Research articles
Tuning dipolar effects on magnetic hyperthermia of Zn0.3Fe2.7O4/SiO2 nanoparticles by silica shell

https://doi.org/10.1016/j.jmmm.2020.167483Get rights and content

Highlights

  • Our approach ensures that magnetic cores are separated by the silica shell and thus dipole interactions can be systematically tuned.

  • Our work reveals that dipole interactions have profound influence on SLP, as they affect both Neel relaxation and Brownian relaxation.

  • Our study suggests that tuning non-magnetic coating thickness is a convenient route to maximizing SLP.

Abstract

Effects of magnetic dipole interactions on specific loss power (SLP) for Zn0.3Fe2.7O4/SiO2 core/shell nanoparticles (NPs) with identical magnetic core size and varying non-magnetic shell thicknesses were systematically investigated. In the case of thin silica shell, NPs form chain-like structures due to strong dipole interactions between NPs. The chain formation induces additional shape anisotropy. With increasing thickness of the silica shell, the dipole interaction and therefore the anisotropy induced by the dipole interactions decrease. Meanwhile, the hydrodynamic size of the NPs shows a counterintuitive reduction with increasing shell thickness, also due to the decrease in interactions. The magnetic anisotropy and hydrodynamic size affect the SLP through changes in the Néel and Brownian relaxation rates, leading to different field-dependent SLP values for NPs with thin and thick shells.

Introduction

Heat dissipation due to hysteresis losses of magnetic nanomaterials in an alternating current (AC) magnetic field are exploited for treating tumors in the so-called magnetic hyperthermia therapy. Nearly 60 years ago, Gilchrist et al. reported the first experimental work on magnetic hyperthermia therapy [1]. Magnetic hyperthermia enables real local heating by embedding the heating sources into tumor tissues [2], [3], [4], [5], [6]. Because heat loss is produced by an external AC magnetic field, the heating effect can penetrate deep into the body [7]. Besides being used as a nano-source of heat, magnetic nanoparticles (NPs) can also serve as a contrast agent for magnetic resonance imaging (MRI) [8], [9], as a carrier for targeted drug delivery [10] or gene delivery [11]. Iron oxide NPs are the most promising candidate for magnetic hyperthermia for clinical applications due to their demonstrated bio-compatibility [12], [13], [14], [15]. Recently, our studies suggested that zinc ferrite NPs exhibited highly efficient AC field heating under low field amplitudes and a biocompatibility comparable to and perhaps even better than iron oxide NPs [16].

High heat loss of NPs in AC field can be realized by maximizing magnetization and tuning the magnetic anisotropy of NPs according to the field amplitude [16], [17], [18]. Magnetization is closely related to material composition, crystal structure, particle size, etc. Magnetic anisotropy, on the other hand, is mainly contributed by the magnetocrystalline anisotropy and shape anisotropy. It is known that dipolar interactions play an important role in the AC field heating. Previous studies suggest that the distance between magnetic NPs significantly affect the magnetic relaxation in an AC field [19], [20]. Much effort has been made to understand the effects of dipole interactions on magnetic hyperthermia, both experimentally [21], [22], [23], [24] and theoretically [25], [26], [27], [28]. Many previous studies relied on changing the solution concentration to change the interparticle distance. However, we point out that the nominal interparticle distance calculated from the concentration does not reflect the true particle spacing as the dispersion of NPs in the solution is inhomogeneous due to interactions. Such inhomogeneous distribution of NPs also applies to magnetic NP fluid injected into the tumor [25], [26], [27], [28].

Magnetic NPs are often surface modified by lipids [29], polymers [30], or silica layers [31] in biomedical applications. Among these modifications, silica coating is widely used because of its excellent biocompatibility, stability with respect to pH and concentration, and easy functionalization [32], [33], [16], [17]. Moreover, the surface of silica has negative charges, and produce sufficient repulsive forces to stabilize the NPs [34], [35], [36]. In this work, we realized a series of monodisperse core/shell NPs with a Zn0.3Fe2.7O4 magnetic core of 18 nm and a uniform silica shell with varying thicknesses ranging from 7 to 14 nm. By changing the shell thickness while keeping the size of the magnetic core fixed, we are able to tune the spacing between the magnetic dipoles keeping the magnitude of the dipole moment fixed, thereby systematically change the magnetic dipole interactions [37]. This allows us to investigate the ensemble structure of the NP dispersion as a result of changing dipole interactions, and their effects on magnetic anisotropy, relaxation, and loss power.

Section snippets

Synthesis of Zn0.3Fe2.7O4 nanoparticles

Zn0.3Fe2.7O4 magnetic NPs were prepared by thermal decomposition method [38]. First, 2.7 mmol Iron(III) acetylacetonate (Fe(acac)3), 0.3 mmol Zinc(II) acetylacetonate hydrate (Zn(acac)2·nH2O), 2 mmol sodium oleate, 4.4 ml oleic acid and 20 ml benzyl ether were added into a four neck flask. Under the protection of argon, the mixture was heated to 120 °C for 30 min, then to 295 °C and kept at that temperature for 2 h. Finally, the mixture was cooled down to room temperature by removing the

Results and discussion

Zn0.3Fe2.7O4 magnetic NPs were synthesized by a modified thermal decomposition method [37], [40], [16]. Compared with the traditional thermal decomposition method, sodium oleate was used to replace the oleyamine and 1,2-hexadecandiol in the original recipe to realizing the one-step synthesis of NPs, effectively increasing productivity. Fig. 1a shows the TEM image of monodisperse Zn0.3Fe2.7O4 NPs, with an average diameter of 18 nm. As shown in the inset in Fig. 1a, NPs are single crystalline,

Conclusion

In this study, the effects of dipole interactions on loss power of magnetic NPs in an AC field were systematically studied, by varying the thickness of non-magnetic shell. With increasing shell thickness, the dipole interactions are weakened, reducing chain formation. This plays dual roles: 1. it decreases the interaction induced effective shape anisotropy, and 2. it decreases the hydrodynamic size. The former affects the Néel relaxation rate while the latter dictates the Brownian relaxation

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by National Natural Science Foundation of China (Grant No. 51771124, 51571146, 51701130), Beijing Natural Science Foundation (Grant No. Z190011), Capacity Building for Sci-Tech Innovation - Fundamental Scientific Research Funds (Grant No. 20530290057).

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    These authors contributed equally to this work.

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