Multicompartment Photonic Microcylinders toward Structural Color

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Multicompartment Photonic Microcylinders toward Structural Color Inks Gun Ho Lee, Tae Yoon Jeon, Jong Bin Kim, Byungjin Lee, Chang-Soo Lee, Su Yeon Lee, and Shin-Hyun Kim Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00873 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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Chemistry of Materials

Multicompartment Photonic Microcylinders toward Structural Color Inks Gun Ho Lee†, Tae Yoon Jeon†, Jong Bin Kim†, Byungjin Lee‡, Chang-Soo Lee‡, Su Yeon Lee*,§ and Shin-Hyun Kim*,† †

Department of Chemical and Biomolecular Engineering (BK21+ Program), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea

‡

Department of Chemical Engineering and Applied Chemistry, Chungnam National University, Daejeon 34134, Republic of Korea §

Division of Advanced Materials, Korea Research Institute of Chemical Technology (KRICT), Daejeon 34114, Republic of Korea ABSTRACT: Structural coloration is promising as an alternative to chemical coloration because it has characteristics of their high color brightness, no fading, and low toxicity. Here, we report a pragmatic micromolding technique to create functional photonic microcylinders which are useful as structural color pigments. Photocurable dispersions of silica particles with interparticle repulsion are molded to spontaneously form regular arrays in confined volumes, which are instantly stabilized by photopolymerization. The resulting photonic microcylinders, released from the mold, exhibit pronounced structural colors from the entire visible range. In addition, multiple compartments can be integrated into single microcylinders through volatile-solvent-mediated sequential molding. As each compartment can be independently rendered to be structurally-colored, transparent, or magneto-responsive, the multicompartment microcylinders show advanced functionalities, such as color-brightness tunability and switchable color properties. These photonic microcylinders will serve as structural color pigments in a wide range of aesthetic coatings and authentication tags.

INTRODUCTION Periodic nanostructures develop structural colors through the reflection of selected wavelengths of visible light.1,2 The structural colors are iridescent and do not fade as long as the periodic structures persist. Moreover, as the colors are developed purely by the structures in the absence of absorptive chemicals, a relatively low toxicity is expected. These features are appealing for use in structural colors in decorative paints, anticounterfeiting patches, colorimetric sensors on skins and eyes, and color-encoded microcarriers for bioassays.3-8 Colloidal crystals have been used as the periodic nanostructures to develop structural colors.9,10 To use the colloidal crystals as an alternative to conventional chemical pigments, it is prerequisite to tailor the crystals to have a granular format that can be suspended in a liquid medium; granules smaller than tens of micrometers are preferred to make coatings that seem uniform to the naked eye. Photonic granules of colloidal crystals have been prepared by droplet templating.11, 12 For example, emulsion drops of colloidal dispersions are microfluidically prepared, and are then shrunken through evaporation of osmotic-pressureinduced pumping to concentrate colloids in the drops.13-18 Alternatively, a colloidal dispersion with repulsive interparticle potential, which spontaneously form crystalline structures without concentration, is emulsified and stabilized by either polymerization of the dispersion medium or formation of protection shell.19, 20 By employing distinct parallel streams of colloidal dispersion, compartmentalized granules have also

been prepared from emulsion templates.21, 22 However, the resulting granules are restricted to isotropic spheres. More importantly, emulsion drops are intrinsically unstable, and require delicate control throughout the whole processes of drop generation and post-treatment, limiting the production throughput. To partially overcome these limitations, photolithography and flow-lithography have been employed.23-26 When the photonic granules are featured by local exposure of collimated ultraviolet (UV) light, various disk-shaped granules can be produced; cylindrical granules can also be prepared by employing cylindrical microfluidic channels.26 However, production throughput still remains low because conventional photolithography is a discontinuous multi-step process and flow-lithography requires a low Reynold number despite being a continuous process. In addition, complex setup and delicate control are required to incorporate multiple compartments in single granules. Furthermore, photonic granules with high aspect-ratio are difficult to produce. Therefore, a robust and scalable method for producing and functionalizing photonic microgranules remains an important challenge for pragmatic application of structural colors. Micromolding has been used to make various micropatterns. 27, 28 In particular, micromolding has enabled the production of colloidal assemblies in confined volumes. For example, colloidal particles are assembled by directional evaporation of dispersion medium to form colloidal crystals in capillary channels.29, 30 With carefully-designed arrays of small holes, controlled numbers of colloidal particles can be trapped in each hole by capillary action, allowing the formation of col-

