Highly Stable Microcapsules of Colloidal Photonic Ink in Nonpolar Medium for Full Color E‐Skin Device

The colloidal dispersion in a nonpolar medium is an essential material for electrophoretic displays (EPD) with low‐power consumption. A uniform‐sized superparamagnetic iron oxide nanoparticle (SPION) is a promising candidate for EPD, which exhibits tunable structural color by Bragg diffraction. In this study, the surface of SPION is charged in a nonpolar medium by inverse micelles of Solsperse‐17k, an oil‐soluble polymeric surfactant. A photonic ink of SPION dispersion exhibits simultaneous magnetochromism and electrochromism. The photonic ink is encapsulated via a complex coacervation process, in which double layers of gelatin/gum Arabic form a stable shell for µ‐capsule. The µ‐capsules show tunable structural colors, which depends upon the size of SPION in photonic ink. The increased surfactant content in photonic ink brings about a decrease in µ‐capsule size due to a reduced surface tension. A lowered gelatin concentration during coacervation results in a smaller µ‐capsule, which exhibits an electrical color tunability. Optical characterization using a confocal microscopy enables 3D visualization of the inner structure of µ‐capsules and the formation of particle chain structure of SPION in H‐field. The encapsulated photonic ink exhibits magnetochromism for 1 year, illuminating the long‐term stability of µ‐capsules developed in this study.


Introduction
Colloidal dispersion in the nonpolar medium has attracted great attention from researchers and engineers due to its numerous applications such as an electrophoretic display (EPD), petroleum industry, and printer toners. EPD is an electronic analog of paper that relies on the reflection of light to show the contrast for display. [1] In the 1990, electrophoretic image film was first developed by E ink corporation, [2] which was commercialized by several companies. The fundamental idea behind EPD is the electrophoretic mobility of oppositely charged colloidal dispersions, respectively, of black and white colors between two parallel transparent electrodes. Voltage bias on each pixel induces black and white reflective contrasts. [2b] From the practical point of view, success in microencapsulation of ink dispersion has triggered the manufacturing process of EPD devices as the encapsulated ink is safe from a potential leakage problem of liquid dispersing medium. Over the past two decades, tremendous advancements have been made in EPD technologies. [3] Fullcolor EPD panels have also been investigated using color filters on black and white particles or by directly using colored dispersions. [4] However, using a color filter or colored particle using organic dye accompanies the inevitable loss of brightness due to the inherent light absorption, and the use of organic dye is subjected to a color bleaching upon exposure to sunlight or room light. Structural colors based on Bragg diffraction can provide better visibility and definition in color display than absorptive colors. Polymers infilled within a thin opal template changed the structural color by electrochemically adjusting the swelling degree in a photonic crystal display developed by Ozin and colleagues. [5] The dispersions of a crystalline colloidal array (CCA) in a liquid medium have also been used to implement EPD. Monodisperse colloidal spheres have the ability to self-assemble into long-range ordered structures with facecentered cubic lattices when their surface is electrostatically stabilized in a liquid medium, exhibiting the structural color. [6] For the color-tunable reflective display, such CCA with interparticle distance at around a half of visible light wavelength (400-700 nm) has shown great promise. [7] As the particle charging is facilitated in a liquid medium with a large dielectric constant ( r ), colloidal dispersions were frequently prepared in water, alcohol, or propylene carbonate for CCA formation. [7b,8a-8c] For instance, Kang et al. reported an EPD of superparamagnetic iron oxide (SPION) microspheres in propylene carbonate. [8a] However, it was discovered that if the surface of the colloidal microsphere is suitably modified, charge-stabilized CCA can be obtained. According to Yin et al., the C18-modified SPION could be stabilized in a hydrophobic medium by adsorbing Aerosol-OT (AOT), an oil-dispersible anionic surfactant in the forms of the inverse micelle. Subsequently, colloidal self-assembly was investigated to tune the magnetochromic color. [9] Charge stabilization can be accomplished using nonpolar polymeric colloidal particles without surface modification. Recently, we used the commercial i-paraffinic fluid in isopar-G (IPG, ExxonMobil) to establish EPD using CCA of poly(t-butyl methacrylate) (PtBMA) μ-spheres stabilized by AOT inverse micelles. [10] The use of core-shell μ-spheres with a high index poly(methyl methacrylate) core was shown to improve van der Waals (VDW) interaction with AOT inverse micelles which led to the enhanced particle charging and greater color tunability. [11] Ge and co-workers recently reported that a weak-polar medium can enhance voltage response of electrically responsive photonic crystal. [12] They also investigated a bistable EPD which is suitable for a low-power consumption display. [13] Microencapsulation of particle dispersion is a prerequisite to expediting the commercialization of color EPD, as the microcapsule (μ-capsule) is an ideal form for the EPD material, which can be coated on the transparent electrode with no leakage concern. As adopted by E ink technology, [2a,14] "complex coacervation" is the key concept of microencapsulation of colloidal dispersions, in which the water-soluble polymer such as gelatin forms a thin layer on the surface of an oil droplet, containing the dispersed particles. [15] pH decrease and subsequent addition of polysaccharides with a lower isoelectric point facilitate the complex coacervation. [14,15] Although microencapsulation of a display ink is an essential process for an EPD device, there has been few attempt for a preparation of structural color-based photonic ink by complex coacervation process. Kim et al. reported the microencapsulation of aqueous CCA by microfluidic technique, [16] and they recently improved the photonic microcapsules. [17] Toward a large-scale preparation of μ-capsules, on the other hand, a bulk process would be more suitable.
