Effects of the surface modification of silver nanoparticles on the surface-enhanced Raman spectroscopy of methylene blue for borohydride-reduced silver colloid

Article abstract of DOI:10.1016/j.molstruc.2010.10.014

The paper further investigated the relationship between the modification of the surface chemistry and the enhancement mechanisms of borohydride-reduced silver particles (BRSC). The bands of residual ions die down while the anomalous bands increase gradually with the increasing of the concentration of Cl ^- and Br^-. It means the residual ions are displaced gradually by the added Cl^- or Br^- and the two halides can lead to the aggregation of the BRSC to a certain extent. However, the most strongly binding anions - I^-, cannot cause any aggregation of silver particles. From the detection of methylene blue (MB), the relationship between the modification of silver surface chemistry and the enhancement mechanisms was discussed. Chloride gives the greatest enhancement while the iodide gives the lowest enhancement among the different kinds of anions. There are also some anomalous bands in the SERS spectra of MB, and these anomalous bands were given rational explanation in this paper.

Full text of DOI:10.1016/j.molstruc.2010.10.014

Journal of Molecular Structure 984 (2010) 396–401
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Effects of the surface modification of silver nanoparticles on the
surface-enhanced Raman spectroscopy of methylene blue for borohydride-reduced
silver colloid
⇑
Xiao Dong, Huaimin Gu , Jian Kang, Xiaojuan Yuan, Jiwei Wu
MOE Key Laboratory of Laser Life Science & Institute of Laser Life Science, College of Biophotonics, South China Normal University, 510631 Guangzhou, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The paper further investigated the relationship between the modification of the surface chemistry and
the enhancement mechanisms of borohydride-reduced silver particles (BRSC). The bands of residual ions
die down while the anomalous bands increase gradually with the increasing of the concentration of Clꢀ
and Brꢀ. It means the residual ions are displaced gradually by the added Clꢀ or Brꢀ and the two halides
can lead to the aggregation of the BRSC to a certain extent. However, the most strongly binding anions –
Iꢀ, cannot cause any aggregation of silver particles. From the detection of methylene blue (MB), the rela-
tionship between the modification of silver surface chemistry and the enhancement mechanisms was
discussed. Chloride gives the greatest enhancement while the iodide gives the lowest enhancement
among the different kinds of anions. There are also some anomalous bands in the SERS spectra of MB,
and these anomalous bands were given rational explanation in this paper.
Received 19 July 2010
Accepted 6 October 2010
Available online 12 October 2010
Keywords:
BRSC
SERS
Modification
Aggregation
Residual ions
Affinity
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
silver surface chemistry. The addition of anions may cause the
aggregation of silver particles, the modification of silver surface
Since the phenomenon of surface-enhanced Raman scattering
(SERS) was first observed in 1974 [1], and was explained partially
by Janmaire in 1977 [2], SERS has excited significant interest in
many research fields [3–10]. Silver island films, silver electrodes,
silver deposited films, and silver colloids are used as SERS-active
substrates [11–17]. Among them, silver colloids are the most com-
monly used SERS-active substrates due to their better enhance-
ment effect and simple preparation. The enhancement of Raman
signal in SERS is related to the so-called ‘‘SERS-active substrates’’
which are various metallic structures with sizes on the order of
tens of nanometers. In general, the explanation is due to two mech-
anisms: the electromagnetic mechanism and the charge transfer
mechanism. Recently, Steven and Narayana studied the competi-
tive binding effect of anions for citrate-reduced silver colloid
[18]. Ramn and his coworkers studied the role of silver nanoparti-
cle surface charge in SERS [19]. Caryn and his coworkers studied
the relationship between the size of silver nanoparticles and the
SERS activity [20]. Jing and his coworkers reported the sculpturing
effect of halideion (Clꢀ) in shape transformation of silver nanopar-
ticles [21].
chemistry (competitive binding effect), the shape transformation
of silver particles and so on. The enhancement of SERS signals have
been reported upon the addition of anions to silver colloids, how-
ever, there are also some reports on the decrease of SERS activity
resulting from the addition of anions. So the effect of the addition
of anions on silver colloid should be further investigated. Although
there are a large amount of literatures on the supplement of
enhancement mechanisms of silver colloids and the optimization
of experimental conditions, the enhancement mechanisms of silver
colloids and the relationship between the enhancement and the
modification of silver particle surface chemistry are not completely
clear yet and should be further investigated.
