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SERS-Active Silver Nanoparticles Prepared by a Simple and Green Method Wenbing Li, Yanyan Guo, and Peng Zhang* Department of Chemistry, New Mexico Tech, Socorro, New Mexico 87801 ReceiVed: January 19, 2010; ReVised Manuscript ReceiVed: February 25, 2010
A very effective, facile and green method to produce silver nanoparticles as surface-enhanced Raman scattering (SERS) substrates is reported. Reduction of silver nitrate with polyethylene glycol 200 without additional steps of introducing other reducing agents or protective agents at room temperature yields silver nanoparticles with particle size less than 5 nm. The as-produced silver nanoparticles can be used as SERS substrates immediately after preparation. The structure and composition of the silver nanoparticles were characterized by UV-visible spectroscopy, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), particle size analyzer (PSA), and Raman spectroscopy, using a 488 nm Ar+ laser and a 632.8 nm HeNe laser. The SERS enhancement of these silver nanoparticles was tested by using tris(2,2′-bipyridyl)ruthenium(II) chloride (Ru(bpy)) and Rhodamine 6G (R6G) in both solid and liquid forms. The silver nanoparticles have been shown to provide elegant SERS signals of Ru(bpy) and R6G, demonstrating the advantage of this new method of preparing Ag-based SERS substrates. Introduction Surface-enhanced Raman scattering (SERS) has been known and intensely studied for over three decades.1-6 In recent years, SERS has been transformed into a useful analytical technique with some significant advantages for sensitive chemical analysis and interfacial studies,7-10 thanks to the ever-growing knowledge of the plasmonic properties of nanomaterials and the constantly improved engineering capability.11-21 It is well-known experimentally that the most intense SERS signals are obtained from molecules adsorbed to microscopically rough silver surfaces, such as silver colloids and silver nanoparticles of various shapes.22-24 The commonly used silver colloids are usually prepared by the reduction of silver nitrate with either sodium citrate or sodium borohydride.25-27 Unfortunately, for analytical purposes, most silver colloids prepared by this strategy suffer from problems including poor stability and reproducibility due to colloid aggregation. Other methods of synthesizing silver nanoparticles involve the use of reducing agent(s) and some kinds of protecting agent to stabilize the produced nanoparticles, both of which may not be environmentally friendly. With the growing emphasis on the “green” aspect of chemical processes, there has been an increasing need to reduce or eliminate the use and/or generation of hazardous substances.28-31 It has been known that polymeric materials contain sizeconfined, nanosized pockets of inter- and intramolecular origin, which can be used in the preparation of nanoparticles.32,33 Dendritic and linear polymers have been successfully applied for nanoparticle synthesis.34,35 In this report, we describe a facile, green method to synthesize silver nanoparticles using only AgNO3 aqueous solution and polyethylene glycol 200 (PEG 200). PEG is a condensation polymers of ethylene oxide and water with the general formula of H(OCH2CH2)nOH, where n is the average number of repeating oxyethylene groups. This polymer is biocompatible, odorless, neutral, lubricating, nonvolatile, and nonirritating. It has been known that ethylene glycol and its polymer, PEG, are environmentally benign materials * To whom correspondence should be addressed. Phone:575-835-6192. Fax: 575-835-5364. E-mail:
[email protected].
Figure 1. Molecular structure of (a) R6G and (b) Ru(bpy).
which have been widely used in the pharmaceutical and biomedical industries as solvent, dispensing agent, ointment and suppository base, and vehicle. In this preparation, PEG serves as both the environmentally benign solvent and reducing agent with its large number of hydroxyl groups, which can facilitate the complexation of silver ions to the molecular matrix and reduce silver ions to form silver nanoparticles at room temperature. It is worthwhile to mention that the polymer chain can act as a protective agent to prevent the thus-formed silver nanoparticles from aggregating, stabilizing the resulting nanoparticles for an extended period of time. Finally, we show that the silver nanoparticles prepared by this method exhibit a very strong SERS activity, using Rhodamine 6G (R6G) and tris(2,2′bipyridyl)ruthenium(II) chloride (Ru(bpy)) as the Raman probes. The results demonstrate the potential of these PEG-reduced Ag nanoparticles as SERS substrates for analytical applications. Experimental Section Materials. Silver nitrate (99.9%), Rhodamine 6G (R6G), and tris(2,2′-bipyridyl)ruthenium(II) chloride (Ru(bpy)) were obtained from Sigma-Aldrich. The molecular structures of R6G and Ru(bpy) are shown in Figure 1. PEG200 was purchased from Fisher Scientific. Reagents and solvents were obtained commercially and used without further purification. R6G and Ru(bpy) stock solutions were prepared by dissolving them in the water separately. Deionized (DI) water, with a resistivity
10.1021/jp100526v 2010 American Chemical Society Published on Web 03/10/2010
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Figure 2. Schematic of the Ag/PEG-NPs synthesis at room temperature.
