pubs.acs.org/Langmuir © 2010 American Chemical Society
High Performance Surface-Enhanced Raman Scattering Substrate Combining Low Dimensional and Hierarchical Nanostructures Hui Wu, Dandan Lin, and Wei Pan* Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Received January 6, 2010. Revised Manuscript Received April 12, 2010 We demonstrate that hierarchical nanostructured metal sub-microtubes with evenly distributed nanoscale pores on sidewalls can be synthesized though electrodeposition of metals on electrospun fiber templates and subsequent wet etching. Due to the hierarchical nanostructure and uniform “hot spots” on sidewalls, these porous sub-microtubes exhibit higher surface-enhanced Raman scattering (SERS) activities than both smooth metal sub-microtubes and nanoporous thin films. The synthetic process is simple, inexpensive, and effective, and therefore is a suitable methodology for large-scale production of reliable and reproducible SERS substrates.
1. Introduction Surface-enhanced Raman scattering (SERS) integrates high levels of sensitivity with spectroscopic precision and thus has tremendous potential for chemical and biomolecular sensing.1,2 SERS substrates are usually constructed with metallic materials with nanoarchitectures.3-7 Generally speaking, there are two different ways to get a nanostructured metallic substrate with SERS activity: top-down and bottom-up. Typical top-down methods include lithography7 and electrochemical roughening of electrodes;8 and electrodeposition of metals9 as well as selforganization of nanoparticles10-14 and solution-phase chemical reactions15-17 belong to the bottom-up category. More recently, some research groups demonstrated that one-dimensional (1-D) metallic nanostructures can serve as high-performance SERS substrates;16-19 meanwhile, some other outstanding work points out that hierarchical structures of nanometals can further enhance *To whom correspondence should be addressed. E-mail: panw@ mail.tsinghua.edu.cn.
(1) Cao, Y. C.; Jin, R. C.; Mirkin, A. Science 2002, 297, 1536. (2) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Chem. Rev. 1999, 99, 2957. (3) Tripp, R. A.; Dluhy, R. A.; Zhao, Y. Nano Today 2008, 3, 3. (4) Schierhorn, M.; Lee, S. J.; Boettcher, S. W.; Stucky, G. D.; Moskovits, M. Adv. Mater. 2006, 18, 2829. (5) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293. (6) Tarabara, V. V.; Nabiev, I. R.; Feofanov, A. V. Langmuir 1998, 14, 1092– 1098. (7) Abu-Hatab, N. A.; Oran, J. M.; Sepaniak, M. J. ACS Nano 2008, 2, 377. (8) Mrozek, M. F.; Wasileski, S. A.; Weaver, M. J. J. Am. Chem. Soc. 2001, 123, 12817. (9) Alsmeyer, Y. W.; McCreery, R. L. Anal. Chem. 1991, 63, 1289. (10) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (11) Schwartzberg, A.; Olson, T. Y.; Zhang, J. Z.; Huser, T.; Talley, C. Anal. Chem. 2006, 78, 4732. (12) Li, X-l.; Xu, W.-Q.; Zhang, J.-H.; Jia, H.-Y.; Yang, B.; Zhao, B.; Li, B-F; Ozaki, Y. Langmuir 2004, 20, 1298–1304. (13) Sun, Y. G.; Wiederrecht, G. P. Small 2007, 3, 1964. (14) Hu, J.-W.; Zhao, B.; Xu, W.-Q.; Fan, Y.-G.; Li, B.-F.; Ozaki, Y. Langmuir 2002, 18, 6839–6844. (15) Schwartzberg, A. M.; Olson, T. Y.; Talley, C.; Zhang, J. Z. J. Phys. Chem. B 2006, 110, 19935. (16) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y.; Yang, P. Nano Lett. 2003, 3, 1229. (17) Lee, S. J.; Morrill, A. R.; Moskovits, M. J. Am. Chem. Soc. 2006, 128, 2200. (18) Wang, T.; Hu, X. G.; Dong, S. J. Small 2008, 4, 781. (19) Wu, Y.; Livneh, T.; Zhang, Y. X.; Cheng, G.; Wang, J.; Tang, J.; Moskovits, M.; Stucky, G. D. Nano Lett. 2004, 4, 2337. (20) Song, W.; Li, W.; Cheng, Y.; Jia, H.; Zhao, G.; Zhou, Y.; Yang, B.; Xu, W.; Tian, W.; Zhao, B. J. Raman Spectrosc. 2006, 37, 755.