In this paper, borohydride-reduced silver colloid (BRSC) was
studied for its highly SERS-active features and particle stability. So-
dium chloride, sodium bromide, and sodium iodide was added to
the colloid respectively, to observe the changes of signal from the
colloid and to investigate the relationship between the modifica-
tion of the surface chemistry and the enhancement mechanisms
of borohydride-reduced silver particles. From detecting the SERS
of MB, we intend to find some general principles between the
enhancement of SERS signal and the modification of the surface
chemistry of silver particles. The experiments were operated under
a wide range of concentrations of halideions. To our knowledge,
there are no published studies on the relationship between the
SERS enhancement and the modification of borohydride-reduced
From previous studies, halideions and several other anions have
great influence on the state of silver colloid and the modification of
⇑
Corresponding author. Tel.: +86 20 85216972.
E-mail address: guhm@scnu.edu.cn (H. Gu).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.10.014

X. Dong et al. / Journal of Molecular Structure 984 (2010) 396–401
397
silver surface chemistry. The purpose of this study is to further
investigate the relationship between the modification of borohy-
dride-reduced silver surface chemistry and the SERS activity of
BRSC and find some general modification mechanisms that could
be used to make the experimental conditions suitable for detecting
some analytes in high efficiency.
2. Experimental
2.1. Instrumentation
A 514.5 nm Argon ion laser was used as excitation source. The
laser spectral line width was further spectrally purified with an
interference type laser line bandpass filter and a notch filter
(HSPF-514.5-1.0, Kaiser Optical Syetemes. Inc., USA) was used to
reject the excitation light while allowing the Raman signal to enter
the spectrometer system.
The Raman system was equipped with a BX-41 Olympus micro-
scope and 45ꢁ objective. The spectrometer system (Acton spec-
tro@2300i, Princeton Acton, USA) was used with a 1200 g/mM
holographic grating. An air-cooled CCD detector (Pixis 256, Prince-
ton Acton, USA) was used for measuring the Raman signal by an
integration method. The Raman spectrum and absorption spec-
trum were obtained at room temperature. The absorption spectra
were detected by spectrophotometer (Lamda 35, PE Company,
USA). The data processing was operated by using Origin 7.5
software.
Fig. 1. SERS spectra of borohydride-reduced silver colloid when adding 0.02 M
(black line), 0.04 M (red line), and 0.06 M (blue line) of NaCl. (For interpretation of
the references to colour in this figure legend, the reader is referred to the web
version of this article.)
(6 ꢁ 10ꢀ2 mol dmꢀ3). It shows that in the case of BRSC, the affinity
of residual ions is weak and these residual ions can be easily dis-
placed by more strongly binding anions, such as Clꢀ. The addition
of Clꢀ leads at least two effects to the colloid: the aggregation of
the colloid and the modification of the surface chemistry of the sil-
ver particles. The aggregation properties of silver nanoparticles are
mostly influenced by their surface charge properties [22]. At the
low concentration of Clꢀ, residual ions affinitying on the silver sur-
face are displaced partially by Clꢀ and the silver colloid is aggre-
gated to some extent. And then, the Raman spectrum of residual
ions which have not been displaced by Clꢀ is observed. With the
increase of Clꢀ concentration, residual ions are displaced gradually
and the residual ions features are replaced by the strong Ag–Cl
band.
2.2. Materials
Silver nitrate (AgNO3), sodium borohydride (NaBH4), and meth-
ylene blue (MB) were purchased from Sigma. Sodium chloride
(NaCl), sodium nitrate (NaNO3), sodium sulfate (Na2SO4), sodium
iodide (NaI), and sodium bromide (NaBr) were purchased from
Sangon. The tri-distilled water was used for all solution
preparation.
