Optical Properties and Applications of Photonic Shells - ACS Applied

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Optical Properties and Applications of Photonic Shells Dan-Bi Myung, and Soo-Young Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04105 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 13, 2019

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ACS Applied Materials & Interfaces

Optical Properties and Applications of Photonic Shells Dan-Bi Myung and Soo-Young Park* Department of Polymer Science & Engineering, Polymeric Nanomaterials Laboratory, School of Applied Chemical Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, 41566 Daegu, Korea KEYWORDS cholesteric liquid crystals, photonic structures, reactive mesogens, chiral dopants, solvent sensors, anti-counterfeiting patches, labeled templates

ABSTRACT

Uniformly sized cholesteric liquid crystal (CLC) shells are fabricated from a reactive CLC mesogen mixture via a microfluidic method with a fine-tuned density of the inner phase of the CLC shell by controlling the mixing ratio of glycerol and water. The solid-state CLC (CLCsolid) shell is obtained after UV curing and chiral-dopant extraction. Stable CLCsolid shells are obtained when the density of the inner phase is comparable to that of the CLC shell during UV curing. The CLCsolid shells display three modes of reflection patterns: central reflection (Rcent), crosscommunications among adjacent CLC shells (Rcomm), and reflection within the shell interior (Rin). The three different modes of reflection of the CLCsolid shells are utilized for solvent

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sensors, anti-counterfeiting patches, and labeled templates for monodispersed droplets using their characteristics of differing swellings of CLCsolid shells in different organic solvents, the complexity of the Rcent, Rcomm, and Rin patterns, and the pores formed in the CLCsolid after chiraldopant extraction, respectively. Thus, CLCsolid shells have intriguing photonic properties and can be applied in many different fields which have previously not been explored with liquid crystalstate CLC shells.

INTRODUCTION Microfluidics has become a general method for producing uniformly sized double emulsions (shells) which have a variety of applications (e.g., controllable encapsulation and release in drug delivery). Cross-linkable polymers, polyelectrolyte complexes, and hydrogels1-3 have been used as shell materials. Recently, cholesteric liquid crystals (CLCs) have been utilized to fabricate a photonic CLC shell.4 The CLC shells produced by microfluidic methods are of interest due to their unique optical reflection modes. Three reflection modes of photonic shells, including reflection from the photonic structure at the center (Rcent), cross-communication among the adjacent CLC shells (Rcomm), and reflection within the shell interior (Rin) are observed for CLC shells.5 Rin has recently been reported in an asymmetric shell, and displays the expected concentric rings. r/r0 (r is the radius of the reflected ring and r0 is the radius of the CLC shell) depends on the number of the reflection within the CLC shell interior, which also governs the color of reflection, and reflection angle according to Bragg’s law.6 The discovery of Rin adds a third mode to the two known modes; Rcent and Rcomm. The complex pattern arising from these three modes makes CLC shells good candidates for anti-counterfeiting applications. However, the liquid crystal (LC)-state of CLC shells hinders their use in real applications. In order to improve the stability of CLC shells (or droplets), a polymer network coating has been applied to


