Macroscopic Assembly of Gold Nanorods into Superstructures with

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Macroscopic Assembly of Gold Nanorods into Superstructures with Controllable Orientations by Anisotropic Affinity Interaction Yun Rong, Liping Song, Peng Si, Lei Zhang, Xuefei Lu, Jiawei Zhang, Zhihong Nie, Youju Huang, and Tao Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03538 • Publication Date (Web): 11 Nov 2017 Downloaded from http://pubs.acs.org on November 13, 2017

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Macroscopic Assembly of Gold Nanorods into Superstructures with Controllable Orientations by Anisotropic Affinity Interaction Yun Rong,a,b† Liping Song,a,b† Peng Si,*c Lei Zhang,a,b Xuefei Lu,a Jiawei Zhang,a,b Zhihong Nie,d Youju Huang,∗a,b Tao Chen*a,b

a，Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China b，University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China c，Department of Structural Biology, Stanford University, Stanford, CA 94305, USA d，Department of Chemistry and Biochemistry, University of Maryland, College Park, MD, USA †

These authors contributed equally to this work.

∗

E-mail: [email protected]

∗

E-mail: [email protected].

∗

E-mail: [email protected].

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ABSTRACT: 2D or 3D highly ordered arrays of anisotropic nanoparticles provide attracting properties that are highly desired by industry. Traditional assembly method such as evaporation usually produces the nanostructure arrays only up to milimeter scale with poor control of nanoparticle orientation, making them hardly applicable for industrial needs. Here we report a facile method to assemble centimeter-scale gold nanorod (Au NR) arrays with highly controlled nanoparticle orientation and high reproducibility. We selectively functionalized the transverse or longitudinal facets of the Au NRs with polyethylene glycol (PEG) molecules and utilized the interfacial polymeric affinity between the PEG domains on Au NRs and the PEGylated substrate to achieve the anisotropic self-assembly. The side-PEGylated Au NRs formed closely packed horizontal arrays while the end-PEGylated Au NRs formed vertically standing arrays on the substrate, respectively. The obtained Au NR arrays with different orientations showed anisotropic surface enhanced Raman scattering (SERS) performance. We showed that the vertically ordered Au NR arrays exhibited 3 times higher SERS signals than the horizontally ordered arrays.

KEYWORDS: Gold nanorods, superstructures, controllable orientations, anisotropic affinity interaction, surface enhanced Raman scattering


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1. INTRODUCTION Assembling anisotropic nanoparticles, especially gold nanorods (Au NRs), into 2D or 3D ordered arrays has attracted increasing interest in recent years1, 2 since the Au NR ordered arrays exhibit unique collective properties such as extremely strong local electromagnetic field3 which yields enhanced optical signals4, 5. 2D/3D plasmonic arrays with different Au NR orientations possess distinctive physical and chemical properties6 that meet a variety of practical needs including biosensors, optoelectrionic devices7, solar cells8 and water splitting cells9. Various techniques including Langmuir-Blodgett deposition10,

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, air/water interfacial assembly12, and

droplet evaporation13-15, have been developed to obtain the Au NRs assemblies. Among them, droplet evaporation has been intensively investigated because of its operability, costeffectiveness16, and wide applicability to various functional nanomaterials17. Generally, there are two strategies: one is to control the evaporation condition of the CTAB stabilized Au NRs during the droplet drying process3, 18; the other is to functionalize the Au NRs surface with specific ligand such as Gemini surfactants13, lipids19 or OH-terminated hexa(ethylene glycol) alkanethiol20 to induce the assembly of the Au NRs. Despite of these efforts, current methods have a number of limitations. First, most studies were only able to assemble Au NRs into micormeterto millimeter-scale films with poorly controlled uniformity, making them difficult for practical device applications21. It is even more difficult to assemble Au NRs with highly controlled diverse orientations with droplet evaporation method. Another challenge is to achieve singlelayer nanocrystals on substrates that can be compatible with practical applications22, because films with such a small thickness tend to crack and have missing nanocrystals in certain regions23. In addition, the assembly procedure of Au NR superstructures usually requires


