Ag Nanostructured Films

In this case, the choice of a proper silver plating solution is vital to the ... SEM image of (C) an ordered hollow Au/Ag nanostructured film (sample ...
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Langmuir 2006, 22, 2605-2609

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Designed Fabrication of Ordered Porous Au/Ag Nanostructured Films for Surface-Enhanced Raman Scattering Substrates Lehui Lu,† Alexander Eychmu¨ller,‡ Atsuko Kobayashi,§ Yoshiaki Hirano,† Kenichi Yoshida,† Yasuo Kikkawa,† Keiko Tawa,§ and Yukihiro Ozaki*,† Department of Chemistry, School of Science and Technology, Kwansei Gakuin UniVersity, Sanda, Hyogo 669-1337, Japan, Technical UniVersity of Dresden, Institute of Physical Chemistry and Electrochemistry, Bergsstrasse 66b, D-01062 Dresden, Germany, and National Institute of AdVanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan ReceiVed September 30, 2005. In Final Form: December 15, 2005 This article reports the designed preparation of two different kinds of novel porous metal nanostructured films, namely, an ordered macroporous Au/Ag nanostructured film and an ordered hollow Au/Ag nanostructured film. Different from previous reports, the presently proposed method can be conveniently used to control film structures by simply varying the experimental conditions. The morphology of these films has been characterized by scanning electron microscopy (SEM), and their performance as surface-enhanced Raman scattering (SERS) substrates has been evaluated by using rhodamine 6G (R6G) as a probe molecule. We show that such porous nanostructured films consisting of larger interconnected aggregates are highly desirable as SERS substrates in terms of high Raman intensity enhancement, excellent stability, and reproducibility. The interconnected nanostructured aggregate, long-range ordering porosity, and nanoscale roughness are important factors responsible for this large SERS enhancement ability.

1. Introduction The need for surface-enhanced Raman scattering (SERS) substrates with good stability, ease of and reproducible preparation, and high-intensity enhancement has been well recognized in its practical applications. The use of colloidal gold and silver nanoparticles has been the most popular methods for SERSactive substrates because of their easy preparation and highintensity enhancement in previous reports.1-6 Unfortunately, the disadvantages of such nanoparticles in terms of stability and reproducibility limit their applications in sensors and chemical assay. To resolve these problems, methods such as selfassembly,7-10 metal evaporation,11,12 deposition,13 and the mirror reaction14 have been developed to prepare 2D metal films on solid supports. Compared with colloidal nanoparticles, the resulting 2D nanoparticle films show significantly better stability, controllability, and reproducibility. However, these 2D nano* Corresponding author. E-mail: [email protected]. † Kwansei Gakuin University. ‡ Technical University of Dresden. § National Institute of Advanced Industrial Science and Technology (AIST). (1) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Chem. ReV. 1999, 99, 2957. (2) Schwartzberg, A. M.; Grant, C. D.; Wolcott, A.; Talley, C. E.; Huser, T. R.; Bogomolni, R.; Zhang, J. Z. J. Phys. Chem. B 2004, 108, 19191. (3) Nie, S.; Emory, S. R. Science 1997, 78, 1667. (4) Faulds, F.; Littleford, R. E.; Graham, D.; Dent, G.; Smith, W. E. Anal. Chem. 2004, 76, 592. (5) Schatz, G. Acc. Chem. Res. 1984, 17, 370. (6) Hu, J. W.; Zhao, B.; Xu, W. Q.; Fan, Y. G.; Li, B. F.; Ozaki, Y. J. Phys. Chem. B 2002, 106, 6500. (7) Freeman, R. G.; Graber, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. C.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (8) Stuart, D. A.; Yonzon, C. R.; Zhang, X. Y.; Lyandres, O.; Shah, N. C.; Glucksberg, M. R.; Walsh, J. T.; Van Duyne, R. P. Anal. Chem. 2005, 77, 4013. (9) Yu, H. Z.; Zhang, J.; Zhang, H. L.; Liu, Z. F. Langmuir 1999, 15, 16. (10) Li, X. L.; Xu, W. Q.; Zhang, J. H.; Jia, H, Y.; Yang, B.; Zhao, B.; Li, B. F.; Ozaki, Y. Langmuir 2004, 20, 1298. (11) Vo-Dinh, T. Trends Anal. Chem. 1998, 17, 557. (12) Rowe, I. E.; Chank, C. V. Phys. ReV. Lett. 1980, 44, 1770. (13) Maxwell, D. J.; Emory, S. R.; Nie, S. M. Chem. Mater. 2001, 13, 1082. (14) Saito, Y.; Wang, J. J.; Smith, D. A.; Batchelder, D. N. Langmuir 2002, 18, 2959.

