Gold aggregates on silica templates and decorated silica arrays for SERS applications

Article abstract of DOI:10.1140/epjd/e2011-10605-7

In this work, we report the fabrication and characterization of size controllable gold nanoparticles (NPs) aggregates for their application in surface enhanced Raman scattering (SERS). Aggregates were prepared using two methodologies: (i) by using silica

Full text of DOI:10.1140/epjd/e2011-10605-7

Eur. Phys. J. D 63, 301–306 (2011)
DOI: 10.1140/epjd/e2011-10605-7
THE EUROPEAN
PHYSICAL JOURNAL D
Regular Article
Gold aggregates on silica templates and decorated silica arrays
for SERS applications
F. Castillo1,a, E. De la Rosa2, and E. P´erez3
1
Doctorado en Ingenier´ıa y Ciencia de Materiales (DICIM), Universidad Aut´onoma de San Luis Potos´ı, Av. Manuel Nava #6,
Zona Universitaria, 78290 San Luis Potos´ı, M´exico
2
´
Centro de Investigaciones en Optica, A.C. A.P. 1-948, Le´on Gto., 37150, Mexico
3
Instituto de F´ısica, Universidad Aut´onoma de San Luis Potos´ı, Av. Manuel Nava #6, Zona Universitaria,
78290 San Luis Potos´ı, Mexico
Received 20 October 2010 / Received in final form 12 January 2011
c
Published online 9 June 2011 – ꢀ EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2011
Abstract. In this work, we report the fabrication and characterization of size controllable gold nanoparticles
(NPs) aggregates for their application in surface enhanced Raman scattering (SERS). Aggregates were
prepared using two methodologies: (i) by using silica particles arrays as a template to agglomerate gold
NPs between the inter-particle interstices, and (ii) by functionalizing silica particles to be used as support
to graft gold nanoparticles and thus to form decorated silica particle arrays. These substrates were used in
the detection of Rhodamine 6G producing an enhancement factor (EF) from 104 to 106 that is associated to
the increment of hot spot (HS) sites, and the fact that plasmon resonance from aggregates and absorption
wavelength of test molecules are closely in resonance with excitation wavelength. The EF was also reduced
when the plasmon resonance was red-shifted as a result of the increment of aggregate size. In spite of this,
the EF is high enough to make these SERS substrates excellent candidates for sensing applications.
1 Introduction
the morphology of Au NPs resulting on irregular par-
ticles and poor reproducibility in the formation of ag-
gregates [18,19]. Zhang et al. reported gold nanoparti-
cles arrays (GNAs) obtained by using Na2S as a reducer
agent during their synthesis that produces Au NPs ag-
gregates [20]. Aggregation process is driven by slightly
variations in reagent (Na2S) concentration. However, the
GNAs produce a broad distribution of sizes and shapes
inducing a broad plasmon band (600–1000 nm). As conse-
quence, they estimated that around 1% of the aggregates
are on resonance with the excitation wavelength. There-
fore, a controllable synthesis of nanoparticles aggregates
is actually the major goal for SERS optimization.
Surface enhanced Raman scattering (SERS) is a high sen-
sitive spectroscopic tool for low concentration detection
of analytes, including single molecule detection [1–4]. The
physical mechanism associated to such enhancement has
been widely studied and reported elsewhere [4–11]. The
most accepted mechanism is based on the electromagnetic
field (EM) enhancement due to the optical field localiza-
tion on metal nanostructures, and the electronic enhance-
ment due to the increase of Raman cross-section [12,13].
Experimental evidences and theoretical simulations indi-
cate that maximum enhancement factor (EF) occurs when
target molecules are located near a metal surface and that
enhancement rapidly attenuates within a few nanometers
away from the surface [14,15].
Recently, it has been reported an extremely high SERS
signal (∼1010) at the junction of two or more nanopar-
ticles, such active sites named hot spot (HS) represent
position of maximum enhancement of EM [16,17]. Since
these reports, several efforts have been done to fabricate
aggregates with well-defined junction sites.
