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Preparation of Au–Ag coreshell nanoparticles and application of bimetallic sandwich in surface-enhanced Raman scattering (SERS)
Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 313–317
Preparation of Au–Ag coreshell nanoparticles and application of bimetallic sandwich in surface-enhanced Raman scattering (SERS) Shuping Xu, Bing Zhao, Weiqing Xu∗ , Yuguo Fan Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, Jilin University, Changchun 130012, PR China Available online 18 December 2004
Abstract Ag-coating Au colloidal nanoparticles have been prepared by Ag deposition on Au core via the chemical reduction of AgNO3 by hydroquinone. The thickness of Ag shell depends on the Au–Ag molar ratio and the reducing time, which were monitored by the ultraviolet–visible (UV–vis) spectrometry and transmission electron microscopy (TEM). A Raman-active molecules, 4-mercaptobenzoic acid (MBA), are adsorbed onto the surface of 16 nm-diameter Au core, and then a 3 nm-thick Ag shell coats the MBA modified Au core to form a Au/MBA/Ag sandwich structure. Surface-enhanced Raman scattering (SERS) spectra show that, in the sandwich structure, 3 nm-thick Ag shell can effectively enhance the SERS signal of MBA. This is probably caused by the electromagnetic coupling of the Au–Ag double metallic layers. © 2004 Elsevier B.V. All rights reserved. Keywords: Coreshell; Nanoparticles; SERS; Bimetallic
1. Introduction Recently, researches on nanoparticles of noble metal have attracted people’s interests in their size-dependent optical, magnetic and catalytic properties, and potential of the emergence of surface-enhanced Raman spectroscopy (SERS) [1–19]. Bimetallic colloids, in which two kinds of metals are contained in one particle, have unique catalytic, electronic, and optical properties distinct from those of the corresponding monometallic particles [20–28]. For example, metal Au and Ag have almost identical lattice constants (0.408 for Au and 0.409 for Ag), and this characteristic leads to a strong tendency toward the alloy formation. Many research groups have done investigations under the control of the size of Agcoating Au or Au-coating Ag colloidal nanoparticles and the formation of the coreshell nanoparticles [29–34]. It has long been observed that an enormous enhancement of Raman scattering intensity occurs for many organic molecules adsorbed onto the surfaces of the rough nanoscale coinage metals with an enhancement factor of 106 or larger [35–44]. Therefore, various techniques have ∗
Corresponding author. Tel.: +86 431 5168505; fax: +86 431 5193421. E-mail address: [email protected] (W. Xu).
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.10.093
been used to prepare nanoscale SERS substrates including physical deposition, [45] electrochemical deposition, [46,47] Langmuir–Blodgett technique, [48] and layer-by-layer selfassembly method [49–52], etc. Gold colloidal nanoparticle is also an important SERS substrate [39–44]. Compared with Au, Ag possesses an extinction coefficient of the surface plasmon band 4 times larger than that of Au due to the calculated result of dielectric constant [30,53,54]. However, problems of instability and irreproducibility of the signal caused by the aggregations of Ag colloid still remain. Many groups have found out that the bimetallic particles of various combinations of metals were good SERS active substrates based on the advantage of two kinds of metals. For example, Lu et al. [55] synthesize larger core-shell Au–Ag nanoparticles with improved monodisperisity. These core-shell nanoparticles possess the monodispersity of Au with the optical properties of Ag from the SERS spectra of rhodamine 6G. In the present study, we synthesized Au–Ag coreshell nanoparticles by reducing AgNO3 with hydroquinone around Au nanoparticles. This method lies in the simplicity and the simultaneous control of both the size and the shape of nanoparticles. Raman-active molecules, 4-mercaptobenzoic acid (MBA), are adsorbed onto the surface of 16 nm-diameter Au core, and then a 3 nm-thick Ag shell coated it to form the
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Au/MBA/Ag sandwich structure. SERS spectra showed that, in the sandwich structure, 3 nm-thick Ag shell can effectively enhance the SERS signals of MBA. This is probably caused by the electromagnetic coupling of the Au–Ag double metallic layers.
