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Aggregation-Based Fabrication and Assembly of Roughened Composite Metallic Nanoshells: Application in Surface-Enhanced Raman Scattering Lehui Lu, Hongjie Zhang,* Guoying Sun, Shiquan Xi, and Haishui Wang Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

Xiaoling Li, Xu Wang, and Bing Zhao Key Laboratory of Supramolecular Structure and Spectroscopy, Jilin University, Changchun 130023, China Received May 1, 2003. In Final Form: August 15, 2003 This paper reports an aggregation-based method for the fabrication of composite Au/Ag nanoshells with tunable thickness and surface roughness. It is found that the resultant roughened composite Au/Ag nanoshells can attract each other spontaneously to form films at the air-water interface. Importantly, such films can be transferred onto the solid substrates without being destroyed and show excellent surface-enhanced Raman scattering (SERS) enhancement ability. Their strong enhancement ability may stem from the unique two-dimensional structure itself.

Introduction In recent years, the self-assembly of nanoparticles of various natures as nanostructures has developed into an increasingly important research area in materials chemistry.1-8 The importance stems largely from the fact that the properties of such nanostructures can be modulated not only through the nature of their constituting units but also through the distances between particles or the morphology of the whole system. In particular, fabrication of nanostructures consisting of a dielectric core with a thin metal shell, termed “nanoshells”, is currently a subject of extensive research due to their unique application in many areas such as nonlinear optics, catalysis, and SERS.4,6-8 Previous studies4,6-7 showed that the plasmon optical resonance of gold nanoshells could be selectively tuned to any wavelength across the visible and the infrared regions of spectrum simply by varying the ratio of the dielectric core to metal shell. Up to now, several techniques have been developed for the fabrication of metal nanoshells on both nanometer and micrometer scales, including chemical reduction,9 electrostatic attraction,10 self-as* To whom correspondence should be addressed. E-mail: [email protected]. Fax: +86-431-5685653. (1) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (2) (a) Rao, C. N.; Kulkarin, G. U.; Thomas, P. J.; Edards, P. P. Chem. Soc. Rev. 2000, 29, 27. (b) Fendler, J. H. Nanoparticles and Nanostructured Films; VCH: Weinheim, Germany, 1998. (3) Caruso, F. Adv. Mater. 2001, 13, 11. (4) (a) Twardowski, M.; Nuzzo, R. G. Langmuir 2002, 18, 55295538. (b) Hills, C. W.; Nasher, M. S.; Frenkel, A. I.; Shapley, J. R.; Nuzzo, R. G. Langmuir 1999, 15, 690. (5) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (6) Lal, S.; Westcott, S. L.; Taylor, R. N.; Jackson, J. B.; Nordlander, P.; Halas, N. J. J. Phys. Chem. B 2002, 106, 5609. (7) Oldenburg, S. J.; Westcott, S. L.; Averitt, R. D.; Halas, N. J. J. Chem. Phys. 1999, 111, 4729. (8) Chen, C. W.; Serizawa, T.; Akashi, M. Chem. Mater. 1999, 11, 1381. (9) Kobayashi, Y.; Salgueirin˜o-Maceira, V.; Liz-Marza´n, L. M. Chem. Mater. 2001, 13, 1630. (10) Dong, A. G.; Wang, Y. J.; Tang, Y.; Ren, N.; Yang W. L.; Gao, Z. Chem. Commun. 2002, 350.

sembly,11 and the combination of self-assembly and seeding.4,6 Here, we present an aggregation-based method for the fabrication of composite Au/Ag nanoshells with tunable thickness and surface roughness. Interestingly, we observed the spontaneous aggregation of such rough composite metallic nanaoshells confined to the air-water interface. The applications of the resulting film in surfaceenhanced Raman scattering (SERS) are investigated. Experimental Section Materials. (3-Aminopropyl)trimethoxysilane (APTMS), tetraethyl orthosilicate (TEOS), HAuCl4, AgNO3, and ascorbic acid were purchased from Aldrich. 4-Aminothiophenol (4-ATP), NaBH4, and sodium citrate were obtained from Sigma. NH4OH, HNO3, and absolute ethanol were purchased from Beijing Chemical Reagents Industry. All chemicals were used as received. Throughout the experiment, doubly distilled water was used. Procedure for Composite Au/Ag Nanoshells. Small colloidal gold (2.6 nm) and silver (4 nm) particles were prepared according to the refs 12 and 13, respectively. Detailed experimental procedures for SiO2@Au nanoparticles were described in our previous report.14a Briefly, uniform silica nanoparticles with a diameter of 84 nm were modified with (3-aminopropyl)trimethoxysilane (APTMS) as indicated in Scheme 1 (step 1). After isolation of the APTMS-modified silica nanoparticles from residual reactants by centrifugation, a solution of colloidal gold was added (step 2). The resultant gold-coated silica nanoparticles were purified and redispersed in water by centrifugation and by sonication, respectively. An excess solution of 4-aminothiophenol (4-ATP) (∼1 mmol) was mixed with 20 mL of a solution of goldcoated silica nanoparticles and allowed to react for 6 h (step 3). Residual 4-ATP molecules were removed by centrifuging and redispersing several times. Then the 4-ATP-modified SiO2@Au (11) Dokoutchaev, A.; James, J. T.; Koene, S. C.; Pathak, S.; Prakash, G. K. S.; Thompson, M. E. Chem. Mater. 1999, 13, 238. (12) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353. (13) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 617. (14) (a) Lu, L. H.; Sun, G. Y.; Xi, S. Q.; Wang, H. S.; Zhang, H. J.; Wang, T. D.; Zhou, X. H. Langmuir 2003, 19, 3074. (b) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Mater. 2001, 13, 2313.

