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Characterization and Magnetic Heating of Commercial Superparamagnetic Iron Oxide Nanoparticles Stefan A. Rovers,*,† Leon A. M. van der Poel,† Carin H. J. T. Dietz,† Jef J. Noijen,‡ Richard Hoogenboom,§ Maartje F. Kemmere,† Klaas Kopinga,‡ and Jos T. F. Keurentjes† Process DeVelopment Group, Department of Chemical Engineering and Chemistry, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, Transport in Permeable Media, Department of Applied Physics, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, Dolphys Medical, Den Dolech 2, 5612 AZ EindhoVen, The Netherlands ReceiVed: April 10, 2009; ReVised Manuscript ReceiVed: June 11, 2009
Commercially available superparamagnetic iron oxide nanoparticles (SPION) of 12 nm are characterized with respect to their physical, magnetic, and heating properties in an alternating magnetic field. For this purpose, a specialized magnetic field setup has been developed and characterized. Particles of the waterbased ferrofluid, EMG705, and particles coated for a hydrocarbon carrier have been investigated using several techniques, including transmission electron microscopy and superconducting quantum interference device magnetometry, showing the superparamagnetic behavior of the particles. The specific absorption rate of the particles has been confirmed to have square dependency on the magnetic field strength. The difference in heating of the two samples could be explained by the difference in particles size distribution. 1. Introduction Superparamagnetic iron oxide nanoparticles (SPION) are of great interest in current research and have been studied for a multitude of applications, including magnetic resonance imaging,1,2 drug targeting,2-4 magnetic separation,5-7 and hyperthermia.8-10 In hyperthermia, an ac magnetic field is used to induce a temperature increase. This magnetic heating of SPION results from two relaxation processes, namely Ne´el and Brown relaxation.7,8,11 Ne´el relaxation is the reorientation of the magnetic moment within the particles in which an anisotropy barrier is crossed, thereby causing a temperature increase. Brown relaxation is the reorientation of the magnetic particle itself in a fluid, resulting in friction between the particle and the fluid. Even though extensive research is carried out on both the synthesis of particles that generate high amounts of thermal energy in an alternating magnetic field8,9 as well as the large scale production of iron oxide nanoparticles,12,13 only a limited number of iron oxide nanoparticles are commercially available. In the present work, two different types of commercially available iron oxide nanoparticles have been studied with regard to physical and heating properties in an alternating magnetic field setup. Furthermore, the experimental setup to study the heating in an alternating magnetic field is discussed and evaluated in detail. 2. Materials and Methods 2.1. Materials. The commercially available iron oxide nanoparticles investigated in this study were purchased from FerroTec, Germany, and were used without further purification. Both water-based ferrofluid (EMG 705) as well as particles for a hydrocarbon carrier (EMG 1200) were investigated. Tetrahydrofuran (THF) (99+%) was purchased from Sigma Aldrich. † Process Development Group, Department of Chemical Engineering and Chemistry. ‡ Transport in Permeable Media, Department of Applied Physics. § Dolphys Medical.
Figure 1. Magnetization setup with (a) photograph of the solenoid and (b) schematic representation with (A) the solenoid, (B) the push-pull oscillator, (C) the dc power supply, and (D) data acquisition.