In this study, microencapsulation of the charge-stabilized SPION dispersions in nonpolar liquid is demonstrated via a complex coacervation, toward a "dye-free" color-tunable photonic crystal display. As a photonic ink, a colloidal dispersion of SPION in a nonpolar mixture of IPG/halocarbon (HC) is prepared using Solsperse-17k (Lubrizol) as a charge control agent, enabling charge stabilization of SPION colloids. The magnetochromic and electrochromic tunings of structural color are demonstrated on photonic ink and μ-capsules, respectively.

Preparation of Photonic Grade SPION Dispersion
The superparamagnetic Fe 3 O 4 @SiO 2 core-shell nanoparticles (SPION) were synthesized using a two-step sol-gel process in a mixed solvent of ethylene glycol (EG) and diethylene glycol (DEG) according to the procedures in a previous literature by Yin et al., and subsequently passivated by an ≈20 nm thick SiO 2 shell. [18] As characterized in Table 1, the SPIONs of three different sizes (SP130, SP140, and SP170) were obtained by controlling the ratio of EG/DEG during Fe 3 O 4 synthesis. Details on SPION synthesis is described in Supporting Information Figure S1. As shown in Figure 1a, the X-ray diffractogram of SP130 showed the diffraction peaks from a typical spinel crystal structure of magnetite. [19] A pristine Fe 3 O 4 nanoparticle exhibits a typical raspberry-like cluster structure ( Figure S1, Supporting Information), while a SPION shows a rounded surface by the SiO 2 shell as shown in Figure 1b,c which are scanning electron microscope (SEM) and transmission electron microscope (TEM) images of SP130, respectively. Average particle size and distribution are important for μ-spheres to be utilized in photonic crystal applications. [20] The size distributions of three SPIONs were characterized by dynamic light scattering data (Figure 1d-f) and SEM, respectively, and the results are summarized in Table 1. A decrease in particle size with higher DEG content during the synthesis is evident in both the hydrodynamic sizes (D h ) and the diameters of dried particles. Formation of the SPION with an appropriate size and narrow distribution enough to show the tunable structural colors under a magnetic (H) field can be confirmed in Figure 1g, where the rainbow-like structural colors are developed near the magnet at the bottom of the vial containing a colloidal dispersion of the SP130 in ethanol. As illustrated in Figure 1h, the magnetic field aligns the dispersed SPION as they are attracted to each other by dipole interaction F = −6 μ 2 /d 4 , where μ is the induced magnetic moment in each SPION and d is the distance between two aligned particles. The magnetically induced attractive force is balanced by the electrostatic repulsion between the μ-spheres stemming from the surface charges of silica surface in a polar solvent, maintaining equilibrium d.
Yin et al. found that the attracted SPIONs form a typical "particle chain" structure to enable Bragg diffraction of light, in which diffraction wavelength ∼2d (Figure 1h). [19] The particle chain can be formed even at a low concentration of colloidal dispersion (<5 wt%), which shows no color in the absence of an H-field. Since the magnetic moment of SPION is stronger at a higher Hfield, d becomes shorter as the colloidal dispersion is nearer to the magnet, resulting in a blue-shifted structural color as shown in Figure 1g. The structural color of SPION is also dependent on the average particle size. Ethanol dispersions of three SPIONs having different average diameters (Φ SPION = 130, 140, and 170 nm) exhibit different structural colors at a similar H-field strength of 60 mT, as shown in Figure 1i, where a dispersion of a larger SPION shows a more red-shifted color. As shown in Figure 1j, the change of structural colors of SPION dispersions with H-field strength can be effectively presented by reflectance spectra, in which the blue shift of peak with increasing H-field is shown. Magnetic field strength versus distance between a magnet and μ-capsules are plotted in the Supporting Information ( Figure S2).

