Magnetically Responsive Assemblies of Polymer-Brush-Decorated

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Magnetically Responsive Assemblies of Polymer-Brush-Decorated Nanoparticle Clusters That Exhibit Structural Color Kohji Ohno,* Motokazu Sakaue, and Chizuru Mori Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

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S Supporting Information *

ABSTRACT: The development of new magnetic materials for applications such as magnetic-driven drug delivery, next-generation display materials, and magnetic resonance imaging is an important objective. To that end, we synthesized monodispersed, magnetically responsive particles grafted with well-defined polymer brushes and investigated the formation of their ordered arrays in organic solvents in response to a magnetic field. To achieve this, we prepared monodispersed magnetic nanoparticle clusters (MNCs) composed of large numbers of superparamagnetic ferrite ZnFe2O4 nanoparticles. The MNCs were subsequently coated with thin silica layers through the hydrolysis of tetraethoxysilane. The colloidal particles were surface-modified with initiating groups for atom transfer radical polymerization (ATRP) using a triethoxysilane derivative with an ATRP initiation site. To demonstrate the ability of the synthesized particles to produce well-defined polymer brushes on their surfaces, the ATRP-initiator-functionalized silica-coated MNCs were subjected to surface-initiated ATRP with methyl methacrylate. This polymerization proceeded in a living fashion to produce graft polymers with targeted molar masses and narrow molar mass distributions. The average graft density was determined to be 0.65 chains/nm2, which confirms the formation of concentrated polymer brushes on the MNCs. The hybrid particles were analyzed by dynamic light scattering and transmission electron microscopy techniques, which revealed excellent uniformity and solvent dispersibility. A suspension of the polymer-brush-decorated MNCs in acetone quickly developed intense structural color in response to approaching a magnet that depended on the strength of the magnetic field.

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INTRODUCTION Surface-initiated living radical polymerization (SI-LRP) is a powerful technique that produces well-defined graft polymers with exceptionally high graft densities.1−6 Such polymeric architectures are often referred to as “concentrated polymer brushes” (CPBs). Surfaces modified with CPBs exhibit a variety of unique physical properties and functions, such as high graft-polymer stretching and anisotropy, low friction, antifouling behavior, and high biocompatibility. These features have been used in a range of applications such as chromatography, 7−9 low-friction surfaces, 10−17 biocarriers,18−20 and cell culture substrates.21−27 One of the advantages of the SI-LRP surface-modification technique is that it can be applied to many solid surfaces of organic and inorganic materials. Of those substrates, various nano- and microparticles have been surface-modified by SI-LRP. We also synthesized polymer-brush-decorated fine particles using a variety of particle substrates including silica,28−30 gold,31,32 iron oxide,33 and zinc sulfide particles.34 We succeeded for the first time in preparing perfectly dispersive, monodispersed particles grafted with high-density polymer brushes by SI-LRP and fabricated two- or three-dimensional colloidal crystals.35−38 In addition, we systematically investigated the physiological properties of the polymer-brush-decorated fine particles regarding blood circulation and biodistribution; these © XXXX American Chemical Society

properties highly depended on the core particle size and brush length.33,39 The key to success lies with well-defined particle structures that can be easily and widely tuned through the versatility and robustness of particle design involving SI-LRP. Meanwhile, magnetically responsive particles attracted our attention because of their potential widespread applications.40−44 Among several synthesis routes to magnetically responsive particles, methods that use heterogeneous polymerization methods, such as the emulsion and suspension polymerizations of vinyl monomers in the presence of iron oxide nanoparticles, are well-known and popular.45−53 Several types of magnetically responsive particles have been prepared using these methods and used for a variety of purposes, including biological applications such as diagnosis and drug delivery as well as organic-synthesis applications as substrates for catalyst immobilization and for the recovery of trace amounts of components. The drawbacks of these methods, however, include a lack of magnetic responsiveness due to the low iron oxide nanoparticle content in a single particle and difficulties associated with the synthesis of monodispersed particles with diameters less than several hundreds of Received: June 19, 2018 Revised: July 19, 2018 Published: July 23, 2018 A

