Article pubs.acs.org/JPCC
Hierarchically Well-Ordered Array of Concentric Silver Nanorings for Highly Sensitive Surface-Enhanced Raman Scattering Substrate Dusik Bae,† Won Joon Cho,† Gumhye Jeon, Jinseok Byun, and Jin Kon Kim* National Creative Research Center for Block Copolymer Self-Assembly, Department of Chemical Engineering, Pohang University of Science and Technology, Kyungbuk 790-784, Korea S Supporting Information *
ABSTRACT: We fabricated a high density array of concentric silver nanorings in a large area (over in.2) with uniform gap distance by utilizing half onion-shaped microdomains prepared by symmetric polystyrene-block-poly(methyl methacrylate) copolymers (PS-b-PMMA) confined within hemispherical cavities in anodized aluminum oxide (AAO) template. Silver nanoparticles with 6 nm height were selectively deposited only on the PS microdomains by thermal evaporation. The gap distance of two neighboring silver nanorings was controlled from 12 to 24 nm by changing the total molecular weight of PS-b-PMMAs. The substrate showed high surface-enhanced Raman scattering (SERS) enhancement factor as high as 4.3 × 107 with good reproducibility (±7%). It could be used for biosensing, detection of trace-level explosive and hazardous chemicals, and reaction monitoring.
R
One method to reduce the gap distance of the concentric rings is to utilize the microdomains obtained by block copolymer self-assembly, because the distance of two different microdomains in a diblock copolymer is on the order of tens nanometer.28−34 Yet, to make the concentric metal rings based on the block copolymer microdomains, round-shaped cavities (or trenches), for instance, tubes or cups, should be prepared. It is well-known that the planar shaped-lamellar microdomains are easily transformed into concentric ring patterns when the block copolymer chains are confined in a cavity.35,36 Ross and coworkers fabricated the concentric ring patterns by spin-coating of cylindrical microdomain-forming polystyrene-block-poly(dimethyl siloxane) copolymer (PS-b-PDMS) on preformed circular trenches prepared by lithography.37 However, the concentric ring pattern was fabricated in a limited area (less than ∼1 cm2) due to the use of lithography to prepare the trenches. Thus, to fabricate the concentric ring patterns in a large area (over in.2), top-down approaches such as lithography are not effective for fabricating the round-shaped cavities. We realize that when an anodized aluminum oxide (AAO) template is used, the round-shaped cavities could be easily prepared in a large area. The diameter of each cavity is simply controlled by changing the anodization condition. In this study, we fabricated a high density array of concentric nanoring patterns of the lamellar microdomains by spin-coating of symmetric polystyrene-block-polymethylmethacrylate copolymer (PS-b-PMMA) on the AAO template. The gap distance
aman spectroscopy has been widely employed for medical,1 environmental, 2 materials, 3 and sensor4,5 research due to providing a fingerprint by which the molecule can be identified. However, very low intensity of the Raman signal should be enhanced.6 After the Raman signal on novel metals was found to be strongly enhanced arising from surfaceenhanced Raman scattering (SERS),7 SERS substrates have been extensively used as label-free immunoassays, 8 in biosensing,9 and surface-enhanced spectroscopy.10,11 The efficiency of a SERS substrate increases through the surface plasmon resonance (SPR), which depends on the size and shape of metal nanoparticles as well as the distance between them.12−14 When the distance between two neighboring metal nanoparticles becomes close, the SERS effect is dramatically increased, which is referred to as the “hot spot”.15,16 SERS is also strongly dependent on the lateral arrangement of metal nanoparticles when they exist in a large area.17,18 In contrast to the random arrangement of metal nanostructures, well-aligned and good lateral ordering of the nanoparticles increases the possibility to generate hot spots, resulting in enhanced SERS signal.19 One of the interesting arrangements of metal nanoparticles is the concentric rings,20 because the gap distance between the rings is easily controlled. By decreasing the gap distance of the concentric rings, an enhanced Raman signal was observed.21−23 However, the gap distance of the concentric rings reported in the literature is on the order of hundred nanometers due to using lithographic technique.24,25 Thus, fabricated SERS substrate showed a relatively small enhancement factor (EF). It is known that the hot spot (and thus higher EF) would be expected at a gap distance of ∼10 nm (or even smaller).15,26,27 © XXXX American Chemical Society
Received: September 11, 2012 Revised: October 31, 2012
A
dx.doi.org/10.1021/jp309008c | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
lamellar domain spacing (L0) of each block copolymer was measured by small-angle X-ray scattering (SAXS) and given in Table 1. Once the lamellar microdomains are confined within hemispherical AAO cavities, the ring patterns of the microdomains are formed at the top of the surface (Figure 1b). Finally, silver was deposited on the block copolymer film through the evaporation. Because of higher selectivity of silver on PS chains as compared to PMMA chains,38 silver nanoparticles are deposited on only PS concentric rings. Thus, a high density array of concentric silver nanorings is fabricated (Figure 1c). Figure 2 gives phase contrast atomic force microscopy (AFM) images of four different PS-b-PMMAs confined in the
between two neighboring concentric nanorings varied from 12 to 24 nm depending on the total molecular weights of PS-bPMMA. Because silver is deposited by evaporation on only PS microdomains of the concentric ring patterns, we could fabricate a high density array of concentric silver nanorings with various gap distances. The substrate showed very high SERS enhancement factor up to 4.3 × 107 with good reproducibility (±7%). This array could be used as biosensors with high sensitivity, chemical sensors for detection of extremely small amounts of explosive chemicals, and monitoring of the chemical reaction.
