ARTICLE pubs.acs.org/Langmuir
Gold Nanoparticle Dimers for Plasmon Sensing Yunan Cheng,†,‡,§ Mang Wang,† Gustaaf Borghs,*,‡,§ and Hongzheng Chen*,† †
MOE Key Laboratory of Macromolecule Synthesis and Functionalization, State Key Laboratory of Silicon Materials and Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China ‡ IMEC, Kapeldreef 75, 3000 Leuven, Belgium § Department of Physics and Astronomy, Katholieke Universiteit Leuven, Celestijnenlaan 200, 3000 Leuven, Belgium
bS Supporting Information ABSTRACT: In this study, gold nanoparticles (GNP) were stabilized for the first time as dimers by a conducting polymer (CP). The morphology of kissing particles was examined by high-resolution transmission electronic microscopy (HRTEM). The broad-band localized surface plasmon resonance (LSPR) tunable by solvent variation and molecular binding was demonstrated by UV vis measurement. The sensitivity of the longitudinal LSPR to the surrounding media or the binding of a biomolecule was 6 times higher than that of the transversal LSPR. A homogeneous bioassay was directly developed from the highly stable GNP-CP dimers with LSPR as prober, and protein sensing with detection limit well below 100 ng/mL was achieved.
’ INTRODUCTION The sensitivity of localized surface plasmon resonance (LSPR) to morphology and surrounding media provides multiple sensing possibilities1 9 and is further increased for particles in close proximity giving rise to coupled plasmonic oscillations.10 15 The band feature of the strongly interacting nanoparticle pairs can be controlled by tuning the intraparticle or interparticle distance15 19 and a “plasmon ruler” could be designed to measure the nanoscale distance.20,21 Only when the interparticle separation within the pair is varied from dielectric proximity to conductive contact are the broad band response and the splitting of the plasmon resonance observed.14,22,23 Even strong field enhancement has been predicted for the so-called kissing cylinders and overlapping nanowires,22,24,25 which may find applications in fluorescence enhancement,4,5,26 surface-enhanced Raman scattering (SERS),8,9,27 29 and the detection of a single molecule.7,30,31 To better understand the physical principles, theoretical modeling of plasmon coupling of metallic nanoparticles at short distances have been carried out extensively.22 25 In view of the limited biological results, questions remain on kissing/overlapping particles with coated biomolecules. So far, most of the experimentally obtained nanoparticle pairs have several nanometers interparticle distance due to either the resolution limit of electron beam lithography or the corresponding biomolecular interlayer.20,21,32,33 There have been a few cases investigating nanoparticle pairs less than 2 nm apart. Laser techniques were used to form a small gap between touching particles.14 The ion beam registration method was used to study randomly dispersed particles.12 However, these methods are costly and the obtained nanoparticle pairs are r 2011 American Chemical Society
prepared only on the supporting substrates, which leaves application in homogeneous sensing system a major challenge. For this reason, we prepared gold nanoparticle (GNP) dimers with interparticle space less than 0.5 nm in this work. As synthesized, the dimer colloids were well dispersed in aqueous solution and highly stable in salt buffers to enable direct development of biosensing systems. More specifically, the conducting polymer (CP) with carboxylic substitutions was used to stabilize GNP, and the protonation of CP by gold resulted in Au-CP link between two particles to coagulate the isolated GNPs into GNP-CP dimers. In our study, the LSPR of GNP-CP dimers, containing two distinct characteristic peaks, were easily tunable by clustering small numbers of dimers in methanol or by connecting two or more dimers with a single biomolecule. The sensitivity of longitudinal LSPR to surrounding media and the binding of biomolecule were 6 times higher than that of transverse LSPR. By using the GNP-CP dimers, we achieved quantitative protein sensing with detection limit well below 100 ng/mL. To the best of our knowledge, this is a new approach to obtain kissing GNPs in aqueous solution and to study their behavior in homogeneous bioassay.