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graphic purification with activated neutral alumina. Ethyl 2bromoisobutyrate (2-(EiB)Br, 98%) was obtained from Tokyo Chemical Industry, Tokyo, Japan. The ATRP initiator having triethoxysilane, (2-bromo-2-methyl)propionyloxypropyltriethoxysilane (BPE), was prepared as described elsewhere.28,29 Magnetic nanoparticle clusters (MNCs) were prepared by the hydrothermal reaction with FeCl3, ZnCl2, sodium citrate, urea, and PAM in water following the method reported by Tang et al.58 The average diameter of the MNCs was 180 nm as measured by transmission electron microscopy (TEM). Pure water of specific resistivity of ∼18 MΩ·cm was obtained using a Milli-Q system (Nihon Millipore Ltd., Tokyo, Japan). All other reagents were used as received from commercial sources. Characterization. Gel-permeation chromatography (GPC) was performed on a high-speed liquid chromatograph (Shodex GPC-101) equipped with one guard (Shodex GPC KF-G) and two analytical (Shodex GPC KF-806L) columns and a differential refractometer (Shodex RI-101), using THF eluent at a flow rate of 0.8 mL/min at 40 °C. The GPC system was calibrated by poly(methyl methacrylate) (PMMA) standards. A dynamic light scattering (DLS) photometer (DLS-7000, Otsuka Electronics, Japan) was used to measure particle size in acetone at 30 °C. A 10 mW He−Ne laser of 633 nm wavelength was used as the light source. The scattered light was collected at a scattering angle of 90°. Data were analyzed based on the non-negative least-squares method. Proton nuclear magnetic resonance (NMR) measurements were performed on a 300 MHz JEOL/ AL300 spectrometer. The transmission electron microscope (TEM) (JEM-2100, JEOL, Japan) was operated at 200 kV. Thermogravimetric analysis (TGA) was performed using thermogravimetry (TGA50, Shimadzu, Japan) under a nitrogen atmosphere. A photonic multichannel analyzer (Hamamatsu Photonics, PMA-11) was combined with a halogen lamp and a fiber detector to obtain reflectance spectra. Silica Coating of the Magnetic Nanoparticle Clusters. A 500 mL round-bottom flask equipped with a mechanical stirrer was charged with a mixture of 28% NH3 (7.39 g) and ethanol (300 mL). A suspension of MNCs (490 mg) in water (48.5 mL) was added dropwise to the flask over 20 min with stirring at 200 rpm while being sonicated, after which a mixture of TEOS (612 mg) and ethanol (15 mL) was added dropwise to the solution over 1 h with stirring at 200 rpm and with occasional sonication in a bath sonicator. The reaction mixture was mechanically stirred for 2 h at room temperature with occasional sonication. The silica-coated MNCs were collecting with a magnet, washed, and redispersed in ethanol to form an ethanol stock suspension. Fixing the ATRP Initiator onto the Silica-Coated Magnetic Nanoparticle Clusters. A 500 mL round-bottom flask was charged with a mixture of silica-coated MNCs (470 mg) in ethanol (100 mL), and then the system was mechanically stirred. A solution of BPE (1.18 g) and ethanol (15 mL) was added dropwise to the particle solution, followed by the dropwise addition of a mixture of 28% NH3 (5.65 g) and ethanol (100 mL) with stirring at 200 rpm while being sonicated. The solution was mixed by a mechanical stirrer for 3 days at room temperature with occasional sonication. The particles were purified by collecting with a magnet, washing, and redispersing in ethanol and DMF. Synthesis of Polymer-Brush-Decorated Magnetic Nanoparticle Clusters by Surface-Initiated ATRP. Polymerization of MMA was conducted with reference to our previous study.28,29 A Pyrex glass tube was charged with CuCl (30 mg), the ATRP-initiatorfixed MNC suspension in DMF (10 wt %, 2 g), MMA (17.7 g), anisole (5.3 g), 2-(EiB)Br (11.5 mg) (as the free initiator), and dNbipy (241 mg) (as the ligand for complexation with the copper). The Pyrex glass tube was equipped with a three-way stopcock. The reaction mixture was immediately degassed by freeze−pump−thaw cycling, and after the degassing, the tube was refilled with argon. The parent reaction solution was divided approximately equally into four Pyrex glass tubes in a glovebox under argon. Each tube was equipped with a three-way stopcock, and the solution was subjected to one freeze−pump−thaw cycling and vacuum-sealed. The tubes were set in

