Self-Assembled Plasmonic Dimers of Amphiphilic Gold Nanocrystals

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Self-Assembled Plasmonic Dimers of Amphiphilic Gold Nanocrystals Lin Cheng,†,‡ Jibin Song,† Jun Yin,† and Hongwei Duan*,† † ‡

School of Chemical and Biomedical Engineering, Nanyang Technological University, 70 Nanyang Drive, Singapore 637457 College of Chemistry and Materials Science, Anhui Normal University, Wuhu, 241000, China

bS Supporting Information ABSTRACT: We report a new strategy to assemble large (>40 nm) gold nanoparticles with isotropic surface chemistry into anisotropic plasmonic dimers by taking advantage of the chain reorganization of the amphiphilic polymer brushes grafted on nanoparticle surfaces in selective solvents. Production of high-purity dimers is of considerable interest for applications requiring strong near-field coupling of surface plasmon resonances. The formation of nanoparticle dimers is confirmed by imaging and spectroscopic characterization at both bulk and single-particle levels. The interparticle plasmonic coupling can be reversibly controlled by modulating the assembly/disassembly of the amphiphilic nanocrystals. The general applicability of surface modification of nanocrystals of diverse chemical compositions and morphologies through tandem “grafting to” and “grafting from” reactions offers the possibility to extend this concept to other types of functional nanocrystals. SECTION: Nanoparticles and Nanostructures

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he development of superstructures of metal and semiconductor nanocrystals is under intense research for technological applications ranging from ultrasensitive detection and bioimaging to optoelectronic devices.1,2 The enormous efforts are mainly driven by the interest in taking advantage of the integrated and collective properties induced by interparticle coupling in the superstructures.1
3 Recent advances in morphology-controlled synthesis of monodisperse nanocrystals of diverse chemical compositions and the ability to tailor their surface chemistry have opened new opportunities to direct the organization of nanocrystals into discrete assemblies with well-defined geometry and association numbers, such as dimeric and trimeric structures.4,5 Current research has focused primarily on sitespecific surface modification of nanocrystals with functionalities such as DNA and small molecules, leading to anisotropically functionalized nanoparticles for further directional linking.2,6
13 For example, metal nanoparticles with discrete numbers of DNA strands could be isolated by gel electrophoresis and highperformance liquid chromatography and further hybridized into dimers, trimers, and other hierarchical structures.6
10 Dimers and trimers of nanocrystals smaller than 20 nm have also been obtained via encapsulation of nanocrystals under controlled aggregation, followed by separation using differential centrifugation.14,15 These model systems are of fundamental interest in understanding interparticle coupling, i.e., coupling of plasmons, at the single-particle level, and elucidating the effect of electromagnetic hot-spots on the enhancement of optical signals such as Raman scattering and fluorescence.16
19 However, it remains a challenge for these self-assembly approaches to produce dimers of large nanoparticles (>20 nm), since the purification techniques undertaken are not effective for large nanoparticles. r 2011 American Chemical Society

Here we report a new strategy to assemble large (>40 nm) gold nanocrystals with isotropic surface chemistry into plasmonic dimers in high yield. Specifically, our results have shown that gold nanocrystals with well-defined amphiphilic mixed polymer brush coatings preferentially form dimers in water due to collapse and aggregation of hydrophobic grafts and reorganization of hydrophilic grafts, as illustrated in Figure 1. Unlike the method of using nanoparticles with anisotropic surface chemistry to build-up higher-order structures, self-assembly of the amphiphilic nanoparticles does not rely on asymmetric functionalization of individual nanoparticles. Furthermore, the ability to assemble large (>40 nm) gold nanoparticles into high-purity dimers are of considerable interest for applications requiring strong near-field coupling of surface plasmon resonances, such as surface-enhanced Raman scattering.20,21 Another interesting feature of this strategy is that the dimer
monomer structural transition and the interparticle plasmonic coupling can be reversibly modulated by directing the assembly/disassembly of the amphiphilic nanocrystals. Amphiphilicity-driven self-assembly of block copolymers has been well-documented, and the impact of structural parameters on morphologies of the resultant assemblies has been systematically studied.22 When metal and semiconductor nanocrystals are grafted with polymer brushes, the hybrid nanoparticles inherit the self-assembly property of their coating materials. Interesting structures including vesicles and filaments with nanoparticles embedded have been reported.23
27 Nonetheless, assemblies composed of defined numbers of nanoparticle building blocks have not been achieved. Generally, nanoparticles with Received: July 25, 2011 Accepted: August 19, 2011 Published: August 19, 2011 2258

