Arrangement at the nanoscale: Effect on magnetic particle hyperthermia

Abstract

In this work, we present the arrangement of Fe₃O₄ magnetic nanoparticles into 3D linear chains and its effect on magnetic particle hyperthermia efficiency. The alignment has been performed under a 40 mT magnetic field in an agarose gel matrix. Two different sizes of magnetite nanoparticles, 10 and 40 nm, have been examined, exhibiting room temperature superparamagnetic and ferromagnetic behavior, in terms of DC magnetic field, respectively. The chain formation is experimentally visualized by scanning electron microscopy images. A molecular Dynamics anisotropic diffusion model that outlines the role of intrinsic particle properties and inter-particle distances on dipolar interactions has been used to simulate the chain formation process. The anisotropic character of the aligned samples is also reflected to ferromagnetic resonance and static magnetometry measurements. Compared to the non-aligned samples, magnetically aligned ones present enhanced heating efficiency increasing specific loss power value by a factor of two. Dipolar interactions are responsible for the chain formation of controllable density and thickness inducing shape anisotropy, which in turn enhances magnetic particle hyperthermia efficiency.

Figure 1

Structural (a) and magnetic (b) characterization of powder samples of 10 and 40 nm Fe₃O₄ MNPs. Green triangles indicate the reference peak positions of bulk magnetite.

Figure 2

Schematic representation of (a) reference sample (random), where the MNPs are fixed at random positions in the agarose matrix and (b) magnetically aligned sample with MNPs’ chain formation (chain) in an external field of 40 mT.

Figure 3

Typical SEM images for 40 nm MNPs with 2 mg/mL concentration and 1 mg/mL agarose content at lower and higher magnification: (a,b) random sample and (c,d) chain sample.

Figure 4. Molecular Dynamics simulations of chain formation of 40 nm MNPs for various concentration values.

(a) Time evolution of the average length of the chains: as the simulation evolves, chains with increasing length appear. The length and density of chains increases with the particle concentration and is stabilized after 200 s (case of 1 and 4 mg/mL) and after 400 s (case of 2 mg/mL). (b) Projection in z-x plane of the randomly oriented MNPs in absence of external magnetic field. (c) Projection in z-x plane of the chain formation of MNPs in an external field of 40 mT. The dimensions of our 3D computational space were x = y = z = L(d₀) = (80d₀) where d₀ is the MNPs diameter of 40 nm. The number of MNPs was set to 380, 760, 1520 for the concentrations of 1, 2 and 4 mg/mL, respectively. (d) Corresponding experimental SEM images for three MNPs concentrations.

Figure 5. FMR spectra of random (blue lines) and chain (red lines) samples recorded at two orthogonal configurations of the applied magnetic field with respect to the sample’s alignment direction.

As schematically shown in the inset, one configuration corresponds to φ = 0° (solid lines), where the field is parallel to the chain axis (arbitrary for the random sample), and a second configuration refers to φ = 90) (dashed lines), where the field is perpendicular to the chain axis. The spectra were taken for chain and random samples with 4 mg/mL concentration of 40 nm MNPs and 1 mg/mL of agarose content.

Figure 6

(a) Major magnetic hysteresis loops at 1 T of 40 nm MNPs at 100 and 300 K with the magnetic field applied parallel to the alignment direction of random (blue lines) and the chain (red lines) samples at 100 and 300 K. (b) Minor hysteresis loops of random (blue line) and chain (red line) samples recorded at 30 mT at 300 K.

Figure 7. MNPs’ alignment influence on magnetic hyperthermia efficiency as expressed by SLP values by varying MNPs’ concentrations for two different configurations (random and chain) at 765 kHz frequency and 30 mT field with MNPs sizes of 40 nm (solid symbol) and 10 nm (open symbol).

For chain samples, SLP values refer to measurements performed with the AC hyperthermia field applied parallel to the alignment direction of the chain samples.