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loidal clusters or particle arrays.31-33 In addition, sequential assembly of colloids in the micromolds enables the production of colloidal clusters composed of two or more distinct colloids.34 Recently, unique configurations of colloidal assembly have been obtained by confining colloids with repulsive potential in cylindrical holes.35 However, no previous works are relevant to producing photonic microgranules; colloidal crystals prepared in capillary channels form a continuous network rather than well-defined granules and colloidal clusters show no photonic effect because they lack of structural periodicity. In this work, we use a simple micromolding technique to produce photonic microcylinders that show pronounced structural colors. Silica particles dispersed in a carefully-selected photocurable resin possess a repulsive interparticle potential due to the formation of a solvation layer on the surface. The particles spontaneously form colloidal crystals and minimize the repulsive energy.36, 37 With the photocurable dispersion, an array of cylindrical holes in an elastomeric mold is filled; these holes are then photopolymerized to produce micropillars containing a regular array of silica particles. The micropillars are released from the substrate, from which silica particles are selectively etched out to make a regular array of cavities in the resulting microcylinders. The porous microcylinders display pronounced structural colors that are tunable in the entire visible range by adjusting the diameter of the silica particles. By employing dispersions containing volatile solvent, microcylinders can be designed to have multiple compartments. After filling the holes in the template, the volatile solvent is selectively evaporated, resulting in the partial occupancy of the holes, where the height is determined by the volume fraction of the nonvolatile dispersion. Therefore, stepwise application of molding and evaporation enables the creation of multiple compartments with well-defined dimensions. Each compartment can be rendered to be structurally-colored with a regular array of cavities, transparent without any structures, or magneto-responsive with aligned magnetic nanoparticles. The micromolding technique is compatible with the roll-to-roll process, potentially serving as a means for high-throughput production of photonic microgranules.38 Moreover, the functionalization of the photonic microgranules with multiple compartments further expands application beyond structural coloration.

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ble dispersions are sandwiched by the mold and oxygenplasma-treated slide glass. The cylindrical holes in the mold are fully infiltrated with the photocurable dispersions by capillary force; dispersions are then irradiated with ultraviolet (UV) light to polymerize the dispersion. After peeling off the PDMS mold, a micropillar array of silica-ETPTA composites whose dimensions are the same as those of the mold is formed on the slide glass (Figure 1b). The micropillars are released from the substrate by blading and yield cylindrical microparticles (Figure 1c). The microcylinders, made from the dispersion of silica particles with diameter of D = 167 nm and volume fraction of ϕ = 0.33, show a faint sky-blue color. When silica particles are selectively etched out by treating the microcylinders with a dilute hydrofluoric acid (HF) solution, the microcylinders show a pronounced blue color (Figure 1d). The difference in color and brightness between the composite and the porous microcylinders is quantitatively characterized with reflectance spectra (Figure 1e). The composite microcylinders have a small peak at 522 nm, whereas the porous microcylinders have a large peak at 476 nm. The small peak in the composite originates from the regular array of silica particles embedded in polymerized ETPTA (pETPTA); the cross-section of the composite microcylinder is shown in Figure 1f. As the refractive index of silica (nsilica = 1.45) is comparable to that of pETPTA (npETPTA = 1.471), the penetration depth of light, or the Bragg length, is much larger than the dimensions of the microcylinders, which results in weak reflection at the stop band. In porous microcylinders, the silica particles are all replaced with air cavities (Figure 1g). The large contrast of refractive index between air (nair = 1) and pETPTA (npETPTA = 1.471) leads to a relatively short Bragg length, which is comparable to the dimension of microcylinders, thereby providing high reflectivity at the stop band. The silica particles, or the air cavities, are aligned to have their hexagonal arrays along the top, bottom, and side surfaces of the cylinder (Figure 1f, 1g, and S2). This arrangement causes the microcylinders to have rotation-independent colors along the cylindrical axis; these colors are the same as those on the top and bottom surfaces. The position of stop band can be estimated using Bragg’s law by considering that the (111) planes of non-close-packed fcc are aligned with the surfaces: λ = 2 =

√

+ (1 − )!

