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Magnetic Behavior of O-Carboxymethylchitosan Bounded With Iron Oxide Particles A. Dusza1 , M. Wojtyniak1 , N. Nedelko1 , A. S´lawska-Waniewska1 , J. M. Greneche2 , C. A. Rodrigues3 , C. Burger3 , C. Stringari3 , and A. Debrassi3 Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland Laboratoire de Physique, UMR CNRS 6087, Université du Maine, 72085 Le Mans Cedex 9, France NIQFAR CCS, Universidade do Vale do Itajaí, CEP 88302-202, Itajaí, SC, Brasil Two magnetic chitosan derivatives of O-carboxymethylchitosan bounded with iron oxide particles have been prepared by in situ coprecipitation and incorporation methods. The investigations were focused on structural and magnetic aspects—morphology of iron oxide particles and overall magnetic behavior of these two magnetic chitosan microspheres. It has been shown that the magnetic properties of the sample prepared by the incorporation method are roughly the same as those of a powder of -Fe2 O3 /Fe3 O4 nanoparticles and both are dominated by strong magnetostatic interparticle interactions. In turn, the sample prepared by the in situ method is more susceptible to the temperature increase (resulted in predominant superparamagnetic behavior at 300 K) which is related to smaller particle sizes and better separation of particles with chitosan molecules. Index Terms—Iron oxide nanoparticles, magnetic O-carboxymethylchitosan, superparamagnetism.
I. INTRODUCTION ITH regard to increasing and aging of populations, many problems in such areas like farming, health protection, and waste disposal require fast and inexpensive solutions. This was the foundation for developing new technologies, which needed to be repeatable and not limited by the high cost or chemical wastes. One of the promising materials for many applications is chitosan, which is a polysaccharide obtained by a deacetylaction of chitin—one of the most abundant natural polymers [1]. Chitosan reveals outstanding biological features such as biodegradability, biocompatibility, hydrophilicity, and antibacterial activity. Many years of chitosan investigations yielded its application as an absorbent of metal ions, dyes, and proteins, in agriculture, biochemistry and biotechnology (see, e.g., [1] and[2] and references therein). The major expectations are however connected with medical applications since in vivo and in vitro experiments did not reveal any toxicity effects [2]. One of the particular subjects of interest are magnetic chitosan derivatives loaded with magnetic nanoparticles, preferably iron oxides, which are considered as the best biocompatible magnetic materials [3], [4]. Magnetic particles stabilized within the chitosan are recently intensively investigated. The magnetic chitosan derivatives are prepared by several methods such as: incorporation of magnetic nanoparticles [5], covalent binding of chitosan onto the surfaces of particles [6], or in situ coprecipitation methods [7], [8]. The requirements of the certain biomedical applications, such as, e.g., increase in a drug incorporation, protection and release control for the drug delivery systems, or high magnetization and specific heating power for magnetic hyperthermia, are important justifications for the development of new nanoparticle–chitosan systems.
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Manuscript received June 19, 2009; revised September 01, 2009; . Current version published January 20, 2010. Corresponding author: A. S´lawska-Waniewska (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2009.2032518
In this paper, the investigations of two kinds of O-carboxymethylchitosan bounded with iron oxide particles are reported. These magnetic chitosan derivatives were produced under the desire to obtain highly magnetized, stable, biocompatible, and inexpensive materials for potential biomedical applications such as drug delivery systems, magnetic hyperthermia, sensing, and separation. Two methods for the material preparation were applied and the obtained samples were characterized by complementary structural and magnetic methods. II. MATERIALS AND METHODS The iron oxide nanoparticles and magnetic chitosan derivatives studied in this work were prepared by the wet chemistry methods. The powder of air dried iron oxide particles was prepared by a co-precipitation method described in [8]—Sample 1. These particles were studied as a reference material for two systems of O-carboxymethylchitosan (O-CMCh) loaded with iron oxide particles—Samples 2 and 3. Sample 2 was prepared by an in situ technique in which the formation of iron oxide particles proceeded in O-CMCh solution [8] and resulted in a water soluble magnetic chitosan derivative in which the polymer molecules were adsorbed onto surfaces of iron oxide particles. Sample 3 was prepared by an incorporation technique in which the previously produced particles (the same as for Sample 1) were bounded into the polymer by the cross-linking reaction [5]; simultaneously the cross-linking process leaded to insolubility of the polymer. The in vitro and in vivo toxicology studies have established that these two magnetic chitosan derivatives does not reveal the toxicity effects and the biochemical parameters of rats treated with these materials were not changed although small deposits of iron in liver and kidneys of rats were observed [9]. The crystal structure of the samples was studied by the X-ray diffraction (XRD) method with Cu K radiation using X-Ray Philips diffractometer. XRD patterns were fitted with the MAUD software. Mössbauer spectra were measured at 300 and 77 K in a transmission geometry using a Co source diffused into the Rh matrix. Fitting of the Mössbauer spectra was performed with the Mosfit program using least-square
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Fig. 1. XRD pattern of iron oxide nanoparticles—Sample 1.