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stringent control24 of the experimental conditions and tedious purification steps25, which leads to poor reproducibility. Here, we developed a selective functionalization approach to fabricate large-scale Au NR arrays with precisely controlled Au NR orientations and assembly layers by manipulating the affinities between the Au NRs and the substrate. We created two different types of Au NR building blocks with the PEG coating only on the transverse surfaces (end-PEGylated Au NR) and longitudinal surfaces (side-PEGylated Au NR), respectively. Such selective PEG functionalization allows the Au NRs to possess anisotropic chemical properties, leading to their self-assembly on the PEGylated substrate with different orientations through polymeric affinity interaction. The end- and side-PEGylated Au NRs formed highly ordered vertical and horizontal arrays on the PEGylated substrate respectively. The size and 3D architecture of the Au NR assemblies can be precisely controlled by adjusting the Au NR concentrations. Compared to conventional evaporative self-assembly methods26-28, our method allows the fabrication of highly uniform Au NR arrays with close to 100 % macroscopic orientation control and controllable larger sizes (up to several centimetres). Due to the affinity between Au NRs and substrate, our method has the advantage of assembling highly ordered Au NRs arrays without tedious multistep reactions and rigorous experimental conditions29 which ensures the reproducibility. The Au NR arrays with different macroscopic orientations exhibit anisotropic SERS performance. The vertically ordered single-layer Au NRs array showed 3-fold higher SERS signals than the horizontally ordered single-layer Au NR arrays.

2. EXPERIMENTAL SECTION


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2.1. Materials and Instrumentation. Hexadecyltrimethylammonium bromide (CTAB, ＞99.0 %) was purchased from Sigma-Aldrich. Sodium borohydride (NaBH4, 99.0 %), silver nitrate (AgNO3, >99.0 %), ascorbic acid (AA, 99.7 %), hydrochloric acid (HCl, 37 wt %). in water), were purchased from Sinopharm Chemical (Shanghai, China). Thiol-terminated methoxy poly (ethylene glycol) (HS-PEG, Mw=2000), PEG silane (Mw=1000 g/mol) and sodium oleate (NaOL, >97.0 %) was purchased from TCI. Hydrogen tetrachloroaurate trihydrate (HAuCl4.3H2O, 99.99 %), was obtained from Aladdin Company in Shanghai. Other reagents and solvents were analytical reagent grade and were used as received from Sinopharm Chemical Reagent. Ultraviolet-visible (UV-vis) absorption spectra were recorded with TU-1810 spectrophotometer from Beijing Purkinje General Instrument Co. Ltd. in transmission mode. Transmission electron microscopy (TEM) was performed on a JEOL JEM-2100F instrument and operated at 200 kV. Scanning electronic microscopy (SEM) measurements were carried out using a JEOL JMS6700F scanning microscope. 2.2. Synthesis of Au NRs: The gold nanorods (Au NRs) was prepared by a previously reported method30. The seed solution was prepared by rapidly injecting 0.6 mL of fresh 0.01 M NaBH4 into the mixture of 5 mL of 0.5 mM HAuCl4 and 5 mL of 0.2 M CTAB solution in a 20 mL scintillation vial under vigorous stirring (1200 rpm). Stirring was stopped after 2 min and the seed solution was aged at room temperature for 30 min before use. To prepare the growth solution, 3.5 g of CTAB and 0.617 g of NaOL were dissolved in 125 mL of warm water. Then 9 mL of 4 mM AgNO3 solution and 125 mL of 1mM HAuCl4 solution were added when the solution was cooled to 30 °C. After 90 min of stirring (700 rpm), 0.75 mL of HCl (37 wt % in water, 12.1 M) was introduced to adjust the pH. After another 15 min of slow stirring at 400 rpm,


