Binary Nanoparticles Coassembly in Bioinspired Block Copolymer Films

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Binary Nanoparticles Coassembly in Bioinspired Block Copolymer Films: A Stepwise Synthesis Approach Using Multifunctional Catechol Groups and Magneto-Optical Properties Hideaki Komiyama,*,† Daisuke Hojo,*,‡ Kazuya Z. Suzuki,†,§ Shigemi Mizukami,†,§ Tadafumi Adschiri,†,∥ and Hiroshi Yabu*,† †

WPI-Advanced Institute for Material Research (WPI-AIMR), ‡New Industry Creation Hatchery Center (NICHe), §Center for Spintronics Research Network (CSRN), and ∥Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan S Supporting Information *

ABSTRACT: The development of hybrid films containing binary nanoparticles is one of the current key objectives in nanotechnology. Different nanoparticles dispersed in a polymer matrix exhibit diverse functions, and their properties are influenced by the interplay between the nanoparticles. The poly(vinyl catechol)-block-polystyrene (PVCa-b-PSt) block copolymer containing a catechol group, which is inspired by the adhesive protein found in the mussel foot, is an attractive platform for the coassembly of binary nanoparticles because the catechol group has various desirable properties, including the ability to coordinate metal oxides and reduce metal ions. In the fabrication of hybrid materials based on PVCa-b-PSt thin films, it is challenging to control the microdomain morphologies and the nanoparticle assembly process. In this work, we investigate the impact of solvent vapor annealing on microphaseseparated nanostructures in lamellae-forming PVCa-b-PSt. We show that a fast solvent vapor annealing in a tetrahydrofuran atmosphere for 10 min induces the formation of perpendicularly oriented lamellar structures within PVCa-b-PSt thin films. We also propose a stepwise approach to create binary nanoparticles coassemblies in PVCa-b-PSt thin films. In the first step of this process, we exploit the metal−coordination properties of PVCa-b-PSt catechol groups to drive the directed self-assembly of magnetite (Fe3O4) nanoparticles for the preparation of magnetic hybrid thin films. The Fe3O4 nanoparticles are dispersed and localized mainly within PVCa microdomains of the lamellar PVCa-b-PSt thin films. In the second step, the catechol group reduces the Ag+ ion in the magnetic hybrid thin films, which leads to the formation of hybrid thin films containing Ag nanoparticles. Plasmonic Ag nanoparticles and magnetic Fe3O4 nanoparticles coassemble in the PVCa microdomains. The resulting plasmonic/magnetic hybrid thin films exhibit an enhanced magneto-optical Kerr effect because of the localized surface plasmon resonance of the Ag nanoparticles near Fe3O4 nanoparticles within the PVCa microdomains. Such magneto-optical (MO) properties make the hybrid thin films interesting for imaging of magnetic fields and MO devices. Our results indicate that PVCa-b-PSt is a promising platform for developing well-ordered hybrid thin films containing different nanoparticles. KEYWORDS: bioinspired block copolymer, catechol, solvent vapor annealing, coassembly, plasmonic effect, magneto-optical Kerr effect

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INTRODUCTION Coassembly of functional nanoparticles has attracted much attention because it opens the possibility to control their functions and influence their properties by exploiting their physical and chemical interactions among different nanoparticles. For example, noble metal nanoparticles and metal oxide nanoparticles coassemblies show an enhanced magnetic response by the plasmonic properties of the noble metal nanoparticles.1−3 Similarly, composite particles of quantum dots and magnetic nanoparticles can be used to fabricate magneto-responsive fluorescent probes.4,5 The fluorescence from quantum dots can be enhanced by the increased plasmonic absorption caused by the presence of the surrounding gold nanoparticles.6,7 To trigger these unique © XXXX American Chemical Society

properties in coassemblies of different nanoparticles in the solid state, rational control of the dimensions, spacing, and arrangement of a variety of nanoparticles on a solid substrate is required. The self-assembly of block copolymers as a platform for the fabrication of nanoparticle−organic hybrid films with wellordered nanostructures has received considerable attention.8−15 This process can be used to obtain the formation of nanoscale body-centered-cubic spheres, hexagonally packed cylinders, and alternating lamellae, with a microphase separation of each block Received: January 25, 2018 Accepted: March 30, 2018 Published: March 30, 2018 A