In this way, Wang et al. have proposed an alternative
method to fabricate nanoparticles substrates by grafting
Au NPs onto functionalized SiO2 particles by using silane-
coupling agents with different functional groups [21], they
can control Au NPs aggregation and selective adsorption
of ClO−4 on functionalized nanoparticles substrates. These
Au NPs decorated SiO2 substrate increase the probabil-
ity of finding a HS until ∼1/50, which can be compared
with 1/1000 using pure metallic colloids [22]. Another
way to fabricate SERS substrates was proposed by Yan
et al. based on the use of a PMMA mask on a glass-gold
slide substrate [23]. A solution of Au NPs was deposited
onto the slide, and after solvent evaporation, a 2D pattern
of nanoparticles aggregates were obtained. This approach
provides a control over the size of the particle aggregates
Kneipp et al. reported strong SERS signals produced
by the aggregation of gold (Au) nanoparticles (NPs)
formed by the addition of sodium chloride (NaCl) to the
gold solution. However, the addition of NaCl destroys
a
e-mail: jjfcr02@gmail.com

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The European Physical Journal D
and their spatial location on the substrate. In this case,
SERS experiments show high reproducibility in different
areas on the substrate and in different substrates fabri-
cated with this approach. However, this method is limited
to produce only 2D arrays.
Box (400 cm3)
Silica particles.
Heat at 275o
Substrate.
Gold nanoparticles.
Substrate.
Peltier cell
9o
9o
In this work, we report the preparation and the sys-
tematic characterization of SERS response of Au NPs ag-
gregates prepared on substrates by two methodologies:
(i) a silicon slide with an arrangement of silica particles
as a template to fabricate Au NPs aggregates between the
inter-particle interstices, (ii) functionalized silica particles
used as support for grafting Au NPs and thus forming dec-
orated silica particle arrays. These substrates were used to
study the SERS response as a function of the aggregates
size. The goal of this work is to extend the already existing
methodologies to control the size and shape of aggregates
for applications as SERS substrates and to analyze the
resonant conditions imposed by a fixed excitation wave-
length on the plasmon resonance of the NPs aggregates.
We use as test molecule rhodamine 6G and the excitation
wavelength was of 514 nm.
Fig. 1. (Color online) Schematic representation of experimen-
tal setup used to obtain aggregates of Au NPs using the col-
loidal crystal template.
2.4 Assembly of Au NPs aggregates templated
by colloidal crystal
The experimental setup consists of a Peltier cell with a
temperature controller surrounded by a small plastic box
to keep out external air flow and contamination, see Fig-
ure 1. The system is tilted 9◦ and temperature inside the
box was kept at 17 ◦C. The silicon substrate (SW) was
cleaned with H2SO4 and rinsed with deionized water and
ethanol. A droplet (20 µL) of silica particles suspension is
spread on the SW substrate. Since the substrate is tilted, a
thin water film was formed while the residual water forms
a droplet at the lower side of the substrate. Evaporation
of water starts from the top of the sample on a horizontal
border where evaporation takes place, and moves to the
bottom until substrate is completely dried. Evaporation
process promotes colloidal crystal arrangement of silica
particles in the SW surfa◦ce. The dried SW substrate was
annealed for 3 h at 275 C to improve crystallinity and
eliminate the residual solvent. An aliquot of 10–40 µL of
gold solution (3.5 × 108−1.418 × 109 nanoparticles) was
deposited onto the colloidal crystal template and dried
under the same experimental condition.
2 Experimental
2.1 Reagents
Tetrachloroauric acid (HAuCl 3H2O), sodium citrate
(HOC(COONa)(CH2COONa)2·2H2O), deionized water,
tetraethyorthosilicate (Si(OC2H5)4, TEOS), 3-amino-
propyl-trimethoxysilane (APTMS), 3-(trimethoxy-silyl)
propyl-methacrylate (MPTMS), ammonium hidroxide
(NH4OH) and rhodamine 6G (C28H31N2O3Cl) ethanol
(CH3CH2OH) were used as received for sample prepa-
ration and SERS characterization. All chemical products
were purchased from Sigma-Aldrich. Silicon wafers (SW)
of 1 cm2 and orientation ꢀ100ꢁ, resistivity 0.01–0.02 Ω cm
and thickness of 500–550 µm were purchased from Silicon
Valley.
