Au CoreâShell Microspheres as a

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Multifunctional Fe3O4@Ag/SiO2/Au Core-shell Microspheres as a Novel SERS-Activity Label via Long-Range Plasmon Coupling Jianhua Shen, Yihua Zhu, Xiaoling Yang, Jie Zong, and Chunzhong Li Langmuir, Just Accepted Manuscript • DOI: 10.1021/la304048v • Publication Date (Web): 03 Dec 2012 Downloaded from http://pubs.acs.org on December 21, 2012

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Multifunctional Fe3O4@Ag/SiO2/Au Core-shell Microspheres as a Novel SERS-Activity Label via Long-Range Plasmon Coupling Jianhua Shen, Yihua Zhu,* Xiaoling Yang, Jie Zong and Chunzhong Li Key Laboratory for Ultraﬁne Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China *Corresponding author: Tel.: +86-21-64252022, Fax: +86-21-64250624 E-mail address: [email protected] (Y. Zhu)

Title Running Head. Multifunctional core-shell microspheres

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Abstract

Noble metallic nanostructures exhibit a phenomenon known as surface-enhanced Raman scattering (SERS) in which the Raman scattering cross-sections are dramatically enhanced for the molecules adsorbed thereon. Due to their wide accessible potential range in aqueous solutions and the high biocompatibility, Au supports are preferred for spectro-electrochemical investigations. But the optical range in SERS spectroscopy is restricted to excitation lines above 600 nm, which is shorter than the Ag supports. In addition, these SERS-activity materials are not easy to separate, and re-used. Herein, the present article reports the novel multifunctional Fe3O4@Ag/SiO2/Au core-shell microspheres which display long-range plasmon transfer of Ag to Au leading to enhanced Raman scattering. The welldesigned microspheres have high magnetization, uniform sphere size. As a result, Fe3O4@Ag/SiO2/Au microspheres have the best enhancement effect in the Raman active research by using Rhodamine-b (RdB) as a probe molecule. The enhancement factor is estimated to be 2.2×104 for RdB from the longrange plasmon transfer of Ag to Au, corresponding to an attenuation of the enhancement by a factor of only 0.672×104 compared to RdB adsorbed directly on the Fe3O4@Ag microspheres. And RdB can be detected down to 10-9 M even without the resonance SERS effect. The unique nanostructure makes the microspheres novel stable and high enhancement effect for Raman detection.

Keywords: K-gold reduction, surface enhanced Raman scattering, Magnetic microsphere, Gold-silver composite


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1. Introduction Noble metal nanomaterials support surface plasmon resonances, collective oscillations of surface electrons that provide means to manipulate light at the nanoscale.1 And plasmonic nanomaterials have long been known for their ability to generate strong optical near-fields, enabling a family of surfaceenhanced molecular spectroscopies such as surface-enhanced Raman scattering (SERS) spectroscopy.2-3 SERS has been intensely explored as a powerful and extremely sensitive analytical technique with applications in biochemistry, chemical production, and environmental monitoring.4-6 And generally substrates based on metals such as Ag, Au or Cu, either with roughened surfaces or in the form of nanoparticles, are required to realize a substantial SERS effect, and this has limited the breadth of practical applications of SERS. Li et al. prepared gold nanoparticles coated with a thin, uniform, fully enclosed and optically transparent shell of silica or alumina so that the gold core generates a large surface enhancement.6 Uzayisenga et al. have synthesized, characterized, tested, and modeled shellisolated nanoparticle-enhanced Raman spectroscopy nanoparticles with a mostly Ag core instead of a Au-core.7 However, these nanoparticles are sometimes separated by complex methods, or they can not be reused and recycled.8 As an important family of separable materials, magnetic particles have gained much attention due to their unique separable feature which makes it possible to realize convenient recycling of novel metals.9 Jun and co-workers presented the SERS-encoded magnetic beads, which provided a large encoding capacity using SERS signals with easy gentle manipulation and separation using magnetic force.10 The core part of the bead was composed of a magnetic nanoparticle-embedded sulfonated polystyrene bead, and the outer part of the bead was coated with Ag NPs and silica shell which was fabricated for further bioconjugation and protection of SERS signaling. And in the other hand, coating the magnetic particles with a gold shell provides an intriguing class of biomaterials, which possess the well-established surface chemistry and biomedical properties.11 Bao et al. prepared Fe2O3/Au core/shell nanoparticles with different Au shell thicknesses by reducing HAuCl4 on the surface of γ-Fe2O3 nanoparticles. And the


