Freestanding and Arrayed Nanoporous Microcylinders for Highly

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Freestanding and Arrayed Nanoporous Microcylinders for Highly Active 3D SERS Substrate Su Yeon Lee, Shin-Hyun Kim, Minsoo P Kim, Hwan Chul Jeon, Hyelim Kang, Hyeong Jun Kim, Bumjoon J. Kim, and Seung-Man Yang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm400298e • Publication Date (Web): 04 Jun 2013 Downloaded from http://pubs.acs.org on June 9, 2013

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Chemistry of Materials

Freestanding and Arrayed Nanoporous Microcylinders for Highly Active 3D SERS Substrate Su Yeon Lee,† Shin-Hyun Kim,‡ Minsoo P. Kim,‡ Hwan Chul Jeon,† Hyelim Kang,† Hyeong Jun Kim,‡ Bumjoon J. Kim,‡ and Seung-Man Yang†,*

†National Creative Research Initiative Center for Integrated Optofluidic Systems and Department of Chemical and Biomolecular Engineering, KAIST, Daejeon, 305-701, Korea ‡Department of Chemical and Biomolecular Engineering, KAIST, Daejeon, 305-701, Korea KEYWORDS: Block copolymers, microphase separation, imprinting, SERS, surface plasmon

ABSTRACT: Surface-enhanced Raman scattering (SERS) has been considered as one of the most promising tools for molecular analysis. To develop practical platforms, a variety of nanoparticles and two-dimensional (2D) nanostructures have been prepared. However, low signal intensity or slow binding kinetics in conventional approaches limits their applications. To overcome these shortcomings, production and usage of 3D nanostructures remain an important yet unmet need. In this paper, we report novel and effective SERS-active materials by fabricating hierarchically-structured SiO2 microcylinders decorated with Au nanoparticles. In order to fully develop 3D nanostructures, while maintaining fast diffusion of analyte molecules, we used self-assembled nanostructures of block-copolymers (BCPs) confined in the microholes of an imprinting mold; the BCPs could provide a template for producing 3D nanostructure composed of nanofibers with sub100 nm diameter through their microphase separation, whereas the imprinting technique provided cylindrical geometry for the local confinement of the BCPs. Microcyinders with nanodomains were then transformed into microcylinders with 3D nanopores via reactive ion etching and subsequently, their nanopores were decorated by Au nanoparticles. The resultant 3D nanopores enable a high loading of Au nanoparticles and formation of abundant hot spots and microcylinders facilitate the fast diffusion of analyte molecules through the nanopores, resulting in significant enhancement of SERS intensity.

INTRODUCTION Label-free detection of chemicals and biomolecules with high sensitivity has great potential in analytical chemistry, medical diagnosis, and drug screening. Surfaceenhanced Raman scattering (SERS) has been intensively studied for decades as a promising strategy for amplifying the weak Raman signals that make up the molecular fingerprint of analyte molecules. This potentially enables label-free identification of the species, even at extremely low concentrations.1 Raman signals from analyte molecules can be increased by several orders of magnitude by absorption onto roughened surfaces or nanoparticles made of noble metal. Such a dramatic amplification is the result of the excitation of localized surface plasmons and formation of charge–transfer complexes on the metal surface. This makes the Raman signal comparable in strength to fluorescence signals, enabling the facile detection of the molecular fingerprint using a conventional spectrometer.2 In order to utilize SERS in practical applications, a variety of new metal nanostructures have been proposed and developed.3,4 For example, anisotropic metal nano-

particles have been synthesized for achieving higher enhancements of the signal.3a,b However, one drawback to this approach is that nanoparticle suspensions sometimes require concentration of the particles in order to acquire a strong signal. As alternatives, two dimensional (2D) arrays of metal nanostructures have been prepared using photolithography, nanosphere lithography, and blockcopolymer lithography to act as SERS-active substrates; here, we classified both the planar and curved surfaces as 2D arrays.3c,d,4 These 2D arrays have higher densities of metal nanostructures on the solid substrates, thereby providing a higher signal intensity,5 although the binding rate of analytes is lower in comparison with the suspension approach. To improve the density of metal nanostructures further, 3D nanostructures have been introduced. These provide a larger surface area of metal nanostructure and potentially have a higher density of hot spots, which are formed in the nanogaps between two neighboring metal nanostructures.6,7 To make such 3D SERS-active substrates, various porous templates have been employed. For example, anodized porous silicon or