2.5 Silica particles decorated with Au NPs
Silica particles were functionalized with APTMS and
MPTMS in different relations (5:95, 30:70 and 50:50). The
treatment produces a silica particle surface terminated
with amine and methyl groups. Organosilane-coated silica
particles were mixed with a suspension of Au NPs lead-
ing to a selective adsorption on the silica particle surface.
Au NPs are attached on amine terminate groups, while
regions with methyl terminated were used as spacers to
control their agglomerations [26].
2.2 Synthesis of gold nanoparticles
1 mL of tetrachloroauric acid (1 mM) was dissolved in
20 mL of deionized water. This solution was boiling under
stirring for 10 min. Then, 25 mg of sodium citrate was
added to the solution that was stirred for 30 min. The
solution underwent a series of color changes and finally
turning into wine red colloidal suspension [24].
3 Results and discussion
2.3 Synthesis of silica particles
3.1 Au NPs characterization
A solution of 10 mL ethanol, 3 mL of water and 0.75 mL
of ammonium hydroxide were stirred at room temperature The particle size was measured by TEM (JEM
and then 1.2 mL of TEOS was added dropwise into the 1230 JEOL). The average size was 15 3 nm obtained by
solution. After 3 h of reaction, the silica particles were measuring more than 500 nanoparticles. Figure 2 shows
centrifuged at 3000 rpm for 30 min and rinsed several the plasmon resonance for these nanoparticles, while the
times [25].
inset shows a typical TEM image.

F. Castillo et al.: Gold aggregates on silica templates and decorated silica arrays for SERS applications
303
Fig. 2. Plasmon resonance of Au NPs solution centered at
527 nm. The inset shows a typical TEM image.
Fig. 4. Red-shift of plasmon resonance band as a function of
aggregate size.
Fig. 5. TEM image of silica core decorated with Au NPs.
Fig. 3. (a) Optical microscope image of colloidal crystal of
silica used as a template, the size of image is ∼50 µm×40 µm.
(b) SEM image of Au NPs aggregates obtained during the de-
position of gold nanoparticles in the template.
APTMS:MPTMS volume ratio used during the function-
alization of silica particle. Contrary to other works [22],
this methodology allows the control the density of Au
NPs adsorbed onto silica particle by a simple variation
of APTMS:MPTMS ratio, an increment of Au NPs den-
sity attached on silica surface was observed by changing
the APTMS:MPTMS ratio from 5:95 to 50:50, such incre-
ment produces hot spot sites, and induces a red-shift of
the plasmon resonance peak, as is observed in Figure 6.
The shifting is the result of the plasmon resonance origi-
nated by the decreasing of distance between nanoparticles
grafted on silica particle at high Au NPs densities. The
maximum shift of ∼16 nm respect to individual plasmon
band was observed for particles functionalized with 50:50
APTMS:MPTMS ratio. This behavior is due to the for-
mation of Au NPs aggregates on silica arrays.
3.2 Aggregates characterization
Figure 3 shows an optical microscope image of colloidal
crystal on SW surface. As expected, the array of silica par-
ticles was regular with an average particle size of ∼1.3 µm.
Au NPs were deposited into the interstices of silica array
forming aggregates. The size of aggregates depends on the
silica particle size and the Au NPs concentration on the
solution deposited on silica particles array. The size in-
crement of aggregate induces a red-shift of the plasmon
resonance band as is shown in Figure 4. The maximum
red-shift observed as a function of Au NPs concentration
was approximately 23 nm respect to the plasmon reso-
nance peak in Figure 2.