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The Journal of Physical Chemistry Letters amphiphilic polymer brushes are prepared either by chemical reduction of a metal precursor in the presence of two types of end-functionalized polymers or through the reaction of a functional group at the joint of block copolymers with nanocrystal surfaces.28,29 However, both of these approaches lack the flexibility to simultaneously control the size of the nanocrystals and the structural parameter of the mixed polymer brushes. In this study, gold nanoparticles coated with mixed polymer brushes of poly(ethylene glycol) (PEG) and poly(methyl methacrylate) (PMMA) were prepared via the tandem “grafting to” (coadsorption of PEG and initiators for atom transfer radical polymerization on gold nanocrystals through Au
S bonds) and “grafting from” (surface initiated polymerization of MMA) reactions.27,30 This approach allows for the integration of chemically distinct hydrophobic and hydrophilic polymers and flexible control over important structural parameters (ratio, molecular weight, and graft density) of the two polymer grafts on the uniform nanocrystal scaffolds synthesized using wellestablished methods. Optimized gold nanoparticles (42 nm in size) with 1215 PEG (Mn = 5 KDa) and 1621 PMMA (Mn = 23.6 KDa, PDI = 1.33) grafts (graft density = 0.5 chain/nm2) have been used as building blocks for the dimeric structure (see Supporting Information). These nanoparticles (Au@PEG/ PMMA) show excellent solubility in the common solvents of PEG and PMMA such as chloroform, dimethylformamide (DMF) and dimethyl sulfoxide. Self-assembly of nanoparticles can be induced by adding selective solvents, i.e., water, for the hydrophilic PEG into DMF solution of Au@PEG/PMMA nanoparticles (5.0 nM). When the volume fraction of water reached 80%, the solution color changes from red to purple, and

Figure 1. Schematic illustration of the self-assembly of amphiphilic gold nanocrystals coated with PEG and PMMA mixed polymer brushes.

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the final solution after removing DMF residue by dialysis against deionized (DI) water is clear and remains colloidally stable for at least 3 months. The interparticle coupling-induced color change and the absence of macroscopic aggregates indicate the formation of nanoparticle clusters in water. Figure 2a is a representative transmission electron microscope (TEM) image of the assembly, showing a dominant population of nanoparticle dimers. TEM images (Figure 2b) at higher magnification reveal a uniform gap of 2
3 nm between the nanoparticles. Statistical analysis (Figure 2c) on 661 of monomers, dimers and multimers in TEM images shows that more than 60% of them are dimers. As depicted in Figure 1, both of the PEG and PMMA grafts should exist in stretched brush conformation in their common solvents. Upon the solvent environment changing from a common solvent, i.e., DMF, to a selective solvent, i.e., water, for PEG, PMMA chains on adjacent nanoparticles aggregate and collapse to form hydrophobic domains at the nanoparticle junction, which drives conformational reorganization of PEG grafts from the junction toward the noncontacting area of the two nanoparticles. The depletion of PEG chains from the junction of the nanoparticle pair increases their grafting density in the noncontacting area.26 Kinetically, when the density of PEG grafts is high enough, participation of additional nanoparticles into the assembly is inhibited due to the steric hindrance of the PEG grafts, leading to the “frozen” dimeric structure. Given the large size of the nanoparticle scaffold, it is expected that not all of PMMA chains could take part in the hydrophobic domains at the nanoparticle junction, and the PMMA away from the contacting area would collapse to form localized domains, which are shielded by the extended PEG grafts. Both the size of the gold nanoparticle and the relative ratio of PEG and PMMA are critical for the preferential formation of the dimers. Our results show that smaller nanoparticles (14 nm) with similar mixed polymer brushes mainly form monomers in water, which probably results from the minimized interparticle association due to the better shielding effect of PEG chains for small nanoparticles. On the other hand, decreasing the fraction of PEG grafts reduces the shielding effect. For example, when the ratio of PMMA and PEG is over 5, unstable large aggregates containing tens of nanoparticles are obtained. UV
vis spectra (Figure 3a) of the dimer solution show a pronounced shoulder at 640 nm in addition to the lower wavelength peak at around 540 nm, which is a characteristic spectroscopic feature of anisotropic gold nanorods.31 Pileni et al.

