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Ag bimetallic hollow nanospheres and its application in surface-enhanced Raman scattering
Applied Surface Science 258 (2011) 212–217
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Facile preparation of Au/Ag bimetallic hollow nanospheres and its application in surface-enhanced Raman scattering Zao Yi a,b , Xibin Xu a,b , Xibo Li b , Jiangshan Luo b , Weidong Wu b , Yongjian Tang b,∗ , Yougen Yi a,∗ a b
College of Physical Science and Technology, Central South University, Changsha 410083, China Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China
a r t i c l e
i n f o
Article history: Received 14 June 2011 Received in revised form 7 August 2011 Accepted 8 August 2011 Available online 11 August 2011 Keywords: Au/Ag bimetallic hollow nanospheres Surface-enhanced Raman scattering Rhodamine 6G
a b s t r a c t In this paper, an Au/Ag bimetallic hollow nanostructure was obtained by using SiO2 nanospheres as sacrificial templates. The nanostructure was fabricated via a three steps method. SiO2 @Au nanospheres were first synthesized by the layer-by-layer technique, and then they were coated with a layer of Ag particles, finally, the Au/Ag bimetallic hollow nanospheres were obtained by dissolution of the SiO2 core by exposure in HF solution. Several characterizations, such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and UV visible absorption spectroscopy were used to investigate the prepared nanostructures. The effectiveness of these Au/Ag bimetallic hollow nanospheres as substrates toward surface-enhanced Raman scattering (SERS) detection was evaluated by using rhodamine 6G (R6G) as a probe molecule. We show that such Au/Ag bimetallic hollow nanospheres structure films which consisting of larger interconnected aggregates are highly desirable as SERS substrates in terms of high Raman intensity enhancement. The Au/Ag bimetallic hollow nanostructured aggregate, interconnected nanostructured aggregate and nanoscale roughness are important factors responsible for this large SERS enhancement ability. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, with the growing interest in hollow and bimetallic noble metals nanospheres, such as gold (Au), silver (Ag), platinum (Pt), and palladium (Pd) have been justified by their fascinating optical [1–3], electronic [4], and catalytic [5] properties, thus leading to a wide range of applications, including surface-enhanced Raman scattering (SERS) [6,7], biosensors [8], and catalysis [5,9]. Therefore, a great deal of effort has been dedicated to the synthesis of metallic hollow spheres. A rich variety of synthesis approaches have been explored for the synthesis of hollow metallic spheres. The template synthesis is a simple and general chemical method for preparing hollow metal and bimetallic submicrometer spheres. The materials including silica colloids [8,10,11], polystyrene colloids [1,12,13], Co nanoparticles [2,5,14], silver nanoparticles [7,8,15,16], ceramic hollow spheres [17], and microemulsion droplets [18,19] are generally employed as templates. Compared with the other templates, silica colloids, owing to its better monodispersity, wider size range (nanometer-to-micrometer), are easy to be synthesized or commercially obtained [20], and, it have been successfully exploited to prepare hollow palladium [21], gold [22], Au/Pt, and
∗ Corresponding authors. Tel.: +86 0816 2480830; fax: +86 0816 2480830. E-mail address: [email protected] (Y. Yi). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.08.033
Au/Pd [23] spheres with controllable size, shape and shell thickness. However, there are few reports about the preparation of Au/Ag bimetallic hollow nanospheres. Metal nanoparticles, especially Au and Ag and their bimetallic forms have been widely employed in SERS [6,7]. SERS is insensitive to humidity, oxygen and other quenchers and considered to be a promising method for sensitive biological identifications and detections [9,24]. It is well known that Raman scattering is enhanced when a molecule makes contact with a metal nanoparticle covered surface [25]. Although there has been considerable debate on the mechanism of SERS, it is generally argued that the basis of surface enhancement is the formation of near-field localized surface plasmon resonance (SPR) due to the coherent oscillations of metal electrons aroused with the interaction of excitation and scattered light [25,26]. This near-field coupling greatly enhances the Raman spectroscopic signals from organic molecules in the proximity of the nanoparticles. The strength of enhancement is correlated to the material, shape, size, and state of aggregation of the nanostructured surface [27]. Therefore, most of the research has been directed toward fabricating nanostructure architectures to achieve maximum Raman enhancement [28]. The results of this research have shown that Raman enhancement is strongly dependent on inter-particle interactions [26,29]. The near-field localized SPR on one particle interacts with that on an adjacent particle, coupling the plasmon oscillations and enhancing the Raman scattering [30].
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Therefore, in this paper, we report a facile silica colloidal templating method to synthesize Au/Ag bimetallic hollow nanospheres with fine monodispersity and controllable size and shape. In order to analyze SERS capability of the Au/Ag bimetallic hollow nanospheres, these samples were self-assembled on glass slide. As we know, we have not been aware of reports on Au/Ag bimetallic hollow nanospheres structured for SERS applications. The application of these Au/Ag bimetallic hollow nanospheres bimetallic structured films as SERS substrates is first investigated by using R6G as a probe molecule. We show that the as-prepared Au/Ag bimetallic hollow nanospheres structured films are extremely efficient SERS substrates in terms of high Raman intensity enhancement, excellent stability, and reproducibility. For the large SERS enhancement ability, the interconnected nanostructured aggregate, nanoscale roughness and Au/Ag bimetallic hollow nanostructured aggregate are considered as important effect factors.