3.4 Surface enhanced Raman scattering (SERS)
3.3 Silica decorated with Au NPs
Au NPs aggregates formed on the interstice of colloidal
Figure 5 shows a typical TEM picture of 1.3 µm silica core silica template and gold decorated silica, both deposited
decorated with Au NPs. Distribution of gold nanopar- on SW substrate, were used to enhance the Raman sig-
ticles grafted over the silica surface depends on the nal of rhodamine 6G (R6G) at concentrations as lower as

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The European Physical Journal D
Fig. 6. Red-shift of resonance plasmon by increasing Au NPs
concentration attached to silica core.
10−6 M. No Raman signal was observed for this concen-
tration without the presence of Au NPs. The adsorption of
R6G molecules onto gold aggregates and decorated silica
were made by depositing a drop (∼20 µL) of rhodamine
on an area ∼1 cm2 and dried at room temperature. Ra-
man signal was recorder by using a Renishaw micro Ra-
man system with 100× objective (Leica, N.A 0.85) at the
excitation wavelength of 514 nm. The incident intensity
is about 20 mW on the sample, and the integration time
was of 10 s. Regions investigated with this objective have
an area of ∼1 µm2. Signal without gold nanoparticles was
measure at a concentration of 1 × 10−2 M and used as
reference. SERS signal was produced by 1.52 × 105 R6G
molecules and reference by 1.88 × 107 molecules.
Fig. 7. (a) R6G SERS spectra of Au NPs aggregates templated
by a colloidal silica crystal. (b) Behavior of intensity of Raman
signal of R6G for different sizes of aggregates.
3.5 SERS on gold nanoparticles aggregates
The Raman spectrum for R6G deposited on aggregates
of gold nanoparticles template-by the colloidal silica crys- excitation wavelength is not completely in resonance with
tal is shown in Figure 7a. Characteristic Raman signal at each plasmon band, an enhancement factor (EF) of ∼106
1646 cm−1 for different aggregates size are shown in the was obtained with SG1. Such EF is two orders of magni-
inset of Figure 7a, where is observed that band positions tude higher than that obtained for a porous thin film of
do not change with the aggregates size. The same behavior gold fabricated in a similar way than our aggregates and
was observed in different parts of the same substrate (data reported recently [27]. Furthermore, it is ∼102 higher than
not showed here), suggesting a homogeneous distribution the response obtained with a substrate with dispersed Au
of aggregates along the SW surface.
NPs (no agglomeration). However, as the size of aggre-
The Raman signal of R6G in these colloidal silica tem- gates increases (sample SG2, SG3, and SG4) the plasmon
plates is increased as the size of Au NPs aggregate de- resonance band moves away from resonance with the ex-
creases. This is shown in Figure 7a for four aggregates citation source. As a consequence, sample SG4 have the
sizes, corresponding to 3.5 × 108, 7 × 108, 1.063 × 109 lowest Raman enhancement, being five times lower than
and 1.41 × 109 particles deposited on silica template and that obtained for SG1. Such strong difference remarks the
called SG1, SG2, SG3, and SG4, respectively. The maxi- importance of Au NPs agglomeration size that defines its
mum signal is observed for sample SG1 and the minimum plasmon resonance. The EF is based on the equilibrium
for sample SG4 (Fig. 7b). The behavior for sample SG1 is between the HS formation and the plasmon band position
attributed to the fact that the plasmon resonance of Au of the NPs agglomeration. This behavior is shown in Fig-
aggregates and the absorption wavelength of test molecule ure 7b where the maximum enhancement of Raman signal
are closely in resonance with the excitation wavelength for occurs when the plasmon resonance of aggregates is close
the sample, which is the opposite case for SG4. Even if the to wavelength of the excitation laser.

F. Castillo et al.: Gold aggregates on silica templates and decorated silica arrays for SERS applications
305
Fig. 9. Behavior of Raman signal intensity of R6G for different
densities of Au NPs on silica surface.
4 Discussion
Fig. 8. R6G SERS spectra of decorated silica for three different
densities of Au NPs adsorbed on silica surface.