DOI: 10.1021/acsanm.8b00141 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials depending on the volume fraction.16,17 The size of microphaseseparated nanostructures can be tuned by varying the degree of polymerization of the macromolecular building blocks. Much effort has been devoted to introducing specific functionalities in block copolymers, including poly(2 (or 4)vinylpyridine) (P2VP or P4VP)- and poly(ethylene oxide) (PEO)-containing block copolymers, which have properties that promote the assembly of nanoparticles, including metal− coordination properties, electrostatic interactions, and hydrogen bond formation ability. Two approaches have been employed to create nanoparticle−block copolymer hybrid films: one uses ex situ synthesized nanoparticles, and the other uses in situ nanoparticles synthesized from appropriate precursors inside one of the block copolymer microdomains.18−28 Russell and co-workers have described the fabrication of ordered CdSe nanoparticle arrays using the directed self-assembly of CdSe nanoparticles synthesized ex situ and a polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) thin film, in which the CdSe nanoparticles were localized in P2VP microdomains.29 The Sohn group reported the in situ synthesis of Au nanoparticles within P4VP microdomains in polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) films by the addition of HAuCl4 as a precursor, followed by reduction with NaBH4 solution in the film, which resulted in the formation of alternating layers of polymers and Au nanoparticles.30 PS-b-P2VP and PS-b-P4VP block copolymers are widely used as structure-directing agents in the fabrication of hybrid films because of the strong interaction of their pyridine moieties with metals, metal cations, and anionic complexes. As an alternative to pyridine groups, the catechol groups, which relates to the adhesive protein of the mussel foot, has a number of important chemical properties, including hydrogen bond formation ability, metal (oxide) coordination, borate complexation, and redox reactions,31 which can be exploited to coassemble different kinds of nanoparticles. In our previous work, we designed and synthesized amphiphilic diblock copolymers of poly(vinyl catechol)-block-polystyrene (PVCab-PSt) containing catechol groups.32,33 We carried out the in situ synthesis of size-controlled Ag nanoparticles in the PVCa microdomains within the films and micelles of PVCa-b-PSt in the presence of Ag+ ions by exploiting the reductive properties of the catechol groups. Very recently, Kim and co-workers described an atomic layer deposition through parallel-cylinderforming PVCa-b-PSt thin films based on the selective interaction of PVCa and metal precursors.34 They also showed that PVCa-b-PSt gives microdomains less than 10 nm in size because of the huge Flory−Huggins interaction parameter between PVCa and PSt macromolecules. Because of the presence of catechol groups, PVCa-b-PSt is also a promising material for the fabrication of nanoparticle hybrid films. However, the ex situ approach based on metal (oxide) coordination and the stepwise approach based on metal (oxide) coordination and reduction remain challenging in the case of PVCa-b-PSt. The morphological evolution of the microdomains in PVCa-b-PSt thin films is a further complication because the microphase-separated nanostructures of PVCa-b-PSt strongly depend on the film-preparation conditions.33 In this work, we studied the effect of solvent vapor annealing on the morphological evolution of PVCa-b-PSt thin films and the stepwise approach for synthesizing binary nanoparticles coassemblies in PVCa-b-PSt. First, we used solvent vapor annealing to generate perpendicularly oriented lamellar PVCa

microdomains in the substrate because this method is known to be effective for amphiphilic diblock copolymer thin films.35−37 For the stepwise synthesis approach of binary nanoparticle coassemblies, magnetite (Fe3O4) and Ag nanoparticles were used to exploit the multiple functions of the catechol groups, including metal−coordination and reductive properties. Besides, coassemblies of Fe3O4 and Ag nanoparticles show fascinating magneto-optical (MO) properties based on the interplay between their magnetic and plasmonic properties. As the first step of the coassembly process, we assembled ex situ synthesized Fe3O4 nanoparticles with PVCa-b-PSt thin films to fabricate magnetic hybrid thin films by exploiting the metal− coordination properties of the catechol group (Scheme 1, top Scheme 1. Fabrication of Magnetic Hybrid Thin Films and Plasmonic/Magnetic Hybrid Thin Films by Bioinspired PVCa-b-PSt Block Copolymer

and left graphics). The Fe3O4 nanoparticles were mainly incorporated into the PVCa microdomains of lamellar PVCa-bPSt thin films after solvent vapor annealing. In the second step, we carried out the in situ synthesis of Ag nanoparticles in the magnetic hybrid thin films to form a coassembly of binary Ag and Fe3O4 nanoparticles in the PVCa microdomains (Scheme 1, right graphic). These plasmonic/magnetic hybrid thin films exhibited an enhanced magneto-optical Kerr effect (MOKE), originating from the interactions between the Fe3O4 nanoparticles and the localized surface plasmon resonance (LSPR) of the neighboring Ag nanoparticles.

