Behdadfar Behshid1, Kermanpur Ahmad1, Sadeghi-aliabadi Hojjat2, Veintemillas-Verdaguer Sabino3, Ruiz Cabello Jesus4, Morales Maria del Puerto3, Mozaffari Morteza5

1 Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran 2 School of Pharmacy, Isfahan Pharmaceutical Research Center, Isfahan University of Medical Sciences, Isfahan, Iran

3 Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049, Madrid, Spain 4 Instituto de Estudios Biofuncionales,Centro de Investigacion Biomodica en Red de Enfermedades Respiratorias

(CIBERES), Universidad Complutense, 28040 Madrid (Spain) 5 Physics Department, Razi University, Kermanshah, Iran

Abstract— Monodispersed aqueous ferrofluids of

superparamagnetic Gd substituted Zn-Fe ferrite (Fe0.7 Zn0.3 Gdy Fe2-y O4, y=0, 0.01, 0.025, 0.04 and 0.05) nanoparticles were synthesized via a direct, efficient and environmentally friendly citric acid assisted hydrothermal method. Synthesized nanoparticles were characterized by X Ray Powder Diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, Transmission Electron Microscopy (TEM) , Dynamic Light Scattering (DLS) method and Nuclear Magnetic Resonance (NMR). Ferrofluids obtained by this way showed good stability at pH=7 and hydrodynamic sizes lower than 100 nm with polydispersity indexes below 0.2 at high concentrations (>25 mg/ml). Longitudinal (r1) and transverse (r2) relaxivities were measured at 1.5 T and 37 ºC. The results showed that the substitution of Gd in the structure of Zn-Fe ferrite has a strong effect on the transverse (r2) relaxivity, increasing its value up to 400 s-1. mM-1. Keywords— Citric acid assisted hydrothermal method, ferrofluid, MRI

I.INTRODUCTION Magnetite (Fe3O4) has been the focus of intense research

since centuries ago due to its unique magnetic properties. In biomedical application magnetite was the most frequently used as core in magnetic functionalized particles that were used in magnetic fluid hyperthermia, as contrast agent for magnetic resonance imaging, in magnetic assisted chemical separation, and as magnetic drug carriers [1, 2]. Partial cation substitution of iron ions in magnetite offers a possibility to improve and control its physical and chemical properties. Cation substitution in spinels may be at one or both A and B sites, with magnetic or non-magnetic ions with different valences. It is possible to change crystal symmetry by substitution and form cation deficit spinels with changing cation valence [3]. Cations incorporated in spinel lattice change magnetic interactions and magnetic anisotropy, influencing the saturation magnetization and coercivity values of the parent spinel [4].

For some specific clinical and industrial applications, magnetite is required to have improved magnetic performance such as high saturation magnetization. Also, small size particles with narrow particle size distribution are sometimes required for bioapplications, which have to be non-toxic and biocompatible [5].

Rare earth substituted spinels have been earlier investigated [6, 7]. It was shown that Curie temperature decreases and coercivity increases with rare earth substitution in Ni-Zn ferrites [6]. In Ho-substituted Fe3O4 the energy barrier height is drastically modified and the blocking temperature shifts towards lower temperatures [7]. Cvejic et al. showed that substitution of some Fe ions by rare earth ions can enhance the saturation magnetization of magnetite and this enhancement for Gd substitution is higher[4]. In addition they mentioned that partial cation substitution of iron ions by the rare earth ions stabilizes magnetite and preserves its structure from transition to hematite, which is important in different applications. Calderon-Ortiz et al.[8] investigated the effect of composition on the corresponding magnetization and Curie temperature in Gd-substituted Mn–Zn ferrite nanocrystals. They showed that the saturation magnetization reaches a maximum with increasing the amount of Gd in the structure of Mn-Zn ferrite nanocrystals. The preparation methods in these works were high energy ball milling [4, 7] or conventional coprecipitation [8]. In this work, isolated superparamagnetic zinc and gadolinium co-substituted magnetite nanoparticles (Fe0.7 Zn0.3 Gdy Fe2-y O4), (y=0, 0.01, 0.025, 0.04 and 0.05), with sizes around 10 nm , high crystallinity and high saturation magnetization have been synthesized in water using citric acid assisted hydrothermal method at temperature 180°C. These nanoparticles are stable in water at pH 7 at high concentrations without using any other organic surfactants and/or materials. The substitution of Gd in the structure of Zn-Fe ferrite has a strong effect on the transverse (r2) relaxivity, increasing its value up to 400 s-1. mM-1.

