12042
Langmuir 2007, 23, 12042-12047
Self-Assembled Silver Nanochains for Surface-Enhanced Raman Scattering Yong Yang,*,†,‡ Jianlin Shi,‡ Taiki Tanaka,† and Masayuki Nogami*,† Department of Materials Science and Engineering, Nagoya Institute of Technology, Showa, Nagoya, 466-8555, Japan, and School of Materials Science and Engineering, East China UniVersity of Science and Technology, Shanghai 200237, People’s Republic of China ReceiVed June 1, 2007. In Final Form: August 27, 2007 Surface-enhanced Raman scattering (SERS) integrates high levels of sensitivity with spectroscopic precision and has tremendous potential for chemical and biomolecular sensing. The key to the wider application of Raman spectroscopy using roughened metallic surfaces is the development of highly enhancing substrates for analytical purposes. Here, we demonstrate a simple strategy for self-assembling silver nanochains on glass substrates for sensitive SERS substrates. The chain length of short Ag nanochains can be controlled by adjusting the concentration of cetyltrimethylammonium bromide (CTAB) and 11-mercaptoundecanoic acid (MUA). CTAB with appropriate concentration serves as the “glue” that can link the {100} facets of two neighboring Ag nanoparticles. MUA is found to be effective in “freezing up” the aggregation of Ag short chains and preventing them from further aggregating into a long chainlike network structure. The surface plasmon bands can be tuned over an extended wavelength range by controlling the length of the nanochains. The Ag monolayer, mainly composed of four-particle nanochains, exhibited the maximum SERS enhancement factor of around 2.6 × 108, indicating that a stronger SERS enhancement can be obtained in these interstitial sites of chainlike aggregated Ag nanoparticles.
Introduction Surface plasmons (SPs) are collective electronic excitations at the interface between metals and dielectrics and are currently being explored for their potential applications in subwavelength optics, data storage, light generation, nonlinear optics, microscopy, and biophotonics.1-4 Especially, the localized surface plasmon resonance in Au, Ag nanoparticles greatly enhances the local electromagnetic field (EM) a few nanometers above surface, resulting in increased Raman scattering. The surface-enhanced Raman scattering (SERS) signal has been found to be enhanced on the order of 1014 in the presence of nanometer-sized Ag “hot particles”, which allows the possibility of studying Raman scattering even at the single-molecule level.5 However, isolated metal nanoparticles usually yield a weak SERS response compared to aggregates, because the latter enables additional strong EM field enhancement in the gap regions between the particles.6,7 Consequently, much recent attention has turned toward the development of SERS substrates based on closely spaced nanoparticles.8 Especially, the local field effects can be enhanced several orders of magnitude in the metal chainlike aggregated * Corresponding author. Tel/Fax: +81 52 7355285. E-mail: nogami@ mse.nitech.ac.jp (M.N.),
[email protected] (Y.Y.). † Nagoya Institute of Technology. ‡ East China University of Science and Technology. (1) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Nature 2003, 424, 824. (2) (a) Kneipp, K.; Kneipp, H.; Kneipp, J. Acc. Chem. Res. 2006, 39, 443. (b) Haynes, C. L.; McFarland, A. D.; Van Duyne, R. P. Anal. Chem. 2005, 77, 338. (3) Maye, M. M.; Luo, J.; Han, L.; Zhong, C. J. Nano Lett. 2001, 1, 575. (4) (a) Yang, Y.; Nogami, M.; Shi, J.; Chen, H.; Ma, G.; Tang, S. Appl. Phys. Lett. 2006, 88, 081110. (b) Yang, Y.; Nogami, M.; Shi, J.; Chen, H.; Ma, G.; Tang, S. J. Phys. Chem. B 2005, 109, 4865. (c) Yang, Y.; Matsubara, S.; Xiong, L.; Hayakawa, T.; Nogami, M. J. Phys. Chem. C 2007, 111, 9095. (5) Xu, H. X.; Bjerneld, E. J.; Ka¨ll, M.; Bo¨rjesson, L. Phys. ReV. Lett. 1999, 83, 4357. (6) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (7) Michaels, A. M.; Nirmal, M.; Brus, L. E. J. Am. Chem. Soc. 1999, 121, 9932. (8) (a) Lu, Y.; Liu, G. L.; Lee, L. P. Nano Lett. 2005, 5, 5. (b) Wang, H.; Levin, C. S.; Halas, N. J. J. Am. Chem. Soc. 2005, 127, 14992. (c) Lee, S. J.; Morril, A. R.; Moskovits, M. J. Am. Chem. Soc. 2006, 128, 2200. (d) Jiang, J.; Bosnick, K.; Maillard, M.; Brus, L. J. Phys. Chem. B 2003, 107, 9964.