Charge Stabilization of SPION in Nonpolar Medium
Although SPIONs are readily charged in polar solvents such as water, [19] alcohol, and propylene carbonate [8a] due to the presence of acidic silanol groups on silica shells, charging them in a nonpolar medium requires the use of oil-miscible surfactant which can form the inverse micelle. [21] Yin et al. reported that AOT in dichlorobenzene could be effectively used as a charge control agent to produce the charged CCA of SPION, which had been rendered hydrophobic. [9] We also prepared the SPION modified by the octadecyl group and observed a structural color from its dispersion in IPG/AOT in H-field. A pristine SPION dispersion was confirmed to show almost no structural color in the same liquid medium (Supporting Information, Figure S3). In this study, our goal was to set up an optimized condition for the dispersion of a pristine SPION in IPG/HC medium using Solsperse-17k, a cationic surfactant containing ethylene diammonium head group and methyl sulfate counter ions with a short-branched polyester tail. [4b] As schematically drawn in Figure 2a, Solsperse-17k will form an inverse micelle in a nonpolar medium with a trace amount of water molecules in the core. Upon collisions between neutral micelles, transfer of counter ions can occur, and there will be oppositely charged inverse micelles mixed (M + M → M + + M − ). [22] In Figure 2b, M + and M − are distinguished by orange-colored core and blue-colored core, respectively. The charged inverse micelles are attracted to the μ-sphere surface, as shown in Figure 2b, owing to VDW attraction with the preadsorbed surfactant molecules on the SiO 2 layer (Figure 2b). Unlike anionic surfactants like AOT, Solsperse-17k can effectively adsorb onto silica surfaces by Coulombic attraction due to its cationic nature. [23] We anticipate a "head-on adsorption" of the cationic surfactant to a negatively charged silica surface on which a trace amount of water molecules may exist. Consequently, without chemical modification of SPION, the charged Fe 3 O 4 @SiO 2 μ-spheres could be successfully produced in nonpolar medium in this study. Presence of Solsperse-17k affected physical properties of liquid medium. As shown in Table 2 and Supporting Information ( Figure S4), viscosities and other physical properties of liquid media with increasing surfactant content were measured at ambient condition.
As plotted in Figure 2c, zeta-potentials of 0.5 wt% of SP170 dispersed in IPG/HC/Solsperse-17k were measured with various surfactant concentrations, showing that the negative surface Figure 2. Schematic illustrations of a) neutral inverse micelle of Solsperse-17k dispersed in oil with cationic ethylene diammonium head and methylsulfate counterions in the core, to produce anionic (blue-colored core) and cationic (orange-colored core) micelle, respectively, by collision, b) spherical SPION dispersed in IPG/HC with anionic inverse micelle adsorbed on the μ-sphere surface by VDW interaction with preadsorbed Solsperse-17k. c) Zeta potential plots of 0.5 wt% of SP170 μ-spheres dispersed in IPG/HC with varied content of Solsperse-17k, d) schematic illustration showing the magnetochromic color changes of SPION via particle chain formation. e) Reflection spectra of 20 wt% of SP170 μ-spheres dispersed in IPG/HC containing 3 wt% Solsperse-17k entrapped between two ITO glass at 100 μm thickness with varying H-field strength, f) structural color changes of the same dispersion by magnetochromism, g) schematic illustration showing the electrochromic color changes of SP170 by electrophoretic movement and self-assembly of the charged μ-spheres, h) reflection spectra of 40 wt% SP170 dispersion with and without voltage bias, i) corresponding electrochromic color changes of the same dispersion. potential as large as −43 mV was obtained at a certain range of surfactant content (1-4 wt%). Interestingly enough, we observed the positive surface charges at both lower (<1%) and higher (>4%) concentration regimes, with asymptotic behavior at extremely high concentrations of surfactant. A different set of experiments confirmed the shape of plots (Supporting Information, Figure S5). In the previous investigations employing AOT as a charge control agent, preferential adsorption of anionic inverse micelles onto the particle surface was the main reason for a negative zeta potential which can also explain the current observation. [22,24] Charge reversal has been reported in several pieces of literature, [25] in which adsorption of cationic micelle and that of sodium counterion in AOT were addressed.
