Langmuir 2002, 18, 5043-5046
5043
Metal-Coated Colloidal Crystal Film as Surface-Enhanced Raman Scattering Substrate† Shoichi Kubo,‡ Zhong-Ze Gu,§,| Donald A. Tryk,⊥ Yoshihisa Ohko,‡ Osamu Sato,*,§ and Akira Fujishima*,‡ Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo 113-8656, Japan, Kanagawa Academy of Science and Technology, KSP Bldg, East 412, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan, National Laboratory of Molecular and Biomolecular Electronics, Southeast University, Nanjing 210096, China, and Department of Applied Chemistry, Graduate Course of Applied Chemistry, Tokyo Metropolitan University, 1-1 Minamiohsawa, Hachiohji, Tokyo 192-0397, Japan Received February 19, 2002. In Final Form: April 26, 2002 A dipping technique was developed to coat colloidal crystal films with metals. It was found that nanosized metallic particles could be simply immobilized onto the monodisperse spheres in the colloidal crystal film. As one of the application of such films, silver-coated colloidal crystal films were used as the surfaceenhanced Raman scattering substrates. Very large enhancements were observed. In addition, it was also found that the enhancement factor varies very little from place to place on a particular substrate and even on different substrates. It can be anticipated that such substrates will find wide applications in Raman analysis and Raman imaging.
During the past few years, colloidal crystals have been studied extensively due to their potential applications as photonic crystals.1,2 In addition to their interesting optical properties, colloidal crystals also have unique structural properties such as three-dimensional periodicity and large surface area, which make them useful for sensor and catalysis applications, as well as others.3,4 Recently, it was also reported that colloidal crystal films could be used as substrates for surface-enhanced Raman scattering (SERS).5 A great enhancement of Raman signal, with a high signal-to-noise ratio, was observed when a metallic inverse opal was used as the Raman substrate, which makes such a film an attractive SERS substrate for sensitive Raman analysis. In this paper, we will show another type of SERS substrate derived from colloidal crystals coated with nanoparticles. It will be shown that this kind of substrate has the advantages of excellent uniformity and reproducibility, in addition to the large SERS enhancement. As the fabrication process of such substrate is very simple and inexpensive, it is anticipated that they may have wide applications in Raman analysis and imaging. The procedure for the fabrication of metal-coated colloidal crystal (SCCC) films is outlined in Figure 1. First, a silica colloidal crystal film was deposited on a glass † This work was supported in part by Kanagawa Prefecture JointResearch Project for Regional Initensive, Japan Science and Technology Corporation. * To whom correspondence should be addressed. E-mail:
[email protected]. ‡ The University of Tokyo. § Kanagawa Academy of Science and Technology. | Southeast University. ⊥ Tokyo Metropolitan University.
(1) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059-2062. (2) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals: Molding the Flow of Light; Princeton University Press: Princeton, NJ, 1995. (3) Velev, O. D.; Kaler, E. W. Adv. Mater. 2000, 12, 531-534. (4) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Adv. Mater. 2000, 12, 693-713. (5) Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Rabolt, J. F.; Lenhoff, A. M.; Kaler, E. W. J. Am. Chem. Soc. 2000, 122, 9554-9555.
