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Jun 26, 2017 - ABSTRACT: Fabrication of films with plasmonic nano- particles (NPs) arrays arranged in chiral configurations of prescribed handedness i...
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Chiral Arrangements of Au Nanoparticles with Prescribed Handedness Templated by Helical Pores in Block Copolymer Films Xuemin Lu,*,† Dong-po Song,‡ Alexander Ribbe,‡ and James J. Watkins*,‡ †

School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Rd., Shanghai 200240, China Department of Polymer Science and Engineering, University of Massachusetts Amherst, 120 Governors Drive, Amherst, Massachusetts 01003, United States

‡

S Supporting Information *

ABSTRACT: Fabrication of films with plasmonic nanoparticles (NPs) arrays arranged in chiral configurations of prescribed handedness is highly attractive for the design of new functional materials; however, this remains a formidable challenge in nanotechnology. In this study, we demonstrated the controlled arrangements of gold (Au) NPs into helical structures templated by helical pores created in cross-linked block copolymer (BCP) films. D- and L-tartaric acid (TA) were used to direct the self-assembly of achiral poly(1,4-butadiene)b-poly(ethylene oxide) BCPs into helical cylindrical morphologies with prescribed handedness, i.e., D or L. Helical pores were generated by BCP cross-linking followed by TA extraction. Helical Au NP arrays, subsequently arranged within the helical pores, exhibited the chiral optical response. The helical structures of NPs arrays and the resulting optical handedness were tunable simply by using either D- or L-porous templates. This simple strategy offers a straightforward pathway for the fabrication of chiral porous BCP films and helical NPs arrays with chiral optical properties.

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INTRODUCTION Homochiral assembly of nanoparticles (NPs) at the nanoscale and mesoscale, which can exhibit response distinctly different toward chiral stimuli, is significantly attractive not only for constructing functional hybrid materials, such as polarization optical devices, photocatalysts, and metamaterials,1−5 but also for understanding the evolution of homochirality in nature. Inherently chiral molecules including DNA, peptides, and lipids and chiral liquid crystal molecules have been reported as chiral porogens, and circularly polarized light stimulation has been used as template to direct the assembly of NPs in solution.6−11 However, the control of chiral assembly of NPs in device-friendly solid-state bulk materials is still a formidable challenge, which undeniably demands extensive explorations. Block copolymers (BCPs), which consist of molecular chain segments of chemically dissimilar repeat units, can form onedimensional (1D), 2D, or 3D ordered periodic nanostructures via self-assembly, according to their compositions and molecular weights. BCPs have been widely used as templates to prepare, host, and direct the assembly of functional additives including NPs, small molecules, and other species.12−36 The spatial arrangement of NP arrays in the BCP matrix depends on the balance between the enthalpic contribution due to the NP/BCP interactions and the entropic penalty mainly arising from polymer chain stretching owing to the incorporation of NPs. A precise control over the arrangement of NP arrays can be successfully achieved by selective incorporation of NPs within a © XXXX American Chemical Society

specific microdomains of the phase-separated BCP, affording well-ordered materials with significantly enhanced mechanical, optical, and electrical properties.37,38 However, helical packing of functional NPs in BCP bulk samples that exhibit chiral physical properties has not been extensively reported. One example is the chiral assembly of NPs templated by BCP with double-gyroid (DG) microphase structure by fine-tuning of the molecular weight and molecular distribution.9,39−41 DG morphology of BCP can be used to fabricate 3D metamaterials due to their 3D interconnected structure and chiral response. Another literature study on the twist packing of NPs was reported by Sanwaria et al. They achieved the twist ensemble of silver NPs stabilized using a polystyrene shell by confining the NPs within a cylindrical domain of a BCP.42 However, the handedness of the NP arrangement, i.e., left and right, appeared in pair, and no chiral optical properties were presented. The self-assembled morphologies available via microphase separation of BCPs are often limited to periodic spherical, cylindrical, lamellar domains, and bicontinuous arrangement within narrow composition limits, and it is difficult to achieve new and morphologically uniform nanostructures. Although theoretical and simulation studies have proved that twist or helical structures can be obtained under certain conditions,43−49 few reports have been presented which involved the construction Received: June 26, 2017

