Microwave-assisted green synthesis of Ag/reduced graphene oxide nanocomposite as a surface-enhanced Raman scattering substrate with high uniformity
Nanoscale Research Letters
Microwave-assisted green synthesis of Ag/reduced graphene oxide nanocomposite as a surface-enhanced Raman scattering substrate with high uniformity
Kai-Chih Hsu 0
Dong-Hwang Chen 0
0 Department of Chemical Engineering, National Cheng Kung University , Tainan 701 , Taiwan
A nanocomposite of silver nanoparticles/reduced graphene oxide (Ag/rGO) has been fabricated as a surface-enhanced Raman scattering (SERS) substrate owing to the large surface area and two-dimensional nanosheet structure of rGO. A facile and rapid microwave-assisted green route has been used for the formation of Ag nanoparticles and the reduction of graphene oxide simultaneously with L-arginine as the reducing agent. By increasing the cycle number of microwave irradiation from 1 and 4 to 8, the mean diameters of Ag nanoparticles deposited on the surface of rGO increased from 10.3 ± 4.6 and 21.4 ± 10.5 to 41.1 ± 12.6 nm. The SERS performance of Ag/rGO nanocomposite was examined using the common Raman reporter molecule 4-aminothiophenol (4-ATP). It was found that the Raman intensity of 4-ATP could be significantly enhanced by increasing the size and content of silver nanoparticles deposited on rGO. Although the Raman intensities of D-band and G-band of rGO were also enhanced simultaneously by the deposited Ag nanoparticles which limited the further improvement of SERS detection sensitivity, the detectable concentration of 4-ATP with Ag/rGO nanocomposite as the SERS substrate still could be lowered to be 10−10 M and the enhancement factor could be increased to 1.27 × 1010. Furthermore, it was also achievable to lower the relative standard deviation (RSD) values of the Raman intensities to below 5%. This revealed that the Ag/rGO nanocomposite obtained in this work could be used as a SERS substrate with high sensitivity and homogeneity.
Reduced graphene oxide; Ag nanoparticles; Microwave; Green synthesis; SERS substrate; Uniformity
Surface-enhanced Raman scattering (SERS) has been
considered as a powerful analytic technology with wide
applications in biomedical sensing, chemical analysis, and
environmental monitoring owing to its extremely high
]. The SERS effect can be resulted by the
electromagnetic mechanism (EM) and chemical mechanism
]. The EM, usually with an enhancement factor
(EF) of 106 to 108, arises from the enhanced local
electromagnetic field due to the surface plasmon resonance of
metal nanostructures which may generate lots of ‘hot
]. The CM, usually with an EF of 10 to 100, is
related to the charge transfer resonances between the
probe molecules and the SERS substrates [
]. Since EM
is the main contributor, the nanoscale characteristics of
metallic substrates such as composition, particle size,
shape, interparticle gap, fissures, and geometry play
important roles in the enhancement of SERS [
The SERS substrates currently developed include
metallic rough surfaces, nanoparticle colloids, and periodic
]. Au and Ag nanostructures are the
materials mostly used because of their excellent ability
to enhance the local electromagnetic field [
Although some top-down nanopatterning techniques such
as lithography can be used for the preparation of SERS
substrates with high reproducibility and homogeneity,
these techniques are limited by low throughput, high
cost, few processable materials, and the difficulty to
fabricate the well-controlled nanostructures with efficient
and abundant hot spots [
]. Thus, most of efforts for
the development of SERS substrates have been focused
on the synthesis of nanoparticle colloids with specific
shapes and the bottom-up fabrication techniques such
as the deposition and self-assembly or aggregation of
nanoparticle colloids [
]. However, it is still a challenge
in controlling the size and morphology of nanoparticles
and their aggregates, the packing degree of assemblies,
and the interparticle gap [
]. Therefore, the
fabrication of reliable SERS substrates with high EF and
homogeneity remains demanded until now.
On the other hand, graphene, also including graphene
oxide (GO) and reduced graphene oxide (rGO), has been
used widely in catalysts, supercapacitors, transparent
electrodes, electrochemical detection, biomedicine, and
so on because of its large specific surface area, high
electron mobility, and unique optical, thermal, and
mechanical properties [
]. Recently, some graphene-based
hybrids have also been fabricated for the use in SERS
]. These hybrid materials show great potential as
SERS substrates because the charge transfer between
adsorbed molecules and graphene leads to CM
mechanism and the noble metal nanoparticles deposited on
graphene result in EM mechanism [
]. Furthermore, it is
also expectable that noble metal nanoparticles can be
deposited on the two-dimensional plate graphene
uniformly due to the flat plane of graphene in nature,
leading to the high uniformity of characteristic Raman signal.