nanometers. Over the previous decade, magnetic nanoparticle clusters (MNCs) with narrow size distributions have been developed by several groups. Yin et al. synthesized MNCs by hydrolysis of iron(III) chloride (FeCl3) with sodium hydroxide (NaOH) using poly(acrylic acid) (PAA) as the surfactant in diethylene glycol at high temperature; the cluster diameters were able to be controlled in the 30−180 nm range by the NaOH concentration, and the prepared clusters exhibited high dispersibilities in water due to the presence of PAA on their surfaces.54−56 Leung et al. synthesized MNCs by the hightemperature hydrolysis reactions of a mixture of FeCl3 and other metal chlorides with sodium acetate in the presence of poly(vinylpyrrolidone) (PVP) as the surfactant in a binary solvent composed of ethylene glycol and diethylene glycol.57 Tang et al. synthesized MNCs using the hydrothermal reaction of metal chloride (metal = Zn, Mg, Mn, and Ni), iron chloride, sodium citrate, urea, and poly(acrylamide) (PAM); the saturation magnetization of the zinc-containing MNCs could be tuned by adjusting the Zn content.58 These clusters exhibit interesting features; in particular, they show superparamagnetic properties at around 25 °C with relatively high saturation magnetizations compared to those of magnetic nanoparticles, which is ascribed to the large cluster sizes. These properties make MNCs promising materials for several applications such as substance separation and magnetic resonance imaging.59−62 In addition, most of the above-mentioned MNCs have narrow diameter distributions that range from several tens to several hundreds of nanometers. Owing to the monodispersities and high dispersibilities of MNCs, they self-assemble into linear arrangements with periodic interparticle distances when the MNC suspensions are exposed to a magnet.63−68 When the interparticle distance was close to the wavelengths of visible light, the suspensions exhibited beautiful structural colors that varied in response to the strength of the external magnetic field. Except for a few studies that used MNCs coated with hydrophobic silane coupling agents,67 most of these studies were performed in water because the MNC surfaces were modified with water-soluble polymers such as PAA, PVP, and PAM. To develop the science and technology of these very interesting MNCs to a higher level and to adapt them to a variety uses, the ability to surface-modify these MNCs is very important and meaningful. Herein, we developed a method for surface-modifying MNCs by grafting them with well-defined polymer brushes using SI-LRP. To avoid aggregation, several precautions were taken to progress the reaction from initiator fixation to polymerization on the MNCs, which are presented in detail. The polymer-brush-decorated MNCs prepared in this study are highly uniform and highly dispersible in the solvents in which the polymer brushes themselves are soluble. We also describe how, owing to their features, the polymer-brushdecorated MNCs form ordered structures that exhibit intense colors that depend on the strength of the external magnetic field, even in an organic solvent.

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EXPERIMENTAL SECTION

Materials. Ammonium hydroxide solution (28% NH3), anisole (99.5%), copper chloride (CuCl, 99.9%), N,N-dimethylformamide (DMF, 99.5%), 4,4′-dinonyl-2,2′-bipyridine (dNbipy, 97%), ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA, 99%), and tetrahydrofuran (THF, 99.5%) were purchased from Wako Pure Chemicals, Osaka, Japan. Methyl methacrylate (MMA, 99%, Nacalai Tesque Inc., Osaka, Japan) was used after chromatoB

