Standing Arrays of Gold Nanorods End-Tethered with Polymer Ligands

transcript

Nanorod Arrays

Alla Petukhova, Jesse Greener, Kun Liu, Dmytro Nykypanchuk, Renaud Nicolaÿ, Krzysztof Matyjaszewski, and Eugenia Kumacheva*

Nanomaterials with vectoral electromagnetic properties have potential applications in solar cells, plasmonic cavity resonators, light polarizers, and biosensing. Here a new, simple, solution-based method for producing nanomaterials comprising vertically aligned standing arrays of gold nanorods (NRs) end-functionalized with polymer ligands is reported. The method utilizes the side-by-side assembly of the NRs into large 2D superlattices, followed by the precipitation of the lattices on a solid substrate. The critical design rules for the self-assembly of superlattices are demonstrated, and they show the generality of the method by forming standing arrays from the NRs end-tethered with poly(N-vinylcarbazole) or with polystyrene molecules.

1. Introduction

The range of applications of self-assembled nanoparticle arrays can be expanded and their performance in functional devices can be greatly enhanced, if control is achieved over their structural characteristics such as directionality, inter-nanoparticle distance, and long-range order.[1] In particular, recently 2D superlattices of inorganic nanorods (NRs), in which the NRs are aligned with their long axes parallel to each other, have attracted great interest, due to the ability to realize the vectorial properties of NRs.[2] For example, the localization of an electric field at the tips of metal NRs and the dependence of the longitudinal surface plasmon resonance on the NR aspect ratio open new opportunities for sensing applications.[3] Polarization-dependent surface

DOI: 10.1002/smll.201101297

Dr. A. Petukhova, Dr. J. Greener, Dr. K. Liu, Prof. E. KumachevaDepartment of Chemistry University of Toronto 80 Saint George street, Toronto, Ontario M5S 3H6, Canada E-mail: ekumache@chem.utoronto.ca

Dr. D. NykypanchukCenter for Functional Nanomaterials Brookhaven National Laboratory Upton, New York 11973, USA

R. Nicolaÿ, Prof. K. MatyjaszewskiDepartment of Chemistry Carnegie Mellon University 4400 Fifth Avenue, Pittsburgh, PA 15213, USA

plasmon-based properties of metal NRs enable routing and manipulation of light and electrical current at the nano-scale.[4] Potential applications of 2D NR superlattices depos-ited on a substrate as standing arrays (SAs) include their use in solar cells, plasmonic cavity resonators, light polarizers, and metamaterials for biosensing.[5]

Semiconductor and metal NRs synthesized by wet chem-istry methods have been organized in a side-by-side manner under the action of capillary forces[6] or electric field,[7] by epitaxial growth on textured surfaces,[8] or by uisng different types of templates.[5a,b] Alternatively, 2D lattices of rod-like particles have been formed in the solution state owing to depletion forces,[9] or by utilizing attraction between the ligands coating the long side of NRs.[10] When the superlat-tices of semiconductor NRs settled under gravity on solid substrates, they formed SAs, in which the NRs were oriented perpendicular to the substrate.[10] Pre-organization of the NRs in the solution phase is conceptually interesting, as it utilizes the concept of amphiphilic ‘colloidal molecules.’ In addition, it is potentially useful as it renders self-assembly independent of the type of a substrate and does not require the use of templates, external fields, or specific evaporation rates of the solvent.