RESULTS AND DISCUSSION

(1)

Micromolding for Production of Single-Colored Photonic Microcylinders. Colloidal particles with repulsive interparticle potential spontaneously form crystals and minimize total repulsive energy.39 Silica particles dispersed in a photocurable resin of exthoxylated trimethylolpropane triacylate (ETPTA) possess a solvation layer on their surface due to the formation of hydrogen bonds between the silanol group on the surface and acrylate group of the resin.40 The solvation layers render the silica particles repulsive above a critical volume fraction at which they overlap. Therefore, silica particles spontaneously form a crystalline structure of face-centered cubic (fcc) lattice.41 The photocurable dispersion of silica particles in ETPTA is molded to produce photonic microgranules (Figure 1a). A polydimethylsiloxane (PDMS) mold containing an array of cylindrical holes is replicated from a lithographicallyfeatured array of cylindrical pillars; the holes have a diameter of 10 µm and a height of 23 µm (Figure S1).42 The photocura-

where the interplane spacing for stacked (111) planes, d111, is a function of D and ϕ, and the effective refractive index, neff, is estimated using the Maxwell-Garnet average of particles of either silica or air and the matrix of pETPTA. For D = 167 nm and ϕ = 0.33, values of λ are 523 nm and 476 nm for np = 1.45 (silica) and np = 1 (air) respectively, which are in good agreement with the reflectance peak positions (Figure 1e). With five distinct dispersions of silica particles with D = 164 nm, 181 nm, 201 nm, 220 nm, and 242 nm at ϕ = 0.33, porous microcylinders showing blue, sky blue, green, Indian red, and maroon colors are respectively prepared (Figure S3a). This redshift of the structural color along with the particle size is attributed to the increase of lattice periodicity as described in the eq 1. In fact, the reflectance spectra have a peak at the stop band whose wavelength follows the eq 1 (Figure S3b and S3c).


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Figure 1. (a) Schematic diagram illustrating the fabrication procedure of single-compartment photonic microcylinders. SilicaETPTA dispersion is molded and exposed to UV light. The polymerized micropillars are released from a substrate to yield composite microcylinders, which are then treated with hydrofluoric acid to yield porous microcylinders. (b) Scanning electron microscopy (SEM) image of silica-ETPTA composite micropillar arrays. The inset shows a SEM image of a single micropillar. (c, d) Optical microscope (OM) images of composite microcylinders before silica etching (c), and porous microcylinders after the etching (d). (e) Reflectance spectra of composite and porous microcylinders. (f, g) Cross-sectional SEM images of the composite (f) and porous microcylinders (g).



Formulation of Structural-Color Inks. To compose structural color inks for coatings, the photonic microcylinders should be suspended in either binder solution or curable resin. As the suspension medium can alter the optical property of the photonic microcylinders by filling out cavities and swelling the matrix, the medium should be carefully selected. When a 15 w/w% aqueous solution of polyvinyl alcohol (PVA) with molecular weight of 13,000-23,000 g/mol is used as the binder solution, the photonic microcylinders maintain their pronounced structural colors after coating (Figure 2a); microcylinders with diameter of 20 µm and length of 20 µm are used. The large PVA molecules do not penetrate through the pETPTA matrix, leaving the cavities of the microcylinders unoccupied after removal of water, as illustrated in Figure 2b. Therefore, microcylinders embedded in a solid film of PVA retain the original color and brightness of the dried powders. By contrast, when a photocurable resin of poly(ethylene)diacrylate (PEGDA) with a molecular weight of 700 g/mol is used as the suspension medium, the cavities are filled with the resin and the pETPTA matrix is slightly swollen, resulting in a red-shift of the color and significant reduction of the reflectivity (Figure 2a); the swelling ratio is estimated at 1.03 according to the reflectance peak position (see Figure S5 and related paragraph in the Supporting Information for detailed calculation). We prepare a dual-color pattern of yin-yang symbol using two different photonic microcylinders suspended in PVA solutions (Figure 2b). To suppress white scattering and enhance color contrast, carbon black nanoparticles are additionally dispersed in the suspensions. The pattern shows bright colors, which remain unchanged even if the observation angle is varied as long as the light impinges parallel to the observation direction (Figure 2c). This angle independence results from the geometry of the photonic microcylinders.

