Article pubs.acs.org/JPCC
Tunable High Aspect Ratio Iron Oxide Nanorods for Enhanced Hyperthermia Raja Das,*,† Javier Alonso,†,‡ Zohreh Nemati Porshokouh,† Vijaysankar Kalappattil,† David Torres,† Manh-Huong Phan,*,† Eneko Garaio,§ José Á ngel García,‡,∥ Jose Luis Sanchez Llamazares,⊥ and Hariharan Srikanth*,† †
Department of Physics, University of South Florida, Tampa, Florida 33620, United States BCMaterials, Edificio No. 500, Parque Tecnológico de Vizcaya, Derio 48160, Spain § Department of Electricity and Electronics, University of Basque Country (UPV/EHU), Leioa 48940, Spain ∥ Department of Applied Physics II, University of Basque Country (UPV/EHU), Leioa 48940, Spain ⊥ Instituto Potosino de Investigación Científica y Tecnológica, Camino a la Presa San José 2055, Col. Lomas 4a, San Luis Potosí, S.L.P. 78216, México ‡
S Supporting Information *
ABSTRACT: Despite magnetic hyperthermia being considered one of the most promising techniques for cancer treatment, until now spherical magnetite (Fe3O4) or maghemite (γFe2O3) nanoparticles, which are the most commonly employed and only FDA approved materials, yield the limited heating capacity. Therefore, there is an increasing need for new strategies to improve the heating efficiency or the specific absorption rate (SAR) of these nanosystems. Recently, a large improvement in SAR has been reported for nanocubes of Fe3O4 relative to their spherical counterpart, as a result of their enhanced surface anisotropy and chainlike particle formation. Considering the proven advantages of high aspect ratio onedimensional (1D) Fe3O4 nanostructures over their spherical and cubic counterparts, such as larger surface area, multisegmented capabilities, enhanced blood circulation time, and prolonged retention in tumors, we propose a novel approach that utilizes this 1D nanostructure for enhanced hyperthermia. Here, we demonstrate that the SAR of iron oxide nanostructures can be enhanced and tuned by altering their aspect ratio. Calorimetric and ac magnetometry experiments performed for the first time on highly crystalline Fe3O4 nanorods consistently show large SAR values (862 W/g for an ac field of 800 Oe), which are superior to spherical and cubic nanoparticles of similar volume (∼140 and ∼314 W/g, respectively). Increasing the aspect ratio of the nanorods from 6 to 11 improves the SAR by 1.5 times. The nanorods are rapidly aligned by the applied ac field, which appreciably increases the SAR values. A detailed analysis of the effect of the alignment of the nanorods in agar indicates an appreciable SAR increase up to 30% when the nanorods are parallel to the field. These findings pave a new pathway for the design of novel high-aspect ratio magnetic nanostructures for advanced hyperthermia.
1. INTRODUCTION Alternating current (ac) hyperthermia using magnetic nanoparticles is one of the most promising emerging therapies for cancer treatment.1 In ac hyperthermia treatment magnetic nanoparticles are targeted or injected directly into the cancer area and the particles are heated by applying an ac magnetic field. This technique is already being employed in some clinics as a supplementary therapy to chemotherapy or radiotherapy for cancer treatment.2 Although encouraging results have been obtained in treating breast carcinoma, brain tumor, etc., concerns have been raised on the cytotoxicity related to the large quantity of magnetic nanoparticles being used in the treatment.3 In addition, research has shown that after the hyperthermia treatment, the nanoparticles tend to get accumulated in liver, spleen, kidneys, and other areas of the body.4 These drawbacks can be circumvented by using the minimum dose of nanoparticles without affecting the level of temperature range needed to kill or deactivate the cancer cells.5 © 2016 American Chemical Society
In order to achieve therapeutic temperature with minimum amount of nanoparticles being used, nanoparticles should have high heating efficiency or specific absorption rate (SAR). Apart from the applied ac magnetic field amplitude and frequency, SAR also depends on the saturation magnetization (MS), size, concentration, and effective anisotropy of the nanoparticles.6 Magnetite (Fe3O4) nanoparticles are widely studied in biomedical applications due to their biocompatibility. Unfortunately, there is a strong decrease in MS upon size reduction to the nanoscale, well below the MS of bulk magnetite (90 emu/ g), as well as a limited therapeutic window of particle size that can be used for cancer treatment. In order to boost the heating efficiency of iron oxide nanoparticles, a less investigated possibility is to tune their effective anisotropy through either Received: February 26, 2016 Revised: April 18, 2016 Published: April 21, 2016 10086
DOI: 10.1021/acs.jpcc.6b02006 J. Phys. Chem. C 2016, 120, 10086−10093
Article
The Journal of Physical Chemistry C