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alumina were used to deposit metal film or metal nanoparticles.7,8 However, their geometry, composed of arrays of vertically aligned long cylindrical pores, achieves limited enhancement of surface area or density of hot spots. Moreover, the long nanopores make the diffusion of analyte molecules through them difficult, resulting in low binding rate of analyte molecules. Therefore, most of previous approaches have not completely exploited the advantage of 3D structures. In this work, in order to fully develop 3D nanostructures, while maintaining fast diffusion of analyte molecules, we have used self-assembled nanostructures of blockcopolymers (BCPs) confined in the microholes of an imprinting mold. The BCPs can provide a template for producing 3D structures composed of fibers with sub-100 nm diameters through their microphase separation, whereas the imprinting technique provides cylindrical geometry for the local confinement of the BCPs on the scale of 1 μm. The combination of these two techniques facilitated the production of 3D nanostructures within a cylindrical shape in the form of an array on a substrate. The resulting microcylinders with nanofibers could then be transformed into nanoporous microcylinders by treatment with oxygen plasma. This resulted in 3D nanostructures that were highly porous and open to the surrounding environment through all surfaces except for the bottom, facilitating diffusion of molecules. In addition, a discrete array of cylinders enabled the fast transport of analyte molecules on the cylinder surface through convective motion, which is difficult to achieve in a continuous nanoporous film, where all transport processes are governed by diffusion from the film surface through the pores. Moreover, our 3D structures were suitable for accommodating metal nanoparticles on the pore surface and producing a high density of hot spots for SERS detection. EXPERIMENTAL SECTION Synthesis of Polystyrene-b-Polydimethylsiloxane-bPolystyrene (PS-b-PDMS-b-PS) Triblock Copolymer. PS-b-PDMS-b-PS triblock copolymers were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. The detailed procedure of RAFT polymerization for the synthesis of the PS-b-PDMS-b-PS was described in the Supporting Information.9 In briefly, styrene monomer, PDMS-RAFT agent, and 2,2'azobisisobutyronitrile (AIBN) (mol of PDMS-RAFT agent: mol of AIBN = 5:1) in benzene were added into a glass ampoule. The reactants were degassed by five freeze-thaw cycles. The inhibitors in the monomers were removed by an alumina column prior to polymerization. The polymerization was performed under vacuum at 70 °C for 12 h. The product was then precipitated by cold methanol. The total number average molecular weight, Mn, and polydispersity index (PDI) of PS-b-PDMS-b-PS were determined as 43,000 g/mol and 1.16, respectively by gel permeation chromatography (GPC) which was calibrated by PS standards. The chemical structure of the product was con-

