Ag-Coated Fe3O4@SiO2 Three-Ply Composite Microspheres

Apr 14, 2010 - Dan Song , Rong Yang , Chongwen Wang , Rui Xiao , Feng Long ..... Shaofeng Zhang , Feng Ren , Wei Wu , Juan Zhou , Lingling Sun ...
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Ag-Coated Fe3O4@SiO2 Three-Ply Composite Microspheres: Synthesis, Characterization, and Application in Detecting Melamine with Their Surface-Enhanced Raman Scattering Haibo Hu, Zhenghua Wang,* Ling Pan, Suping Zhao, and Shiyu Zhu Anhui Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science, Anhui Normal UniVersity, Wuhu 241000, People’s Republic of China ReceiVed: January 6, 2010; ReVised Manuscript ReceiVed: March 24, 2010

Ag nanoparticles with average sizes of 20 nm were well-dispersed on the surfaces of Fe3O4@SiO2 composite microspheres through a simple wet-chemical method employing the Ag-mirror reaction. The as-synthesized Ag-coated Fe3O4@SiO2 three-ply composite microspheres are monodisperse and bifunctional, with ferromagnetic and surface-enhanced Raman scattering (SERS) properties. The products were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray analysis (EDX). SERS signals of typical analytes such as rhodamine 6G (Rh 6G) were observed on Ag nanoparticles from the Ag-coated Fe3O4@SiO2 microspheres, even though the concentration of the analyte was as low as 1 × 10-15 M (Rh 6G). The Ag-coated Fe3O4@SiO2 microspheres were applied to detecting melamine, and strong SERS signals were obtained with melamine concentration of 1 × 10-6 M. This work may provide a potential and unique technique to detect melamine. 1. Introduction In recent years, much attention has been attracted by the fabrication of magnetic nanoparticles and nanocomposites because they have both fundamental and practical values due to their potential applications in areas such as ferrofluids, medical imaging, drug targeting and delivery, cancer therapy, separations, and catalysis.1-5 Especially, magnetic nanocomposites which contain two or more different nanoscale functionalities are of special interest. Due to their particular structure and interface interactions, these magnetic nanocomposites can exhibit novel physical, chemical, and biologic properties that will be essential for future technological applications. The magnetic nanocomposites with core-shell structures are wellknown composite systems that have shown multiple properties such as adsorptive, optical, magnetic, antibacterial, and catalytic properties compared to their individual single-component materials.6-9 For example, Maceira et al. have synthesized semiconductor-coated magnetic silica spheres with both optical and magnetic properties.6 Liu and co-workers have shown that Chitosan modified magnetite nanoparticles can remove above 85% heavy metal ions such as Pb2+, Cu2+, and Cd2+ from wastewater.7 Wang et al. reported that Ag immobilized magnetic nanoparticles could easily kill Escherichia coli (gram-negative bacteria), Staphylococcus epidermidis (gram-positive bacteria), and Bacillus subtilis (spore bacteria) and could easily be removed from water with a magnetic field to avoid contamination of surroundings because they have superparamagnetic and antibacterial properties.8 Therefore, the synthesis of multifarious magnetic nanocomposites with multiple properties attracts more and more attentions. Since surface-enhanced Raman spectroscopy (SERS), which can enhance Raman signal intensity by up to 6 orders of magnitude depending on the preparation of the nanometer sized metallic structures (typically Ag and Au),10,11 was discovered by Fleischmann and Hendra,12 it arouses intense interest because * To whom correspondence should be addressed. Phone: +86-5533869303. Fax: +86-553-3869302. E-mail: [email protected].

