Gold NanocubeâNanosphere Dimers: Preparation, Plasmon Coupling

Article pubs.acs.org/JPCC

Gold Nanocube−Nanosphere Dimers: Preparation, Plasmon Coupling, and Surface-Enhanced Raman Scattering Daedu Lee and Sangwoon Yoon* Department of Chemistry, Dankook University, 152 Jukjeon-ro, Suji-gu, Yongin, Gyeonggi 448-701, Korea S Supporting Information *

ABSTRACT: We explore plasmon coupling and surface-enhanced Raman scattering (SERS) from nanogaps defined by different surface shapes. To this end, we assemble heterodimers where gold nanocubes (AuNCs, edge length 84 nm) are linked to gold nanospheres (AuNSs, diameter 55 nm) through their vertices or edges via 1,8-octanedithiol (C8DT). Regioselective disintegration of trimethylammonium bilayers on the vertices and edges of AuNCs using acetonitrile permits the formation of AuNC−AuNS dimers in high yield. Strong plasmonic interactions between the AuNCs and AuNSs produce the longitudinal plasmon coupling band at 790 nm, significantly red-shifted from the surface plasmon resonance band of the isolated AuNCs (λ = 581 nm) or AuNSs (λ = 534 nm). Localized electric fields confined to the nanogaps between the AuNCs and AuNSs also generate a strong SERS signal. We observe the Raman spectrum for C8DT from the AuNC−AuNS dimers with a 1 × 1010 enhancement factor (EF), which is much larger than that for nanoassemblies consisting of only AuNSs, such as core− satellites (EF = ∼108) and clusters (EF = ∼107). Comparison with finite difference time domain simulations reveals the nature of the plasmon coupling and the local field enhancement in the AuNC−AuNS dimers.

1. INTRODUCTION Assembling noble metal nanoparticles is a fascinating means of controlling and enhancing their plasmonic properties.1 The optical response of noble metal nanoparticles is governed by their surface plasmon resonance (SPR), the resonant excitation of conduction electrons within the nanoparticles into a collective oscillatory motion. The resonance frequency for isolated nanoparticles is inherently determined by their size, shape, material, and environment.2 However, this resonance frequency changes drastically as the nanoparticles form assembled structures. The shift in resonance frequency depends on the interactions between the nanoparticles in close proximity. Therefore, nanoparticle assembly provides a means of tuning the resonance frequency in a flexible and fine manner for a wide range of wavelengths. Furthermore, electric fields intensely localize at the interstitial sites in nanoassemblies, which makes them excellent platforms for exploring many nearfield enhancement effects, such as surface-enhanced Raman scattering (SERS), surface-enhanced fluorescence (SEF), and plasmonic photocatalytic activity.3−6 Many assemblies have been explored ranging from dimers, the simplest assembly structure, to aggregates, a completely uncontrolled complex assembly structure. The grand challenges facing many researchers are (i) assembling nanoparticles into well-defined structures in high yield and (ii) fundamentally understanding the plasmonic properties of the resultant nanoassemblies. Mixing nanoparticles with their surfaces functionalized with molecular linkers inevitably produces a mixture of assemblies. The understanding of how various © 2015 American Chemical Society

parameters that define the nanogaps (e.g., interparticle distance, shape, composition) influence the plasmon coupling, quantum effects, and SERS is far from perfect.7−9 Recently, several groups successfully prepared molecularly linked nanoparticles with controlled stoichiometry via asymmetric functionalization or finely controlled electrostatic interactions.10−13 Our group also reported new assembly methods to prepare core−satellites and dimers in high yield (>90%).14,15 We measured the plasmon coupling for the assemblies as a function of their interparticle distances. Despite significant progress recently made, the assembly of nanoparticles with different shapes has been relatively overlooked. The assembly of nanospheres with nonspherical anisotropic nanoparticles is particularly interesting. Directional plasmon coupling is feasible depending on where the nanospheres are attached to the anisotropic nanoparticles. For example, Thomas and co-workers observed a shift only in the longitudinal plasmon mode for gold nanorods with gold nanospheres (AuNSs) adsorbed on their ends.16 Similarly, attaching AuNSs to the side of the gold nanorods is anticipated to shift the transverse mode. We can also explore the effect of the nanogap shape on the plasmon coupling and near-field enhancement. The nanogaps between nanospheres and anisotropic nanoparticles possessing sharp curves or pointy Received: January 12, 2015 Revised: March 4, 2015 Published: March 18, 2015 7873

