Synthesis of Gold Nanostar Arrays as Reliable, Large-Scale

Article pubs.acs.org/JPCC

Synthesis of Gold Nanostar Arrays as Reliable, Large-Scale, Homogeneous Substrates for Surface-Enhanced Raman Scattering Imaging and Spectroscopy L. Osinkina, T. Lohmüller,* F. Jac̈ kel,*,† and J. Feldmann Photonics and Optoelectronics Group, Department of Physics and Center for Nanoscience (CeNS), Ludwig-Maximilians-Universität München, Amalienstr. 54, 80799 Munich, Germany S Supporting Information *

ABSTRACT: We report a two-step approach for the fabrication of quasihexagonal ordered arrays of star-shaped gold nanoparticles (gold nanostars) as reliable, large-scale homogeneous substrates for surfaceenhanced Raman scattering (SERS) spectroscopy by a combination of block-copolymer micelle nanolithography and electroless deposition. The applicability of this platform for SERS imaging is demonstrated by pHsensitive Raman measurements of 4-mercaptobenzoic acid adsorbed to the nanostars in the array. A homogeneous enhancement factor of ∼105 was observed on the nanostar monolayer over an area of several hundreds of square micrometers.

■

individually addressed with polarized light.21,24 Nanostars can be synthesized in solution with a high yield (>90%).25,26 Lines of spherical nanoparticles27 and polymer-coated, nanostars28 were produced and probed for their SERS performance, but patterning these star-shaped particles in a monolayer with a high lateral order has been a challenge. Although a number of strategies have been developed to generate robust and reliable substrates that show a high and homogeneous SERS performance over a large surface area29−34 including 3D assemblies,35 none of these examples take advantage of the extraordinary SERS performance of nanostars. Here we present a reliable and reproducible approach for the synthesis of gold nanostar arrays with high lateral order. The samples display a homogeneous SERS enhancement over an area of several hundreds of square micrometers. We furthermore demonstrate the applicability of this SERS-active platform for chemical imaging and spectroscopy by performing pH-sensitive Raman mapping of mercaptobenzoic acid (MBA) molecules that are adsorbed to the nanostar array.

INTRODUCTION Raman spectroscopy has found widespread applications as an analytical method for the investigation of chemical or biological samples due to the capability of providing a chemical fingerprint of the analyte.1,2 Acquiring spectral information from only a few or even single molecules distributed in a sample is difficult because most molecules have only a very small Raman scattering cross section (typically 10−29 to 10−30 cm2).3 Raman signals can be amplified by several orders of magnitude by taking advantage of surface-enhanced Raman scattering (SERS).4,5 Precious metal nanoparticles or rough metal films, for example, are known to increase the scattering intensity of molecules in their close vicinity.6,7 Ever since the discovery of SERS, there has been huge progress in understanding the origin of the enhancement.8−11 Recent studies suggested that the plasmonic coupling between nanoparticles can generate strongly enhanced electromagnetic field with the potential to boost the Raman signal intensity of molecules that are positioned right at the nanoparticle junction by several orders of magnitude.12−14 Coupled nanostructures, however, may be impractical for analytical applications due to the need of delivering the molecules of interest precisely to the small plasmonic “hotspot”. Another example of materials suitable to increase the Raman signal intensity is highly branched gold or silver structures such as star-shaped gold nanoparticles or “nanostars” that display superior performance for SERS.15−21 Here, a strong plasmon excitation at the sharp tips of the nanostar spikes causes a strongly enhanced eletromagnetic field that is localized at the tip region,22 similar to the “lightning rod effect”.23 A large increase in the Raman signal intensity on the order of ∼107 has been reported from molecules located at the tips of single gold nanostars, and it has been demonstrated that each tip can be © XXXX American Chemical Society