■
RESULTS AND DISCUSSION A schematic of the fabrication of a hierarchically well-ordered array of concentric silver nanorings in a large area is given in Figure 1. First, an AAO template with hemispherical cavities
Figure 1. Fabrication of a hierarchically well-ordered array of concentric silver nanorings by confining the lamellar microdomains of PS-b-PMMA in the cavities inside an AAO template. (a) PS-bPMMA was spin-coated on an AAO template with hemispherical cavities and thermally annealed at high temperature. (b) Concentric ring patterns of the microdomains were generated through the confinement of block copolymer microdomains in the cavities. (c) Silver was selectively deposited on the PS microdomains with a thermal evaporator.
Figure 2. Phase contrast AFM images of four different PS-b-PMMAs confined in the hemispherical cavities. Yellow and brown concentric ringes correspond to PMMA and PS phases, respectively. The total number of concentric rings in each cavity for four different block copolymers is (a) 16 of SML-36, (b)14 of SML-51, (c) 12 of SML-66, and (d) 10 of SML-98.
having a diameter (D) of 430 nm and a center-to-center distance between the cavities of 500 nm was prepared in a large area (the details are given in the Experimental Section and section 1 of the Supporting Information). Next, symmetric PSb-PMMA in toluene solution was spin-coated on the cavities and annealed at high temperature (170 °C) to obtain the equilibrium morphology. To change the gap distance between lamellar microdomains, we used four different molecular weights of PS-b-PMMAs whose molecular characteristics are given in Table 1. Because of the almost symmetric volume fraction in all block copolymers used in this study, the equilibrium morphology should be lamellar microdomains. The
hemispherical cavities. Thus, all of the images represent the surface morphology at the top of the cavity. PS concentric ring patterns are also clearly observed by field-emission scanning electron microscopy (FE-SEM) when PMMA blocks are completely removed (see Figure S2 in the Supporting Information). PS-b-PMMA morphology inside the cavity was half-onion structure, confirmed by cross-sectional transmission electron microscopy (TEM) (see Figure S3 in the Supporting Information). Yellow and brown rings in the AFM images in Figure 2 correspond to PMMA and PS microdomains, respectively. The outer rim of the AAO cavities with six edges is clearly observed. Because PMMA is more hydrophilic than PS, PMMA microdomains are located at the wall of the AAO cavities.39 Then, alternating PS and PMMA rings are formed to satisfy the commensurability betweeen L0 and D. Because D/L0 of all of the block copolymers is close to integer (see Table 1), the PS microdomain is located at the center of the cavity. As seen in Figure 2, all of the block copolymers show the concentric ring patterns consisting of alternative PMMA and PS microdomains from the edge to the center of the cavity. The total number (n) of the rings in each cavity for SML-36, SML-51, SML-66, and SML-98 is 16, 14, 12, and 10, respectively. These values are consistent with the estimated ones (1 + D/L0) by the commensurability condition.36,37 Because of the fixed D, the gap distance between each ring
Table 1. Molecular Characteristics of Symmetric PS-bPMMAs Employed in This Study symbol SML36 SML51 SML66 SML98
Mn of PS block
Mn of PMMA block
Mw/ Mna
f PSb
L0 (nm)
D/L0
18 000
18 000
1.07
0.52
28.8
14.9
25 000
26 000
1.06
0.51
32.9
13.0
33 000
33 000
1.09
0.52
38.4
11.2
50 000
48 000
1.13
0.53
47.3
9.1
a
Mn and Mw are the number- and weight-average molecular weights, respectively. bVolume fraction is obtained from weight fraction and known density of PS (1.05 g cm−3) and PMMA (1.15 g cm−3). B
dx.doi.org/10.1021/jp309008c | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
increases gradually with increasing molecular weight of PS-bPMMA. Figure 3 gives FE-SEM images of concentric silver nanorings after depositing silver with a thermal evaporator. Although
Figure 4. SERS spectra of CV on the concentric silver nanorings with various gap distances.