Fig. 2 shows a series UV–vis spectra of BRSC with different con-
centration of NaCl. The absorption maximum of this kind of silver
colloid is at 492 nm and the full width at half-maximum (FWHM)
is about 50 nm. The absorption spectrum can provide information
on the average particle size and its FWHM can be used to estimate
particle dispersion. From Van Hyning’s study [15], the average size
of silver nanoparticles is about 15 nm. The UV–vis absorption spec-
trum of colloid is closely related to the localized surface plasmon
2.3. Silver colloid and sample preparation
The silver colloid was prepared by Dirk and Charles borohy-
dride-reduced method [14,15]. (a) 8.5 mg AgNO3 was dissolved
in 50 mL H2O to yield 10ꢀ3 M AgNO3 solution. (b) 11.349 mg
NaBH4 was dissolved in 150 mL H2O to yield 2 ꢁ 10ꢀ3 M NaBH4
solution. (c) 50 mL AgNO3 solution was added dropwise to
150 mL of ice-cold NaBH4 solution and the mixture was stirred vig-
orously. (d) The stirring continued for 1 h until glassy yellow col-
our was obtained. The silver colloid was stored in an amber
bottle and was laid in dark place. The UV/vis absorption spectrum
of the colloid has a maximum absorption band at 392 nm with a
full width half-maximum (FWHM) of 50 nm.
The sample that was used for Raman studies was MB. The con-
centration of MB solution was 1 ꢁ 10ꢀ5 mM (to final concentra-
tion). The solution was stored at ambient temperature.
3. Results and discussion
Fig. 1 shows the SERS spectra of BRSC to which a range of so-
dium chloride with different concentrations were added. It can
be seen clearly that as the concentration of sodium chloride in-
creases, the intensity of the band at 241 cmꢀ1 which is assigned
to the Ag–Cl bonds increases distinctively, while the bands at
1363, 1512, 1575, 1650 cmꢀ1 which are assigned to the bands of
residual ions (borohydride ions, its hydrolytic products, the bo-
rates, or nitrate) affinitying on the silver surface decrease and dis-
appear at the highest concentration of sodium chloride
Fig. 2. UV–vis absorption spectrum of the borohydride-reduced silver colloid and
the spectra when adding a wide range of concentrations of NaCl, respectively.

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X. Dong et al. / Journal of Molecular Structure 984 (2010) 396–401
resonances (LSPR) which are collective oscillations of the conduc-
tion electrons confined to metallic nanoparticles and metallic
nanostructures [23–25]. The LSPR is excited when light interacts
with the free electrons of the metallic nanostructure, which lead
to strong enhancements of the local electromagnetic fields sur-
rounding the nanoparticles [25]. The position and the intensity of
the maximum absorption are the characteristic of the type of metal
colloid and highly reflect the dimension, distribution and shape of
the nanostructures [25]. From the figure, it can be observed clearly
that the intensity of the surface plasmon absorption band de-
creases and the FWHM is broadened with the increase of NaCl con-
centration. However, the addition of NaCl at a low concentration
(0.01–0.04 M) does not alter the position of the absorption bands
dramatically. But the plasmon resonance wavelength is red-shifted
(from 392 nm to 405 nm) when adding NaCl at the highest concen-
tration (0.05 M). From the literatures [26–28], the shift of the plas-
mon resonance wavelength as a function of particle separation
reflects the interparticle coupling on the particle plasmon reso-
nance and the aggregating condition of silver colloid. The reso-
nance wavelength of two coupled particles in close proximity is
red-shifted from that of individual particles. It can be concluded
Clꢀ and the repulsive force between particles is reduced, which
leads to the aggregation of particles.
