Chiral and Achiral Nanodumbbell Dimers: The Effect of Geometry on

Chiral and Achiral Nanodumbbell Dimers: The Effect of Geometry on Plasmonic Properties Kyle W. Smith,† Hangqi Zhao,§ Hui Zhang,§ Ana Sánchez-Iglesias,⊥ Marek Grzelczak,⊥,∥ Yumin Wang,§ Wei-Shun Chang,† Peter Nordlander,*,‡,§ Luis M. Liz-Marzán,*,⊥,∥,¶ and Stephan Link*,†,§ †

Department of Chemistry, ‡Department of Physics and Astronomy, and §Department of Electrical and Computer Engineering, Laboratory for Nanophotonics, Rice University, Houston, Texas 77005, United States ⊥ CIC biomaGUNE, Paseo de Miramón 182, 20009 Donostia-San Sebastian, Spain ∥ Ikerbasque, Basque Foundation for Science, 48013 Bilbao, Spain ¶ Biomedical Research Center in Bioengineering, Biomaterials and Nanomedicine, Ciber-BBN, 20009 Donostia-San Sebastian, Spain S Supporting Information *

ABSTRACT: Metal nanoparticles with a dumbbell-like geometry have plasmonic properties similar to those of their nanorod counterparts, but the unique steric constraints induced by their enlarged tips result in distinct geometries when self-assembled. Here, we investigate gold dumbbells that are assembled into dimers within polymeric micelles. A single-particle approach with correlated scanning electron microscopy and dark-field scattering spectroscopy reveals the effects of dimer geometry variation on the scattering properties. The dimers are prepared using exclusively achiral reagents, and the resulting dimer solution produces no detectable ensemble circular dichroism response. However, single-particle circular differential scattering measurements uncover that this dimer sample is a racemic mixture of individual nanostructures with significant positive and negative chiroptical signals. These measurements are complemented with detailed simulations that confirm the influence of various symmetry elements on the overall peak resonance energy, spectral line shape, and circular differential scattering response. This work expands the current understanding of the influence self-assembled geometries have on plasmonic properties, particularly with regard to chiral and/or racemic samples which may have significant optical activity that may be overlooked when using exclusively ensemble characterization techniques. KEYWORDS: chiroptical activity, single-particle spectroscopy, localized surface plasmons, self-assembly, gold nanoparticles for arbitrary geometries of nanoparticle assemblies.10−13 Work toward understanding the optical response of metal nanostructures has also benefited greatly from their large optical cross sections, which have enabled detection at the single-particle level and hence opened the possibility for a direct correlation between structure and optical response.14−17 Among the many different nanoparticle shapes that can be synthesized,18−25 colloidal gold nanorods have been a subject of particular interest in recent years due to their robust chemical synthesis,26,27 highly tunable optical properties,28 and potential application as nanoscale sensors with high sensitivity.29,30 Selfassembly of gold nanorods has mainly yielded either purely side-by-side or end-to-end configurations4,31−35 with notable exceptions using structural scaffolds.36−38 These predominant

C

ontrolled self-assembly of nanoparticles into precise geometries has been a long-standing goal of the nanoscience community for the simple reason that, just as in macroscale materials, the organization of the individual building blocks strongly affects the overall properties. For many applications of nanoparticles, functionality arises from the optical properties of the individual nanoparticles as well as from their collective behavior.1−6 Thus, extensive research has gone into understanding the strong impact of the optical properties of individual nanostructures and the resulting changes due to various geometric arrangements of nanoparticles into larger assemblies. This effort has been enormously successful with regard to metallic nanoparticles for which the optical response is dominated by surface plasmons, coherent oscillations of conduction band electrons coupled to incident light.7−9 Plasmon hybridization provides an intuitive description of coupled plasmonic modes arising from closely spaced nanostructures, and full electromagnetic simulations are capable of achieving quantitative agreement with the measured spectra © 2016 American Chemical Society

Received: March 31, 2016 Accepted: May 12, 2016 Published: May 12, 2016 6180

DOI: 10.1021/acsnano.6b02194 ACS Nano 2016, 10, 6180−6188

Article

www.acsnano.org

Article

ACS Nano

RESULTS AND DISCUSSION AuNDs were dimerized using a diblock copolymer method previously reported.53 The clustering process resulted in a variety of different dimer configurations as the dominant product compared to monomers and larger aggregates. One common feature of all observed dimers was a side-by-side dimerization rather than a tip-to-tip assembly, due to the specific solvent composition of the initial mixture (tetrahydrofuran/dimethylformamide).57 Upon addition of water, hydrophobic polystyrene molecules located on the tips collapsed, leading to the preferential interactions between the tips and simultaneously exposing side-located surfactant molecules (cetyltrimethylammonium bromide, CTAB) to the medium. The limited solubility of CTAB in tetrahydrofuran, however, induces further side-to-side interactions, leading to the final parallel geometry. Note that the self-assembly of AuNDs in the mixture containing only dimethylformamide results in a chain-like configuration. The dimers can be broadly grouped into categories of aligned parallel, twisted, and twisted with planar displacement (Figure 1a). Transmission electron microscopy (TEM) analysis of the initial AuNDs yielded the following average particle dimensions: length 67.7 ± 3.2 nm, width at center 15.9 ± 1.3 nm, width at the tips 26.2 ± 1.5 nm (Figure 1b). Unlike previous results of AuND assembly by this method, crossed dimers with an approximate twist angle of 90° were a minority product. The AuNDs used here had a smaller aspect ratio and less pronounced tip growth, resulting in lower steric constraints for driving assembly into a predominantly crossed configuration. Accordingly, the AuND dimers in Figure 1c had lower uniformity and adopted a wider variety of possible configurations. The structural diversity was beneficial though for this single-particle investigation because we were able to study and compare the optical properties of a variety of different dimer geometries within the same sample without extensive searching. Smaller aspect ratio AuNDs were furthermore necessary to observe their plasmon resonances within the detection range of our Si detector. Ensemble UV−visible extinction spectroscopy of the AuND solution before and after dimerization was consistent with sideby-side assembly and the presence of significant heterogeneity (Figure 1d,e). Side-by-side assembly of gold nanorods has been reported to yield a red shift of the transverse mode and a blue shift of the longitudinal mode.4,32 Here, we observed that the transverse mode undergoes a significant red shift of 13 nm upon dimerization, while the longitudinal mode experiences a slight red shift of 4 nm with a shoulder at longer wavelengths. The red shift in the transverse mode is in agreement with the observed side-by-side assembly behavior, while the lack of a blue shift in the longitudinal mode can be attributed to the presence of small numbers of larger aggregates. The adsorption of the diblock copolymer surrounding the dimer may also contribute to the observed red shift, due to a larger refractive index of polystyrene (n = 1.55) compared to the solvent (n = 1.33) and the well-known sensitivity of plasmon resonances to local changes in the refractive index.29,58,59 To understand these changes quantitatively, single-particle studies are needed to directly connect structural geometry and the resulting optical properties. Ensemble CD characterization showed that neither the isolated AuNDs nor the dimerized sample showed a detectable CD signal even though we expect many of the dimer configurations observed in the TEM images to be chiral