autoclave with a Teflon lining and heated to 200 °C for 6 h under autogenous pressure. After cooling to room temperature, the black precipitate was washed with ethanol thrice and transferred to hexane for storing. 2.2. Phase Transfer to Water. As-prepared oleic acidcoated iron oxide nanorods were dried in air. 100 mg of dried powder was dispersed in 10 mL of ethanol containing 500 mg of tetramethylammonium hydroxide (TMAH). The ratio of particles/ethanol/TMAH was kept 1:10:5 in all the cases. The above solution was sonicated for 30 min followed by washing with water. After washing, the obtained nanorods were dispersed in water. The morphology and the size of the nanorods showed no variation due to the phase transferring of the nanorods from organic to aqueous medium. 2.3. Characterization of the Samples. Crystal structure of the nanoparticles was analyzed using a Bruker AXS D8 X-ray diffractometer (XRD). To characterize their morphology, a FEI Morgagni 268 transmission electron microscope (TEM) operating at 60 kV was used. The nanostructures of the rods were further characterized with high-resolution TEM. The HRTEM images were obtained in a FEI Tecnai F-30 high resolution transmission electron microscope (HRTEM). Magnetic measurements were performed in a commercial physical property measurement system (PPMS) from Quantum Design with a vibrating sample magnetometer. Hyperthermia experiments were performed by (a) calorimetric methods using a 4.2 kW Ambrell Easyheat Li3542 system (glass vial of 16 mm × 50 mm), with ac fields varying between 0 and 800 Oe at a constant frequency of 310 kHz, and (b) using a homemade ac magnetometer setup,19 with fields between 0 and 400 Oe at a frequency of 310 kHz. In these experiments, the samples were dissolved either in water or in a 2% weight agar, with concentrations of 1 and 3 mg/mL, put into a vial inside the ac coil, and the evolution of temperature vs time was recorded for different ac fields.
shape or surface modifications.7,8 Interestingly, Boubeta et al. have shown an improvement in the SAR of Fe3O4 nanocubes relative to spherical nanoparticles, as a result of the enhanced surface anisotropy and chainlike particle formation.8 Their Monte Carlo simulation has also revealed an increase of the hysteresis loop area with increasing length of the chain.8 But under what condition such chains stay experimentally stable has remained an important open question, and so the optimum length of the chains for obtaining the best value of SAR needs to be studied further. Anisotropic one-dimensional (1D) magnetic nanostructures have drawn considerable attention due to their high surface to volume ratio, which drastically influences physical and chemical properties. Kolhar et al. have reported that the 1D nanostructures present an increase of specific attachment and a reduction in nonspecific attachment to their target as compared to their spherical counterparts.9 On the other hand, Gen et al. have demonstrated that anisotropic nanostructures offer enhanced blood circulation time and prolonged retention in the tumor site when compared to spherical nanostructures.10 In vivo studies have also indicated no or low toxicity of nanorod-shaped 1D nanostructures.11−14 Recent study showed that Fe3O4 nanorods have a higher relaxivity R2 value compared to spherical particles due to strong induced magnetic field.15 Apart from their biomedical applications, these nanostructures are very interesting systems for the study of morphology-dependent magnetic properties. Considering the advantages of 1D nanostructures, it would be interesting to study SAR of magnetite nanorods. While altering the length of a chain composed of nanocubes8 is a challenging experimental task, the effect of the chain length on SAR can be easily assessed by varying the length of Fe3O4 nanorods. By changing the aspect ratio of the nanorods, the effect of surface anisotropy on SAR can also be investigated in a systematic way. Over the past years most attention has been paid to the synthesis of iron oxide nanoparticles, but the focus has been mainly on spherical (0D) and thin film (2D) nanostructures. Since synthesizing 1D magnetite with controlled morphology is a difficult task,16 there have been only a few reports on the synthesis of 1D Fe3O4 nanostructures.16−18 To the best of our knowledge, there is no study on the morphology-dependent magnetic properties and hyperthermia response of 1D magnetite nanostructures. We present here the results of the first comprehensive study of the chemical synthesis, magnetic properties, and hyperthermia response of highly crystalline and tunable aspect ratio nanorods of Fe3O4. We show that the nanorods possess enhanced saturation magnetization and heating efficiency relative to their spherical and cubic counterparts, and that the SAR of the nanorods can be tuned by varying their aspect ratio. Our study paves the way for development of novel anisotropic magnetic nanostructures for enhanced hyperthermia.
