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: akirafu@fchem.chem.t.u-tokyo.ac.jp.
‡ 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) Procházka, M.; Moješ, P.; Vlcková, B.; Turpin, P.-Y. J. Phys. Chem.
B 1997, 101, 3161-3167.
(10) Rivas, L.; Sanchez-Cortes, S.; Garcı́a-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
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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
Garcı́a-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) Garcı́a-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.
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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+