10.1021/la034738g CCC: $25.00 © 2003 American Chemical Society Published on Web 09/18/2003

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Scheme 1. Preparation Sequence of Composite Au/Ag Nanoshells

nanoparticles redispersed in water was added to an excess amount of silver colloid (∼40 mL) (step 4). The yellow colored colloidal silver changed to blue-gray within 30 min, and after 2 h the formation of a blue-gray precipitate was observed, indicative of the aggregation of small silver nanoparticles. Afterward, the precipitate was washed and resuspended by sonication. Both steps 3 and 4 were repeated to fabricate more compact and thicker Au/Ag nanoshells. After the each step, all samples were centrifuged (Sigma, 3K30) at speed of 4000 rpm, washed using water, and redispersed in water. Through the procedures, the pH was adjusted to an appropriate value (∼4.0-5.0) by adding HNO3 or NH3‚H2O. Procedure for Film Consisting of Composite Au/Ag Nanoshells at the Air-Water Interface. After purification involving centrifuging and redispersing, in the presence of 1.5 × 10-3 M NaCl, the freshly prepared composite Au/Ag nanoshells were placed in a centrifuge tube (10 mL) without any disturbance. After 2 h, the film consisting of composite Au/Ag nanoshells was spontaneously formed at the air-water interface, which could be seen even with naked eyes. The film can be transferred onto the copper grid, quartz substrates for TEM, XPS, and Raman measurement, respectively. Characterization. TEM and XPS measurement were performed on a transmission electron microscope (JEOL 2000-FX) and X-ray photoelectron spectrometer (VG ESCA MKII), respectively. SERS spectra were measured with a Renishaw 1000 model confocal microscopy Raman spectrometer. The radiation wavelength is 514.5 nm, and the laser power is 100 µW.

Results and Discussion Figure 1a,b shows representative TEM micrographs from steps 1 and 2 of the procedure described in Scheme 1. As evident from the figure, modifications of silica nanoparticles with APTMS and centrifugation/redispersion steps have no effect on their size and morphology as determined by TEM images. These APTMS molecules bond to the surface of the silica nanoparticles and extend their amine groups outward. When a solution of colloidal gold is mixed with the APTMS-modified silica nanoparticles, these small gold nanoparticles can bond covalently to the APTMS molecules via the amine groups and are well separated from each other. The Au coverage of silica nanoparticles was evaluated to be approximately 25% from the magnified TEM images, consistent with previous reported coverage of gold nanoparticles onto aminemodified surface of solid sustrates.15 The SiO2@Au nanoparticles with high Au coverage can be obtained by a reaction of HAuCl4 and ascorbic acid on the surface of gold nanoparticles.14 (15) (a) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, J. Science 1995, 267, 1629. (b) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 21148.

Figure 1. TEM micrographs of (a) APTMS-modified SiO2 nanoparticles and (b) SiO2@Au nanoparticles. Inset: corresponding magnified TEM images.

