Journal of the Korean Physical Society, Vol. 73, No. 1, July 2018, pp. 125 129 Cube-Shaped Cetyltrimethyl Ammonium Bromide-Coated Nickel Ferrite Nanoparticles for Hyperthermia Applications Ashfaq Ahmad, Hongsub Bae and Ilsu Rhee Department of Physics, Kyungpook National University, Daegu 41566, Korea (Received 5 December 2017, in final form 12 February 2018) Cetyltrimethyl ammonium bromide (CTAB)-coated nickel ferrite (NiFe 2O 4) nanoparticles were synthesized using the high-temperature thermal decomposition method. The hydrophobic particles became water-soluble after the coating with CTAB, a positively charged ligand. The morphology and the phases of the nanoparticles were characterized using X-ray diffraction and transmission electron microscopy (TEM). TEM images demonstrated that the particles were cube-shaped; the average length of their side was 32.6 nm. Inductively coupled plasma spectroscopy measurements were performed to confirm the chemical composition of the particles. The particles exhibited superparamagnetic behavior with negligible coercive force. Magnetic heating of the aqueous suspensions of nanoparticles was performed in the presence of a radio-frequency magnetic field of 4.4 ka/m at a frequency of 216 khz. The 1.5-mg/mL sample reached the hyperthermia target temperature of 42 C and exhibited a high specific absorption rate (SAR) value of 152 W/g. These findings show that the investigated nanoparticles are suitable for magnetic hyperthermia applications and have the advantage of low dosage owing to their high SAR values. PACS numbers: 81.05.Ni, 76.60.Es, 61.46.Df, 87.61. c Keywords: Cetyltrimethyl ammonium bromide-coated nickel ferrite nanoparticle, Cube-shaped particles, High specific absorption rate DOI: 10.3938/jkps.73.125 I. INTRODUCTION Nanoparticles have attracted significant research interest owing to their potential for various applications, including biomedical applications, such as magnetic hyperthermia [1,2], magnetic resonance imaging [3,4], biosensors [5,6], and nanodrug delivery [7,8]. Among magnetic nanoparticles, inverse spinel ferrite nanoparticles have been extensively studied owing to their shape-dependent physical properties and unique size [3]. Owing to their incomplete blood vessels, cancer cells are not as resistant to heat as healthy cells. The blood vessels of cancer cells are leaky owing to their rapid angiogenesis. Therefore, they cannot efficiently release input heat. Cancer cells can be eliminated by heating them to a temperature of 42 C. Normal cells can endure up to 47 C. In magnetic hyperthermia, using magnetic nanoparticles, cancer cells are heated to temperatures above 42 C [9]. Magnetic nanoparticles generate heat in the presence of an alternating magnetic field owing to loss mechanisms, and the generated heat can be used to kill cancerous tissues. For this process, magnetic nanoparticles that have high specific absorption rate (SAR) are preferred. The SAR represents the amount of dissipated E-mail: ilrhee@knu.ac.kr heat per unit mass of the magnetic component of the nanoparticles. Therefore, higher specific absorption rates can minimize the dosage required to increase the temperature to the therapeutic limit. The SAR depends on the particles size, shape, chemical composition, magnetic properties, concentration in liquid media, and frequency and strength of the applied field [10]. Ferrite-based MFe 2 O 4 nanoparticles, where M can be Ni, Mn, Zn, Gd, or Co, are ideal candidates for magnetic hyperthermia applications owing to their small size, high magnetization, and simple synthesis [11,12]. The saturation magnetization and the coercivity of ferrite nanoparticles are known to vary with the particles size due to their large surface-to-volume ratio. Several studies have reported on the relation between the morphology and the magnetic properties of ferrites [13]. Nickel ferrite is a soft ferrite material with a face-centered cubic inverse spinel structure; however, several researchers have reported that nickel ferrite with a mixed spinel structure can also exist [14]. The magnetic properties of nickelferrite nanoparticles have shown strong dependences on the distribution of ions and the crystal structure, and the particles can exist in a paramagnetic, ferrimagnetic or superparamagnetic state depending on the lattice structure and the particle size [15]. In nickel ferrite, nickel ions occupy only octahedral sites while iron ions occupy pissn:0374-4884/eissn:1976-8524 -125- c 2018 The Korean Physical Society
