pubs.acs.org/Langmuir © 2010 American Chemical Society
Spatially Controlled SERS Patterning Using Photoinduced Disassembly of Gelated Gold Nanoparticle Aggregates Jung Su Park, Jun Hee Yoon, and Sangwoon Yoon* Department of Chemistry, Dankook University, 126 Jukjeon-dong, Suji-gu, Yongin, Gyeonggi 448-701, Korea Received September 8, 2010. Revised Manuscript Received October 22, 2010 Controlling the assembly of the nanoparticles is important because the optical properties of noble metal nanoparticles, such as the surface plasmon resonance (SPR) and surface-enhanced Raman scattering (SERS), are critically dependent on interparticle distances. Among many approaches available, light-induced disassembly is particularly attractive because it enables spatial modification of the optical properties of nanoparticle assemblies. In this study, we prepare gold nanoparticle (AuNP) aggregates in a gel matrix. Irradiation of the gelated AuNP aggregates at 532 nm leads to the disassembly of the aggregates, changing the color (SPR) from dark blue to red and extinguishing the SERS signal along the irradiated pattern, which opens the possibility of facile fabrication of spatially controlled SERS-generating microstructures. The photoinduced disassembly of the AuNP aggregates in solution is also investigated using UV-vis spectroscopy and transmission electron microscopy.
1. Introduction Noble metal nanoparticles are becoming increasingly important due to the unique optical properties arising from surface plasmons, the collective oscillation of conduction electrons.1-4 Resonant excitation of surface plasmons gives rise to strong absorption of light and produces intense electromagnetic fields around the nanoparticles. Such high energy confined in a nanometerscale space leads to enhanced photochemistry and photobiology, permitting a wide range of applications of noble metal nanoparticles from plasmonics, catalysis, and energy to biology and medicine.5-8 The surface plasmon resonance (SPR) of noble metal nanoparticles depends on the size, shape, material, and assembly of the nanoparticles as well as the dielectric constant of the surrounding media.9 The assembly of nanoparticles is particularly important because the SPR changes drastically, depending on the interparticle spaces among the nanoparticles of the assembly.10-13 Furthermore, nanogaps in the assembly produce surface-enhanced Raman scattering (SERS), which can be used for the sensing or spectroscopy *To whom correspondence should be addressed. E-mail: sangwoon@ dankook.ac.kr. (1) Sardar, R.; Funston, A. M.; Mulvaney, P.; Murray, R. W. Langmuir 2009, 25, 13840. (2) Odom, T. W.; Pileni, M.-P. Acc. Chem. Res. 2008, 41, 1565. (3) Odom, T. W.; Nehl, C. L. ACS Nano 2008, 2, 612. (4) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293. (5) Liz-Marzan, L. M. Langmuir 2006, 22, 32. (6) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834. (7) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Acc. Chem. Res. 2008, 41, 1578. (8) Zhang, J. Z. J. Phys. Chem. Lett. 2010, 1, 686. (9) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (10) Zhong, Z.; Patskovskyy, S.; Bouvrette, P.; Luong, J. H. T.; Gedanken, A. J. Phys. Chem. B 2004, 108, 4046. (11) Jensen, T.; Kelly, L.; Lazarides, A.; Schatz, G. C. J. Cluster Sci. 1999, 10, 295. (12) Lazarides, A. A.; Kelly, K. L.; Jensen, T. R.; Schatz, G. C. THEOCHEM 2000, 529, 59. (13) Lazarides, A. A.; Schatz, G. C. J. Phys. Chem. B 2000, 104, 460. (14) Schwartzberg, A. M.; Grant, C. D.; Wolcott, A.; Talley, C. E.; Huser, T. R.; Bogomolni, R.; Zhang, J. Z. J. Phys. Chem. B 2004, 108, 19191. (15) Michaels, A. M.; Jiang, J.; Brus, L. J. Phys. Chem. B 2000, 104, 11965. (16) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Phys. Rev. Lett. 1997, 78, 1667.