substrate by the vertical deposition method.6 The monodispersed silica spheres were purchased from Catalysts & Chemicals Industries Co., Ltd. (Japan). The size of the silica spheres is 300 nm. Then, the substrate was immersed into an alcoholic colloidal solution containing 17 wt % silver nanoparticles and 13 wt % polymer species, which was purchased from Nippon Paint Co., Ltd. (Japan). The average size of the silver nanoparticles is 10 nm, and the polymer species stabilize the silver nanoparticles in colloidal solution. Finally, the substrate was raised at a constant rate. During this procedure, both the silver nanoparticles and the polymer molecules are infiltrated into the voids of colloidal crystal film completely by capillary forces and convection fluxes driven by evaporation.7 Following this treatment, the samples were calcined at 300 °C for 1 h in air to remove the polymer species filled in the voids and immobilize the silver particles onto the surfaces of the silica spheres. The cavities were formed in the voids due to removal of the polymer species. Figure 2 exhibits the optical images of the SCCC film. Uniform color can be observed over centimeter distances. Detailed spectral information on the film was derived from UV-vis measurements. From the spectrum (Figure 3), strong plasmon absorption of silver was observed at 440 nm, indicating that the silver nanoparticles were immobilized onto the spheres.8-10 The absorption peak at 645 nm comes from the stop band of the colloidal crystal,4,6 indicating that the ordered structure of the opal film remains after the coating process. This conclusion is also supported by the scanning electron microscopy (SEM) image observation, which are shown in Figure 4. Hex(6) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132-2140. (7) Gu, Z.-Z.; Kubo, S.; Fujishima, A.; Sato, O. Appl. Phys. A 2002, 74, 127-129. (8) Siiman, O.; Bumm, L. A.; Callaghan, R.; Blatchford, C. G.; Kerker, M. J. Phys. Chem. 1983, 87, 1014-1023. (9) Procha´zka, M.; Mojesˇ, P.; Vlckova´, B.; Turpin, P.-Y. J. Phys. Chem. B 1997, 101, 3161-3167. (10) Rivas, L.; Sanchez-Cortes, S.; Garcı´a-Ramos, J. V.; Morcillo, G. Langmuir 2001, 17, 574-577.
10.1021/la020176+ CCC: $22.00 © 2002 American Chemical Society Published on Web 05/25/2002
5044
Langmuir, Vol. 18, No. 13, 2002
Letters
Figure 1. Schematic diagram of the fabrication of SCCC films.
Figure 3. The UV-vis spectrum of an SCCC film.
Figure 2. Optical images of the SERS substrates: 300-nm SiO2 opal film (left); SCCC film (right). The width of the glass slides was 25 mm.
agonal arrangements of silica spheres were observed in the images both before and after coating. p-Toluenethiol was selected as a model compound for the SERS measurements. The procedure to modify the substrates with p-toluenethiol is as follows. First, substrates were soaked in a 1 mM solution of p-toluenethiol in absolute ethanol for 24 h. Then, the substrates were taken out and dried under a stream of nitrogen. Raman spectra were measured in ambient air with a Reninshaw System 2000 imaging microscope (Reninshaw, U.K.). The 514.5 nm Ar+ laser was used as the excitation source. The laser light was focused onto the sample using a 50× objective lens mounted on an Olympus BH-2 microscope, with a spot size of approximately 2 µm. The results are shown in Figure 5. The peaks at 1080 and 1580 cm-1 are the characteristic peaks of p-toluenethiol, indicating that the compound was adsorbed on the substrates. The 2560 cm-1 peak, which cannot be observed in the spectrum, is the characteristic S-H stretching band of p-toluenethiol and can be observed in the powder sample. This result means that the molecules of p-toluenethiol form a mono-
layer on the silver surface via a bond between the thiol group and silver surface.11 To evaluate the SERS effect of the SCCC film, two types of commonly used substrates, one a flat silver film on slide glass prepared by vacuum deposition and two, a silver film roughened by the electrochemical oxidation-reduction cycle (ORC) method,12,13 were also modified with p-toluenethiol by the same treatment described above and used for SERS measurement. The spectra derived from the three types of substrates are shown in Figure 5. The broad background in the 1000-1700 cm-1 is often observed in SERS spectra and is attributed to vibrations of degraded compound formed on the metal surface by laser-induced degradation of the adsorbate.14 The 1700 cm-1 peak observed in the spectrum of SCCC film is also considered to be from the degraded compound. It is apparent that, among the three substrates, the SCCC substrate provided the strongest Raman enhancement with the smallest background signal. The intensity derived from the SCCC substrate was about 40 times larger than that from the flat silver film and about three times larger than that from the ORC-treated substrate. (11) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (12) Cai, W. B.; Ren, B.; Li, X. Q.; She, C. X.; Liu, F. M.; Cai, X. W.; Tian, Z. Q. Surf. Sci. 1998, 406, 9-22. (13) Taylor, C. E.; Pemberton, J. E.; Goodman, G. G.; Schoenfisch, M. H. Appl. Spectrosc. 1999, 53, 1212-1221. (14) Garrell, R. L. Anal. Chem. 1989, 61, 401A-411A.