structure and are responsible for the enhanced SERS signals. Chains of metallic nanoparticles have been prepared by physical confinement in ion-beam-etched tracks in glass9 or porous anodic alumina10 and by using linear macromolecular or supramolecular templates11 such as polyelectrolytes, DNA, and carbon nanotubes. Recently, we reported the fabrication of chainlike aggregates of gold nanoparticles by cetyltrimethylammonium bromide (CTAB).12a However, the formation mechanism is still elusive, and the controllable creation of nanoparticles pairs and 1D chain structure with definite chain length has also remained a challenge. Here, we report the preparation of a 1D short chainlike aggregate of Ag nanoparticles using an improved strategy based on controlled ligand exchange of citrate ions adsorbed onto the surface of Ag nanoparticles, because the Ag nanoparticles are SERS-active substrates. We show that the CTAB with appropriate concentration serves as the “glue” that can link the {100} facets of two neighboring Ag nanoparticles, which leads to an anisotropic distribution of the residual surface charges, and this extrinsic electric dipole formation is responsible for the linear organization of the Ag nanoparticles into short chains. Furthermore, 11mercaptoundecanoic acid (MUA) is found to be effective in “freezing up” the aggregation of Ag short chains and preventing them from further aggregating into a long chainlike network structure. The surface plasmon bands of these Ag nanochains can be tuned over an extended wavelength range by controlling the length of the nanochains, and this provides the opportunity to study the function of local field enhancement with the SERS (9) Penninkhof, J. J.; Polman, A.; Sweatlock, L. A.; Maier, S. A.; Atwater, H. A.; Vredenberg, A. M.; Kooi, B. J. Appl. Phys. Lett. 2003, 83, 4137. (10) Nagle, L.; Ryan, D.; Cobbe, S.; Fitzmaurice, D. Nano Lett. 2003, 3, 51. (11) (a) Minko, S.; Kiriy, A.; Gorodyska, G.; Stamm, M. J. Am. Chem. Soc. 2002, 124, 10192. (b) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775. (c) Ugarte, D.; Chatelain, A.; de Heer, W. A. Science 1996, 274, 1897. (12) (a) Yang, Y.; Matsubara, S.; Nogami, M.; Shi, J.; Huang, W. Nanotechnology 2006, 17, 2821. (b) Yang, Y.; Matsubara, S.; Nogami, M.; Shi, J. Mater. Sci. Eng. B 2007, 140, 172. (c) Lin, S.; Li, M.; Dujardin, E.; Christian G.; Mann, S. AdV. Mater. 2005, 17, 2553. (d) Shipway, A. N.; Lahav, M.; Gabai, R.; Willner, I. Langmuir 2000, 16, 8789.
10.1021/la701610s CCC: $37.00 © 2007 American Chemical Society Published on Web 10/27/2007
Self-Assembled SilVer Nanochains for SERS
Langmuir, Vol. 23, No. 24, 2007 12043
Figure 1. TEM images of Ag colloids: (a) sol-1, (b) sol-2, (c) sol-3, and (d) sol-4. All scale bars represent 50 nm. (e-h) Particle number distribution in Ag nanochains for related TEM images. The criterion to judge whether a group of Ag particles form a chain or not is that the distance between neighboring particles is less than the chain length of CTAB molecules.