4-Aminothiophenol (4-ATP), consisting of -NH2 and -SH groups, are bifunctional molecules. When mixed with SiO2@Au nanoparticles, these bifunctional molecules bond to the surface of the small gold particles immobilized in silica nanopartilces via -SH groups, extending their -NH2 groups outward as a new termination of the nanoparticles surface. After the addition of 4-ATP-modified SiO2@Au nanoparticles to excess silver colloid, small silver nanoparticles can bond covalently to the 4-ATP molecules via the -NH2 groups. It is crucial to modify SiO2@Au nanoparticles with 4-ATP molecules because the 4-ATP modification quality of SiO2@Au nanoparticles has a direct effect on the resulting composite Au/ Ag nanoshells. We failed to prepare more compact and thicker composite Au/ Ag nanoshell using SiO2@Au nanoparticles without 4-ATPmodification. This may be attributed to the absence of covalent interaction between small Ag nanoparticles and the -NH2 groups under the above condition. Typical TEM images of composite Au/Ag nanoshells with three and five silver layers are shown in Figure 2. As seen from the magnified TEM image (Figure 2c), rough aggregates with stringlike structures were formed in great abundance on the surface of silica nanoparticles. These rough aggregates attached to silica nanoparticles are robust, which can be confirmed by the fact that the centrifugation, sonication, and redispersion steps did not destroy such structures. Also, it was found that the thickness and surface roughness of composite Au/Ag nanoshells were gradually increased with the increase of silver layers on the surface of goldcoated silica nanoparticles. The high roughness inherent in composite Au/Ag nanoshells due to the fabrication process is more desirable for SERS application, which will provide an additional enhancement mechanism.6,16 Moreover, the composite Au/Ag nanoshells with tuned ratio of the dielectric silica core to Au/Ag shell are easily produced by repeating steps 3 and 4. This characteristic of composite Au/Ag nanoshells can be beneficial for SERS because control of silver layer allows for the optimization of the SERS enhancement at a particular pump frequency.6 Previous studies4,15b had shown that the repulsive electrostatic interactions between the colloidal particles apparently prevented higher coverage of colloidal particles on the surface of substrates being achieved. Thus, it is difficult to fabricate compact and thicker composite Au/ Ag nanoshell simply through a self-assembly procedure. As for our present work, one possible explanation for successful fabrication of compact and thicker composite (16) Bozhevolnyi, S. I.; Markel, V. A.; Coello, V.; Kim, W.; Shalaev, V. M. Phys. Rev. B 1998, 58, 11441.

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Figure 3. TEM image of aggregates of composite Au/Ag nanoshells with five layers of silver nanoparticles at larger scale.

Figure 2. Representative TEM images of composite Au/Ag nanoshells with (a) three layers and (b) five layers of silver nanoparticles. (c) Magnified TEM images corresponding to (b).

Au/Ag nanoshell involves the aggregation of small silver nanoparticles on the surface of 4-ATP-modified SiO2@Au. In colloidal silver solution, the surface-adsorbed citrate cause the silver nanoparticles to be charged, and the ionic strength of the solution creates a Debye-Hu¨ckel screening length of only a few atomic distances.17 The resulting repulsive double-layer interaction between the particles makes the colloidal silver stable against aggregation. However, the addition of 4-ATP-modified SiO2@Au and electrolyte (HNO3 or NH3‚H2O) reduces the repulsion between particles and, as a result, increases the van der Waals attractive forces between them which allow particles to stick when they collide.17 During the heterogeneous aggregation, the attached silver nanoparticles on the surface of SiO2@Au grow and start to merge with neighboring particles. In the procedure, the silver shell on the surface of silica nanoparticles resembles a fractal network of an aggregated colloid, which results in the formation of a rough stringlike structure on the surface of SiO2@Au. Brust et al. also observed similar experimental phenomenon during C60-mediated aggregation of gold nanoparticles.18 Our explanation can be evidenced by the existence of peal-necklace aggregates observed in solution and on the surface of composite Au/Ag nanoshells. Although there exists some advantage for the application of composite Au/Ag nanoshells in SERS, a major problem is the tendency of their aggregation, which makes the metal nanoshells unstable in solution and often results in the poor reproducibility of the SERS spectra. At the same time, the aggregation is a prerequisite for strong SERS enhancement. Our approach to resolving this enigma is to prepare composite Au/Ag nanoshell films that combines the above-mentioned desirable features. We found that, in the presence of 1.5 × 10-3 M NaCl, when the freshly prepared composite Au/Ag nanoshells was placed a centrifuge tube without any disturbance for (17) Hurd, A. J.; Schaefer, D. W. Phys. Rev. Lett. 1985, 11, 1043. (18) Brust, M.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. J. Am. Chem. Soc. 1998, 120, 12367.