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antigens in test solution were separated by Fe2O3/Au magnetic nanoparticles capped with antibody expediently, and the separation efficiency was detected by immunoassay analysis based on SERS.12 Although Au shells have wide accessible potential range in aqueous solutions and high biocompatibility,13-15 the optical range in SERS spectroscopy is restricted to excitation lines above 600 nm, which is shorter than the Ag supports.16 Therefore, this limitation would impair exploitation of the molecular resonance enhancement when using SERS-active Au materials. Conversely, due to its higher bandgap, Ag displays a much broader enhancement profile ranging from 400 nm to the near infrared region. For this reason, and despite the narrow potential range and low biocompatibility, Ag supports also have been wildly used.17-20 Furthermore, it is widely known that surface plasmon efficiency is greater for Ag than it is for Au, and it is also much cheaper than Au. But most SERS applications employ Au instead of Ag because Ag is more mobile and more easily oxidized, properties which adversely influence SERS signal stability and reproducibility. If Ag nanostructures can be stabilized by a chemically and electrically inert shell, these weaknesses can be managed.7 So Feng et al. developed an Ag–silica–Au hybrid device that displayed a long-range plasmon transfer of Ag to Au leading to enhanced Raman scattering of molecules largely separated from the optically excited Ag surface.16 And recently, the core-shell of Ag-Au hybrid nanoparticles have been extensively studied for SERS.21-23 Herein, the present article reports the novel multifunctional Fe3O4@Ag/SiO2/Au core-shell microspheres which display long-range plasmon transfer of Ag to Au leading to enhanced Raman scattering. And the microspheres possess the precise control of the size, morphology, surface chemistry, and assembly process of each component: Fe3O4 microspheres are prepared by the solvothermal reaction; the Ag shells coated on Fe3O4 utilized the thermal reduction; and the final nanocomposite microspheres possess the core of silica-protected Fe3O4@Ag, and in situ growth gold shell on the outer shell by layerby-layer electrostatic self-assembly and the K-gold solution reduction (Figure 1a). The well-designed microspheres have high magnetization, uniform sphere size. The SERS activities of these microspheres have been tested by using Rhodamine-b (RdB) as a probe molecule. And the unique nanostructure makes the microspheres novel stable and high enhancement effect for Raman detection. ACS Paragon Plus Environment

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2. Experimental Section 2.1. Reagents and Materials. Poly(allylamine hydrochloride) (PAH) was purchased from SigmaAldrich

Chemicals

Co.