DOI: 10.1021/acs.jpcc.5b00314 J. Phys. Chem. C 2015, 119, 7873−7882

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The Journal of Physical Chemistry C tips are expected to generate stronger electric fields than nanosphere assemblies due to the lightning rod effect.17,18 In this article, we report the assembly of nanocube− nanosphere heterodimers. Gold nanocubes (AuNCs) are an excellent assembly building block for plasmonic applications. They exhibit strong multiple plasmon resonances.19 The electric fields are strongly confined in the vertices and edges of the nanocubes, which makes them superior plasmonic nanoantennae to comparably sized round nanospheres.20,21 These enhanced local fields have been fully exploited for SERS and two-photon photoluminescence enhancement.22−25 Despite their excellent plasmonic properties, however, attempts to use nanocubes in assembly are rare. The major obstacle lies with the difficulty in modifying the nanocube surfaces. Nanocubes are usually surrounded by a cetyltrimethylammonium (CTA) bilayer, which is too robust to penetrate or displace in contrast to the easily replaceable citrates on nanospheres.26 Therefore, previous approaches relied on either electrostatic interaction using the positive surface charges of the CTA bilayers on the nanocubes or covalent bonding to the surface where the CTA bilayers are presumably less ordered.17,22−24,27 These uncontrolled methods exhibited low yields for the desired nanoassemblies, which complicated accurately measuring the plasmon coupling and near field enhancement effects occurring in the nanocube assemblies. In this study, we aim to prepare AuNC−AuNS heterodimers in high yield. The AuNCs and AuNSs are covalently bonded via dithiol linkers after preferentially destroying the CTA bilayers at the AuNC vertices or edges using a polar aprotic solvent (acetonitrile). Electrostatic repulsion is used judiciously to prevent unlinked monomers from forming and to thus increase the dimer yield. Preparing these AuNC−AuNS dimers in high yield allows us to explore the plasmon coupling and SERS activity at the junction between the AuNCs and AuNSs. Comparison with finite difference time domain (FDTD) simulations reveals the nature of the plasmon coupling and local field enhancement in the AuNC−AuNS dimers.

Figure 1. (a, b) TEM images and (c) size distribution for AuNCs. The edge length and the radius of curvature (rc) for a AuNC are defined in (b).

The AuNSs were synthesized using the method of Puntes and co-workers.29 Seed AuNSs are grown to the desired size via the successive reduction of HAuCl4 by citrate. We obtained citrate-stabilized AuNSs 55.4 ± 4.8 nm in diameter (N = 113). The TEM images and size distribution for the prepared AuNSs are available in the Supporting Information (Figure S1). 2.2. Assembly of AuNC−AuNS Dimers. The prepared AuNCs and AuNSs must be reacted to produce strongly coupled dimers. In pursuing this goal, we had to resolve two issues: (i) how to link AuNCs with AuNSs and (ii) how to prevent multimers and unlinked monomers from forming. For the first challenge, it is conceivable that electrostatic interactions or molecular linkers are used for the linkage. Electrostatic interactions provide a facile route to assemble oppositely charged nanoparticles such as AuNSs (capped with citrate anions) and AuNCs (capped with positive CTA). However, multiple interactions between particles form multimers and aggregates. Furthermore, even if a dimer forms, the CTA bilayers on the AuNCs limit the interparticle gap distance to >∼3.5 nm. Thus, molecular linkers are preferable. However, when using molecular linkers, one should consider that modifying the AuNC surface is extremely difficult due to the impregnable CTA bilayer.26 In our assembly strategy, we use dithiol linkers to connect AuNSs to AuNCs. The impregnable CTA bilayers are circumvented using acetonitrile, which reportedly disintegrates them.30 We note that the CTA bilayers are relatively disordered at the curved areas of the AuNCs (such as vertices and edges), thus more readily removed by acetonitrile. For the second challenge, the high AuNC−AuNS dimer yield requires minimizing the formation of multimers and unlinked monomers. We use electrostatic repulsion to prevent a third particle from adsorbing onto already formed dimers. Electrostatic repulsion also prohibits AuNCs from adsorbing on the positively charged glass surface, lowering the chance for the AuNC monomer formation. Figure 2 illustrates the assembly procedure we developed based on the above strategy. The detailed procedure is provided

2. RESULTS AND DISCUSSION 2.1. Preparation of AuNCs and AuNSs. AuNCs and AuNSs were prepared separately and fully characterized before being assembled into dimers. To synthesize the AuNCs, we adopted the method developed by Huang and co-workers.28 Briefly, reducing Au3+ with NaBH4 produces Au seed particles. These seed particles then grow into cube-shaped particles via the additional reduction of Au3+ by ascorbic acid in the presence of cetyltrimethylammonium chloride (CTAC) and NaBr. The synthesis typically yields 20 mL of colloidal AuNCs dispersed in water at a concentration of 4.5 pM. The concentration was determined from the AuNC extinction coefficient (ε581 nm = 5.2 × 1011 M−1 cm−1), which was measured by combining transmission electron microscopy (TEM) and inductively coupled plasma mass spectrometry (ICP-MS). Details on how we measured the concentration and the extinction coefficient of AuNCs are provided in the Supporting Information. Figure 1 presents the TEM images and the size distribution of the prepared AuNCs. The resultant AuNC’s edge averages 84.0 ± 3.7 nm in length (N = 413). The narrow size distribution (RSD = 4%) indicates that the prepared AuNCs are very homogeneous. The more magnified image reveals that the AuNCs have round corners with a ∼10 nm radius of curvature (rc). 7874