2. Experimental 2.1. Preparation of the Au–Ag coreshell nanoparticles Gold colloids were prepared by following the procedures developed by Frens [56]. All the glassware used were cleaned with freshly prepared aquaregia (HCl:HNO3 = 3:1 (v/v)). As soon as the 100 mL of a 10−3 mol/L HAuCl4 solution started boiling, 9.5 mL of a 1% (m/m) trisodium citrate aqueous solution was added to it. After the mixture had been boiled for 20 min, the heating was stopped. An ultraviolet–visible (UV–vis) spectrum of the Au colloids thus prepared was measured; a surface plasmon resonance peak appeared at 518 nm (the bottom curve shown in Fig. 1). The average diameter of Au particles was estimated to be ∼16 nm according to their transmission electron microscopy (TEM) images. While the Au colloids were stirred at the normal temperature, different molar ratios (1.0:0.0, 1.0:0.2, 1.0:0.4, 1.0:1.6, 1.0:0.8, 1.0:1.0, 1.0:1.2, 1.0:1.4, 1.0:1.6, 1.0:1.8, 1.0:2.0 and 1.0:3.0, respectively) of hydroquinone (1 × 10−3 mol/L) to AgNO3 (1 × 10−3 mol/L) were added dropwise in 100 mL of 1 × 10−4 mol/L Au colloids. The mixture reacted for 20 min under stirring. Then, the Au colloids were coated by Ag to form the Au–Ag coreshell colloidal nanoparticles. Finally, they were rinsed by centrifugation and resuspension with ultrapure water twice in order to remove the unreacted hydroquinone and AgNO3 .
2.2. Preparation of the Au/MBA/Ag sandwich structure Twenty microlitres of the MBA solution (1 × 10−3 mol/L) as a Raman-active probes were added to 1.0 mL of the Au colloids, and then the mixture reacted for 12 h under stirring. MBA are adsorbed onto the surface of Au particles with thiol groups [57,58]. Then, the MBA modified Au colloids were rinsed by centrifugation and resuspension with ultrapure twice in order to remove the unreacted MBA. Ag shell was conducted with AgNO3 reduced by hydroquinone in the dark. The molar ratio of hydroquinone (1 × 10−3 mol/L) to AgNO3 (1 × 10−3 mol/L) (v/v) was 1. The Ag-coating Au core step was completed within 20 min. Finally, they were rinsed by centrifugation and resuspension with ultrapure water twice. 2.3. SERS, UV–vis and TEM measurements SERS spectra were measured with a confocal microscopic Raman spectrometer (Renishaw 1000 model) with a CCD detector and a holographic notch filter. Radiation of 514.5 nm from an air-cooled argon ion laser (Spectra-Physics model 163-C4260) was used for excitation. Laser power at the sample position was not more than a few milliwatts. The microscope attachment was based on a Leica DMLM system, and a 50× objective was used to focus the laser beam onto a spot of approximately 1 m in diameter. The colloidal nanoparticles were held in a glass capillary. The laser focuses on the inner side of the capillary. All the spectra reported were the results of a single 10 s accumulation. UV–vis spectra were recorded on a Shimadzu UV-3100 spectrophotometer. TEM was measured with a Hitachi H-8100 IV operating at 200 kV.
3. Results and discussion 3.1. Au–Ag coreshell nanoparticles
Fig. 1. UV–vis absorption spectra of the process of Au–Ag coreshell particles (the molar ratio is 1.0:1.0) growth with the reacting time from 6 min to 27 min after hydroquinone added. They were recorded at 3 min intervals after hydroquinone (1 × 10−3 mol/L) and AgNO3 (1 × 10−3 mol/L) added into Au colloids. The upper three curves are the absorption spectra of Au–Ag coreshell colloids with the Ag reducing time of 1.0 h, 1.5 h and 2.0 h, respectively.
The growth of the Au–Ag coreshell nanoparticles were determined by UV–vis spectrometry. Fig. 1 shows a series of absorption spectra, which were recorded at 3 min intervals after hydroquinone (1 × 10−3 mol/L) and AgNO3 (1 × 10−3 mol/L) were added into Au colloid. The molar ratio of hydroquinone (1 × 10−3 mol/L) to AgNO3 (1 × 10−3 mol/L) (v/v) was 1. From the decreasing absorption peak at 288 nm assigned to hydroquinone, we can know the reducer is enough throughout the reacting process. Within 24 min, a continuous shift from 520 to 510 nm is observed in Fig. 1. These shifts provide strong evidence for the reduced Ag shell growth, which has been investigated theoretically according to the traditional Mie theory and with the aid of the dielectric data [20,28,30,59–65]. After the reacting time of 24 min, an increasing absorption peak at 338 nm gradually appears, which is due to the surface plasmon band of
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Fig. 2. UV–vis absorption spectra of Au–Ag coreshell nanoparticles with different molar ratios of Au–Ag added. From the bottom up, the Au–Ag molar ratios is 1.0:0.0 (pure Au colloid), 1.0:0.2, 1.0:0.4, 1.0:1.6, 1.0:0.8, 1.0:1.0, 1.0:1.2, 1.0:1.4, 1.0:1.6, 1.0:1.8, 1.0:2.0 and 1.0:3.0, respectively. These absorption spectra were recorded when the reducing Ag time was 20 min.
