Article pubs.acs.org/Langmuir
Colloidal Polarization of Yolk/Shell Particles by Reconfiguration of Inner Cores Responsive to an External Magnetic Field Ayako Okada, Daisuke Nagao,* Takuya Ueno, Haruyuki Ishii, and Mikio Konno* Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba Aramaki-aza, Aoba-ku, Sendai 980-8579, Japan S Supporting Information *
ABSTRACT: Yolk/shell particles, which were hollow silica particles containing a movable magnetic silica core (MSC), were prepared by removing a middle polystyrene layer from multilayered particles of MSC/polystyrene/silica shell with heat treatment followed by a slight etching with a basic solution. An ac electric field was applied to the suspension of the yolk/shell particles to form pearl chains (1D structure) of yolk/shell particles. Observation with an optical microscope showed that the MSCs in the silica compartment of the pearl chains had a zigzag structure under the electric field. An external magnetic field applied to the suspension could form a novel structure of doublet MSC in the shell compartment of the quasi-pearl chain structure. Application of a magnetic field was also performed for 2D hexagonally close-packed assemblies of the yolk/shell particles, which could two-dimensionally form a doublet structure of MSCs as if they were polarized in the compartment. Switching on/off the magnetic field successfully controlled the positional ordering of cores in the consolidated silica shell.
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INTRODUCTION Since miniaturization of electronic devices such as processors and memories gradually approaches the limit of enhancing their performance, next-generation devices operating with novel principles have been intensively demanded. Assemblies of monodisperse particles as building blocks, most of which can form colloidal crystals, are a promising candidate to create nextgeneration devices with high performance. In the field of colloidal assembling that can be regarded as a phase transition from liquid to crystal (or glass), a number of experimental results and simulations have been reported on anisotropic particles1−4 as well as spherical particles.5−8 It was shown in some simulations that complete photonic band gaps should exist for crystals composed of anisotropic particles such as dumbbells.9−12 Applications of external fields to colloidal suspension have also been used to explore new colloidal crystals with anisotropic particles such as silica dumbbells,13 polymer dumbbells,14 spindle magnetic particles,15 and Janus particles.16 An increasing requirement for colloidal crystals is the switching capability in tuning their optical or electrical properties with external stimuli, which will extend their applications as chemical17 or biological sensors,18 displays, and printings.19,20 For example, polymer-based thermosensitive photonic crystals were developed to enable tuning their optical properties by temperature-induced phase transition of polymers such as poly(N-isopropylacrylamide).21,22 Magnetic particles with low polydispersity were also employed as a component of © 2013 American Chemical Society
rewritable photonic paper on which an aqueous solution of hygroscopic salt was used as an ink.23 Switching the properties of responsive colloidal crystals reported so far, however, required additional space for reconfiguration of building blocks24−29 or accompanied volume phase transitions of building blocks or their matrices.21,22 When colloidal building blocks are employed as components highly packed in devices, neither the additional space for their reconfigurations nor the volume phase transition of building blocks is desirable. It is also preferable for the building blocks to be consolidated in devices because the optical or electrical properties of particles dispersed in media will to a certain extent have distributions wider than those of particles fixed in a specific position. For the requirements mentioned above, the present work experimentally shows a method to assemble yolk/shell particles,30−32 which are also called rattle-typed particles, as building blocks. The configuration of inner cores incorporated into the yolk/shell particles can be changed under external fields without any variation in space occupied with the building blocks consolidated in suspension or medium. The original idea of using the yolk/shell particles as building blocks was proposed by several researchers before.33,34 Camargo et al. predicted in their theoretical calculation that a switching Received: May 1, 2013 Revised: June 12, 2013 Published: June 18, 2013 9004