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RESULTS AND DISCUSSION The PVCa-b-PSt (Mn = 19.9 kg mol−1, Mw/Mn = 1.49, weight fraction of PVCa block = 0.33) was synthesized via reversible− addition−fragmentation chain transfer (RAFT) polymerization of 3,4-dimethoxystyrene and styrene, followed by deprotection of the methoxy groups with BBr3, according to our previously reported procedure (see Experimental Section).32 Details of the polymer characterization of the PVCa-b-PSt can be found in Figures S1−S5 of the Supporting Information. Atomic force microscopy (AFM) was used to gain insight into the surface morphologies of the thin films treated with solvent vapor annealing. Figure 1 shows the AFM height image of the PVCab-PSt thin films prepared by spin-coating 1 wt % tetrahydrofuran (THF) solution of the polymer on a Si wafer after solvent vapor annealing using various solvents. The selfassembly features are poorly defined in the as-spun thin film due to fast solvent evaporation (Figure 1a). After solvent vapor annealing in the presence of toluene, chloroform, and methanol for 10 min at room temperature, the surface morphologies B

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Figure 1. AFM height images of PVCa-b-PSt thin films (a) before and after solvent vapor annealing with (b) toluene, (c) chloroform, and (d) methanol for 10 min. AFM height images of PVCa-b-PSt thin films annealed with THF for (e) 10 min, (f) 30 min, and (g) 60 min. The scan area is 2 μm × 2 μm. FFT images are shown in the inset.

cases. Note that THF can completely dissolve PVCa-b-PSt, whereas chloroform cannot. This result indicates that PVCa is more soluble in THF than in chloroform because of the smaller χPVCa−solvent value. Therefore, THF, which can dissolve both PVCa and PSt, is a suitable solvent for the creation of ordered structures in PVCa-b-PSt thin films. We further investigated the thin film annealed with THF for 10 min using transmission electron microscopy (TEM) and grazing incidence small-angle X-ray scattering (GISAXS). Figure 2a shows a bright-field (BF-)TEM image of the solvent-vapor-annealed thin film, in which PVCa microdomains are stained with OsO4 to enhance contrast against the PSt microdomains. A fingerprint pattern similar to that seen in the AFM image was also observed in the BF-TEM image. GISAXS was used to further investigate the internal structure and orientation of the fingerprint pattern in the thin film obtained after solvent vapor annealing for 10 min. Figure 2b,c shows the GISAXS two-dimensional image and the corresponding inplane intensity profile of the thin film. The profile exhibits two scattering peaks at 0.146 and 0.290 nm−1 in the qy vector, corresponding to a 1:2 in ratio, which indicates lamellar morphology. The periodicity estimated from the first peak position is 43.1 nm, which is in good agreement with the domain spacing obtained from AFM and TEM observations. The fingerprint pattern can therefore be assigned to a lamellar structure perpendicularly oriented with respect to the substrate. Compared to the thermal annealing of PVCa-b-PSt, the solvent vapor annealing with THF is very fast (10 min). Kim et al. have used such prolonged thermal annealing (12 h) at 170 °C to induce ordered PVCa microdomains.34 Scheme 1 (top and left graphics) illustrates the fabrication of PVCa-b-PSt thin films containing Fe3O4 nanoparticles (magnetic hybrid thin films). Oleic-acid-ligand-stabilized Fe3O4 nanoparticles dispersed in cyclohexane were synthesized according to the reported method.40 Details of the characterization of the resulting Fe3O4 nanoparticles are shown in Figure S6. The diameter (9.4 ± 1.2 nm) estimated from TEM imaging is similar to the crystal size (10.0 nm) calculated by Scherrer’s

changed, but well-defined microphase-separated structures were not observed (Figure 1b−d). On the other hand, after exposure to THF vapor for 10 min, a clear fingerprint pattern was observed (Figure 1e). The domain spacing was found to be ∼42 nm, as determined from the corresponding fast Fourier transform (FFT) image. Upon 30 min annealing, the fingerprint pattern became order, as shown in Figure 1f, but dewetting of the thin film was occasionally observed on the substrate. Further annealing for 60 min resulted in film dewetting and deterioration of the fingerprint pattern (Figure 1g). As depicted in Figure 1, the surface morphologies are strongly affected by the solvent. We also considered the polymer−solvent interaction parameters (χ) for the PVCa and PSt macromolecules for each solvent. The selectivity of solvents for the PVCa-b-PSt can be defined by the equation χ = Vs(δs − δp)2/RT + 0.34, where Vs is the molar volume of the solvent, R is the gas constant, T is the temperature in kelvin, and δs and δp are the solubility parameters of the solvent and polymer, respectively.38 The solubility parameters for the PVCa and PSt macromolecules, as determined from the Hoy solubility parameter method,39 were 26.5 and 19.0 MPa1/2, which was consistent with the literature value (18.8 MPa1/2).38 The resulting χ values are listed in Table 1. Methanol (a polar solvent) has PVCa selectivity, whereas nonpolar solvents like toluene and chloroform and the polar aprotic solvent THF possess PSt selectivity. Although the χ value for chloroform is similar to that of THF, the surface morphologies of the PVCab-PSt thin films were found to be entirely different in the two Table 1. Polymer−Solvent Interaction Parameter (χ) for PVCa-b-PSt solvent

molar volume (cm3 mol−1)

δs (MPa0.5)

