Facile Gold-coated Maghemite Nanoparticles Fabrication

Quanguo He §, Jingke Huang , Rong Hu, Lei Zeng

Green Packaging and Biological Nanotechnology Laboratory, Hunan University of Technology, Zhuzhou 412007, China

Email: hequanguo@126.com

Abstract—The maghemite (γ-Fe2O3) nanoparticles were obtained via high temperature pyrolysis with the precursor of magnetite that was synthesized via chemical coprecipitation. Then the (3-aminopropyl) triethoxysilane (APTES) was used for the surface functionalization of γ-Fe2O3 nanoparticles. Finally the gold-coated maghemite (γ-Fe2O3/Au) nanoparticles are prepared via sonochemical synthesis of a solution mixture of hydrochloroauric acid (HAuCl4) and APTES-coated γ-Fe2O3 nanoparticles with further drop-addition of sodium citrate. The γ-Fe2O3/Au nanoparticles were characterized by X-ray powder diffraction (XRD), ultraviolet–visible spectroscopy (UV-Vis), transmission electron microscopy (TEM) and superconducting quantumn inteference devices (SQUID). The γ-Fe2O3/Au nanoparticles with double properties of magnetism and colloidal gold may have intensive application in immunology, biochemistry, biomedical and molecular biology.

Keywords-Maghemite; γ-Fe2O3/Au Nanoparticle; Gold Coating; Sonochemical Synthesis; Surface Modification

I. INTRODUCTION Magnetic nanoparticles (MNPs) have been increasingly

receiving considerable attention because their inherent magnetic properties enable them desirable for information storage, magnetic resonance imaging contrast agent, and macroscopic quantum tunnelling associated with size quantization and electronic quantum confinement effects [1-5], and so on. The maghemite (γ-Fe2O3) nanoparticles attract intensive interest due to their technological and fundamental importance. It is also an important material widely used in many industries including electronics, catalysis, dyes, biotechnology, and medicine. The use of maghemite depends strongly upon its inherent physiochemical properties such as density, crystallinity, particle size, and surface area. The other hand, Au nanoparticles (i.e. colloidal gold) are a fascinating material from both nanotechnology and biology perspectives, since they are harmless to the living body [6–10], they could combine firmly with biomolecules via the Au–S bonding [11–13], and possess an absorption band in the visible spectra because of the surface plasmon (SP) [14]. Gold nanoparticles also exhibit fascinating size dependent electric, optical properties (quantum size effect), thus endow themselves with versatile applications from catalysis to biology [15–20].

If gold is combined with MNPs appropriately, the biomolecules through direct or indirect Au–S conjugation

would be not only easily detected, but also selectively separated and manipulated by an external magnetic field. Some similar GoldMagnetic nanoparticles (GoldMag NPs) were reported to exhibit good biocompatibility and affinity via amine/thiol terminal groups bonding. For examples, Hye-Young Park et al. reported a kind of Fe Oxide@Au NPs for the protein binding and bioseparation [21]. Yoshiteru Mizukoshi et al. reported the composite of Au /γ-Fe2O3 for the separation of glutathione [22]. So the air-stable gold-coated maghemite nanoparticles are expected a wide range of potential applications in immobilization of proteins and enzymes [23], bioseparation [24-25], immunoassays [26], drug delivery and biosensors [27-28]. Consequently, tailing a MNPs surface with gold shell is a popular challenge in the realization of these purposes.

Hereby, we report a facile sonochemical synthesis of air-stable gold-coated maghemite nanocomposites. The results showed that the average particle diameter of γ-Fe2O3 was about 20-30 nm. The nanocomposites’ size and morphology changed little as compared with parent maghemite ones. After gold coating, the γ-Fe2O3/Au nanoparticles presented a SP peak of gold nanostructure at 588 nm. Various characterizations were performed to check corresponding properties variations.

