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Langmuir 2007, 23, 4606-4611
Plasmon Coupling in Layer-by-Layer Assembled Gold Nanorod Films Ste´phanie Vial, Isabel Pastoriza-Santos, Jorge Pe´rez-Juste, and Luis M. Liz-Marza´n* Departamento de Quı´mica Fı´sica and Unidad Asociada CISC, UniVersidade de Vigo, 36310, Vigo, Spain ReceiVed December 28, 2006. In Final Form: February 6, 2007 A systematic study of the optical effects derived from plasmon coupling in mono- and multilayers of gold nanorods is presented. The monolayers were prepared using the standard polyelectrolyte-assisted layer-by-layer (LbL) method and gold nanorods coated with either poly(N-vinyl pyrrolidone) or homogeneous silica shells. Such plasmon coupling leads in general to extensive red-shift and broadening of the longitudinal plasmon bands, which are discussed on the basis of recently reported theoretical modeling. Whereas for PVP-coated rods, strong interactions were observed for high-density monolayers and closely spaced multilayers, increasingly efficient screening is observed for thicker silica shells.
Introduction Many practical applications of (coinage) metal nanoparticles are directly related to their striking optical properties, arising from localized surface plasmon resonances.1-4 Such applications can be found in fields such as nonlinear optics, optical recording, biolabeling, biosensing, or therapy and often require the immobilization of nanoparticles on a transparent substrate. For this reason, development of efficient immobilization techniques and control over all parameters influencing the optical response of deposited nanoparticles are crucial for an efficient use of these systems within practical devices. In the context of exploiting the optical response of metal nanoparticles, the main parameters are particle size and shape, dielectric environment (refractive index of medium/solvent and of coating shells), and interparticle distance.4 Control over particle shape has been dramatically improved over the past decade, with geometries including spheres,5 single-crystal or penta-twinned rods,6-10 flat platelets,11-14 nanocages,15,16 polyhedrons,17,18 or even nanostars,19,20 with better or worse success regarding simultaneous control over particle size. A number of studies have focused * Corresponding +34 986812556.
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(1) Haes, A. J.; Stuart, D. A.; Nie, S.; Van Duyne, R. P. J. Fluoresc. 2004, 14, 355. (2) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (3) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (4) Liz-Marza´n, L. M. Langmuir 2006, 22, 32. (5) Rodrı´guez-Ferna´ndez, J.; Pe´rez-Juste, J.; Garcı´a, de Abajo, F. J.; LizMarza´n, L. M. Langmuir 2006, 22, 7007. (6) Pe´rez-Juste, J.; Pastoriza-Santos, I.; Liz-Marza´n, L. M.; Mulvaney, P. Coord. Chem. ReV. 2005, 249, 1870. (7) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann., S. J. Mater. Chem. 2002, 12, 1765. (8) Chen, H.; Wang, J.; Yu, H.; Yang, H.; Xie, S.; Li, J. J. Phys. Chem. B 2005, 109, 2573. (9) Wang, Z. L.; Mohamed, M. B.; Link, S.; El-Sayed, M. A. Surf. Sci. 1999, 440, L809. (10) Sun, Y. G.; Xia, Y. AdV. Mater. 2002, 14, 833. (11) Jin, R.; Cao, C. Y.; Hao, E.; Me´traux, G.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487. (12) Germain, V.; Li, J.; Ingert, D.; Wang, Z. L.; Pileni, M. P. J. Phys. Chem. B 2003, 107, 8717. (13) Pastoriza-Santos, I.; Liz-Marza´n, L. M. Nano Lett. 2002, 2, 903. (14) Lofton, C.; Sigmund, W. AdV. Funct. Mater. 2005, 15, 1197. (15) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (16) Chen, J.; Saeki, F.; Wiley, B.; Cang, H.; Cobb, M.; Li, Z.; Au, L.; Zhang, H.; Kimmey, M.; Li, X.; Xia, Y. Nano Lett. 2005, 5, 473. (17) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. Angew. Chem., Int. Ed. 2004, 43, 3673. (18) Sa´nchez-Iglesias, A.; Pastoriza-Santos, I.; Pe´rez-Juste, J.; Garcı´a, de Abajo, F. J.; Liz-Marza´n, L. M. AdV. Mater. 2006, 18, 2329.