Letters
Figure 4. SEM images of SiO2 opal films (a) before coating and (b) after coating with silver. The hexagonal arrangement can be observed in both images.
Figure 5. Raman spectra of p-toluenethiol on silver substrates: SCCC substrate, solid line; ORC-treated substrate, dotted line; smooth film of silver, dashed line. Inset: the spectrum of solid p-toluenethiol.
Three possible reasons should be considered as being responsible for the large signal enhancement. The first is the large surface area of the SCCC film. As the SCCC film is a three-dimensional porous film, the surface area is much larger than that of a flat film or an ORC-treated film. As a result, the number of molecules absorbed on the surface of the SCCC substrate is larger than the number absorbed on the other two substrates. A second reason should lie in the specific structural properties of the SCCC film. As shown in Figure 1, the basic framework of the SCCC film is a collection of close-packed spheres, which naturally form many crevices in the film. Crevices of this type are very efficient for the enhancement of Raman
Langmuir, Vol. 18, No. 13, 2002 5045
Figure 6. Raman spectra of p-toluenethiol measured at different points on substrates: (a) SCCC substrate; (b) an ORCtreated substrate.
scattering according to the theoretical result reported by Garcı´a-Vidal and Pendry.15 Additionally, a third reason is that the spheres in SCCC film have surfaces with roughness on the nanometer order after the immobilization of metal nanoparticles. Such roughness can also enhance the Raman scattering.8-10,16,17 In addition to the great enhancement of the Raman signal, it was also found that the dispersion of the signal measured at different points is very small. The spectra measured at different points on the SCCC substrate and ORC-treated substrate are exhibited in Figure 6. From the spectra, it can be deduced that all of the spectra measured at different positions on the SCCC substrate have almost the same intensity and shape, while they are quite dispersed for the ORC-treated substrate. The normalized standard deviations of the peak at 1580 cm-1 are 0.04 for SCCC substrates and 0.71 for ORC-treated substrate. It is apparent that the normalized standard deviation for the SCCC substrates is much smaller than that for the ORC-treated substrates. This means that same results can be obtained at any point on the substrate, which is very important for techniques such as SERS imaging.18,19 This uniformity of the Raman intensity presumably arises from the structural uniformity of the SCCC film. As the opal films are composed of monodisperse (15) Garcı´a-Vidal, F. J.; Pendry, J. B. Phys. Rev. Lett. 1996, 77, 11631166. (16) Bergman, D. J. Phys. Rep. 1978, 43, 377-407. (17) Shalaev, V. M. Phys. Rep. 1996, 272, 61-137. (18) Yang, X. M.; Ajito, K.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. 1996, 100, 7293-7297. (19) Yang, X. M.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Raman Spectrosc. 1998, 29, 725-732.
5046
Langmuir, Vol. 18, No. 13, 2002
spheres, the structure factor is the same at any point on the same film or on different films. In addition, high uniformity can also be expected for different SCCC substrates. In conclusion, we have developed a SERS substrate by taking advantage of a colloidal crystal, which can be fabricated both simply and inexpensively. This type of
Letters
substrate has two remarkable advantages. The first is large SERS enhancement, and second is a very high degree of uniformity of the Raman signal intensity. It can be expected that such substrates will have wide applications in Raman imaging and analysis. LA020176+
