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Antibonding Plasmon Modes in Colloidal Gold Nanorod Clusters Marek Grzelczak,*,† Stefano A. Mezzasalma,† Weihai Ni,‡ Yury Herasimenka,§ Luigi Feruglio,† Tiziano Montini,†,§ Jorge Perez-Juste,‡ Paolo Fornasiero,†,§ Maurizio Prato,† and Luis M. Liz-Marzan*,‡ †
Department of Chemical and Pharmaceutical Sciences, University of Trieste, P.le Europa 1, 34127 Trieste Italy Departamento de Química Física, Universidade de Vigo, 36310 Vigo, Spain § Life Science Department, University of Trieste, V. Giorgieri 1, 34127 Trieste, Italy ‡
bS Supporting Information ABSTRACT:
The optical response of nanoplasmonic colloids in disperse phase is strictly related to their shape. However, upon self-assembly, new optical features, for example, bonding or antibonding modes, emerge as a result of the mutual orientations of nanoparticles. The geometry of the final assemblies often determines which mode is dominating in the overall optical response. These new plasmon modes, however, are mostly observed in silico, as self-assembly in the liquid phase leads to cluster formation with a broad range of particle units. Here we show that low-symmetry clustering of gold nanorods (AuNRs) in solution can also reveal antibonding modes. We found that UV light irradiation of colloidal dispersions of AuNRs in N-methyl-2-pyrrolidone (NMP), stabilized by poly(vinylpyrrolidone) (PVP) results in the creation of AuNRs clusters with ladderlike morphology, where antibonding modes can be identified. We propose that UV irradiation induces formation of radicals in solvent molecules, which then promote cross-linking of PVP chains on the surface of adjacent particles. This picture opens up a number of relevant questions in nanoscience and is expected to find application in light induced self-assembly of particles with various compositions and morphologies.
’ INTRODUCTION Nanoparticle self-assembly has become an exciting field of nanoscience for a number of reasons, including its relationship with crystallization processes from colloidal dispersions into either confined geometries1,2 or extended crystals.3 Particularly attractive is the clustering of plasmonic nanoparticles, which possess different optical features to those of their corresponding building blocks.4 Such differences find an intriguing explanation in terms of plasmon hybridization,5,6 which can also apply to anisotropic nanoparticle building blocks like gold nanorods (AuNRs), and to the larger number of conformational states they are able to produce as compared to spherical nanoparticles.7,8 In a simple example, when AuNRs self-assemble in tip-to-tip or side-to-side orientations, significant red- or blue-shifts in the longitudinal localized surface plasmon resonance (LSPR) band are registered, respectively.9 16 So far, plasmon hybridization concepts have been mostly exploited to describe (the simplest possible) pairwise conformations, but in order to meet more complex geometrical or topological structures with larger unit numbers, they will surely require ad-hoc generalizations. In addition, the concept of plasmon hybridization was built in analogy to the notion of molecular orbitals (MO), lending itself r 2011 American Chemical Society
to a further search for similarities and cross-fertilizations either in polymer theory, where scaling17 and statistical mechanics18 approaches can solve a wealth of issues, or in supramolecular chemistry (e.g., π π interactions).19 A crucial requisite to obtain “plasmonic molecules” is a certain degree of control over the self-assembly of plasmonic nanoparticles as building blocks,20 generally requiring a nontrivial interplay of attractive and repulsive forces. Among the usually accessible stimuli, responsible for establishing nanoparticle attraction forces, light has been proven to cause self-assembly through a couple of generally recognized mechanisms. Light can either interact with photosensitive molecules (located at the colloid surface or solid liquid interface),21 25 or with the dispersant (the bulk medium in which colloids diffuse by Brownian motion).26 28 The latter case, requiring light-sensitive solvents, has been poorly investigated so far. We show here that UV light irradiation of N-methyl pyrrolidone Special Issue: Colloidal Nanoplasmonics Received: September 24, 2011 Revised: October 31, 2011 Published: November 01, 2011 8826
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Langmuir solutions containing AuNRs stabilized with polyvinylpyrrolidone (PVP) induces nanoparticle self-assembly into clusters with ladderlike morphology. Such an unusual clustering is reflected in strong plasmon coupling with detectable antibonding modes. Elucidating the interparticle interaction mechanism behind such unusual assembly morphology is out of the scope of this paper, and will be further approached by a forthcoming study of theoretical nature. We have thus concentrated on providing evidence for the occurrence of this photoinduced phenomenon and discussing the optical effects derived from the assembly and plasmon hybridization, with an emphasis on the identification of antibonding modes.