it has wide potential application in many fields.13,14 Various synthesis methods of such metals have been reported which include vapor deposition,15 chemical reactions,16 and ion-beamsputtered technique.17 However, more work is still needed to simplify the synthesis method and to improve the sensitivity. In this work, a sort of three-ply composite microsphere was prepared by coating Ag nanoparticles on the surfaces of Fe3O4@SiO2 composite microspheres through the well-known Ag-mirror reaction. The prepared Ag-coated Fe3O4@SiO2 microspheres possess both ferromagnetic and SERS properties. On the one hand, they show obvious ferromagnetic property at room temperature and can be easily magnetized by an external magnetic field. On the other hand, they can serve as an efficient SERS substrate. The typical analyte Rhodamine 6G (Rh 6G) which has been full-characterized by SERS was selected as the model to validate the effect of this substrate; the result indicates that this substrate is ultrasensitive. Furthermore, this SERS substrate was applied to detect melamine and the SERS signals of melamine are strong even though the concentration of melamine is as low as 1 × 10-6 M. 2. Experimental Section All reagents were commercially available from Shanghai Chemical Regents Co. with analytical grade and were used without further purification. 2.1. Monodisperse Fe3O4 Microspheres Synthesis. The synthesis was carried out according to a previous report with a little modification.19 In a typical procedure, 1.35 g of FeCl3 · 6H2O was dissolved in 40 mL of ethylene glycol to form a clear solution, then 1.0 g of polyethylene glycol 20000 and 3.6 g of NaAc · 3H2O were added. The mixture was stirred until the reactants were fully dissolved. After that, the mixture was transferred into a Teflon-lined autoclave with a capacity of 50 mL and heated at 200 °C for 8 h. The products were collected and rinsed with deionized water and ethanol several times each, then dried under vacuum at 60 °C for 6 h for further use.

10.1021/jp100141c  2010 American Chemical Society Published on Web 04/14/2010

Ag-Coated Fe3O4@SiO2 Three-Ply Composite Microspheres

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Figure 1. Synthesis route of Ag-coated Fe3O4@SiO2 composite microspheres.

2.2. Magnetic Fe3O4@SiO2 Composite Microspheres Synthesis. The synthesis was carried out according to the noted Sto¨ber method with a little modification.20 Typically, 0.2 g of prepared Fe3O4 microspheres was dispersed in a mixture of 20 mL of ethanol and 4 mL of deionized water by ultrasonication for about 10 min. Then under continuous mechanical stirring, 1 mL of ammonia solution (25%) and 0.8 mL of tetraethyl orthosilicate (TEOS) were consecutively added to the mixture. The reaction was allowed to proceed at room temperature for 3 h under continuous mechanical stirring. The resulting products were collected and washed, and then dried under vacuum at 60 °C for 3 h for further use. 2.3. Monodisperse Ag-Coated Fe3O4@SiO2 Composite Microspheres Synthesis. First, 0.05 g of Fe3O4@SiO2 composite microspheres was dispersed into 30 mL of a 0.1 M Ag(NH3)2+ solution and the solution was stirred by a mechanical stirrer for 0.5 h to ensure sufficient adsorption of Ag(NH3)2+ by the Fe3O4@SiO2 composite microspheres. Then, the microspheres were collected and washed with deionized water two times. Next, the microspheres were dispersed into 30 mL of a 0.5 M glucose solution. The solution was heated with a water bath at 50 °C for 1 h. During the heating process, the solution was also stirred by a mechanical stirrer. The final products were collected and washed, then dried under vacuum at 60 °C for 3 h. The whole synthesis procedure is shown in Figure 1. 2.4. Preparation of SERS Substrate. A piece of copper foil (8 mm × 8 mm) was washed by ultrasonication in ethanol and acetone one time each. Then a small copper box without a cover (5 mm × 5 mm × 1 mm) was fabricated by tailoring the copper foil. The Ag-coated Fe3O4@SiO2 composite microspheres (about 0.005 g) were put into the copper box and pressed. 2.5. Characterization Methods. The structure and morphology of samples were characterized by X-ray diffraction (XRD; XRD-6000), scanning electron microscopy (SEM; Hitachi S-4800), and transmission electron microscopy (TEM; JEOL2010 with energy dispersive X-ray analysis (EDXA) system). Infrared (IR) spectra of the samples were obtained on a Vectortm 22 Fourier transform infrared (FT-IR) spectrometer (Bruke, Germany). The magnetic properties of the samples were investigated by using a vibrating sample magnetometer (VSM) with an applied field between -5000 and 5000 Oe at room temperature (BHV-55, Riken, Japan). Rh 6G and melamine were detected via SERS with a Labram-HR confocal laser microRaman spectrometer equipped with an argon ion laser with excitation of 514.5 nm. An air-cooled CCD was used as the detector, the accumulation time was 20 s, and the incident power was 3 mW. The spot size of the laser was 1 µm in diameter, using a 50× objective.