parallel arrangements often conserve the large intrinsic optical anisotropy of the nanorods, useful for many applications such as polarization control and sensing.34,39,40 However, at the same time, these side-by-side and end-to-end geometries are also limiting the potential of assembled nanorods. For instance, both of these assembled configurations are achiral, while there is much promise for the strong optical activity and enantioselective catalytic properties of chiral plasmonic nanostructures. So far, only a few chiral geometries based on self-assembled nanoparticles have been realized, such as twisted nanorods,41−43 although applications of chiral plasmonic nanostructures have recently come into intense focus.44−50 Gold nanodumbbells (AuNDs) have a slightly modified nanorod geometry, with spherically enlarged tips, but retain very similar optical properties compared to their nanorod counterparts.51,52 The enlarged tips provide additional steric constraints that influence the self-assembly of AuNDs, thereby rendering them interesting candidates as building blocks for bottom-up fabrication. Previous work demonstrated an effective scheme for the self-assembly of AuND dimers into a dominant “cross-bones” shape.53 This twisting of one AuND orientation relative to the other in a dimer configuration significantly alters the resulting optical properties because the constituent plasmon modes are highly dipolar and oriented along the longitudinal axes of the AuNDs. While the coupling of these longitudinal modes is relatively simple for side-by-side and tip-to-tip alignments, when significant twist angles and asymmetric alignments are introduced, their influence on the optical response leads to more complex coupled plasmon resonances and potential mixing of several fundamental modes.54 In particular, geometric chirality is introduced in a side-by-side dimer of AuNDs with a shared central transverse axis for a twist angle between 0 and 90°. Chirality also occurs in such a dimer through a planar shift of one AuND with respect to the other with any nonzero twist angle.55,56 Competition between steric hindrance introduced by the tip geometry of the AuNDs and hydrophilic packing forces produces a much larger variety of chiral AuND dimers compared to nanorod dimers. However, it is difficult to observe such behavior using ensemble measurements because without a particular driving force it is equally likely to obtain both chiral enantiomers and potentially a wide range of chiral nanostructures. Here, we examined AuND dimers with a variety of geometries obtained through clustering inside polymeric micelles using ensemble and single-particle optical characterization techniques. Traditional visible−near-infrared extinction and circular dichroism (CD) spectroscopy were employed for ensemble characterization of the colloidal AuNDs before and after dimerization, while correlated structural and optical singleparticle measurements were performed using scanning electron microscopy (SEM) as well as standard dark-field and circular differential scattering (CDS) spectroscopy. We find that AuND dimerization yields a racemic mixture of chiral enantiomers and achiral nanostructures, which in ensemble measurements show no observable CD signal. The strong optical activity of the chiral dimer assemblies is, however, revealed at the singleparticle level, where always only one enantiomer is investigated at a time. The direct correlation between optical activity and dimer geometry is furthermore validated by electromagnetic simulations. 6181


Article

ACS Nano

The polarization-dependent scattering spectra were acquired by rotating a linear polarizer in the detection path of the microscope and collecting spectra at 30° intervals. The greatest intensity spectrum of these polarized measurements was obtained when nearly aligned with the dominant mode axis (0° in figure legends) and plotted in comparison to the orthogonal collection polarization (90° in figure legends). The CDS measurements were performed by switching between lefthanded and right-handed circularly polarized excitation light (LHC and RHC, respectively) and taking the difference of the scattering spectra collected under each excitation condition. The reported CDS spectra in this paper are limited to the spectral window of 600−850 nm because of the quarter-wave plate (Edmund Optics, #65-919) and IR filter (Thorlabs, FGS550) used (Figure S1). Further details of this experimental setup and the CDS technique can be found in the Methods section, Supporting Information (SI), and previous work.61 While ensemble CD measurements sum the scattering and absorption contributions, CDS isolates the scattering from the total extinction, where, for particles of sufficient size, scattering is a major contributor to the CD spectrum.62 We report all CDS measurements in units of %CDS calculated as %CDS (λ) = 100 ×

ScaLHC(λ) − ScaRHC(λ) ScaLHC/RHC(λmax )

where ScaLHC and ScaRHC are the scattering spectra collected with LHC and RHC polarized excitation and ScaLHC/RHC(λmax) is the maximum intensity taken from either the ScaLHC or ScaRHC spectrum at the peak resonance wavelength depending on which one has a larger value. This definition of %CDS allows direct comparison with simulations by normalization to the maximum scattering intensity and furthermore avoids confusion with the commonly used g-factor that is based on extinction measurements. To account for reported CDS artifacts due to orientated samples and imperfect circular polarization, we used single nanorods as an achiral control sample to perform a correction for false CDS signal, taking into account particle orientation and anisotropy (Figure S2). This correction procedure yielded a limit of detection of 10% of the initial scattering intensity, as indicated for all CDS spectra with a pink envelope overlaid on the plot. Additionally, finite-difference time-domain (FDTD) simulations were performed on the AuND structures investigated here to further validate and interpret the measured plasmonic behavior. The AuND was modeled as two spheroidal tips connected by an object with parabola-shaped surfaces. The relevant dimensions were the length, L, the center rod width, W, and outer sphere radius, R, which were determined for each AuND via correlated SEM imaging and are summarized in Table S1. The simulations also included a detailed threedimensional geometry that is described in detail in Table S2. A diagram of these simulation parameters is shown in Figure S3. Dark-field spectroscopy of single AuNDs demonstrates that their plasmonic properties were very similar to those of single nanorods despite the additional structural feature of enlarged tips (Figure 2a). The scattering from the longitudinal mode of the AuND was highly polarized, as shown by the modulation of the scattering intensity with respect to the detection polarization (Figure 2b). Simulations showed very good agreement with the experimental spectrum, and the charge plot of the longitudinal mode confirms the same dipolar nature indicated by the experimental polarization dependence. The blue shift of

Figure 1. Structural and spectral characteristics of AuNDs and AuND dimers. (a) Cartoon depiction of the dimer formation scheme through polymer capture, resulting in parallel (top), twisted (middle), and twisted with planar displacement (bottom) dimers. (b,c) TEM images of the AuNDs before and after dimerization, respectively. Scale bars are 100 nm. (d,e) Ensemble UV−vis extinction spectra of the AuND and dimer samples, respectively. The AuNDs show maxima at 521 and 812 nm. The maxima for AuND dimers are at 534 and 816 nm. (f,g) Ensemble CD spectra of the AuND and dimer samples, respectively. No peaks are observed in either case.