-126- Journal of the Korean Physical Society, Vol. 73, No. 1, July 2018 Fig. 1. (Color online) TEM image of the CTAB-coated nickel-ferrite nanoparticles. The histogram shows the size distribution of one hundred particles observed in TEM images. both octahedral and tetrahedral sites due to the chemical structure of the particles. The unit cell of nickel ferrite contains 8 metal ion of Ni 2+ and 16 Fe 3+ surrounded by 32 oxygen atoms. These are distributed into tetrahedral and octahedral lattice sites in the inverse spinel symmetry [3]. The net magnetization occurs due to the antiparallel orientation of nickel and iron ions in octahedral and tetrahedral sites. The inverse spinel arrangements of the metallic and the oxygen ions cause the super-exchange interaction to tune the magnetic properties. The magnetic properties of the ferrite nanoparticles depend on several factors, such as the spin-canting effect and the dipolar interactions on the surfaces of the particles. In addition to the particle size, the synthesis technique plays an important role in the crystallinity and the composition of nickel-ferrite nanoparticles [14]. In this study, the thermal decomposition technique [16,17] was employed to synthesize cube-shaped NiFe 2 O 4 nanoparticles. These nanoparticles were coated with cetyltrimethyl ammonium bromide (CTAB), which is a biocompatible material. The magnetic, structural, and magneto-thermal properties of the particles were studied, and their potentials for magnetic hyperthermia applications were evaluated. II. EXPERIMENTAL METHODS The high-temperature thermal decomposition method was used to synthesize cube-shaped nickel-ferrite nanoparticles. First, 2-mmol Fe(III) acetylacetonate and 1-mmol Ni acetylacetonate were mixed with 25 ml of benzyl, 2 ml of oleic acid, 3 ml of squalene, and 2 ml of olylamine in a three-neck round-bottom flask. The mixture was stirred while increasing the temperature to 100 C in an argon atmosphere; it was maintained at this temperature for 1 h to remove water molecules. Then, the temperature was increased to 270 C to reflux the mixture, and it was maintained at this temperature for 1 h. The mixture was then allowed to naturally cool to room temperature. The synthesized particles were separated using a magnet and washed several times using ethanol. The as-prepared hydrophobic particles were made to be water-soluble by using a surfactant (CTAB); 500 mg of CTAB was added to water and stirred for 20 min at 60 C to form a solution. Then, 200 mg of particles dispersed in 10 ml of chloroform were added to the CTAB solution to generate an oil-in-water microemulsion. The mixture was stirred until the chloroform was completely evaporated, leaving a water-dispersible black aqueous solution. Prior to the CTAB coating, the particles would not dissolve in water; however, after the CTAB coating, the particles were well dispersed in water for weeks, which indicates the surface optimization of the nanoparticles. The crystallinity and the inverse spinel structure of the samples were analyzed using X-ray diffraction (XRD; X pert PRO, PANalytical). The particle shape and size distribution were revealed using a transmission electron microscope (HT 7700, Hitachi Ltd). The stoichiometry of the nanoparticles was analyzed using inductively coupled plasma spectroscopy (ICP; IRISAP, Thermo Jarrell Ash). A vibrating sample magnetometer (VSM; MPMS, Quantum Design) was employed to measure their magnetic properties. A Fourier-transform infrared spectrometer (FT-IR; Nicolet 380, Thermo Scientific USA) was employed to confirm the presence of the CTAB coating on the surfaces of the particles. Magnetic hyperthermia measurements were performed using an induction heating system (Osung High Tech, OSH-120-B). The temperature of the aqueous solution of the particles was measured using a CALEX infrared thermometer (PyroUSB CF, Calex Electronics Limited).
Cube-Shaped Cetyltrimethyl Ammonium Bromide-Coated Nickel Ferrite Ashfaq Ahmad et al. -127- Fig. 2. (Color online) XRD patterns of the CTAB-coated nickel-ferrite nanoparticles. The identified indices shown in the figure match those observed for the inverse spinel crystalline structure of ferrite. Fig. 3. (Color online) FT-IR spectra of the CTAB-coated nickel-ferrite nanoparticles. The absorption bands correspond to the characteristic vibrations and stretching modes. III. RESULTS AND DISCUSSION Figure 1 shows a transmission electron microscopy (TEM) image of the CTAB-functionalized nickel-ferrite nanoparticles. In addition, the size distribution of one hundred nanoparticles observed in TEM images is presented in this figure. The nanoparticles had cubicshapes; the average length of a side was 32.6 nm. Figure 2 shows the XRD patterns of the CTAB-coated nickel-ferrite nanoparticles. The observed diffraction peaks correspond to the characteristic crystallographic (2 2 0), (3 1 1), (4 0 0), (5 1 1), and (4 4 0) planes, which match with those observed in ferrite nanoparticles [JCPDS No. 22-1086]. The peaks are sharp, which indicates good crystallinity and large particle size. The coating status of the nanoparticles (coated with CTAB) was investigated by analyzing FT-IR spectra (Fig. 3). The peaks around 560 cm 1 correspond to metal-oxygen vibrations (Ni-O and Fe-O). The characteristic band around 1630 cm 1 reflects the presence of NH 2 groups on the nanoparticles surfaces [18]. The peak around 2900 cm 1 emerges owing to the stretching vibrations of the C H bonds. The peak at 3435 cm 1 reflects the presence of O H molecules on the nanoparticles surfaces [19]. All these peaks confirm the presence of a CTAB coating on the surfaces of the nanoparticles. Hysteresis measurements on the CTAB-coated nickelferrite nanoparticles were performed