Fabrication of SERS-Active Patterned Gold Nanoparticle Films by

Dec 17, 2008 - An approach to fabricate patterned gold (Au) nanoparticle (NP) films for surface-enhanced Raman scattering (SERS) active substrates is ...
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J. Phys. Chem. C 2009, 113, 618–623

Fabrication of SERS-Active Patterned Gold Nanoparticle Films by Electron Irradiation and Postpyrolysis Yong Nam Kim, Seung Hwa Yoo, and Sung Oh Cho* Department of Nuclear and Quantum Engineering, Korea AdVanced Institute of Science and Technology (KAIST), 373-1 Guseong, Yuseong, Daejeon 305-701, Republic of Korea ReceiVed: October 11, 2008; ReVised Manuscript ReceiVed: NoVember 17, 2008

An approach to fabricate patterned gold (Au) nanoparticle (NP) films for surface-enhanced Raman scattering (SERS) active substrates is presented on the base of electron irradiation and subsequent pyrolysis of solid precursor films, which is a mixture of hydrogen tetrachloroaurate (HAuCl4), ethylene glycol, and poly(diallyldimethyl ammonium chloride) (PDDA). Electron irradiation decomposed HAuCl4 to form Au NP seeds and simultaneously induced cross-link of PDDA. Due to the cross-linking behavior of PDDA under electron irradiation, the precursor film could be patterned by selectively irradiating an electron beam onto the film using a metal mask. Subsequent pyrolysis process resulted in the growth of Au seeds while completely decomposing organic materials, leading to the formation of a patterned film comprising high-purity Au NPs. The size of the Au NPs could be readily controlled from 25 to 92 nm by changing the thickness of the precursor film. The prepared Au NP films exhibited NP size-correlated SERS enhancement factors and had the maximum enhancement factor of 6.6 × 105 for thiophenol. Introduction Metal nanoparticles (NPs) have been extensively investigated in recent years due to their unique physical and chemical properties and potential applications in nanotechnology and biotechnology. In particular, synthesis of noble metal NPs such as gold (Au) and silver attracted great interest for various applications to optics,1,2 chemical and biological sensors,3,4 catalysis,5,6 and surface-enhanced Raman scattering (SERS)7,8 because of their size- and shape-dependent surface plasmonic properties.5,9 SERS is a powerful tool for the detection of tiny amount of biochemical molecules adsorbed on noble metal surfaces.10,11 The intensity of Raman scattering from a biochemical molecule is dramatically increased near nanostructured metal surfaces. The increase in the Raman intensity is caused by the extremely high local electromagnetic fields that arise from localized surface plasmon resonance (LSPR), which is activated when the frequency of an incident photon is resonant with the collective oscillation frequency of the conduction electrons. While considerable effort for SERS-active substrates has been directed toward the size- and shape-controlled synthesis of noble metal NP arrays, fabrication of a position-controlled assembly or patterning of metal NPs on a solid substrate is still challenging. However, such patterned noble metal NPs are indispensable for the applications to laboratory-on-a-chip devices that allow to detect different molecules simultaneously using one substrate.12,13 Among a few fabrication approaches for SERS-active patterned metal NPs, microcontact printing,14,15 chemical interactions with surface functionalization,13,16 and direct electron beam lithography17,18 are widely used. However, microcontact printing method is not always successful because of the hydrophobic nature of polydimethylsiloxane.19 In the case of the chemical surface interactions, the procedure is relatively complicated and the final products can have impurities because NPs should be treated with functional molecules and the * Corresponding author. Telephone: +82-42-350-3823. Fax: +82-42350-3810. E-mail: [email protected].

