ARTÍCULO ORIGINAL

Engineering iron oxide nanoparticles for biomedicine and bioengineering applications

Diseño de nanopartículas magnéticas para aplicaciones en biomedicina y bioingeniería

Jeaneth Patricia Urquijo^1,2, Herley Casanova², Alvaro L. Morales^1,* , Roberto D. zysler³

¹Grupo de Estado Sólido, Instituto de Física, Facultad de Ciencias Exactas y Naturales, Universidad de Antioquia.Calle 70 N°. 52-21. AA. 1226. Medellín, Colombia.

²Grupo de Coloides, Instituto de Química, Facultad de Ciencias Exactas y Naturales, Universidad de Antioquia. Calle 70 N°. 52-21. AA. 1226. Medellín, Colombia

³Centro Atómico Bariloche / CONICET. CP. 8400. S.C. de Bariloche, Argentina

^*Autor de correspondencia: teléfono:: + 57 + 4 + 4309327, correo electrónico: jhmarca65@gmail.com (J. Marín)

(Recibido el 19 de junio de 2013. Aceptado el 19 de febrero de 2014)

Abstract

In the present study the one-step coprecipitation method is used to obtain magnetic nanoparticles at controlled pH of 10 and 12, and surfactant concentration of1% and 3%(m/m). The surfactant is sodium polyacrylate(PS), biocompatible and biodegradable, necessary attributes for biological applications. The magnetic nanoparticles have a magnetite core, and a shell of maghemite surrounded by a shell of polymer. The maghemite layer is smaller for large surfactant concentration(3%) and pH 10. The TEM images confirm the particle size distribution in the average range of 5-10 nm. Mossbauer results at 80 K showed line shapes dominated by magnetic relaxation effects with sextets and combinations of sextets and doublets for pH 12. The doublet features dominated the samples obtained at pH 10.

The interactions of the surfactant with the nanoparticle surface, mainly with the Fe³⁺, is strong showing at least two surfactant layers, one layer directly over the nanoparticle surface and another layer resting over the inner layer. FTIR confirmed the attachment of the surfactant to the magnetic nanoparticle surface. The nanoparticles showed superparamagnetic behavior at room temperature and ferromagnetic properties at 5 K. The saturation magnetization presented lower values than reported bulk systems due to the presence of a large layer of maghemite. The very close partióle size for all samples gave indication that the partióle growth was dominated by the surface properties of the nanoparticles and that the pH and surfactant concentration did not affect importantly the growth process.

Keywords:Magnetic nanoparticles, sodiumpoiyacrylate, Mossbauer spectroscopy, thermo magnetic measurements, FTIR, TEM

Resumen

Se usó el método de coprecipitación en un solo paso controlando el pH a 10 y 12 y en concentraciones de poliacrilato(PS) de 1% y 3%(m/m). El surfactante es biocompatible y biodegradable, atributos necesarios para su uso en aplicaciones biológicas. Las nanopartículas magnéticas están formadas por una coraza interna de magnetita, una capa de maghemita y una capa externa del polímero. La capa de maghemita es pequeña para la concentración de 3% y pH 10. Las imágenes de TEM confirman la distribución de tamaños de partícula en el rango promedio de 5-10 nm. Los resultados Mossbauer a 80 K mostraron formas de línea dominadas por efectos de relajación magnética en forma de sextetos y combinanciones de sextetos y dobletes; estos dominaron a pH 10. Las interacciones del polímero con la superficie de las nanopartículas. principalmente con el Fe³⁺, es fuerte mostrando al menos dos capas del polímero sobre ellas. Las medidas magnéticas muestran un comportamiento superparamagnético a temperatura ambiente y ferrimagnético a 5 k. La magnetización de saturación presentó valores menores que las repotadas para volúmenes grandes debido a la caapa de maghemita presente. El tamaño de partícula obtenido para todas las muestras es muy cercano entre si indicando que el crecimiento de las partículas fue dominado por las propiedades de la superficie de estas y en menor grado por las condiciones de concentración y pH usadas.

Palabras clave:Nanopartículas magnéticas, poliacrilato de sodio, espectroscopia Mossbauer, medidas magnética térmicas

Introduction

Magnetic nanoparticles, covered with polymers, based on iron oxides, particularly mixtures of magnetite and maghemite, have been synthesized by different routes, coprecipitation, sol-gel, ionic liquids, etc, for more than one decade [1-3], in order to acquire enough knowledge to apply them in industry and biomedicine [4]. The colloidal stability is of primary importance to ensure the proper performance of the ferrofluids and also the further functionalization of the physical- chemical system. Issues about biocompatibility and biodegradability have been also addressed in order to use this systems in biology and medicine.

