Rationally designed SERS active silica coated silver nanoparticles

Article abstract of DOI:10.1039/c0cc05338d

A reproducible method for the successful silica coating of silver nanoparticles is reported. A selection of tri-functional reporter molecules were designed to allow controlled synthesis of silica shelled, dye coded, SERS active silver nanoparticles.

Full text of DOI:10.1039/c0cc05338d

View Article Online / Journal Homepage / Table of Contents for this issue
This article is part of the
Surface Enhanced Raman
Spectroscopy web themed issue
Guest editors: Professors Duncan Graham,
Zhongqun Tian and Richard Van Duyne
All articles in this issue will be gathered together online at
www.rsc.org/sers.

Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 4415–4417
www.rsc.org/chemcomm
COMMUNICATION
Rationally designed SERS active silica coated silver nanoparticleswz
Louise Rocks, Karen Faulds and Duncan Graham*
Received 2nd December 2010, Accepted 22nd February 2011
DOI: 10.1039/c0cc05338d
A reproducible method for the successful silica coating of silver
nanoparticles is reported. A selection of tri-functional reporter
molecules were designed to allow controlled synthesis of silica
shelled, dye coded, SERS active silver nanoparticles.
The silica shell is considered to ‘‘physically sequester’’ the
nanoparticle core allowing exploitation of the optical, electrical,
and catalytic properties of the core.12 Silica growth can be
achieved using Stober synthesis.13 This involves ammonium
¨
hydroxide-catalysed hydrolysis and condensation of alkoxy-
silanes in low molecular-weight alcohols. The main limitation
to the silica coating of metallic nanostructures is the inherent
vitreophobic nature of noble metals. The relative chemical
instability of silver, in comparison to gold, poses an additional
obstacle in the silica coating of silver nanoparticles. Ammonium
hydroxide readily oxidises silver to form soluble complex
ions resulting in the formation of excess core-free silica
particles.14 Furthermore, transferring metallic nanoparticles
Surface enhanced Raman scattering (SERS) has been employed
in the development and improvement of numerous biological
detection systems.1,2 The output consists of narrow spectral
bands highlighting the multiplexing capabilities of this techni-
que.3 One approach that can be exploited to improve the
sensitivity of this technique is to make use of the resonance
contribution from a dye molecule which has an electronic
transition close to the excitation frequency. This is known
as surface enhanced resonance Raman scattering (SERRS).
Sensitivity of this detection system is excellent and in some
cases can rival that of fluorescence.4,5
into an alcohol/water solution, as per the Stober method, may
¨
result in multiple silver cores within the silica shell due to the
reduction in dielectric constant. For this reason vitreophilising
agents, namely mercapto and amino functionalised silane
coupling agents, have been employed to facilitate the silica-
coating of both silver and gold nanoparticles.12,15 Self assembled
monolayer coverage of the silane coupling agents enhances the
stability of the core in the harsh conditions necessary for silica
growth and provides a vitreophilic surface for the growth of a
silica shell via alkoxysilane condensation.
Metallic nanoparticles, especially the coinage metals, have
proven to be useful substrates for SE(R)RS analysis. Decreasing
the size of the particles to the nanometre range results in
significant absorption in the UV/visible range which can
influence the optical properties in both near and far field.6
The vast majority of biological cellular SE(R)RS analysis
has utilised gold nanoparticles, due to the biocompatible
nature of the material, their mobility and versatility.7 Whilst
the SERS enhancement of silver nanoparticles is typically
greater than that observed with gold analogues,8 there has
been some concern regarding the cytotoxic properties of silver
and as such their biocompatibility.9 The requirement for
aqueous compatible nanoparticles for use within a biological
environment has led to the development of core-shell systems
to isolate the inner metal core from the outer biological
environment. The use of nanoparticle cores, both metallic
and semi-conductor, functionalised with polyethylene glycol
(PEG) shells has been shown to be widely applicable to cellular
analysis.10,11 Similarly, silica is being used increasingly for the
shelling of nanoparticles to provide a robust substrate with
consistent surface chemistry.
Raman reporter molecules such as fluorescein isothiocyante
have previously been incorporated into the silica coated
nanoparticles to produce SERS active probes for biological
applications.16 These methods often employ mixed monolayer
coverage of Raman active molecules and vitreophilising
precursors. Competition for surface binding of the afore-
mentioned molecules can result in reduced reproducibility of
SERS signal, since the amount of reporter molecules per
nanoparticle may not be homogenous. Successful silica
encapsulation of gold nanospheres has been achieved via
covalent attachment of silane precursors to complete monolayers
of Raman reporters17 and also monolayer coverage of terminal
alkoxysilane Raman reporters.18 Similarly, a resonant molecule
capable of providing a vitreophillic surface should provide a
suitable precursor for silver/ silica core/shell nanotags with
increased sensitivity.
A series of tri-functional linkers based on benzotriazole dyes
were synthesised for conjugation to the surface of silver
nanoparticles (Fig. 1).19 Benzotriazole dyes have previously
been synthesised and designed specifically to complex directly
to the surface of silver nanoparticles through the benzotriazole
moiety.20 Alkoxy silane functionality can be introduced to the
dyes (Dye 1) to provide a vitreophillic surface. Thus, they can
Centre for Molecular Nanometrology, WestCHEM, Department of
Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral
Street, Glasgow, UK. E-mail: duncan.graham@strath.ac.uk;
Tel: +44 1415484701
w This article is part of a ChemComm web-based themed issue on
Surface Enhanced Raman Spectroscopy.
z Electronic supplementary information (ESI) available: SEM images,
UV-visible experimental procedures. See DOI: 10.1039/c0cc05338d
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4415–4417 4415