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the droplet surface, although the unstable LC state was still used.7, 8 Stable CLC shells, obtained by partial cross-linking of small amounts of reactive mesogens in the CLC mixture, have been reported, although a large quantity of fluid LCs remained in the CLC shell.9 Recently, a complete solid-state CLC (CLCsolid) droplet and shell have been reported, prepared using a commercially available reactive mixture (RMM) mixed with a chiral dopant.10 After UV curing and chiral dopant extraction, they displayed a well-developed CLC structure and clear photonic color. They were stable and swelled in suitable organic solvents, which increased the helical pitch and accordingly changed the colors of the Rcent and Rcomm.10, 11 However, a photonic pattern arising from Rin of CLCsolid shells has not been previously reported. Manufacturers and brand owners combat counterfeiting by employing one of several anticounterfeiting technologies.12 When CLCsolid shells are assembled into a transparent solid film, the photonic pattern generated by the three reflection modes can be used in anti-counterfeiting applications. Additionally, materials can be encapsulated into the core of the shell. The most typical example is the encapsulation of small drug molecules.13, 14 The shell of the CLCsolid shell acts as a membrane, as the extraction of the chiral dopant forms pores through which small materials can penetrate into the core of the CLCsolid shell. The authors of this study have previously demonstrated encapsulation and release of Rhodamine 6G into empty CLC solid shells by controlling solvent power and temperature.10 Similarly, polymerizable monomers with crosslinkers can be encapsulated by and cured within the core of the CLCsolid shell to make monodisperse solid droplets. The monodisperse solid droplets can be extracted from the CLCsolid shell by swelling of the solid inner droplets through the use of a suitable solvent (good solvent for the solid inner droplet and relatively poor solvent for the CLCsolid shell). This approach to producing monodisperse droplets has a number of advantages as the monodisperse droplets can


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be prepared from the CLCsolid shell templates when required, without specialized equipment. Additionally, the CLCsolid shells can be applied as labels, distinguishing the type of encapsulated droplet by the reflected photonic color. In this study, CLCsolid photonic shells were prepared from a reactive CLC mixture doped with the chiral dopant (S)-4-cyano-4´-(2-methylbutyl)biphenyl (CB15), using a microfluidic method combining co-flow and flow focusing geometry. In order to make stable droplets after UV curing, the density of the inner phase was controlled by changing the mixing ratio of a solution of glycerol and water. The three different reflection modes of the CLCsolid shells, namely, Rcent, Rcomm, and Rin, were utilized as solvent sensors, anti-counterfeiting patches, and labeling templates for monodispersed droplets. The swelling of the CLCsolid photonic shells in organic solvents was investigated, with respect to the change in reflected color of the three modes, for applications in solvent sensing. Patches containing CLCsolid shells were studied to determine unique photonic optical properties for applications in anti-counterfeiting. The CLCsolid shells were used as a template for producing a variety of monodispersed droplets, where the reflected color served as a photonic label. Thus, since the Rin was fully discussed, its application was first demonstrated in this article with solvent sensors, anti-counterfeiting patches, and labeled templates for monodispersed droplets, which cannot be realized with CLC state (or partially polymerized CLC). The combination of Rcent, Rcomm, and Rin increases the detection limit in sensor because the reflected color range with swelling is different with the reflection mode. Thus, the intriguing photonic properties of the CLCsolid shells enable applications in a number of different fields which have not been explored for the LC-state CLC shells.


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Figure 1. ((a), (b)) TOM and (c) ROM images of ((a),(c)) top- and (b) side- views of the CLC shells at  = (i) 29, (ii) 42 and (iii) 59 vol%; cartoons represent the direction of observation; g is the direction of gravity; the scale bar corresponds to 100 m; (r, r) = (103, 14), (112, 13), and (155, 17) m for (i), (ii), and (iii), respectively, where r is the outer radius and r is thickness of the shell.

RESULS AND DISCUSSION Preparation of CLC shells Overall preparation methods of CLC and CLCsolid shells and their applications are summarized in Scheme 1. The asymmetry of the CLC shell was controlled using the mixing ratio of the glycerol/water solution of the inner phase. Figure 1a and b shows the transmission optical microscopy (TOM) images of top- (Figure 1a) and side- (Figure 1b) views of the CLC shells, respectively. The columns ((i), (ii), (iii)) represent shells with glycerol/water cores with differing quantities of glycerol (); the CLC shells were suspended in the same glycerol/water mixture as that in the core. CLC shells with top-thin, symmetrical, and bottom-thin geometries were produced at  = 29, 42, and 59 vol%, respectively. The density of the mixture at  = 42 vol%


