J. Phys. Chem. C 2008, 112, 15643–15646
15643
Relaxation Processes of Superparamagnetic Iron Oxide Nanoparticles in Liquid and Incorporated in Poly(methyl methacrylate) Stefan A. Rovers,*,† Richard Hoogenboom,‡ Maartje F. Kemmere,† 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, and Dolphys Medical, Den Dolech 2, 5612 AZ EindhoVen, The Netherlands ReceiVed: June 26, 2008; ReVised Manuscript ReceiVed: July 16, 2008
Commercially available superparamagnetic iron oxide nanoparticles (SPION) of 12 nm are investigated with respect to the contribution of both Ne´el and Brown relaxation to the magnetic heating of these particles. For this purpose experiments have been performed to heat the particles, suspended in a liquid as well as incorporated in a poly(methyl methacrylate) (p(MMA)) matrix, using an alternating (ac) magnetic field of 745 kHz at three different field strengths up to 2850 A m-1. It is shown that the specific absorption rates (SAR) of the particles in the ferrofluid are identical within experimental error to the SAR of the particles incorporated in p(MMA) at all measured field strengths. As Brown relaxation is not possible if the SPION are incorporated in the polymer matrix, it can be concluded that Ne´el relaxation is the only relaxation process that contributes to heating the ferrofluid at 745 kHz as well. This is confirmed by calculation of the relaxation times for Ne´el and Brown relaxation. Furthermore, calculations suggest that Ne´el relaxation is also the predominant relaxation process for small superparamagnetic particles at lower frequencies. 1. Introduction Superparamagnetic iron oxide nanoparticles (SPION) are of great interest 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.8,11 Ne´el relaxation is the reorientation of the magnetic moment within the particles, in which an anisotropy barrier is exceeded, thereby causing a temperature increase. Brown relaxation is the reorientation of the magnetic particle itself in a fluid, resulting in friction of the particle with the fluid. The Ne´el relaxation time is given by11
τN ) τ0e(KV/kbT)
(1)
and the Brown relaxation time by6
3ηVH τB ) kbT
(2)
where τ0 ) 10-9 s, K is the anisotropy constant of 8 kJ m-3,8 V is the volume of the particle core [m3], kb is the Boltzmann constant [J K-1], T is the temperature [K], η is the viscosity of the carrier liquid [kg m-1 s-1], and VH is the hydrodynamic volume of the particle [m3]. If SPION are dispersed in a highly viscous medium, e.g. a gel, the particles are not able to rotate. Therefore, Brown relaxation is generally excluded in this case.6 However, in low viscous media the relative contribution of Ne´el * To whom correspondence should be addressed. † Eindhoven University of Technology. ‡ Dolphys Medical.
and Brown relaxation is not entirely evident. Generally, the contribution of Ne´el relaxation is found to be the predominant, though not exclusive, relaxation mechanism. The decrease of the specific absorption rate (SAR) in a gel compared to a low viscous fluid is in the former case attributed to the loss of Brown relaxation.6 The most frequently used method to distinguish between the two relaxation processes is measuring the temperature increase of both the ferrofluid and the gel containing the iron oxide nanoparticles.9 Furthermore, a distinction has been made using calculations of the specific absorption rates based on magnetic susceptibility measurements of particles in ferrofluid and incorporated in gel.8 In the present work, the heating of commercially available magnetic nanoparticles in a liquid and incorporated in a polymer matrix is investigated at different magnetic field strengths. In the case of polymer incorporated particles, Brownian motion can be completely excluded, allowing the direct assessment of the relaxation process in the ferrofluid by a direct comparison of the temperature increase. 2. Materials and Methods 2.1. Materials. The iron oxide nanoparticles used in this study, aqueous ferrofluid EMG705, were used as purchased from Ferrotec, Germany. Methyl methacrylate (MMA) (99.5+%) and lauric acid (99.5+%) were purchased from Sigma Aldrich. Tetramethylammonium hydroxide (25% in water) and potassium persulfate (99+%) were purchased from VWR International. 2.2. Distribution of EMG705 Particles in Polymer. EMG705 nanoparticles were distributed in the polymer by mixing the aqueous ferrofluid with a p(MMA) latex and subsequent freeze-drying using a Labconco Freezone 4.5 in combination with a Chemstar 1402N vacuum pump, operated at 84 × 10-3 mbar. Thereafter, the resulting powder was compounded into cylindrical bars of approximately 10 mm length with a diameter of 3 mm, using a preheated custombuilt double-screw compounder with a volume of 5 cm3, set at
10.1021/jp805631r CCC: $40.75 2008 American Chemical Society Published on Web 09/12/2008
15644 J. Phys. Chem. C, Vol. 112, No. 40, 2008
Rovers et al.