greater than 18.0 MΩ · cm (Millipore Milli-Q system), was used in the preparation of all aqueous solutions. All glassware used in the following procedures was cleaned with freshly prepared aqua regia (3:1 of HCl:HNO3) and rinsed thoroughly by tap water first and then DI water prior to use. Preparation of Ag/PEG Nanoparticles. The schematic of the formation of Ag nanoparticles in PEG (Ag/PEG-NPs) is shown in Figure 2. Silver nanoparticles are synthesized by slowly adding AgNO3 aqueous solution into polyethylene glycol (PEG200) under rigorous stirring. In brief, to prepare 1.0 mM Ag/PEG-NPs, 0.2 mL of 50 mM AgNO3 aqueous solution was added slowly to 9.8 mL of PEG solution under vigorous stirring. The mixture was clear without any color at the beginning. The color of the mixture turned light yellow with a few minutes of stirring at room temperature. After 6 h of stirring at room temperature, the color of the mixture changed to yellow. The yellow solution was then kept in a refrigerator at 4 °C before being used in various measurements. Different concentrations of Ag/PEG-NPs were prepared in a similar way. Extinction Spectra Measurements. The extinction spectra were recorded by a Shimadzu 2550 spectrophotometer (Shimadzu Corporation Japan), using a 1-cm path length quartz cell at room temperature. The measurement of extinction spectra was carried out by directly measuring the Ag/PEG-NPs mixture solution. TEM and EDX Measurements. A drop of suspension containing Ag nanoparticles was deposited on a Formvarcovered carbon-coated copper grid (Electron Microscopy Sciences, PA). The samples were allowed to dry at room temperature overnight. A JEOL 2010 high-resolution transmission electron microscope (HRTEM) was used to obtain the TEM images and EDX spectra at 200 kV accelerating voltage. PSA Measurements. The silver nanoparticles were characterized by a Nanotrac particle size analyzer (PSA) from Microtrac Inc. The measurement of PSA is based on the dynamic light scattering at single-angle detection. The average values of the particle size and polydispersity, defined as a relative width of the size distribution, were determined from the PSA measurements. Raman Spectra Measurements. For the liquid samples, an Ar+ laser (Melles Griot) with multiple lines was used. The laser beam was focused by a 10× objective onto the sample contained in a 1-cm path length quartz cell. Raman signals were collected by the same objective, through a notch filter, and coupled into a spectrometer (SpectraPro-2300i, Acton Research, MA) equipped with an air-cooled CCD detector (Spec-10, Roper Scientific, NJ). Laser intensities at the samples were approximately 9.2 mW for the 488-nm line. For the solid samples, the Ag/PEGNPs solution was mixed thoroughly with R6G or Ru(bpy) solution of a certain concentration. Then a drop of the mixture
Figure 3. Extinction spectra of Ag/PEG prepared with different reaction times (10 min, 20 min, 30 min, 40 min, 1 h, 2 h, 3 h, and 6 h).
was cast onto a tilted, clear coverslip, and let dried. Raman measurements were carried out on a LabRAM Raman microscope (HORIBA Jobin Yvon Inc., NJ). Laser intensities at the samples were set at approximately 130 µW for the 632.8-nm HeNe laser used in all measurements. Between different Raman sessions, the 520.7-cm-1 peak of a silicon wafer was used to calibrate the spectrograph for possible fluctuation of the Raman system. Exposure time for all measurements was 1 s. Each spectrum was the average of 10 scans. Results and Discussion Extinction Spectra of Ag Nanoparticles. UV-vis spectroscopy is used to characterize the synthesized Ag/PEG-NPs. The spontaneous formation of silver nanoparticles can be attributed to the direct redox reaction between PEG and AgNO3, because there was no other reducing agent in the mixture. With the abundance of hydroxyl (-OH) groups, PEG200 serves both as a mild reducing agent to form Ag nanoparticles and a protecting agent to stabilize the resulting Ag nanoparticles and further prevent them from aggregation. Figure 3 shows the extinction spectra of 1.0 mM Ag/PEG prepared with different reaction times (10 min, 20 min, 30 min, 1 h, 2 h, and 6 h). The spectra show a distinct band positioned around 420 nm with a full width at half-maximum (fwhm) of approximately 80 nm, which is characteristic of Ag nanoparticles due to the surface plasmon absorption.36 It has been known that the surface plasmon absorption band is very sensitive to the Ag nanoparticle size, the solvent, and the chemisorbed solute molecules. The maximum of the measured extinction spectrum of the Ag/PEG-NPs solution can provide information on the average particle size, whereas fwhm can also be used to estimate particle dispersion.37 As expected, the extinction peak around 420 nm increases with increasing reaction time as a result of an increase in the amount of Ag NPs that are formed in the solution. As can be seen in Figure 3, the Ag/PEG-NPs have good monodispersed distribution of the particle size when 0.20 mL of silver nitrate solution (50 mM) is added to 9.80 mL of PEG. All Ag NPs are prepared under vigorous stirring at room temperature and found to be highly SERS-active after preparation. TEM, PSA, and EDX of Ag Nanoparticles. The synthesized Ag nanoparticles can be isolated by centrifugation and be resuspended in deionized water or polar solvents such as ethanol for further uses. To obtain a more detailed understanding of the Ag nanoparticles and their extinction spectra, we examine
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Figure 4. TEM image and PSA histogram of as-synthesized Ag NPs (a, b) by 0.5 mM AgNO3 with PEG, (c, d) by 1.0 mM AgNO3 with PEG, (e, f) by 2.0 mM AgNO3 with PEG, and (g, h) by 1.0 mM AgNO3 with PEG, after storing at 4 °C for 3 weeks. Scale bars: 20 nm.