’ EXPERIMENTAL SECTION Materials. All salts and reagents, including gold salts, monomer (anthralinic acid), protein A (prA), human IgG (h-IgG), and bovine Received: March 5, 2011 Revised: May 2, 2011 Published: May 16, 2011 7884
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Figure 1. Typical procedures to produce GNP-CP dimers. (1) Electrostatic encapsulation of citrate GNP by water-soluble CP. (2) Coagulation of GNP-CP composite nanoparticles by hydrophobization. (3) Regrouping of GNP-CP dimers via hydrophilization. serum albumin (BSA), were purchased from Sigma-Aldrich (St. Louis, MO, USA), unless otherwise specified. Instrumentation. Optical measurements of CP and GNP were conducted on a UV vis spectrophotometer UV-2450 (SHIMADZU, Kyoto, Japan). High-resolution transmission electric microscopy (HRTEM) photos of nanoparticles were taken on a Tecnai F30 operating at 300 kV (FEI, Oregon, USA). All sonications were performed using a Branson 2510 sonicator (Branson, Danbury, CT, USA). Synthesis of GNP-CP Composite Nanoparticles. First, citrate GNPs were synthesized according to the procedure described by Frens.34 A first solution consisting of 0.01% (w/v) HAuCl4 was prepared. A second solution consists of 1% (w/v) sodium citrate. A 50 mL of boiling HAuCl4 solution was mixed with 0.89 mL of sodium citrate solution to result in approximately 20 nm particles. The exact size and the size distribution were verified by HRTEM. Second, water-soluble CP was synthesized following a method described by Patrick.35 Generally speaking, anthralinic acid (14.4 mM) dissolved in 10 mL of water along with ammonium persulfate (14.4 mM) was added into ferric chloride (72 mM) previously dissolved in 100 mL of 1.2 M HCl under magnetic stirring. The mixture was allowed to react for 5 h at room temperature with magnetic stirring. The reaction was then stopped by adding NaOH pellets in order to raise the pH to 12 and the mixture was filtered. Finally, the gold colloid was added to polymer solution. The volume ratio of polymer solution and gold colloid was kept at 5:1 to ensure an excess amount of water-soluble CP. The mixture was incubated for 15 min and adjusted to pH 2.0 by adding drops of HCl. The further incubated mixture was centrifuged at 5000 rpm for 15 min, and the precipitates were resuspended in 0.01 M phosphate buffer (PB) at pH 7.0. The particle size and the coating thickness were characterized by HRTEM. Synthesis of GNP-CP Dimers. GNP-CP colloids were added to methanol. The volume ratio of methanol and GNP-CP solution was kept as 10:1 to ensure an excess amount of methanol. The mixture was incubated at room temperature for 30 min until a stable color of dark blue was observed and centrifuged at 5000 rpm for 15 min. The precipitates were washed two times by 0.01 M HCl and centrifuged at 2000 rpm for 30 min until the supernatants turned from ultralight red to colorless. The pellets were finally resuspended in 0.01 M phosphate buffer (PB) at pH 7.0 for characterization and further use. Stability Study. GNP-CP dimer colloids were diluted in 0.01 M PBS buffer (pH 7.0) with different concentrations of NaCl (0.1 M, 0.2 M, 0.3 M, 0.4 M, 0.5 M, 0.6 M, 0.7 M, 0.8 M, 0.9 M, 1 M). The mixtures were incubated for 1 h at room temperature and their UV vis spectra were measured.