particle films lack advanced characteristics such as porous structures and periodicity that have been shown to improve material performance.15,16 Recent studies in this field indicated that 3D highly ordered macroporous metal nanostructured films prepared by a colloidal crystal-templating method could provide an excellent platform.17,18 For instance, Velev et al.17,18 successfully used latex microspheres as a template to fabricate SERSactive porous gold nanoparticle films. These 3D gold nanostructured films supported by a glass slide are stable and can be easily and reproducibly fabricated. Importantly, because of their unique structural characters including long-range ordering and tunable porosity, excellent SERS enhancement can be achieved by judiciously controlling the experimental conditions, which may give an excellent alternative for the design of novel nanostructured biosensors. To the best of our knowledge, however, there are only a few reported examples of such nanostructured films for SERS applications, and these examples mainly focus on macroporous gold nanostructures.17,18 We have not been aware of reports on other macroporous nanostructures such as silver, another good candidate, or other porous nanostructures (for example, hollow structures) for SERS applications. Our motivation in the present study is to construct novel metal nanostructured films with advanced features including longrange ordering and porosity. It is expected that such novel nanostructured films can exhibit unique optical properties. On the basis of this idea, we successfully design two different kinds of ordered porous Au/Ag nanostructured films. A significant advantage of the proposed method is that an ordered macroporous nanostructured film and an ordered hollow nanostructured film can be selectively fabricated by simply changing the experimental conditions. The application of these films as SERS substrates is (15) Moody, R. L.; Vo-Dinh, T.; Fletcher, W. H. Appl. Spectrosc. 1987, 41, 966. (16) Van Duyne, R. P.; Hulteen, J. C.; Treichel, D. A. J. Chem. Phys. 1993, 99, 2101. (17) Tessier, P. T.; Velev, O. D.; Kalambur, A. T.; Rabolt, J. F.; Lenhoff, A. M.; Kaler, E. W. J. Am. Chem. Soc. 2000, 122, 9554. (18) Kuncicky, D. M.; Christesen, S. D.; Velev, O. D. Appl. Spectrosc. 2005, 59, 401.