(Figure 1f,g). It is unlikely that chiral geometries originate during deposition onto a substrate and subsequent drying, as a previous study employing a similar polymer capture method to aggregate nanoparticles demonstrated that the three-dimensional character of aggregates was largely preserved within the polymer shell.60 Therefore, the existence of a racemic mixture of optically active chiral dimer geometries in solution is a likely conclusion that we tested in detail using single-particle measurements. Correlated single-particle analysis was performed with darkfield microscopy-based polarization-dependent scattering spectroscopy, CDS measurements, and SEM structural characterization. In dark-field scattering measurements, circularly polarized light was passed through a large numerical aperture condenser for total internal reflection excitation at the sample. 6182


Article

ACS Nano

Figure 2. Single-particle AuND characterization. (a) Structural characterization of a single AuND. SEM image of a AuND (left). Scale bar is 100 nm. Model used in FDTD calculations (right). The AuND had dimensions of L = 70 nm, W = 16 nm, and R = 15 nm. (b) Correlated single-particle scattering spectra of the AuND in (a) collected with orthogonal detection polarizations overlaid (left) and the simulated spectrum (right; inset: calculated charge plot). (c) Corresponding single AuND CDS spectrum (left). The experimental spectrum is shown with a pink envelope that represents the error of these measurements (i.e., 10% of the scattering intensity). The simulated CDS spectrum (right) confirms the achiral geometry of a single AuND. Dashed lines at zero CDS are included as guides for the eye.

Figure 3. Single-particle characterization of a parallel side-by-side AuND dimer. (a) Structural characterization of the dimer. SEM image of a AuND dimer (left). Scale bar is 100 nm. Model used in FDTD calculations (right). The dimensions of the constituent particles were L = 84 nm, W = 14 nm, and R = 15 nm for one AuND and L = 88 nm, W = 14 nm, and R = 15 nm for the other AuND with both particles aligned parallel. (b) Correlated singleparticle scattering spectra of the AuND dimer in (a) collected with orthogonal detection polarizations overlaid (left) and the simulated spectrum (right; inset: charge plot). (c) Corresponding CDS spectrum of the AuND dimer (left). The simulated CDS spectrum (right) confirms the achiral geometry of this side-by-side AuND.

the resonance maximum in the simulations is due to the neglect of the substrate, as is discussed further below and in the SI. No significant CDS was observed experimentally for individual AuNDs (Figure 2c and Figure S4). FDTD simulations agreed with this observation, consistent also with the behavior of single nanorods.61 This result is not surprising as the AuND has rotational symmetry and therefore is achiral by definition. Due to increased computational complexity, the simulated scattering and CDS spectra shown here considered dark-field rather than total internal reflection excitation and nanostructures surrounded by an isotropic dielectric medium. In the Supporting Information (Figure S4), we show that these approximations have only a very minor effect on the CDS response for individual AuNDs. We next describe the results of the three dimer configurations described in Figure 1a: parallel, twisted, and twisted with planar displacement. Two parallel AuNDs represent an achiral geometry for sideby-side dimers (Figure 3a). In the specific experimental case studied in Figure 3, SEM reveals a small size mismatch between the two AuNDs, but this deviation from a perfect parallel homodimer does not induce chirality unless the size mismatch is also accompanied by a twist. This parallel dimer is observed to have a highly polarized scattering response (Figure 3b), similar to two side-by-side nanorods. The simulated charge plot shows that the coupled plasmon mode originates from two parallel aligned dipoles, corresponding to an antibonding mode

in plasmon hybridization theory.11 Expectedly, this achiral dimer exhibits no CDS activity in either experimental or calculated spectra. The other achiral case of AuND dimers, where the AuNDs are perpendicular to each other with a twist angle of 90°, was considered in simulations only and also had no CDS signal (Figure S5). The population of these ideal achiral dimers was small and not encountered in the singleparticle experiments where SEM imaging was performed after optical measurements. Twisted side-by-side AuND assemblies were found to be the most abundant chiral configuration and allowed us to characterize dimers having both left or right handedness. Chirality in this case was induced by a significant twist along the AuND shared transverse axis (Figure 4a,d). The scattered light was strongly modulated in the detection polarization, indicating that the dimers retain highly polarized, dipolar scattering modes despite twisting and nonperfect center-tocenter alignment (Figure 4b,e). The dipolar nature of these scattering modes was further indicated by the charge plots calculated at the simulated maximum scattering wavelength, showing two parallel aligned dipoles. While the two-dimensional projection of the dipolar mode may appear achiral, when the three-dimensional configuration is considered, it is clear that the charge oscillation is indeed chiral. The coupled scattering modes of the two twisted AuNDs shown here result in a significant CDS signal that is clearly above the experimental 6183


Article

ACS Nano

Figure 4. Single-particle characterization of the two enantiomers belonging to the twisted side-by-side AuND dimer geometry. (a,d) Structural characterization of the two AuND dimers. The experiment sections show SEM images. Scale bars are 100 nm. The theory sections show the models used in the FDTD calculations. Panel (a) uses identical dimensions of L = 80 nm, W = 16 nm, and R = 15 nm for both AuNDs in a lefthanded geometry with a twist of 15°. Panel (d) uses identical dimensions of L = 84 nm, W = 16 nm, and R = 15 nm for both AuNDs in a righthanded geometry with a twist of 28°. A more detailed geometry of both dimers is given in the SI. (b,e) Correlated single-particle scattering spectra of the AuND dimers in (a) and (d), respectively, collected with orthogonal detection polarizations overlaid (left) and simulated spectra (right; insets: charge plots). (c,f) Corresponding experimental and simulated CDS spectra of the two AuND dimers showing opposite signs, as expected for two enantiomers.

