Synthesis of gold-coated iron oxide nanoparticles

Synthesis of gold-coated iron oxide nanoparticles

Journal of Non-Crystalline Solids 356 (2010) 1233–1235 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids j o u r n a l h o...

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Journal of Non-Crystalline Solids 356 (2010) 1233–1235

Contents lists available at ScienceDirect

Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l

Synthesis of gold-coated iron oxide nanoparticles E. Iglesias-Silva a,b, J.L. Vilas-Vilela a, M.A. López-Quintela b, J. Rivas b, M. Rodríguez a, L.M. León a,⁎ a b

Department of Physical-Chemistry, University of the Basque Country, Faculty of Science and Technology, Barrio Sarriena, s/n E-48940, Leioa, Bizkaia, Spain Laboratory of Magnetism and Nanotechnology (NANOMAG), Institute of Technological Research, University of Santiago de Compostela, E-15782 Santiago de Compostela, A Coruña, Spain

a r t i c l e

i n f o

Article history: Received 22 July 2009 Received in revised form 15 April 2010 Available online 20 May 2010 Keywords: Iron oxide; Gold; Core–shell; Optical properties; Magnetic properties

a b s t r a c t A microemulsion method has been used for the synthesis of iron oxide nanoparticles and its aqueous stabilization. Iron oxide nanoparticles of 9±2 nm in size were then coated with gold in a second step. For this purpose HAuCl4 was reduced employing glucose as a reducing agent. The mild conditions promote the reduction of Au ions adsorbed onto the γ-Fe2O3 particles in order to ensure a controlled shell growth of gold, and avoiding the formation of new gold nuclei. Different molar ratios of HAuCl4/glucose were investigated. The obtained core–shell structures have been characterized by TEM, UV spectroscopy, XRD and Magnetometry measurements. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The last advances in the atomic scale investigation gives us a new discipline called Nanomedicine which applies techniques proceeding from the Nanotechnology in human health [1–6]. Such techniques allow the obtaining of structures similar in size to biomolecules which are capable to interact with human cells. Most of the applications involve the bioferrofluids development, that means, biocompatible colloidal suspensions of magnetic nanoparticles [7–10] which can be coated by organic or inorganic materials allowing its functionalization for specific applications. The aim of our study is the synthesis of core– shell nanoparticles, with a controlled particle size and size distribution, in order to obtain biocompatible colloidal suspensions, and the study of their magnetic properties. The encapsulation at nanometric level of iron oxide particles with noble metals, like gold or silver, allows to obtain biocompatible nanoparticles because of the good tolerance of the human body to these noble metals. In a previous publication [11] the synthesis of silver coated iron oxide nanoparticles was reported and in this work the results obtained for gold-coated nanoparticles are presented.

2. Chemical and experimental section

Aldrich Chemical, clorhidric and nitric acids were obtained from Merck, and solution of 10% tetrametylamonium hidroxide (TMAOH), from Fluka. Gold (III) chloride trihydrate was obtained from Alfa Aesar. All of them were used as received. A microemulsion method was used to the synthesis of iron oxide nanoparticles and its aqueous stabilization [12,13], confirming the formation of the nanoparticles the black colour acquired by the mixture. The prepared iron oxide particles were then separately coated with gold. The amount of HAuCl4 used was calculated assuming a complete covering of the magnetic cores (of 9 nm size) with a 2 nm gold shell. For the reduction a mild reducing agent, glucose, was employed in order to ensure a controlled shell growth of gold onto iron oxide nanoparticles and avoiding the formation of new gold nuclei. The mild conditions promote the reduction of Au (I) ions adsorbed onto iron oxide particles. We investigated different molar ratio of glucose/HAuCl4 to analyze his influence in the shell growth. The synthesis was carried out at room temperature. As the particles were gradually coated by gold, the black iron oxide particles turned reddish or purple depending on the size of the shell. The final solution was washed using a chloroform/methanol (1:1) mixture. Then, the nanoparticles were isolated by precipitation with acetone and, after magnetic separation, they were washed several times with acetone and ethanol followed by drying under vacuum to obtain the final product.

2.1. Synthesis of iron oxide core-Au shell nanoparticles 2.2. Characterization Ferrous (III) chloride, ferrous (II) sulfate, cyclohexane, cyclohexilamine, Brij-97 and D(+)-glucose anhydrous were purchased from ⁎ Corresponding author. Tel.: +34 94 601 27 10; fax: +34 946 015 535. E-mail address: [email protected] (L.M. León). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.04.022

Particle size distributions were characterized by Transmission Electron Microscopy (TEM) with a 200 kV ultrahigh-resolution analytical electron microscope JEOL JEM-2010, and the microanalysis was purchased by a Microanalysis EDS, Oxford Inca Energy 200.

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Fig. 1. UV–Vis absorption of γ[email protected] at different R.