impurities restrict the applicability to SERS.20 Direct electron beam lithography (EBL) through which individual nanostructures are produced is a good technique to fabricate metal NPs arrays; however, this technique requires expensive and timeconsuming processes and thus scale up is limited.21 Here, we report a facile and straightforward approach to fabricate patterned Au NP films by selective electron irradiation and postpyrolysis of a precursor material in film form. The precursor material consists of hydrogen tetrachloroaurate (HAuCl4), ethylene glycol (EG), and poly(diallyldimethyl ammonium chloride) (PDDA). Due to the cross-linking behavior of PDDA, the precursor material could be easily patterned by selective electron irradiation onto the precursor using a metal mask. In addition to the pattern formation, electron irradiation decomposed HAuCl4 to form Au NP seeds, which were then grown up by the subsequent pyrolysis process. The size of produced Au NPs can be controlled by changing the thickness of a precursor film. In addition to the size tunability, the final products of Au NPs had high purity, which facilitates the applications of the patterned Au NP films to SERS-active substrates. Experimental Methods Chemicals. Hydrogen tetrachloroaurate(III) hydrate (HAuCl4 · nH2O, n ) 3.5, Kojima Chemical Co., Ltd.), PDDA (20 wt % aqueous, Mw ) 400000-500000 g · mol-1, SigmaAldrich Co.), ethylene glycol (EG; 99%, Junsei Chemical Co., Ltd.), thiophenol (TP, or benzenethiol; 98%, Junsei Chemical Co., Ltd.), and methanol (>99.9%, Merck Ltd.) were used without further purification. Precursor Film Preparation. Polished p-type Si(100) wafers (1-30 Ωcm-1, 500-µm-thick) cut in 1 × 1 cm were treated in piranha solution (7:3 mixture of concentrated sulfuric acid with 35% hydrogen peroxide) for 1 h in ultrasonic conditions to derive a hydroxyl surface, and were then used as substrates after thoroughly rinsed with pure water. All substrates were kept in

10.1021/jp809021f CCC: $40.75  2009 American Chemical Society Published on Web 12/17/2008

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SCHEME 1: Schematic Representation of the Process for the Fabrication of Patterned Au NP Films

TABLE 1: TOC Analysis of Pristine Sample 1 min Irradiated Sample, and 5 min Irradiated Sample sample

TC (mg/L)

IC (mg/L)

TOC-blank (mg/L)

max TOC (mg/L)

solubility (%)

pristine 1 min irradiation 5 min irradiation

3.633 4.752 1.681

0.388 0.416 0.459

2.648 3.738 0.625

3.066 4.598 1.533

86.4 81.3 40.8

pure water prior to use to maintain the hydrophilicity of the surface, which is an essential factor for uniform film formation. To prepare the precursor solution, 500 mM HAuCl4 aqueous solution was prepared by dissolving 1 g of HAuCl4 · nH2O in 4.966 mL of pure water, and then an aliquot of 10 µL HAuCl4 aqueous solution was added into a mixture of 1 mL of EG and 20 µL of PDDA with stirring, which results in yellowish solution. The as-prepared Au precursor solution was spin-coated on Si substrates at 600 rpm for 60 s. The drop amount of Au precursor solution was varied to prepare a film with a different thickness. After spin-coating, all the samples were immediately dried in vacuum ambient (∼10-3 Torr) for 15 min. Electron Irradiation and Pyrolysis. The precursor films were irradiated with an electron beam produced from a thermionic electron gun with a tantalum filament cathode. All the experiments were carried out at ambient temperature, particularly in vacuum of less than 2 × 10-5 Torr. The beam energy was 20 keV and the total electron fluence was 8.29 × 1016 cm-2. For the irradiation experiments, precursor materials coated on Si substrates were attached to a sample stage of the electron irradiation device.22,23 The sample stage was kept to 0 °C by a refrigerating bath circulator (Jeio Tech, Republic of Korea) to prevent the precursor from being deformed by irradiation-induced heating. A 200 mesh copper TEM grid was used as a mask for the pattern formation. After the irradiation, the samples were developed with pure water to remove unexposed parts. Finally, the samples were pyrolyzed in air at 600 °C for 30 min to remove residual organics. A schematic representation of the process for the fabrication of the patterned Au NP film is shown in Scheme 1. Characterization. The morphology and structure of the samples were characterized by a field-emission scanning electron microscope (FESEM, S4800, Hitachi, Japan). The thickness of the precursor film was determined from the cross-sectional FESEM images of the film. The crystalloid of the samples was analyzed with an X-ray diffractometer (XRD, D/MAX-RC, Rigaku, Japan) using Cu KR radiation. The chemical composition of the samples was characterized with an X-ray photoelectron spectrometer (XPS, AXIS NOVA, Kratos, UK) equipped with a monochromatic Al KR X-ray source. All the XPS spectra were referenced to C 1s at 285 eV. UV-vis spectra were acquired using an external reflection mode of UV-vis spectrometer (UV-3101PC, Shimadzu, Japan). Total organic carbon (TOC) measurements of aqueous solution were obtained with a TOC analyzer (Vcsn, Shimadzu, Japan). TOC-blank measurements were obtained by calculating the difference between Total Carbon (TC) and Inorganic Carbon (IC) measurements while subtracting TOC of the bare Si, 0.598.

