Confinement of Polymer-Tethered Gold Nanowires in Polymeric

Mar 19, 2014 - State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun...
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Confinement of Polymer-Tethered Gold Nanowires in Polymeric Colloids Jiangping Xu†,‡ and Wei Jiang†,* †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: Driven by the interfacial tension, polymertethered ultrathin gold nanowires (AuNWs) are successfully confined in polymeric nanocolloids, such as spheres, vesicles, and cylinders. The confined morphology of the AuNWs significantly depends on the size and the shape of the confined space. A series of structured hybrid colloids containing elastically or inelastically distorted AuNWs are obtained. Uncoiling the elastically deformed AuNWs can be realized through solvent swelling of the polymer shell. Thus, the hybrid colloids can be applied for mechanical energy storage and release. The elastic potential and the resilience of a confined AuNW can be estimated to be 3.2 × 10−17 J and 1.6 × 10−9 N, respectively. Moreover, the hydrophobicity of the tethered molecular shell on the AuNWs has been studied to reveal the influence of the interfacial tension to the coiling of the AuNWs, which offers an important guideline for manipulating nanomaterials in emulsion droplets.

1. INTRODUCTION Confinement of filaments in a finite space to form a higherorder structure is common in biology.1 For a geometrical perspective, the formation of these complex structures is ascribed to the spatial confining interactions between the flexible fibers and the stiff containers. For example, the ring of tubulin forms when a tubulin rod grows to a length exceeding the diameter of a bounding vesicle.2 Thus, the confined space provides a versatile approach to reversibly change the shape of the nanofilaments. This interesting phenomenon brings new inspirations for fabricating smart devices. In modern nanotechnology, many attempts have been conducted to mimic this amazing process to create nanodevices using synthetic materials.3−15 However, the manipulation of the nanomaterials to achieve hierarchical structures and novel functions remains a big challenge. Coiling inorganic filaments into rings or helices has attracted much attention because these structures have potential applications in nanoengineering. Carbon nanotubes and metal nanowires such as gold nanowires, palladium nanowires, and so on, can be curved to rings or helices.4−8,11,12 It has been reported that ring-structured carbon nanotube bundles can be obtained by direct synthesis or sonication. The strong π−π stacking interactions among the multiple loops was regarded as the driving force for the coiling action.6,8,16,17 Chen and coworkers recently reported some profound work in bending carbon nanotubes and metal nanowires by using emulsion droplets as templates.5,10−12,18 They demonstrated that the interfaces between the nanowires and the solvents played a © 2014 American Chemical Society

critical role in the shape transformation of these nanostructures. The main driving force for the curving of the nanowires was the interfacial tension between the hydrophobic nanowire and the water, while the resistant of this process was the resilience of the curly nanowires.12 Although a general method applied for coiling nanofilaments has been developed by Chen’s group,12 the coils they obtained may not very stable due to the high mobility of the small molecular ligands on the surfaces of the filaments.18 To improve the stability and processability of the coils, these nanofilaments can be introduced in the matrix of polymers. Yet, the confinement of these filaments in polymeric colloids with various shapes and their coiling/uncoiling processes have rarely been reported.5,11 More importantly, the elastic potential can be stored in the coils during the bending process. How many energies the coils have is essential for their application as smart devices. However, the elastic potential has not been quantitatively estimated before. In addition, the confinement of nanofibers in vesicles and cylinders will be more intriguing, since these colloids can offer various spaces to confine the nanofibers, leading to the formation of novel structures. The confined assembly behavior of nanofibers in such geometries may bring new opportunity to understand confinement phenomena in nature and inspire the designation of functional nanodevices. However, to the best of our knowledge, the Received: December 13, 2013 Revised: February 18, 2014 Published: March 19, 2014 2396

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Scheme 1. Confinement of AuNWs in Various Geometries via Emulsion-Based Self-Assemblya

a

Route A: As the organic solvent is evaporating, the droplets which contain homopolymers shrink to compress the AuNWs to spherical coils; Route B: The block copolymers and AuNWs co-assembly into hybrid vesicles and cylinders.