enhancement. The Ag monolayer, mainly composed of fourparticle nanochains, exhibited the maximum SERS enhancement factor of around 2.6 × 108, indicating that a stronger SERS enhancement can be obtained in these interstitial sites of chainlike aggregated Ag nanoparticles. Experimental Section AgNO3 and poly(diallyldimethylammomium chloride) (PDDA) was purchased from Tokyo Chemical Co. CTAB (99%) and MUA were obtained from Sigma. All other reagents were from Aldrich and were used as received. Ultrapure deionized water (Narnstead Nanopure H2O purification system) was used throughout the experiments. We prepared four different chainlike silver nanoparticle colloids. The first Ag nanoparticle colloid was prepared by thermal reduction of 10 mM AgNO3 aqueous solution in the presence of 15 mM trisodium citrate at 90 °C (referred to as sol-1). After the solution had turned green within 30 s, it was cooled quickly in the ice bath. The citrate ions act as both reductant and stabilizer of silver particles in solution. Synthesized sol-1 (20 mL) was reacted with 0.2, 0.6, and 1 mL of 0.1 mM CTAB aqueous solution at room temperature for 15 min (referred to as sol-2, sol-3, and sol-4, respectively). Then, 1 mL of 0.2, 0.6, 1 mM MUA was also added into sol-2, sol-3, and sol-4, respectively, in order to prevent the excess aggregation of CTAB-modified silver colloids.12 The silver nanoparticles were selfassembled on PDDA-modified (1% w/w) glass slides13 by immersing the slides in the solution for 24 h and withdrawing at a speed of 10 mm‚min-1, followed by extensive rinsing with water along the fixed direction.14 Four self-assembled monolayers of silver nanoparticles were deposited by alternate immersion into the silver colloids. Finally, these films were treated at 100 °C for 60 min in a stream of 20% H2-80% N2 in order to partially remove some organic agents. Then these films are stored in vacuum for 15 days. The absorption optical spectra of these silver nanochain monolayers were recorded using a Jasco Ubest 570 UV-vis-NIR spectrophotometer. All the spectra were recorded in air at room temperature. The microstructure and morphology of silver nanoparticles in silver (13) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. J. Phys. Chem. B 2001, 105, 3441. (14) Yang, Y.; Xiong, L.; Shi, J.; Nogami, M. Nanotechnology 2006, 17, 2670.
colloids was measured with a JEOL JEM-2000EXII transmission electron microscopy (TEM) operating at 200 kV. Those samples were prepared by dropping the colloid onto a carbon-coated Cu grid underlying tissue paper, leaving behind a film. The surface morphology and roughness of the silver nanochain monolayers were monitored with an atomic force microscope (Seiko II, SPA-300HV) operated in dynamic force mode. Far-field SERS measurements were performed by a Jasco NFS-220 M scanning near-field optical microspectrometer. All samples for SERS measurement were prepared by casting 10 µL of 5 nM Rhodamine 6G (R6G) in ethanol onto a Ag monolayer and allowing the solvent to evaporate. The referenced sample was prepared by casting 10 µL of 100 mM R6G (R6G distribution area 78.5 mm2) in ethanol onto glass and allowing the solvent to evaporate.
Results and Discussions Figure 1a-d shows TEM images of sol-1, sol-2, sol-3, and sol-4, respectively. Well-dispersed Ag nanoparticles with a spherical shape and diameter of around 30 nm were observed in sol-1 prepared without CTAB (Figure 1a). In contrast, we found that colloids prepared with CTAB formed chainlike, aggregated Ag nanoparticles, with the number of aggregated particles increasing upon increasing the amount of CTAB. Nanochains mainly composed of two, three, and four Ag nanoparticles are obtained in sol-2, sol-3, and sol-4, respectively (Figure 1b-d). Figure 1e-h shows the statistical results for the particle number distributions in different Ag nanochains, also indicating that the nanochains in sol-2, sol-3, and sol-4 are mainly composed of two, three, and four Ag nanoparticles. Hence, it is clear that CTAB concentration plays an important role in determining the nature of the aggregation. Figure 2a shows the optical absorbance spectra of Ag colloids modified by CTAB and MUA. The Ag sol-1 without CTAB and MUA exhibits one narrow absorbance band at 408 nm, which is attributed to the surface plasmon resonance (SPR) band of monodisperse Ag nanoparticles. Ag sol-2, sol-3, and sol-4 exhibit two absorbance bands; one is near 408 nm and the other absorbance band red-shifts with the increase of CTAB volume,
12044 Langmuir, Vol. 23, No. 24, 2007
Yang et al.