After TOC-blank measurement, solubility was calculated by dividing TOC-blank by theoretical maximum TOC. Raman spectra were acquired using a micro-Raman spectrometer (RM1000, Renishaw, U.K.) equipped with a confocal Raman microscope (Leica). 632.8 nm radiation from a He-Ne laser was used as the excitation source. The typical laser power at the sampling position was 5 mW with an average spot size of 20 µm in diameter using a 50X magnification of objective lens and the data acquisition time was 30 s. To obtain the SERS spectra, TP was selected as a probe molecule since it is adequate for SERS measurement due to its distinct Raman characteristics and strong affinity for Au surfaces to form self-assembled monolayers.24 The TP molecules were adsorbed on the asprepared samples by immersion into 5 mM TP methanolic solution for 12 h. Prior to recording of the SERS spectra, the samples were repeatedly rinsed with methanol to ensure monomolecular coverage of the TP. The SERS spectra were collected from more than six different points of the sample and then were averaged. The Raman spectrum of neat TP was measured using a capillary tube with an inner diameter of 1.15 mm and an outer diameter of 1.55 mm. Results and Discussion The FESEM images of the electron-irradiated film and the postpyrolyzed film are shown in Figure 1. As can be seen in Figure 1a, electron irradiation induced square pattern corresponding to the mask shape on the precursor film. Highresolution FESEM image displayed that tiny NPs with mostly less than 10 nm in size were embedded in the irradiated region (left inset of Figure 1a). We also found that the irradiated materials were strongly adsorbed to Si substrates and could not be removed in the developing process, although nonirradiated materials were readily removed by water washing. Thus, a clear pattern of the precursor film was created after the water washing. TOC analysis indicated that the solubility of irradiated films was less than that of the pristine film and furthermore the solubility of the irradiated films decreased with increasing irradiation time (Table 1). This reflects that electron irradiation induced cross-link of the precursor polymeric materials and the degree of cross-link increases with irradiation time.25 When the patterned precursor films were pyrolyzed, NPs of several tens nm were formed while keeping the square pattern (Figure 1b). The pattern has the same size and shape with the mask, suggesting that two-dimensional (2D) NP pattern with a desired shape and size can be fabricated by using a proper mask. The crystalloid of the samples in parts a and b of Figure 1 was characterized by XRD. Figure 2 shows the XRD patterns

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Figure 1. FESEM images of (a) the irradiated film using a 200 mesh TEM grid as a mask, and (b) the postpyrolyzed film. Inset in parts a and b are magnified images of each pattern.

Figure 2. XRD patterns of (a) the pristine sample, (b) the irradiated sample, and (c) the postpyrolyzed sample.

of (a) the pristine sample, (b) the irradiated sample, and (c) the postpyrolyzed sample. No Au peaks were observed for the pristine sample (Figure 2a). However, very weak and broad peak corresponding to Au(111) peak appeared in the irradiated sample (Figure 2b). Combining this result with the FESEM image shown in the left inset of Figure 1a, we can conclude that tiny Au crystals with the size of mostly less than 10 nm were produced inside the precursor materials by the electron irradiation. These small Au crystals might serve as seeds for further growth of Au NPs during the subsequent pyrolysis process.26,27 For the pyrolyzed sample, clear diffraction peaks at 38.2°, 44.4°, 64.6°, and 77.6° were observed, which correspond to Au(111), Au(200), Au(220), and Au(311), respectively (Figure 2c). The chemical composition of the pristine and postpyrolyzed films was characterized using XPS. The XPS spectrum of the pristine film displayed C, N, O, Cl, and Au peaks, which were originated from the precursor materials (Figure 3a). After the pyrolysis, however, N and Cl peaks disappeared and the intensity of C peak was remarkably decreased, while Si peak newly appeared and the intensity of O peak was greatly increased (Figure 3c). This suggests that PDDA polymer was almost completely decomposed by the pyrolysis process: C, N, and Cl are the elements comprising PDDA. We note here that EG in the precursor material was inevitable to well mix PDDA and HAuCl4 aqueous solution and correspondingly for the preparation of a uniform precursor film using spin coating. Since PDDA is a cationic polyelectrolyte, it interacts with AuCl4- to form an insoluble deposit.28 However, EG prevents the interaction between PDDA and AuCl4- and hence the precursor materials can keep enough low viscosity for spin coating. EG was easily