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of 37% formaldehyde was added to the stirred mixture to begin the reduction of silver onto the SiO2 @Au nanoshells. This step was followed by the addition of ammonium hydroxide (typically 20–50 l of 28–30% NH4 OH). The addition of NH4 OH into the sample solution caused a rapid increase in the pH of the solution, which facilitated in the reduction of Ag+ to Ag0 that deposited onto the surface of the seed particles, forming silver nanoshells. SiO2 @Au/Ag nanoshells precipitate were redispersed in 4% HF solution for 30 min to remove the SiO2 spheres, yielding Au/Ag bimetallic hollow nanospheres. After stirring for 30 min at room temperature, the precipitate was collected by centrifuging at 8000 rpm for 10 min and washed with deionized water twice. To compare the SERS ability of the Au/Ag bimetallic hollow nanospheres with Ag hollow nanospheres, the Ag hollow nanospheres were fabricated by using the procedure of Chen et al. [31]. 2.3. Preparation of SERS film on the glass substrate
2. Materials and methods 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 samples SiO2 nanoparticles were synthesized according to the well known Stöber process [18]. An aqueous ammonium solution (28 wt%, 3 ml) and tetraethylorthosilicate (TEOS) (1.5 ml) were added together in a 100 ml flask containing 50 ml of ethanol (99.5 wt%). The mixed solution was stirred vigorously for 8 h and then centrifuged at 10,000 rpm for 15 min. The supernatant was discarded and the precipitate was redispersed in 50 ml of ethanol. Following addition of another 3 ml ammonium aqueous (28 wt%) and 50 l of 3-aminopropyltriethoxysilane (APTES), the resulting solution was stirred for 24 h. The solution was centrifuged at 10,000 rpm for 15 min. The supernatant was removed and the precipitate washed by deionized water, then washed by ethanol twice. Finally, the precipitate (amine-functionalized silica nanoparticles) was redispersed and stored in 5 ml of ethanol for further use. First, aqueous solutions of small Au seeds (∼5 nm) were prepared by reduction of HAuCl4 with tetrakishydroxymethylphosphonium chloride (THPC) as described by Duff et al. [24]. The Au seed solution displayed no signs of degradation or aggregation after six months in fridge. To attach Au seeds to amine-functionalized silica particles, 1 ml of amine-functionalized silica nanoparticle solution and 5 ml of Au seed suspension were mixed with vigorous stirring for 1 h at room temperature that led the Au seeds attach to the dielectric nanoparticle surfaces via molecular linkages. To remove the free Au seeds, the solution was centrifuged at 2000 rpm (repeated at least three times), the supernatant was removed, and the precipitate (silica@Au seed nanoparticles) was redispersed in 1 ml of deionized water. To grow the Au shell, first a growth solution was prepared by mixing 25 ml deionized water, 6.28 g K2 CO3 and 2.5 ml HAuCl4 solution (6 mM). 20 l of silica@Au seed nanoparticle solution was added to 27.5 ml of the growth solution, after which 10 l of formaldehyde were rapidly dropped into the solution. After stirring for 6 h at room temperature, the precipitate was collected by centrifuging at 8000 rpm for 10 min and washed with deionized water twice. The final precipitate was redispersed in 1 ml of deionized water, yielding silica@Au nanoshells. In brief, the SiO2 @Au nanoshells were mixed with a fresh 0.2 mM solution of silver nitrate (AgNO3 ) and stirred vigorously. Then, 60 l
Glass slides (thickness, 1 mm, which were cut into 1 cm × 4 cm pieces) were used as the support for the assembly of above samples. The clean glass slides were prepared by immersing them into chromic acid overnight and subsequently sonicating them in piranha solution (H2 O2 :H2 SO4 = 3:7) for 30 min at 70 ◦ C. (Caution: piranha solution is a powerful oxidizing agent and reacts violently with organic compounds. It should be handled with extreme care.) After thoroughly rinsing the slides with distilled water, they were blown dry with N2 . The clean glass slides were immersed into a 2% (v/v) solution of 3-(aminopropyl)-triethoxysilane (APTES) in anhydrous ethanol overnight, washed by sonication with ethanol and water, and dried at 120 ◦ C for 2 h. The amine-terminated glass slides were subsequently immersed into above samples solution for 5 h with vertical angles in vials to form a self-assembled monolayer of nanoparticles film by electrostatic interactions. At last, the slide was taken out from the vial and rinsed with water. These slides were dried naturally at room temperature for at least 24 h. 50 m of 1.0 × 10−8 M R6G aqueous solution was dropped onto the as-prepared nanoparticles films, and the solvent was allowed to evaporate under ambient conditions. Finally, a Raman spectrometer was used to measure the SERS activities of these films. In this experiment, more than 5 SERS-active substrates of each sample were prepared, and 10 different points on each substrate were selected to detect the R6G probes, to verify the stability and reproducibility of these SERS-active substrates. 2.4. Characterization The images of transmission electron microscopy (TEM) were obtained by a JEM-2010 microscope which it using an accelerating voltage of 120.0 kV and the samples were prepared by placing a drop of products aqueous dispersion on carbon-coated copper grid. The optical properties of these nanoparticles were measured by a Perkin-Elmer Lambda 12 spectrophotometer. The Scanning electron microscopy (SEM) images were recorded by using a Leica Cambridge S440 field emission scanning electron microscope with an accelerating voltage of 5.0 kV. SERS spectra were measured with a Renishaw 2000 model confocal microscopy Raman spectrometer with a CCD detector and a holographic notch filter (Renishaw Ltd., Gloucestershire, U.K.). The microscope attachment was based on a Leica DMLM system, and a 50× objective was used to focus the laser beam onto a spot with approximately 1 m in diameter. Radiation of 514.5 nm from an air-cooled argon ion laser was used for the SERS excitation. The laser power at the samples’ position was typically 7.2 mW. All of the Raman spectra were recorded in 20 s. All the spectra were baseline corrected and noise filtered.
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Fig. 1. SEM and TEM images of samples: (A) SiO2 nanospheres (sample A); (B) Au seeds (sample B); (C) SiO2 @Au nanoshells (sample C); (D) SiO2 @Au/Ag nanoshells (sample D); (E) Au/Ag bimetallic hollow nanospheres(sample E); (F) Au/Ag bimetallic hollow nanospheres (sample E).
