Morphological Control in Aggregates of Amphiphilic Cylindrical Metal

Apr 4, 2013 - interest because it provides a high degree of freedom in tailoring the properties of ... the morphological control of AuNR aggregates ha...
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Morphological Control in Aggregates of Amphiphilic Cylindrical Metal−Polymer “Brushes” Yiyong Mai,†,‡ Lin Xiao,‡ and Adi Eisenberg*,‡ †

School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, 200240 Shanghai, P. R. China ‡ Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec H3A 2K6, Canada S Supporting Information *

ABSTRACT: Controlled self-assembly of gold nanorods (AuNRs) into nanostructures of various morphologies has attracted considerable interest because it provides a high degree of freedom in tailoring the properties of the nanostructures by the coupling of the optical and electronic properties of the individual AuNRs. This paper presents a new strategy for making AuNR aggregates of tunable morphologies. In this approach, the surface of AuNRs is chemically coated with an amphiphilic diblock copolymer. The coating gives the AuNRs a cylindrical brush structure. By varying the nature of the common solvent or the interparticle electrostatic repulsion, the self-assembly of the amphiphilic cylindrical AuNR−polymer “brushes” can produce water-soluble aggregates of controllable morphologies, including singlerod ellipsoidal micelles, curved circular lamellae, and nanospheres. The AuNRs in the various aggregates generate different surface plasmon resonance (SPR) absorption patterns, with the longitudinal SPR band in the near-infrared spectral window shifting as the aggregate morphology changes.

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INTRODUCTION Gold nanorods (AuNRs) have emerged as a subject of great interest in view of their unique physical properties.1−3 Controlled assembly of AuNRs into structures of various morphologies provides additional freedom in tailoring their properties by the coupling of the optical and electronic properties of the individual AuNRs.4−14 Such assembled structures might be potentially useful in optoelectronics, sensing, imaging, and biomedical applications.1−14 Therefore, the morphological control of AuNR aggregates has also attracted considerable attention, although it still presents a considerable challenge.2,3 Assembled structures of AuNRs can be obtained, for example, by addition of appropriate connectors (e.g., DNA) to solutions of the AuNRs4,7 or by modification of the AuNR surface with small molecules (e.g., biotin) along with adding a particular linker (e.g., streptavidin).5,8,9,11 However, each of these approaches generally gives rise to AuNR aggregates of only a single specific morphology. In contrast, chemical binding of polymer chains to AuNRs may make them similar to amphiphilic macromolecules.12−14 This similarity, in turn, can allow for morphological control and variability of the assemblies of the polymer-tethered nanorods, through adjustment of the polymer composition or of some environmental assembly parameters such as the nature of common solvent or the content of precipitant, among others. 12,13 To our knowledge, the morphological control and variability in © 2013 American Chemical Society

aggregates of polymer-tethered AuNRs have been achieved by only two strategies, in which homopolymers were linked to the ends of nanorods. Mirkin and co-workers synthesized AuNRs terminated with polypyrrole at one end, namely Au-Ppy rods. By changing the Au/Ppy block ratio, the templated assembly of the Au-Ppy rods generated different structures including bundles and tubes.12 Kumacheva and colleagues prepared AuNRs coated with cetyltrimethylammonium bromide (CTAB) on the side and terminated with polystyrene (PS) chains at both ends. The self-assembly of the PS-AuNR-PS “triblocks” in different solutions with varied water contents, driven by hydrophobic interaction of the PS, produced a variety of structures including rings, chains, bundles, and spheres.13 The present paper describes a very different approach to morphologically tunable aggregates of polymer-tethered AuNRs. A new type of polymer-linked AuNR structure, namely AuNRs with chemically coated amphiphilic diblocks on the entire surface, are prepared by a full-surface CTAB ligand replacement with a thioctate ester (TE)-terminated polystyrene270-block-poly(acrylic acid) 15 copolymer (TE-PS270 -bPAA15)15 (Figure 1). The coating of the diblocks makes the AuNRs structurally analogous to cylindrical polymer brushes.16 The self-assembly of the amphiphilic cylindrical AuNR− Received: February 1, 2013 Revised: March 27, 2013 Published: April 4, 2013 3183