II. EXPERIMENTAL PROCEDURES

A. Chemicals and reagents Ferric chloride (FeCl3·6H2O, 99.0%) is purchased from

Tianjin Bodi Chemicals Co., Ltd. Ferrous chloride (FeCl2· 4H2O, 99.0%) is purchased from Tianjin Shuangchuan Chemicals Co., Ltd. Sodium hydroxide (NaOH), ethanol (95.0%), hydrochloric acid and sodium citrate are purchased from Hunan Huihong Chemicals Co., Ltd. All chemicals used are of analytical grade. Water (T = 25 ℃ 18.2 MΩ) is purified by SUPER WATER-II water purification systems. (3-aminopropyl)triethoxysilane (APTES) was purchased from Sigma. Hydrogen tetrachloroaureate (III) trihydrate (HAuCl4, Au ≥ 47.8%) was a product of Shanghai Platinum Group Metals Chemicals Co., Ltd.

B. Instruments and characterization Transmission electron microscopy (TEM) images were

carried out on the Hitachi H-600 electron microscope (20 kV)

§Correspondence,Email:hequanguo@126.com Tel: +86-731-22182107

978-1-4244-4713-8/10/$25.00 ©2010 IEEE

for measuring morphology. X-ray powder diffraction (XRD) spectra were made on the Bruker Advanced-D8 powder diffractometer for analyzing the crystal phase of samples. The ultraviolet–visible spectroscopy (UV–Vis) and fourier transform infrared (FT-IR) spectrometer were used to compare the optical properties for each process of preparing the γ-Fe2O3/Au nanoparticles. The magnetic properties were characterized by superconducting quantumn inteference devices (SQUID). And muffle furnace is used to oxidize the Fe3O4 nanoparticles into γ-Fe2O3 nanoparticles by high temperature pyrolysis.

C. Experiment Principle Firstly, Fe3O4 nanoparticles were synthesized by chemical

coprecipitation as previously reported in [29]. Then the Fe3O4 nanoparticles were calcined as precursors in muffle furnace at high temperature. After that Fe3O4 could be oxidized into γ-Fe2O3 nanoparticles under atmosphere. The chemical reaction equation (1) was as follows:

3 4 2 3400( )o

OxidationC

Fe O Fe O at high temperatureγ⎯⎯⎯⎯→ − （1）

Secondly, APTES was used to modify the surface of γ-Fe2O3. As depicted in Fig. 1, the hydroxyl groups on γ-Fe2O3 surface were combined with APTES by Si-O bonds. Finally, gold-coated γ-Fe2O3 nanoparticles were prepared through sonochemical reaction involved the former synthesis of APTES-coated γ-Fe2O3 nanoparticles as seeds and a subsequent reduction of HAuCl4 in the presence of reductive agent.

Figure 1. Synthetic chemistry for γ-Fe2O3/Au nanoparticle preparation.

D. Synthesis of γ-Fe2O3 /Au nanoparticles 1) The synthesis of γ-Fe2O3 nanoparticles. The γ-Fe2O3

nanoparticles were prepared through high temperature pyrolysis. The Fe3O4 nanoparticles as precursors were obtained via chemical coprecipitation of Fe(II) and Fe(III) chlorides (Fe(II)/Fe(III) ratio = 1:2) with 1.5 mol NaOH. Dark precipitates were gradually produced during the reaction and then separated by magnetic field. The product was washed with ethanol two times and deionized water three times. The dried product was collected after vacuum drying. Then the dried presursors were oxidized in muffle furnace at 400 °C for 2 hours and cooled down to room temperature, the γ-Fe2O3 nanoparticles were obtained finally.

2) The synthesis of APTES-coated γ-Fe2O3 nanoparticles. For the surface functionalization of γ-Fe2O3 nanoparticles, 25 ml of ferrofluid (5 g of γ-Fe2O3/L of ethanol solution) was diluted to 150 ml with absolute ethanol and sonicated for 30

min. Then 0.4 ml of APTES was dropped into the mixture and the mixture was stirred at room temperature for about 7 h. After that the solid product was magnetically separated, washed with ethanol five times and then redispersed in ethanol (1 g of APTES-γ-Fe2O3/L of ethanol solution).