3. Results and discussion Fig. 1 shows the SEM and TEM images of these samples. As shown in Fig. 1(A) (sample A), the as-prepared aminated SiO2 particles are spherical shape with a relatively uniform size of 200 nm. Fig. 1(B) (sample B) is the TEM image of Au seeds, which are spheroidal particles with average size of 5 nm. The Au seeds are electronegative and the surface of SiO2 particles is positive because of adsorbed amidogen. The Au seeds are easy adsorbed to the surface of SiO2 particles by electrostatic adsorption. Fig. 1(C) (sample C) is the SiO2 particles absorbed with fine Au seeds as the “seed” for the further packing or growth of gold shell. The SiO2 @Au nanoshells have a diameter of 250 nm with Au shells of about 25 nm of thickness. Fig. 1(D) (sample D) is the TEM images of SiO2 @Au/Ag nanoshells after the SiO2 @Au nanoshells solution is added to 27.5 ml of the AgNO3 growth solution. The SiO2 @Au/Ag nanoshells have a thickness of about 47 nm Au/Ag nanoshells. The pictures (E) and pictures (F) of Fig. 1 show the typical TEM and SEM images of the Au/Ag bimetallic hollow nanospheres (sample E). As shown in Fig. 1(F), the Au/Ag bimetallic hollow nanospheres are packed into out-of-order structure, but almost all the com-
posite spheres are squashed. This should arise from that the shell thickness is too thin to sustain the interfacial tension during the self-assembly process. Some Au atoms are come out from Ag shell along with SiO2 core removal. Counting on 200 nanoparticles in Fig. 1(F), the average outer and inner diameters of the Au/Ag bimetallic hollow nanospheres are measured to be approximately 200 and 294 nm. The image of Fig. 1(E) shows a typical TEM image of Au/Ag bimetallic hollow nanospheres, where there is strong contrast difference in all of the spheres with a bright center surrounded by a much darker edge, confirming their hollow architecture. It is well-known that, UV–vis spectroscopy can investigate the surface plasmon resonance (SPR) property that is commonly used to monitor the growth of the Au/Ag bimetallic hollow nanospheres. The UV–vis spectra of the as-prepared sample aqueous solution are shown in Fig. 2. The initial Au seeds solution has a surface plasmon resonance peak at 501 nm. After formation of composite layers on the SiO2 particles, the peak shift to 591 nm, because of enhanced interactions between the gold nanoparticles and changes in the dielectric medium surrounding the gold nanoparticles [32]. After a layer of Ag particles grows on the SiO2 @Au nanoshells, as shown in Fig. 2(C), the SPR peak of SiO2 @Au/Ag nanoshells becomes more
Fig. 2. UV–vis spectra of the as-prepared: (A) sample B; (B) sample C; (C) sample D; (D) sample E.
Fig. 3. EDX image of the as-prepared: sample E.
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Fig. 5. SERS spectra of 1.0 × 10−8 M R6G on sample E: (A) without KCl solution and (B) with KCl solution.
Fig. 4. Structure of rhodamine 6G.
prominent and red shifted up to a maximum of 782 nm. This is accompanied by a substantial broadening of the SPR peak. Such systematic red shift is consistent with literature reports for gold and silver nanoshells, prepared by different procedures on silica microspheres [33]. This may be attributed to the reason that the Ag shell has more strong characteristic absorption in the UV–vis–NIR spectroscopy, and the Ag shell in the as-prepared Au/Ag nanostructure is thick enough to hide the characteristic SPR peak of the SiO2 @Au nanoshells. This can be another support for the formation of the Au/Ag bimetallic nanostructure. SiO2 core removal by HF also affects the peak position-a red shift that is observed relative to the core–shell (Au/Ag bimetallic hollow nanospheres) particles (986 nm vs. 782 nm). The red-shift of the SPR peak has a contribution from both Ag as well as Au plasmon absorptions. Another fact may also be responsible for the hollow cavity of the nanospheres, which is in accordance with the literature reported previously [34]. By combining with the change of SPR, it can be indicated that Ag is successfully grown on the surface of the SiO2 @Au nanoshells, in order to form an Au/Ag bimetallic nanostructure, finally forming an Au/Ag bimetallic hollow nanospheres structure. 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 ITO glass substrate), indicating that the hollow metals nanostructure is made up of metallic Au and Ag. By combining with EDX, it can be indicated that Ag is successfully grown on the surface of the SiO2 @Au nanoshells, forming an Au/Ag bimetallic nanostructure. To evaluate the effectiveness of the four above-mentioned kinds of Au/Ag nanostructured films for SERS applications, rhodamine 6G (R6G) was used as a touchstone because it is a dye molecule which often used in SERS studies because of its enormous intensity enhancement and adsorption onto silver particles. R6G is a cationic dye with strong absorption in the visible and a high fluorescence yield. The molecule consists of two chromophores, one dibenzopyrene chomophore (xanthene), and one carboxyphenyl group tilted by about 90◦ with respect to the xanthene ring (see Fig. 4). Therefore the -stems of the two chromophores of R6G are not conjugated [35]. Previous SERS studies suggested that the presence of chlo-
ride ions had an activation effect, which has been considered to create “active sites” for Raman enhancement [36]. A similar activation effect is also observed in the present study. We found that the SERS signal intensities were increased by about 7 times in the presence of 10 mM KCl (Fig. 5). The bands which show strong resonance enhancements correspond to C–C stretching in the xanthene plane are found at 1648, 1574, 1505, and 1362 cm−1 . Since all of these bands involve some characters of xanthene in-plane C–C stretching, they are expected to show resonance enhancements. The other bands at 1598 and 1311 cm−1 are assigned to C C stretching; 1186 cm−1 are assigned to aromatic C–H bending, respectively [37]. Fig. 6 compares the 514.5 nm excitation SERS spectra of 1.0 × 10−8 M R6G in the presence of 10 mM KCl obtained from four different substrates. From the four right bars that compare the plateau with the intensity of the strongest peak (aromatic C–C stretching at ca.1648 cm−1 ) in curves B–E, we find that the Au/Ag bimetallic hollow nanospheres structured films exhibit the highest SERS enhancement ability. It is shown that the SERS peaks intensity in Fig. 6(C) is an order of magnitude stronger than the corresponding ones in Fig. 6(B). The SERS intensity at 1648 cm−1 for the Au/Ag
Fig. 6. SERS spectra of 1.0 × 10−8 M R6G on different substrates. (B) Sample B; (C) sample C; (D) sample D; (E) sample E.