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Figure 1. Preparation of AuNRs with chemically attached amphiphilic TE-PS270-b-PAA15 diblocks by a full-surface ligand exchange of the CTABstabilized AuNRs as well as the self-assembly of the copolymer-coated AuNRs (cylindrical “brushes”) in solutions using various common solvents. (a) A TEM image of the CTAB-stabilized AuNRs; (b) single-rod ellipsoidal micelles, average dimensions: a = 25.3 ± 3.3 nm, b = 16.0 ± 1.6 nm, c = 21.6 ± 3.2 nm, d = 7.3 ± 1.0 nm; (c) nanospheres; (d) watch-glass-like lamellae. The TEM micrograph in (d) was obtained with 60° tilting.

polymer “brushes” in solution can be induced to generate a range of water-soluble aggregates of well-defined controllable morphologies, including single-rod ellipsoidal micelles (Figure 1b), nanospheres (Figure 1c), and curved circular (watch-glasslike) lamellae (Figure 1d). The morphological tunability results from the block copolymer coating on the AuNRs and is thus more closely related to the morphogenic characters of amphiphilic diblock copolymers. In the resultant aggregates, the hydrophobic PS blocks form a continuous layer surrounding the AuNRs, which provides strong protection against some species which can damage the AuNRs in aqueous media, as has been demonstrated for gold nanoparticles (spheres or rods) with PS-b-PAA copolymers physically adsorbed over the entire surface.17−19 Such protection has not been seen on assembled AuNR structures in aqueous solutions prepared by other methods. The nanospheres have a supermicelle structure20,21 (see the inset of Figure 1c, to be discussed later). To our knowledge, the nanospheres of the supermicelle structure and the lamellae are new members in the family of AuNR aggregates. Furthermore, it is also of interest that the AuNRs in the different aggregates produce different surface plasmon resonance (SPR) absorption patterns, with the

longitudinal SPR band shifting in the near-infrared (NIR) spectral region (generally ca. 650−1200 nm) as the aggregate morphology varies. Nanostructures with SPR bands in the NIR spectral window are desirable for in vivo applications, owing to the small attenuation of light by blood or soft tissue in this spectral range.1−3

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PREPARATION OF THE TE-PS270-B-PAA15-COATED AUNRS The CTAB-stabilized AuNRs were synthesized by the seedmediated growth method following the procedure developed by Nikoobakht and El-Sayed.22 The experimental details are given in the Supporting Information. Figure 1a presents the transmission electron microscopy (TEM) image of the CTAB-coated AuNRs of a mean diameter of 14.5 ± 1.9 nm and a length of 45.4 ± 5.0 nm. As the cylindrical surfaces of AuNRs are densely capped by the CTAB molecules through Au−N interaction, replacement of this surfactant layer with other ligands such as thiolated species presents a challenge.2,6 In many cases chemical binding occurs only to the relatively bare end surfaces.5,8,13 Sonication and heating were found to facilitate a full-surface ligand replacement, for example, by some 3184