cores has occurred. Nanotags prepared using Dye 4 exhibited
minimal agglomeration of the nanoparticle cores.
The SE(R)RS properties of the four tri-functional precursors
were investigated, post-silica coating of the silver nanoparticles,
using 3 excitation wavelengths (Fig. 2c). The intensity of the
peak at 1615 cmꢁ1 was standardized against cyclohexane
intensity at 800 cmꢁ1. No significant peaks were observed from
any of the samples using 785 nm excitation.
The SERS enhancement was found to be greater for the
nanotags synthesised from Dyes 1 and 3 (with alkoxysilyl and
amino functionality) which is most likely due to formation of
multiple cores within the sample. In the case of Dye 1, this
is most likely due to premature siloxane bridging between
adjacent nanoparticle-dye conjugates in the alkaline alcohol
solution prior to addition of TEOS. Dye 3 was observed to
induce aggregation of nanoparticles in an aqueous environment
prior to silica coating (ESI, Fig. S2z). This is most likely a
result of electrostatic interactions between the protonated
primary amine of Dye 3 with the bare silver nanoparticles.
The UV visible data illustrated in Fig. 2a and b indicates Dyes
2 and 4, with carboxyl and hydroxyl functionality, respectively,
have increased reproducibility with respect to monodispersity
of the nanotags.
Fig. 1 Tri-functional benzotriazole dyes designed as precursors for
silica encapsulation of silver nanoparticles.
be considered to act as precursors for the development of an
integrated network of siloxane linkages via the Stober method.
¨
Silver nanoparticles (AgNP) were synthesised according to
the previously published method by Fabrinkos et al.21 Dye 1
was added to colloidal suspensions to a final concentration of
1 ꢀ 10ꢁ7 M. Silica coating was achieved using a modified
Stober method (see supporting informationz). Due to the
¨
oxidizing capabilities of ammonium hydroxide, a more sterically
hindered amine base, triethylamine, was used as a catalyst.
Previous work has indicated that even one methyl substitute
on ammonium hydroxide is enough to reduce the rate of
oxidation of silver nanoparticles.22 Tetraethyl orthosilicate
(TEOS) was introduced to the nanoparticle–precursor conjugates
in small aliquots over a three hour period until a final
concentration of 5.4 mM was achieved. Continuous agitation
of the samples was employed throughout the shelling process.
Silica coating was successful using the alkoxysilyl functional
Dye 1, however the formation of multiple silver cores was
evident (ESI, Fig. S1z).
SEM images of Ag@SiO2 nanotags synthesised from Dyes
2–4 illustrate that shelling of the silver nanoparticles was
successful using the tri-functional precursors (ESI, Fig. S3z).
The samples synthesised from Dye 3 exhibited numerous
nanoparticle cores within the silica shell. The nanotags
synthesised from Dyes 2 and 4 showed evidence of monomers
and smaller aggregates within shells. This is in agreement with
the data observed in Fig. 2. All four rationally designed dyes
are capable of initiating silica shelling of silver nanoparticles,
however Dyes 2 and 4 produce nanotags with reduced
polydispersity. The SERS intensity of nanotags synthesised
from Dye 2 is approximately twice that observed from the Dye
4 analogues. Thus the carboxyl functionalised benzotriazole
dye is determined to be the optimal dye for silica coating of
silver nanoparticles. Similarly, non resonant Raman reporters
with terminal carboxyl groups should prove to be equally
effective precursors.
Dyes 2–4 also possess functional groups capable of undergoing
condensation with TEOS. These dyes were added to colloidal
suspensions of AgNP to a final concentration of 1 ꢀ 10ꢁ7
M
prior to silica coating via the outlined modified Stober method
¨
(see supporting informationz). The resulting AgNP@SiO2
(core@shell) ‘‘nanotags’’ were washed and analysed by UV
visible spectroscopy (Fig. 2a and b). The data was normalised
to take into account the variation in nanoparticle concentration
within the samples. Conjugates prepared from Dye 2 did not
exhibit significant aggregation of the AgNP. Dampening of
the surface plasmon and increased absorbance at longer
wavelengths were observed in the nanotags prepared from
Dyes 1 and 3. This suggests aggregation of the nanoparticle
Earlier work in this field has employed mixed monolayers of
vitreophilising precursors and dye molecules to synthesise
SE(R)RS active nanotags. We have demonstrated that certain
dyes can act as both a resonant molecule for SERRS and
a precursor for the silica shelling of silver nanoparticles.
Fig. 2 A comparison of the (a) UV visible (b) full width at half height measurements and (c) SERRS spectra observed at 2 wavelengths from
nanotags synthesised using the 4 benzotriazole dyes identified in Fig. 1. Spectra are the average of triplicate samples (the error bars are ꢂ1 standard
deviation).
c
4416 Chem. Commun., 2011, 47, 4415–4417
This journal is The Royal Society of Chemistry 2011