The co-assembly of inorganic NRs and functional organic materials in superlattices is another very attractive feature that can render interesting and potentially, useful applica-tions of SAs. For example, ZnO and ZnO-TiO2 NRs were functionalized with conductive π-conjugated polymers,[11] dyes[12] and inorganic quantum dots[13] for photovoltaic applications. A combination of metal (e.g., gold NRs) and

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Our group has recently reported the self-assembly of polymer-tethered gold NRs in small clusters chains, rings, spheres and raft-like structures.[3,15] The approach to NR self-organization in a variety of structures utilized the reduc-tion in the quality of solvent for the ligands coating either the long side or the ends of the NRs, thereby inducing cor-responding regiospecific attraction forces between the NRs. Here we report a new, simple, and cost-efficient method for producing SAs comprising thousands of gold NRs. The method utilizes solution-based side-by-side assembly of the NRs into large 2D superlattices, which settle on a substrate in the most stable configuration with their long face facing the substrate, thereby forming SAs. We demonstrate the gen-erality of the approach for two exemplary systems, namely, gold NRs end-tethered with either poly(N-vinylcarbazole), or polystyrene molecules. Whereas polystyrene was used as a model polymer, poly(N-vinylcarbazole) has useful character-istics including a conductivity, photoconductivity, photo- and electroluminescence, and redox properties.[16] For example, photoconductivity of poly(N-vinylcarbazole) can be activated by UV-radiation and can be shifted into visible and near infrared spectral range by using sensitizers such as electron acceptors or dyes.[16] The co-assembly of poly(N-vinylcar-bazole) and gold NRs leads to potential applications of the resulting SAs in photoelectronics.

2. Results and Discussion

2.1. End-to-End Assembly of Polymer-Tethered Gold NRs

Figure 1. TEM images and absorbance spectra of the self-assembled chains of NRs in the DMF/water mixture at a water concentration of Cw = 15 wt%. a,c) TEM images of the chains of NRs end-tethered with PVK and PS, respectively. Scale bars are 100 nm. The inset in (c) shows a high-magnification image of an individual NR with PS ligands at both ends. Scale bar is 50 nm. b,d) Absorbance spectra of the chains of PVK-tethered (and PS-tethered) NRs assembled for 5 (1) and 360 (2) min, respectively.

We synthesized gold NRs with a mean length of 46 ± 3 nm and diameter of 10 ± 1.5 nm, which were stabilized with cetyltri-methylammonium bromide (CTAB) mol-ecules.[17] In the first series of experiments, we verified that thiol-terminated polymer ligands replaced CTAB molecules at the NR ends. It is established that preferential attachment of CTAB to the longitudinal {100} and {110} faces of the NRs makes the {111} faces at their ends deprived of CTAB and favors the replacement of CTAB with thiol-terminated molecules.[18] The site-specific replacement of CTAB at the NR ends occured only for a well-defined amount of the polymer ligands introduced in the NR solution: an excessive amount of the poly mer ligand can lead to the replace-ment of CTAB at both the ends and the long side of the NRs.

We used thiol-terminated poly(N-vinylcarbazole) and thiol-terminated poly-styrene (‘PVK’ or ‘PS’, respectively) for site-specific functionalization of the NR

ends. To verify that PVK and PS molecules bind to the ends of gold NRs, we conducted CTAB-polymer ligand exchange and examined the self-assembly of polymer-functionalized NRs in the solvent poor for the polymer ligands. First, fol-lowing the exchange of CTAB with PS or PVK at polymer concentration of 0.1 wt% (see Experimental Section), we transferred the NRs into N,N-dimethylformamide (DMF), a good solvent for CTAB and both polymers. In DMF, the PVK and PS polymers have the Flory-Huggins interaction parameters, χ, of 0.48 and 0.40, respectively,[19] and the values of Flory characteristic ratios, C∞, are 15.9 and 9.85, respec-tively,[20] indicating a higher rigidity of PVK. (We note that although DMF is a good solvent for CTAB and polymer lig-ands, they would remain attached to the NR surface, if the enthalpy gain due to the strong binding to the Au is higher than entropy gain due to desorption and dissolution). Next, we added to the NR solution a DMF/water mixture to reach the total concentration of water of 15 wt%. With the addition of water, the mixture became a poor solvent for the polymer ligands but remained a good solvent for CTAB ligands. To minimize unfavorable polymer interactions with the solvent and reduce the surface energy of the system, the poly mer molecules formed a physical ‘bond’ between the neighboring NRs. The dominant attachment of PS and PVK to the NR ends was established by monitoring NR organization in the end-to-end manner in chains.