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nanoshells structured, Au/Ag bimetallic hollow nanospheres structured possess porous structure. The presence of these porous structures can increase the surface area of the substrates, which allows more R6G molecules to adsorb per unit surface of the substrates [45]. According to Tian et al. [26], a large SERS enhancement can be achieved at these crevice sites because of the enormous electromagnetic field. For instance, Xia et al. reported that an electromagnetic enhancement of 2 × 107 was present between two nanopsheres separated by 1.8 nm [46]. This redistribution can also yield superior SERS enhancement. After removing SiO2 spheres, samples E was obtained. Samples E appears porous structure and interconnected aggregate structures as indicated in Fig. 1(E), and the surface areas of samples E are increased further because of the removal of SiO2 spheres. The surface area ratio of sample E to sample D was roughly calculated as follows: Sh 4(r1 2 + r2 2 ) = =1+ Ss 4r1 2 Fig. 7. Comparison of SERS intensities between (E) sample E and (F) sample F, Ag hollow nanospheres structured film. The inset shows the SEM of Ag hollow nanospheres. The concentration of R6G was 1.0 × 10−8 M.
bimetallic hollow nanospheres structured film is about 2.3 times higher than that for the SiO2 @Au/Ag nanoshells structured film and about 6 times higher than that for the SiO2 @Au nanoshells structured film (Fig. 6(C–E)). More importantly, it is noted that the SERS performance of such Au/Ag bimetallic hollow nanospheres structured film has been stable for at least 4 months. Fig. 7 present SERS spectra of R6G molecules on sample E and an Ag hollow nanospheres structured film (sample F), respectively. The inset of Fig. 7 is the SEM of Ag hollow nanospheres. The average outer and inner diameters of the Ag hollow nanospheres are measured to be approximately 200 and 294 nm, the same as the Au/Ag bimetallic hollow nanospheres. However, the SERS intensity at 1648 cm−1 for the Au/Ag bimetallic hollow nanospheres structured film is about 1.4 times higher than that for the Ag hollow nanospheres structured film. This SERS enhancement may be related to several factors. First, extremely intense local electromagnetic fields generated in the gaps between adjacent Au nanoparticles can strongly enhance the Raman scattering of probe molecules located in the gaps between the closely spaced Au nanoparticles [37], so the SERS intensity from the SiO2 @Au nanoshells structured film is stronger than that observed on the Au nanoparticles structured film. Second, since the intrinsic activity of Ag is much higher than Au [38] and the surfaces of bimetals provide more possibilities for molecules to deposit on the boundaries between Ag and Au domains [39], the SERS intensity at 1648 cm−1 for the SiO2 @Au/Ag nanoshells structured film is about 2.6 times higher than that for the SiO2 @Au nanoshells structured film. Excited by an incident light, the collective surface plasmons are localized at these hollow nanostructured and interconnected nanostructured aggregates, leading to the formation of a local field in this region that is many orders of magnitude higher than the incident light [40]. The localized resonant plasmon modes can contribute to larger SERS enhancement [22,41–43]. The number of SERS active sites in samples D–E increase because of the presence of a large number of hollow nanostructured and interconnected nanoparticle junctions, and thus it is not surprising to observe the multifold increase in SERS intensity enhancement compared with that of the SiO2 @Au/Ag nanoshells structured film. In this case, nanoscale roughness is another important factor contributing to SERS enhancement because it plays an important role by providing pathways for the hot electrons to the probe molecules, which results in SERS enhancement by chemical effects [44]. Moreover, from the microstructure viewpoint, different from the SiO2 @Au/Ag
r 2 2
r
=1+
100 100 + 47
2
≈ 1.46
Sh and Ss are the surface areas of samples E and D, respectively. r1 and r2 are the radius of a SiO2 @Au/Ag nanoshells and a SiO2 nanoparticles, respectively. Our calculations indicates that the surface area of samples E increased by less than 1.46 times compared with that of sample D. Interestingly, as mentioned above, the SERS intensity at 1648 cm−1 for samples E is about 2.3 times higher than that of sample D. This result reveals that the higher SERS enhancement of sample E relative to that of sample D may not be explained only by the increase in surface area, but also the surfaces of bimetals provide more possibilities for molecules to deposit on the boundaries between Ag and Au domains (Fig. 7) [39] because some Au atoms were came out from Ag shell along with SiO2 core removal. 4. Conclusions In conclusion, Au/Ag bimetallic hollow nanospheres with fine monodispersity and controllable size and shape were successfully synthesized using a modified SiO2 nanoparticles-templating method at room temperature. The as-prepared Au/Ag bimetallic hollow nanospheres structured films are used for the first time as SERS substrates and exhibit good SERS enhancement ability, excellent reproducibility, and stability, which may find practical application for routine SERS analysis. It was found that the Au/Ag bimetallic hollow nanostructured aggregate, interconnected nanostructured aggregate and nanoscale roughness are important factors which responsible for this large SERS enhancement ability. Acknowledgments This 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] H.P. Liang, L.J. Wan, C.L. Bai, L. Jiang, J. Phys. Chem. B 109 (2005) 7795. [3] J.H. Lee, A. Mahmoud, V. Sitterle, J. Sitterle, J.C. Meredith, J. Am. Chem. Soc. 131 (2009) 5048. [4] 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. [5] X.M. Feng, C.J. Mao, G. Yang, W.H. Hou, J.J. Zhu, Langmuir 22 (2006) 4384. [6] G.V.P. Kumar, S. Shruthi, B. Vibha, B.A.A. Reddy, T.K. Kundu, C. Narayana, J. Phys. Chem. C 111 (2007) 4388. [7] R. Güzel, Z. Üstünda˘g, H. Eks¸i, S. Keskin, B. Taner, Z.G. Durgun, A.I. Turan, A.O. Solak, J. Colloid Interface Sci. 351 (2010) 35. [8] F.Y. Cheng, C.T. Chen, C.S. Ye, Nanotechnology 20 (2009) 425104. [9] Y.T. Xia, W.S. Lu, L. Jiang, Nanotechnology 21 (2010) 85501.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Facile preparation of Au/Ag bimetallic hollow nanospheres and its application in surface-enhanced Raman scattering Zao Yi a,b , Xibin Xu a,b , Xibo Li b , Jiangshan Luo b , Weidong Wu b , Yongjian Tang b,∗ , Yougen Yi a,∗ a b
