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Controlled Fabrication of Nanopillar Arrays as Active Substrates for Surface-Enhanced Raman Spectroscopy Chuanmin Ruan,†,‡ Gyula Eres,§ Wei Wang,‡ Zhenyu Zhang,§,| and Baohua Gu*,‡ Oak Ridge Institute for Science and Education, Oak Ridge, Tennessee 37831, EnVironmental Sciences and Materials Science Technology DiVisions, Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee 37831, and Department of Physics and Astronomy, The UniVersity of Tennessee, KnoxVille, Tennessee 37996 ReceiVed December 15, 2006. In Final Form: February 19, 2007 Highly ordered gold nanopillar arrays were fabricated using anodized aluminum oxide (AAO) templates. Nanopillars with a dimension of 110 ( 15 nm in vertical height and 75 ( 10 nm in base diameter were formed with a density of 150 µm-2. The ordered nanopillar arrays give reproducible surface-enhanced Raman scattering (SERS) at a detection limit of 10-8 M using thionine as probing molecules. The enhancement by the Au nanopillar arrays was comparable with or better than that of dispersed gold nanoparticle SERS substrates. This work demonstrates a new technique for producing highly ordered and reproducible SERS substrates potentially applicable for chemical and biological assay.
Introduction Over the past decade, surface-enhanced Raman spectroscopy (SERS) has evolved into a powerful technique for both qualitative and quantitative chemical and biological analyses.1-3 As a vibrational spectroscopic technique, SERS provides characteristic fingerprints of molecular structure, because Raman bands and their relative intensities are determined by the molecular geometry of the chemical bonds. Ensemble SERS is 106-108 times more sensitive than conventional Raman and allows detection of molecules at nanomolar concentration levels. However, one main challenge of using SERS as a routine analytical and diagnostic tool is the fabrication of homogeneous, reproducible, and stable SERS substrates. To address these problems, extensive research has been focused on preparing reproducible SERS substrates in recent years.4-8 For example, self-assembled monolayer technology has been used to fabricate Au or Ag nanoparticle films on silicon or glass surfaces,9 although several shortcomings were also noted. Au or Ag nanoparticles are usually not uniform and tend to form heterogeneous aggregates upon drying, due to strong particle-particle interactions. It is thus difficult to precisely align these nanoparticles to form an ordered and uniform monolayer or multilayer on a large scale by the self-assembly technique. Lu * Corresponding author. Phone: (865)-574-7286, E-mail:
[email protected]. † Oak Ridge Institute for Science and Education. ‡ Environmental Sciences Division, ORNL. § Materials Science Technology Division, ORNL. | The University of Tennessee. (1) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Chem. ReV. 1999, 99, 2957-2976. (2) Kneipp, K.; Kneipp, H.; Kneipp, J. Acc. Chem. Res. 2006, 39, 443-450. (3) Ruan, C. M.; Wang, W.; Gu, B. H. Anal. Chem. 2006, 78, 3379-3384. (4) Freeman, R. G.; Graber, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. C.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629-632. (5) Stuart, D. A.; Yonzon, C. R.; Zhang, X.; Lyandres, O.; Shah, N. C.; Glucksherg, M. R.; Walsh, J. T.; Van Duyne, R. P. Anal. Chem. 2005, 77, 40134019. (6) Lee, S. J.; Morrill, A. R.; Moskovits, M. J. Am. Chem. Soc. 2006, 128, 2200-2201. (7) Wang, H. H.; Liu, C. Y.; Wu, S. B.; Liu, N. W.; Peng, C. Y.; Chan, T. H.; Hsu, C. F.; Wang, J. K.; Wang, Y. L. AdV. Mater. 2006, 18, 491-495. (8) Perney, N. M. B.; Baumberg, J. J.; Zoorob, M. E.; Charlton, M. D. B.; Mahnkopf, S.; Netti, C. M. Opt. Exp. 2006, 14, 847-857. (9) Orendorff, C. J.; Gole, A.; Sau, T. K.; Murphy, C. J. Anal. Chem. 2005, 77, 3261-3266.