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thiolated small molecules.18,19,23 However, no reports have been found on the preparation, by full-surface ligand exchange, of AuNRs with chemically attached amphiphilic block copolymers, possibly due to the difficulties in the synthesis of appropriate end-capped amphiphilic copolymers and in finding a common solvent which can dissolve both the CTABstabilized AuNRs and the amphiphilic copolymers. In the present work, a thioctate ester-terminated amphiphilic diblock copolymer (TE-PS270-b-PAA15)15 was employed as the coating agent and a “mix-and-heat” method was used for the full-surface ligand replacement.18,19 Briefly, a mixture of the CTAB-coated AuNRs and the TE-PS270-b-PAA15 copolymer in N,Ndimethylformamide (DMF) was sonicated for 0.5 h and then heated at 110 °C for 2 h under a nitrogen atmosphere; subsequently, the mixture was slowly cooled to room temperature and incubated for 3 days (see the experimental section 1.3 in the Supporting Information). The AuNR product was purified by dialysis and centrifugation and then dissolved in either DMF or dioxane or tetrahydrofuran (THF) for the subsequent self-assembly. It should be pointed out that CTABcoated AuNRs show good solubility in DMF but poor solubility in dioxane or THF;13 the evidently improved solubility of the copolymer modified nanorods in dioxane or THF is a result of the attachment of the copolymer to the AuNRs. The dynamic light scattering (DLS) plot shows a single-peak size distribution with an average hydrodynamic diameter (Dh) of 108 nm for the TE-PS270-b-PAA15 coated AuNRs in DMF (Figure S1, with a brief discussion of the copolymer chain dimensions given in the caption). The Dh is much larger than the dimensions (both diameter and length) of the CTABstabilized AuNRs obtained by TEM; this increase is consistent with the linking of the TE-PS270-b-PAA15 chains to the surface of the AuNRs as the corona. To observe the polymer coating on the AuNRs directly by TEM in the collapsed state, the samples were prepared by adding water to the DMF solution of the copolymer-coated nanorods, followed by dialysis of the solution against water to remove the DMF. Figure 1b presents a representative image of the copolymer coated AuNRs, in which a polymer layer is clearly seen. Since the single nanorods with the polymer coating exhibit a nearly ellipsoidal shape in TEM images, they are referred to as single-rod ellipsoidal micelles. The average number of the copolymer chains in an ellipsoidal micelle is calculated to be ∼440 according to the average dimensions of the single-rod micelles (see section 3.1 in the Supporting Information). From TEM images such as that in Figure 1b, one can see that for most of the polymer-coated nanorods the entire surface is covered by the diblocks, with a somewhat thinner coating at one or both ends compared with the coating thickness on the sides. This difference arises from the fact that for the same grafting density the coating on the hemispherical rod ends should be thinner than that on the sides because of the difference in curvature (see the calculation in section 3.2 in the Supporting Information). The polymer coating is not found in TEM images for AuNRs prepared in the control experiment using PS270-b-PAA15 without the TE end group15 as the coating agent. This difference confirms the key role of the TE group in the chemical binding of the TE-PS270-bPAA15 chains to AuNRs.

nanorods in DMF, dioxane, or THF. The initial molar concentrations of the nanorods in the organic solutions were ∼3 nM (estimated by ultraviolet−visible (UV−vis) spectrometry, see the experimental section 1.3 in the Supporting Information; the molar concentration equals the number concentration/6.02 × 1023). Self-assembly occurred at a water content of ∼9 wt % in DMF, at ∼5 wt % in dioxane, and at ∼6 wt % in THF; the onsets of aggregation were obtained from the turbidity diagrams of the AuNR solutions upon water addition (Figure 2). In all cases, more water was added slowly until the

Figure 2. Turbidity diagrams of the solutions of the copolymer-coated AuNRs in DMF, dioxane, and THF upon water addition. Absorbance readings were recorded at λ = 480 nm; at this wavelength the absorption of the AuNRs has a minimal effect on the attenuation of light. At each point of water addition, the solutions were left to stir for 5 min.

water content reached ca. 50 wt %. The aggregates were quenched in a 5-fold excess of water, and then the solutions were dialyzed against water to remove the organic solvents (see the experimental section 1.4 in the Supporting Information). It is seen in Figure 2 that for each sample there is only one turbidity transition, and the turbidity does not show any obvious changes after the addition of ca. 15 wt % water. TEM observation shows that at a water content of ca. 15 wt % the resultant aggregates are similar to those prepared at 50 wt % water content followed by quench and dialysis. In addition, it is known that for the self-assembly of PS-b-PAA block copolymers in the same organic solvents upon water addition, the PS-b-PAA aggregates form in a similar water content window (5−15 wt %).24 Therefore, it is reasonable to suggest that the AuNR aggregates form at water contents of 5−15 wt %. On the other hand, for block copolymers with a glassy PS hydrophobic block at room temperature (Tg,PS ∼ 100 °C in bulk), such as TE-PS270-b-PAA15 in this study, the aggregates are generally frozen after the removal of the organic solvents from the PS regions, which occurs during water addition and dialysis at high water contents (>50 wt %); thus, the drying of the aggregates on the TEM grids does not affect the aggregation process.25 It was found that the morphology of the resultant aggregates is controlled by the nature of the common solvent. When DMF was used as the common solvent, the self-assembly of the TEPS270-b-PAA15-coated AuNRs produced single-rod ellipsoidal micelles along with smaller amounts of doublets, triplets, and

