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Hierarchical Porous Plasmonic Metamaterials for Reproducible Ultrasensitive Surface-Enhanced Raman Spectroscopy Xinyi Zhang, Yuanhui Zheng, Xin Liu, Wei Lu, Jiyan Dai, Dang Yuan Lei,* and Douglas R. MacFarlane* Since the first report of the detection of a single molecule,[1,2] surface-enhanced Raman spectroscopy (SERS) has emerged as one of the most versatile tools for sensing and imaging chemical and biological analytes.[3–12] SERS-based molecular detection can be achieved by using metal (mostly silver and gold) nanostructures as plasmonic antennas to amplify the Raman signals. A unique feature of these metal nanostructures is their ability to support localized surface plasmons (LSPs), i.e., light-driven coherent oscillations of conduction electrons, which can generate an enormous electromagnetic (EM) field enhancement in the close vicinity of the metal surface. The EM field enhancement is particularly strong at nanoscopically sharp corners or tips,[2,5,6,13–15] interparticle gaps,[1,7–12] and nanopores,[16,17] typically referred to as “hot” spots. In general, it is believed that the EM field-induced SERS enhancement is proportional to the fourth power of the ratio of the localized EM field at the location of the analyte molecule to the incident excitation field.[18] The SERS enhancement factor has been often observed on the order of 104–106 and can be as high as 108–1014,[19] which allows the technique to be sensitive enough to detect single molecules.[20–22] Although extremely high SERS sensitivity is possible for some existing nanostructures, their practical application is frequently limited by their structural instability and/or poor reproducibility of the enhanced signals. For example, wide SERS enhancement distributions have been reported ranging from 2 × 107 to 2 × 109 for polymer-coated silver nanoparticle junctions[23] and from 2.8 × 104 to >4.1 × 1010 for a silver film-over-nanosphere (SFON) substrate.[24] The “hottest” SERS-active sites (enhancement factor >108) for the SFON substrate accounted for less than 0.1% of the total “hot” spots, but contributed 47% of the overall SERS intensity.[24] Dr. X. Zhang, Prof. D. R. MacFarlane School of Chemistry Monash University Clayton, VIC 3800, Australia E-mail: [email protected] Dr. Y. Zheng School of Chemistry The University of New South Wales Sydney, NSW 2052, Australia Dr. X. Liu, Dr. W. Lu, Prof. J. Dai, Prof. D. Y. Lei Department of Applied Physics The Hong Kong Polytechnic University Hong Kong, China E-mail: [email protected]
DOI: 10.1002/adma.201404107
Adv. Mater. 2014, DOI: 10.1002/adma.201404107
To date, periodic nanohole arrays with controllable size, shape, and spacing in metal films[6,24] and their complementary counterparts, ordered nanoparticle arrays,[10,13,23–25] have been investigated as one class of promising SERS substrates with excellent structural reproducibility and pronounced signal enhancement. While the enhanced Raman scattering by the nanohole arrays is mainly attributed to excitations of propagating surface plasmons via grating coupling,[6,24] the nanoparticle arrays share the similar enhancement mechanism as individual nanostructures with excitations of LSPs as discussed above. In addition, dealloyed nanoporous metals have also been exploited as attractive Raman-active structures because of their large surface area and three-dimensional (3D) bicontinuous porous configuration.[26–31] Such unique features not only allow for excitations of LSPs but also provide a large number of molecular binding sites. However, the overall SERS enhancement observed from these substrates are usually either poor in reproducibility or insufficient for single-molecule detection due to the limitation of control over the structural parameters such as pore geometry and order. The aforementioned three types of SERS substrates, including individual nanoparticles with sharp geometric features, periodic nanohole or nanoparticle assembly arrays, and the recently developed nanoporous structures, are all singlescale architectures. On the one hand, the individual nanoparticles could possess corners, tips, or gaps of nanometer or even sub-nanometer dimensions, acting as electromagnetically “hottest” spots, but the spatial occupation of such features per unit area is often very low. On the other hand, the nanoporous structure exhibits a large amount of pores of a few tens of nanometers, yet it usually lacks the generation of extremely SERS-active sites. The structural and enhancement reproducibility associated with these two types of substrates are often inferior to periodic nanoholes or nanoparticles. Understanding such issues in the development of SERS technique, we focused our effort on hierarchical multiscale SERS substrates that combine the best features of these existing substrates. Metamaterials are artificial, engineered materials with periodically or quasi-periodically arrangements of subwavelength-size components. Such materials possess unique properties and functions that largely depend on the size and arrangement of components.[32,33] In this work, we design and fabricate a new class of 3D plasmonic metamaterials-based SERS substrates, namely, hierarchically ordered porous gold membranes consisting of close-packed arrays of nanohole channels and uniformly distributed mesopores over the bulk. The large number of nanoholes themselves can efficiently harvest the incident light by
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Figure 1. Fabrication and characterization of hierarchically ordered porous metamaterials. a) Schematic illustration of the preparation procedures: 1) Preparation of the negative poly(methyl methacrylate) (PMMA) template from porous anodic alumina; 2) Injection of lyotropic liquid-crystal precursor into the PMMA template; 3) Formation of lyotropic liquid-crystal within the PMMA template; 4) Electrodeposition of gold; and 5) Removal of the PMMA template. b,c) SEM images of oblique (b) and plain (c) views of an as-prepared membrane and d,e) its high-resolution images of plain (d) and cross-sectional views (e). f) TEM image of cross-sectional view of the membrane and its corresponding electronic diffraction pattern (inset). g) TEM image of the region indicated in (f) with the HRTEM image of a single mesopore shown in the inset.