College of Physical Science and Technology, Central South University, Changsha 410083, China Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China
a r t i c l e
i n f o
Article history: Received 14 June 2011 Received in revised form 7 August 2011 Accepted 8 August 2011 Available online 11 August 2011 Keywords: Au/Ag bimetallic hollow nanospheres Surface-enhanced Raman scattering Rhodamine 6G
a b s t r a c t In this paper, an Au/Ag bimetallic hollow nanostructure was obtained by using SiO2 nanospheres as sacrificial templates. The nanostructure was fabricated via a three steps method. SiO2 @Au nanospheres were first synthesized by the layer-by-layer technique, and then they were coated with a layer of Ag particles, finally, the Au/Ag bimetallic hollow nanospheres were obtained by dissolution of the SiO2 core by exposure in HF solution. Several characterizations, such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and UV visible absorption spectroscopy were used to investigate the prepared nanostructures. The effectiveness of these Au/Ag bimetallic hollow nanospheres as substrates toward surface-enhanced Raman scattering (SERS) detection was evaluated by using rhodamine 6G (R6G) as a probe molecule. We show that such Au/Ag bimetallic hollow nanospheres structure films which consisting of larger interconnected aggregates are highly desirable as SERS substrates in terms of high Raman intensity enhancement. The Au/Ag bimetallic hollow nanostructured aggregate, interconnected nanostructured aggregate and nanoscale roughness are important factors responsible for this large SERS enhancement ability. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, with the growing interest in hollow and bimetallic noble metals nanospheres, such as gold (Au), silver (Ag), platinum (Pt), and palladium (Pd) have been justified by their fascinating optical [1–3], electronic [4], and catalytic [5] properties, thus leading to a wide range of applications, including surface-enhanced Raman scattering (SERS) [6,7], biosensors [8], and catalysis [5,9]. Therefore, a great deal of effort has been dedicated to the synthesis of metallic hollow spheres. A rich variety of synthesis approaches have been explored for the synthesis of hollow metallic spheres. The template synthesis is a simple and general chemical method for preparing hollow metal and bimetallic submicrometer spheres. The materials including silica colloids [8,10,11], polystyrene colloids [1,12,13], Co nanoparticles [2,5,14], silver nanoparticles [7,8,15,16], ceramic hollow spheres [17], and microemulsion droplets [18,19] are generally employed as templates. Compared with the other templates, silica colloids, owing to its better monodispersity, wider size range (nanometer-to-micrometer), are easy to be synthesized or commercially obtained [20], and, it have been successfully exploited to prepare hollow palladium [21], gold [22], Au/Pt, and
∗ Corresponding authors. Tel.: +86 0816 2480830; fax: +86 0816 2480830. E-mail address: [email protected] (Y. Yi). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.08.033
Au/Pd [23] spheres with controllable size, shape and shell thickness. However, there are few reports about the preparation of Au/Ag bimetallic hollow nanospheres. Metal nanoparticles, especially Au and Ag and their bimetallic forms have been widely employed in SERS [6,7]. SERS is insensitive to humidity, oxygen and other quenchers and considered to be a promising method for sensitive biological identifications and detections [9,24]. It is well known that Raman scattering is enhanced when a molecule makes contact with a metal nanoparticle covered surface [25]. Although there has been considerable debate on the mechanism of SERS, it is generally argued that the basis of surface enhancement is the formation of near-field localized surface plasmon resonance (SPR) due to the coherent oscillations of metal electrons aroused with the interaction of excitation and scattered light [25,26]. This near-field coupling greatly enhances the Raman spectroscopic signals from organic molecules in the proximity of the nanoparticles. The strength of enhancement is correlated to the material, shape, size, and state of aggregation of the nanostructured surface [27]. Therefore, most of the research has been directed toward fabricating nanostructure architectures to achieve maximum Raman enhancement [28]. The results of this research have shown that Raman enhancement is strongly dependent on inter-particle interactions [26,29]. The near-field localized SPR on one particle interacts with that on an adjacent particle, coupling the plasmon oscillations and enhancing the Raman scattering [30].
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Therefore, in this paper, we report a facile silica colloidal templating method to synthesize Au/Ag bimetallic hollow nanospheres with fine monodispersity and controllable size and shape. In order to analyze SERS capability of the Au/Ag bimetallic hollow nanospheres, these samples were self-assembled on glass slide. As we know, we have not been aware of reports on Au/Ag bimetallic hollow nanospheres structured for SERS applications. The application of these Au/Ag bimetallic hollow nanospheres bimetallic structured films as SERS substrates is first investigated by using R6G as a probe molecule. We show that the as-prepared Au/Ag bimetallic hollow nanospheres structured films are extremely efficient SERS substrates in terms of high Raman intensity enhancement, excellent stability, and reproducibility. For the large SERS enhancement ability, the interconnected nanostructured aggregate, nanoscale roughness and Au/Ag bimetallic hollow nanostructured aggregate are considered as important effect factors.