Figure 2. (a) OM images and reflectance spectra of photonic microcylinders in air, PVA film, and polymerized PEGDA film. Color and spectrum of PVA are comparable to those of air. (b) Schematic illustration of the composition of porous microcylinder-embedded PVA coating. PVA molecules form a matrix for the porous microcylinders and pores in the microcylinders remain occupied with air after drying of water. Carbon black nanoparticles are additionally embedded in the matrix to reduce white scattering. Photograph in the inset shows a yin-yang pattern prepared using two different photonic microcylinders. (c) Series of photo-

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graphs showing no color change with variation of observation angle, where the white light impinges on the pattern parallel to the observation direction, as illustrated in the cartoon. (d) Series of photographs showing a blue-shift of colors along with the angle between the incident light and observation direction, where the light vertically impinges on the pattern.

The randomly-oriented cylinders can reflect the incident light to the surface-normal direction on the side surface, almost regardless of the direction of the incident light, in a manner similar to that of spheres.15 On the other hand, the pattern shows a blue-shift of the color when the angle between the incident light and the observation direction is increased (Figure 2d); the upper part of the pattern (yin) turns from red to green and the bottom part (yang) turns from green to blue. This angle dependency is caused by angle-dependent path difference among the scattered lights in Bragg diffraction. As reflection angle is the same as incidence angle in the reflection condition, half the angle between the incident light and the observation direction, θ/2, is the incidence angle on the curved (111) plane of fcc (Figure 2d). Therefore, the angle dependency of the specular reflection is inevitable in any geometry including cylinders and spheres. This angle-dependent blue-shift is also observed in the liquid suspension of photonic microcylinders (Figure S6 and Movie S1 of the Supporting Information). Solvent-Mediated Sequential Molding for Production of Multicompartment Photonic Microcylinders. The photonic microcylinders can be designed to have multiple compartments. To create the multiple compartments in a controlled manner, we use the silica-ETPTA dispersion diluted by a volatile solvent of ethanol. By applying a negative pressure and subsequently blading out the residual dispersion, the holes in the PDMS mold are fully filled with the dispersion. The ethanol is selectively evaporated from the dispersion at 70°C for 15 min, which leaves behind an ethanol-free silica-ETPTA dispersion that partially occupies the holes (Figure 3a). The dispersion that remains in the holes is solidified by UV irradiation and the voids on the top of the solidified resin are further subjected to a molding process to create secondary compartments. To form microcylinders composed of structurallycolored and transparent compartments, we follow the molding procedure with silica-free ETPTA as a second solution. In the first dispersion, to control the axial dimension of the photonic compartment, three different compositions of ethanol are used; 0, 30, and 70% in volume fraction. The resulting microcylinders made from the three dispersions have different axial lengths of the photonic and transparent compartments (Figure 3b-d); silica particles are removed by HF treatment after the molding. Not only the length but also the color of the photonic compartments is varied unexpectedly, although the same silicaETPTA dispersion with D = 198 nm and ϕ = 0.33 in the ethanol-free base is used for all three dispersions. It can be clearly seen in the images that the color blue-shifts along with the fraction of ethanol. This can be further confirmed from the reflectance spectra (Figure 3e). The blue-shift of the stop band can occur only when the interparticle distance is reduced so that the volume fraction of silica particles increase; the size of the silica particles is invariable. From the stop band positions of 509 nm and 452 nm for the microcylinders made from dispersions with 30% and 70% ethanol, the values of ϕ are estimated at 0.4 and 0.5 using eq 1. Because ETPTA is not


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volatile, the increase of ϕ is attributed to diffusion of ETPTA into the PDMS matrix. Although ETPTA itself does not swell the PDMS, it can diffuse into a nanoporous matrix of PDMS in the presence of ethanol because ethanol has a volume swelling ratio of 1.125 for PDMS.43 The axial lengths of the photonic compartments are 13 µm and 5 µm for the microcylinders made from dispersions with 30% and 70% ethanol, respectively; these values are 57% and 22% of total length of 23 µm (Figure 3f); that is, the lengths are slightly smaller than expected from the volume fraction of silica-ETPTA. This discrepancy is also attributed to the diffusion of ETPTA into the PDMS matrix. The increase of ϕ from 0.33 to 0.4 and 0.5 causes reductions in the dispersion volume of as much as 0.17 and 0.33. These reductions yield lengths of 58% and 20%, which roughly agree with the measured lengths. To further confirm the reduction of interparticle distance in the presence of ethanol in the inks, cross-sections of two distinct photonic microcylinders—one prepared without ethanol and the other prepared with 70% ethanol—are observed, as shown in Figure S7. It can be clearly seen that the microcylinders formed from 70% ethanol is more porous and has a shorter inter-pore distance than that without ethanol. The top surface of the photonic compartments is concave as shown in Figure S8. The silicaETPTA dispersion on the surface of PDMS in the air surrounding has a contact angle of 72⁰, which forms the concave interface in the holes during the first step of the molding.

microcylinders in (b-d). (f) Height (left y-axis) and reflectance peak position (right y-axis) of photonic compartments as a function of the volume fraction of ethanol in the dispersion.