their LSPs, generating a strong EM enhancement in the close vicinity of the nanohole edges over the whole gold substrate. The integration of ordered mesoporous nanostructures into the periodic array of the nanoholes generates a landscape of plasmon modes and offers a cascaded multiscale EM field enhancement. This results in strong Raman-active sites over the whole substrate. Moreover, the mesopores provide efficient binding sites for the capture of analyte molecules at the “hot” spots. With these two advantages, we demonstrate that the 3D plasmonic metamaterials exhibit a significantly enhanced Raman intensity by a factor of up to 30-fold compared with the commercial Klarite substrate and a detection limit down to 10−13 m for non-resonant benzenethiol molecules. The 3D hierarchically ordered porous metamaterials were fabricated through a dual-templating approach utilizing reverse
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porous poly(methyl methacrylate) (PMMA) as a hard template and non-ionic surfactant octaethylene glycol monohexadecyl ether (C16EO8) as a lyotropic liquid-crystal (LLC) precursor. Figure 1a illustrates the synthetic procedures for the fabrication of the porous gold metamaterials. Briefly, the negative PMMA nanorod array cast from anodic aluminum oxide (AAO) template was used as a sacrifice template, where the precursor solution was loaded to prepare the hexagonal LLC mesophases in the nanovoids of the PMMA template. This was followed by the electrodeposition of gold and subsequent removal of the PMMA. The details of the procedures are shown in the Experimental Section. The surface and bulk morphological features of the hierarchical porous metamaterials were characterized by electron microscopy. The top and cross-sectional views of scanning electron microscopy (SEM) images shown in Figure 1b–e
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COMMUNICATION Figure 2. SERS measurements and electromagnetic modeling. a) Measured SERS spectra of benzenethiol adsorbed on the commercial Klarite substrate, the mesoporous film, the nanohole membrane, and the hierarchical porous gold metamaterial. The width and depth of the pyramid-shape microholes in the Klarite substrate are 1.6 µm and 0.8 µm, and the edge-to-edge distance between adjacent holes is 0.2 µm (Figure S1, Supporting Information). The wall thickness and hole diameter of nanohole membrane are 25 and 80 nm, while those of the mesoporous film are 3.5 and 2.5 nm (Figure S1, Supporting Information), respectively. The geometric parameters of the hierarchical porous metamerials structure inherit those of the mesoporous film and the nanohole membrane. b) Normalized electric field (E-field) amplitude distribution at the surface for the mesoporous film (b1), the nanohole membrane (b2), and the hierarchical porous gold metamaterial (b3) with excitation configuration (k and E) shown in the inset of (b2). The color bar is linear and deliberately restricted to [0 2] for better viewing but note that the field magnitude is by far larger around the nanohole edges in (b3). The black scale bars are 6, 20, and 20 nm from (b1) to (b3), respectively. c) Raman intensities at peaks of 996 (squares), 1071 (circles), and 1570 cm−1 (diamonds) and the simulated EM enhancement factors (open circles) at the excitation wavelength 782 nm for the three structures. The error bars correspond to the standard deviations of five SERS measurement. n0 is a conceptual line density of molecules adsorbed along the edges of mesopres and nanoholes, and is used for normalization purpose.
reveal that the gold porous membrane retains the dimensions, morphology, and structure of the alumina template, exhibiting a uniform, highly ordered, hexagonal-arranged array of nanoholes over a large area. The wall thickness and the nanohole diameter measured from Figure 1d are about 25 and 80 nm, respectively. The mesoporous feature of the metamaterial is revealed by the transmission electron microscopy (TEM) images of the cross-sectional view (Figure 1f,g), which show that the domains on the inner wall appear darker than the void parts. The inset in Figure 1f shows the corresponding electron diffraction (ED) pattern in which the bright concentric rings can be indexed as the face-centered cubic (FCC) lattice structure of gold. The high-magnification TEM images of the region indicated in Figure 1f exhibit well-organized mesopores with diameter ranges from 2 to 3 nm (Figure 1g). A typical highresolution TEM (HRTEM) image of an individual mesopore (inset in Figure 1g) exhibits the lattice spacings of 0.23 and 0.20 nm, corresponding to the {100} and {111} atomic planes of gold, respectively. The hierarchical porous features are further confirmed by the top-view TEM images of the membrane (Figure S1, Supporting Information). To evaluate the SERS performance of the hierarchical porous gold metamaterials, a non-resonant Raman analyte, benzenethiol, was used as a model analyte. Figure 2a compares the Raman spectra of benzenethiol adsorbed on four substrates: a commercial Klarite substrate, a mesoporous gold film, a nanohole gold membrane, and the hierarchical porous gold metamaterial. All the Raman spectra were collected from 1 µm2 area of
Adv. Mater. 2014, DOI: 10.1002/adma.201404107
the samples excited at 782 nm. The characteristic Raman peaks at 417, 693, 996, 1022, 1071, and 1570 cm−1 for benzenethiol[34] were observed for all the four samples. The SERS sensitivity increases from the Klarite substrate, the mesoporous gold film (and the nanohole gold membrane) to the hierarchical porous gold metamaterial. More specifically, the Raman intensity collected from the hierarchical porous metamaterial is as much as 30 times that of the Klarite substrate (Figure S2, Supporting Information), demonstrating a significantly improved SERS sensitivity of the rationally designed metamaterial substrate. More importantly, the observed SERS signal intensities at the three representative Raman shifts (i.e., 996, 1071, and 1570 cm−1) for the hierarchical porous metamaterial are substantially higher than those for the mesoporous film and nanohole membrane (Figure 2a), thereby unambiguously revealing the dual enhancement effect arising from combination of nanoholes and mesopores in a single structure. It is well known that the SERS signal intensity scales as the product of the EM field intensities at the excitation and Raman scattering wavelengths. Therefore, to understand the underlying physical mechanism responsible for the observed SERS responses, we have adopted the full-wave finite element method to calculate the electric near-field distribution for each structure at the laser excitation wavelength and extracted the theoretical SERS enhancement factor (EF) normalized with respect to the incident field and the number of the probe molecules bound within the “hot” spots. Figure 2b shows maps of the electric field distribution with field amplitude normalized to the
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Figure 3. Examination of SERS reproducibility and detection limit. a) Spot-to-spot Raman intensity variation at 1071 cm−1 for the porous gold metamaterial (top) and the commercial Klarite substrate (bottom). b) Substrate-to-substrate Raman intensity variation at 1071 cm−1 measured for six porous metamaterial substrates. Each intensity value (error bar) represents the average (standard deviation) of twelve independently measured results at different spots. For all the measurements in panels (a) and (b), the benzenethiol concentration and loading time are 1 × 10−6 m and 12 h, respectively. c) Raman intensity of benzenethiol adsorbed on porous metamaterials at peaks of 996, 1022, 1071, and 1570 cm−1 as a function of benzenethiol concentration (in logarithmic scale). The inset shows the same graph for solution concentration ranging from 10−13 to 10−8 m. The markers are the experimental data points and the solid lines are the fitted curves. d) SERS spectra of porous gold metamaterials loaded with benzenethiol of analyte concentration ranging from 10−13 to 10−10 m. The benzenethiol loading time in panels (c) and (d) is 12 h.