Two experimental approaches for the preparation of Au
NPs aggregates have been presented. Template of silica
particles and Au NPs decorated silica particles provide
systematic procedure for the preparation of size-controlled
aggregates. The EF was found between 104 and 106 and it
is associated to the generation of HS as a result of the ade-
quate separation between NPs. The agglomeration of NPs
increases the HS sites that in turn increase the EF, but
an excess of Au NPs gets them closer breaking the con-
dition for optimum enhancement of the electromagnetic
field. In addition, an excess of NPs could promote coales-
cence. The electromagnetic field of agglomerated NPs is
coupled resulting on a red-shift of the plasmon resonance.
This property could be an advantage because it provides
a mechanism to move the plasmon resonance to a position
closer to the resonance with the excitation wavelength. In
our particular case, plasmon position moves away from
resonance producing a decreasing effect on the EF. These
results remark the importance of aggregate size control in
order to match the plasmon resonance spectrum with ex-
citation wavelength for the detection of low molecular con-
centration. This matching condition could be maintained
by changing the excitation wavelength, which is not al-
ways accessible for the common commercial Raman appa-
ratus. However, it is possible to obtain good SERS signal
using gold aggregates whose plasmon resonance spectrum
partially matches the excitation wavelength.
3.6 SERS from silica decorated with Au NPs
Figure 8 shows the Raman spectrum of rhodamine 6G
deposited on three different samples of decorated silica
system. As observed, the Raman signal decreases from
sample Sh1 to sample Sh3. The former one corresponds
to the sample with the lowest density of Au NPs obtained
from APTMS:MPTMS ratio of 5:95. This result shows
that a variation in the APTMS:MPTMS ratio produces
also a variation in the NPs aggregates, therefore this ra-
tio not only defines the distance separation between NPs
on the silica particle, but also the NPs aggregates when
these particles form arrays on the SW. The reduction of
Raman signal for larger content of MPTMS, and then
high concentration of Au NPs on the silica particle, forms
larger aggregate size and the aggregate plasmon band is
red-shifted. The Raman signal intensity of 1646 cm−1
peak was measured at different densities of attached Au
NPs, and its values are shown in the inset of Figure 8.
The calculated EF was ∼8.8 × 104 for Sh1 and 2.3 × 104
for Sh3.
Figure 9 shows the Raman signal intensity as a func-
tion of plasmon position peak of the NPs aggregates
for samples Sh1, Sh2, and Sh3. As was discussed above
for the aggregates formed on the silica templates, the
plasmon resonance band was also red-shifted as the ag-
5 Conclusion
glomerate size increases with the MPTMS content, and In conclusion, control size agglomeration of Au NPs by
the resonance with excitation wavelength was also re- using a silica template and decorating silica particles has
duced, producing a decreasing the Raman signal intensity. been demonstrated. The EF was greater for agglomerate
However, these systems can produce an enhancement of prepared with silica template than for the NPs decorated
Raman signal though the excitation wavelength does not silica particles, as a result of the increase of HS sites. In
match very well with the position of plasmon resonance both cases, the EF diminishes with the excess of NPs; such
of each sample.
effect is produced by the red shift of plasmon resonance

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The European Physical Journal D
band away from resonance with excitation wavelength.
The methodologies for SERS substrate preparation are
simple, reliable and not too expensive, and moreover pro-
duce a high enough enhancement factor for sensing low
concentration of analytes.