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firmed by 1H-NMR (see Figure S3 in the Supporting Information). Preparation of Microcylinder Arrays of BCPs. After cleaning a silicon wafer (100) by performing ultrasonic treatment, a 2 wt % solution of PS-b-PDMS-b-PS in chloroform was spin-coated onto a silicon wafer at 3000 rpm to produce a continuous film of the BCP. The imprinting mold was prepared using standard soft lithography with a combination of soft-PDMS in the body of the mold and hard-PDMS on the surface, which enabled the achievement of high shape definition, while maintaining the softness of the mold.10 The PDMS mold with cylindrical holes was placed onto the BCP film and heated up to 140°C and maintained for 3 h under vacuum. Following this, the sample was further annealed with two steps of heat treatment at 140°C for 24 h and at 90°C for 12 h. After cooling the sample down to room temperature, the PDMS mold was released from the substrate, resulting in the microcylinder arrays of BCPs on the substrate. Reactive Ion Etching (RIE) of BCP Templates and Deposition of Au Nanoparticles on Nanopores. To produce the nanoporous structures within each microcylinder, RIE was performed on the microcylinder arrays with O2 gas under 100 mTorr for 10 min. Gold nanoparticles were synthesized by gold precursor reduction method.15 In 500 mL flask equipped with a condenser, 137 mL aqueous solution of 1 mM HAuCl4·3H2O (hydrogen tetrachloroaurate (Ⅲ) trihydrate) was boiled at 100°C using oil bath. Into this solution, 13.7 mL aqueous solution of 38.8 mM sodium citrate was rapidly injected and the mixture was maintained for 10 min. After cooling to room temperature, the solution was diluted with distilled water into 0.012 wt/wt% suspension. Prior to deposition of the Au nanoparticles on the nanopores, the SiO2 pores were functionalized with amine groups to induce positive charge. For this, the nanoporous microcylinder arrays were soaked in 100 mL of ethanol solution containing 10 μL aminopropyltrimethoxysilane (APTMS) and 5 mL of ammonia solution for 12 h. After washing with ethanol, the nanoporous microcylinder arrays were then immersed in an aqueous suspension of Au nanoparticles for 15, 30, and 45 min, respectively and the substrate was washed with distilled water several times. Characterization. The morphologies of surface and cross-section of sample were observed by scanning electron microscope (SEM, Philips-XL30SFEG). The size and zeta potential of synthesized Au nanoparticles were measured by transmission electron microscope (TEM, Philips Tecnai F20) and Zetasizer Nano ZS90 (Malvern Instruments), respectively. The absorption spectrum of the Au particles dispersed in water was characterized by UV/VIS–NIR spectrometer (Jasco V-570). The Raman spectra were measured using a high-resolution dispersive Raman microscope (Horiba JobinYvon, LabRAM HR UV/Vis/NIR), in which a 633 nm laser with a power of 4.25 mW was focused on the sample surface with spot diameter of 1 μm.

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Chemistry of Materials

Figure 1. Schematic illustration of the method to fabricate array of nanoporous SiO2 microcylinders decorated with Au nanoparticles. The microphase separation produces a template for nanoporous structure, whereas microimprinting technique provides isolated volume for the microphase separation. Subsequent plasma treatment with oxygen and deposition of gold nanoparticles produce surface-enhanced Raman scattering (SERS)-active substrate.

RESULTS AND DISCUSSION The procedures for preparing the 3D SERS-active substrate are schematically illustrated in Figure 1. We have used PS-b-PDMS-b-PS triblock-copolymer with Mn = 43 kg/mol, volume fraction of PDMS (fPDMS)=0.23 as a BCP template to produce 3D nanostructure. A continuous film of the BCP was spin-coated with a thickness of 200 nm (Figure 1, step 1), where the thickness was carefully controlled in order to minimize residual film on the wafer during the subsequent imprinting procedure. When the initial film thickness was smaller than 200 nm, holes on the mold were not fully occupied although the residual film can be completely dewetted, as shown in Figure S4 in the Supporting Information. The PDMS mold had cylindrical holes 1 μm in both diameter and height, forming square arrays with a periodicity of 2 μm. The imprinting of the PS-b-PDMS-b-PS was carried out by annealing the film with the mold at 140°C under vacuum. As this temperature was above the glass transition temperatures of both the PS (Tg ~ 100°C) and PDMS (Tg ~ −125°C), the BCP PS-b-PDMS-b-PS climbed up the holes in the PDMS mold by capillary action. It was observed that the PS-b-PDMSb-PS polymers completely filled the holes within 3 h, leaving ultra-thin film behind outside of the holes on the Si substrate.11 After the filling of the mold was complete, the sample was annealed at 140 °C for 24 h to promote microphase separation of the PS-b-PDMS-b-PS within each hole. And then, the sample was further annealed at a lower temperature of 90 °C for an additional 12 h to obtain a higher degree of separated nanodomains (Figure 1, step 2). As the Flory-Huggins interaction parameter (χ) is inversely proportional to temperature (T), annealing at a lower temperature can impose a higher driving force for phase separation. Such stepwise thermal annealing at two distinct temperatures can efficiently induce phase separation of BCP nanostructures at relatively short annealing time. After cooling the sample down to room temperature, the PDMS mold was released from the substrate, leaving the microcylinder arrays of BCPs over a large area (Figure 1, step 3; Figures 2a and b). The size and the aspect ratio of the fabricated microcylinders were well matched with those of the holes in the mold, which indicates that the mold patterns were perfectly transferred to the microcylinders of PS-b-PDMS-b-PS BCPs. The Si substrate with pattern of microcylinder arrays exhibited diffraction colors, as shown in the inset of Figure 2a, indicating high uniformity of the pattern on a macroscopic area (~ 2 × 2 cm2). It was found that the PS-b-PDMS-b-PS (Mn