We also previously observed the charge reversal phenomenon, in which PtBMA μ-spheres were dispersed in IPG/HC/AOT. [10] Although a further study is necessary, strong adsorption of cationic Solsperse-17k molecules onto SPION surface is believed to be responsible for positive zeta potential at low surfactant concentration with methylsulfate counterions transferred to adjacent inverse micelles near particle surface, which is followed by preferential adsorption of anionic inverse micelles at the increased surfactant content to reverse the surface charge. As the surfactant concentration further increases, the surface will be equally saturated with anionic and cationic micelles to show an asymptotic decrease of surface potential. [24] As schematically illustrated in Figure 2d, H-field can also induce a particle chain structure of a charge-stabilized SPION dispersion in a nonpolar medium owing to a balanced magnetic attractive force with a repulsive force between particles enabled by the surface charges. [9] Using a 20 wt% dispersion of pristine SP130 in IPG/HC/Solsperse-17k with surfactant concentration at 3 wt%, a development of diffraction peak and its blue shift by the increased H-field strength was confirmed as shown in Figure 2e. Figure 2f shows a magnetochromic structural color of the dispersion in a 100 μm thick cell at ≈30 mT H-field. Unlike magnetochromism in which the structural color originates from the light diffraction at the particle chain, which can exist at a low concentration of SPION (<5%), electrochromism requires a much higher content of SPION. [8a,20] As the concentration of the charged SPION increases as high as 40 wt%, the dispersion begins to show the structural color which is tunable by E-field, as schematically shown in Figure 2g. [8a] As predicted by zeta potential measurements, 40% dispersion of SP130 with 3 wt% Solsperse-17k in IPG/HC becomes negatively charged, and that confined between two transparent electrodes exhibited a weak orange color. As shown in Figure 2h, the reflectance spectrum without voltage bias showed a broad peak having a peak wavelength ( peak ) at ≈600 nm, while a blue shift in a peak was observed upon voltage bias. At 3 V, peak shifted to 560 nm due to the electrophoretic migration of the negatively charged dispersion to an anode (top indium tin oxide (ITO) glass in Figure 2i). As shown in Figure 2b, substantial amount of Solsperse-17k is supposed to be adsorbed on the SPION surface within a photonic ink with high content (>20%) of SPION, and thus the bulk concentration of Solsperse-17k in IPG/HC will be lowered compared to that used for zeta potential measurement in which much lower SPION was dispersed. As an evidence, resistivities of IPG/Solsperse-17k with and without 2 wt% of SPION were measured to be 0.07 and 0.08 μS, respectively, confirming that Solsperse-17k concentration is lowered in the bulk solution. Therefore, the surface charge of SPION in an electrochromic photonic ink cannot be directly estimated from the results shown in Figure 2c. Nevertheless, development of structural color and its electrical tuning imply that the charge stabilization of SPION in nonpolar medium is successfully accomplished by adsorption of Solsperse-17k inverse micelles. Broad peaks with and without E-field imply that the alignment of SPION dispersion is quasi-amorphous, as reported previously. [8a,20] An advantageous feature of quasi-amorphous photonic ink is that structural color has little angle dependency, which was confirmed in the Supporting Information ( Figure S6). [8a,10,26] The changes in structural colors by voltage bias are shown in Figure 2i. It is noteworthy that the use of Solsperse-17k simplifies the preparation procedures of the charged SPION. When AOT is used as a charge stabilizing agent, the surface of SPION has to be modified by an alkyl silane compound. [9] No structural color was developed for an untreated SPION dispersed in IPG/HC/AOT due to poor adsorption of AOT inverse micelles onto the polar SIO 2 surface ( Figure S3, Supporting Information). On the other hand, an IPG/HC/Solsperse-17k dispersion of an untreated SPION exhibited E-tunable structural color due to strong adsorption of Solsperse-17k molecules on the SiO 2 surface to facilitate subsequent adsorption of charged inverse micelles as depicted in Figure 2b.

