Abstract
The Linear Response Theory (LRT) is a widely accepted framework to analyze the power absorption of magnetic nanoparticles for magnetic fluid hyperthermia. Its validity is restricted to low applied fields and/or to highly anisotropic magnetic nanoparticles. Here, we present a systematic experimental analysis and numerical calculations of the specific power absorption for highly anisotropic cobalt ferrite (CoFe2O4) magnetic nanoparticles with different average sizes and in different viscous media. The predominance of Brownian relaxation as the origin of the magnetic losses in these particles is established, and the changes of the Specific Power Absorption (SPA) with the viscosity of the carrier liquid are consistent with the LRT approximation. The impact of viscosity on SPA is relevant for the design of MNPs to heat the intracellular medium during in vitro and in vivo experiments. The combined numerical and experimental analyses presented here shed light on the underlying mechanisms that make highly anisotropic MNPs unsuitable for magnetic hyperthermia.
Highlights
The specific power absorption (SPA), known as specific absorption rate (SAR) or specific loss power (SLP), quantifies the power absorbed by a system of MNPs due to magnetic losses, taking place when an alternate magnetic field (AMF) it is applied to the sample
We performed numerical simulations within the ‘classical’ Specific Power Absorption (SPA) (CSPA) model, applied to magnetic colloids by Rosensweig[7], which considers that both Néel and Brown relaxations are the main mechanisms for magnetic relaxation
Our investigation on the heating capability of highly anisotropic Co-ferrite nanoparticles confirmed that the SPA values up to ≈1300–1400 W/g obtained in low-viscosity media originate in a purely Brownian relaxation mechanism
Summary
The specific power absorption (SPA), known as specific absorption rate (SAR) or specific loss power (SLP), quantifies the power absorbed by a system of MNPs due to magnetic losses, taking place when an alternate magnetic field (AMF) it is applied to the sample. Magnetic losses are the main physical phenomena involved in magnetic hyperthermia treatments (MHT) to target and kill cancerous cells The physics behind this mechanism of heating is related to the structural and magnetic parameters of the MNPs (namely the effective anisotropy constant Keff, saturation magnetization MS, average particle size 〈d〉) and to the viscosity of the medium (η). When the magnetic anisotropy of MNPs is such that the energy barrier required to flip the magnetic moments is much larger than thermal energy at room temperature, the Brownian rotation is the predominant mechanism for magnetic relaxation[4] In this situation, the hydrodynamic diameter of the MNPs and the viscosity of the medium are key parameters to determine the SPA. The lack of Brownian relaxation would explain the absence of heating observed in our experiments
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