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Feb 18, 2010 - A Simple Method To Construct Bifunctional Fe3O4/Au Hybrid Nanostructures and. Tune Their Optical ... electron microscopy (SEM), energy-...
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J. Phys. Chem. C 2010, 114, 4297–4301

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A Simple Method To Construct Bifunctional Fe3O4/Au Hybrid Nanostructures and Tune Their Optical Properties in the Near-Infrared Region Yang Wang,† Yuhua Shen,*,†,‡ Anjian Xie,*,†,‡ Shikuo Li,† Xiufang Wang,† and Yan Cai† School of Chemistry and Chemical Engineering, Anhui UniVerity, Hefei 230039, People’s Republic of China, State Key Laboratory of Coordination Chemistry, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed: October 18, 2009; ReVised Manuscript ReceiVed: January 6, 2010

In this paper, we have developed a simple, facile, and efficient approach to synthesize bifunctional Fe3O4/Au hybrid nanostructure using L-cysteine as a linker. The morphology, composition, crystallinity, and magnetism of the as-prepared nanocomposites were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), powder X-ray diffraction (XRD), and superconducting quantum interference device (SQUID). The results indicated that the spherical composite particles showed a monodispersity and supermagnetism. The surface of the composite spheres contained plentiful Au nanoparticles (NPs) smaller than 10 nm. And the absorption peak of the composite spheres could be conveniently tuned over a broad spectral range spanning from the visible to the near-infrared (NIR) by simply controlling the diameter of Fe3O4. This novel bifunctional hybrid nanostructure will open up an exciting opportunity in biomedical applications. Introduction Core/shell nanostructures have stimulated great interest in the past decades because of their enhanced optical, tunable surface properties, electronic properties, and catalytic properties, and consequent wide potential applications.1–4 Generally, the shell layer coated on the cores could protect the core from oxidation and enhance their stability and compatibility,5 and the outer layer materials could also provide a platform for surface modification and functionalization,6 and even provide a natural vehicle for obtaining the hybrid, multifunctional materials.7 Most of all, the physical and chemical properties of the core/shell nanostructures can be tailored conveniently by controlling their composition and the relative sizes of the core to shell. Over the past few years, Au NPs coated on various magnetic cores such as Fe3O4@Au, γ-Fe2O3@Au, have attracted extensive attention due to the low reactivity, high chemical stability, biocompatibility, and good affinity for binding to amine (-NH2) or thiol (-SH) terminal groups of the outer Au layer.8 These materials of magnetic cores with an Au shell have been largely used in the fields of protein separation,9–11 drug delivery,12 cell separation,13 catalysis,14,15 detection,16 biological sensing and probing,17,18 and targeted photothermal (PT) therapy.19,20 Especially in PT therapy, in which Au NPs are attached to targets, the absorbed light energy is quickly transformed into heat and eventually leads to irreparable target damage through thermal denaturation and coagulation or through mechanical stress caused by sudden bubble formation.21 The combination of targeted delivery, magnetic resonance imaging diagnosis, and NIR photothermal ablation would greatly increase the treatment efficacy and simultaneously minimize the damage to normal cells and tissues. * To whom correspondence should be addressed. Tel.: +86-551-5108090. Fax: +86-551-5107342. E-mail: [email protected] and [email protected]. † Anhui Univerity. ‡ Nanjing University.