in turn, on the optical effects derived from changes in the dielectric environment, either through dispersion (immersion) in different solvents6 or through growth of (inorganic) shells around the particles.21-24 However, detailed studies regarding the optical changes due to interparticle interactions are much more rare and basically restricted to spherical particles. For example, Brust et al.25 have examined multilayer films made of small gold nanoparticles where interparticle distance was dictated by the length of hydrocarbon chains within dithiols used as linker molecules, whereas Ung et al.26 studied interactions at longer separation distances using silica shells with controlled thickness as rigid interparticle spacers. Apart from a general observation of red-shift and broadening of the plasmon band as the particles come closer together and consequently the interactions increase, these results also provide the idea that the range of dipolar coupling is of the order of the diameter of the spheres, which readily dictates a rule of thumb regarding the minimum separation within a device for the metallic nanospheres to maintain their singleparticle response. However, there is a lack of experimental systematic studies regarding interparticle interactions between nonspherical particles, and this is a crucial point for practical uses, since the efficiency of a device will improve by increasing the density of nanoparticles on the support (for example in a biosensor we can increase the density of recognition sites), but one has to be aware that below a certain distance the singleparticle response will no longer be available, therefore affecting the measurements. The most popular (and better characterized until now) nonspherical metal nanoparticle systems are gold nanorods,6 which display orientation-dependent optical responses, with a tight control of the plasmon resonance frequencies, mainly as a function of aspect ratio (length over thickness). The wetchemical synthesis for gold nanorods has been optimized over the last few years, allowing the dimensions to be tailored, thus offering a tool to fabricate nanoparticle-based devices operating in a wide wavelength range. Another advantage is the sensitivity (19) Yamamoto, M.; Kashiwagi, Y.; Sakata, T.; Mori, H.; Nakamoto, M. Chem. Mater. 2005, 17, 5391. (20) Nehl, C. L.; Liao, H.; Hafner, J. H. Nano Lett. 2006, 6, 683. (21) Liz-Marza´n, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (22) Liu, M.; Guyot-Sionnest, P. J. Mater. Chem. 2006, 16, 3942. (23) Pastoriza-Santos, I.; Pe´rez-Juste, J.; Liz-Marza´n, L. M. Chem. Mater. 2006, 18, 2465. (24) Pastoriza-Santos, I.; Pe´rez-Juste, J.; Carregal-Romero, S.; Herve´s, P.; Liz-Marza´n, L. M. Chem. Asian J. 2006, 1, 730. (25) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425. (26) Ung, T.; Liz-Marza´n, L. M.; Mulvaney, P. J. Phys. Chem. B 2001,105, 3441.
10.1021/la063753t CCC: $37.00 © 2007 American Chemical Society Published on Web 03/17/2007
Plasmon Coupling in Gold Nanorod Films