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first investigated by using R6G as a probe molecule. We show that the as-prepared films are extremely efficient SERS substrates in terms of high Raman intensity enhancement, excellent stability, and reproducibility. The interconnected nanostructured aggregate, long-range ordering porosity, and nanoscale roughness are considered to be important effect factors for this large SERS enhancement ability. 2. Experimental Section Preparation of a Gold-Coated Silica Film. Citrate-stabilized 3-nm gold nanoparticles were prepared according to ref 19. The 267-nm silica spheres were synthesized by the modified Sto¨ber method20 and were functionalized by 3-aminopropyltrimethoxysilane (APTMS).21 The APTMS-modified silica spheres were fabricated onto the glass substrates by a solvent evaporation method.22 Next, the resultant 3D silica films were immersed in the gold nanoparticle solution. To fabricate different porous nanostructured films, a gold concentration of 0.02 mM and an immersion time in the gold colloidal solution of 8 h were required to obtain the hollow nanostructured film, and a gold concentration of 0.3 mM and an immersion time in the gold colloidal solution of 16 h were needed to prepare the macroporous nanostructured film. For comparison, the disordered silica film was fabricated by adding a small amount of larger APTMSmodified silica spheres (300 nm in diameter) during the assembly of the 267-nm APTMS-modified silica spheres. Preparation of the Silver Plating Solution and Porous Au/Ag Nanostructured Films. Typically, 1.2 mL of 28-30% NH3‚H2O was slowly added to 50 mL of AgNO3 (0.12 M) solution under vigorous stirring to obtain a transparent solution, which was followed by quickly mixing 50 mL of solution containing 5.0 g of potassium sodium tartrate. Then the resultant Au-coated silica films were immersed in the plating solution for 8 min. Finally, the as-prepared Au/Ag-coated silica films were dipped into a 4% HF solution for 4 min to remove the silica spheres. Before use for SERS examination, the samples were washed briefly with deionized water and ethanol and dried with N2 gas at room temperature. The disordered silica film was also used as a template to prepare a disordered hollow Au/Ag nanostructured film. Fabrication of Other SERS Substrates. To compare the SERS ability of the as-prepared samples with those previously reported, the mirror reaction and self-assembly methods were used to prepare 2D Ag films as control experiments. The Ag film from the mirror reaction was fabricated by using the procedure of Saito et al.14 Fiftysix nanometer silver colloids were synthesized by the citrate-reduction method of Lee et al.23 and assembled onto the APTMS-modified glass slides according to the procedure of Natan et al.7 Instruments. Scanning electron microscopy (SEM) images were taken in a field-emission SEM (Hitachi S-5000) operated at an acceleration voltage of 20 kV. Raman spectra were obtained with an NRS-2100 model Raman spectrometer (JASCO) by using a 514.5nm excitation laser. The laser power at the samples’ position was typically 7.2 mW. The data acquisition time was 10 s with two accumulations.

3. Results and Discussion 3.1. Fabrication and Morphology of Ordered Porous Au/ Ag Nanostructured Films. The present synthetic procedure builds upon the recent success in the fabrication of ordered porous nanostructures.24-34 To facilitate the incorporation of the nanoparticles and the selective fabrication of different nanostructures, (19) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 2000, 12, 306. (20) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (21) Lu, L. H.; Capek, R.; Kornowski, A.; Gaponik, N.; Eychmu¨ller, A. Angew. Chem., Int. Ed. 2005, 44, 5997. (22) Jiang, P.; Bertone, J. F.; Kwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (23) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (24) Brown, P. V.; Wiltzius, P. Nature 1999, 402, 603. (25) Yang, P. D.; Deng, T.; Zhao, D. Y.; Feng, P. Y.; Pine, D.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. Science 1998, 282, 2244.