In recent years, many research efforts have been focused on developing novel gold nanostructures to achieve surface plasma resonance in the NIR region. Halas and co-workers have developed 10 nm thick gold nanoshells supported on 110 nm diameter silica cores with a NIR absorption peak and demonstrated their use in photothermal ablation of cancer cells and tissue.22 Wang et al. have prepared Fe3O4@polymer@Au shell core-shell nanostructures by NH2OH reduction and seedinduced growth methods and found that the composites display magnetization and near-IR absorption.23 Guo et al. have synthesized Fe3O4/Au hybrid nanostructures by using 3-aminopropyltrimethoxysilane as a linker to adsorb Au nanoparticles which also displayed near-IR absorption.24 El-Sayed and coworkers have demonstrated that gold nanorods 20 nm in diameter and 78 nm in length have a longitudinal absorption mode in the NIR region and can also serve as a photothermal therapeutic agent.25 And Aurod-Fe3O4 “nano-pearl-necklaces” of 15 nm diameter Fe3O4 nanoparticles and gold nanorods 56 nm in length for photothermal therapy were also reported.26 However, it remains a challenge to develop a simple method to construct multifunctional nanostructures to facilitate targeted delivery and photothermal therapy while retaining a strong NIR absorption. Herein, we demonstrate a simple, facile, and efficient approach to fabricate Fe3O4/Au bifunctional hybrid microsized spheres. First, the magnetic core was synthesized by a solvothermal method in polyol medium, and then was modified by L-cysteine. The chloroauric acid ions (AuCl4-) were then easily attracted onto the surface of L-cysteine-functionalized Fe3O4 cores owing to static interactions. The superparamagnetic Fe3O4/ Au hybrid spheres were fabricated at last by using L-ascorbic acid as a reductive agent. The spherical Fe3O4/Au composite particles showed a supermagnetism and novelty tuned optical properties from the visible to the NIR by simply controlling the diameter of Fe3O4. To the best of our knowledge, this is the first report about using L-cysteine as a linker to construct Fe3O4/ Au bifunctional hybrid microsized spheres and researching the

10.1021/jp9099804  2010 American Chemical Society Published on Web 02/18/2010

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influence of the Fe3O4 diameter on Au nanoshell optical properties in the NIR region. This Fe3O4/Au hybrid nanostructure will be used as potential photothermal therapeutic agents and for catalysis, biological sensing, probing, and so on. Experimental Section Materials. Tetrachloroauric acid tetrahydrate (HAuCl4 · 4H2O), sodium hydroxide (NaOH), sodium acetate (NaAc), iron(III) chloride hexahydrate (FeCl3 · 6H2O), ethylene glycol (EG), polyethylene glycol (PEG), and ethanol were obtained from Shanghai Chemical Reagent Co. Ltd. (China), and L-cysteine and L-ascorbic acid were purchased from Sinopharm Chemical Reagent Co. Ltd. (China). All chemicals were A.R. grade and used without further purification. Double distilled water was used in our experiments. Synthesis of Fe3O4/Au Hybrid Spheres. The Fe3O4 nanoparticles with high saturation magnetization were synthesized by a solvothermal method in polyol medium according to the procedure published by Li and co-workers.27 With different reaction times, different diameters of Fe3O4 nanoparticles such as 330, 380, 420, and 450 nm were synthesized, respectively. The as-prepared magnetic Fe3O4 nanoparticles (20 mg) were then dissolved in 20 mL of deionized water and the pH value was adjusted to 4.0-5.0 by addition of 2 M HCl solution. A 5 mL sample of 1 g/L L-cysteine was added dropwise into the Fe3O4 NPs suspension. The mixture was then sonicated for 30 min, followed by addition of 5 mL of chloroauric acid (3 × 10-3 M HAuCl4 · 4H2O), and the mixed solution was vigorous stirring for another 30 min. Then 5 mL of 1.0 wt % L-ascorbic acid was quickly added and reaction was allowed for 3 h under rapid stirring. At last, the products were separated magnetically and washed with deionized water and ethanol several times to eliminate organic and inorganic impurities, and then dried in vacuum at 40 °C for 24 h. Characterization. UV-vis spectra were measured with a TU-1901 model UV-vis double beam spectrophotometer (Beijing Purkinje General Instrument Co., Ltd., China). FTIR spectra were performed and recorded with a Fourier-transform infrared spectrophotometer Nicolet 870 between 4000 and 400 cm-1, with a resolution of 4 cm-1. Field emission scanning electron microscopy (FESEM) measurements were taken with a Hitachi S4800 scanning electron microscope. Transmission electron microscopy measurements were performed on JEM model 100SX electron microscopes (Japan Electron Co.) operated at an accelerating voltage at 80 kV. The phase structure and phase purity of the as-synthesized products were examined by X-ray diffraction (XRD), using a MAP18XAHF instrument, with the X-ray diffractometer using Cu Ka radiation (λ ) 1.5 Å) at a scan rate of 0.04° 2θ s-1. The accelerating voltage and applied current were 36 kV and 20 mA, respectively (MAC Science, Japan). The magnetic properties were measured with a Quantum Design Magnetic Properties Measurement System (MPMS) XL-7 Superconducting Quantum Interference Device (SQUID). The products were dispersed in ethanol for absorption experiments. Results and Discussion The morphology and structure of the as-synthesized produces were investigated by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). Figure 1a-d shows the TEM images of four diameters of Fe3O4 nanospheres with ∼330, ∼380, ∼420, and ∼450 nm, respectively. It can be easily seen from the images that the Fe3O4 nanospheres have a relatively smooth surface and disperse compactly possibly because of the magnetic dipole-dipole