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of 37% formaldehyde was added to the stirred mixture to begin the reduction of silver onto the SiO2 @Au nanoshells. This step was followed by the addition of ammonium hydroxide (typically 20–50 l of 28–30% NH4 OH). The addition of NH4 OH into the sample solution caused a rapid increase in the pH of the solution, which facilitated in the reduction of Ag+ to Ag0 that deposited onto the surface of the seed particles, forming silver nanoshells. SiO2 @Au/Ag nanoshells precipitate were redispersed in 4% HF solution for 30 min to remove the SiO2 spheres, yielding Au/Ag bimetallic hollow nanospheres. After stirring for 30 min at room temperature, the precipitate was collected by centrifuging at 8000 rpm for 10 min and washed with deionized water twice. To compare the SERS ability of the Au/Ag bimetallic hollow nanospheres with Ag hollow nanospheres, the Ag hollow nanospheres were fabricated by using the procedure of Chen et al. [31]. 2.3. Preparation of SERS film on the glass substrate
2. Materials and methods 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 samples SiO2 nanoparticles were synthesized according to the well known Stöber process [18]. An aqueous ammonium solution (28 wt%, 3 ml) and tetraethylorthosilicate (TEOS) (1.5 ml) were added together in a 100 ml flask containing 50 ml of ethanol (99.5 wt%). The mixed solution was stirred vigorously for 8 h and then centrifuged at 10,000 rpm for 15 min. The supernatant was discarded and the precipitate was redispersed in 50 ml of ethanol. Following addition of another 3 ml ammonium aqueous (28 wt%) and 50 l of 3-aminopropyltriethoxysilane (APTES), the resulting solution was stirred for 24 h. The solution was centrifuged at 10,000 rpm for 15 min. The supernatant was removed and the precipitate washed by deionized water, then washed by ethanol twice. Finally, the precipitate (amine-functionalized silica nanoparticles) was redispersed and stored in 5 ml of ethanol for further use. First, aqueous solutions of small Au seeds (∼5 nm) were prepared by reduction of HAuCl4 with tetrakishydroxymethylphosphonium chloride (THPC) as described by Duff et al. [24]. The Au seed solution displayed no signs of degradation or aggregation after six months in fridge. To attach Au seeds to amine-functionalized silica particles, 1 ml of amine-functionalized silica nanoparticle solution and 5 ml of Au seed suspension were mixed with vigorous stirring for 1 h at room temperature that led the Au seeds attach to the dielectric nanoparticle surfaces via molecular linkages. To remove the free Au seeds, the solution was centrifuged at 2000 rpm (repeated at least three times), the supernatant was removed, and the precipitate (silica@Au seed nanoparticles) was redispersed in 1 ml of deionized water. To grow the Au shell, first a growth solution was prepared by mixing 25 ml deionized water, 6.28 g K2 CO3 and 2.5 ml HAuCl4 solution (6 mM). 20 l of silica@Au seed nanoparticle solution was added to 27.5 ml of the growth solution, after which 10 l of formaldehyde were rapidly dropped into the solution. After stirring for 6 h at room temperature, the precipitate was collected by centrifuging at 8000 rpm for 10 min and washed with deionized water twice. The final precipitate was redispersed in 1 ml of deionized water, yielding silica@Au nanoshells. In brief, the SiO2 @Au nanoshells were mixed with a fresh 0.2 mM solution of silver nitrate (AgNO3 ) and stirred vigorously. Then, 60 l
Glass slides (thickness, 1 mm, which were cut into 1 cm × 4 cm pieces) were used as the support for the assembly of above samples. The clean glass slides were prepared by immersing them into chromic acid overnight and subsequently sonicating them in piranha solution (H2 O2 :H2 SO4 = 3:7) for 30 min at 70 ◦ C. (Caution: piranha solution is a powerful oxidizing agent and reacts violently with organic compounds. It should be handled with extreme care.) After thoroughly rinsing the slides with distilled water, they were blown dry with N2 . The clean glass slides were immersed into a 2% (v/v) solution of 3-(aminopropyl)-triethoxysilane (APTES) in anhydrous ethanol overnight, washed by sonication with ethanol and water, and dried at 120 ◦ C for 2 h. The amine-terminated glass slides were subsequently immersed into above samples solution for 5 h with vertical angles in vials to form a self-assembled monolayer of nanoparticles film by electrostatic interactions. At last, the slide was taken out from the vial and rinsed with water. These slides were dried naturally at room temperature for at least 24 h. 50 m of 1.0 × 10−8 M R6G aqueous solution was dropped onto the as-prepared nanoparticles films, and the solvent was allowed to evaporate under ambient conditions. Finally, a Raman spectrometer was used to measure the SERS activities of these films. In this experiment, more than 5 SERS-active substrates of each sample were prepared, and 10 different points on each substrate were selected to detect the R6G probes, to verify the stability and reproducibility of these SERS-active substrates. 2.4. Characterization The images of transmission electron microscopy (TEM) were obtained by a JEM-2010 microscope which it using an accelerating voltage of 120.0 kV and the samples were prepared by placing a drop of products aqueous dispersion on carbon-coated copper grid. The optical properties of these nanoparticles were measured by a Perkin-Elmer Lambda 12 spectrophotometer. The Scanning electron microscopy (SEM) images were recorded by using a Leica Cambridge S440 field emission scanning electron microscope with an accelerating voltage of 5.0 kV. SERS spectra were measured with a Renishaw 2000 model confocal microscopy Raman spectrometer with a CCD detector and a holographic notch filter (Renishaw Ltd., Gloucestershire, U.K.). The microscope attachment was based on a Leica DMLM system, and a 50× objective was used to focus the laser beam onto a spot with approximately 1 m in diameter. Radiation of 514.5 nm from an air-cooled argon ion laser was used for the SERS excitation. The laser power at the samples’ position was typically 7.2 mW. All of the Raman spectra were recorded in 20 s. All the spectra were baseline corrected and noise filtered.
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Fig. 1. SEM and TEM images of samples: (A) SiO2 nanospheres (sample A); (B) Au seeds (sample B); (C) SiO2 @Au nanoshells (sample C); (D) SiO2 @Au/Ag nanoshells (sample D); (E) Au/Ag bimetallic hollow nanospheres(sample E); (F) Au/Ag bimetallic hollow nanospheres (sample E).