evaporated in the irradiation chamber under a vacuum environment, suggesting that almost no EG was remained in the precursor film before the electron irradiation. In addition, since precursor materials that were not irradiated with electrons were washed out, some regions of Si substrate were exposed to air. The exposed Si regions were liable to be oxidized during the pyrolysis process and this might be the reason for the strong increase in the O peak as well as new appearance of Si peak in the XPS spectrum. Obtained binding energies of Si 2p peak (103.9 eV) and O 1s peak (533.1 eV) are coherent with the value in SiO2,29 reflecting that the exposed regions of Si substrate were oxidized and residual C is not chemically bonded onto the Si substrate. The peaks of Au 4f7/2 (83.4 eV) and Au 4f5/2 (86.8 eV) in the pristine film (Figure 3b) shifted by 0.6 and 0.9 eV, respectively, toward lower binding energies relative to pure Au atoms. However, the position of Au 4f spectrum for the postpyrolyzed film showed the same as those of pure Au (84 and 87.7 eV), revealing that Au NPs were not covered with any materials (Figure 3d).30 Therefore, Au NPs of high purity were produced by the electron irradiation and postpyrolysis process. The size of Au NPs fabricated in the pattern could be changed simply by changing the thickness of precursor film while keeping the parameters of electron irradiation and pyrolysis. Figure 4 shows the FESEM images of the Au NPs produced from the precursor films of different thickness and the histogram of the produced NP size distribution. When the precursor film thickness was increased, the average NP size also increased: the average NP sizes were 25 nm, 40 nm, 67 nm, and 92 nm with the standard deviation of 7 nm, 9 nm, 28 nm, and 34 nm for ∼200 particles at the precursor film thickness of 156 nm, 392 nm, 588 nm, and 824 nm, respectively. However, the monodispersity of the NPs was deteriorated and the particle density was decreased, which might come from random Ostwald ripening process under pyrolysis.31 In addition, the interparticle spacing was also increased with increasing the precursor film thickness. It is well-known that the optical properties of NPs depend on the particle size and thus NP size-correlated SERS spectra of the patterned Au NP films were investigated. First, sizedependent plasmonic properties of the NP films were measured using UV-vis extinction spectra. The sizes of produced NPs are much smaller than the wavelength of the light source in the SERS measurement. Consequently, we represented the extinction spectra as the absorption spectra of the NP films by neglecting the contribution of scattering effect. The absorption

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Figure 3. XPS spectra of (a) the pristine and (c) the postpyrolyzed samples. (b and d) XPS spectra of Au 4f corresponding to parts a and c, respectively.

Figure 4. FESEM images of different size of Au NPs inside pattern and histogram of size distribution. Average sizes of Au NPs are (a) 25 nm, (b) 40 nm, (c) 67 nm, and (d) 92 nm.

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Figure 5. UV-vis extinction spectra for the Au NPs of different average sizes.

spectra of the nontransparent samples, due to Si substrates, were acquired by the relationship A ) -log(R/R0), where A is absorbance, R is the reflectance of an as-prepared sample, and R0 is the reflectance of a Si substrate.32,33 Figure 5 shows that the plasmon resonance peak increased from 569 nm to 597 nm, 632 nm, and 673 nm as the average NP size increased from 25nm to 40 nm, 67 nm, and 92 nm, respectively. The bandwidth of the peak increased with increasing the average NP size, which is attributed to the increased particle distribution for larger NPs. SERS spectra of the patterned Au NP films with different particle sizes are shown in Figure 6a. Both the Raman shifts and the peak intensity of the probe TP molecule were calibrated using the Si peak at 520 cm-1 as an internal standard. To calculate the enhancement factors (EFs) of the SERS signals from the patterned Au NP films, the Raman spectrum of neat TP was also measured. The SERS EF is given by the equation EF ) (ISERS/Ineat)(Nneat/NSERS), where Ineat and ISERS are the corresponding Raman and SERS intensity of the TP peak at 1573 cm-1, respectively, while Nneat and NSERS are the total numbers of TP molecules exposed under the laser spot in the bulk and on the SERS substrate. Nneat is given by FAh/M, where F is the density of bulk TP (1.073 g cm-3), A is the sampling area (ca. 20 µm in diameter), h is the penetration depth of the focused beam (ca. 72 µm),34 and M is the molecular weight of bulk TP (110.18 g mol-1), respectively. NSERS was calculated from NSERS ) CASσ, where C is the number of particles in the unit area which was estimated from the FESEM images (Figure 4), and S is the average surface area of the particles exposed to laser while assuming spherical shape of the particles, respectively. Thus, CAS gives the total surface area of the Au NPs illuminated by laser, whereas σ is the packing density of TP (6.8 × 1014 cm-2)35 for monolayer coverage of TP molecules on the Au surface. For the calculation of ISERS, the average Raman intensities measured at six different positions of the patterned Au NP films were used (Figure 6b). On the basis of the above analysis, the EFs of the SERS signals were 1.1 × 105, 2.8 × 105, 6.6 × 105, and 2.7 × 105 for the patterned Au NP films with the average NP size of 25 nm, 40 nm, 67 nm, and 92 nm, respectively. The EF value was a maximum for the NP film with the average particle size of 67 nm, because the plasmon resonance peak of the NP film was closest to the wavelength of the Raman excitation light. It should be noted that the change in the EF values in Figure 6 might also be affected by the change in the interparticle spacing as well as the difference in the particle size. This is because, when the average particle size of the NP film was increased, the interparticle spacing was also increased, as shown in Figure 4. Compared to other Raman EF obtained from various Au