’ EXPERIMENTAL SECTION Synthesis of Gold Nanorods. AuNRs were prepared using the well-known seeded-growth method.29 A seed solution of gold nanoparticles was prepared by borohydride (30 mM, 0.3 mL) reduction of HAuCl4 (0.25 mM, 5 mL) in aqueous cetyltrimethylammonium bromide (CTAB) solution (0.1 M). An aliquot of seed solution (24 μL) was added to a growth solution (10 mL) containing CTAB (0.1 M), HAuCl4 (0.5 mM), ascorbic acid (0.8 mM), AgNO3 (0.12 mM), and HCl (18.6 mM). Mixtures were left undisturbed for 4 h at 27 °C. Surface Functionalization of AuNRs with Polyvinylpyrrolidone. Surface functionalization of AuNRs with PVP was achieved by slight modifications of the method described by Carbo-Argibay et al.30 Typically, a suspension of as-prepared AuNRs (10 mL) was centrifuged at 8000 rpm for 15 min, and the precipitate was redispersed in Milli-Q water (5 mL). Subsequently, 5 mL of CTAB-coated gold nanorods was mixed with an aqueous PVP solution (1.2 mM, 5 mL) and stirred overnight at 25 30 °C. The mixture was then centrifuged at 4500 rpm for 60 min, followed by removal of the supernatant. To remove the rest of the solvent (approximately 50 μL), remaining in contact with the precipitated particles, centrifuge tubes were placed in a water bath at 50 °C for 30 min under argon/nitrogen flow. Care should be taken to avoid complete evaporation since dryness causes particle aggregation. The precipitate was subsequently redispersed in fresh, anhydrous N-methylpyrrolidone (NMP) or in an NMP solution containing the desired PVP concentration (see the Supporting Information), under sonication (5 10 s), until the particle concentration reached the value of 0.125 mM, as determined by UV vis spectroscopy (A400 nm = 0.3).31 Contact with water was prevented by storing the solutions up to 3 4 days in the dark, under vacuum. Self-Assembly. The fresh AuNR solution (3.5 mL) was transferred into a quartz cuvette (path length 10 mm) and placed under a highpressure UVA Hg lamp (Helios, Italquarzt s.r.l.). The lamp emission spectrum features a main peak at 360 nm and a weaker one at 254 nm, with a set of secondary peaks in the range of 250 550 nm. The lamp was then placed at a distance of 40 cm above the reactor, so as to emit 95 W/m2 of radiant flux at a temperature between 35 and 40 °C. After the irradiation period (30 90 min), the cuvette was left undisturbed for aging (25 300 h), during which time the self-assembly process took place. For transmission electron microscopy (TEM) characterization, a solution of assembled AuNRs (50 μL) was drop-casted on a carboncoated nickel grid supported on filter paper. Clustering of Spherical Nanoparticles. Spherical gold nanoparticles (AuNPs) with diameter of 55 ( 5 nm were prepared according to the method reported by Rodríguez-Fernandez et al.31 Surface functionalization was performed according to the procedure described above (AuNR@PVP). To obtain spherical particle clusters, Au@PVP solutions (3.5 mL) in NMP were placed in a quartz cuvette and irradiated with UV-light (90 min). The solution was then left undisturbed for 25 h. Instrumentation. Optical characterization was carried out by UV/ vis/NIR spectroscopy with a Varian Cary 5000 spectrophotometer using 10 mm path length quartz cuvettes. TEM measurements were performed in
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a TEM Philips EM208 instrument operating at an accelerating voltage of 100 kV. Raman spectra were recorded using a Renishaw Invia microspectrometer, equipped with a He Ne NIR laser line (785 nm) and a 50 objective. Raman measurements were carried out with a laser power of 18 mW at the sample, and a collection time of 20 s. FDTD Simulations. The plasmon coupling in Au nanorod assemblies was theoretically modeled using the finite-difference time-domain (FDTD) method. The dielectric function of Au used in the calculation was described by the free electron Drude model with parameters set to match the standard experimental values.32 The refractive index of the surrounding medium, water, was taken to be 1.33. Based on the TEM measurements, each nanorod was modeled as a cylinder with semispherical caps at both ends. The diameter and length of the nanorods were set at 13 and 61 nm, respectively. Besides the single nanorod, assemblies formed by two to five nanorods were investigated. For the nanorod assemblies, the gap distance between neighboring nanorods in the assembly was set at 1.5 nm. Neighboring nanorods were offset to each other by 31 nm in the assemblies, in agreement with the TEM measurements. The incident light was set perpendicular to the plane of the nanorod assembly with its polarization direction along the long axis of the nanorod. The mesh size used in the calculation was taken to be 0.5 nm. The E-field enhancement profiles at surface plasmon resonance wavelengths were plotted in a logarithmic scale.