Figure 3. UV-vis-NIR absorbance spectra of Ag sol-3 at different time intervals before and after the addition of MUA.
Figure 2. (a) UV-vis-NIR absorbance spectra of Ag colloids: (A) sol-1, (B) sol-2, (C) sol-3, and (D) sol-4. (b) The simulated extinction spectra of Ag nanochains upon varying the aggregated particle number (N) in one chain.
at 560, 620 and 710 nm, respectively. The second absorbance band is attributed to the SPR coupling band of linear-aggregated Ag nanoparticles.13,15a The polarization of the conduction electron oscillations in adjacent Ag nanoparticles causes a new red-shifted plasmon absorbance band attributed to the coupling of the plasmon absorbance of the particles. The established electric field alignment description of the point-dipole model15b for a metal chain suggests the presence of two SPR modes: a longitudinal and a transverse plasmon resonance along and perpendicular to the chain-axis, respectively. The transverse modes are located around the SPR position of a single-particle dipole mode (410 nm), and the positions of the longitudinal modes depend on the number N of aggregated particles. We simulated spectral features (Figure 2b) using Gans’ theory,16 which is applied to ellipsoids and nanorods, and found that the longitudinal modes of two, three, and four Ag chainlike aggregates were located at 500, 610, and 720 nm, respectively. By taking into account the chain polydispersity (Figure 1) and variation in the interparticle separations, our experimental result is consistent with the simulated results. (15) (a) Thomas, K. G.; Barazzouk, S.; Ipe, B. I.; Joseph, S. T. S.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 13066. (b) Maier, S.; Kik, P. G.; Atwater, H. A. Phys. ReV. B 2003, 67, 205402. (16) (a) Papavassiliou, G. C. Prog. Solid State Chem. 1979, 12, 185. (b) Link, S.; Mohamed, M. B.; El-sayed, M. A. J. Phys. Chem. B 1999, 103, 3073. (c) Khlebtsov, N. G.; Melnikov, A. G.; Bogatyrev, V. A.; Dykman, L. A. Photopolarimetry in Remote Sensing (NATO Science Series II) Mathematics, Physics, and Chemistry; Vidden, G., Yatskiv Y. S., Mishchenko, M. I., Eds.; Kluwer: Dordrecht, 2004; p 265.
Figure 4. TEM image of Ag sol-3 after the addition of CTAB for 180 min.
The function of MUA is indicated in the absorbance spectral change of Ag sol-3 as a function of time, as shown in Figure 3. After 0.6 mL of CTAB was added into the citrate-stabilized Ag sol-1 for 15 min, first two SP bands around 400 and 615 nm were observed. With the aging time increased to 90 min, three SPR bands around 420, 530, and 810 nm appeared, which indicated that more monodisperse Ag nanoparticles self-assembled into linear-aggregated Ag nanoparticles units. Interestingly, other SPR bands except the SPR band at 420 nm broadened and red-shifted to the near-infrared region around 980 nm after aging for 180 min. Their TEM image (Figure 4) showed that Ag nanoparticles aggregated into a long chainlike network structure. It indicated that the CTAB-modified Ag colloid was not stable and the aggregation extent would increase with the increased aging time. However, we found that this solution was very stable after MUA was added into this solution, and two SPR bands at 400 and 620 nm did not show any change until the aging time of 180 min. It indicates that MUA can “freeze up” the aggregation of Ag short chains and prevent them from further aggregating into a long chainlike network structure. The aggregation of the Ag nanoparticles can be explained by the preferential binding of CTAB molecules on a certain facet of silver nanoparticles and the Ag nanoparticles electrostatic
Self-Assembled SilVer Nanochains for SERS
Langmuir, Vol. 23, No. 24, 2007 12045
Scheme 1. Schematic Diagram Illustrating the Aggregation Behavior of Ag Sol-3 Modified by CTAB and MUAa
a
The {111} and {100} facets are shown in gray and green, respectively.