nonresonant SERS molecules, 1,2-di(4-pyridyl)ethylene (BPE) and 4-aminothiophenol (4-ATP), were also investigated, because these do not give any resonance at 514.5 nm. As shown in Figures S6−S8 in the Supporting Information, the effects of the gap distance on the SERS intensity (and EF value) for two nonresonant SERS molecules are similar to those of CV, although the SERS intensity (and the EF) was slightly decreased (20−30%) as compared to those of CV. This indicates that the SERS substrates employed in this study have excellent SERS properties for both resonant and nonresonant SERS molecules. The increase of the Raman intensity for both resonant and nonresonant molecules with decreasing d is consistent with the red-shift of a peak position at ∼500 nm in UV−visible spectra. The peak position at 446 nm for d = 24 nm was red-shifted to 516 nm for d = 12 nm (see Figures S9−11). Because the excitation wavelength was 514.5 nm, the red-shift arises from better plasmon coupling with decreasing d. Figure 5 shows plots of the calculated EF for SERS substrates with concentric silver nanorings versus the gap distance. The
Figure 3. FE-SEM images of concentric silver nanorings prepared by evaporating silver on the PS microdomains in four different block copolymers: (a) SML-36, (b) SM-51, (c) SML-66, and (d) SML-98.
some of evaporated silvers could be deposited on the entire block copolymer films, the deposited silvers on the PMMA microdomains quickly move to the PS microdomains due to the lower affinity of silver with PMMA as compared to PS.38 The deposited silvers were aggregated to form tiny silver nanoparticles with a diameter of ∼10 nm to reduce the surface energy (see Figures S4 and S5 in the Supporting Information). Thus, the concentric silver nanorings consisted of disconnected tiny silver nanoparticles, not continuous silver strips. The disconnected silver nanoparticles could increase additionally the SERS due to hot spot generation between the silver nanoparticles.40,41 The number of concentric silver nanorings for SML-36, SML-51, SML-66, and SML-98 is 8, 7, 6, and 5, respectively. This is exactly the same as the number of PS rings given in Figure 2, indicating that the silver is only deposited on the PS microdomains. The gap distance between concentric silver nanorings is equal to the width of lamellar microdomains of the PMMA block. In this study, we could control the gap distance ranging from 12 to 24 nm by changing the total molecular weight of PS-b-PMMA from 36 000 to 98 000. However, we could not decrease further the gap distance (less than 10 nm), because PS-b-PMMA with lower molecular weights (less than ∼25 000) could not microphase separate due to χN < 10.5, in which χ is the segmental interaction parameter of PS and PMMA, and N is the total number of segments. Thus, the formation of lamellar microdomains is impossible. Also, it is practically impossible to decrease dramatically the number of silver rings (say, 2−3 rings corresponding to the gap distance larger than ∼30 nm) due to huge bending force and entropy penalty of the block copolymer chains required for the bigger sized ring formation inside a cavity. Figure 4 shows SERS spectra of crystal violet (CV) molecules42,43 adsorbed on the concentric silver nanorings with different gap distances (d), measured with a Raman microscope at 514.5 nm excitation and a laser spot of ∼1.54 μm2. The SERS spectra show characteristic peaks of CV at 1172, 1371, and 1619 cm−1. The SERS signals increased with decreasing the gap distance from 24 to 12 nm, because of strong E-field localization at shorter gap distance. Two
Figure 5. EFs of CV molecules on SERS substrates with high density array of concentric silver nanorings as a function of gap distance (d).