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Figure 1. Transmission electron micrographs of (a) magnetic nanoparticle clusters (MNCs) prepared by the hydrothermal decomposition of iron chloride and zinc chloride, (b) fragmented MNCs formed following surface-initiated atom transfer radical polymerization (ATRP) with the ATRPinitiator-bearing MNCs, and (c) silica-coated MNCs prepared by silica-coating the MNCs shown in (a) using a modified Stöber method. a shaking oil bath (Personal H-10, TAITEC, Japan), and the polymerization was conducted at 60 °C. Each sample was cooled to room temperature after the predetermined time. NMR and GPC analyses were conducted to determine monomer conversion and polymer molar masses. The polymer-grafted MNCs were collected from the remaining reaction mixture by centrifugation. The centrifugation−redispersion cycle was repeated three times using acetone and twice using THF to purify polymer-grafted MNCs.

which causes particle aggregation because of the increase in system viscosity due to the production of free polymers. In addition, the free polymers produced from 2-(EiB)Br provide a good measure of the graft polymers concurrently produced on the SiP surfaces; in other words, characteristics such as the molar masses of the free polymers are recognized (or inferred) to be almost identical to those of the grafted polymers. Figure 2 displays a kinetic plot for the MMA polymerization with the

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RESULTS AND DISCUSSION Synthesis of ATRP-Initiator-Fixed Magnetic Nanoparticle Clusters. To synthesize MNCs, we selected the method reported by Tang et al.58 They claimed that their synthetic strategy had several advantages. First, the synthesis can be performed in water using the raw reaction materials that are less expensive than those used in other methods. Second, the prepared MNCs exhibit relatively high saturation magnetizations due to the presence of small amounts of metals such as Zn and Mg. Third, the MNCs are uniform in size and can be tuned by adjusting the reaction conditions. Following their method, we prepared zinc ferrite nanoparticle clusters with diameters of about 180 nm that had a narrow size distribution, as shown in Figure 1a. Subsequently, we attempted to modify the surfaces of the prepared MNCs with ATRP-initiating sites, followed by SI-ATRP of MMA from the modified particles. However, as Figure 1b shows, the clusters disintegrated following polymerization, even though some polymers were grafted onto their surfaces. This is ascribable to polymer chains grown from the insides of the MNCs as well as from the MNC surfaces, which disconnect the cluster-forming physical linkages between the nanoparticles. To overcome this problem, we silica-coated the MNCs using a modified Stöber method with TEOS and NH3 in ethanol aided by sonication to retain high MNC dispersibility. Figure 1c display a TEM image of MNCs coated with a thin silica layer of about 10 nm in thickness. Subsequently, the surfaces of the silica-coated MNCs were modified with the ATRP-initiator-bearing silane coupling agent (BPE) in a similar way to that reported previously by us for silica particles with the exception being that sonication was used to maintain a homogeneous reaction system. The BPEmodified, silica-coated MNCs exhibited very high dispersibilities in DMF; consequently, they were subjected to SI-ATRP with MMA in DMF/anisole as the (mixed) solvent. SI-ATRP from Initiator-Fixed MNCs. The coppermediated SI-ATRP of MMA was performed in DMF/anisole in the presence of 2-(EiB)Br. As we reported previously,28,29 2(EiB)Br plays an important role in producing an adequate amount of Cu(II) species at the beginning of polymerization, thereby controlling the polymerization through the persistent radical effect;69−71 it also reduces particle−particle coupling,

Figure 2. Plot of ln([M]0/[M]) as a function of polymerization time (t) for the solution polymerization of methyl methacrylate (MMA) with the ATRP-initiator-fixed, silica-coated magnetic nanoparticle clusters with an average diameter of 200 nm (0.78 wt %) in a mixture of N,N-dimethylformamide (7.8 wt %) and anisole (20.8 wt %) at 60 °C; [MMA]0/[ethyl 2-bromoisobutylate]0/[Cu(I)Cl]0/[4,4′-dinonyl2,2′-bipyridine]0 = 3000/1/5/10. The initial concentration of MMA is 70 wt %.