the pure Ag nanoparticles. This peak suggests that, with the reacting time increasing, the reduced Ag slowly aggregate to form the new Ag nuclei and grow larger. The upper three curves in Fig. 1 are the absorption spectra of Au–Ag coreshell colloids with the Ag reducing time of 1.0 h, 1.5 h and 2.0 h, respectively. Based on the slowly changed absorption peak of 510 nm in the three curves, we can know, after 1.5 h the Ag shell grows slowly and a large amount of the fresh Ag nanoparticles heterogeneous mix into the Au–Ag coreshell colloids. So we rinsed the mixing colloid solution by centrifugation and resuspension twice at the reacting time of 20 min in order to remove the unreacted hydroquinone and AgNO3 and to avoid Ag nucleation. Fig. 2 shows the absorption spectra of Au–Ag coreshell colloidal nanoparticles with different Au–Ag molar ratios (from 1.0:0.2 to 1.0:3.0). These absorption spectra were recorded when the reducing Ag time was 20 min. As the Au–Ag molar ratio is increasing and near to 1.0:1.0, there is only one surface plasmon peak between 520 to 510 nm due to Ag-coating Au coreshell nanoparticles. The blue shift of the 518 nm peak suggests the formation of the Ag shell around the Au core within 20 min. The larger the molar ratio is, the thicker the Ag shell is. On the other hand, when Au–Ag molar ratio is above 1.0:1.0, a new absorption band at 334 nm emerges. This can be explained that with the molar amount of AgNO3 increasing in the limit volume, there are more and more chances for the Ag nucleation. In the mixed colloid solution, not only Au cores but also Ag cores as activator can be coated by Ag shell and rapidly grow up. So the surface plasmon band of Ag appears and is increasing with the amount of AgNO3 . From Fig. 2, we can know an Au–Ag molar ratio of 1.0:1.0 is proper for the coreshell nanoparticles growth. Finally, the treatment of the centrifugation and resuspension is necessary to purify the Au–Ag coreshell nanoparticles. TEM images can help us to see the shape and size of the Au/Ag coreshell nanoparticles. Fig. 3(a) is the TEM image of Au colloid. It shows that the average diameter of Au
Fig. 3. TEM images of (a) Au nanoparticles and (b) Au–Ag coreshell nanoparticles in the molar ratio of 1:1 added. The average diameter of Au nanoparticles is ∼16 nm; that of Au–Ag coreshell nanoparticles is ∼22 nm. All the statistical result is among over 200 nanoparticles.
nanoparticles is ∼16 nm. Fig. 3(b) is that of Au–Ag coreshell colloidal nanoparticles with the Ag reducing time of 20 min. Their average diameter is ∼22 nm. All the statistical results are among over 200 nanoparticles. It is observed that both the Au colloid and the Au–Ag coreshell nanoparticles are uniformly spherical. So we think the formation process of the Au–Ag coreshell nanoparticles is that Ag cations are reduced catalytically in the presence of Au nanoparticles, and then the reduced Ag aggregate onto the surface of Au core, and finally form the coreshell structure. These results are in accord with UV–vis spectral evidence. 3.2. Au/MBA/Ag bimetallic sandwich structure for SERS substrate In the process of synthesis, the SERS-active molecules were added and partially adsorbed on the surface of Au core.
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strate. Using this Au/MBA/Ag sandwich structure, we observed the significant SERS signals of MBA. The maximum enhancement is obtained when the Ag shell is 3 nm. This novel method lies in the simplicity and the simultaneous control of both the size and the shape of nanoparticles. The sandwich structure has the advantage of restricting the adsorbate far away from the solvent disturbance and preventing the analyte from the oxidation by air. Further work is in progress to implement other analyte in SERS measurement by using Au–Ag sandwich structure.
Acknowledgment Fig. 4. SERS spectra of MBA in Au/MBA/Ag sandwich structure when coated with the different thickness of the Ag shell controlled by reducing time of 20 min (b), 40 min (c), and before Ag reduction (a).
The MBA modified Au core was then coated by Ag shell to form the Au/MBA/Ag bimetallic sandwich structure. We found that this sandwich structure was able to enhance the SERS signals of the probe molecules remarkably and the intensity of SERS signal is almost relative to the thickness of Ag shell. Fig. 4 shows the SERS spectra when the Ag reducing time is 0 min (a), 20 min (b), 40 min (c). As the thickness of Ag shell increases, SERS signals of MBA are enhanced quickly; when the thickness of Ag shell reaches ∼3 nm (according to the TEM results), the SERS signals are strongest; while when it is beyond ∼3 nm, SERS signals decrease (Fig. 4(c)). Strong SERS bands at 1586 and 1076 cm−1 are assigned to the ν8a and ν12 aromatic ring vibrations, respectively, and those at 1373 and 841 cm−1 are assigned to ν(COO− ) and δ(COO− ), respectively [66]. The remarkable SERS spectra illuminate that the bimetallic sandwich nanoparticles can be utilized as an effective SERS substrate with the proper thickness of Ag shell. The Ag shell of Au/MBA/Au nanoparticle plays an important role in enhancing the MBA signals in the SERS detection. This can be possibly explained by the electromagnetic coupling of double metallic layers [67]. However, SERS bands, such as the band of 1376 cm−1 shifts to 1367 cm−1 . These shifts indicate that the vibrational modes of the adsorbate are sensitive to both Ag aggregation and the increasing thickness of Ag shell. In Fig. 4(b) initially, a lot of Ag aggregates easily enhance SERS signals of MBA, mainly because Ag shell holds higher enhancing capability. As the reaction time increases, Ag shell turns thicker. Some vibration modes, such as C C stretch and δ(C H), are confined under the Ag shell coating. As the Ag shell grows over the proper thickness, most vibrational modes are concealed from Ag substrate’s background as Fig. 4(c) shown. In summary, we have synthesized a sort of Au–Ag coreshell nanoparticles and provided a new effective SERS sub-
This work is supported by the National Natural Science Foundation of China (20375014, 20273022).
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Preparation of Au–Ag coreshell nanoparticles and application of bimetallic sandwich in surface-enhanced Raman scattering (SERS) Shuping Xu, Bing Zhao, Weiqing Xu∗ , Yuguo Fan Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, Jilin University, Changchun 130012, PR China Available online 18 December 2004
Abstract Ag-coating Au colloidal nanoparticles have been prepared by Ag deposition on Au core via the chemical reduction of AgNO3 by hydroquinone. The thickness of Ag shell depends on the Au–Ag molar ratio and the reducing time, which were monitored by the ultraviolet–visible (UV–vis) spectrometry and transmission electron microscopy (TEM). A Raman-active molecules, 4-mercaptobenzoic acid (MBA), are adsorbed onto the surface of 16 nm-diameter Au core, and then a 3 nm-thick Ag shell coats the MBA modified Au core to form a Au/MBA/Ag sandwich structure. Surface-enhanced Raman scattering (SERS) spectra show that, in the sandwich structure, 3 nm-thick Ag shell can effectively enhance the SERS signal of MBA. This is probably caused by the electromagnetic coupling of the Au–Ag double metallic layers. © 2004 Elsevier B.V. All rights reserved. Keywords: Coreshell; Nanoparticles; SERS; Bimetallic
1. Introduction Recently, researches on nanoparticles of noble metal have attracted people’s interests in their size-dependent optical, magnetic and catalytic properties, and potential of the emergence of surface-enhanced Raman spectroscopy (SERS) [1–19]. Bimetallic colloids, in which two kinds of metals are contained in one particle, have unique catalytic, electronic, and optical properties distinct from those of the corresponding monometallic particles [20–28]. For example, metal Au and Ag have almost identical lattice constants (0.408 for Au and 0.409 for Ag), and this characteristic leads to a strong tendency toward the alloy formation. Many research groups have done investigations under the control of the size of Agcoating Au or Au-coating Ag colloidal nanoparticles and the formation of the coreshell nanoparticles [29–34]. It has long been observed that an enormous enhancement of Raman scattering intensity occurs for many organic molecules adsorbed onto the surfaces of the rough nanoscale coinage metals with an enhancement factor of 106 or larger [35–44]. Therefore, various techniques have ∗
Corresponding author. Tel.: +86 431 5168505; fax: +86 431 5193421. E-mail address: [email protected] (W. Xu).