8. E.C. Le Ru, E. Blackie, M. Mayer, P.G. Etchegoin, J. Phys.
Chem. C. 111, 13794 (2007)
9. A. Campion, P. Kambhampati, Chem. Soc. Rev. 27, 241
(1998)
10. F.J. Garc´ıa-Vidal, J.B. Pendry, Phys. Rev. Lett. 77, 1163
(1996)
11. J. Grand, M. Lamy de la Chapelle, J.-L. Bijeon, P.-M.
Adam, A. Vidal, P. Royer, Phys. Rev. B 72, 033407 (2005)
12. M.T. Sun, S.B. Wang, Y.J. Liu, Y. Jia, H.X. Xu, J. Raman
Spectrosc. 39, 402 (2008)
13. J.R. Lombardi, R.L. Birke, J. Chem. Phys. 126, 244709
(2007)
14. L. Gunnarsson, E.J. Bjerneld, H.X. Xu, S. Petronis, B.
Kasemo, M. Ka¨ll, Appl. Phys. Lett. 78, 802 (2001)
15. Wei Wang, Zhipeng Li, Baohua Gu, Zhenyu Zhang,
Hongxing Xu, ACS Nano 3, 3493 (2009)
16. A.M. Michaels, J. Jiang, L.J. Brus, Phys. Chem. B 104,
11965 (2000)
The financial support of CONCyTEG grant 09-04-K662-072,
CONACyT grant 49482, and PROMEP are acknowledged.
F. Castillo thanks to CONACYT for the PhD scholarship
(221405). Authors thanks to Q.F.B. Ma. de Lourdes Gonz´alez
Gonza´lez (IF-UASLP) for technical support in the synthesis of
gold and silica particles; M.C. Claudia El´ıas Alfaro and Ing.
Fernando Rodr´ıguez Ju´arez, from (IMUASLP), for her advis-
ing and support on the TEM and SEM characterization of gold
and silica particles, and to Miguel Angel Vidal Borbolla from
IPICYT, for facilities to use Raman spectrometer.
17. A.M. Schwartzberg et al., J. Phys. Chem. B 108, 19191
(2004)
References
18. K. Kneipp, H. Kneipp, R. Manoharan, E.B. Hanlon, I.
Itzkan, R.R. Dasari, M.S. Feld, Appl. Spectrosc. 52, 12
(1998)
19. Rongchao Jin, Angew. Chem. Int. Ed. 49, 2826 (2010)
20. J. Zhang et al., J. Phys. Chem. B 108, 19191 (2004)
21. W. Wang et al., Anal. Chim. Acta 567, 121 (2006)
22. W. Wang et al., Anal. Chim. Acta 567, 121 (2006)
23. B. Yan et al., ACS Nano 3, 1190 (2009)
24. J. Turkenvich, P.C. Stevenson, J. Hillier, Discuss. Faraday
Soc. 11, 55 (1951)
1. S. Nie, S.R. Emory, Science 275, 1102 (1997)
2. K. Kneipp, Y. Wang, H. Kneipp, L.T. Perelman, I. Itzkan,
R.R. Dasari, M.S. Feld, Phys. Rev. Lett. 78, 1667 (1997)
3. M. Moskovits, Rev. Mod. Phys. 57, 783 (1985)
4. Hongxing Xu, J. Aizpurura, M. K¨all, P. Apell, Phys. Rev.
E 62, 3 (2000)
5. G.C. Schatz, R.P. Van Duyne, Electromagnetic mech-
anism of surfaceenhanced spectroscopy, in Handbook of
Vibrational Spectroscopy, edited by J.M. Chalmers, P.R.
Griffiths (John Wiley & Sons, Ltd., 2002), pp. 759–774
6. R. Aroca, Surface-enhanced Vibrational Spectroscopy
(John Wiley & Sons, Chichester, 2006)
25. D. Kandpal, S. Kalele, S.K. Kulkarni, Pramana J. Phys.
69, 2 (2000)
26. S.L. Westcott, S.J. Oldenburg, T.R. Lee, N.J. Halas,
Langmuir 14, 19 (1998)
27. P.M. Tessier, O.D. Velev, A.T. Kalambur, J.F. Rabolt,
A.M. Lenhoff, E.W. Kaler, J. Am. Chem. Soc. 122, 9554
(2000)
7. E.C. Le Ru, P.G. Etchegoin, Principles of Surface
Enhanced Raman Spectroscopy, and related plasmonic ef-
fects (Elsevier, Amsterdam, 2009)