was added to the solution and kept standing at 25 °C for 5 h. The final dark-red AuNWs were centrifuged at 4500 rpm for 15 min, washed by ethanol and finally redispersed in 5.0 mL of chloroform. Then, a ligand exchange approach was applied to modify the AuNWs with thiolated polymers through Au−S bond.27,28 Hydrophobic PS-tethered AuNWs (PS-AuNWs) and binary polymer brush (PS and PEO) tethered AuNWs (PSPEOAuNWs) were prepared through this method (Scheme 2). In a

encapsulation of anisotropic nanowires in such confined geometries to achieve hierarchical structures has rarely been involved in the literatures.3,13,14,19,20 Herein, we report the confinement of polymer-tethered ultrathin gold nanowires (AuNWs)21 in polymeric nanocolloids via the emulsion-based self-assembly (Scheme 1).22−24 3D confinement of AuNWs can be achieved by encapsulating them in spherical polymeric colloids.25,26 Elastic and inelastic deformation of the AuNWs has been achieved by altering the dimension of the confined space. In the case of elastic deformation, the uncoiling of the AuNWs is readily realized by solvent swelling of the polymer shell and the elastic potential of the coils can be released. Moreover, the AuNWs can also be confined in vesicle walls (3D confinement) and cylindrical micelles (2D confinement) to obtain novel structures. In addition, the hydrophobicity of the tethered molecular shell on the AuNWs has been studied to reveal the influence of the interfacial tension to the coiling process of the AuNWs. This result offers an important guideline for manipulating AuNWs in emulsion droplets.

Scheme 2. Synthesis of Polymer-Tethered AuNWs by Ligand Exchangea

2. EXPERIMENTAL SECTION 2.1. Materials. Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, purity: 99.99%) and homopolystyrene (hPS, Mn = 123 000 g mol−1, Mw/Mn = 1.08) were purchased from Alfa Aesar. Oleylamine (OA, purity: 80−90%), triisopropylsilane (TIPS, purity: 99%), and sodium dodecyl sulfate (SDS, purity: 99%) were obtained from Aladdin. Amphiphilic diblock copolymer polystyrene-b-poly(ethylene oxide) (PS 356 -bPEO148, Mn = 37 000 g mol−1 for PS block and Mn = 6 500 g mol−1 for PEO block, Mw/Mn = 1.06), thiol terminated polystyrene (PS2K-SH, Mn = 2000 g mol−1, Mw/Mn = 1.15, thiol functionality >95%), and thiol terminated poly(ethylene oxide) methyl ether (PEO2K-SH, Mn = 2000 g mol−1, Mw/Mn = 1.05, thiol functionality >95%) were purchased from Polymer Source Inc., Canada. Other chemicals were supplied by Beijing Chemical Factory. All of the materials were used after receiving without further purification. The glassware were cleaned by aqua regia and rinsed with deionized water prior to the experiments. 2.2. Synthesis of Gold Nanowires. OA-capped gold nanowires (OA-AuNWs, diameter: 1.8 nm) were synthesized following a previously reported method.21 In a typical experiment, 0.50 mL of OA and 15.0 mg of HAuCl4·3H2O were added into 12.50 mL of hexane. Then 0.75 mL of TIPS

a

The OA are replaced by the thiolated polymers through the Au−S bond.

typical route for preparing PS-AuNWs, 8.0 mg of PS2K-SH was first dispersed in 3.0 mL chloroform and then blended with 5.0 mL as-synthesized AuNWs chloroform solution. After ultrasonic treatment in ice bath for 10 min, the solution was stirred for 24 h at 200 rpm. Then, 4.0 mL of methanol was added dropwise to the solution to induce the precipitation of the product. The mixture was kept overnight at −20 °C and separated by centrifugation (10000 rpm, 30 min), discarded the red or pink supernatant. Repeated precipitation-centrifugation procedure (at least 5 times) was conducted until no unbound PS2K-SH was left. Finally, the resulting PS-AuNWs were dispersed in chloroform to obtain a brown solution (1.0 mg mL−1). 2397