of the nanorod longitudinal plasmon band toward environmental changes, which renders them very attractive for sensing applications.27 Regarding interparticle coupling in gold nanorod systems, large red-shifts of the longitudinal plasmon band were reported for cases where end-to-end assembly was dominant,28-30 whereas El-Sayed and co-workers31 have recently reported that blue-shifts arise from salt-induced aggregation in solution, where side-to-side assembly is more probable. These authors carried out computational modeling based in the discrete dipole approximation (DDA),32 which explains both types of effects. Another related paper has been published by Wang, Dong, and colleagues,33 where the layer-by-layer method was applied to build multilayer films of gold nanorods, where coupling-related new bands are reported, but only briefly discussed, whereas emphasis was laid on SERS effects. We concentrate in this paper on a systematic investigation of the optical effects resulting from an increase of nanorod density on a glass support, either allowing very high densities and even particle-particle contacts, or preventing such contacts through assembly of silica-coated nanorods, so that minimum spacing is determined by the shell thickness. The corresponding optical effects are explained on the basis of the theoretical results reported in ref 31. Experimental Section Materials. Tetrachloroauric acid (HAuCl4‚3H2O), sodium borohydride, ascorbic acid, sodium chloride (NaCl), HCl, NH4OH (32%), cetyltrimethyl ammonium bromide (CTAB), poly(allylamine hydrochloride) (PAH, Mw 15 000), and tetraethylorthosilicate (TEOS) were purchased from Aldrich. Poly(vinylpyrrolidone) (PVP, MW 10 000, 40 000, and 360 000) was supplied by Fluka. Poly(styrenesulfonate) (PSS, MW 70 000) and poly(diallyldimethylammonium chloride) (PDDA, MW 200 000-350 000, 20 wt %) were procured from Sigma. Poly(styrenesulphonate) (PSS, Mw 14 900) was procured from Polysciences. All chemicals were used as received. Pure grade ethanol and Milli-Q grade water were used to make up all solutions. Synthesis of Gold Nanorods. Gold nanorods were synthesized according to the protocols described by Nikoobakht et al.34 and Liu et al.35 First, a gold seed solution was prepared by borohydride reduction of 0.25 mM HAuCl4 in an aqueous 0.1 M CTAB solution. Subsequently, 24 µL of the seed solution was added to a growth solution (10 mL) containing 0.1 M CTAB, 0.5 mM HAuCl4, 0.019 M HCl, 0.8 mM ascorbic acid, and silver nitrate (0.08 mM). Prior to LbL assembly, the metal particles were coated with poly(vinylpyrrolidone) (PVP, Mw 40 000) by first centrifuging (8000 rpm, 20 min) and redispersing in Milli-Q water, followed by dropwise addition of a PVP aqueous solution, previously sonicated during 15 min ([Au]/[PVP] ratio ) 1), under vigorous stirring. The mixture was gently stirred overnight and then centrifuged (3500 rpm, 3h) and redispersed in ethanol under sonication. Silica Coating. Deposition of uniform silica shells on gold nanorods was accomplished by use of a recently reported method,23 based on a combination of polyelectrolyte wrapping and hydrolysis of tetraethoxysilane in an isopropanol-water mixture. In brief, after removing excess surfactant by centrifugation (8000 rpm, 20 min), (27) Wang, C.; Ma, Z.; Wang, T.; Su, Z. AdV. Funct. Mater. 2006, 16, 1673. (28) Thomas, K. G.; Barazzouk, S.; Ipe, B. I.; Joseph, S. T. S.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 13066. (29) Joseph, S. T. S.; Ipe, B. I.; Pramod, P.; Thomas, K. G. J. Phys. Chem. B 2006, 110, 150. (30) Correa-Duarte, M. A.; Pe´rez-Juste, J.; Sa´nchez-Iglesias, A.; Liz-Marza´n, L. M. Angew. Chem., Int. Ed. 2005, 44, 4375. (31) Jain, P. K.; Eustis, S.; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 18243. (32) Draine, B. T.; Goodman, J. Astrophys. J. 1993, 405, 685. (33) Hu, X.; Cheng, W.; Wang, T.; Wang, Y.; Wang, E.; Dong, S. J. Phys. Chem. B 2005, 109, 19385. (34) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (35) Liu, M.; Guyot-Sionnest, P. J. Phys. Chem. B 2004, 108, 5882.