= 43 kg/mol, fPDMS = 0.23) BCPs exhibited the formation of PDMS nanodomains in continuous PS matrix as they were casted into film, where the spacing between PDMS domains was 27 nm as we confirmed from the grazing incidence x-ray scattering measurements (Figure S3). In addition, we observed the interior of the PS-b-PDMS-b-PS microcylinders, as shown in Figure S5; TEM images show disordered cylindrical domain of PDMS which are embedded in continuous PS domain. To fabricate the 3D porous structures within each microcylinder, RIE was carried out on the arrays for 10 min, transforming them into porous SiO2 structures (Figure 1, step 4). The RIE removed all the organic molecules of the PS and PDMS nanodomains through oxidization of the carbon and hydrogen into CO2 and H2O, while oxidizing the silicon atoms in the PDMS into SiO2. As shown in Figures 2c and d, each resulting microcylinder become highly porous structures composed of nanofibers. All the microcylinder surfaces were open to the surrounding environment through the nanopores, except for the bottom. The removal of the organic BCP molecules and the formation of the SiO2 structures were confirmed by the change in the contact angle of water drop on the substrate; the water drops spread on the porous cylinder arrays after RIE due to hydrophilicity of SiO2 surface, while dense cylinder arrays

Figure 2. (a, b) Scanning electron microscopy (SEM) images of microcylinder arrays made of PS-b-PDMS-b-PS triblockcopolymer. The inset of (a) shows high uniformity of the 2 pattern on a macroscopic area (~ 2 × 2 cm ). (c, d) SEM images of nanoporous SiO2 microcylinder arrays which were prepared by reactive ion etching (RIE) with oxygen.

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Figure (a-c) SEM images of nanoporous SiO2 The microcylinder before 3.RIE showed the contact angle of 130°. resultant arrays decorated with Au nanoparticles; three different deposition times of (a) 15, (b) 30, and (c) 45 min were employed. SEM images of dense microcylinder arrays decorated with Au nanoparporous microcylinder arrays were able to provide a(d) superticles for 30 min. The each inset of (a-d) is corresponding magnified image. (e) Raman spectra of benzenethiol (BT) adsorbed on hydrophobic surface through simple vapor deposition of a Au nanoparticles of nanoporous SiO microcylinder with three different deposition times. The spectrum from dense microcylin2 fluoro-terminated silane onto the porous SiO2 structure. der included for comparison. (f) Influence of deposition time of Au nanoparticles on SERS signal intensity of BT at 994, 1017, Theis hierarchical porous structures, consisting of na−1 1071, and 1571 cm , where the signal is normalized to that for deposition time of 45 min.

nopores formed by microphase separation and 1 μm pores produced by imprinting, facilitated the trapping of air within the structures, thereby making the superhydrophobic surface. (see Figure S6 in the Supporting Information). Superhydrophobicity on SERS-active surfaces can localize a significant concentration of analyte molecules in a small area through evaporation of a water droplet, enabling the measurement of SERS signal from an extremely low concentration of analyte.12

The 3D nanoporous microcylinders were decorated with Au nanoparticles to produce the SERS-active substrate. For efficient deposition of the gold nanoparticles on the nanopores, the electrostatic attraction between the Au and the SiO2 was utilized. 13 To accomplish this, the SiO2 pores were functionalized with amine groups by the formation of a self-assembled monolayer of APTMS, resulting in positive charge under neutral pH conditions.14 Au nanoparticles with a diameter of 13 ± 3 nm were synthesized and stabilized by citric acid, resulting in a negative charge with a zeta potential of −31.7 mV under neutral pH conditions.15 TEM images and the absorption spectrum of the Au particles dispersed in water are shown in the inset of Figure 3a and Figure S7 of the Supporting Information. The substrate with the nanoporous microcylinder arrays was incubated in a 0.012 wt/wt% aqueous suspension of Au nanoparticles in order to deposit them in the nanopores via electrostatic attraction. Since the SERS activity depends on the density of metal nanoparticles or interparticle distance, as well as the size and shape of the nanoparticles, deposition time is extremely important for achieving a high density of hot spots for SERS-based molecular detection. Therefore, the incubation time was controlled at 15, 30, and 45 min. Deposition during the initial 15 min achieved incomplete