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SELF-ASSEMBLY OF THE TE-PS270-B-PAA15-COATED AUNRS The self-assembly of the copolymer-coated AuNRs was performed by slow addition of water to solutions of the 3185

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small nanospheres containing 4−8 nanorods, as shown in Figure 1b. Statistics (based on ca. 300 aggregates) show that the numerical percentages of single-rod micelles, doublets and triplets, and small nanospheres are ∼70%, ∼23%, and ∼7%, respectively. The percentage of the single-rod micelles can be increased to ∼90% through the addition of a small amount of NaOH (NaOH:COOH ∼ 1:1 molar ratio) into the initial DMF solution of the AuNRs. This increase is probably due to the fact that the addition of NaOH causes an increase in the degree of ionization of the PAA chains26 in the copolymer-coated AuNRs, and consequently an increase in the interparticle repulsion among the individual particles, favoring the formation of single-rod ellipsoidal micelles upon water addition. When dioxane was employed as the common solvent, nanospheres with multiple AuNRs randomly distributed inside were obtained, as shown in Figure 1c. Tilting TEM studies confirm the spherical nature of the aggregates (Figure S2). The nanospheres have an average diameter of 124 ± 60 nm and are stable in water; 10 min sonication or one month storage after the addition of NaOH (NaOH:COOH ∼ 1:1 molar ratio) to their aqueous solution did not cause any observable changes in their morphology or size, as confirmed by TEM. Based on these experimental facts, a supermicelle structure can be proposed for the nanoshperes,20,21 as illustrated in the inset of Figure 1c. In the supermicelle, the PS chains are linked at one end to the outer surface of the micelles via the PAA chains, and at the other end via the TE group to one of the AuNRs. The PS chains span the distance between the AuNRs and the interface of the supermicelle. One can inquire whether this arrangement is physically realistic in terms of the block lengths and the detailed morphological parameters. It is known that the average distance from any point inside a sphere to the nearest point on the surface is R/4 (where R is the radius of the sphere).21 Since the average radius of the supermicelles (nanospheres) is ca. 60 nm, the average distance from a nanorod in the supermicelle to the nearest point on the surface is ca. 15 nm. Both this value and the average radius are between an estimated unperturbed endto-end distance of ca. 3 nm and a fully stretched length of ca. 70 nm for the PS blocks in the TE-PS270-b-PAA15 corona of the AuNRs. Therefore, the connection of any point within the supermicelle to the surface by a PS chain is realistic. It is also reasonable that such an arrangement of the copolymer chains applies to the doublets, triplets, and small nanospheres mentioned previously. However, in the nanosphere sample there is a small amount (ca. 10 num. %) of nanospheres with radii (90−120 nm) much larger than the fully stretched length of the PS blocks (70 nm). For these large nanospheres, it seems reasonable that some of the PAA blocks are present in the interior of the micelle cores due to imperfect segregation.25 At present, we do not have independent experimental evidence for the presence of PAA blocks inside the micelle core. When the common solvent was THF, the self-assembly of the TE-PS270-b-PAA15-coated AuNRs yielded watch-glass-like lamellae. Figure 1d presents a TEM micrograph obtained with 60° tilting of the lamellae, which lie on the surface with their concave side up. The normal TEM image (without tilting) of the same aggregates is given in Figure 3a1; for ease of comparison, the TEM image in Figure 1d is shown again in Figure 3a2. Some lamellae are also found to lie on the surface with their concave side down (Figure 3b). More TEM images are given in Figure S3. The projections of the lamellae on the TEM images taken without tilting, such as those in Figure

Figure 3. TEM images of the watch-glass-like lamellae: (a1, b1) images obtained without tilting; (a2, b2) images obtained with 60° tilting.