Ding et al. has reported that the Au/rGO hybrid had good
uniformity as a SERS substrate. The relative standard
deviation (RSD) of Hg2+ Raman signal at 1,618 cm−1
was 12.8% [
In this work, the fabrication of Ag/rGO
nanocomposite as a SERS substrate with high EF and homogeneity
was attempted. Ag was chosen because of its lower cost
as compared to Au. Furthermore, to achieve the goals of
high EF and homogeneity, it was desired to deposit
plenty of Ag nanoparticles with uniform size on the
substrate. Noteworthily, microwave irradiation which offers
rapid and uniform heating of solvents, reagents, and
intermediates can provide uniform nucleation and growth
]. So this technique has been used for the
synthesis of many metal nanoparticles [
to reduce or eliminate substances hazardous to human
health and the environment, the development of green
chemical processes and products is becoming more
and more important in the past decade [
L-arginine (i.e., one of the most common natural amino
acids) has been demonstrated to be useful for the green
synthesis of some metal and metal oxide nanoparticles
because it not only played a role of reducing agent but also
acted as a capping agent [
]. Accordingly, here, we
developed a facile and rapid microwave-assisted green
route for the formation of Ag nanoparticles and the
reduction of graphene oxide simultaneously using L-arginine as
the reducing agent to yield the Ag/rGO nanocomposite.
The average size and density of the Ag nanoparticles could
be controlled by adjusting the cycle number of microwave
irradiation. By the detection of the common Raman
reporter molecules, 4-aminothiophenol (4-ATP), the
resulting Ag/rGO nanocomposites were demonstrated to be
suitable SERS substrates with high sensitivity and
Graphite powder (99.9%) was obtained from Bay Carbon,
Bay City, MI, USA. Potassium manganite (VII) and sodium
nitrate were purchased from J.T. Baker, Phillipsburg, NJ,
USA. Sulfuric acid was supplied by Panreac, Barcelona,
Spain. Hydrogen peroxide was a product of Showa,
Minato-ku, Japan. Sulfuric acid was obtained from Merck,
Whitehouse Station, NJ, USA. L-arginine was supplied
by Sigma-Aldrich, St. Louis, MO, USA. Silver nitrate
was obtained from Alfa Aesar, Ward Hill, MA, USA.
4-Aminothiophenol was the product of Aldrich. All
chemicals were of guaranteed or analytical grade reagents
commercially available and used without further
purification. The water used throughout this work was the
reagent grade water produced by a Milli-Q SP
ultra-purewater purification system of Nihon Millipore Ltd.,
GO was prepared from purified natural graphite by a
modified Hummers method [
nanocomposite was synthesized by a facile, rapid, and green process
according to our previous work on the synthesis of
silver/iron oxide nanocomposite [
]. Firstly, 15 mg of
graphite oxide was dispersed in 20 mL of deionized water
by ultrasonication to form a stable GO colloid solution
and then mixed with 10 mL of solution containing AgNO3
(300 mM) and L-arginine (60 mg/mL). Next, the solution
was transferred into a Teflon beaker and then reduced by
different cycles of microwave irradiation (2.45 GHz,
900 W). Each cycle included 50s ‘on’ and 10s ‘off’ for three
times. The product was collected by centrifugation and
then washed several times with deionized water. The
resulting nanocomposites were referred to as 1C, 4C, and
8C according to cycle number of microwave irradiation.
Following the above procedures in the absence of AgNO3,
rGO was prepared to confirm the reduction of GO and
for comparison with Ag/rGO nanocomposite.