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SERS activity.20 However, there has hardly been any effort reported on integrating low-dimensional characteristics with hierarchical structures of metallic substrates, which might be a promising way to create materials with outstanding SERS activities. In this Letter, we designed and fabricated a metallic substrate with a novel 1-D hierarchical sub-micro/nanoarchitecture which can be described as “sub-microtubes with nanopores on sidewalls”. Our synthetic method involves both top-down and bottom-up strategies: first, we fabricated metal sub-microtubes by electrodeposition (bottom-up) and then obtained secondary hierarchical structures on tube walls by chemical etching (top-down). The synthesized hierarchical metallic sub-microtubes were proved to be highperformance SERS substrates. In recent years, metallic sub-micro/nanotubes have captured the imagination of material scientists for their well-defined structures and structurally related properties. Considering their high specific surface areas and low dimensional configurations, metal sub-microtubes are expected to have high SERS activities. However, there have been limited methods for synthesis of metal sub-microtubes. Up to now, metal sub-microtubes are mainly produced by a template method, which involves deposition of desired material inside a porous membrane with uniform cylindrical pores.21 Nevertheless, disadvantages of this prevalent method, such as high cost, low efficiency, complex preparation procedure of the templates, and difficulty in removing them, have restricted the SERS application. Electrospun fibers could be better templates for processing SERS active metallic sub-microtubes, since they can be easily produced and removed.22-26 Moreover, electrospun fibers are extremely long, implying that the resulting sub-microtubes easily form uniform and continuous net webs in large scale, which benefits the fabrication of stable and homogeneous SERS substrate. Considering this, we developed a strategy to fabricate metallic sub-microtubes by electrodepositing on electrospun nanofiber templates. (21) Zhang, L. S.; Zhang, P. X.; Fang, Y. J. Colloid Interface Sci. 2007, 311, 502. (22) Li, D.; Xia, Y. Adv. Mater. 2004, 16, 1151. (23) Bognitzki, M.; Hou, H.; Ishaque, M.; Frese, T.; Hellwig, M.; Schwarte, C.; Schaper, A.; Wendorff, J. H.; Greiner, A. Adv. Mater. 2000, 12, 637. (24) Ca~namares, M. V.; Chenal, C.; Birke, R. L.; Lombardi, J. R. J. Phys. Chem. C 2008, 112, 20295–20300. (25) Kudelski, A. Chem. Phys. Lett. 2005, 414, 271–275. (26) Wu, H.; Zhang, R.; Liu, X.; Lin, D.; Pan, W. Chem. Mater. 2007, 19, 3506.
Published on Web 04/20/2010
DOI: 10.1021/la1000649
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Figure 1. (a) Schematic illustration of the experimental process used to fabricate metal sub-microtubes. (b) SEM image of the synthesized Cu sub-microtubes. Inset: a clear picture of an open end. (c) TEM image of a single Cu sub-microtube. Inset: SAED pattern of the tube.