3a1,b1, show nearly round aggregates. The average diameter of the projections of the lamellae in the micrographs is 220 ± 60 nm. Similar disk-like structures are frequently observed in the study of colloidal phenomena owing to their lower total edge energy relative to those of other two-dimensional shapes.27 TEM samples prepared by freeze-drying give similar images, suggesting that the drying of the aggregates on the TEM grids does not influence the morphology of the watch-glass-like lamellae. Atomic force microscopy (AFM) imaging confirms the watch-glass-like structure and gives an average lamella thickness of ca. 50 nm (Figure S4). Since the single-rod ellipsoidal micelles have an average dimension of ca. 50 nm × 30 nm, the ∼50 nm thickness suggests that some stacking of the nanorods takes place in the lamellae. A consideration of the TEM and AFM results, along with the arrangement of copolymer chains in the supermicelles discussed previously, suggests a possible structure as illustrated at the bottom of Figure 1d for the watch-glass-like lamellae. In the schematic illustration, the AuNRs are surrounded by a PS layer; the PS chains are linked at one end to the outer surface of the lamella via the PAA chains and at the other end via the TE group to one of the nanorods. A tentative explanation can be suggested for the formation of the curvature leading to the watch-glass shape of the lamellae. During lamella formation, the average amount of PS chains per unit volume is possibly different in different layers of the lamellae. Such a difference might be induced by differences in the number density of AuNRs in the “upper” and “lower” sections. At equilibrium in a THF swollen state in the presence of water, the layer with the higher PS content would contain more organic solvent. As the solvent is eliminated (during quenching or dialysis), this layer would tend to shrink more, giving the lamellae a watch-glass shape. The larger the difference of the average amount of PS chains per unit volume in the “upper” and “lower” parts of the lamella, the greater the curvature. Watch-glass-like lamellae were also found in the self-assembly of other block copolymer or small molecule amphiphile systems.28 The proposed mechanism suggests that flat disks (lamellae) can bend due to thermal fluctuation and form vesicles. Whether a bent disk can produce a vesicle (closed lamella) or not depends on the edge energy of the lamella and 3186

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the energy of bending.28 In the present study, vesicles are not found, suggesting that the energy of bending is larger than the edge energy of the watch-glass-like lamellae, probably owing to the presence of rigid AuNRs in the lamellae, which are likely to act as a reinforcing filler.

periphery of lamellae, growth occurs preferentially in the plane rather than in three dimensions. The solvent effect on the aggregate morphology is also found in the self-assembly of the copolymer-coated AuNRs using mixed solvents as the common solvents, such as DMF−dioxane or DMF−THF mixtures in various ratios. In both mixed solvent systems, as the DMF content increases, small nanospheres (which are much smaller than the nanospheres prepared using dioxane as the common solvent), doublets, triplets, and single-rod micelles appear gradually, with an increase in the percentage of single-rod micelles. The morphogenic effect of the electrostatic repulsion among the PS-b-PAA-coated AuNRs is confirmed by adding various amounts of NaOH (5 μL water solution) to the initial dioxane solutions (0.2 mL) of the copolymer-coated AuNRs, followed by the addition of water (Figure S5). At a NaOH to COOH molar ratio of 1:4, watch-glass-like lamellae are formed; when the ratio was 1:2, a mixture of watch-glass-like lamellae and small nanospheres is obtained; at a NaOH to COOH ratio of 1:1, a mixture of single-rod micelles, doublets, triplets, and small nanospheres is observed. Clearly, the morphology of the aggregates is also electrostatically tunable.