Fig. 3 A comparison of the (a) UV visible and (b) SERRS spectra observed (514 nm excitation) from AgNP (yellow), nanotags synthesised from
full monolayer of Dye 2 (red), mixed monolayer APTMS/Dye 2 (blue) and mixed monolayer MPTMS/Dye 2 (green). Spectra are an average of
triplicate samples.
To highlight the increased reproducibility of the SERS enhance-
ment of nanotags synthesised from monolayers of the
rationally designed dyes in comparison to mixed monolayers,
vitreophilising precursors 3-aminopropyl trimethoxysilane
(APTMS) and 3-mercaptopropyl trimethoxysilane (MPTMS)
were added to the silver colloidal suspension to a final con-
centration of 1 ꢀ 10ꢁ6 M. The samples were agitated for a
period of 0, 15 or 60 min prior to addition of Dye 2 to a final
concentration of 1 ꢀ 10ꢁ7 M.
suitable site of growth for the silica greatly enhanced the
reproducibility of both the synthesis and the SERS. This has
resulted in the synthesis of reproducible SERS active silver/
silica, core/shell nanotags which will find use in a wide range of
applications where gold has previously been used but offering
the alternative optical properties of silver.
Notes and references
1 D. Graham and K. Faulds, Chem. Soc. Rev., 2008, 37, 1042.
2 X. Han, B. Zhao and Y. Ozaki, Anal. Bioanal. Chem., 2009, 394,
1719.
3 K. Faulds, F. McKenzie, W. E. Smith and D. Graham, Angew.
Chem., 2007, 119, 1861.
4 G. Sabatte, R. Keir, M. Lawlor, M. Black, D. Graham and
W. E. Smith, Anal. Chem., 2008, 80, 2351.
5 K. Faulds, R. P. Barbagallo, J. T. Keer, W. E. Smith and
D. Graham, Analyst, 2004, 129, 567.
6 M. Quinten, Z. Phys. D: At., Mol. Clusters, 1995, 35, 217.
7 J. Kneipp, H. Kneipp, M. McLaughlin, D. Brown and K. Kneipp,
Nano Lett., 2006, 6, 2225.
8 R. J. Stokes, A. Macaskill, P. J. Lundahl, W. E. Smith, K. Faulds
and D. Graham, Small, 2007, 3, 1593.
9 P. V. AshaRani, G. Low Kah Mun, M. P. Hande and
S. Valiyaveettil, ACS Nano, 2008, 3, 279.
10 X. H. Gao, Y. Y. Cui, R. M. Levenson, L. W. K. Chung and
S. M. Nie, Nat. Biotechnol., 2004, 22, 969.
Irreversible aggregation of the silver nanoparticles was
induced in samples incubated with the silane precursors for
15 and 60 min (ESI, Fig. S4a and bz). Addition of Dye 2
simultaneously with the silane precursors did not induce
significant aggregation of the silver nanoparticles, however
variation between sample replicates was significantly increased
(ESI, Fig. S4cz). Raman measurements were also obtained
from all conjugates pre-silica coating at 514 nm excitation
(ESI, Fig. S5z). The significant variation in SERS intensity
observed in the samples incubated with APTMS/MPTMS for
a period of 15 or 60 min can largely be attributed to the