Langmuir, Vol. 23, No. 8, 2007 4607 the particles were dispersed in Milli-Q water and added dropwise under vigorous stirring to an aqueous solution of polystyrenesulphonate (PSS, Mw 14 900; 2 g/L, 6 mM NaCl). PSS adsorption was allowed to proceed for 3 h. After a washing step to remove excess PSS, the process was repeated with poly(allylamine hydrochloride) (PAH). PSS/PAH coated gold nanorods were then mixed with PVP (Mw 10 000, 4 g/L) in water and stirred overnight. The mixture was centrifuged at 2800 rpm for 3 h and the precipitate redispersed in isopropanol (1.5 mL) in an ultrasonic bath. Aqueous ammonia, and TEOS in isopropanol were then added dropwise under vigorous stirring, followed by gentle stirring for 2 h. The shell thickness was mainly determined through the nanorod and TEOS concentrations. As an example, for a thickness of 12 nm, the final concentrations were 0.85 mM Au, 10.55 M water, 0.2 M ammonia, and 5.57 mM TEOS. LbL Assembly. Microscope glass slides were used as substrates, so as to allow UV-vis-NIR spectroscopy measurements. The substrates were sonicated for 10 min in water and then in ethanol, subsequently thoroughly cleaned using piranha solution (H2SO4: H2O2)7:3), rinsed with deionized water, and dried under an air stream. The slides were stored in water until use. For the assembly of nanorods, the slides were first immersed in an aqueous solution of positively charged PDDA (1 mg/mL in 0.5 M NaCl aqueous solution) for 20 min, then in an aqueous solution of the polyanion PSS (Mw 70 000, 1 mg/mL in 0.5 M NaCl aqueous solution) for 10 min, and finally in PDDA solution for 10 min. At this stage, the slides are positively charged, favoring the electrostatic interactions with negatively charged nanorods. To obtain a monolayer of gold nanorods, the pretreated slides were immersed into the corresponding colloid for a suitable period time (see below). Multilayer films were prepared by sequential adsorption of PDDA (20 min) and Au nanorods (1 h), denoted (PDDA/Au)n for n layers. After each adsorption step, the substrates were rinsed with water and dried under an air stream.
Results and Discussion The strategy used for the assembly of gold nanorods was the well-known polyelectrolyte-assisted layer-by-layer assembly.36 This technique, mainly based on electrostatic interactions between polyelectrolytes and nanoparticles, has been often applied for the assembly of metal (among many others) nanoparticles,26,33,37 and renders random, uniform deposition derived from a strong interaction and fast deposition. A modification of this technique, where the polyelectrolytes were assembled on high aspect ratio nanorods, rather than on the substrate was reported as well.38 Unless indicated, the results reported here were obtained using Au nanorods with average length of 62 nm and average aspect ratio of 3.3. Shown in Figure 1 are a representative TEM image and the corresponding UV-vis-NIR spectra, for the CTABstabilized nanorods in water and after exchange of CTAB for PVP, both in water and ethanol. Exchange of CTAB for PVP was required to generate a uniform negative charge on the nanorods’ surface23 and thereby facilitate LbL assembly. However, since the long-term stability of PVP capped nanorods in water was compromised, they were transferred into ethanol. From the spectra in Figure 1, it is clear that the exchange of capping agent does not lead to aggregation, since basically no changes can be seen when the particles are dispersed in water, while a small red-shift of about 8 nm is observed upon transfer into ethanol due to the change in solvent refractive index from 1.333 up to 1.359).6 1. Au-PVP Monolayers. Monolayers of gold nanorods were hence formed by adsorption of the negatively charged particles generated upon PVP capping, on a glass substrate covered by (36) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (37) Gao, M. Y.; Richter, B.; Kirstein, S.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102, 4096. (38) Gole, A.; Murphy, C. J. Chem. Mater. 2005, 17, 1325.
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Figure 1. Representative TEM image and UV-vis-NIR spectra of gold nanorods (aspect ratio 3.3), stabilized with CTAB in water (green), and with PVP in both water (red) and ethanol (blue).
Figure 2. UV-vis spectra of a monolayer of PVP-coated gold nanorods on a PDDA-modified glass substrate, as a function of deposition time.
the cationic polyelectrolyte PDDA, and UV-vis spectra were recorded as a function of deposition time, as summarized in Figure 2. Since the nanorod concentration in the ethanolic dispersion was found to strongly influence the deposition rate, the concentration of PVP coated gold nanorods was maintained constant for all experiments ([Au] ) 0.5 mM). At the early assembly stages, the UV-vis spectra invariably show two surface plasmon bands centered at 513 and 715 nm, as compared to 514 and 771 nm for the same rods in water. The blue-shift of the longitudinal surface plasmon band is due to the change in the average refractive index of the environment after deposition on
Vial et al.