’ RESULTS AND DISCUSSION In our experiments, AuNRs were dispersed in N-methyl-2pyrrolidone (NMP) solutions (Figure 1), using poly(vinylpyrrolidone) (PVP) as a stabilizer, which creates a polymer layer at the metal liquid interface.33 The total amount of PVP partitions between full coverage of the AuNR surface and a fraction that remains in NMP solution, giving rise to a polymer solution where polymer molecules fluctuate and tend to swell (NMP behaves, in the present temperature domain, as a good solvent for PVP). Whether equilibrium of the adsorbed polymer interface with the disperse system is achieved or not will depend on the observation time scale. Normally, equilibration times of macromolecules are much longer than typical collision times in colloids. However, the stabilization process was carried out by saturating the Au surfaces with PVP chains, setting before irradiation a constrained equilibrium state leading to nanoparticle repulsion.34 For an adlayer thickness within ∼(1 5) nm, an estimate of the polymer molar concentration near the surface yields ∼(10 2 10 1) aM for any of the employed amounts of PVP in solution. More precisely, the homopolymer adlayers consist of a dense region attached to the surface, rich in monomer sequences and in distributions of molecular loops and tails, with the former stemming from the surface and returning to it and the latter dangling loosely in the bulk phase.35 A first set of self-assembly experiments were conducted with NMP solutions containing AuNR@PVP, which gradually aggregated upon irradiation by UV-light, as reflected in the registered dramatic optical changes (Figure 1a), which are indicative of plasmon coupling.12 During the first hours of assembly, the longitudinal LSPR initially damped and broadened, suggesting the presence of a mixture of AuNR monomers and clusters with a small number of units. The LSPR subsequently red-shifted by 450 nm and its intensity increased, while the initially symmetric LSPR bands gradually became asymmetric, with a pronounced shoulder at lower wavelengths. TEM analysis of the colloids showed that only AuNR clusters (no free nanorods) were present at the final stages of the assembly process, which means that the new band (shoulder) can safely be ascribed to an intrinsic optical feature of the developing 8827
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Figure 1. UV-light induced AuNR assembly. (a) UV vis-NIR spectra showing the changes in the longitudinal LSPR band over time (the chemical structures of PVP and NMP are given in the inset). (b, c) Histograms for the interparticle gap distance and pairwise offset (in % of the longitudinal nanoparticle length). (d, e) TEM images demonstrating the formation of assemblies at different elapsed times: (d) 5 h, corresponding to spectrum 1 in (a); (e) 25 h, corresponding to spectrum 2.
structures. During the self-assembly process, the solutions were left undisturbed,36 observing a gradual red-to-purple color change, and precipitation within two weeks. Cluster conformations were characterized by TEM (Figure 1d,e) at various times, showing that they comprised few units at early times, mainly including monomers, dimers, and trimers (Figure 1d), whereas within ∼25 h they branched into more extended networks (Figure 1e and Figure S1a, Supporting Information). Interestingly, the geometrical distributions were consistently dominated by side-to-side contact conformations, with varying values of the longitudinal pairwise offset. By analysis of >50 clusters, we can estimate the average value of both the offset and interparticle separation, obtaining values of ∼50% and ∼1.5 nm, respectively (Figure 1b,c; see the Supporting Information for detailed description of offset determination). The above-described measurements and observations turned out to be reproducible within at least 10 independent experiments. It is also worth mentioning, that the low-symmetry nanoparticle clustering here presented is a promising route toward the design of templatefree chiral plasmonic molecules and macromolecules.37 40 We have additionally found that the assembly protocol presented here can be easily extended to the formation of spherical NP chains (a representative image is shown in Figure S2, Supporting Information), but we shall not discuss this further in the present manuscript. We briefly discuss here on the mechanisms potentially contributing to the formation of ladderlike architectures. In the absence of irradiation, no changes in the aggregation or dispersion state were detected; therefore, the corresponding driving forces are