Magnetoresponsive Photonic Microcylinders with ColorBrightness Tunability. The anisotropic shape of the photonic microcylinders does not provide any special photonic effect when they are randomly suspended in a liquid medium; that is, no difference is expected from isotropic microspheres. However, the shape matters if the orientation of the microcylinders can be collectively controlled. To render the photonic microcylinders magneto-responsive and manipulate their orientation, we first produce a thin compartment containing barium ferrite (BaFe12O19) nanoparticles and then create a thick photonic compartment on the top of the magnetic layer (Figure 4a). The dispersion used for the magnetic compartment is prepared by diluting the ETPTA dispersion containing 6 w/w% barium ferrite (BaFe12O19) nanoparticles with D = 80 nm and silica particles with D = 310 nm at ϕ = 0.1 with 70% ethanol. The barium ferrite nanoparticles are ferromagnetic and have high stability against corrosion; they maintain their magnetic property even after HF treatment.44 The silica particles are dispersed to prevent severe agglomeration of the barium ferrite nanoparticles by increasing the viscosity and filling the interstices among the barium ferrite nanoparticles. After the holes in the mold are filled with the dispersion, ethanol is evaporated. The mold is sandwiched between a pair of disk magnets to align the ferromagnetic barium ferrite nanoparticles along the external magnetic field. Subsequent UV irradiation fixes the aligned nanoparticles in the polymerized matrix; this causes which renders the compartment to have a net magnetic moment.26 The photonic compartment is produced on top of the magnetic layer with a silica-ETPTA dispersion of D = 198 nm at ϕ = 0.33, and the microcylinders are finally treated with HF to remove silica particles. The resulting microcylinders have a thin magnetic compartment and a thick photonic compartment, as shown in the optical microscope image taken at a transmission mode in which the magnetic compartments are opaque and the photonic compartments are transparent (Figure 4b); the microcylinders are suspended in ethanol. The microcylinders show a pronounced red color in a reflection mode (Figure 4c). The ethanol fills out the cavities and swells the microcylinders, resulting in a reflection peak at 621 nm, which is in good agreement with the value of λ = 617 nm estimated using eq 1 with the swelling ratio of 1.08. The orientation of the microcylinders can be controlled using the external magnetic field. Because the net magnetic moment is set to the direction of the cylindrical axis, the microcylinders are aligned parallel to the direction of the external field.

Figure 3. (a) Schematic diagram illustrating the preparation procedure for photonic compartments that partially occupy cylindrical holes in a mold. A photocurable dispersion diluted with ethanol is infiltrated into the holes and residual dispersion is removed by blading. After complete evaporation of ethanol, the dispersion remaining in the holes is irradiated with UV. (b-d) OM images of microcylinders composed of photonic and transparent compartments, in which three different fractions of ethanol are used for the production of photonic compartments: (b) 0%, (c) 30%, and (d) 70%. Insets are magnified views. (e) Reflectance spectra of



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is sandwiched by two photonic compartments with different colors (Figure 5a). The bottom photonic compartment is first prepared with silica-ETPTA dispersion with D = 198 nm at ϕ = 0.33 diluted with 70% ethanol. The magnetic compartment is then formed with the magnetic dispersion diluted with 70% ethanol; after evaporation of ethanol, the external magnetic field is applied before UV irradiation. The top photonic compartment is finally formed with the ethanol-free silica-ETPTA dispersion with D = 198 nm at ϕ = 0.33. Removal of silica particles forms two photonic compartments colored green and red in ethanol (Figure 5b and 5c).

Figure 4. (a) Schematic illustration of microcylinders composed of thick photonic and thin magneto-responsive compartments. Barium ferrite nanoparticles whose magnetic dipoles are aligned along the cylindrical axis are embedded in the magnetoresponsive compartments. (b,c) OM images of the magnetoresponsive photonic microcylinders taken in transmittance mode (b) and reflectance mode (c). (d-g) Sets of illustrations and OM images showing the microcylinders aligned under an external magnetic field: four different directions of the external field are employed as denoted in each panel. (h) Reflectance spectra of microcylinders at two different states: horizontal alignment, as shown in (d-f), and vertical alignment, as shown in (g). (i) Photographs of microcylinder suspension in ethanol under external magnetic field: the field direction is denoted in the panels.