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(1.087 mg/mL) is comparable to that of the CLC (1.089 mg/mL) (Figure S1 in the Supporting Information). The top views of the CLC shells (Figure 1a) appear symmetrical regardless of ; however, the side views of the CLC shells (Figure 1b) reveal the asymmetry. The top-thin and bottom-thin shells were produced due to lower and higher densities of the inner phase with respect to that of the CLC, respectively. The relatively symmetrical shell was produced at  = 42 vol%, although a slight asymmetry in the structure was unavoidable. Figure 1c depicts the reflected light optical microscopy (ROM) images of the top views of the CLC shells with glycerol/water core mixtures of  = 29, 42, and 59 vol%. Rcent and Rcomm are observed for all CLC shells; however, Rin is only observed for the top-thin shells prepared at  = 29 vol%. The top-thin asymmetric shell is necessary for the generation of Rin,5as the light enters and escapes via the thin part of the shell. Figure 2a shows the ROM images of the top-thin CLC shells prepared at  = 29 vol%, with different focal planes. The green Rcomms are observed when the focal plane is close to the top of the shell, as Rcomm from the outer surface occurs close to the top as shown in Figure 3c. When the focal plane is nearer the bottom of the shell, typical concentric rings are observed at Bragg angles of 45 and 60. The observed rings are assigned numbers (m, n) of changing direction (m) and reflection (n).5 These rings are designated as (1, 2), and (1, 3), respectively. The (1, 2) ring is the least reflected by the inner surface and the strongest due to a smaller loss of intensity by reflection. Rin is not visible for the symmetric CLC shell prepared at  = 42 vol% glycerol, even at entirely different focal planes, as shown in Figure 2b. Thus, the Rin results from the reflection from the interior surface of the bottom of the asymmetric CLC shell.


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Middle phase (O)

Water + Glycerol + Fe3O4

RMM 727 + CB15

Continuous phase (W)

Water + Glycerol + PVA

Changing of reflection pattern

Solvent sensor

Solvent

Shrinkage

Swelling

Monomer mixture

UV curing 10 min, 12 cm

Infiltration of AA or NIPAM monomer

Changing of continuous phase Curing 6cm, 10min

Extraction of CB15 with acetone

Acetone

Oil Hydrogel

Washing

Hydrogel

with acetone

Hydrogel droplets

CLCsolid shells

Anti-counterfeiting patch

Scheme 1. Schematics of overall preparation methods of the CLC and CLCsolid shells and their applications.

Figure 2. ROM images of the top views of the CLC shells at  = (a) 29 and (b) 42 vol% with the focal planes depicted in insets as yellow dotted lines; the scale bar applies to all figures; (r, r) = (103, 14), and (155, 13) m for (a) and (b), respectively.

Figure S2 in the Supporting Information shows the TOM images of the CLC shells in water, prepared at  = 42 vol% glycerol, with different shell thicknesses, and single CLC droplets


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prepared for comparison. The single CLC droplets display distinct Rcent with radial Rcomm streaks. Streaks are observed rather than spots as Rcomm occurs inside the droplets along the radial direction at 45 from the incident beam.9 However, the thin CLC shell does not produce the streaks as only inner communication occurs in the thin shell. This is one of the advantages of using CLC shells as optical materials compared with single CLC droplets. Grainy CLC shells with oily streaks are stabilized to form CLC shells with uniform orientations. The stabilization of the CLC shell is strongly dependent on the shell thickness. Figure S2 in the Supporting Information shows the CLC shells after stabilization. Shells with thicknesses of 28, 16, and 8 µm require 13, 10, and 6 days to become stable, respectively. Thicker shells require longer stabilization times.

(a) (i)

(ii)

(iii)

100 μm

(b)

(c)

Rin

Rcent

Rcomm

Surface

45 °

45 °

Figure 3. ROM images of the (a) top and (b) side views of the CLCsolid shells in acetone, prepared at  = 42 vol%, with the focal planes in (a) located at the (i) top, (ii) middle, and (iii) bottom; the insets in (a) are schematics of the CLCsolid shells with yellow lines representing the focal planes; the scale bar applies to all figures, (c) a schematic of the Rcent, Rcomm, and Rin; the side view in (b) was obtained by abrupt rotation of the square capillary tube by 90 after observing (a); (r, r) = (140, 14) m for (a) and (b).