Microencapsulation of Nonpolar SPION Dispersion
Using a concentrated SPION (>20 wt%) dispersed in IPG/HC/Solsperse-17k as an oil phase, complex coacervation of gelatin/gum Arabic on the oil-water interface was performed for encapsulation of the photonic ink. Although IPG/HC has a relative permittivity ( / 0 ) as low as 2.2, the value increases upon adding Solsperse-17k, as shown in Figure 3a. A higher / 0 implies a lower surface tension at the oil-water interface, enabling a stable dispersion of the oil phase in aqueous media. In the first stage of complex coacervation, during which gelatin deposition occurs (Figure 3b), the aqueous phase was kept at a high pH to ensure the negatively charged gelatin. As schematized in Figure 3c, a significant portion of surfactant molecules will self-assemble at the oil-water interface facing cationic head groups to the water phase, which can result in 1) lowering of surface tension of oil droplet and 2) preferential adsorption of anionic gelatin by Coulombic attraction between opposite charges. The entire experimental procedures for complex coacervation are schematically illustrated in Figure 3d-g, in which the consecutive drawings show the formation of d) oil droplets containing the charged SPIONs, e) gelatin layer deposition at pH ≈6, f) deposition of anionic gum Arabic on cationic gelatin at pH < 4, and g) crosslinking of hydroxyl and amine substituents in both layers by glutaraldehyde (GA). The enlarged drawings, respectively, illustrate d) a charged SPION with adsorbed Solsperse-17k and inverse micelles on the surface, molecular structures of e) gelatin at pH 6, and f) gum Arabic at pH 4, and g) crosslinked gelatin and gum Arabic by GA by reaction with hydroxyl and amine functional groups in both polymers. Upon completion of crosslinking reaction, stable μ-capsules were obtained.
Four μ-capsules were prepared at different preparation conditions, and the physical properties and preparation conditions are summarized in Table 3. As discussed in Figure 3a, the increased concentration of Solsperse-17k not only affects the charge stabilization of SPION μ-spheres, but also changes the dielectric constant of liquid medium, which is related to the stability of oil droplet in water. The effect of surfactant contents on complex coacervation was investigated for IPG/HC dispersions of SPION while fixing other experimental conditions (e.g., gelatin/gum Arabic concentrations, pH conditions, stirring speed, mixing volumes, and temperature variation). The SPION contents were kept at 20 wt%, while two different sized SPIONs (SP140 and SP170) were used. Within the range of Solsperse-17k content (3-10 wt%) investigated in this study, three different μ-capsules (MC-180, MC-270, and MC-650) were successfully prepared.
As shown in Figure 4a-c, the increased surfactant content resulted in a decrease in the average μ-capsule size. The average sizes were calculated from the distributions, as shown in Figure 4d. A decrease in μ-capsule size with increased surfactant content can be attributed to a lowered interfacial tension between the oil and water phases. Figure 4e shows that all three μ-capsules exhibited magnetochromism by applying H-field. MC-180 containing SP170 showed the structural colors changing from orange to green, while MC-270 and MC-650 changed from light brown to blue since they contained smaller-sized SP140. Therefore, it was confirmed that the initial stage of complex coacervation is schematically illustrated showing gelatin adsorption from aqueous phase onto the oil-water interface, c) during adsorption of gelatin, some of the Solsperse-17k molecules will assemble at the oil-water interface while the majority of them will form inverse micelles which are predominantly adsorbed on SPION surface. Schematic illustrations of entire complex coacervation procedure, in which d) SPIONcontaining IPG/HC is added to aqueous gelatin solution to form oil droplet, e) negatively charged gelatins are adsorbed onto the surface of oil droplet at pH 6, f) aqueous gum Arabic is added, and adsorbed on the gelatin-coated oil droplet upon lowering pH down to 3.5, g) glutaraldehyde promotes crosslinking of both gelatin and gum Arabic to complete complex coacervation. The enlarged illustrations, respectively, show d) a SPION with adsorbed Solsperse-17k, molecular structures of b) gelatin, c) gum Arabic above their pK a values, and d) crosslinked gelatin and gum Arabic by glutaraldehyde.
structural colors of μ-capsules are predominantly influenced by SPION size rather than by surfactant content. In Figure 4f, enlarged images of 180 μm capsules are shown with low (10 mT) and high (60 mT) H-field intensities. More photographs are shown in the Supporting Information ( Figure S7). Appearance of structural color from the μ-capsule implies that the colloidal stability is successfully maintained after the complex coacervation process.