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Figure 1. (a−c) TEM images of OA-AuNWs, PS-AuNWs, and PSPEO-AuNWs, respectively.

characterization. The other one is the redispersion of the hybrid colloids in chloroform solution. The colloids were first collected by centrifugation and dried in vacuum. Then, 0.1 mL of chloroform was added to rediseperse the nanocolloids. A drop of the solution was put on the copper grids and dried in a vacuum oven for 3 h at 30 °C. Then, the samples were used for TEM characterization. 2.5. Characterization. The grafting density of the polymer tethered AuNWs was determined by the combination of TGA (STARe system, Mettler Toledo), 1H NMR (400 MHz, Bruker), and TEM (JEM-1011, JEOL). The morphologies of the nanocolloids were examined by TEM. The sizes of the nanocolloids were examined by DLS (Zetasizer Nano ZS, Malvern). The size and morphology of the emulsion droplets were investigated by optical microscope (ZEISS AXIO Imager.A2m).

A similar procedure was employed to prepare PSPEOAuNWs. Typically, 8.0 mg of PS2K-SH and 8.0 mg of PEO2KSH were simultaneously dispersed in 3.0 mL of chloroform, followed by blending with 5.0 mL of as-synthesized AuNWs. After the same ligand exchange and purification processes described above, the resulting product was dispersed in chloroform to get a brown solution (1.0 mg mL−1). It is worth to note that in this case, the PSPEO-AuNWs are more difficult than PS-AuNWs to be separated by centrifugation, due to the amphiphilicity of the tethered polymers. The details about the characterization of the polymer tethered AuNWs were shown in Figure S1 (Supporting Information). The TEM images of the three types of AuNWs are displayed in Figure 1 and the grafting densities of the polymer-tethered AuNWs are shown in Table 1. Table 1. Characteristics of Polymer-Tethered AuNWsa samples

wPolymer (%)

σPS (chain/nm2)

σPEO (chain/nm2)

PS-AuNW PSPEO-AuNW

41.86 50.00

1.90 2.19

− 0.43

3. RESULTS AND DISCUSSION 3.1. Preparation of PS-AuNWs and PSPEO-AuNWs. The OA-capped AuNWs were prepared using the reported approach.21 The total length of a single wire is up to 2 μm (Figure 1a). The small molecular ligands OA on the surface of the NWs are enthalpically incompatible with the polymer matrix (i.e., PS) used in the present study. To improve the compatibility of AuNWs to the polymeric colloids, polymertethered AuNWs were fabricated by ligand exchange of OA by thiolated polymers, PS2K-SH (for PS-AuNWs) and PEO2K-SH (for PSPEO-AuNWs). Figure 1b shows the TEM image of PSAuNWs with length up to 2 μm, though small fraction of them breakup during the modification process. The grafting density of PS is 1.90 chains/nm2, which is obtained by analyzing the TGA and TEM results (Table 1 and Figure S1a, details of the calculation can be found in the Supporting Information). The PSPEO-AuNWs were fabricated by simultaneously adding PS2K-SH and PEO2K-SH to exchange the OA molecules on the OA-AuNWs. The grafting densities of the PS and PEO can be figured out by the combination of TGA (Figure S1b) and 1H NMR (Figure S1c). As shown in Table 1, the grafting densities of PS and PEO are 2.19 chains/nm2 and 0.43 chains/nm2, respectively (Supporting Information). The grafting densities of PS in PS-AuNWs and PSPEO-AuNWs are similar due to the same concentration of PS2K-SH in the ligand exchange process. However, the grafting density of PEO is much lower than that of the PS, probably because the solubility of PEO in chloroform is lower than that of PS. 3.2. Confinement of PS-AuNWs in Spherical Colloids. PS-AuNWs (Figure 1b) were dissolved in chloroform and then emulsified with SDS aqueous solution under ultrasonication for 30 s (droplets size D = 2.4 ± 0.8 μm, Table S1 and Figure S2). Then the emulsion was stirred at 100 rpm in an open vial to

a

Note: wPolymer is the weight fraction of polymers grafting on the AuNWs obtained from TGA; σPS and σPEO are the grafting densities of PS and PEO on the AuNWs, respectively.