3) The synthesis of γ-Fe2O3/Au nanoparticles. Under ultrasonic irradiation of 90 Hz condition, 15 ml of the APTES-coated γ-Fe2O3 nanoparticles and 3 ml HAuCl4 solution (0.5 % mass fraction) were mixed in the round bottom flask of 100 mL. After 30 min, 20 mM sodium citrate was then dropped into the mixture solution until the colour changed from yellow to black. The dark material was precipitated and separated by a magnetic field and centrifugation. The precipitated product was washed with ethanol three times and redispersed in ethanol. The nanoparticle solution appeared dark purple.

III. RESULTS AND DISCUSSION

A. Morphology and Structure Fig. 2 showed the representative TEM micrograph of γ-

Fe2O3 (Fig. 2(A)) and γ-Fe2O3/Au nanoparticles (Fig. 2(C)). In Fig. 2(A), particles analysis indicated that the average diameter of the γ-Fe2O3 seeds was about 20-30 nm. Comparing the particles in Fig. 2(A) and Fig. 2(C), the Au-coated particles appeared much darker than pure γ-Fe2O3, and the average size of particles changes to about 30-40 nm. From the particle size analysis, it’s proved that particles were enlarged due to gold coating. The selected-area electron diffraction (SAED) pattern indicated the crystalline characteristics of γ-Fe2O3 (Fig. 2(B)) and γ-Fe2O3/Au (Fig. 2(D)) nanoparticles. The SAED pattern of γ-Fe2O3 nanoparticles could be indexed to (220), (311), (400), (422), (511) and (440), and the SAED pattern of γ-Fe2O3/Au nanoparticles could be indexed to (111), (200), (220), (311) and (222), which illustrating pure γ-Fe2O3 and Au-coated structures, respectively.

A

D C

B

Figure 2. Representative TEM images of γ-Fe2O3 (A) and γ-Fe2O3/Au (C) in a low concentration of ethanol solution, the selected-area electron diffraction

(SAED) pattern of the sample γ-Fe2O3 (B) and γ-Fe2O3/Au (D)

In Fig. 3, the curve 1 shows that the diffraction peaks location of γ-Fe2O3 at 2θ = 30.4°, 35.7°, 43.2°, 53.7°, 57.3° and 62.8°, which can be indexed to the (220), (311), (400), (422), (511) and (440) planes in a cubic phase (REF:00-039-1346), respectively. In the curve 2 the only peak at 2θ = 35.7° (the highest peak for γ-Fe2O3) corresponding to the structural planes (311) for the γ-Fe2O3 existing did not completely vanish but its intensity decreased, which due to penetration of X-rays inside the core indicates presence of γ-Fe2O3. Main diffraction peaks we obtained 2θ and structural planes corresponding to Au only (peaks at 2θ indicating indices corresponding to 111, 200, 220, 311, and 222 of the pure gold crystal planes).31 The results affirmed the above-mentioned results of TEM and SAED again, indicating both preparation of the maghemite nanopaticles and gold-coated ones is a success.

Figure 3. XRD diffraction peaks of γ-Fe2O3 (1) and Au-coated γ-Fe2O3(2) NP

B. Optical properties The FT-IR spectra of uncoated and APTES-coated and

Au-coated γ-Fe2O3 nanoparticles were analyzed with a FTIR spectrometer. As shown in Fig. 4(A 1), the Fe-O and -OH stretching vibration were at 581 cm-1 and 3380 cm-1, The intensity around 3417 cm-1 band in Fig. 4(A 2) probably contributed to the amino groups on the APTES-coated γ-Fe2O3 nanoparticles, which would be overlapped by the O-H stretching vibration band. The outstanding features in Fig. 4(A 2) were the appearance of the peaks at 1105, 1385, and 2921 cm-1, respectively. The Si-O stretching vibration observed at 1105 cm-1 significantly reveals that the covalent bonds of Fe-O-Si were formed after APTES modification. The bands around 2921 and 1385 cm-1 were assigned to -CH2 and C-N stretching vibration. The results provided the evidences that APTES had been bonded on the γ-Fe2O3 nanoparticles surface through silanization reaction with -OH groups. After glod-coating the acorrelative stretching vibration peaks were all weaken.