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Figure 2. Assembly scheme. Step 1: amine coating of a glass slide using APTMS. Step 2: adsorption of negatively charged citrate-capped AuNSs (55 nm) on the glass slide. Step 3: thiol functionalization of the AuNSs using alkanedithiol. Step 4: adsorption of the AuNCs (84 nm) to the AuNSs via covalent bonds between the thiol on the AuNS and vertices or edges of the AuNC. (a) and (b) illustrate the formation of alkanedithiol SAMs on the AuNSs and CTA bilayer disintegration at the curved areas of the AuNCs by CH3CN, respectively. The two-sided arrow in the last step represents the electrostatic repulsion between the positively charged AuNCs and amine-coated glass surface.

in the Experimental Section. Briefly, a glass slide (25 mm × 12 mm) is thoroughly cleaned and coated with (3-aminopropyl)trimethoxysilane, APTMS (step 1). Immersing the glass slide in a AuNS solution (11 pM, 5 mL) for 12 h leads to the adsorption of the citrate-capped AuNSs (55 nm) on the aminecoated glass surfaces via electrostatic interactions (step 2).31 The adsorbed AuNS surfaces are modified with thiol in step 3. The 1,8-octanedithiol (C8DT) self-assembled monolayer (SAM) forms on the AuNS surface as illustrated in Figure 2a, after immersing the AuNS-anchored glass slide in an ethanol solution of dithiol (1 mM, 5 mL) for 6 h.32 In step 4, the AuNCs (84 nm) are attached to the AuNSs via the C8DT SAMs. The AuNC solution (4.5 pM, 5 mL) is centrifuged, and the precipitate is redispersed in a 90% (v/v) acetonitrile solution. Acetonitrile preferentially disintegrates the CTA bilayers at the AuNC vertices and edges (Figure 2b). Immersing the glass slide from step 3 in this AuNC/acetonitrile solution (5 mL) for 2 h leads to the covalent bonding between the vertices or edges of the AuNCs and the terminal thiol group of the C8DT SAMs on the AuNSs, completing the formation of AuNC−AuNS dimers. Electrostatic repulsion prevents the CTA-capped AuNCs from adsorbing on the amine-coated glass slide and thus minimizes the AuNC monomer formation. The final AuNC−AuNS dimers can be prepared either as adsorbed on the glass slide or as a colloidal dispersion in a solution by simply sonicating the glass slide after step 4. Scanning electron microscopy (SEM) and TEM images indicate that the AuNC−AuNS dimers were successfully produced. Figures 3a and 3b show typical SEM and TEM images of the AuNC−AuNS dimers adsorbed on glass slides and dispersed in solution, respectively. More images are available in the Supporting Information (Figure S2). Inspecting 655 particles from three sets of experiments reveals that the AuNC−AuNS dimers constitute 71% of the total particles (Figure 3c). Additionally, the observed monomers (18%) are all nanospheres, suggesting that the electrostatic repulsion does prevent the positively charged CTA-capped AuNCs from adsorbing on the amine-coated glass surfaces (see step 4 in Figure 2). Trimers (6%) are in the form of either one AuNC bridging two neighboring AuNSs or two AuNCs attached to one AuNS. Multimers (5%) mostly consist of a single AuNS with multiple AuNCs, presumably induced by the destabilized AuNCs due to the acetonitrile. Close examination of the SEM images also reveals that most nanocubes adsorb on nanospheres via their vertices or edges (Figures 3d,e). These results

Figure 3. Representative (a) SEM and (b) TEM images of the prepared AuNC−AuNS dimers on glass slides and in solution, respectively. (c) Percent yields for the AuNC−AuNS nanoparticle assemblies prepared on glass slides based on 655 particles. (d, e) Magnified SEM images showing the nanocubes adsorbed on the nanospheres at their vertex and edge.

strongly suggest that acetonitrile removes the CTA bilayers from the vertices or edges of AuNCs, where they are less ordered, promoting the covalent bonding of the terminal thiol on the AuNSs to these regions of the AuNC surfaces. Without acetonitrile, the CTA-protected AuNCs do not bind to the thiol-functionalized AuNSs on the glass slides (Figure S3 in the Supporting Information). 2.3. Plasmon Coupling. Because AuNCs and AuNSs interact with each other in close proximity, new resonance bands arise from their surface plasmon coupling. The UV−vis extinction spectrum for the AuNC−AuNS dimers dispersed in solution exhibits two distinctive bands (Figure 4a). One band appears at 564 nm, between the SPR bands of isolated AuNSs (λ = 534 nm) and AuNCs (λ = 581 nm). The other band appears at 790 nm, which is greatly red-shifted from the SPR bands of the component nanoparticles. 7875