2.3. Preparation of the Hybrid Nanocolloids. Emulsionbased self-assembly method was employed to prepare the hybrid nanocolloids (Scheme 1). Typically, 100 μL of chloroform solution containing polymer-tethered AuNWs and hPS or PS356-b-PEO148 (Table S1) was added to 1.0 mL of SDS aqueous solution in a vial and then emulsified via ultrasonic treatment (40 kHz, 150 W) for 30 s. Followed by being stirred in the open vial at 100 rpm for 24 h to completely evaporate the chloroform. As the removal of organic solvent, the droplets which contain hPS shrink to bend the AuNWs in spherical colloids (Route A, Scheme 1); or the droplets which contain PS356-b-PEO148 breakup to release the AuNWs and block copolymers due to the interfacial instability.29,30 In this case, AuNWs and PS356-b-PEO148 coassemble into hybrid vesicles and cylinders which are filled with curly AuNWs (Route B, Scheme 1). Finally, the hybrid colloids were separated by centrifugation (15000 rpm, 30 min) and redispersed in water. 2.4. Swelling of the Hybrid Colloids to Uncoil the AuNW Coils. Two strategies were employed to swell the polymer shells of the colloids. One is the chloroform vapor treatment: the hybrid colloids were placed on copper grids used for TEM characterization. Then put the samples in a vial with saturated chloroform vapor. After being annealed for 3 h at 25 °C, the samples were taken out of the vial and dried in a vacuum oven for 3 h at 30 °C before being used for TEM 2398

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Figure 2. (a) TEM image of the coils formed by neat PS-AuNWs (0.5 mg mL−1). The arrows in (a) indicate the kinks on the AuNWs. The coils in part (a) cannot bounce back to straight wires when (b) annealed in chloroform vapor or (c) redispersed in chloroform solution.

thoroughly evaporate the chloroform. As the volatile solvent in the droplet gradually evaporates, the size of the droplet decreases. When droplets reach a critical size, the trapped AuNWs are forced to bend into coils.1,12 The driving force for the coiling action is the interfacial tension between water and the hydrophobic PS-AuNWs. The solvation energy of the AuNWs in water (Ewater) and chloroform (Eoil) are different. If their difference is larger than the induced strain (Estrain) of the AuNWs, they can be bent in the oil droplets. The ΔG of this process can be expressed as eq 1, where A is the interfacial area, γoil‑NW and γNW‑water are the interfacial tension of the chloroform−AuNW interface and AuNW−water interface, respectively. To simplify the estimation of the strain energy, the complex AuNW coils are regarded as regular rings. Thus, the strain energy of a single AuNW can be expressed as eq 2,1 where Y is the Young’s modulus of the AuNW, I is the area moment of inertia, r is the radius of the AuNW, κ = 1/R is the curvature of the coil with radius R, and l is the length of the AuNW. As demonstrated in the literature, the interfacial tension is strong enough to bend the AuNWs (r = 0.9 nm), while the induced resilience resists the coiling action until the resilience exceeds the driving force or when chloroform thoroughly evaporated and the droplets become solid.12

Figure 3. Size distribution profiles (DLS results) of the hybrid colloids obtained (a) at different initial concentrations of PS-AuNWs and (b) at different emulsification conditions.

ΔG = Eoil − Ewater + Estrain = A(γoil ‐ NW − γNW ‐ water) + Estrain < 0

Estrain = YIκ 2l /2 = πYlr 4 /8R2

(1)

