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gold–silver nanofibers
Journal of Colloid and Interface Science 382 (2012) 28–35
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Surface-enhanced Raman scattering-active substrates of electrospun polyvinyl alcohol/gold–silver nanofibers Xiaofei Li, Minhua Cao, Han Zhang, Lin Zhou, Si Cheng ⇑, Jian-Lin Yao, Li-Juan Fan ⇑ Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
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Article history: Received 7 March 2012 Accepted 25 May 2012 Available online 7 June 2012 Keywords: Au–Ag alloy Electrospinning Surface-enhanced Raman scattering
a b s t r a c t Surface-enhanced Raman scattering (SERS)-active substrates of polyvinyl alcohol/gold–silver (PVA/Au– Ag) nanofibers were prepared using a simple approach involving electrospinning. The tunable surface plasmon resonance (SPR) of gold–silver alloy (Au–Ag alloy) nanoparticles (NPs) was achieved by controlling the feed ratio between gold and silver precursors. A higher concentration of Au–Ag alloy NPs could be obtained than the conventional methods, using 1 wt% of PVA as the stabilizer. The Au–Ag alloy structure was demonstrated by HRTEM and STEM-EDX. After the electrospinning, the Au–Ag alloy NPs were successfully embedded in PVA nanofibers, as shown in the SEM and TEM images. Raman spectra displayed an apparent enhancement in the signal of 4-mercaptobenzoic acid (4-MBA), pyridine, and thiophenol molecules pre-absorbed from their ethanol solution onto the PVA/Au–Ag nanofibers. Different SERS effects were achieved by varying the Au content or excitation wavelength. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Noble metallic nanostructures usually have an interesting property known as surface plasmon resonance (SPR), which arises from the collective oscillation of conduction electrons on the surface of metals and resulting in a variety of optical signal enhancements, such as surface-enhanced Raman scattering (SERS), surface-enhanced fluorescence, and surface-enhanced infrared absorption. These phenomena have allowed for powerful analytical techniques in sensor applications [1–3]. SERS is one of these phenomena, and it is highly dependent on the properties of the metal, such as gold, silver, and copper, as well as the size and shape of the nanoparticles. The optical properties of Au and Ag NPs have been studied in great detail [4]. Generally, there was a lower enhancement of Raman signal in the visible light region for Au than that for Ag [5]. However, Au NPs have many advantages over Ag NPs, such as easy preparation, high homogeneity, long-term stability, and biocompatibility [6]. To combine the high signal enhancement of Ag NPs with the advantages of the Au materials, the synthesis of bimetallic (alloy or core-shell) nanostructures consisting of Au and Ag has been a popular research topic, especially as substrates with improved SERS activity. It is now commonly known that the optical properties can be tuned by varying the composition of the metals. In this aspect, the gold–silver alloy nanoparticles (Au–Ag alloy NPs) displayed higher sensitivity ⇑ Corresponding authors. E-mail addresses: [email protected] (S. Cheng), [email protected] (L.-J. Fan). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.05.048
for SERS than their monometallic nanocrystals [7]. Basically, the SPR bands of Au–Ag alloy NPs were demonstrated to be continuously tunable in a wide range of wavelengths, from 400 nm to 520 nm [8]. To our best knowledge, the SERS effect has been mainly studied in three types of nanostructure systems: colloidal dispersions of metal particles, nanostructured metal film, and one-dimensional (1D) nanostructure. The nanostructure films provided SERS substrates with several advantages over colloidal systems, such as portability and compatibility with a wider range of substrates. Recently, 1D nanostructure systems have also been proven to be applicable as SERS substrates [9–14]. Among the 1D nanostructures, electrospun polymer nanofiber mats embedded with monometallic nanocrystals, such as PVA/Ag and PMMA/Au nanofiber, were demonstrated to be SERS substrates with high performance [11,12]. One of the major advantages of the electrospun nanofibers is the large surface-area-to-volume ratio. This might provide the materials with more binding sites for absorbing analyte molecules, which results in higher sensitivity than that of conventional films obtained by spin-coating or drop-casting [10,11]. In addition, electrospinning can produce uniform and continuous webs composed by extremely long nanofibers, in large scale, which is favorable for fabricating stable and homogeneous SERS substrates [10,14]. Herein, we present a simple process for fabricating stable and portable SERS substrates by electrospinning (Fig. 1). Firstly, Au– Ag alloy NPs were obtained by co-reduction of their salts with sodium citrate, the reducing agent, in dilute PVA aqueous solution. Then, Au–Ag alloy NPs were centrifuged and added into another
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2.2. Preparation of poly(vinyl alcohol)/gold–silver nanofibers (PVA/ Au–Ag nanofibers)
Fig. 1. Schematic diagram of the process for fabricating PVA/Au–Ag nanofibers via electrospinning.
PVA aqueous solution with relatively higher concentration. Finally, the mixture was electrospun into nanofibers, which could be used as SERS-active substrates. Comparing this to conventional methods in preparing hybrid materials with nanoparticles [8], several remarkable advantages are worth mentioning in this strategy. One is that PVA chains act as the stabilizer in the first step, making nanoparticles obtained in higher concentration possible [15], which is realized by protecting the clusters from coalescing with each other [16] and inhibiting the formation of the insoluble metal halide by-product. The synthesis of Au@Ag core-shell NPs and monometallic NPs (Au, Ag) in polymer solution has been reported previously [15,17–19]. The aggregation and precipitation of particles could be greatly reduced, even after further condensation, by centrifugation. Thus, much denser nanoparticles would be introduced onto the fibers, which is desirable in forming hot-spots for SERS. In addition, good compatibility between the particles and the polymer matrix could be achieved after mixing, since PVA is not only used as the stabilizer for particles, but also the main component for electrospinning. Another merit for this strategy is the tunability of optical properties for the alloy NPs, as mentioned in the previous paragraph, which is favorable for achieving high SERS activity toward different analytes. This would not be possible if polymer/monometallic nanocrystals (Au or Ag) were used as SERS substrates, where there is only one SPR band. As an initial study of the application of these fibers as SERS substrates, the detection of different organic molecules using different ratios of Au–Ag was carried out. A desirable SERS effect was achieved in these systems. In addition, the electrospun PVA/Au– Ag nanofibers were demonstrated to have higher SERS sensitivity than the drop-cast film. 2. Experimental 2.1. Materials Silver nitrate (AgNO3, 99%), chloroauric acid (HAuCl4, >99.5%), 4mercaptobenzoic acid (4-MBA), pyridine, thiophenol, and sodium citrate were all from China Medicine (Group) shanghai Chemical Reagent Corporation. Polyvinyl alcohol (PVA, Mw = 170,000 g/ mol, 88% hydrolyzed) was supplied by Beijing Eastern Petrochemical Co. Ltd. All were used as received. Ultra-filtered water (18.2 MX cm) used in all reactions was obtained by Millipore purification system (Simplicity).