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L1 and T1 band correspond to the dipolar plasmon coupling between the AuNC and the AuNS along and across the interparticle axis, thus representing longitudinal and transverse dipolar plasmon coupling band, respectively. In contrast, the L2 band arises from the multipolar coupling between the AuNC and the AuNS induced by the longitudinal polarization excitation. Figure 4d illustrates the plasmon coupling scheme for the AuNC−AuNS dimers in the framework of the plasmon hybridization model.33,34 The in-phase dipolar plasmon coupling along the interparticle axis (L1) gives rise to the lowest energy resonance band for the dimer. Both the out-ofphase transverse dipolar coupling (T1) and the longitudinal multipolar plasmon coupling (L2) contribute to the higher energy resonance band. The other hybridization modes such as longitudinal antibonding dipole mode and transverse bonding dipole mode are probably damped by the interband transition of Au nanoparticles in the UV region. We obtained a similar extinction spectrum for AuNC−AuNS dimers formed using the AuNC edge (Figure S5, Supporting Information). The only difference is that the transverse dipolar coupling and the longitudinal multipolar coupling is more disparate in energy for those edge-linked dimers than for the vertex-linked dimers, resulting in the broader resonance band with a shoulder in the 600 nm wavelength region for the former. 2.4. SERS. The large red-shift of the longitudinal plasmon coupling band relative to the SPR bands of the individual component nanoparticles (Δλ > 200 nm) suggests that the AuNCs and AuNSs strongly interact with each other as they form dimers. In addition, the dimer assembly via the AuNC vertices is likely to enhance local electric fields significantly at the gap junction due to the lightning rod effect. The resonance plasmon coupling wavelength for the AuNC−AuNS dimers is also in the near-IR region, which is perfectly suited for many biomedical applications. These conditions consistently indicate that AuNC−AuNS dimers must be an excellent SERS substrate. This led us to explore the SERS activity of the AuNC−AuNS dimers in comparison with other nanoassemblies. We acquired Raman spectra for AuNC−AuNS dimers prepared on glass slides (Figure 5a). The observed Raman peaks correspond to the vibrational modes of C8DT SAMs linking the AuNC and AuNS as a dimer.35,36 Notably, the strong Raman peak of the C−S stretching mode from the trans form of C8DT (720 cm−1), marked with a star, and the absence of a peak from the gauche form (620−660 cm−1) indicate that C8DT constructs highly ordered SAM structures between AuNCs and AuNSs.35,36 The intense Raman signal observed for the AuNC−AuNS dimers, despite the small number of C8DT molecules confined to the nanogap region between the AuNC vertex and AuNS surface, confirms that strong SERS occurs in the dimers. For the calculation of the SERS enhancement factor (EF), we obtained the Raman spectra from several spots on the glass slides supporting the AuNC−AuNS dimers and averaged the intensity of the νT(C−S) Raman peak (ISERS in Table S1 and Figure S7, Supporting Information). From the measurements, we conservatively estimate the SERS EF for the AuNC− AuNS dimers at (1.2 ± 0.2) × 1010. The numerical parameters used in this calculation are listed in the Supporting Information with references (Table S1). The SERS EF obtained for the AuNC−AuNS dimers, on the order of 1010, is significantly larger than is typical for other nanoassembly structures. For comparison, we prepared two

Figure 4. (a) UV−vis extinction spectrum for the AuNC−AuNS dimers dispersed in solution (solid line). For comparison, the UV−vis extinction spectra for the AuNSs (dotted line) and AuNCs (dashed line) used to assemble these dimers are included after being normalized to the 564 nm band intensity. The inset is a photograph of the sample containing the dispersed AuNC−AuNS dimers, illuminated with white light. (b) Simulated extinction spectrum for a AuNC−AuNS dimer with the AuNC bound to the AuNS through its vertex with a gap distance of 1 nm (black line). For assignment of the bands, extinction spectra obtained by excitation of the dimer with the light polarization parallel (red line) or perpendicular (blue line) to the interparticle axis are included. (c) Calculated charge density distribution for the T1, L1, and L2 bands labeled in (b). (d) Plasmon coupling diagram for the AuNC−AuNS dimers derived from the simulations.