interactions. As reported in some literature,17,18 these quasispherical Ag nanoparticles in Ag colloids created by the citratereduction method can be ascribed to a mixture of pentagons, triangles, hexagons, and quasitetragons, likely corresponding in three dimensions to decahedron (or related twin-crystal icosahedron), tetrahedron (or triangular plate), truncated tetrahedron, and cube of Ag nanoparticles.17 The Ag colloids are also a mixture of single crystalline, single-twinned, and multiply twinned structures, the same as most solution-phase syntheses of Ag nanoparticles. Here the aggregation behavior of truncated tetrahedron-shaped Ag nanoparticles is given as an example of how to form stable chainlike Ag aggregates by CTAB and MUA in sol-3, as shown in the schematic illustrations (Scheme 1). Alkyltrimethylammonium bromides are an effective stabilizer for metal nanoparticles and surfactants that assist in the synthesis of metal nanorods of various sizes. Previous work19 about CTABassisted synthesis of gold nanorods by El-Sayed et al. has shown that the gold nanorods are capped with a bilayer of CTAB and the CTAB surfactant preferentially binds to the {100} facets rather than the {111} end facets of gold nanoparticles. The CTAB with appropriate concentration serves as the “glue” that can link the {100} facets of two neighboring Ag nanoparticles12a (route I), which leads to an anisotropic distribution of the residual surface charges. The opposite {100} facet A in the Ag nanopair has higher negative potential than other {100} facets of this Ag nanoparticle such as facets B and C. As shown in route II, the next CTAB molecules preferentially bind to this facet A, and other citrate-stabilized Ag nanoparticles are adsorbed on these CTAB molecules orientated at facet A to self-assemble into linear, chainlike aggregates by the extrinsic electric dipole formation.12 Upon increasing the reaction time, the long chainlike network aggregates will form, as shown in Figure 4. While MUA is added into CTAB-modified Ag sol-3 in time, the MUA can “freeze up” the aggregation of Ag short chains and prevent them from further aggregating into the long chainlike network structure because the MUA can bind to Ag nanoparticles by thiol agent to form a new protecting layer and the carboxylate-terminated agent12c (17) (a) Xia, Y.; Halas, N. J. MRS Bull. 2005, 30, 338. (b) Wiley, B.; Sun, Y.; Chen, J.; Cang, H.; Li, Z.; Li, X.; Xia, Y. MRS Bull. 2005, 30, 356. (18) Wiley, B.; Herricks, T.; Sun, Y.; Xia, Y. Nano Lett. 2004, 4, 1733. (19) Nikoobakht, B.; El-Sayed, M. A. Langmuir 2001, 17, 6368.
can prevent further binding on Ag nanoparticles by CTAB molecules, as shown in route III. The packing topography of two-dimensional Ag nanochains on the substrates was investigated by the tapping mode of an atomic force microscopy (AFM). Figure 5a-d shows AFM images of those monolayers prepared from sol-1, sol-2, sol-3, and sol-4, respectively. Monodisperse Ag nanoparticles with uniform shape and diameter of about 30 nm are clearly seen in the monolayer prepared using sol-1 (Figure 5a). Figure 5b-d shows the silver nanochains mainly composed of two, three and four nanoparticles, respectively. The gap between those adjacent Ag nanoparticles in one short nanochain was found to be less than 2 nm. Interestingly, those nanochains self-assemble in a manner resembling a two-dimensional array in a certain directional arrangement due to the function of water flux with the fixed direction in all washing procedure. The particle densities of those Ag nanochains monolayers are estimated to be around 625, 200, 640, and 650 µm-2, respectively. Thus, our simple method using different concentrations of CTAB and MUA is appropriate for self-assembling silver short nanochains with definite aggregation numbers. Figure 6 shows optical absorbance spectra of Ag monolayers prepared from sol-1, sol-2, sol-3, and sol-4. The well-dispersed Ag nanoparticle monolayer exhibits an absorbance band at 425 nm, which is attributed to the SPR band of individual Ag nanoparticles. Besides the SPR band at 425 nm, monolayers composed of different silver nanochains exhibit several bands, at 670, 710 and 820, respectively. The second absorbance band can be also attributed to the SPR coupling band of linearaggregated Ag nanoparticles. Compared with the absorbance spectra (Figure 2) of Ag colloids, these second absorbance bands in Ag monolayers red-shift due to the increase of the matrix refractive index. Therefore, the surface plasmon bands can be tuned over an extended wavelength range by controlling the length of the nanochains. One of the major goals of the present study is to prepare SERS-active substrates that can be useful for investigating the molecules adsorbed on them. Figure 7 shows far-field SERS spectra of 5 nM R6G on the monolayers of Ag nanochain excited at 532.0 nm. The Ag nanochain monolayers exhibited very strong
12046 Langmuir, Vol. 23, No. 24, 2007
Yang et al.
