⇦Chinese Physics Letters, 2019, Vol. 36, No. 3, Article code 037501 Effect of Magnetic Anisotropy on Magnetic Thermal Induction of Mn$_{0.3}$Zn$_{0.3}$Co$_{x}$Fe$_{2.4-x}$O$_{4}$ Nanoparticles * Chen-Hui Lv (吕晨辉)1†, Li-Chen Wang (王利晨)1†, Zheng-Rui Li (李峥睿)1, Xiang Yu (俞翔)1, Yan Mi (米岩)1, Ruo-Shui Liu (刘若水)1, Kai Li (李凯)2, Dan-Li Li (李丹丽)2, Shu-Li He (贺淑莉)1** Affiliations 1Department of Physics, Capital Normal University, Beijing 100048 2Department of Chemistry, Capital Normal University, Beijing 100048 Received 21 December 2018, online 23 February 2019 *Supported by the National Natural Science Foundation of China under Grant Nos 51571146, 51771124 and 51701130.
†Chen-Hui Lv and Li-Chen Wang contributed equally to this work.
**Corresponding author. Email: shulihe@cnu.edu.cn Citation Text: Lv C H, Wang L C, Li Z R, Yu X and Mi Y et al 2019 Chin. Phys. Lett. 36 037501    Abstract Mn$_{0.3}$Zn$_{0.3}$Co$_{x}$Fe$_{2.4-x}$O$_{4}$ series magnetic nanoparticles are prepared by the high-temperature organic solvent method, and Mn$_{0.3}$Zn$_{0.3}$Co$_{x}$Fe$_{2.4-x}$O$_{4}$@SiO$_{2}$ composite nanoparticles are prepared by the reverse microemulsion method. The as-prepared samples are characterized, and the results show that the magnetic anisotropy constant of nanoparticles increases with the cobalt content, and the magnetic thermal induction shows a trend of increasing first and then decreasing. The optimal magnetic thermal induction is obtained at $x=0.12$ with a specific loss power of 2086 w/g$_{\rm metal}$, which is a bright prospect in clinical magnetic hyperthermia. DOI:10.1088/0256-307X/36/3/037501 PACS:75.50.Gg, 75.60.Ej, 76.60.Es © 2019 Chinese Physics Society Article Text Traditional hyperthermia has become a sophisticated treatment method for cancers in the field of biomedicine.[1-4] Cancer cells will be killed while normal cells can survive in the process of hyperthermia, due to the fact that the tolerance of normal cells to temperature is higher than that of cancer cells.[5] In 1957, Gilchrist first proposed the use of thermotherapy by placing magnetic material in an alternating magnetic field to kill tumor cells.[6] Two problems must be considered in the preparation of such nanomaterials: (1) improving the specific loss power (SLP) as far as possible and ensuring safety within the human body; (2) ensuring a good biocompatibility and considerable killing effect on tumor cells. Ferrite magnetic nanoparticles are the most commonly used materials in magnetic hyperthermia due to their low toxicity.[7] Therefore, obtaining a higher SLP value of ferrite nanoparticles is one of the important research directions to improve heat production efficiency. Choosing the right size to protect nanoparticles from aggregation, obtaining the highest possible saturation magnetization ($M_{\rm s}$) of the nanoparticles, and tuning the magnetic anisotropy are the three main ways normally used.[8] A previous work reported that the soft and hard magnetic core shell structure could be used to tune the magnetic anisotropy, in which the size of the core shell of the samples should be precisely controlled.[8] On the other hand, samples with high magnetic thermal induction can be efficiently tuned with ion-doped soft and hard ferrite nanoparticles, which is also more conducive to the application of various functions in biomedical systems. In this Letter, $M_{\rm s}$ is enhanced by doping Mn$^{2+}$ ions and Zn$^{2+}$ ions in Fe$_{3}$O$_{4}$ nanoparticles with an anti-spinel structure, due to the fact that the magnetic moments of Mn$^{2+}$ and Zn$^{2+}$ are larger than that of Fe$^{2+}$, while the magnetic anisotropy is improved by doping Co$^{2+}$ ions, because CoFe$_{2}$O$_{4}$ is a traditional hard magnetic material. Moreover, an SiO$_{2}$ coating is applied to the as-prepared samples due to its excellent water solubility and stability. Meanwhile, coating with silica can transform