the AuNWs (indicated by arrows in Figure 2a), implying the inelastic deformation of the NWs.31 As the PS matrix of the colloid can be swollen by good solvent, the confined effect can be eliminated and consequently the AuNWs can be freed. If the deformation is elastic, the AuNWs will bounce back to straight wires due to the resilience, while they will preserve the deformation and cannot bounce back if the deformation is inelastic. As a result, we use chloroform to treat the hybrid colloids for checking whether the curved PS-AuNWs can spontaneously bounce back or not. As shown in Figure 2b−c, when the coils are treated in chloroform vapor or redispersed in chloroform solution, the AuNWs only mildly stretch but cannot spontaneously bounce back to straight wires, indicating that the deformation of AuNWs in this case is inelastic. The resilience of the wires is released by the emergence of the yielding points. The confinement of AuNWs in nanospheres is identified to the 3D confinement. In this case, the size of the confined space plays a critical role in the confined morphology of the AuNWs. The larger the confined space, the less constraint of the AuNWs. Thus, the deformation type (elastic or inelastic) may be changed by tuning the size of the colloids. Three simple

(2)

When the volume of the droplet reduces to increase the concentration of the AuNWs, the viscosity in the droplets increases dramatically and eventually vitrified colloids are formed to lock the morphology of the AuNWs, storing the elastic potential in the colloids (Figure 2a). Because the droplets are stabilized by SDS, they will not coalesce to larger droplets during the solvent evaporation under gentle stirring. Thus, all the substances in a droplet will finally form a single colloid after removal of the organic solvent. As the initial concentration of the PS-AuNWs in the droplets is low (i.e., 0.5 mg mL−1), the sizes of the resulting nanospheres are very small (hydrodynamic diameter Dh = 100.3 nm, Figure 3a). During the evaporation of chloroform, the exerted compressive force continues to bend the AuNWs. The driving force (interfacial tension) not only bends the AuNWs, but also overcomes the elastic tension, leading to severely curved AuNWs random coils in a finite space (most of the coils contain a single AuNW, Figure 2a). The compressive force is so large that the yielding of the AuNWs can be observed. Many kinks can be found along 2399

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Figure 4. (a) Size distribution profiles of hybrid colloids obtained at different concentrations of hPS (1.5 to 5.0 mg mL−1). (b) TEM image of a typical sphere obtained by blending hPS (3.0 mg mL−1) with PS-AuNWs (0.5 mg mL−1). The less constrained AuNWs in part (b) can bounce back when being annealed in chloroform vapor (c) or redispersed in liquid chloroform (d).

Additionally, adding hPS, which can form spherical colloids (Figure S5a), to the droplets with PS-AuNWs will enlarge the hybrid colloids and thus reduce the confined effect. It will gradually decrease the proportion of the inelastically distorted AuNWs and finally change the AuNWs from inelastic to elastic deformation. As the concentration of hPS increases from 1.5 to 5.0 mg mL−1, the diameter of the colloids increases from 185.5 to 313.8 nm (Figure 4a and S6). When the hPS concentration is low (1.5 mg mL−1), some of the AuNWs are still inelastically distorted (Figure S7a). However, when the hPS concentration reaches 3.0 mg mL−1, almost all of the AuNWs are elastically deformed. Evidences are shown in Figure 4b-d and S7b, where chloroform treatment can make the curly AuNWs to spontaneously bounce back to straight wires. This implies that the elastic potential can be stored in the coils and released when the polymeric matrix is swelled.5 It is worth to note that the morphology of the elastically deformed AuNWs is much more regular than that of the inelastically deformed AuNWs. As shown in Figure 4b, the PS-AuNWs inside the colloids curve to helices and networks with uniform interval (∼12 nm), which locate close to the polymer−water interface. This regularity probably results from the isotropic resilience of the elastically deformed AuNWs. In this case, the first loop of the AuNW coil acts as a template to guide the assembly of the subsequent loops.5 However, in the case of inelastic deformation, the anisotropic resilience of the distorted AuNWs will result in irregular coils, as shown in Figure 2a. As stated above, the deformation type of the AuNWs is controlled by tuning the size of the confining space. The elastic potential is stored in the elastically deformed AuNWs. As the coils may have applications in nanoengineering, it is important to figure out how many mechanical energies are stored in the coils. To simplify the calculation, we regard the coils as regular rings. According to eq 2, the elastic potential of the elastically deformed AuNWs can be estimated, presuming the bending energy is completely converted to the elastic potential. The Young’s modulus of the AuNW is about 100 GPa,32,33 the radius of the AuNW is r = 0.9 nm and the length is l = 2 μm. Taking the AuNW coils in Figure S3b for example, the average radius of the coils obtained from TEM images is R = 40 nm, then the elastic potential is calculated to be 3.2 × 10−17 J. The resilience of the AuNW is F = −∂Estrain/∂R = π Ylr4/4R3 = 1.6 × 10−9 N. As a consequence, the elastic potential can be stored in the coils and released when the polymer shell is swelled. Although the energy change in this coiling-decoiling process is much less than that provided by chemical reaction or mechanical agitation, it is enough to actuate the movement of nanoparticles. Thus, these hybrid coils may be employed as