Polyvinylpyrrolidone

(PVPk-30),

Chloroauric

acid

tetrahydrate

(AuCl3·HCl·4H2O), ethanol and Rhodamine-b were purchased from Sinopharm Chemical Reagent Co., and all other chemicals were purchased from Shanghai Chemical Reagent Co. All chemicals were used as received. Ultrapure water (18 MΩ cm) was used for all experiments. Fe3O4 was synthesized by solvent-thermal method as reported.24-26 2.2. Instruments. Scanning electron microscope (SEM) images were taken by using a JEOL JSM6360LV microscopy equipped with an energy dispersive X-ray analyzer (EDX), the surfaces of the samples were sputtered with gold before testing. Transmission electron microscope (TEM) images were taken on a JEOL JEM 2011 microscope (JEOL, Japan) at an acceleration voltage of 200 kV, the specimen was prepared by drop casting the sample dispersion onto a carbon-coated copper grid, followed by drying under room temperature. The crystalline structure was investigated by X-ray power diffraction (RIGAK, D/MAX 2550 VB/PC, Japan) and UV-vis absorption spectra were recorded by Cary 500 (Varian, USA) at room temperature. Raman spectra were recorded on an inVia Raman microprobe (Renishaw Instruments, England) with 785 nm laser excitation. 2.3. Synthesis and Preparation. The Ag-embedded microspheres were prepared according to the solution-phase reduction which was adopted to protected growth under PVP on the surface of magnetic Microspheres. The prepared Fe3O4 particles (0.2 g) were dispersed in 24 ml of ethylene glycol containing silver nitrate (0.1 g) and PVPk-30 (2.5 g) and then stirred for 12 h at 100 oC.10 After completing the reaction, silver nanoparticles-embedded Fe3O4 particles (Fe3O4@Ag) were washed with acetone and water several times to remove ethylene glycol and PVPk-30. Finally, the product was dried at room temperature for further uses. The shell of SiO2 was synthesized via the sol-gel process. And an aqueous dispersion of the abovementioned Fe3O4@Ag particles (10 mL, 0.02 g/mL) was added to a three-neck round-bottom flask ACS Paragon Plus Environment

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charged with absolute ethanol (40 mL) and concentrated ammonia solution (0.14 mL, 28%) under mechanical stirring for 15 min at room temperature. Then 0.14 mL TEOS was added dropwise in 2 min, and the reaction proceeded for 6 h under continuous mechanical stirring. The resultant core-shell Fe3O4@Ag/SiO2 microsphere product was separated and collected with a magnet. Gold coated on the composite microspheres were prepared using the interfacial growth method by our group which was adopted to embed the Au seed particles and reduce the K-gold solution with formaldehyde. Fe3O4@Ag/SiO2 microspheres (40 mg) were soaked in PAH solution (60 mL, 0.2 wt %). After 30 min under ultrasonication, the precipitate was washed, and then immersed in HAuCl4 aqueous solution. Separated with a magnet and washed, the resulting microspheres were redispersed in 0.1 M NaBH4 solution for reduction of AuCl4-. Finally, the gold nanoshells were prepared by reduction of Kgold solution with formaldehyde (10mL, 37%). To prepare the K-gold solution, 0.6 mL of 1 wt% HAuCl4 was added to 40 mL of water containing 0.01 g of K2CO3 under magnetic stirring. The resultant Fe3O4@Ag/SiO2/Au microsphere product was separated, collected and dried at room temperature for further uses. 2.4. Measurements of SERS Activity of the Fe3O4@Ag/SiO2/Au Devices. Because the Fe3O4@Ag/SiO2/Au core/shell particles have the core Fe3O4 with magnetic properties and the shell Ag and Au are well known SERS active materials, the Fe3O4@Ag/SiO2/Au microspheres would be collected by magnet and also have SERS activity. The different concentrations of RdB aqueous solution were configured into 0.2 M, 10-4 M, 10-5 M, 10-6 M, 10-7 M, 10-8 M, and 10-9 M, respectively. To analyze both of these characteristics, we put the microspheres in a vial, and the solution of RdB (10 mL) was mixed with the microspheres in the vial. After 10 min, magnet was placed under the vial to collect the particles, and then in situ SERS activity of the solution at the position of onto the magnet was detected. The Raman spectra were recorded on a Renishaw Invia system, equipped with Peltier chargecoupled device (CCD) detectors and a Leica microscope. Samples were excited with a 785 nm (diode) laser line. Spectra were collected in Renishaw continuous mode with accumulation times of 10 s (unless