We performed FDTD simulations to find the origin of the new resonance bands. We reproduced experimental conditions in the simulations. The vertex of a AuNC (84 nm) and the surface of a AuNS (56 nm) was spaced by 1.0 nm (two grids of the 0.5 nm mesh) to simulate the gap distance of the dimers linked by the C8DT SAMs (∼1.3 nm).15 Then, the electromagnetic wave was allowed to propagate through the center of the dimer at the origin with a polarization parallel (longitudinal excitation) or perpendicular (transverse excitation) to the interparticle axis of the AuNC−AuNS dimer (Figure S4, Supporting Information). Figure 4b shows that the simulated total extinction spectrum closely resembles the measured one. The simulation also reveals that the longer wavelength resonance band originates from the excitation of the dimers with a light polarization along the interparticle axis (longitudinal plasmon coupling, labeled L1). The resonance band near the SPR bands of the component nanoparticles is mostly due to transverse plasmon coupling (T1). It stands to reason that weak coupling between the AuNC and the AuNS in the transverse direction yields their own monomer SPR bands, which lie in this spectral region. We also note that there is an additional contribution to the band from the longitudinal plasmon coupling, designated L2. The nature of each plasmon coupling band is clearly revealed by the charge density distribution calculation (Figure 4c). The 7876


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Figure 5. Representative SEM images, selected Raman spectra, and UV−vis extinction spectra for the nanoassemblies supported on glass slides: (a) AuNC (84 nm)−AuNS (55 nm) dimers, (b) AuNS core (92 nm)−AuNS satellite (55 nm) assemblies, and (c) AuNS clusters that consist of one 55 nm AuNS and three 58 nm AuNSs. In the Raman spectra, νT, ρ, and δ denote the stretching (of trans form), rocking, and bending vibrational modes, respectively. The red dashed line in the UV−vis spectra marks the wavelength of the Raman excitation laser used to acquire the Raman spectra.

Figure 6. FDTD simulation of local electric field distribution around (a) on-vertex and (b) on-edge AuNC−AuNS dimers with a gap distance of 1 nm upon excitation with transverse (middle column) and longitudinal (right column) polarization. The calculated local field enhancement factor for SERS, |Emax/E0|4, is inscribed on each panel. The scale bars represent 50 nm.

nanoassemblies consisting of only spherical nanoparticles: core−satellites and clusters. We employed the masked desilanization method to assemble the AuNSs into the desired structures on glass slides. 14 Figures 5b and 5c show representative SEM images for the resulting nanoassemblies. The core−satellite assembly structures consist of a AuNS core (92 nm) surrounded by 7 ± 1 satellite AuNSs (55 nm). The

clusters contain three or four AuNSs of similar size. In our experiments, the clusters form with one AuNS (55 nm) at the center and 3 ± 1 citrate-capped AuNSs (58 nm) linked to this center via C8DT SAMs. We obtained Raman spectra for these assembly structures. The overall spectral features are identical to the SERS spectrum of the AuNC−AuNS dimers, indicating that SERS is also observed for C8DT using these nanosphere 7877


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Figure 7. FDTD simulation of electric field distribution (middle column) and extinction spectrum (blue line, right column) for (a) a AuNC−AuNS dimer, (b) a core−satellite structure, and (c) a cluster. The electric field distribution was calculated at the peak wavelength of the extinction spectrum using the polarization of light denoted by E0. The presented spatial distribution of electric fields is a 2-dimensional slice through the 3-dimensional model. The scale bars represent 50 nm. The calculated local field enhancement factor for SERS, |Emax/E0|4, is inscribed on each electric field distribution panel. Experimentally measured UV−vis extinction spectra for each nanoassembly using unpolarized light are reproduced from Figure 5 and included in the right column for comparison (gray line).

FDTD simulations corroborate the local field enhancement at the nanogap junction for AuNC−AuNS dimers. Figure 6 presents the electric field distribution for the AuNC−AuNS dimers as induced by transverse and longitudinal plasmon excitations. The simulation results indicate that the strongly enhanced electric field is confined to the very narrow nanogap region (1 nm) between the AuNC and AuNS upon longitudinal plasmon coupling excitation regardless of whether the AuNC binds to AuNS via its vertex or edge. The maximum local field enhancement relevant to SERS, |Emax/E0|4, corresponds to ∼1011, roughly consistent with our experimental EF. Similar FDTD simulations for core−satellites and clusters reveal that AuNC−AuNS dimers are more effective SERS substrates than the nanoassembly structures consisting of AuNSs. We calculated the extinction spectrum for each nanoassembly structure and then obtained the electric field distribution induced by excitation at the resonant wavelength. The polarization of light (E0) was set such that the greatest possible plasmon coupling is achieved. Figure 7 shows that the peak position of the simulated extinction spectrum for the chosen polarization matches the most red-shifted plasmon coupling band in the measured UV−vis spectrum. Upon resonant excitation at those wavelengths, the electric field is strongly localized in the nanogaps between the nanoparticles.

