Aggregation behaviour of magnetic nanoparticle suspensions investigated bymagnetorelaxometry

Abstract

The aggregation behaviour of magnetic nanoparticles (MNP) is a decisive factor for theirapplication in medicine and biotechnology. We extended the moment superposition modeldeveloped earlier for describing the Néel relaxation of an ensemble of immobilized particleswith a given size distribution by including the Brownian relaxation mechanism. Theresulting cluster moment superposition model is used to characterize the aggregation ofmagnetic nanoparticles in various suspensions in terms of mean cluster size, aggregatefraction, and size dispersion. We found that in stable ferrofluids 50%–80% of largermagnetic nanoparticles are organized in dimers and trimers. The scaling of therelaxation curves with respect to MNP concentration is found to be a sensitiveindicator of the tendency of a MNP suspension to form large aggregates, which maylimit the biocompatibility of the preparation. Scaling violation was observed inaged water based ferrofluids, and may originate from damaged MNP shells. Inbiological media such as foetal calf serum, bovine serum albumin, and humanserum we observed an aggregation behaviour which reaches a maximum at aspecific MNP concentration. We relate this to agglutination of the particles bymacromolecular bridges between the nanoparticle shells. Analysis of the scaling behaviourhelps to identify the bridging component of the suspension medium that causesagglutination.