190 °C and rotating at 100 rpm. The p(MMA) latex was made by emulsion polymerization at 80 °C using 50 g of water, 5 g of tetramethylammonium hydroxide, 1.8 g of lauric acid, 70 g of methyl methacrylate, and 1 g of potassium persulfate and was performed under an argon atmosphere with a reaction time of 1 h. The resulting p(MMA) particles were measured by dynamic light scattering (DLS) and were found to have a size of 55 ( 14 nm. 2.3. Characterization. The size of the superparamagnetic iron oxide particles used in this work was determined using transmission electron microscopy (TEM). A single drop of diluted ferrofluid (100× w/w) was placed on a carbon-coated copper grid. Furthermore, the distribution of the particles within the poly(methyl methacrylate) was studied with TEM as well. A Leica RM2165 rotary microtome was used to cut slices with an approximate thickness of 50 nm, and the samples were placed on a carbon-coated copper grid. TEM analysis was performed with a FEI Tecnai G2 Sphera cryo-TEM operated at 200 kV. Dynamic light scattering was use to investigated the p(MMA) latex particle size as well as the presence of clusters of the iron oxide nanoparticles in the ferrofluid. A Coulter N4 Plus particle size analyzer was used measuring three times for 900 s at 20 °C at an angle of 90°. The presence of clusters was confirmed by cryo-TEM analysis. Therefore, cryo-TEM samples were prepared on carbon-coated copper grids by injection into liquid ethanol using a Vitrobot Mark III. The magnetization of the iron oxide nanoparticles and the particles incorporated in p(MMA) was determined at room temperature with a heliumcooled MPMS 50 SQUID (superconducting quantum interference device) magnetometer. 2.4. Temperature Measurements. Heating experiments were performed to determine the amount of heat generated by the iron oxide nanoparticles, both in the ferrofluid and in the nanocomposite. In case of the ferrofluid, a Luxtron Fluoroptic temperature probe was placed in a glass tube with an inner diameter of 6 mm and a height of 40 mm containing 0.5 g of EMG705 ferrofluid. The temperature of the nanocomposite was measured by placing three cylindrical bars around and in direct contact with the Luxtron probe. Subsequently, the samples were placed in a custom-built setup generating an ac magnetic field with a frequency of 745 kHz. All samples were measured three times at different magnetic field strengths (2010, 2440, and 2850 A m-1). 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 as measured by DSC analysis using the equation
SAR )
100Cp dT x dt
( )
ini
(3)
where SAR is the amount of heat generated per gram of iron oxide [W giron oxide-1], Cp is the specific heat of the sample, 4.02 and 1.65 J °C-1 gsample-1 for the ferrofluid and nanocomposite, respectively, x is the iron oxide content of the sample [wt %], and (dT/dt)ini is the initial temperature increase [°C s-1]. 3. Results and Discussion 3.1. Characterization. The commercially available superparamagnetic iron oxide nanoparticles are characterized using TEM, magnetization measurements, and DLS. TEM analysis shows that the core size of the EMG705 ferrofluid particles is in the superparamagnetic range, with an average size of 12.1
Figure 1. Images of EMG705 by (a) conventional TEM of the nanoparticles and (b) cryo-TEM of the ferrofluid.
( 3.0 nm12 (Figure 1a). Moreover, magnetization measurements of the particles have illustrated zero coercivity and remanence (data not shown). Furthermore, DLS analysis of the ferrofluid shows that the particles in the fluid are agglomerated in clusters of 118 ( 48 nm. These clusters have been confirmed by cryoTEM analysis of the EMG705 ferrofluid (Figure 1b). The iron oxide nanoparticles incorporated in p(MMA) are observed to be in clusters of the same size order as in the ferrofluid (Figure 2). These clusters are well distributed throughout the p(MMA) matrix. The decrease in specific absorption rate after immobilization of the nanoparticles in gel is often attributed to the loss of Brown relaxation upon immobilization.6,9 However, the effect of the
Superparamagnetic Iron Oxide Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 40, 2008 15645
Figure 4. Heating of p(MMA) containing 30 wt % EMG705 nanoparticles at different field strengths, with the corresponding initial heating slopes.