II.Experimental Materials All raw materials, including Fe(NO3)3.9H2O, ZnCl2, NH4OH 25% and C6H8O7.H2O were purchased from Merck Co. and GdCl3.6H2O was purchased from Sigma-Aldrich Co. with minimum purities of 99%. Nanoparticle synthesis Required amounts of Fe3+, Zn2+ and Gd3+ salts (0.3 mmol of Zn2+ and 2.7 mmol of (Fe3+ + Gd3+)) were dissolved in 20 mL of distilled water. After 10 min stirring, a solution of

Á. Jobbágy (Ed.): 5th European IFMBE Conference, IFMBE Proceedings 37, pp. 1113–1118, 2011. www.springerlink.com

Gd Substituted Zn-Fe Ferrite Nanoparticles as High T2 MRI Agents

NH4OH 25% was added slowly to reach a medium pH of 9. Vigorous stirring continued for another 10 min and a reddish brown slurry was obtained. The slurry was then centrifuged and washed three times with deionized distilled water to remove any excess ions. 2 mmol of citric acid was then added to the precursors. The mixture was stirred vigorously for 10 min and then transferred into a 500 ml volume teflon-lined autoclave. The autoclave was kept at 180 ˚C for 20 h and then free-cooled to room temperature. Ferrofluids were prepared by washing the precipitate with acetone, mixing it with 10 ml of deionized distilled water and sonication for 10 min. The black suspension was centrifuged for 10 min with a speed of 6000 rpm. The supernatant was the desired magnetic ferrofluid and the remaining nanoparticles aggregated at the bottom of the tube were discarded. Characterization The morphology, particle size and their size distribution were investigated using a transmission electron microscope (JEOL-2000 FXII) operating at 200 keV. A drop of the ferrofluid was deposited on a carbon coated copper grid and leaved to evaporate at room temperature. Mean particle size was calculated from TEM data by measuring at least 300 particles. Data were fitted to a log-normal distribution and then mean size and polydispersity degree were obtained from the fitting. Fourier transform infrared spectra were recorded between 3600 and 400 cm-1 in a Jasco FTIR-680 plus spectrophotometer. Samples were prepared by diluting the nanoparticles in KBr at 2% by weight and pressing it into a pellet. Thermogravimetric analysis of the powders was carried out in a SEIKO TG/ATD 320 U, SSC 5200. The analysis was performed from room temperature to 900ºC at 10ºC/min in an air flow. The analysis of the weight loss percentage allows the quantification of the coating. Colloidal properties of the aqueous suspensions (mean hydrodynamic size, polydispersity index and zeta potential) were obtained by dynamic light scattering (DLS) method using a Malvern instrument Zetasizer (DTS Ver. 5.02). Diluted samples dispersed in deionized distilled water at 1 mg Fe/ml were used for this study. Hydrodynamic size was measured at pH 7 and the intensity data were analyzed to obtain the Z-average size (Cumulants mean) and the intensity, volume and number distributions. Z-potential was measured with 0.01 M concentration of KNO3 at different pH values between 2 and 12.

Magnetic measurements were carried out by a vibrating sample magnetometer (MLVSM9 MagLab 9 T, Oxford Instrument). Samples were prepared by packing the powders into pellets. Magnetization curves were recorded at room temperature by first saturating the sample in a field of

5 T. The magnetization values were normalized to the amount of iron to yield the specific magnetization (emu/g Fe3O4). Saturation magnetization values (Ms) were evaluated by extrapolating to infinite field the experimental results obtained in the high field range where the magnetization linearly increases with 1/H. Relaxometric properties were investigated for each sample by measuring T1 and T2 protons relaxation times at different dilutions between 0.05 and 0.5 mMolar of Fe. The relaxation time measurements were carried out in a Minispec MQ60 (Brucker) at 37 ºC with a magnetic field of 1.5 T. From the graph of the Fe-concentration dependent relaxation times, the relaxivities r1 and r2 were determined for each sample.

