Dynamic susceptibility investigation of biocompatible ...

Dynamic susceptibility investigation of biocompatible magnetic fluids: The surface coating effect P. C. Morais, J. G. Santos, L. B. Silveira, and A. C. Oliveira Citation: J. Appl. Phys. 97, 10Q911 (2005); doi: 10.1063/1.1853152 View online: http://dx.doi.org/10.1063/1.1853152 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v97/i10 Published by the American Institute of Physics.

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JOURNAL OF APPLIED PHYSICS 97, 10Q911 共2005兲

Dynamic susceptibility investigation of biocompatible magnetic fluids: The surface coating effect P. C. Morais,a兲 J. G. Santos, L. B. Silveira, and A. C. Oliveira Universidade de Brasília, Instituto de Física, Núcleo de Física Aplicada, 70919-970, Brasília-Distrito Federal, Brazil

共Presented on 9 November 2004; published online 17 May 2005兲 In this study room-temperature dynamic susceptibility measurements were used to investigate magnetite nanoparticles surface-coated with dextran and dimercaptosuccinic acid, both dispersed as biocompatible magnetic fluids. Multicomponent susceptibility curves were associated with the presence of monomers and dimers in the sample. Transmission electron microscopy data have been used to support the analysis carried out with the susceptibility data. The field dependence of the peak position of the imaginary component of the dynamical susceptibility was analyzed within the picture of an asymmetric double-well potential for the relaxation of the magnetic moment associated with the magnetite nanoparticle. The nanoparticle size dependence of the susceptibility peak position was taken into account in the data analysis. Nanoparticle-size parameters obtained from the analysis of the susceptibility data of the two samples 共3.7 and 6.1 nm兲 are in very good agreement with the values obtained from the fitting of the electron microscopy data 共3.1 and 5.6 nm兲. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1853152兴 In recent years the design and synthesis of biocompatible magnetic fluids have attracted intense interest, with special emphasis on their applications in biomedicine.1 Biocompatible magnetic fluids 共BMFs兲 are especially designed material structures consisting of a core magnetic nanoparticle surfacecoated with biocompatible molecules and dispersed in physiological medium as an ultrastable colloid.2 The core nanoparticle is usually a metal-oxide, especially cubic ferrites, which is conveniently synthesized using chemical coprecipitation reactions.3 The present study reports on the magnetic investigation of dimercaptosuccinic acid 共DMSA兲 and dextran-coated magnetite nanoparticles peptized as BMFs. Dynamical susceptibility 共DS兲 measurements were used to draw conclusions regarding the response of the surfacecoated magnetite nanoparticles upon the application of a radio-frequency field superimposed to a steady magnetic field. Here, special emphasis is on the field 共H兲 dependence of the peak frequency 共f p兲, due to their influence on the magnetohyperthermia therapy.4–6 Magnetohyperthermia is realized upon the application of external, low amplitude, ac magnetic field to the target tissue, which is previously labeled with magnetic nanoparticles. The efficiency of the onsite heat dissipation, due to the interaction of the ac field with the magnetic moment, is maximized at the peak frequency 共f p兲. The present study describes how an external steady field 共H兲 modulates the system’s magnetic response. Discussion of the DS data of both samples will be focused on the features observed in the imaginary component of the susceptibility curves. The parameters obtained from the fitting of the field dependence of the peak frequency will be compared with the standard magnetite parameters and the size Author to whom correspondence should be addressed; FAX: 共⫹55兲 612723151/61-2736655; electronic mail: [email protected]