details for the EF calculation have been given previously10 (see also section S5 of the Supporting Information). Because the size of the error bar is definitely smaller than the size of the data points shown in blue rectangles, the values of the standard deviation (STD) of the EF of all of the samples are given in Table S1. C
dx.doi.org/10.1021/jp309008c | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
EF of a SERS substrate with d larger than 17 nm was ∼5.4 × 106. At d ≈ 14 nm, EF was ∼8 times increased (∼3.4 × 107). The largest EF obtained in this study was ∼4.3 × 107 at d ≈ 12 nm. Although the critical gap distance of d for SERS substrate with silver nanoparticles is known to be ∼10 nm,15,26,27 a large increase in SERS intensity for high density array of concentric silver nanorings was observed for d ≈ 14 nm. This might be because the nanorings fabricated in this study contain concentrically arranged silver nanoparticles, which can increase the resonant frequency of collective oscillation arising from very small gap distance betweeen silver nanoparticles.44,45 However, the contribution of the gap distance in the concentric silver nanorings to EF of a SERS substrate dominates over that of the silver nanoparticle assembly. We also performed an additional SERS experiment by using another laser with a longer excitation wavelength (633 nm) to investigate the effect of the laser excitation wavelength on EF of the substrates. As shown in Figures S12 and S13, the change of EFs with d obtained by this laser is similar to that obtained by using a laser with a shorter wavelength (514.5 nm). However, the values of EFs of all of the substrates by using the laser with a wavelength of 633 nm are distinctly smaller than those obtained by another laser with a wavelength of 514.5 nm. This result clearly demonstrates that the SERS effect would be maximized when the excitation wavelength matches well with the plasmonic resonance of the silver nanostructures, consistent with the results given in ref 12. Additionally, we found that the measured Raman intensity is quite reproducible (less than ±7% over 50 experiments). This is because a large number of silver nanorings (and thus the hot spots), at least larger than 1000, contributed Raman spectra within the laser spot (∼1.54 μm2) and good uniformity in the gap distance. In conclusion, we first prepared the concentric ring patterns by confinement of block copolymer microdomains inside the hemispherical cavities of the AAO template, and then the concentric silver nanorings by selective deposition of silver on the PS microdomains. The Raman signal intensity (and EF) increased with decreasing the gap distance between silver nanorings. Because the gap distance depends on the total molecular weights of the block copolymer, EF is easily tuned. Also, the SERS intensity was quite reproducible because the high density array of concentric silver nanorings was prepared in a large area. The SERS substrate fabricated in this study could be used for biosensing, detection of trace-level explosive and hazardous chemicals, and reaction monitoring.
pores was etched, and a dimple layer remained. The second anodization was performed at 0 °C for 15 s. Finally, the pore diameter was controlled by pore-widening in 0.1 M H3PO4 aqueous solution at 30 °C. At a widening time of 150 min, the pore diameter was 430 nm. The center-to-center distance between neighboring pores was 500 nm (the details are given section 1 of the Supporting Information). The concentric ring patterns of block copolymer microdomains were prepared by spin coating from various concentrations of PS-b-PMMA in toluene on the AAO template. The pores in the AAO template were completely filled with block copolymers, and the sample was annealed at 170 °C for 5 days in vacuum, followed by quenching to room temperature. The silver was selectively deposited on PS microdomains in concentric ring patterns up to 3 nm thickness by using thermal evaporation at a rate of 0.01 nm s−1 under high vacuum.38,48 Characterization. The top view of the concentric ring patterns was measured by AFM (Digital Instrument) with tapping mode and a field emission scanning electron microscope (FE-SEM, Hitachi, S-4800) operating at 1 kV. The inner morphologies inside the cavities were measured by a crosssectional transmission electron microscope (TEM, Hitach Ltd., S-7600) operating at 80 kV. Sample preparation for the crosssectional TEM measurement is described in section S3 of the Supporting Information. Raman intensity was measured in a backscattering geometry by using a JY LabRam HR fitted with a liquid-nitrogen cooled CCD detector. The spectra were collected under ambient conditions using the 514.5 nm line of an Ar-ion laser with 0.05 mW irradiating the sample surface. The acquisition time was 20 s. The radius of the laser spot was 0.70 μm; thus, the laser spot area was 1.54 μm2. The SERS molecules employed in this study, CV, BPE, and 4-ATP, were purchased from Aldrich Chemical Co.