Refractive Index Sensing. Glycerol was added into GNP-CP dimer colloids in 0.1 M PBS (pH 7.0) to make final concentrations ranging from 10% to 50%. The mixtures were incubated for 15 min and their UV vis spectra were measured. Glycerol concentrations of 0%, 10%, 20%, 32%, 40%, and 52% correspond, respectively, to an index of refraction of 1.330, 1.345, 1.357, 1.373, 1.384, and 1.401. Protein Binding Study. 100 μL of human IgG solution at concentrations of 10 500 μg/mL was added into 900 μL of a GNPCP dimer solution to make a final protein concentration of 1 50 μg/mL. The mixture was incubated for 15 min, and the UV vis spectra were tested. Preparation of GNP-CP Dimer-IgG Conjugates. The procedure was adapted as we recently reported.36 100 μL of human IgG solution with concentration of 100 μg/mL was added into 900 μL of a GNP-CP dimer solution. Then, 150 μL of bovine serum albumin (BSA) at concentration of 4 mg/mL was added. The mixture was incubated at room temperature for 1 h and then centrifuged at 5000 rpm for 3 min. After decantation of the solution, the nanoparticle residue was resuspended in 1 mL of 0.01 M PBS. The probes were ready for use in the assay. Affinity of GNP-CP Dimer-IgG Conjugates for prA. The binding of prA onto GNP-CP dimer-IgG conjugate was evaluated by UV vis absorption. Hereto, equal volumes of GNP-CP dimer-IgG conjugates and prA solution in 0.01 M PBS were mixed together for 15 min. The concentration of prA was varied from 100 to 1000 ng/mL. The samples were measured by UV vis absorption without further purification.
’ RESULTS AND DISCUSSION Synthesis and Characterization of GNP-CP Dimers. Synthesis and Mechanisms. GNP-CP dimers were synthesized from the
protocol schematically described in Figure 1. Three typical procedures included electrostatic encapsulation of citrate GNP by watersoluble CP, coagulation of GNP-CP composite nanoparticles by hydrophobization, and regrouping of GNP-CP dimers via hydrophilization. Figure 2A shows the optical properties of the pristine CP. As schematically drawn in the insets, the characteristic peaks of CP at 214 and 281 nm are assigned to the π π* transition, and the peaks at 460 and 486 nm are related to electron transfer along the polymer chain, all of which are highly altered by the conjugation extent of the polymer.37 39 Figure 2B shows the extinction 7885
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Figure 2. Optical properties of (A) water-soluble CP in HCl, (B) GNP before and after CP coating, and (C) GNP-CP as clusters in methanol and dimers in PBS buffer. The inset schemes show electron transfer mechanisms corresponding to characteristic peaks.
properties of GNP before and after CP coating. The characteristic peaks of pristine CP are not present for GNP-CP composite materials anymore. The plasmon resonance of GNP-CP is redshifted for about 3 nm compared to that of pristine GNP. These observations demonstrate the successful coating of a polymer layer around GNP. The attractions between GNP and CP are mainly electrostatic forces, which are schematically described in the Supporting Information (Figure S1 and Figure S2). Figure 2C shows the optical properties of GNP-CP composite materials during solvent transfer. A strong absorption at about 300 nm is observed for GNP-CP in methanol. On one hand, the peak is red-shifted compared to the absorption of prestine CP at 281 nm. As reported,39 the red shift of such an absorption band is caused by the increased extent of conjugation. In our study, the
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CP shell gets deprontonated by methanol (Figure S1), which decreases the steric hindrance along the polymer chain, decreases the torsion angel between adjacent phenyl rings, and consequently increases the conjugation extent of the polymer. On the other hand, the intensity of the 300 nm peak is remarkably large, indicating a possible charge transfer from CP to GNP. Researchers have fabricated a digital memory device from CP/GNP. The transition from the OFF to the ON state was attributed to an electric-field-induced charge transfer between CP nanofibers and GNP. Under a sufficient electric field, the electrons residing on the imine nitrogen of the CP may gain enough energy to surmount the interface between the CP nanofibers and the GNP and move onto the GNP.40 The GNP-CP clusters in methanol contain particles with close proximity, in between which an ultrathin organic interlayer is possibly formed (schematically shown in Figure S3, Figure S8, and demonstrated by Figure S7). At the same time, the energy level of CP may be lowered due to deprotonation by methanol. As such, the energy barrier between CP and GNP may become smaller, which enables the charge transfer between the two