Electric-Field-Assisted Assembly of Polymer-Tethered Gold Nanorods

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Electric-Field-Assisted Assembly of PolymerTethered Gold Nanorods in Cylindrical Nanopores Ke Wang, Seon-Mi Jin, Jiangping Xu, Ruijing Liang, Khurram Shezad, Zhigang Xue, Xiaolin Xie, Eunji Lee, and Jintao Zhu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b00487 • Publication Date (Web): 07 Apr 2016 Downloaded from http://pubs.acs.org on April 8, 2016

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Electric-Field-Assisted Assembly of Polymer-Tethered Gold Nanorods in Cylindrical Nanopores Ke Wang,† Seon-Mi Jin,‡ Jiangping Xu,† Ruijing Liang,† Khurram Shezad,† Zhigang Xue,*,† Xiaolin Xie,† Eunji Lee,*,‡ and Jintao Zhu*,†

†

Key Laboratory of Materials Chemistry for Energy Conversion and Storage (HUST) of Ministry

of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ‡

Graduate School of Analytical Science and Technology, Chungnam National University,

Daejeon 305764, Republic of Korea

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ACS Paragon Plus Environment

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ABSTRACT: In this report, we demonstrate the confined assembly of polymer-tethered gold nanorods in anodic aluminum oxide (AAO) channels with the assistance of electric field (EF). Various interesting hybrid assemblies, such as single-, double-, triple-, or quadruple-helix, linear, and hexagonally packed structures are obtained by adjusting pore size in AAO channels, ligand length, and EF orientation. Correspondingly, surface plasmonic property of the assemblies can thus be tuned. This strategy, by coupling of external-field and cylindrically confined assembly, is believed to be a promising approach for generating ordered hybrid assemblies with hierarchical structures, which may find potential applications in photoelectric devices, biosensors, and data storage devices.

KEYWORDS: Gold nanorods, Electric field, Cylindrical confinement, Confined assembly, Helix structure

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The organization of functional nanoparticles (NPs) into one dimensional (1D), 2D and 3D hierarchical structures can be applied to fabricate advanced materials and devices with enhanced properties (e.g., plasmonic coupling and plasmon-exciton).1-5 Great progress has been achieved in controlled assembly of NPs by using template,6, 7 external field,8, 9 biomolecule recognition,10, 11 and directional chemical binding.2 Compared to isotropic NPs, the anisotropic nanorods (NRs) exhibit unique optical, electronic or magnetic responses.12-16 Especially, 1D assembly of NRs has showed wide applications in sensors, catalysis, photoelectric devices and data storage, due to the localized surface plasmon resonance (LSPR) and large surface energy of the ordered structures. Recently, confined assembly provides a facile route to fabricate 1D assembly.17-19 Both soft and hard spheres have been employed to assemble into 1D assemblies with various interesting structures (e.g., linear chain, zig-zag, and double-helical structures) in strong 2D confinement (e.g., anodic aluminum oxide (AAO) channels or carbon nanotubes) by tuning particle and confining dimension.20, 21 Yet, to the best of our knowledge, confined assembly of anisotropic NPs has been rarely reported. Moreover, only limited types of well-ordered structures of NRs assemblies have been obtained in the weak confinement.22, 23 It is still highly desirable to generate 3


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hierarchical NRs assemblies with unique structures under 2D strong confinement.24

Here, we report a facile yet effective strategy to control the assembly of polystyrene-tethered AuNRs (AuNRs@PS) under cylindrical confinement with the assistance of electric field (EF) (Scheme 1). Our results show that AuNRs@PS can assemble into a variety of well-ordered structures by tailoring the orientation of EF, molecular weight of PS (Mw-PS), and pore size of AAO channel (DC). Moreover, plasmonic properties of the hybrid assemblies can be tuned by manipulating interparticle distance. These findings are helpful to understand confined assembly of NRs. Also, this strategy provides a robust route to prepare 1D NRs assemblies with potential applications in optoelectronic devices and data storage.