Mn$_{0.3}$Zn$_{0.3}$Co$_{x}$Fe$_{2.4-x}$O$_{4}$ (MZCFO) nanoparticles into an aqueous phase so that it can be better applied in biomedicine. The magnetic thermal induction of the composite nanoparticles was studied, and a high SLP value of 2086 w/g$_{\rm metal}$ was obtained in the Mn$_{0.3}$Zn$_{0.3}$Co$_{0.12}$Fe$_{2.28}$O$_{4}$ nanoparticles with a mean size of 22 nm. MZCFO nanoparticles were prepared using the high-temperature organic solvent method. The reagents are as follows: manganese acetylacetonate (Mn(acac)$_{2}$), zinc acetylacetonate (Zn(acac)$_{2}$), cobalt acetylacetonate (Co(acac)$_{2}$), iron acetylacetonate (Fe(acac)$_{3}$) and 1,2-dodecanediol were purchased from Alfa Aesar; and oleic acid (90%), oil ammonia (70%) and benzyl ether (99%) were purchased from Acros. An appropriate amount of precursor was added to a round-bottomed three-necked flask, and dibenzyl ether, oleic acid and oil ammonia were sequentially added by pipette. The solution in the three-necked flask was stirred by a magnetic stirrer. The temperature was raised to 200$^{\circ}\!$C and maintained for 1 h under argon gas protection. The temperature was then raised to 300$^{\circ}\!$C and maintained for 2 h, and lowered to room temperature afterwards. Nanoparticles with 10 nm were separated by centrifugation. The nanoparticles could then be grown to 22 nm by the seed-mediated method.[9] In this study, MZCFO@SiO$_{2}$ composite nanoparticles were prepared by the reverse microemulsion method as reported previously.[10] The reagents used here included cyclohexane, CO-520, ammonia (NH$_{3}\cdot$H$_{2}$O) (30%) and TEOS. All the reagents were purchased from Acros. Here, 20 ml cyclohexane and 1.15 ml CO-520 were pipetted into a small round-bottomed flask, and 20 mg MZCFO nanoparticles were added to the flask and ultrasonically mixed. The flask was placed on a magnetic stirrer, then 0.15 ml aqueous ammonia and 0.1 ml TEOS were added dropwise, and stirred at room temperature for 24 h. MZCFO@SiO$_{2}$ composite nanoparticles were obtained by centrifugation after 24 h. The content of Co$^{2+}$ ions was adjusted by changing the content of cobalt acetylacetonate in the preparation of MZCFO. The properties of the as-prepared MZCFO ($x=0$, 0.05, 0.1, 0.12 and 0.15) magnetic nanoparticles were characterized comprehensively. The samples' morphology was analyzed by transmission electron microscopy (TEM, Hitachi H7650 (120 kV)). An X-ray diffractometer (XRD, Bruker D8) with Cu K$\alpha$ radiation ($\lambda=1.5418$ Å) in the $\theta$–$2\theta$ scan mode at room temperature was used to characterize the purity and structure of samples. The magnetic properties of the samples were characterized by a commercial superconducting quantum interference device magnetometer (SQUID, Quantum Design). A HYPER5 machine (MSI Company) was employed to measure the hyperthermia performance.
cpl-36-3-037501-fig1.png
Fig. 1. XRD patterns of MZCFO nanoparticles.
Figure 1 shows the XRD patterns of the MZCFO magnetic nanoparticles prepared by the high-temperature organic solvent method. It can be seen from the XRD patterns that the MZCFO magnetic nanoparticles crystallize in a single cubic phase. The morphology of nanoparticles could be obtained by TEM, as shown in Fig. 2. Figures 2(a)–2(e) exhibit the TEM images of the MZCFO. As can be seen from the figure, the magnetic nanoparticles have good mono-dispersion with a particle size of about 22 nm, and the shape is regular. Figures 2(f)–2(j) display the TEM images of the MZCFO@SiO$_{2}$ composite nanoparticles. It can be seen that no empty shells exist in the SiO$_{2}$-coated composite nanoparticles, and the shell thickness is uniform.
cpl-36-3-037501-fig2.png
Fig. 2. The TEM image of the MZCFO composite nanoparticles: (a) $x=0$, (b) $x=0.05$, (c) $x=0.1$, (d) $x=0.12$, (e) $x=0.15$. MZCFO@SiO$_{2}$: (f) $x=0$, (g) $x=0.05$, (h) $x=0.1$, (i) $x=0.12$, (j) $x=0.15$.