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0.625 mL of 0.064 M ascorbic acid (AA) was added and the solution was stirred vigorously for 30 s. Finally, 0.2 mL seed solution was injected into the growth solution. The resulting mixture was stirred for 30 s and left undisturbed at 30 °C for 12 hours for NRs growth. The resulting nanocrystals were washed twice by centrifugation prior to characterization and re-dispersed in water for further use. 2.3. PEGylation of silicon substrate: The PEG monolayer was grafted onto the silicon wafers by the method reported by Papra et al31. The silicon wafers were cleaned by sonication in ethanol/water (1:1, v/v) for 5 min prior to use. Oxidation of the silicon wafers was carried out by treatment with a mixture of hydrogen peroxide (30 %, w/v) and sulfuric acid (96 %) (20: 80, v/v) for 10 min at 120 °C. This mixture should be used with extreme caution due to its high oxidizing power and risk of explosion. The samples were washed 3 times in water, sonicated in water for 10 min, blown dry with air, and immediately immersed in a solution of 3 mM PEG silane (Mw=1000 g/mol) in toluene with 0.8 mL/L of concentrated HCl) for 18 h at room temperature. Afterward the wafers were washed once in toluene, twice in ethanol, and twice in water and then sonicated in water for 2 min to remove nongrafted material. The wafers were blown dry with air and stored dry under ambient conditions. 2.4. Asymmetric Functionalization of Au NRs: Preparation of end-PEGylated Au NRs. 4 mL of the GNR solution was centrifuged twice (9000 rpm for 7 min). After the removal of as much of the supernatant as possible, the precipitate (about 10 µL) was diluted to 1 mL with deionized water and then mixed with the HS-PEG solution (10 mM, 10 µL). The mixture was gentle stirred in 30 °C water bath for 24 h. The final rod concentrations were adjusted to 5, 15, 25, and 35 nM (ultrasonic bath for 1 min) for selfassembly.


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Preparation of Side-PEGylated GNRs. 4 mL of the GNR solution was centrifuged for the twice (9000 rpm, 30 °C for 7 min). After the removal of as much of the supernatant as possible, the precipitate (about 10 µL) was diluted to 1 mL with deionized water and then mixed with the HSC6H12OH in solution (10 mM, 10 µL). The mixture was gentle stirred in 30 ℃ water bath for 24 h. The solution was centrifuged (9000 rpm, 30 °C for 7 min) to remove the excess HS-C6H12OH. The precipitate redisperse in 1 mL of deionized water and mixed with HS-PEG solution (10 mM, 500 µL). The final rod concentrations were adjusted to 5, 15, 25, and 35 nM (ultrasonic bath for 1 min) for self-assembly. 2.5. Self-Assembly of the asymmetric functionalized Au NRs: 20 µL of the concentrated GNR solution was dropped onto a clean silicon wafer for slow evaporation at 25 °C in a 10 mL sample vial sealed with the lid. 2.6. SERS property of the Self-Assembled arrays: The arrays were soaked in aqueous solutions of 10-6 M 4-MPy solution overnight for adsorption of the 4-MPy molecules. Raman scattering spectroscopy was conducted on thesaurus arrays using a Confocal Laser Scanning Microscope (Renishaw) Raman Spectrometer excited with a solid-state laser (λ=633 nm), 10 % of the intensity and exposure time was 10 seconds.


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3. RESULTS AND DISCUSSION

Figure 1. Schematic illustration of the fabrication of precisely controlled self-assembled Au NR structures on the substrate. Au NRs with ∼80 nm length and ∼20 nm diameter (Fig. S1) were used for the assembly. It is important to note that the aspect ratio of Au NRs has certain impact on the distribution of sulfhydryl molecules. As was reported, the end {111} facet of Au NR is more exposed and accessible to chemical linkers than the longitudinal {100} facet.32, 33 For the Au NRs with high aspect ratio (approximate to nanowire), despite of the lattice structure difference between their longitudinal and transverse surfaces, the small surface area of the transverse end might decrease the odds of chemical linkers binding on the {111} facet. The as synthesized Au NRs were capped by a bilayer of positively charged CTAB molecules34, producing strong positive repulsion that protects Au NRs against aggregation. However, the repulsion also prevented Au NRs from forming closely packed self-assemblies. Therefore the CTAB molecules have to be replaced in order to facilitate the nanorod self-assembly process35. The method for asymmetric functionalization of the Au NRs was illustrated in Fig.S2. The CTAB bilayer is less ordered and


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dense at the ends than on the side surface of Au NRs owing to the larger curvature at the ends36. In addition, Au-S bond is much stronger than the static electricity interaction between the Au NRs and CTAB molecule.3,

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When a small amount of thiol-terminated methoxypoly