2.2. Magnetic Field. 2.2.1. Setup. In this work, a custom built setup was used to generate an alternating magnetic field with a nominal field strength of 2850 A m-1 with a maximum frequency of 745 kHz, which decreases slightly with decreasing field strength. This setup consisted of three basic elements, see Figure 1. Part A consisted of a gold-coated hollow copper solenoid of 33 turns around a polycarbonate tube with an internal diameter of 60 mm. Two capacitors were placed in series across the solenoid in order to create an inductor-capacitor (LC) circuit. The center of the solenoid was at earth potential for radio frequency (RF). A RF signal was generated by a solid state pushpull oscillator (part B) containing four IRFP3600 MOSFETS, which was connected to two taps on the solenoid, located one turn above and below the center, respectively. The frequency of this oscillator was determined by the LC circuit containing the solenoid. The dc voltage of this oscillator was supplied by a Delta Electronics SM7020 D power supply (70 V, 20 A), part C. A feedback circuit was used to control the average current through the MOSFETS. The output power of the oscillator, and hence the magnetic field strength, was
10.1021/jp903333r CCC: $40.75 2009 American Chemical Society Published on Web 07/22/2009
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adjusted by changing the setting of the power supply. The solenoid was kept at room temperature by flowing high electrical resistance cooling fluid through the tube of the coil. The temperature of the samples, placed in the alternating magnetic field, was measured using a fluoroptic temperature probe and recorded, part D. 2.2.2. Field Characterization. The magnetic field strength and frequency were measured using a custom built pickup coil, consisting of a single turn with a diameter of 31.5 mm, connected to a Tektronix TDS210 Oscilloscope. In the lateral direction, the pickup coil was located in the center of the solenoid of the setup. In the axial direction, the position of the pickup coil has been varied in order to determine the magnetic field strength relative to the axial center of the solenoid. 2.3. Characterization of the Particles. The presence of surfactants that stabilize the commercially available iron oxide nanoparticles was studied using a TA Instruments Q500 thermogravimetric analyzer (TGA), with a heating rate of 10 °C min-1 under N2 flow (60 mL min-1). Furthermore, the core size of the iron oxide nanoparticles was determined by transmission electron microscopy (TEM). TEM samples were prepared by diluting EMG705 ferrofluid (100× w/w) or suspending EMG1200 particles in THF (0.03 wt %) and placing a single drop on a carbon-coated copper grid. In this work, a FEI Tecnai G2 Sphera TEM operating at 200 kV was used. Furthermore, agglomeration of the nanoparticles was investigated by dynamic light scattering (DLS) using a Coulter N4 Plus. The commercially available superparamagnetic iron oxide particles were characterized using a small-angle X-ray diffraction technique to determine the class of iron oxide as well as the crystallinity of the particles. Moreover, the peak broadening effect, due to a small particle size, was used to calculate the crystal size. For this purpose, the full width at half the peak maximum of the six most significant peaks was corrected for the peak broadening due to the inconstancy of the Lorentz polarization. Thereafter, the Scherrer’s equation was used to calculate the crystallite size:
dcrys )
0.93λ B1/2 cos θ
(1)
where dcrys is the crystallite size [nm], λ the used X-ray wavelength [nm], B1/2 the peak width a half-maximum [-], and cos θ the Bragg’s angle. The measurements were performed on a Rigaku Geigerflex diffractometer (copper, 40 kV, 25 mA, λ ) 1.54056 Å). The magnetization of the commercial iron oxide nanoparticles was determined at room temperature with a helium cooled MPMS 50 SQUID (superconducting quantum interference device) magnetometer. 2.4. Temperature Measurements. Heating experiments were performed to determine the heat generated by both types of commercial iron oxide nanoparticles, EMG705 and EMG1200. Therefore, EMG1200 nanoparticles were suspended in THF (27 wt %) using an overhead mixer. Subsequently, 0.5 g of ferrofluid was placed in a glass tube with an inner diameter of 6 mm and a height of 40 mm. The temperature was measured by placing a Luxtron Fluoroptic temperature probe in the ferrofluid. Thereafter, the samples were placed in the center of the custom built magnetic field setup, and the heating was measured at seven different magnetic field strengths. The amount of heat per gram of iron oxide generated, i.e., the specific absorption rate (SAR), was calculated based on the initial heating rate of the sample, the iron oxide content and the specific heat using
Figure 2. Magnetic field strength and frequency of the setup in time.
eq 2.