obviously to be sought among any light soft matter interaction taking place in this ternary system. The more promising physical models for a well-posed quantitative description seem to be provided by polymer interfaces, which are modified upon irradiation, as well as interactions of light with each solid or liquid phase. Regarding interfacial contributions, the kinetics of in situ formation of radicals from the solvent, which can initiate photo-cross-linking reactions on the stabilizer chains, is expected to play an important role. This is justified by the high sensitivity of NMP toward UV-light, producing radicals,41 as well as the ability of PVP to cross-link in the presence of radicals.42 Specifically, NMP decomposition leads to loss of CO and gain of reactive diradicals, returning pyrrole (among other compounds) upon a dark reaction series.41,43 We examined NMP alone before and after light irradiation, by means of NMR and gas chromatography coupled to mass spectrometry (GC-MS). The main product from exposure to UV was N-methyl pyrrole (Figures S3 and S4, Supporting Information), suggesting a variation of solvent chemistry and production of radicals as intermediate steps in this process. It has been shown on the other hand that •OH groups from UV-irradiated water can react with PVP molecules and yield macroradicals, leading to intermolecular cross-linking of polymer chains.44,45 On this basis, we suggest that NMP radicals induce formation of PVP macroradicals, which then cross-link with other PVP molecules on the AuNR surface or in proximity to AuNR-PVP interfaces, thereby increasing the effective AuNR attraction. To further support the above interpretation, we studied the influence of radicals on PVP cross-linking. Exposing PVP films to 8828
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Langmuir radicals arising from either UV light irradiation or Fenton reagents (see the Supporting Information for details) was found to promote formation of insoluble polymeric species, likely due to cross-linking of PVP, which were characterized by Raman spectroscopy (see Figure 2). It has been shown that PVP molecules may cross-link upon (i) monomer bridging by C O C groups and (ii) partial pyrrolidone ring transformation into succinimide side chains.42 The latter display an additional symmetric stretching mode from two CdO groups and are thus easily detectable by Raman analysis (Figure 2). In fact, the corresponding peak
Figure 2. Raman spectra of a PVP film before and after cross-linking, induced by either UV-light or Fenton reagents. The symmetric mode of the succinimide byproduct can be observed in both cases.
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appeared at 1768 cm 1 in the spectra acquired from films treated with radicals (Figure 2). Because less than 10% of the initial PVP is oxidized to succinimide, whereas most of it undergoes crosslinking reactions,42 peak intensities were relatively low. The rise of this peak supports anyway the hypothesis that the environment around AuNRs promotes cross-linking between stabilizer molecules.46 Assuming that the •OH radicals are responsible for particles clustering, the presence of extra PVP molecules in solution should drastically affect the kinetics of nanoparticle assembly. UV vis spectroscopy revealed that when the amount of dissolved PVP was increased, the self-assembly kinetics was slowed down (Figure 3a). Thus, we monitored the time evolution of the absorbance ratio at two relevant wavelengths, Af(t,λf)/Ai(t,λi), for various free PVP concentrations, at fixed irradiation time (90 min) and AuNR concentration. The two wavelengths were chosen according to the initial and final LSPR positions, λi = 880 nm and λf = 1350 nm, so that the time-derivative of their intensity ratio would yield a (relative) measure of self-assembly kinetic rate. Whereas a pronounced LSPR red-shift (∼ 450 nm) was observed with the lower PVP concentration (inset in Figure 3a), it was gradually inhibited by increasing PVP amounts and fully quenched at the largest value (1 mM). TEM analysis was also consistent with this observation (see Figure S6, Supporting Information). These results indicate that the free PVP molecules can effectively scavenge •OH radicals preventing cross-linking of the stabilizing agent, which in turn quench clustering of the gold nanorods. The effect of increased viscosity which might lower the collision frequency between AuNRs is also taken into account and disscused in detail in the Supporting Information (section S7). Another important factor that confirms a mechanism based on radical induced self-assembly is the UV irradiation time of the NMP solution. We expect that the amount of photoinduced radicals should increase with increasing irradiation time. When irradiation time of UV-light was increased (∼30, 60, 75, 90 min; see
Figure 3. Effect of PVP concentration (a) and UV exposure time (b) on the NR assembly rate. (a) Decreasing PVP concentration in the polymer solution and (b) gradually prolonging the irradiation time (from 30 to 90 min) progressively speeds the self-assembly rate up. In (b), the final spectra are quite reproducible regardless of the exposure time (inset). 8829
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Figure 4. FDTD simulation of ladderlike AuNRs clusters. (a) Extinction spectra for the different clusters, showing that, upon increase in the number of AuNR units, the longitudinal LSPR band red-shifts and splits into high and low energy bands. (b) Magnified view of the high-energy bands. (c) Plot of LSPRmax versus number of units in the cluster. Low energy band (bonding) progressively red-shifts while high-energy band (antibonding) remains unchanged. (d h) Electric near field enhancement maps for clusters of increasing number of AuNR units, at their corresponding LSPR maximum wavelength. The direction and polarization of incident light are indicated in (d).