the glass slide and drying (from 1.333 down to almost 1, for the rods in air but in contact with the glass support).6 When the deposition time was prolonged from 5 min up to 1 h, both plasmon bands remained basically at the same position, though with a significant absorbance increase, indicating that a larger number of particles were deposited on the substrate, but still well separated from each other, so that plasmon coupling was still negligible. However, when the deposition time reached 2 h, broadening and red-shift were observed, along with a further absorbance increase of the longitudinal plasmon band, and these effects became more and more evident for longer deposition times (up to 14 h). According to what has been reported for Au nanoparticles with other shapes,26,39 as well as calculations for Au nanorod assemblies,31 these effects point to increasingly stronger coupling between adjacent particles as the average interparticle distance decreases, which was confirmed by scanning electronic microscopy (see Figure 3). From the SEM images, we can see that after 1h of assembly time the gold nanorods are randomly distributed onto the substrate and consistently well separated from each other, which results in the absence of interparticle interactions, whereas for longer times, as more nanorods are deposited (always filling up empty surface, rather than on previously deposited nanorods, because of the charge difference), the distance between rods gets smaller and the number of contact points gradually increases, which are indistinctly head-to-tail, side-to-side, or tip-to-side, leading to increased broadening of the absorbance bands. Image analysis reveals that for deposition times of 1, 3.5, and 14 h, the surface coverage amounts to 6%, 13%, and 25%, respectively, revealing that filling up a surface monolayer with randomly oriented nanorods is very difficult, since additional rods hardly fit in empty spaces and repulsion from like-charged rods screens attraction to the oppositely charged substrate. Interpretation of the optical effects can be made in the following terms. If we consider a single monolayer of particles, the interactions between neighboring nanorods can be classified into head-to-tail, end-to-side, and side-to-side. Head-to-tail interactions arise from coupling between longitudinal plasmons, producing a red-shift of the longitudinal plasmon resonance, as has been shown from both theory and experiments,28-31 with a longer shift as interparticle distance decreases. On the other hand, sideto-side interactions are attributed to the mutual coupling between transverse and longitudinal plasmon resonances of the interacting nanorods, thus affecting both resonances. Focusing on the effect over the longitudinal plasmon band, the coupling leads in this case to a blue-shifted resonance. Additionally, in both cases, an increase in the number of assembled particles (dimers, trimers, tetramers, etc.) enhances the magnitude of the respective shifts. Finally, end-to-side interactions produce the coupling of the longitudinal plasmon band of one nanorod with the transverse plasmon of its neighbor, producing a red-shift of the longitudinal plasmon resonance, as demonstrated by Jain et al.31 from DDA simulations. From the relative orientation of neighboring rods in the SEM images (Figure 3), one can see that end-to-side should be the dominating interactions. In our experimental results, we indeed see from Figures 2 and 3 that, as the density of particles within a monolayer increases, a progressive broadening and red-shift in the position in the longitudinal plasmon band occurs (from 712 nm up to 838 nm). For clarification, we have normalized the absorption spectra at the maximum of the longitudinal resonance (see Figure S1 in the Supporting Information), which clearly reveals that the broadening (39) Malikova, N.; Pastoriza-Santos, I.; Schierhorn, M.; Kotov, N. A.; LizMarza´n, L. M. Langmuir 2002, 18, 3694.
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Figure 3. Representative SEM images of LbL assembled nanorod monolayers on PDDA-covered glass slides, for selected deposition times, as indicated in the labels. Three different magnifications are shown for each time, defined by scale bars on the top images, showing homogeneous deposition and an increase of surface coverage for longer deposition times.
Figure 4. UV-vis spectra of LBL-assembled Au nanorod monolayers (number of layers as labeled), separated by single PDDA layers.
Figure 5. Normalized UV-vis-NIR spectra of one monolayer after 1 h deposition time (black) and four Au nanorod monolayers separated by: single PDDA monolayers (red), PDDA/PSS bilayers (blue), and PDDA/PSS/PDDA/PSS multilayers (green).