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property to control propagation of light will be obtained by applying external magnetic fields to yolk/shell particles incorporating a movable magnetic core.33 Although some optical measurements indirectly supported the motion of inner cores in a suspension of yolk/shell particles,35 no one directly observed the motion of inner cores and experimentally showed the possibility of reconfiguration of inner cores with an external field. Recently, we first verified the inner cores randomly moving in a silica compartment by direct observation of micrometer-sized silica yolk/shell particles under an external electric field.36 New types of assembling building blocks imitating atoms and molecules37 have been extensively studied to explore novel functions generated from colloidal crystals. For instance, selfassembling of large and small particles with controlled surface charges led to CsCl or NaCl structure composed of the two particles.38,39 Permanent strings of various colloidal particles, similarly to biopolymer such as γ-DNA, could be successfully produced using external electric fields and a double-layer repulsion followed by a thermal heating step or seeded growth.40,41 In the present work, where the position of inner cores incorporated into yolk/shell particles is manipulated by applying external fields to be moved toward specific directions, reconfiguration of the inner cores freely movable in the shell compartment can be regarded as an analogue of polarized materials such as dielectric and magnetic materials under external fields. The reconfiguration is allowed to offer different positional orderings of cores incorporated into the consolidated shell, creating a variety of possibilities for finding new colloidal structures. In the present article, such reconfigurations under an external magnetic field are directly observed with an optical microscope (OM), which also experimentally verifies the colloidal structure of yolk/shell particles proposed by Camargo et al.33
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Figure 1. Schematic procedure for preparation of yolk/shell particles used for observation with an optical microscope. The second formation of PSt shell was performed at a core−shell particle concentration of 0.05 vol %. Second, the doubly PSt-coated silica particles were coated with silica. A suspension of the PSt-coated particles was added to a solution containing poly(allylamine hydrochloride) (PAH) and NaCl. The concentrations of the doubly PSt-coated particles, PAH, and NaCl were 0.1 vol %, 0.71 kg/m3, and 36 mol/m3 in the mixture, respectively. After two centrifugation steps to remove nonadsorbed PAH and NaCl, particles were redispersed into 20 mL of an ethanolic solution containing 0.14 g of polyvinylpyrrolidone (PVP). Two more centrifugation steps were used to remove excess PVP, and particles were redispersed into 10 mL of ethanol. To the ethanolic solution, 3.69 mL of ammonia solution, silica sources of TEOS (296 μL) and aminopropyltriethoxysilane (19 μL) were added. PSt particles coated with silica were dried, followed by heat treatment for 4 h in air in an oven at 500 °C. Particles after heat treatment were immersed in 30 mL of a NaOH solution (0.5 mM) at a particle concentration of 0.05 wt % to slightly etch the silica component of the particles. The obtained yolk/shell particles were redispersed in deionized water and used for optical observation without any particle classification. Although the amount of yolk/shell particles was small, that was enough to observe their movements and assemblies in water. Characterization. The yolk/shell particles obtained with the above method were observed with FE-TEM (Hitachi, HD-2700). The sample cell used for particle observation under external fields consisted of a capillary (0.1 × 1 mm rectangular cross section, VITRO COM) and two 50 μm diameter copper wires (99.99%, NIRACO) threaded along the side walls. The capillary was filled with an aqueous suspension of particles, and the ends of the capillary were sealed with glue. The ac field was applied by connecting the copper wires to a function generator (GWINTEK, SFG-2004) and amplifier (NF Circuit Design Bloc, HSA4011). The electric field strength (peak to peak) was measured with a digital oscilloscope (GWINTEK, GDS-1062A). An external magnetic field with 100 mT was used for application to the suspension, which was generated with a magnetic coil fabricated by Toei Scientific Industrial Co. The concentration of yolk/shell particles in the suspensions was set at a low weight fraction of 0.2 wt % to acquire clear images of 1D structure in observation with an optical microscope. A 2D hexagonally close-packed structure was observed in the capillary at 1.2 wt % of yolk/shell particles.