χPVCa−solvent

χPSt−solvent

toluene chloroform THF methanol

106.9 80.7 81.7 40.7

18.2 19.0 19.4 29.7

3.26 2.14 1.97 0.51

0.37 0.34 0.35 2.22 C

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Figure 2. (a) TEM image of OsO4-stained PVCa-b-PSt thin film after solvent vapor annealing with THF for 10 min. (b) GISAXS 2D image and (c) in-plane profile of PVCa-b-PSt thin film after solvent vapor annealing with THF for 10 min.

of Fe3O4 nanoparticles relative to the PVCa block (w = 5, 10, and 20 wt %). The dark lines, bright lines, and black dots represent OsO4-stained PVCa microdomains, PSt microdomains, and Fe3O4 nanoparticles, respectively. For the thin film with w = 5 wt %, the Fe3O4 nanoparticles were mainly localized at the PVCa microdomains and the interface between PVCa and PSt microdomains, which indicates that the Fe3O4 nanoparticles were assembled with PVCa-b-PSt during the microphase separation process, driven by the metal−coordination properties of the catechol groups. The successful directed self-assembly of Fe3O4 nanoparticles in the PVCa microdomains was confirmed by AFM observations (Figure 4, bottom panels). We found that the Fe3O4 nanoparticles were selectively arranged along darker microdomains. In the AFM height images, the PVCa microdomains are visualized as darker regions, and they are several nanometers below the surface of the PSt microdomains. When w was increased to 10 wt %, a larger amount of Fe3O4 nanoparticles was incorporated and localized in the PVCa microdomains mainly, but a lower ordering in the lamellar structure was observed in AFM height images as compared to the PVCa-b-PSt thin film with w = 5 wt %. With a further increase in w to 20 wt %, the microphaseseparated nanostructure disappeared. At the highest concentration of Fe3O4 nanoparticles, PVCa-b-PSt could not facilitate ligand exchange with oleic acid in solution because of the saturation of the catechol groups. In this case, the microphaseseparated nanostructures of PVCa-b-PSt were no longer correlated with the directed self-assembly of the Fe3O4 nanoparticles. We also demonstrated an assembly of Fe3O4 nanoparticles on the solvent-vapor-annealed PVCa-b-PSt thin film by immersion of the annealed film in cyclohexane solution containing Fe3O4 nanoparticles. However, the amount of Fe3O4 nanoparticles was small because Fe3O4 nanoparticles were incorporated only on the surface of PVCa microdomains in PVCa-b-PSt thin film. On the other hand, in the present directed self-assembly of Fe3O4 nanoparticles, Fe3O4 nanoparticles were incorporated not only on the surface of PVCa microdomains but also inside PVCa microdomains. The incorporation of large amount of Fe3O4 nanoparticles leads to good MO response in MOKE measurement. We have previously reported the in situ reduction of Ag+ ions to form size-controlled Ag nanoparticles in PVCa microdomains in PVCa-b-PSt, in which we exploited the reductive porperties of the catechol group.32,33 In the solvent-vaporannealed thin films described in this work, Ag nanoparticles

equation in XRD analysis, indicating that the resulting nanoparticles are single-crystal Fe3O4. Different amounts of cyclohexane solution containing 0.23 wt % Fe3O4 nanoparticles were subsequently added to the THF solution of PVCa-b-PSt, and the resulting solution was shaken for 3 h. Because of their stronger metal oxide affinity, the catechol groups of PVCa-bPSt are expected to replace the oleic acid ligands on the Fe3O4 nanoparticles. The final solution was spin-coated on a clean Si wafer (for AFM observations) and on a poly(vinyl alcohol) (PVA)-coated Si wafer (for TEM observations) and solventvapor-annealed with THF for 10 min. Figure 3 shows AFM

Figure 3. AFM height images of Fe3O4 nanoparticle/PVCa-b-PSt hybrid thin films (w = 5 wt %) (a) before and (b) after solvent vapor annealing with THF for 10 min. The scan area is 2 μm × 2 μm.

height images of the magnetic hybrid thin films with 5 wt % fraction of Fe3O4 nanoparticles relative to the PVCa block (w) before and after solvent vapor annealing. A lamellar structure is generated after solvent vapor annealing, although its grain size is smaller than that of PVCa-b-PSt thin films without Fe3O4 nanoparticles, as shown in Figure 1, due to the lower mobility of nanoparticle-coordinated polymer chains during solvent vapor annealing. We therefore conclude that solvent vapor annealing is an effective way to improve the microphaseseparated nanostructures not only in neat PVCa-b-PSt thin films but also in ex situ synthesized nanoparticle hybrid thin films. The influence of the Fe3O4 nanoparticle concentration on the microphase-separated nanostructures in the PVCa-b-PSt thin films was also investigated by TEM and AFM. Figure 4 (top panels) shows the BF-TEM images of the annealed magnetic hybrid thin films containing different weight fractions D

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Figure 4. BF-TEM (top panels) and AFM height (bottom panels) images of Fe3O4 nanoparticle/PVCa-b-PSt hybrid thin films with various composition ratios (w): (a) w = 5 wt %, (b) w = 10 wt %, and (c) w = 20 wt %. The thin films shown in the TEM images are stained with OsO4. The scan area of the AFM images is 2 μm × 2 μm.