strategies, including tuning the initial concentration of PSAuNWs, altering the initial droplets size, and adding hPS to the droplets, are used to control the colloid size in the present study. These strategies are based on the alteration of the amount of substances inside the emulsion droplets. As the organic solvent evaporating, the droplets may solidify before the compressive force overcoming the yielding point of the AuNWs. Thus, elastically deformed AuNWs can be possibly obtained. Increasing the initial concentration of PS-AuNWs from 0.5 to 1.0 mg mL−1, while keeping the initial droplets size unchanged (the emulsification conditions remain unchanged), leads to larger nanospheres containing more AuNWs after the removal of chloroform (Dh = 123.4 nm, Figure 3a and Figure S3). These AuNWs can partially bounce back to release the elastic potential upon solvent treatment, indicating that the AuNWs are less constrained and some of them are elastically deformed (Figure S3d). It is worth noting that the coils obtained at high concentration (1.0 mg mL−1) are nearly spherical (Figure S3b), while those obtained at low concentration (0.5 mg mL−1) are elliptical in shape (Figure S3a). This can be ascribed to the resilience exerted by the AuNWs in these two cases are different. At high concentration, the elastically deformed AuNWs exert isotropic resilience to maintain a spherical morphology. However, as the AuNWs suffer stress exceeding the yielding point at low concentration, kinks emerge to produce unevenly internal stress distribution. Less counteraction at the poles (near the kinks) and more at the equator results in the elliptical shape. Furthermore, increasing the initial size of the emulsion droplets, while keeping the concentration of PS-AuNWs unchanged (0.5 mg mL−1), will result in more PS-AuNWs in a droplet. In our experiment, the droplet size is controlled by the shearing force during the emulsification. Droplets with different sizes are obtained at different emulsification conditions. The average sizes of the droplets are 22.3 μm when being emulsified by magnetic bar stirring at 1000 rpm for 5 min, 12.0 μm when being stirred at 2200 rpm for 5 min, and 6.2 μm when being emulsified by ultrasonication for 15 s, respectively. After removal of the chloroform, the larger droplets will result in larger colloidal particles containing more AuNWs which are less constrained (Figure S4). This result is similar to the case of changing initial concentration of PS-AuNWs (Figure S3). As a consequence, the size of the confined space can be well tuned by controlling the size of the emulsion droplet, which is dominated by the emulsification condition. Thus, the confined morphology of the AuNWs can be well controlled. 2400

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which is mostly like a metal shielding net, has potential application in the nanoscience for electrostatic screening. Furthermore, the confining AuNWs in the vesicle wall can act as a stiff fibrous filler to reinforce the mechanical property of the vesicle. Besides being confined in the vesicles, the PS-AuNWs can also be embedded in cylindrical micelles (Figure 5d), where the confinement arises from the length mismatch between the AuNWs and the cylinders. In this case, the confinement of AuNWs in cylinders can be regarded as 2D confinement. It is different from the 3D confinement of AuNWs in spherical colloids. As shown in Figure 5d, the long AuNWs are folded back and forth along the length direction of the cylinders (200−800 nm). Although the AuNWs are confined in the length direction, the constraint in this direction is much less than the constraint suffered in nanospheres, as the length of the cylinders is larger than the diameter of spheres. The back-andforth morphology is less energetically expensive than the spherical coil. As a result, the AuNWs prefer to fold back and forth rather than curve to coils. Since the AuNWs are seriously bent at both ends of the cylinders, they bounce back to reshape the cylinders’ terminals to ellipsoids and the fingerprint-like structures are observed. This strategy offers an important platform to fold AuNWs, which may potentially be applied in nanoengineering for nanodevice fabrication. 3.4. Influence of the Ligands to the Coiling of AuNWs. The property of ligands on the surface of AuNWs plays a critical role in bending the AuNWs, since the strength of the interfacial tension is dominated by the ligands. When the assynthesized OA-AuNWs are applied in the emulsion droplets, they can be curved to irregular rings as the droplets shrink (Figure 6a). The diameters of the rings (∼100−400 nm) are much larger than that of the coils shown in Figure 2a and the diameter of AuNWs is thicker (2.5 to 10 nm) than that of the as-synthesized AuNWs (1.8 nm) due to the coalescence of