Figure 2. Typical XRD patterns of (a) Fe3O4 microspheres, (b) Fe3O4@SiO2 composite microspheres, and (c) Ag-coated Fe3O4@SiO2 composite microspheres.

3. Results and Discussion 3.1. Characterization of the Obtained Samples. The phase and purity of the as-obtained samples were examined by XRD. Figure 2a shows a typical XRD pattern of the obtained Fe3O4 sample; all of the diffraction peaks could be readily indexed to the orthorhombic phase of Fe3O4 (JCPDS card No. 75-1609), which matched well with the result of Li et al.19 Figure 2b is a typical XRD pattern of the Fe3O4@SiO2 sample, which shows almost the same feature as that shown in Figure 2a: no diffraction peaks corresponding to SiO2 were observed because the prepared SiO2 is amorphous. Figure 2c shows a typical XRD pattern of the Ag-coated Fe3O4@SiO2 sample. In addition to the diffraction peaks that correspond to Fe3O4, there also exist three other diffraction peaks (labled with the symbol #). These diffraction peaks could be easily indexed to the cubic phase of Ag (JCPDS card No. 04-0783). The samples were also characterized by IR. Figure 3a shows the IR spectrum of Fe3O4 microspheres, in which only a peak at 582 cm-1 can be seen. This peak is ascribed to the Fe-O stretching vibration.21 Figure 3b shows the IR spectrum of the Fe3O4@SiO2 sample. In addition to the peak at 582 cm-1 ascribed to Fe-O stretching vibration, there is a new strong band around 1093 cm-1 that originates from the Si-O bond of silica.21 This result indicates that SiO2 is immobilized on the surfaces of Fe3O4 microspheres. Figure 3c shows the IR spectrum of the Ag-coated Fe3O4@SiO2 sample. Because Ag nanoparticles do not have absorption in the infrared region, the IR spectrum of the Ag-coated Fe3O4@SiO2 sample is almost the same as that of the Fe3O4@SiO2 sample. The size and shape of the samples were examined by SEM and TEM. Panels a and b of Figure 4 show the SEM images of the Fe3O4 sample, from which it can be clearly seen that the sample is composed of many nearly monodisperse spherical particles with a diameter of about 400 nm. Furthermore, Figure

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Figure 3. IR spectra of (a) Fe3O4 microspheres, (b) Fe3O4@SiO2 composite microspheres, and (c) Ag-coated Fe3O4@SiO2 composite microspheres.

Figure 5. (a) TEM image of Fe3O4@SiO2 composite microspheres, (b, c) TEM images of Ag-coated Fe3O4@SiO2 composite microspheres, and (d) EDX spectrum of Ag-coated Fe3O4@SiO2 composite microspheres.

Figure 4. (a, b) SEM images of Fe3O4 microspheres, (c, d) SEM images of Fe3O4@SiO2 composite microspheres, and (e, f) SEM images of Ag-coated Fe3O4@SiO2 composite microspheres.

4b indicates that the magnetic microspheres are composed of many smaller particles and their surfaces are not smooth. SEM images of the Fe3O4@SiO2 sample are shown in Figure 4c,d, which indicate that the Fe3O4@SiO2 composite microspheres have a smooth silica shell. Panels e and f of Figure 4 show the SEM images of the Ag-coated Fe3O4@SiO2 sample. Many little Ag nanoparticles with a diameter of about 20 nm are adhered to the surfaces of the Fe3O4@SiO2 microspheres. The TEM images (Figure 5a-c) further confirm the above results. It can be clearly seen from Figure 5a that the Fe3O4@ SiO2 composite microspheres have the core-shell structure. TEM images of Figure 5b,c show distinctly that the Ag-coated Fe3O4@SiO2 composite microspheres have three-ply structure and the outermost layer is composed of many tiny Ag nanoparticles. Figure 5d is the EDX spectrum of the Ag-coated Fe3O4@SiO2 composite microspheres, in which Fe, O, Si, and Ag are all present. The C and Cu were derived from the carboncoated copper TEM grid. Figure 6 shows the magnetic hysteresis loops of the asobtained Fe3O4 microspheres, Fe3O4@SiO2 composite micro-