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MORPHOGENIC EFFECT OF THE COMMON SOLVENT OR THE INTERPARTICLE ELECTROSTATIC REPULSION ON THE AGGREGATES The effect of the common solvent on the morphology of PS-bPAA block copolymer aggregates has been well studied.24,25 The morphogenic effect arises from the influence of the common solvent on both hydrophobic and hydrophilic segments of the PS-b-PAA copolymer.24,25 For the aggregates of the PS-b-PAA-coated AuNRs, the morphological control through the nature of the common solvent can be also understood by consideration of the similar influence factors. It is known that the solubility parameter (δ) of PS (δ = 16.6−20.2) is closer to those of THF (δ = 18.6) and dioxane (δ = 20.5) than to that of DMF (δ = 24.8); thus, the degree of swelling of the PS corona of the nanorods and the mobility of the PS chains are higher in THF or dioxane than in DMF.24,25 This facilitates the aggregation of the nanorods in THF or dioxane. Moreover, the window of water contents in which the aggregates are still labile is broader in THF or dioxane than in DMF.24,25 These factors provide more opportunities for morphological transitions before the aggregates are frozen at high water contents. Therefore, using dioxane or THF as the common solvent, the PS-b-PAA-coated AuNRs have the opportunity to form supermicelles or lamellae, which contain multiple nanorods. More importantly, the interparticle repulsion of the PS-b-PAA-coated AuNRs depends on the interactions between the solvent and the PAA corona. It should be recalled here that the self-assembly of the copolymer-coated AuNRs occurs at a water content of ∼9 wt % in DMF and at ∼5 wt % in dioxane or THF. With so little water in the mixed solvent, the dielectric constant of the organic solvent affects the ionization of the PAA block.24,25 If DMF, which has a relatively high dielectric constant (ε = 38.2), is the common solvent, the PAA blocks are partially charged (although the degree of ionization is low);24,25 the electrostatic repulsion among the copolymer-coated nanorods is relatively strong before the onset of self-assembly upon water addition. The strong repulsion is unfavorable for the aggregation of the nanorods. Thus, singlerod ellipsoidal micelles are primarily formed, along with smaller amounts of doublets, triplets, and small nanospheres (containing 4−8 nanorods). It should be recalled that the effect of the interparticle electrostatic repulsion is proved by the addition of NaOH into the initial DMF solution of the copolymer-coated AuNRs, which results in an increase in the percentage of single-rod ellipsoidal micelles. On the other hand, in a common solvent of low dielectric constant, such as dioxane (ε = 2.2), the charge on the PAA blocks is much smaller and possibly negligible;24,25 the repulsion among the nanorods is weak and insufficient to prevent the nanorods from aggregating. Thus, the nanospheres containing multiple AuNRs are formed. Using THF (ε = 7.5) as the common solvent, which has a dielectric constant between those of DMF and dioxane,25 the repulsion among the nanorods is intermediate, leading to the formation of lamellae. Since the repulsion is weaker near the

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SURFACE PLASMON RESONANCE ABSORPTION OF THE AUNRS IN THE VARIOUS AGGREGATES IN WATER One of the physical properties of AuNRs is the generation of two separate SPR bands, which are related to their width and length and are known as the transverse and longitudinal SPR bands, respectively.1−3 The SPR bands of AuNRs in close proximity to each other are greatly influenced by their arrangement geometry because of plasmonic coupling. The side-by-side geometry usually shows a blue-shift in the longitudinal mode (LSPR) and a red-shift in the transverse mode (TSPR) while the end-to-end geometry shows a red-shift in the LSPR with little change in the TSPR.2,3,7 It is interesting to study the SPR absorption of the AuNRs in the aggregates of various morphologies in aqueous solution in the present system. Figure 4 shows the SPR absorption spectra recorded by a UV−vis spectrometer. For the sake of comparison, curve A shows the SPR spectrum of the CTAB-stabilized AuNRs in water (the spectra of the copolymer-coated AuNRs in DMF, dioxane, or THF are shown in Figure S6, since they are similar to that of the CTAB-stabilized AuNRs in water). Curve B represents the absorption spectrum of the sample shown in Figure 1b, which contains ∼70% of single-rod ellipsoidal micelles, ∼23% of doublets and triplets, and ∼7% of small nanospheres containing 4−8 nanorods. Compared with curve A, curve B exhibits a slight red-shift in the TSPR and a slight blue-shift in the LSPR. The band shift is considered to be induced by the plasmonic coupling of the AuNRs in the doublets, triplets, and small nanospheres, in which the nanorods are preferentially arranged side-by-side (Figure S7).13 When the number percentage of the single-rod ellipsoidal micelles is increased to ∼90%, both TSPR and LSPR bands show slight red-shifts (TSPR: ca. 7 nm; LSPR: ca. 5 nm, compared with curve A). These shifts might be due to a decrease in SPR energy with increasing refractive index of the surrounding medium.2,17,18 The SPR spectrum of the AuNRs in the nanospheres is given in curve C, in which the TSPR is redshifted and both bands become broad because of the strong plasmonic coupling of the nanorods in the nanospheres. At this 3187