Researchers in [3] have synthetized magnetite, in acrylic acid anion at concentrations smaller than 0.5%(m/m), obtaining sizes between 12-20 nm and confirming by FTIR and TGA that the anion has been attached to the surface in a chelating bidentate configuration. Authors in [5] obtained almost spherical 5.5 nm diameter magnetite particles from a sol-gel method. The nanoparticles were covered with two substances oleic and dodecanoic acids with a concentration of magnetite in the range 63-78% and saturation magnetization ranging from 83,2-96,0 emu/g. these high values are explained as an effect of the acids preventing surface spin disorder. Also, due to their high magnetite concentration they found that interparticle dipolar interactions were important.

Further research by authors in [6] obtained poly(methyl methacrylate) or polystyrene/ magnetite nanocomposites, the polymers were added after the magnetic particles were formed. For the 32% wt and 6%wt iron oxide samples the saturation magnetization was 13 and 3 emu/g and coercitivity 240 and 220 Oe respectively. The particles showed a cubic morphology with 20 nm average size. In [7] it was prepared stable magnetite-copolymer complexes with magnetite percentage running from 6.9% to 45.5 % wt. They obtained first the 9 nm nanoparticles and afterwards they were coated with the copolymer, two-step process, by an elaborate technique, being able to control its amount. Saturation magnetization were in the range 67-75 emu/g and carboxylic acid groups were the dominant binding mechanism to the particle surface.

Studies in [8] have simulated theoretically the interparticle interactions in field cooling(FC) measurements and they found a flattening of the FC curve at low temperatures as a signal of such interactions, for monodisperse particles, or weak interacting particles, the FC curve increases monotonically as the temperature decreases. They also produce 4 nm average maghemite nanoparticles by microwave plasma method and from them obtained monodisperse and compacted samples for which the FC experimental curves were in agreement with the computer simulations.

Tetramethyl ammonium hydroxide (TMAOH)- functionalized 8 nm nanoparticles were obtained in [9] using the one-step coprecipitation method. They found, by oxidizing the nanoparticles to drive the transformation from magnetite to maghemite, a decrease in the saturation magnetization and this effect also helped to improve colloidal stability because magnetic interaction between them decreased. In reference [10], researchers have tested the adsorption behaviour and wettability of synthetic magnetite and mineral particles. They obtained iron oxide nanoparticles by a two step route where in the first step they used a primary surfactant and sodium polyacrylate (PS) was used as a second layer surfactant at pH 8.5. The primary surfactant helped the adsorption of PS by reducing the electrostatic repulsion between the particles and PS. Their results indicate that the wettability of both synthetic magnetite coated with a primary surfactant and magnetite mineral can be improved by adsorbing a hydrophilizing agent such as polyacrylate.

In the present study the one-step coprecipitation method is used to obtain magnetic nanoparticles at controlled pH of 10 and 12, and surfactant concentrations of 1%(m/m) and 3%(m/m). The surfactant was sodium polyacrylate. PS is an anionic polymer, with negative charge carboxylic groups, with a uniform distribution of ionizable groups along its chain and highly soluble in water, it is also biodegradable and biocompatible. The PS has been used generally as a secondary cover on magnetic nanoparticles, here it is used as the first layer on them. PS provide the required electrosteric properties to stabilize the nanoparticle fluid, at the same time the surfactant adhere to the nanoparticle surface providing a way to further functionalize the system. Furthermore, the surfactant is intended as a mean of controlling the nanoparticle growth.

Experimental

The magnetic nanoparticles were obtained by the coprecipitation method [11], with appropriate modifications, starting with precursors FeCl₃.6H₂O (>=99.0%), FeCl₂.4H₂O (>=99%) y NaOH (>=99%), obtained from Merck®, and sodium polyacrylate commercial grade. All solutions were prepared and the synthesis ran under nitrogen flux in order to avoid oxidation. Also, deionized water was used in the solution preparation. A 0.06 M solution of Fe²⁺, a 0.12 M solution of Fe³⁺, and a 2M solution of NaOH were poured into a solution of the surfactant with automatic pH control using a Metrohm 907. Magnetic stirring of the solution was kept during all the synthesis time at room temperature 21-23 C.