components. Additionally in Figure 2C, the characteristic extinction of GNPCP in HCl (647 nm) is blue-shifted from that in methanol (704 nm), which is due to the improved dispersion of gold clusters (Figure S5). After GNP-CP clusters are washed by HCl, GNP-CP dimers with very narrow size distribution (Figure S5-D) are obtained. Furthermore, the intensity of LSPR at around 530 nm keeps increasing when the colloids are transferred from methanol to acidic and neutral buffers. This, on one hand, is related to the increased amount of single particles polarized with light and, on the other hand, indicates the formation of strongly interacting particle pairs. Morphology. The size and the coating thickness of Au particles are provided in Figure 3. Citrate GNP has a diameter of 17 ( 1 nm (Figure 3A). The GNP-CP has a coating thickness of 5 6 nm (Figure 3B). The coating thickness of GNP-CP in methanol decreases to an average value of 4 nm (Figure 3C), indicating a layer of organic coating is removed. As previously reported,35 GNP is possibly covered by a single layer of headattached linear CP chains in which a second layer of such chains is semiembedded (Figure S2). It is likely our case. If the first layer of CP was 4 nm, a second layer with half body exposed would make 2 nm and generate a total thickness of 6 nm, which is consistent with the result from Figure 3B. As dispersed in a PBS buffer, the two particles of the dimer are separated by 0 0.5 nm (Figure S5, Figure S7, and Figure S8) while the organic layer has a thickness of about 3.5 nm (Figure 3D). We reason a 0.5 nm loss as CP strands fall due to the loss of the second layer support (Figure S4). As shown in Figure 3D, most of the dimers have zero interparticle spacing, but some dimers are observed to have an organic interlayer with thickness of less than 1 nm. Therefore, with the polymer approach, the smallest gap within the dimer could be zero, but the largest gap might be depending on the polymer, which is approaching 1 nm in our case. Figure 3 also gives direct evidence about the dispersity of particle colloids. The particles are coagulated as clusters in methanol (Figure 3C) and dispersed as dimers in PBS buffers (Figure 3D). As discussed in the Supporting Information (Figure S7 and Figure S8), methanol first dissolves the CP layer around GNP, leaving the bare GNP to aggregate. The CP molecules reassemble around the GNP clusters as a monolayer. Subsequently, HCl attacks the CP layers and separates the 7886
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Figure 3. Morphology of gold colloids. (A) Citrate GNP with an average diameter of 17 nm. (B) A single GNP-CP nanocomposite particle with coating thickness of 5 6 nm. (C) GNP-CP clusters in methanol. (D) GNP-CP dimers with an average coating thickness of 3.5 nm in PBS buffer.
Figure 4. Optical properties of GNP-CP dimers in different buffer conditions.
GNP-CP as single particles and dimer particles (Figure S5). After purification, the single particles are removed (Figure S6), leaving monodispersed GNP-CP dimers (Figure S5-D). Combining the purification efficiency (Figure S5-C) and the poly
dispersity index (Figure S5-D), the dimer yield is estimated as about 63.0%. Stability. Figure 4 shows the optical properties of GNP-CP dimers in different concentrations of salt buffers. It is found that the dual LSPR peaks stayed stable for NaCl concentrations up to 0.8 M, demonstrating good stability of the GNP-CP dimer colloids. In salt buffers with concentrations over 0.7 M, the LSPR peaks decreased and shifted to the infrared range, indicating particle aggregations. When 1.0 M NaCl was added, the sample precipitated and the LSPR peaks almost vanished. Compared to the citrate GNP, which is only stable below 25 mM salts buffer as we recently studied,36 such high stability enables GNP-CP dimers to be further developed into homogeneous bioassay. Compared to previously reported metallic nanoparticle dimers limited on substrates, the GNP-CP dimers synthesized in aqueous solution and stable in salts buffer enable direct application for in situ biomedical detections. Optical Properties of GNP-CP Dimers. Figure 2C shows the broad and splitted LSPR absorbance of the dimer colloids. The splitting of the surface plasmon energy is related to the formation of strongly interacting nanoparticle pairs, where the interparticle dipole dipole interaction is shunted and the plasmon polaritons exhibit multipolar behavior when a conductive contact is formed.14 Similar results have been reported with gold nanodisc 7887
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Figure 6. LSPR shift of GNP-CP dimer colloids as a function of h-IgG concentration. The inserted drawing represents the protein binding on a GNP-CP dimer.
