Multi-core iron oxide nanoparticles: on their hierarchical architecture and coherence of their cores

Abstract text (incl. figure legends and references)

Multi-core iron oxide nanoparticles (IONPs), usually consisting of the spinel compounds magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃), possess superparamagnetic behavior, high saturation magnetization, good biocompatibility, and interactive surface characteristics. These properties predestine IONPs for biomedical applications, for example as carrier for drug delivery in the human body, as heat mediator for hyperthermia cancer treatment, or as contrast agent for magnetic resonance imaging. However, a profound description of the hierarchical structure of multi-core IONPs, in particular the crystallographic interconnection and coherence of the individual cores which strongly influence their magnetic properties, has not been provided so far.

The aim of this study was to describe the correlations between the internal structure of the IONPs and their magnetic properties. The IONPs investigated here were commercially available, dextran-coated multi-core IONPs (synomag-D, micromod Partikeltechnologie GmbH, Rostock, Germany), which were synthesized using a polyol method [1]. Their structural characteristics and magnetic properties were investigated using transmission electron microscopy (TEM), X-ray diffraction (XRD) and alternating gradient magnetometry (AGM).

High-resolution TEM (HRTEM) showed that the IONPs consist of several cores with specific orientation relationships. HRTEM in combination with geometric phase analysis [2] disclosed that larger cores are further fragmented into small domains that have a mutual misorientation of less than 1°. The fragmentation of the cores was confirmed by XRD that revealed a dependence of the line broadening on the magnitude of the diffraction vector, which is typically observed for slightly misoriented, partially coherent nanocrystallites [3]. The particle size and the core size distributions of the IONPs were determined in a statistical manner by applying a multi-stage semi-automatic segmentation routine to several low-magnification high-angle annular dark-field scanning TEM images [4] and correlated with the microstructure characteristics. The structural properties determined in a correlative manner are related to the magnetic properties of the multi-core IONPs that were obtained from the AGM measurements.

References

[1] H. Gavilán et al., Part. Part. Syst. Charact. 34 (2017) 1700094.

[2] M. Hytch et al., Ultramicroscopy 74 (1998) 131-146.

[3] D. Rafaja et al., J. Appl. Crystallogr. 37 (2004) 612-620.

[4] S. Neumann et al., CrytEngComm 22 (2020) 3644-3655.