the correspondingly increasing average slopes in Figure 3b and Supporting Information Figure S8), the self-assembly kinetics was observed to proceed faster (lowest PVP concentration in solution was used, so as to reduce the quenching effect due to stabilizer excess). This is again consistent with identifying radical production as one of the driving forces for this process. Note that the final UV vis spectra were almost coincident in all cases (Figure 3b, inset), revealing a similar extent of aggregation, even for different irradiation times, with the only difference that the equilibrium state was reached at longer times for shorter irradiation times. Such observations also indicate a competition/interplay between the amount of PVP in solution and aging time. A yet unresolved question is why AuNRs form ladderlike assemblies. According to the mechanism presented above, one could expect “perfect” sideto-side assembly, as the particles would tend to maximize the surface contact. We propose that the longitudinal offset is the result of equilibrium between solvent particle and particle particle interactions. It is reasonable to assume that the high solvating ability of NMP toward polymeric PVP can compete with the slowly forming covalent cross-links between PVP molecules. However, we are not yet in a position to distinguish between primary-like (coagulation) or secondary-like (flocculation) association mechanisms, though a certain analogy with polymer flocculation may occur.47 Albeit not really surprising, the evolution of the optical response during AuNRs assembly is very interesting because the shape of the final plasmon band contains certain features that require detailed discussion. To better interpret the observed optical
response, finite-difference time-domain (FDTD) calculations were carried out for clusters containing different numbers of units, ranging from 1 to 5. The spatial dimensions and their distribution, including nanoparticle length, width, interparticle distance and pairwise longitudinal offset, were determined from TEM statistical analysis (Figure 1 and Supporting Information Figure S1). The simulation of the extinction spectra (Figure 4a) was performed by averaging over four different polarization angles to simulate the situation in solution, where the AuNRs are in constant Brownian motion. The results showed that an increase in the number of NR units in the cluster leads to a progressive red-shift of the longitudinal LSPR, up to ∼250 nm. The difference in the magnitude of the red-shift, between calculated (∼250 nm) and experimental (∼450 nm) spectra (Figure 1a), originates from the different geometrical features of simulated clusters and real elongated networks, which possess a higher overall aspect ratio (Figure 1e), further shifting the LSPR. Since simulation of an arbitrary particle network would require a large computational effort, we limited our numerical simulations to a maximum of five units and these showed a satisfactory agreement with our experimental observations. Similar to the longitudinal, the transverse LSPR band was observed to progressively red shift when the number of units in the chain was increased (Figure S9, Supporting Information), though in a much smaller extent. However, it is remarkable that a new distinct feature can be observed in the simulated spectra of trimers, tetramers, and pentamers, as compared to monomers and dimers. These spectra display well-defined peaks located at ∼900 nm, that is, at higher 8830
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Figure 5. (a) Electric near field enhancement for clusters at the wavelength corresponding to the antibonding modes. (b,c) Contribution of the antibonding mode to the overall spectra from solution. (b) Theoretical overall spectrum (red) resulting from simulated spectra for clusters with different number of units (2 5). (c) UV vis-NIR spectrum of the solution at the final stage of the assembly. Dotted lines correspond to the λmax of the initial stage of the assembly.
energies than the corresponding longitudinal LSPR (Figure 4b). These features can be properly explained in terms of longitudinal antibonding modes, which become visible when the symmetry of the clusters is lowered.12 The origin of such symmetry breaking is often ascribed to the heterogeneity of nanorods,48 their angular offset,49 or even to small interparticle separations.12 In our view, whenever clusters contain more than two nanorods or, equivalently, when their symmetry becomes highly perturbed, optically inactive high-energy transitions will gain a nonzero dipole. Another indication for the antibonding character of the highenergy peak is its almost unaltered spectral position upon increase of the number of units within the clusters (Figure 4c). Opposite to the bonding mode, the antibonding mode is not sensitive to changes in the aspect ratio of the cluster, as it lacks net dipole moment in the aligned particles, resulting in the inability of this mode to couple to the far field.6 Thus, the energy difference between both modes is progressively enhanced upon increasing the number of units in the chain. Strong field enhancements at AuNR gaps, achieved by excitation at maximum LSPR, λ = 955 nm (dimer), 1016 nm (trimer), 1075 nm (tetramer), and 1117 nm (pentamer), also indicate that these modes are of the bonding type, with parallel alignment of each AuNR dipole48 (Figure 4d h). On the contrary, when the field enhancement maps are plotted for illumination at the wavelengths of the new intermediate bands, at λ = 944 nm (trimer), 934 nm (tetramer), and 936 nm (pentamer), weaker enhancements in the gap between the particles are consistently obtained, in agreement with the proposed antibonding character (Figure 5a).