SAR )
100Cp dT x dt
( )
(2)
ini
where SAR is the amount of heat generated per gram of iron -1 oxide [W giron oxide], Cp the specific heat of the sample, 3.42 and -1 for the EMG705 ferrofluid and EMG1200 1.54 J °C1- gsample particles suspended in THF, respectively, x the iron oxide content of the sample [wt %], and (dT)/(dt)ini the initial temperature increase [°C s-1]. 3. Results and Discussion 3.1. Characterization of the Field. The magnetic field, generated by the magnetic field setup, has been characterized using the single turn pickup coil. To verify the time needed for the setup to create and maintain a stable alternating magnetic field for further measurements and heating experiments, the magnetic field strength and frequency have been determined as a function of time. A steady magnetic field amplitude has been found approximately 11 min after the setup is started, Figure 2. As a considerable amount of power is supplied to the setup, the temperature of the setup, and in particular that of the uncooled capacitors, is significantly increased. Therefore, the capacitance and the loss factor of the capacitors changes, changing the resonance frequency and the magnetic field amplitude of the setup during the initial stages of the experiment. Consequently, all heating measurements described in this paper have been started after 20 min of starting up the setup. Furthermore, the magnetic field strength was determined in the axial direction relative to the center of the solenoid. As expected, the maximum field strength has been found at the center of the solenoid, Figure 3. The field strength as a function of axial position relative to the center can be calculated by eq 3, derived from the Biot-Savart law.14
Hmax√l2 + 4r2 H) 2l
[√
(l - 2x)
+
(l - 2x)2 + 4r2
(2x + l)
√(2x + l)2 + 4r2
]
(3)
where Hmax is the maximum field strength [A m-1], l is the length the solenoid (0.19 m), r the radius of the solenoid (0.038 m), and x is the position from the center [m]. The
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Figure 5. Transmission electron microscopy image of (a) EMG705 and (b) EMG1200 nanoparticles. Figure 3. Magnetic field strength of the setup at different axial positions relative to the center, with the calculated field strength from the Biot-Savart law.
Figure 6. Particle size distribution of EMG705 and EMG1200 nanoparticles obtained by TEM analysis.
Figure 4. Thermogravimetric analysis of EMG705 and EMG1200 nanoparticles.
experimentally found magnetic field strength corresponds well with the calculations based on the Boit-Savart law, Figure 3. To maintain the field strength during measurements with 95 and 98% of the maximum field strength, the sample has to remain within a 8.9 and 6.1 cm window, respectively. During the measurements described in this paper, all samples were within 1 cm of the center of the solenoid and, therefore, the magnetic field strength is considered to be constant. 3.2. Characterization of the Particles. Thermogravimetric analysis (TGA) has been performed to investigate the presence of stabilizing surfactants on the initial iron oxide nanoparticles. The TGA results show that the EMG705 particles appear to have a single surfactant that is lost in between approximately 120 and 220 °C, Figure 4. Because of the hydrophilic nature of the iron oxide, the surfactant is presumably oriented in a double layer and contains either an amine, alcohol, or acid group.15-18 In contrast, TGA analysis of the particles for organic media, EMG1200, shows a secondary plateau from approximately 270 and 350, indicating the presence of a second surfactant. The surfactant content of EMG705 and EMG1200 nanoparticles is 21% and 16%, respectively. The particle size distribution of the iron oxide nanoparticles has been determined using transmission electron microscopy. TEM images show nearly spherical particles, Figure 5. Both types of particles, the water-based EMG705 as well as the particles for hydrocarbon carrier, EMG1200, have a similar particle size in the superparamagnetic range.19 The TEM images have been analyzed to determine the particle size distribution, from which it has been found that both types of particles have
TABLE 1: Particle and Crystal Size of Commercial Iron Oxide Nanoparticles EMG705 EMG1200
