Abstract
The widespread use of magnetic nanoparticles in the biotechnical sector puts new demands on fast and quantitative characterization techniques for nanoparticle dispersions. In this work, we report the use of asymmetric flow field-flow fractionation (AF4) and ferromagnetic resonance (FMR) to study the properties of a commercial magnetic nanoparticle dispersion. We demonstrate the effectiveness of both techniques when subjected to a dispersion with a bimodal size/magnetic property distribution: i.e., a small superparamagnetic fraction, and a larger blocked fraction of strongly coupled colloidal nanoclusters. We show that the oriented attachment of primary nanocrystals into colloidal nanoclusters drastically alters their static, dynamic, and magnetic resonance properties. Finally, we show how the FMR spectra are influenced by dynamical effects; agglomeration of the superparamagnetic fraction leads to reversible line-broadening; rotational alignment of the suspended nanoclusters results in shape-dependent resonance shifts. The AF4 and FMR measurements described herein are fast and simple, and therefore suitable for quality control procedures in commercial production of magnetic nanoparticles.
Highlights
We report the use of asymmetric flow field-flow fractionation (AF4) and ferromagnetic resonance (FMR) to study the properties of a commercial magnetic nanoparticle dispersion
We would like to point out that it is not possible to conclude from Transmission Electron Microscopy (TEM) alone whether these aggregates are present in the dispersed state, but our fractionation study suggests that these are preparation artefacts caused by drying induced aggregation
Since FMR spectra are in some sense fingerprints, sensitive to the physical properties and to the colloidal state of any nanoparticle dispersion, we suggest its use as a tool for quality control, and monitoring in commercial production of magnetic nanoparticles
Summary
We use asymmetric flow field-flow fractionation (AF4), ferromagnetic resonance (FMR) and AC- and DCmagnetomery to analyze a commercial magnetic nanoparticle system (nanoPET Pharma GmbH, FeraSpin). We show that AF4 and FMR are effective in characterizing and classifying a nanoparticle dispersion as Nanoscale a bimodal, both with respect to its physical and magnetic properties. TEM bright field (BF) images of the particles were used to measure the size and aspect ratio of the nanoparticles and nanoclusters. SEM analysis of magnetic nanoparticles was performed using a Zeiss Supra 40VP system equipped with a field emission gun at 2–5 kV. FeraSpin L colloidal nanoclusters were prepared by electrostatic immobilisation of the nanoparticles on Si substrates, in order to prevent agglomeration of the particles during the drying process. The sample injection onto the channel was made at a flow rate of 0.20 mL min−1 for 1 min. Freeze dried samples for AC-susceptibility measurements were prepared by dissolving 50 mM mannitol in the nanoparticle dispersions. The magnetic AC-moment was converted to dimensionless SI units (χ) assuming a density of 5 g cm−3 and a Fe/FexOy mass ratio of 0.7 (assuming γ-Fe2O3)
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