et al.10 reported an improved approach for fabricating high-density, uniform silver nanoparticle films by self-assembling hydrophobic Ag nanoparticles at the air/water interface, followed by transferring the particle monolayer onto a solid substrate. However, the technique was found to be only suitable for assembling metal particles with sizes less than 20 nm in diameter. Moreover, a sorbed organic surfactant layer used in the phase transfer process could potentially make the SERS film ineffective or complicate the measurements of target molecules due to background scattering. Recently, nanosphere lithography has also been exploited to fabricate highly ordered arrays with improved uniformity.8,12-15 The technique has shown some success, but improvement of SERS sensitivity of these ordered array substrates is still marginal. Here, we report a technique for fabricating highly ordered gold nanopillar arrays by vacuum deposition of gold onto anodic aluminum oxide (AAO) templates. We demonstrate the effectiveness and reproducibility of such nanopillar arrays as a SERS substrate for detecting thionine dye at ultralow concentrations. Because of its highly ordered nanoporous structure, AAO has been used as a robust template for fabricating a variety of nanostructured materials such as metals,16,17 semiconductors,18 carbon nanotubes,19,20 and polymers.21 Additionally, the pore size and interpore distances can be readily tunned during the anodization. Both electroless and electrochemical deposition have been used to fill template pores with metals to create ordered (10) Lu, Y.; Liu, G. L.; Lee, L. P. Nano Lett. 2005, 5, 5-9. (11) Krug, J. T.; Wang, G. D.; Emory, S. R.; Nie, S. J. Am. Chem. Soc. 1999, 121, 9208-9214. (12) Ormonde, A. D.; Hicks, E. C. M.; Castillo, J.; Van Duyne, R. P. Langmuir, 2004, 20, 6927-6931. (13) Lyandres, O.; Shah, N. C.; Yonzon, C. R.; Walsh, J. T., Jr.; Glucksberg, M. R.; Van Duyne, R. P. Anal. Chem. 2005, 77, 6134-6139. (14) Liu, G. L.; Lee, L. P. Appl. Phys. Lett. 2005, 87, 074101-3. (15) Liu, L. H.; Eychmu¨ller, A.; Kobayashi, A.; Hirano, Y.; Yoshida, K.; Kikkawa, Y.; Tawa, K.; Ozaki, Y. Langmuir 2006, 22, 2605-2609. (16) Sander, M. S.; Gao, H. J. J. Am. Chem. Soc. 2005, 127, 12158-12159. (17) Sander, M. S.; Prieto, A. L.; Gronsky, R.; Sands, T.; Stacy, A. M. AdV. Mater. 2002, 14, 665-667. (18) Chik, H.; Xu, J. M. Mater. Sci. Eng., R: 2004, 43, 103-138. (19) Li, J.; Papadopoulos, C.; Xu, J. M.; Moskovits, M. Appl. Phys. Lett. 1999, 75, 367-369. (20) Davydov, D. N.; Sattari, P. A.; AlMawlawi, D.; Osika, A.; Haslett, T. L.; Moskovits, M. J. Appl. Phys. 1999, 86, 3983-3987. (21) Yu, B. Z.; Gao, Y.; Li, H. L. J. Appl. Polym. Sci. 2004, 91, 425-430.
10.1021/la0636356 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/11/2007
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nanowire or nanopillar arrays.22,23 Furthermore, thermal vacuum evaporation onto AAO templates has been reported to create even better and more homogeneous nanopillar arrays than the electrochemical deposition technique.24 For example, Masuda and co-workers25,26 obtained an ordered gold nanodot array by using vacuum evaporation techniques, whereas Barreca et al.27 produced nanotube arrays by RF-sputtering of gold onto AAO films at low temperatures. In this study, we report an improved technique for fabricating ordered gold nanopillar arrays on a large surface area by vacuum evaporation and demonstrate the application of these arrays as a sensitive SERS substrate. Experimental Procedure Annealed, high-purity aluminum foils (99.99%) from Alfra Aesar (Ward Hill, MA) were used for anodization. Oxalic acid dihydrate and sulfuric acid (98%) were purchased from EMD Chemicals, Inc. (Gibbstown, NJ). Phosphoric acid (85.6%) was purchased from J. T. Baker (Phillipsburg, NJ). Thionine, methanol, potassium chromate, and acetone were purchased from Sigma (St. Louis, MO). Deionized water (18.2 MΩ·cm-1) was used throughout the experiments. The aluminum foil was first degreased with acetone under an ultrasonic bath for 10 min. It was electropolished in a mixed solution of (20% H2SO4 + 80% H3PO3 + 2% K2CrO4) under a constant voltage of 9 V and a temperature of 90-100 °C for 10 min. During this process, the aluminum was used as the anode, and a platinum plate as the cathode. To obtain ordered nanopore arrays, we used a two-step anodizing process.25,28 The foil was anodized first in 0.3 M oxalic acid at 33 V at 0-5 °C for 14 h. It was then immersed in a mixed solution of 5 wt % phosphoric acid and 1.8 wt % chromic acid (1:1 in volume) at 60 °C for 3 h to remove the alumina layer. In the second step, the sample was again anodized for 2 h under the same conditions and then immersed in a 5 wt % phosphoric acid solution at 30 °C for 30 min for pore-widening.28 The anodized alumina film (with Al support underneath) was cleaned with distilled water and methanol, then dried under a stream of nitrogen gas prior to vacuum deposition of a gold layer. The deposition of gold was performed using electron beam evaporation in a vacuum of 80 nm could not be fabricated if the deposition rate was too high to quickly close the pores.26 Obviously, the nanopillar height can also be affected by the pore size and depth of the AAO template. The cone shape of the nanopillars was a result of the vapor deposition process, during which the template pore opening became progressively coated with gold. This process reduced the diameter of the holes until the holes were filled with gold and the top layer of the template completely covered by a continuous Au coating. This technique thus offers a convenient way of fabricating highly ordered nanostructures over a large surface area that can be used as active SERS substrates. No nonhomogeneous fillings were observed over the entire AAO surface, as illustrated in Figure 2A. The gold nanopillar arrays were subsequently examined using thionine as a probing molecule for SERS. Thionine is one of the
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Figure 2. Comparison of SEM images of anodized aluminum oxide (AAO) template and the resulting gold nanopillar arrays at low (A,C) and high resolution (B,D), respectively.