Figure 6. Magnetic hysteresis loops of (a) Fe3O4 microspheres, (b) Fe3O4@SiO2 composite microspheres, and (c) Ag-coated Fe3O4@SiO2 composite microspheres.

spheres, and Ag-coated Fe3O4@SiO2 composite microspheres. All of them show ferromagnetic behavior at room temperature. The magnetic saturation (Ms) values of the Fe3O4 microspheres, Fe3O4@SiO2 composite microspheres, and Ag-coated Fe3O4@ SiO2 composite microspheres are 80.0, 36.1, and 34.5 emu · g-1, respectively. It is noticed that the Ms value of Fe3O4 microspheres is higher than that of Fe3O4@SiO2 composite microspheres and Ag-coated Fe3O4@SiO2 composite microspheres, because in the Fe3O4@SiO2 composite microspheres and Agcoated Fe3O4@SiO2 composite microspheres the Fe3O4 is coated with a layer of amorphous SiO2. The small decrease of the Ms value of Ag-coated Fe3O4@SiO2 composite microspheres compared to that of Fe3O4@SiO2 composite microspheres can be attributed to the slight increase of size and mass due to the adherence of Ag nanoparticles on the surface of the magnetic composites. Such an excellent magnetic property means that all of the prepared samples have strong magnetic responsivity and can be separated easily from solution with the help of an external magnetic force.

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Figure 7. SEM images of Ag-coated Fe3O4@SiO2 composite microspheres obtained with different concentrations of Ag(NH3)2+: (a) 0.1 M and (b) 0.01 M.

Figure 9. SERS spectra of different concentrations of melamine methanol solutions adsorbed on Ag nanoparticles of the Ag-coated Fe3O4@SiO2 composite microspheres: (a) 1 × 10-6 M, (b) 1 × 10-7 M, (c) 1 × 10-8 M, and (d) 1 × 10-9 M.

Figure 8. SERS spectrum of a 1 × 10-15 M R6G ethanol solution adsorbed on Ag nanoparticles of the Ag-coated Fe3O4@SiO2 composite microspheres.

3.2. Effect of Ag(NH3)2+ Concentration on the Formation of Ag-Coated Fe3O4@SiO2 Composite Microspheres. It must be noticed that the concentration of Ag(NH3)2+ solution plays an important role in the formation of Ag-coated Fe3O4@SiO2 composite microspheres. Along with the concentration of Ag(NH3)2+ solution decreases, the amount of Ag nanoparticles is decreased and their distribution becomes nonuniform on the surfaces of Ag-coated Fe3O4@SiO2 composite microspheres. This result can be obtained from the comparison of the sample prepared with Ag(NH3)2+ concentration of 0.1 M (Figure 7a) and the sample prepared with Ag(NH3)2+ concentration of 0.01 M (Figure 7b). Higher concentrations of Ag(NH3)2+ proved to be crucial for augmenting the yield of the Ag nanoparticles and their uniformity. 3.3. Application of Ag-Coated Fe3O4@SiO2 Composite Microspheres in SERS. The prepared Ag-coated Fe3O4@SiO2 SERS substrate was first used to detect the typical SERS active analyte such as Rh 6G to test its effect. Figure 8 shows the SERS spectrum obtained by adding 25 µL of Rh 6G ethanol solution with a concentration of 1 × 10-15 M on the substrate. Four peaks exist in this spectrum and these peaks are characteristic peaks of Rh 6G.16,18,22 The peak at 1188 cm-1 is associated with C-C stretching vibrations, while the peaks at 1303, 1356, and 1575 cm-1 are associated with aromatic C-C stretching vibrations.23,24 Although the concentration of Rh 6G is very low, the signals of the SERS are still strong. These results indicate that this substrate for SERS detection is effective and ultrasensitive. It is reported that there are two major mechanisms that contribute to the enhancement effect:16,18 one is the electromagnetic effect associated with large local fields due to resonances occurring in the microstructures on the metal surface, the other is the chemical effect involving a scattering process associated with chemical interaction between the molecule and the metal surface. The enhancement due to the former is believed