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DOI: 10.1021/acs.macromol.7b01364 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules of helical structure with clear handedness in BCP film. In a recent study, extensive research efforts have been devoted to introduce helical morphologies via the synthesis of BCPs containing chiral repeat units within the backbone. For example, Ho’s group employed a chiral repeat unit, namely poly(styrene)-b-poly(Llactide), in one segment of a BCP system to achieve helical nanostructures.50−52 The chirality of lactide entity plays a significantly important role in controlling the handedness of the helical structure. Recently, we reported the fabrication of helical morphologies in achiral, linear BCP, poly(ethylene oxide)-bpoly(tert-butyl acrylate) (PEO-b-PtBA), through the addition of enantiomers of tartaric acid (TA).53 Hydrogen bond interactions between TA and PEO block enhanced the phase segregation strength of the composite, and the chirality of the additive induced the conformational chirality in the composite. The weak interaction of the hydrogen bond makes it possible to fabricate chiral porous film with controlled handedness in a simple and facile way. Gold (Au) NPs are used extensively for various applications in nanoscience and nanotechnology; therefore, their arrangement in well-ordered arrays is highly desirable. This may be accomplished by self-assembling BCP films. Moreover, Au NPs have excellent optical properties, and they do not deteriorate with time. Herein, we reported for the first time a new approach for the chiral arrangement of gold (Au) NPs into helices using a chiral nanoporous BCP film as template. Chiral pores were constructed through additive-driven self-assembly of BCPs into a chiral cylinder phase followed by cross-linking and selective removal of the chiral additives. The as-obtained helical structures exhibited the chiral optical response as confirmed by circular dichroism (CD) and vibrational circular dichroism (VCD). The resulting chiral porous BCP film was employed to control helical arrangement of Au NPs by backfilling the porous structure. The Au NPs presented clear helical structure with wellcontrolled handedness and optical properties determined by the chirality of the porous BCP film.

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radiation (0.154 nm). The sample-to-detector distance was 5 m, and the X-ray beam size was 0.22 × 0.18 mm2. PBdEO was blended with TA at a given mass ratio in THF, then drop-cast on glass slides, dried at room temperature, and finally baked at 50 °C. Further the samples were thermally annealed at 60 °C for 16 h. After being scraped from the glass slides, these dried bulk samples were then placed in the center of metal washers, sandwiched by Kapton film, and placed on a vertical holder. Transmission electron microscopy (TEM) measurements were conducted using a JEOL 2000FX transmission electron microscope operated at accelerating voltage of 200 kV. Composite samples were embedded in epoxy resin and cured overnight at room temperature. Thin sections with size of approximately 50 nm for TEM were prepared using a Leica Ultracut UCT microtome equipped with a Leica EM FCS cryogenic sample chamber operated at −80 °C. The TEM tomography was performed using a Tecnai F20 FEG transmission electron microscope (FEI, The Netherlands) operated at 200 kV. The sample grids were placed in a Fischione 2020 single-tilt holder (Export, PA). Single-tilt series was collected at the increment of 1° over a ± 60° range using a FEI Eagle 4K × 4K CCD camera and imaging software Explore3D. Images were aligned using Au particles, and the final tomograms were reconstructed using the R-weighted algorithm of the IMOD software. Thermogravimetric analysis (TGA) was performed by heating 2 mg samples at 10 °C/min to 800 °C in nitrogen to detect the loading amount of Au NPs in the porous film using a TA TGA Q5000IR instrument. Infrared (IR) spectra were recorded on a Nicolet 6700 Fourier transform (FT) IR spectrometer. The solution samples of PBdEO and TA, PBdEO blended with D-TA, and PBdEO blended with L-TA were prepared in THF with the concentration of about 20 mg mL−1, and 100 μL solution was drop-cast on the CaF2 substrate. All the samples were thermally annealed under the conditions similar to those of SAXS and TEM. The films were peeled off the substrate and submitted to FTIR observation. VCD measurements were performed using a Chiral IR-2X (BioTools, USA) in a rotating model. Thin films of PBdEO, PBdEO blended with D-TA, and PBdEO blended with L-TA were prepared in THF with the concentration of about 20 mg mL−1 and then drop-casted on CaF2 substrate. The thickness of the obtained film was about 15 μm. The film was thermally annealed under same condition as that for SAXS and TEM observation. The CD spectra were acquired on a circular dichroism spectrometer (JASCO J-815, Japan), and UV−vis spectra were recorded on a PerkinElmer Lambda 20 UV−vis spectrometer. The solution samples of PBdEO and TA, PBdEO blended with D-TA, and PBdEO blended with L-TA were prepared in THF with the concentration of about 5 mg mL−1. The path length of the quartz cuvettes was 1.0 mm. The bulk film of PBdEO blended with D- or L-TA was prepared by drop-casting the THF solution on quartz slides, which were then dried at room temperature followed by thermal annealing at 50 °C for 72 h. Thickness of the obtained film was ca. 500 nm determined by a UV−vis interferometer (model F20, Filmetrics, Inc.). Further the samples were measured directly by the CD and UV−vis instrument.