method and Lorentzian shapes of the lines. The values of isomer shifts were quoted relatively to that of -Fe at 300 K. The static magnetic measurements were performed with a vibrating sample magnetometer (Oxford Instruments Ltd) over the temperature range 5–300 K. III. RESULTS AND DISCUSSION The X-ray diffraction pattern of iron oxide nanoparticles—Sample 1—is presented in Fig. 1. All the diffraction peaks can be attributed to a spinel structure of magnetite (Fe O ) and/or maghemite ( -Fe O ). Magnetite is an inverse spinel with general formula FeA (Fe Fe )B O , ions can be oxidized leading to formation of but the Fe maghemite [10]. The oxidation of the magnetite during and post-preparation is a well-established process for nanoparticles [11]. The XRD patterns of magnetite and maghemite are very similar preventing thus an accurate separation of these two phases in a granular Fe-oxide sample [12]. The experimental spectrum of Sample 1—Fig. 1—shows reflections typical of maghemite and magnetite, both displaying the same pattern. It was fitted using two components-maghemite and magnetite and the Debye-Scherrer formula was used to estimate the average crystallite size [12]. The fitted crystallographic data are: the lattice parameter 0.8309(5) and 0.8705(5) nm, the average crystallite size 15(2) and 97(10) nm and the area ratio 75(5)% and 25(5)% for the first and the second component, respectively. Obtained parameters indicate that -Fe O is a dominant phase in Sample 1, but the mixture of these two oxides and certain deviations from stoichiometry prevent from more accurate analysis. To get a better insight into the nanoparticle morphology the Mössbauer spectrometry was applied. It is the best method that allows to distinguish between different iron oxides by investigating the oxidation degree and distribution of cations into octahedral and tetrahedral sites. The Mössbauer spectra of Samples 1, 2, and 3 at 77 and 300 K are shown in Fig. 2, respectively. All the spectra are composed of a magnetically split component superimposed on a paramagnetic doublet; the main difference consists in the intensity ratio of these two subspectra. The refined values of hyperfine parameters are listed in Table I. The isomer shift values of both components are consistent with the presence of Fe ions [13]. The lack of a clear component characteristic of octahedrally coordinated Fe ions in an intermediate valence
Fig. 2. Mössbauer spectra at 77 and 300 K of Samples 1—(a), 2—(b), and 3—(c).
state of magnetite (with the isomer shift 0.67 mm/s) clearly indicates that in Samples 1, 2, and 3 the maghemite is the dominant phase, whereas the admixture of the magnetite can only be of an order of 5%–10% (see Table I, sextet with the highest ). The magnetically split sextets exhibit broad and asymmetric lines and can thus be attributed to Fe located in both tetrahedral and octahedral sites but the estimation of their respective ratios would require application of an external magnetic field. The broadening of magnetic lines originate from relaxation effects of the smallest particles, different local environments of Fe cations resulted from nonstoichiometry and/or poor crystallinity as well as surface effects (surface disorder, spin canting). The second component, which is the quadrupolar doublet, can be ascribed to particles in a superparamagnetic state. For each sample the relative intensity of this component decreases with decreasing temperature, as expected for the thermally activated processes. Mössbauer spectra of Samples 2 and 3 are similar to the ones of Sample 1 (no additional components, close values of the hyperfine parameters) indicating similar morphology of iron oxide particles not only in Samples 1 and 3 (expected because of the preparation methods and containing the same particles) but also in Sample 2, prepared with the independent process. Simultaneously the different intensity ratios of magnetic and paramagnetic subspectra point to different sizes and/or various dispersion of particles in the samples studied. Higher contribution of the paramagnetic doublet indicates that more particles are in the superparamagnetic state. This implies that nanoparticles in O-CMCh matrix are better separated from each other and easier to undergo transition to the superparamagnetic state. The superparamagnetic fraction is the most significant in Sample 2, where at
DUSZA et al.: MAGNETIC BEHAVIOR OF O-CARBOXYMETHYLCHITOSAN BOUNDED WITH IRON OXIDE PARTICLES
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TABLE I REFINED VALUES OF HYPERFINE PARAMETERS ( —THE ISOMER SHIFT, —THE QUADRUPOLAR SPLITTING, —THE HYPERFINE FIELD)
1
B
Fig. 3. Temperature evolutions of ZFC-FC magnetization for Samples 1, 2, 3.