the surface morphology of Ag nanoparticles by TEM and the particle sizes by PSA. As shown in Figure 4 of the typical TEM images and PSA results, the particle sizes of Ag/PEG-NPs are less than 5 nm. Ag/PEG-NPs are well-dispersed from each other at AgNO3 concentration of 0.5 to 2.0 mM in PEG. Overall, the particles are isotropic in shape. These results indicate that we can prepare silver nanoparticles of less than 5 nm in a broad range of AgNO3 concentrations in PEG. Figure 4 also shows the typical TEM images of as-synthesized Ag/PEG-NPs after storing at 4 °C for 4 weeks. We observe that the dispersed nanoparticles are fairly stable and do not result in the growth of particle size or aggregation even after weeks of storage under 4 °C. This is most likely due to that fact that, after the formation of Ag NPs, PEG serves as a protective agent to prevent the aggregation of Ag nanoparticles. EDX results are the other evidence to support the formation of Ag/PEG-
NPs. Figure 5 shows the typical EDX spectrum of as-synthesized 1.0 mM Ag NPs. It clearly indicates the presence of the silver element in the final silver nanoparticles. SERS Activity of the Ag/PEG-NPs. To characterize the SERS activity of the Ag/PEG-NPs, the Ag NPs solution that is produced by reaction of 0.20 mL of a 50.0 mM silver nitrate solution with 9.8 mL of PEG is selected as a representative model. For the solid samples, results of Raman measurements of the Raman probes (Ru(bpy) and R6G) mixed with the Ag/ PEG-NPs are shown in Figure 6. It can be seen that the Ag/ PEG-NPs display very strong SERS activity. Figure 6a shows the SERS spectra of different concentrations of R6G absorbed on the Ag/PEG-NPs, excited by a 632.8-nm laser. The marked peaks in the SERS spectra correspond to the Raman bands of R6G. The spectra are very similar to those in the literature,38,39 while the concentration of R6G in this study is much lower
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Figure 5. Typical EDX spectra of the Ag/PEG-NPs. Cu and carbon peaks appear due to scattering caused by the carbon film and Cu of the TEM grid.
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Figure 7. SERS spectra of Ag/PEG-NPs with different concentrations of Ru(bpy) excited by 488-nm laser.
Conclusions We have demonstrated a facile and green protocol at room temperature for the synthesis of stable, SERS-active silver nanoparticles, based on the reduction of silver nitrate by PEG, serving as both a reducing and a stabilizing agent. The procedure does not require high temperature. No toxic reagents, surfactant, or organic solvents were involved in the whole process. This method offers numerous benefits, ranging from green reaction to readiness of being integrated into biologically relevant systems. TEM and PSA of the PEG-reduced NPs show that the silver nanoparticles are predominantly spherical with narrow distribution. The as-synthesized silver nanoparticles are stable for weeks without aggregation. They also demonstrate very high SERS activity, both in liquid and solid forms. We thus believe that Ag/PEG-NPs hold tremendous potential in analytical applications. Acknowledgment. Support from the Natural Science Foundation (CHE-0632071) and the Water Innovation Fund of the State of New Mexico is gratefully acknowledged. We thank the Department of Earth and Planetary Science, University of New Mexico, for access to the TEM facility. References and Notes Figure 6. SERS spectra of Ag/PEG-NPs with different concentrations of (a) R6G and (b) Ru(bpy) excited by 632.8-nm laser.
than those reported previously. Figure 6b shows the SERS spectra of different concentrations of Ru(bpy) mixed with the Ag/PEG-NPs. Compared to the classical Lee-Meisel silver sol system,25 the Ag/PEG-NPs as substrate can detect the SERS of Ru(bpy) down to a concentration of 10-14 M. These results demonstrate the superior SERS activity of the Ag/PEG-NPs toward different probe molecules. From the analytical point of view, it would be ideal if such Ag/PEG-NPs can be used in liquid settings. To this end, we mix different concentrations of pure Ru(bpy) solution with the Ag/PEG-NPs solution, and the SERS spectra are taken directly on the solution. As shown in Figure 7, the SERS intensity of Ru(bpy) in the Ag/PEG-NPs solution is very high even when the concentration of Ru(bpy) in the mixture is ∼10-8 M. Since such high SERS signals are achieved in solution, they represent the average SERS enhancement. This result indicates that the Ag/PEG-NPs can be used in the analytical environment for routine measurements. Further studies are underway to apply such Ag nanoparticles for the detections of polycyclic aromatic hydrocarbons and explosives.
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