Figure 5. Sensitivity of GNP-CP dimers to refractive index change. (A) Optical response of GNP-CP dimer colloids to the variation of glycerol concentration. (B) Red-shift of transversal and longitudinal LSPR of GNP-CP dimer colloids as a function of refractive index. The sensitivity of longitudinal LSPR is about 6 times higher than that of transversal LSPR.
pairs.15 As the distance between two metal nanostructures decreased below 2 nm, the resonance line splitting was observed. As revealed in Figure 2C, the dual LSPR extinction for clusters in methanol not only displays an assembly induced red shift compared to the well-dispersed dimers in PBS buffer, but also shows intensity changes. The dimers in PBS buffer have two LSPR peaks with almost equal intensities, which is different from the clustered particles in methanol. The results are consistent with the previous reports on overlapping gold nanoparticles.14 There is also an interesting absorption peak at 300 nm, which occurs only for particle clusters in methanol. This peak, stemming from transitions in the conjugated polymer, is not present for polymers in the presence of dimers nor dimers decorated with IgG. However, the peak reappears upon binding prA to the IgG, which causes a clustering of dimers as will be shown further in the paper. The exact reason for the increased absorption, due to quenching or because of increased light focusing due to an antenna function of the clustered dimers, is not clear at this moment. However, the optical properties of GNP-CP dimers highly tunable by solvent variation may provide a versatile approach for sensing systems. Plasmon Sensing. Refractive Index Sensing. As shown in Figure 5, the LSPR sensitivity of GNP-CP dimer to surrounding media is demonstrated by changing the refractive index, which is
realized by varying glycerol concentration from 10% to 50%. Once mixed with glycerol, a red-shift and intensity increase of LSPR absorbance is observed for dimer colloids (Figure 5A). More interestingly, the wavelength shift of longitudinal LSPR is very significant. As summarized in Figure 5B, the red-shift of longitudinal LSPR as refractive index increase is about 6 times larger than that of the transversal LSPR. This is consistent with other reports11,12,41 that the coupling between two particles in dimer is much stronger for longitudinal than transverse polarization. By adding surrounding media, the coupling for longitudinal polarization is therefore more affected than for transverse polarization. The response of GNP-CP dimers to refractive index change allows for further studies of particle based bioconjugation. The distinguished sensitivity of two LSPR peaks may provide multiple approaches in the sensing of protein binding. Bioconjugation Studies. As expected, the same behavior is found in the study of protein binding to GNP-CP dimers (Figure 6). When adding h-IgG, the particle extinction exhibits the similar red-shift. The LSPR shift is plotted as a function of h-IgG concentration. Also, here the longitudinal LSPR is the most sensitive. As protein concentration varies from 2 to 10 μg/mL, the longitudinal LSPR has a quite linear response. However, the transversal LSPR gives a significant shift only after 5 μg/mL of added proteins. Furthermore, the LSPR shift reaches saturation for h-IgG concentration of 10 μg/mL, related to 35.1 IgG molecules on a single particle (Figure S9). As calculated theoretically, 46.2 IgG molecules should be bound to a 17 nm GNP as a full coverage. The loss of more than 9 molecules per particle is caused by the interconnection neck between two particles and therefore the loss of surface area. As such, the GNP-CP dimers have almost a full coverage of h-IgG, showing a strong coupling efficiency of CP with biomolecules, which is consistent with other reports on the easy manipulating and monitoring of biomolecular interactions with conjugated conducting electroactive polymers.42,43 The high loading efficiency of protein enables further observation of GNP-CP dimer-IgG bioconjugates in an immunoassay. Bioaffinity Detection. Figure 7 reveals the GNP-CP dimer behavior in an immunoassay, where we selected h-IgG and 7888
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Figure 7. Optical sensitivity of GNP-CP dimer-IgG to prA. The inset graph plots wavelength shift of infrared resonance (green) and the intensity increase of 300 nm absorbance (red) as a function of prA concentrations. The inset drawing shows bioaffinity-induced assembly of GNP-CP dimers.