2. Experimental Section The synthetic method is illustrated in Figure 1a. First, polymer fiber templates were prepared by electrospinning. Second, a thin layer of gold serving as a working electrode was sputtered on the fibers. Then the fibers were coated with metal by a conventional electrodeposition process. Finally, the products were washed in a certain organic solvent to remove the polymer core; metal hollow tubes were left as solid after dissolving the templates. Electrospinning. The solution used for electrospinning was made by dissolving polyvinyl butyral (PVB) powders in ethanol with a concentration of 6 wt %. The viscosity was 0.2 Pa 3 S. In a typical electrospinning process, a small piece of stainless steel mesh (20 20 mm2) was employed as the fiber collector. The applied voltage was 20 kV, while the distance between the spinning tip and collector was 20 cm. Within a short collection time of about 2 min, suspended PVB fibers as a nonwoven mat were deposited on the mesh. Fabrication of Metal Sub-Microtubes. The metal mesh electrode with adherent fibers was sputtered with a thin layer of gold and then immersed into an organic-free copper electroplating bath containing 0.07 M CuSO4 with a pH of 1.9. The fibers were coated by a Cu layer deposited at a constant cell voltage of 1.0 V. After deposition, PVB/Cu coaxial fibers could be obtained. The products were rinsed with distilled water and then were dried in air. To remove the templates, the coated fibers were immersed into excess ethanol at room temperature for 12 h, and the samples were washed several times to ensure complete core removal. Complete removal of the PVB template was also confirmed by thermoanalysis (TG/DSC, STA 409, German) in the temperature range of 25-600 °C (see the Supporting Information). No breakdown occurred in the metal tube after removal of the template (Figure 1b). Fabrication of Porous Sub-Microtubes. Cu-Ni alloys were first electrodeposited onto PVB fiber templates from a solution containing 0.02 M CuSO4, 0.25 M NiSO4, and 0.8 M amino acetic acid. Electrodeposition was conducted at a constant cell voltage of 0.8 V. The coated fibers were then immersed in alcohol to remove 6866 DOI: 10.1021/la1000649
Figure 2. Raman spectra of 1.0 10-6 CV adsorbed on different substrates.
PVB templates, and Cu-Ni alloy sub-microtubes were formed as a result. After been washed in dilute sulfuric acid (0.1M) at room temperature for 12 h, Ni was selectively etched from the alloy submicrotubes; and porous Cu sub-microtubes were left as solid. The products were then washed in deionized water for several times and dried in air for further characterization. Characterization. The morphology of the tubes was examined by transmission electron microscopy (TEM, JEOL JEM-2010) and scanning electron microscopy (SEM, JEOL JSM-6301 F). For SERS effect characterization, the samples were prepared and kept in air for 3 days before testing. In order to detect the Raman spectrum from single molecules, it was originally necessary to take advantage of the additional boost in intensity provided by a molecular resonance, and therefore, various dye molecules such as rhodamine 6G (R6G) and crystal violet (CV) were employed.25 In the Langmuir 2010, 26(10), 6865–6868
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Figure 3. (a) TEM image of a single Cu-Ni alloy microdube. (b-d) TEM images of Cu sub-microtubes with porous sidewalls in different magnifications.
present research, we selected CV to examine the SERS activity. During the experiments, the sub-microtubes were dipped into CV solution with a certain concentration for 1 h and then rinsed with distilled water and dried in air for 2 h before testing. The Raman scattering spectra were examined and recorded using a microscopic confocal Raman spectrometer (Renishaw, RM2000) with a 633 nm air-cooled argon ion laser as excitation source; the laser power was 4.7 mW.