assembly structures. The Raman intensity, however, is markedly different between the AuNC−AuNS dimers and the AuNS assemblies. The latter produces weaker Raman signals than the former despite having more hot spots. The EF of the core− satellites and the clusters are measured at (4.4 ± 1.0) × 108 and (2.5 ± 1.1) × 107, respectively (Table S1). Because these nanoassemblies all had the same interparticle distances corresponding to the C8DT SAMs, we believe that it is the nanoparticle shape at the nanogap junction that causes the differing SERS EFs. The nanogaps in the AuNC−AuNS dimers are formed between the sharp curvature of the AuNCs (vertices and edges) and round AuNS surfaces, whereas smooth surfaces form the gap for core−satellite and AuNS clusters. Therefore, the local electric field is more strongly enhanced in the AuNC− AuNS dimers, which significantly increases the SERS EF. The resonant interaction between the Raman excitation wavelength (785 nm) and the plasmon coupling band may also contribute to enhancing the Raman signal. However, the effect seems less important because the core−satellites support larger SERS than the clusters even though their plasmon coupling band is farther away from the Raman excitation laser wavelength (Figure 5). FDTD simulations also show that even with the resonant excitation for all three nanoassemblies, AuNC−AuNS dimers yield the largest SERS EF (vide inf ra). 7878


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The Journal of Physical Chemistry C The maximum local field enhancement for the core−satellites and the clusters yields ∼1010 and ∼109, respectively. Although these values are higher than the measured ones presumably due to the resonant excitation contrary to the off-resonance interaction between the plasmon coupling band and the Raman excitation laser wavelength in experiments, they are still lower than the value for the cube−sphere dimers (∼1011). Thus, the FDTD simulation results suggest consistently with the experiments that the cube−sphere dimers should be better SERS substrates than the core−satellites or the clusters. Finally, we note that the SERS EF of the AuNC−AuNS dimers we obtained is very large considering that it was measured from the ensembles of dimers. While many single particle measurements routinely yield EFs in the range of 1011− 1014, the ensemble-averaged value is usually much lower because of the inhomogeneous hot spot characteristics.37,38 For example, in case the ensemble contains a mixture of nanoassemblies where some structures have numerous hot spots while others have none, averaging the EF results in the lower value than is obtained from a single nanoassembly structure. The variation in the gap distance among nanoassemblies also contributes to lowering the EF in the ensemble measurements. We believe that the high purity of the AuNC− AuNS dimers and the well-defined small gap distance (∼1.3 nm C8DT SAMs) in our experiments lead to the large ensembleaveraged SERS EF comparable to the EF of a single nanoassembly.

4. EXPERIMENTAL SECTION 4.1. Materials. The following compounds were purchased and used without further purification: gold(III) chloride trihydrate (HAuCl4·3H2O, ≥99.9%, Aldrich), cetyltrimethylammonium chloride (CTAC, C16H33N(CH3)3Cl, 95.0%, TCI), sodium borohydride (NaBH4, 99%, Aldrich), L-ascorbic acid (C6H8O6, 99%, Aldrich), sodium bromide (NaBr, ≥99%, Aldrich), sodium citrate tribasic dehydrate (C6H9Na3O9, ≥99.0%, Aldrich), (3-aminopropyl)trimethoxysilane (APTMS, C 6 H 17 NO 3 Si, 97%, Aldrich), 1,8-octanedithiol (C8DT, C8H18S2, ≥97%, Aldrich), HCl (35.0−37.0%, Duksan Chemical), RBS detergent solution (35 concentrate, Fluka), acetonitrile (CH3CN, 99.5%, Duksan Chemical), ethanol (≥99.9%, Duksan Chemical), and water (J.T. Baker, HPLC grade). 4.2. Synthesis of AuNCs. We adopted the method developed by Huang and co-workers using a scale-up factor of 5.28 Au seeds were prepared by reducing Au3+ with NaBH4. An ice-cold aqueous solution of NaBH4 (0.02 M, 0.450 mL) was rapidly added to a 10 mL HAuCl4 (2.5 × 10−4 M) and CTAC (0.10 M) solution while stirring at 600 rpm. Upon this addition, the solution turned brown, indicating that Au seed particles 3−4 nm in size formed.28 The solution was mixed further for 2 min and placed in a 30 °C water bath for 1 h to decompose the remaining unreacted borohydride. We prepared two growth solutions to grow the seed particles, labeled A and B. In vial A, CTAC (0.32 g) was fully dissolved in 9.625 mL of deionized water at 27−28 °C. HAuCl4 (0.01 M, 0.250 mL), NaBr (0.01 M, 0.010 mL), and ascorbic acid (0.04 M, 0.090 mL) were then sequentially added to vial A while stirring at 500 rpm. In vial B, CTAC (1.6 g) was dissolved in 48.125 mL of deionized water; HAuCl4 (0.01 M, 1.250 mL), NaBr (0.01 M, 0.050 mL), and ascorbic acid (0.04 M, 0.450 mL) were sequentially added while stirring at 700 rpm. The growth solution color changed from yellow to colorless as the ascorbic acid reduced the Au3+ to Au+. Finally, 0.025 mL of the seed solution was added to the growth solution A while stirring at 500 rpm. After 10−13 s, the color turned bright pink, and 0.125 mL of solution A was transferred to solution B while stirring at 700 rpm. After stirring for 20 s, the solution was left unperturbed at 27−28 °C for 30 min to promote particle growth. The final solution (50 mL) was divided into 10 vials and centrifuged twice at 890g for 10 min to remove residual CTAC; each precipitate was redispersed in 2 mL of water and combined into one flask later. 4.3. Synthesis of AuNSs. We synthesized the AuNSs via the method developed by Puntes and co-workers with slight modification.15,29 This synthesis is based on the repeated growth of seed particles in the presence of HAuCl4 and citrate until the desired size is achieved. The Au seed particles were synthesized by mixing sodium citrate (60 mM) and HAuCl4 (25 mM) solutions in 150 mL of water at 100 °C while stirring at 1000 rpm. The solution exhibited a light red-wine color as the seed particles formed. To grow the seed particles, we prepared stock solutions of HAuCl4 (25 mM) and sodium citrate (60 mM) that were added to the seed solution as detailed in the Supporting Information. Table S2 summarizes the reaction conditions and characteristics for the 55.4, 58.3, and 92.4 nm AuNSs used to assemble the AuNC−AuNS dimers, AuNS core−satellites, and AuNS clusters. 4.4. Assembly of AuNC−AuNS Dimers. AuNC−AuNS dimers were prepared on glass slides via a four-step process.