Figure 6. (a) Raman spectra and (b) relative Raman intensities of the patterned Au NP films with average particle sizes of 25 nm, 40 nm, 67 nm, and 92 nm, using TP as a probe molecule. Raman scattering intensities are the average of six measurements.

substrates,36 which are generally ranged from 104 to 106, the patterned Au NP films fabricated here have reasonably enhanced Raman signal. Furthermore, as can be seen in Figure 6b, the variation of the measured Raman intensities at different positions of the NP films decreased as the average NP size increased. For the NP film of the maximum EF value, the intensity variation was around (15%, indicating that the fabricated films exhibit comparatively uniform SERS behavior. Conclusion A strategy to fabricate patterned Au NP films was presented based on electron irradiation and postpyrolysis of a precursor film that consist of HAuCl4, EG, and PDDA. The cross-linking behavior of the mixture precursor film allows a desired pattern by selectively irradiating an electron beam onto the film. The size of Au NPs can be controlled by changing the thickness of precursor film and thus the corresponding plasmon band of the NPs can be tuned. The patterned Au NP films exhibited good enough SERS effect, promising the applications to biochips. Although electron irradiation is used for the fabrication of the patterned Au NP film, the presented irradiation technique is far from conventional EBL technique. In EBL, each nanostructure in a pattern or an array is formed by electrons passing through a mask with nanometer-sized holes or by direct writing of a focused electron beam. Consequently, EBL is a timeconsuming process and limits large-area formation of nanostructured arrays. However, in this strategy, electron irradiation is required not directly for the formation of NPs but for the formation of NP assembly pattern. NPs in the pattern are produced at the subsequent pyrolysis process, where many NPs

Gold Nanoparticle Films are simultaneously produced in parallel and thus large-area NP films can be prepared with relatively high speed. The presented technique, if proper precursor materials are chosen, can also be used to produce patterned monolayer arrays of other metal NPs, which are important for micro/nanoelectronics, photonic, and nanoelectromechanical devices as well as biological and chemical sensors, molecular sensor arrays, and transducers. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (No. 2008-00393). This research was also supported by Electric Power Research Institute (EPRI) under the Project Agreement EP-P19394/C9578. We gratefully acknowledge the support and encouragement of the EPRI project manager, Ken Barry. References and Notes (1) Dirix, Y.; Bastiaansen, C.; Caseri, W.; Smith, P. AdV. Mater. 1999, 11, 223. (2) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (3) Taton, T. A.; Mirkin, C. A.; Letsinger, R. L. Science 2000, 289, 1757. (4) Jiang, C.; Markutsya, S.; Pikus, Y.; Tsukruk, V. V. Nat. Mater. 2004, 3, 721. (5) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (6) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343. (7) Campion, A.; Kambhampati, P. Chem. Soc. ReV. 1998, 27, 241. (8) Kwon, K.; Lee, K. Y.; Lee, Y. W.; Kim, M.; Heo, J.; Ahn, S. J.; Han, S. W. J. Phys. Chem. C 2007, 111, 1161. (9) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545. (10) Keating, C. D.; Kovaleski, K. M.; Natan, M. J. J. Phys. Chem. B 1998, 102, 9404. (11) Dick, L. A.; Haes, A. J.; Van Duyne, R. P. J. Phys. Chem. B 2000, 104, 11752. (12) Henleya, S. J.; Silva, S. R. P. Appl. Phys. Lett. 2007, 91, 023107. (13) Hering, K. K.; Mo¨ller, R.; Fritzsche, W.; Popp, J. Chem. Phys. Chem. 2008, 9, 867.