EXPERIMENTAL SECTION
Chemicals. Styrene (St, 99%), sodium chloride, potassium persulfate (KPS, 95%), and p-styrenesulfonate (NaSS, 80%) were obtained from Wako Pure Chemical Industries (Osaka, Japan). The inhibitor for St monomer was removed by inhibitor removal columns. Polyvinylpyrrolidone (PVP, Mw = 360 000 g/mol) was purchased from Tokyo Chemical Industry. Poly(allylamine hydrochloride) (PAH, Mw = 15 000 g/mol) was obtained from Aldrich. Silane coupling agents of 3-aminopropyltrimethoxysilane (APTMS, 95%) and 3-methacryloxypropyltrimethoxysilane (MPTMS, 95%) were purchased from Chisso Corp. and Shinetsu Chemical, respectively. Chemicals except for styrene were used as received. Particle Synthesis. Multilayered particles (magnetic silica core/ PSt/silica shell) were prepared with a combination of soap-free emulsion polymerization, silica coating, and pyrolysis of the polymer components. A schematic procedure for preparation of yolk/shell particles is presented in Figure 1. The magnetic silica cores (MSCs) were prepared according to our method previously reported.42,43 In the preparation of multilayered particles first, a suspension of MSCs surface modified with 3-methacryloxypropyltrimethoxysilane (MPTMS) was bubbled with nitrogen for 30 min. The surfacemodification was performed at a MSC concentration of 0.3 vol %, a MPTMS concentration of 2 mM, and a reaction volume of 300 mL. To the suspension of MPTMS-modified particles, KPS and NaSS were added. After stirring for 20 min at 70 °C, styrene monomer (50 mM) was added as an inner shell source to the suspension. Formation of PSt shell was conducted at [KPS] = 2 mM, [NaSS] = 0.25 mM, and [MSC] = 0.15 vol %. To further increase the thickness of PSt shell, another polymerization of St was conducted at a concentration of 200 mM with KPS initiator (2 mM) in the presence of NaCl (16 mM).
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RESULTS AND DISCUSSION Figure 2a shows TEM image of yolk/shell particles used for observation with an optical microscope (OM). As shown in the Figure 1, the particles are composed of a magnetoresponsive silica core (MSC) and outer silica shell, which were fabricated by removing a middle PSt layer from multilayered particles of MSC/PSt/silica shell with heat treatment followed by slight etching with a basic solution. The inset of Figure 2a shows a magnified TEM image of approximately 10 nm magnetite/γmaghemite nanoparticles homogeneously incorporated into silica particles. Figure 2b presents a TEM image showing that 9005
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Figure 2. TEM images of yolk/shell particles observed with high (a) and low magnifications (b) and their magnetization curve (c) measured with a vibrating sample magnetometer.
direction without any particle overlapping as shown Figure 3b. Some particle chains being arranged side by side coalesced to become a thick chain. The yolk/shell particles in a swinging chain were slightly moving due to Brownian motion. When a swinging chain approached another swinging chain, MSCs in the approaching shells attracted each other to form zigzag structure of MSC in a double chain.24,29 The result indicated that MSCs were attracted strongly enough to keep the polarized cores in a close proximity under the ac electric field (see Movie 1, Supporting Information). Magnetic interactions between the MSCs of yolk/shell particles were examined by applying a 100 mT magnetic field. Figure 4a and 4b shows OM images of yolk/shell particles at 3
the yolk/shell particles have no aggregation in their fabrication with the combination of heat treatment and alkaline etching. The MSC had an average size of 930 nm larger than the visible wavelength used in the OM observation and also had sufficient void between MSC and silica shell for clearly observing the motion of MSC under external fields. The saturation magnetization of MSC was 0.8 emu/g (see Figure 2c), which can estimate a content of magnetic component to approximately 1.1 wt % using a saturation magnetization (∼70 emu/g) of the nanoparticles as prepared. We reported that application of an ac electric fields is effective for building blocks to form assembled pearl chains that facilitated observation of inner cores randomly moving under the electric field.36 Figure 3 shows OM images of yolk/shell
Figure 3. OM images of yolk/shell particles observed at 0 (a) and 30 s (b) after initiation of an ac electric field with 50 V/mm and 1 kHz.