TEM [nm]
XRD crystals [nm]
12.1 ( 3.0 11.4 ( 2.6
13.4 9.2
a rather broad particle size distribution, Figure 6 and Table 1. The EMG705 ferrofluid has a small fraction of relatively larger particles, whereas EMG1200 has a small fraction of smaller particles, Figure 6. Furthermore, agglomerates have been found for both types of particles in fluid using dynamic light scattering, with a size of 118 ( 48.4 and 175 ( 67.3 nm, respectively. X-ray diffraction has been used to determine the type of iron oxide of the commercial nanoparticles. The diffraction spectra of both samples correspond to magnetite, Fe3O4, spectra of the International Centre for Diffraction Data, Figure 7. From the X-ray diffraction spectra, the crystal size of the iron oxide nanoparticles has been calculated using eq 1. The mean area diameter corresponds, within experimental error, with the TEM data, Table 1. The magnetization of both types of iron oxide nanoparticles has been measured using a SQUID magnetometer. The magnetization curves clearly shows the superparamagnetic behavior of both types of nanoparticles, Figure 8. The magnetization can be fitted using the Langevin equation:20,21
( ( )
M ) Ms coth
kBT mH kBT mH
where m is the magnetic moment:
)
(4)
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J. Phys. Chem. C, Vol. 113, No. 33, 2009 14641 [m]. In the strong field the limit, eq 4 combined with eq 5 reduces to21,23
1 H
(6)
6 MskBT π µ M d3 0 b
(7)
M ) Ms - R where
R)
Moreover, for weak magnetic fields, the Langevin equation gives the initial susceptibility 3
χi )
π µ0MsMbd 18 kBT
(8)
where
χi ≡
Figure 7. X-ray diffraction pattern of (a) EMG705 and (b) EMG1200 nanoparticles with corresponding spectra of the International Centre of Diffraction Data.
Figure 8. Magnetization curve of EMG705 nanoparticles.
πµ0Mbd3 m) 6
where Ms is the saturation magnetization of the particles, kB the Boltzmann constant, µ0 is the permeability of free space (4π × 10-7), Mb is the saturation magnetization of bulk magnetite (4.2 × 105 A m-1),22 and d is the particle diameter
H)0
(9)
Using the magnetization curves of the nanoparticles, the initial susceptibility, the saturation magnetization, and the coefficient R have been calculated to be 22.2, 366 kA m-1 and 5.5 × 109 A2 m-2 for the EMG705 particles and 14.4, 365 kA m-1 and 8.2 × 109 A2 m-2 for the EMG1200 particles, respectively. Using the limit for strong fields, the particle size has been calculated using eq 7 to be 9.7 and 8.5 nm for EMG705 and EMG1200, respectively. However, calculating the particle size using the limit for weak fields, eq 8, d ) 13.6 and 11.7 nm have been found for EMG705 and EMG1200 nanoparticles, respectively. The observed difference between the values calculated using the strong and weak field limitation can be explained by the fact that the approach to saturation is more sensitive to the smaller particles that are present, whereas the main contribution to the magnetization at weak fields originates from the larger particles.23,24 These results are in good agreement with the particle size obtained from TEM and X-ray diffraction. 3.3. Influence of Field Strength on the Specific Absorption Rate. Samples of the commercially available iron oxide nanoparticles have been heated in an alternating magnetic field at different magnetic field strengths. The temperature increase of the water-based ferrofluid, EMG705, and the nanoparticles for hydrocarbon carriers, EMG1200, suspended in THF, have been measured using an fluoroptic temperature probe, Figure 9. From the initial heating rate, the specific absorption rate has been calculated using eq 2. In addition, the specific absorption rate can be calculated based on particle properties using25
SAR )
(5)
( ∂M ∂H )
2(πmHfτN)2 τNkBTV(1 + (2πf)2τN2)
(10)
where f is the frequency of the alternating field [Hz] and τN is the Ne´el relaxation time. The Ne´el relaxation time is used in eq 10 because Ne´el relaxation is the only relaxation process that contributes to the heating of the suspended particles at this frequency.26 The Ne´el relaxation time is given by:11
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Figure 10. Heating of EMG705 and EMG1200 ferrofluids, at different field strengths.