The black product was washed using a dialysis membrane until the conductivity of the water was the same as the conductivity of deionized water. The nanoparticles embedded in the polymer were dried using mechanical vacuum, a part of the solutions were converted to gel, by adding 0.2g/ ml agar per ml of magnetic solution, to preserve the particle distribution in the fluid. The same synthesis was repeated without the surfactant to obtain a set of nanoparticles as a reference (Pph12, Pph10). The synthesis was carried out for pH of10 and 12 and surfactant concentrations of 1% and 3% (m/m), the samples were named as concentration-surfactant-pH, e.g. 1PS12, etc. In order to assess the amount of iron in the samples. i.e. the amount of magnetite/maghemite, Atomic absorption spectroscopy (AAS) was used.

To corroborate the phase composition, magnetite and maghemite, of the product obtained Microraman (MR) measurements were taken in a Horiba Jobin Yvon, model Labram HR, with CCD detector with resolution of1024x256 pixels, spectral range of 100-500 cm^-1 and a 633 nm He/ Ne laser. The crystalline structure of the spinel phases was probed by X-ray diffractograms taken with CuKa radiation in the range 10-80 2θ, step 0.02 2θ, 3 seconds per step. The XRD analysis was performed with program powderx [12] and observed peaks were assigned according to the JCPDS cards [13].

The morphology and particle size distribution were investigated by transmission electron microscopy(TEM) with a Philips CM200 UT microscope, operating at 200 kV. Magnetic information on the samples was obtained by means of the Mossbauer technique in a Wissel spectrometer with MR-260 transducer, ⁵⁷Co (Rh) source, and a closed-cycle cryostat, the hyperfine parameters were extracted by program DISTRI [14]. All spectra were fitted with hyperfine field and quadrupole splitting distributions due to the asymmetric line shape found. The sextet spectra for the pH 12 samples were fitted using a modification of the model by Berry et al. cited in [15] which consist of five iron sites, Fe³⁺ (F3T)from tetrahedral sites, Fe³⁺ (F3O)from octahedral sites, a mixed- valence Fe275+ (F32O)from octahedral sites, Fe²⁺ (F2O)from octahedral sites, and a Fe³⁺ (F3S)sextet coming from the surface of the particles. This site was included because the spectra showed a feature different from the spectra reported in [15].

The interaction of the surfactant with the surface of the nanoparticles was sensed by both Fourier Transform Infrared Spectroscopy, in a Perkin Elmer equipment, and thermogravimetic analysis, in a T.A Instruments Q100 v. 9.9. Further, magnetic characteristics of the samples were analyzed by magnetic field versus magnetic moment measurements in a Quantum Design PPMS, Vibrating Sample Magnetometer, in the temperature range 5-300 K.

Results and discussions

Information on the crystalline structure, morphology and composition of the nanoparticles were obtained from XRD, fig. 1, MR, fig. 2, and TEM, fig. 3.

XRD shows spinel peaks, belonging to both magnetite and maghemite not separable by this technique. The peaks are broad due to the small particle size of the samples, this broadening leads to crystallite sizes, by using the Scherrer's formula with an error of 10%, of 10 nm and 9 nm for 1PS12 y 3 PS12 samples respectively.

MR results shows that the nanoparticles were formed as a mixture of magnetite and maghemite. and they suffered further oxidation due to the drying process. The assignment of strongest peak positions for all samples was based on the literature [16]. The uncovered sample (Pph12) shows a similar spectra with peaks corresponding to magnetite and maghemite. The main structure around 600-800 cm^-1 is broad due to particle size distribution in the samples and to overlapping phases, and it consist of three peaks at around 665 cm^-1, 680 cm^-1 and 709 cm^-1. The first peak corresponds to magnetite, the third peak has been assigned in [16] to iron oxidation at the magnetite B-sites, i.e. non-stoichiometric magnetite, but other authors assigned it to maghemite [17]. The peak at 680 cm^-1 has not been reported in the literature as a peak belonging to magnetite or maghemite but it may come from the interface separating the magnetite core from the maghemite layer. Peaks around 374 cm^-1 and 627 cm^-1 belong to maghemite.