Figure 5. AFM images of Ag monolayers prepared using (a) sol-1, (b) sol-2, (c) sol-3, and (d) sol-4. All scale bars represent 100 nm. Section analysis data are also given for the blue dotted lines of images.
Raman signals with little background noise. The observed Raman band at 1645, 1560, 1510, and 1355 cm-1 are attributed to ν(C-C) stretching vibrations and agree well with literature values.8,20 Two other Raman bands at 1186 and 1530 cm-1 can be also attributed to R6G,20 while the band at 1080 cm-1 can be assigned to the residual coated molecules.8 Without SERS originated from Ag nanostructures, relatively weak, slightly shifted peaks were found for 100 mM R6G on bare glass (spectrum A). The well-dispersed Ag nanoparticle monolayer deposited on glass also showed a very weak and poorly resolved Raman spectrum (spectrum B), even worse than that of 100 mM R6G adsorbed on glass. We note that the Ag nanochain monolayers exhibit much stronger and better resolved spectra, when compared with the SERS spectra obtained by using randomly absorbed (20) (a) Hildebrandt, P.; Srockburger, M. J. Phys. Chem. B 1984, 88, 5935. (b) Hayazawa, N.; Inouye, Y.; Sekkat, Z.; Kawata, S. Chem. Phys. Lett. 2001, 335, 369.
monodisperse Ag NPs monolayer. Very interestingly, the SERS signals from R6G are observed to increase with the increasing aggregation number of Ag nanochains, indicating that the Ag nanochains monolayers with more interstitial sites between Ag nanoparticles are more powerful as SERS-active substrates. The surface enhancement factor (SEF) was calculated for R6G on the Ag nanochain samples according to the equation14,21 SEF ) (Isurf/Nsurf)/(Ibulk/Nbulk), where Isurf and Ibulk denote the integrated intensities for the 1355 cm-1 band of the 5 nM R6G adsorbed on the Ag surface and 100 mM R6G on glass, respectively, whereas Nsurf and Nbulk represent the corresponding number of R6G molecules excited by the laser beam. The SEF of monodisperse Ag nanoparticle monolayer is evaluated to be 1.3 × 106, the same order of magnitude as the reported Ag nanoparticles with the diameter of 33 nm excited at 632.8 nm.22 The SEF increases with the increase of the Ag nanochain length (the inset of Figure 7), and the maximal SERS enhancement
Self-Assembled SilVer Nanochains for SERS
Figure 6. UV-vis-NIR absorbance spectra of self-assembled Ag monolayers prepared using (A) sol-1, (B) sol-2, (C) sol-3, and (D) sol-4.
Langmuir, Vol. 23, No. 24, 2007 12047
laser excitation frequency is about half the vibrational Stokes shift of the coupled SPR frequency maximum for the adsorbate covered surface.24 The larger SERS enhancement should be observed when the laser excitation frequency is closer to the coupled SPR frequency. Here, the enhancement can be attributed to the coupling of dipole plasmon and local field enhancement at these interstitial sites of Ag nanochains that served as SERS “hot spots”.9,11,25 Coupled surface plasmon are localized in these junctions between nanochains, and these junctions act as electromagnetic “hot spots”. Such hot spots support extremely intense local electromagnetic fields and thus exhibit a large contribution to SERS. Recently theoretical studies26 and experimental results4,27 on special metal chainlike aggregates suggest that the local field effects can be enhanced several orders of magnitude in the metal chainlike aggregated structure and are responsible for the enhanced SERS signals. Even the maximum electric field enhancement of 1013 can be obtained by an array of truncated tetrahedron dimers.28 Moreover, the chain length dependency of the SERS enhancement factor can be attributed to the “lightning rod” effect,29 by which the electric field enhancement increases as the chain length of the nanochains increases. The local fields are affected by the aggregated particle number in nanochains through the depolarization tensor and can be strongly enhanced by these surface plasmon coupling modes at the highly localized surface plasmon.