2.2.1. Synthesis of Au–Ag alloy NPs A series of Au–Ag alloy NPs were synthesized in PVA aqueous solution with different initial Au/Ag molar ratios (0:1, 1:3, 1:2, 1:1, 2:1, 3:1, 1:0), which are denoted as Ag, Au1Ag3, Au1Ag2, Au1Ag1, Au2Ag1, Au3Ag1, and Au, respectively. This synthesis was performed by reducing HAuCl4 and AgNO3 with sodium citrate in the presence of 1 wt% PVA. The concentration of HAuCl4 and AgNO3 aqueous solution was 2 mM. For example, 6 mL of HAuCl4 and 6 mL of AgNO3 solution were used for the Au1Ag1 alloy system. The two salt solutions and the PVA solution were mixed up, followed by heating the mixture to boiling. Afterward sodium citrate aqueous solution (1 wt%, 5 mL) was injected quickly into the reaction system, and the reaction mixture was refluxed for 2 h. The resulting mixture was cooled to room temperature and then purified by using the washing-centrifugation cycle three times. The centrifugation was set at 6000 rpm, 25 min each time, to remove the excess metal salt dissolved in water. The purified Au–Ag alloy NPs were redispersed in 0.6 mL ultra-filtered water and saved for further use. To optimize the preparation process of Au–Ag alloy NPs, Au1Ag1 NPs in different PVA solution (5 wt%, 10 wt%) were prepared with the similar procedure. The concentration of HAuCl4 and AgNO3 in reaction solution was maintained at 2 mM. 2.2.2. Preparation of electrospinning solution Au–Ag alloy colloids solution (0.6 mL) was added to a PVA aqueous solution (10 wt%, 2.4 mL). The PVA/Au–Ag alloy mixture was under vigorous stirring for 5 h. The resulting homogenous gel was used for electrospinning or drop-casting. 2.2.3. Electrospinning In a typical electrospinning process, PVA/Au–Ag alloy mixture was placed in a syringe pump and electrospun at a positive voltage of 12 kV. The collector distance between the target and the needle tip was 10 cm. A grounded aluminum foil was used as the collector plate. The flow rate was maintained as 0.3 mL/h, and the electrospinning time was 2 h. 2.2.4. Preparation of PVA/Au–Ag film For the preparation of drop-casting films, 0.6 mL of PVA/Au–Ag alloy mixture was dropped onto a 18 mm 18 mm cleaned slide glass and dry under ambient conditions to form thin film [11]. 2.3. Characterization and Instrumentation UV–vis absorption spectra of the Au–Ag alloy NPs and PVA/Au– Ag nanofiber in the 350–650 nm regions were obtained with UV– vis spectroscopy (Shimadzu UV-3150, Japan). Au–Ag alloy solution after twice centrifugation at 6000 rpm was dropped on a copper grid for alloy NPs’ transmission electron microcopy (TEM, TecnaiG220, FEI company, US) characterization. A copper grid was placed on the grounded aluminum foil to collect nanofibers, for observing the distribution of Au–Ag alloy NPs in PVA fiber by TEM. High-resolution transmission electron microscopy (HRTEM, TecnaiG2 F20, FEI company, US) measurements were taken at an operation voltage of 200 kV. The high-angle annual dark-field scanning transmission electron microscopy (HAADF-STEM) imaging was carried out on a TecnaiG2 F20 with a Schottky electron source and an operation voltage of 200 kV. Scanning transmission electron microscopy (STEM) images were obtained by using an electron probe with an approximate diameter of 0.2 nm. Energy-dispersive X-ray spectroscopy (EDX) line-scan profiles were taken on 40 dots with each sample by using a probe diameter of ca. 0.5 nm and with 4 s acquisition time for each spectrum. The size and morphology of
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electrospun nanofibers were observed on scanning electron microscope (SEM, Hitachi S-4700, Japan). The pre-treatment for SEM measurement, the sample being sputter-coated with a layer of gold, was omitted based on the consideration of the observation that Au–Ag alloy NPs might be interfered. Thus, the sample was directly adhered onto the stage by double-sided adhesive carbon conductive tape. For EDX measurements, a drop of the nanoparticle solution was drop-casted onto a silicon chip, and the sample was dried in a dehumidifying cabinet. The wide-angle X-ray diffraction (XRD, PANalytical Xpert-Pro, the Netherlands) patterns were operated at a voltage of 45 kV and a current of 40 mA with Cu Ka radiation in the 2h range from 30° to 80°. The 632.8 nm and 514.5 nm radiations were used as the excitation source for the Raman experiments. A long working distance 50 objective was used to collect the Raman scattering signal. The slit and pinhole employed were 100 and 400 lm, respectively. Data acquisition time was approximately 5 s. We prepared 2 mM 4-mercaptobenzoic acid (4-MBA), 0.1 M pyridine, and 1 mM thiophenol in ethanol solvent, which are used as the analyte molecules. The Raman spectra were obtained for samples prepared by dropping 20 lL 2 mM 4-MBA, 0.1 M pyridine, or 1 mM thiophenol onto a series of PVA/Au–Ag nanofibers’ membranes with the size of 18 mm 18 mm, and then dried at room temperature to evaporate ethanol present in all three solutions. SERS spectra of analyte molecules from 10 randomly selected positions on nanofibers were collected, and the average value was used as Raman intensity of sample. 3. Result and discussion 3.1. Preparation and characterization Fig. 1 shows the procedure of fabricating PVA/Au–Ag nanofibers. The whole process can be divided into three steps: the preparation of Au–Ag NPs in the presence of dilute PVA as the stabilizer, the mixing of NPs with relatively high concentration PVA solution, and electrospinning. 3.1.1. Optimization of the concentration of PVA stabilizer Au–Ag alloy NPs were obtained in the first step by co-reduction of AgNO3 and HAuCl4, using sodium citrate as a reducing agent in the presence of 1 wt% PVA aqueous solution. PVA was used as the stabilizer in this step, which was based on the consideration that metal nanoparticles in high concentration have a tendency to aggregate. PVA solution was chosen to form a polymeric layer at the surface of the particles to prevent aggregation and precipitation of Au–Ag alloy NPs, even in high concentrations. In the conventional methods, the Au–Ag alloy NPs were reduced in dilute aqueous solution in order to avoid aggregation and precipitation of AgCl [20]. However, such low concentration nanoparticles in aqueous solution will result in less nanoparticles on the nanofibers, which is not in favor of forming hot-spots for SERS. Different concentrations of PVA as stabilizer were tried in this step. Fig. 2a–c shows the photographs and TEM images of Au1Ag1 alloy NPs, obtained in 1 wt%, 5 wt%, 10 wt% PVA solution. Well-dispersed Au– Ag alloy NPs in relatively high concentration were obtained only in 1 wt% PVA solution, displaying orange color in normal light (Fig. 2a). The NPs were aggregated to a certain degree in 5 wt% PVA solution (Fig. 2b) and 10 wt% PVA solution, having purple color and violet color, respectively. The UV–vis absorption spectra (Fig. 2d) were consistent with TEM image and particle colors. The absorption peak of Au1Ag1 particles reduced in 1 wt% PVA solution lies in between the pure Ag NPs (420 nm) and Au NPs (520 nm), indicating the formation of the alloy. However, the peaks for Au1Ag1 NPs obtained in 5% and 10% PVA solution were similar to
Fig. 2. TEM images of Au1Ag1 alloy nanoparticles reduced in (a) 1 wt%, (b) 5 wt%, (c) 10 wt% PVA aqueous solutions, and (d) the corresponding UV–vis absorption spectra.
those of pure Au NPs. Thus, in the case of using 5% and 10% PVA as the stabilizer, excess PVA molecules might be absorbed onto the surface of NPs, which slowed down the process of alloy formation and even stopped the co-reduction process. In addition, excess PVA might also cause the particles’ adherence to each other [21]. Therefore, a conclusion that 1% PVA was the best choice for stabilizing NPs and was used in the following experiments for preparing different alloy NPs could be made at this point. Au–Ag alloy NPs were reduced in dilute aqueous solution in the literature, which was based on the consideration of avoiding aggregation and precipitation of AgCl [8]. The highest concentration of HAuCl4 and AgNO3 to obtain nanoparticles without aggregation in the literature was 0.3 mM [8]. If the concentration of HAuCl4
Fig. 3. TEM images of Au–Ag alloy nanoparticles with various compositions. The insets show the corresponding photos of Au–Ag alloy nanoparticles: (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
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Fig. 4. HRTEM images of Au–Ag alloy nanoparticles: (a) Au1Ag3, (b) Au1Ag2, (c) Au1Ag1, (d) Au2Ag1, and (e) Au3Ag1. The inset is fast Fourier-transformed (FFT) patterns of their corresponding nanoparticles.
and AgNO3 was increased to 2 mM, aggregated alloy NPs would be found due to the absence of the PVA stabilizer (Fig. 1S in Supporting information). However, stable Au–Ag alloy NPs without aggregation could be synthesized in 1 wt% PVA solution if the concentration of HAuCl4 and AgNO3 was 2 mM. UV–vis spectroscopy study was carried out for further confirming that the concentration of Au–Ag alloy NPs obtained in the presence of PVA was higher than that of NPs obtained by the conventional method in the literature. For comparison, Au– Ag alloy NPs were also prepared in aqueous solution according to previous report [8]. Both spectra for Au1Ag1 NPs prepared by our method and according to previous report [8], respectively, are shown in Fig. 2S. Apparently, the absorbance of NPs prepared in 1 wt% PVA solution, even after the solution was diluted by a factor of 4, was still much stronger than that for NPs prepared in aqueous solution. Thus, conclusion that the concentration of Au1Ag1 NPs synthesized in 1 wt% PVA solution was much larger than the concentration of NPs prepared in aqueous solution can be made.