EXPERIMENTAL SECTION

Preparation of Chiral Block Coploymer Film. Poly(butadiene)b-poly(ethylene oxide) (PBdEO, Mw: PBd22K-PEO9K, PDI: 1.08) was purchased from Polymer Source Inc., Canada. Au-OH NPs with a core diameter of approximately 2 nm were synthesized by single-phase reduction reaction according to the Brust−Schiffrin method.54 The choice of the Au NP was mainly dependent on the pore size of the obtained pore structure. An appropriate amount of PBdEO BCP was weighed and dissolved in anhydrous tetrahydrofuran (THF), which was followed by the addition of TA solutions in the same solvent to form about 2% (w/v) stock solutions. THF solution (100 μL) was drop-cast through a 0.4 μm PTFE filter onto a clean substrate. After solvent evaporation, the dried films were annealed at 60 °C for 16 h. Preparation of Porous Block Copolymer Films. Porous BCP film was prepared by extracting the TA molecules from TA/BCP hybrid film after thermal annealing treatment. The TA/BCP hybrid films were cross-linked by exposing them to 254 nm ultraviolet (UV) light for 2 h, and then the resulting films were immersed in aqueous solution of lithium bromide (LiBr, 0.1 g mL−1) for 2 h to remove TA molecules by destroying the hydrogen bonding. The obtained film was cleaned several times using water and ethanol and dried at 40 °C for 1 day. Arrangement of Gold Nanoparticles in Porous Block Copolymer Film. The obtained porous film was immersed in Au NP solution in ethanol (6−7 mg/25 mL), and then the solution was bubbled using N2 for more than 4 h. Prior to the characterization, the Au/BCP film was cleaned using pure ethanol and dried at temperature below 40 °C for 24 h. Characterization. Small-angle X-ray scattering (SAXS) measurements were performed using a Ganesha SAXS-LAB system with Cu Kα

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RESULTS AND DISCUSSION We first investigated the effect of TA loading on the phase structure of PBdEO to ensure the formation of cylindrical structure of the PEO minor phase. We fixed the amount of PBdEO and tuned the amount of TA enantiomers. SAXS profiles shown in Figure 1 and TEM result (Figure S1) reveal that the native BCP exhibits a cylindrical structure of the PEO segments comprising the minor phase distributed in the majority phase of PBd. Introduction of TA molecules enhances the phase separation strength of PBdEO and yields a well-ordered cylindrical structure in the PBdEO/TA hybrid as indicated by the appearance of highly ordered scattering peaks. This indicates that when the TA loadings are 16 and 21 wt %, scattering peaks with relative q ratio of 1:√7 and 1:√2:2 appear, respectively. The domain spacing of the cylinder structure formed in the TA/ BCP film increased to 49 and 54 nm for samples containing 16 B