2 h, the aggregation of composite Au/Ag nanoshells confined to two dimensions occurred at the air-water interface. The aggregation process can be rationalized as follows: Particle aggregation (considering doublet formation) at the interface depends primarily on the particle pair interaction potential.19 In this case, the interactions that stabilize the aggregates may be thought to result from the repulsive electrostatic forces and attractive van der Waals interactions.20-22 In the presence of high-level stray electrolytes (1.5 × 10-3 M NaCl), the resultant shielding sharply diminished the repulsion between the charged nanoshells, and the van der Waals attractive forces prevail, allowing the nanoshells to attract when they collide.17 Nanoshells in the bulk phase may be assumed to migrate toward the interface by the Brown motion,19 whereas the trapping of nanoshells is known to occur as the result of the surface tension of the air-water interface.17,23 When a single nanoshell comes into collision with existing clusters by the Brownian motion at the airwater interface, it attracts itself to them. Similar collision between one cluster and another cluster also allows many clusters to merge and aggregate among themselves. When more and more nanoshells are trapped, the number of trapped nanoshells and clusters increases and nanoshell films grow up. Finally, through nanoshell-cluster and cluster-cluster collisions, the growth of the cluster leads to heterogeneous, highly ramified networks formed throughout the whole interface.24 Importantly, the resultant aggregates can be transferred to solid substrates without destruction (Figure 3). As observed in Figure 3, these aggregates possess an open low-density structure. Similar results have been observed in the aggregation of silica microspheres and metal colloids at the air-water interface.17,24 X-ray photoelectron spectra (XPS) of such aggregates consisting of composite Au/Ag nanoshells with five layers (19) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Adv. Mater. 2000, 12, 693. (20) Williams, D. F.; Berg, J. C. J. Colloid Interface Sci. 1992, 152, 218. (21) Wickman, H. H.; Korley, J. N. Nature 1998, 393, 445. (22) Onoda, G. Y. Phys. Rev. Lett. 1985, 55, 226. (23) Chan, D. Y. C.; Henry, J. D.; White, L. R. J. Colloid Interface Sci. 1981, 79, 410. (24) (a) Witten, T. A.; Sander, L. M. Phys. Rev. Lett. 1981, 47, 1400. (b) Hu, J. W. Zhao, B.; Xu, W. Q.; Fan, Y. G.; Li, B. F.; Ozaki, Y. J. Phys. Chem. B 2002, 106, 6500.

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Figure 5. SERS spectra of R6G molecules obtained from (a) the solution of composite Au/Ag nanoshells with five layers of silver nanoparticles and aggregates of composite Au/Ag nanoshells with (b) three layers and (c) five layers of silver nanoparticles, respectively. The concentration of R6G molecules in (a) and (b-c) are 100 nmol/L and 1 nmol/L, respectively.

molecules as probes. Figure 5 compare the SERS signal intensities of an R6G molecule obtained from the solution of composite Au/Ag nanoshells (Figure 5a) and from their aggregates (Figure 5b,c). As indicated in the figure, these aggregates transferred onto the silicon substrate show a significantly strong SERS signal corresponding to the Raman band for the R6G molecule relative to the solution of composite Au/Ag nanoshells. The above results indicate that the aggregates consisting of composite Au/Ag nanoshells prepared as presented are highly efficient SERS active substrates, and their strong enhancement ability is closely related to the unique two-dimensional structure itself. Conclusion Figure 4. XPS spectra for aggregates of composite Au/Ag nanoshells with five layers of silver nanoparticles: (a) Ag 3d orbital; (b) Au 4f orbital.

of silver nanoparticles show the significant Ag 3d signal corresponding to the binding energy of metallic Ag and very weak Au 4f signal characterative of metallic Au (Figure 4). The results reveal that the surface composition of as-prepared samples is dominated by Ag atoms. According to previous theoretical and experimental studies, these aggregates consisting of rough composite Au/Ag nanoshells could be desirable SERS active substrates.25 The application of aggregates of composite Au/ Ag nanoshells in SERS was investigated by using R6G (25) (a) Stockman, M. I.; Shalaev, V. M.; Moskovits, M.; Botet, R. George, T. F. Phys. Rev. B 1992, 46, 2821. (b) Creighton, J. A.; Blatchford, C. G.; Albecht, M. G. J. Chem. Soc., Faraday Trans. 1979, 275, 790.

In summary, composite metallic nanoshells were fabricated on nanosized silica spheres by an aggregationbased method. The thickness and surface roughness of such nanoshells could be simply controlled by aggregation of silver nanoparticles onto the surface of silica nanoparticles. Importantly, these rough composite Au/Ag nanoshells could spontaneously form aggregates confined to the air-water interface. Such aggregates of composite Au/Ag nanoshells exhibited excellent surface-enhanced optical properties, which will play an important role in the extension of novel SERS application. Acknowledgment. This work is supported by the National Natural Science Key Foundation of China (Grant No. 20171043) and the National Key Project for Fundamental Research of Rare Earth Functional Materials. LA034738G