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3.1.2. Au–Ag alloy nanoparticles After obtaining the optimized concentration of PVA stabilizer, a series of the Au–Ag alloy NPs together with pure Au and pure Ag NPs were prepared with the same procedure. As seen in Fig. 3, the diameter of Ag NPs was about 25 nm, larger than that of Au NPs (10 nm), and all alloy particles were in the range of 20– 25 nm. The difference in the diameter of NPs was due to the different formation kinetics. Similar results have been reported in the literature [22]. The possible mechanism was proposed in the literature according to the theory of nucleation and growth. The diameters of all alloy particles were much closer to those of Ag NPs, in our study, which can also be explained by the growth process. Different molar ratios of Au and Ag starting solutions resulted in different NPs colors, from light yellow to deep purplish red (the inset of TEM images in Fig. 3). Fig. 4 shows the HRTEM images of the Au–Ag alloy NPs with different compositions, which revealed the formation of crystalline multiple twined nanoparticles for all samples. The lattice spacing in the NPs is clearly visible in the areas presented, which corresponded well with the interplanar spacing of the corresponding alloy structures of face-centered cubic (fcc) structure phase. High-angle annular dark-field (HAADF) scanning transmission electron microcopy (STEM) imaging, in conjunction with energydispersive X-ray spectroscopic (EDX) elemental line profiling (Fig. 5), revealed that all the nanoparticles with different Au/Ag ratios have a Au–Ag alloy structure. These alloy structures could also be proved by UV–vis spectra in Fig. 5, based on the fact that each absorption spectrum has only one plasmon band and that the wavelength at the maximum absorbance shifted in a linear fashion depending on the composition. For Au1Ag3 NPs (Fig. 5b), the Ag was much richer than the Au along the EDX scan line, and for Au3Ag1 NPs, there was an alloy structure with more Au. As for Au1Ag1 NPs, there was a more random alloy structure observed. The Au/Ag ratios obtained from the EDX were different as a result of the different feeding ratios. Multiple single-particle EDX analysis of Au–Ag alloy NPs was carried out by EDS equipped on SEM, and it showed the molar ratio of Au and Ag. The results of the EDX measurements are
Fig. 5. Cross-sectional compositional STEM-EDS line-scan profiles (corresponding to red vertical lines in insets) recorded from one nanoparticle: (a) Au1Ag3, (b) Au1Ag2, (c) Au1Ag1, (d) Au2Ag1, and (e) Au3Ag1. The insets are their typical HAADF-STEM images. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. UV–vis absorption spectra of Au–Ag alloy nanoparticles with varying gold fractions (left). Positions of maximum surface plasmon bands (kmax) plotted against the mole fractions of gold in alloy nanoparticles (right): (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
summarized in Table 1S. The measured atomic ratio of Au and Ag from measurement is very close to the feeding ratio, further confirming that these particles were an Au–Ag alloy and not individual compounds [23]. UV–vis spectra and the SPR peak positions of pure Au NPs, Ag NPs, and the Au–Ag alloy NPs are shown in Fig. 6. The absorbance of pure Au and pure Ag NPs, synthesized by a method similar to those obtained from literatures [24,25], has characteristic peaks around 520 nm and 420 nm, respectively. All the NPs displayed one absorption peak between 420 nm and 520 nm instead of two separated ones. In addition, red-shifting in the absorbance was found by increasing the Au/Ag molar ratio and the maximum wavelength (kmax) had a linear relationship with the Au mole fraction. All these phenomena are consistent with the formation of alloyed nanostructures [8,26]. The crystal structure of the Au–Ag NPs was also determined by XRD. The XRD peaks of as-prepared nanoparticles can also be indexed as fcc structure, as shown in Fig. 7. The peaks obtained for the alloy NPs were also on the same 2h values of the Ag and Au patterns since the crystalline lattice constants of Au (4.078 Å) and Ag (4.086 Å) are extremely similar to each other (JCPDS 4-0784 and 40783). The diffraction angles at 36.8°, 42.7°, and 62.1° could correspond respectively to Bragg’s reflections of the (1 1 1), (2 0 0), and (2 2 0) planes of a face-centered cubic lattice of Au and Ag. 3.1.3. PVA/Au–Ag nanofibers Electrospinning had drawn growing attention in fabricating fibers with diameters at the nano- or micro-scale due to relatively low cost, simplicity, and versatility [27]. In addition, electrospun polymer/Ag and polymer/Au nanofibers had also been demonstrated to be a kind of effective SERS substrate [11]. TEM images of PVA nanofibers with NPs produced by electrospinning are
Fig. 7. XRD pattern of Au–Ag nanoparticles: (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
Fig. 8. TEM images of PVA/Au–Ag nanofibers: (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
shown in Fig. 8. The diameter of the fibers was between 100 and 200 nm. Many Au–Ag alloy NPs were embedded in the nanofibers, as demonstrated in SEM images (Fig. 3S) with many bright spots on the surface of the nanofibers. In addition, aggregated Au–Ag NPs were also found in TEM images, which are favorable for enhancing SERS effect [11,28]. To make it more suitable for electrospinning, the pre-prepared alloy NPs were blended with a higher concentration PVA solution (10 wt%). During electrospinning process, PVA solution with high viscosity, as an organic additive, would induce the aggregation of individual Au–Ag NPs. Similar phenomenon has been observed previously [11]. The absorption spectra and the SPR peak positions of PVA nanofibers, shown in Fig. 9, contain Au–Ag alloy NPs with different ratio between Au and Ag. Similar to the NPs in solution (Fig. 6), redshifting in the absorbance was found by increasing the Au/Ag molar ratio, and the maximum wavelength had a linear relationship with the Au mole fraction. However, for each Au/Ag composition, slight red-shifting and broadening of the spectra were found in the nanofibers (Fig. 9). In comparison with the corresponding PVA/Au–Ag alloy solutions (Fig. 6), the broadening and red-shift could be attributed to the aggregation of Au–Ag alloy nanoparticles [11,29].
Fig. 9. UV–vis absorption spectra of PVA/Au–Ag nanofibers with varying gold fractions (left). Positions of maximum surface plasmon bands (kmax) plotted against the mole fractions of Au in alloy nanoparticles (right): (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
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Fig. 10. Photographs of PVA/Au–Ag alloy nanofibers before (bottom) and after (top) drop analyte molecules ethanol solution: (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
Fig. 11. SERS spectra of 4-MBA absorbed on PVA/Au–Ag nanofibers at 514.5 nm (A) and 632.8 nm (B) excitation: (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
Fig. 12. SERS spectra of pyridine absorbed on PVA/Au–Ag nanofibers at 514.5 nm (A) and 632.8 nm (B) excitation: (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
Fig. 13. SERS spectra of thiophenol absorbed on PVA/Au–Ag nanofibers at 514.5 nm (A) and 632.8 nm (B) excitation: (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
3.2. Surface-enhanced Raman spectroscopy The Ag and Au nanoparticles displayed different optical properties which can be explained by the electromagnetic mechanism
associated with the SERS effect [30]. To achieve a good SERS effect, selecting an adequate excitation wavelength depending on the metal employed is important. Ag colloids had a higher activity in the visible region, while the Au colloids were more active in the red
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Intensity (a.u.)
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a b 1000
1200
1400
1600
1800
Raman shift (cm-1) Fig. 14. SERS spectra of 4-MBA absorbed from its ethanol solution (2 10 onto (a) PVA/Au–Ag nanofibers and (b) drop-cast PVA/Au–Ag film.
3
M)
light region. However, the activity of both metals greatly decreases in the near infrared region. The preparation of Au–Ag alloy NPs allows for the combination of the SERS activities of both metals in a broader range of the electromagnetic spectrum [31].