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characteristic peaks of TA molecules and PEO chains.56,57 On the other hand, we noticed that the VCD signals did not show obvious change irrespective of the variation in the relative ratio of TA to PEO (Figure S3). For the PBdEO/TA hybrid film, the effect of loading level of TA molecules on the chiral signals was also investigated as shown in Figure 2 and Figure S4. Different from those of PEO/TA system, the chiral signals show obvious dependence on the TA loading level in the hybrid film. Clearly, at a loading of 10 wt %, weak signals appear at 1770 and 1450 cm−1 corresponding to the CO and C−H groups (Figure S4). When the TA loading is increased to 16 wt %, chiral signals attributed to TA molecules appear in VCD spectrum, and the intensity is obviously amplified compared to the PEO/TA film; even the relative ratio of TA to the PEO segments is at the same level. (Figure 2a).55,56 Noteworthy, the signal at 1775 cm−1 presented an obvious difference with the change of TA loading level. When the TA loading was 16 wt %, only a single strong broad peak appeared at 1775 cm−1. When the TA loading was increased to 21%, two groups of split peaks ((1699, 1722 cm−1), (1734, 1760 cm−1)) appeared, and the intensity was also obviously weaker than that of 16 wt % samples even the 21 wt % sample has more TA molecules (Figure 2b). The signal splitting of CO group was attributed to the coexistence of hydrogen bonds of TA-PEO and TA-TA molecules. As shown in Figure S2, two groups of peak appeared corresponding to the CO group for TA enantiomers. For the 16 wt % loading film, hydrogen bonding mainly occurred between PEO and TA, which led to the disappearance of the TA−TA hydrogen bond signal. With the increase in the TA loading, i.e., for the 21 wt % film, the splitting peaks indicated the coexistence of TA−PEO and TA−TA hydrogen bonds. For the TA/PEO system, the chiral signals did not show obvious dependence on the TA loading level. This difference can be possibly attributed to the formation of helical structure in the PBdEO/TA film. Previously, Ho et al. proposed a twist-shift model to understand the formation of helical phase in a chiral BCP system.51,52 According to the model, the chiral unit leads to not only the formation of helical conformation but also the handedness of the helical phase structure. Owing to the effect of specific helical steric hindrance on molecular packing and microdomain stacking, the microphase of cylindrical structure gets twisted and shifts toward the others during morphology evolution to minimize the system energy. Herein, on the basis of the density functional theory, we calculated the energy level of PEO/TA under different states: one was helical state and the other was not. We assumed the initial energy level of PEO and TA molecules to be zero. The energy level for the combination of

Figure 1. SAXS profiles of PBdEO with different TA loading. The samples were prepared by drop-casting a solution onto a clean glass substrate and thermally annealed at temperature of 60 °C for 16 h.

and 21 wt % of TA, respectively, compared to 35 nm for neat PBdEO. Further increase of TA loading in the BCP matrix led to a change in the microstructure of the PBdEO/TA hybrid. Cylindrical structure was retained at a TA loading of 21 wt %, while a lamellar structure with characterized peaks ratio of 1:2:4:5 was obtained when TA loading was up to 25 wt %. Based on the above-mentioned results, samples with 16 and 21 wt % TA loading were selected to control the formation of cylindrical structure in the PBdEO/TA hybrid film in the following study. The hydrogen bonds between TA and PEO segments led to the formation of supramolecule PBdEO/TA, which was then supposed to cause the chirality transfer from TA molecules to the polymer main chains through the domino effect. The chirality transfer from TA molecules to PBdEO/TA complex in the film was verified by VCD which is proved to be an effective tool to determine the chirality of polymer chains. For comparison, the VCD spectra of pure D- and L-TA molecules as well as PEO/TA hybrid are presented in Figures S2 and S3 of the Supporting Information. For pure D- and L-TA enantiomers, strong chiral signals appear at 1775 cm−1 attributed to the stretching of CO group and at 1252, 1292, and 1340 cm−1 ascribed to the bending of C−H bonds. The mirror character of these peaks indicated the opposing chirality of D- and L-TA molecules. Moreover, obvious split effect at 1775 cm−1 was presented. This split Cotton effect was attributed to the existence of intramolecular and intermolecular hydrogen bonds. For the PEO/TA hybrid film, the VCD profiles show the characteristic peaks similar to those of pure TA samples. This may be due to the overlap of most of the