Fig. 3 shows a series of Raman spectra of BRSC to which increas-
ing amounts of NaBr and NaI was added. Addition of Brꢀ at low
concentration leads to the aggregation of silver nanoparticles,
and then the bands of residual ions are observed. With the increase
of Brꢀ concentration, the bands at 613, 770, 1180, 1311, 1363,
1509, 1573, 1650 cmꢀ1 nearly disappear and are replaced by some
new bands. The new bands at 1003, 1234, 1394, 1445 cmꢀ1 may be
attributed to some known compound – they could be impurities.
However, addition of Iꢀ does not cause any obvious change of silver
colloid. The addition of Iꢀ does not cause any aggregation of silver
particles deduced from Fig. 4. It is not yet clear from literatures
why Clꢀ and Brꢀ can cause slight aggregation but the most strongly
binding halide Iꢀ cannot lead any aggregation. Fig. 4 shows the
comparison of affinity among the three kinds of halide ions. It
can be observed that the intense band of Ag–Cl at 241 cmꢀ1 is re-
moved by addition of Brꢀ and the anomalous bands appear. Addi-
tion of Iꢀ cause dramatic loss of Ag–Cl band and anomalous bands.
The results may be interpreted that the affinity of Clꢀ is weaker
than that of Brꢀ and Iꢀ. It is ascribed to that Clꢀ may have weaker
electrostatic interactions with borohydride-reduced silver nano-
particles than Brꢀ and Iꢀ, which have the highest affinity and low-
est solubility [18].
The bands of residual ions in the BRSC without being treated by
anions is so weak that they nearly could not be seen. However, lots
of analytes give very weak signals with untreated colloids and it is
often necessary to add extra anions to aggregate the colloids before
using them, and then intense SERS signals of residual ions can be
observed. By this process some anomalous bands which do not cor-
respond to the analyte under study also appear, which may be
attributed to the impurities or the interaction between residual
ions and Ag particles. Fig. 5a shows the SERS spectra of MB when
adding NaCl, NaNO3, and Na2SO4 to the colloid, respectively. All
of the sodium salts are in optimal concentration. Through compar-
ison of the three spectra of MB, it is noted that the addition of an-
ions can enhance the SERS signal to some extent. Among the three
kinds of anions, Clꢀ gives the greatest enhancement, which may be
attributed to the greatest electronic interaction between Clꢀ and
Ag particles. The modification of the silver surface chemistry
makes the MB molecules to adsorb on silver surface easily, and
that the addition of NaCl at
a low concentration (0.01–
0.04 mol dmꢀ3) mainly causes the modification of the silver sur-
face chemistry but causes little aggregations, however, aggregation
occurs when concentration of NaCl increases to 0.05 mol dmꢀ3. It
may be attributed to that all of the residual ions are displaced by
Fig. 4. Spectrum of BRSC when adding NaCl with high concentration (red line) to
the colloid, spectrum of BRSC when adding a high concentration of NaBr to the
mixture of NaCl and colloid (black line), spectrum of BRSC when adding a high
concentration of NaI to the mixture of NaCl and colloid (blue line), and spectrum of
BRSC when adding a high concentration of NaI to the mixture of NaBr and colloid
(grey line). (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
Fig. 3. (a) SERS spectra of borohydride-reduced silver colloid when adding 0.04 M
(red line), 0.06 M (black line), and 0.08 M (blue line) of NaBr. (b) SERS spectra of
borohydride-reduced silver colloid when adding 0.04 M (red line), 0.06 M (black
line), and 0.08 M (blue line) of NaI. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)

X. Dong et al. / Journal of Molecular Structure 984 (2010) 396–401
399
Fig. 6. Aggregation protocol of silver colloids for MB when adding Clꢀ as
aggregating agent.
Fig. 5. (a) SERS spectra of MB with borohydride-reduced silver colloid when
appropriate concentration of NaCl (black line), NaNO3 (blue line), and Na2SO4 (grey
line) was added respectively. (b) Normal Raman spectrum of 100 mM MB aqueous
solution. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
then, greater enhanced signal can be observed. However, the band
at 241 cmꢀ1 which is assigned to Ag–Cl bonds is also observed
when adding Clꢀ to the colloid. It means the MB molecules cannot
displace Clꢀ which has a high affinity to the silver surface and form
Ag–Cl bond with silver. The structural formula of the process is
presented in Fig. 6. The concentration of Clꢀ is 0.02 M, which gives
the lowest intensity to Ag–Cl band (Fig. 1). However, it gives the
greatest enhancement to MB bands, and the intensity of the Ag–
Cl band is in proportion to that of MB bands (Fig. 7). So, it can be
also concluded that MB molecule has positive charge and can eas-
ily adsorb on the silver surface which has been treated by Clꢀ.