Figure 5. STEM-EDX observation of Fe3O4 nanoparticle/PVCa-b-PSt thin film (w = 5 wt %) after immersion in a 0.2 M AgNO3 solution for 2 h: HAADF-STEM images in (a) low and (b) high magnification; (c) Os M mapping; (d) Ag Lα mapping; (e) O Kα mapping; (f) Fe Kα mapping; and (g) merged image of the elemental mappings.

The magnetic hybrid thin films were prepared using a procedure similar to that described in the previous section (w = 5 wt %). The magnetic hybrid thin films were immersed in 0.2 M AgNO3 solution for 2 h and then rinsed with water. Figure 5a,b shows high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the magnetic hybrid thin film after the in situ synthesis of Ag nanoparticles. The lamellar structure was maintained after the Ag nanoparticle synthesis. The energy-dispersive X-ray spectroscopy (EDS) elemental mappings for Os, Ag, O, and Fe (Figure 5c−g) revealed that the Ag nanoparticles were synthesized selectively in the PVCa microdomains of the magnetic hybrid thin films. Both Ag and Fe3O4 nanoparticles were well dispersed and showed no agglomeration in the PVCa microdomains. Some of the Ag nanoparticles were close to the Fe3O4 nanoparticles, indicating that the plasmonic contribution of the Ag nano-

were successfully synthesized in the lamellar PVCa microdomains by immersion in a 0.2 M AgNO3 solution. Ag nanoparticles were exclusively observed in PVCa microdomains, as shown in the AFM height images, and the LSPR of the Ag nanoparticles were clearly observed in UV−vis spectra (Figure S7). In this study, we performed the in situ synthesis of Ag nanoparticles in magnetic hybrid thin films, in order to coassemble Ag and Fe3O4 nanoparticles in the PVCa microdomains, which we indicate as plasmonic/magnetic hybrid thin films. This process is illustrated in Scheme 1 (right graphic). The stepwise approach is useful for assembling large amount of binary nanoparticles compared to a one-step approach (simultaneous assembly of binary nanoparticles). We also examined the effect of the plasmonic properties of the Ag nanoparticles on the MOKE of the Fe3O4 nanoparticles. E

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number of Fe3O4 nanoparticles by TEM imaging. For AgNO3 immersion times of 2, 6, and 12 h, the nanoparticle densities were 15.6 × 108, 19.5 × 108, and 21.1 × 108 cm−2, respectively (Figure 6e), indicating that the number of Ag nanoparticles is controlled by the immersion time. Further immersion time for 24 h resulted in nonspecific adsorption of Ag nanoparticles on the thin film surface. We also confirmed the increment of Ag nanoparticles in the magnetic hybrid thin films using UV−vis spectroscopy (Figure 6f). The LSPR assigned to Ag nanoparticles was clearly observed at a wavelength of 433 nm. With increasing immersion times, the LSPR peak was found to increase in intensity, but its wavelength remained constant. This indicates that the immersion time increases the amount of Ag nanoparticles but does not affect the nanoparticle diameter. The confinement of the Ag nanoparticles in the PVCa microdomains is responsible for maintaining the nanoparticle diameter fixed, despite the longer synthesis time. Finally, the magnetic responses of the plasmonic/magnetic hybrid films and magnetic hybrid thin films were measured using polar MOKE magnetometry and vibrating sample magnetometry (VSM) at room temperature. For polarMOKE measurements, a laser wavelength of 408 nm, which is close to the LSPR wavelength of the Ag nanoparticles in the thin films (433 nm), was used. The magnetization of the magnetite (Fe3O4) nanoparticles perpendicular to the surface contributes to the MO Kerr rotation. Figure 7a shows the MO Kerr rotation angle versus the applied magnetic field (Kerr hysteresis loop) for the magnetic/plasmonic hybrid thin film (w = 5 wt % of Fe3O4 nanoparticle, Ag nanoparticle synthesis of 12 h) and the magnetic hybrid film (w = 5 wt % of Fe3O4 nanoparticle). In the Kerr hysteresis loop of the magnetic/ plasmonic hybrid thin film, the linear dependence on the magnetic field of paramagnetic Ag nanoparticles was subtracted to extract the response from the Fe3O4 nanoparticles. Both hybrid thin films exhibited a good MO response and a typical superparamagnetic behavior. The MO Kerr rotation angle of the plasmonic/magnetic hybrid thin film was found to be 1.2 times larger than that of the magnetic hybrid thin film at the magnetic field of ±3 kOe. We confirmed the reproducibility of this enhancement. VSM measurements indicated that the magnetization of the Fe3O4 nanoparticles did not change after the Ag nanoparticle synthesis (Figure 7b). Therefore, we concluded that MOKE on Fe3O4 nanoparticles located near Ag nanoparticles was enhanced by the plasmonic contribution, which increases the electric field component of the electromagnetic waves on the Ag nanoparticle surface. The 1.2-fold increase seems to be reasonable because the degree of enhancement of the MO response is strongly dependent on the wavelength of the incident light in the presence of plasmonic media and the number of plasmonic nanoparticles closed to magnetic nanoparticles.2,41,42 In our experiments, however, the wavelength of the incident light was slightly different from the LSPR wavelength, and the number of Ag nanoparticles adjacent to Fe3O4 nanoparticles was relatively smaller than that reported in the literatures.2,41,42 Bisio et al. reported that in MOKE measurements in which the wavelengths of incident light and the LSPR wavelength exactly match, the MO Kerr rotation angle of a bilayer film of large numbers of Ag and Fe3O4 nanoparticles shows an almost 5-fold increase as compared to that a monolayer film of Fe3O4 nanoparticles.2