protein A. The Fc fragment of h-IgG has a high affinity to protein A. As is well-known,44 each protein A molecule can bind with at least two IgG molecules with high affinity (1010 1011 M 1). We have experimentally demonstrated that each protein A binds with approximately two to three h-IgG molecules.39 Therefore, once protein A is added to GNP-CP dimer-IgG conjugates, it combines at least two dimers and causes an aggregation. Interestingly, the clusters have controllable size (Figure S10). As shown in Figure 7, upon addition of protein A, the dual LSPR of GNP-CP dimer-IgG red-shifted to the infrared and merged into a broad extinction, accompanied by the repeat occurrence of a strong peak at 300 nm. The infrared resonance is related to particle assembly caused by immune affinity (inset scheme of Figure 7). The strong absorption at 300 nm is similarly found for GNP-CP clusters in methanol (Figure 3B). Both the wavelength shift of infrared resonance (Figures S11-A) and the intensity increase at 300 nm absorption (Figures S11-B) are plotted as a function of prA concentration (Figure S12) and show linear response between 100 ng/mL and 500 ng/mL (inset of Figure 7). When the prA concentration reaches 1000 ng/mL, the aggregation is most likely too large and almost no plasmonic effects are detected. Also, the particles start to precipitate from the solution, leaving only a broad shoulder at the longer wavelength. The 300 nm absorption also fades into a distinguishable level. For a comparison, we carried out the control experiments with isolated GNP (Figure S13). The GNP dimers show advantages over isolated GNPs from at least two aspects. First, when coated with IgG molecules, GNP-CP exhibits a red-shift which is far less sensitive than that of GNP-CP dimers. Second, when detecting prA, the bioconjugation of GNP-CP-IgG has no response but
aggregation until 1000 ng/mL of prA is added. As the concentration of prA continues rising, particle precipitation is observed. Compared to the bioconjugation of GNP-CP dimer-IgG, which has a detection limit of prA sensing below 100 ng/mL, the GNPCP-IgG displays very poor sensitivity. Therefore, the high sensitivity of GNP-CP dimers to surrounding media and biomolecular binding is proven to be unique and is believed to be a powerful tool in plasmon sensing areas.
’ CONCLUSION To conclude, we have developed a simple and versatile method to synthesize the stable kissing GNP-CP dimers by using CP as a coating layer. Due to the surface modification by CP, the kissing GNP-CP dimers exhibited an excellent stability in salt buffer so that direct application in homogeneous bioassay was enabled. As expected, GNP-CP dimers showed the interesting broadband LSPR property with two distinct characteristic peaks, which were further tunable by changing solvents or by connecting two or more dimers with a single biomolecule. In general, the sensitivity of longitudinal LSPR, either to surrounding media or to molecular binding, was 6 times higher than that of transversal LSPR. By using the GNP-CP dimers, we achieved quantitative detection of protein with detection limit well below 100 ng/mL. ’ ASSOCIATED CONTENT
bS
Supporting Information. Discussions on synthesis, mechanism, and surface chemistry of GNP-CP dimers; calculations of binding protein on GNP-CP dimer; and control experiments
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Langmuir with isolated GNP. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*
[email protected] (H. Chen),
[email protected] (G. Borghs).