a兲

0021-8979/2005/97共10兲/10Q911/3/$22.50

dispersity obtained from the transmission electron microscopy 共TEM兲 data. The two biocompatible magnetic fluids used in this study 共based on dextran- and DMSA-coated magnetite兲 contained about 5 ⫻ 1016particle/ cm3. After precipitation in alkaline medium, fresh magnetite nanoparticles were surface-coated to produce stable BMF samples at physiological condition 共pH7 and 0.9% sodium chloride兲. Typical BMF sample preparation routine can be found in Ref. 7. The TEM micrographs were obtained using a Jeol-1011 system whereas the DS measurements were performed using a home-made Robinson oscillator operating in the megahertz region 共10– 40 MHz兲. Susceptibility measurements 共real and imaginary components兲 were recorded at room temperature under different external steady fields 共0 – 4 kG兲. Figure 1 shows typical imaginary components of the DS of both samples 共dextran and DMSA coated兲 recorded at H = 0.6 kG. At room temperature and in the frequency range of our investigation, the susceptibility imaginary component of DMSA-coated and dextran-coated BMFs were curve fitted using two and three gaussian-shaped curves 共fitting not shown兲, respectively. Regarding the field dependence of the DS peak positions 共see Fig. 1兲, the symbols in Fig. 2 represent the data 共peak frequency versus applied steady magnetic field兲 obtained from the dextran-coated sample 共circles兲 and from the DMSA-coated sample 共squares兲. The f p vs H data shown in Fig. 2 correspond to the lowest-frequency susceptibility peak 共left-hand-side feature兲. The vertical error bars in Fig. 2 are on the order of the symbol 共circles and squares兲 sizes. The particle size histograms, obtained from the TEM micrographs, are shown in Fig. 3 for the dextran- 共left panel兲 and for the DMSA-coated sample 共right panel兲. The solid lines in Fig. 3 represent the best curve fitting of the experimental data using the log-normal distribution function.8

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© 2005 American Institute of Physics

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FIG. 1. Typical room-temperature imaginary component of the susceptibility of the dextran-coated sample 共circles兲 and the DMSA-coated sample 共squares兲, under an external steady field of 0.6 kG.

The first aspect regarding the room-temperature susceptibility data shown in Fig. 1 is the multipeak structure related to both BMF samples. The same BMF samples have been frozen down to 77 K, under zero-field condition, in order to perform the DS measurements. The low-temperature susceptibility data of the zero-field-frozen BMF samples allowed identification of only one peak in the imaginary component of the susceptibility curves. This single peak 共imaginary component of the susceptibility curve兲 observed in both BMF samples at 77 K is more likely related to the lowestfrequency susceptibility peak 共left-hand-side feature兲 observed at room temperature. We argue that at room temperature and under zero external field most of the magnetic units in suspension in the magnetic fluid samples are composed of isolated nanoparticles. By freezing the samples under zerofield condition the isolated nanoparticles will be trapped in the solid matrix. However, application of an external field, even weak fields, induces particle-particle coupling in nanoparticle-based arrays. In support of this picture, monomers and dimers have been identified through static magnetic birefringence measurements9–11 and magnetic-resonance experiments.12 Furthermore, two types of dimer structures have been identified, namely, coherent and fanning, being the fanning configuration more likely than the coherent one.9,10 Fanning and coherent modes represent the lowest energy configurations of a dimer structure; the fanning showing lower total energy 共Zeeman plus magnetostatic兲 than the coherent one.9 It has been observed that the magnetic permeability of the fanning configuration peaks at higher fields than the coherent configuration.11 Therefore, based on the birefringence and resonance experiments, the structures observed in the right-hand side 共higher-frequency values兲 of the susceptibility curves shown in Fig. 1 can be associated with dimers. Indeed, concerning the peaks observed in Fig. 1, the highest-frequency peak 共more intense than the middlefrequency peak兲 is likely related to the fanning configuration whereas the middle-frequency peak is likely related to the coherent configuration. The dynamical susceptibility associated with the nanoparticle system follows the usual Debye form, ␹共␻兲 = ␹0共1 − i␻␶兲−1, with the Néel relaxation described by ␶ = ␶0 exp共␯ / kT兲, where ␶0 is the relaxation time of the nanoparticle’s magnetic moment 共typically 10−9 s兲 and ␯ = KV 共K is the magnetocrystalline anisotropy and V the nanoparticle

FIG. 2. Field dependence of the dynamical susceptibility peak taken from the imaginary component and associated with the dextran-coated sample 共circles兲 and the DMSA-coated sample 共squares兲. The solid lines represent the best fit using Eq. 共2兲.