III.Results and discussion Fig.1 shows XRD patterns of the samples

(Fe0.7Zn0.3GdyFe2-yO4, y= 0, 0.01, 0.025, 0.04, 0.05). The excellent matching between the diffraction peaks and those corresponding to the spinel structure, suggested that Gd3+ (ionic radius 0.97 Å) have substituted for Fe3+ ones (0.64Å) in the host ferrite structure. The average crystallite sizes, was estimated by Scherrer’s formula from (311) peak, varied between 10 and 12 nm (table 1). The corresponding lattice parameter was slightly increased from 0.832 to 0.842 nm for y, from 0 to 0.05, respectively (table 1).

Fig. 1 XRD patterns of Fe0.7Zn0.3GdyFe2-yO4 with different y values

Fig. 2 shows TEM image of the sample with y=0 which evidences the nanocrystalline nature and uniformity in size of the particles. Mean particle size of the sample was calculated from TEM data by measuring at least 300 particles. Data were fitted to a log-normal distribution (Fig.3). The mean particle size was 9.3 nm with a standard deviations of 1.9 nm, which is in very good agreement with the average crystallite size estimated from Scherrer’s formula. This similitude between these two values

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evidences the formation of the particles which are single crystals.

Fig. 2 TEM image of the sample with y=0

Fig. 3 Nanoparticli diameter histogram for sample with y=0

Figures 4 and 5 show TEM images and size distribution of the sample with y=0.025 respectively. As can be seen this sample has uniform particles with a mean particle size of 9.5 nm with a standard deviation of 1.7 nm. This value is comparable with the mean crystallite size of the sample, which shows each particle is a crystallite too. TEM investigations indicate the monodispersed character in size and shape of the nanoparticles prepared by the citric acid assisted hydrothermal method.

Fig. 4 TEM images for the sample with y=0.025

Fig. 5 Nanoparticli diameter histogram for sample with y=0.025

Fig. 6 shows room temperature M–H curve for the sample with y=0.01 as a typical curve. As can be seen the nanoparticles are superparamagnetic at room temperature (zero remanence and coercivity), with a saturation magnetization of 82.00 emu/g. Similarly all of the samples had the same curve and are superparamagnetic at room temperature, which their saturation magnetizations tabulated in table 1. As a general trend the higher the Gd fractions in the Zn–Fe ferrite, the lower the saturation magnetization (Fig.7). An exception was observed for y=0.01, where an increase in the saturation magnetization (82.00 emu/g) was observed with respect to the pure Zn-Fe ferrite (80.17emu/g). This rise in magnetization can be explained by substitution of Fe3+ by Gd3+ ions having a higher magnetic moment in B-sites [8].

Fig. 6 room temperature M–H curve for the sample with y=0.01

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Fig. 7 Saturation magnetization versus y for Fe0.7Zn0.3GdyFe2-yO4

nanoparticles

The presence of citrate ions on nanoparticles surfaces was revealed by infrared spectroscopy and it was shown in figure 8 for sample with y=0.01 as a typical curve. The intense and broad band around 3420 cm-1 (No.1) shows the structural OH. The 1690-1760 cm-1 strong peak assignable to the C=O vibration (symmetric stretching) from the COOH group of citric-acid shifts to an intense band at about 1600cm-1 (No.3) for the substituted magnetite nanoparticles coated with citric acid revealing the binding of a citrate ion to the nanoparticle surface. The band at 1375 cm-1 (No.4) indicates the asymmetric stretching of CO from the COOH group. The band at 545 cm-1 (No.5) is the characteristic peak of magnetite [9].