RESULTS AND DISCUSSION In a typical experiment, AuNRs were synthesized through seed-growth method25 and functionalized with thiol-terminated PS (PS-SH) by ligand exchange procedure.26 The AAO membrane was then immersed in AuNRs@PS dispersion, and the AuNRs@PS filled the channel due to capillary force as the solvent evaporated. Subsequently, AAO membrane was annealed under saturated chloroform vapor for 24 h. During the annealing process, a direct current EF with tunable direction was applied. Isolated AuNRs@PS assemblies were obtained after removal of the AAO template by aqueous sodium hydroxide solution.

EF played a key role in determining the ordered assembly of AuNRs@PS under 2D

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confinement (Figure S1 in the Supporting Information (SI)). In the absence of EF, disordered structures with random orientation of AuNRs and no special packing style were obtained when fused in the nanopores (Figure S1a and S2), which seems to be inconsistent with that of PS-tethered Au nanospheres in our previous report.20 The reason can be ascribed to the anisotropic shape of NRs, consisting of the long axis and short axis which enhanced steric hindrance for rotation and shift of AuNRs under 2D strong confinement, and energy barrier due to thermodynamically metastable state (Scheme S1). External field, for instance EF, can provide the driving force to re-orientate anisotropic NRs.27-29 When NRs are putted in an EF, due to the dipolarization of NRs, the positive and negative charges around the end of the NRs are created (Scheme S2). The charges will be attracted with electrode, provide the driving force to rotate NRs, and make the NRs along the electric field line when EF-induced torque is greater than thermal excitation energy.30,

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To order NRs, we applied parallel EF on the confined assembly of

AuNRs@PS. With low electric field intensity (E) (e.g., 0.125 × 105 or 0.625×105 V/m), disorder structures were still observed (Figure S1b and S1c). When E was greater than 1.25 × 105 V/m, ordered assemblies with special packing style (which will be discussed in the following part) and AuNRs parallel to EF were obtained (Figure S1e-f). Relationship between electric field force (F) and E can be written as:

F = E2 (εAu@ PSl 2 ) / k

(1)

Where ε Au @ PS and l are permittivity and length of AuNRs@PS, respectively. While k is 5


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the Coulomb's constant (see SI for more details). Clearly, F exponentially increases with that of E. Thus, E could effectively influence the EF force, and confined assembly of AuNRs@PS. Generally, the EF plays at least two important roles in the formation of ordered structures under cylindrical confinement: i) initially assisting rotation of AuNRs; ii) keeping orientation of AuNRs along the EF line to eliminate random rotation in thermal motion in order to weaken the steric hindrance. Presumably, this will enhance the driving force for orientation, and reduce energy barrier to make the system into thermodynamically stable state.

Furthermore, direction of the applied EF significantly influences the confined assembly of AuNRs@PS. Interesting hybrid assembles with varied orientation of AuNRs were observed when tilting the EF (Figure 1 and 2). Clearly, orientation of AuNRs was induced along the EF line. Moreover, orientation of EF could further affect effective diameters of AuNRs@PS to adjust the confinement strength, defined as ratio of the pore size to NRs size (DC/DNR), which determined packing style of the assembly. Meanwhile, confinement strength has been proved to be critical parameter in governing confined assembly.19, 20, 32-35 In this study, we defined DNR-S and DNR-L as the overall effective diameters of AuNRs@PS when it was parallel or perpendicular to the long axis of AAO channel, respectively (Scheme S3). Characteristics of pore size DNR-S and DNR-L of AuNRs coated with different Mw-PS are shown in Table 1. When EF is parallel to the AAO channel, DNR-S is considered as the effective diameter, since AuNRs@PS orientate along the long axis of the channel. When DC is 24.6 nm, side-by-side 6