3. Results and discussion Fig. 1 shows the SEM and TEM images of these samples. As shown in Fig. 1(A) (sample A), the as-prepared aminated SiO2 particles are spherical shape with a relatively uniform size of 200 nm. Fig. 1(B) (sample B) is the TEM image of Au seeds, which are spheroidal particles with average size of 5 nm. The Au seeds are electronegative and the surface of SiO2 particles is positive because of adsorbed amidogen. The Au seeds are easy adsorbed to the surface of SiO2 particles by electrostatic adsorption. Fig. 1(C) (sample C) is the SiO2 particles absorbed with fine Au seeds as the “seed” for the further packing or growth of gold shell. The SiO2 @Au nanoshells have a diameter of 250 nm with Au shells of about 25 nm of thickness. Fig. 1(D) (sample D) is the TEM images of SiO2 @Au/Ag nanoshells after the SiO2 @Au nanoshells solution is added to 27.5 ml of the AgNO3 growth solution. The SiO2 @Au/Ag nanoshells have a thickness of about 47 nm Au/Ag nanoshells. The pictures (E) and pictures (F) of Fig. 1 show the typical TEM and SEM images of the Au/Ag bimetallic hollow nanospheres (sample E). As shown in Fig. 1(F), the Au/Ag bimetallic hollow nanospheres are packed into out-of-order structure, but almost all the com-
posite spheres are squashed. This should arise from that the shell thickness is too thin to sustain the interfacial tension during the self-assembly process. Some Au atoms are come out from Ag shell along with SiO2 core removal. Counting on 200 nanoparticles in Fig. 1(F), the average outer and inner diameters of the Au/Ag bimetallic hollow nanospheres are measured to be approximately 200 and 294 nm. The image of Fig. 1(E) shows a typical TEM image of Au/Ag bimetallic hollow nanospheres, where there is strong contrast difference in all of the spheres with a bright center surrounded by a much darker edge, confirming their hollow architecture. It is well-known that, UV–vis spectroscopy can investigate the surface plasmon resonance (SPR) property that is commonly used to monitor the growth of the Au/Ag bimetallic hollow nanospheres. The UV–vis spectra of the as-prepared sample aqueous solution are shown in Fig. 2. The initial Au seeds solution has a surface plasmon resonance peak at 501 nm. After formation of composite layers on the SiO2 particles, the peak shift to 591 nm, because of enhanced interactions between the gold nanoparticles and changes in the dielectric medium surrounding the gold nanoparticles [32]. After a layer of Ag particles grows on the SiO2 @Au nanoshells, as shown in Fig. 2(C), the SPR peak of SiO2 @Au/Ag nanoshells becomes more
Fig. 2. UV–vis spectra of the as-prepared: (A) sample B; (B) sample C; (C) sample D; (D) sample E.
Fig. 3. EDX image of the as-prepared: sample E.
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Fig. 5. SERS spectra of 1.0 × 10−8 M R6G on sample E: (A) without KCl solution and (B) with KCl solution.
Fig. 4. Structure of rhodamine 6G.
prominent and red shifted up to a maximum of 782 nm. This is accompanied by a substantial broadening of the SPR peak. Such systematic red shift is consistent with literature reports for gold and silver nanoshells, prepared by different procedures on silica microspheres [33]. This may be attributed to the reason that the Ag shell has more strong characteristic absorption in the UV–vis–NIR spectroscopy, and the Ag shell in the as-prepared Au/Ag nanostructure is thick enough to hide the characteristic SPR peak of the SiO2 @Au nanoshells. This can be another support for the formation of the Au/Ag bimetallic nanostructure. SiO2 core removal by HF also affects the peak position-a red shift that is observed relative to the core–shell (Au/Ag bimetallic hollow nanospheres) particles (986 nm vs. 782 nm). The red-shift of the SPR peak has a contribution from both Ag as well as Au plasmon absorptions. Another fact may also be responsible for the hollow cavity of the nanospheres, which is in accordance with the literature reported previously [34]. By combining with the change of SPR, it can be indicated that Ag is successfully grown on the surface of the SiO2 @Au nanoshells, in order to form an Au/Ag bimetallic nanostructure, finally forming an Au/Ag bimetallic hollow nanospheres structure. 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 ITO glass substrate), indicating that the hollow metals nanostructure is made up of metallic Au and Ag. By combining with EDX, it can be indicated that Ag is successfully grown on the surface of the SiO2 @Au nanoshells, forming an Au/Ag bimetallic nanostructure. To evaluate the effectiveness of the four above-mentioned kinds of Au/Ag nanostructured films for SERS applications, rhodamine 6G (R6G) was used as a touchstone because it is a dye molecule which often used in SERS studies because of its enormous intensity enhancement and adsorption onto silver particles. R6G is a cationic dye with strong absorption in the visible and a high fluorescence yield. The molecule consists of two chromophores, one dibenzopyrene chomophore (xanthene), and one carboxyphenyl group tilted by about 90◦ with respect to the xanthene ring (see Fig. 4). Therefore the -stems of the two chromophores of R6G are not conjugated [35]. Previous SERS studies suggested that the presence of chlo-
ride ions had an activation effect, which has been considered to create “active sites” for Raman enhancement [36]. A similar activation effect is also observed in the present study. We found that the SERS signal intensities were increased by about 7 times in the presence of 10 mM KCl (Fig. 5). The bands which show strong resonance enhancements correspond to C–C stretching in the xanthene plane are found at 1648, 1574, 1505, and 1362 cm−1 . Since all of these bands involve some characters of xanthene in-plane C–C stretching, they are expected to show resonance enhancements. The other bands at 1598 and 1311 cm−1 are assigned to C C stretching; 1186 cm−1 are assigned to aromatic C–H bending, respectively [37]. Fig. 6 compares the 514.5 nm excitation SERS spectra of 1.0 × 10−8 M R6G in the presence of 10 mM KCl obtained from four different substrates. From the four right bars that compare the plateau with the intensity of the strongest peak (aromatic C–C stretching at ca.1648 cm−1 ) in curves B–E, we find that the Au/Ag bimetallic hollow nanospheres structured films exhibit the highest SERS enhancement ability. It is shown that the SERS peaks intensity in Fig. 6(C) is an order of magnitude stronger than the corresponding ones in Fig. 6(B). The SERS intensity at 1648 cm−1 for the Au/Ag
Fig. 6. SERS spectra of 1.0 × 10−8 M R6G on different substrates. (B) Sample B; (C) sample C; (D) sample D; (E) sample E.