particles might affect the MO response of the Fe 3 O 4 nanoparticles. The diameter of the Ag nanoparticles was controlled to be ∼7 nm, which is slightly smaller than the diameter of the Fe3O4 nanoparticles. A detailed investigation of the HAADF-STEM images revealed that the Ag nanoparticles have brighter contrast compared to the Fe3O4 nanoparticles, owing to the larger electron density localized on the Ag atoms. Not all the catechol groups in the PVCa block were involved in the coordination with the Fe3O4 nanoparticles because the PVCa microdomains retained their reductive properties even in the magnetic hybrid thin films. It should be noted that the binary nanoparticles were successfully coassembled in the PVCa microdomains through a stepwise approach using the metal−coordination and reduction properties of the catechol group. Moreover, the density of the Ag nanoparticles was controlled by the immersion time in the AgNO3 solution. Figure 6a−d shows BF-TEM images of the magnetic hybrid

Figure 6. BF-TEM images of Fe3O4 nanoparticle/PVCa-b-PSt thin films (w = 5 wt %) (a) before and after immersion in a 0.2 M AgNO3 solution for (b) 2, (c) 6, and (d) 12 h. (e) Relashonship between the density of the nanoparticles and the immersion time in the AgNO3 solution. (f) UV−vis absorption spectra of plasmonic/magnetic hybrid thin films.

thin films after immersion in the AgNO3 solution for 0, 2, 6, and 12 h, respectively. The number of nanoparticles in the PVCa microdomains clearly increases with the immersion time (see also the AFM height images in Figure S8). The density of the Fe3O4 nanoparticles in the magnetic hybrid film (0 h, w = 5 wt %) was 7.2 × 108 cm−2, from which we determined the F

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Figure 7. (a) Polar MOKE loops and (b) VSM M−H loops of plasmonic/magnetic hybrid thin films (red curves) and magnetic hybrid thin film (black curves).

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Aldrich. Anhydrous 1,4-dioxane and dichloromethane, boron tribromide (BBr3), styrene (St), and 2,2′-azobis(isobutyronitrile) (AIBN) were purchased from Wako Pure Chemical Industries, Ltd., Japan. Inhibitor in DMSt and St was removed by extraction with NaOH solution before use. AIBN was purified by recrystallization in methanol. Measurements. NMR spectra were recorded on a Buruker AVANCE III 400 MHz spectrometer using tetramethylsilane (δ = 0.00 ppm) as an internal standard. Molecular weights of the polymers were determined using GPC performed on a Tosoh HLC-8320GPC, with THF as an eluent and polystyrene as a standard. UV−vis spectra were recorded on a JASCO V-670 spectrometer. AFM observations were carried out using a Park NX-10 in noncontact mode. BF-TEM images were taken using a Hitachi High-Technologies H-7650 at an accelerating voltage of 100 kV. HAADF-STEM observations with EDS were performed using a JEOL JEM-ARM200F at an accelerating voltage of 200 kV. GISAXS was measured using a Rigaku Nano-Viewer setup with a two-dimensional PILATUS detector. X-ray measurements were performed using a Cu Kα radiation beam (λ = 1.541 Å). Polar MOKE measurements were performed using a NEOARK BH-PI920TW2 magnetometer at room temperature. The wavelength of the incident laser was 408 nm, and the diameter of the laser beam was approximately 80 μm. VSM measurements were carried out using a TOEISI PV-M20-5 magnetometer at room temperature. Synthesis of PVCa-b-PSt. The synthetic route to the polymer is shown in Scheme 2.32 Preparation of PDMSt Macro-CTA. RAFT polymerization was used to prepare the poly(3,4-dimethoxystyrene) macro-chain-transfer agent (PDMSt macro-CTA). DMSt (1.43 g, 8.72 mmol), CPDTTC RAFT agent (30.1 mg, 82 μmol), and AIBN (4.8 mg, 29 μmol) were added to a test tube and backfilled three times with nitrogen. Dry 1,4-dioxane (0.3 mL) was added to the test tube using a syringe, and the solution was subjected to four freeze−pump−thaw cycles. The reaction was