When NOꢀ3 and SO24ꢀ are added to the colloid as aggregating agents,
the band at 241 cmꢀ1 cannot be seen. It means NO3ꢀ and SO24ꢀ have
a lower affinity to silver surface and cannot form the intense Ag-
anion bond. Fig. 5b shows the normal Raman spectrum of
100 mM MB aqueous solution. The bands are observed at 447,
496, 598, 670, 776, 812, 861, 907, 950, 1035, 1071, 1163, 1298,
1393, 1474, 1627 cmꢀ1. Many bands in the SERS spectra have a
small shift in position relative to their corresponding normal Ra-
man spectrum. For example, the bands at 447, 496 cmꢀ1 which
can be assigned to C–N–C skeletal bending [29,30] in normal Ra-
man spectrum shift to 450, 483 cmꢀ1 in SERS spectra, respectively.
These bands at 1393 and 1627 cmꢀ1 which can be assigned to C–N
Fig. 7. SERS spectra of MB when adding different concentrations of Clꢀ to the
colloid.
stretching and C–C stretching in normal Raman spectrum shift to
1388, 1620 cmꢀ1 in SERS spectra. In comparison with the normal
Raman spectrum, some bands split in the SERS spectra. For in-
stance, the band at 1474 cmꢀ1 in Fig. 5b splits into two peaks at
1439 and 1476 cmꢀ1. The band at 1344 cmꢀ1 in SERS spectrum
of MB when adding Clꢀ as aggregating agent and the band at
1511 cmꢀ1 in SERS spectrum when adding NO3ꢀ as aggregating
agent cannot be observed in normal Raman spectrum. The shifts
and splits of these Raman bands indicate that MB molecules may
be chemisorbed on the silver nanoparticles surface and chemical
effects is the main mechanism responsible for the relative shifts
[29,31].
Fig. 7 shows the intensity of the SERS signal of MB, on addition
of different concentration of Clꢀ. Noticeable enhancement is
achieved on addition of 0.02 mol dmꢀ3 Clꢀ (final concentration).
As the concentration of Clꢀ increases continually, the SERS signal
is reduced. This phenomenon indicates that, when using positively

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X. Dong et al. / Journal of Molecular Structure 984 (2010) 396–401
charged analytes, the addition of Clꢀ at a much lower concentra-
tion does lead to a great enhancement in SERS signal. However, it
can be observed from Fig. 2 that the addition of NaCl at a low con-
centration (0.01–0.04 mol dmꢀ3) mainly causes the modification of
the silver surface chemistry but causes little aggregation, and
aggregation occurs when the concentration of NaCl increases to
0.05 mol dmꢀ3. The experiment results imply that the modification
of silver surface caused by Clꢀ changes the binding affinity of MB
and it leads to a variation in SERS intensity that can be attributed
to the ‘‘activation’’ effect in SERS but not due to the aggregation
of silver particles. The optimal concentration of Clꢀ is
0.02 mol dmꢀ3. When adding more Clꢀ to the colloidal solution,
the SERS intensity of MB drops, and it may be due to the collapse
of colloidal particles. At the same time, the band of Ag–Cl bonds
is in proportion to MB bands. The result is reflected in the
241:1627 cmꢀ1 ratio, which is 0.886, 0.916, 0.805, 0.722, corre-
sponding to 0.1, 0.2, 0.3, 0.4 mol dmꢀ3 NaCl. The reason has been
discussed above. Fig. 8 shows the SERS spectra of MB when adding
Brꢀ and Iꢀ to the colloid. It is observed that Brꢀ and Iꢀ give low
enhancement though they have a high affinity for silver surface.