3.2.1. Detection of different organic molecules As an initial study on application of these hybrid nanofibers as SERS substrates in detecting organic molecules, 20 lL ethanol solutions of 4-MBA (2 mM), pyridine (0.1 M), and thiophenol (1 mM) was dropped onto a series of PVA/Au–Ag nanofibrous membranes. The Raman spectra were taken on the dry membrane after the ethanol evaporation under room temperature. As Fig. 10 demonstrates, the PVA/Au–Ag nanofibers are transparent after the evaporation of ethanol solvent, which is favorable for obtaining strong SERS signal [32]. The SERS spectra of 4-MBA on the various PVA/Au–Ag nanofiber are shown in Fig. 11. Distinctive peaks could be assigned as the m(CC) ring-breathing modes (1079 cm 1), m(CC) ring-stretching (1586 cm 1), and d(CH) bends (1138 cm 1 and 1182 cm 1) [29]. The most intensive Raman signal for 4-MBA molecule was achieved with PVA/Au1Ag2 nanofibers. When the excitation was set at 514.5 nm, the Raman intensities at 1586 cm 1 and 1079 cm 1 from PVA/Au1Ag2 nanofibers were 6.5-fold and 5.5-fold as compared to their respective intensities from PVA/Ag nanofibers. When the excitation was set at 632.8 nm, the Raman intensities at 1586 cm 1 and 1079 cm 1 from PVA/Au1Ag2 nanofibers were 4.5-fold and 4-fold of those from PVA/Ag nanofibers, respectively. Similar results were obtained when using pyridine as the analyte molecules. As seen in Fig. 12, the characteristic bands of pyridine were present, including the symmetric breathing mode at 1012 cm 1 and the trigonal breathing mode at 1039 cm 1 [33]. For the 514.5 nm excitation (Fig. 12B), the band at 1012 cm 1 was the strongest and the band at 1036 cm 1 was moderately strong. The signals from absorbed pyridine at 1012 cm 1 and 1036 cm 1 on PVA/Au1Ag2 nanofibers were about 1.5 times and 2.5 times of those on the PVA/Ag nanofibers, respectively, while at the excitation of 632.8 nm about 1.5-fold and 2-fold were achieved. For thiophenol molecule (Fig. 13), distinct characteristic Raman bands can also be seen as m(CC) stretching (997 cm 1, 1022 cm 1, 1575 cm 1) and m(CS) at 1077 cm 1. These results were consistent with previous literatures [34]. The Raman intensities from the thiophenol molecules absorbed on PVA/Au1Ag3 nanofibers were about 3.5-fold, 3.5-fold, and 2-fold at 997 cm 1, 1077 cm 1, and
1575 cm 1, respectively, when compared to those of PVA/Ag fibers excited at 514.5 nm. However, only about 1.2, 1.3, and 0.9 times enhancement could be achieved at 997 cm 1, 1077 cm 1, and 1575 cm 1, respectively, when the excitation was at 632.8 nm. In short, stronger SERS signals for three analyte molecules absorbed on all the PVA/Au–Ag nanofibers could be obtained as compared to those of PVA/Au nanofibers, and some demonstrated even stronger signals than those of PVA/Ag nanofibers. The SERS signals were dependent on the Au/Ag ratio but do not increase monotonously with the increase in the Au fraction; the strongest SERS signals were achieved when the Au/Ag molar ratio was 1:2 or 1:3. The difference in SERS activity could be attributed to the various extent of matching between the laser wavelength and the materials’ composition [4,35]. For the three organic molecules, there were almost no signals obtained from PVA/Au nanofibers but all related peaks appeared for PVA/Ag nanofibers (curve g in Fig. 11–13). Previous literature noted that not all sizes of Au NPs had good Raman enhancement [36] and that Ag NPs were more SERS active [4]. In addition, for the range of Au content of the PVA/Au–Ag nanofiber from 50% to 100%, negligible SERS signals can be observed at the 514 nm excitation. The observations of the relationship between the Au content, excitation wavelength, and SERS intensities in our system were generally consistent with the results presented from another paper [37]. 3.2.2. Comparison between nanofiber and film In general, the Raman spectrum is most enhanced when the molecule is directly bonded to the SERS-active surface. The large surface-area-to-volume ratio of the PVA/Au–Ag nanofibers provides more binding sites to adsorb analyte molecules, and thus, it has higher sensitivity than the conventional films by spin-coating or drop-casting. To evaluate the SERS performance of the PVA/ Au–Ag nanofibers, two types of SERS substrates with the same composition were prepared for comparison. The first one (substrate a) is electrospun PVA/Au1Ag3 nanofibers mat. The second one (substrate b) is a drop-cast film formed by dropping the asprepared PVA/Au1Ag3 alloy solution on a glass slide and dried in air under room temperature. The SERS spectra of 2 mM 4-MBA molecules obtained from the two substrates are shown in Fig. 14. The electrospun nanofibers showed better SERS sensitivity in comparison with drop-cast film because the electrospun nanofibers had a higher surface-area-to-volume ratio and larger absorption capacity of the analyte molecules [11]. 4. Conclusion In summary, we have demonstrated a facile approach to prepare electrospun PVA nanofibers embedded with Au–Ag alloy NPs intended for SERS-active substrates. Au–Ag alloy NPs were obtained by co-reduction of AgNO3 and HAuCl4, with sodium citrate as a reducing agent in the presence of dilute PVA aqueous solution. Then, the SERS-active substrates of PVA/Au–Ag nanofibers were successfully prepared by electrospinning. HRTEM and STEM-EDX confirmed that these nanoparticles were all alloy structures with different Au/Ag ratio. SEM and TEM images demonstrated that the Au–Ag alloy NPs were embedded in the PVA nanofibers. The tunability in UV–vis absorbance, in both Au–Ag alloy NPs and PVA/ Au–Ag nanofibers, was also achieved by varying the alloy composition. Different SERS signals of PVA/Au–Ag nanofiber could be observed with different analyte molecules. The electrospun PVA/ Au–Ag nanofiber substrate showed the better SERS sensitivity in comparison with the drop-cast film. This facile method to prepare SERS-active substrates, with tunable SPR to achieve the highest signal enhancement in various detecting systems, may be useful in the large-scale production of sensing materials in the future.