Figure 1a shows a typical transmission electron micro-scopy (TEM) image of the chain of NRs that were modified with PVK. Inspection of the image revealed PVK globules localized between the ends of the neighboring NRs, sug-gesting that PVK was attached to the NR ends. By analyzing scanning electron microscopy (SEM) images of the chains we determined the mean volume of the PVK globules, VPVK, to be 419 nm3. The number of PVK molecules, NPVK, attached to the NR end was estimated as NPVK = (ρPVKVPVKNAV)/2Mn

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Figure 2. Absorbance spectra of the PVK-tethered NRs in the THF solution at CPVK = 0.01 wt% acquired at: a) different sonication times of the NR solution and b) different aging times without sonication of the system. PVK absorbs in the range from 230 to 345 nm.[16,30]

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0: 0 min

1: 1 min

2: 2 min

3: 3 min

5: 5 min

(0)(1)

(3)(5)

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Figure 3. SAXS patterns of the PVK-tethered NRs, after self-assembly in THF solution.

attached to the NR end[15a] for ρPS = 1.05 g/cm3 and Mn = 20 000 g/mol. Furthermore, the inset in Figure 1c highlights the site-specific attachment of PS molecules to the ends of the individual NR, whereas the long sides of the NR remain polymer-free.

The solution-based self-assembly of the NRs in chains was monitored by acquiring time-dependent absorbance spectra of the NRs (Figure 1 b,d). With increasing self-assembly time, the longitudinal plasmonic band of the NRs broadened and red-shifted, consistent with previous reports on the end-to-end assembly of gold NRs.[22] A red shift of 45 and 73 nm was observed after 360 min long self-assembly of the NRs teth-ered with PVK and PS, respectively.

2.2. Monitoring Side-by-Side Assembly of Polymer-Tethered Gold NRs

The side-by-side assembly of PVK- or PS-tethered NRs was induced by adding an aqueous solution of CTAB-stabilized NRs to the solution of PVK or PS in tetrahydrofuran (THF), a good solvent for both polymers and a poor solvent for CTAB molecules.[19,23] In contrast to the self-assembly exper-iments described above, the concentration of polymers in their respective THF solutions was dramatically reduced: the concentration of PVK, CPVK, was 0.001≤ CPVK ≤ 0.01 wt% and the concentration of PS, CPS, was 0.00001≤CPS ≤ 0.0001 wt%.

Gold NRs stabilized with CTAB were introduced into the PVK or PS solution in THF. Immediately, a distinct color change from pinkish-red to blue was observed. The absorbance spectrum of this system exhibited a broad band in the range from 650 to 1200 nm (Figure 2a), suggesting the formation of large NR aggregates.[24] The system was sonicated for various time intervals and subsequently, incubated for 24 h. After sonication, the NR solution became transparent and acquired a grey-blue color. With the sonication time increasing from 1 to 5 min, the transverse plasmon peak blue-shifted from 586 to 577 nm and increased in intensity. In addition, the near-IR band disappeared, indicating that a large number of NRs assembled in the side-by-side fashion.[25] By contrast, without sonication, the time-dependent absorbance spectra remained identical to those acquired immediately after the addition of the NRs to the PVK solution in THF (Figure 2b).

We also characterized the self-assembly of NRs in solution by using small angle X-ray scattering (SAXS). The SAXS pat-tern (structure factor S(q)) from the NR assemblies showed four Bragg’s peaks, q* = 0.0262 Å−1, q1 = 0.0579 Å−1 (the dom-inant peak), q2 = 0.1015 Å−1 and q3 = 0.1122 Å−1 (Figure 3). Analysis of the peak position ratios among q1, q2, and q3 reveals the ratio qx/q1 =

√1 :

√3 :

√4 , which corresponds to

the lattice of 2D hexagonal-close-packed (h.c.p.) cylinders. The peak position at q1 = 0.0579 Å−1 was therefore attributed to the periodicity in <10> directions, from which the inter-partcle distance was calculated as d = ( 2√