(ethylene glycol) (HS-PEG) was added into the Au NRs solution, the majority of the thiols preferentially adsorbed onto the transverse surface of Au NRs34, forming end-PEGylated Au NRs. It is worth noting that the PEG molecular weight has certain impact on the assembly of Au NRs according to previous work.37-39 Generally, nanoparticles (NPs) functionalized with polymers of low molecular weight often have poor assembly due to insufficient polymeric affinity interaction. NPs conjugated with high molecular weight polymers can result in less ordered assembly behaviors. Therefore, the PEG molecular with moderate weight was used in this work for Au NR assembly. The formation of PEGylated Au NRs was validated by the FTIR absorption spectra (Fig.S3), which showed two new peaks at 2887 and 1108 cm-1, attributing to the symmetric stretching of CH2-O and the stretching vibration of ether bonds of the PEG, respectively. We also observed the decrease in peaks at 2849 and 2917 cm-1, which attribute to the symmetric and asymmetric (CH2)- vibrations of CTAB. The side-PEGylated Au NRs were prepared following two step ligand exchange process. First, we protected the transverse surfaces of Au NRs by reacting Au NRs with 6-mercapto-1-hexanol (6-HS-C6H12OH). The process was monitored by the FTIR (Fig.S3), showing a small peak at 3482 cm-1, corresponding to the vibration of -OH in 6-HSC6H12OH. After that, Au NRs were reacted with SH-PEG to form side-PEGylated Au NRs by exchanging the CTAB molecules at the longitudinal surfaces of the Au NRs with thiolterminated PEG. The end-PEGylated and side-PEGylated Au NRs were used as the building blocks to fabricate Au NR self-assembled arrays with precisely controlled orientations (Figure


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1). The PEGylated silicon wafer was used as a substrate to form the polymeric affinity interaction with the PEG domains selectively functionalized on Au NRs. Grafting of PEG onto the silicon substrate was achieved by hydrolysis of the PEG-terminated silanes, resulting an ultra-thin hydrophilic and stable PEG monolayer on the silicon substrate. A droplet (typically 20 µL) of PEG functionalized Au NRs solution was dropped on the PEG functionalized substrate and left to dry in a sealed vial in room temperature (Fig.S4) for over 24 hours. As the water evaporated, the contact line of the droplet underwent stick-slip motion and fomed a coffee ring like stain (Fig. S5a and Fig. S6a) on the substrate.

Figure 2. SEM images of the vertically (a) and horizontally (b) aligned Au NRs array obtained from the end-PEGylated Au NRs and the side -PEGylated Au NRs on the substrate, respectively.

Fig. 2 shows high magnification SEM images of the self-assembly structures formed by endand side-PEGylated Au NRs, respectively. In the vertical array formed by end-PEGylated Au NRs, the nanorods stood vertically on the substrate in a closely packed quasi-hexagonal mode, with an inter-rods distance of ~4 nm (Fig. 2a and Fig. S5d). This distance well matches the thickness of CTAB bilayers. A 2D superlattice structure of approximately 1500 µm2 was formed by the end-PEGylated Au NRs (Fig. S5a). In contrast to the end-PEGylated Au NRs, side-


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PEGylated Au NRs lied on the substrate in an end-to-end manner, forming closely packed parallel rows each containing thousands of Au NRs (Fig. 2b and Fig. S6).

Figure 3. Schematic illustration of evaporation mediated self-assembly process and the force between the Au NRs and the substrate.

The macroscopic orientation of the Au NR assemblies can be precisely controlled by selective functionalization of Au NRs followed by evaporation mediated self-assembly. The self-assembly structures of the of PEG functionalized Au NRs rely on the equilibrium of the driving force40. The self-assembly process is affected by relatively weak forces at molecule scale such as Van der Waals forces and large length scale force such as capillary forces. Fig. 3 shows the schematic illustration of the self-assembly process. During drying, the solvent evaporate more quickly at the edge than that in the canter and the solvent from the edge is replenished by solvent from the interior. The outward solvent flow carries dispersed Au NRs to the edge (Figure 3a). As a result, the concentration of Au NRs at the edge gradually increases and becomes much higher than that