Fig. 8 FTIR spectrum of sample with y=0.01

Quantification of the coating was carried out by TG and DTA analyses. Figure 9 shows TG and DTA curves of the sample with y=0.01 as a typical curve. A weight loss of about 4% with a an endothermic peak at 34 ˚C can be ascribed to the removal of physically absorbed water and citric acid molecules on the Fe3O4 nanoparticles. Due to the presence of one step weight loss in neat citric acid around 200°C [10], The weight loss of about 5.5% with a sharp exothermic peak at 260 ˚C can be associated with the removal of chemically attached citric acid molecules from the surface of Fe3O4 nanoparticles. The weight loss of about 1.9% beyond 400 ˚C with an exothermic peak at 590 ˚C is associated with the phase transformation of Fe3O4–Fe2O3.

Fig. 9 TG and DTA graphs of sample with y=0.01

Table 1 contains colloidal properties of the ferrofluids prepared from all samples. The hydrodynamic size distribution in intensity data at pH 7 was obtained using the DLS. The Z-average size for sample with y=0.01was 75.14 nm (Fig.10) resulting from a monomodal distribution and the polydispersity index was 0.13. As the polydispersity index can vary from 0.01 up to 0.7 [11], we can conclude that the resulting ferrofluids prepared from these magnetic nanoparticles are rather monodispersed. Zeta potential measurements at different pH values between 2 and 12 (Fig.11 for sample with y=0.01 as a typical curve), show that the ferrofluids at pH 7 have a negative surface charge between -30 and -25 mV, confirming the presence of citrate ions on the surface of magnetite nanoparticles and assuring a long term stability of the ferrofluids at pH 7. The isoelectric points for the synthesized ferrofluids change between pH of 2 and 3.

Fig.10 Hydrodynamic size distribution of dispersed nanoparticles of

sample with y=0. 01 in aqueous media

72747678808284

0 0.02 0.04 0.06

Ms(

emu/

g)

y

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Fig. 11 Variation of the zeta potential for sample with y=0.01 versus

pH

Longitudinal (r1) and transverse (r2) relaxivities were measured at 1.5 T and 37ºC and the data are collected in Table 1. The results show that the substitution of Gd in the structure of Zn-Fe ferrite has a strong effect on the transverse (r2) relaxivity, increasing its value more than 400 s-1. mM-1 (Fig. 12).

Table 1 measured characteristics for Fe0.7Zn0.3GdyFe2-yO4 nanoparticles

Fig. 12 transverse (r2) relaxivities versus y for Fe0.7Zn0.3GdyFe2-yO4

nanoparticles

y Ms

(emu/g)

Lattice Parameter

(nm)

Crystallite Size(nm)

XRD

Hydrodynamic Size in

water(nm) PDI

CA percentage

(%)

CA desorption

Temperature (°C)

r1 (s-1mM-1)

r2 (s-1mM-1)

0 80.17 0.832 10 92.1 0.144 9.5 266.5 13.8 343 0.01 82 0.839 10 75.14 0.130 11 265.7 18 451 0.025 81.99 0.839 11 53.73 0.172 11.4 265.6 16.9 463 0.04 75.84 0.842 10 59.57 0.150 10.8 270.9 15.7 427 0.05 73.19 0.842 12 66.6 0.194 11.6 271.3 15.6 431

III.Conclusion Citric acid assisted hydrothermal route was used to

prepare Gd-substituted Zn-Fe ferrite. Superparamagnetic monodispersed nanoparticles with the mean size of about 10 nm and high saturation magnetizations were successfully synthesized by this method. This is a facile, low energy and environmental friendly method which leads to the formation of single phase magnetic nanoparticles at 180°C, using citric acid as the only surfactant in the process. Moreover, ferrofluids prepared directly from the resulting particles present monodispersed hydrodynamic sizes with mean sizes below 100 nm and the polydispersity indexes below 0.2. The observed changes in MS for Fe0.7 Zn0.3 Gdy Fe2-yO4 family are influenced by different values of y. The maximum saturation magnetization of Gd substituted Zn–Fe ferrite nanocrystals was strongly dependent on the y value and varied from 80 to 82 emu/g when y was increased from 0.0 to 0.05, respectively. the obtained Gd substituted Zn-Fe ferrite nanoparticles can be useful for MRI application,

through manipulation with chemical composition and synthesis procedure.

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