3 )( 2πq1

) = 12.5(nm), where d is the distance between the centroid of the two nearest-neighbor particles.[10c] The peak at q* = 0.0262 Å−1 corresponds to a spacing d* = 24 nm, which was calculated as d* = (2π/q*). This peak is indicative of 3D ordering in the system, and most likely is a second order peak from the lamella-like stacking of 2D nanosheets, with a repeat distance of 48 nm (2 × d*). Thus we conclude that the SAXS data con-firm that in the THF solution the PVK-tethered NRs assem-bled into close-packed hexagonal 2D lattices.

2.3. Formation of SAs of Polymer-Modified NRs

Following sonication and incubation of the NR solution for 24 h, we observed the formation of precipitate on the bottom of the vial. The supernatant was removed and the precipitate

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Figure 4. a) Absorbance spectra of: 1) the supernatant and 2) the superlattices of PVK-functionalized NRs in THF solution after removing the supernatant, 3) the dried SAs deposited on a glass substrate, at CPVK = 0.01 wt%; b–d) SEM images of SAs of NRs: b) SAs of PVK-tethered NRs on a carbon grid at CPVK = 0.01 wt%. Inset shows the top view of the fragment of the SA; c) SAs of PS-tethered NRs on a carbon grid, CPS = 0.0001 wt%; d) multilayer SAs on a silicon wafer produced by spin-coating a solution of superlattices of PVK-tethered NRs at CPVK = 0.01 wt%. The arrows show the bottom layer of SAs of NRs. Scale bars in (b–d) are 100 nm. Scale bar in inset is 40 nm.

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was re-dispersed in THF. We examined absorbance properties of the supernatant and of the precipitate. The spectrum of the supernatant showed two bands at 536 and 810 nm (Figure 4a), which were characteristic of the transverse and longitudinal plasmonic bands, respectively, of the individual NRs.[17] The presence of the individual PVK-tethered NRs in the superna-

Figure 5. a) SEM image and b) absorbance spectrum of disordered CTAB-stabilized NRs aggregates produced at CSH-PVK = 0.0006 wt%; c) SEM image and d) absorbance spectrum of PVK-tethered NRs at CSH-PVK = 0.1 wt%. Scale bars in (a,c) are 100 nm. The spectra were acquired in THF solution.

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tant was also confirmed by SEM imaging. The spectrum of the dispersed precipitate featured a single transverse plasmon band, suggesting a close-to-perfect side-by-side alignment of the NRs.

Figure 4b shows an SEM image of the precipitate of the PVK-modified NRs deposited on a carbon grid. The superlat-tices contained close-packed side-by-side assembled NRs aligned perpendicular to the substrate. We refer to these lattices as to ‘standing arrays’ (SAs). The SAs exhib-ited a long-range order over micrometer-large areas. The packing density of the NRs in the SAs was up to 1012 NRs/cm2. Figure 4c shows SAs formed from super-lattices of PS-tethered NRs when the solu-tion was drop-casted onto a solid substrate. Similar results were obtained for SAs of polymer-tethered NRs on glass, mica and indium-tin oxide (ITO) glass substrates.

A single transverse plasmon band was excited in the SA of PVK-tethered NRs deposited on a glass substrate, indicating that the side-by-side alignment of the NRs

in solution-based superlattices was largely preserved in the SAs (Figure 4a). The plasmon coupling between the polymer-tethered NRs in the SA led to the broad-ening and the red-shift of the transverse plasmonic band, in comparison with that of the individual polymer-tethered NRs.[22b,25]

Multi-layer SAs with the area exceeding ca. 4 cm2 were obtained by spin-coating the solution of NR superlattices on a silicon wafer (Figure 4d). The extent of order in such arrays was compromised, in comparison with the SAs that settled on a solid substrate under gravity, yet, the majority of NRs in the spun-cast films had a vertical orientation.