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in the centre. When the Au NR concentration at the edge increases to a certain point, the longrange interactive force between the Au NRs, induced by the surface tension of the solvent and the attractive interaction between the nanoparticles, causes the Au NRs to coalesce and form close-packed arrays on the substrate. The polymer chains of PEG at the end or side of the Au NRs could anchor Au NRs on the PEGylated substrate and facilitate the accumulation of Au NRs into aggregates and by interacting with each other. For the end-PEGylated Au NRs, their PEG could tangle with the PEG on the substrate (Figure 3b), assembling the Au NRs on the substrate vertically. For the side-PEGylated Au NRs, their longitudinal side is covered with PEG, leading Au NRs to assemble on the substrate horizontally. Then the lateral capillary force acts on the partially immersed Au NRs and brings them more together (Fig. 3c and Fig. 3e). To verify the Au NR self-assembly structure was formed as a result of polymeric affinity interaction between PEGylated Au NRs and PEGylated substrate, control evaporation experiments were conducted with either non-PEGylated Au NRs or non-PEGylated substrate. Both conditions induced disordered Au NR aggregates with random orientations (Fig. S7 and Fig. S8), indicating the PEGylation on both Au NRs and substrate play critical roles on controlling ordered Au NR selfassembly. The evaporation mediated self-assembly process is also influenced by the concentration of the Au NRs. For both end- and side-PEGylated Au NRs, when the Au NR concentration was very low (such as 10-12 M), random deposition of the Au NRs on the substrate was observed (Fig. S9). At this concentration, the Au NRs are too far apart from each other that the attractive forces between them are too weak to trigger the self-assembly. The size and the layer of vertical and horizontal aligned Au NRs array can be tuned by changing the concentration of the Au NRs. For the end-PEGylated Au NRs, they assembled into small individual islands composed of vertically


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aligned single-layer arrays at concentration as low as 5 nM. These islands had an average diameter of 300 nm and randomly scattered on the substrate (Fig. 4a). When the concentration increased to 10~20 nM, the islands enlarged to over 1 µm in their sizes (Figure 4b and 4c), but remained single-layer structure. However, when the Au NR concentration reached 40 nM, the assembly array became a 3D structure with at least 4 layers (Fig. 4d).

Figure 4. SEM images of vertically aligned Au NRs array obtained from different concertation of end-PEG coated Au NRs. The concertation of the samples in (a), (b), (c) and (d) are 5 nM, 10 nM, 20 nM and 40 nM, respectively.


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Figure 5. SEM images of vertically aligned Au NRs array obtained from different concertation of side-PEGylated Au NRs. The concertation of the samples in (a), (b), (c) and (d) are 5 nM, 10 nM, 20 nM and 40 nM, respectively.

For the side-PEGylated Au NRs, at low concentration (5 nM), the Au NRs assembled side by side into small bundles, where Au NRs and randomly spread on the substrate as single layer (Fig. 5a). When the concentration increased to 20 ~30 nM, the number of the Au NRs in the bundles increased to tens and the bundles rearranged into rows which were almost parallel to each other (Fig. 5b and Fig. 5c). When the concentration of the Au NRs reached 50 nM, the assembly formed a multi-layer structure (Figure 5d).


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Figure 6. Raman spectrum of 4-Mpy powder deposited on silicon wafer (a). SERS spectra of 4Mpy adsorbed on horizontally (b) and vertically (c) aligned Au NRs arrays.

Precise control over the orientation and spatial arrangement of the Au NRs assembly possess the ability that translates the plasmonic property from individual Au NRs to the macroscopic materials by the coupling between Au NRs. The plasmonic property of the Au NRs assembly was measured by surface enhanced Raman scattering (SERS), which was performed with 633 nm excitation and 4-Mercaptopyridine (4-MPy) was selected as a model analyte. The SERS signals of single-layer horizontal and vertical Au NR arrays were compared with the net 4-MPy powder and shown in Figure 6. Owing to the strong coupling between the assembled Au NRs in both vertically and horizontally aligned Au NRs arrays, as revealed by Fig. 6 (b) and (c), the C-S stretching mode at 1092 cm-1 and the ring-breathing vibration band at 1002 cm-1 of 4-Mpy were largely enhanced because of the absorption on the Au surface via the Au–S covalent bonding41. Other typical Raman band of 4-Mpy such as at 1197 and 1613 cm-1 were also enhanced, which can hardly be observed in the spectrum of 4-Mpy powder. Interestingly, the absolute intensity of


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the peaks associated with the vertically aligned Au NRs arrays was 3 fold higher than the peaks associated with horizontal arrays, which is accorded with the previous report42,

43

. These

differences could be explained by the electromagnetic field distribution around the Au NRs in the different orientation arrays. This observation also indicates the importance of the geometric orientation control of the Au NRs assembly structure for the SERS application.