incident field for the three structures (with the same unit cell area for the latter two structures). Although the mesoporous film has a larger number density of plasmonic “hot” spots, the field strength is significantly weaker than that of the nanohole structure. This is due to efficient excitation of the LSPs in the nanoholes while the dimensions of mesopores are simply too small to support strong plasmon resonances. Intriguingly, the integration of ordered mesopores into the periodic nanohole array generates a landscape of plasmon modes and further amplifies the EM field both in the vicinity of the nanoholes and at the interstitial spaces between nanoholes. This is achieved by a two-step amplification process of plasmonic near fields in the porous gold metamaterial, namely, effective harvesting of incident energy by subwavelength nanoholes and subsequent concentration of evanescent fields by the mesopores, resulting in a cascaded multiscale EM focusing effect as corroborated by the simulated largest field strength. Note that in our calculations the second step amplification effect is naturally taken into account by the effective gold dielectric response of the hierarchical porous metamaterial. The SERS enhancement factors extracted from Figure 2b for the three structures are compared in Figure 2c, which shows excellent agreement with the measured Raman intensities.
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Reliable SERS reproducibility is of equal importance as the SERS sensitivity, and this is critical for the design of SERSbased sensors allowing for quantification of a specific analyte. To examine the reproducibility of the hierarchical porous metamaterials, we undertook a thorough statistical analysis to quantify the variation in the SERS signal intensity at different locations on one substrate (spot-to-spot variation) and between different substrates (substrate-to-substrate variation). Figure 3a shows the spot-to-spot variation distribution of the captured Raman intensities at the 1071 cm−1 peak for a randomly selected hierarchical porous metamaterial substrate (top) and a commercial Klarite substrate (bottom), respectively. For quantitative comparison, we calculated the average Raman signal intensity and the coefficient of variation for both substrates. We find that the average signal intensity for the hierarchical porous metamaterial structure is ca. 62 200 counts with a coefficient of variation of 21%, while the commercial substrate has an average signal intensity of 4350 counts and a coefficient of variation of 45%. Such distinctive differences clearly indicate not only a substantially higher SERS sensitivity but also better signal reproducibility associated with the hierarchical porous substrate. To test the substrate-to-substrate SERS reproducibility, six hierarchical porous metamaterial substrates with the same
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nominal geometric parameters were loaded with benzenethiol of the same concentration and a series of Raman spectra were measured at 12 different spots of each substrate to obtain an average Raman intensity. Figure 3b compares the substrate-tosubstrate variation at 1071 cm−1 in the Raman spectra for the six substrates, which exhibits remarkable reproducibility. The detection limit is one of the most important parameters for evaluating the overall performance of an SERS substrate. To determine the detection limit of the hierarchical porous metamaterials, six identical substrates were exposed, respectively, overnight to benzenethiol solutions with concentrations varying from 10−13 to 10−8 m. The corresponding Raman spectra are provided in Figure S3, Supporting Information. The SERS intensity of four different Raman shifts at 996, 1022, 1071, and 1570 cm−1 as a function of benzenethiol concentration is plotted in Figure 3c. The results show that the Raman intensities of the benzenethiol decrease with decreasing the loading concentration and that the benzenethiol loading follows the first-order adsorption kinetics. The logarithmic plot of SERS intensity versus loading concentration of benzenethiol displays a small slope of 0.322 ± 0.002, indicating that the hierarchical porous gold metamaterial offers sufficient binding sites for the efficient capture of the analyte from the loading solution. Figure 3d shows the Raman spectra of benzenethiol adsorbed on the hierarchical porous metamaterials with analyte loading concentration of 10−13–10−10 m. Even at a low concentration of 10−13 m, the signature Raman peaks of benzenethiol at 996, 1071, and 1570 cm−1 can still be clearly observed. The signal-to-noise ratios for these Raman peaks were determined to be 3.3, 3.2, and 1.7. Hence the combination of the joint EM field enhancement and a large number of binding sites in the hierarchical porous metamaterials results in a detection limit as low as 1 × 10−13 m. Ideal SERS substrates for chemical identification and biosensing have to be extremely sensitive and highly reproducible. This requires new nanofabrication approaches that allow the production of plasmonic nanostructures with a large number of uniformly distributed hot spots. We have demonstrated here that a dual-template guided synthetic strategy can address this important nanofabrication need as this technique can precisely control the size and composition of hierarchical porous metamaterials consisting of close-packed arrays of nanoholes and uniformly distributed mesopores over the bulk. The rational combination of periodic nanoholes and mesopores enables a cascade EM field enhancement arising from strong excitation of localized surface plasmon fields near the edges of the nanoholes and further concentration of the fields by the uniformly distributed mesopores. The electromagnetic simulations provide the physical interpretation for our experimental results and clearly evidence a cascaded field enhancement on the multiscale porous metamaterials. Moreover, the mesopores provide sufficient binding sites to efficiently capture the analytes from the analyte loading solution. Consequently, the hierarchical porous metamaterials exhibit strong SERS activities that are remarkably superior to the commercial Klarite substrate in terms of both signal intensity and reproducibility for the detection of aromatic molecules and yield a detection limit of 10−13 m. Their performance could be further improved by optimizing the Raman measurement conditions (for example, the
excitation wavelength and power density of the laser) and tailoring the composition and structure of the metamaterials. The work demonstrated here represents a novel proof-of-concept approach leading the way to a plethora of hierarchical porous metamaterials with a range of exceptional electrical, optical, catalytic, and sensing properties.