■
ASSOCIATED CONTENT
S Supporting Information *
Experimental details and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
■
Author Contributions †
EXPERIMENTAL SECTION Fabrication of a High Density Array of Concentric Silver Nanorings. Four different symmetric PS-b-PMMAs were purchased from Polymer source Inc. and used as received. The molecular characteristics of all of the polymers are given in Table 1. The lamellar domain spacings of PS-b-PMMAs were measured by synchrotron small-angle X-rays scattering at the 4C1 beamline of Pohang Accelerator Laboratory with an X-ray radiation source of λ = 0.1608 nm, and a 2-D CCD camera (Princeton Instruments, SCX-TE/CCD-1242) was used to collect the scattered X-rays.46 The sample thickness and the exposure time were 1.0 mm and 100 s, respectively. AAO was prepared by two-step anodization.47 First, anodization was performed in 0.1 M phosphoric acid aqueous solution at 0 °C to obtain the perfect hexagonal pore arrangement over a large area. Thick oxide layer with irregular
These authors contributed equally.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the National Creative Research Initiative Program of the National Research Foundation of Korea and a grant (code no. 2011-0031635) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, Korea. We appreciate the assistance of Nam Suk Lee in experiments of TEM at the National Center for Nanomaterials and Technology (NCNT). Small-angle X-ray scattering was performed at PAL beamline (4C1) supported by POSCO and NRF. D
dx.doi.org/10.1021/jp309008c | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
■
Article
Resonances in Ring/Disk Plasmonic Nanocavities. ACS Nano 2010, 4, 1664−1670. (23) Lee, S. J.; Guan, Z.; Xu, H.; Moskovits, M. Surface-Enhanced Raman Spectroscopy and Nanogeometry: The Plasmonic Origin of SERS. J. Phys. Chem. C 2007, 111, 17985−17988. (24) Chang, Y. T.; Tzuang, D. C.; Wu, Y. T.; Chan, C. F.; Ye, Y. H.; Hung, T. H.; Chen, Y. F.; Lee, S. C. Surface Plasmon on Aluminum Concentric Rings Arranged in a Long- Range Periodic Structure. Appl. Phys. Lett. 2008, 92, 253111−253114. (25) Hou, Y.; Xu, J.; Wang, P.; Yu, D. Surface-Enhanced Raman Spectroscopy on Coupled Two-Layer Nanorings. Appl. Phys. Lett. 2010, 96, 203107−203109. (26) Wang, H.; Levin, C. S.; Halas, N. J. Nanosphere Arrays with Controlled Sub-10-nm Gaps as Surface-Enhanced Raman Spectroscopy Substrates. J. Am. Chem. Soc. 2005, 125, 14992−14993. (27) Wang, H.; Liu, C.; Wu, S.; Liu, N.; Peng, C.; Chan, T.; Hsu, C.; Wang, J.; Wang, Y. Highly Raman-Enhancing Substrates Based on Silver Nanoparticle Arrays with Tunable Sub-10 nm Gaps. Adv. Mater. 2006, 18, 491−495. (28) Leibler, L. Theory of Microphase Separation in Block Copolymers. Macromolecules 1980, 13, 1602−1617. (29) Bates, F. S.; Fredrickson, G. H. Block Copolymer Thermodynamics: Theory and Experiment. Annu. Rev. Phys. Chem. 1990, 41, 525−557. (30) Han, C. D.; Baek, D. M.; Kim, J. K.; Ogawa, T.; Sakamoto, N.; Hashimoto, T. Effect of Volume Fraction on the Order-Disorder Transition in Low Molecular Weight Polystyrene-block-Polyisoprene Copolymers. 1. Order-Disorder Transition Temperature Determined by Rheological Measurements. Macromolecules 1995, 28, 5043−5062. (31) Ryu, D. Y.; Jeong, U.; Kim, J. K.; Russell, T. P. Closed-Loop Phase Behaviour in Block Copolymers. Nat. Mater. 