J. Phys. Chem. C, Vol. 113, No. 2, 2009 623 (14) Song, W.; Li, W.; Cheng, Y.; Jia, H.; Zhao, G.; Zhou, Y.; Yang, B.; Xu, W.; Tian, W.; Zhao, B. J. Raman Spectrosc. 2006, 37, 755. (15) Wang, B.; Chen, K.; Jiang, S.; Reincke, F.; Tong, W.; Wang, D.; Gao, C. Biomacromolecules 2006, 7, 1203. (16) Kim, K.; Park, H. K.; Kim, N. H Langmuir 2006, 22, 3421. (17) Laurent, G.; Fe´lidj, N.; Lau Truong, S.; Aubard, J.; Le´vi, G.; Krenn, J. R.; Hohenau, A.; Leitner, A.; Aussenegg, F. R. Nano Lett. 2005, 5, 253. (18) Laurent, G.; Fe´lidj, N.; Grand, J.; Aubard, J.; Le´vi, G.; Hohenau, A.; Aussenegg, F. R.; Krenn, J. R. Phys. ReV. B 2006, 73, 245417. (19) Xu, C.; Taylor, P.; Fletcher, P. D. I.; Paunov, V. N. J. Mater. Chem. 2005, 15, 394. (20) Jung, H. Y.; Park, Y. K.; Park, S.; Kim, S. K. Anal. Chim. Acta 2007, 602, 236. (21) Grand, J.; Kostcheev, S.; Bijeon, J. L.; de la Chapelle, M. L.; Adam, P. M.; Rumyantseva, A.; Le´rondel, G.; Royer, P. Synth. Met. 2003, 139, 621. (22) Cho, S. O.; Lee, E. J.; Lee, H. M.; Kim, J. G.; Kim, Y. J. AdV. Mater. 2006, 18, 60. (23) Lee, H. M.; Kim, Y. N.; Kim, B. H.; Kim, S. O.; Cho, S. O. AdV. Mater. 2008, 20, 2094. (24) Haynes, C. L.; Yonzon, C. R.; Zhang, X.; Van Duyne, R. P. J. Raman. Spectrosc. 2005, 36, 471. (25) Psˇeja, J.; Hrncˇirˇ´ık, J.; Kupec, J.; Charva´tova´, H.; Hruzı´k, P.; Tupy´, M. J. Polym. EnViron. 2006, 14, 231. (26) Srnova´-sˇloufova´, I.; Vlcˇkova´, B.; Bastl, Z.; Hasslett, T. L. Langmuir 2004, 20, 3407. (27) Sudeep, P. K.; Kamat, P. V. Chem. Mater. 2005, 17, 5404. (28) Li, C.; Shuford, K. L.; Chen, M.; Lee, E. J.; Cho, S. O. ACS Nano 2008, 2, 1760. (29) Bensalem, A.; Bozon-Verduraz, F.; Delamar, M.; Bugli, G. Appl. Catal., A 1995, 121, 81. (30) Laihoa, T.; Leiroa, J. A.; Lukkari, J Appl. Surf. Sci. 2003, 212213, 525. (31) Schwind, M.; Ågren, J. Acta Mater. 2001, 49, 3821. (32) Driskell, J. D.; Lipert, R. J.; Porter, M. D. J. Phys. Chem. B 2006, 110, 17444. (33) Leverette, C. L.; Jacobs, S. A.; Shanmukh, S.; Chaney, S. B.; Dluhy, R. A.; Zhao, Y. P. Appl. Spectrosc. 2006, 60, 906. (34) Lu, J.; Chamberlin, D.; Rider, D. A.; Liu, M.; Manners, I.; Russell, T. P. Nanotechnology 2006, 17, 5792. (35) McFarland, A. D.; Young, M. A.; Dieringer, J. A.; Van Duyne, R. P. J. Phys. Chem. B 2005, 109, 11279. (36) Bhuvana, T.; Pavan Kumar, G. V.; Kulkarni, G. U.; Narayana, C. J. Phys. Chem. C 2007, 111, 6700.

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