Experimental Section Anodic aluminum oxide (AAO) templates were prepared by anodizing high purity aluminum foils (99.999%) in oxalic acid solutions. A gold layer of about 6 nm thickness was deposited on one side of an AAO template as the electrode by using a vacuum evaporation apparatus. The reverse porous poly(methyl methacrylate) (PMMA) templates were prepared from the AAO by a replication method.[35] Typically, the alumina membrane was then immersed into methyl methacrylate monomer containing benzoyl peroxide (5 wt% ). Polymerization was carried out at 85 °C for 12 h. The AAO template was then removed in NaOH solution, and a replicated negative type of PMMA template was obtained. A twostep method was used to prepare ordered hierarchical porous gold metamaterials.[36] First, a homogeneous binary oligomeric non-ionic surfactant solution is prepared and loaded into the PMMA template and the sample was sealed and maintained at 65 °C for 12 h. The PMMA template with thicknesses ca. 10 µm and surface area about 0.5 cm2 was soaked with the prepared C16EO8 solution by distributing about 0.2 mL of the solution over the surface of the template. A 50–55 wt% C16EO8 solution was used for the formation of lyotropic liquid-crystalline phase. Then, the template was cooled down to room temperature to allow the formation of ordered and stable hexagonal LLC in the void space of the template. Second, the LLC phases act as mesostructure templates, and gold was deposited in the void space of the LLC-loaded template by electrodeposition. A 30 wt% AuCl3 solution was used as the Au plating solution. The LLC-loaded PMMA template was immersed into 0.2 mL Au plating solution and electrodeposition was carried out at room temperature (ca. 25 °C) at 50 µA cm−2 by a galvanostatic method. A Pt wire was used as a counter electrode. At last, the porous Au membranes were obtained by dissolving the PMMA with acetone, followed by cleaning with piranha solution to remove the residues. The nanohole gold membranes and mesoporous gold films were prepared with similar methods by using the PMMA templates without LLCs and gold coated glass substrates, respectively. All the samples were then exposed to UV-ozone at an oxygen flow rate of 3 L min− 1 for 10 min. After the UV-ozone treatment, the samples were immersed in 2 mL of different concentrations of benzenethiol in ethanol solution and left for 12 h, upon which the samples were rinsed with ethanol and then dried. All the chemical reagents in this study were of analytical grade and were supplied by Sigma–Aldrich (Australia). The morphology and microstructure of the porous gold metamaterials were investigated by SEM (JEOL JSM-6300) and TEM (JEOL-2011 and JEOL-2100F). Both tansmission electron microscopes were operated at 200 kV. The Raman spectra were recorded using a Confocal microRaman System (Renishaw RM 2000) equipped with a near-IR diode laser at a wavelength of 782 nm (laser power: 1.15 mW and laser spot size: 1 µm). All Raman spectra were collected by fine-focusing a 50× microscope objective and the data acquisition time was 10 s. The plasmonic electric-field calculations of the studied metal nanostructures were performed using the finite element method (FEM) implemented in a commercially available electromagnetic solver (COMSOL Multiphysics). The nanohole membrane and the hierarchical porous metamaterial were modeled as periodic structures with rectangle shape unit cell, which consists of a central nanohole surrounded by four quarters and has an area of 105 nm × 105 3 nm with 105 nm being the interhole distance. The mesoporous film was modeled with 3 × 3 unit cells (with an area of 6 × 6 3 nm2 for each unit cell and 6 nm being the interpore distance) in order to fully take into account the electromagnetic interaction between neighboring mesopores. The
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www.MaterialsViews.com unit cell was discretized using tetrahedral mesh elements with mesh size down to 1 nm around air nanoholes or mesopores in the three structures. The simulation domain was surrounded up and down by two perfectly matched layers to absorb the outward propagating radiation while the periodic boundary conditions were applied to the side boundaries. The electric-field profile was calculated in frequency domain, i.e., at the laser excitation wavelength. The dielectric function of gold was extracted from the Johnson & Christy data[37] and used for the mesoporous film and nanohole membrane. The dielectric function for the hierarchical porous metamaterial was calculated in the framework of Bruggeman theory:[38] f
ε a − ε eff ε − ε eff + (1 − f ) m =0 ε a + 2ε eff ε m + 2ε eff
(1)
where f denotes the volume fraction of air in the structure and εm is the permittivity of gold (εa = 1 for air). Based on the mesopore diameter (2.5 nm) and interpore distance (6 nm), we calculated the volume fraction of air to be 31.5% for the hierarchical porous metamaterials. In our calculations, three assumptions were made to simplify the experimental scenario but without loss of the underlying physics. First, the surface density of the adsorbed molecules is the same for the three substrates; second, only the molecules adsorbed around the edges of nanoholes or mesopores contribute to the detected Raman signals;[18,39] and third, the dielectric response of the porous metamaterials can be treated as an effective medium through Bruggeman theory.[40] Based on these assumptions, sophisticated normalization procedures were applied with respect to the number density of “hot” spots and their respective dimensions in each structure. The theoretical SERS enhancement factor (EF) was calculated from the normalized electric near-field enhancement and the number of probe molecules adsorbed within the “hot” spots of each nanostructure. Based on the three assumptions mentioned in the main text, the SERS EF for the three structures can be simply given as: E Eo
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× l × n0
(2)
where E and E0 are the electric field amplitude at the structure surface and that of the incident light, respectively. Here l represents the total length of the circumferences of nanoholes or mesopores in a unit cell of the same area 105 nm × 105 3 nm and n0 with a unit of number/nm is the line density of molecules adsorbed along the circumferences of nanoholes or mesopores. As only the molecules adsorbed at the edges of nanoholes or mesopores significantly contribute to the measured Raman signal intensity, here we used the maximum field amplitude to calculate the field enhancement. For the mesoporous film, E/E0 = 1.68, ⎛105 × 105 ⎞ l = ⎝ 6 × 6 ⎠ × 2 × π × 2.5 = 4810 nm, we had an SERS enhancement factor of 3.83 × 104 n0. For the nanohole membrane and the hierarchical porous metamaterial, E/E0 = 3.53 and 4.78, l = 2 × π × 80 = 502 nm for both, we obtained SERS enhancement factors of 7.79 × 104 n0 and 2.62 × 105 n0, respectively.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements X.Z. and Y.Z. contributed equally to this work. This work was supported by the Australian Research Council through Grant No. DP120104334. D.R.M. is grateful to Australian Research Council for the Laureate Fellowship. Y.Z. thanks The University of New South Wales for
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the Vice-chancellor's Research Fellowship (Project ID RG134604). D.Y.L. acknowledges the financial support from the Hong Kong Research Grants Council (509513) and the Natural Science Foundation of China (11304261). Received: September 8, 2014 Revised: November 16, 2014 Published online:
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[35] T. Yanagishita, K. Nishio, H. Masuda, Adv. Mater. 2005, 17, 2241. [36] X. Zhang, W. Lu, J. Dai, L. Bourgeois, N. Hao, H. Wang, D. Zhao, P. A. Webley, Angew. Chem. Int. Ed. 2010, 49, 10101. [37] P. B. Johnson, R. W. Christy, Phys. Rev. B 1972, 6, 4370. [38] J. Sancho-Parramon, V. Janicki, H. Zorc, Opt. Express 2010, 18, 26915. [39] R. T. Hill, J. J. Mock, Y. Urzhumov, D. S. Sebba, S. J. Oldenburg, S.-Y. Chen, A. A. lazarides, A. Chilkoti, D. R. Smith, Nano Lett. 2010, 10, 4150. [40] A. V. Kabashin, P. Evans, S. Pastkovsky, W. Hendren, G. A. Wurtz, R. Atkinson, R. Pollard, V. A. Podolskiy, A. V. Zayats, Nat. Mater. 2009, 8, 867.