According to plasmon hybridization, the high-energy peak corresponds to the antibonding mode and the low-energy peak to the bonding mode. The scattering intensity from the antibonding mode is less modulated than the previously discussed AuND dimers because the individual dipoles of the constituent AuNDs are poorly aligned, thus yielding a weaker net dipole moment. This effect is observed to an even higher degree in the bonding mode, whose scattering is observed to be almost isotropically polarized in the detection path. This polarization dependence of the scattering is also revealed in the charge plot calculated at the resonance energy of the bonding mode. This twisted dimer with planar displacement was also observed to display significant CDS signal (Figure 5c). While the experimental spectrum revealed only a monosignate CDS line shape corresponding to the stronger antibonding mode, the simulated CDS spectrum has a small positive peak at the energy of the bonding mode, resulting in a bisignate line shape. The bonding mode is clearly observed in the simulation to have an opposite chirality to that of the high-energy mode, in agreement with predications from plasmon hybridization theory but with significantly lower CDS amplitude. Unfortunately, the magnitude of this positive peak was unresolvable in our experimental setup, which is partially limited by the low incident light intensity of the white light spectrum (Figure S1) after passing through an IR filter to avoid excessive heating of the polarization optics. Symmetry breaking in nanoparticle samples can come from the geometry of the nanostructure itself but may also be induced by symmetry breaking in the excitation geometry and

threshold and is confirmed by FDTD simulations (Figure 4c,f). The sign of the CDS signal is determined by the handedness of the scattering mode. This behavior can be intuitively understood by considering that the twist of the electric field in the excitation matches the twist in charge distribution of the plasmon mode as the field travels along the twist axis of the dimer, resulting in stronger coupling and thus stronger scattering by one handedness of circularly polarized light than the other. Both of the dimers shown in Figure 4 showed approximately monosignate line shapes, though with opposite signs. The monosignate nature of the CDS line shape originates from a single plasmonic mode dominating the scattering response. Plasmon hybridization predicts two modes for nanoparticle dimers, with a dominant antibonding mode having a large net dipole for a mostly parallel arrangement of rod-shaped nanostructures and a low-energy bonding mode that has a much weaker or even no net dipole.10 The bonding mode is present in twisted dimers due to the symmetry breaking and is chiral with the opposite handedness of the antibonding mode, but its dipole is too small to contribute to the scattering and CDS spectra. This observation is consistent with the CDS spectra of chiral nanorod dimers having small twist angles.61 The final configuration to be discussed is the twisted dimer with planar displacement (Figure 5a). This case is interesting because its more significant symmetry breaking results in two distinct plasmon modes that have different chirality, dipole magnitudes and orientations, and resonance energies. These two modes manifest themselves spectrally as a dominant highenergy scattering peak with a low-energy shoulder (Figure 5b). 6184


Article

ACS Nano

Figure 6. Histogram of CDS maxima for all single-particle measurements performed. Peak CDS for 34 particles is included with a mean %CDS signal of −3.2% and a standard deviation of 13%.

supporting our conclusion. This conclusion is particularly interesting when considering the molecular analogy, where a racemic mixture of chiral molecules with both enantiomers present would produce no optical rotation or CD, yet examination of an individual molecule would indeed always reveal structural chirality. However, the CD responses of typical chiral molecules are so weak (at least 4 orders of magnitude smaller in amplitude compared to optical absorption) that probing a single molecule is not feasible using traditional methods. The significantly larger chiroptical activity of chiral plasmonic nanostructures allows us to access this single-particle regime. A common metric of optical activity is the chiral dissymmetry g-factor defined as64,65

Figure 5. Single-particle characterization of a twisted AuND dimer with significant planar displacement. (a) Structural characterization of the dimer. SEM image of a AuND dimer (left). Scale bar is 100 nm. Model used in FDTD calculations (right) for this dimers, which consisted of AuNDs with identical dimensions of L = 82 nm, W = 14 nm, and R = 15 nm with a twist of 17° and a right-handed geometry. A more detailed geometry of the dimer is given in the SI. (b) Correlated single-particle scattering spectra of the AuND dimer in (a) collected with orthogonal polarization conditions overlaid (left) and the calculated scattering spectrum (right; inset: charge plots). (c) Corresponding CDS spectrum of the AuND dimer (left). The simulated CDS spectrum (right) shows a bisignate line shape with peaks corresponding to the maxima of the fundamental dimer modes indicated by the vertical dashed lines in (b).

g=

2(εL − εR ) εL + εR

where εL and εR are the extinction coefficients for LHC and RHC polarized excitation, respectively. Our measurements only probe the scattering response of the AuND dimers, and our results are therefore reported in %CDS. Nevertheless, we can make comparisons to previous work by converting %CDS to

the substrate creating a nonuniform environment.63 In an effort to further understand the role of symmetry breaking on the CDS line shape for this AuND dimer configuration with a significant net dipole moment of the bonding mode and to exclude such extrinsic effects, simulations including a substrate and evanescent wave excitation were performed (Figure S6). The substrate enhanced the relative scattering intensity of the bonding mode but did not significantly change the bisignate nature of the CDS spectrum, while the excitation condition primarily affected the CDS amplitudes. We furthermore note that the FDTD calculations without the substrate using darkfield excitation, as shown in Figures 4 and 5, consistently predicted CDS magnitudes lower than those experimentally observed by a factor of 2−5. This difference is primarily due to these specific simulation conditions, as illustrated in Figure S6, and potentially other subtle structural symmetry-breaking features that are unresolved via SEM imaging. Due to the strong CDS observed from individual dimers and no CD signal measured in ensemble experiments, the colloidal dispersion of AuND dimers must be racemic. A histogram of the maximum single-particle CDS signals gives insight into the distribution of the optical activity of our AuND dimers (Figure 6). The histogram shows an approximately even distribution of positive and negative CDS signals ranging between ±25% CDS,

gscat =

2(ScaLHC − ScaRHC) , (ScaLHC + ScaRHC)

keeping in mind the distinct measure-

ment techniques and their physical interpretations. We observe a maximum value of gscat ≈ ±0.25 at the resonance wavelength compared to, for example, ref 63, which reported a maximum gfactor of ∼0.4 for a nanorod dimer with extrinsic chirality. Also, multilayer lithographically prepared chiral arrangements of nanodiscs were observed to have a maximum g-factor of 0.14.46 The measured magnitude of the optical activity in our chiral nanostructures is of comparable magnitude to other recent work.