CONCLUSIONS We have described the application of the solvent vapor annealing and nanoparticle assembly approach in a bioinspired block copolymer PVCa-b-PSt containing multifunctional catechol groups to the fabrication of nanoparticle−polymer hybrid films. We examined the effect of different solvents (THF, toluene, chloroform, and methanol) in the solvent vapor annealing process and found that THF is a suitable solvent for the creation of microphase-separated nanostructures because it can induce swelling of both PVCa and PSt in PVCa-b-PSt. AFM, TEM, and GISAXS measurements revealed that PVCa-bPSt thin films form lamellar microdomains oriented perpendicularly to the substrate after solvent vapor annealing with THF for 10 min at room temperature. We also successfully fabricated well-ordered magnetic hybrid thin films through the directed self-assembly of Fe3O4 nanoparticles with PVCa-b-PSt based on the ligand exchange of oleic acid with PVCa-b-PSt, which was possible by the strong corrdination properties of the catechol group to metal oxides. Fe3O4 nanoparticles were found to be highly dispersed and mainly incorporated in the lamellar PVCa microdomains at low Fe3O4 nanoparticle concentrations. At higher concentrations, the ordered microphase-separated nanostructures deteriorated, owing to the fact that the number of Fe3O4 nanoparticles was too large for a complete ligand exchange to occur. Moreover, by exploiting the reductive properties of the catechol group, in situ synthesis of Ag nanoparticles could be carried out in magnetic hybrid thin films to obtain plasmonic/magnetic hybrid thin films. A stepwise approach allowed us to coassemble binary Ag and Fe3O4 nanoparticles within the PVCa microdomains. The results of TEM observations and UV−vis measurements indicated that the density of Ag nanoparticles in the PVCa microdomains increases with the immersion time in AgNO3 solution. The resulting plasmonic/magnetic hybrid thin films exhibited a larger MO Kerr rotation angle than the magnetic hybrid thin films in MOKE measurements because of the LSPR induced by Ag nanoparticles in the vicinity of the Fe3O4 nanoparticles. The hybrid thin films are interesting for potential applications in highly sensitive magnetic fields imaging and miniaturized MO devices. We believe that the present work represents an important reference for the future development of coassembly approaches via multifunctional PVCa-b-PSt block copolymers.

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Scheme 2. Synthesis of PVCa-b-PSt Block Copolymer

EXPERIMENTAL SECTION

Materials. 3,4-Dimethoxystyrene (DMSt) and 2-cyano-2-propyldodecyl trithiocarbonate (CPDTTC) were purchased from SigmaG