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stated otherwise). Power at the sample was varied at 150 mW. A long working distance 50× objective was used to collect the Raman scattering signal. 3. Results and Discussion 3.1. Characterization of Fe3O4@Ag/SiO2/Au nanostructures. This core-shell structured Fe3O4@Ag/SiO2/Au has an Ag-embedded Fe3O4 magnetic core, SiO2 coated, and a layer of Au nanoparticles shell (Figure 1a). And Figure 1a shows the fabrication process of the core-shell microspheres. Fe3O4 particles with a diameter of approximately 390 nm were first synthesized through a robust solvothermal reaction (Figure 1b and S1).24 The core-shell Fe3O4@Ag microspheres are uniform with a size of about 410 nm, and the Ag nanoparticles layer is 10 nm thick (Figure 1c and Figure S2). Scanning electron microcopy (SEM) images of Fe3O4@Ag/SiO2 (Figure 1d and S3) show that the microspheres coated with SiO2 shell having diameters of roughly 440 nm, and the shells are composed of SiO2 with 15 nm thick and smooth surface. Finally, the whole Fe3O4@Ag/SiO2/Au microspheres with a size of approximately 460 nm are coated with Au nanoparticles 10 nm in thickness, and the shells are composed of primary Au nanoparticles with the rough surface (Figure 1e and S4). With the increase of the shells, the distribution of the particle size are constantly deteriorated. As revealed by transmission electron microscopy (TEM), Figure 2a displays the TEM photograph of Fe3O4@Ag microspheres with mean diameter of about 410 nm, which is consistent with the SEM results. Figure 2b is an enlarged image of a single microsphere, which clearly shows the shell is composed of smaller particles. In the inset of Figure 2b, the smaller particles are Ag nanocrystals of about 3 nm. In panels c and d of Figure 2, one can see that the silica nanoparticles coated with Fe3O4@Ag microspheres are perfectly spherical with smooth particle surfaces and represent clear core-shell structures, and the silica layer is about 15 nm in thickness, which further displays a uniform with a diameter of roughly 440 nm and it is similar to the SEM images (Figure 1). The surfaces of the core-shell Fe3O4@Ag/SiO2 microspheres are highly negatively charged, so we successfully demonstrate the in situ growth of gold seed particles and interfacial growth of gold nanoshell on the outer shell, as we previously reported.25,26 ACS Paragon Plus Environment

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Figure 2e and 2f show the gold particles layer on the composite microsphere, which are composed of Au nanoparticles with the uniform size of approximately 10 nm. And the size of the Fe3O4@Ag/SiO2/Au microspheres is ca. 460 nm (Figure 2e, 1e and S4), the different layers uniformly coated on the microspheres. To verify the formation of Ag and Au nanocrystals in the microspheres, Figure 3 shows X-ray diffraction (XRD) patterns of Fe3O4, Fe3O4@Ag, Fe3O4@Ag/SiO2 and Fe3O4@Ag/SiO2/Au. Wide-angle XRD patterns show the characteristic broad diffraction peaks indexed to the spinel Fe3O4, amorphous silica and cubic-phase Ag and Au nanoparticles in the composite microspheres.27 And the specific XRD of Fe3O4@Ag, characterized by four strong pecks positioned at 2θ values of 38.2o, 44.3o, 64.5o, and 77.5o, which correspond to the [111], [200], [220], and [311] lattice planes of the facecentered cubic (fcc) phase of Ag (JCPDS No.04-0783), respectively.28 The intensity of these four peaks of Fe3O4@Ag/SiO2 significantly weakens due to the silica layer. And it is found that the XRD pattern for Fe3O4@Ag/SiO2/Au exhibits broader characteristic peaks than those for the Fe3O4@Ag/SiO2 microspheres although the characteristic peaks for Au and Ag were too close to distinguish. It reveals that Ag and Au shells are successfully fixed in the microspheres, further proving the attachment of AgSiO2-Au and the well-retained magnetite phase. Figure 4 shows diffuse reflectance UV/Vis spectroscopy of Fe3O4@Ag, Fe3O4@SiO2/Au and Fe3O4@Ag/SiO2/Au. The starting Fe3O4@Ag microspheres have an absorption peak at 448 nm, the peak at 553 nm observed on the Fe3O4@SiO2/Au microspheres were caused by both surface plasmon coupling between closely spaced gold nanoparticles.25 And after depositing Au on the Fe3O4@Ag/SiO2 microspheres, the absorption peak leads to a shift to 481 nm. Thus, SERS measurements with 481 nm excitation have been extended to adsorbents on Au surfaces. In addition, the room-temperature magnetization saturation of as prepared Fe3O4 particles values is measured to be 45.9 emu/g, and the remanence and coercivity is 3.0 emu/g and 35.5 Oe, respectively. And Fe3O4@Ag/SiO2/Au ACS Paragon Plus Environment