2002, 1, 114−117. (32) Kim, J. K.; Lee, J. I.; Lee, D. H. Self-Assembled Block Copolymers: Bulk to Thin Film. Macromol. Res. 2008, 16, 267−292. (33) Bang, J.; Jeong, U.; Ryu, D. Y.; Russell, T. P.; Hawker, C. J. Block Copolymer Nanolithography: Translation of Molecular Level Control to Nanoscale Patterns. Adv. Mater. 2009, 21, 1−24. (34) Kim, J. K.; Yang, S. Y.; Lee, Y. M.; Kim, Y. S. Functional Nanomaterials Based on Block Copolymer Self-Assembly. Prog. Polym. Sci. 2010, 35, 1325−1349. (35) Shin, K.; Xiang, H.; Moon, S. I.; Kim, T.; McCarthy, T. J.; Russell, T. P. Curving and Frustrating Flatland. Science 2004, 306, 76. (36) Ma, M.; Titievsky, K.; Thomas, E. L.; Rutledge, G. C. Continuous Concentric Lamellar Block Copolymer Nanofibers with Long Range Order. Nano Lett. 2009, 9, 1678−1683. (37) Jung, Y. S.; Jung, W.; Ross, C. Nanofabricated Concentric Ring Structures by Templated Self-Assembly of a Diblock Copolymer. Nano Lett. 2008, 8, 2975−2981. (38) Lopes, W. A. Nonequilibrium Self-Assembly of Metals on Diblock Copolymer Templates. Phys. Rev. E 2002, 65, 031606− 031619. (39) Sun, Y.; Steinhart, M.; Zschech, D.; Adhikari, R.; Michler, G. H.; Gösele, U. Diameter-Dependence of the Morphology of PS-b-PMMA Nanorods Confined Within Ordered Porous Alumina Templates. Macromol. Rapid Commun. 2005, 26, 369−375. (40) Ding, P.; Liang, E. J.; Hu, W. Q.; Cai, G. W.; Xue, Q. Z. Tunable Plasmonic Properties and Giant Field Enhancement in Asymmetric Double Split Ring Arrays. Photonics Nanostruct. Fundam. Appl. 2011, 9, 42−48. (41) Lahiri, B.; Khokhar, A. Z.; De La Rue, R. M.; McMeekin, S. G.; Johnson, N. P. Asymmetric Split Ring Resonators for Optical Sensing of Organic Materials. Opt. Express 2009, 17, 1107−1115. (42) Watanabe, T.; Pettinger, B. Surface-Enhanced Raman Scattering from Crystal Violet Adsorbed on a Silver Electrode. Chem. Phys. Lett. 1982, 89, 501−507. (43) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Single Molecule Detection Using SurfaceEnhanced Raman Scattering (SERS). Phys. Rev. Lett. 1997, 78, 1667− 1670.
REFERENCES
(1) Kneipp, J.; Kneipp, H.; Wittig, B.; Kneipp, K. Following the Dynamics of pH in Endosomes of Live Cells with SERS Nanosensors. J. Phys. Chem. C 2010, 114, 7421−7426. (2) Vo-Dinh, T. SERS Chemical Sensors and Biosensors: New Tools for Environmental and Biological Analysis. Sens. Actuators, B: Chem. 1995, 29, 183−189. (3) Chen, Y.-C.; Young, R. J.; Macpherson, J. V.; Wilson, N. R. Single-Walled Carbon Nanotube Networks Decorated with Silver Nanoparticles: A Novel Graded SERS Substrate. J. Phys. Chem. C 2007, 111, 16167−16173. (4) Maher, R. C.; Maier, S. A.; Cohen, L. F.; Koh, L.; Laromaine, A.; Dick, J. A. G.; Stevens, M. M. Exploiting SERS Hot Spots for DiseaseSpecific Enzyme Detection. J. Phys. Chem. C 2009, 114, 7231−7235. (5) Han, B.; Choi, N.; Kim, K. H.; Lim, D. W.; Choo, J. Application of Silver-Coated Magnetic Microspheres to a SERS-Based Optofluidic Sensor. J. Phys. Chem. C 2011, 115, 6290−6296. (6) Nie, S.; Emory, S. Probing Single Molecules and Single Nanoparticles by Surface- Enhanced Raman Scattering. Science 1997, 275, 1102−1106. (7) Fleischmann, M.; Hendra, P.; McQuillan, A. Raman Spectra of Pyridine Adsorbed at a Silver Electrode. Chem. Phys. Lett. 1974, 26, 163−166. (8) Schultz, S.; Smith, D. R.; Mock, J. J.; Schultz, D. A. Single-Target Molecule Detection with Nonbleaching Multicolor Optical Immunolabels. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 996−1001. (9) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Biosensing with Plasmonic Nanosensors. Nat. Mater. 