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stacking of NRs is limited while triple-helix structure from AuNRs@PS5k is obtained due to strong confinement (Figure 1a), where AuNRs showed a 12 ± 2° tilt angle to the direction of parallel EF. Increment in Mw-PS from 5 to 50 kg/mol enlarges DNR-S, resulting in enhanced confinement. Thus, hybrid assembly changes from triple-helix to two-NR layers, double-helix, and finally to linear chain (Figure 1b-d), where DC/DNR-S presents the value of 1.75 to 1.00. Importantly, enlargement of DC increases the DC/DNR-S, leading to some complex structures due to weaker confinement. For example, when DC = 61.2 nm, hexagonally packed structure from AuNRs@PS5k was obtained (Figure 1e). In this case, increasing Mw-PS from 5 to 50 kg/mol gives rise to the stacking of more complex NRs assemblies (Figure 1f-h). 3D tomography reconstruction reveals the hexagonally packed structure from confined assembly of AuNRs@PS20k (Figure S3 and Movie S1). The compact nature can be ascribed to the collapsed NRs during sample preparation. Increment of DC to 96.3 nm further weakens the confinement strength, where multilayer packed structure was observed for AuNRs@PS5k assembly (Figure 1i). However, switch of EF orientation from parallel to perpendicular leads to significant different hybrid assembly of AuNRs@PS (Figure 2), where DNR-L acts as the effective diameter. In this case, since AuNRs@PS filled the channels before applying EF, assemblies could still be formed although DC (24.6 nm) is narrower than DNR-L. Surprisingly, when DNR-S of AuNRs@PS5k (DNR-S = 14.1 nm) is close to the radius of channel (DC/2 = 12.3 nm), helical two-NR layer-like structure was observed (Figure 2a). Yet, when DNR-S (e.g., AuNRs@PS12k, AuNRs@PS20k and AuNRs@PS50k) is beyond the radius of DC, assemblies with single-linear chain like structure was 7


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obtained (Figure 2b-d). Notably, the assemblies from AuNRs@PS12k and AuNRs@PS20k stacked as single-linear chain with side-by-side orientation of AuNRs at a certain angle, where side-by-side distances are 2.7 ± 0.9 nm and 2.9 ± 0.6 nm, respectively, while the assembly from AuNRs@PS50k stacked as single-linear chain with end-to-end orientation of AuNRs with end-to-end distance of 14.2 ± 3.6 nm. Presumably, though AuNRs couldn’t lie down in the channel by totally perpendicular orientation, they still kept the orientation to the axis of EF line as far as possible. Under confinement strength of narrow channel and applied EF, the orientation of AuNRs kept a maximum angle to the axis of EF line. This phenomenon has not been observed for spherical NPs due to the isotropy of the particles. More interestingly, the maximum angle changed with the variation of Mw-PS. For instance, when Mw-PS increased from 12 to 20 kg/mol, DNR-S enlarged from 16.0 to 22.9 nm which is narrower than DC (24.6 nm), implying there is free space for AuNRs@PS to tilt in the channel, and maximum angle was 45 ± 6° and 56 ± 5°, respectively (Figure 2b and 2c). Yet, when Mw-PS increased to 50 kg/mol, DNR-S increased to 24.6 nm (close to DC), indicating that there is no free space for AuNRs@PS to tilt, and maximum angle appeared to be 90 ± 2° (Figure 2d). This is the reason why AuNRs@PS50k assembly still stacked as single-linear chain with end-to-end packing under perpendicular EF.

Increase of DC over DNR-L lies down AuNRs in the channel and forms various interesting assemblies under perpendicular EF. For example, when DC reaches 61.2 nm, triple-helix structure from AuNRs@PS5k is observed (Figure 2e), where the tilt angle to the direction of perpendicular EF is 11 ± 3° and side-by-side distance is 5.8 ± 2.3 nm. Raising Mw-PS from 5 to 50 kg/mol 8