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microspheres can be separated with a magnet below 25 s (Figure S4), which is important for the promising applications ranging from electromagnetic devices to biomedicine, catalysis, sensor, energy conversion, and so on. 3.2. SERS activity of Fe3O4@Ag/SiO2/Au nanostructures. The Fe3O4@Ag/SiO2/Au nanostructures can be readily used as SERS substrates for molecular sensing with sensitivity and specificity. Because the core-shell microspheres have the core Fe3O4 with magnetic properties and the novel metals (Ag and Au) were well-known SERS active material, the Fe3O4@Ag/SiO2/Au microspheres would be collected by magnet and also have SERS activity. To analyze both of these characteristics, we put the microspheres in a vial, a magnet was placed under the vial to collect the particles, and then in suit SERS activity of the solution at the position of onto the magnet was detected (Figure 5). Herein, RdB was selected as the probe molecule, because of its wellestablished Raman spectral data and large Raman scattering cross section, and it had no optical absorption in the near-infrared region. Thus, it acted as a non-electronic resonant Raman probe in this range. The SERS response of this molecule could be assigned to the substrate properties and their variation with gold-coverage without additional contributions from itself..29 Figure 6 presents the SERS spectra change dramatically with the different nanostructures for the SERS active shells. As shown, the Fe3O4@Ag/SiO2 core-shell microspheres exhibit weak enhancement because of the electromagnetic damping of silica shells on Ag (curve c of Figure 6), the Fe3O4@SiO2/Au microspheres possess weaker enhancement, which display that surface plasmon efficiency is greater for Ag than Au. And then, the deposition of Au shells causes a large increase in the intensity, which is larger than the other structure microspheres. And characteristic bands including νCH(ip) (in-plane) (1192 cm-1) and νCC (1355, 1503, 1580 and 1652 cm-1) are shown in Figure 6 and Table S1. The large SERS enhancement obtained from Fe3O4@Ag/SiO2/Au nanostructures is a process that involves electromagnetic enhancement due to the interaction of the excition wavelength with plasmon ACS Paragon Plus Environment

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excitations in the nanostructures, as well as chemical enhancement of the RdB molecules adsorbed on the gold-coated nanomaterials. Due to the long-range interparticle plasmon coupling, the plasmon resonance wavelength of the Fe3O4@Ag/SiO2/Au nanostructures can shift to the range of nearinfrared and the nanostructures can be on-resonance using NIR laser excitation such as 785 nm.29,30 In addition, the SERS intensity of RdB on Fe3O4@Ag/SiO2/Au nanostructures also has a large increase due to the long-range interparticle Plasmon coupling.16 The overall enhancement of the Raman signal is of great importance, so the SERS enhancement factor (EF) was defined as Table S1.31-33 And the EF was estimated to be 2.2×104 for RdB from the Fe3O4@Ag/SiO2/Au nanostructures, corresponding to an attenuation of the enhancement by a factor of only 0.672×104 compared to RdB adsorbed directly on the Fe3O4@Ag microspheres for the bands at 1652 cm-1. Figure 7 shows concentration-dependent SERS spectra of RdB with typical aromatic ring vibrations, e.g., 1355, 1503, 1580 and 1652 cm-1. Low concentrations (10-9 M) of RdB in solution can be detected even without the resonance SERS effect. The Fe3O4@Ag/SiO2/Au substrates exhibited no obvious decrease in the SERS spectra after a month. Five samples were prepared utilizing the same method to check the reproducibility of the SERS active substrates. Both of the relative standard deviations were found to be below 5%, which were acceptable. Accordingly, the present method may also be used in the relatively quantitative detection of biomolecules in organisms with the advantages of separability, simplicity and sensitivity. 4. Conclusion In conclusion, we report a simple and effective method for the novel multifunctional Fe3O4@Ag/SiO2/Au core-shell microspheres which display long-range plasmon transfer of Ag to Au leading to enhanced Raman scattering. The preparation route is developed to create SERS active substrates, which has proved to be efficient and reliable. In addition, the well-designed microspheres have high magnetization, uniform sphere size. And the microspheres successfully lead to an intense SERS signal. Due to the rough surface of the Fe3O4@Ag/SiO2/Au nanostructures, there are multiplied hot spots between the closely core-shell microspheres, which exhibit the enhancement ability of SERS ACS Paragon Plus Environment