Optical Characterizations of Multiemulsion SPION μ-Capsule
For more rigorous structural characterizations of μ-capsule, MC-180 was visualized by three different optical imaging techniques. Figure 5a shows a reflection OM image of MC-180 contained in water, where each capsule appears to possess multigranular textures. A transmission OM image showed dark regions of SPION  dispersion surrounded by a bright gelatin shell (Figure 5b). However, a magnified transmission image in Figure 5c revealed an interesting "multiemulsion" internal structure in a μ-capsule.
To characterize the internal structure of a μ-capsule more precisely, a laser scanning confocal microscopy (LSCM) analysis was performed in a reflection mode, from which a typical crosssectional image is shown in Figure 5d. It was confirmed that many tiny emulsions are formed inside a μ-capsule. Since a reflection mode LSCM does not require a fluorescent dye, a higher index region appeared darker in an LSCM image. When a magnet is placed near the μ-capsule, an LSCM image exhibits an interesting stripe patterns in the multiemulsions in a μ-capsule aligned parallel to the H-field direction, as shown in Figure 5e,f, which can be attributed to the formation of "particle chain" structures of SPIONs in the presence of H-field as schematically illustrated below each LSCM image. A high refractive index of SPION is attributed to the appearance of stripe patterns in a reflection LSCM. [27] Inset color images in Figure 5d,e are the stereoscopy images of a μ-capsule in the absence and the presence of an H-field, respectively, showing the development of structural color by particle chain formation. More LSCM images are shown in the Supporting Information ( Figure S8).

Long-Term Stability of Molded μ-Capsule Film
The most advantageous feature of μ-capsule is that the photonic film can be prepared simply by spreading them on a substrate. As a demonstrative example, replica molding of μ-capsule was carried out. An aqueous dispersion of MC-180 was spread on a chameleon-shape mold and flattened using a bar-coater as illus-trated in Figure 6a, and a molded film is shown in Figure 6b. After water evaporation, the development of magnetochromic structural color from the dried μ-capsules in a chameleon-shape mold was confirmed, which indicates that the SPION dispersion is well maintained within μ-capsules (Figure 6c). A flexible film of μcapsules was obtained by adding optically clear adhesive (OCA) on molded MC-180 and subsequent photocuring (Figure 6d). Flexibility and magnetochromism of the cured film are demonstrated in Figure 6e,f and also shown in movie clips in the Supporting Information. As shown in Figure 6f, the MC-180 exhibited gradual changes in structural color with the increased H-field strength, implying that the charged SPION dispersion in each capsule is well preserved without significant particle agglomeration or drying of the dispersant.
To examine the lifetime of the μ-capsules developed in the current investigation, the molded MC-180 μ-capsule as shown in Figure 6f was kept at an ambient condition for 1 year, and the resulting magnetochromism is shown in Figure 6g in which the enlarged images of the molded μ-capsules show the magnetochromic structural color changes without substantial deterioration. The long-term stability of the various μ-capsules kept in water, as shown in Figure 4e, was also confirmed to be as good as a molded film, and the results are summarized in the Supporting Information ( Figure S9).

Magnetochromism and Electrochromism of Single Emulsion μ-Capsule
When the gelatin concentration in the aqueous phase is decreased during the complex coacervation, a smaller-sized μ-capsule was produced, probably due to a reduced solution viscosity providing more vigorous agitation. Shown in Figure 7a is MC-70, 70 μm average-sized μ-capsule containing SP130 prepared with a half of gelatin concentration compared to other μ-capsules. At 60 mT H-field, a violet structural color appeared because the smallest-sized SPION (SP130) was dispersed in oil phase (Figure 7b). Shown in Figure 7c,d are the OM images of air-dried μ-capsules maintaining a spherical shape upon drying. The mechanical stability of μ-capsule was good enough to withstand vacuum condition as shown in Figure 7e of an SEM image in which a single μ-capsule stayed intact. More images are summarized in the Supporting Information ( Figure S10).
3D image analysis revealed that a single pool of oil phase was formed within a μ-capsule, as confirmed in a series of LSCM images without ( Figure 7f) and with (Figure 7g,h) H-field from different directions, respectively. Just as shown in the LSCM images of multiemulsion μ-capsules, stripe patterns appeared along the H-field due to the formation of SPION particle chain structure.
More LSCM images with different magnifications are shown in the Supporting Information ( Figure S11).