From these detections it can be easily calculated that silver colloid
with appropriate concentration of anions can enhance the Raman
signal several orders of magnitude: The enhancement factor of
SERS can be calculated from the formulation [12,29,32].
volume, and the Nsol is the number of MB molecules in aqueous
solution in laser activation volume. Nads and Nsol are directly pro-
portional to the concentration of MB in colloid and in aqueous solu-
tion, respectively. From the calculation, the enhancement factors for
MB with optimal concentration of Clꢀ, NO3ꢀ, SO4ꢀ2, Brꢀ, and Iꢀ are
1.72 ꢁ 107, 1.45 ꢁ 107, 6.27 ꢁ 106, 1.02 ꢁ 107 and 4.9 ꢁ 106, respec-
tively (for the SERS/Raman band at 1626/1626 cmꢀ1). The result
shows that Iꢀ, the most strongly binding anion, gives the least
enhancement for SERS. Clꢀ gives the greatest enhancement among
the series anions. From previous studies [33,34], the aggregation
of silver particles (electromagnetic mechanism) plays a key role in
SERS while the modification of silver surface chemistry (charge
transfer mechanism) only enhance the SERS signal a few fold. How-
ever, in this study the strongly binding anions, such as halides, give
greater enhancement factors than 105. This can be attributed to the
cooperative effect of the two mechanisms for halides. From the
above discussion, the addition of Iꢀ does not cause any aggregation.
So, it can be concluded that the poor enhancement of Iꢀ may be due
to its poor aggregation effect after adding MB to the colloid. The
problem why the most strongly binding anions cause the least
aggregation of silver particles is not clear yet and need to be further
investigated. The poor enhancement of SO24ꢀ may be also due to its
poor aggregation effect. From what has been discussed above,
aggregating agents mainly lead to two effects on the greatest
enhancement of SERS. As for the weak binding anions (weaker than
the residual ions on the silver particles), the effect of anions is to
aggregate the silver particles. As for the strong binding anions
which can displace the residual ions on the silver particles, the main
effect is to cause the modification of silver surface chemistry, but
the aggregation effect also occurs. So it may be due to the coopera-
tive effect of the two effects.
G ¼ ðIAg=NadsÞ=ðIsol=Nsol
Þ
where IAg and Isol are the intensities of the same band for MB in the
SERS spectrum and in the normal Raman spectrum, respectively.
Nads is the number of MB molecules in the colloid in laser activation
4. Conclusions
From this study, it is found that relative affinity of residual ions
in BRSC is weaker than that of halideions and the bands of residual
ions can be removed by the added halideions. The addition of the
most strongly binding anions-Iꢀ does not cause any aggregation
of silver particles. The addition of Clꢀ leads two effects on the col-
loid: the aggregation of the colloid and the modification of the sur-
face chemistry of the silver particles. The addition of NaCl at a low
concentration mainly causes the modification of the silver surface
chemistry but causes little aggregation. From the SERS of MB, it is
found that the Clꢀ with optimal concentration which gives the
greatest enhancement mainly leads to the modification of silver
surface chemistry, and the surface modification makes the MB
molecules to adsorb on silver surface easily and lead to great
enhancement. However, the aggregation effect also occurs. So it
may be due to the cooperative effect of the two effects. For the
weak binding anions, such as NOꢀ3 and SOꢀ4 2, the only effect of an-
ions is to aggregate the silver particles. The purpose of this study is
to further investigate the effect of anions on silver colloid and clar-
ify the interaction mechanism between anions and borohydride-
reduced silver colloid. It will help to find some general principles
that could be used to make the experimental conditions suitable
for detecting some analytes in high efficiency.
Acknowledgement
This research is supported by the National Natural Science
Foundation of China (60678050).
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Fig. 8. (a) SERS spectra of MB when adding increasing concentrations of Brꢀ to the
colloid. (b) SERS spectra of MB when adding increasing concentrations of Iꢀ to the
colloid.
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