Magnetic nanoparticles (MNPs), and in particular iron oxide nanoparticles (mainly magnetite and maghemite), are being widely used in the form of aqueous colloids for biomedical applications. In such colloids, nanoparticles tend to form assemblies, either aggregates, if the union is permanent, or agglomerates, if it is reversible. These clustering processes have a strong impact on the MNPs’ properties that are often not well understood. In this review, the causes and consequences of MNPs aggregation/agglomeration are reviewed and discussed. Special attention has been paid to the impact of the MNPs aggregation/agglomeration on their magnetic properties and heating properties, when exposed to an alternating magnetic field in the frame of magnetic hyperthermia. In addition, a model system with MNPs of two different sizes coated with three different molecules oleic acid, meso-2, 3-dimercaptosuccinic acid and poly(maleic anhydride-alt-1-octadecene) has been characterized and the results used to support the ideas reviewed.

Uniform iron oxide magnetic nanoparticles, with sizes in the range 9–22 nm, have been synthesized by thermal decomposition of an iron oleate complex in 1-octadecene, controlling reaction parameters related to the nucleation and growth processes. After transferring to water through a ligand substitution process, nanoparticles display very good magnetic and magneto-thermal properties. The relationship between these properties and the size and size distribution of the particles is discussed. The colloidal stability of the nanoparticles dispersed in common biological buffers has also been studied.

Iron oxide nanoparticles have found biomedical applications as therapeutic and/or diagnostic agents.

Purpose Tumor cells can be effectively inactivated by heating mediated by magnetic nanoparticles. However, optimized nanomaterials to supply thermal stress inside the tumor remain to be identified. The present study investigates the therapeutic effects of magnetic hyperthermia induced by superparamagnetic iron oxide nanoparticles on breast (MDA-MB-231) and pancreatic cancer (BxPC-3) xenografts in mice in vivo. Methods Superparamagnetic iron oxide nanoparticles, synthesized either via an aqueous (MF66; average core size 12 nm) or an organic route (OD15; average core size 15 nm) are analyzed in terms of their specific absorption rate (SAR), cell uptake and their effectivity in in vivo hyperthermia treatment. Results Exceptionally high SAR values ranging from 658 ± 53 W*g_Fe ⁻¹ for OD15 up to 900 ± 22 W*g_Fe ⁻¹ for MF66 were determined in an alternating magnetic field (AMF, H = 15.4 kA*m⁻¹ (19 mT), f = 435 kHz). Conversion of SAR values into system-independent intrinsic loss power (ILP, 6.4 ± 0.5 nH*m²*kg⁻¹ (OD15) and 8.7 ± 0.2 nH*m²*kg⁻¹ (MF66)) confirmed the markedly high heating potential compared to recently published data. Magnetic hyperthermia after intratumoral nanoparticle injection results in dramatically reduced tumor volume in both cancer models, although the applied temperature dosages measured as CEM43T90 (cumulative equivalent minutes at 43°C) are only between 1 and 24 min. Histological analysis of magnetic hyperthermia treated tumor tissue exhibit alterations in cell viability (apoptosis and necrosis) and show a decreased cell proliferation. Conclusions Concluding, the studied magnetic nanoparticles lead to extensive cell death in human tumor xenografts and are considered suitable platforms for future hyperthermic studies.