As mentioned above, the plasmon band of the colloid gradually becomes asymmetric upon clustering, which indicates a wide distribution of the optically active nanostructures. Apart from a combination of the contribution from multiple optical signatures (from dimers to pentamers and higher), the presence of the “dark” modes might additionally perturb the optical response. Thus, we need to confirm that the antibonding plasmons account for the final spectra registered from solution. Recently, El-Sayed and Eustis proposed a numerical method to obtain the distribution of aspect ratios from the experimental LSPR bands.50 This method helps to correlate a statistical distribution of aspect ratios from numerical data with the shape of the absorbance spectra from solution. Although this method was originally applied to evaluate the importance of aspect ratio on the ensemble spectrum, one can extend it to the evaluation of the contribution of the optical response from clusters containing different numbers of units. Thus, the product of these spectra helps determine the contribution of the antibonding modes to the overall spectra. We first normalized the numerical spectra by volume of gold nanorods, simply dividing each spectrum by the number of units in the corresponding cluster (Figure 4c). Next, each resulting spectrum was multiplied by the corresponding frequency factor, population of each type of cluster containing from 1 to 5 units, in the sample, estimated by TEM analysis (Figure S10, Supporting Information). The sum of all resulting spectra yields the overall optical response (Figure 5b, red line), resulting in a spectrum with two major bands. The main band coincides with the bonding mode from the trimers, which are the dominating cluster population in the 8831
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Langmuir experimental sample (highest frequency factor), while the second band, at high energy, arises from a contribution of the antibonding modes of all clusters. Since the amount of dimers is just 6%, their contribution to the antibonding mode is rather minor. Although the experimental spectrum (Figure 5c) also displays two bands (dominating broad band in NIR and shoulder at higher energy), there is a notable discrepancy with the simulated one. The reason for this discrepancy relies on the presence of extended networks made from clusters (see Figure 1e), while no interactions between clusters are considered in the simulated overall spectrum. However, it is important to recall that detailed TEM analysis of the fully assembled cluster sample allows us to elude the presence of single gold nanorods, so it is justified to assume that the higher energy shoulder in the absorbance spectrum is an intrinsic feature of the clusters. On the basis of the analysis of the simulations, it is finally hypothesized that high-energy antibonding modes do contribute to the final asymmetric shape of the LSPR band.
’ CONCLUSIONS In summary, we experimentally detected a UV induced selfassembly phenomenon, yielding unexpected nanoladder conformations of gold nanorod assemblies. The physical and chemical origin of this novel phenomenon lies in an interfacial cooperation between radical formation induced by solvent irradiation with UV light, leading to cross-linking at the length scale of the adsorbed polymer layer, and anisotropic interparticle interactions that lead to mutual offset in the assembly. Careful in situ monitoring of the optical response upon assembly allowed us to detect antibonding modes that result from plasmon coupling. Given the low symmetry of this clustering process, this method is encouraging toward the design of chiral plasmonic molecules and macromolecules. In addition, after recalling the molecular nature of plasmon hybridization models, these ladderlike conformations suggest an analogy to π π interactions taking place in aromatic molecules.13 Although such a “supramolecular” particle clustering certainly requires further experimental and theoretical work, the findings here presented point out an interesting starting point for a novel unification of polymer and colloid sciences with photochemistry and nanoplasmonics. ’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental procedures; full series of simulated spectra and electric field enhancement maps; NMR and GC-MS; effects of PVP and irradiation time on selfassembly; viscosity measurements; self-assembly of spherical particles. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (M.G.);
[email protected] (L.M.L.-M.).
’ ACKNOWLEDGMENT Prof. Titus Jenny is gratefully acknowledged for stimulating discussion. This work has been funded by the Spanish Ministerio de Ciencia e Innovacion (MAT2010-15374), and by the EU (NANODIRECT, grant number CP-FP 213948-2).
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