■

MATERIALS AND METHODS

Preparation of Micelle Monolayer. Arrays of single gold nanoparticles were fabricated by block-copolymer micelle nanolithography (BCMN).36,37 Diblock copolymer molecules (polystyrene(1056)-block-poly(2-vinylpyridine)(495), Polymer Source, Montreal, Canada) were dissolved in toluene p.a. (Merck, Germany) at a concentration of 3 mg/mL and stirred Special Issue: Ron Naaman Festschrift Received: December 10, 2012 Revised: February 28, 2013

A

dx.doi.org/10.1021/jp312149d | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

Figure 1. Schematic description of the preparation of nanostar array substrates: (a) Arrays of gold nanoparticles are prepared on top of a glass or silica substrate by block copolymer micelle nanolithography (BCMN). A monolayer of block copolymer micelles loaded with a metal precursor is formed via dip-coating. The micelles are arranged in a quasi-hexagonal order on the substrate. A short plasma treatment is applied to remove the polymer film and form small gold nanoparticles. (b) Schematic representation of the nanostar growth from individual gold particle seeds. (c) SEM micrographs of particle seeds before growth. (d) SEM micrograph of a nanostar array on a silica substrate after nanostar growth.

room temperature. Two-dimensional SERS maps were acquired with a T64000 triple grating Raman system (Horiba Scientific, France). The measurements were performed in water using a 647 nm Argon/Krypton gas laser line (Coherent) and a 63× water immersion objective (Zeiss SMT, Germany). The laser power density on the sample was 3.98 kW/cm2. The laser excitation wavelength was chosen to be resonant with the plasmon excitation of the particles but nonresonant with the electronic excitation of the MBA molecules. Reference measurements in bulk were performed on the same setup using a 400 mM solution of MBA in ethanol and a laser power density of 31.8 kW/cm2. The radius of the laser spot was 1 μm, which corresponds to a laser spot area of 3.14 μm2 and a confocal volume of 12.6 μm3.

at room temperature for 12 h. In toluene, the molecules start to microphase-segregate into spherical micelles with the polyvinylpyridine-block forming the micellar core. Gold(III)chloride hydrate (Sigma Aldrich) was added to the polymer solution at a loading rate L = 0.5 with respect to the polymer concentration. The solution was stirred until all metal salt was dissolved. A self-assembled monolayer of micelles was formed on the surface of a glass or silica substrate by dip-coating. To this end, substrates were immersed into the polymer solution and retracted with a constant rate of 15 mm/min using a motorized stage (MTS50, Thorlabs). Growth of Nanostars. The micelle-coated samples were activated for 2 min using air plasma (Harrick PDC-32G, 0.2 mbar, 18 W). This short plasma treatment was applied to remove only a part of the polymer matrix and to reduce the metal precursor salt inside the micellar core. During this process, small gold nanoparticles (particle size ∼6 nm) were formed, which serve as seeds for the growth of nanostars by electroless deposition. Gold nanostars were prepared by incubating the samples in an aqueous solution of 9.2 × 10−2 mol/L cetyltrimethylammoniumbromide (CTAB), 6.13 × 10−4 mol/L gold(III)-chloride hydrate, 1.23 × 10−3 mol/L ascorbic acid, and 6.13 × 10−5 mol/L silver nitrate (all chemicals from Sigma-Aldrich). After 1 h, the substrates were rinsed in water and dried at room temperature to stop the reaction. Finally, the samples were again treated with air plasma for 45 min to remove all residual organic molecules and capping agents from the surface of the nanostars. SEM Imaging. SEM measurements were performed using an Ultra 55 field-emission scanning electron microscope (FESEM, Zeiss SMT, Germany) operated at accelerating voltages of 5−25 kV. Samples prepared on glass were covered with a carbon layer prior to SEM measurements. SERS Measurements. The nanostar substrates were incubated overnight with a 10 mM solution of 4-mercaptobenzoic acid (MBA; Sigma Aldrich, USA) in ethanol. After incubation, the samples were rinsed with ethanol and dried at