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Acknowledgments This work is supported by the National Natural Science Foundation of China (Nos. 51003069, 20973120, 20974075), Natural Science Foundation of Jiangsu Higher Education Institutions of China (No. 10KJB430014), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.05.048. References [1] F. Yan, T. Vo-Dinh, Actuators B 121 (2007) 61. [2] J.N. Anker, W.P. Hall, O. Lyandres, N.C. Shah, J. Zhao, R.P. Van Duyne, Nat. Mater. 7 (2008) 442. [3] M. Mulvihill, A. Tao, K. Benjauthrit, J. Arnold, P. Yang, Angew. Chem. Int. Ed. 47 (2008) 6456. [4] S.E. Hunyadi, C.J. Murphy, J. Mater. Chem. 16 (2006) 3929. [5] J.J. Laserna, Anal. Chim. Acta. 283 (1993) 670. [6] S. Lee, S. Kim, J. Choo, S.Y. Shin, Y.H. Lee, H.Y. Choi, S. Ha, K. Kang, C.H. Oh, Anal. Chem. 79 (2007) 916. [7] Y.C. Liu, C.C. Yu, T.C. Hsu, Electrochem. Commun. 9 (2007) 639. [8] S. Link, Z.L. Wang, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 3529. [9] I. Yoon, T. Kang, W. Choi, J. Kim, Y. Yoo, S.W. Joo, Q.H. Park, H. Ihee, B. Kim, J. Am. Chem. Soc. 131 (2009) 758. [10] H. Wu, D.D. Lin, W. Pan, Langmuir 26 (2010) 6865. [11] D. He, B. Hu, Q.F. Yao, K. Wang, S.H. Yu, ACS Nano 3 (2009) 3993. [12] B. Guo, G. Han, M. Li, S. Zhao, Thin Solid Films 518 (2010) 3228.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Surface-enhanced Raman scattering-active substrates of electrospun polyvinyl alcohol/gold–silver nanofibers Xiaofei Li, Minhua Cao, Han Zhang, Lin Zhou, Si Cheng ⇑, Jian-Lin Yao, Li-Juan Fan ⇑ Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
a r t i c l e
i n f o
Article history: Received 7 March 2012 Accepted 25 May 2012 Available online 7 June 2012 Keywords: Au–Ag alloy Electrospinning Surface-enhanced Raman scattering
a b s t r a c t Surface-enhanced Raman scattering (SERS)-active substrates of polyvinyl alcohol/gold–silver (PVA/Au– Ag) nanofibers were prepared using a simple approach involving electrospinning. The tunable surface plasmon resonance (SPR) of gold–silver alloy (Au–Ag alloy) nanoparticles (NPs) was achieved by controlling the feed ratio between gold and silver precursors. A higher concentration of Au–Ag alloy NPs could be obtained than the conventional methods, using 1 wt% of PVA as the stabilizer. The Au–Ag alloy structure was demonstrated by HRTEM and STEM-EDX. After the electrospinning, the Au–Ag alloy NPs were successfully embedded in PVA nanofibers, as shown in the SEM and TEM images. Raman spectra displayed an apparent enhancement in the signal of 4-mercaptobenzoic acid (4-MBA), pyridine, and thiophenol molecules pre-absorbed from their ethanol solution onto the PVA/Au–Ag nanofibers. Different SERS effects were achieved by varying the Au content or excitation wavelength. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Noble metallic nanostructures usually have an interesting property known as surface plasmon resonance (SPR), which arises from the collective oscillation of conduction electrons on the surface of metals and resulting in a variety of optical signal enhancements, such as surface-enhanced Raman scattering (SERS), surface-enhanced fluorescence, and surface-enhanced infrared absorption. These phenomena have allowed for powerful analytical techniques in sensor applications [1–3]. SERS is one of these phenomena, and it is highly dependent on the properties of the metal, such as gold, silver, and copper, as well as the size and shape of the nanoparticles. The optical properties of Au and Ag NPs have been studied in great detail [4]. Generally, there was a lower enhancement of Raman signal in the visible light region for Au than that for Ag [5]. However, Au NPs have many advantages over Ag NPs, such as easy preparation, high homogeneity, long-term stability, and biocompatibility [6]. To combine the high signal enhancement of Ag NPs with the advantages of the Au materials, the synthesis of bimetallic (alloy or core-shell) nanostructures consisting of Au and Ag has been a popular research topic, especially as substrates with improved SERS activity. It is now commonly known that the optical properties can be tuned by varying the composition of the metals. In this aspect, the gold–silver alloy nanoparticles (Au–Ag alloy NPs) displayed higher sensitivity ⇑ Corresponding authors. E-mail addresses: [email protected] (S. Cheng), [email protected] (L.-J. Fan). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.05.048
for SERS than their monometallic nanocrystals [7]. Basically, the SPR bands of Au–Ag alloy NPs were demonstrated to be continuously tunable in a wide range of wavelengths, from 400 nm to 520 nm [8]. To our best knowledge, the SERS effect has been mainly studied in three types of nanostructure systems: colloidal dispersions of metal particles, nanostructured metal film, and one-dimensional (1D) nanostructure. The nanostructure films provided SERS substrates with several advantages over colloidal systems, such as portability and compatibility with a wider range of substrates. Recently, 1D nanostructure systems have also been proven to be applicable as SERS substrates [9–14]. Among the 1D nanostructures, electrospun polymer nanofiber mats embedded with monometallic nanocrystals, such as PVA/Ag and PMMA/Au nanofiber, were demonstrated to be SERS substrates with high performance [11,12]. One of the major advantages of the electrospun nanofibers is the large surface-area-to-volume ratio. This might provide the materials with more binding sites for absorbing analyte molecules, which results in higher sensitivity than that of conventional films obtained by spin-coating or drop-casting [10,11]. In addition, electrospinning can produce uniform and continuous webs composed by extremely long nanofibers, in large scale, which is favorable for fabricating stable and homogeneous SERS substrates [10,14]. Herein, we present a simple process for fabricating stable and portable SERS substrates by electrospinning (Fig. 1). Firstly, Au– Ag alloy NPs were obtained by co-reduction of their salts with sodium citrate, the reducing agent, in dilute PVA aqueous solution. Then, Au–Ag alloy NPs were centrifuged and added into another
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2.2. Preparation of poly(vinyl alcohol)/gold–silver nanofibers (PVA/ Au–Ag nanofibers)
Fig. 1. Schematic diagram of the process for fabricating PVA/Au–Ag nanofibers via electrospinning.
PVA aqueous solution with relatively higher concentration. Finally, the mixture was electrospun into nanofibers, which could be used as SERS-active substrates. Comparing this to conventional methods in preparing hybrid materials with nanoparticles [8], several remarkable advantages are worth mentioning in this strategy. One is that PVA chains act as the stabilizer in the first step, making nanoparticles obtained in higher concentration possible [15], which is realized by protecting the clusters from coalescing with each other [16] and inhibiting the formation of the insoluble metal halide by-product. The synthesis of Au@Ag core-shell NPs and monometallic NPs (Au, Ag) in polymer solution has been reported previously [15,17–19]. The aggregation and precipitation of particles could be greatly reduced, even after further condensation, by centrifugation. Thus, much denser nanoparticles would be introduced onto the fibers, which is desirable in forming hot-spots for SERS. In addition, good compatibility between the particles and the polymer matrix could be achieved after mixing, since PVA is not only used as the stabilizer for particles, but also the main component for electrospinning. Another merit for this strategy is the tunability of optical properties for the alloy NPs, as mentioned in the previous paragraph, which is favorable for achieving high SERS activity toward different analytes. This would not be possible if polymer/monometallic nanocrystals (Au or Ag) were used as SERS substrates, where there is only one SPR band. As an initial study of the application of these fibers as SERS substrates, the detection of different organic molecules using different ratios of Au–Ag was carried out. A desirable SERS effect was achieved in these systems. In addition, the electrospun PVA/Au– Ag nanofibers were demonstrated to have higher SERS sensitivity than the drop-cast film. 2. Experimental 2.1. Materials Silver nitrate (AgNO3, 99%), chloroauric acid (HAuCl4, >99.5%), 4mercaptobenzoic acid (4-MBA), pyridine, thiophenol, and sodium citrate were all from China Medicine (Group) shanghai Chemical Reagent Corporation. Polyvinyl alcohol (PVA, Mw = 170,000 g/ mol, 88% hydrolyzed) was supplied by Beijing Eastern Petrochemical Co. Ltd. All were used as received. Ultra-filtered water (18.2 MX cm) used in all reactions was obtained by Millipore purification system (Simplicity).