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nanoshells structured, Au/Ag bimetallic hollow nanospheres structured possess porous structure. The presence of these porous structures can increase the surface area of the substrates, which allows more R6G molecules to adsorb per unit surface of the substrates [45]. According to Tian et al. [26], a large SERS enhancement can be achieved at these crevice sites because of the enormous electromagnetic field. For instance, Xia et al. reported that an electromagnetic enhancement of 2 × 107 was present between two nanopsheres separated by 1.8 nm [46]. This redistribution can also yield superior SERS enhancement. After removing SiO2 spheres, samples E was obtained. Samples E appears porous structure and interconnected aggregate structures as indicated in Fig. 1(E), and the surface areas of samples E are increased further because of the removal of SiO2 spheres. The surface area ratio of sample E to sample D was roughly calculated as follows: Sh 4(r1 2 + r2 2 ) = =1+ Ss 4r1 2 Fig. 7. Comparison of SERS intensities between (E) sample E and (F) sample F, Ag hollow nanospheres structured film. The inset shows the SEM of Ag hollow nanospheres. The concentration of R6G was 1.0 × 10−8 M.
bimetallic hollow nanospheres structured film is about 2.3 times higher than that for the SiO2 @Au/Ag nanoshells structured film and about 6 times higher than that for the SiO2 @Au nanoshells structured film (Fig. 6(C–E)). More importantly, it is noted that the SERS performance of such Au/Ag bimetallic hollow nanospheres structured film has been stable for at least 4 months. Fig. 7 present SERS spectra of R6G molecules on sample E and an Ag hollow nanospheres structured film (sample F), respectively. The inset of Fig. 7 is the SEM of Ag hollow nanospheres. The average outer and inner diameters of the Ag hollow nanospheres are measured to be approximately 200 and 294 nm, the same as the Au/Ag bimetallic hollow nanospheres. However, the SERS intensity at 1648 cm−1 for the Au/Ag bimetallic hollow nanospheres structured film is about 1.4 times higher than that for the Ag hollow nanospheres structured film. This SERS enhancement may be related to several factors. First, extremely intense local electromagnetic fields generated in the gaps between adjacent Au nanoparticles can strongly enhance the Raman scattering of probe molecules located in the gaps between the closely spaced Au nanoparticles [37], so the SERS intensity from the SiO2 @Au nanoshells structured film is stronger than that observed on the Au nanoparticles structured film. Second, since the intrinsic activity of Ag is much higher than Au [38] and the surfaces of bimetals provide more possibilities for molecules to deposit on the boundaries between Ag and Au domains [39], the SERS intensity at 1648 cm−1 for the SiO2 @Au/Ag nanoshells structured film is about 2.6 times higher than that for the SiO2 @Au nanoshells structured film. Excited by an incident light, the collective surface plasmons are localized at these hollow nanostructured and interconnected nanostructured aggregates, leading to the formation of a local field in this region that is many orders of magnitude higher than the incident light [40]. The localized resonant plasmon modes can contribute to larger SERS enhancement [22,41–43]. The number of SERS active sites in samples D–E increase because of the presence of a large number of hollow nanostructured and interconnected nanoparticle junctions, and thus it is not surprising to observe the multifold increase in SERS intensity enhancement compared with that of the SiO2 @Au/Ag nanoshells structured film. In this case, nanoscale roughness is another important factor contributing to SERS enhancement because it plays an important role by providing pathways for the hot electrons to the probe molecules, which results in SERS enhancement by chemical effects [44]. Moreover, from the microstructure viewpoint, different from the SiO2 @Au/Ag
r 2 2
r
=1+
100 100 + 47
2
≈ 1.46
Sh and Ss are the surface areas of samples E and D, respectively. r1 and r2 are the radius of a SiO2 @Au/Ag nanoshells and a SiO2 nanoparticles, respectively. Our calculations indicates that the surface area of samples E increased by less than 1.46 times compared with that of sample D. Interestingly, as mentioned above, the SERS intensity at 1648 cm−1 for samples E is about 2.3 times higher than that of sample D. This result reveals that the higher SERS enhancement of sample E relative to that of sample D may not be explained only by the increase in surface area, but also the surfaces of bimetals provide more possibilities for molecules to deposit on the boundaries between Ag and Au domains (Fig. 7) [39] because some Au atoms were came out from Ag shell along with SiO2 core removal. 4. Conclusions In conclusion, Au/Ag bimetallic hollow nanospheres with fine monodispersity and controllable size and shape were successfully synthesized using a modified SiO2 nanoparticles-templating method at room temperature. The as-prepared Au/Ag bimetallic hollow nanospheres structured films are used for the first time as SERS substrates and exhibit good SERS enhancement ability, excellent reproducibility, and stability, which may find practical application for routine SERS analysis. It was found that the Au/Ag bimetallic hollow nanostructured aggregate, interconnected nanostructured aggregate and nanoscale roughness are important factors which responsible for this large SERS enhancement ability. Acknowledgments This 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] H.P. Liang, L.J. Wan, C.L. Bai, L. Jiang, J. Phys. Chem. B 109 (2005) 7795. [3] J.H. Lee, A. Mahmoud, V. Sitterle, J. Sitterle, J.C. Meredith, J. Am. Chem. Soc. 131 (2009) 5048. [4] 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. [5] X.M. Feng, C.J. Mao, G. Yang, W.H. Hou, J.J. Zhu, Langmuir 22 (2006) 4384. [6] G.V.P. Kumar, S. Shruthi, B. Vibha, B.A.A. Reddy, T.K. Kundu, C. Narayana, J. Phys. Chem. C 111 (2007) 4388. [7] R. Güzel, Z. Üstünda˘g, H. Eks¸i, S. Keskin, B. Taner, Z.G. Durgun, A.I. Turan, A.O. Solak, J. Colloid Interface Sci. 351 (2010) 35. [8] F.Y. Cheng, C.T. Chen, C.S. Ye, Nanotechnology 20 (2009) 425104. [9] Y.T. Xia, W.S. Lu, L. Jiang, Nanotechnology 21 (2010) 85501.
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