The similar Raman spectra for plain and covered spectra points to the fact that the dominant oxidation process is during the drying stage. The MR spectra of pH 10 samples showed more noisy spectra due to the larger amount of polymer covering the particles. The amount of polymer stuck to the surface of the particles was evaluated by AAS. The amount of magnetite in the dry samples was, for pH 12 samples 93.5(1%) and 83.1(3%) and for pH 10 samples 49.8(1%) and 15.3(3%). As a result, the amount of polymer decreases in the order 3PS10, 1PS10, 3PS12, 1PS12. The amount of surfactant deposited on the particles can be understood considering the negatively charge increase on the particle surface and on the polymer surface as the pH increases leading to a larger electrostatic repulsion. Both surfaces, iron oxide and polymer, are negatively charged since they are well above the zero charge point (ZCP). Researchers in [ 18] had reported a 95% polymer ionization at pH 8.

Of course, additional binding mechanisms are acting in those systems like steric repulsion, Van der Waals forces, surface complexation and formation of hydrogen bridges. The layers over the nanoparticles will play an important role for applications since functionalizing substances can be added on them.

TEM adds information on the morphology particle size and its distribution. Figure 3 shows the outcome for selected samples. All micrographs show the nanoparticles embedded in the respective polymer which makes difficult the assessment of the morphology, i.e. the particle size and its distribution. The main features present in the micrographs are isolated nanoparticles of irregular form, aggregated particles, and polymer surrounding the particles. Nevertheless, the analysis was performed to obtain the particle size distribution and average size of the nanoparticles. The frequency plot for the particle size, only isolated particles were taken into account, 46 particles for 3PS10 and 114 for 1PS10, was adjusted to a log-normal distribution [19] in this way the average particle size and standard deviation were obtained. A systematic error comes from the blurring of the images due to the polymeric layer on the particles making the results for the averages just approximate. Average sizes are 6 ± 2 nm for 1PS10, 5 ± 2 nm for 3PS10. These averages are essentially the same indicating that at these high concentrations the particle size comes out nearly the same in spite of the fact that the surfactant concentration is rather different. These sizes are in agreement with the crystallite size given by XRD (see below a comparison of crystallite sizes).

The magnetic properties of the nanoparticles were investigated by MS, fig. 4, Hysteresis measurements, fig. 5, and FC/ZFC measurements, fig. 6.

The Mossbauer hyperfine parameters, for dry samples, at 80 K are shown in Table 1. The spectra are composed of sextets for samples obtained at pH 12, a very small doublet is present. The sextets show dominant components of Fe³⁺ and smaller components of mixed valence Fe²⁷⁵⁺ (IS=0.54- 0.74 mm/s). The fact that these sites are present tells that the samples are ferrimagnetic at this temperature, they present the Verwey transition at 125 K, and present good crystallinity. The sextets are dominated by magnetic relaxation effects due to the broader and asymmetric lines; this is related to the small particle size and its distribution.

For the sample 1PS10 a superparamagnetic doublet dominates the spectrum and a small magnetic component exist. For the sample with 3% surfactant concentration and pH 10, it can be noticed a pure doublet with two clear peaks. It must be emphasized that the Mossbauer spectra line shape could be affected by interparticle interactions, due to the particle density in the polymer as shown by AAS, i.e. for the highest particle densities sextets are found and for the lower ones superparamagnetic doublets and small magnetic components are observed.

These results mean that although the particles are small in size or superparamagnetic, recall the Scherrer's crystallite size, due to the relative closeness at high particle densities they interact and produce a magnetic signal. As a consequence, the Mossbauer line shape is attributed mainly to the presence of interparticle interactions and not to a large size magnetic particles in agreement with [20]. In general, for large iron oxide particles. the crystallite size derived from XRD is smaller than the size given by TEM so the particles are polycrystalline [ 21]. The general trend showed by the Mossbauer spectra is that for pH 12 samples the spectra showed magnetic components, and for the pH 10 samples superparamagnetic behavior dominated; this trend point to the presence of particles of nanometer size. Since the sextets showed mainly Fe³⁺ components a different way of preparing the sample was tried, i.e. instead of drying the sample it was converted to a gel to preserve the original phase composition of the nanoparticles. The result is showed in Fig. 4 for the sample 3PS12g where the Fe²⁺ peak is reveled. In this way the presence of magnetite was further confirmed and the oxidation effect on the samples during the drying process. Next, the magnetic behavior of the nanoparticles was analyzed by hysteresis measurements, figure 5 and table 2 [22-25].