Therefore, when the external magnet is rotated clockwise while maintaining the field direction parallel to the substrate, the microcylinders collectively rotate counterclockwise (see Figure 4d-f and Movie S2). The external field whose direction is perpendicular to the substrate, erects all the microcylinders (see Figure 4g and Movie S2). It can be clearly seen that the color brightness increases when the orientation is changed from parallel to perpendicular. The higher reflectivity on the top surfaces than on the side surface is the result of the longer optical path along the axis direction than the radial direction; the Bragg length at the stop band is larger than the dimensions of the microcylinders in the ethanol suspension. Although top surface has a smaller area than the side surface, they both can fully contribute to the reflection, whereas only the narrow central region on the side surface is eligible for reflection. There is no orientation-dependent color change because hexagonal arrays of cavities are aligned parallel to the top, bottom, and side surfaces. The orientation-dependent reflectivity change is also confirmed by reflectance spectra and photographs for the bulk suspension (Figure 4h, 4i, and Movie S3). Color-Switchable Photonic Microcylinders. Through three step of molding, the microcylinders can be further designed to have three compartments. To make magneto-responsive microcylinders with two distinct colors, a magnetic compartment

Figure 5. (a) Schematic illustration of microcylinders composed of magneto-responsive compartments sandwiched by two photonic compartments with different structural colors. (b, c) OM images of the microcylinders taken in transmittance mode (b) and reflection mode (c). (d-g) Sets of illustrations and OM images showing the microcylinders aligned under an external magnetic field, where four different directions of the external field are employed as denoted: side-surfaces up (d, e), green-coloredcompartments up (f), and red-colored-compartments up (g).

Although the bottom and top compartments are made of the same silica dispersions as those in the ethanol-free base, the diffusion of ETPTA into the PDMS mold during the formation of the bottom compartments with the ethanol-diluted dispersion leads to a blue-shift of the reflection color, resulting in two distinct colors of green and red in the bottom and top compartments, respectively. The orientation of the dualcolored photonic microcylinders can be controlled by external magnetic field (Figure 5d-g). When either the top or bottom surface is aligned to face the direction of view, one color is selected from the two, enabling color switching.

CONCLUSION In summary, we report a pragmatic micromolding technique to produce photonic microcylinders for structural coloration. The silica particles dispersed in a photocurable resin spontaneously form a regular array in the cylindrical holes of the mold; this array is instantly captured by photopolymerization, yielding photonic microcylinders. The microcylinders exhibit pronounced structural colors in the absence of any chemical pigments; colors can be selected from the entire visible range by adjusting the size of the silica particles. The photonic microcylinders can be further formulated to compose structural color inks for decorative paintings, color cosmetics, and anticounterfeiting patches. Volatile-solvent-mediated sequential


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molding enables the production of photonic microcylinders with two or more compartments. Each compartment can be functionalized to be structurally-colored with a regular array of cavities, transparent without any nanostructures, and magneto-responsive with aligned magnetic nanoparticles. Therefore, advanced functionality in structural coloration can be achieved through combinations of distinct compartments. For example, photonic microcylinders with tunable color brightness can be prepared by integrating photonic and magnetic compartments. In addition, color-switchable microcylinders can be created by sandwiching a magnetic compartment between two photonic compartments with different colors. These photonic granules can potentially serve as active color pigments for user-interactive anti-counterfeiting materials and reflection-mode displays.45, 46 Moreover, two or three photonic compartments can be integrated into single microcylinders, which are potentially useful for structural color mixing and encoding of microcarriers for bio-analysis (Figure S9).3, 47