The particle size and composition were determined by
transmission electron microscopy (TEM) and
energydispersive X-ray (EDX) spectroscopy on a high-resolution
field emission transmission electron microscopy (HRTEM,
JEOL Model JEM-2100 F, Akishima-shi, Japan). The
HRTEM image and selected area electron diffraction
(SAED) pattern were obtained by a JEOL Model
JEM2100 F electron microscope at 200 kV. The Ag content of
Ag/rGO nanocomposite was also determined by
dissolving the sample in a concentrated HCl solution and
analyzing the solution composition using a GBC SensAA
Dual M/A Series Flame/Furnace atomic absorption
spectrometer (AAS). The UV-Vis absorption spectra of the
resultant colloid solutions were monitored by a JASCO
model V-570 UV/Vis/NIR spectrophotometer, Oklahoma
City, OK, USA. The crystalline structures were
characterized by X-ray diffraction (XRD) analysis on a Shimadzu
model RX-III X-ray diffractometer, Kyoto, Japan, at 40 kV
and 30 mA with CuKα radiation (λ = 0.1542 nm). Raman
scattering was performed on a Thermo Fisher Scientific
DXR Raman Microscopy, Waltham, MA, USA, using a
532-nm laser source, and a × 10 objective was used to
focus the laser beam onto the sample surface and to
collect the Raman signal. The XPS measurements were
performed on a Kratos Axis Ultra DLD photoelectron
spectrophotometer, Chestnut Ridge, NY, USA, with an
achromatic Mg/Al X-ray source at 450 W.
For the study on the SERS property, 0.1 mL of
solution containing Ag/rGO nanocomposite (3 mg/mL) was
dropped on the glass slide and then dried in a vacuum
oven at 35°C to obtain the SERS-active substrate. Next,
the SERS-active substrate was immersed in 40 mL of
4ATP solution for 2 h, then washed with deionized water
to remove free molecules and dried in air. Finally, the
SERS spectrum of 4-ATP was analyzed by the Thermo
Fisher Scientific DXR Raman microscopy using a
532nm laser source.
Results and discussion
Figure 1 shows the TEM and HRTEM images of Ag/rGO
nanocomposites 1C, 4C, and 8C. It was found that Ag
nanoparticles have been uniformly deposited on rGO
successfully. The mean diameters of Ag nanoparticles
increased as 10.3 ± 4.6, 21.4 ± 10.5, and 41.1 ± 12.6 nm when
the cycle numbers of microwave irradiation were 1, 4, and
8, respectively. The mean diameters of Ag nanoparticles
were determined by 300 Ag nanoparticles deposited on
rGO. Their HRTEM images all indicated the interlayer
spacing of 0.23 to 0.24 nm which related to the (111)
plane of face-centered cubic (fcc) Ag. Furthermore, the
SAED patterns of Ag/rGO nanocomposites 4C and 8C
showed the characteristic rings for the (111), (200), (220),
and (311) planes of fcc Ag. For Ag/rGO nanocomposite
1C, the characteristic rings for the (220) and (311) planes
of fcc Ag were not significant, probably due to the
less Ag content. The EDX analysis of Ag/rGO
nanocomposite 8C is indicated in Figure 1g. The presence
of Ag confirmed the deposition of Ag nanoparticles.
As for the signal of Cu, it was from the copper grid.
Furthermore, to confirm the composition, the Ag content
of Ag/rGO nanocomposites was also determined by AAS.
The weight percentages of Ag in the Ag/rGO
nanocomposites 1C, 4C, and 8C were determined to be 37.4%,
69.6%, and 91.6%, respectively. These results revealed that
the average size and content of Ag nanoparticles could
be controlled by adjusting the cycle number of
The UV-Vis absorption spectra of Ag/rGO
nanocomposites 1C, 4C, and 8C were shown in Figure 2a, in
which the spectra of GO and rGO were also indicated
for comparison. The spectrum of GO exhibited the
characteristic peaks at 233 and 300 nm, which related to the
absorption of C-C and C = O bonds, respectively [
The characteristic peak of rGO in this work was
observed at 260 nm, which was slightly lower than the
characteristic peak of highly reduced GO (approximately
268 nm) [
]. This result demonstrated the partial
reduction of GO in this work. The successful deposition of
Ag nanoparticles on the rGO surface was confirmed by
the peaks around 447 nm. With increasing the cycle
number of microwave irradiation, the surface plasmon
resonance (SPR) bands were redshifted and broadened
due to the larger size and aggregation of Ag
nanoparticles. This might be due to the substrate effect and the
increase in the surface coverage of rGO by Ag
The XRD patterns of GO, rGO, and Ag/rGO
nanocomposite 1C, 4C, and 8C were shown in Figure 2b. The
sharp peak at 2θ = 10.56° was due to the (001) plane of
GO. However, this peak was not observed in the other
XRD patterns, revealing GO has been reduced to rGO.