Figure 9. Heating of (a) EMG705 aqueous ferrofluid (25 wt % particles) and (b) EMG1200 nanoparticles in THF (27 wt %) at different field strengths, with corresponding initial heating slopes.
( )
τN ) τ0 e
KV kBT
(11)
where τ0 ) 10-9 s, K is the anisotropy constant of magnetite of 8 kJ m-3,8,25 V the volume of the particle core [m3], kB the Boltzmann constant [J K-1], and T the temperature [K]. Using an average particle size of 12 nm and a frequency of 745 kHz,27 a square frequency dependency is expected from eq 10 for (2πf)2τN2 , 1. Therefore, the specific absorption rate has been divided by the frequency squared to determine the dependency of the specific absorption rate on the magnetic field strength. For both commercial types of particles, a quadratic field dependence has been observed, Figure 10, in accordance with eq 10. However, the specific absorption rate of the EMG1200 particles has been found to be approximately 1.55 times lower than that of the EMG705 particles. Using eqs 10 and 11 and the particle size distribution, Figure 6, the specific absorption rate has been calculated to estimate the effect of the difference in particles size distribution. Even though the absolute values resulting from these calculations are approximately 10 times higher than the measured values, the calculated relative ratio between the samples of 1.6 agrees with the observed ratio of 1.55. The difference in observed, and calculated values might be due to agglomeration of the particles as observed by DLS, which reduces the heating of the particles.28,29 4. Conclusion Commercially available iron oxide nanoparticles have been characterized in detail and were investigated with respect to
heating in an alternating magnetic field. A custom built magnetic field setup has been developed and characterized, demonstrating that it provides a stable alternating magnetic field for all measurements. The types of investigated nanoparticles, waterbased ferrofluid, EMG705, and particles coated for a hydrocarbon carrier, EMG1200, have a similar particles size with a broad size distribution in the superparamagnetic size range. From the magnetization of the EMG705 particles, described by the Langevin equation, the particle size could be calculated, which was in good agreement with the particle size determined by transmission electron microscopy and X-ray diffraction. Both samples of commercial iron oxide nanoparticles consisted of magnetite with approximately 20 wt % of surfactant to stabilize the particles in suspension. The heating of the particles in fluid, occurring by Ne´el relaxation, has been measured at various magnetic field strengths, showing a quadratic dependence of heating on the field strength. The difference in heating of the EMG705 and EMG1200 particles could be explained by the dependence of the specific absorption rate on the particle size. Due to the presence of a fraction of smaller particles in the EMG1200 particles and a fraction of larger particles in the EMG705 particles, the specific absorption rate of EMG705 is higher. Acknowledgment. This research has been financially supported by SenterNovem and carried out with the support of the Soft-Matter Cryo-TEM Research Unit, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology. Furthermore, the authors express their gratitude towards Marco Hendrix of the Materials and Interface Chemistry group, Department of Chemical Engineering and Chemistry, and Reinoud Lavrijsen of the Physics of Nanostructures group, Department of Applied Physics, Eindhoven University of Technology, for their measurements of the X-ray diffraction spectra and SQUID measurements, respectively. References and Notes (1) Jung, C. W.; Jacobs, P. Magn. Reson. Imaging 1995, 13, 661. (2) Gupta, A. K.; Gupta, M. Biomaterials 2005, 26, 3995. (3) Zhang, J. L.; Srivastava, R. S.; Misra, R. D. K. Langmuir 2007, 23, 6342. (4) Alexiou, C.; Jurgons, R.; Schmid, R. J.; Bergemann, C.; Henke, J.; Erhardt, W.; Huenges, E.; Parak, F. J. Drug Targeting 2003, 11, 139. (5) Melville, D.; Paul, F.; Roath, S. Nature 1975, 255, 663. (6) Bahaj, A. S.; James, P. A. B.; Moesschler, F. D. J. Appl. Phys. 1998, 83, 6444.