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Hierarchical Porous Plasmonic Metamaterials for Reproducible Ultrasensitive Surface-Enhanced Raman Spectroscopy Xinyi Zhang, Yuanhui Zheng, Xin Liu, Wei Lu, Jiyan Dai, Dang Yuan Lei,* and Douglas R. MacFarlane* Since the first report of the detection of a single molecule,[1,2] surface-enhanced Raman spectroscopy (SERS) has emerged as one of the most versatile tools for sensing and imaging chemical and biological analytes.[3–12] SERS-based molecular detection can be achieved by using metal (mostly silver and gold) nanostructures as plasmonic antennas to amplify the Raman signals. A unique feature of these metal nanostructures is their ability to support localized surface plasmons (LSPs), i.e., light-driven coherent oscillations of conduction electrons, which can generate an enormous electromagnetic (EM) field enhancement in the close vicinity of the metal surface. The EM field enhancement is particularly strong at nanoscopically sharp corners or tips,[2,5,6,13–15] interparticle gaps,[1,7–12] and nanopores,[16,17] typically referred to as “hot” spots. In general, it is believed that the EM field-induced SERS enhancement is proportional to the fourth power of the ratio of the localized EM field at the location of the analyte molecule to the incident excitation field.[18] The SERS enhancement factor has been often observed on the order of 104–106 and can be as high as 108–1014,[19] which allows the technique to be sensitive enough to detect single molecules.[20–22] Although extremely high SERS sensitivity is possible for some existing nanostructures, their practical application is frequently limited by their structural instability and/or poor reproducibility of the enhanced signals. For example, wide SERS enhancement distributions have been reported ranging from 2 × 107 to 2 × 109 for polymer-coated silver nanoparticle junctions[23] and from 2.8 × 104 to >4.1 × 1010 for a silver film-over-nanosphere (SFON) substrate.[24] The “hottest” SERS-active sites (enhancement factor >108) for the SFON substrate accounted for less than 0.1% of the total “hot” spots, but contributed 47% of the overall SERS intensity.[24] Dr. X. Zhang, Prof. D. R. MacFarlane School of Chemistry Monash University Clayton, VIC 3800, Australia E-mail: [email protected] Dr. Y. Zheng School of Chemistry The University of New South Wales Sydney, NSW 2052, Australia Dr. X. Liu, Dr. W. Lu, Prof. J. Dai, Prof. D. Y. Lei Department of Applied Physics The Hong Kong Polytechnic University Hong Kong, China E-mail: [email protected]
DOI: 10.1002/adma.201404107
Adv. Mater. 2014, DOI: 10.1002/adma.201404107
To date, periodic nanohole arrays with controllable size, shape, and spacing in metal films[6,24] and their complementary counterparts, ordered nanoparticle arrays,[10,13,23–25] have been investigated as one class of promising SERS substrates with excellent structural reproducibility and pronounced signal enhancement. While the enhanced Raman scattering by the nanohole arrays is mainly attributed to excitations of propagating surface plasmons via grating coupling,[6,24] the nanoparticle arrays share the similar enhancement mechanism as individual nanostructures with excitations of LSPs as discussed above. In addition, dealloyed nanoporous metals have also been exploited as attractive Raman-active structures because of their large surface area and three-dimensional (3D) bicontinuous porous configuration.[26–31] Such unique features not only allow for excitations of LSPs but also provide a large number of molecular binding sites. However, the overall SERS enhancement observed from these substrates are usually either poor in reproducibility or insufficient for single-molecule detection due to the limitation of control over the structural parameters such as pore geometry and order. The aforementioned three types of SERS substrates, including individual nanoparticles with sharp geometric features, periodic nanohole or nanoparticle assembly arrays, and the recently developed nanoporous structures, are all singlescale architectures. On the one hand, the individual nanoparticles could possess corners, tips, or gaps of nanometer or even sub-nanometer dimensions, acting as electromagnetically “hottest” spots, but the spatial occupation of such features per unit area is often very low. On the other hand, the nanoporous structure exhibits a large amount of pores of a few tens of nanometers, yet it usually lacks the generation of extremely SERS-active sites. The structural and enhancement reproducibility associated with these two types of substrates are often inferior to periodic nanoholes or nanoparticles. Understanding such issues in the development of SERS technique, we focused our effort on hierarchical multiscale SERS substrates that combine the best features of these existing substrates. Metamaterials are artificial, engineered materials with periodically or quasi-periodically arrangements of subwavelength-size components. Such materials possess unique properties and functions that largely depend on the size and arrangement of components.[32,33] In this work, we design and fabricate a new class of 3D plasmonic metamaterials-based SERS substrates, namely, hierarchically ordered porous gold membranes consisting of close-packed arrays of nanohole channels and uniformly distributed mesopores over the bulk. The large number of nanoholes themselves can efficiently harvest the incident light by
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Figure 1. Fabrication and characterization of hierarchically ordered porous metamaterials. a) Schematic illustration of the preparation procedures: 1) Preparation of the negative poly(methyl methacrylate) (PMMA) template from porous anodic alumina; 2) Injection of lyotropic liquid-crystal precursor into the PMMA template; 3) Formation of lyotropic liquid-crystal within the PMMA template; 4) Electrodeposition of gold; and 5) Removal of the PMMA template. b,c) SEM images of oblique (b) and plain (c) views of an as-prepared membrane and d,e) its high-resolution images of plain (d) and cross-sectional views (e). f) TEM image of cross-sectional view of the membrane and its corresponding electronic diffraction pattern (inset). g) TEM image of the region indicated in (f) with the HRTEM image of a single mesopore shown in the inset.