It is important to remember that the samples were prepared as gels to keep the partióle distribution in the fluid, this will influence the results and they may be different from some of the results shown above for powder samples for which an extra oxidation took place. The main characteristic at 300 K was the superparamagnetic behavior of the samples in contrast to 5 k where they behaved as ferrimagnetic. The small coercive fields. measured at room temperature, are probably due to a tiny magnetic component coming from aggregation of particles and a small number of large particles. The magnetic saturation is the largest for 1PS12 and decreases for 3PS12, but for the pH 10 samples the magnetic saturation is very small. The magnetic saturation is considerable smaller than reported bulk values. The coercive field for bulk samples is not comparable with its counterpart for nanoparticles because they come from different physical mechanisms; for the bulk it is a consequence of the pinning of domain walls while for the nanoparticles it comes from the anisotropy energy barriers. These smaller magnetic saturations as compared with reported values ([5, 7]) are due to the maghemite layer surrounding the magnetite core in agreement with [ 9].

At 5 K the samples behaved as ferrimagnetic showing large values of the susceptibility and the coercive fields. From the measured initial susceptibility an estimation of the magnetic particle size, corresponding to the largest particles present in the distribution, can be made using the formula,, where K_b, X_i, μ₀, M_s, M_b, stand for the Boltzmann constant, temperature, initial susceptibility. vacuum permeability, saturation magnetization for nanoparticles, saturation magnetization for bulk magnetite. The magnetic particle size is presented in table 3 from which it can be deduce that the particle size obtained from these samples is very similar in spite of the fact that the surfactant concentrations are very different.

This result indicates that it is the surface properties of the nanoparticles the dominant fact by which the surfactants bind to it, in this way the chelanting mechanism by which the surfactants stick to the Fe³⁺ sites is confirmed. For this kind of superparamagnetic particles the sizes given by XRD, TEM, and magnetic measurements are very close to each other [26].

The FC/ZFC magnetization measurements, fig. 6, give a more detailed information on the particle size distribution and interparticle interactions. The temperature T_max , at the maximum of the ZFC magnetization curve, is related to the blocking temperature distribution; and the irreversible temperature T_i, where the FC curve starts to deviate from the ZFC curve, is related to the energy barrier distribution in the sample, table 4.

A larger difference between T_max and T_i means a broader particle size distribution. The derivative gives the features of the magnetic energy barrier distribution highly correlated to the particle size distribution and also gives the temperature T_b at the maximum of the barrier distribution. In Table 4 it is seen large differences between T_max and T_i for the pH 12 samples and the opposite behavior for the pH 10 samples. With respect to interparticle interactions the pH 12 samples show strong forces and for the pH 10 samples very weak interactions are present. These interactions are also reflected in the increase of T_max for pH 12 samples; on the other hand, the flattening of the FC curve at low temperatures is indicating strong interactions between nanoparticle moments, recall the high particle densities from AAS for pH 12 samples [27,28].

It is worth noting that the 80 K Mossbauer spectra showed in fig. 4 was composed of magnetic components in some cases and magnetic-superparamagnetic components for the other cases. A correlation can be drawn with the FC/ZFC magnetic measurement although the samples for the Mossbauer technique were in powder form while for magnetic measurments were gels this may bring small differences in the magnetic behavior of the samples. Sextets were obtained for the samples for which the 80 K temperature (T80) was below T_max , i.e. pH 12 samples. For 1PS10 T_max and T_i are well below T80 and a doublet should be expected, instead a sextet-doublet line shape appears pointing to the different sample preparation for which the powdering process allows the particle aggregation and further oxidation to maghemite. For 3PS10 a doublet was obtained as expected where the drying process did not affected its properties probably due to the fact that it had the largest amount of polymer covering it.

The interaction of the magnetic particles with the polymer was studied using FTIR, fig. 7, and TGA, fig. 8. table 5 summarizes the FTIR results for the four covered samples.

By analyzing the change in the asymmetric stress mode of the polyacrylate carboxylic group COO- it is noticed a shift in the absorption bands to higher wave number, i.e. from 1550 cm^-1 in the pure sample to 1570 cm^-1 for the pH 10 samples, precluding a large interaction with the magnetic nanoparticles surface [29]. The spinel peaks for these samples are less clear due to the large amount of polymer on the surface of the nanoparticles, see results from AAS, for this reason they cannot be determined. The spectra in fig. 7 was normalized in order to enhance the differences in phase composition, for this reason transmittances are not given.