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of molecules even at a single-molecule level.14-18 Therefore, controlling the assembly of noble metal nanoparticles is essential for tuning their optical properties. Various physical and chemical methods have been used to control the assembly of metal nanoparticles. An array of triangular nanoparticles was devised using a nanosphere liftoff lithography technique.19 Silver nanowire bundles are another example of the assembly of nanostructures.20 Chemical methods typically involve nanoparticle aggregates in a solution. DNA (de)hybridization or biotin-streptavidin pairing between surface-functionalized nanoparticles leads to separable aggregates.21,22 The thermal or pH responses of capping materials on the nanoparticles are also used for reversible aggregation.23-26 Using light is another attractive means to this end.27-30 In particular, because light can be spatially steered, a pattern of assembled or disassembled nanoparticles can be produced with a submicrometer spatial resolution. Localized nanocrystal growth using light has been demonstrated.27,28 Erasable writing media have been developed using the color differences between dispersed and aggregated metal nanoparticles induced by the photoisomerization of capping agents on nanoparticle surfaces.29 In this (17) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (18) Wustholz, K. L.; Henry, A.-I.; McMahon, J. M.; Freeman, R. G.; Valley, N.; Piotti, M. E.; Natan, M. J.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2010, 132, 10903. (19) Jensen, T. R.; Malinsky, M. D.; Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2000, 104, 10549. (20) Lee, S. J.; Morrill, A. R.; Moskovits, M. J. Am. Chem. Soc. 2006, 128, 2200. (21) Aslan, K.; Luhrs, C. C.; Perez-Luna, V. H. J. Phys. Chem. B 2004, 108, 15631. (22) Storhoff, J. J.; Lazarides, A. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L.; Schatz, G. C. J. Am. Chem. Soc. 2000, 122, 4640. (23) Zhu, M.-Q.; Wang, L.-Q.; Exarhos, G. J.; Li, A. D. Q. J. Am. Chem. Soc. 2004, 126, 2656. (24) Si, S.; Mandal, T. K. Langmuir 2007, 23, 190. (25) Li, D.; He, Q.; Cui, Y.; Li, J. Chem. Mater. 2007, 19, 412. (26) Sistach, S.; Rahme, K.; Perignon, N.; Marty, J.-D.; de Viguerie, N. L.; Gauffre, F.; Mingotaud, C. Chem. Mater. 2008, 20, 1221. (27) Duffus, A.; Camp, P. J.; Alexander, A. J. J. Am. Chem. Soc. 2009, 131, 11676. (28) Bjerneld, E. J.; Svedberg, F.; Kall, M. Nano Lett. 2003, 3, 593. (29) Klajn, R.; Wesson, P. J.; Bishop, K. J. M.; Grzybowski, B. A. Angew. Chem., Int. Ed. 2009, 48, 7035. (30) Sun, S.; Mendes, P.; Critchley, K.; Diegoli, S.; Hanwell, M.; Evans, S. D.; Leggett, G. J.; Preece, J. A.; Richardson, T. H. Nano Lett. 2006, 6, 345.
Published on Web 11/02/2010
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Letter
Letter, we report on the photoinduced disassembly of gold nanoparticle (AuNP) aggregates and its application to the fabrication of spatially controlled SERS-generating microstructures.
2. Materials and Methods AuNPs were synthesized by the reduction of Au3þ precursor ions with citrate.31 A sodium citrate solution (34 mM, 50 mL) was added to a boiling solution of HAuCl4 3 3H2O (0.27 mM, 950 mL) with vigorous stirring. The solution was heated until the color changed from yellow to red. After cooling, the AuNP solution was centrifuged at 10 000g for 10 min in order to remove the residual citrate, and then it was redispersed into deionized water. The AuNPs were characterized using transmission electron microscopy (TEM) and UV-vis spectroscopy. AuNP aggregates were produced by the addition of p-aminothiophenol (pATP) in ethanol (4.4 μM, 2 mL) to an aqueous solution of AuNPs (2.0 nM, 2 mL). Chemisorption of the pATP molecules on the AuNP surfaces by the formation of strong Au-S bonds displaced the citrate anions, leading to the aggregation.32,33 A series of pATP concentrations were used to find the optimal conditions for the formation of AuNP aggregates that maintained their stability during the experiments without precipitation. Figure S1 in the Supporting Information shows that the AuNP aggregates were stable for at least 48 h when 4.4 μM pATP was used, which corresponds to a surface coverage of 0.51 for the AuNPs with a diameter of 14.5 nm.32 The laser irradiation experiments were performed approximately 12 h after the addition of pATP. The gelated AuNP aggregates were prepared by the addition of a 0.5 mL 2% agarose solution at 90 °C to the 3 mL AuNP aggregate solution. The second harmonic generation of a Nd:YAG laser (Spectra Physics, GCR 11) was used to induce the disassembly of the AuNP aggregates. The 532 nm, 8 ns pulses running at 10 Hz were directed to the AuNP aggregate samples (1.5 mL) in a 10 mm 10 mm quartz cuvette. The laser beam (diameter 7 mm) was intentionally unfocused to illuminate the largest possible area of the sample. The resulting changes in the AuNP aggregates by irradiation were measured using UV-vis spectroscopy and TEM. For the patterning of SERS-generating structures, masks were fabricated by etching a chromium film on quartz using an electron beam. The gelated AuNP aggregates were irradiated through the mask. For Raman mapping of the irradiated AuNP aggregates, Raman spectra were acquired from 40 30 spots over a 650 μm 350 μm region using a Raman microscope (Kaiser, Raman MicroProbe). A laser at 785 nm was focused onto the spots through a 10 objective and Raman scattering was collected and directed to a spectrometer. An exposure time for each Raman spectrum was 1 s.