For the XRD patterns of Ag/rGO nanocomposites 4C
and 8C, the characteristic peaks at 2θ = 38.42°, 44.62°,
64.72°, and 77.68° related to the (111), (200), (220), and
(311) planes of fcc Ag, respectively, confirming the
formation of Ag nanoparticles on rGO. Nevertheless, for
Ag/rGO nanocomposite 1C, only the (111) plane of Ag
could be found easily. This might be due to the less Ag
Figure 3 shows the C1s XPS spectra of GO and Ag/
rGO nanocomposites 1C, 4C, and 8C. As illustrated in
Figure 3a, the C1s XPS spectrum of GO at 280 to
292 eV showed the characteristic peaks of C-C, C-O-H,
C-O-C, C = C, C = O, and O-C = O. They could be
attributed to the presence of epoxy, hydroxyl, and
carbonyl groups, respectively [
]. From Figure 3b,c,d, with
increasing the cycle number of microwave irradiation,
the peak intensity of C1s which related to oxygenated
functional groups (C-O-H and C-O-C) showed a
significant decrease, confirming that most of the epoxide,
hydroxyl, and carbonyl functional groups were removed
and the degree of reduction of could be enhanced. It
was noted that two new characteristic peaks of C-N and
O-C = O were observed, and the intensity of C-N and
O-C = O could be enhanced with increasing the cycle
number of microwave irradiation. This could be
reasonably attributed to the increase of arginine capped on the
surface of Ag/rGO nanocomposites.
Figure 1 TEM and HRTEM images of Ag/rGO nanocomposites. 1C (a, b), 4C (c, d), and 8C (e, f). The insets indicate the SAED patterns.
(g) The EDX spectrum of Ag/rGO nanocomposite 8C.
Figure 4 shows the XPS signature of the Ag 3d doublet
(3d5/2 and 3d3/2) for the Ag nanoparticles deposited on
rGO. The Ag 3d5/2 and 3d3/2 peaks of Ag/rGO
nanocomposites 1C appeared at 368 and 374 eV, respectively,
which shifted to the lower binding energy compared
with the characteristic peaks for silver metal at 368.2
and 374.2 eV. In addition, the Ag 3d5/2 binding energies
have values of 368.2, 367.4, and 367.8 eV for Ag, Ag2O,
and AgO (with average oxidation states of 0, +1, and +2,
]. As a result, slight oxidation on the
surface of Ag nanoparticles might be the reason for the
negative shift of Ag 3d3/2 and Ag 3d5/2 binding energy.
Moreover, from Figure 4, the binding energy of 3d3/2
and Ag 3d5/2 increased with increasing the cycle number
of microwave irradiation. The results were due to the
electron transfer from metallic Ag to the graphene
sheets owing to the smaller work function of Ag (4.2 eV)
than graphene (4.48 eV) and also proved that the
content of Ag nanoparticles could be controlled via
adjusting the cycle number of microwave irradiation.
Figure 5a shows the typical SERS spectra of 10−4 M
4-ATP acquired from rGO and Ag/rGO nanocomposites
1C, 4C, and 8C. For rGO, only two prominent peaks
corresponding to the G and D bands were observed clearly
and no evident Raman peaks of 4-ATP could be found.
However, for Ag/rGO nanocomposites, the characteristic
peaks of 4-ATP were observed clearly. This demonstrated
that the Ag/rGO nanocomposites possessed significant
SERS property. Their SERS intensities at 1,140 cm−1 were
indicated in Figure 5b. It was obvious that the peak
intensity increased significantly with increasing the cycle
number of microwave irradiation. It is known that increasing
the number density of Ag nanoparticles on the surface of
graphene sheets as hot spots for strong localized EM fields
produced by the gap between neighboring Ag
]. Also, the SERS intensity of Ag nanoparticles
usually increased with the increase of particle size. The
optimal size of spherical Ag nanoparticles for SERS was about
50 nm [
]. In this work, the mean diameters of Ag
nanoparticles increased from 10.3 ± 4.6 to 41.1 ± 12.6 nm when
the cycle numbers of microwave irradiation increased from
1 to 8. Thus, the cycle number effect of microwave
irradiation could be attributed to the larger size and higher
content or number density of Ag nanoparticles.