surface coverage of the porous structures, as shown in Figure 3a (see inset of Figure 2d for comparison with the porous structure without Au nanoparticles). Extending the deposition time to 30 or 45 min increased the density of Au nanoparticles in the pores, as shown in Figures 3b and c; 2D nanostructure was formed between microcylinders from residual layer. The plasmonic coupling between the Au nanoparticles embedded in the porous structure can influence the wavelength of plasmonic absorption. To investigate this coupling effect, we measured the reflectance spectrum of the Au nanoparticle-decorated microcylinders as shown in Figure S8. The dip position in the reflectance, 650 nm, corresponds to plasmonic wavelength. By comparison, the plasmonic wavelength of the same nanoparticles (diameter of 13 nm) in aqueous phase is 520 nm as shown in Figure S7; this is the coupling-free plasmonic absorption.16 Therefore, we can confirm that the Au nanoparticles in the porous structure are coupled in terms of localized surface plasmonic resonance. To investigate the SERS activities of microcylinders with Au nanoparticles deposited for the three different time periods, the arrays were immersed in an ethanolic solution of 2 mM benzenethiol (BT) for 12 h. During this incubation, BT molecules were immobilized on the surface of the Au nanoparticles within the pores of the microcylinder arrays. After washing with ethanol several times, the substrates were dried and the Raman signals from the BT molecules were then measured using a high-resolution dispersive Raman microscope. A laser with a wavelength of 633 nm and a power of 4.25 mW was focused on one microcylinder with a spot size 1 μm in diameter and the Raman signal was collected during integration time of 5 s; for each measurement, single microcylinder was included

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Chemistry of Materials to incomplete coverage in the inner volume of microcylinder. Although the EF value is comparable, larger surface area of 3D structure definitely provides much higher intensity of SERS signals.

Figure 4. (a) Schematics for release of microcylinders from substrate to be freely-suspended microparticles. (b, c) SEM images of nanoporous silica particles.

within the spot, thereby excluding any contribution from residual 2D nanostructure. The SERS spectra obtained from BT on microcylinders are summarized in Figure 3e. The measurements were repeated several times for different microcylinders and the deviation in intensity was found to be less than 5% (see Figure S9 in the Supporting Information). The spectra can be seen to exhibit characteristic peaks of BT molecules at 994, 1017, 1071, and 1571 cm-1, with the intensity of each peak depending significantly on the Au deposition time. Deposition for 45 min induced a signal almost 10 times stronger than that for 15 min for all four peaks as shown in Figure 3f, where the intensities are normalized to maximum values. The high level of sensitivity after the 45 min deposition time was attributed to the formation of dense hot spots, which normally exist in nanogaps between two metal structures. As the deposition time increased, the density of Au nanoparticles increased, shortening the interparticle distance. Because the Au nanoparticles all had the same surface charge, severe aggregation was prevented, even at long deposition times or high densities of particles in the nanopores. The enhancement factor (EF) can be roughly obtained by estimating surface area of Au nanoparticles in porous microcylinders and packing density of BT on Au surfaces. To estimate the area, we assumed that the porous microcylinder is composed of long nanocylinders whose diameter is same to that of PDMS domains as 15 nm; this provides the total length of the nanocylinders as 1.022 mm by considering the volume fraction of PDMS, 23%, in each microcylinder. Assuming the surface of nanocylinders is fully and densely covered by hemispheres of Au nanoparticles, we estimated the total surface of Au nanoparticles which are available for SERS measurement. For packing density of BT molecules in Au surface, we used reference value of 6.8 × 1014 molecules/cm2.3c,17 From these, we obtained the EF as 0.65 × 105 at 1071 cm-1 for deposition time of 45 min; this value is comparable to previous substrates with 2D and pseudo3D structures.3c,d,4,5 But, the value is possibly underestimated due to overestimation of the surface area; the actual surface area of Au nanoparticles would be smaller due