■

RESULTS AND DISCUSSION A schematic overview of the two-step approach to create an array of hexagonally ordered gold nanostars is shown in Figure 1a,b. The nucleation sites for nanostar growth were first ordered on top of a glass or silica substrate by self-assembly of block copolymer micelle arrays that were loaded with a gold precursor salt (Figure 1a). The organic component was partially removed by a short, 2 min plasma treatment, leaving an ordered array of nanoparticles embedded in the residual of the polymer matrix. The fine adjustment of the plasma protocol was thereby essential to ensure that the particles on the substrate were kept in position.29 Particles that would otherwise lift-off or move during the growth process would cause a defect of the lateral order. In general, the plasma treatment has to be long enough to form the gold particle seeds and to remove enough of the polymer film to enable a homogeneous nanostar growth. At the same time, the plasma treatment has to be short enough to avoid too much of the polymer being etched away, which would cause the stabilizing effect of micellar matrix to vanish. The nanoparticle array was analyzed by SEM. The spacing between the particles is dictated by the molecular B



Article

Figure 2. (a) Reflected light micrograph of a silicon substrate partially covered by a nanostar array. The area covered by the particles appears red. (b) 2D map of the MBA Raman signal in the area corresponding to the white square from panel a. (c) SEM image of an area corresponding to the white square from panel b. (d) SEM micrograph of a single nanostar illustrating how the particle surface area was estimated. First, the particle was approximated as a sphere, considering only the size of the particle core. Then, each tip was counted individually for each nanostar. The total surface area of a single particle was then calculated by adding the values for a spherical particle with the values for all tips. (e) Averaged SERS spectrum of MBA from the area covered with a nanostar array. At pH 7, the characteristic band at 1710 cm−1 corresponds to CO stretching vibrations. (f) Averaged SERS spectrum of MBA at pH 10. The characteristic band at 1423 cm−1 corresponds to COO− stretching vibrations.

overall density of nanostars can therefore be changed by adjusting the initial density of particle seeds. For the experiments shown here, the particle spacing of 100 nm was chosen to avoid coalescence of the particles into aggregates but at the same time to have the highest density of well-separated, individual nanostars possible. This way, the overall Raman enhancement measured from a single spot on the substrate could be attributed to the exact number of stars and the total number of nanostar tips in that particular area. The applicability of the nanostar platform for SERS imaging and spectroscopy was demonstrated by Raman mapping of MBA functionalized to the particles. Mapping was performed in a region where some of the particles were mechanically removed from the substrate to achieve a strong imaging contrast, and each Raman image was recorded with 144 pixels and a pixel size of 2.5 × 2.5 μm (Figure 2b,c). Two strong characteristic bands at 1070 and 1588 cm−1, both arising from C−C ring deformation mode, dominate the Raman spectrum of MBA (Figure 2e,f). The order of magnitude of the signal intensity was found to be homogeneous over an area of several hundreds of square micrometers. Notably, no signal was observed from the area where the stars had been removed (Figure 2b). The Raman response of MBA is also sensitive to the pH of the surrounding medium.39 The intensity of the band at 1710 cm−1 corresponding to CO stretching vibrations of nondissociated carboxylic group increases with increasing the acidity of solution. On the contrary, at basic pH, this band disappears and a new band at 1423 cm−1 corresponding to COO− stretching vibration of dissociated carboxylic group is