to be a few orders of magnitude more than the latter. According to Garcia-Vidal and Pendry’s model,25 the smaller metallic particles give higher enhancement. But when the particle size is less than 15 nm, the enhancement saturates because the separation between metal nanoparticles is equal to their diameter. In this work, the average size of the Ag nanoparticles coated on Fe3O4@SiO2 microspheres is close to this size, which may be responsible for the ultrasensitivity of the Ag-coated Fe3O4@ SiO2 SERS substrate. Melamine (2,4,6-triamino-1,3,5-triazine) is a nitrogen-rich chemical commonly used to produce melamine resin, a synthetic heat-tolerant polymer. Although melamine is not inherently a carcinogen and toxic chemical, illegal large-dose adulteration in routine dairy products can result in urinary calculi, acute renal failure, and even infant death. Thus developing an accurate and rapid on-site melamine screen method is a very important and significant task. Herein, we used Ag-coated Fe3O4@SiO2 composite microspheres as a SERS substrate to detect melamine. Figure 9 shows the SERS spectra obtained by adding 25 µL of melamine methanol solution with different concentrations on the substrate. The SERS spectrum of the 1 × 10-6 M melamine methanol solution (Figure 9a) clearly shows the peaks at 686 and 582 cm-1.26 The most prominent peak around 686 cm-1 is assigned to the ring breathing mode II and involves in-plane deformation of the triazine ring in melamine molecules.27 Spectra b-d in Figure 9 show the SERS spectra of the melamine methanol solution with concentrations of 1 × 10-7, 1 × 10-8, and 1 × 10-9 M, respectively. In these spectra only the weak peak at 686 cm-1 can be seen. These results demonstrate that melamine can be well detected with the Ag-coated Fe3O4@SiO2 SERS substrate in concentrations as low as 1 × 10-6 M. 4. Conclusions In summary, bifunctional and monodisperse Ag-coated Fe3O4@SiO2 composite microspheres with both ferromagnetic and SERS properties have been successfully synthesized through a simple solution-phase method employing the Ag-mirror reaction. The Ag-coated Fe3O4@SiO2 composite microspheres can be used as a SERS substrate to detect melamine with a high sensitivity even when the concentration is as low as 1 × 10-6 M. This method is effective and convenient for detecting melamine at very low concentration and with a short detection time. Acknowledgment. Financial support form the National Natural Science Foundation of China (20701001) and Anhui

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Key Laboratory of Controllable Chemistry Reaction & Material Chemical Engineering is gratefully acknowledged. References and Notes (1) Ledezma, R.; Bueno, D.; Ziolo, R. F. Macromol. Symp. 2009, 28384, 307. (2) Zhang, L.; Wang, W. Z.; Zhou, L.; Shang, M.; Sun, S. M. Appl. Catal., B 2009, 90, 458. (3) Hou, C. H.; Hou, S. M.; Hsueh, Y. S.; Lin, J.; Wu, H. C.; Lin, F. H. Biomaterials 2009, 30, 3956. (4) Zhao, W. R.; Chen, H. R.; Li, Y. S.; Li, L.; Lang, M. D.; Shi, J. L. AdV. Funct. Mater. 2008, 18, 2780. (5) Wang, L.; Neoh, K. G.; Kang, E. T.; Shuter, B.; Wang, S. C. AdV. Funct. Mater. 2009, 19, 2615. (6) He, X. X.; Chen, Y. J.; Wang, K. M.; Wu, P.; Gong, P.; Huo, H. L. Nanotechnology 2007, 18, 285604. (7) Liu, X. W.; Hu, Q. Y.; Fang, Z.; Zhang, X. J.; Zhang, B. B. Langmuir 2009, 25, 3. (8) Salgueirino-Maceira, V.; Correa-Duarte, M. A.; Spasova, M.; LizMarzan, L. M.; Farle, M. AdV. Funct. Mater. 2006, 16, 509. (9) Guo, S. J.; Dong, S. J.; Wang, E. Chem.sEur. J. 2008, 14, 4689. (10) Nie, S. M.; Emery, S. R. Science 1997, 275, 1102. (11) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R.; Feld, M. S. Phys. ReV. Lett. 1997, 78, 1667. (12) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163. (13) Shadi, I. T.; Chowdhry, B. Z.; Snowden, M. J.; Withnall, R. Chem. Commun. 2004, 1436.

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