To evaluate the effect of the 3D porous structure, dense microcylinder arrays were prepared using the same procedure but with some modifications to the RIE step, with only 30 s treatment carried out as opposed to the 10 min used previously. This short etching time resulted in only the molecules on the surface being oxidized. The formation of SiO2 on the surface of the microcylinders was confirmed by the reduction in the contact angle of water drop. After the RIE treatment, an APTMS self-assembled monolayer was again produced on the microcylinders, followed by 30 min deposition of Au nanoparticles. As shown in Figure 3d, the Au nanoparticles fully covered all surfaces of the microcylinders. The SERS spectra of BT molecules immobilized on the Au particle-coated dense microcylinders were measured, as shown in the green curve in Figure 3e. The intensity of peaks from the dense microcylinders was observed to be at least 10-fold lower than those from the porous microcylinders with the same time of Au nanoparticle deposition. It was confirmed that the 3D nanoporous structures, which had a much higher surface area for Au particle deposition, had a significant advantage over the 2D structure. More importantly, the nanopores had a higher probability of forming hot spots, where two particles are separated by a nanogap, owing to their specific geometry. These results indicate that the Au nanoparticle-decorated porous microcylinders, which delivered much better results than the 2D arrays, could be used as an effective and promising 3D SERS substrate. Microcylinder arrays facilitate the transport of analyte molecules onto the surface of a 3D porous structure through convective motion, in comparison to a continuous film of porous structures. However, substrate-based SERS-active materials have intrinsic limitations in the binding kinetics of analyte molecules due to lack of mobility, whereas fast molecular binding can be achieved by suspension-based SERS-active materials.6c,18 In this work, the 3D porous microcylinders mounted on the substrate could be released as freely-suspended microparticles to achieve high mobility and therefore faster binding of the analyte, as shown in Figure 4a. In this regard, ultrasonication was applied for several minutes, which provided stress on the microcylinders and caused them to detach to release the microcylinders from the substrate (Figure 4b). The microcylinders were safely released without deterioration of the structure as shown in Figures 4b and c, where the bottom surfaces are intact. This proves that ultra-sonication has no influence on structural stability. These highly-porous microparticles can be used as SERS-active materials after deposition of Au nanoparticles. In addition, they are potentially useful in various applications including catalysts, adsorbents, and drug carriers, owing to their high surface area and high mobility; these will be further studied as our future work.

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CONCLUSIONS In summary, we have fabricated a 3D SERS-active substrate by depositing Au nanoparticles into hierarchicallystructured SiO2 microcylinders. A combination of BCP microphase separation and microimprinting techniques enabled the production of nanoporous SiO2 structures within microscale cylinder arrays. Using the nanoporous SiO2 microcylinders as a supporting architecture, negatively charged Au nanoparticles were deposited on the positively-charged nanopores via electrostatic attraction. The resulting Au nanoparticle-decorated porous microcylinders exhibited a SERS signal from analyte molecules 10 times higher than that obtained using a surface composed of dense microcylinders. This was attributed to the more substantial loading of Au nanoparticles and the higher density of hot spots. In addition, the current approach provides the means for patterning of microcylinders, and so could be used for on-chip incorporation of the microstructures and in-situ detection of analyte molecules.19 Therefore, this facile and straightforward approach to fabricating 3D SERS substrates provides great potential for robust, cost-effective, and ultrasensitive label-free detection of chemicals and biomolecules.

ASSOCIATED CONTENT Supporting Information. Experimental details for synthesis of block copolymer, optical microscope images for superhydrophobic surface and TEM image of gold nanoparticles are included. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENT This work was supported by a grant from the Creative Research Initiative Program of the Ministry of Education, Science and Technology for “Complementary Hybridization of Optical and Fluidic Devices for Integrated Optofluidic Systems.” The authors also appreciate partial support from the Brain Korea 21 Program.