stems almost exclusively from red-shifting effects, indicating that side-to-side interactions are hardly noticeable. This can be due to a negligible contribution compared to other interactions (as suggested from the relative intensities obtained in the calculations reported in ref 31) or to the shorter interparticle distances required for the onset of side-to-side interaction. 2. Au@PVP Multilayer Films. The build-up of multilayer structures, even if they are not densely packed, is more likely to involve interparticle interactions than in the case of single
monolayers, since contact between particles located in successive layers is highly probable.26,33,39 In the present work, Au nanorod multilayer films were built up using the LbL approach, through alternate deposition of oppositely charged species (PDDA and PVP-capped nanoparticles) onto glass substrates. To separately account for interlayer interactions, rather than interparticle interactions within single layers, each nanorod monolayer was prepared allowing deposition to proceed for 1 h, which was previously found to be the limit to avoid plasmon coupling within
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Figure 6. TEM images of LBL assembled multilayer Au nanorod films with four nanorod layers separated by: a single PDDA layer (a); PDDA/PSS bilayers (b), and PDDA/PSS/PDDA/PSS multilayers (c).
a single monolayer, using 0.5 mM gold nanorod dispersions in ethanol. This was combined with PDDA deposition (for 20 min) to build up to four (PDDA/Au) bilayers, and the optical response was investigated by UV-vis-NIR spectroscopy (Figure 4). Although in principle the effects appear to be similar to those described for denser nanorod monolayers (red-shift and broadening, along with absorbance increase), closer inspection reveals that the absorbance band also becomes less symmetric after deposition of the second layer, in such a way that a new band seems to develop at higher wavelengths, while the original band remains basically at the same position, but with a clear overlap between them. The original longitudinal plasmon band still remains because many particles within the different monolayers do not suffer from any coupling, since in each monolayer the particles are well separated, thus preserving their single-particle optical response. This effect is in accordance with the trend reported by Hu et al. for a similar system33 and was more clearly observed when nanorods with higher aspect ratio were assembled (see Figure S2 in the Supporting Information). If the density of particles in each monolayer were higher, the initial longitudinal band should disappear, as observed for the single monolayer experiment at longer deposition times (Figure 2). In this case, as we have also seen for Au rod monolayers, the optical response of the films seems to originate mainly from head-to-tail and head-to-side interactions, but now between neighboring monolayers. The development of new bands from plasmon coupling has been reported for Au nanoparticle multilayers with different geometries, including nanoplates, in which case the assignment to interlayer interactions was based on experiments where coupling was screened by separating monolayers with thicker spacing.39 We carried out similar experiments to gradually screen the interactions between adjacent gold nanorod layers, just by increasing the thickness of the polymeric spacing. Repeated depositions of PDDA and PSS bilayers allowed a gradual increase of the separation distance between successive gold nanorod layers, since the thickness of a PDDA/PSS bilayer has been reported to be 2-3 nm.40-42 The results are summarized in Figure 5, where the spectrum of one monolayer after 1 h of deposition time and the final spectra after deposition of four nanorod monolayers are compared, for those cases where PDDA/PSS bilayers and PDDA/PSS/PDDA/PSS multilayers were used as spacers between successive nanorod monolayers. For the sake of an easier comparison, the maximum absorbance was normalized to 1, so that the broadening and position of the maximum can be compared for the different samples. It is clear from this plot that if the separating polymeric region is increased, the (40) Lvov, Y.; Decher, G.; Mohwald, H. Langmuir 1993, 9, 481. (41) Decher, G.; Schmitt, J. Prog. Colloid Polym. Sci. 1992, 89, 160. (42) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3422.