Figure 6a indicates the optical image of an area of
0.5 mm × 0.3 mm for the Ag/rGO nanocomposite 8C
substrate. The corresponding two-dimensional SERS mapping
(at 1,140 cm−1) after 4-ATP adsorption was shown in
Figure 6b. It was found that the SERS intensities at
different positions had no significant differences. To further
investigate the uniformity, a series of SERS spectra
randomly collected from 30 spots of the Ag/rGO
nanocomposite 8C substrate at 10−5 M 4-ATP were shown in
Figure 6c. The RSD values of the intensities for three main
vibrations at 1,140, 1,389, and 1,434 cm−1 were calculated
to be 5.08%, 4.79%, and 4.6%, respectively, as indicated in
Figure 6d,e,f. Such low RSD values were significantly
better than some previous works with lower RSD values
and revealed that the resulting Ag/rGO nanocomposite
8C had outstanding uniformity as a SERS substrate
]. This could be attributed to the fact that Ag
nanoparticles were deposited uniformly on the flat surface
of rGO so the closely packed Ag nanoparticles might offer
a great deal of uniform hot spots for SERS to enhance the
Raman signal of adsorbed molecules. This result revealed
that the Ag/rGO nanocomposites could be regarded as an
excellent SERS-active substrate with highly uniformity.
Figure 7 shows the SERS spectra of different
concentrations of 4-ATP adsorbed on Ag/rGO nanocomposites
1C, 4C, and 8C. The SERS spectrum of 4-ATP on the
Ag/rGO nanocomposite exhibited four b2 vibration modes
at 1,140, 1,389, 1,434, and 1,574 cm−1, which could be
assigned to ν(C-C), ν(C-C) + δ(C-H), δ(C-H) + ν(C-C),
δ(C-H), respectively, and one a1 vibration mode of the p,
p'-dimercaptoazobenzene molecule at 1,074 cm−1 related
to ν(C-S) [
]. It was obvious that the intensities of the five
bands in the SERS spectrum of 4-ATP on the Ag/rGO
nanocomposites were enhanced significantly.
The apparent EF of the characteristic Raman signal at
1,140 cm−1 in the SERS spectrum of 4-ATP could be
estimated according to the following relation [
EF ¼ ISERSCNRS=INRSCSERS
where ISERS and INRS are the SERS intensities on the
SERS-active and non-SERS-active substrates, respectively,
and CSERS and CNRS are the corresponding analyte
concentrations used. The EF values at 1,140 cm−1 for the Ag/rGO
nanocomposites 1C and 4C substrates at 10−8 M 4-ATP
were found to be 1.97 × 107 and 9.04 × 107, respectively.
Also, the EF value at 1,140 cm−1 for the Ag/rGO
nanocomposite 8C substrate at 10−10 M 4-ATP was further
raised to 1.27 × 1010. This demonstrated the EF values
for the Ag/rGO nanocomposites could be enhanced
by increasing the size and content of Ag nanoparticles on
the surface of rGO.
It was mentionable that the closely packed Ag
nanoparticles on the surface of rGO not only enhanced the
Raman signal of 4-ATP significantly but also enhanced
the Raman intensities of D-band and G-band of rGO
simultaneously as shown in Figure 7. This limited the
further improvement of SERS detection sensitivity.
However, in spite of this, the detectable concentration of
4ATP with the Ag/rGO nanocomposite 8C as the SERS
substrate still could be lowered to be about 10−10 M and
the EF value could be raised to 1.27 × 1010. They were
better than some previous works [
to the above results, the Ag/rGO nanocomposite indeed
could be used as a SERS substrate with high EF and
Ag/rGO nanocomposite has been synthesized via a rapid
and facile green process. By the use of L-arginine and
microwave irradiation, Ag nanoparticles were deposited
uniformly on the surface of rGO. The size and content
of Ag nanoparticles could be controlled via adjusting the
cycle number of microwave irradiation. The Ag/rGO
nanocomposite has been demonstrated to be useful as
the SERS substrate with high sensitivity and uniformity
owing to the uniform deposition of Ag nanoparticles on
the flat surface of rGO, offering a lot of hot spots for
SERS. Although the Raman intensities of D-band and
Gband of rGO were also enhanced and limited the further
improvement of SERS detection sensitivity, the
detectable concentration of 4-ATP with Ag/rGO
nanocomposite as the SERS substrate still could be lowered to be
10−10 M and the EF value could be raised to 1.27 × 1010.
In addition, the RSD values of the intensities could be
decreased to below 5%.
The authors declare that they have no competing interests.
KCH carried out the experiments and drafted the manuscript. DHC guided
the study and modified the manuscript. Both authors read and approved the
KCH is currently a PhD student of the National Cheng Kung University
(Taiwan). DHC is a distinguished professor of Chemical Engineering
Department at National Cheng Kung University (Taiwan).
This work was performed under the auspices of the National Science Council
of the Republic of China, under contract number NSC 102-2221-E-006-221-MY3,
to which the authors wish to express their thanks.
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