Preparation of asymmetrical CLCsolid shells


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UV curing and chiral dopant extraction were performed to fabricate the CLCsolid shells from the top-thin, symmetrical, and bottom-thin CLC shells prepared at  = 29, 42, and 59 vol%, respectively. However, the asymmetrical top-thin and bottom-thin CLC shells ruptured frequently (Figure S3a in the Supporting Information), due to shrinkage of the shell during UV curing, which led to stress concentration at the thin part of the CLC shell. The relatively symmetric shells did not rupture. Thus, the optical properties of the CLCsolid shells prepared from the symmetric CLC shell at  = 42 vol% were investigated, as shown in Figure S3b in the Supporting Information. The Rcent shifts from red to orange to blue, and Rcomm shifts from green to blue to UV (not visible), after sequential UV curing and dopant extraction, respectively, and Rin is observed after UV curing (Figure S3a ii). The Rcent, Rcomm, and Rin of the CLCsolid shells are blue-shifted after sequential UV curing and dopant extraction due to the decreased pitch caused by UV curing and extraction of the chiral dopant. Figure 3a shows the ROM images of the CLCsolid shells in acetone prepared at  = 42 vol% (symmetrical CLC shell) with different focal planes. When the focal plane is at the top of the CLCsolid shell, the distinct blue hexagonal Rcomm is observed (Figure 3a i). When the focal plane is near the bottom of the shell, only the green Rin is observed (Figure 3a iii). At the focal plane near the middle of the shell, both Rcomm and Rin can be clearly observed (Figure 3a ii). The appearance of Rin indicates that asymmetry is introduced into the CLCsolid shells during UV curing and dopant extraction. The asymmetry of the CLCsolid shells is evident in the TOM side-view of the CLCsolid shells (Figure 3b). Figure 3c presents a schematic of the three modes of reflection of asymmetric CLC solid shells. The Rcomm and Rin of (1, 2) occur at the top and bottom of the asymmetric CLCsolid shell, respectively. Both modes occur at a Bragg reflection angle of 45 and are described by the equation  = nPcos ( = 45) where , n, and  are wavelength at the photonic band gap, reflective index, and Bragg


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angle, respectively. Thus, if the values of n and P are equal, the same color should be observed. Before UV curing and dopant extraction, Rcomm and Rin are both green (Figure 1c i). After UV curing and dopant extraction, Rcomm and Rin are blue and green, respectively, as shown in Figure 3a i-iii. This result indicates that the thin top of the asymmetric CLCsolid shell has less pitch than the thick bottom. Shrinkage of the shell occurred during UV curing with the stress of deformation concentrated at the thin part of the CLC shell. The Rin of (1, 2) is observed slightly inside of the Rcomm at  = 45 because the inner droplet is smaller than the outer droplet. Thus, the CLCsolid shell has an asymmetric structure in terms of shape and pitch of helix. This asymmetry of the CLCsolid shell structure results in a more diverse reflection pattern than that of the symmetric CLC shell. The effect of curing speed on the structure of the CLCsolid shell (prepared at  = 26 wt% and  = 42 vol%) was also studied with different UV intensity. Figure S4 in the Supporting Information shows the TOM and ROM images of the top and side views of the CLCsolid shells, prepared at  = 42 vol% glycerol, with different UV intensities of 33, 80, and 133 μW/cm2. Top and side views reveal the internal structure of the CLCsolid shell after UV curing and dopant extraction. The structures of the CLCsolid shells are similar each other at different UV intensity, indicating that the UV intensity little affect the structure of the CLC solid shell. This may be because the thin shell is cured fast enough within the tested UV intensities not to affect the structural change. Figure 4a to c shows the scanning electron microscopy (SEM) images of the outer (Figure 4a and b) and knife-cut cross-sectional (Figure 4c) surfaces of the dried CLCsolid shells with shell thicknesses of 30 (Figure 4a), 10 (Figure 4b) and 28 (Figure 4c) m, respectively. The thickest dried CLCsolid shells (30 m) are perfect spheres (Figure 4a) while the thinnest CLCsolid shells (10 m) are dimpled spheres (Figure 4b). The thicker CLCsolid shells have a rigid structure