room temperature it reaches 80%. For this sample it can be estimated that the average blocking temperature, i.e., the temperature at which the magnetically split and unsplit components represent 50% each of the spectral area, is around 128 K. Thus, the Mössbauer results show that in Sample 2 the nanoparticles are smaller and/or dipolar interactions between particles are significantly reduced due to larger interparticle distances than in Samples 1 or 3. But magnetically coupled agglomerates of particles, relatively stable against the temperature changes, are present in each of the samples and are responsible for the hyperfine split component. For all the samples the temperature dependences of magnetization measured in the applied field of 0.01 T in the zerofield-cooled ZFC (lower curves)-field-cooled FC (upper curves) regimes are shown in Fig. 3. Samples 1 and 3 exhibit similar characteristics—FC curves are only slightly dependent on temperature, ZFC curves do not exhibit any maximum, and both curves are separated in the whole temperature range. These similarities are expected and originate from the preparation techniques of both samples. The observed slight changes of FC magnetization with the temperature suggest that the magnetic behavior of the sample is dominated by large, magnetically coupled and easily saturated agglomerates. The lack of maximum in ZFC curves also suggests strong interparticle interactions. The slight decrease of FC magnetization with decreasing temperature below K can be considered as indicative of a spinglass-like state of particles in agglomerates, i.e., the state often observed in nanoparticle systems with strong magnetostatic interactions ([14] and references therein). The magnetic behavior of Sample 2 differs from those of Samples 1 and 3. In the ZFC curve a broad maximum is observed and is associated with the temperature dependent blocking of the magnetic moment of particles the size of which determines a magnetic anisotropy comparable to the thermal energy. The peak temperature is at K and it corresponds to the average blocking temperature of roughly independent
particles which undergoes the superparamagnetic transition. This value is lower than the one estimated from the Mössbauer investigations ( K) and this is related to different time scales of both experiments. For all the samples the magnetization curves have been systematically measured and the loops recorded at room temperature are shown in Fig. 4. None of the samples was saturated at 1 T which can be due to a surface spin disorder and spin canting structure in the nanoparticle shells. Additionally, each of the loop exhibits at least slight hysteresis even at 300 K. The room temperature loop shapes are similar for Samples 1 and 3 and different for Sample 2. The saturation magnetization of Sample 1, estimated from the approach to saturation of the high field portion of the hysteresis loop, was 45 emu/g, which is much smaller than the one of the bulk maghemite (76 emu/g). The size dependent reduction of magnetization in ferrimagnetic particles is a known phenomenon and is attributed to the surface effects (spin canting, surface disorder, and competing antiferromagnetic interactions) and poor crystallinity [14]. The values of magnetization for Samples 2 and 3 at 300 K were around 21 emu/g, thus the weight fraction of nanoparticles in the O-CMCh samples was around 45%. For Sample 2, which displays the predominant superparamagnetic behavior at room temperature, the anhysteretic magnetization curve at room temperature was fitted to the Langevin function modified with log-normal distribution of particle sizes [15]:
(1) is the satwhere is the magnetic moment of a particle and uration magnetization of the sample, is the temperature, and is the Boltzmann constant. The results of the fit are shown in the inset in Fig. 4 together with the experimental curve. The effective moment of the superparamagnetic particle was estimated to be around 292 emu/cm while the average diameter, evaluated by assuming a spherical shape of the particles, was found to be around 7 nm with the standard deviation of . The obtained large width of the distribution is consistent with the ZFC-FC dependences and is related to the presence of a certain amount of agglomerates in addition to isolated particles. Thus, the average size of nanoparticles in Sample 2 is much smaller
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Fig. 4. Hystereis loops at 300 K. The Inset shows the magnetization curve for Sample 2 (points) and the fit to the Langevin function (solid line).
and incorporation methods were studied as potential inexpensive and nontoxic materials for biomedical applications. The structural and magnetic investigations have shown that nanoparticles consist mainly of maghemite. In the material produced by the in situ method the magnetic nanoparticles are smaller and better separated one from each other than in the material prepared by the incorporation technique. The magnetic microspheres of O-CMCh/ Fe O prepared by the in situ method displays at room temperature the predominant superparamagnetic behaviour and thus are very promising material for nanomedicine—e.g., as a contrast agent for MRI, in the site-specific drug delivery or local anticancer therapy. Additionally, due to the high magnetization, both magnetic chitosan derivatives can be used for magnetic field—assisted extraction of cells and macromolecules as well as magnetic separation. ACKNOWLEDGMENT This work was supported in part by the CNRS-PAS collaboration project. REFERENCES
Fig. 5. Coercive field versus temperature for Samples 1, 2, and 3.
than particles in Samples 1 and 3 ( nm, as estimated from XRD). It may indicate that in the in situ method the chemisorption, which causes the adsorption of chitosan molecules onto surfaces of particles, may simultaneously prevent the growth of iron oxide particles. , estiThe temperature dependences of the coercive field mated from the isothermal loop measurements, are presented in Fig. 5. The dependences for Samples 1 and 3 are similar one to each other. The observed progressive decrease of the coercivity reflects the intrinsic temperature dependences of the anisotropy and saturation magnetization as well as the exdependence trinsic thermally activated processes. The for Sample 2 is evidently different. It can be separated into two regions: (i) a low temperature part where a rapid decrease is connected with higher susceptibility of roughly uncoupled particles and their transition to the superparamagnetic state and (ii) at K a weak decrease of the coercivity is associated with the large, magnetically coupled agglomerates, relatively stable against temperature fluctuations. These results are consistent with the outcome of the analysis of the Mössbauer spectra. IV. CONCLUSION Two magnetic chitosan derivatives of O-carboxymethyl-chitosan bounded with iron oxide particles prepared by the in situ
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