ATRP-initiator-functionalized MNCs and 2-(EiB)Br in DMF/ anisole. The plot is almost linear passing through the origin, which reveals that the propagating radical concentration was almost constant throughout the polymerization period. Figure 3 displays the number-average molar masses (Mn) and dispersity index (Mw/Mn) of the free polymers produced from 2-(EiB)Br during the polymerization process corresponding to the kinetic plot shown in Figure 2. The line in Figure 3 indicates the theoretical Mn value (Mn,theo), which is calculated by (MMA molar mass) × (monomer conversion) × (initial MMA concentration)/(initial initiator concentration available both in solution and on MNC surface). The initiation sites available on the surfaces of MNCs were determined based on the grafting density of polymer, as discussed below. The free polymer molar masses increase in proportion to the monomer conversion and are in good agreement with the theoretical values. The dispersity index Mw/Mn of around 1.1−1.2 C

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Figure 4. Time-dependent graft densities of the poly(methyl methacrylate) grown from the surfaces of ATRP-initiator-fixed, silica-coated magnetic nanoparticle clusters with an average diameter of 200 nm. The dotted line is provided for guidance. Figure 3. Evolution of number-average molar mass (Mn) and dispersity index (Mw/Mn) of the free polymer as a function of monomer conversion during the solution polymerization of methyl methacrylate (MMA) with ATRP-initiator-fixed, silica-coated magnetic nanoparticle clusters with an average diameter of 200 nm (0.78 wt %) in a mixture of N,N-dimethylformamide (7.8 wt %) and anisole (20.8 wt %) at 60 °C; [MMA]0/[ethyl 2-bromoisobutylate]0/ [Cu(I)Cl]0/[4,4′-dinonyl-2,2′-bipyridine]0 = 3000/1/5/10. The initial concentration of MMA is 70 wt %.

grafts is in the concentrated-brush regime, which can be demonstrated by the sufficiently high graft density. The PMMA-MNCs obtained in this study were well dispersed in good solvents for PMMA, such as acetone, THF, and toluene. To evaluate the dispersibility and the hydrodynamic size of the hybrid particles in more detail, some dilute PMMA-MNC dispersions in acetone were subjected to DLS. A plot of the average hydrodynamic diameter, Dh, of the PMMA-MNCs against the Mw of the free PMMA produced concurrently during the synthesis of the PMMA-MNCs is displayed in Figure 5. The diameters of the compact core− shell and the fully stretched core−shell models are also shown.32 The compact core−shell model consists of a bulk PMMA shell and an MNC core, and the fully stretched core−

indicates a narrow molar mass distribution. By considering the data displayed in Figures 2 and 3, we conclude that this polymerization proceeded in a living manner. We believe that polymerizations from the MNC surfaces also proceeded in a similar fashion to give graft polymers with targeted molar masses and with narrow molar mass distributions as well as producing MNCs grafted with well-defined PMMA brushes (PMMA-MNC). Characterizing the PMMA-MNCs. The PMMA-MNCs were subjected to thermogravimetric analysis (TGA) to determine the mass (w) of the polymer grafted onto the MNCs (Figure S1). The MNC density was calculated as follows: The mixed solvents of varying densities were prepared using chloroform and bromoform. The PMMA-MNCs (graft polymer Mn = 236000) were dispersed in the mixtures, after which they were centrifuged at 1000 rpm for 10 min. The PMMA-MNCs was not sedimented in solvents of densities higher than 1.80 g/mL, and therefore the overall density of the PMMA-MNCs approximated 1.80 g/mL on average. Because the density of PMMA is known to be 1.2 g/mL and the TGA result reveals that the PMMA content was 58 wt %, the density of the MNCs was calculated to be 5.65 g/mL on average. The following equation gives the graft density (σ): σ = (w/M n)A v /(πdc 2)

Figure 5. Average hydrodynamic diameter (Dh) of silica-coated magnetic nanoparticle clusters grafted with poly(methyl methacrylate) (PMMA-MNCs) as a function of the weight-average molar mass Mw of the PMMA free chains that were concurrently produced from the free initiator during surface-initiated polymerization. The Dh values were determined by dynamic light scattering in dilute acetone suspensions at 30 °C. The average diameter of silica-coated magnetic nanoparticle clusters cores is 200 nm, as determined by transmission electron microscopy (see Figure 1c). The broken and dotted lines display the diameters of the fully stretched and compact core−shell models, respectively (see text).32 The numbers in parentheses are the relative standard deviations in particle size. The solid line is provided for guidance.