DOI: 10.1021/acsanm.8b00141 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials carried out for 15 h at 60 °C. After cooling to room temperature, the reaction mixture was purified by reprecipitation from methanol for three times. The precipitate was collected by centrifugation and dried under vacuum to afford PDMSt macro-CTA as a yellow solid (yield = 0.83 g, 58%). 1H NMR (400 MHz, CDCl3): δ 6.72−5.87 ppm (aromatic group), 3.89−3.52 ppm (methoxy group), 2.11−1.19 ppm (polymer backbone), 0.87 ppm (methyl group in CPDTTC RAFT agent) (see Figure S1). Mn(NMR) = 7.8 kg mol−1. GPC (eluent: THF): Mn = 8.2 kg mol−1, Mw = 8.8 kg mol−1, and PDI = 1.04 (see Figure S4). Preparation of PDMSt-b-PSt. Poly(3,4-dimethoxystyrene-blockstyrene) (PDMSt-b-PSt) was synthesized by a method similar to the one described for PDMSt macro-CTA, using PDMSt macro-CTA (0.55 g, 71 μmol), St (14.8 g, 142 mmol), AIBN (9.3 mg, 57 μmol), and dry 1,4-dioxane (18 mL). The reaction proceeded to completion in 24 h at 60 °C. After cooling to room temperature, the reaction mixture was purified by reprecipitation from methanol for three times. The precipitate was collected by filtration and dried under vacuum to give PDMSt-b-PSt as a white solid (yield = 2.69 g). 1H NMR (400 MHz, CDCl3): δ 7.22−5.86 ppm (aromatic group), 3.88−3.51 ppm (methoxy group), 2.13−1.18 ppm (polymer backbone), 0.88 ppm (methyl group in CPDTTC RAFT agent) (see Figure S2). Mn(NMR) = 21.2 kg mol−1. GPC (eluent: THF): Mn = 36.9 kg mol−1, Mw = 49.9 kg mol−1, and PDI = 1.35 (see Figure S4). Preparation of PVCa-b-PSt. PDMSt-b-PSt was treated with BBr3 to convert the methoxy groups to hydroxyl groups. To a solution of PDMSt-b-PSt (1.07 g, 2.9 μmol) in dry dichloromethane (200 mL) was added dropwise dichloromethane solution containing BBr3 (1.0 M, 5 mL) at 0 °C. The solution was stirred overnight at room temperature. The reaction mixture was then treated with a large volume of aqueous 1 M HCl for three times. The precipitate was collected by filtration and dried under vacuum to give PVCa-b-PSt as a white solid (yield = 0.79 g, 74%). 1H NMR (400 MHz, DMF-d6): δ 8.79−8.35 ppm (aromatic group), 7.45−5.88 ppm (aromatic group), 2.45−1.12 ppm (polymer backbone), 0.88 ppm (methyl group in CPDTTC RAFT agent) (see Figure S3). Mn(NMR) = 19.9 kg mol−1. GPC (eluent: THF): Mn = 19.6 kg mol−1, Mw = 29.2 kg mol−1, and PDI = 1.49 (see Figure S4). Synthesis of Oleic-Acid-Ligand-Stabilized Fe3O4 Nanoparticles. Oleic-acid-ligand-stabilized Fe3O4 nanoparticles were synthesized according to a method previously reported in the literature.40 The precursor (iron(III) acetylacetonate, 0.28 g, 0.80 mmol), the modifier (oleic acid, 3.39 g, 12.0 mmol), and the solvent (oleylamine, 3.20 g, 12.0 mmol) were heated at 120 °C under reduced pressure with 1,2-hexadecanediol (0.62 g, 2.40 mmol), a reducing agent, for 2 h as a pretreatment. A portion of this solution (3.5 mL) was placed in a pressure-resistant Hastelloy vessel (inner volume, 5 mL) with a Hastelloy sphere and subjected to a solvothermal reaction at 300 °C for 3 h. The organic-ligand-stabilized nanoparticles were extracted from cyclohexane (5 mL). The product was precipitated from the cyclohexane solution using ethanol (4 mL) and was separated by centrifugation. The nanoparticles were characterized using TEM and XRD measurements (Figure S6). The diameter of the nanoparticle was found to be 9.4 ± 1.2 nm by TEM observation. Thin Film Preparation. Solvent Vapor Annealing for PVCa-bPSt Thin Films. A 1 wt % THF solution of PVCa-b-PSt was spincoated onto a cleaned Si wafer and a quartz substrate for AFM observation and UV−vis spectroscopy, respectively, at 2000 rpm for 30 s. Solvent annealing was carried out in a glass dish, where the samples were oriented vertically, facing the inside of the glass dish. For TEM specimens, PVCa-b-PSt thin films were prepared on a poly(vinyl alcohol) (PVA; Mn = 10 kg mol−1) film as a sacrificial layer, which was obtained by spin-coating a 5 wt % aqueous solution on a Si wafer at 1000 rpm for 60 s. After solvent vapor annealing, the sample was immersed in Milli-Q water to dissolve the PVA sacrificial layer, and the floated PVCa-b-PSt thin film was scooped up with TEM grid. The specimens were stained with OsO4 for 2 h. Preparation of Magnetic Hybrid Thin Films. Solutions of PVCa-bPSt were prepared in THF with various amounts of a 0.23 wt % cyclohexane solution containing Fe3O4 nanoparticles (5, 10, and 20 wt

% relative to the PVCa block). The resulting solution was spin-coated on a PVA-coated Si wafer and a cleaned Si wafer at 2000 rpm for 30 s and solvent-vapor-annealed with THF for 10 min. Preparation of Plasmonic/Magnetic PVCa-b-PSt Composite Thin Films. Fe3O4 nanoparticle/PVCa-b-PSt hybrid thin films were immersed in a 0.2 M AgNO3 aqueous solution. After immersion, the thin films were washed several times with Milli-Q water and then dried at room temperature.

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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/acsanm.8b00141. Additional supporting figures, NMR spectra, GPC curves, and DSC curves of the polymers, TEM image and XRD patterns of Fe3O4 nanoparticles, AFM images and UV−vis absorption spectra of plasmonic hybrid thin films, AFM images of plasmonic/magnetic hybrid thin films (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.K.). *E-mail: [email protected] (D.H.). *E-mail: [email protected] (H.Y.). ORCID

Hideaki Komiyama: 0000-0001-5544-9305 Hiroshi Yabu: 0000-0002-1943-6790 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Grants-in-Aid for Scientific Research (Nos. 16K21218 (H.K.), 17H01223, and 16K12071 (H.Y.)) from JSPS. H.K. thanks the supports by Izumi Science and Technology Foundation, ATI Research Grants 2016, and WPI-AIMR Fusion Research Proposal. We are grateful to Profs. Atsushi Muramatsu and Kiyoshi Kanie from Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, for the use of the Nanoviewer. We thank Center for Integrated Nano Technology Support, Tohoku University, for the STEM-EDS observation.

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REFERENCES

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DOI: 10.1021/acsanm.8b00141 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsanm.8b00141 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX