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Porous Ag and Au hybrid nanostructures: Synthesis, morphology, and their surface-enhanced Raman scattering properties
Physica B 433 (2014) 138–143
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Physica B journal homepage: www.elsevier.com/locate/physb
Porous Ag and Au hybrid nanostructures: Synthesis, morphology, and their surface-enhanced Raman scattering properties Jingwu Tang a,b, Yougen Yi a,n, Jie Wu a, Yongjian Tang c a
School of Physics and Electronics, Central South University, Changsha 410083, China School of Physics and Electronic Science, Hunan University of Science and Technology, Xiangtan 411201, China c Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 16 September 2013 Received in revised form 8 October 2013 Accepted 10 October 2013 Available online 25 October 2013
In this paper, an ordered porous Ag and Au hybrid nanostructures were obtained by using ordered SiO2 nanospheres film as sacrificial templates. Different from previous reports, the different atom ratio of Ag and Au can be conveniently used to control film structures by simply varying the experimental conditions. The morphology of these nanostructures has been characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), and their performance as surfaceenhanced Raman scattering (SERS) substrates has been evaluated by using rhodamine 6G (R6G) as probe molecule. It was found that the enhancement ability of the ordered porous Ag and Au hybrid nanostructures were related with the atom ratio of Ag and Au. The highest enhancement factor can be achieved at the average value of 4.76 109 from the sample when the atom ratio of Au/Ag is 1:1. & 2013 Elsevier B.V. All rights reserved.
Keywords: Porous Ag and Au hybrid nanostructure Surface-enhanced Raman scattering Rhodamine 6G
1. Introduction Recently, growing interest in porous metal and bimetallic comprised of noble metals such as silver (Ag), gold (Au), palladium (Pd), and platinum (Pt) has been justified by their fascinating optical [1–3], catalytic [4], and electronic [5] properties, leading to a wide range of applications, such as biosensors [6], catalysis [4,7] and surface-enhanced Raman scattering (SERS) [8,9]. In addition to their unique optical properties, colloidal crystal films have interesting structural properties such as three dimensional (3D) periodicity and large surface areas which make them desirable as template materials. Consequently, a number of strategies have been developed to incorporate metal nanoparticles into 3D ordered colloidal crystal films [10–12]. For example, Gu et al. [13] reported a dipping method for the fabrication of metal-coated 3D ordered colloidal crystal films of silica microspheres. The 3D ordered colloidal crystal films were immersed into a mixture of 10 nm gold or silver nanoparticles and supporting polymer followed by lifting the 3D ordered colloidal crystal films out of the solution at a constant speed. During the lifting process, simultaneously both nanoparticles and supporting polymer infiltrated the voids within the 3D ordered colloidal crystal films. Finally, the resulting film was calcined at 300 1C to remove the polymer and to immobilize the nanoparticles on the surface of the silica spheres. It was found that the as-prepared silver-coated 3D
n
Corresponding author. Tel./fax: þ 86 8 1648 0830. E-mail address: [email protected] (Y. Yi).
0921-4526/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2013.10.027
ordered colloidal crystal films possess an excellent SERS enhancement ability, and the gold-coated 3D ordered colloidal crystal films could be used as refractive index sensors. Metal nanoparticles, especially Au and Ag and their bimetallic forms have been widely employed in SERS [14,15]. SERS is insensitive to humidity, oxygen and other quenchers and considered to be a promising method for sensitive biological identifications and detections [9,16]. Recent studies in this field indicated that 3D highly ordered macroporous metal nanostructured films prepared by a colloidal templating method could provide an excellent platform [17,18]. For instance, Velev et al. [19] successfully used polystyrene (PS) nanospheres as a template to fabricate SERS active porous gold nanoparticle films. To the best of our knowledge, however, there are only a few reported examples of such nanostructured films for SERS applications, and these examples mainly focus on macroporous gold nanostructures [19,20]. We have not been aware of reports on ordered porous Ag and Au hybrid nanostructures for SERS applications. Therefore, in this paper, an ordered porous Ag and Au hybrid nanostructures were obtained by using ordered SiO2 nanospheres film as sacrificial templates. The application of these ordered porous Ag and Au hybrid nanostructure films as SERS substrates is investigated by using R6G as a probe molecule. We show that the ordered porous Ag and Au bimetallic nanostructure films are extremely efficient SERS substrates in terms of high Raman intensity enhancement and reproducibility. It was found that the enhancement ability of the ordered porous Ag and Au hybrid nanostructures were related with the atom ratio of Ag and Au.
J. Tang et al. / Physica B 433 (2014) 138–143
This work will be of great significance in understanding the SERS enhancement mechanism and in the fabrication of nanoparticle films for biosensing.
2. Experimental
139
available for deposition on the silica@Au nanoshells surface can be controlled by the volume of the silver nitrate solution. Then, the 3D SiO2@Au and Ag film was immerged in 4% HF solution for 30 min to remove the SiO2 spheres, yielding ordered porous Ag and Au hybrid nanostructures. Then, the 3D ordered porous Ag and Au hybrid nanostructure film washing with deionized water twice.
2.1. Materials All chemicals (analytical grade) were supplied by Aldrich and used without further purification. Deionized and doubly distilled water was used throughout the experiments. 2.2. Preparation of a gold-coated silica film The SiO2 nanoparticles were synthesized according to the well known Stöber process [21] and were functionalized by 3-aminopropyltrimethoxysilane (APTMS) [22]. Citrate-stabilized 3-nm gold nanoparticles were prepared according to Natan et al. [23]. The APTMS-modified SiO2 nanoparticles were fabricated onto the glass substrates by a solvent evaporation method [24]. Next, the resultant 3D SiO2 nanoparticles films were immersed in the gold nanoparticle solution. To grow the Au shell, first a growth solution was prepared by mixing 30 ml deionized water, 7.25 mg K2CO3 and 3.5 ml HAuCl4 solution (5 mM). The 3D SiO2@Au seed nanoparticle film was added to 27.5 ml of the growth solution, after which 10 μl of formaldehyde were rapidly dropped into the solution. After reaction for 6 h at room temperature, the 3D SiO2@Au film washing with deionized water twice.
2.4. SERS measurements The sample was immersed in a 1 10 9 M aqueous solution of rhodamine 6G (R6G) for about 30 min, and then gently washed with distilled water and dried with high-purity flowing nitrogen. In this experiment, more than three SERS-active substrates of samples were prepared, and at least 10 different points on each substrate were selected to detect the R6G probes, to verify the stability and reproducibility of these SERS-active substrates. 2.5. Characterization The Scanning electron microscopy (SEM) images were recorded using a Leica Cambridge S440 field emission scanning electron microscope with an accelerating voltage of 5.0 kV. Energy dispersive X-ray spectra (EDX) were recorded on an Oxford INCA energy spectroscope. Raman spectra were obtained with a Renishaw inVia micro-Ramanspectroscopy system. Laser radiation of 514.5 or 632.8 nm was used for excitation. The Raman band of a silicon wafer at 520 cm 1 was used to calibrate the spectrometer. All of the spectra reported were the results of a single accumulation of 20 s.
2.3. Preparation of porous Au and Ag nanostructured films In brief, the 3D SiO2@Au film was mixed with a fresh 0.2 mM solution of silver nitrate and stirred vigorously. Then, 60 μl of 37% formaldehyde was added to the mixture to begin the reduction of silver onto the 3D SiO2@Au film. This step was followed by the addition of ammonium hydroxide (typically 20–50 μl of 28–30% NH4OH). The addition of NH4OH into the sample solution caused a rapid increase in the pH of the solution, which facilitated in the reduction of Ag þ to Ag0 that deposited onto the surface of the seed particles, forming silver nanoshells. The amount of silver
3. Results and discussion The procedure that we have developed for fabricating ordered porous Ag and Au hybrid nanostructure films on flat substrates is shown schematically in Fig. 1. In the first step, 3D silica spheres films were fabricated on a glass substrate that was vertically placed in a slowly evaporating ethanol solution of APTMSmodified silica spheres with diameters of 200 nm. After drying in air, the 3D film was immersed in a dilute solution of very small
Fig. 1. Schematic illustration of the templating technique to the ordered porous Ag and Au hybrid nanostructures.
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J. Tang et al. / Physica B 433 (2014) 138–143
gold seeds (about 3 nm in diameter) for 5 h. During this step, gold seeds were immobilized on the surface of the APTMS-modified silica spheres via the amine group. Subsequently, the resulting 3D film (silica@Au seed nanoparticles) was immersed in a mixture of HAuCl4/formaldehyde for gold plating to obtain a silica@Au nanoshells 3D film with an adjustable size of the gold nanoparticles. A similar procedure to fabricate the high quality macroporous metal films was developed earlier by Jiang et al. [25]. Then, the resulting 3D film (silica@Au nanoshells) was immersed in a mixture of AgNO3/NH4OH/formaldehyde for Ag plating to obtain a SiO2@Au/Ag nanoshells 3D film with an adjustable thickness of the Ag nanoshells. The last, SiO2@Au/Ag nanoshells 3D film was redispersed in 4% HF solution to remove the SiO2 spheres, yielding ordered porous Ag and Au hybrid nanostructure films. 3D films of APTMS-modified silica spheres of up to several square centimeters are easily prepared by this solvent evaporation method. Fig. 2 show typical scanning electron microscopy (SEM) images of a 3D silica spheres film (Fig. 2a and b) and of ordered
porous Ag and Au hybrid nanostructure film (Fig. 2c and d). As seen from Fig. 2a and b, the microspheres crystallize in a hexagonal ordered packing with the (1 1 1) facet parallel to the glass substrate. Fig. 2c and d show a typical SEM image of the resultant template free ordered porous Ag and Au hybrid nanostructure film. In Fig. 2c, open voids and interconnected walls consisting of larger interconnected aggregates form a pore structure. These pores are well ordered in a hexagonal packing arrangement, and such an ordered structure can be extended to several hundreds of micrometers. From the higher magnification image of Fig. 2d, it is clearly observed that there are three dark regions inside each pore corresponding to the air spheres of the underlying layer, indicating that these spheres are indeed close packed. The chemical composition of hybrid nanostructure is determined by energy-dispersive X-ray spectroscopy (EDX) (Fig. 3). The EDX spectrum with two main peaks (Au and Ag) is observed (other peaks originated from substrate), indicating that the hollow
Fig. 2. SEM images of the samples: (a)–(b) 3D silica spheres film; (c)–(d) ordered porous Ag and Au hybrid nanostructure film.
Fig. 3. EDX spectra of the ordered porous Ag and Au hybrid nanostructure films prepared under different silver nitrate solution: (a) 20 ml (sample A); (b) 40 ml (sample B); (c) 60 ml (sample C).
J. Tang et al. / Physica B 433 (2014) 138–143
metals nanostructure is made up of metallic Au and Ag. When the volume of the silver nitrate solution is 20 ml (Fig. 3a), the Au peak possesses a higher intensity than the Ag peak, which is consistent with the composition (the atom ratio of Au/Ag is 4: 1). After the volume of silver nitrate solution increased (40 ml), the Au peak decreased greatly and the intensity of the Ag peak increased as shown in Fig. 3b (the atom ratio of Au/Ag is 1:1), indicating the thickness of Ag nanoshells becomes thickly. With addition of the volume of silver nitrate solution (60 ml), the Au peak is much weaker than the Ag peak (Fig. 3c, (the atom ratio of Au/Ag is 1:4)). By comparing with EDX, it can be indicated that Ag is successfully grown on the surface of the SiO2@Au nanoshells, forming Au/Ag bimetallic nanostructure. To evaluate the effectiveness of the ordered porous Ag and Au hybrid nanostructure films for SERS applications, rhodamine 6G (R6G) was used in SERS studies because of its enormous intensity enhancement and adsorption onto noble metals particles. R6G is a cationic dye with strong absorption in the visible and a high fluorescence yield [26]. The molecule consists of two chromophores, one dibenzopyrene chomophore (xanthene), and one carboxyphenyl group tilted by about 901 with respect to the xanthene ring (see Fig. 4). Therefore the π-stems of the two chromophores of R6G are not conjugated [27]. Fig. 5 shows the SERS spectra that the R6G molecules are adsorbed onto (a) the cleaned glass substrate; (b) sample B. Fig. 5(a) shows the SERS of the R6G deposited film that is prepared by dropping 5 10 2 ml R6G aqueous solution (1 10 4 M) onto the cleaned glass substrate. As expected, we cannot distinguish any signal. Fig. 5 (b) shows the SERS of the 5 10 2 ml R6G aqueous solution (1 10 9 M) is deposited on sample C. Several strong bands at 1649, 1598, 1574, 1535, 1506, 1361 and 1310 cm 1 are observed on the substrate. The bands at 1649, 1574, 1506 and 1361 cm 1 are assigned to aromatic C–C stretching; 1598 cm 1 is assigned to C¼ C stretching, respectively [15]. Fig. 6 compare the 514.5 nm excitation SERS spectra of 1.0 10 8 M R6G obtained from three different substrates. From the three right bars that compare the plateau with the intensity of the strongest peak (aromatic C–C stretching at ca.1649 cm 1) in curves A–C, we find that the sample B exhibit the highest SERS
141
Fig. 5. Raman spectra of R6G on (a) the cleaned glass substrate and on (b) sample C.
Fig. 6. SERS spectra of 10 nM R6G on different substrates. (A) sample A; (B) sample B; (C) sample C. The incubation time was 30 min. The excitation wavelength is 514.5 nm.
Fig. 4. Structure of R6G.
enhancement ability. It is shown that the SERS peaks intensity in Fig. 6 B is an order of magnitude stronger than the corresponding ones in Fig. 6 A. The SERS intensity at 1649 cm 1 for the sample B is about 2.45 times higher than that for the sample C and about 9.17 times higher than that for the sample A (Fig. 6). This SERS enhancement may be related to several factors. First, extremely intense local electromagnetic fields generated in the gaps between adjacent nanoparticles can strongly enhance the Raman scattering of probe molecules located in the gaps between the closely spaced nanoparticles. Second, since the intrinsic activity of Ag is much higher than Au [28] and the surfaces of bimetals provide more possibilities for molecules to deposit on the boundaries between Ag and Au domains [29], the SERS intensity at 1649 cm 1 for the sample B is higher than that for the sample A and sample C. One of the classical ways to evaluate the SERS activity for a given substrate is to calculate the enhancement factor (EF). Although there are debates and large uncertainties associated with EF calculations, this remains an interesting means to compare the SERS activity of different substrates [30,31]. The EF is defined as the ratio of inelastic scattering intensity per molecule between the presence and absence of the SERS structure. The Raman
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J. Tang et al. / Physica B 433 (2014) 138–143
Table 1 Enhancement factors (G) calculated from the SERS spectra (A)–(C) as shown in Fig. 6. Raman shift (cm 1)
1361
1506
1574
1649
Average
G factor (sample A) (sample B) (sample C)
6.84 107 5.63 109 9.36 107
7.25 107 2.76 109 8.54 107
7.35 107 2.43 109 7.28 108
8.13 107 8.21 109 7.01 108
7.39 107 4.76 109 4.02 108
can be found to increase in a series: 514.5 4488 4568 4632.8 4 782 nm. It is conceivable that the 514.5 nm excitation source will yield a highest SERS activity.
4. Conclusion
Fig. 7. Enhancement factors (G) with different SERS substrates according to the different excitation wavelengths: 488, 514.5, 568, 632.8 and 782 nm.
In conclusion, porous Ag and Au hybrid nanostructures with different atom ratio of Ag and Au can be conveniently used to control film structures by simply varying the experimental conditions. The application of these ordered porous Ag and Au hybrid nanostructure films as SERS substrates is investigated by using R6G as a probe molecule. It was found that the enhancement ability of the ordered porous Ag and Au hybrid nanostructures were related with the atom ratio of Ag and Au. The enhancement factors (G) of the ordered porous Ag and Au hybrid nanostructures different excitation wavelengths can be found to increase in a series: 514.5 4488 4568 4632.8 4782 nm. These porous Ag and Au hybrid nanostructures exhibit high SERS activity and may have potential applications in investigating the surface chemistry in a particular reaction using SERS, for example, biosensing.
enhancement factor can be written as [32]: G¼
I SERS N Ref I Ref N SERS
ð1Þ
ISERS is the enhanced intensity of the adsorbed R6G molecules on the SERS substrate. The value of ISERS mainly arises from a single molecule layer covering a nanoparticle array, from which other additional molecules layers of analytes on the SERS substrate, as previously reported [33], do not contribute to Raman gain and can be neglected. NSERS is the number of the single-layer molecules covering the SERS substrate under the laser spot. NRef is the number of the bulk molecules excited by laser on the surface of the regular substrate. IRef is the spontaneous Raman scattering intensity from the bulk R6G molecules under the laser spot on the blank glass substrate. In order to obtain the values of these four parameters, we follow the same procedures based on the published literatures [34]. Using the 100 objective lens, we determine the area of the laser spot size at around 1 μm2. We also calculate the area of a single molecule of R6G to be approximately 1 106 nm2. Thus, the value of NSERS under the laser excitation is approximately 10 molecules. The focusing scope of laser beam is approximately 5 μm, thus scattering from all R6G molecules underneath the laser spot is detected. Assuming a uniform distribution of R6G over the droplet area of 2 mm2, the value of NRef is approximately 1.5 106 molecules. Therefore, from Eq. (1), the enhancement factor G is simply (1.5 104) (ISERS/IRef). The enhancement factors of each assigned Raman peak as measured from different SERS substrates are also shown in Table 1. The results show that the highest enhancement factor can be achieved at the average value of 4.76 109 from the sample B, which obtained from the optimized SERS substrate when the atom ratio of Au/Ag is 1:1. Here, we have studied the relationship of enhancement factors (G) and excitation wavelengths, as shown in Fig. 7. The enhancement factors (G) of the ordered porous Ag and Au hybrid nanostructures different excitation wavelengths
Acknowledgments The work was supported by the National Natural Science Foundation of China (No. 10804101), the State Key Development Program for Basic Research of China (Grant No.2007CB815102), and the Science and Technology Development Foundation of Chinese Academy of Engineering Physics (Grant No.2007B08007). References [1] Z.J. Liang, A. Susha, F. Caruso, Chem. Mater. 15 (2003) 3176. [2] T.C. He, C.S. Wang, X. Pan, Y.L. Wang, Phys. Lett. A 373 (2009) 592. [3] J.H. Lee, M.A. Mahmoud, V. Sitterle, J. Sitterle, J.C. Meredith, J. Am. Chem. Soc. 131 (2009) 5048. [4] X.M. Feng, C.J. Mao, G. Yang, W.H. Hou, J.J. Zhu, Langmuir 22 (2006) 4384. [5] Z.L. Liu, B. Zhao, C.L. Guo, Y.J. Sun, F.G. Xu, H.B. Yang, Z. Li, J. Phys. Chem. C 113 (2009) 16766. [6] F.Y. Cheng, C.T. Chen, C.S. Yeh, Nanotechnology 20 (2009) 425104. [7] Y.T. Xi, W.S. Lu, L. Jiang, Nanotechnology 21 (2010) 085501. [8] D.H. Zhang, H.B. Yang, Physica B 415 (2013) 44. [9] R. Guzel, Z. Ustundag, H. Eks, S. Keskin, B. Taner, Z.G. Durgun, A.A.I. Turan, A.O. Solak, J. Colloid Interface Sci. 351 (2010) 35. [10] L.H. Lu, A. Eychmuller, A. Kobayashi, Y. Hirano, K. Yoshida, Y. Kikkawa, K. Tawa, Y. Ozaki, Langmuir 22 (2006) 2605. [11] J.H. Zhang, P. Zhan, H.Y. Liu, Z.L. Wang, N. Ming, Mater. Lett. 60 (2006) 280. [12] J.H. Zhang, J.B. Liu, S.Z. Wang, P. Zhan, Z.L. Wang, N.B. Ming, Adv. Funct. Mater. 14 (2004) 1089. [13] Z.Z. Gu, R. Horie, S. Kubo, Y. Yamada, A. Fujishima, O. Sato, Angew. Chem. Int. Ed. 41 (2002) 1154. [14] Z. Yi, J.B. Zhang, Y. Chen, S.J. Chen, J.S. Luo, Y.J. Tang, W.D. Wu, Y.G. Yi, Trans. Nonferrous Met. Soc. China 21 (2011) 2049. [15] Z. Yi, S.J. Chen, Y. Chen, J.S. Luo, W.D. Wu, Y.G. Yi, Y.J. Tang, Thin Solid Films 520 (2012) 2701. [16] A.P. Zhang, J.Z. Zhang, Physica B 404 (2009) 2435. [17] L.H. Lu, A Eychmüller, Acc. Chem. Res. 2 (2008) 244. [18] J. Ye, N. Verellen, W.V. Roy, L. Lagae, G. Maes, G. Borghs, P.V. Dorpe, ACS Nano 4 (2010) 1457. [19] D.M. Kuncicky, S.D. Christesen, O.D. Velev, Appl. Spectrosc. 59 (2005) 401. [20] L.H. Lu, I. Randjelovic, N. Gaponik, R. Capek, A. Eychmüller, Chem. Mater. 17 (2005) 5731.
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Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Porous Ag and Au hybrid nanostructures: Synthesis, morphology, and their surface-enhanced Raman scattering properties Jingwu Tang a,b, Yougen Yi a,n, Jie Wu a, Yongjian Tang c a
School of Physics and Electronics, Central South University, Changsha 410083, China School of Physics and Electronic Science, Hunan University of Science and Technology, Xiangtan 411201, China c Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 16 September 2013 Received in revised form 8 October 2013 Accepted 10 October 2013 Available online 25 October 2013
In this paper, an ordered porous Ag and Au hybrid nanostructures were obtained by using ordered SiO2 nanospheres film as sacrificial templates. Different from previous reports, the different atom ratio of Ag and Au can be conveniently used to control film structures by simply varying the experimental conditions. The morphology of these nanostructures has been characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), and their performance as surfaceenhanced Raman scattering (SERS) substrates has been evaluated by using rhodamine 6G (R6G) as probe molecule. It was found that the enhancement ability of the ordered porous Ag and Au hybrid nanostructures were related with the atom ratio of Ag and Au. The highest enhancement factor can be achieved at the average value of 4.76 109 from the sample when the atom ratio of Au/Ag is 1:1. & 2013 Elsevier B.V. All rights reserved.
Keywords: Porous Ag and Au hybrid nanostructure Surface-enhanced Raman scattering Rhodamine 6G
1. Introduction Recently, growing interest in porous metal and bimetallic comprised of noble metals such as silver (Ag), gold (Au), palladium (Pd), and platinum (Pt) has been justified by their fascinating optical [1–3], catalytic [4], and electronic [5] properties, leading to a wide range of applications, such as biosensors [6], catalysis [4,7] and surface-enhanced Raman scattering (SERS) [8,9]. In addition to their unique optical properties, colloidal crystal films have interesting structural properties such as three dimensional (3D) periodicity and large surface areas which make them desirable as template materials. Consequently, a number of strategies have been developed to incorporate metal nanoparticles into 3D ordered colloidal crystal films [10–12]. For example, Gu et al. [13] reported a dipping method for the fabrication of metal-coated 3D ordered colloidal crystal films of silica microspheres. The 3D ordered colloidal crystal films were immersed into a mixture of 10 nm gold or silver nanoparticles and supporting polymer followed by lifting the 3D ordered colloidal crystal films out of the solution at a constant speed. During the lifting process, simultaneously both nanoparticles and supporting polymer infiltrated the voids within the 3D ordered colloidal crystal films. Finally, the resulting film was calcined at 300 1C to remove the polymer and to immobilize the nanoparticles on the surface of the silica spheres. It was found that the as-prepared silver-coated 3D
n
Corresponding author. Tel./fax: þ 86 8 1648 0830. E-mail address: [email protected] (Y. Yi).
0921-4526/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2013.10.027
ordered colloidal crystal films possess an excellent SERS enhancement ability, and the gold-coated 3D ordered colloidal crystal films could be used as refractive index sensors. Metal nanoparticles, especially Au and Ag and their bimetallic forms have been widely employed in SERS [14,15]. SERS is insensitive to humidity, oxygen and other quenchers and considered to be a promising method for sensitive biological identifications and detections [9,16]. Recent studies in this field indicated that 3D highly ordered macroporous metal nanostructured films prepared by a colloidal templating method could provide an excellent platform [17,18]. For instance, Velev et al. [19] successfully used polystyrene (PS) nanospheres as a template to fabricate SERS active porous gold nanoparticle films. To the best of our knowledge, however, there are only a few reported examples of such nanostructured films for SERS applications, and these examples mainly focus on macroporous gold nanostructures [19,20]. We have not been aware of reports on ordered porous Ag and Au hybrid nanostructures for SERS applications. Therefore, in this paper, an ordered porous Ag and Au hybrid nanostructures were obtained by using ordered SiO2 nanospheres film as sacrificial templates. The application of these ordered porous Ag and Au hybrid nanostructure films as SERS substrates is investigated by using R6G as a probe molecule. We show that the ordered porous Ag and Au bimetallic nanostructure films are extremely efficient SERS substrates in terms of high Raman intensity enhancement and reproducibility. It was found that the enhancement ability of the ordered porous Ag and Au hybrid nanostructures were related with the atom ratio of Ag and Au.
J. Tang et al. / Physica B 433 (2014) 138–143
This work will be of great significance in understanding the SERS enhancement mechanism and in the fabrication of nanoparticle films for biosensing.
2. Experimental
139
available for deposition on the silica@Au nanoshells surface can be controlled by the volume of the silver nitrate solution. Then, the 3D SiO2@Au and Ag film was immerged in 4% HF solution for 30 min to remove the SiO2 spheres, yielding ordered porous Ag and Au hybrid nanostructures. Then, the 3D ordered porous Ag and Au hybrid nanostructure film washing with deionized water twice.
2.1. Materials All chemicals (analytical grade) were supplied by Aldrich and used without further purification. Deionized and doubly distilled water was used throughout the experiments. 2.2. Preparation of a gold-coated silica film The SiO2 nanoparticles were synthesized according to the well known Stöber process [21] and were functionalized by 3-aminopropyltrimethoxysilane (APTMS) [22]. Citrate-stabilized 3-nm gold nanoparticles were prepared according to Natan et al. [23]. The APTMS-modified SiO2 nanoparticles were fabricated onto the glass substrates by a solvent evaporation method [24]. Next, the resultant 3D SiO2 nanoparticles films were immersed in the gold nanoparticle solution. To grow the Au shell, first a growth solution was prepared by mixing 30 ml deionized water, 7.25 mg K2CO3 and 3.5 ml HAuCl4 solution (5 mM). The 3D SiO2@Au seed nanoparticle film was added to 27.5 ml of the growth solution, after which 10 μl of formaldehyde were rapidly dropped into the solution. After reaction for 6 h at room temperature, the 3D SiO2@Au film washing with deionized water twice.
2.4. SERS measurements The sample was immersed in a 1 10 9 M aqueous solution of rhodamine 6G (R6G) for about 30 min, and then gently washed with distilled water and dried with high-purity flowing nitrogen. In this experiment, more than three SERS-active substrates of samples were prepared, and at least 10 different points on each substrate were selected to detect the R6G probes, to verify the stability and reproducibility of these SERS-active substrates. 2.5. Characterization The Scanning electron microscopy (SEM) images were recorded using a Leica Cambridge S440 field emission scanning electron microscope with an accelerating voltage of 5.0 kV. Energy dispersive X-ray spectra (EDX) were recorded on an Oxford INCA energy spectroscope. Raman spectra were obtained with a Renishaw inVia micro-Ramanspectroscopy system. Laser radiation of 514.5 or 632.8 nm was used for excitation. The Raman band of a silicon wafer at 520 cm 1 was used to calibrate the spectrometer. All of the spectra reported were the results of a single accumulation of 20 s.
2.3. Preparation of porous Au and Ag nanostructured films In brief, the 3D SiO2@Au film was mixed with a fresh 0.2 mM solution of silver nitrate and stirred vigorously. Then, 60 μl of 37% formaldehyde was added to the mixture to begin the reduction of silver onto the 3D SiO2@Au film. This step was followed by the addition of ammonium hydroxide (typically 20–50 μl of 28–30% NH4OH). The addition of NH4OH into the sample solution caused a rapid increase in the pH of the solution, which facilitated in the reduction of Ag þ to Ag0 that deposited onto the surface of the seed particles, forming silver nanoshells. The amount of silver
3. Results and discussion The procedure that we have developed for fabricating ordered porous Ag and Au hybrid nanostructure films on flat substrates is shown schematically in Fig. 1. In the first step, 3D silica spheres films were fabricated on a glass substrate that was vertically placed in a slowly evaporating ethanol solution of APTMSmodified silica spheres with diameters of 200 nm. After drying in air, the 3D film was immersed in a dilute solution of very small
Fig. 1. Schematic illustration of the templating technique to the ordered porous Ag and Au hybrid nanostructures.