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uniformly sized silica spheres were first functionalized with 3-aminopropyltrimethoxysilane (APTMS) and closely packed on a glass slide. The resultant APTMS-functionalized silica crystal template was soaked in the aqueous solution containing gold nanoparticles. By controlling the soak time and the concentration of the gold solution, we could fabricate different kinds of nanostructures. Then, the silica template with gold nanoparticles was immersed in a freshly prepared silver plating solution for several minutes. In this case, the choice of a proper silver plating solution is vital to the successful fabrication of porous films with high SERS performance. Our initial attempt to use the plating solution prepared according to ref 10 was less successful, probably because of a faster reaction between [Ag(NH3)2]+ ions and glucose. Subsequently, the silica crystal template was removed to obtain tough, self-supporting porous nanostructured films. Similar seeding/electroless deposition methods were used by Jiang et al. to obtain macroporous metal nanostrutured films33 and by Chen et al. to prepare hollow silver nanostructures.34 However, there are several important differences between the method described in this article and those previously reported. First, in our experiment 3-nm citrate-stabilized gold nanoparticles were chosen instead of others. The use of such nanoparticles has several advantages. (1) They can be prepared easily and reliably in this size range and are desirable for further metal plating and the preparation of uniform nanostructures. (2) No further heat is required to remove organics from the gold surface, which is of great importance to the successful preparation of ordered hollow nanostructured films. (3) Although no heating treatment is performed to strengthen the template in our work, tough freestanding porous Au/Ag nanostructured films can be fabricated and transferred from a glass slide onto other substrates for various applications (different from the case in ref 33). (4) Close-packed APTMS-modified silica films supported on a glass slide were used as templates, and small gold nanoparticles of the desired size are well separated on the surface of the silica spheres in only one step, which yields an alternative for the fabrication of other novel nanostructures (different from the case in ref 34). Importantly, by using the presently proposed method we can selectively fabricate an ordered macroporous Au/Ag nanostructured film and an ordered hollow Au/Ag nanostructured film by simply controlling the experimental conditions. Also, the resultant porous Au/Ag nanostructued films are much different in terms of the constituent particle size, morphology, and composition. In fact, the morphology, structures, and resulting properties of the porous nanostructured films that are prepared by the crystaltemplating method strongly depend on the combination of experimental procedures such as the modification of the template and the choice of the nanoparticle seed and the plating solution.21 Figure 1A presents a scanning electron microscopy (SEM) image of a Au/Ag-coated silica film (sample A). The hexagonally ordered arrangement of the original silica spheres is observed. The corresponding higher-magnification SEM image clearly indicates the formation of rough Au/Ag nanoshells on the surface of the silica spheres (Figure 1B). An SEM image of a typical (26) Holland, B. T.; Blanford, C. F.; Stein, A. Science 1998, 281, 538. (27) Wijnhoven, J. E. G. J.; Vos, W. L. Science 1998, 281, 802. (28) Park, S. H.; Xia, Y. N. Chem. Mater. 1998, 10, 1745. (29) Wang, D. Y.; Caruso, R. A.; Caruso, F. Chem. Mater. 2001, 13, 364. (30) Johnson, S. A,; Ollivier, P. J.; Mallouk, T. E. Science 1999, 283, 963. (31) Blanco, A.; Chomski, E.; Grabtchak, S.; Ibisate, M.; John, S.; Leonard, S. W.; Lopez, C.; Meseguer, F.; Miguez, H.; Mondia, J. P.; Ozin, G. A.; Toader, O.; van Driel, H. M. Nature 2000, 405, 437. (32) Yan, H. W.; Blanford, C. F.; Holland, B. T.; Parent, M.; Smyrl, W. H.; Stein A. AdV. Mater. 1999, 11, 1003. (33) Jiang, P.; Cizeron, J.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 7957. (34) Chen, Z.; Zhan, P.; Wang, Z. L.; Zhang, J. H.; Zhang, W. Y.; Ming, N. B.; Chan, C. T.; Shen, P. AdV. Mater. 2004, 16, 417.

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Figure 1. SEM image of (A) a Au/Ag-coated silica film (sample A) and (B) a corresponding higher-magnification image. SEM image of (C) an ordered hollow Au/Ag nanostructured film (sample B) and (D) a corresponding higher-magnification image.

hollow Au/Ag nanostructured film made by removing silica spheres of sample A is shown in Figure 1C (sample B). The image reveals that after the removal of the silica template both the long-range ordering and the spherical shape are well preserved from the original template. A higher-magnification SEM image shown in Figure 1D elucidates the fact that these relatively uniform Au/Ag shells consist of many larger interconnected aggregates, which results in the formation of a rough surface, and its thickness is estimated to be about 56 nm. The hollow feature of the asprepared samples can be evidenced by the broken Au/Ag shell as illustrated in Figure 1D. APTMS-functionalized silica crystals can also serve as templates for the fabrication of macroporous Au/Ag nanostructured films by a proper combination of experimental conditions. Figure 2A shows a typical SEM image of the resultant templatefree macroporous Au/Ag nanostructured film (Sample C). As illustrated in Figure 2A, open voids and interconnected walls consisting of larger interconnected aggregates form a pore structure. These pores are well ordered in a hexagonal packing arrangement, and such an ordered structure can be extended to several hundreds of micrometers. From the higher-magnification image of Figure 2B, it is clearly observed that there are three dark regions inside each pore corresponding to the air spheres of the underlying layer, indicating that these spheres are indeed close-packed.17,18,21,24-34 3.2. Applications. To evaluate the effectiveness of the three above-mentioned kinds of porous Au/Ag nanostructured films for SERS applications, rhodamine 6G (R6G) is used as a touchstone because it is a dye molecule often used in SERS studies because of its enormous intensity enhancement and absorbability onto silver particles. Previous SERS studies suggested that the presence of chloride ions had an activation effect, which has been considered to create “active sites” for Raman enhancement.13,35 A similar activation effect is also observed in the present study. We found that the SERS signal (35) Hildebrandt, P.; Stockburger, M. J. Phys. Chem. 1984, 88, 5935.