CONCLUSIONS In summary, we performed experimental and theoretical singleparticle characterization of self-assembled AuND dimers and analyzed the influence of dimer geometry on their plasmonic properties. Dimers grouped into parallel, twisted, and twisted with planar displacement highlighted the effects of these geometries on the plasmon resonance position, the scattering line shape, and the CDS spectra. Chiral dimers with significant optical activity were identified by single-particle spectroscopy, despite the absence of a CD signal from the racemic mixture in ensemble measurements. The molecular analogue of this observation is not feasible and can only be obtained here 6185


Article

ACS Nano

Dark-Field Microscopy. Single-particle scattering measurements were performed on a home-built inverted dark-field microscope.61,67 Light generated from a tungsten−halogen lamp and filtered through a visible band-pass filter (304−785 nm, Thorlabs, FGS550) was directed through an inverted microscope body (Zeiss Axio Observer m1) with an oil-immersion dark-field condenser (Zeiss, NA = 1.4) to create a total internal reflection excitation condition for nanoparticles deposited onto indexed,68 indium tin oxide coated microscope slides (Fisher Scientific). Scattered light was collected with a 50× air-space objective (Zeiss, NA = 0.8) and passed to a hyperspectral detection system consisting of an imaging spectrograph with a slit aperture (Princeton Instrument, Acton SpectraPro 2150i with Pixis 400 thermoelectrically cooled back-illuminated CCD) mounted on a computer-controlled translation stage (Newport Linear Actuator model LTA-HL). This system allows for the collection of spectra of several nanoparticles within the region of the slit, which is scanned across the sample area of interest in an automated manner. Polarization-dependent measurements were performed by placing a linear polarizer (Thorlabs, LPVIS100) in the detection path of the dark-field microscope and collecting the scattering spectra of nanoparticles under different linear polarizations in 30° intervals. The 0 and 90° orientations shown in Figures 2−5 were determined by selecting the highest intensity spectrum (0°) and overlaying it with the spectrum collected under orthogonal detection polarization conditions (90°). CDS measurements were performed by using left-handed and righthanded circularly polarized excitation, which was generated by passing the unpolarized light from the tungsten−halogen lamp through a linear polarizer (Thorlabs, LPVIS100) and subsequently through a quarter-wave plate (Edmund Optics, #65-919) oriented with its axis ±45° relative to the linear polarizer. Each day that CDS measurements were performed, an achiral nanorod sample was measured to determine the parameters for the correction procedure. More information on the correction procedure to account for distortions in the excitation polarization can be found in the SI. Simulations. Simulations of AuNDs were performed using the FDTD method (Lumerical FDTD solutions). The AuND was modeled as two spherical tips connected by an object with parabolashaped surfaces, and the sizes were extracted from the corresponding SEM images. For AuND dimers, the interparticle distance and twist angle were chosen to approximate the dimer configuration apparent from SEM images and then varied to optimize the agreement with the experimental observations. The dielectric function of gold was taken from Johnson and Christy.69 For simplicity, all calculations in the main text were performed on single nanostructures embedded in uniform surrounding medium (n = 1) and averaging six incident directions to simulate a dark-field excitation condition, which was found to give results that were similar to the total internal reflection excitation condition used in the experiments (Figures S4 and S6). Performing simulations in a uniform dielectric can result in a shift of the resonance position relative to experimental results of nanoparticles measured on dielectric substrates due to charge screening effects,13,70 which limited the ability of perfect resonance matching between simulation and experiment. For simulation of CDS, circularly polarized excitation was employed, and the scattering spectrum was obtained by integrating the Poynting vector in the far-field. The CDS spectra were calculated as the difference of scattering for left-handed and right-handed circular polarized excitations. Charge density plots were calculated at the resonance maxima with images created by cutting a plane across the nanostructure and employing the electromagnetic boundary conditions to that interface. In the simulations for the total internal reflection excitation, the nanoparticle was placed on a glass substrate (n = 1.45) in air (n = 1). Circular polarized light was incident on the substrate with an angle of 69°, which corresponds to experimental condition. The results from four incident light directions were averaged to simulate the experimental light cone in the microscope. Simulations for the darkfield excitation with a substrate basically employed the same procedure, except that the excitation was from above the substrate.

because of the intense optical activity of chiral plasmonic nanomaterials and the use of single-particle microscopy techniques. This work advances the current understanding of how the spatial arrangement of self-assembled nanoparticles impacts their optical properties and opens the door for more detailed characterizations of the optical activity of chiral plasmonic nanomaterials. Finally, the fact that self-assembly of plasmonic nanoparticles in solution leads to a racemic mixture sets new experimental goals to further develop synthetic protocols for template-free chiral plasmonic clusters in the liquid-phase and chirality-based separation methods for nanoparticle samples.

METHODS Materials. Gold(III) chloride trihydrate (HAuCl4), sodium borohydride, hexadecyltrimethylammonium bromide (CTAB), silver nitrate (AgNO3), hydrochloric acid (HCl), ascorbic acid, tetrahydrofuran (THF), and N,N-dimethylformamide (DMF) were purchased from Sigma-Aldrich. Thiol-terminated polystyrene (PS509-SH) and poly(styrene-b-acrylic acid) (PS403-b- PAA62) were purchased from Polymer Source. Synthesis of Gold Nanorods.66 Gold seeds were prepared by borohydride (10 mM, 0.3 mL) reduction of HAuCl4 (0.25 mM, 5 mL) in aqueous CTAB solution (100 mM). The seed solution was aged at room temperature for 30 min before use. An aliquot of seed solution (50 μL) was added to a growth solution (25 mL) containing CTAB (100 mM), HAuCl4 (0.5 mM), ascorbic acid (0.8 mM), AgNO3 (0.12 mM), and HCl (19 mM). The mixture was left undisturbed for 2 h at 27 °C. The solution was then centrifuged twice (8000 rpm, 30 min) to remove excess silver salt, ascorbic acid, and HCl and redispersed in CTAB solution (100 mM) to obtain a final concentration of gold equal to 2.5 mM. TEM analysis of the nanorods revealed an average aspect ratio of 3.3 ± 0.3 nm (length 47.3 ± 2.5 nm, width 14.6 ± 0.9 nm). Synthesis of AuNDs.51 For the growth of AuNDs, CTAB (100 mM, 25 mL) was mixed with HAuCl4 (50 mM, 0.125 mL) and stored for 5 min at 27 °C to allow for complexation of gold ions, followed by addition of KI (10 mM, 14.25 μL) and ascorbic acid (100 mM, 0.1 mL). Finally, the gold nanorod seed solution (2.5 mM, 1.5 mL) was added under stirring. After 30 min, the dispersion was centrifuged two times (7000 rpm, 15 min) and redispersed in water ([Au] ∼ 10 mM, 1 mL). TEM analysis of the AuNDs yielded the following average dimensions: length 67.7 ± 3.2 nm, width in the middle 15.9 ± 1.3 nm, and width on the tips 26.2 ± 1.5 nm. Ligand Exchange.53 To a tetrahydrofuran solution of PS509-SH (1.43 mg/mL, 10 mL) were added AuNDs (10 mM, 1 mL) dropwise under sonication. The solution was left for 15 min in a sonic bath. To ensure ligand exchange, the resulting mixture was left undisturbed for 12 h at room temperature and then was centrifuged twice (6000 rpm, 30 min). The AuNDs were finally dispersed in a mixture of THF and DMF (3:1 w/w) to a final gold concentration equal to 5 mM. Self-Assembly of AuNDs and Polymer Encapsulation.3 To produce dimers, water (0.2 mL) was added to the colloidal AuNDs (1.8 mL, THF/DMF 3:1 w/w) under magnetic stirring. In the final mixture, the concentration of gold was 0.25 mM. The solution was left undisturbed for 1 h under ambient conditions. To quench further AuND assembly and avoid the formation of larger clusters, a solution of PS403-b-PAA62 (6 mg/mL, 0.2 mL, THF/DMF 3:1 w/w) was added. Subsequently, the water content was increased up to 35 wt %, followed by increasing the temperature up to 70 °C, which was maintained for 1 h. The final solution was centrifuged two times (4500 rpm, 20 min) and dispersed in pure water. Characterization. Optical characterization was carried out by UV−vis spectroscopy with an Agilent 8453 spectrophotometer. TEM images were acquired with a JEOL JEM-1400PLUS transmission electron microscope operating at an acceleration voltage of 120 kV. SEM imaging was carried out on a FEI Quanta 400 ESEM FEG using an electron beam energy of 15 keV. 6186