3. RESULTS AND DISCUSSION 3.1. Reaction Mechanism and Characterization. Fe3O4 nanorods of tunable aspect ratio were synthesized using the hydrothermal method with iron pentacarbonyl as a precursor and oleic acid as a capping agent.17 The details of the synthesis are mentioned in the Experimental Details section. The aspect ratio of the nanorods was tuned by varying the amount of hexadecylamine in the reaction. In the hydrothermal condition oleic acid condenses with hexadecylamine and forms water as a byproduct in the reaction. This water hydrolyzes iron oleate which is formed due to the reaction of iron pentacarbonyl with oleic acid. In our case, the amount of hexadecylamine was varied while keeping all other reaction parameters constant (Table 1). Importantly, we have found that when the amount of hexadecylamine is optimum, long rods with high aspect ratio are formed but lower or higher amounts of hexadecylamine
2. EXPERIMENTAL DETAILS 2.1. Synthesis of Fe3O4 Nanorods. Iron oxide nanorods were synthesized using a previously reported method by Sun et al.17 In a typical synthesis 1.2 g of hexadecylamine (HAD) and 4 mL of oleic acid (OA) were mixed in 16 mL of 1-octanol. The above solution was heated to 55 °C and stirred for 30 min to ensure formation of a clear solution. After that, the solution was cooled to room temperature, where 4 mL of iron pentacarbonyl was added and magnetically stirred for another 60 min. Then, the solution was transferred to a 45 mL
Table 1. Amount of Hexadecylamine Used To Obtain Different Aspect Ratios of Fe3O4 Nanorodsa
a
10087
sample label
hexadecylamine (g)
average length (nm)
average width (nm)
aspect ratio
S1 S2 S3
1.2 0.6 0.44
41 65 56
7 5.7 10
5.8 11 5.6
All the reactions were carried out at 200 °C for 6 h. DOI: 10.1021/acs.jpcc.6b02006 J. Phys. Chem. C 2016, 120, 10086−10093
Article
The Journal of Physical Chemistry C
Figure 1. Scheme for the synthesis of tunable aspect ratio Fe3O4 nanorods. All the reactions were performed in an autoclave with a Teflon liner at 200 °C and varying the oleic acid (OA) to hexadecylamine (HDA) ratio. Shown are TEM images of the as-synthesized Fe3O4 nanorods. Scale bar is 200 nm. Photographs reveal the highly monodisperse nature of Fe3O4 nanorods (sample S1), which were dispersed in hexane in (A) absence and (B) presence of a permanent magnet.
3.2. Magnetic Properties of Fe3O4 Nanorods. Temperature dependence of zero-field-cooled (ZFC) magnetization in an applied field of 20 Oe is shown in Figure 3A for all the nanorod samples. The absence of maximum in ZFC magnetization in the measured temperature range indicates the blocking temperature of the synthesized nanorods exceeds 330 K. The ZFC magnetization vs temperature (M−T) curves show a sharp change of magnetization at ∼120 K for all samples (S1, S2, and S3), which is the signature of the thermally activated first-order Verwey transition (TV).20,21 The Verwey transition in Fe3O4 is related to the structural transition from high temperature cubic to low temperature monoclinic structure.20−23 In Fe3O4 spherical nanoparticles (below ∼50 nm diameter) due to their poor crystallinity and surface defects this transition is often not observable.22,23 For comparison, in the present study we have also synthesized and magnetically characterized the 20−25 nm spherical Fe3O4 nanoparticles, which possess the same volume of the nanorods (S1 and S2). These nanoparticles were synthesized using the thermal decomposition method.24 As one can see in inset of Figure 3A, there is no signature of the Verwey transition in the M−T curve for the spherical nanoparticles. The poor crystallinity and presence of disordered surface spins in the spherical nanoparticles have been suggested to be the main reason for the absence of the Verwey transition.23 The presence of the Verwey transition in our hydrothermally grown nanorods reveals the excellent crystallinity of the samples, consistent with both the XRD and HRTEM data (Figure 2). This is also in accordance with the previous reports showing that the hydrothermal method yields nanoparticles with better crystallinity than other synthesis methods do.25,26 Figure 3B shows the room temperature M−H loops of samples S1, S2, and S3. The magnetization values have been normalized to the total mass of the sample. It can be observed
result in rods of lower aspect ratio. The scheme of the reaction is illustrated in Figure 1. TEM and HRTEM images of the nanorod samples with varying aspect ratios (denoted as S1, S2, and S3 in Table 1) are shown in Figures 1 and 2A,B. TEM images show the formation of nanorods with a narrow size distribution. The average length and width of the nanorods were 41 nm × 7 nm, 65 nm × 5.7 nm, and 56 nm × 10 nm for samples S1, S2, and S3, respectively. HRTEM image of S1 (Figure 2B) shows clear lattice fringes in the Fe3O4 or γ-Fe2O3 nanorods. The interplane distance (d) value is calculated to be ∼0.24 nm, which corresponds to the (222) plane of Fe3O4 with a cubic structure. The SAED pattern (inset to Figure 2B) displays a dot pattern, indicating the single crystalline nature of the nanorods. The diffraction spots of the SAED pattern can also be indexed to (511) and (311) planes of the cubic spinel structure. Powder XRD was carried out to check the crystalline phase of the materials. As one can see clearly in Figure 2C, all the XRD peaks can be indexed to Fe3O4 or γ-Fe2O3. No peaks of FeO or other phases of iron oxide were detected within the instrument resolution. The formation of cubic Fe3O4 or γ-Fe2O3 indicates that the capping with oleic acid prevents the nanorods from surface oxidation. Here we note that XRD alone is not a conclusive tool to discriminate between Fe3O4 and γ-Fe2O3 crystalline phases because the main difference consists of a few low intensity diffractions (