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studies show that the AuNRs in the different aggregates produce different SPR absorption patterns, with the LSPR band shifting in the NIR spectral region as the aggregate morphology changes.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, supporting figures, and calculations. 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]. Notes

The authors declare no competing financial interest.

Figure 4. UV−vis absorption curves of the various AuNR aggregates in dilute aqueous solutions: (A) CTAB-coated AuNRs in Figure 1a; (B) single-rod ellipsoidal micelles along with smaller amounts of doublets, triplets, and small nanospheres in Figure 1b; (C) nanospheres in Figure 1c; (D) watch-glass-like lamellae in Figure 1d.

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ACKNOWLEDGMENTS The authors thank the Natural Science and Engineering Research Council of Canada (NSERC) for financial support as well as the Center for Self-Assembled Chemical Structures (CSACS) for the use of facilities.

point, it should be recalled that the mean diameter of the nanospheres is ca. 125 nm, ∼3 times the average length of the AuNRs (ca. 45 nm). Thus, in the nanoshperes, the nanorods have chance to adopt the end-to-end arrangement in addition to the side-by-side array, leading to an overall SPR absorption as seen in curve C. Curve D gives the SPR spectrum of the AuNRs in the watch-glass-like lamellae; both TSPR and LSPR bands of the AuNRs are red-shifted and become broad also due to the strong plasmonic coupling of the nanorods. The membranous structure of the lamellae leads to a narrower LSPR peak of the AuNRs with a larger red-shift than that produced by the AuNRs in the nanospheres. The LSPR bands of the AuNRs in all of the aggregates are located in the NIR spectral range and shift as the aggregate morphology varies. The aggregates are stable in water, and the AuNRs within the aggregates are protected in aqueous solutions by the surrounding PS layer. These features suggest that the assemblies might have potential applications in, for example, sensing, imaging, and photothermal therapy in aqueous media.1−3

