Magnetic hyperthermia in brick-like Ag@Fe3O4 core–shell nanoparticles

https://doi.org/10.1016/j.jmmm.2015.08.081Get rights and content

Highlights

  • The heat emission of multifunctional Ag@Fe3O4 brick-like nanoparticles was studied.

  • Specific absorption rate is comparable to the most efficient plain nanoparticles.

  • Brick-like nanoparticles are candidates for optical and hyperthermia applications.

Abstract

Heating efficiency of multifunctional Ag@Fe3O4 brick-like nanoparticles under alternating magnetic field was investigated by means of specific absorption rate (SAR) measurements, and compared with equivalent measurements for plain magnetite and dimer heteroparticles. The samples were synthesized by thermal decomposition reactions and present narrow size polydispersity and high degree of crystallinity. The SAR values are analyzed using the superparamagnetic theory, in which the basic morphology, size and dispersion of sizes play key roles. The results suggest that these novel brick-like nanoparticles are good candidates for hyperthermia applications, displaying heating efficiencies comparable with the most efficient plain nanoparticles.

Introduction

Iron oxide nanoparticles, such as magnetite (Fe3O4), are very interesting materials for biomedical applications because of their high biocompatibility and peculiar magnetic properties [1]. Owing to their reduced size, the nanoparticles (NPs) behave as superparamagnets, i.e., quasi-static magnetic measurements at room temperature display the Langevin isotherm without hysteresis. Although NPs do not behave as conventional bulk ferromagnets, they can be guided by a static magnetic field within the human body allowing, for instance, specific drug delivery. Contrary to bulk ferromagnets, once the field is removed, the absence of magnetic remanence prevents problems associated to particle aggregation. In addition, one can take advantage of the rather slow magnetic dynamics of the superparamagnetic regime to recover the remanence in a rather controlled way by applying a high-frequency alternating magnetic field. In this situation, the NPs release energy, giving rise to a localized heating. This effect has been intensively studied in the last years, and applied as a promising cancer treatment known as magneto-hyperthermia [2], [3]. The treatment is based on a local heating of a tumor-affected tissue up to 41–45 °C; in this temperature range, the damage for normal cells is reversible while in tumor cells it is irreversible [4]. In contrast to larger particles showing conventional ferromagnetism with multi-domain structures [5], the superparamagnetic NPs have been pointed out to possess better properties for hyperthermia therapy [6].

Moreover, the magnetic NPs coupled with an optically active noble metal, such as silver or gold, would give rise to a wide spectrum of desirable synergistic and complementary effects [7]. In this case, the biocompatibility is a problem yet to be solved. While the magnetite colloids are biocompatible [8], [9], silver NPs are well-known by their bactericide effect, revealing important problems of biocompatibility [10]. Therefore, it seems appropriate for medical applications to eliminate the contact of silver with the tissues. The simplest possibility is a core–shell heteroparticle Ag@Fe3O4. However, as the bactericide effect of silver [11] is due to release of silver ions [12], conventional core–shell heteroparticles [13], [14] with an amorphous and thin magnetite shell does not solve the biocompatibility problem [15]. Recently, we synthetized brick shaped Ag@Fe3O4 NPs with larger shell-to-core ratio and compact structure [16]. The compact cubic magnetic shell could, in principle, solve the lack of biocompatibility of silver. These brick-like NPs can represent a significant advance for in-vivo applications of silver-magnetite heterodimers. Hyperthermia and biocompatibility studies are essential for future application of these complex NPs.

The main objective of this work is to report the heating efficiency of these novel Ag@Fe3O4 brick-like heteroparticles for hyperthermia purposes. The efficiency study was performed by means of specific absorption rate (SAR) measurements, which, in principle, can be predicted by the superparamagnetic theory. However, these nanomaterials can exhibit physical properties that are different from their bulk counterparts [17], [18], hindering appropriate theoretical predictions [19], [20]. In this work, we present a systematic study of the structural and magnetic properties, as well as SAR, of plain nanoparticles, dimer and novel brick-like heteroparticles, synthesized under similar conditions. We shed some light on the physical origin of the heating emission process of the particles by means of basic magnetic characterization and conventional superparamagnetic formalism.

Section 2.1 deals with the synthesis of the nanoparticles under study, namely, plain magnetite (P), silver-magnetite heteroparticles dimers (D) or brick like core–shell nanoparticles (BL). The structure of the particles is described in Section 2.2. Section 2.3 is dedicated to the basic magnetic characterization related to the radiofrequency heat emission application of the particles, which is properly discussed in Section 3. Finally, in Section 4, the conclusions and perspectives are presented.

Section snippets

Synthesis

The general process used in this work to produce magnetite nanoparticles is the thermal decomposition reaction [21]. The synthesis consists on a decomposition of an organometallic precursor in presence of surfactants [22].

For typical magnetite synthesis, the iron precursor in the form of Fe (III) acetylacetonate (3 mmol), a reducing agent 1,2-hexadecanediol (1 mmol) and a mixture of surfactants consisting of oleylamine (3 mmol) and oleic acid (3 mmol) were added in a benzyl ether solution (20 ml),

Magneto-hyperthermia

The magneto-hyperthermia effect was evaluated by means of Specific Absorption Rate (SAR). This parameter is aimed to evaluate the heat emission of a magnetic material when a resonant alternating magnetic field is applied, namely,SAR=CpmSmFTt,where Cp is the solution specific heat, mS the total ensemble mass, mF the magnetite mass and ΔTt the initial slope of the field induced heating curve.

The heating curves were measured on a test tube containing a mixture of toluene (2 ml) and the magnetic

Conclussions

Heat emission efficiency of multifunctional Ag@Fe3O4 brick-like nanoparticles under an alternating magnetic field were systematically studied, and compared to plain magnetite and dimer nanoparticles produced by slight modifications of the synthesis routes.The specific absorption rates were analyzed using structural and magnetic data by means of classical superparamagnetic theory considering the size distribution of the nanoparticles. The results suggest that these novel brick-like nanoparticles

Authors information

The authors declare no competing financial interest.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgments

This work has been funded by the brazilian agencies FAPESP and CNPq. We thank TEM facilities of C2NANO-Brazilian Nanotechnology National Laboratory (LNNano) at Centro Nacional de Pesquisa em Energia e Materiais (CNPEM)/MCT (# 14825 and 14827). R.L.R. acknowledges FAPESP grant 2013/13275-8. D.M acknowledges FAPESP grant 2011/01235-6.

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