absorbance becomes noticeably less broadened, due to further screening of interparticle interactions, in a similar way to what has been previously reported for gold nanoplates.39 The increase in monolayer separation distance seems to screen mainly the weaker interparticle coupling, arising from interactions between particles separated by longer distances. Therefore, only the more intense coupling remains, which also leads to a red-shift in the longitudinal plasmon band, as compared to the spectrum of a single monolayer, but much narrower than that for closely spaced multilayers. The morphology of the multilayer films was characterized using SEM, as shown in Figure 6. The images reveal that, with a single separating polyelectrolyte layer, it is difficult to distinguish the successive gold nanorod layers, thus confirming that the particles are indeed very close to each other. On the contrary, when 3 or 5 polyelectrolyte monolayers are deposited between each subsequent gold rod layers, the contrast between them gradually gets more blurred, revealing the increased separation between neighboring layers, which contributes to screening of interparticle interactions. 3. Au@ SiO2 Multilayer Films. From the results presented in the previous section, we learn that increasing physical separation between gold nanorods can efficiently screen plasmon coupling and thus retain the optical response of single nanorods within multilayer films. However, even when successive layers are separated by five polyelectrolyte monolayers, some residual broadening is observed (Figure 5), which suggests that an alternative procedure is required for an efficient screening. Based on our own experience with silica coating of metal nanoparticles,21,23 and previous studies related to the study of plasmon coupling in gold nanoparticle multilayer films,26 we decided to carry out the construction of multilayer films through assembly of gold nanorods coated with silica shells of various thickness. The coating was performed through a recently reported procedure,23 which allows a fine control of shell thickness, and LbL assembly was used in the same way as for PVP-capped rods. The colloidal stability of gold nanorods (no aggregation) during the silica coating was monitored through UV-vis-NIR spectroscopy and TEM (examples are shown in the Supporting Information, Figure S3). As expected, growth of increasingly thick silica shells was observed to promote a gradual red-shift of the longitudinal surface plasmon band, due to the corresponding increase in local refractive index around the particles, while the global shape of the spectrum remains almost completely unaltered, except for a relative absorbance increase at lower wavelengths due to the contribution of Rayleigh scattering from the silica shell.21,23 For the sake of consistency, these experiments were also carried out with gold nanorods of aspect ratio 3.3, so that a proper comparison with the previous section can be made. Particles were used with silica shell thicknesses of 4, 9, and 12 nm, so as to evaluate plasmon coupling as a function of minimum
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more red-shifted for monolayers of nanorods coated with thicker shells, due to the above-discussed local refractive index effect, which is more obvious in the dry films than in solution, because of the larger refractive index difference between silica and air than between silica and water or ethanol. Additionally, although in all films a red-shift is observed when the number of deposited layers is increased, the slope is clearly smaller for thicker shells, with only 4 nm shift for 12 nm thick silica shells, indicating that interparticle interactions are almost fully suppressed for distances of the order of the nanorod thickness.
Conclusions
Figure 7. (a) SEM image of a (PDDA/ Au@SiO2)4 nanorod multilayer (silica shell thickness ) 9 nm). (b) Effect of silica shell thickness (as indicated in the legend) on the evolution of longitudinal surface plasmon band position during the construction of a multilayer film with four nanoparticle layers.
interparticle distance (defined by twice the shell thickness). Although the obtained multilayer structure was in all cases far from compact (see a representative SEM image in Figure 7a), the UV-vis-NIR spectra measured for the films confirm a gradual screening of the coupling for increasing shell thickness, as summarized in Figure 7b, where the maximum positions of the respective longitudinal surface plasmon bands are plotted for the first four monolayers of each sample. In this plot, we can see that for the first (noncompact) monolayer, the plasmon band is
The influence of average interparticle distance on the optical response of gold nanorods was studied through the systematic construction (via LbL assembly) and characterization of monolayers and multilayers on transparent glass substrates. The control of nanorod density in monolayers was achieved by varying deposition time, and interlayer separation in multilayer films was varied through the assembly of an increasing number of polyelectrolyte monolayers. Finally, well-defined interparticle distances were imparted through homogeneous coating of the nanorods with silica shells. In all cases, plasmon coupling (leading to extensive red-shift and broadening of the longitudinal surface plasmon band) could be efficiently screened by keeping the particles separated at distances longer than the nanorod short axis. These results, which are in agreement with previous results for gold nanoparticles of different shapes, can be explained through recently reported theoretical modeling, and should be taken into account when designing devices based on gold nanorod assemblies, if the single particle optical response is to be maintained. Acknowledgment. This work has been supported by the Spanish Ministerio de Educacio´n y Ciencia, through Grant Nos. MAT2004-02991 and NAN2004-09133-C03-03. The authors would like to thank Jacinto Pe´rez-Borrajo for assistance during SEM measurements. Supporting Information Available: Additional resources including Figures S1-S4. This material is available free of charge via the Internet at http://pubs.acs.org. LA063753T