Electrochromism of the encapsulated SPION photonic ink was also investigated. The MC-70 μ-capsules were confined between two ITO electrodes of 10 × 10 mm 2 area at 100 μm thickness using polyurethane (PU) binder. As shown in Figure 7i,j, a thin layer of ivory-colored MC-70 changed to bluish violet structural color at the center region of ITO upon 30 V DC bias. Appearance of structural color originates from an electrophoretic movement of SPIONs in μ-capsule as illustrated in Figure 7k,l. Reversible color changes were confirmed under a repeated voltage bias between 0 and 30 V as shown in a movie clip in the Supporting Information. Electrochromism of μ-capsules at a relatively high voltage can be attributed to a relatively thick-film thickness (≈120 μm) and an existence of μ-capsule shell and binder which cause substantial voltage drop. Occurrence of electrochromic color only at the center area might be due to a thinner center thickness caused by an evaporation-induced outward migration of PU binder during the fabrication of the ITO cell. Without using the binder, we could not observe an electrochromic color change of the microcapsule since the air gap from the interstices obstructs color changes as well as it causes a large voltage drop.
A field-induced color tuning of the encapsulated photonic ink, with a long shelf-life, shows its strong potential to be utilized in various applications such as anticounterfeiting techniques, color changing wall-coating materials, and flexible EPD devices.

Conclusion
Fe 3 O 4 @SiO 2 core-shell microspheres of SPION in three different sizes (130, 140, and 170 nm) were synthesized using two-step procedures. Upon addition of Solsperse-17k, a polymeric charge control agent, a SPION dispersion in a nonpolar mixture of IPG/HC exhibited negative surface charges as large as −40 mV without further surface modification. This is due to the preferential adsorption of the cationic Solsperse-17k molecules on the SiO 2 surface and subsequent adsorption of its inverse micelles. Owing to an appropriate size and surface charges, a photonic ink of SPION dispersion in IPG/HC/Solsperse-17k exhibited tunable structural colors, respectively, by magnetochromism and electrochromism. Complex coacervation process was carried out in which addition of photonic ink to an aqueous gelatin solution was followed by addition of gum Arabic at a lowered pH and subsequent crosslinking reaction to produce spherical μ-capsules. Successful microencapsulation of photonic ink was confirmed by magnetochromic color tuning of μ-capsules, in which 130 nm sized SPION showed violet structural color, and those with 140 nm SPION and 170 nm SPION, respectively, showed blue and green colors in the presence of H-field. Increase in Solsperse-17k contents in photonic ink resulted in a reduction of μ-capsule size, and 3D imaging by LSCM revealed that μ-capsules possess multiemulsion structure to assure an excellent mechanical strength, good enough to apply replica molding and photocuring, and also to show long-term stability as long as 9 months. Decreased gelatin content in aqueous phase during complex coacervation resulted in the formation of a single emulsion microcapsule with a smaller capsule size. The single emulsion μ-capsule showed an electrochromism under 30 V DC voltage as well as magnetochromism, which shows a promising applicability to color-tunable E-skin devices. Since the μ-capsules were prepared with lower gelatin concentration in the aqueous phase during the coacervation process, a single oil emulsion capsule was obtained as shown by LSCM images f) without H-field and g,h) with H-fields having different field directions. Schematic illustrations of single emulsion capsules with aligned SPIONs inside are shown below each LSCM image. Photographs of SP130 μ-capsules with PU binder confined between two ITO glasses at the voltage bias of i) 0 and j) 30 V, showing a E-driven structural color. Schematic drawings of the μ-capsules k) without E field and l) with electrophoretic alignment by voltage bias.