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compared with that of the thinner shells, which preserves the spherical shape during drying. The cross section (Figure 4c) shows both the asymmetry and distinct layering of the structure. The

pitch of the shell was measured at the bottom, middle, and top of the shell. The measured pitches were 458, 510, and 530 nm at the thick (28 m), middle (25.6 m), and thin (17.3 m) parts of the CLCsolid shell, respectively. Thus, it was confirmed that the different colors of Rcomm and Rin at a Bragg angle of 45 were due to the difference in pitch at thick and thin parts of the CLC solid shell. Figure 4. SEM micrographs of ((a), (b)) the dried CLCsolid shells (prepared at  = 42 vol%) with shell thickness of (a) 30 and (b) 10 m, and (c) the knife-cut surface of the dried CLCsolid shells with insets of the enlarged (i) thick (28 m), (ii) middle (25.6 m), and (iii) thin (17.3 m) parts of the shell in red square boxes; the knife-cut surface image of (c) was obtained from the crosslinked PAA film containing the CLCsolid shells.

Effect of swelling on the optical properties of asymmetric CLCsolid shells The swelling of the asymmetric CLCsolid shells in different organic solvents was investigated. Figure S5a and b in the Supporting Information shows the ROM images of the CLCsolid shells ( = 42 vol%) and UV-vis spectra of the CLCsolid films (inset in Figure S5a in the Supporting Information) in several organic solvents. The wavelength of the photonic band gap (PGB) was determined from the peak position in the UV-vis spectrum. Solvent quality can be controlled using its solubility parameter. A good solvent swells the CLCsolid shell, increases the pitch of


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helix, and red-shifts the color of the reflections. Rin is also dependent on the pitch of the helix, thus, its color changes with the solvent. Rcent changes from blue (in water, PGB = 450 nm) to orange (PGB = 610 nm), red (PGB = 685 nm), red (PGB = 700 nm), and green (PGB = 550 nm) in the good solvents acetone, tetrahydrofuran (THF), pyridine, and acrylic acid, respectively. These solvents have solubility parameters of approximately 21 MPa1/2 (Figure S5c in the Supporting Information). Rin of CLCsolid shells in water (poor solvent) is in the UV range. When the shell swells, the wavelength of Rin shifts into the visible range, and the color changes from blue to green, and then red, as the swelling increases in good solvents. Thus, Rin can be used for the determination of solvent quality. In cases where Rcent is invisible (when the solvent is too good for CLCsolid and its pitch shifts the wavelength into the infrared range), R in can be used to


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determine solvent quality. In this situation, the wavelength of Rin of (1, 2) is shortened by

√

compared with that of Rcent, and only Rin is visible. For example, the Rcent of the CLCsolid shells (prepared at  = 42 vol%) in pyridine (Figure S5a, vii in the Supporting Information) is barely visible, while the red Rin of (1, 2) is clearly visible. Figure 5. (a) ROM images of the CLCsolid shells (prepared at  = 42 vol%) in pyridine/water mixtures at  (pyridine content) = (i) 0, (ii) 80, (iii) 85, (iv) 90, (v) 95, and (vi) 100 vol%; insets are images of the film samples; the scale bars correspond to 100 m; (r, r) = (101, 6), (115, 10), (234, 12), (120, 14), and (124, 16) m for (i), (ii), (iii), (iv), (v) and (vi), respectively, (b) UV-vis spectra of the CLC films ((a), insets), and (c) the PBG of (b) as a function of , and (d) the thickness of the CLCsolid shells as a function of solvent exchange cycle when the solvent is changed from acetone to pyridine; insets in (d) represent the TOM and ROM images in acetone