(1)

where dc is the diameter of the MCN core and Av is Avogadro’s number. In this calculation, the molar masses of the free polymers were adopted because, as mentioned above, the free polymers are recognized to provide a good measure of the properties of the graft polymers produced concurrently, and both polymers have almost identical molar masses. As shown in Figure 4, the graft density was not dependent on polymerization time, being almost constant, at approximately 0.65 chains/nm2. It is important that the layer of polymer D

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Figure 6. Transmission electron micrographs of transferred films of silica-coated magnetic nanoparticle clusters end-grafted with poly(methyl methacrylate) brushes (PMMA-MNCs); the average diameter of silica-coated magnetic nanoparticle clusters cores is 200 nm, as determined by transmission electron microscopy (see Figure 1c). The PMMA-MNCs were synthesized by the surface-initiated atom transfer radical polymerization of MMA from the MNCs for (a) 3, (b) 6, (c) 12, and (d) 24 h.

MNCs dispersed in aqueous solutions.56,58 Owing to the balance of interparticle electrostatic repulsive forces and magnetic attractive forces, the MNCs formed an ordered structure in the direction of the magnetic field, with constant interparticle distances that almost correspond to visible-light wavelengths of some hundreds of nanometers. Consequently, the suspensions diffract visible light and exhibit structural color that can be tuned by adjusting the strength of the applied magnetic field. However, most studies relating to such magnetically tunable structural colors with MNC suspension were performed in aqueous systems due to the difficulties associated with the synthesis of MNCs that are dispersible in solvents other than water. However, a good feature of aqueous systems is that they guarantee long-range repulsive electrostatic interactions between MNCs. One study, reported by Yin et al., described the magnetic assembly of MNCs in 1,2-dichlorobenzene (DCB) as a nonpolar solvent.67 A charge-control agent, namely sodium bis(2-ethylhexyl) sulfosuccinate (AOT), was added to the MNC suspension in DCB to produce micelles in the suspension, which reduced the energy barriers between ion pairs on the MNC surfaces, since AOT micelles entrap the existing counterions away from the MNCs and, hence, enhance the surface charges on the MNCs. As a result, Yin et al. successfully established long-range electrostatic repulsions between the MNCs to produce magnetically responsive structural colors, even in DCB. In this study, we used the characteristic physical properties of concentrated polymer brushes that exhibit strong compression repulsion to broaden the applicability of MNCbased structural color.1 With this in mind, we attempted to

shell model consists of a shell of PMMA chains radially stretched in an all-trans conformation and an MNC core. The Dh of the PMMA-MNCs increases as the Mw value increases. In addition, the Dh value lies between the diameters of the two models throughout all Mw values examined here. Figure 5 also shows the relative standard deviation, δ, of the Dh value. It should be emphasized that the δ value is approximately 6% for most samples. This indicates that high dispersibility of the particles are retained in all synthesis processes from BPE immobilization to surface-initiated polymerization. We previously reported the fabrication of monolayers of polymer-brush-decorated fine particles.28,35 By using this procedure, a toluene suspension of PMMA-MNCs was deposited onto the surface of pure water in a Petri dish. When the toluene evaporated, a thin film was formed at the air−water interface. The thin film was subjected to TEM analysis after transferring the film onto a copper grid. Figure 6 displays the TEM micrographs of the monolayers of PMMAMNC with PMMA graft polymers of varying molar mass. The MNC cores are visible as dark circles, and they are well dispersed with almost constant interparticle distance throughout each film without any aggregation. However, PMMA chains which form fringes around the MNC cores are not observed because they have significantly lower electron densities. The average center-to-center distances between the adjacent particles increased with increasing molar mass of grafted chains. Magnetically Responsive Assemblies of PMMAMNCs. Yin et al. and Tang et al. reported magnetically tunable structural-color systems through the assembly of E