Figure 4. FT-IR Spectra comparison of γ-Fe2O3 nanoparticles (A 1), APTES-coated γ-Fe2O3 nanoparticles (A 2) and Au-coated γ-Fe2O3 nanoparticles (A

3); UV–vis spectra of γ-Fe2O3 nanoparticles (B 1), the APTES-coated γ-Fe2O3 nanoparticles (B 2), the mixture before reaction (B 3) and γ-Fe2O3 /Au (B 4)

(λ max = 588 nm)

Through comparing the UV-visible absorption date in Fig. 4(B), it suggested that the Au was coated on the maghemite thus the coated nanoparticles would enhance its optical properties. Although there are no strong absorption peak, it can be seen that the absorption of APTES-coated γ-Fe2O3 (curve 2) nanoparticles was stronger than that in γ-Fe2O3 ones (curve 1). Before or after the ultrasonic gold reductive reaction the absorption peak changed sharply. As showed in the curve 3 and curve 4, after the reaction there is a strong peak appeared at about 588 nm which was bigger than the typical 10 nm Au nanoparticles absorption peak of 520 nm. Einstein shift appeared due to the size increased after reaction on the base of γ-Fe2O3. Such a 588 nm absorption is contributed to SP of gold, since colloidal-gold-like gold-coated maghemite nanoparticle was formed after gold coating. The results were also consistent with the TEM results.

C. Magnetic properties The SQUID magnetometry reveals that the γ-Fe2O3

nanoparticles and Au-coated γ-Fe2O3 nanoparticles were both have good magnetic behaviour. Fig. 5(A) and (B) show that the hysteresis curve was given at different temperature of T=300 K(close to room temperature) and T=5 K, respectively. The saturation magnetization (Ms) of γ-Fe2O3 particles at T=5 K (curve 3) was found to be 110 emu·g-1 which was higher than that of 100 emu·g-1 at T=300 K (curve 1). The Ms of γ-

A

B

Fe2O3 /Au nanoparticles was found to be 65 emu·g−1 at 300 K (curve 2) and 70 emu·g−1 at 5 K (curve 4), respectively. All particles had relatively lower values of Ms at 300 K. The result was consistent with the relationship between Ms and temperature.32 By comparing Ms values in figure 4 (A) and (B) both at T=5 K and T=300 K, the corresponding Ms of γ-Fe2O3 (curve 1, 3) was higher than that of γ-Fe2O3 /Au (curve 2, 4).

Figure 5. Hysteresis loops of γ-Fe2O3 nanoparticles (curve 1, 3) and γ-Fe2O3/Au nanoparticles (curve 2, 4) at T=5K and T=300K, Photo of γ-Fe2O3

nanoparticles (A), γ-Fe2O3 /Au nanoparticles (B) and magnetic isolation photo of γ-Fe2O3 and γ-Fe2O3 /Au

Furthermore, the Fig. 5(C) shows that the γ-Fe2O3 nanoparticles (1) and after metal gold coating (2) is dispersed in ethanol solution. In the middle of Fig. 5(C), both the γ-Fe2O3 and Au-coated γ-Fe2O3 were still have a good magnetic response and it can be easily separated by magnetic separation. Meanwhile it also shows that the metalized Au coating affected the bulk magnetic properties of maghemite

little. Moreover the colour changes of the products were accorded with the results of UV-visible absorption.

IV. CONCLUSIONS In summary, the γ-Fe2O3 nanoparticles were synthesized

via high temperature pyrolysis at 400 °C, then APTES-functionalized γ-Fe2O3 was also obtained, at final step the maghemite NPs was used as core for reductive deposition of Au on their surface to completing gold coating under sonication. The particle size of γ-Fe2O3 was about 20±5 nm, and that of γ-Fe2O3/Au was about 30±5 nm. TEM and XRD were used to verify the compositions and structures of the former and the latter. By analyzing of surface compositions of element, the APTES-coating and gold-coating process was a success. All the characteristic stretching vibration bands of APTES-coated γ-Fe2O3 and gold-coated one were found and compared via spectral analysis. After gold coating, the γ-Fe2O3/Au nanoparticles presented a SP peak of gold nanostructure at 588 nm. Furthermore, magnetic characterization showed strong magnetic responses before or after gold-coating. The results reveal that the gold-coated maghemite nanopaticles were prepared via a simple method, and such gold-magnetic nanopaticles would offer a competitive alternative material used for immunoassays, biochemical sensors and targeted drug delivery.