intensities were increased by about 6 times in the presence of 10 mM KCl (Figure 3). Figure 4 compares the 514.5-nm excitation SERS spectra of 50 nM R6G in the presence of 10 mM KCl obtained from four different substrates. In this case, a Ag film is prepared by the mirror reaction as a control experiment because such a film is a good SERS substrate according to previous reports.14,36 SERS signals corresponding to R6G are observed in each case, but clearly the porous Au/Ag nanostructured films exhibit the highest SERS enhancement ability. The SERS intensity at 1362 cm-1 for the porous Au/Ag nanostructured film is about 2.5 times higher than that for the Au/Ag-coated silica film and about 8 times higher than that for the Ag film obtained by the mirror reaction (Figure 4C, D). Importantly, it is noted that the SERS performance of such ordered porous nanostructued films has been stable for at least 3 months. One of the advanced characteristics of the as-prepared films is their ordered structure, thus it is important to investigate the effect of the ordered structure on the SERS enhancement. Figure 5A and B presents SERS spectra of R6G molecules on sample B and a nonordered hollow Au/Ag nanostructured film, respectively. A multifold increase was observed in the SERS intensity for R6G molecules on sample B, relative to the nonordered hollow Au/Ag nanostructured film. The possible reasons for the high Raman intensity enhancement are explained as follows. From the nanostructure viewpoint, the SERS enhancement efficiencies depend on the particle size.13 As-prepared samples A-C mainly consist of nanoparticles in the size range of 50-80 nm; such a particle size is very efficient for SERS enhancement excitation at 514.5 nm.13 As observed in Figures 1 and 2, these nanoparticles further interconnect to form many porous aggregates similar to the Ag nanoparticle film from the mirror reaction. Excited by incident light, the collective surface plasmons are localized at these interconnected nanostructured aggregates, leading to the formation of a local field in this region that is many orders of magnitude higher than (36) Wang, Z. J.; Pan, S. L.; Krauss, T. D.; Du, H. Rothberg, L. J. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 8638.

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Figure 4. SERS spectra of 50 nM R6G on different substrates. (A) Ag film prepared by the mirror reaction; (B) sample A; (C) sample C; and (D) sample B. The incubation time was 30 min.

Figure 2. SEM image of (A) an ordered macroporous Au/Ag nanostructured film (sample C) and (B) a corresponding highermagnification image. Figure 5. Comparison of SERS intensities between (A) sample B and (B) a disordered hollow Au/Ag nanostructured film. The concentration of R6G was 20 nM. The incubation time was 30 min.