EXPERIMENTAL SECTION Preparation of photocurable dispersions. Monodisperse silica particles are synthesized by two-step sol-gel reactions: 1) seed formation by two-phase method and 2) seed growth by Stöber method.48, 49 After washing the silica particles with ethanol several times, silica particles are completely dried at 70°C in convection oven and weight of the dried silica powder is measured. The silica particles are dispersed in ethanol, and ETPTA (Mw 428, Sigma-Aldrich) containing 1 w/w% photoinitiator of 2-hydroxyl-2-methyl-l-phenyl-1-propanone (Darocur 1173, Ciba Chemicals) is added. The weight of the added ETPTA is determined from the weight of the dried silica powder to set a target volume fraction of silica particles in the final ethanol-free dispersion; densities of ETPTA and silica particles are 1.11 g/mL and 2 g/mL, respectively. After complete mixing of the dispersion, ethanol is evaporated in a 70°C convection oven for 1 day. Preparation of photonic microcylinders with single compartment. The silica-ETPTA dispersion is dropped onto a glass slide treated with oxygen plasma (PDC-001, Harrick Plasma). The dispersion is covered with a PDMS mold with a square array of cylindrical holes with diameter of 10 µm and depth of 23 µm or diameter of 20 µm and depth of 20 µm. After the holes are completely filled with the dispersion, the whole mold is irradiated with UV (CoolWave UV Curing System, Nordson) for 1 min to polymerize the ETPTA. The mold is peeled off and the micropillars are released from the glass substrate by blading. The microcylinders are treated with 2 w/w% hydrofluoric acid (50%, Sigma-Aldrich) for 12 h to etch out the silica particles. To formulate photonic inks, the microcylinders are suspended in either 15 w/w% aqueous solution of PVA (Mw 13,000 - 23,000 g/mol, Sigma-Aldrich) containing 0.05 w/w% carbon black nanoparticles (24 nm, KCB) or PEGDA (Mw 700 g/mol, Sigma-Aldrich) containing 1 w/w% Darocur 1173. Preparation of photonic microcylinders with multiple compartments. The silica-ETPTA dispersion diluted with ethanol is dropped onto the PDMS mold. The mixture infiltrates the holes of the PDMS mold under vacuum and residual dispersion is removed by blading. The mold is incubated at 70°C in a convection oven for 15 min to selectively evaporate

ethanol; mold is then irradiated with UV. To make a transparent compartment in the unfilled parts of the holes, silica particle-free ETPTA containing a photoinitiator is molded following the same protocol as that used for single-compartment microcylinders. To make structurally-colored compartment, silica-ETPTA dispersions are used. To produce three distinct compartments in each microcylinder, two silica-ETPTA dispersions containing ethanol are used, stepwise; finally, ethanol-free silica-ETPTA dispersion is used. To make magnetoresponsive compartments, 6 w/w% barium ferrite nanoparticles (Mw 1111.46 g/mol, Sigma-Aldrich) with average diameter of 80 nm are dispersed in ETPTA. Silica particles with diameter of 310 nm are additionally dispersed in ETPTA at a volume fraction of 0.1; dispersion is then mixed with ethanol. After filling the holes in the mold with the dispersion and evaporating ethanol, the mold is placed between two rectangular disk-shaped neodymium magnets for 1 h to align the barium ferrite nanoparticles; mold is then irradiated with UV for 1 min. Characterization. Photonic microcylinders are observed with an optical microscope in reflection (Eclipse L150, Nikon) and transmission modes (Eclipse Ti, Nikon). Reflectance spectra are measured using a fiber-coupled spectrometer (USB 4000, Ocean Optics Inc.) installed in the optical microscope in reflection mode. The cross-sections of photonic microcylinders are observed using scanning electron microscopy (SEM, S-4800, Hitachi) after coating with osmium tetraoxide.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. A PDMS mold; internal structure of photonic microcylinders; particle diameter- and solvent-dependent structural colors; a lattice model for swelling; iridescence of microcylinder suspension; increased void fraction of porous microcylinders made from ethanol-diluted dispersion; contact angle of ETPTA drop on PDMS surface and OM images showing the boundary between two compartments; photonic microcylinders with multiple compartments (PDF) Angle-dependent structural color of microcylinder suspension (AVI) Collective motion of microcylinders under external magnetic field (AVI) Real-time tuning of color brightness (AVI)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S.-H.K.) and [email protected] (S.Y.L.)

Author Contributions G.H.L. carried out all experiment; T.Y.J., J.B.K., B.L. and C.-S.L. helped the characterization; S.Y.L. and S.-H.K. designed and supervised the research.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Global Research Laboratory (NRF-2015K1A1A2033054) through the National Research



Foundation (NRF), funded by the Ministry of Science, ICT and Future Planning (MSIP).

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Multicompartment Photonic Microcylinders toward Structural Color

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