2.2.1. Synthesis of Au–Ag alloy NPs A series of Au–Ag alloy NPs were synthesized in PVA aqueous solution with different initial Au/Ag molar ratios (0:1, 1:3, 1:2, 1:1, 2:1, 3:1, 1:0), which are denoted as Ag, Au1Ag3, Au1Ag2, Au1Ag1, Au2Ag1, Au3Ag1, and Au, respectively. This synthesis was performed by reducing HAuCl4 and AgNO3 with sodium citrate in the presence of 1 wt% PVA. The concentration of HAuCl4 and AgNO3 aqueous solution was 2 mM. For example, 6 mL of HAuCl4 and 6 mL of AgNO3 solution were used for the Au1Ag1 alloy system. The two salt solutions and the PVA solution were mixed up, followed by heating the mixture to boiling. Afterward sodium citrate aqueous solution (1 wt%, 5 mL) was injected quickly into the reaction system, and the reaction mixture was refluxed for 2 h. The resulting mixture was cooled to room temperature and then purified by using the washing-centrifugation cycle three times. The centrifugation was set at 6000 rpm, 25 min each time, to remove the excess metal salt dissolved in water. The purified Au–Ag alloy NPs were redispersed in 0.6 mL ultra-filtered water and saved for further use. To optimize the preparation process of Au–Ag alloy NPs, Au1Ag1 NPs in different PVA solution (5 wt%, 10 wt%) were prepared with the similar procedure. The concentration of HAuCl4 and AgNO3 in reaction solution was maintained at 2 mM. 2.2.2. Preparation of electrospinning solution Au–Ag alloy colloids solution (0.6 mL) was added to a PVA aqueous solution (10 wt%, 2.4 mL). The PVA/Au–Ag alloy mixture was under vigorous stirring for 5 h. The resulting homogenous gel was used for electrospinning or drop-casting. 2.2.3. Electrospinning In a typical electrospinning process, PVA/Au–Ag alloy mixture was placed in a syringe pump and electrospun at a positive voltage of 12 kV. The collector distance between the target and the needle tip was 10 cm. A grounded aluminum foil was used as the collector plate. The flow rate was maintained as 0.3 mL/h, and the electrospinning time was 2 h. 2.2.4. Preparation of PVA/Au–Ag film For the preparation of drop-casting films, 0.6 mL of PVA/Au–Ag alloy mixture was dropped onto a 18 mm 18 mm cleaned slide glass and dry under ambient conditions to form thin film [11]. 2.3. Characterization and Instrumentation UV–vis absorption spectra of the Au–Ag alloy NPs and PVA/Au– Ag nanofiber in the 350–650 nm regions were obtained with UV– vis spectroscopy (Shimadzu UV-3150, Japan). Au–Ag alloy solution after twice centrifugation at 6000 rpm was dropped on a copper grid for alloy NPs’ transmission electron microcopy (TEM, TecnaiG220, FEI company, US) characterization. A copper grid was placed on the grounded aluminum foil to collect nanofibers, for observing the distribution of Au–Ag alloy NPs in PVA fiber by TEM. High-resolution transmission electron microscopy (HRTEM, TecnaiG2 F20, FEI company, US) measurements were taken at an operation voltage of 200 kV. The high-angle annual dark-field scanning transmission electron microscopy (HAADF-STEM) imaging was carried out on a TecnaiG2 F20 with a Schottky electron source and an operation voltage of 200 kV. Scanning transmission electron microscopy (STEM) images were obtained by using an electron probe with an approximate diameter of 0.2 nm. Energy-dispersive X-ray spectroscopy (EDX) line-scan profiles were taken on 40 dots with each sample by using a probe diameter of ca. 0.5 nm and with 4 s acquisition time for each spectrum. The size and morphology of
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electrospun nanofibers were observed on scanning electron microscope (SEM, Hitachi S-4700, Japan). The pre-treatment for SEM measurement, the sample being sputter-coated with a layer of gold, was omitted based on the consideration of the observation that Au–Ag alloy NPs might be interfered. Thus, the sample was directly adhered onto the stage by double-sided adhesive carbon conductive tape. For EDX measurements, a drop of the nanoparticle solution was drop-casted onto a silicon chip, and the sample was dried in a dehumidifying cabinet. The wide-angle X-ray diffraction (XRD, PANalytical Xpert-Pro, the Netherlands) patterns were operated at a voltage of 45 kV and a current of 40 mA with Cu Ka radiation in the 2h range from 30° to 80°. The 632.8 nm and 514.5 nm radiations were used as the excitation source for the Raman experiments. A long working distance 50 objective was used to collect the Raman scattering signal. The slit and pinhole employed were 100 and 400 lm, respectively. Data acquisition time was approximately 5 s. We prepared 2 mM 4-mercaptobenzoic acid (4-MBA), 0.1 M pyridine, and 1 mM thiophenol in ethanol solvent, which are used as the analyte molecules. The Raman spectra were obtained for samples prepared by dropping 20 lL 2 mM 4-MBA, 0.1 M pyridine, or 1 mM thiophenol onto a series of PVA/Au–Ag nanofibers’ membranes with the size of 18 mm 18 mm, and then dried at room temperature to evaporate ethanol present in all three solutions. SERS spectra of analyte molecules from 10 randomly selected positions on nanofibers were collected, and the average value was used as Raman intensity of sample. 3. Result and discussion 3.1. Preparation and characterization Fig. 1 shows the procedure of fabricating PVA/Au–Ag nanofibers. The whole process can be divided into three steps: the preparation of Au–Ag NPs in the presence of dilute PVA as the stabilizer, the mixing of NPs with relatively high concentration PVA solution, and electrospinning. 3.1.1. Optimization of the concentration of PVA stabilizer Au–Ag alloy NPs were obtained in the first step by co-reduction of AgNO3 and HAuCl4, using sodium citrate as a reducing agent in the presence of 1 wt% PVA aqueous solution. PVA was used as the stabilizer in this step, which was based on the consideration that metal nanoparticles in high concentration have a tendency to aggregate. PVA solution was chosen to form a polymeric layer at the surface of the particles to prevent aggregation and precipitation of Au–Ag alloy NPs, even in high concentrations. In the conventional methods, the Au–Ag alloy NPs were reduced in dilute aqueous solution in order to avoid aggregation and precipitation of AgCl [20]. However, such low concentration nanoparticles in aqueous solution will result in less nanoparticles on the nanofibers, which is not in favor of forming hot-spots for SERS. Different concentrations of PVA as stabilizer were tried in this step. Fig. 2a–c shows the photographs and TEM images of Au1Ag1 alloy NPs, obtained in 1 wt%, 5 wt%, 10 wt% PVA solution. Well-dispersed Au– Ag alloy NPs in relatively high concentration were obtained only in 1 wt% PVA solution, displaying orange color in normal light (Fig. 2a). The NPs were aggregated to a certain degree in 5 wt% PVA solution (Fig. 2b) and 10 wt% PVA solution, having purple color and violet color, respectively. The UV–vis absorption spectra (Fig. 2d) were consistent with TEM image and particle colors. The absorption peak of Au1Ag1 particles reduced in 1 wt% PVA solution lies in between the pure Ag NPs (420 nm) and Au NPs (520 nm), indicating the formation of the alloy. However, the peaks for Au1Ag1 NPs obtained in 5% and 10% PVA solution were similar to
Fig. 2. TEM images of Au1Ag1 alloy nanoparticles reduced in (a) 1 wt%, (b) 5 wt%, (c) 10 wt% PVA aqueous solutions, and (d) the corresponding UV–vis absorption spectra.
those of pure Au NPs. Thus, in the case of using 5% and 10% PVA as the stabilizer, excess PVA molecules might be absorbed onto the surface of NPs, which slowed down the process of alloy formation and even stopped the co-reduction process. In addition, excess PVA might also cause the particles’ adherence to each other [21]. Therefore, a conclusion that 1% PVA was the best choice for stabilizing NPs and was used in the following experiments for preparing different alloy NPs could be made at this point. Au–Ag alloy NPs were reduced in dilute aqueous solution in the literature, which was based on the consideration of avoiding aggregation and precipitation of AgCl [8]. The highest concentration of HAuCl4 and AgNO3 to obtain nanoparticles without aggregation in the literature was 0.3 mM [8]. If the concentration of HAuCl4
Fig. 3. TEM images of Au–Ag alloy nanoparticles with various compositions. The insets show the corresponding photos of Au–Ag alloy nanoparticles: (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
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Fig. 4. HRTEM images of Au–Ag alloy nanoparticles: (a) Au1Ag3, (b) Au1Ag2, (c) Au1Ag1, (d) Au2Ag1, and (e) Au3Ag1. The inset is fast Fourier-transformed (FFT) patterns of their corresponding nanoparticles.
and AgNO3 was increased to 2 mM, aggregated alloy NPs would be found due to the absence of the PVA stabilizer (Fig. 1S in Supporting information). However, stable Au–Ag alloy NPs without aggregation could be synthesized in 1 wt% PVA solution if the concentration of HAuCl4 and AgNO3 was 2 mM. UV–vis spectroscopy study was carried out for further confirming that the concentration of Au–Ag alloy NPs obtained in the presence of PVA was higher than that of NPs obtained by the conventional method in the literature. For comparison, Au– Ag alloy NPs were also prepared in aqueous solution according to previous report [8]. Both spectra for Au1Ag1 NPs prepared by our method and according to previous report [8], respectively, are shown in Fig. 2S. Apparently, the absorbance of NPs prepared in 1 wt% PVA solution, even after the solution was diluted by a factor of 4, was still much stronger than that for NPs prepared in aqueous solution. Thus, conclusion that the concentration of Au1Ag1 NPs synthesized in 1 wt% PVA solution was much larger than the concentration of NPs prepared in aqueous solution can be made.
