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Dynamic Magnetic Properties of Optimized Magnetic Nanoparticles for Magnetic Particle Imaging

Magnetic particle imaging (MPI) requires optimized tracers with monodisperse magnetic nanoparticle (MNP) cores. Organic phase routes to core synthesis are most promising, but require a subsequent phase transfer step to add a biocompatible, water-soluble coating. This transfer process can potentially produce dimers or clusters of magnetic cores that are not resolved by routine hydrodynamic size measurements. Here, we present systematic dynamic magnetic investigations on nominally single-core MNPs with respect to their suitability as tracers for MPI. Two MNP suspensions with similar core and hydrodynamic diameters show differences in their ac susceptibility (ACS) and magnetic particle spectroscopy (MPS) data. The maximum in the imaginary part of the ACS at ~1 kHz for both samples is caused by the Brownian relaxation of multicore (estimated to be dimers) particles with hydrodynamic diameters of more than 70 nm, whereas the maximum at ~150 kHz is caused by the Néel relaxation of single-core particles. The different relative portions of both particle types cause a different frequency dependence of the harmonic spectra.