The concentrations of PVK and PS in the THF solution of 0.001 ≤ CPVK ≤ 0.01 wt% and 0.00001 ≤ CPS ≤ 0.0001 wt%, respectively, were critical for producing large-area SAs. At 0 ≤ CPVK < 0.001 wt%, the PVK-tethered NRs formed aggre-gates comprising small, randomly ori-ented clusters of side-by-side aligned NRs (Figure 5a). The corresponding absorb-ance spectrum of these structures in the THF solution is shown in Figure 5b. A broad low-intensity plasmonic band in the

spectral range of 500–700 nm was suggestive of a random ori-entation of NRs in the aggregates (Figure 5b). Addition of the polymer solution in THF to the NRs to the concentra-tion CPVK > 0.01 wt% or CPS > 0.0001 wt% yielded individual, stable NRs, as revealed by TEM imaging and the corre-sponding absorbance spectrum (Figure 5c,d, respectively).

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Figure 6. Reflection-mode FTIR absorption spectra of a) SAs of PVK-functionalized NRs; b) PVK and c) CTAB. Spectrum (a) was acquired for SAs deposited on a Si surface. The spectrum of Si was used as the reference.

/ a. u

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The presence and the location of the PVK in superlattices of PVK-tethered NRs were verified by acquiring reflection FTIR spectra of SAs deposited onto a Si substrate, as well as the spectra of films of PVK and CTAB (Figure 6). In the low frequency window, a peak at 1600 cm−1 (characteristic of the aromatic C=C bonds) was present in the spectra of SAs and PVK (Figure 6, spectra (a) and (b), respectively).[26] In the high frequency window, the spectra of SAs and PVK featured peaks at 2955, 2920, and 2855 cm−1, which were characteristic of vibrations of CH2 (anti-symmetric), CH (anti-symmetric) and CH (symmetric) groups, respectively, of PVK. Two other peaks at 3060 and 3010 cm−1 corresponded to CH in the aro-matic rings of PVK, although their relative intensities were reduced compared to the spectrum of pure PVK. In contrast, the IR spectrum of CTAB showed no band at 1600 cm−1 and its CH2 anti-symmetric and symmetric peaks in the high fre-quency window (at 2925 and 2850 cm−1, respectively) were distinct from the aliphatic CH/CH2 peaks of SH-PVK.[26] We stress that although CTAB was present in the SAs, it coated the {110} and {100} faces of the NRs long sides and in the reflection mode these faces were largely hidden from the IR probe light.

2.4. Discussion of the Mechanism of the Formation of SAs

Based on the evolution of absorbance spectra of NR solu-tions, we conclude that the side-by-side association of NRs took place in solution, rather than due to attraction forces imposed during the drying process. The formation of SAs occured upon the precipitation of the 2D superlattices, with the large face of the sheets facing the solid surface, that is, in their most stable configuration. The alignment of NRs in the superlattices was largely preserved when they settled on solid substrates as SAs: the transverse plasmonic band of the NRs in the SAs underwent an insignificant broadening and change in intensity upon precipitation and drying of the NR assemblies (Figure 4a). Because the SAs were formed by gravity-driven precipitation of the superlattices, they could be readily obtained on various substrates. Thus we conclude that

solution-based self-assembly of poly mer-terminated gold NRs in superlattices is an efficient method for producing SAs on solid substrates.

In contrast to our earlier work on solution-based formation of 2D superlat-tices of semiconductor NRs,[10a,b] the self-assembly of polymer-tethered gold NRs required two critical conditions: i) the son-ication of the dispersion of NR aggregates formed upon the addition of the polymer solution to CTAB-stabilized NRs and ii) the presence of a very small amount of the polymer in the solution (and eventu-ally, on the NR ends).