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3.1.2. Au–Ag alloy nanoparticles After obtaining the optimized concentration of PVA stabilizer, a series of the Au–Ag alloy NPs together with pure Au and pure Ag NPs were prepared with the same procedure. As seen in Fig. 3, the diameter of Ag NPs was about 25 nm, larger than that of Au NPs (10 nm), and all alloy particles were in the range of 20– 25 nm. The difference in the diameter of NPs was due to the different formation kinetics. Similar results have been reported in the literature [22]. The possible mechanism was proposed in the literature according to the theory of nucleation and growth. The diameters of all alloy particles were much closer to those of Ag NPs, in our study, which can also be explained by the growth process. Different molar ratios of Au and Ag starting solutions resulted in different NPs colors, from light yellow to deep purplish red (the inset of TEM images in Fig. 3). Fig. 4 shows the HRTEM images of the Au–Ag alloy NPs with different compositions, which revealed the formation of crystalline multiple twined nanoparticles for all samples. The lattice spacing in the NPs is clearly visible in the areas presented, which corresponded well with the interplanar spacing of the corresponding alloy structures of face-centered cubic (fcc) structure phase. High-angle annular dark-field (HAADF) scanning transmission electron microcopy (STEM) imaging, in conjunction with energydispersive X-ray spectroscopic (EDX) elemental line profiling (Fig. 5), revealed that all the nanoparticles with different Au/Ag ratios have a Au–Ag alloy structure. These alloy structures could also be proved by UV–vis spectra in Fig. 5, based on the fact that each absorption spectrum has only one plasmon band and that the wavelength at the maximum absorbance shifted in a linear fashion depending on the composition. For Au1Ag3 NPs (Fig. 5b), the Ag was much richer than the Au along the EDX scan line, and for Au3Ag1 NPs, there was an alloy structure with more Au. As for Au1Ag1 NPs, there was a more random alloy structure observed. The Au/Ag ratios obtained from the EDX were different as a result of the different feeding ratios. Multiple single-particle EDX analysis of Au–Ag alloy NPs was carried out by EDS equipped on SEM, and it showed the molar ratio of Au and Ag. The results of the EDX measurements are
Fig. 5. Cross-sectional compositional STEM-EDS line-scan profiles (corresponding to red vertical lines in insets) recorded from one nanoparticle: (a) Au1Ag3, (b) Au1Ag2, (c) Au1Ag1, (d) Au2Ag1, and (e) Au3Ag1. The insets are their typical HAADF-STEM images. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. UV–vis absorption spectra of Au–Ag alloy nanoparticles with varying gold fractions (left). Positions of maximum surface plasmon bands (kmax) plotted against the mole fractions of gold in alloy nanoparticles (right): (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
summarized in Table 1S. The measured atomic ratio of Au and Ag from measurement is very close to the feeding ratio, further confirming that these particles were an Au–Ag alloy and not individual compounds [23]. UV–vis spectra and the SPR peak positions of pure Au NPs, Ag NPs, and the Au–Ag alloy NPs are shown in Fig. 6. The absorbance of pure Au and pure Ag NPs, synthesized by a method similar to those obtained from literatures [24,25], has characteristic peaks around 520 nm and 420 nm, respectively. All the NPs displayed one absorption peak between 420 nm and 520 nm instead of two separated ones. In addition, red-shifting in the absorbance was found by increasing the Au/Ag molar ratio and the maximum wavelength (kmax) had a linear relationship with the Au mole fraction. All these phenomena are consistent with the formation of alloyed nanostructures [8,26]. The crystal structure of the Au–Ag NPs was also determined by XRD. The XRD peaks of as-prepared nanoparticles can also be indexed as fcc structure, as shown in Fig. 7. The peaks obtained for the alloy NPs were also on the same 2h values of the Ag and Au patterns since the crystalline lattice constants of Au (4.078 Å) and Ag (4.086 Å) are extremely similar to each other (JCPDS 4-0784 and 40783). The diffraction angles at 36.8°, 42.7°, and 62.1° could correspond respectively to Bragg’s reflections of the (1 1 1), (2 0 0), and (2 2 0) planes of a face-centered cubic lattice of Au and Ag. 3.1.3. PVA/Au–Ag nanofibers Electrospinning had drawn growing attention in fabricating fibers with diameters at the nano- or micro-scale due to relatively low cost, simplicity, and versatility [27]. In addition, electrospun polymer/Ag and polymer/Au nanofibers had also been demonstrated to be a kind of effective SERS substrate [11]. TEM images of PVA nanofibers with NPs produced by electrospinning are
Fig. 7. XRD pattern of Au–Ag nanoparticles: (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
Fig. 8. TEM images of PVA/Au–Ag nanofibers: (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
shown in Fig. 8. The diameter of the fibers was between 100 and 200 nm. Many Au–Ag alloy NPs were embedded in the nanofibers, as demonstrated in SEM images (Fig. 3S) with many bright spots on the surface of the nanofibers. In addition, aggregated Au–Ag NPs were also found in TEM images, which are favorable for enhancing SERS effect [11,28]. To make it more suitable for electrospinning, the pre-prepared alloy NPs were blended with a higher concentration PVA solution (10 wt%). During electrospinning process, PVA solution with high viscosity, as an organic additive, would induce the aggregation of individual Au–Ag NPs. Similar phenomenon has been observed previously [11]. The absorption spectra and the SPR peak positions of PVA nanofibers, shown in Fig. 9, contain Au–Ag alloy NPs with different ratio between Au and Ag. Similar to the NPs in solution (Fig. 6), redshifting in the absorbance was found by increasing the Au/Ag molar ratio, and the maximum wavelength had a linear relationship with the Au mole fraction. However, for each Au/Ag composition, slight red-shifting and broadening of the spectra were found in the nanofibers (Fig. 9). In comparison with the corresponding PVA/Au–Ag alloy solutions (Fig. 6), the broadening and red-shift could be attributed to the aggregation of Au–Ag alloy nanoparticles [11,29].
Fig. 9. UV–vis absorption spectra of PVA/Au–Ag nanofibers with varying gold fractions (left). Positions of maximum surface plasmon bands (kmax) plotted against the mole fractions of Au in alloy nanoparticles (right): (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
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Fig. 10. Photographs of PVA/Au–Ag alloy nanofibers before (bottom) and after (top) drop analyte molecules ethanol solution: (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
Fig. 11. SERS spectra of 4-MBA absorbed on PVA/Au–Ag nanofibers at 514.5 nm (A) and 632.8 nm (B) excitation: (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
Fig. 12. SERS spectra of pyridine absorbed on PVA/Au–Ag nanofibers at 514.5 nm (A) and 632.8 nm (B) excitation: (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
Fig. 13. SERS spectra of thiophenol absorbed on PVA/Au–Ag nanofibers at 514.5 nm (A) and 632.8 nm (B) excitation: (a) Ag, (b) Au1Ag3, (c) Au1Ag2, (d) Au1Ag1, (e) Au2Ag1, (f) Au3Ag1, and (g) Au.
3.2. Surface-enhanced Raman spectroscopy The Ag and Au nanoparticles displayed different optical properties which can be explained by the electromagnetic mechanism
associated with the SERS effect [30]. To achieve a good SERS effect, selecting an adequate excitation wavelength depending on the metal employed is important. Ag colloids had a higher activity in the visible region, while the Au colloids were more active in the red
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Intensity (a.u.)
34
a b 1000
1200
1400
1600
1800
Raman shift (cm-1) Fig. 14. SERS spectra of 4-MBA absorbed from its ethanol solution (2 10 onto (a) PVA/Au–Ag nanofibers and (b) drop-cast PVA/Au–Ag film.
3
M)
light region. However, the activity of both metals greatly decreases in the near infrared region. The preparation of Au–Ag alloy NPs allows for the combination of the SERS activities of both metals in a broader range of the electromagnetic spectrum [31].