3. CONCLUSIONS Here, we reported the assembly and plasmon properties of AuNC−AuNS heterodimers. These dimers were prepared in high yield by exploiting the collapse of CTA bilayers at the AuNC vertices and edges using acetonitrile and electrostatic repulsion between the AuNCs and glass surface. This strategy yielded the AuNC−AuNS dimers where the AuNC vertices or edges were covalently linked to the AuNS surfaces through C8DT SAMs. The SEM and TEM analysis revealed that over 70% of the particles formed AuNC−AuNS dimers. The nanogap formed between the AuNC vertex or edge and round AuNS surface defines the plasmonic properties for the AuNC−AuNS dimers. We observed two extinction bands at 564 and 790 nm in the UV−vis extinction spectra for the AuNC−AuNS dimers. From the comparison with our FDTD simulation results, the former and latter peaks were attributed to the transverse and longitudinal plasmon coupling modes, respectively. We also found that the AuNC−AuNS dimers were an excellent SERS substrate. The Raman spectra observed for C8DT in the nanogap between the AuNCs and AuNSs yielded an estimated SERS EF of 1 × 1010. We measured the EFs for other nanoassemblies consisting of only AuNSs, such as core− satellites and clusters. The EF was 2 or 3 orders of magnitude larger for the AuNC−AuNS dimers than for these AuNS nanoassemblies, which suggests that the AuNC vertex or edge forming the nanogap plays an important role in enhancing the Raman scattering intensity. The FDTD simulation indicated that the local electric field was strongly confined to the nanogap between the AuNC vertex or edge and round AuNS surface with an enhancement factor of ∼1011, consistent with our experimental observations. 7879


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The Journal of Physical Chemistry C Step 1. Amine-coating of glass slides. A glass slide (25 mm × 12 mm, Marienfeld, Germany) was cleaned by immersing in a 1:1 (v/v) mixture of RBS detergent solution and deionized water at 90 °C and sonicating for 5 min. After washing with water, the glass slide was immersed in a methanol and HCl solution (1:1, v/v) for 30 min to protonate the silanol groups on its surface. The glass slide was then washed and dried in an oven at 100 °C for 2 h. The glass slide was immersed in a 1% APTMS solution in ethanol for 30 min to functionalize its surface with amine. The residual APTMS on the glass surfaces was thoroughly removed after the reaction by repeatedly washing, rinsing, and sonicating with ethanol. The amine-coated glass slide was dried in an oven at 120 °C for 3 h. Step 2. Adsorption of AuNSs on the glass slides. Citrate-capped AuNSs (55.4 nm) were anchored to the amine-functionalized glass slide via electrostatic interactions. To ensure that the amines on the glass slide were protonated, the AuNS solution (11 pM, 5 mL) was adjusted to a pH of ∼4.0 by adding 10 μL of an HCl solution (2% v/v). The glass slide was then immersed in the AuNS solution for 12 h. Unreacted AuNSs were removed by washing the glass slide with water and ethanol (99.9%) over three times. Step 3. Thiolf unctionalization of AuNSs. The adsorbed AuNS were surfacefunctionalized with thiol. Immersing the AuNS-anchored glass slide in an ethanol solution of C8DT (1 mM, 5 mL) for 6 h formed a dithiol SAM. Step 4. Adsorption of AuNCs onto the AuNSs. To covalently bind thiols on the AuNS to the AuNC surface, the CTA bilayer on the AuNC must be at least partially disintegrated. The AuNC solution (4.5 pM, 5 mL) was centrifuged at 890g for 10 min; the precipitate was redispersed in a 5 mL mixture of acetonitrile and water (9:1 v/v). The glass slide from step 3 was immersed in the AuNC/acetonitrile solution for 2 h while stirring at 500 rpm. Desorption of AuNC− AuNS dimers. The characterization and optical measurements of the AuNS−AuNC dimers were mostly performed using dimers adsorbed on the glass slide. When dimers in a dispersed colloidal state were required (such as for the TEM sampling), the glass slide from the final step was immersed in an acetonitrile solution and sonicated for 60 s (B2510-DTH, Bronson). 4.5. Assembly of AuNS Core−Satellites and Clusters. The assembly procedure for core−satellites and clusters containing a few AuNSs was described in a prior publication.14 This assembly method is based on masked desilanization. Step 1. Amine-coating of glass slides. The glass was cleaned and aminecoated with APTMS as described above. Step 2. Adsorption of the f irst AuNSs on the glass slides. Amine-coated glass slides were immersed in AuNS solutions (92.4 nm, 10 pM, 5 mL for core− satellites and 55.4 nm, 22 pM, 5 mL for clusters) for 12 h. Step 3. Masked desilanization of glass slides. The amine coating on the glass slide was removed by immersing the AuNS-anchored glass slides in NaOH solution (1 mM, 5 mL) for 5 h. Step 4. Thiolf unctionalization of the f irst AuNSs. The surface of the AuNSs adsorbed on the glass slides was thiol-functionalized by immersing in a 1 mM ethanol solution of C8DT for 12 h. Step 5. Adsorption of the second AuNSs onto the f irst AuNSs. The citrate-capped AuNSs were adsorbed on the thiol-functionalized AuNSs. The glass slide was immersed in a AuNS solution (55.4 nm, 67 pM, 5 mL for core−satellites and 58.3 nm, 74 pM, 5 mL for clusters) for 8 h. 4.6. Measurements. TEM (JEM-2100F, JEOL) measures the shape and size of the AuNCs and AuNSs. The AuNC− AuNS dimers prepared on a glass slide were monitored by SEM (S-4800, Hitachi). The dispersed AuNC−AuNS dimers in