We focus first on the role of sonica-tion and discuss the role of polymer lig-ands later. When CTAB-coated NRs were introduced in the solution of the poly mer

in THF, two concurrent processes took place: polymer adsorption to the NR ends and the association of the NRs, due to the attraction forces between CTAB ligands in a poor solvent. The first process would suppress NR association in the end-to-end manner, since THF is a good solvent for PVK, whereas the second process would result in NR association in both side-by-side and end-to-end modes. Based on the inspec-tion of Figure 5a, we conclude that the rate of NR associa-tion exceeded the rate of adsorption of PS or PVK to the NR surface. Marked end-to-end NR association of small clusters of laterally adhering NRs yielded nanostructures with lack of long-range order.

The sonication step led to disintegration of the NR clus-ters and assisted in polymer adsorption to the NR ends. The ability of SH-terminated molecules to replace CTAB from the (111) plane at the ends of gold NRs has been reported by several research groups[15c,18,22a] and is supported by the results shown in Figure 1 in the present work. Based on the evolution of absorbance spectra (Figure 2a), the optimized sonication time was 3–5 min. Without sonication, 24 h long incubation of NRs in the polymer solution did not result in the formation of the superlattices, as the NRs were trapped in the aggregate.

A well-defined, small volume fraction of the polymer attached to the NRs was a critical design rule in the formation of NR lattices. Reduction in the polymer concentration in the system below a critical value did not lead to sufficient CTAB replacement at the NR ends, which resulted in both end- to-end and side-by-side association of the individual NRs and their small clusters (Figure 7a). At the optimized amount of polymer tethers, the NR ends were polymer-capped, however, attraction between the CTAB ligands on the long side of NRs dominated over repulsion between the polymer tethers and resulted in the formation of the superlattices. The minimiza-tion in energy and the narrow size distribution of the NRs led to their side-by-side assembly “in registry”, thereby leading to the orientational and positional order of the NRs in the lattices (Figure 7b). Above the optimized amount of the poly mer, repulsion between the polymer ligands dominated over association between the CTAB-coated NR sides, and

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Figure 7. Illustration of the effect of the amount of polymer ligand on the NR ends on their behavior in solution.

the NR self-assembly was suppressed (Figure 7c). The super-lattices resembled lamellae structures formed by the self-assembly of triblock copolymers with a rigid central block and two flexible side blocks.[27] Assuming that the copolymers and the NRs share certain general characteristics, we propose that the formation of large NR lamellae vs. small “hockey puck” structures was favored due to the small volume frac-tion of PVK or PS in the polymer-tethered NRs.[28]

3. Conclusion

In summary, we report a simple, straightforward, and cost-effective method for solution-based assembly of SAs of gold NRs functionalized with a conductive, photoactive polymer. The method can be used for the assembly of other types of NRs end-functionalized with different types of polymers, as long as the polymer ligand protects NRs from end-to-end association and does not suppress attraction between the lig-ands attached to the long NR side.

4. Experimental Section

Synthesis and Self-Assembly of Gold NRs: Gold NRs were syn-thesized using a method described elsewhere.[17] (see Supporting Information). To remove excess of CTAB, the NRs were centrifuged twice (8,500 r.p.m., 30 min, Eppendorf centrifuge 5417R) and redispersed in deionized water. After the second centrifugation cycle, the concentration of NRs was ∼1.0 mg/mL. Subsequently, ligand exchange was conducted, in order to replace CTAB at the NR

ends with SH-PVK or SH-PS. Thiol-terminated poly(N-vinylcarbazole) (Mn = 19 200 g/mol; Mw/Mn = 1.34) was synthesized by RAFT polymerization.[28,29] The experimental details of the syntheses of SH-PVK are provided in the Supporting Information. Thiol-terminated poly styrene (Mn = 12 000 g/mol and 20 000 g/mol; Mw/Mn = 1.07) was supplied by Polymer Source, Canada.

a) Side-by-Side Assembly of Polymer-Tethered NRs: To trigger side-by-side asso-ciation of the NRs, the NR solution was added to 10 mL of the solution of PVK or PS in THF. The resulting NR solution was sonicated for various time intervals. Subsequent 24 h long incubation of the solution at room tem-perature led to the formation of close-packed arrays of side-by-side assembled NRs on the bottom of the vial. The supernatant containing individual NRs and free polymer was carefully removed and the arrays were redispersed by adding THF.