Figure 2. TEM images of the assembly at low (a) and high (b) magnification. The dimers are highlighted by black ellipses. Inset in panel a is a photograph of a DMF solution of the gold nanocrystal (left) and the dimer dispersion in water (right). (c) The statistical fractions of monomer, dimer, and multimeric structures. 2259

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The Journal of Physical Chemistry Letters

Figure 3. Self-assembly and disassembly of the amphiphilic nanoparticles. (a) UV
vis spectra of the amphiphilic nanoparticle in DMF (red line) and the dimers assembled from gold nanoparticles at different concentrations: black line (5.0 nM), blue line (3.0 nM), and green line (2.0 nM). (b) Absorption at the longitudinal peak (640 nm) of the dimers as a function of the volume fraction of DMF in the solution.

reported an equation derived from the discrete dipole approximation method to correlate the longitudinal peak of gold nanorods to their aspect ratios.32 Here, an aspect ratio of 2.3 is obtained from the longitudinal peak of 640 nm, further confirming the formation of high-purity dimers in the solution. Unlike gold nanorods, the longitudinal peak of the nanoparticle dimer is weaker than the transverse peak, which should be due to the presence of the gap of 2
3 nm in width between the nanoparticles. This is consistent with the spectroscopic profile of highpurity dimers of metal nanoparticles previously reported.14,17 The optimal starting concentration of gold nanoparticles in DMF is in the range of 4.0
8.0 nM, and more dimers are obtained at a higher concentration within this range (Figure 3a). This is understandable because higher concentration of gold nanoparticles increases the chance of interparticle association in the selfassembly process. The dimer
monomer structures can be reversibly tuned by changing solvent quality between selective solvents and common solvents for PEG and PMMA. As shown in

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Figure 4. Imaging and spectroscopic detection of single monomers and dimers of the amphiphilic gold nanoparticles by dark-field microscopy. (a) Dark-field image: single gold nanoparticle appears green, and dimers are yellow/orange in color. (b) Scattering spectra of the single particle: monomer (red line) and dimer (blue line).

Figure 3b, adding 95% (vol) of DMF leads to complete dissociation of the dimers into individual nanoparticles, suggested by the disappearance of the 640 nm longitudinal peaks (see Supporting Information) and a color change from purple to red. One interesting finding is that the dimer remains stable until the volume fraction of DMF reaches 85%, and quickly dissociates in the range of 85
95%. The nanoparticle dispersion can be concentrated and reused for self-assembly. The process of recovery-assembly can be repeated three times without affecting the colloidal stability of the nanoparticle. However, each cycle of recovery led to 10
15% loss of the nanoparticles. The use of 42 nm gold nanoparticles as building blocks allows for detecting the scattering spectra at the single-particle level by a dark-field microscopy. When illuminated by unpolarized white light through a dark-field condenser, particles spread on a coverslip are clearly visible, and the image (Figure 4a) collected by a colored charge-coupled device (CCD) reveals green monomers and yellow/orange dimers.6 Representative scattering spectrum (Figure 4b) of the dimers shows a characteristic 2260

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The Journal of Physical Chemistry Letters two-peak scattering profile, resembling the ensemble-averaged extinction spectrum in Figure 3a. The pronounced red-shift of the scattering spectra of dimers relative to that of the monomers is obviously due to the interparticle plasmon coupling, and has been observed in other studies on metal nanoparticle dimers.6,7 The higher scattering intensity of dimers results from their larger scattering cross-section than monomers.6,7,16 The ability to detect color and spectroscopic variations at the single-particle level demonstrates the possibility to develop gold nanoparticlebased ultrasensitive nanosensors by taking advantage of the stimuli-responsive assembly/disassembly of gold nanoparticles with smart polymer brush coatings. Note that the scattering image and spectra can be collected continuously without photobleaching, offering great advantages over fluorescence-based assays. In summary, we have presented a straightforward approach to assembling isotropic nanocrystals with amphiphilic polymer brush coatings into anisotropic dimeric structures in high purity. The steric hindrance generated by reorganization of flexible hydrophilic polymer brushes during the interparticle association is critical for the morphological selectivity in the assembly. These plasmonic dimers can potentially serve as a new class of nanoplatforms for enhancing optical signals and plasmonic sensors. The general applicability of surface modification of nanocrystals through two-step surface modification opens up the possibility to extend this concept to other types of functional nanocrystals.33