solution were sampled on a TEM grid (Ultrathin Carbon TypeA 01822-F, Ted Pella) 30 times to obtain sufficient density and subjected to TEM. The SPR and plasmon coupling were measured using a UV−vis spectrometer (Lambda 25, PerkinElmer). The Raman spectra were obtained using a Raman microscope (Raman MicroProbe, Kaiser). A 785 nm laser was focused on a sample through a 50× objective unless noted otherwise. The Raman scattering was back-collected using the same objective and transmitted to a spectrometer (HoloSpec, Kaiser). The SERS spectra from the nanoassemblies were obtained using assemblies adsorbed on the glass slides. The Raman spectra for C8DT were measured using a neat solution in a quartz cell with a 10× objective. 4.7. FDTD Simulation. The FDTD simulations were performed using a calculation package from Lumerical Solution, Inc. (FDTD Solutions ver. 8.9). A AuNC−AuNS dimer was set so that the AuNC (edge length = 84.0 nm, corner curvature radius = 10.0 nm) and the AuNS (diameter = 56.0 nm) were spaced with 1.0 nm between either the AuNC vertex or edge and AuNS surface. To simulate core−satellite and cluster nanoassemblies, AuNSs in diameters of 92 nm (core), 56 nm (satellite), 56 nm (cluster core), and 58 nm (cluster satellite) were used. We adopted the dielectric constant for the AuNC and AuNS from Johnson and Christy.39 Other calculation parameters include 1500 × 1500 × 1500 nm3 for the simulated region, 500 fs for the simulated time (1000 fs for core− satellites), and 1.3441 and 1.3614 for the refractive index of acetonitrile (for AuNC−AuNS dimer) and ethanol (for core− satellites and clusters), respectively. To improve calculation accuracy, we set an override region for a 150 × 150 × 300 nm3 box with a mesh size of 0.5 nm with the dimer in the center of the box. The extinction cross sections were calculated for a plane wave with a 400−1100 nm wavelength, propagating along the x, y, and z axes (Figure S4, Supporting Information). The local electric field distribution and the charge density distribution were calculated for the excitation of light at the wavelengths corresponding to the maximum transverse and longitudinal coupling bands with parallel or perpendicular polarization to the interparticle axis.

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ASSOCIATED CONTENT

S Supporting Information *

Measurements of concentration and extinction coefficient of AuNCs, characterization data for the prepared AuNSs, more SEM and TEM images for AuNC−AuNS dimers, results of the control experiments on the assembly of AuNC−AuNS dimers without acetonitrile, details of the FDTD simulations, comparison of simulated extinction spectra between the vertexand the edge-linked AuNC−AuNS dimers, calculations of the SERS EFs for AuNC−AuNS dimers, AuNS core−satellites, and AuNS clusters, and synthetic procedures for AuNSs with various sizes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.Y.). Notes

The authors declare no competing financial interest. 7880


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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2013R1A1A2008336).

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