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.10.093
been used to prepare nanoscale SERS substrates including physical deposition, [45] electrochemical deposition, [46,47] Langmuir–Blodgett technique, [48] and layer-by-layer selfassembly method [49–52], etc. Gold colloidal nanoparticle is also an important SERS substrate [39–44]. Compared with Au, Ag possesses an extinction coefficient of the surface plasmon band 4 times larger than that of Au due to the calculated result of dielectric constant [30,53,54]. However, problems of instability and irreproducibility of the signal caused by the aggregations of Ag colloid still remain. Many groups have found out that the bimetallic particles of various combinations of metals were good SERS active substrates based on the advantage of two kinds of metals. For example, Lu et al. [55] synthesize larger core-shell Au–Ag nanoparticles with improved monodisperisity. These core-shell nanoparticles possess the monodispersity of Au with the optical properties of Ag from the SERS spectra of rhodamine 6G. In the present study, we synthesized Au–Ag coreshell nanoparticles by reducing AgNO3 with hydroquinone around Au nanoparticles. This method lies in the simplicity and the simultaneous control of both the size and the shape of nanoparticles. Raman-active molecules, 4-mercaptobenzoic acid (MBA), are adsorbed onto the surface of 16 nm-diameter Au core, and then a 3 nm-thick Ag shell coated it to form the
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Au/MBA/Ag sandwich structure. SERS spectra showed that, in the sandwich structure, 3 nm-thick Ag shell can effectively enhance the SERS signals of MBA. This is probably caused by the electromagnetic coupling of the Au–Ag double metallic layers.
2. Experimental 2.1. Preparation of the Au–Ag coreshell nanoparticles Gold colloids were prepared by following the procedures developed by Frens [56]. All the glassware used were cleaned with freshly prepared aquaregia (HCl:HNO3 = 3:1 (v/v)). As soon as the 100 mL of a 10−3 mol/L HAuCl4 solution started boiling, 9.5 mL of a 1% (m/m) trisodium citrate aqueous solution was added to it. After the mixture had been boiled for 20 min, the heating was stopped. An ultraviolet–visible (UV–vis) spectrum of the Au colloids thus prepared was measured; a surface plasmon resonance peak appeared at 518 nm (the bottom curve shown in Fig. 1). The average diameter of Au particles was estimated to be ∼16 nm according to their transmission electron microscopy (TEM) images. While the Au colloids were stirred at the normal temperature, different molar ratios (1.0:0.0, 1.0:0.2, 1.0:0.4, 1.0:1.6, 1.0:0.8, 1.0:1.0, 1.0:1.2, 1.0:1.4, 1.0:1.6, 1.0:1.8, 1.0:2.0 and 1.0:3.0, respectively) of hydroquinone (1 × 10−3 mol/L) to AgNO3 (1 × 10−3 mol/L) were added dropwise in 100 mL of 1 × 10−4 mol/L Au colloids. The mixture reacted for 20 min under stirring. Then, the Au colloids were coated by Ag to form the Au–Ag coreshell colloidal nanoparticles. Finally, they were rinsed by centrifugation and resuspension with ultrapure water twice in order to remove the unreacted hydroquinone and AgNO3 .
2.2. Preparation of the Au/MBA/Ag sandwich structure Twenty microlitres of the MBA solution (1 × 10−3 mol/L) as a Raman-active probes were added to 1.0 mL of the Au colloids, and then the mixture reacted for 12 h under stirring. MBA are adsorbed onto the surface of Au particles with thiol groups [57,58]. Then, the MBA modified Au colloids were rinsed by centrifugation and resuspension with ultrapure twice in order to remove the unreacted MBA. Ag shell was conducted with AgNO3 reduced by hydroquinone in the dark. The molar ratio of hydroquinone (1 × 10−3 mol/L) to AgNO3 (1 × 10−3 mol/L) (v/v) was 1. The Ag-coating Au core step was completed within 20 min. Finally, they were rinsed by centrifugation and resuspension with ultrapure water twice. 2.3. SERS, UV–vis and TEM measurements SERS spectra were measured with a confocal microscopic Raman spectrometer (Renishaw 1000 model) with a CCD detector and a holographic notch filter. Radiation of 514.5 nm from an air-cooled argon ion laser (Spectra-Physics model 163-C4260) was used for excitation. Laser power at the sample position was not more than a few milliwatts. The microscope attachment was based on a Leica DMLM system, and a 50× objective was used to focus the laser beam onto a spot of approximately 1 m in diameter. The colloidal nanoparticles were held in a glass capillary. The laser focuses on the inner side of the capillary. All the spectra reported were the results of a single 10 s accumulation. UV–vis spectra were recorded on a Shimadzu UV-3100 spectrophotometer. TEM was measured with a Hitachi H-8100 IV operating at 200 kV.
3. Results and discussion 3.1. Au–Ag coreshell nanoparticles
Fig. 1. UV–vis absorption spectra of the process of Au–Ag coreshell particles (the molar ratio is 1.0:1.0) growth with the reacting time from 6 min to 27 min after hydroquinone added. They were recorded at 3 min intervals after hydroquinone (1 × 10−3 mol/L) and AgNO3 (1 × 10−3 mol/L) added into Au colloids. The upper three curves are the absorption spectra of Au–Ag coreshell colloids with the Ag reducing time of 1.0 h, 1.5 h and 2.0 h, respectively.
The growth of the Au–Ag coreshell nanoparticles were determined by UV–vis spectrometry. Fig. 1 shows a series of absorption spectra, which were recorded at 3 min intervals after hydroquinone (1 × 10−3 mol/L) and AgNO3 (1 × 10−3 mol/L) were added into Au colloid. The molar ratio of hydroquinone (1 × 10−3 mol/L) to AgNO3 (1 × 10−3 mol/L) (v/v) was 1. From the decreasing absorption peak at 288 nm assigned to hydroquinone, we can know the reducer is enough throughout the reacting process. Within 24 min, a continuous shift from 520 to 510 nm is observed in Fig. 1. These shifts provide strong evidence for the reduced Ag shell growth, which has been investigated theoretically according to the traditional Mie theory and with the aid of the dielectric data [20,28,30,59–65]. After the reacting time of 24 min, an increasing absorption peak at 338 nm gradually appears, which is due to the surface plasmon band of
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Fig. 2. UV–vis absorption spectra of Au–Ag coreshell nanoparticles with different molar ratios of Au–Ag added. From the bottom up, the Au–Ag molar ratios is 1.0:0.0 (pure Au colloid), 1.0:0.2, 1.0:0.4, 1.0:1.6, 1.0:0.8, 1.0:1.0, 1.0:1.2, 1.0:1.4, 1.0:1.6, 1.0:1.8, 1.0:2.0 and 1.0:3.0, respectively. These absorption spectra were recorded when the reducing Ag time was 20 min.
the pure Ag nanoparticles. This peak suggests that, with the reacting time increasing, the reduced Ag slowly aggregate to form the new Ag nuclei and grow larger. The upper three curves in Fig. 1 are the absorption spectra of Au–Ag coreshell colloids with the Ag reducing time of 1.0 h, 1.5 h and 2.0 h, respectively. Based on the slowly changed absorption peak of 510 nm in the three curves, we can know, after 1.5 h the Ag shell grows slowly and a large amount of the fresh Ag nanoparticles heterogeneous mix into the Au–Ag coreshell colloids. So we rinsed the mixing colloid solution by centrifugation and resuspension twice at the reacting time of 20 min in order to remove the unreacted hydroquinone and AgNO3 and to avoid Ag nucleation. Fig. 2 shows the absorption spectra of Au–Ag coreshell colloidal nanoparticles with different Au–Ag molar ratios (from 1.0:0.2 to 1.0:3.0). These absorption spectra were recorded when the reducing Ag time was 20 min. As the Au–Ag molar ratio is increasing and near to 1.0:1.0, there is only one surface plasmon peak between 520 to 510 nm due to Ag-coating Au coreshell nanoparticles. The blue shift of the 518 nm peak suggests the formation of the Ag shell around the Au core within 20 min. The larger the molar ratio is, the thicker the Ag shell is. On the other hand, when Au–Ag molar ratio is above 1.0:1.0, a new absorption band at 334 nm emerges. This can be explained that with the molar amount of AgNO3 increasing in the limit volume, there are more and more chances for the Ag nucleation. In the mixed colloid solution, not only Au cores but also Ag cores as activator can be coated by Ag shell and rapidly grow up. So the surface plasmon band of Ag appears and is increasing with the amount of AgNO3 . From Fig. 2, we can know an Au–Ag molar ratio of 1.0:1.0 is proper for the coreshell nanoparticles growth. Finally, the treatment of the centrifugation and resuspension is necessary to purify the Au–Ag coreshell nanoparticles. TEM images can help us to see the shape and size of the Au/Ag coreshell nanoparticles. Fig. 3(a) is the TEM image of Au colloid. It shows that the average diameter of Au
Fig. 3. TEM images of (a) Au nanoparticles and (b) Au–Ag coreshell nanoparticles in the molar ratio of 1:1 added. The average diameter of Au nanoparticles is ∼16 nm; that of Au–Ag coreshell nanoparticles is ∼22 nm. All the statistical result is among over 200 nanoparticles.