ACKNOWLEDGMENT The authors gratefully acknowledge the financial support

from the Chinese 863 High-Tech Project (2006AA03Z357), the National Natural Science Foundation of China (60871007, 20505020), and the Scientific Research Fund of Hunan Provincial Education Department (08A013).

REFERENCES [1] J.L. Dormann, D. Fiorani, “Magnetic Properties of Fine Particles,”

North-Holland, Amsterdam, in press. [2] T. Hyeon, S.S. Lee, J. Park, Y. Chung, H.B. Na, “Synthesis of highly

crystalline and monodisperse maghemite nanocrystallites without a size-selection process,” J. Am. Chem. Soc., vol. 123, pp. 12798-12801, November 2001.

[3] A.O. Caldeira, A.J. Leggett, “Influence of Dissipation on Quantum Tunneling in Macroscopic Systems,” J. Phys. Rev. Lett., vol. 46, pp. 211-214, 1981.

[4] J. Tejada, X.X. Zhang, Li Balcells, “Nonthermal viscosity in magnets: Quantum tunneling of the magnetization (invited),” J. Appl. Phys., vol. 73, pp. 6709, May 1993.

[5] L. Zhang, G.C. Papaefthymiou, J.Y. Ying, “Synthesis and Properties of γ-Fe2O3 Nanoclusters within Mesoporous Aluminosilicate Matrices,” J. Phys. Chem. B, vol. 105, pp. 7414-7423, July 2001.

[6] R.C. Mucic, J.J. Storhoff, C.A. Mirkin, R.L. Letsinger, “DNA-directed synthesis of binary nanoparticle network materials,” J. Am Chem Soc., vol. 120, pp. 12674-12675, November 1998.

[7] J.J. Storhoff, A.A. Lazarides, R.C. Mucic, C.A. Mirkin, R.L. Letsinger, G.C. Schatz, “What controls the optical properties of DNA-Linked gold nanoparticle assemblies?” J. Am Chem Soc., vol. 122, pp. 4640-4650, April 2000.

[8] W. Fritzshe, “DNA-gold conjugates for the detection of specific molecular interactions ,”Rev. Mol. Biotechnol., vol. 82, pp. 37, November 2001.

A

C

B

[9] T. Liu, J. Tang, L. Jiang. “Sensitivity enhancement of DNA sensors by nanogold surface modification,” Biochem. Biophys. Res. Commun., vol. 295, pp.14, June 2002.

[10] L. Ren, G.M.Chow, Mat Sci Eng C-Biomimetic Supramol Syst., C23,113 (2003).

[11] R.G. Nuzzo, D.L. Allara, “Adsorption of bifunctional organic disulfides on gold surfaces,” J. Am Chem Soc., vol. 105, pp. 4481-4483, June 1983.

[12] C.A. Mirkin, R.L. Letsinger, R.C. Mucic, “A DNA-based method for rationally assembling nanoparticles into macroscopic materials,” J.J. Storhoff, Nature., vol. 382, pp. 607-609, August 1996.

[13] C.H. Kiang, “Phase transition of DNA-linked gold nanoparticles,” Physica A., vol. 321, pp. 164-169, December 2003.

[14] J.A. Creighton, D.G. Eadon, “Ultraviolet–visible absorption spectra of the colloidal metallic elements,” J. Chem Soc Faraday Trans, vol. 87, pp. 3881-3891, 1991.

[15] M.C. Daniel, D. Autruc, “Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology,” Chem. Rev., vol.104, pp.293-346, 2004.