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REFERENCES

(1) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857− 13870. (2) Huang, X.; Neretina, S.; El-Sayed, M. A. Adv. Mater. 2009, 21, 4880−4910. (3) Vigderman, L.; Khanal, B. P.; Zubarev, E. R. Adv. Mater. 2012, 24, 4811−4841. (4) Dujardin, E.; Hsin, L. B.; Wang, C. R. C.; Mann, S. Chem. Commun. 2001, 1264−1265. (5) Caswell, K. K.; Wilson, J. N.; Bunz, U. H. F.; Murphy, C. J. J. Am. Chem. Soc. 2003, 125, 13914−13915. (6) Khanal, B. P.; Zubarev, E. R. Angew. Chem., Int. Ed. 2007, 46, 2195−2198. (7) Park, H.-S.; Agarwal, A.; Kotov, N. A.; Lavrentovich, O. D. Langmuir 2008, 24, 13833−13837. (8) Jain, T.; Westerlund, F.; Johnson, E.; Moth-Poulsen, K.; Bjørnholm, T. ACS Nano 2009, 3, 828−834. (9) Ni, W.; Mosquera, R. A.; Pérez-Juste, J.; Liz-Marzán, L. M. J. Phys. Chem. Lett. 2010, 1, 1181−1185. (10) Shao, L.; Woo, K. C.; Chen, H.; Jin, Z.; Wang, J.; Lin, H. ACS Nano 2010, 4, 3053−3062. (11) Nepal, D.; Park, K.; Vaia, R. A. Small 2012, 8, 1013−1020. (12) Park, S.; Lim, J.-H.; Chung, S.-W.; Mirkin, C. A. Science 2004, 303, 348−351. (13) (a) Nie, Z.; Fava, D.; Kumacheva, E.; Zou, S.; Walker, G. C.; Rubinstein, M. Nat. Mater. 2007, 6, 609−614. (b) Fava, D.; Nie, Z.; Winnik, M. A.; Kumacheva, E. Adv. Mater. 2008, 20, 4318−4322. (14) Song, J.; Cheng, L.; Liu, A.; Yin, J.; Kuang, M.; Duan, H. J. Am. Chem. Soc. 2011, 133, 10760−10763. (15) The copolymers of TE-PS270-b-PAA15 and PS270-b-PAA15 without the TE end group were prepared in previous work: Mai, Y.; Eisenberg, A. J. Am. Chem. Soc. 2010, 132, 10078−10084. The polydispersity indexes (PDI) of the two copolymers are around 1.15. The molecular structure of TE-PS270-b-PAA15 is given in p S1 of the Supporting Information. (16) (a) Djalali, R.; Li, S.; Schmidt, M. Macromolecules 2002, 35, 4282−4288. (b) Zhang, M.; Müller, A. H. E. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 3461−3481. (c) Rzayev, J. ACS Macro Lett. 2012, 1, 1146−1149. (17) Kang, Y.; Taton, T. A. Angew. Chem., Int. Ed. 2005, 44, 409− 412.

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CONCLUSIONS This paper describes a new approach to morphologically controllable assemblies of polymer-tethered AuNRs. Amphiphilic diblock copolymer-coated AuNRs of a cylindrical brush structure were prepared by a full-surface ligand exchange of the CTAB-stabilized nanorods with TE-PS270-b-PAA15 chains. DLS and TEM results demonstrate the presence of the copolymer chains on the AuNRs. By varying the nature of the common solvent or the amount of added base, the self-assembly of the copolymer-coated AuNRs generates water-soluble aggregates of various morphologies, including primarily single-rod ellipsoidal micelles, watch-glass-like lamellae, and nanospheres. The electrostatic repulsion among the PS 270-b-PAA 15-coated AuNRs, which can be adjusted by changing the dielectric constant of the common solvent or the amount of added base, plays a crucial role in the morphological control of the aggregates. A strong interparticle repulsion results in the formation of single-rod ellipsoidal micelles, an intermediate interparticle repulsion gives watch-glass-like lamellae, and a weak repulsion leads to the generation of nanospheres. UV−vis 3188

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(18) Chen, H.; Abraham, S.; Mendenhall, J.; Delamarre, S. C.; Smith, K.; Kim, I.; Batt, C. A. ChemPhysChem 2008, 9, 388−392. (19) Tan, L.; Xing, S.; Chen, T.; Chen, G.; Huang, X.; Zhang, H.; Chen, H. ACS Nano 2009, 3, 3469−3474. (20) Mai, Y.; Eisenberg, A. Acc. Chem. Res. 2012, 45, 1657−1666. (21) Duxin, N.; Liu, F.; Vali, H.; Eisenberg, A. J. Am. Chem. Soc. 2005, 127, 10063−10069. (22) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957− 1962. (23) Yu, C.; Nakshatri, H.; Irudayaraj, J. Nano Lett. 2007, 7, 2300− 2306. (24) Yu, Y.; Zhang, L.; Eisenberg, A. Macromolecules 1998, 31, 1144− 1154. (25) Mai, Y.; Eisenberg, A. Chem. Soc. Rev. 2012, 41, 5969−5985. (26) Zhang, L.; Yu, K.; Eisenberg, A. Science 1996, 272, 1777−1779. (27) Holder, S. J.; Sommerdijk, N. A. J. M. Polym. Chem. 2011, 2, 1018−1028. (28) Yu, K.; Eisenberg, A. Macromolecules 1998, 31, 3509−3518.

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