Figure 1. TEM images of as-prepared Fe3O4 nanoparticles with different diameters: (a) ∼330, (b) ∼380, (c) ∼420, and (d) ∼450 nm. Fe3O4/Au hybrid nanospheres with different diameters: (e) ∼330, (f) ∼380, (g) ∼420, and (h) ∼450 nm.

attractions.28 Figure 1e-h displays the TEM images of the composite particles which correspond to the above Fe3O4 nanospheres (a-d), respectively. In comparing Figure 2a-d with Figure 2e-h, the diameter of the magnetite nanospheres was not obviously changed. But, it is interesting to observe that plentiful dark nanoparticles with a size of about 10 nm were coated on the core surface of Fe3O4. Comparing panels a and e of Figure 1 representatively, the spheres distribution changes from compactness to sparseness; this may be caused by the small sized diamagnetic particles coated on the surface of Fe3O4 spheres, which decrease the magnetization of Fe3O4. The representative SEM image and size distribution were shown in images a and b of Figure 2. The as-prepared composite particles exhibit a spherical shape with a diameter in the range of 330-340 nm (Figure 2a). From Figure 2b (partial high magnification of panel a), it can be clearly found that the spheres present a rough surface consistent with plentiful NPs smaller than 10 nm, which coincides well with the TEM results (Figure 1e-h). The spectrum of electron dispersive spectroscopy (EDS) of the spherical particles is shown in Figure 2c. There the peaks of Au, Fe, and O elements present, and the atomic ratio of Fe/O is near 3:4, suggesting that the hybrid spheres may contain Fe3O4 and Au.

Bifunctional Fe3O4/Au Hybrid Nanostructures

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Figure 4. Room temperature magnetization curves of (a) Fe3O4 nanoparticles and (b) Fe3O4/Au hybrid nanoparticles.

Figure 2. SEM images of as-prepared hybrid nanospheres: (a) 330 nm diameter of Fe3O4/Au hybrid nanopheres; (b) partial highmagnification of panel a; and (c) EDX analysis of the Fe3O4 /Au hybrid nanopheres.

Figure 5. UV/visible absorption spectroscopy of as-prepared Fe3O4/ Au hybrid nanopheres with different diameters of Fe3O4 spheres with reducing 5 mL of HAuCl4: (1) 450, (2) 420, (3) 380, and (4) 330 nm.

Figure 3. XRD patterns of (a) Fe3O4 and (b) Fe3O4/Au hybrid nanoparticles.

Figure 3 shows the X-ray diffraction patterns of the asprepared Fe3O4 nanoparticles and Fe3O4/Au hybrid spheres. Six major reflections appearing in curve a located at about 30.4°, 35.4°, 43.2°, 53.4°, 57.2°, and 62.7° can be assigned to diffraction of the Fe3O4 crystal with inverse spinel structure from the (220), (311), (400), (422), (511), and (440) (JCPDS card no. 19-0629), respectively. No other peaks were observed in curve a, indicating that the spheres are pure Fe3O4 crystalline phase. Compared with curve a, there are other diffraction peaks at 38.1°, 44.3°, and 64.6° present in curve b, which corresponds to the (111), (200), and (311) planes of the gold crystal with a cubic phase (JCPDS card no. 04-0784).29 And it can be found that the characteristic peak (2θ ) 35.4°) of Fe3O4 becomes weak in curve b due to the heavy atom effect of Au.30 The above results suggested that composite particles are Fe3O4/Au hybrid nanostructures, and dark nanoparticles coated on the core surface are possibly Au NPs. This further confirmed the hypothesis supposed by EDS results. The magnetic properties of the as-prepared Fe3O4 NPs and the Fe3O4/Au hybrid NPs were studied by using a superconducting quantum interference device (SQUID) magnetometer at room temperature (300 K). As shown in Figure 4, for the two samples, the coercivity force was almost negligible at 300 K, which indicated that Fe3O4 NPs and Fe3O4/Au hybrid NPs were superparamagnetic at room temperature. And it can be calculated that the saturation magnetization (Ms) of Fe3O4 NPs was about 75 emu/g (curve a), and the Ms of Fe3O4/Au hybrid particles decreased to 67 emu/g (curve b). We think the decrease

Figure 6. FTIR spectrum of (a) L-cysteine; (b) L-cysteine-functionalized Fe3O4; and (c) Fe3O4.

in magnetization is not only due to the increased mass of the nanoparticle introduced by the gold nanoshell but also the diamagnetic contribution of the Au NPs on the surface of the Fe3O4 core.31 The strong magnetization of the hybrid NPs should facilitate protein separation, photothermal therapeutics, catalysis, biological sensing, and so on. UV/visible absorption spectroscopy experiments were carried out to confirm that the as-prepared Fe3O4/Au composite spheres display the NIR absorption of Au NPs. From Figure 5, it can be found that the composite spherical particles with a core diameter of 450 nm showed absorption at 625 nm without absorption in the NIR region (curve 1). When the diameter of Fe3O4 decreased to 420 and 380 nm, the peak of the composite spheres slightly shifted to longer wavelength (683 and 706 nm, seen in curves 2 and 3), still no obvious NIR absorption was observed. Once the diameter of Fe3O4 decreased to 330 nm, the Fe3O4/Au composite spheres showed a strong absorption in the NIR region of 730 nm (seen in curve 4). Generally, spherical Au NPs with a diameter of 5 nm present the surface plasmon band at 520 nm in ethanol, but it is very sensitive to the size, shape, interparticle distance, and environment (dielectric properties) of the Au NPs.32 We think the following three reasons resulted in the peak of the composites red-shifting: (1) The rough

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SCHEME 1: Schematic Depiction of the Fabrication of the Fe3O4 /Au Hybrid Nanostructure

surface of Fe3O4/Au composites (shown in Figure 1) enhanced the local electromagnetic field, which influenced the optical properties of the composite spheres.33,34 (2) The strong localized electromagnetic field produced by the short gaps between the Fe3O4 and Au particles caused the absorption to be redshifted.35,36 (3) The absorption of Fe3O4/Au composites was significantly red-shifted by the diameter decrease of the Fe3O4 core. Comparing with previous work, Wang et al. report that the maximum absorption peak of Fe3O4@polymer@Au coreshell nanostructures presented a red-shift by increasing the thickness of the Au shell with a 258 nm diameter Fe3O4 core,23 we find an interesting phenomenon that the ratio decrease of Fe3O4 core radius/Au shell thickness could make the surface plasmon resonance wavelength shifted from the visible to the NIR region. However, Jain et al reported that the plasmon peak of Au displayed a red-shift along with the silica core radius increase of the SiO2@Au nanocomposites.37 Oldenburg et al indicated that when the diameter of the silica core was 120 nm, the optical resonances of the Au shell shifted into the NIR spectral range as the thickness of gold nanoshells decreased.38 The above results demonstrated that the plasmon peak of Au NPs could be tuned from the visible to the NIR region with the ratio increase of SiO2 core radius/Au shell thickness, which was different from our experiment results owing to the change of core composition. From the above discussion, we could conclude that core composition and a different core radius/shell thickness ratio of the nanocomposites can influence the surface plasmon resonance wavelength of the Au nanoshell. To our knowledge, this is the first report about simply controlling the core diameter of Fe3O4, and the absorption peak of Fe3O4/Au composites can be conveniently tuned over a broad spectral range spanning from the visible to the near-infrared (NIR). To testify to the adsorption of L-cysteine on the Fe3O4 spheres surface, Figure 6 shows typical FTIR spectra of (a) L-cysteine (b) L-cysteine-functionalized Fe3O4, and (c) Fe3O4. The broad bands of two samples (b and c) at about 585 cm-1 both corresponded to Fe-O vibration modes.39 Compared to the uncoated magnetic nanoparticles, FTIR spectra of L-cysteinefunctionalized Fe3O4 (Figure 6b) exhibited the characteristic absorption peaks of the hydrosulfide group, the amino group, and the carbonyl group of L-cysteine, such as 1061, 1621, and 1375 cm-1 due to the C-N, -CO-, and -CH- stretching vibrations and the band at 2540 cm-1 assigned to the S-H stretching vibration.40,41 This indicated that L-cysteine was bonded on the surface of Fe3O4 spheres through the condension interaction between the -COOH group of L-cysteine and the -OH group on the Fe3O4 surface. Uncoated magnetic NPs (Figure 6c) also showed a O-H strong absorption peak at around 3405 cm-1 due to the OH groups on the surface of Fe3O4 spheres.27 On the basis of the above results, multifunctional structures of the Fe3O4/Au hybrid nanoparticles with a rough surface were fabricated as schematically outlined in Scheme 1. (1) The

monodispersed Fe3O4 NPs were prepared according to a modified procedure described previously and the as-prepared magnetic Fe3O4 spheres were hydrophilic.27 (2) L-cysteine was bound to the surface of the Fe3O4 spheres by the condension reaction between the COOH group of L-cysteine and OH groups on the surface of Fe3O4 spheres. (3) The chloroauric acid ions (AuCl4-) were successfully adsorbed on the surface of Fe3O4 due to electrostatic interaction between positively charged L-cysteine-functionalized Fe3O4 and negatively charged chloroauric acid ions (the L-cysteine has a positive charge when the pH of the solution is at 4.0-5.0; the isoelectric point of L-cysteine is 5.07). (4) L-ascorbic acid was used as a reductive agent to obtain Au NPs. The gold nanoparticles were successfully adsorbed on the surface of Fe3O4 spheres due to strong coordinative interactions between the NH2 and SH groups of L-cysteine and Au NPs.14 Conclusions In summary, a simple, feasible, and efficient route to obtain bifunctional hybrid nanostructures based on Fe3O4 spheres has been explored. Our studies demonstrate that both the strong magnetization of the magnetite spheres and the NIR optical absorption of the gold NPs are retained in the bifunctional hybrid nanostructures. The combined functionalities will open up many exciting opportunities in targeted delivery, diagnosis, and therapeutics. Furthermore, this experiment also suggested a simple way to synthesize various bifunctional or multifunctional composite nanomaterials through simply linking two or several kinds of nanomaterials by chemical bonds. Further research on several other important applications such as biosensors, nanodevices, and electrocatalysis is underway. Acknowledgment. This work is supported by the National Science Foundation of China (Grants 20871001, 20671001, 20731001), the Major Program of Anhui Provincial Education Departmen (Grant ZD2007004-1), the Research Fund for the Doctoral Program of Higher Education of China (20070357002), and the Functional Material of Inorganic Chemistry of Anhui Province. References and Notes (1) Schartl, W. AdV. Mater. 2000, 12, 1899. (2) Caruso, F. AdV. Mater. 2001, 13, 11. (3) Skumryev, V.; Stoyanov, S.; Zhang, Y.; Hadjipanayis, G.; Givord, D.; Nogues, J. Nature 2003, 423, 850. (4) Zeng, H.; Li, J.; Wang, Z. L.; Liu, J. P.; Sun, S. H. Nano Lett. 2004, 4, 187. (5) Goia, D. V.; Matijevic, E. New J. Chem. 1998, 22, 1203. (6) Ge, J. P.; Hu, Y. X.; Zhang, T. R.; Yin, Y. D. J. Am. Chem. Soc. 2007, 129, 8974. (7) Velikov, K. P.; Moroz, A.; Van Blaaderen, A. Appl. Phys. Lett. 2002, 80, 49. (8) Daniel, M. C.; Astruc, D. D. Chem. ReV. 2004, 104, 293. (9) Bao, J.; Chen, W.; Liu, T. T.; Zhu, Y. L.; Jin, P. Y.; Wang, L. Y.; Liu, J. F.; Wei, Y. G.; Li, Y. D. ACS Nano 2007, 1, 293.

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