9490
Langmuir 2003, 19, 9490-9493
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.; Salgueiriño-Maceira, V.; Liz-Marzán, 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
Roughened Composite Metallic Nanoshells
Langmuir, Vol. 19, No. 22, 2003 9491
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.
9492
Langmuir, Vol. 19, No. 22, 2003
Lu et al.
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-Hückel 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.
Roughened Composite Metallic Nanoshells
Langmuir, Vol. 19, No. 22, 2003 9493
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