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for RdB with the detection limit of 10-9 M, and the EF was estimated to be 2.2×104 for the bands at 1652 cm-1. The proposed nanostructures provide stable, sensitive and reusable SERS substrates. Furthermore, the low signal interference of this effective method has shown a potential application for Fe3O4@Ag/SiO2/Au nanostructures in different analytes, and the nanostructures investigated here could be promising candidates for SERS sensor chips. Finally, we envision from these results that these coreshell microspheres are expected to apply the ranges from electromagnetic devices to biomedicine, catalysis, sensor, energy conversion, and so on. ASSOCIATED CONTENT Supporting Information. Figures S1-S5 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author * E-mail address: [email protected] (Y. Zhu) Acknowledgment. We thank the National Natural Science Foundation of China (20925621, 20976054, and 21176083), the Special Projects for Nanotechnology of Shanghai (11nm0500800) the Fundamental Research Funds for the Central Universities (WD1013015 and WD1114005), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT0825), and the Shanghai Leading Academic Discipline Project (project number: B502) for financial supports.


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(10) Jun, B. H.; Noh, M. S.; Kim, G.; Kang, H.; Chung, W. J.; Kim, Y. K.; Cho, M. H.; Jeong, D. H.; Lee, Y. S. Protein separation and identification using magnetic beads encoded with surface-enhanced Raman spectroscopy. Anal. Biochem. 2009, 391, 24. (11) Cui, Y.; Wang, Y.; Hui, W.; Zhang, Z.; Xin, X.; Chen, C. The Synthesis of GoldMag NanoParticles and their Application for Antibody Immobilization. Biomed. Microdevices 2005, 7, 153. (12) Bao, F.; Yao, J. L.; Gu, R. A. Synthesis of magnetic Fe2O3/Au core/shell nanoparticles for bioseparation and immunoassay based on surface-enhanced Raman spectroscopy. Langmuir 2009, 25, 10782. (13) Shen, J.; Yang, X.; Zhu. Y.; Kang, H.; Cao, H.; Li, C. Gold-coated silica-fiber hybrid materials for application in a novel hydrogen peroxide biosensor. Biosens. Bioelectron. 2012, 34, 132. (14) Park, H. Y.; Schadt, M. J.; Wang, L.; Lim, I. I. S.; Njoki, P. N.; Kim, S. H.; Jang, M. Y.; Luo, J.; Zhong, C. J. Fabrication of Magnetic Core@Shell Fe Oxide@Au Nanoparticles for Interfacial Bioactivity and Bio-separation. Langmuir 2007, 23, 9050. (15) Feng, J. J.; Gernert, U.; Sezer, M.; Kuhlmann, U.; Murgida, D. H.; David, C.; Richter, M.; Knorr, A.; Hildebrandt, P.; Weidinger, I. M. Novel Au−Ag Hybrid Device for Electrochemical SE(R)R Spectroscopy in a Wide Potential and Spectral Range. Nano Lett. 2009, 9, 298. (16) Feng, J. J.; Gernert, U.; Hildebrandt, P.; Weidinger, I. M. Induced SER-Activity in nanostructured Ag–silica–Au supports via long-range plasmon coupling. Adv. Funct. Mater. 2010, 20, 1954. (17) Wackerbarth, H.; Klar, U.; Gunther, W.; Hildebrandt, P. TRANSMISSIBLE SPONGIFORM ENCEPHALOPATHIES IN HUMANS. Appl. Spec. 1999, 53, 283. (18) Todorovic, S.; Jung, C.; Hildebrandt, P.; Murgida, D. H. Conformational transitions and redox potential shifts of cytochrome P450 induced by immobilization. J. Biol. Inorg. Chem. 2006, 11, 119.