nanoparticles is ∼16 nm. Fig. 3(b) is that of Au–Ag coreshell colloidal nanoparticles with the Ag reducing time of 20 min. Their average diameter is ∼22 nm. All the statistical results are among over 200 nanoparticles. It is observed that both the Au colloid and the Au–Ag coreshell nanoparticles are uniformly spherical. So we think the formation process of the Au–Ag coreshell nanoparticles is that Ag cations are reduced catalytically in the presence of Au nanoparticles, and then the reduced Ag aggregate onto the surface of Au core, and finally form the coreshell structure. These results are in accord with UV–vis spectral evidence. 3.2. Au/MBA/Ag bimetallic sandwich structure for SERS substrate In the process of synthesis, the SERS-active molecules were added and partially adsorbed on the surface of Au core.
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strate. Using this Au/MBA/Ag sandwich structure, we observed the significant SERS signals of MBA. The maximum enhancement is obtained when the Ag shell is 3 nm. This novel method lies in the simplicity and the simultaneous control of both the size and the shape of nanoparticles. The sandwich structure has the advantage of restricting the adsorbate far away from the solvent disturbance and preventing the analyte from the oxidation by air. Further work is in progress to implement other analyte in SERS measurement by using Au–Ag sandwich structure.
Acknowledgment Fig. 4. SERS spectra of MBA in Au/MBA/Ag sandwich structure when coated with the different thickness of the Ag shell controlled by reducing time of 20 min (b), 40 min (c), and before Ag reduction (a).
The MBA modified Au core was then coated by Ag shell to form the Au/MBA/Ag bimetallic sandwich structure. We found that this sandwich structure was able to enhance the SERS signals of the probe molecules remarkably and the intensity of SERS signal is almost relative to the thickness of Ag shell. Fig. 4 shows the SERS spectra when the Ag reducing time is 0 min (a), 20 min (b), 40 min (c). As the thickness of Ag shell increases, SERS signals of MBA are enhanced quickly; when the thickness of Ag shell reaches ∼3 nm (according to the TEM results), the SERS signals are strongest; while when it is beyond ∼3 nm, SERS signals decrease (Fig. 4(c)). Strong SERS bands at 1586 and 1076 cm−1 are assigned to the ν8a and ν12 aromatic ring vibrations, respectively, and those at 1373 and 841 cm−1 are assigned to ν(COO− ) and δ(COO− ), respectively [66]. The remarkable SERS spectra illuminate that the bimetallic sandwich nanoparticles can be utilized as an effective SERS substrate with the proper thickness of Ag shell. The Ag shell of Au/MBA/Au nanoparticle plays an important role in enhancing the MBA signals in the SERS detection. This can be possibly explained by the electromagnetic coupling of double metallic layers [67]. However, SERS bands, such as the band of 1376 cm−1 shifts to 1367 cm−1 . These shifts indicate that the vibrational modes of the adsorbate are sensitive to both Ag aggregation and the increasing thickness of Ag shell. In Fig. 4(b) initially, a lot of Ag aggregates easily enhance SERS signals of MBA, mainly because Ag shell holds higher enhancing capability. As the reaction time increases, Ag shell turns thicker. Some vibration modes, such as C C stretch and δ(C H), are confined under the Ag shell coating. As the Ag shell grows over the proper thickness, most vibrational modes are concealed from Ag substrate’s background as Fig. 4(c) shown. In summary, we have synthesized a sort of Au–Ag coreshell nanoparticles and provided a new effective SERS sub-
This work is supported by the National Natural Science Foundation of China (20375014, 20273022).
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