Another route to analyze the interaction particle- polymer is to take the difference between the asymmetric and symmetric stretch absorptions, Δv = vas(COO^-) - vs(COO^-), which in this case are larger than in the pure polymer leading preferentially to the formation of a chelating bidentate configurations attached to a single Fe³⁺ on the surface of the particles [18, 30, 31]. The phase composition magnetite/maghemite is also confirmed by FTIR results. TGA gives complementary information about the interaction between the particle surface and the polymer, the results are shown in fig. 8. The pure polyacrylate presents a one step-like curve, from 150-500 C, for which the first sector, 20-150 C, is the loss mass due to physisorbed water, a second sector, 150-400 C, where dehydration and the carboxilate groups dissociation occurs, and the main decomposition occurs in the temperature range 400-500 C.

On the other hand, the covered samples show a two step-like curve which mean that the decomposition takes on at different times due to different polymer layers, with different chemical properties, that surround the magnetic particles. The inner layers decomposed at later times so it can be seen a mayor decomposition in the temperature range 600-750 C. The results also correlate with AAS since samples obtained at pH 12 had a thinner polymer layer but strongly bound to the surface of the particle exhibiting a slower mass loss in such a way that at a temperature of 400 C the mass loss is around 13% while for pH 10 the corresponding quantity is 40%.

Conclusions

The one step coprecipitacion, with surfactant included, produced magnetic nanoparticles covered with significant amount of polymer, especially for the pH 10 samples, as shown by absorption spectroscopy measurements. The magnetic nanoparticles have a magnetite core, and possibly a layer of non-stiochiometric magnetite, surrounded by a maghemite layer and on top of that a surfactant layer, as indicated by Raman, XRD, FTIR, TGA, and magnetic measurements. The TEM images confirm the crystallinity of the nanoparticles, their particle size distribution in the average range of 5-10 nm.

Mossbauer results at 80 K showed line shapes dominated by magnetic relaxation effects with sextets and combinations of sextets and doublets due to the asymmetric lineshapes and very large broadening of sextets. The hyperfine parameters revealed the presence of Fe³⁺, and small amounts of Fe²⁺ for the dried samples in contrast to gel samples which showed larger content of Fe²⁺, this indicates a sample oxidation during the drying process. The doublet features dominated the samples obtained at pH 10. Also, the Mossbauer spectra showed effects of interparticle interactions leading to the presence of sextets, especially for pH 12 samples, due to high density of particles embedded in the surfactant which is increased by the drying process. This fact can be understood remembering that the particle size, from XRD and TEM, is in the average range of 5-10 nm ; in this way, it is expected a doublet component at 80 K which is missing.

The interactions of the surfactant with the nanoparticle surface, mainly with the Fe³⁺, is strong showing at least two surfactant layers, one layer directly over the nanoparticle surface and another layer resting over the inner surfactant layer. This is reflected thorough a step-like structure in the TGA technique. The frequency shifts, intensity and line shape changes in FTIR confirmed the attachment of the surfactant to the magnetic nanoparticle surface. The main mechanism of binding was the chelating mechanism, due to the fact that both particles and surfactant present a negative charge which leads to repulsion forces between them.

The magnetic behavior was evaluated, in gel form, by moment versus temperature and magnetic field measurements. The nanoparticles showed superparamagnetic behavior at room temperature and ferrimagnetic properties at 5 K. The saturation magnetization presented lower values than reported bulk values and nanosystems prepared by a different route, these occurred due to the presence of a large layer of maghemite. A magnetic diameter was extracted from the initial susceptibility which shows very close values for all samples indicating that a these one-step synthesis conditions, especially large concentrations 1-3%(m/m), the chemical reaction proceeds in a way that the surface properties of the nanoparticles dominate the kinetics. The FC/ZFC plots confirmed the superparamagnetic nature of the iron oxide particles and showed that the distribution of moments was broad for pH 12 samples while it was narrow for pH 10 samples. The magnetic information deduced from Mossbauer spectroscopy confirms the magnetic information extracted from magnetic measurements.

The very close particle size obtained from XRD, TEM, and magnetic measurements gave indication that the particle growth was dominated by the surface properties of the nanoparticles and that the different concentrations and pH did not play a fundamental rol.

The obtained PS-covered nanoparticles may be easily functionalized for biomedicine and bioengineering applications since PS presents charged active sites along the polymeric chain where organic substances can be inmovilized for the applications. Additionaly, PS is biocompatible and biodegradable. Furthermore, the nanoparticles superparamagnetic character makes them appropriate to apply the active susbstance and to control the nanoparticle flux by an external magnetic field.

Acknowledgments

The authors are grateful to Colciencias, Colombian research council, and CODI, Sustainability Program for Solid State Group 2011-2012, Universidad de Antioquia, for financial support.

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