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strengthens the confinement effect, forming different kind of helix structures. For example, double-helix structure forms from AuNRs@PS50k (Figure 2h), which is confirmed through 3D tomography reconstruction (Figure 3 and Movie S2), where the tilt angle to the direction of perpendicular EF is 11 ± 2° and side-by-side distance is 12.7 ± 4.8 nm. Similarly, further increase of pore size to 96.3 nm weakens the confinement strength, leading to more complex assemblies (Figure 2i-l). 3D tomography reconstruction precisely explored the assembly from AuNRs@PS5k (Figure 4 and Movie S3). Clearly, quadruple-helix structure, where AuNRs formed four individual helix lines, is obtained, while tilt angle to the direction of perpendicular EF is 17 ± 3° and side-by-side distance is 6.8 ± 0.7 nm (Figure 2i). Surprisingly, tubular-like instead of solid structure were obtained (see cross-sectional xz view in Figure 4), while formation mechanism of the tubular structure is still under investigation. As mentioned above, increase of Mw-PS to 50 kg/mol further enhances the confinement strength, resulting in the formation of interesting stacking assembly of AuNRs@PS50k. 3D reconstruction (Figure 2l, S4 and Movie S4) reveals the unique structures with AuNRs lying down in the channel to form four-NR layers with 23 ± 5° tilt angle to direction of EF, and an end-to-end linear chain of AuNRs with 24.7 ± 5.7 nm distance across the four-NR layer. Formation of the unique assembly certainly reflects the combination effect of dual-confinement and applied EF, where the inner end-to-end linear chain structures formed under confined effect provided by the four-NR layer.

In short, experimental parameters, including DC, Mw-PS, and orientation of EF which can change effective diameters of AuNRs@PS, contribute significantly to the confinement strength, 9


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and the subsequent hybrid assemblies (Figure 1 and 2). We showed that the confinement played a critical role in the assembly of AuNRs@PS as well as block copolymer or spherical NPs.20, 36-39 For instance, similar assemblies are expected when AuNRs@PS were under same confinement strength (Figure 5). As shown in Figure 5a-c, linear-chain structures were observed with different maximum angles although different Mw-PS was used. Interestingly, the confinement strength (DC/DPro) is set to similar value (0.92, 0.97 and 1.00), where we define the projected diameter (DPro) as the effective diameter of AuNRs@PS (Scheme S3). While, under different EF orientations (parallel and perpendicular), similar triple-helix structures were observed from AuNRs@PS5k even different DC (24.6 nm and 61.2 nm) was employed (Figure 5d and 5e). The confinement strength in both assemblies are similar (DC/DNR-S = 1.75 and DC/DNR-L = 1.80). Moreover, even if all the factors (e.g., Mw-PS, DC, and orientation of EF) are totally different, AuNRs@PS could still keep similar stacking mode due to the compatible confinement strength (Figure 5f and 5g). In contrast, when confinement strength was sufficiently weaken by increasing DC up to 210.2 nm, the above mentioned assemblies cannot be obtained regardless of the value of Mw-PS and orientation of EF (Figure S5). Thus, confinement strength plays the critical role in the formation of the unique structure and affecting NRs stacking style.

Furthermore, PS brush is also crucial to the assembly of AuNRs@PS. One interesting thing is that, in all the helix structures (Figure 1 and 2), AuNRs did not arrange along the EF line strictly, but along the helix-line with end-to-end packing. This could be ascribed to the strong interaction between PS brushes surrounding the end of AuNRs since grafting density of PS is higher on the 10


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end of AuNRs.3 In addition, although the lengths of hybrid assembly depending on the uncontrolled filling process are broad distributed, it’s still observed that AuNRs@PS with high Mw-PS could form longer assemblies since PS with longer chain-length have stronger interaction to avoid the breakage during or after the removal of AAO template (Figure S6a, b). On the other hand, PS with low Mw-PS (e.g., PS5k) would have a rigid characteristic, leading to the reduction of interparticle space and thus closely ordered multilayer structure. Yet, PS with high Mw-PS would behave more flexible and provide broader interparticle space for AuNRs tilting. In this case, the defect density of assemblies would increase while correlation length of the ordering structure would decrease, especially in weak confinement (Figure S6c, d). For instance, increasing Mw-PS from 5 to 50 kg/mol can certainly raise DNR-S and confinement effect, yet the assemblies of AuNRs@PS12k, AuNRs@PS20k and AuNRs@PS50k still showed a certain degree of disorder while the tilt angles to the direction of parallel EF are 21 ± 9°, 27 ± 10° and 33 ± 11°, respectively (Figure 1j-l and S7e). Moreover, Mw-PS could affect the interparticle distance of NRs, and thus optical property of the assemblies (e.g., LSPR absorption peak). For instance, end-to-end distance of AuNRs increased from 4.5 ± 1.4 to 26.3 ± 3.2 nm when increasing Mw-PS from 5 to 50 kg/mol, while setting DC at 61.2 nm and under parallel EF. An 87 nm blue-shift of longitudinal SPR, due to the plasmon coupling, was observed with the increased interparticle distance (Figure S7e, f).