weight of the diblock copolymer, the polymer concentration, and the dip-coating velocity.38 For the experimental conditions reported here, the size of the particle seeds was set to be ∼6 nm and the separation distance was set to be ∼100 nm (Figure 1c). Gold nanostars were grown from the nanoparticle template by electroless deposition (Figure 1b). The synthetic conditions to grow nanostars were optimized to ensure that stars were grown only from the particle seeds on the substrate and not in solution. Because of the dip-coating process, only half of the substrate was patterned with gold particle seeds, whereas the other half above the dipping-edge was not patterned and served as an internal reference. Any nanostar found on the blank side of the substrate could therefore only originate from solution. During the experiment, the growth of nanostars on the substrate was already visible by the naked eye due to a color change of only the nanopatterned area of the sample. No color change of the reaction mixture was observed at the same time, indicating that no nanostars were formed in solution. SEM imaging was performed to confirm the nanostar growth only from particle seeds and to analyze the yield and shape of the particles (Figure 1d). We found that 68% of the particles were star-shaped, 24% were just grown without having sharp tips and protrusions, and 8% of the particle seeds were not grown at all. On average, the nanostars were grown to a mean core size of 29 nm having two tips per star and a mean tip length of 10 nm (data derived from over 240 particles). Only nanoparticle protrusions that were longer than 5 nm were considered as nanostar tips for this analysis. In theory, BCML renders it possible to generate nanoparticle pattern with a variety of spacing ranging between 25 and 250 nm.38 The C



Article

for SERS imaging was confirmed by pH-sensitive Raman mapping of MBA molecules adsorbed to the particles. The homogeneous EF of ∼105 over an area of several hundreds of square micrometers illustrates the great potential of this nanostar substrate for label-free chemical analysis and spectroscopy.

observed. This behavior could be reliably demonstrated and observed by SERS spectra taken on MBA-functionalized nanostar arrays at different pH values (Figure 2e,f). At pH 7, the 1710 cm−1 band was prominent while the 1423 cm−1 band was vanishing (Figure 2e). At pH 10, the intensity of the 1423 cm−1 band increased while the 1710 cm−1 band almost disappeared (Figure 2f). We compared the SERS signals of MBA measured on the nanostar array with the intensity of the Raman signal measured in bulk to determine the average enhancement factor (EF). The EF was calculated as the ratio of the excitation power (P), the integrated Raman intensity (IR) normalized with acquisition time, and the amount of molecules contributing to the signal (N) from both bulk and the nanostar substrate by using the following formula: EF =

■

ASSOCIATED CONTENT

S Supporting Information *

Discussion of stability and reuse of the introduced SERS-active substrates. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

IR(SERS)·PBULK ·NBULK

*(T.L.) Tel: +49 (0)892 180 3318; E-mail: t.lohmueller@lmu. de. (F.J.) E-mail: [email protected].

IR(BULK)·PSERS·NSERS

Present Address

The SERS intensity (IR(SERS)) was taken from an average of the whole Raman image. NBULK was calculated as the amount of MBA molecules localized in the confocal volume of the laser spot. NSERS was calculated by considering a monolayer of MBA molecules on the particle surface. The surface area of a single nanostar was calculated to be 1415 nm2 on average with one MBA molecule occupying an area of 0.19 nm2.40 For a particle spacing of 100 nm, ∼317 nanostars were measured simultaneously in a laser spot with an area of 3.14 μm2. With IR(SERS) = 3.17 × 104 photons per minute and IR(BULK) = 2.91 × 104 photons per minute the measured average EF was found to be 1.1 × 105 with a maximum EF of 2.6 × 105. Compared with literature values, it is important to point out that in many studies it is considered that only molecules situated in the area of highest field enhancement (in the case of nanostars on the nanostar tips) contribute to the SERS signal.13,21,41,42 The area occupied by molecules in this case equals half of the area of a sphere with a radius that equals the radius of curvature of a single nanostar tip. The radius of curvature of the nanostar tips was 5.5 nm (average value derived from 50 tips). Considering only molecules that are located at the tips of the nanostar spikes, the local Raman EF on the nanostar platform was found to be 2.2 × 108. This value is in good agreement with what has been calculated and measured for the tips of single gold nanostars17,21 but has never been demonstrated for a monolayer of star-shaped particles assembled in an array and over a large surface area. It is worth mentioning that the nanostar substrates were stable enough to be reused for new measurements even after several months of storage under ambient conditions. This was demonstrated by cleaning an old sample with air plasma and functionalizing it again with MBA. SERS measurements revealed that the magnitude of the EF did not change between the two separate measurements (Figure S1, Supporting Information). SEM measurements of single nanostars before and after the plasma treatment and the laser exposure revealed that neither one of these two processes had any effect on the shape of the nanostars. (Figure S2, Supporting Information)