volume兲. However, particle-particle interaction as well as application of an external field deforms the symmetrical double-well potential. Under the asymmetrical double-well potential the relaxation of the magnetic moment is described by ␶ = ␶0 exp共␯ / kT兲sech共E / 2kT兲, where ␯ and E are the energy barrier height and the asymmetry parameter, respectively. Therefore, in our experiments, the imaginary component of the susceptibility curve peaks at frequencies given by f共V , E兲 = f 0 exp共−␯ / kT兲cosh共E / 2kT兲. Whereas ␯ describes the magnetocrystalline anisotropy 共KV兲, E is due to the interaction between the nanoparticle magnetic moment 共m兲 and the applied field 共H兲. Note that we are not including particleparticle interaction in our model picture, due to the average particle-particle distance 共about 30 nm兲 compared to the typical particle diameter 共3.1 and 5.6 nm兲. Regarding the polydispersity in particle volume 共V兲, the energy barrier height 共␯兲 and the asymmetry parameter 共E兲 need to be described using distribution functions. The log-normal distribution function, P共V兲, has been widely used to describe size polydispersity,8 whereas a flat distribution function, G共E兲, has been used to describe the asymmetry parameter.13 Then, the field dependence of the peak frequency 共f p兲 reads

f P共H兲 =

冕冕

f共V,E兲P共V兲G共E兲dEdV.

共1兲

FIG. 3. Particle size histograms of the dextran-coated sample 共left panel兲 and the DMSA-coated sample 共right panel兲. The solid lines represent the best curve fitting of the data using the log-normal distribution function.

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The nonflat distribution function G共E兲 = sech␤共E / 2kT兲, with ␤ = 3, however, has been introduced as an excellent alternative for fitting the field dependence of the DS peak frequency.14 Using the nonflat description Eq. 共1兲 is reduced to f P共H兲 =

冕

A exp共− KV/kT兲tanh共E/2kT兲P共V兲dV.

共2兲

The solid lines in Fig. 2 represent the best fit of the experimental data according to Eq. 共2兲. In contrast, the use of a flat G共E兲 function replaces, in Eq. 共2兲, the asymptotic tanh共E / 2kT兲 by the divergent sinh共E / 2kT兲, not accounting for the saturation behavior observed in Fig. 2. At this point two aspects of the nonflat distribution function, G共E兲, should be emphasized. First, from the mathematical point of view Eq. 共2兲 is exactly obtained from Eq. 共1兲, as long as G共E兲 = sech3共E / 2kT兲. Second, from the physical point of view the nonflat distribution function, G共E兲, deviates very little from the Boltzmann distribution function. In other words, the good agreement between the data and the model proposed in the present study, which uses the nonflat distribution function, indicates that the asymmetry parameter 共E兲 may follow a classical distribution function. Though empirical, the present approach represents a step forward as compared to the use of a flat distribution function. The parameters obtained from the fitting of the DS data 共solid lines in Fig. 2兲 are in excellent agreement with the data provided by the TEM micrographs. The log-normal fit shown in Fig. 3 for the dextran-coated 共DMSA-coated兲 sample gives 3.1± 0.3 nm 共5.6± 0.2 nm兲 and 0.26± 0.02 共0.22± 0.01兲 for the mean particle diameter and diameter dispersion, respectively. The particle polydispersity parameters obtained from the fitting of the TEM data were used as initial values 共guess values兲 in the fitting procedure 共least-squares fit兲 of the DS data using Eq. 共2兲. The fitting of the DS data 共solid lines兲 shown in Fig. 2 was performed with 3.7± 0.5 nm 共6.1± 0.5 nm兲 and 0.27± 0.04 共0.25± 0.03兲 for the mean particle diameter and diameter dispersion of the dextran-coated 共DMSA-coated兲 sample, respectively. Furthermore, the average anisotropy values we found from the DS analysis of the dextran-coated and DMSA-coated samples were 共1.2± 0.4兲 ⫻ 104 and 共1.6± 0.3兲 ⫻ 104 J / m3, respectively. Considering the uncertainties, the anisotropy values we found are in excellent agreement with the value reported for the anisotropy of bulk magnetite 共1.9⫻ 104 J / m3兲.15 Finally, the average magnetization obtained from the DS analysis of both samples was 110± 8 emu/ g. Though upshifted from the saturation magnetization value reported in the literature16 共84 emu/ g for magnetite兲 the fitted value still falls in the expected range. The observed difference in saturation magnetization, i.e., the value reported in the literature versus the value found from the susceptibility data may indicate the presence of a nonmagnetic surface layer in the magnetite nanoparticles. Similar investigation, namely, the field dependence of the imaginary peak susceptibility, has been reported in the literature with no model picture supporting quantitative explanation for the data.17 Note that measurements available from the literature present data recorded in the range of 0 to about 1 kG. In such a low-end side of magnetic

