Research articlesMagnetic and power absorption measurements on iron oxide nanoparticles synthesized by thermal decomposition of Fe(acac)3
Graphical abstract
Introduction
One of the promising techniques in cancer treatment, aside drug therapy, is magnetic hyperthermia [1], [2], [3]. This clinical protocol is based on the idea to induce tumor cells death by locally increasing the temperature of ill tissue, when they are previously loaded with magnetic nanoparticles (MNPs) and exposed to alternating (AC) magnetic field [4], [5], [6]. The underlying physical process is governed by the transformation of magnetic to thermal energy in each single-domain MNP through Brownian and Néel relaxation processes. The amount of generated heat power depends on the experimental conditions (field amplitude, H0 and frequency, f), as well as on physical parameters of a system such as: viscosity of a medium, the chemical composition of MNPs, particle size and shape, the effective anisotropy of material, size and anisotropy dispersion, the interparticle interactions [7], [8], [9], [10], [11], [12].
Nanoparticles of magnetite (Fe3O4) or maghemite (γ-Fe2O3) are among the most studied systems for this purpose due to their low toxicity for the human body [13], [14], [15], [16]. There is some controversy about the optimum size and shape of iron oxide nanoparticles for maximum heating, and this is probably related to the fact that detailed models incorporating clustering effects, dipolar interactions and the more realistic mathematical Landau-Lifshitz-Gilbert equation to describe relaxation modes are still lacking. From the experimental point of view, it was found that Fe3O4 nanoparticles of ∼15–25 nm size result in good heating rates under physiologically relevant AC magnetic field conditions [17], [18]. Alternative approaches and systems, such as perovskite-based nanoparticles (with TC in the range 13–82 °C) have also been investigated [19], as well as some core-shell structures and exchange-bias-coupled nanoparticles [20], [21]. Finally, other ferromagnetic nanoparticles (e.g. FeCo or FePt alloys) [22], [23] have been studied as potential heating mediators. Investigations on such systems can be interesting from the fundamental point of view, but their use can be limited by the biocompatibility.
In order to obtain ferrofluid with the best magnetic hyperthermia performances, different synthesis methods have been tested [24], [25], [26]. Well crystallized NPs, with narrow particle size distribution and the size close to the critical one determined by the transition from superparamagnetic (SPM) to ferromagnetic state, are desired. High-temperature decomposition of iron salts in organic media is a quite well known method for production of well crystallized iron oxide nanoparticles with narrow size distribution [27], [28], [29], [30], [31], [32], [33]. The main drawback of this non-green synthesis method is that nanocrystals are passivated with hydrophobic ligands that hinder their bioapplications. To make them biocompatible, the transformation from nonpolar to polar medium has to be done. It is possible through ligand exchange or ligand addition reactions in solution [34], [35], [36]. Unfortunately, these processes can bring to significant aggregation of NPs, their partial oxidation and disruption of heating capability. These issues should be properly resolved.
To investigate how the heating ability of NPs depends upon size, shape, degree of aggregation, surface functionalization and magnetic interparticle interactions, both, theoretical and experimental approaches are used [35], [37], [38], [39], [40], [41]. Some studies are focused on the evolution of the heating efficiency of NPs with their size [9], [12], [42], and the others on the influence of shape and coating of NPs [32], [43], [44]. Slight deviation from the expected chemical composition can also influence heating ability of nanoparticles dispersed in a fluid, as well as the oxidation process which can take place and, thus hinder the stability of magnetic fluids in time [45], [46]. Some recent studies have revealed that type of surfactant molecule, used to stabilize the surface of nanoparticles, can be important in determining the resulting crystal and magnetic structure of nanoparticles [45], [47]. It has also been shown that the ligand exchange process can invoke changes of the magnetic ordering in the surface layer and consequently change the surface anisotropy contribution to the effective anisotropy constant [47]. Influence of dipolar interactions on the heating ability diverges. It can be noticed either the decrease or the increase of the heating power due to interactions [35], [48].
In this paper, single crystalline iron oxide nanoparticles were synthesized using thermal decomposition of iron(III) acetylacetonate. The effects of different reaction conditions on the structural and magnetic properties of nanocrystals were examined. The hyperthermic properties of superparamagnetic iron oxide nanoparticles were investigated too. For the ferrofluid with nanoparticles ∼12 nm in size and with a narrow size distribution, the measurements of the specific absorption rate (SAR) were performed in a wide range of experimental conditions: the amplitude, H0 and the frequency, f of applied AC magnetic field. We have found quadratic field and frequency dependence of the SAR. On the contrary, de la Presa et al. [43] obtained the nearly quadratic f-dependence of the SAR in NPs around 8 nm in size, but not in the systems of bigger NPs with an average size around 11 and 13 nm. Therefore, we discussed our result taking into account value of the magnetic anisotropy constant. A ligand exchange process was performed on selected nanoparticles in a bipolar solvent using meso-2,3-dimercaptosuccinic acid (DMSA) molecules. Experimental results obtained for thus modified nanoparticles are presented in the Supplementary Information (SI)).
Section snippets
Synthesis of magnetic iron oxide nanoparticles
Iron oxide nanocrystals were synthesized by thermal decomposition of iron acetylacetonate (Fe(acac)3), in two different solvents, 1-octadecene (boiling point ∼315 °C) and 1-eicosene (boiling point ∼341 °C). The classical protocol during synthesis was followed [27]. A mixture of oleic acid (OA, C17H33COOH), oleylamine (OM, C18H35NH2) and 1,2-dodecandiol (1,2-DDDO, C12H26O2) was added into a round bottom three neck flask, previously filled with: i) 1-octadecene (samples S0 and T0), or ii) 1
The effect of reaction parameters on the particle size
Fig. 1, Fig. 2 show TEM images of the as-synthesized iron oxide nanocrystals coated with nonpolar oleic acid (OA), or oleic acid and trioctylphosphine oxide (OA + TOPO). The histograms of particle size and the evolution of the reaction temperature in time are also shown in these figures. Good size dispersion inferred from the narrow particle size distribution was found for samples S0 (synthesized in 1-octadecene) and E0 (synthesized in 1-eicosene). The mean particle size of samples S0 and E0
Conclusions
In this work, iron oxide nanocrystals were synthesized by thermal decomposition method of Fe(acac)3 in the presence of surfactants and a reducing agent. Keeping the concentration of iron ions in solvent constant (1 mol of Fe3+ in 80 ml of solvent), we found that a heating rate to reflux can be the most important factor in determining the particle size. Nearly all nanoparticles showed spherical shape with slight deviations, the pronounced faceted morphology being observed in sample synthesized
Acknowledgements
This work was supported by the Ministry of Education, Science and Technological Development (MESTD) of the Republic of Serbia through the project No. 45015. N.J.O. would like to thank MESTD for the postdoctoral fellowship at the Institute of Nanoscience of Aragon, University of Zaragoza, Spain, and Prof. Goya for kind hospitality in his laboratory. G.F.G. and M.P.C. thank financial support from the DGA (Gobierno de Aragon, Project E26) and Spanish Ministerio de Economia y Competitividad
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