3. Results and Discussion We first investigated the photoinduced disassembly of AuNP aggregates in a solution. The pATP-derived AuNP aggregates in ethanol/water were irradiated by a 532 nm pulsed laser at 50 mW (13 mJ pulse-1 cm-2). The resulting changes were probed by UV-vis spectroscopy. Figure 1 presents the evolution of the UV-vis spectra of the AuNP aggregates as the irradiation time increases. The UV-vis spectrum of the AuNP aggregates shows a characteristic band in the long wavelength region near 700 nm, arising from the interactions between the surface plasmons of AuNPs in close proximity.10-13 This surface plasmon coupling band is sensitive to the interparticle spacing. As the number of shots on the AuNP aggregates increases, the surface plasmon coupling band gradually decreases while the SPR band of dispersed AuNPs at 525 nm is restored. This observation indicates (31) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (32) Yoon, J. H.; Park, J. S.; Yoon, S. Langmuir 2009, 25, 12475. (33) Basu, S.; Pande, S.; Jana, S.; Bolisetty, S.; Pal, T. Langmuir 2008, 24, 5562.
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Figure 1. UV-vis spectra of AuNP aggregates in solution, irra-
diated by a 532 nm pulsed laser at 50 mW (13 mJ pulse-1 cm-2) for the period of time indicated in the figure. The UV-vis spectrum of dispersed AuNPs before aggregation (dotted line) is included for comparison. The arrows are visual guides to indicate the direction of the changes in the SPR band (∼525 nm) and in the surface plasmon coupling band (∼700 nm) with the irradiation time. The color of the AuNP aggregates solution before and after the irradiation (61 s) is displayed in the inset.
Figure 2. TEM images of (a) the dispersed AuNPs initially prepared, (b) the AuNP aggregates induced by the addition of pATP, and (c) the redispersed AuNPs after irradiation for 1 min. The distributions of particle sizes are shown on the right.
that the AuNP aggregates are disassembled into separate AuNPs due to the irradiation. The color of the solution also changes from the dark blue of the AuNP aggregates to the red of dispersed AuNPs, as shown in the inset of Figure 1. Further irradiation past 1 min leads to the decrease and slight blue-shift of the SPR band at 525 nm, suggesting that smaller, fragmented nanoparticles are produced (Supporting Information). We focus on the disassembly process in this study because the fragmentation of dispersed AuNPs has been extensively investigated in previous studies.34-41 (34) Park, J. S.; Yoon, J. H.; Kim, H. J.; Huh, Y.-D.; Yoon, S. Bull. Korean Chem. Soc. 2010, 31, 819. (35) Kurita, H.; Takami, A.; Koda, S. Appl. Phys. Lett. 1998, 72, 789.