particles observed before and after application of an ac electric field with a peak-to-peak strength of 50 V/mm and a frequency of 1 kHz. The direction of electric field in the observation capillary was indicated with the arrow. The concentration of yolk/shell particles was 0.2 wt % in aqueous suspension. Figure 3a shows the initial state of yolk/shell particles dispersed in solution. During the 30 s application, the yolk/shell particles gradually formed the pearl chain structure toward the field
Figure 4. OM images of yolk/shell particles observed at 3 (a) and 30 s (b) after initiation of a magnetic field with 100 mT. 9006
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and 30 s after initiation of magnetic field application. The direction of the magnetic field was horizontal to the OM image frame, which is indicated with the arrow. Most of the two adjacent MSCs of yolk/shell particles in close proximity attracted each other within 3 s to form “doublet structure” accompanying the steric hindrance of outer spherical shells, although some shells as shown in Figure 4b formed “zigzag structure” containing cores almost in a line parallel to the magnetic field. Continuous application of the magnetic field gradually formed a quasi-pearl chain structure where two MSCs of adjacent yolk/shell doublets were located so as to keep the MSC−MSC distance close while maintaining each doublet structure (see Movie 2, Supporting Information). The structure of the quasi-pearl chains was readily released by turning off the magnetic field. For the yolk/shell particles containing a magnetic core proposed by Camargo et al.,33 who predicted the alignment of cores in yolk/shell particles toward a specific direction along the applied field line, they probably assumed inhomogeneous magnetic fields applied to the magnetic cores with weakly magnetic interparticle interaction. On the other hand, since attractive magnetic forces between the MSCs in the present work was stronger than those caused by inhomogeneity in the magnetic field, such interesting patterns of doublet MSC structure or zigzag shell structure were formed and maintained under the field application. An additional application of electric field (1 kHz, 50 V/mm) under the fixed 100 mT magnetic field was also performed to strengthen attractive forces between MSCs. After initiation of the electric field application, MSCs in the quasi-pearl chain formed by the magnetic field application took a reconfiguration more quickly toward the zigzag MSC structure (see Movie 3, Supporting Information) than the corresponding multilayered particles of MSC/polystyrene/silica shell or the anisotropic magnetic particles already reported by Sacanna et al.29 The quicker reconfiguration was probably because the MSCs were freely movable in the silica compartment. The additional application of magnetic field, however, did not lead to complete 2D structure of the yolk/shell particles because of the low particle density on the observation plane. Observation of yolk/shell particles 2D hexagonally close packed was achieved by forced convections induced by solvent absorption with hydrophilic gel to localize the yolk/shell particles on a glass plate. Figure 5a shows an OM image of yolk/shell particles hexagonally close packed with the forced convections under no magnetic field. Most MSC incorporated into the yolk/shell particles still have Brownian motion at the close-packed state. A 100 mT magnetic field was applied in the direction parallel to the lines of yolk/shell particles illustrated in the scheme. Within 20 s after initiation of the magnetic field, as shown in Figure 5b, almost 50% MSC moved within the shell compartment were attracted to the right side along the field direction whereas the rest moved were attracted to the left side. The reconfiguration of MSCs fixed to the right and left sides could form the 2D doublet structure (see Movie 4, Supporting Information). The 2D doublet structure of MSCs could be released to the original state of Brownian motion by turning off the magnetic field, suggesting that the positional ordering of MSCs can be controlled by the external magnetic field. Figure 6 shows a similar structure of 2D doublets observed under switching on/off a magnetic field in the capillary used in the experiments of Figures 3 and 4. The iterative application
Figure 5. OM images of close-packed yolk/shell particles observed at 0 (a, c) and 20 s (b, d) after initiation of a magnetic field with 100 mT. Close-packed structure of yolk/shell particles (a) was rotated by 30° to obtain the structure of c.
indicated that the MSCs in the silica compartment could reversibly respond to an external magnetic field. Another observation of 2D close-packed particles fabricated by the forced convections was made with the 30°-inclined 2D array shown in Figure 5c. As illustrated in Figure 5d, application of magnetic field to the 2D array forced the movable cores to be arranged in the structure of lined doublets inclined by 30° from the direction of applied field (also see Movie 5, Supporting Information). The structure of MSCs in Figure 5d can be expected from the following equation F=
3(1 − 3 cos2 α)m2 r d4
(1)
where F is the dipole force exerted on a MSC induced by an adjacent MSC, α is the angle between the external magnetic field and the line connecting the center of the two particles, d is the distance between the center of particles, m is the magnetic moment of MSC, and r is the unit vector parallel to the line pointed from the center of MSC to that of adjacent MSC. A critical angle of 54.09° derived from the above equation shows that the dipole−dipole interaction is attractive when 0° ≤ α < 54.09° and repulsive when 54.09°≤ α
