Microwave-assisted green synthesis of Ag/reduced graphene oxide nanocomposite as a surface-enhanced Raman scattering substrate with high uniformity
© Hsu and Chen; licensee Springer. 2014
Received: 26 February 2014
Accepted: 17 April 2014
Published: 28 April 2014
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.
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 sensitivity . The SERS effect can be resulted by the electromagnetic mechanism (EM) and chemical mechanism (CM) . 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 spots’ [3, 4]. 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 [4–6]. 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 [1, 3, 7].
The SERS substrates currently developed include metallic rough surfaces, nanoparticle colloids, and periodic nanostructures . Au and Ag nanostructures are the materials mostly used because of their excellent ability to enhance the local electromagnetic field [8, 9]. 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 [1, 3]. 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 [1, 3]. 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 [1, 3, 10, 11]. 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 [12–19]. Recently, some graphene-based hybrids have also been fabricated for the use in SERS [4, 20–24]. 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 conditions . So this technique has been used for the synthesis of many metal nanoparticles [27, 28]. Moreover, 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 [29, 30]. Recently, 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 [28, 31–34]. 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 outstanding uniformity.
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-pure-water purification system of Nihon Millipore Ltd., Tokyo, Japan.
GO was prepared from purified natural graphite by a modified Hummers method . Ag/rGO 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 energy-dispersive 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 JEM-2100 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 4-ATP 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 532-nm laser source.
Results and discussion
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 content.
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 4-ATP 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 [22, 42, 43]. According to the above results, the Ag/rGO nanocomposite indeed could be used as a SERS substrate with high EF and homogeneity.
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 G-band 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%.
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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