Figure 3 shows the histogram of the size distribution of MZCFO nanoparticles and MZCFO@SiO$_{2}$ nanoparticles, in which the Gauss fitting curves are given. It can be seen from the diagrams that the mean size of MZCFO nanoparticles prepared in this work is 22 nm, and the mean size of MZCFO@SiO$_{2}$ composite nanoparticles is 28 nm. It can be concluded that the thickness of the SiO$_{2}$ shell is 3 nm and the coating is very uniform. The (220) or (311) crystal planes of the nanoparticles can be obtained from high-resolution TEM, as shown in Fig. 4. It can be seen from the figure that all the nanoparticles are in a single crystal state.
cpl-36-3-037501-fig3.png
Fig. 3. Size distribution of MZCFO (a)–(e), and MZCFO@SiO$_{2}$ (f)–(j).
Figure 5 displays the $M$–$H$ curves of MZCFO nanoparticles collected at 10 K and performed on SQUID-VSM, with the inset showing the variation of $M_{\rm s}$ of MZCFO based on the $M$–$H$ curves. It can be found from the $M$–$H$ curves that the saturation magnetization of Mn$_{0.3}$Zn$_{0.3}$Fe$_{2.4}$O$_{4}$ nanoparticles prepared by Mn$^{2+}$ and Zn$^{2+}$ doping is significantly improved, compared to Fe$_{3}$O$_{4}$ with the same size. The reason for this is not only due to the larger magnetic moment value of Mn$^{2+}$ (5 $µ_{\rm B}$) compared to the value of Fe$^{2+}$ (4 $µ_{\rm B}$), but also due to the parallel arrangement of Fe$^{3+}$ formed at the B site resulting from the replacement of Fe$^{3+}$ at the A site by partial doping of Zn$^{2+}$. The saturation magnetization of the series samples decreases slightly with the increase of the Co$^{2+}$ ion doping amount, which is why the magnetic moment value (3 $µ_{\rm B}$) of Co$^{2+}$ ions is smaller than that of Fe$^{2+}$ (4 $µ_{\rm B}$). The values of the magnetic anisotropy constant $K$ of the series samples at 10 K can be calculated according to the $H_{\rm c}$ values at 10 K of the series samples. At the same time, the magnetic anisotropy constant at 300 K can be derived according to $$ \frac{K_{\rm u}(T=0)}{K_{\rm u}(T=300\,{\rm K})}=\Big[\frac{M_{\rm s}(T=0)}{M_{\rm s}(T=300\,{\rm K})}\Big]^{\gamma}, $$ where $\gamma=3$.[11] The magnetic anisotropy constant at 300 K can be estimated, and the results are listed in Table 1.
cpl-36-3-037501-fig4.png
Fig. 4. High-resolution TEM images of the MZCFO: (a) $x=0$, (b) $x=0.05$, (c) $x=0.1$, (d) $x=0.12$, (e) $x=0.15$. MZCFO@SiO$_{2}$ composite nanoparticles: (f) $x=0$, (g) $x=0.05$, (h) $x=0.1$, (i) $x=0.12$, (j) $x=0.15$.
Table 1. Coercivity $H_{\rm c}$ measured at 10 K and anisotropy constant $K$ measured at 10 K, estimated at 300 K of MZCFO.
$x=0$ $x=0.05$ $x=0.1$ $x=0.12$ $x=0.15$
$H_{\rm c}$ (kA/m), $T=10$ K 7.3 37.5 106.8 128 161.2
$K$ (10$^{4}$ J/m$^{3}$), $T=10$ K 0.5 2.56 7.06 8.29 10.18
$K$ (10$^{4}$ J/m$^{3}$), $T=300$ K 0.06 0.29 0.84 1.09 1.36
It can be concluded from Table 1 that the magnetic anisotropy of the nanoparticles is successfully improved by doping Co$^{2+}$ ions in the MnZn ferrite nanoparticles. The heating curves of the composite nanoparticle with 1 mg/ml can be measured by a magnetocalorometer under different alternating magnetic fields. Figures 6(a)–6(e) show the temperature change $\Delta T$ ($^{\circ}\!$C) versus time $T$ (s) for MZCFO@SiO$_{2}$ composite magnetic nanoparticles with $x=0$, 0.05, 0.1, 0.12 and 0.15. It can be derived from the figure that the SLP values rise first, and then drop with the increase of the Co$^{2+}$ ion doping amount, and reach a peak of 2086 W/g$_{\rm metal}$ at $x=0.12$, as shown in Fig. 6(f).