3. Results and Discussion Figure 1b shows SEM images of the synthesized Cu submicrotubes. As observed, sub-microtubes with smooth surfaces were obtained. TEM images (Figure 1c) confirm the hollow structure of the products. As observed, the tubes were smooth, with inner diameters in the range of 220-270 nm, and the wall thickness was about 20 nm and was remarkably uniform along the length of a single tube. The upper left inset of Figure 1c presents the selected-area electron diffraction (SAED) pattern of the Cu sub-microtubes, which shows diffuse rings, indicating that the Cu sub-microtubes are polycrystalline. To investigate the SERS sensitivity of the copper sub-microtubes, Raman spectra of 1.010-6 M CV adsorbed on the surface of copper sub-microtubes were measured. The prepared smooth Cu microtube showed some SERS activity as shown in Figure 2b, while the CV molecules on the surface of the flat electrodeposited Cu plate did not generate detectable Raman signals (Figure 2a) when they were excited under the same conditions. The SERS activity of synthesized Cu sub-microtubes may be attributed to their nanoscale curve morphology on the sub-microtubes.15-17 The trace gold element inside the tubes is not responsible for the SERS activity (see the Supporting Information). However, the enhancement of the Raman signal is not so strong. Generally Langmuir 2010, 26(10), 6865–6868
speaking, for SERS phenomena, the metal substrate features responsible for its operation must be small with respect to the wavelength of the exciting light. This normally means that the SERSactive systems must ideally possess nanostructures in the 5-100 nm range. Although the thickness of the tube wall (∼20 nm) is in that range, the diameter of the sub-microtube is larger than 100 nm. To generate greater SERS signals, we designed a hierarchical porous structure on the smooth sidewall. In our experiment, Cu-Ni alloy sub-microtubes with smooth sidewalls (Figure 3a) were first prepared using a similar process. A top-down strategy, in which the Ni component was selectively removed from the alloy submicrotubes by etching in dilute sulfuric acid, was employed to make pores in the tube sidewalls. Figure 3b shows is a typical TEM image of the etched samples, showing continuous submicrotubes with rough sidewalls, on the surfaces of which wellproportioned pores distributed uniformly. TEM images with higher magnifications are given in Figure 3c and d, which displays a single etched tube with an open end. As observed, the structure of the tube sidewall can also be described as the aggregation of many tiny nanoparticles with a size of ∼5 nm; this novel architecture is beneficial to SERS activity as will be discussed later. The chemical composition of the porous tubes was studied by energy dispersive spectroscopy (EDS, taken from TEM), and it was determined that only copper was present in the sample; no Ni can be detected in the tubes, indicating that the etching process is complete. The unique surface morphologies are expected to result in high SERS activities of the synthesized Cu sub-microtubes. Results showed that the absolute Raman intensity of 1.0 10-6 M CV absorbed on hierarchical Cu sub-microtubes is more than 10 times higher than that for smooth Cu sub-microtubes (Figure 2d). Moving the laser beam along all tubes always generates Raman spectra DOI: 10.1021/la1000649
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Figure 4. SERS spectra of different concentrations of CV on hierarchical Cu sub-microtubes: (a) 1.0 10-7 and (b) 1.0 10-8.
with similar intensities, indicating the uniformity of the Cu submicrotubes. These results confirm that the synthesized Cu submicrotubes indeed exhibit an excellent capability to enhance Raman signals of molecules adsorbed on their surfaces. A SERS signal of CV molecules can be observed for concentrations as dilute as 1.0 10-8 M, showing the great sensitivity of this SERS technique (Figure 4). The high SERS activity is possibly beneficial from the novel aggregated-nanoparticle structure. Some former study demonstrated that aggregates of nanoparticles induce strong SERS enhancement due to the large electromagnetic (EM) fields at the junctions of the nanoparticles.27-29 However, aggregated metal particles tend to coagulate, making them unstable, resulting in poor reproducibility of SERS signals. Luckily, our synthesized hierarchical Cu sub-microtubes have the advantage of interparticle structure, while conglomerations of nanoparticles are avoided. As mentioned previously, the porous sub-microtubes can be considered as an aggregation of Cu nanoparticles; in such ensembles of nanoparticles, it is theoretically shown that the highly localized plasmon modes could be generated at the gaps between the nanoparticles. These sites are often referred to as “hot spots”.30-32 Because these hierarchical sub-microtubes have a close internanoparticle distance (0-3 nm as shown in Figure 3d), these metallic layers are expected to show larger local EM field enhancement mainly at the nanopores between the nanoparticles. It is worth mentioning that considering the SERS signals are highly reproducible along the tubes, it is strongly suggested that the so-called “hot spots” are evenly distributed over the sub-microtubes. Besides “hot spots”, the 1-D configuration of the hierarchal tubes is another important cause of SERS enhancement.15-17 As a (27) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (28) Schuck, P. J.; Fromm, D. P.; Sundaramurthy, A.; Kino, G. S.; Moerner, W. E. Phys. Rev. Lett. 2005, 94, 017402. (29) Aravind, P. K.; Nitzan, A.; Metiu, H. Surf. Sci. 1981, 110, 189. (30) Wang, H.; Levin, C. S.; Halas, N. J. J. Am. Chem. Soc. 2005, 127, 14992. (31) Tsai, D. P.; Kovacs, J.; Wang, Z.; Moskovits, M.; Shalaev, V. M.; Suh, J. S.; Botet, R. Phys. Rev. Lett. 1994, 72, 4149. (32) Zhang, P.; Haslett, T. L.; Douketis, C.; Moskovits, M. Phys. Rev. B 1998, 57, 15513.