aggregation as observed in the UV visible spectra. When
APTMS and Dye 2 were added simultaneously, the SERS
intensity was 2.75 times greater than in the absence of a silane
precursor. However, the relative standard deviation of the
sample replicates increased from 14.6% to 62.7%. When
MPTMS and Dye 2 were added simultaneously, the preferential
binding of sulfur to the silver surface results in minimal
binding of Dye 2 and thus the SERS intensity was depleted.
The resulting AgNP@SiO2 ‘‘nanotags’’ were washed and
analysed by UV visible and Raman spectroscopy (Fig. 3).
While the greatest enhancement is observed from nanotags
prepared from an APTMS/Dye 2 mixed monolayer, the
relative standard deviation of the SERS intensity is 3 times
greater than in the absence of a silane precursor (92.5% cf.
28.3%). Samples prepared using a mixed monolayer coverage
of dye and MPTMS had reduced SERS intensity but increased
variation in SERS intensity at 1615 cmꢁ1 when compared to
samples prepared in the absence of a silane precursor (102.1%
cf. 28.3%).
11 X. M. Qian, X. H. Peng, D. O. Ansari, Q. Yin-Goen, G. Z. Chen,
D. M. Shin, L. Yang, A. N. Young, M. D. Wang and S. M. Nie,
Nat. Biotechnol., 2008, 26, 83.
12 S. P. Mulvaney, M. D. Musick, C. D. Keating and M. J. Natan,
Langmuir, 2003, 19, 4784.
13 W. Stober, A. Fink and E. Bohn, J. Colloid Interface Sci., 1968,
¨
26, 62.
14 T. Ung, L. M. Liz-Marzan and P. Mulvaney, Langmuir, 1998, 14,
3740.
15 L. M. LizMarzan, M. Giersig and P. Mulvaney, Langmuir, 1996,
12, 4329.
16 X. Liu, M. Knauer, N. P. Ivleva, R. Niessner and C. Haisch, Anal.
Chem., 2009, 82, 441.
17 B. Kustner, M. Gellner, M. Schutz, F. Schoppler, A. Marx,
¨
¨
¨
¨
P. Strobel, P. Adam, C. Schmuck and S. Schlucker, Angew. Chem.,
¨
Int. Ed., 2009, 48, 1950.
18 M. Schutz, B. Kustner, M. Bauer, C. Schmuck and S. Schlucker,
¨
¨
Small, 2010, 6, 733.
¨
19 G. McAnally, C. McLaughlin, R. Brown, D. C. Robson,
K. Faulds, D. R. Tackley, W. E. Smith and D. Graham, Analyst,
2002, 127, 838.
Four tri-functional molecules have been synthesised to
stabilise a silver nanoparticle core, act as a Raman reporter
and provide a suitable precursor for an integrated network of
20 D. Graham, G. McAnally, J. C. Jones and W. Ewen Smith, Chem.
Commun., 1998, 1187.
21 V. Fabrikanos, S. Athanassiou and K. Leiser, Z. Naturforsch. B:
Chem. Sci., 1963, 18, 612.
22 Y. Kobayashi, H. Katakami, E. Mine, D. Nagao, M. Konno and
L. M. Liz-Marzan, J. Colloid Interface Sci., 2005, 283, 392.
silica polymers via Stober condensation. The use of a silane
¨
precursor was found to be unnecessary, however use of a
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4415–4417 4417