cpl-36-3-037501-fig5.png
Fig. 5. The magnetic hysteresis loops of as-synthesized MZCFO. Inset: variation of $M_{\rm s}$ of the MZCFO series sample with Co$^{2+}$ ion doping amount at 10 K.
cpl-36-3-037501-fig6.png
Fig. 6. (a)–(e) Heating curves of MZCFO@SiO$_{2}$ under different AC fields. (f) Variation of SLP value of the MZCFO@SiO$_{2}$ series sample with Co$^{2+}$ ion doping amount under magnetic field of 27 kA/m.
The reason for such a trend is mainly based on the Rosensweig theory.[12] For superparamagnetic nanoparticles, the heat production under alternating magnetic fields is mainly caused by Brownian relaxation and Neel relaxation.[13-15] Brownian relaxation mainly refers to the relaxation caused by the body rotation of the particle, and Neel relaxation is the relaxation caused by the magnetic moment rotation inside the magnetic nanomaterial against the magnetic anisotropy due to the thermal disturbance, $$ {\tau_{N}=\tau }_{0}e^{KV/k_{\rm B}T}, $$ where $\tau_{0}$ is the time constant, $K$ is the magnetic anisotropy constant, $V$ is the volume of the nanoparticles, and $T$ is the temperature. It can be seen from the formula that at a certain temperature, the magnetic anisotropy of magnetic nanoparticles with a certain size will directly affect the process of the Neel relaxation,[16] and thereby affect the SLP value as a metric of heat production. The results of theoretical simulations in the literature have declared that the SLP values of magnetic nanoparticles increase first, and then decrease with the change of magnetic anisotropy constant,[17] and the maximum SLP value is obtained at a corresponding $K$ value. The experimental results in this study are in good agreement with the theoretical model. In summary, monodisperse $M_{x}$Fe$_{3-x}$O$_{4}$ ($M$=Mn$^{2+}$, Zn$^{2+}$) magnetic nanoparticles have been prepared by the high-temperature organic solvent method. The saturation magnetization is significantly improved by Mn$^{2+}$ and Zn$^{2+}$ ion doping, compared with Fe$_{3}$O$_{4}$. To obtain samples with a high SLP, Co$^{2+}$ ion doping is used to modulate the magnetic anisotropy of the series samples with the highest SLP value of 2086 w/g$_{\rm metal}$ for $x=0.12$. In this work, the magnetocaloric properties of magnetic nanoparticles have been successfully modulated by changing the magnetic anisotropy, and the results are expected to be applied to clinical magnetic hyperthermia. References Magnetic particle hyperthermia – Properties of magnetic multicore nanoparticles administered to tumor tissueCellular responses to hyperthermia (40–46 ° C): Cell killing and molecular eventsOld and new facts about hyperthermia-induced modulations of the immune systemEnhanced Magnetic Fluid Hyperthermia by Micellar Magnetic Nanoclusters Composed of Mn x Zn 1– x Fe 2 O 4 Nanoparticles for Induced Tumor Cell ApoptosisSelective Inductive Heating of Lymph NodesWater-Soluble Iron Oxide Nanocubes with High Values of Specific Absorption Rate for Cancer Cell Hyperthermia TreatmentExchange-coupled magnetic nanoparticles for efficient heat inductionMonodisperse MFe 2 O 4 (M = Fe, Co, Mn) NanoparticlesFe 3 O 4 @SiO 2 Core/Shell Nanoparticles: The Silica Coating Regulations with a Single Core for Different Core Sizes and Shell ThicknessesHeating magnetic fluid with alternating magnetic fieldNanoscale Magnetism Control via Surface and Exchange Anisotropy for Optimized Ferrimagnetic HysteresisLow frequency hysteresis loops of superparamagnetic nanoparticles with uniaxial anisotropyRelaxation of the magnetization in uniaxial single-domain ferromagnetic particles driven by a strong ac magnetic fieldMagnetic particle hyperthermia—biophysical limitations of a visionary tumour therapyEffect of the distribution of anisotropy constants on hysteresis losses for magnetic hyperthermia applications
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