To prepare large-area standing arrays of NRs on a substrate, a glass cylinder with the height and diameter of 2 and 5 cm, respec-tively, was attached to a silicon wafer, using a Krytox Perfluorinated Polyether Grease (DuPont Corp.) as an adhesive. A solution of

NR arrays was poured into the cylinder, which was covered to sup-press the evaporation of THF. Following the sedimentation of the arrays to the bottom of the tube, the cover was removed to allow THF evaporation.

Spin-coating a solution of NR lattices on silicon wafers was car-ried out for 100 s at 500 r.p.m. (Spin coater Laurell, model WS-400B-6NPP/Lite). Subsequently to spin-coating, the substrate was cut into small pieces for further imaging of SAs.

b) End-to-End Assembly of Polymer-Tethered NRs: The end- to-end assembly of the NRs carrying PVK or PS at their ends was car-ried out as described elsewhere.[15a,c] First, a solution (∼1.0 mg/mL) of gold NRs stabilized with CTAB was mixed with 10 mL of a 0.1 wt% solution of PVK or PS in THF. The resulting mixture was sonicated for 60 min, the solution was incubated for at least, 24 h and sub-sequently, purified by several 30-min-long centrifugation cycles at 10 000 r.p.m. The supernatant was removed, the modified NRs were dried and redispersed in DMF. The self-assembly of the polymer- tethered NRs was mediated by the drop-wise addition of the DMF/water mixture to the total content of water in the system of 15 wt%. The resulting mixture was incubated for 24 h.

Characterization of NRs Assemblies: SEM and TEM images were obtained on a Hitachi S-5200 microscope at an accelerating voltage of 10.0 kV and a current of 15 mA. A droplet of the solution con-taining NR assemblies was applied to a carbon grid or silicon wafer and the solvent was allowed to evaporate. Absorbance spectra of the assemblies—both in solutions and the dry state—were acquired by UV–visible spectroscopy using a Varian UV–vis–NIR Cary 5000 spectrophotometer (to obtain the spectra of the dried sample, the SAs, were deposited on a glass substrate using a pro-cedure described above). FTIR spectra were recorded in the reflec-tion-mode using a Bruker VERTEX 70 Series FTIR Spectrometer. The

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NR arrays were deposited on the silicon wafer from their concen-trated solution and THF was allowed to evaporate. The spectrum of the silicon wafer was used as the background or reference. The spectra of PVK and CTAB were acquired from the powder samples obtained by evaporation of the corresponding solutions in THF.

SAXS experiments were performed at the National Synchrotron Light Source’s X-9 beamline. The scattering data were collected with a charge coupled device (CCD) area detector at wavelength λ = 0.9184 Å. The data are presented as the variation in structure factor S(q) versus scattering vector, q = (4π/λ)sin(θ/2), where θ is the scattering angle. The values of q were calibrated with silver behenate (q = 0.1076 Å−1). S(q) was calculated as Ia(q)/Ip(q), where Ia(q) and Ip(q) are background-corrected azimuthally aver-aged 1D scattering intensities for a system under consideration and un-aggregated nanorods, respectively. The peak positions in S(q) are determined by fitting a Lorentzian form.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

The authors thank funding from NSERC Canada through the CRC program and Discovery Grant (NSERC) and funding from NSF (DMR 09-69301). KL thanks funding of the Post-DoctoralFellow program aware from the Ontario Ministry of Economic Development and Innovation. Research was carried out in part at the Center for Func-tional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. The authors thank Dr. Oleg Gang for helpful discussions.

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Received: June 29, 2011 Revised: November 16, 2011 Published online: January 9, 2012

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Standing Arrays of Gold Nanorods End-Tethered with Polymer Ligands

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