Figure 7. Variation of SERS intensity of 4-Mpy acquired from 10 different spots on the vertically aligned Au NRs array (a) and horizontally aligned Au NRs array (b).

To further assess the reproducibility of the Au NRs arrays as SERS substrates, a thorough statistical analysis was undertaken to quantify the variations in the SERS signal intensity from 10 different spots over the assembly structures. The obtained spectra showed excellent consistency for the 10 measurements (Fig. 7). The intensity change of the Raman band at 1002 cm-1 was used to calculate the relative standard deviation (RSD) of the intensity from 10 points (Fig. S10). The RSD values were measured to be 6.7 % and 5.1 % for the vertically and horizontally aligned Au NRs arrays respectively, indicating the Au NR arrays had remarkably good uniformity. These results demonstrate that our evaporation mediated self-assembly method was capable of fabricating highly uniform Au NR arrays for high-performance SERS applications.


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4. CONCLUSIONS In conclusion, we developed a facile approach for the fabrication Au NRs arrays with precisely controlled orientation and spatial architecture. We have successfully achieved vertically and horizontally aligned Au NR arrays on the PEGyated substrate by selectively grafting PEG molecules only on the transverse and longitudinal surfaces of Au NR respectively. The Au NRs formed highly ordered self-assembly on the substrate due to the polymeric affinity interaction between PEGylated Au NRs and PEGylated substrate during droplet evaporation. By tuning the Au NR concentration, we were also able to precisely control the assembly layers of the 3D Au NR superlattice. The uniform alignment of Au NRs could couple the external electromagnetic fields which lead to a strong confinement of light within nanoscale gaps, hence enabling their exciting applications as novel SERS substrates with high sensitivity and reproducibility. Au NR arrays with different orientations led to different factors of enhancement because of the different densities of the hot spot and couple modes. We believe our facile approach for Au NR controlled assembly can be readily extended to fabricate ordered arrays of many other anisotropic nanostructured materials, which may provide interesting plasmonic properties and enable unprecedented applications.

Supporting Information TEM image and SEM image of Au NRs, schematic illustration of the asymmetric functionalization of CTAB-Au NRs, Reflection-mode FTIR absorption spectra of CTAB-Au NRs, end PEG coated Au NRs and side PEG coated Au NRs. SEM images of vertically and horizontally aligned Au NRs array with different magnification, SEM images of the assembly of the CTAB stabilized Au NRs on the PEG functionalized Si substrate, SEM images of the


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assembly of the CTAB stabilized Au NRs, end PEG coated Au NRs and side PEG coated Au NRs on Si substrate without PEG, The histogram of the SERS intensity at 1002 cm-1 of 4-Mpy acquired from 10 different spots on the of the vertically aligned Au NRs array and horizontally aligned Au NRs array ,this material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENTS We thank the Natural Science Foundation of China (51473179), Ningbo Science and Technology Bureau (2015C110031), Youth Innovation Promotion Association of Chinese Academy

of

Sciences

(Grant

No.

2016268

and

2017337).

Key Research Program of Frontier Science, Chinese Academy of Sciences (QYZDB-SSWSLH036), the project of International Cooperation Foundation of Ningbo.

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Table of Contents Graphic

Macroscopic Assembly of Gold Nanorods into Superstructures with Controllable Orientations by Anisotropic Affinity Interaction Yun Rong, Liping Song, Peng Si, Lei Zhang, Xuefei Lu, Jiawei Zhang, Zhihong Nie, Youju Huang, Tao Chen


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Macroscopic Assembly of Gold Nanorods into Superstructures with

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