Figure 3. SERS spectra of thionine at concentrations of (a) 10-8, (b) 10-7, (c) 10-6, and (d) 10-5 M using ordered gold nanopillars as a SERS substrate. The spectra were collected using laser excitation at 785 nm (∼1.5 mW at the exit of a 50× microscope objective) without baseline corrections.
cationic phenothiazine dyes and has been shown previously to be SERS-active on roughened gold electrodes.29,30 Without SERS, poorly resolved Raman spectra of thionine could be obtained only at a relatively high concentration (>10-3 M) because of the strong fluorescence of thionine and the low sensitivity of conventional Raman spectroscopy. Figure 3 shows SERS spectra of thionine with a concentration varying from 10-5 to 10-8 M on the nanopillar substrate. The observed Raman bands are in good agreement with those in the ensemble Raman spectrum of thionine on roughened Au electrodes.29 Typical Raman band (29) Hutchinson, K.; Hester, R. E. J. Chem. Soc., Faraday Trans. 1 1984, 80, 2053-2071. (30) Virdee, H. R.; Hester, R. E. Laser Chem. 1988, 9, 401-416.
assignments for thionine of 10-5 M are as follows: The band at 1617 cm-1 with a shoulder at 1595 cm-1 is assigned to the ν(C-C) ring stretching vibration.30 The band at 1512 cm-1 is attributed to asymmetric skeletal deformation of C-C-C possibly accompanying C-C stretching.31 The strong asymmetric stretching vibration of C-N appears at 1413 cm-1.30 The band at 1321 cm-1 is due to Ar-N stretching.32 The bands at 1123 cm-1 and 1032 cm-1 are out-of-plane and in-plane bending of the C-H bond,30,33 and that at 613 cm-1 is assigned to skeletal deformation resulting from C-S-C. The peak at 480 cm-1 results from the C-N-C skeletal deformation mode. The intensity of most Raman bands decreases with decreasing concentration of thionine in the range from 10-5 to 10-8 M (Figure 3). However, at low concentrations, the spectra become poorly resolved. For example, a shoulder at ∼1595 cm-1 is observable at the thionine concentrations of 10-5 and 10-6 M but disappears at the concentration below 10-7 M (Figure 3A,B). We note that, when compared with the SERS spectra obtained by using dispersed Au nanoparticles,34 the use of the ordered Au nanopillar substrates gives much better resolved spectra. As shown in Figure 4, even at a concentration of 10-8 M, an intense spectrum of thionine was obtained with well-resolved Raman bands by using the Au nanopillar substrate. In contrast, a poorly resolved SERS spectrum (Figure 4B) was obtained by using dispersed colloidal Au nanoparticles as a SERS substrate.34 These Au colloids were approximately 50-60 nm in diameter, and previous studies have shown that dispersed Au nanoparticles with a size less than 20 nm or greater than 80 nm could not give SERS signal at concentrations below 10-7 M.34 Nonetheless, the observed band positions and assignments on both substrates are in general agreement with previously published results,30 except for the vibrational bands at 1607 cm-1 and 1430 cm-2. These (31) Xu, W.; Aydin, M.; Zakia, S.; Akins, D. L. J. Phys. Chem. B 2004, 108, 5588-5593. (32) Sa´ez, E. I.; Corn, R. M. Electrochim. Acta 1993, 8, 1619-1625. (33) Palafox, M. A.; Gil, M.; Nunez, J. L.; Tardajos, G. Int. J. Quantum Chem. 2002, 89, 147-171. (34) Ruan, C. M.; Wang, W.; Gu, B. J. Raman Spectrosc. 2007, 38, in press.
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Figure 5. Good reproducibility as demonstrated by the overlapping spectra achieved by using an ordered Au nanopillar substrate. Peak intensities varied