’ ACKNOWLEDGMENT The authors are grateful to NSFC (Grants No. 50990063 and 51011130028) and the Major State Basic Research Development Program (2007CB613400) for financial support. Yunan Cheng thanks China Scholarship Council (CSC) and FWO-Flanders for financial support. Thanks also go to Xi Yang for useful discussions on LSPR, and Dr. Minmin Shi and Dr. Gang Wu for suggestions on biosensing experiments. ’ REFERENCES (1) Liz-Marzan, L. M. Tailoring surface plasmons through the morphology and assembly of metal nanoparticles. Langmuir 2006, 22, 32–41. (2) Sardar, R.; Funston, A. M.; Mulvaney, P.; Murray, R. W. Gold nanoparticles: past, present, and future. Langmuir 2009, 25 (24), 13840–13851. (3) Lee, S.; Mayer, K. M.; Hafner, J. H. Improved localized surface plasmon resonance immunoassay with gold bipyramid substrates. Anal. Chem. 2009, 81, 4450–4455. (4) Bardhan, R.; Grady, N. K.; Cole, J. R.; Joshi, A.; Halas, N. J. Fluorescence enhancement by Au nanostructures: nanoshells and nanorods. ACS Nano 2009, 3 (3), 744–752. (5) Bardhan, R.; Grady, N. K.; Halas, N. J. Nanoscale control of nearinfrared fluorescence enhancement using Au nanoshells. Small 2008, 4 (10), 1716–1722. (6) Vogelgesang, R.; Dmitriev, A. Real-space imaging of nanoplasmonic resonances. Analyst 2010, 135, 1175–1181. (7) Mayer, K. M.; Hao, F.; Lee, S.; Nordlander, P.; Hafner, J. H. A Single molecule immunoassay by localized surface plasmon resonance. Nanotechnology 2010, 21, 255503–255511. (8) Wang, W.; Li, Z.; Gu, B.; Zhang, Z.; Xu, H. Ag@SiO2 core shell nanoparticles for probing spatial distribution of electromagnetic field enhancement via surface-enhanced Raman scattering. ACS Nano 2009, 3 (11), 3493–3496. (9) Liang, H.; Li, Z.; Wang, W.; Wu, Y.; Xu, H. Highly surfaceroughened ‘‘flower-like’’ silver nanoparticles for extremely sensitive substrates of surface-enhanced raman scattering. Adv. Mater. 2009, 21, 4614–4618. (10) Li, Z.; Shegai, T.; Haran, G.; Xu, H. Multiple-particle nanoantennas for enormous enhancement and polarization control of light emission. ACS Nano 2009, 3 (3), 637–642. (11) Slaughter, L. S.; Wu, Y. P.; Willingham, B. A.; Nordlander, P.; Link, S. Effects of symmetry breaking and conductive contact on the plasmon coupling in gold nanorod dimers. ACS Nano 2010, 4 (8), 4657–4666. (12) Funston, A. M.; Novo, C.; Davis, T. J.; Mulvaney, P. Plasmon coupling of gold nanorods at short distances and in different geometries. Nano Lett. 2009, 9 (4), 1651–1658. (13) Nordlander, P.; Oubre, C.; Prodan, E.; Li, K.; Stockman, M. I. Plasmon hybridization in nanoparticle dimers. Nano Lett. 2004, 4 (5), 899–903. (14) Atay, T.; Song, J. H.; Nurmikko, A. V. Strongly interacting plasmon nanoparticle pairs: from dipole-dipole interaction to conductively coupled regime. Nano Lett. 2004, 4 (9), 1627–1631. (15) Jain, P. K.; Huang, W. Y.; El-Say, M. A. On the universal scaling behavior of the distance decay of plasmon coupling in metal nanoparticle pairs: a plasmon ruler equation. Nano Lett. 2007, 7 (7), 2080–2088.
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dx.doi.org/10.1021/la200840m |Langmuir 2011, 27, 7884–7891
