetic nanoparticles

“…Single cores are mostly in the range of 5-7 nm diameter; they are partly aggregated showing a log-normal distribution. Including the carboxydextran shell the particles hydrodynamic diameters are broadly distributed between about 10 and 100 nm [5].…”