2008, 7, 442−453. (10) Cho, W. J.; Kim, Y.; Kim, J. K. Ultrahigh-Density Array of Silver Nanoclusters for SERS Substrate with High Sensitivity and Excellent Reproducibility. ACS Nano 2012, 6, 249−255. (11) Yap, F. L.; Thoniyot, P.; Krishnan, S.; Krishnamoorthy, S. Nanoparticle Cluster Arrays for High-Performance SERS through Directed Self-Assembly on Flat Substrates and on Optical Fibers. ACS Nano 2012, 6, 2056−2070. (12) Mulvihill, M. J.; Ling, X. Y.; Henzie, J.; Yang, P. Anisotropic Etching of Silver Nanoparticles for Plasmonic Structures Capable of Single-Particle SERS. J. Am. Chem. Soc. 2010, 132, 268−274. (13) Porter, M. D.; Lipert, R. J.; Siperko, L. M.; Wang, G.; Narayanan, R. SERS as a Bioassay Platform: Fundamentals, Design, and Applications. Chem. Soc. Rev. 2008, 37, 1001−1011. (14) Kneipp, J.; Kneipp, H.; Kneipp, K. SERS- A Single-Molecule and Nanoscale Tool for Bioanalytics. Chem. Soc. Rev. 2008, 37, 1052− 1060. (15) Qin, L.; Zou, S.; Xue, C.; Atkinson, A.; Schatz, G. C.; Mirkin, C. A. Designing, Fabricating, and Imaging Raman Hot Spots. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 13300−13303. (16) Le Ru, E. C.; Blackie, E.; Meyer, M.; Etchegoin, P. G. Surface Enhanced Raman Scattering Enhancement Factors: A Comprehensive Study. J. Phys. Chem. C 2007, 111, 13794−13803. (17) Ko, H.; Singamaneni, S.; Tsukruk, V. V. Nanostructured Surfaces and Assemblies as SERS Media. Small 2008, 4, 1576−1599. (18) Halas, N. J.; Lal, S.; Chang, W. S.; Link, S.; Nordlander, P. Plasmons in Strongly Coupled Metallic Nanostructures. Chem. Rev. 2011, 111, 3913−3961. (19) Genov, D. A.; Sarychev, A. K.; Shalaev, V. M.; Wei, A. Resonant Field Enhancements from Metal Nanoparticle Arrays. Nano Lett. 2004, 4, 153−158. (20) Pasquale, A. J.; Reinhard, B. M.; Negro, L. D. Concentric Necklace Nanolenses for Opitcal Near-Field Focusing and Enhancement. ACS Nano 2012, 6, 4341−4348. (21) Wi, J. S.; Sengupta, S.; Wilson, R. J.; Zhang, M.; Tang, M.; Wang, S. X. Raman-Active Two-Tiered Ag Nanoparticles with a Concentric Cavity. Small 2011, 7, 3276−3280. (22) Sonnefraud, Y.; Verellen, N.; Sobhani, H.; Vandenbosch, G. A. E.; Moshchalkov, V. V.; Van Dorpe, P.; Nordlander, P.; Maier, S. A. Experimental Realization of Subradiant, Superradiant, and Fano E
dx.doi.org/10.1021/jp309008c | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
(44) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: the Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107, 668− 677. (45) Chang, W. S.; Willingham, B.; Slaughter, L. S.; Medina, S. D.; Swanglap, P.; Link, S. Radiative and Nonradiative Properties of Single Plasmonic Nanoparticles and Their Assemblies. Acc. Chem. Res. 2012, 45, 1936−1945. (46) Bolze, J.; Kim, J.; Huang, J. Y.; Rah, S.; Youn, H. S.; Lee, B.; Shin, T. J.; Ree, M. Current Status of the Synchrotron Small-Angle Xray Scattering Station BL4C1 at the Pohang Accelerator Laboratory. Macromol. Res. 2002, 10, 2−12. (47) Li, A. P.; Müller, F.; Birner, A.; Nielsch, K.; Gösele, U. Fabrication and Microstructuring of Hexagonally Ordered TwoDimensional Nanopore Arrays in Anodic Alumina. Adv. Mater. 1999, 11, 483−487. (48) Lopes, W. A.; Jaeger, H. M. Hierarchical Self-Assembly of Metal Nanostructures on Diblock Copolymer Scaffolds. Nature 2001, 414, 735−738.
F
dx.doi.org/10.1021/jp309008c | J. Phys. Chem. C XXXX, XXX, XXX−XXX