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Figure 7. (a) Photographic image of a suspension of poly(methyl methacrylate)-brush-decorated magnetic nanoparticle clusters in acetone with a magnet in close proximity. (b) Reflectance spectra of the same suspension; the spectra became red-shifted with increasing magnet-to-suspension distance.

create structural color using a suspension of the MNCs grafted with PMMA brushes in acetone. Because the PMMA brushes are uncharged or neutral, electrostatic interactions between the hybrid particles in acetone are negligible. Figure 7a shows a photographic image of a suspension of PMMA-MNCs in acetone in the presence of a magnet (neodymium magnet, 30 × 20 × 10 mm3); the color of the suspension depended on the distance between the magnet and the suspension. Figure 7b displays reflectance spectra of the same suspension. The spectra became red-shifted with increasing distance between the magnet and the suspension, that is, as the strength of the external magnetic field decreased. This is probably a result of the periodic interparticle distance, which determines structural color, increasing with decreasing magnetic field strength. In addition, the response to the magnetic field was extremely quick; the color quickly and repeatedly appeared and disappeared according to the movement of the magnet, as seen in Video S1. We believe that these phenomena are ascribed to the ordered arrangement of the PMMA-MNCs in acetone in the direction of external magnetic field and that the concentrated polymer brushes contribute to the quick response because they produce strong repulsive forces between particles that result in rapid particle redispersion as the magnet is moved away from the suspension. To the best of our knowledge, this is the first report demonstrating that suspensions of MNCs exhibit structural color in an organic solvent unaided by electrostatic interactions between the MNCs. We are currently investigating the effects of the lengths and densities of the polymer brushes, types of polymer brush and solvent, particle concentration, and the strength of the magnetic field on the structural color created by polymer-brush-decorated MNC suspensions, which will be reported in the future.

accomplished in acetone, an organic solvent, despite most MNC-based structural colors reported so far being reported in aqueous systems. We consider the use of polymer brushes as the key to success because they provide strong repulsive forces between particles that are well balanced with the attractive magnetic forces that form ordered structures with constant interparticle distances. This approach opens up new possibilities for the use of polymer-brush-decorated MNCs in next-generation display materials. In addition, because the structural parameters of the polymer-brush-decorated MNCs, including length, density, and type of graft polymer as well as the diameter of MNC core, can easily be controlled, we expect that these hybrid particles will find a variety of unique applications. For instance, MNCs grafted with hydrophilic polymer brushes may be applicable to a variety of biomedical applications such as magnet-driven drug delivery and magnetic resonance imaging by combining the magnetic properties of MNCs and the biocompatibilities of hydrophilic polymer brushes. Such work is currently underway and will be published separately.

CONCLUSIONS Narrowly size-distributed, silica-coated MNCs of average diameter of 200 nm were prepared and modified with a silane coupling agent to immobilize ATRP-initiating sites on the particle surfaces. The particles were then subjected to surfaceinitiated ATRP with MMA in a living fashion. This synthesis procedure provided well-defined PMMA-brush-decorated MNCs with high dispersibilities and a high grafting density of 0.65 chains/nm2. The high uniformity, high dispersibility, and good magnetic responsiveness of the PMMA-MNCs resulted in structural color from a particle suspension that was easily tuned by the external magnetic field. This was

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b02073. Figure S1 (DOCX) Video S1 (AVI)

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AUTHOR INFORMATION

*E-mail: [email protected] (K.O.). ORCID

Kohji Ohno: 0000-0002-1812-3354 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by Grants-in-Aid for Scientific Research (No. 15H03866 and 16K13607) from Japan Society for the Promotion of Science (JSPS). F

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DOI: 10.1021/acs.langmuir.8b02073 Langmuir XXXX, XXX, XXX−XXX