Figure 2. (a) VCD of PBdEO/TA film with 16 wt % TA. (b) VCD of PBdEO/TA film with 21 wt % TA. All films were measured in rotating model. C

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immersed in dimethyl sulfoxide (DMSO) solution for 4 h. TA molecules remained in the hybrid film after immersion in aqueous solution of LiBr; however, got resolved in DMSO and then detected by NMR spectroscopy due to high sensitivity. Figure S6 exhibits that no TA signal appear in the DMSO solution. These results indicated the complete removal of TA molecules from PBdEO/TA film. This film after TA removal was further characterized by the CD measurement. Here, the sample with 16 wt % TA loading was used. The residual PBdEO films retained a weak CD signal at the wavelength of 227 nm. According to the results of FTIR and NMR spectroscopy, we confirmed the complete removal of TA molecules from PBdEO, and no chromophore existed in the PBdEO film. The residual CD signal can be attributed to the helical arrangement of PBd segments in the hybrid film. This result is also consistent with that of VCD shown in Figure 2. The construction of chiral porous BCP film was carried out by leaching TA molecules from BCP/TA hybrid film. The formation of porous structure in the BCP film was first investigated by SAXS. Figure 4 demonstrates that the hexagonal

PEO and TA molecules under helical state was calculated to be −71 mJ cm−2, lower than that without helical state. Thus, we concluded that the formation of helical phase structure is helpful to the interaction between TA and PEO segments in PBdEO/TA hybrid, which caused the difference from the pure PEO/TA system. Undeniably, a lot more systematic explorations and further simulation study are still required to comprehensively understand the interactions between TA and PEO segments in the helical phase structure. Besides the chiral signals of TA molecules, a new mirror peak at the wavenumber of 1642 cm−1 also appeared, which corresponds to the CC bond of PBd segments as shown in Figure 2a. This result indicated that the PBd segments were also arranged in a chiral conformation in the hybrid film. Compared to the 16 wt % TA loading, the signal of the CC group in the film with 21 wt % TA loading becomes weak, indicating the weakening of chiral assembly of PBd chains. CD results further confirmed the chirality of the BCP/TA film. In order to avoid the possible effect of anisotropic arrangement of polymer chains, in particular the possible orientation of CO group in the hybrid film, all samples were measured at different angles and nearly identical CD spectra were obtained: one peak appeared at a wavelength of 221 nm corresponding to that of TA molecules, indicating the chiral nature of the hybrid film (Figure S5). We further removed the TA molecules from PBdEO film by immersing the PBdEO/TA hybrid film in LiBr water or ethanol/water mixture for about 2 h. The enantiomer TA molecules could be removed by destroying the hydrogen bonds between PEO segments and TA molecules. Complete removal of TA molecules from PBdEO film was verified by the disappearance of characteristic peaks of TA molecules in the FTIR spectra as shown in Figure 3. The absorption peak of the

Figure 4. SAXS profiles of porous PBdEO films after removal of TA. The parent film contained 16 wt % TA.

structures are retained irrespective of the configuration (D- or L) of the original BCP film, although the d-spacing decreased slightly after removal of TA molecules compared to BCP films before the removal of TA molecules. Figures 5a and 5b show the TEM images of porous films originated from D- and L-BCP films, respectively. TEM images reveal the formation of porous structures in the films prepared from 16 wt % TA-doped BCPs after extraction of TA (Figures 5a and 5b). All the samples were not stained and directly submitted for TEM observation. The brighter area indicates the position of pore structure. Clear helical structures are observed in the porous films. The regularity of the microstructure originated from D-BCP films was better than that originated from L-BCP films. According to our previous report,53 the handedness of the helical pore structure corresponds to the chirality of the D- or L-enantiomer TA added to the BCP prior to extraction. More direct proof of the pore structure comes from the cross-sectional TEM images of the porous film. Figure 5c exhibits the appearance of brighter areas in the cross-sectional TEM images due to the formation of porous structure. The formed pore was around the cylindrical structure, and the pore size was approximately 15 nm. Au NPs stabilized with 4-mercaptophenol were localized within the porous chiral BCP film by immersing the film in an

Figure 3. FTIR spectrum of TA/BCP film after immersion in aqueous solution of LiBr at different time. The TA loading in the film was 16 wt %.