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gold seeds (about 3 nm in diameter) for 5 h. During this step, gold seeds were immobilized on the surface of the APTMS-modified silica spheres via the amine group. Subsequently, the resulting 3D film (silica@Au seed nanoparticles) was immersed in a mixture of HAuCl4/formaldehyde for gold plating to obtain a silica@Au nanoshells 3D film with an adjustable size of the gold nanoparticles. A similar procedure to fabricate the high quality macroporous metal films was developed earlier by Jiang et al. [25]. Then, the resulting 3D film (silica@Au nanoshells) was immersed in a mixture of AgNO3/NH4OH/formaldehyde for Ag plating to obtain a SiO2@Au/Ag nanoshells 3D film with an adjustable thickness of the Ag nanoshells. The last, SiO2@Au/Ag nanoshells 3D film was redispersed in 4% HF solution to remove the SiO2 spheres, yielding ordered porous Ag and Au hybrid nanostructure films. 3D films of APTMS-modified silica spheres of up to several square centimeters are easily prepared by this solvent evaporation method. Fig. 2 show typical scanning electron microscopy (SEM) images of a 3D silica spheres film (Fig. 2a and b) and of ordered
porous Ag and Au hybrid nanostructure film (Fig. 2c and d). As seen from Fig. 2a and b, the microspheres crystallize in a hexagonal ordered packing with the (1 1 1) facet parallel to the glass substrate. Fig. 2c and d show a typical SEM image of the resultant template free ordered porous Ag and Au hybrid nanostructure film. In Fig. 2c, open voids and interconnected walls consisting of larger interconnected aggregates form a pore structure. These pores are well ordered in a hexagonal packing arrangement, and such an ordered structure can be extended to several hundreds of micrometers. From the higher magnification image of Fig. 2d, it is clearly observed that there are three dark regions inside each pore corresponding to the air spheres of the underlying layer, indicating that these spheres are indeed close packed. The chemical composition of hybrid nanostructure is determined by energy-dispersive X-ray spectroscopy (EDX) (Fig. 3). The EDX spectrum with two main peaks (Au and Ag) is observed (other peaks originated from substrate), indicating that the hollow
Fig. 2. SEM images of the samples: (a)–(b) 3D silica spheres film; (c)–(d) ordered porous Ag and Au hybrid nanostructure film.
Fig. 3. EDX spectra of the ordered porous Ag and Au hybrid nanostructure films prepared under different silver nitrate solution: (a) 20 ml (sample A); (b) 40 ml (sample B); (c) 60 ml (sample C).
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metals nanostructure is made up of metallic Au and Ag. When the volume of the silver nitrate solution is 20 ml (Fig. 3a), the Au peak possesses a higher intensity than the Ag peak, which is consistent with the composition (the atom ratio of Au/Ag is 4: 1). After the volume of silver nitrate solution increased (40 ml), the Au peak decreased greatly and the intensity of the Ag peak increased as shown in Fig. 3b (the atom ratio of Au/Ag is 1:1), indicating the thickness of Ag nanoshells becomes thickly. With addition of the volume of silver nitrate solution (60 ml), the Au peak is much weaker than the Ag peak (Fig. 3c, (the atom ratio of Au/Ag is 1:4)). By comparing with EDX, it can be indicated that Ag is successfully grown on the surface of the SiO2@Au nanoshells, forming Au/Ag bimetallic nanostructure. To evaluate the effectiveness of the ordered porous Ag and Au hybrid nanostructure films for SERS applications, rhodamine 6G (R6G) was used in SERS studies because of its enormous intensity enhancement and adsorption onto noble metals particles. R6G is a cationic dye with strong absorption in the visible and a high fluorescence yield [26]. The molecule consists of two chromophores, one dibenzopyrene chomophore (xanthene), and one carboxyphenyl group tilted by about 901 with respect to the xanthene ring (see Fig. 4). Therefore the π-stems of the two chromophores of R6G are not conjugated [27]. Fig. 5 shows the SERS spectra that the R6G molecules are adsorbed onto (a) the cleaned glass substrate; (b) sample B. Fig. 5(a) shows the SERS of the R6G deposited film that is prepared by dropping 5 10 2 ml R6G aqueous solution (1 10 4 M) onto the cleaned glass substrate. As expected, we cannot distinguish any signal. Fig. 5 (b) shows the SERS of the 5 10 2 ml R6G aqueous solution (1 10 9 M) is deposited on sample C. Several strong bands at 1649, 1598, 1574, 1535, 1506, 1361 and 1310 cm 1 are observed on the substrate. The bands at 1649, 1574, 1506 and 1361 cm 1 are assigned to aromatic C–C stretching; 1598 cm 1 is assigned to C¼ C stretching, respectively [15]. Fig. 6 compare the 514.5 nm excitation SERS spectra of 1.0 10 8 M R6G obtained from three different substrates. From the three right bars that compare the plateau with the intensity of the strongest peak (aromatic C–C stretching at ca.1649 cm 1) in curves A–C, we find that the sample B exhibit the highest SERS
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Fig. 5. Raman spectra of R6G on (a) the cleaned glass substrate and on (b) sample C.