UV–visible spectra were measured with a Hewlett-Packard 8452A Diode-Array Spectrophotometer. The crystalline structure of the powders was studied by X-ray diffraction (XRD) with a Philips PW-1710 X-ray diffractometer using Cu Kα radiation with a wavelength of 1,54056 Å. Magnetization measurements were recorded with a Quantum Design PPMS Model 6000 magnetometer.

3. Results and discussion The UV–VIS spectra of the obtained iron oxide nanoparticles, shows an absorption band at a wavelength near to 520 nm. This result can be transformed to obtain the optical band gap Eg using Tauc's expression [14]: (αhν) = A(hν − Eg)n, where α is the absorption coefficient, A is a constant, and n is equal to ½ for allowed direct transitions and 2 for allowed indirect transitions. The linear region in the plot of (αhν)1 / n vs. (hν), has been extrapolated to obtain a value for the indirect or direct band gap. In this case, both of them can be adjusted to a straight line, so it´s difficult to make conclusions. However, the obtained band gap allowed direct transitions is near the theoretical value of the maghemita [15] (2,5 eV), so we can conclude that is the iron oxide we have. The absorption peak near 520 nm, due to the surface plasmon resonance band of the gold, confirms the presence of gold coating the iron oxide particles. Fig. 1 shows the behaviour of the plasmon band with the change in the relation between the reductor and the gold salt (R). It can be seen how, as R increases, the plasmonic bands go to right and they have higher intensity, which can be related with a higher size of nanoparticles. Moreover, as it can be seen in the Fig. 1, depending

Fig. 3. X-ray diffraction patterns for the nanoparticles.

on the size of the shell, the nanoparticle dispersions display different colours, from violet to red. Fig. 2 shows the images obtained by transmission electronic microscopy (TEM). As it can be seen, the coated nanoparticles are spherical and their monodispersity being improved as the reductor amount decreases. The obtained sizes are between 13 nm and 11 nm for the lowest R value, this tendency agreeing with the UV–Vis results. This behaviour could be due to the relative lower amount of reductor available for coating the iron oxide nanoparticles. Besides, in consequence, the width of the gold shell would be more homogeneous improving the size distribution. The theoretical calculated gold shell (2 nm) gives a calculated size for the core–shell nanoparticles in very good agreement with the experimental ones as it has been determined by HRTEM. Moreover, the results of microanalysis carried out for some particles has revealed the presence of Fe and Au in the nanoparticles corroborating the coating of the iron oxide cores. XRD diffractograms for different nanoparticles are shown in Fig. 3 along with the patterns of gold and NaCl (this has been used to precipitate the nanoparticles and in occasions appears like a rest). It should be noticed that the diffraction peaks from γ-Fe2O3 are not observed in the γ[email protected] nanoparticles, however the peaks that appear are agree with gold pattern, suggesting a complete coating of the iron oxide by gold [16,17]. Magnetic properties of the γ[email protected] nanoparticles were derived from zero field cooled–field cooled (ZFC-FC) magnetization as a function of temperature from 4 to 300 K under an applied field of 1000 Oe. The Fig. 4 shows the results for γ[email protected] samples using different relations between reductor and gold salt. A clear decrease of the magnetization (at least 6 times smaller than 37.4 emu/g obtained for the uncoated ones) is found for the coated nanoparticles. This result agrees

Fig. 2. a. γ-Fe2O3 nanoparticles (9 ± 2 nm); b, c, and d. γ[email protected] nanoparticles, obtained at different R values: b: (1:2)(13 ± 3 nm); c: (1:4)(12 ± 2 nm); and d: (1:8)(11 ± 1 nm).

E. Iglesias-Silva et al. / Journal of Non-Crystalline Solids 356 (2010) 1233–1235

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UV–visible spectra, Transmission Electron Microscopy (TEM), X-ray diffraction (XRD) and magnetometry (a preliminary study of the magnetic properties shows a large decrease of the magnetization for the coated magnetite nanoparticles in comparison with the uncoated ones).

Acknowledgments Financial support from the Basque Government (Actimat Consortium, ETORTEK 05/01), and the MEC, Spain (MAT2005-07554-C02-01; NAN2004-09195-C04-01) is gratefully acknowledged.

References

Fig. 4. Magnetization measurements of γ[email protected] nanoparticles with different R.

with recent results for Ag [11] or Au [18] coated iron oxide, and it could be explained taking into account that the coated nanoparticles contain less magnetic material per gram than the original ones and the decreased coupling of the magnetic moments as a result of the increased interparticle spacing of magnetic cores. The blocking temperature, however, remain constant (TB = 25 K) indicating that the original nanoparticles remain unmodified. 4. Conclusions Iron oxide nanoparticles of 9 ± 2 nm has been prepared by the microemulsion method and, in a second step, the nanoparticles have been coated by a shell of Au. The characterization of the nanoparticles shows how the inicial nuclei have been completely coated by the metal, and depending on the amount of reductor different shells size have been obtained as can be confirmed by the results obtained from

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