−COOH group at 1750 cm−1 was used to characterize the leach of TA from the PBdEO film. After immersing the hybrid film in aqueous solution of LiBr for about 100 min, the adsorption peak at 1750 cm−1 disappeared completely. Moreover, a broad adsorption band at 3400 cm−1 ascribed to the −OH group also disappeared after immersing treatment for 100 min. Further confirmation of the TA removal was carried out by NMR spectroscopy. The PBdEO/TA film was immersed in aqueous solution of LiBr for 2 h, and then the obtained film was further D

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Figure 5. (a) TEM image of D-TA originated porous PBdEO film with projection parallel to the cylinder axis. (b) TEM image of L-TA originated porous PBdEO film with projection parallel to the cylinder axis. (c) Cross-sectional TEM image of D-TA originated porous PBdEO film with projection perpendicular to the cylinder structure. The bright area in the image corresponds to the location of porous structure. All the samples were not stained.

Figure 6. TEM image of chiral arrangement of Au NPs in a porous PBdEO film prepared using D-TA with projection (a) perpendicular to the cylinder axis and (b) parallel to the cylinder axis. The doping amount of TA in the parent BCP film is 16 wt %. (c) 3D tomography of chiral arrangement of Au NPs in a porous PBdEO film using D-TA. (d) TEM image of chiral arrangement of Au NPs in a porous PBdEO film prepared using L-TA with projection perpendicular to the cylinder axis.

5c. First Au NPs with different diameters were used to fill the pore structure. We found that it was difficult to deposit the NPs with size larger than 5 nm. In order to make the depositing

ethanol dispersion of the NPs. Porous BCP film originated from TA 16 wt % parent PBdEO was first used as template. Pore size was about 10 nm according to the TEM result shown in Figure E

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Figure 7. (a) CD optical response of Au NP/PBdEO films. (b) UV−vis spectrum of Au NP/PBdEO films. The parent PBdEO film contained 16 wt % TA.

The controlled arrangement of Au NP was ascribed to the interactions between the NPs and PEO segments. After leakage of TA molecules from the hybrid film, we speculated that the PEO segments were located at the surface of pore structure. At the initial stage of immersion treatment, the Au NPs were adsorbed on the inner surface of the pore through hydrogen bonds between PEO segments and −OH groups of Au NPs. Moreover, the Au NP could be further filled into the pore through space confinement, and therefore the arrangement of the Au NP could inherit the helicity of the pore in the PBdEO film. According to the literature,24,25,42,57−60 metal NPs can be arranged in different configurations by using BCP as template, which was generally carried out by directly doping them into the BCP matrix. However, these studies do not report the chiral optical response, though twist packing was obtained.42 This can be attributed to the in-pair presence of right and left handedness structure for most of these cases. In this study, the chiral response of the Au/BCP hybrid films was verified by CD measurement as shown in Figure 7a. In order to attribute the chiral response more clearly, UV−vis spectra of PBdEO/Au hybrid film, pure PBdEO film, and Au NPs ethanol solution were detected. Figure S9 exhibits the UV adsorption peak at the wavelength of 223 nm for pure BCP sample. For the Au NPs in ethanol, a broad adsorption was observed as shown in Figure S9, though no obvious peak appeared. After deposition of Au NPs into the porous BCP film, a broad adsorption peak at the wavelength of 271 nm appeared as shown in Figure 7b. For the PBdEO/Au hybrid film, evident broad CD signals with mirror image appeared at a wavelength of 271 nm. Previous studies proved that the helical response on metal NPs is mainly from the following two aspects: one is due to the chiral capping molecules on the NPs surface, and the other is the chiral ensemble of metal NP included in chiral space.9,39−41,61 In this study, CD response was not obtained for the pure Au NPs in the ethanol solution. Combined with the UV results of PBdEO and Au NPs, we can possibly attribute the CD signals of the BCP/Au hybrid film to the helical arrangement of Au NP in the porous channels of BCP film. Owing to the weak surface plasmon resonance property of 2 nm Au NP and the strong adsorption of the hybrid film, the CD signals were very weak. We also performed the experiment by using Au NPs with large size in