REFERENCES (1)(a) Nie, S. M.; Emery, S. R. Science 1997, 275, 1102. (b) Cao, Y. W. C.; Jin, R. C.; Mirkin, C. A. Science 2002, 297, 1536. (c) Shanmukh, S.; Jones, L.; Driskell, J.; Zhao, Y. P.; Dluhy, R.; Tripp, R. A. Nano Lett. 2006, 6, 2630. (d) Kneipp, K.; Kneipp, H.; I, I.; Dasari, R. R.; Feld, M. S. Chem. Rev. 1999, 99, 2957. (2) (a) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R.; Feld, M. S. Phys. Rev. Lett. 1997, 78, 1667. (b) Fleischm.M; Hendra, P. J.; Mcquilla.A. J. Chem. Phys. Lett. 1974, 26, 163. (c) Jeanmaire, D. L.; Vanduyne, R. P. J. Electroanal. Chem. 1977, 84, 1. (3) (a) Mulvihill, M. J.; Ling, X. Y.; Henzie, J.; Yang, P. D. J. Am. Chem. Soc. 2010, 132, 268. (b) Li, W. Y.; Camargo, P. H. C.; Au, L.; Zhang, Q.; Rycenga, M.; Xia, Y. N. Angew. Chem. Int. Ed. 2010, 49, 164. (c) Yu, Q. M.; Guan, P.; Qin, D.; Golden, G.; Wallace, P. M. Nano Lett. 2008, 8, 1923. (d) Jang, S. G.; Choi, D. G.; Heo, C. J.; Lee, S. Y.; Yang, S. M. Adv. Mater. 2008, 20, 4862.