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comparison, we fabricated flat Cu-Ni alloy films on conductive glass using a similar electrodeposition process. After chemical etching, Cu films with highly roughened surfaces can be fabricated (see supporting information; Figure 3 for SEM image). Figure 2c displays the Raman spectrum of CV molecules with a same concentration (1.0 10-6 M) on porous Cu planar film. It can be seen that the SERS signal for porous Cu sub-microtubes is about 10 times higher than that of planar porous films. Obviously, the Raman excitation beam covered more surface area on the planar porous film than on the sub-microtubes since they have a very similar surface morphology, so sub-microtubes outperforming the planar nanoporous films is due trivially to a surface area increase. This experimental result indicates that the 1-D geometry is an important factor for SERS activity. However, it is difficult to determine the enhancement factor for SERS of the porous Cu sub-microtubes due to the unresolved intensity of the Raman signal of the CV molecules on a bare Cu plate. It is worth mentioning that in this paper we chose Cu as the example SERS substrate, since Cu is cheap and easy for fabrication. However, this preparation method toward porous microdube is flexible and not limited to copper. For example, we believe porous Ag sub-microtubes can be fabricated from electrospun nanofibertemplated Ag/Au alloy sub-microtubes using an etching process described in refs 33 and 34.
4. Conclusions In conclusion, we prepared anisotropic and hierarchical Cu sub-microtubes with evenly distributed nanoscale pores on sidewalls by electrodeposition on polymer nanofibers templates followed with chemical etching. Benefiting from high surface area and evenly distributed nanopores, the synthesized hierarchical Cu sub-microtubes have high SERS activities. The concentration study with CV revealed that the minimum concentration which can be detected on these hierarchical sub-microtubes was 1.0 10-8 M. This synthetic process is simple, inexpensive, and effective, and therefore is a suitable methodology for large-scale production of reliable and reproducible SERS substrates. The novel process for preparation of metallic sub-micrometer or sub-microtubes, and porous metallic sub-micrometer or sub-microtubes can be applied as a potential technique for microfluids, microreactors, microfiltration, and also some medical uses. Acknowledgment. This study was supported by the National Natural Science Foundation of China (Grant No. 50872063). Supporting Information Available: TG-DSC curves of fiber samples before and after removal of template; EDS spectra (from TEM) of Cu-Ni alloy and porous Cu submicrotubes. This material is available free of charge via the Internet at http://pubs.acs.org. (33) Jia, F. L.; Yu, C. F.; Deng, K. J.; Zhang, L. Z. J. Phys. Chem. C 2007, 111, 8424. (34) Dixon, M. C.; Daniel, T. A.; Hieda, M.; Smilgies, D. M.; Chan, M. H. W.; Allara, D. L. Langmuir 2007, 23, 241.
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