†

F. Jäckel: Department of Physics and Stephenson Institute for Renewable Energy, University of Liverpool, Oliver Lodge Building, Oxford Street, Liverpool, L69 7ZE, United Kingdom. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support by the German Research Foundation (DFG) through the Nanosystems Initiative Munich (NIM), the Elite Network of Bavaria (International Doctorate Program NanoBioTechnology), by the ERC through the Advanced Investigator Grant HYMEM, and by the DFG through the SFB 1032 is gratefully acknowledged.

■

REFERENCES

(1) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Surface-Enhanced Raman Scattering and Biophysics. J. Phys.: Condens. Matter 2002, 14, R597−R624. (2) Sharma, B.; Frontiera, R. R.; Henry, A. I.; Ringe, E.; Van Duyne, R. P. SERS: Materials, Applications, and the Future. Mater. Today 2012, 15, 16−25. (3) Kato, Y.; Takuma, H. Absolute Measurement of RamanScattering Cross Sections of Liquids. J. Opt. Soc. Am. 1971, 61, 347−350. (4) Ervin, K. M.; Koglin, E.; Sequaris, J. M.; Valenta, P.; Nürnberg, H. W. Surface Enhanced Raman Spectra of Nucleic Acid Components Adsorbed at a Silver Electrode. J. Electroanal. Chem. Interfacial Electrochem. 1980, 114, 179−194. (5) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Ultrasensitive Chemical Analysis by Raman Spectroscopy. Chem. Rev. 1999, 99, 2957−2976. (6) Jeanmaire, D. L.; Van Duyne, R. P. Surface Raman Spectroelectrochemistry: Part I. Heterocyclic, Aromatic, and Aliphatic Amines Adsorbed on the Anodized Silver Electrode. J. Electroanal. Chem. Interfacial Electrochem. 1977, 84, 1−20. (7) Albrecht, M. G.; Creighton, J. A. Anomalously Intense Raman Spectra of Pyridine at a Silver Electrode. J. Am. Chem. Soc. 1977, 99, 5215−5217. (8) Nie, S.; Emory, S. R. Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science 1997, 275, 1102−1106. (9) 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.

■

CONCLUSIONS In conclusion, we have demonstrated a reliable and highly reproducible fabrication method to create highly ordered arrays of nanostars by block copolymer nanolithography and seeded growth. The applicability of the nanostar arrays as a platform D