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(4) (a) Lee, W.; Lee, S. Y.; Briber, R. M.; Rabin, O. Adv. Funct. Mater. 2011, 21, 3424. (b) Yap, F. L.; Thoniyot, P.; Krishnan, S.; Krishnamoorthy, S. ACS Nano 2012, 6, 2056. (c) Cho, W. J.; Kim, Y.; Kim, J. K. ACS Nano 2012, 6, 249. (d) Sawai, Y.; Takimoto, B.; Nabika, H.; Ajito, K.; Murakoshi, K. J. Am. Chem. Soc. 2007, 129, 1658. (e) Hirai, Y.; Yabu, H.; Matsuo, Y.; Ijiro, K.; Shimomura, M. Chem. Commun. 2010, 46, 2298. (5) (a) Stiles, P. L.; Dieringer, J. A.; Shah, N. C.; Van Duyne, R. R. Annu. Rev. Anal. Chem. 2008, 1, 601. (b) Gunnarsson, L.; Bjerneld, E. J.; Xu, H.; Petronis, S.; Kasemo, B.; Kall, M. Appl. Phys. Lett. 2001, 78, 802. (c) Litorja, M.; Haynes, C. L.; Haes, A. J.; Jensen, T. R.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 6907. (d) Zhang, L.; Lang, X.; Hirata, A.; Chen, M. ACS Nano 2011, 5, 4407. (e) Adams, S. M.; Campione, S.; Caldwell, J. D.; Bezares, F. J.; Culbertson, J. C.; Capolino, F.; Ragan, R. Small 2012, 8, 2239. (f) Caldwell, J. D.; Glembocki, O.; Bezares, F. J.; Bassim, N.; Rendell R. W.; Feygelson, M.; Ukaegbu, M.; Kasica, R.; Shirey, L.; Hosten C. ACS Nano 2011, 5, 4046. (6) (a) 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. (b) Wang, Y.; Becker, M.; Wang, L.; Liu, J. Q.; Scholz, R.; Peng, J.; Gosele, U.; Christiansen, S.; Kim, D. H.; Steinhart, M. Nano Lett. 2009, 9, 2384. (c) Hwang, H.; Kim, S. H.; Yang, S. M. Lab Chip 2011, 11, 87. (d) Jeon, H. C.; Heo, C. J.; Lee, S. Y.; Park, S. G.; Yang, S. M. J. Mater. Chem. 2012, 22, 4603. (7) (a) Sun, Y. H.; Liu, K.; Miao, J.; Wang, Z. Y.; Tian, B. Z.; Zhang, L. N.; Li, Q. Q.; Fan, S. S.; Jiang, K. L. Nano Lett. 2010, 10, 1747. (b) Ko, H.; Tsukruk, V. V. Small 2008, 4, 1980. (c) Ji, N.; Ruan, W. D.; Wang, C. X.; Lu, Z. C.; Zhao, B. Langmuir 2009, 25, 11869. (8) (a) Chan, S.; Kwon, S.; Koo, T. W.; Lee, L. P.; Berlin, A. A. Adv. Mater. 2003, 15, 1595. (b) Lee, S. J.; Guan, Z. Q.; Xu, H. X.; Moskovits, M. J. Phys. Chem. C. 2007, 111, 17985. (c) Qin, L. D.; Zou, S. L.; Xue, C.; Atkinson, A.; Schatz, G. C.; Mirkin, C. A. Proc. Natl. Acad. Sci. USA 2006, 103, 13300. (d) Oh, Y. J.; Jeong, K. H. Adv. Mater. 2012, 24, 2234. (9) (a) Paek, K.; Chung, S.; Cho, C. H.; Kim, B. J. Chem. Commun. 2011, 47, 10272. (b) Lim, J.; Yang, H.; Paek, K.; Cho, C. H.; Kim, S.; Bang, J.; Kim, B. J. J. Polym. Sci. Part A: Polym. Chem. 2011, 49, 3464. (10) Odom, T. W.; Love, J. C.; Wolfe, D. B.; Paul, K. E.; Whitesides, G. M. Langmuir 2002, 18, 5314. (11) Lee, S. Y.; Jeong, J. R.; Kim, S. H.; Kim, S.; Yang, S. M. Langmuir 2009, 25, 12535. (12) De Angelis, F.; Gentile, F.; Mecarini, F.; Das, G.; Moretti, M.; Candeloro, P.; Coluccio, M. L.; Cojoc, G.; Accardo, A.; Liberale, C.; Zaccaria, R. P.; Perozziello, G.; Tirinato, L.; Toma, A.; Cuda, G.; Cingolani, R.; Di Fabrizio, E. Nat. Photonics 2011, 5, 683. (13) McConnell, M. D.; Kraeutler, M. J.; Yang, S.; Composto, R. J. Nano Lett. 2010, 10, 603. (14) Jang, S. G.; Kim, S. H.; Lee, S. Y.; Jeong, W. C.; Yang, S. M. J. Colloid. Interf. Sci. 2010, 350, 387. (15) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735. (16) Kang, H.; Heo, C. J.; Jeon, H. C.; Lee, S. Y.; Yang, S. M. ACS Appl. Mater. Interfaces 2013, DOI: 10.1021/am400019v (17) Wan, L. J.; Terashima, M.; Noda, H.; Osawa, M. J. Phys. Chem. B 2000, 104, 3563 (18) (a) Han, Y.; Sukhishvili, S.; Du, H.; Cefaloni, J.; Smolinsko, B. J. Nanosci. Nanotechno. 2008, 8, 5791. (b) Jeon, H. C.; Han, S. G.; Park, S. G.; Yang, S. M. RSC Adv. 2012, 2, 2334. (19) (a) Huh, Y. S.; Chung, A. J.; Cordovez, B.; Erickson, D. Lab Chip 2009, 9, 433. (b) Wang, M.; Jing, N.; Chou, I. H.; Cote, G. L.; Kameoka, J. Lab Chip 2007, 7, 630. (c) Piorek, B. D.; Lee, S. J.; Santiago, J. G.; Moskovits, M.; Banerjee, S.; Meinhart, C. D. Proc. Natl. Acad. Sci. USA 2007, 104, 18898.

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Su Yeon Lee, Shin-Hyun Kim, Minsoo P. Kim, Hwan Chul Jeon, Hyelim Kang, Hyeong Jun Kim, Bumjoon J. Kim, and Seung-Man Yang* Chem. Mater. Freestanding and Arrayed Nanoporous Microcylinders for Highly Active 3D SERS Substrate

Chemistry of Materials

Hierarchically-structured SiO2 microcylinders decorated with Au nanoparticles were prepared as 3D surface-enhanced Raman scattering (SERS)-active substrates. A combination of microphase separation of block-copolymers and microimprinting techniques enables the production of hierarchically-structured nanoporous SiO2 microcylinders. Such 3D porous structures provide high loading of Au nanoparticles and facilitate formation of hot spots, resulting in significant enhancement in SERS activity.

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