TABLE 1: Initial Heating and Specific Absorption Rate of Iron Oxide Nanoparticles in Ferrofluid and Incorporated in p(MMA) at Different Magnetic Field Strengths Figure 2. TEM image of EMG705 nanoparticles incorporated in p(MMA).
Figure 3. Heating of EMG705 ferrofluid containing 33 wt % iron oxide nanoparticles at different field strengths, with the corresponding initial heating slopes.
distribution and agglomeration of the particles in the gel has never been studied in these investigations. Nonetheless, in the current work the particles in both the ferrofluid and the p(MMA) are present as ∼100 nm clusters, allowing a direct comparison of the specific absorption rate. 3.2. Temperature Measurements. Heating experiments are performed to assess the contribution of Ne´el and Brown relaxation to the heating of SPION in a ferrofluid by a magnetic field. The heating of SPION in a ferrofluid and incorporated in a p(MMA) matrix are measured three times using a fluoroptic fiber in an alternating magnetic field at different field strengths. The heating experiments show a significant temperature increase of both the ferrofluid and SPION containing p(MMA) depending on the magnetic field strength (Figures 3 and 4). Initial heating rates have been found to be identical within experimental error for the three repetitive measurements for each sample and field strength. Maximum initial heating rates of 0.64 and 1.42 °C s-1 are found for the ferrofluid and the nanocomposite, respectively, by applying the maximum field strength of 2850 A m-1. As
field strength [A m-1]
initial heating [°C s-1]
specific absorption rate [W giron oxide-1]
ferrofluid
2010 2440 2850
0.242 ( 0.001 0.382 ( 0.003 0.642 ( 0.003
2.94 4.66 7.82
incorporated in p(MMA)
2010 2440 2850
0.533 ( 0.006 0.844 ( 0.001 1.42 ( 0.02
2.94 4.65 7.83
expected, decreasing the field strength results in lower initial heating rates for both the ferrofluid and the composite. The calculated specific absorption rates at various field strengths are identical for both samples (Table 1). Since the iron oxide particles that are incorporated in p(MMA) cannot rotate, it is concluded that the heating contribution of Brown relaxation can be excluded in the ferrofluid as well. In order to verify experimentally observed data with known theory of Ne´el and Brown relaxation, calculations of the relaxation times have been performed. Assuming a hydrodynamic particle size of 3 times the particle core size measured by TEM as determined previously for similar particles,8 the relaxation times for Ne´el and Brown relaxation in ferrofluid can be calculated from eqs 1 and 2. Using T ) 300 K and η ) 1.01 × 10-3 kg m-1 s-1, the calculation results in τ ) 6.07 × 10-9 and 1.84 × 10-5 s for Ne´el and Brown relaxation, respectively. Therefore, Brown relaxation is not able to follow the fast change in magnetic field at a field frequency of 745 kHz (τ ) 2.14 × 10-7 s). On the basis of these calculations, it can be concluded that Ne´el relaxation will be the main relaxation process in the ferrofluid, which is in accordance with the heating experiments. Moreover, the relaxation process with the shortest relaxation time tends to predominate, which suggests that Ne´el relaxation will be the predominating relaxation process at lower frequencies as well. 4. Conclusion Experimental results of heating superparamagnetic iron oxide nanoparticles in a ferrofluid as well as incorporated in poly(methyl methacrylate) reveal identical specific absorption rates for both samples at three different magnetic field strengths.
15646 J. Phys. Chem. C, Vol. 112, No. 40, 2008 Because of the exclusion of Brown relaxation for the particles incorporated in p(MMA), it is concluded that heating by Brown relaxation can also be neglected in the ferrofluid, and thus, Ne´el relaxation is the only occurring relaxation process that contributes to heating. At a frequency of 745 kHz this is confirmed by calculations of the relaxation times which show that Brown relaxation is not able to follow the fast change of field direction. Moreover, these calculations also suggest that for 12 nm sized particles in low viscous media Ne´el relaxation will also predominate at lower frequencies due to the significantly shorter relaxation times. 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. 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.
Rovers et al. (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 (London) 1975, 255, 663. (6) 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. (7) Bahaj, A. S.; James, P. A. B.; Moesschler, F. D. J. Appl. Phys. 1998, 83, 6444. (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, Jr., P. A. 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) 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.
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