CONCLUSIONS In summary, we have demonstrated an interesting and robust route to fabricate 1D hierarchical

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hybrid assemblies of AuNRs@PS by coupling of 2D confinement and applied electric field. EF markedly helped AuNRs to overcome the steric hindrance to assemble into ordered structures, and controllable orientation of AuNRs in the hybrid structures. Moreover, confinement strength (DC/DNR-L or DC/DNR-S) is proved to be a critical factor in governing stacking style of the NRs assemblies. The structure and surface plasmonic properties of hybrid assemblies can be tailored by controlling molecular weight of PS brushes, orientation of EF and pore size of AAO membrane. This strategy is applicable for fabricating 1D NR hierarchical assemblies, and the functional hybrid assemblies with unique structures and properties may find promising applications in photoelectric devices, biosensors, catalysis and data storage.

EXPERIMENTAL SECTION

Materials: Anodic aluminum oxide (AAO) membranes with pore size of 24.6 ± 6.7 nm, 61.8 ± 9.2 nm, 96.3 ± 7.5 nm were purchased from Puyuan Nanotech, Co., Ltd., China. AAO membranes with pore size of 210.2 ± 16.7 nm were purchased from Whatman, Inc. These membranes are freestanding disks with a diameter of 13 mm and a thickness of 60 µm, with double-pass channels (Figure S8 and S9). Sodium borohydride (NaBH4), ascorbic acid, hydrogen tetrachloroauratetrihydrate

(HAuCl4·3H2O,

purity:

99.9%),

AgNO3,

and

cetyltrimethylammonium bromide (CTAB, purity ≥ 99%) were obtained from Sigma-Aldrich. PS5k-SH (Mw/Mn = 1.15), PS12k-SH (Mw/Mn = 1.4), PS20k-SH (Mw/Mn = 1.09), and PS50k-SH (Mw/Mn = 1.06) were purchased from Polymer Source, Inc., Canada. Deionized water (Millipore Milli-Q grade) with resistivity of 18.0 MΩ was used in all the experiments. All the glassware was 12


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cleaned by aquaregia and rinsed with deionized water prior to the experiments.

AuNRs Synthesis: AuNRs were synthesized according to the published protocols. The seed solution of AuNRs was prepared by reducing HAuCl4·3H2O (2.5 mL, 1 mM) mixed with 5 mL of a 0.2 M aqueous solution of CTAB, 2.5 mL deionized H2O and fresh NaBH4 (0.6 mL, 10 mM) in an ice-cold water. For the preparation of a growth solution, 19 mL of a 0.1 M CTAB solution was mixed with 1 mL of a 10 mM aqueous solution of HAuCl4·3H2O, 0.2 mL of 10 mM aqueous solution of AgNO3. Followed by the addition of 0.12 mL of 0.1 M aqueous solution of ascorbic acid, the dark yellow solution turned colorless. Finally, 0.32 mL of a 5 min-aged seed solution of nanoparticles was added to the growth solution. The NRs were purified using twice 30 min-long centrifugation cycles at 10,000-12,000 rpm. At the end of each centrifugation cycle, the supernatant was removed and the precipitated NRs were redispersed in deionized water.