at room temperature by using a VSM (Fig. 4). The saturation magnetization and the coercive force were found to be 61 emu/g and 30 Oe, respectively. This value of the saturation magnetization is larger than other reported results [20, 21]. This can be attributed to the good crystallinity and the uniform size distribution of our particles. The low coercive force and the moderate saturation magnetiza- Fig. 4. (Color online) Hysteresis curve of the CTAB-coated nickel-ferrite nanoparticles. The inset shows a negligible coercive force to confirm the superparamagnetic behavior of the particles. tion make these nanoparticles suitable for hyperthermia applications. The heating efficiency of the CTAB-coated nanoparticles was investigated using a radio-frequency (RF) field with a frequency of 216 khz and a field strength of 4.4 ka/m. The temperature as a function of the heating time for five different nanoparticles concentrations in the aqueous solutions is shown in Fig. 5. The nanoparticle concentration was in the range from 1.5 mg/ml to 10 mg/ml. First, the temperature rapidly increases; then, it saturates after 1000 s, where the heat generated by the nanoparticles is balanced by the heat loss to the
-128- Journal of the Korean Physical Society, Vol. 73, No. 1, July 2018 Fig. 5. (Color online) Heating curves for the aqueous solutions of the CTAB-coated nickel-ferrite nanoparticles for various nanoparticle concentrations in the presence of an alternating magnetic field. The sample with a concentration of 1.5 mg/ml reached the hyperthermia target temperature of 42 C. Fig. 6. (Color online) SAR and saturation temperature as functions of the nanoparticle concentration. The saturation temperature increases with increasing nanoparticle concentration, as expected. The SAR decreases with increasing nanoparticle concentration in the aqueous solution owing to the enhanced dipole interactions between nanoparticles. aqueous solution. The target hyperthermia temperature of 42 C could be achieved using a moderate particle concentration of 1.5 mg/ml. The concentration dependence of the saturation temperature is shown in Fig. 6, along with the concentration dependence of the SAR (which will be discussed below). Several factors, including the magnetization, particles size, chemical composition, etc., determine the heating capabilities of magnetic particles in the presence of an alternating magnetic field [22]. The magnetic particles in the presence of the field generate heat owing to magnetic loss mechanisms, which can be divided into two types, hysteresis and relaxation losses. Hysteresis loss occurs in multi-domain particles while relaxation loss is dominant in superparamagnetic particles. For our nanoparticles, the coercive force is very small; therefore, we can ignore the hysteresis loss heat contribution. The relaxation losses are divided into Néel loss, which occurs owing to the rotation of the magnetic moment in the presence of an alternating magnetic field, and Brownian loss, which occurs owing to the physical rotation of the nanoparticles in an aqueous media. Therefore, the saturation magnetization and the particle size are important factors that determine the heating capability of nanoparticles in the presence of an alternating magnetic field. The magneto-thermal properties of magnetic nanoparticles can be quantified by using the SAR, which is defined as the amount of heat loss per unit mass of the magnetic components of the particles. When the particles are dispersed in an aqueous solution, the SAR can be calculated as [23] SAR = C W m W m Ni+Fe dt dt, (1) where C W = 4.18 J g 1 K 1 is the specific heat of water, m W is the mass of water (1 g), dt dt is the initial temperature increase (the value can be obtained using the heating curve), and m Ni+Fe is the mass of the magnetic components (nickel and iron) in the particles, as determined using an ICP. The concentration dependence of the SAR is shown in Fig. 6. The 1.5-mg/mL sample showed the largest SAR value of 152 W/g. The SAR decreased with increasing concentration of particles. This behavior can be explained through dipolar interactions [23,24]. The separation between particles decreases with increasing particle concentration; hence, the dipolar interactions between particles increase, which cause a reduction in the Néel loss [25]. The reduced Néel loss decreases the generation of heat in the nanoparticles. IV. CONCLUSION CTAB-coated cube-shaped nickel-ferrite nanoparticles were synthesized using thermal decomposition and characterized to reveal their potentials for magnetic hyperthermia applications. The average length of one side of the cube-shaped particles was 32.6 nm. The particles exhibited the inverse spinel crystalline structure of ferrite. The presence of the biocompatible CTAB coating, which prevents direct contact of the toxic particles with tissues in clinical applications, was confirmed using FT-IR spectra. The nickel-ferrite nanoparticles exhibited superparamagnetic behavior with negligible coercive force. The heating capability of the nanoparticles in the
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