their LSPs, generating a strong EM enhancement in the close vicinity of the nanohole edges over the whole gold substrate. The integration of ordered mesoporous nanostructures into the periodic array of the nanoholes generates a landscape of plasmon modes and offers a cascaded multiscale EM field enhancement. This results in strong Raman-active sites over the whole substrate. Moreover, the mesopores provide efficient binding sites for the capture of analyte molecules at the “hot” spots. With these two advantages, we demonstrate that the 3D plasmonic metamaterials exhibit a significantly enhanced Raman intensity by a factor of up to 30-fold compared with the commercial Klarite substrate and a detection limit down to 10−13 m for non-resonant benzenethiol molecules. The 3D hierarchically ordered porous metamaterials were fabricated through a dual-templating approach utilizing reverse
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porous poly(methyl methacrylate) (PMMA) as a hard template and non-ionic surfactant octaethylene glycol monohexadecyl ether (C16EO8) as a lyotropic liquid-crystal (LLC) precursor. Figure 1a illustrates the synthetic procedures for the fabrication of the porous gold metamaterials. Briefly, the negative PMMA nanorod array cast from anodic aluminum oxide (AAO) template was used as a sacrifice template, where the precursor solution was loaded to prepare the hexagonal LLC mesophases in the nanovoids of the PMMA template. This was followed by the electrodeposition of gold and subsequent removal of the PMMA. The details of the procedures are shown in the Experimental Section. The surface and bulk morphological features of the hierarchical porous metamaterials were characterized by electron microscopy. The top and cross-sectional views of scanning electron microscopy (SEM) images shown in Figure 1b–e
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COMMUNICATION Figure 2. SERS measurements and electromagnetic modeling. a) Measured SERS spectra of benzenethiol adsorbed on the commercial Klarite substrate, the mesoporous film, the nanohole membrane, and the hierarchical porous gold metamaterial. The width and depth of the pyramid-shape microholes in the Klarite substrate are 1.6 µm and 0.8 µm, and the edge-to-edge distance between adjacent holes is 0.2 µm (Figure S1, Supporting Information). The wall thickness and hole diameter of nanohole membrane are 25 and 80 nm, while those of the mesoporous film are 3.5 and 2.5 nm (Figure S1, Supporting Information), respectively. The geometric parameters of the hierarchical porous metamerials structure inherit those of the mesoporous film and the nanohole membrane. b) Normalized electric field (E-field) amplitude distribution at the surface for the mesoporous film (b1), the nanohole membrane (b2), and the hierarchical porous gold metamaterial (b3) with excitation configuration (k and E) shown in the inset of (b2). The color bar is linear and deliberately restricted to [0 2] for better viewing but note that the field magnitude is by far larger around the nanohole edges in (b3). The black scale bars are 6, 20, and 20 nm from (b1) to (b3), respectively. c) Raman intensities at peaks of 996 (squares), 1071 (circles), and 1570 cm−1 (diamonds) and the simulated EM enhancement factors (open circles) at the excitation wavelength 782 nm for the three structures. The error bars correspond to the standard deviations of five SERS measurement. n0 is a conceptual line density of molecules adsorbed along the edges of mesopres and nanoholes, and is used for normalization purpose.
reveal that the gold porous membrane retains the dimensions, morphology, and structure of the alumina template, exhibiting a uniform, highly ordered, hexagonal-arranged array of nanoholes over a large area. The wall thickness and the nanohole diameter measured from Figure 1d are about 25 and 80 nm, respectively. The mesoporous feature of the metamaterial is revealed by the transmission electron microscopy (TEM) images of the cross-sectional view (Figure 1f,g), which show that the domains on the inner wall appear darker than the void parts. The inset in Figure 1f shows the corresponding electron diffraction (ED) pattern in which the bright concentric rings can be indexed as the face-centered cubic (FCC) lattice structure of gold. The high-magnification TEM images of the region indicated in Figure 1f exhibit well-organized mesopores with diameter ranges from 2 to 3 nm (Figure 1g). A typical highresolution TEM (HRTEM) image of an individual mesopore (inset in Figure 1g) exhibits the lattice spacings of 0.23 and 0.20 nm, corresponding to the {100} and {111} atomic planes of gold, respectively. The hierarchical porous features are further confirmed by the top-view TEM images of the membrane (Figure S1, Supporting Information). To evaluate the SERS performance of the hierarchical porous gold metamaterials, a non-resonant Raman analyte, benzenethiol, was used as a model analyte. Figure 2a compares the Raman spectra of benzenethiol adsorbed on four substrates: a commercial Klarite substrate, a mesoporous gold film, a nanohole gold membrane, and the hierarchical porous gold metamaterial. All the Raman spectra were collected from 1 µm2 area of
Adv. Mater. 2014, DOI: 10.1002/adma.201404107
the samples excited at 782 nm. The characteristic Raman peaks at 417, 693, 996, 1022, 1071, and 1570 cm−1 for benzenethiol[34] were observed for all the four samples. The SERS sensitivity increases from the Klarite substrate, the mesoporous gold film (and the nanohole gold membrane) to the hierarchical porous gold metamaterial. More specifically, the Raman intensity collected from the hierarchical porous metamaterial is as much as 30 times that of the Klarite substrate (Figure S2, Supporting Information), demonstrating a significantly improved SERS sensitivity of the rationally designed metamaterial substrate. More importantly, the observed SERS signal intensities at the three representative Raman shifts (i.e., 996, 1071, and 1570 cm−1) for the hierarchical porous metamaterial are substantially higher than those for the mesoporous film and nanohole membrane (Figure 2a), thereby unambiguously revealing the dual enhancement effect arising from combination of nanoholes and mesopores in a single structure. It is well known that the SERS signal intensity scales as the product of the EM field intensities at the excitation and Raman scattering wavelengths. Therefore, to understand the underlying physical mechanism responsible for the observed SERS responses, we have adopted the full-wave finite element method to calculate the electric near-field distribution for each structure at the laser excitation wavelength and extracted the theoretical SERS enhancement factor (EF) normalized with respect to the incident field and the number of the probe molecules bound within the “hot” spots. Figure 2b shows maps of the electric field distribution with field amplitude normalized to the
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Figure 3. Examination of SERS reproducibility and detection limit. a) Spot-to-spot Raman intensity variation at 1071 cm−1 for the porous gold metamaterial (top) and the commercial Klarite substrate (bottom). b) Substrate-to-substrate Raman intensity variation at 1071 cm−1 measured for six porous metamaterial substrates. Each intensity value (error bar) represents the average (standard deviation) of twelve independently measured results at different spots. For all the measurements in panels (a) and (b), the benzenethiol concentration and loading time are 1 × 10−6 m and 12 h, respectively. c) Raman intensity of benzenethiol adsorbed on porous metamaterials at peaks of 996, 1022, 1071, and 1570 cm−1 as a function of benzenethiol concentration (in logarithmic scale). The inset shows the same graph for solution concentration ranging from 10−13 to 10−8 m. The markers are the experimental data points and the solid lines are the fitted curves. d) SERS spectra of porous gold metamaterials loaded with benzenethiol of analyte concentration ranging from 10−13 to 10−10 m. The benzenethiol loading time in panels (c) and (d) is 12 h.