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The TEM images in Figure 2 show the morphological changes of the AuNP aggregates upon irradiation. The original, welldispersed AuNPs have a monodisperse size distribution with an average diameter of 14.5 ( 1.4 nm. Figure 2b shows networked AuNPs, a typical morphological feature of AuNP aggregates. Irradiation of the AuNP aggregates for 1 min disassembles the aggregates, yielding well-separated, individual AuNPs with a diameter of 14.4 ( 2.3 nm, similar to that of the original nanoparticles. Additionally, larger AuNPs (30.6 ( 4.7 nm) are observed in Figure 2c. These particles are presumably produced by the fusion or melting of closely located AuNPs in the aggregates, as observed in previous studies.41,42 It is not clear yet how photoinduced disassembly occurs at a molecular level. The most probable mechanism is the surfaceplasmon-mediated thermal desorption.43 Excited surface plasmons of AuNPs by resonant photoexcitation relax to equilibrium by energy exchange with lattice phonons within 1 ps.44-46 The excited phonons then lead to the desorption of the pATP molecules from the AuNP surfaces. The adsorption sites left empty by the desorption are quickly filled with citrates that are abundantly present in the solution. The restored negative charges on the surfaces separate the AuNPs. One interesting observation related to the disassembly mechanism is that once the AuNPs are dispersed by laser irradiation, they do not reaggregate, suggesting that the desorbed pATP does not readsorb onto the surfaces of the AuNPs (Supporting Information). Because the addition of a fresh solution of pATP to the disassembled AuNPs still induces aggregation, it seems unlikely that the surfaces of AuNPs are annealed or changed by irradiation. More likely is that pATP, when desorbed, converts to a species that does not interact with the AuNPs. Previous studies on the photodesorption of self-assembled monolayers on two-dimensional gold surfaces have shown that the thiolate molecules desorb in the oxidized sulfonate (SO3-) forms.47,48 We confirmed that the addition of sulfanilic acid (NH2C6H4SO3-), an oxidized form of pATP, to a solution of AuNPs did not induce aggregation. However, attempts to identify the exact form of photodesorbed pATP using various spectroscopic methods failed due to the small amount of pATP used to induce the aggregation. The photoinduced disassembly of AuNP aggregates in solution provides knowledge that can be applied to the fabrication of patterned SERS-active assembly structures, combining the spatial controllability of light with the tunable optical properties of the (dis)assembly of AuNPs. Gel matrices are an appropriate medium for this study because they provide the stiffness that supports the structure of the AuNP aggregates and, at the same time, the (36) Mafune, F.; Kohno, J.-y.; Takeda, Y.; Kondow, T. J. Phys. Chem. B 2002, 106, 7575. (37) Mafune, F.; Kohno, J.-y.; Takeda, Y.; Kondow, T. J. Phys. Chem. B 2002, 106, 8555. (38) Mafune, F.; Kohno, J.-y.; Takeda, Y.; Kondow, T. J. Phys. Chem. B 2003, 107, 12589. (39) Takami, A.; Kurita, H.; Koda, S. J. Phys. Chem. B 1999, 103, 1226. (40) Peng, Z.; Walther, T.; Kleinermanns, K. Langmuir 2005, 21, 4249. (41) Fujiwara, H.; Yanagida, S.; Kamat, P. V. J. Phys. Chem. B 1999, 103, 2589. (42) Chen, C.-D.; Yeh, Y.-T.; Wang, C. R. C. J. Phys. Chem. Solids 2001, 62, 1587. (43) Extended discussion on the photoinduced disassembly mechanisms is provided in the Supporting Information. (44) Ahmadi, T. S.; Logunov, S. L.; El-Sayed, M. A. J. Phys. Chem. 1996, 100, 8053. (45) Hodak, J. K.; Martini, I.; Hartland, G. V. J. Phys. Chem. B 1998, 102, 6958. (46) Perner, M.; Bost, P.; Lemmer, U.; Plessen, G. v.; Feldmann, J.; Becker, U.; Mennig, M.; Schmitt, M.; Schmidt, H. Phys. Rev. Lett. 1997, 78, 2192. (47) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342. (48) Hutt, D. A.; Cooper, E.; Leggett, G. J. J. Phys. Chem. B 1998, 102, 174.
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Figure 3. UV-vis spectra of gelated AuNP aggregates, irradiated
by a 532-nm pulsed laser at 100 mW (26 mJ 3 pulse-1 3 cm-2) for the period of time indicated in the figure. The UV-vis spectrum of gelated dispersed AuNPs before aggregation (dotted line) is included for comparison. The arrows are visual guides to indicate the direction of the changes in the SPR band (∼ 520 nm) and in the surface plasmon coupling band (∼700 nm) with the irradiation time. The change in color of the irradiated region (dotted circle) is shown in the inset.