Article

ACS Nano It was checked that all simulations converged to ensure the accuracy and reliability of the simulation results.

(11) Nordlander, P.; Oubre, C.; Prodan, E.; Li, K.; Stockman, M. I. Plasmon Hybridization in Nanoparticle Dimers. Nano Lett. 2004, 4, 899−903. (12) Oubre, C.; Nordlander, P. Finite-Difference Time-Domain Studies of the Optical Properties of Nanoshell Dimers. J. Phys. Chem. B 2005, 109, 10042−10051. (13) Wu, Y.; Nordlander, P. Finite-Difference Time-Domain Modeling of the Optical Properties of Nanoparticles near Dielectric Substrates. J. Phys. Chem. C 2010, 114, 7302−7307. (14) Arbouet, A.; Christofilos, D.; Del Fatti, N.; Vallée, F.; Huntzinger, J. R.; Arnaud, L.; Billaud, P.; Broyer, M. Direct Measurement of the Single-Metal-Cluster Optical Absorption. Phys. Rev. Lett. 2004, 93, 127401. (15) van Dijk, M. A.; Tchebotareva, A. L.; Orrit, M.; Lippitz, M.; Berciaud, S.; Lasne, D.; Cognet, L.; Lounis, B. Absorption and Scattering Microscopy of Single Metal Nanoparticles. Phys. Chem. Chem. Phys. 2006, 8, 3486−3495. (16) Sönnichsen, C.; Franzl, T.; Wilk, T.; von Plessen, G.; Feldmann, J.; Wilson, O.; Mulvaney, P. Drastic Reduction of Plasmon Damping in Gold Nanorods. Phys. Rev. Lett. 2002, 88, 77402. (17) Olson, J.; Dominguez-Medina, S.; Hoggard, A.; Wang, L.-Y.; Chang, W.-S.; Link, S. Optical Characterization of Single Plasmonic Nanoparticles. Chem. Soc. Rev. 2015, 44, 40−57. (18) O’Brien, S.; Brus, L.; Murray, C. B. Synthesis of Monodisperse Nanoparticles of Barium Titanate: Toward a Generalized Strategy of Oxide Nanoparticle Synthesis. J. Am. Chem. Soc. 2001, 123, 12085− 12086. (19) Tang, Z.; Kotov, N. A.; Giersig, M. Spontaneous Organization of Single CdTe Nanoparticles into Luminescent Nanowires. Science 2002, 297, 237−240. (20) Sun, Y.; Xia, Y. Shape-Controlled Synthesis of Gold and Silver Nanoparticles. Science 2002, 298, 2176−2179. (21) Jin, R.; Cao, Y. C.; Hao, E.; Métraux, G. S.; Schatz, G. C.; Mirkin, C. A. Controlling Anisotropic Nanoparticle Growth through Plasmon Excitation. Nature 2003, 425, 487−490. (22) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. Anisotropic Metal Nanoparticles: Synthesis, Assembly, and Optical Applications. J. Phys. Chem. B 2005, 109, 13857−13870. (23) Nie, Z.; Xu, S.; Seo, M.; Lewis, P. C.; Kumacheva, E. Polymer Particles with Various Shapes and Morphologies Produced in Continuous Microfluidic Reactors. J. Am. Chem. Soc. 2005, 127, 8058−8063. (24) Grzelczak, M.; Pérez-Juste, J.; Mulvaney, P.; Liz-Marzán, L. M. Shape Control in Gold Nanoparticle Synthesis. Chem. Soc. Rev. 2008, 37, 1783−1791. (25) Sanchez-Gaytan, B. L.; Park, S.-J. Spiky Gold Nanoshells. Langmuir 2010, 26, 19170−19174. (26) Jana, N. R.; Gearheart, L.; Murphy, C. J. Wet Chemical Synthesis of High Aspect Ratio Cylindrical Gold Nanorods. J. Phys. Chem. B 2001, 105, 4065−4067. (27) Pérez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzán, L.; Mulvaney, P. Gold Nanorods: Synthesis, Characterization and Applications. Coord. Chem. Rev. 2005, 249, 1870−1901. (28) Link, S.; El-Sayed, M. A. Spectral Properties and Relaxation Dynamics of Surface Plasmon Electronic Oscillations in Gold and Silver Nanodots and Nanorods. J. Phys. Chem. B 1999, 103, 8410− 8426. (29) Zijlstra, P.; Paulo, P. M. R.; Orrit, M. Optical Detection of Single Non-Absorbing Molecules Using the Surface Plasmon Resonance of a Gold Nanorod. Nat. Nanotechnol. 2012, 7, 379−382. (30) Sönnichsen, C.; Alivisatos, A. P. Gold Nanorods as Novel Nonbleaching Plasmon-Based Orientation Sensors for Polarized Single-Particle Microscopy. Nano Lett. 2005, 5, 301−304. (31) Caswell, K. K.; Wilson, J. N.; Bunz, U. H. F.; Murphy, C. J. Preferential End-to-End Assembly of Gold Nanorods by Biotin− Streptavidin Connectors. J. Am. Chem. Soc. 2003, 125, 13914−13915.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b02194. Detailed description of the CDS correction procedure applied to account for linear dichroism in the singleparticle CDS measurements and additional simulations performed to observe the effect of geometry on the CDS signal (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was funded by the Robert A. Welch Foundation (C1664 to S.L. and C-1222 to P.N.), the Army Research Office (MURI W911NF-12-1-0407 to S.L. and P.N.), and National Science Foundation (CHE1507745 to S.L.). K.W.S. acknowledges that this material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program (0940902). L.M.L.-M. acknowledges financial support from the European Research Council (ERC Advanced Grant #267867, Plasmaquo) and the Spanish Ministerio de Economiá y Competitividad (MAT2013-46101-R). REFERENCES (1) Nie, Z.; Petukhova, A.; Kumacheva, E. Properties and Emerging Applications of Self-Assembled Structures Made from Inorganic Nanoparticles. Nat. Nanotechnol. 2010, 5, 15−25. (2) Chan, W. C. Quantum Dot Bioconjugates for Ultrasensitive Nonisotopic Detection. Science 1998, 281, 2016−2018. (3) Mayer, K.; Hafner, J. Localized Surface Plasmon Resonance Sensors. Chem. Rev. 2011, 111, 3828−3857. (4) Wang, L.; Zhu, Y.; Xu, L.; Chen, W.; Kuang, H.; Liu, L.; Agarwal, A.; Xu, C.; Kotov, N. A. Side-by-Side and End-to-End Gold Nanorod Assemblies for Environmental Toxin Sensing. Angew. Chem., Int. Ed. 2010, 49, 5472−5475. (5) Fan, J. A.; Wu, C.; Bao, K.; Bao, J.; Bardhan, R.; Halas, N. J.; Manoharan, V. N.; Nordlander, P.; Shvets, G.; Capasso, F. SelfAssembled Plasmonic Nanoparticle Clusters. Science 2010, 328, 1135− 1138. (6) Nie, Z.; Fava, D.; Kumacheva, E.; Zou, S.; Walker, G. C.; Rubinstein, M. Self-Assembly of Metal-Polymer Analogues of Amphiphilic Triblock Copolymers. Nat. Mater. 2007, 6, 609−614. (7) Wang, H.; Brandl, D. W.; Nordlander, P.; Halas, N. J. Plasmonic Nanostructures: Artificial Molecules. Acc. Chem. Res. 2007, 40, 53−62. (8) Aubry, A.; Lei, D. Y.; Maier, S. A.; Pendry, J. B. Interaction between Plasmonic Nanoparticles Revisited with Transformation Optics. Phys. Rev. Lett. 2010, 105, 233901. (9) Halas, N. J.; Lal, S.; Chang, W.-S.; Link, S.; Nordlander, P. Plasmons in Strongly Coupled Metallic Nanostructures. Chem. Rev. 2011, 111, 3913−3961. (10) Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. A Hybridization Model for the Plasmon Response of Complex Nanostructures. Science 2003, 302, 419−422. 6187