incident field for the three structures (with the same unit cell area for the latter two structures). Although the mesoporous film has a larger number density of plasmonic “hot” spots, the field strength is significantly weaker than that of the nanohole structure. This is due to efficient excitation of the LSPs in the nanoholes while the dimensions of mesopores are simply too small to support strong plasmon resonances. Intriguingly, the integration of ordered mesopores into the periodic nanohole array generates a landscape of plasmon modes and further amplifies the EM field both in the vicinity of the nanoholes and at the interstitial spaces between nanoholes. This is achieved by a two-step amplification process of plasmonic near fields in the porous gold metamaterial, namely, effective harvesting of incident energy by subwavelength nanoholes and subsequent concentration of evanescent fields by the mesopores, resulting in a cascaded multiscale EM focusing effect as corroborated by the simulated largest field strength. Note that in our calculations the second step amplification effect is naturally taken into account by the effective gold dielectric response of the hierarchical porous metamaterial. The SERS enhancement factors extracted from Figure 2b for the three structures are compared in Figure 2c, which shows excellent agreement with the measured Raman intensities.
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Reliable SERS reproducibility is of equal importance as the SERS sensitivity, and this is critical for the design of SERSbased sensors allowing for quantification of a specific analyte. To examine the reproducibility of the hierarchical porous metamaterials, we undertook a thorough statistical analysis to quantify the variation in the SERS signal intensity at different locations on one substrate (spot-to-spot variation) and between different substrates (substrate-to-substrate variation). Figure 3a shows the spot-to-spot variation distribution of the captured Raman intensities at the 1071 cm−1 peak for a randomly selected hierarchical porous metamaterial substrate (top) and a commercial Klarite substrate (bottom), respectively. For quantitative comparison, we calculated the average Raman signal intensity and the coefficient of variation for both substrates. We find that the average signal intensity for the hierarchical porous metamaterial structure is ca. 62 200 counts with a coefficient of variation of 21%, while the commercial substrate has an average signal intensity of 4350 counts and a coefficient of variation of 45%. Such distinctive differences clearly indicate not only a substantially higher SERS sensitivity but also better signal reproducibility associated with the hierarchical porous substrate. To test the substrate-to-substrate SERS reproducibility, six hierarchical porous metamaterial substrates with the same
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nominal geometric parameters were loaded with benzenethiol of the same concentration and a series of Raman spectra were measured at 12 different spots of each substrate to obtain an average Raman intensity. Figure 3b compares the substrate-tosubstrate variation at 1071 cm−1 in the Raman spectra for the six substrates, which exhibits remarkable reproducibility. The detection limit is one of the most important parameters for evaluating the overall performance of an SERS substrate. To determine the detection limit of the hierarchical porous metamaterials, six identical substrates were exposed, respectively, overnight to benzenethiol solutions with concentrations varying from 10−13 to 10−8 m. The corresponding Raman spectra are provided in Figure S3, Supporting Information. The SERS intensity of four different Raman shifts at 996, 1022, 1071, and 1570 cm−1 as a function of benzenethiol concentration is plotted in Figure 3c. The results show that the Raman intensities of the benzenethiol decrease with decreasing the loading concentration and that the benzenethiol loading follows the first-order adsorption kinetics. The logarithmic plot of SERS intensity versus loading concentration of benzenethiol displays a small slope of 0.322 ± 0.002, indicating that the hierarchical porous gold metamaterial offers sufficient binding sites for the efficient capture of the analyte from the loading solution. Figure 3d shows the Raman spectra of benzenethiol adsorbed on the hierarchical porous metamaterials with analyte loading concentration of 10−13–10−10 m. Even at a low concentration of 10−13 m, the signature Raman peaks of benzenethiol at 996, 1071, and 1570 cm−1 can still be clearly observed. The signal-to-noise ratios for these Raman peaks were determined to be 3.3, 3.2, and 1.7. Hence the combination of the joint EM field enhancement and a large number of binding sites in the hierarchical porous metamaterials results in a detection limit as low as 1 × 10−13 m. Ideal SERS substrates for chemical identification and biosensing have to be extremely sensitive and highly reproducible. This requires new nanofabrication approaches that allow the production of plasmonic nanostructures with a large number of uniformly distributed hot spots. We have demonstrated here that a dual-template guided synthetic strategy can address this important nanofabrication need as this technique can precisely control the size and composition of hierarchical porous metamaterials consisting of close-packed arrays of nanoholes and uniformly distributed mesopores over the bulk. The rational combination of periodic nanoholes and mesopores enables a cascade EM field enhancement arising from strong excitation of localized surface plasmon fields near the edges of the nanoholes and further concentration of the fields by the uniformly distributed mesopores. The electromagnetic simulations provide the physical interpretation for our experimental results and clearly evidence a cascaded field enhancement on the multiscale porous metamaterials. Moreover, the mesopores provide sufficient binding sites to efficiently capture the analytes from the analyte loading solution. Consequently, the hierarchical porous metamaterials exhibit strong SERS activities that are remarkably superior to the commercial Klarite substrate in terms of both signal intensity and reproducibility for the detection of aromatic molecules and yield a detection limit of 10−13 m. Their performance could be further improved by optimizing the Raman measurement conditions (for example, the
excitation wavelength and power density of the laser) and tailoring the composition and structure of the metamaterials. The work demonstrated here represents a novel proof-of-concept approach leading the way to a plethora of hierarchical porous metamaterials with a range of exceptional electrical, optical, catalytic, and sensing properties.