Fig. 6. SERS spectra of 10 nM R6G on different substrates. (A) sample A; (B) sample B; (C) sample C. The incubation time was 30 min. The excitation wavelength is 514.5 nm.
Fig. 4. Structure of R6G.
enhancement ability. It is shown that the SERS peaks intensity in Fig. 6 B is an order of magnitude stronger than the corresponding ones in Fig. 6 A. The SERS intensity at 1649 cm 1 for the sample B is about 2.45 times higher than that for the sample C and about 9.17 times higher than that for the sample A (Fig. 6). This SERS enhancement may be related to several factors. First, extremely intense local electromagnetic fields generated in the gaps between adjacent nanoparticles can strongly enhance the Raman scattering of probe molecules located in the gaps between the closely spaced nanoparticles. Second, since the intrinsic activity of Ag is much higher than Au [28] and the surfaces of bimetals provide more possibilities for molecules to deposit on the boundaries between Ag and Au domains [29], the SERS intensity at 1649 cm 1 for the sample B is higher than that for the sample A and sample C. One of the classical ways to evaluate the SERS activity for a given substrate is to calculate the enhancement factor (EF). Although there are debates and large uncertainties associated with EF calculations, this remains an interesting means to compare the SERS activity of different substrates [30,31]. The EF is defined as the ratio of inelastic scattering intensity per molecule between the presence and absence of the SERS structure. The Raman
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Table 1 Enhancement factors (G) calculated from the SERS spectra (A)–(C) as shown in Fig. 6. Raman shift (cm 1)
1361
1506
1574
1649
Average
G factor (sample A) (sample B) (sample C)
6.84 107 5.63 109 9.36 107
7.25 107 2.76 109 8.54 107
7.35 107 2.43 109 7.28 108
8.13 107 8.21 109 7.01 108
7.39 107 4.76 109 4.02 108
can be found to increase in a series: 514.5 4488 4568 4632.8 4 782 nm. It is conceivable that the 514.5 nm excitation source will yield a highest SERS activity.
4. Conclusion
Fig. 7. Enhancement factors (G) with different SERS substrates according to the different excitation wavelengths: 488, 514.5, 568, 632.8 and 782 nm.
In conclusion, porous Ag and Au hybrid nanostructures with different atom ratio of Ag and Au can be conveniently used to control film structures by simply varying the experimental conditions. The application of these ordered porous Ag and Au hybrid nanostructure films as SERS substrates is investigated by using R6G as a probe molecule. It was found that the enhancement ability of the ordered porous Ag and Au hybrid nanostructures were related with the atom ratio of Ag and Au. The enhancement factors (G) of the ordered porous Ag and Au hybrid nanostructures different excitation wavelengths can be found to increase in a series: 514.5 4488 4568 4632.8 4782 nm. These porous Ag and Au hybrid nanostructures exhibit high SERS activity and may have potential applications in investigating the surface chemistry in a particular reaction using SERS, for example, biosensing.
enhancement factor can be written as [32]: G¼
I SERS N Ref I Ref N SERS
ð1Þ
ISERS is the enhanced intensity of the adsorbed R6G molecules on the SERS substrate. The value of ISERS mainly arises from a single molecule layer covering a nanoparticle array, from which other additional molecules layers of analytes on the SERS substrate, as previously reported [33], do not contribute to Raman gain and can be neglected. NSERS is the number of the single-layer molecules covering the SERS substrate under the laser spot. NRef is the number of the bulk molecules excited by laser on the surface of the regular substrate. IRef is the spontaneous Raman scattering intensity from the bulk R6G molecules under the laser spot on the blank glass substrate. In order to obtain the values of these four parameters, we follow the same procedures based on the published literatures [34]. Using the 100 objective lens, we determine the area of the laser spot size at around 1 μm2. We also calculate the area of a single molecule of R6G to be approximately 1 106 nm2. Thus, the value of NSERS under the laser excitation is approximately 10 molecules. The focusing scope of laser beam is approximately 5 μm, thus scattering from all R6G molecules underneath the laser spot is detected. Assuming a uniform distribution of R6G over the droplet area of 2 mm2, the value of NRef is approximately 1.5 106 molecules. Therefore, from Eq. (1), the enhancement factor G is simply (1.5 104) (ISERS/IRef). The enhancement factors of each assigned Raman peak as measured from different SERS substrates are also shown in Table 1. The results show that the highest enhancement factor can be achieved at the average value of 4.76 109 from the sample B, which obtained from the optimized SERS substrate when the atom ratio of Au/Ag is 1:1. Here, we have studied the relationship of enhancement factors (G) and excitation wavelengths, as shown in Fig. 7. The enhancement factors (G) of the ordered porous Ag and Au hybrid nanostructures different excitation wavelengths
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