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(19) Murgida, D. H.; Hildebrandt, P. Proton-Coupled Electron Transfer of Cytochrome c. J. Am. Chem. Soc. 2001, 123, 4062. (20) Hrabakova, J.; Ataka, K.; Heberle, J.; Hildebrandt, P.; Murgida, D. H. Long distance electron transfer in cytochrome c oxidase immobilised on electrodes. A surface enhanced resonance Raman spectroscopic study. Phys. Chem. Chem. Phys. 2006, 8, 759. (21) Kruss, S.; Srot, V.; van Aken, P. A.; Spatz, J. P. Au-Ag hybrid nanoparticle patterns of tunable size and density on glass and polymeric supports. Langmuir 2012, 28, 1562. (22) Pinkhasova, P.; Yang, L.; Zhang, Y.; Sukhishvili, S.; Du, H. Differential SERS Activity of Gold and Silver Nanostructures Enabled by Adsorbed Poly(vinylpyrrolidone). Langmuir 2012, 28, 2529. (23) Xia, W.; Sha, J.; Fang, Y.; Lu, R.; Luo, Y.; Wang, Y. Gold Nanoparticles Assembling on Smooth Silver Spheres for Surface-Enhanced Raman Spectroscopy. Langmuir 2012, 28, 5444. (24) Zhu, Y.; Shen, J.; Zhou, K.; Chen, C.; Yang, X.; Li, C. Multifunctional Magnetic Composite Microspheres with in Situ Growth Au Nanoparticles: A Highly Efficient Catalyst System. J. Phys. Chem. C 2011, 115, 1614. (25) Shen, J.; Zhu, Y.; Zhou, K.; Yang, X.; Li, C. Tailored anisotropic magnetic conductive film assembled from graphene-encapsulated multifunctional magnetic composite microspheres. J. Mater. Chem. 2012, 22, 545. (26) Shen, J.; Zhu, Y.; Yang, X.; Li, C. Magnetic composite microspheres with exposed {001} faceted TiO2 shells: a highly active and selective visible-light photocatalyst. J. Mater. Chem. 2012, 22, 13341. (27) Deng, Y.; Cai. Y.; Sun, Z.; Liu, J.; Liu, C.; Wei, J.; Li, W.; Liu, C.; Wang, Y.; Zhao, D. Multifunctional Mesoporous Composite Microspheres with Well-Designed Nanostructure: A Highly Integrated Catalyst System. J. Am. Chem. Soc. 2010, 132, 8466.