Figure 4. (a) Optical microscopy images of masks used in the photoinduced SERS patterning. The letters DKU are transparent in the left mask and opaque in the right one. (b) Two- and threedimensional SERS intensity maps for the 1078 cm-1 mode of pATP, obtained from the gelated AuNP aggregates irradiated through the masks shown in (a). Representative SERS spectra of pATP from the nonirradiated region (red) and from the irradiated region (blue) are also shown.
softness that allows for the mobility of the nanoparticles.49 The gelated AuNP aggregates were prepared with an agarose gel and, (49) The mobility of AuNPs in a gel matrix was tested by adding a solution of pATP to the gelated dispersed AuNPs. See the Supporting Information for details.
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subsequently, subjected to irradiation at 532 nm, 26 mJ pulse-1 cm-2.50 Figure 3 presents the changes in the UV-vis spectra of the gelated AuNP aggregates as the irradiation time increases. The irradiation lowers the intensity of the surface plasmon coupling band at 695 nm and increases the SPR band near 520 nm, indicating the breakup of the aggregates into separated nanoparticles. The initial changes in the UV-vis spectra appear more dramatic in Figure 3 than in Figure 1 because the disassembled nanoparticles are not mixed in the gel. The inset in Figure 3 shows that the irradiated spot turned red. The color changes produced by localized laser-induced disassembly of gelated AuNP aggregates are useful for color patterning, as demonstrated in the Supporting Information (Figure S10). Since the aggregation state of AuNPs influences not only surface plasmon coupling, as described above, but also SERS properties, we attempted spatially controlled patterning of SERSgenerating structures using photoinduced disassembly of AuNP aggregates. The gelated AuNP aggregates were prepared in a quartz cell, as described in section 2. The laser light impinges on the aggregates through a mask, placed on the surface of the cell. Figure 4a shows the optical microscopy images of the masks. Laser light passes through the letters “DKU”, with a width of 50 μm, in the mask shown on the left while it is blocked by the letters on the right. After irradiation, Raman spectra were collected at 40 30 points. Figure 4b shows the Raman intensity maps for the 1078 cm-1 mode of pATP. The Raman signal of pATP is enhanced in the masked region by a factor of 3.4 104 due to the interstitial sites present in the AuNP aggregates (Supporting Information). The SERS signal, however, decreases markedly upon irradiation. Figure 4b shows the decrease in the SERS signal along the irradiated patterns. The aggregated AuNPs are dispersed by irradiation, causing the SERS signal to decrease. This work demonstrates that it is possible to make any SERS-generating patterns on the micrometer scale, using the spatial controllability of light and gelated AuNP aggregates. (50) The optimal laser power was determined by the power-dependence experiments. See the Supporting Information for details.
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4. Conclusion A facile approach to the spatial patterning of SERS-active substrates was demonstrated. Color-tunable and SERS-generating stable AuNP aggregates were formed by the addition of pATP to a AuNP solution. Laser irradiation of the AuNP aggregates at 532 nm led to the disassembly of the aggregates, decreasing the surface plasmon coupling band near 700 nm and restoring the SPR band of the dispersed AuNPs near 520 nm in the UV-vis spectra. TEM images further confirmed the disassembly of the aggregates to well-separated, individual AuNPs. Photoinduced disassembly also occurred in the gelated AuNP aggregates. The irradiated spot turned red and the UV-vis spectra showed changes consistent with the changes in solution. Patterned SERS-active structures were fabricated by irradiating the gelated AuNP aggregates in a spatially controlled fashion. The disassembly of the AuNP aggregates significantly decreased the SERS signal in the irradiated region. Any SERS patterns can be produced using the photoinduced disassembly of gelated AuNPs, which is potentially useful in developing SERS-based sensors. Acknowledgment. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2007-0054329 and 2010-0007764). We also gratefully acknowledge the support from the Korea Research Institute of Standards and Science (KRISS). We thank H. K. Hong at the Korea Electronics Technology Institute (KETI) for the fabrication of the optical masks. Supporting Information Available: Formation of stable AuNP aggregates, fragmentation of AuNPs, optimal conditions for the gel experiments, example of color pattering, calculation of the SERS enhancement factor for gelated AuNP aggregates, and extended discussion on the mechanism of the photoinduced disassembly of AuNP aggregates. This material is available free of charge via the Internet at http://pubs.acs.org.
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