Surface Modification of AuNRs: Entire surface of AuNRs was modified by PS-SH through two-step ligand-exchange approach. In a typical experiment, 0.4 mL of the concentrated (~0.8 mg/mL) aqueous solution of NRs was added to 10 mL of a 0.2 mg/mL solution of PS5k-SH in THF. The solution was sonicated for 30 min and incubated for at least 24 h. The modified NRs were washed with chloroform three times to remove CTAB and free PS-SH, and then the precipitate was dissolved in 4 mL of THF. Purification procedure was performed by using three 30-min-long centrifugation cycles at 10,000-12,000 rpm. Subsequently, PS-SH (4 mg of PS5k-SH) in THF was added to the resulting PS-coated AuNRs solution, followed by same sonication, 13


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incubation, and purification process as carried out above. Finally, AuNRs with high grafting density were thus obtained. AuNRs were very stable in THF and no significant change was found by dispersing the NRs in THF for at least two months. DNR-L and DNR-S, containing the AuNRs core and the polymer PS shell, can be measured from end-to-end distance and side-by-side distance between the center of AuNRs in the TEM images of AuNRs@PS monolayer packing film (Figure S10).

Preparation of AuNRs Assemblies: The AAO membrane was immersed in a 1 mL PS-tethered AuNRs (AuNRs@PS) in THF (~3 mg/mL). Slow evaporation of THF allowed the AuNRs@PS to enter into the nanopores of the membrane, and the AAO membrane was then annealed under saturated chloroform vapor for more than 24 h, in the presence of an applied direct current (DC) electric field (with 400 µm gaps between the opposing electrodes). Two different EF orientations, either parallel or perpendicular, were applied on the 2D channel of AAO membrane. AuNRs hybrid assemblies were obtained after chloroform solvent vapor annealing, and the AAO template was removed by aqueous sodium hydroxide solution.

ASSOCIATED CONTENT Supporting Information Available: Supporting information, including characterization and calculation details, additional figures S1-S10, additional schemes S1-S3, and movies S1-S4, is available free of charge via the Internet at http://pubs.acs.org/journal/ancac3.

AUTHOR INFORMATION 14


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Corresponding Author *E-mail: [email protected] (Z. X.); [email protected] (E. L.); [email protected] (J. Z.) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We gratefully acknowledge funding for this work provided by the National Basic Research Program of China (973 Program, 2012CB821500), National Natural Science Foundation of China (51525302), and Natural Science Foundation of Hubei Scientific Committee (2015BHE001). We also thank HUST Analytical and Testing Center for allowing us to use its facilities. S. Jin and E. Lee acknowledge support by the National Research Foundation of Korea (NRF) funded by the Korea Government (MEST, 2013R1A1A2061197).

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Figures:

Scheme 1. Illustration showing the strategy for confined assembly of polymer-tethered AuNRs in AAO cylindrical channel with the assistance of EF. Hybrid assemblies with various structures, e.g., linear chain, side row, single-helix, triple-helix and hexagonally packed structures, were obtained.

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Figure 1. TEM images of AuNRs@PS assemblies under parallel EF by adjusting pore size (DC) and molecular weight of PS (Mw-PS). The images display AuNRs@PS5k, AuNRs@PS12k, AuNRs@PS20k, and AuNRs@PS50k assemblies in AAO channel with pore size of (a-d) 24.6 nm, (e-h) 61.2 nm and (i-l) 96.3 nm, respectively. Insets are the cartoons showing the assemblies of AuNRs@PS.

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Figure 2. TEM images of AuNRs@PS assemblies under EF, which are perpendicular to the cylindrical channel, by adjusting DC and Mw-PS. The images display AuNRs@PS5k, AuNRs@PS12k, AuNRs@PS20k, and AuNRs@PS50k assemblies in AAO channel with pore size of (a-d) 24.6 nm, (e-h) 61.2 nm and (i-l) 96.3 nm, respectively. Insets are the cartoons (a, b, e) and 3D reconstruction (h, i, l) showing the assemblies of AuNRs@PS.

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Figure 3. 3D reconstruction obtained by the tomography of double helical assembly of AuNRs@PS50k, when DC = 61.2 nm. The 3D volume rendering shows the entire structure, in which the reconstructed density is color coded, that is, the AuNRs appear golden (high density) and the ligand PS appears grey (low density). From the tomogram, some of the AuNRs are segmented to visualize their spiral arrangement. The helical assemblies of NRs were represented in red and green, respectively.