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(28) Chen, D. H.; Chen, C. J. Formation and characterization of Au–Ag bimetallic nanoparticles in water-in-oil microemulsions. J. Mater. Chem. 2002, 12, 1557. (29) Li, D.; Li, D. W.; Li, Y.; Fossey, J. S.; Long, Y. T. Cyclic electroplating and stripping of silver on Au@SiO2 core/shell nanoparticles for sensitive and recyclable substrate of surface-enhanced Raman scattering. J. Mater. Chem. 2010, 20, 3688. (30) Lu, Y.; Liu, G. L.; Kim, J.; Mejia, Y. X.; Lee, L. P. Nanophotonic Crescent Moon Structures with Sharp Edge for Ultrasensitive Biomolecular Detection by Local Electromagnetic Field Enhancement Effect. Nano Lett. 2005, 5, 119. (31) Cai, W. B.; Ren, B.; Li, X. Q.; She, C. X.; Liu, F. M.; Cai, X. W.; Tian, Z. Q. Investigation of surface-enhanced Raman scattering from platinum electrodes using aconfocal Raman microscope: dependence of surface roughening pretreatment. Surf. Sci. 1998, 406, 9. (32) Yu, H. Z.; Zhang, J.; Zhang, H. L.; Liu, Z. F. Surface-Enhanced Raman Scattering (SERS) from Azobenzene Self-Assembled “Sandwiches”. Langmuir 1999, 15, 16. (33) Ngo, Y. H.; Li, D.; Simon, G. P.; Garnier, G. Gold Nanoparticle–Paper as a Three-Dimensional Surface Enhanced Raman Scattering Substrate. Langmuir 2012, 28, 8782.


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Figure captions Figure 1. (a) Schematic diagram and structural model of the multifunctional composite Fe3O4@Ag/SiO2/Au microspheres. SEM images of (b) Fe3O4 particles, (c) Fe3O4@Ag microspheres, (d) Fe3O4@Ag/SiO2 microspheres and (e) Fe3O4@Ag/SiO2/Au microspheres. Figure 2. TEM images of (a and b) Fe3O4@Ag microspheres, (c and d) Fe3O4@Ag/SiO2 microspheres, (e and f) Fe3O4@Ag/SiO2/Au microspheres. Insets (a, c and e) are the structural models for the Fe3O4@Ag, Fe3O4@Ag/SiO2 and Fe3O4@Ag/SiO2/Au microspheres, respectively. Insets (b and f) are enlarged TEM images. Figure 3. The wide-angle XRD patterns of Fe3O4 particles, Fe3O4@Ag microspheres, Fe3O4@Ag/SiO2 microspheres and Fe3O4@Ag/SiO2/Au microspheres. The black, red, and green numbers indicate the peak of Fe3O4, Ag and Au, respectively. Figure 4. UV-vis spectra of Fe3O4@Ag, Fe3O4@SiO2/Au and Fe3O4@Ag/SiO2/Au microspheres. Figure 5. Schematic illustration of in suit SERS detection of RdB absorbed on Fe3O4@Ag/SiO2/Au microspheres. Enlarge part A are the schematic illustration of the RdB molecules on Fe3O4@Ag/SiO2/Au microspheres in the vial and the Raman experiments. Figure 6. Raman spectra of 0.2 M RdB (a) in aqueous solution. SERS spectra of 1×10-6 M RdB molecules on (b) Fe3O4@Ag microspheres, (c) Fe3O4@Ag/SiO2 microspheres, (d) Fe3O4@SiO2/Au microspheres and (e) Fe3O4@Ag/SiO2/Au microspheres. Acquisition parameters: 150 mW of 785 nm excitation, 10 s integration. Figure 7. (a) Representative 785 nm excited SERS spectra of RdB adsorbed on the Fe3O4@Ag/SiO2/Au microspheres. (b) A logarithmic plot of RdB concentration vs. logarithmic signal intensity for the bands at 1652 cm-1.


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Figure 1.


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Figure 2.


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Figure 3.


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Figure 4.


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Figure 5.


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Figure 6.


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Figure 7.


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TOC: Paper Title: Multifunctional Fe3O4@Ag/SiO2/Au Core-shell Microspheres as a Novel SERS-Activity Label via Long-Range Plasmon Coupling Authors: Jianhua Shen, Yihua Zhu,* Xiaoling Yang, Jie Zong and Chunzhong Li

Summary An effective novel SERS-activity label based on Fe3O4@Ag/SiO2/Au core-shell microspheres with ease of operation, good separation efficiency, and rapid detection has been introduced in this paper. The microspheres possess the precise control of the size, morphology, surface chemistry, and assembly process of each component. And the unique nanostructure makes the microspheres novel stable and high enhancement effect for Raman detection.


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