Experimental Section Anodic aluminum oxide (AAO) templates were prepared by anodizing high purity aluminum foils (99.999%) in oxalic acid solutions. A gold layer of about 6 nm thickness was deposited on one side of an AAO template as the electrode by using a vacuum evaporation apparatus. The reverse porous poly(methyl methacrylate) (PMMA) templates were prepared from the AAO by a replication method.[35] Typically, the alumina membrane was then immersed into methyl methacrylate monomer containing benzoyl peroxide (5 wt% ). Polymerization was carried out at 85 °C for 12 h. The AAO template was then removed in NaOH solution, and a replicated negative type of PMMA template was obtained. A twostep method was used to prepare ordered hierarchical porous gold metamaterials.[36] First, a homogeneous binary oligomeric non-ionic surfactant solution is prepared and loaded into the PMMA template and the sample was sealed and maintained at 65 °C for 12 h. The PMMA template with thicknesses ca. 10 µm and surface area about 0.5 cm2 was soaked with the prepared C16EO8 solution by distributing about 0.2 mL of the solution over the surface of the template. A 50–55 wt% C16EO8 solution was used for the formation of lyotropic liquid-crystalline phase. Then, the template was cooled down to room temperature to allow the formation of ordered and stable hexagonal LLC in the void space of the template. Second, the LLC phases act as mesostructure templates, and gold was deposited in the void space of the LLC-loaded template by electrodeposition. A 30 wt% AuCl3 solution was used as the Au plating solution. The LLC-loaded PMMA template was immersed into 0.2 mL Au plating solution and electrodeposition was carried out at room temperature (ca. 25 °C) at 50 µA cm−2 by a galvanostatic method. A Pt wire was used as a counter electrode. At last, the porous Au membranes were obtained by dissolving the PMMA with acetone, followed by cleaning with piranha solution to remove the residues. The nanohole gold membranes and mesoporous gold films were prepared with similar methods by using the PMMA templates without LLCs and gold coated glass substrates, respectively. All the samples were then exposed to UV-ozone at an oxygen flow rate of 3 L min− 1 for 10 min. After the UV-ozone treatment, the samples were immersed in 2 mL of different concentrations of benzenethiol in ethanol solution and left for 12 h, upon which the samples were rinsed with ethanol and then dried. All the chemical reagents in this study were of analytical grade and were supplied by Sigma–Aldrich (Australia). The morphology and microstructure of the porous gold metamaterials were investigated by SEM (JEOL JSM-6300) and TEM (JEOL-2011 and JEOL-2100F). Both tansmission electron microscopes were operated at 200 kV. The Raman spectra were recorded using a Confocal microRaman System (Renishaw RM 2000) equipped with a near-IR diode laser at a wavelength of 782 nm (laser power: 1.15 mW and laser spot size: 1 µm). All Raman spectra were collected by fine-focusing a 50× microscope objective and the data acquisition time was 10 s. The plasmonic electric-field calculations of the studied metal nanostructures were performed using the finite element method (FEM) implemented in a commercially available electromagnetic solver (COMSOL Multiphysics). The nanohole membrane and the hierarchical porous metamaterial were modeled as periodic structures with rectangle shape unit cell, which consists of a central nanohole surrounded by four quarters and has an area of 105 nm × 105 3 nm with 105 nm being the interhole distance. The mesoporous film was modeled with 3 × 3 unit cells (with an area of 6 × 6 3 nm2 for each unit cell and 6 nm being the interpore distance) in order to fully take into account the electromagnetic interaction between neighboring mesopores. The
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www.MaterialsViews.com unit cell was discretized using tetrahedral mesh elements with mesh size down to 1 nm around air nanoholes or mesopores in the three structures. The simulation domain was surrounded up and down by two perfectly matched layers to absorb the outward propagating radiation while the periodic boundary conditions were applied to the side boundaries. The electric-field profile was calculated in frequency domain, i.e., at the laser excitation wavelength. The dielectric function of gold was extracted from the Johnson & Christy data[37] and used for the mesoporous film and nanohole membrane. The dielectric function for the hierarchical porous metamaterial was calculated in the framework of Bruggeman theory:[38] f
ε a − ε eff ε − ε eff + (1 − f ) m =0 ε a + 2ε eff ε m + 2ε eff
(1)
where f denotes the volume fraction of air in the structure and εm is the permittivity of gold (εa = 1 for air). Based on the mesopore diameter (2.5 nm) and interpore distance (6 nm), we calculated the volume fraction of air to be 31.5% for the hierarchical porous metamaterials. In our calculations, three assumptions were made to simplify the experimental scenario but without loss of the underlying physics. First, the surface density of the adsorbed molecules is the same for the three substrates; second, only the molecules adsorbed around the edges of nanoholes or mesopores contribute to the detected Raman signals;[18,39] and third, the dielectric response of the porous metamaterials can be treated as an effective medium through Bruggeman theory.[40] Based on these assumptions, sophisticated normalization procedures were applied with respect to the number density of “hot” spots and their respective dimensions in each structure. The theoretical SERS enhancement factor (EF) was calculated from the normalized electric near-field enhancement and the number of probe molecules adsorbed within the “hot” spots of each nanostructure. Based on the three assumptions mentioned in the main text, the SERS EF for the three structures can be simply given as: E Eo
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× l × n0
(2)
where E and E0 are the electric field amplitude at the structure surface and that of the incident light, respectively. Here l represents the total length of the circumferences of nanoholes or mesopores in a unit cell of the same area 105 nm × 105 3 nm and n0 with a unit of number/nm is the line density of molecules adsorbed along the circumferences of nanoholes or mesopores. As only the molecules adsorbed at the edges of nanoholes or mesopores significantly contribute to the measured Raman signal intensity, here we used the maximum field amplitude to calculate the field enhancement. For the mesoporous film, E/E0 = 1.68, ⎛105 × 105 ⎞ l = ⎝ 6 × 6 ⎠ × 2 × π × 2.5 = 4810 nm, we had an SERS enhancement factor of 3.83 × 104 n0. For the nanohole membrane and the hierarchical porous metamaterial, E/E0 = 3.53 and 4.78, l = 2 × π × 80 = 502 nm for both, we obtained SERS enhancement factors of 7.79 × 104 n0 and 2.62 × 105 n0, respectively.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements X.Z. and Y.Z. contributed equally to this work. This work was supported by the Australian Research Council through Grant No. DP120104334. D.R.M. is grateful to Australian Research Council for the Laureate Fellowship. Y.Z. thanks The University of New South Wales for
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the Vice-chancellor's Research Fellowship (Project ID RG134604). D.Y.L. acknowledges the financial support from the Hong Kong Research Grants Council (509513) and the Natural Science Foundation of China (11304261). Received: September 8, 2014 Revised: November 16, 2014 Published online:
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