fields a linear dependence of the peak frequency shift has been observed,17 similar to the data reported in this study. In conclusion, magnetite-based biocompatible magnetic fluid samples were investigated using room-temperature dynamical susceptibility measurements. The multipeak component observed in the susceptibility curves can be explained in terms of the presence of monomers and dimers 共fanning and coherent兲 in the magnetic fluid samples; the dimers induced by a weak magnetic field due to the Robinson oscillator superimposed to the external steady field. We found, in addition, that the fanning configuration of the dimer is more likely to occur than the coherent one, indicating that the DMSA-coated magnetite nanoparticles built up essentially the fanning dimer whereas the dextran-coated magnetite nanoparticles form both fanning and coherent dimers. Considering the uncertainty related to the polydispersity parameters obtained from both dynamical susceptibility and transmission electron microscopy data, the present investigation highlights the capability of the susceptibility measurements in assessing the mean size and size dispersion of magnetic nanoparticles in magnetic fluid samples. In addition, the field dependence of the main susceptibility peak allows estimation of the saturation magnetization 共110± 8 emu/ g兲 and magnetocrystalline anisotropy 共1.2± 0.4⫻ 104 and 1.6± 0.3 ⫻ 104 J / m3兲 values associated with molecular-coated magnetite nanoparticles. Indeed, the field dependence of the main susceptibility peak has been described via a relaxation picture of the magnetic moment of an isolated nanoparticle in an asymmetric double-well potential. ACKNOWLEDGMENTS

This work was supported by the Brazilian agencies CNPq, FINEP, and FINATEC. C. C. Berry and A. S. G. Curtis, J. Phys. D 36, R198 共2003兲. T. Goetze, C. Gansau, N. Buske, M. Roeder, P. Gornert, and M. Bahr, J. Magn. Magn. Mater. 252, 399 共2002兲. 3 P. C. Morais, V. K. Garg, A. C. Oliveira, L. P. Silva, R. B. Azevedo, A. M. L. Silva, and E. C. D. Lima, J. Magn. Magn. Mater. 225, 37 共2001兲. 4 A. Jordan et al., Int. J. Hyperthermia 13, 587 共1997兲. 5 C. W. Jung, J. M. Rogers, and E. V. Groman, J. Magn. Magn. Mater. 194, 210 共1999兲. 6 M. H. A. Guedes et al., J. Magn. Magn. Mater. 272–276, 2406 共2004兲. 7 Y. S. Kang, S. Risbud, J. F. Rabolt, and P. Stroeve, Chem. Mater. 8, 2209 共1996兲. 8 B. M. Lacava, R. B. Azevedo, L. P. Silva, Z. G. M. Lacava, K. Skeff Neto, N. Buske, A. F. Bakuzis, and P. C. Morais, Appl. Phys. Lett. 77, 1876 共2000兲. 9 A. F. Bakuzis, M. F. da Silva, P. C. Morais, L. S. F. Olavo, and K. Skeff Neto, J. Appl. Phys. 87, 2497 共2000兲. 10 K. Skeff Neto, A. F. Bakuzis, P. C. Morais, A. R. Pereira, R. B. Azevedo, L. M. Lacava, and Z. G. M. Lacava, J. Appl. Phys. 89, 3362 共2001兲. 11 P. C. Morais, K. Skeff Neto, A. F. Bakuzis, M. F. da Silva, and N. Buske, IEEE Trans. Magn. 38, 3228 共2002兲. 12 P. C. Morais, G. R. R. Gonçalves, A. F. Bakuzis, K. Skeff Neto, and F. Pelegrini, J. Magn. Magn. Mater. 225, 84 共2001兲. 13 P. C. Morais, A. L. Tronconi, and K. Skeff Neto, J. Appl. Phys. 55, 3744 共1984兲. 14 A. F. R. Rodriguez, A. C. Oliveira, P. C. Morais, D. Rabelo, and E. C. D. Lima, J. Appl. Phys. 93, 6963 共2003兲. 15 K. P. Belov, Phys. Usp. 36, 380 共1993兲. 16 R. C. O’Handley, Modern Magnetic Materials: Principles and Applications 共Wiley, New York, 2000兲, p. 125. 17 P. C. Fannin, S. W. Charles, and T. Relihan, J. Magn. Magn. Mater. 162, 319 共1996兲. 1 2

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