’ EXPERIMENTAL SECTION Uniform 13 nm gold nanocrystals were synthesized by citrate reduction of HAuCl4 in DI water. Typically, a DI water solution (2 mL) of sodium citrate (102 mg) was rapidly added into a boiling aqueous solution of HAuCl4 (30 mg in 200 mL of DI water) under vigorous stirring. The heat source was removed 15 min later, and the dispersion was cooled to room temperature. Large Au nanoparticles with diameter of 42 nm were synthesized using 13 nm nanoparticles as seeds. Briefly, a freshly prepared aqueous HAuCl4 solution (15 mg in 100 mL of DI water) was heated to boiling, followed by the successive injection of the asprepared seed solution (10 mL, 13 nm Au nanoparticles) and sodium citrate solution (0.6 mL, 40 mM) under vigorous stirring. The mixed solution was then heated for another 30 min before being cooled to room temperature. Amphiphilic gold nanoparticles were synthesized using the tandem “grafting to” and “grafting from” reaction we recently developed.27,30 In the “grafting to” reaction, a solution of thiolated-PEG (PEG-SH) (30 mg) and 2,20 -dithiobis[1-(2-bromo-2-methyl-propionyloxy)] ethane (DTBE) (15 mg) in DMF (2 mL) was added slowly into 100 mL of the original 42 nm Au nanoparticles in water. After stirring 12 h, the solution was centrifuged (4000g for 15 min) to recover the nanoparticles (Au@PEG/DTBE). The supernatant was discarded, and the nanoparticles were redispersed in DMF. The redispersion
centrifugation process was repeated three times to completely remove unattached DTBE. Au@PEG/DTBE was redispersed in 2 mL of DMF for further uses. In the “grafting from” reaction, MMA (0.4 mL) and Au@PEG/DTBE (6 nM) were mixed in DMF (2 mL). After being degassed for 0.5 h by nitrogen, CuBr (4 mg) and N,N,N0 ,N0 ,N00 -pentamethyldiethylene-triamine (PMDETA, 15 mg) were added, and the reaction solution was kept in 40 °C water bath for 10 h. After being purified by centrifugation, the amphiphilic 42 nm Au@PEG/PMMA nanoparticles were stored in DMF. To prepare dimers of

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amphiphilic gold nanoparticles, 800 μL of DI water was added dropwise into 200 μL of DMF solution of Au@PEG/PMMA (5 nM) in 10 min under gentle stirring. Then, the obtained solution was further dialyzed against DI water for 24 h. To trigger disassembly of the dimer, DMF (e.g., 0.9 mL) was added dropwise in an aqueous solution of dimers (0.1 mL).

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental details and supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT H.D. thanks the program of Nanyang Assistant Professorship for financial support. This work is partially supported by the National Nature Science Foundation of China (20904001), and the NSF of Anhui Province (090414198) to L.C. ’ REFERENCES (1) Rosi, N. L.; Mirkin, C. A. Nanostructures in Biodiagnostics. Chem. Rev. 2005, 105, 1547–1562. (2) Choi, C. L.; Alivisatos, A. P. From Artificial Atoms to Nanocrystal Molecules: Preparation and Properties of More Complex Nanostructures. Annu. Rev. Phys. Chem. 2010, 61, 369–389. (3) Nie, Z. H.; Petukhova, A.; Kumacheva, E. Properties and emerging applications of self-assembled structures made from inorganic nanoparticles. Nat. Nanotechnol. 2010, 5, 15–25. (4) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics. Angew. Chem., Int. Ed. 2009, 48, 60–103. (5) Kwon, S. G.; Hyeon, T. Colloidal Chemical Synthesis and Formation Kinetics of Uniformly Sized Nanocrystals of Metals, Oxides, and Chalcogenides. Acc. Chem. Res. 2008, 41, 1696–1709. (6) S€onnichsen, C.; Reinhard, B. M.; Liphardt, J.; Alivisatos, A. P. A Molecular Ruler Based on Plasmon Coupling of Single Gold and Silver Nanoparticles. Nat. Biotechnol. 2005, 23, 741–745. (7) Reinhard, B. M.; Shikholeslami, S.; Mastroianni, A.; Alivisatos, A. P.; Liphardt, J. Use of Plasmon Coupling to Reveal the Dynamics of DNA Bending and Cleavage by Single EcoRV Restriction Enzymes. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 2667–2672. (8) Xu, X. Y.; Rosi, N. L.; Wang, Y. H.; Huo, F. W.; Mirkin, C. A. Asymmetric Functionalization of Gold Nanoparticles with Oligonucleotides. J. Am. Chem. Soc. 2006, 128, 9286–9287. (9) Aldaye, F. A.; Sleiman, H. F. Dynamic DNA Templates for Discrete Gold Nanoparticle Assemblies: Control of Geometry, Modularity, Write/Erase and Structural Switching. J. Am. Chem. Soc. 2007, 129, 4130–4131. (10) Shuford, K. L.; Meyer, K. A.; Li, C. C.; Cho, S. O.; Whitten, W. B.; Shaw, R. W. Computational and Experimental Evaluation of Nanoparticle Coupling. J. Phys. Chem. A. 2009, 113, 4009–4014. (11) Novak, J. P.; Feldheim, D. L. Assembly of PhenylacetyleneBridged Silver and Gold Nanoparticle Arrays. J. Am. Chem. Soc. 2000, 122, 3979–3980. (12) Sardar, R.; Heap, T. B.; Shumaker-Parry, J. S. Versatile Solid Phase Synthesis of Gold Nanoparticle Dimers Using an Asymmetric Functionalization Approach. J. Am. Chem. Soc. 2007, 129, 5356–5357. 2261

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