Superparamagnetic Iron Oxide Nanoparticles (7) Hergt, R.; Hiergeist, R.; Zeisberger, M.; Glo¨ckl, G.; Weitschies, W.; Ramirez, L. P.; Hilger, I.; Kaiser, W. A. J. Magn. Magn. Mater. 2004, 280, 358. (8) Hergt, R.; Hiergeist, R.; Hilger, I.; Kaiser, W. A.; Lapatnikov, Y.; Margel, S.; Richter, U. J. Magn. Magn. Mater. 2004, 270, 345. (9) Chan, D. C. F.; Kirpotin, D. B.; Bunn, P. A., Jr. In Scientific and Clinical Applications of Magnetic Carriers; Ha¨feli, U., Ed.; Plenum Press: New York, 1997. (10) Hiergeist, R.; Andra¨, W.; Buske, N.; Hergt, R.; Hilger, I.; Richter, U.; Kaiser, W. J. Magn. Magn. Mater. 1999, 201, 420. (11) Ne´el, L. Ann. Geophys. 1949, 5, 99. (12) Park, J.; An, K.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Hwang, N. M.; Hyeon, T. Nat. Mater. 2004, 3, 891. (13) Lee, Y.; Lee, J.; Bae, C. J.; Park, J. G.; Noh, H. J.; Park, J. H.; Hyeon, T. AdV. Funct. Mater. 2005, 15, 503. (14) Crummett, W. P.; Western, A. B. UniVersity Physics: Models and Applications; WCB/McGraw-Hill: New York, 1994. (15) Boal, A. K.; Das, K.; Gray, M.; Rotello, V. M. Chem. Mater. 2002, 14, 2628. (16) Cheng, F. Y.; Su, C. H.; Yang, Y. S.; Yeh, C. S.; Tsai, C. Y.; Wu, C. L.; Wu, M. T.; Shieh, D. B. Biomaterials 2005, 26, 729. (17) Molday, R. S.; Mackenzie, D. J. Immunol. Methods 1982, 52, 353. (18) Zhang, L.; He, R.; Gu, H. C. Appl. Surf. Sci. 2006, 253, 2611.
J. Phys. Chem. C, Vol. 113, No. 33, 2009 14643 (19) Dutz, S.; Hergt, R.; Mu¨rbe, J.; Mu¨ller, R.; Zeisberger, M.; Andra¨, W.; To¨pfer, J.; Bellemann, M. E. J. Magn. Magn. Mater. 2007, 308, 305. (20) Parvin, K.; Ma, J.; Ly, J. J. Appl. Phys. 2004, 95, 7121. (21) Rosensweig, R. E., Ferrohydrodynamics; Cambridge University Press: Cambridge, 1985. (22) Kim, T.; Shima, M. J. Appl. Phys. 2007, 101, 09M516. (23) Krekhova, M.; Lattermann, G. J. Mater. Chem. 2008, 18, 2842. (24) Chantrell, R. W.; Popplewell, J.; Charles, S. W. IEEE Trans. Magn. 1978, MAG-14, 975. (25) Hergt, R.; Andra¨, W.; d’Ambly, C. G.; Hilger, I.; Kaiser, W. A.; Richter, U.; Schmidt, H. G. IEEE Trans. Magn. 1998, 34, 3745. (26) Rovers, S. A.; Hoogenboom, R.; Kemmere, M. F.; Keurentjes, J. T. F. J. Phys. Chem. C 2008, 112, 15643. (27) Due to a decrease in temperature of the setup at decreased field strength, the frequency of the setup decreases slightly with decreasing field strength. (28) Bu¨scher, K.; Helm, C. A.; Gross, C.; Glo¨ckl, G.; Romanus, E.; Weitschies, W. Langmuir 2004, 20, 2435. (29) Rovers, S. A.; Dietz, C. H. J. T.; van der Poel, L. A. M.; Hoogenboom, R.; Kemmere, M. F.; Keurentjes, J. T. F. 2009, submitted for publication.
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