process of Au NP easier, 2 nm Au NPs were selected as target NPs. Depending on the amount of Au NPs deposited in the porous film, the hybrid film exhibited various colors by controlling the immersion time: brown for less than 30 min immersion and black for more than 6 h. Based on TGA results, the deposition amount of Au NPs could reach up to 13 wt % after 12 h immersion in the Au NP ethanol solution (Figure S7). The arrangement of Au NPs in the helical porous film was characterized by TEM. Figure 6a demonstrates that for the DTA 16% parent PBdEO film clear single-band helical arrangement of Au NPs with a pitch of approximately 40 nm was observed in the BCP film after infusion of the Au NPs. Different from those templated by biomolecules, the chiral structure of NP presented herein involves the aggregation of NPs, almost similar to those prepared by using circular light at stimulations.10 Crosssectional TEM image shown in Figure 6b further confirms the location of Au NP in the porous PBdEO film. Clearly, all the Au NPs are located in the PEO cylinder phase; however, the distribution of these NPs is not homogeneous, and it is just at the edge of the PEO cylinders as shown in Figure 6b. This is consistent with the distribution of the pore in the PEO phase as shown in Figure 5c. 3D tomography further confirmed the handedness of the helices as shown in Figure 6c. Clearly, for the D-porous PBdEO film, the Au NPs arranged in a left handed chiral configuration which is consistent with the handedness of the phase structure formed in the D-TA doped hybrid film.53 Porous film parented from L-TA 16 wt % PBdEO was also used as template to arrange the Au NPs, and similar helical arrangement of Au NPs with right handed chirality was obtained as shown in Figure 6d. The pitch of the helical structure was almost equal to that obtained by D-TA 16 wt % parent PBdEO film. Porous films obtained from TA 21 wt % parent PBdEO film were also used as template to arrange Au NPs. Figure S8 indicates that helical arrangement of Au NP can also be found in the porous film; however, the regularity of the arrangement is much lower compared to that obtained by using TA 16 wt % parent PBdEO film. This can be ascribed to the less regularity of the microstructure in the BCP film. This result is also consistent with the VCD results as shown in Figure 2. F

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Macromolecules

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diameter, i.e., 5 nm. Unfortunately, a little amount of Au NPs could be deposited into the pore structure in the BCP film. This may be due to the large size of the Au NPs relative to the pore structure. Considering the importance of chiral arrangement of metal NPs in bulk or film for constructing the chiral optical device, using a chiral porous film or bulk material as template provides an effective platform for their related applications.

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CONCLUSION Herein, we described a simple strategy for the fabrication of porous block copolymer (BCP) films with controllable chiral pore and for the chiral assembly of gold nanoparticles (Au NPs). In the PBdEO/TA hybrid film, BCP main chains exhibited chiral conformation irrespective of direct or indirect interaction with the TA molecules. By removing the chiral additives, chiral porous BCP film with controllable handedness could be obtained. NPs backfilled into the chiral pores could be arranged in the same handedness to the BCP template. The helical structures of NP arrays and the resulting optical handedness were tunable simply by using either right- or left-handed porous templates. This simple strategy offers a straightforward pathway for the fabrication of chiral porous BCP films and helical NP arrays with attractive physical properties.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01364. TEM image of neat PBdEO, VCD spectra of neat TA molecules, homopolymer PEO with different TA loading, and 10 wt % TA loading PBdEO film, 1H NMR of DMSO after immersion of TA/PBdEO, and UV−vis spectra of PBdEO film and Au NP in ethanol (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(X.L.) E-mail [email protected]. *(J.J.W.) E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Science foundation of China (21374060, 21574081) and the NSF Center for Hierarchical Manufacturing at the University of Massachusetts Amherst (CMMI-1025020). X. M. Lu thanks Ms Ruibin Wang for the help in the VCD measurements.

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REFERENCES

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DOI: 10.1021/acs.macromol.7b01364 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b01364 Macromolecules XXXX, XXX, XXX−XXX