3.2.1. Detection of different organic molecules As an initial study on application of these hybrid nanofibers as SERS substrates in detecting organic molecules, 20 lL ethanol solutions of 4-MBA (2 mM), pyridine (0.1 M), and thiophenol (1 mM) was dropped onto a series of PVA/Au–Ag nanofibrous membranes. The Raman spectra were taken on the dry membrane after the ethanol evaporation under room temperature. As Fig. 10 demonstrates, the PVA/Au–Ag nanofibers are transparent after the evaporation of ethanol solvent, which is favorable for obtaining strong SERS signal [32]. The SERS spectra of 4-MBA on the various PVA/Au–Ag nanofiber are shown in Fig. 11. Distinctive peaks could be assigned as the m(CC) ring-breathing modes (1079 cm 1), m(CC) ring-stretching (1586 cm 1), and d(CH) bends (1138 cm 1 and 1182 cm 1) [29]. The most intensive Raman signal for 4-MBA molecule was achieved with PVA/Au1Ag2 nanofibers. When the excitation was set at 514.5 nm, the Raman intensities at 1586 cm 1 and 1079 cm 1 from PVA/Au1Ag2 nanofibers were 6.5-fold and 5.5-fold as compared to their respective intensities from PVA/Ag nanofibers. When the excitation was set at 632.8 nm, the Raman intensities at 1586 cm 1 and 1079 cm 1 from PVA/Au1Ag2 nanofibers were 4.5-fold and 4-fold of those from PVA/Ag nanofibers, respectively. Similar results were obtained when using pyridine as the analyte molecules. As seen in Fig. 12, the characteristic bands of pyridine were present, including the symmetric breathing mode at 1012 cm 1 and the trigonal breathing mode at 1039 cm 1 [33]. For the 514.5 nm excitation (Fig. 12B), the band at 1012 cm 1 was the strongest and the band at 1036 cm 1 was moderately strong. The signals from absorbed pyridine at 1012 cm 1 and 1036 cm 1 on PVA/Au1Ag2 nanofibers were about 1.5 times and 2.5 times of those on the PVA/Ag nanofibers, respectively, while at the excitation of 632.8 nm about 1.5-fold and 2-fold were achieved. For thiophenol molecule (Fig. 13), distinct characteristic Raman bands can also be seen as m(CC) stretching (997 cm 1, 1022 cm 1, 1575 cm 1) and m(CS) at 1077 cm 1. These results were consistent with previous literatures [34]. The Raman intensities from the thiophenol molecules absorbed on PVA/Au1Ag3 nanofibers were about 3.5-fold, 3.5-fold, and 2-fold at 997 cm 1, 1077 cm 1, and
1575 cm 1, respectively, when compared to those of PVA/Ag fibers excited at 514.5 nm. However, only about 1.2, 1.3, and 0.9 times enhancement could be achieved at 997 cm 1, 1077 cm 1, and 1575 cm 1, respectively, when the excitation was at 632.8 nm. In short, stronger SERS signals for three analyte molecules absorbed on all the PVA/Au–Ag nanofibers could be obtained as compared to those of PVA/Au nanofibers, and some demonstrated even stronger signals than those of PVA/Ag nanofibers. The SERS signals were dependent on the Au/Ag ratio but do not increase monotonously with the increase in the Au fraction; the strongest SERS signals were achieved when the Au/Ag molar ratio was 1:2 or 1:3. The difference in SERS activity could be attributed to the various extent of matching between the laser wavelength and the materials’ composition [4,35]. For the three organic molecules, there were almost no signals obtained from PVA/Au nanofibers but all related peaks appeared for PVA/Ag nanofibers (curve g in Fig. 11–13). Previous literature noted that not all sizes of Au NPs had good Raman enhancement [36] and that Ag NPs were more SERS active [4]. In addition, for the range of Au content of the PVA/Au–Ag nanofiber from 50% to 100%, negligible SERS signals can be observed at the 514 nm excitation. The observations of the relationship between the Au content, excitation wavelength, and SERS intensities in our system were generally consistent with the results presented from another paper [37]. 3.2.2. Comparison between nanofiber and film In general, the Raman spectrum is most enhanced when the molecule is directly bonded to the SERS-active surface. The large surface-area-to-volume ratio of the PVA/Au–Ag nanofibers provides more binding sites to adsorb analyte molecules, and thus, it has higher sensitivity than the conventional films by spin-coating or drop-casting. To evaluate the SERS performance of the PVA/ Au–Ag nanofibers, two types of SERS substrates with the same composition were prepared for comparison. The first one (substrate a) is electrospun PVA/Au1Ag3 nanofibers mat. The second one (substrate b) is a drop-cast film formed by dropping the asprepared PVA/Au1Ag3 alloy solution on a glass slide and dried in air under room temperature. The SERS spectra of 2 mM 4-MBA molecules obtained from the two substrates are shown in Fig. 14. The electrospun nanofibers showed better SERS sensitivity in comparison with drop-cast film because the electrospun nanofibers had a higher surface-area-to-volume ratio and larger absorption capacity of the analyte molecules [11]. 4. Conclusion In summary, we have demonstrated a facile approach to prepare electrospun PVA nanofibers embedded with Au–Ag alloy NPs intended for SERS-active substrates. Au–Ag alloy NPs were obtained by co-reduction of AgNO3 and HAuCl4, with sodium citrate as a reducing agent in the presence of dilute PVA aqueous solution. Then, the SERS-active substrates of PVA/Au–Ag nanofibers were successfully prepared by electrospinning. HRTEM and STEM-EDX confirmed that these nanoparticles were all alloy structures with different Au/Ag ratio. SEM and TEM images demonstrated that the Au–Ag alloy NPs were embedded in the PVA nanofibers. The tunability in UV–vis absorbance, in both Au–Ag alloy NPs and PVA/ Au–Ag nanofibers, was also achieved by varying the alloy composition. Different SERS signals of PVA/Au–Ag nanofiber could be observed with different analyte molecules. The electrospun PVA/ Au–Ag nanofiber substrate showed the better SERS sensitivity in comparison with the drop-cast film. This facile method to prepare SERS-active substrates, with tunable SPR to achieve the highest signal enhancement in various detecting systems, may be useful in the large-scale production of sensing materials in the future.
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Acknowledgments This work is supported by the National Natural Science Foundation of China (Nos. 51003069, 20973120, 20974075), Natural Science Foundation of Jiangsu Higher Education Institutions of China (No. 10KJB430014), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.05.048. References [1] F. Yan, T. Vo-Dinh, Actuators B 121 (2007) 61. [2] J.N. Anker, W.P. Hall, O. Lyandres, N.C. Shah, J. Zhao, R.P. Van Duyne, Nat. Mater. 7 (2008) 442. [3] M. Mulvihill, A. Tao, K. Benjauthrit, J. Arnold, P. Yang, Angew. Chem. Int. Ed. 47 (2008) 6456. [4] S.E. Hunyadi, C.J. Murphy, J. Mater. Chem. 16 (2006) 3929. [5] J.J. Laserna, Anal. Chim. Acta. 283 (1993) 670. [6] S. Lee, S. Kim, J. Choo, S.Y. Shin, Y.H. Lee, H.Y. Choi, S. Ha, K. Kang, C.H. Oh, Anal. Chem. 79 (2007) 916. [7] Y.C. Liu, C.C. Yu, T.C. Hsu, Electrochem. Commun. 9 (2007) 639. [8] S. Link, Z.L. Wang, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 3529. [9] I. Yoon, T. Kang, W. Choi, J. Kim, Y. Yoo, S.W. Joo, Q.H. Park, H. Ihee, B. Kim, J. Am. Chem. Soc. 131 (2009) 758. [10] H. Wu, D.D. Lin, W. Pan, Langmuir 26 (2010) 6865. [11] D. He, B. Hu, Q.F. Yao, K. Wang, S.H. Yu, ACS Nano 3 (2009) 3993. [12] B. Guo, G. Han, M. Li, S. Zhao, Thin Solid Films 518 (2010) 3228.
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