Article

ACS Nano (32) Park, H.-S.; Agarwal, A.; Kotov, N. A.; Lavrentovich, O. D. Controllable Side-by-Side and End-to-End Assembly of Au Nanorods by Lyotropic Chromonic Materials. Langmuir 2008, 24, 13833−13837. (33) Nie, Z.; Fava, D.; Rubinstein, M.; Kumacheva, E. "Supramolecular” Assembly of Gold Nanorods End-Terminated with Polymer “Pom-Poms”: Effect of Pom-Pom Structure on the Association Modes. J. Am. Chem. Soc. 2008, 130, 3683−3689. (34) Huang, H.; Liu, X.; Hu, T.; Chu, P. K. Ultra-Sensitive Detection of Cysteine by Gold Nanorod Assembly. Biosens. Bioelectron. 2010, 25, 2078−2083. (35) Thomas, K. G.; Barazzouk, S.; Ipe, B. I.; Joseph, S. T. S.; Kamat, P. V. Uniaxial Plasmon Coupling through Longitudinal Self-Assembly of Gold Nanorods. J. Phys. Chem. B 2004, 108, 13066−13068. (36) Lan, X.; Chen, Z.; Dai, G.; Lu, X.; Ni, W.; Wang, Q. Bifacial DNA Origami-Directed Discrete, Three-Dimensional, Anisotropic Plasmonic Nanoarchitectures with Tailored Optical Chirality. J. Am. Chem. Soc. 2013, 135, 11441−11444. (37) Querejeta-Fernández, A.; Chauve, G.; Methot, M.; Bouchard, J.; Kumacheva, E. Chiral Plasmonic Films Formed by Gold Nanorods and Cellulose Nanocrystals. J. Am. Chem. Soc. 2014, 136, 4788−4793. (38) Guerrero-Martínez, A.; Auguié, B.; Alonso-Gómez, J. L.; Džolić, Z.; Gómez-Graña, S.; Ž inić, M.; Cid, M. M.; Liz-Marzán, L. M. Intense Optical Activity from Three-Dimensional Chiral Ordering of Plasmonic Nanoantennas. Angew. Chem., Int. Ed. 2011, 50, 5499− 5503. (39) Zhao, Y.; Alù, A. Manipulating Light Polarization with Ultrathin Plasmonic Metasurfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 205428. (40) Zhu, Y.; Kuang, H.; Xu, L.; Ma, W.; Peng, C.; Hua, Y.; Wang, L.; Xu, C. Gold Nanorodassembly Based Approach to Toxin Detection by SERS. J. Mater. Chem. 2012, 22, 2387−2391. (41) Kuzyk, A.; Schreiber, R.; Fan, Z.; Pardatscher, G.; Roller, E.-M.; Högele, A.; Simmel, F. C.; Govorov, A. O.; Liedl, T. DNA-Based SelfAssembly of Chiral Plasmonic Nanostructures with Tailored Optical Response. Nature 2012, 483, 311−314. (42) Kuzyk, A.; Schreiber, R.; Zhang, H.; Govorov, A. O.; Liedl, T.; Liu, N. Reconfigurable 3D Plasmonic Metamolecules. Nat. Mater. 2014, 13, 862−866. (43) Ma, W.; Kuang, H.; Wang, L.; Xu, L.; Chang, W.-S.; Zhang, H.; Sun, M.; Zhu, Y.; Zhao, Y.; Liu, L.; et al. Chiral Plasmonics of SelfAssembled Nanorod Dimers. Sci. Rep. 2013, 3, 1934. (44) Pendry, J. B. A Chiral Route to Negative Refraction. Science 2004, 306, 1353−1355. (45) Fan, Z.; Govorov, A. O. Plasmonic Circular Dichroism of Chiral Metal Nanoparticle Assemblies. Nano Lett. 2010, 10, 2580−2587. (46) Hentschel, M.; Schäferling, M.; Weiss, T.; Liu, N.; Giessen, H. Three-Dimensional Chiral Plasmonic Oligomers. Nano Lett. 2012, 12, 2542−2547. (47) Zhao, Y.; Belkin, M. A.; Alù, A. Twisted Optical Metamaterials for Planarized Ultrathin Broadband Circular Polarizers. Nat. Commun. 2012, 3, 870. (48) Plum, E.; Zhou, J.; Dong, J.; Fedotov, V. A.; Koschny, T.; Soukoulis, C. M.; Zheludev, N. I. Metamaterial with Negative Index due to Chirality. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 035407. (49) Lieberman, I.; Shemer, G.; Fried, T.; Kosower, E. M.; Markovich, G. Plasmon-Resonance-Enhanced Absorption and Circular Dichroism. Angew. Chem., Int. Ed. 2008, 47, 4855−4857. (50) Valev, V. K.; Baumberg, J. J.; Sibilia, C.; Verbiest, T. Chirality and Chiroptical Effects in Plasmonic Nanostructures: Fundamentals, Recent Progress, and Outlook. Adv. Mater. 2013, 25, 2517−2534. (51) Grzelczak, M.; Sánchez-Iglesias, A.; Rodríguez-González, B.; Alvarez-Puebla, R.; Pérez-Juste, J.; Liz-Marzán, L. M. Influence of Iodide Ions on the Growth of Gold Nanorods: Tuning Tip Curvature and Surface Plasmon Resonance. Adv. Funct. Mater. 2008, 18, 3780− 3786. (52) Stender, A. S.; Wei, X.; Augspurger, A. E.; Fang, N. Plasmonic Behavior of Single Gold Dumbbells and Simple Dumbbell Geometries. J. Phys. Chem. C 2013, 117, 16195−16202.