Conclusions
Figure 7. (a) SERS spectra of 100 mM R6G adsorbed on (A) glass and 5 nM R6G adsorbed on Ag nanochain monolayers prepared using (B) sol-1, (C) sol-2, (D) sol-3, and (E) sol-4 (excited wavelength, 532 nm; exposure time, 5 s; focus area diameter, 2 µm; power, 4 mW). The inset is the chain length (N) dependency of the SERS enhancement factor.
factor observed on Ag nanochain monolayer (sol-4) is estimated to be 2.6 × 108, about 2 orders of magnitude higher than that of monodisperse Ag nanoparticle monolayer. This SEF is 40fold less than that of single hemoglobin molecules attached to aggregated Ag nanoparticles colloids6 because analyte molecules in colloids have a higher chance of adsorbing on those hot spots than those in film-based SERS substrates. Generally, when the colloidal substrate is converted into a film-based substrate, the SERS sensitivity drops 10-100 times.23 Therefore, it is not surprising that most single molecule SERS was observed in colloidal substrates rather than film-based substrates. It is wellknown that the strongest SERS enhancement occurs when the (21) Chaney, S. B.; Shanmukh, S.; Dluhy, R. A.; Zhao, Y. P. Appl. Phys. Lett. 2005, 87, 031908. (22) Orendorff, C. J.; Gearheart, L.; Jana, N. R.; Murphy, C. J. Phys. Chem. Chem. Phys. 2006, 8, 165. (23) Jana, N. R.; Pal, T. AdV. Mater. 2007, 19, 1761.
In summary, we successfully applied one simple method to fabricate the linear chains of silver nanoparticles. An appropriate concentration of CTAB serves as the “glue” that can link the {100} facets of two neighboring Ag nanoparticles, which leads to an anisotropic distribution of the residual surface charges, and this extrinsic electric dipole formation is responsible for the linear organization of the Ag nanoparticles into short chains. The 11mercaptoundecanoic acid (MUA) is found to be effective in “freezing up” the aggregation of Ag short chains and preventing them from further aggregating into a long chainlike network structure. By controlling the concentration of CTAB and MUA, the aggregated number of Ag linear chains can be tuned. The self-assembled Ag monolayer mainly composed of four-particle nanochains exhibited the maximum SERS enhancement factor of around 2.6 × 108. The enhancement of localized electromagnetic field arises from the localized surface plasmon coupling at the interstitial sites of Ag nanochains and is responsible for the SERS enhancement. Importantly, the self-assembled silver nanochains exhibited strongly enhanced SERS properties, which will present new promising applications in the fields of sensors, biotechnology, and nanodevices. LA701610S (24) McFarland, A. D.; Young, M. A.; Dieringer, J. A.; Van Duyne, R. P. J. Phys. Chem. B 2005, 109, 11279. (25) (a) Svedberg, F.; Li, Z.; Xu, H.; Ka¨ll, M. Nano Lett. 2006, 6, 2639. (b) Imura, K.; Okamoto, H.; Hossain, M. K.; Kitajima, M. Nano. Lett. 2006, 6, 2173. (26) Hao, E.; Schatz, G. C. J. Chem. Phys. 2004, 120, 357. (27) Maier, S.; Kik, P. G.; Atwater, H. A.; Meltzer, S.; Harel, E.; Koel, B. E.; Requicha, A. A. G. Nat. Mater. 2003, 2, 229. (28) Zou, S.; Schatz, G. C. Chem. Phys. Lett. 2005, 403, 62. (29) Wang, D. S.; Kerker, M. Phys. ReV. B 1981, 24, 1777.