Article

(10) Moskovits, M. Surface Roughness and the Enhanced Intensity of Raman Scattering by Molecules Adsorbed on Metals. J. Chem. Phys. 1978, 69, 4159−4161. (11) LeRu, 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. (12) Xu, H.; Bjerneld, E. J.; Käll, M.; Börjesson, L. Spectroscopy of Single Hemoglobin Molecules by Surface Enhanced Raman Scattering. Phys. Rev. Lett. 1999, 83, 4357−4360. (13) Ringler, M.; Klar, T. A.; Schwemer, A.; Susha, A. S.; Stehr, J.; Raschke, G.; Funk, S.; Borowski, M.; Nichtl, A.; Kürzinger, K.; et al. Moving Nanoparticles with Raman Scattering. Nano Lett. 2007, 7, 2753−2757. (14) Michaels, A. M.; Jiang, J.; Brus, L. Ag Nanocrystal Junctions as the Site for Surface-enhanced Raman Scattering of Single Rhodamine 6G Molecules. J. Phys. Chem. B 2000, 104, 11965−11971. (15) Yang, M.; Alvarez-Puebla, R. A.; Kim, H.-S.; Aldeanueva-Potel, P.; Liz-Marzán, L. M.; Kotov, N. SERS-Active Gold Lace Nanoshells with Built-in Hotspots. Nano Lett. 2010, 10, 4013−4019. (16) Laurier, K. G. M.; Poets, M.; Vermoortele, F.; De Cremer, G.; Martens, J. A.; Uji-i, H.; De Vos, D. E.; Hofkens, J.; Roeffaers, M. B. J. Photocatalytic Growth of Dendritic Silver Nanostructures as SERS Substrates. Chem. Commun. 2012, 48, 1559−1561. (17) Rodríguez-Lorenzo, L.; Á lvarez-Puebla, R. A.; De Abajo, F. J. G.; Liz-Marzán, L. M. Surface Enhanced Raman Scattering Using StarShaped Gold Colloidal Nanoparticles. J. Phys. Chem. C 2010, 114, 7336−7340. (18) Nalbant Esenturk, E.; Hight Walker, A. R. Surface-Enhanced Raman Scattering Spectroscopy via Gold Nanostars. J. Raman Spectrosc. 2009, 40, 86−91. (19) Barbosa, S.; Agrawal, A.; Rodríguez-Lorenzo, L.; PastorizaSantos, I.; Alvarez-Puebla, R. A.; Kornowski, A.; Weller, H.; LizMarzán, L. M. Tuning Size and Sensing Properties in Colloidal Gold Nanostars. Langmuir 2010, 26, 14943−14950. (20) Khoury, C. G.; Vo-Dinh, T. Gold Nanostars For SurfaceEnhanced Raman Scattering: Synthesis, Characterization and Optimization. J. Phys. Chem. C 2008, 112, 18849−18859. (21) Hrelescu, C.; Sau, T. K.; Rogach, A. L.; Jäckel, F.; Feldmann, J. Single Gold Nanostars Enhance Raman Scattering. Appl. Phys. Lett. 2009, 94, 153113. (22) Nehl, C. L.; Liao, H.; Hafner, J. H. Optical Properties of StarShaped Gold Nanoparticles. Nano Lett. 2006, 6, 683−688. (23) Liao, P. F. Lightning Rod Effect in Surface Enhanced Raman Scattering. J. Chem. Phys. 1982, 76, 751−752. (24) Hrelescu, C.; Sau, T. K.; Rogach, A. L.; Jäckel, F.; Laurent, G.; Douillard, L.; Charra, F. Selective Excitation of Individual Plasmonic Hotspots at the Tips of Single Gold Nanostars. Nano Lett. 2011, 11, 402−407. (25) Sau, T. K.; Rogach, A. L.; Döblinger, M.; Feldmann, J. One-Step High-Yield Aqueous Synthesis of Size-Tunable Multispiked Gold Nanoparticles. Small 2011, 7, 2188−2194. (26) Senthil Kumar, P.; Pastoriza-Santos, I.; Rodríguez-González, B.; Javier García de Abajo, F.; Liz-Marzán, L. M. High-Yield Synthesis and Optical Response of Gold Nanostars. Nanotechnology 2008, 19, 1−6. (27) Pazos-Pérez, N.; Ni, W.; Schweikart, A.; Alvarez-Puebla, R. A.; Fery, A.; Liz-Marzán, L. M. Highly Uniform SERS Substrates Formed by Wrinkle-Confined Drying of Gold Colloids. Chem. Sci. 2010, 1, 174−178. (28) Mueller, M.; Tebbe, M.; Andreeva, D. V.; Karg, M.; AlvarezPuebla, R. A.; Pazos-Pérez, N.; Fery, A. Large-Area Organization of pNIPAM-Coated Nanostars as SERS Platforms for Polycyclic Aromatic Hydrocarbons Sensing in Gas Phase. Langmuir 2012, 28, 9168−9173. (29) Lohmueller, T.; Bock, E.; Spatz, J. P. Synthesis of QuasiHexagonal Ordered Arrays of Metallic Nanoparticles with Tuneable Particle Size. Adv. Mater. (Weinheim, Ger.) 2008, 20, 2297−2302. (30) Lee, W.; Lee, S. Y.; Briber, R. M.; Rabin, O. Self-Assembled SERS Substrates with Tunable Surface Plasmon Resonances. Adv. Funct. Mater. 2011, 21, 3424−3429.