nanodevices to provide mechanical energy, which has potential applications in nanospring, nanomotor, and many other smart devices. 3.3. Confinement of PS-AuNWs in Vesicles and Cylindrical Micelles. Vesicles and cylinders are the most common geometries in block copolymer self-assembly in selective media. More interestingly, these particular geometries can offer various confined spaces to bend AuNWs into different styles. Thus, confining AuNWs in these assemblies are of most interest in nanosciences. As demonstrated by Zhu and Hayward, the interfacial instability induced block copolymer self-assembly could be applied to prepare various nanocolloids with functional nanoparticles.29,30,34 This novel method was introduced to confine AuNWs in the vesicle walls and cylindrical micelles in the present study (Figure S5b−c). Diblock copolymer PS356-b-PEO148 was added to the droplets containing PS-AuNWs. As the droplets shrink and then burst, the PS356-b-PEO148 self-assemble into vesicles or cylindrical micelles (depends on the concentration of block polymer and SDS,34 Table S1) and encapsulate the PS-AuNWs in the hydrophobic domains. In the vesicle wall, the AuNWs are bent because of the confining effect of the thin and curve membrane. In this case, the AuNWs are under 3D confinement. The morphology of the AuNWs in vesicle walls is affected by the length of AuNWs and the size of vesicles. As shown in Figure 5a−b, two kinds of

Figure 5. (a) Half-filled and (b) full-filled vesicles observed when the concentration of AuNWs is 0.5 mg mL−1. (c) Densely filled vesicles obtained by increasing the concentration of AuNWs to 1.0 mg mL−1. (d) The AuNWs are confined in cylinders.

hybrid vesicles are observed at low concentration of PS-AuNWs (0.5 mg mL−1). When the diameter of vesicles is large (>350 nm), half-filled vesicles can be obtained (Figure 5a). However, the AuNWs can completely surround the vesicles to form nanocages when the size of vesicle is small (∼150 nm) (Figure 5b). The number of AuNWs in the vesicles can be remarkably increased by increasing the concentration of AuNWs to obtain densely filled nanocages (Figure 5c). This novel structure,

Figure 6. (a) Irregular rings formed by OA-AuNWs in emulsion droplets. (b) Hairy spheres formed by neat amphiphilic PSPEOAuNWs. (c, d) PSPEO-AuNWs locate at the interfaces of the spheres and the cylinders, respectively. 2401