(53) Grzelczak, M.; Sánchez-Iglesias, A.; Mezerji, H. H.; Bals, S.; Pérez-Juste, J.; Liz-Marzán, L. M. Steric Hindrance Induces Crosslike Self-Assembly of Gold Nanodumbbells. Nano Lett. 2012, 12, 4380− 4384. (54) Auguié, B.; Alonso-Gómez, J. L.; Guerrero-Martínez, A.; LizMarzán, L. M. Fingers Crossed: Optical Activity of a Chiral Dimer of Plasmonic Nanorods. J. Phys. Chem. Lett. 2011, 2, 846−851. (55) Shen, X.; Zhan, P.; Kuzyk, A.; Liu, Q.; Asenjo-Garcia, A.; Zhang, H.; Garcia de Abajo, F. J.; Govorov, A.; Ding, B.; Liu, N. 3D Plasmonic Chiral Colloids. Nanoscale 2014, 6, 2077−2081. (56) Solís, D. M.; Taboada, J. M.; Obelleiro, F.; Liz-Marzán, L. M.; García de Abajo, F. J. Toward Ultimate Nanoplasmonics Modeling. ACS Nano 2014, 8, 7559−7570. (57) Fava, D.; Nie, Z.; Winnik, M. A.; Kumacheva, E. Evolution of Self-Assembled Structures of Polymer-Terminated Gold Nanorods in Selective Solvents. Adv. Mater. 2008, 20, 4318−4322. (58) Lee, K.-S.; El-Sayed, M. A. Dependence of the Enhanced Optical Scattering Efficiency Relative to that of Absorption for Gold Metal Nanorods on Aspect Ratio, Size, End-Cap Shape, and Medium Refractive Index. J. Phys. Chem. B 2005, 109, 20331−20338. (59) Chen, H.; Kou, X.; Yang, Z.; Ni, W.; Wang, J. Shape- and SizeDependent Refractive Index Sensitivity of Gold Nanoparticles. Langmuir 2008, 24, 5233−5237. (60) Urban, A. S.; Shen, X.; Wang, Y.; Large, N.; Wang, H.; Knight, M. W.; Nordlander, P.; Chen, H.; Halas, N. J. Three-Dimensional Plasmonic Nanoclusters. Nano Lett. 2013, 13, 4399−4403. (61) Wang, L.-Y.; Smith, K. W.; Dominguez-Medina, S.; Moody, N.; Olson, J. M.; Zhang, H.; Chang, W.-S.; Kotov, N.; Link, S. Circular Differential Scattering of Single Chiral Self-Assembled Gold Nanorod Dimers. ACS Photonics 2015, 2, 1602−1610. (62) Bustamante, C.; Tinoco, I.; Maestre, M. F. Circular Differential Scattering Can Be an Important Part of the Circular Dichroism of Macromolecules. Proc. Natl. Acad. Sci. 1983, 80, 3568−3572. (63) Lu, X.; Wu, J.; Zhu, Q.; Zhao, J.; Wang, Q.; Zhan, L.; Ni, W. Circular Dichroism from Single Plasmonic Nanostructures with Extrinsic Chirality. Nanoscale 2014, 6, 14244−14253. (64) Kuhn, W. The Physical Significance of Optical Rotatory Power. Trans. Faraday Soc. 1930, 26, 293−308. (65) Nakanishi, K.; Berova, N.; Woody, R. Circular Dichroism: Principles and Applications, 1st ed.; VCH Publishers, Inc.: New York, 1994. (66) Liu, M.; Guyot-Sionnest, P. Mechanism of silver(I)-Assisted Growth of Gold Nanorods and Bipyramids. J. Phys. Chem. B 2005, 109, 22192−22200. (67) Byers, C. P.; Hoener, B. S.; Chang, W.-S.; Yorulmaz, M.; Link, S.; Landes, C. F. Single-Particle Spectroscopy Reveals Heterogeneity in Electrochemical Tuning of the Localized Surface Plasmon. J. Phys. Chem. B 2014, 118, 14047−14055. (68) Slaughter, L.; Chang, W.-S.; Link, S. Characterizing Plasmons in Nanoparticles and Their Assemblies with Single Particle Spectroscopy. J. Phys. Chem. Lett. 2011, 2, 2015−2023. (69) Johnson, P. B.; Christy, R. W. Optical Constants of the Noble Metals. Phys. Rev. B 1972, 6, 4370−4379. (70) Knight, M. W.; Wu, Y.; Lassiter, J. B.; Nordlander, P.; Halas, N. J. Substrates Matter: Influence of an Adjacent Dielectric on an Individual Plasmonic Nanoparticle. Nano Lett. 2009, 9, 2188−2192.

6188


Chiral and Achiral Nanodumbbell Dimers: The Effect of Geometry on

Recommend Documents