(31) 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. (32) Su, Q.; Ma, X.; Dong, J.; Jiang, C.; Qian, W. A Reproducible SERS Substrate Based on Electrostatically Assisted APTES-Functionalized Surface-Assembly of Gold Nanostars. ACS Appl. Mater. Interfaces 2011, 3, 1873−1879. (33) Abalde-Cela, S.; Ho, S.; Rodríguez-González, B.; Correa-Duarte, M. A.; Á lvarez-Puebla, R. A.; Liz-Marzán, L. M.; Kotov, N. A. Loading of Exponentially Grown LBL Films with Silver Nanoparticles and Their Application to Generalized SERS Detection. Angew. Chem. 2009, 121, 5430−5433. (34) Joseph, V.; Gensler, M.; Seifert, S.; Gernert, U.; Rabe, J. P.; Kneipp, J. Nanoscopic Properties and Application of Mix-and-Match Plasmonic Surfaces for Microscopic SERS. J. Phys. Chem. C 2012, 116, 6859−6865. (35) Alvarez-Puebla, R. A.; Agarwal, A.; Manna, P.; Khanal, B. P.; Aldeanueva-Potel, P.; Carbó-Argibay, E.; Pazos-Pérez, N.; Vigderman, L.; Zubarev, E. R.; Kotov, N. A. Gold Nanorods 3D-Supercrystals as Surface Enhanced Raman Scattering Spectroscopy Substrates for the Rapid Detection of Scrambled Prions. Proc. Natl. Acad. Sci. 2011, 108, 8157−8161. (36) Spatz, J. P.; Mössmer, S.; Hartmann, C.; Möller, M.; Herzog, T.; Krieger, M.; Boyen, H.-G.; Ziemann, P.; Kabius, B. Ordered Deposition of Inorganic Clusters from Micellar Block Copolymer Films. Langmuir 2000, 16, 407−415. (37) Glass, R.; Möller, M.; Spatz, J. P. Block Copolymer Micelle Nanolithography. Nanotechnology 2003, 14, 1153−1160. (38) Lohmueller, T.; Aydin, D.; Schwieder, M.; Morhard, C.; Louban, I.; Pacholski, C.; Spatz, J. P. Nanopatterning by Block Copolymer Micelle Nanolithography and Bioinspired Applications. Biointerphases 2011, 6, MR1−MR12. (39) Michota, A.; Bukowska, J. Surface-Enhanced Raman Scattering (SERS) of 4-Mercaptobenzoic Acid on Silver and Gold Substrates. J. Raman Spectrosc. 2003, 34, 21−25. (40) Gole, A.; Sainkar, S. R.; Sastry, M. Electrostatically Controlled Organization of Carboxylic Acid Derivatized Colloidal Silver Particles on Amine-Terminated Self-Assembled Monolayers. Chem. Mater. 2000, 12, 1234−1239. (41) Le Ru, E. C.; Etchegoin, P. G.; Meyer, M. Enhancement Factor Distribution Around a Single Surface-Enhanced Raman Scattering Hot Spot and Its Relation to Single Molecule Detection. J. Chem. Phys. 2006, 125, 204701. (42) Dieringer, J. A.; Lettan, R. B.; Scheidt, K. A.; Van Duyne, R. P. A Frequency Domain Existence Proof of Single-Molecule SurfaceEnhanced Raman Spectroscopy. J. Am. Chem. Soc. 2007, 129, 16249−16256.

E


Synthesis of Gold Nanostar Arrays as Reliable, Large-Scale

Recommend Documents