[16] J.P. Wilcoxon, J.E. Martin, F. Parsapour, B. Wiedenman, D.F. Kelley, “Photoluminescence from nanosize gold clusters,” J. Chem. Phys., vol. 108, pp. 9137-9143,1998.

[17] M. Haruta, “Catalysis of gold nanoparticles deposited on metal oxides,” Cattech, vol. 6, pp. 102-115, June 2002.

[18] S. Link, M.A. El-Sayed, “Optical properties and ultrafast dynamics of metallic nanocrystals,” Annu. Rev. Phys. Chem., vol. 54, pp. 331–366, October 2003.

[19] S. Link, M.A. El-Sayed, “Spectral properties and relaxation dynamics of surface plasmon electronic oscillations in gold and silver nanodots and nanorods,” J. Phys. Chem. B, vol. 103, pp. 8410-8426, September 1999.

[20] S. Link, M.A. El-Sayed, “Size and temperature dependence of the plasmon absorption of colloidal gold nanoparticles,” J. Phys. Chem. B, vol. 103, pp. 4212-4217, May 1999.

[21] H.Y. Park, M.J. Schadt, L.Y Wang, I.S. Lim, P.N. Njoki, S.H. Kim, et al. “Fabrication of Magnetic Core@Shell Fe Oxide@Au Nanoparticles for

Interfacial Bioactivity and Bio-separation,” J. Langmuir, vol. 23, pp. 9050-9056, July 2007.

[22] Y. Mizukoshi, S. Seino, K. Okitsu, T. Kinoshita, Y. Otome, T. Nakagawa, T.A. Yamamoto, Ultrasonics Sonochemistry., vol. 12, pp. 191-195, 2005.

[23] D.H. Chen, M.H. Liao, “Preparation and characterization of YADH-bound magnetic nanoparticles,” J. Mol. Catal. B Enzym., vol. 16, pp.283-291, February 2002.

[24] S. Rooth, “Biological and biomedical aspects of agnetic fluid technology,” J. Magn. Magn. Mater., vol. 122, pp. 329-334. 1993.

[25] S.V. Sonti, A. Bose, “Cell Separation Using Protein-A-Coated Magnetic Nanoclusters,” J. Colloid Interface Sci., vol. 170, pp. 575, 1995.

[26] L Wang, J Bao, L Wang, F Zhang, Y D Li. “One-Pot Synthesis and Bioapplication of Amine-Functionalized Magnetite Nanoparticles and Hollow Nanospheres,” Chem. Eur. J.,vol. 12, pp. 6341–6347, 2006.

[27] S.R. Rudge, T.L. Kurtz, C.R. Vessely, L.G. Catterall, D.L.Williamson, “Preparation, characterization, and performance of magnetic iron–carbon composite microparticles for chemotherapy,” Biomaterials, vol. 21, pp.1411, May 2000.

[28] A.R. Varlan, W. Sansen, L.A. Van, M. Hendrickx, “Covalent enzyme immobilization on paramagnetic polyacrolein beads,” Biosens. Bioelectron., vol. 11, pp. 443, August 1996.

[29] W. Wu, Q.G. He, H. Chen, J.X Tang, L.B. Nie, “Sonochemical Synthesis, Structure and Magnetic Properties of Air-stable Fe3O4/Au Nanoparticles,” Nanotechnol., vol. 18, pp.145609, March 2007.

[30] J.M. Perez, F.J. Simeone, Y. Saeki, L. Josephson and R. Weissleder, “Viral-Induced Self-Assembly of Magnetic Nanoparticles Allows the Detection of Viral Particles in Biological Media,” J. Am. Chem. Soc., vol. 125, pp. 10192-10193, August 2003.

[31] M. Mandal, S. Kundu, S. K. Ghosh, S. Panigrahi, T.K. Sau, S.M. Yusuf and T. Pal, “Magnetite nanoparticles with tunable gold or silver shell,” J. Colloid and Interface Sci., vol. 286, pp. 187-194, February 2005.

[32] R.C. O’Handley, “Modern magnetic materials—principles and applications”, J. John Wiley & Sons Press, New York, 1999.