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Figure 4. 3D reconstruction obtained by tomography showing quadruple-helix assembly of AuNRs@PS5k with DC = 96.3 nm. 3D volume rendering shows the entire structure, in which reconstructed density is color coded according to the color line, that is, (a-d) AuNRs appear golden (high density) while PS appears grey (low density). From the tomography, some of the AuNRs are segmented to visualize their spiral arrangement. The helical assemblies of NRs were represented in red, green, purple and blue, respectively. Slices show sections through center of the 3D volume in three directions in space.

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Figure 5. Representative TEM images of AuNRs@PS hybrid assemblies with same packing mode under similar confinement strength. (a-c) assemblies with linear chain-like structure from AuNRs@PS with different Mw-PS; (d, e) assemblies with triple-helix structure from AuNRs@PS5k in AAO channel with different DC and under EF with different orientations; (f, g) assemblies with double-helix structure from AuNRs@PS with different Mw-PS, in AAO channel with different DC and under EF with different orientations. Insets are the cartoons showing the assemblies of AuNRs@PS.

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Table 1. Characteristics of AuNRs@PS in the cylindrical confinement.

a

AuNRs@PS5k

AuNRs@PS12k

AuNRs@PS20k

AuNRs@PS50k

7.0 ± 1.0

14.1 ± 0.9

16.0 ± 1.0

22.9 ± 1.6

24.6 ± 1.8

27.9 ± 3.9

34.0 ± 1.6

37.9 ± 1.2

45.2 ± 1.5

56.4 ± 2.8

wPS

-

28.70%

42.23%

47.82%

62.29%

d

-

1.56

1.18

0.89

0.64

DNR-S

b

DNR-L

c

AuNRs

∑

DC/DNR-S

DC/DNR-L

e

DC (nm)

24.6 ± 1.7

1.75

1.54

1.07

1.00

61.2 ± 3.1

4.34

3.83

2.67

2.49

96.3 ± 1.7

6.83

6.02

4.21

3.91

210.2 ± 6.2

14.91

13.14

9.17

8.55

24.6 ± 1.7

0.72

0.92

0.97

1.00

61.2 ± 3.1

1.80

1.61

1.35

1.08

96.3 ± 1.7

2.83

2.54

2.13

1.71

210.2 ± 6.2

6.18

5.55

4.65

3.73

e

DC (nm) *

*

*

Note: aDNR-S and bDNR-L refer to the overall effective diameters of AuNR@PS when it is parallel or perpendicular to the AAO channel along EF line, respectively. These values were obtained by calculating more than 100 samples in TEM images. cWPS denotes weight ratio of tethered PS to total AuNRs@PS obtained by TGA. dΣ presents grafting density of PS on AuNRs. eDC represents average pore size of AAO membrane determined by SEM measurement. The data marked with star are the values of DC/DPro, which will be further explained in text.

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The table of contents entry:

Title: Electric-Field-Assisted Assembly of Polymer-Tethered Gold Nanorods in Cylindrical Nanopores Ke Wang, Seon-Mi Jin, Jiangping Xu, Ruijing Liang, Khurram Shezad, Zhigang Xue,* Xiaolin Xie, Eunji Lee,* and Jintao Zhu*

TOC Graph:

We present a facile yet effective route to fabricate 1D hierarchical assemblies of AuNRs@PS under cylindrical confinement with assistance of electric field (EF). Interesting assemblies, e.g., linear chain, helix, and hexagonally packed structures, can be generated by tuning molecular weight of PS, orientation of EF and confinement size. This strategy is applicable for generating hybrid assemblies with unique structure, potentially useful in photoelectric devices, biosensors, catalysis and data storage.

Keyword: Gold nanorods, Electric field, Cylindrical confinement, Confined assembly, Helix structure 29


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Electric-Field-Assisted Assembly of Polymer-Tethered Gold Nanorods

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