Experimental Section
Synthesis of Fe 3 O 4 @SiO 2 Core-Shell Microsphere: 3.28 g Ferric chloride hexahydrate (FeCl 3 ⋅6H 2 O, JUNSEI) was dissolved in 40 mL ethylene glycol (EG, 99.5%, DAEJUNG)/diethylene glycol (DEG, 99%, DUKSAN) in 250 mL three-neck round bottom flask (RBF) as the main reactor. Using a syringe pump (KD Scientific), 1.27 g sodium citrate (99.5% DAEJUNG) and 6.0 g sodium acetate (99.0% Sigma-Aldrich) were dissolved in 80 mL EG containing 4 mL deionized (DI) water and slowly added to the reactor for 20 min with vigorous stirring in another vial. The light brown dispersion was transferred to a 200 mL Teflon (Dupont) liner and placed in a stainless steel reactor. The reactor temperature was elevated to 200°C in an oven (LI-VDO01, LK lab), where the solvothermal reaction proceeded for 7 h. The reaction was quenched by cooling, brownish SPION dispersion was washed with ethanol and dried in a vacuum. The EG/DEG mixture ratio controlled the size of SPION. In a bath sonicator (Powersonic 410, Hwashin), 0.26 g of SPION was dispersed in 160 mL ethanol for less than 30 min and transferred to 500 mL three-neck RBF with mechanical stirring at 400 rpm. Using a syringe pump for silica shell formation, 16 mL aqueous ammonium hydroxide (NH 4 OH, 25.0-30.0%, SAMCHUN) and 24 mL DI water were added, and 0.4 mL tetraethylorthosilicate (TEOS, Sigma-Aldrich) was slowly added for 30 min. Upon completion of the solgel reaction, the slurry was washed with ethanol and DI water and dried in a vacuum oven. The dried SPION was characterized by a field-emission SEM (FE-SEM; SU-8010, Hitachi), a TEM (JEM-1010, JEOL), and a size analyzer (ZEN-3690, Malvern Panalytical) for shape and size analysis.
As a color-tunable photonic ink, the dried SPION powder was redispersed in either ethanol or IPG (ExxonMobil)/HC (Sigma-Aldrich) mixture (1:1 by volume) containing Solsperse-17k (Lubrizol). Solution properties of a photonic ink were measured using a viscometer (SV-10 AND) and a dielectric constant measurement system (Model 871, Sunray tech) at room temperature, respectively, and the zeta potential of SPION dispersion was analyzed by a zeta potential analyzer (ZEN-3690, Malvern Panalytical).
Encapsulation of SPION Dispersion by Complex Coacervation: As an oil phase, 20 wt% SPION dispersion in IPG/HC/Solsperse-17k was prepared. As an aqueous phase, 3 g of bovine-gelatin (225 bloom, Type B, Sigma-Aldrich) was dissolved in 50 mL DI water in a jacketed flask at 55°C, kept around pH 6. 2 g gum Arabic was dissolved in 50 mL water at the same temperature as the gelatin solution. 1 mL photonic ink containing 2 g SPION in IPG/HC/Solsperse-17k was prepared and slowly added to gelatin solution over 20 min using a syringe pump under mechanical stirring, typically at 200 rpm. The gum Arabic solution was added for 20 min, and 100 mL of warm water was added for 5 min for dilution. pH was decreased to 4 by adding 14% acetic acid. The temperature of the reaction vessel was dropped to 5°C and maintained for 30 min. Solution pH was readjusted to 10 by adding aqueous sodium hydroxide solution, and 3 mL aqueous glutaraldehyde (25%, Sigma-Aldrich) was added to crosslink gelatin and gum Arabic overnight. μ-capsules were sedimented by a magnet and washed several times with DI water. μ-capsules were preserved in water after washing. For a molded film fabrication, aqueous dispersion of μ-capsules with SP170 was spread on a 100 μm thick chameleon-shaped mold (Surlyn, Dupont) on a glass slide, and flattened using a film applicator. Upon water evaporation, optically clear adhesive (NOA-68, Norland) was applied, and subsequently cured by exposure to UV light (200 W, Spectroline) for 10 min.
Optical Characterization of μ-Capsules: The observation of μ-capsules was conducted using a reflection microscope (BA310, Motic) or a transmission microscope (Bimence). Photographs were taken using a digital camera equipped in iphone-11 (Apple). For visualization of the inner structure of μ-capsules, LSCM (TCS SP5, Leica Microsystems) was used under reflection mode using a 488 nm Ar-laser as a probe beam through an objective lens (40x, Leica Microsystems).
Electrochromic Experiment of μ-Capsules: The wet single emulsion μ-capsules containing 40 wt% SP130 dispersed in IPG/HC/10 wt% Solsperse-17k was mixed with PU binder, and were spread in the middle of 120 μm thick O-shaped spacer (Surlyn, Dupont) on a bottom ITO, and flattened using a film applicator. As most of water dried out, a top ITO electrode was covered and pressed gently. After overnight drying in 50°C oven, the assembly was cooled down to ambient temperature, and 0-30 V DC voltage was repeatedly applied using a power supply (DP30, Toyotech) every 5 s between two ITO's to induce electrophoretic migration of SPION in the capsule.

Supporting Information
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