Figure 3. SERS spectra of 50 nM R6G (A) without and (B) with KCl solution on sample B. The incubation time was 30 min.

the incident light.36,37 The localized resonant plasmon modes can contribute to larger SERS enhancement.7,36-41 The number (37) Garcia-Vidal, F. J.; Pendry, J. B. Phys. ReV. Lett. 1996, 77, 1163. (38) Jackson, J. B.; Halas, N. J. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 17930. (39) Campion, A.; Kambhampati, P. Chem. Soc. ReV. 1998, 27, 241. (40) Mulvaney, S. P.; Keating, C. D. Anal. Chem. 2000, 72, 145R.

of SERS active sites in samples A-C increase because of the presence of a large number of interconnected nanoparticle junctions,2 and thus it is not surprising to observe the multifold increase in SERS intensity enhancement compared with that of the Ag nanoparticle film from the mirror reaction. In this case, nanoscale roughness is another important factor contributing to SERS enhancement because it plays an important role by providing pathways for the hot electrons to the probe molecules, which results in SERS enhancement by chemical effects.1 Moreover, from the microstructure viewpoint, different from the Ag nanoparticle film from the mirror reaction, samples A-C possess ordering porous structure. As indicated in Figure 1A, long-range ordering porous structures are formed between Au/ Ag-coated silica spheres. The presence of these porous structures can increase the surface area of the substrates, which allows more R6G molecules to adsorb per unit surface of the substrates.18 Furthermore, we can approximately regard Au/Ag-coated silica spheres within films as large metal spheres. These large metal spheres come into contact with one another, resulting in the construction of many periodic crevices. According to Pendry et al.,37 a large SERS enhancement can be achieved at these crevice (41) Michaels, A. M.; Jiang, J.; Brus, L. J. Phys. Chem. B 2000, 104, 11965.

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the surface area of samples B and C increased by less than 1.5 times compared with that of sample A. Interestingly, as mentioned above, the SERS intensity at 1362 cm-1 for samples B and C is about 2.5 times higher than that of sample A. This result reveals that the higher SERS enhancement of samples B and C relative to that of sample A may not be explained only by the increase in surface area. A detailed SERS enhancement mechanism will be investigated in future work. In addition to high-intensity enhancement, it was also found that the SERS spectra are highly reproducible on the porous nanostructured films. Figure 6 demonstrates the SERS spectra of R6G measured at different points on the ordered hollow Au/ Ag nanostructured film. The deviation of the peak intensity at 1362 cm-1 is calculated to be about 2-8% from area to area on the film.

4. Conclusions Figure 6. SERS spectra of 20 nM R6G on the different points of sample B. The incubation time was 30 min.

sites because of the enormous electromagnetic field. For instance, Xu et al. reported that an electromagnetic enhancement of 1010 was present between two nanopsheres separated by 1 nm.42 Furthermore, in the periodic metal structure a photon density of states redistribution may readily occur according to Gaponenko.43 This redistribution can also yield superior SERS enhancement. After removing silica spheres, samples B and C were obtained. Samples B and C retained the original features of sample A such as long-range ordering porous structure and interconnected aggregate structures as indicated in Figures 1B and 2, and the surface areas of samples B and C are increased further because of the removal of silica spheres. Our calculations44 indicates that (42) Xu, H. X.; Bjerneld, E. J.; Ka¨ll, M.; Bo¨rjesson, L. Phys. ReV. Lett. 1999, 83, 4357. (43) Gaponenko, S. G. Phys. ReV. B 2002, 65, 140303R.

In summary, we have reported how ordered macroporous Au/ Ag nanostructured films and ordered hollow Au/Ag nanostructured films are successfully designed using a modified silica crystal-templating method at room temperature. The as-prepared ordered porous Au/Ag nanostructured films are used for the first time as SERS substrates and exhibit good SERS enhancement ability, excellent reproducibility, and stability, which may find practical application for routine SERS analysis. Acknowledgment. We thank Dr. Harumi Sato of Kwansei Gakuin University for the XRD examination. LA052659U (44) The surface area ratio of sample B to sample A was roughly calculated as follows. Sh/Ss ) 4π(r12 + r22)/4π(r12) ) 1 + (r2/r1)2 ) 1 + (134/(134 + 56))2 ≈ 1.5. Sh and Ss are the surface areas of samples B and A, respectively. r1 and r2 are the radii of a Au/Ag-coated silica sphere and a silica sphere, respectively.