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them.18,20 Adding hPS or PS-b-PEO to the droplets still results in irregular AuNWs rings or cages (Figure S8). On the other hand, when the amphiphilic AuNWs (PSPEO-AuNWs, Figure 1c) are used, they loosely aggregate to form hairy spheres (Dh = 142.0 nm, Figure 6b). They cannot be strictly confined in the cores of the nanocolloids even if they are blended with hPS (Figure 6c) or PS356-b-PEO148 (Figure 6d). Instead, the PSPEO-AuNWs surround the spheres or cylinders at the interfaces with tethered PS chains stretching inward to the core and the PEO chains facing outward. As the droplets shrink, the interfacial tension between the ligands and water leads to the coiling of AuNWs. Since the AuNWs can counteract the coiling action due to the resilience, there must be sufficient compressive force to bend the AuNWs. When OA-AuNWs are employed, though the OA are small surfactants, the interfacial tension offers enough compressive force to bend the AuNWs. This is also recently reported by Chen and co-workers.12 However, the OA-AuNWs easily fuse to be thicker and stronger during the droplets shrink.18 Thus, it is too difficult for the interfacial tension to severely bend the AuNWs to very small coils. Instead, large rings are obtained (Figure 6a).12,18 As the resilience is anisotropic during the coalescence of AuNWs, the consequent AuNW rings are irregular in shape. Additionally, when the OA-AuNWs are added to the spheres and vesicles, irregular structures are also observed because of the incompatibility between OA and the PS matrix. In these cases, the AuNWs cannot uniformly distribute in the PS matrix due to the enthalpic repulsion between OA and PS. In order to preserve the coalescence of AuNWs and to offer the compatibility between AuNWs and PS matrix, a strong ligand (PS-SH) is needed. Replacing OA by PS-SH offers not only fine compatibility between AuNWs and PS matrix, but also larger interfacial tension to bent the AuNWs in the nanocolloids, because the PS shell (surface tension: ∼ 44 mJ/m2) is more hydrophobic than OA molecules (surface tension: ∼ 31 mJ/m2). The interfacial tension between the tethered PS and the surrounding water is remarkably increased. Thus, the γNW‑water will be more positive, which means stronger driving force to coil the PS-AuNWs according to eq 1. As a result, the AuNWs can be severely bent to random coils and even the yielding phenomenon can be observed. After grafting hydrophilic PEO to the PS-AuNWs, the bending force is significantly reduced due to the decrease of interfacial tension. The PEO chains offer amphiphilicity to the AuNWs. Consequently, the γNW‑water is more negative, which means weaker driving force provided by the AuNW−water interface (eq 1). In this case, the PSPEO-AuNWs alone can be slightly curved to hairy spheres (Figure 6b). When they are blended with hPS or PS-b-PEO, they prefer to curve on the interfaces due to the amphiphilic nature of them (Figure 6c−d). On the basis of the experimental results above, we conclude that the driving force from the interfacial tension between hydrophobic ligands and water is strong enough to bend the AuNWs. However, the OA molecules on the AuNWs could dissociate when the AuNWs approach each other, resulting in the coalescence of the AuNWs, which makes them stronger and harder to bend.18 Thus, a strong ligand (thiolated polymer in the present study) is necessary to prevent the coalescence of the AuNWs. The tethered hydrophobic PS as well offer more interfacial tension to bend the AuNWs, which makes them to be bent tightly. The diameter of the coils (∼100 nm) is much smaller than that of the rings (476 nm) obtained from OA-

AuNWs in the literature.18 Thus, more mechanical energy can be stored in such coils, which may be applicable in nanoengineering. The hydrophilic PEO chains, which reduce the interfacial tension, result in the inefficient coiling of AuNWs. Thus, the hydrophobicity of the ligands, which dominates the interfacial tension, plays a critical role to tightly coil the nanowires.

4. CONCLUSIONS In summary, the confinement of polymer tethered AuNWs in various geometries has been studied through a versatile emulsion-based self-assembly approach. A series of hybrid nanocolloids, such as spheres, cylinders, and vesicles, containing distorted AuNWs are obtained. The curving morphology of the AuNWs significantly depends on the size and the shape of the confining space. Removing the compressive force exerted on the elastically distorted AuNWs by solvent treating can make them spontaneously bounce back and release the elastic potential. The role of the tethered molecules shell to the coiling action has also been demonstrated, which shows that the hydrophobicity of the shell plays a critical role in bending the AuNWs. This result offers an important guideline for the manipulation of the nanomaterials in emulsion droplets. These hybrid colloids may be applied in nanoengineering to fabricate smart nanodevices for energy storage and release.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1 (details of the ingredients), Figure S1−S8, showing the weight loss profiles, NMR spectra, optical microscopy and TEM images, and influence of the droplet size on morphology, and a discussion of the calculations. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China for General Program (21374118). REFERENCES

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