(bottom right corner) and pyridine (upper left corner); (r, r) = (128, 19) and (108, 12) m for the CLCsolid shells in pyridine and acetone, respectively. The solubility parameter can be systematically controlled using the pyridine/water mixing ratio, where pyridine and water are good and poor solvents, respectively. The pyridine content in


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the mixture is denoted as , in vol%. Figure 5a-c shows the ROM images of the CLCsolid shells (prepared at  = 42 vol%), UV-vis spectra, and PBG of the CLCsolid films (Figure 5a, inset) at different values of . The Rcent continuously shifts from blue to red as  increases. Simultaneously, Rin shifts from the UV region to blue, green, yellow, and red as  increases. Thus, Rin can be used for determination of the solubility parameter of the solvent as depicted in Figure S5 in the Supporting Information. The CLCsolid shell is quite stable due to its solid state, and thus the swelling and shrinking, and corresponding reflection color change can be repeated by exchange of solvents. The repeatability of the swelling was investigated using pyridine and acetone, as shown in Figure 5d. The colors of Rcent and Rin can be alternated from orange to infrared (IR, not visible) for Rcent, and green to red for Rin, by changing the solvent from acetone to pyridine for up to 5 cycles, although the repeatability would be expected to continue.


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Figure 6. (a) Size distribution of the cross-linked PAA hydrogel droplets with TOM images of hexagonal packed droplets (right inset) and the process of escaping the CLCsolid shells (left inset), and (b) the diameter ratio (d/d0) of the cross-linked PAA hydrogel droplets as a function of pH, with a reference diameter (d0) at pH = 12; (c) the size distribution of the cross-linked P(NIPAMco-AA) hydrogel droplets with TOM image, (d) diameter of the cross-linked P(NIPAM-co-AA) droplets as a function of the temperature-change cycle between 25 C and 85 C, and (e) the diameter ratio (d/d0) of the cross-linked P(NIPAM-co-AA) hydrogel droplets as a function of pH with a reference diameter (d0) at pH = 12.


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Application of asymmetric CLCsolid shells as templates for solid single emulsion droplets The CLCsolid shells can be used as templates for monodisperse single droplets. The single droplets can subsequently be extracted from the CLCsolid shell. Cross-linked polyacrylic acid (PAA) droplets were produced by encapsulation of an acrylic acid (AA) mixture (see the Experimental Section) into CLCsolid shells and subsequent UV-curing followed by removal of the solid shell.15, 16 Figure 6a shows the size distribution and TOM images of the cross-linked PAA droplets. Good solvents (e.g. acetone) for PAA were used for removal of the CLC solid shell. Swelling of CLCsolid occurs in a narrow range of solvent solubility parameters. The solubility parameter of acetone (19.9 MPa1/2) is outside the range of good solvents for CLCsolid shells. Thus, the swollen cross-linked single droplets in the CLCsolid shell can be extracted from the relatively unswollen CLCsolid shell, as shown in the left-side inset of Figure 6a. The resulting PAA hydrogel droplets are 235  5 m in size and have a narrow size distribution. PAA is a weak anionic polyelectrolyte which swells and shrinks at high and low pHs, respectively.17, 18 Figure 6b shows the dependency of the size of the produced PAA hydrogel droplets on pH. pH values were controlled by buffer solutions. A step-wise increase in size of the PAA hydrogel droplets is observed beginning at pH = 6 as the pH increases; the droplet diameter increases more than two-fold, corresponding to more than an eight-fold increase in volume.19 Thus, the produced PAA hydrogel droplets show a strong pH dependence. Similarly, the CLCsolid shells can be used as templates for producing monodisperse crosslinked P(NIPAM-co-AA) single droplets (NIPAM = N-isopropylacrylamide). P(NIPAM-co-AA) is known to have a dual response to temperature and pH.20-22 Homo cross-linked P(NIPAM) droplets were initially prepared. However, the swelling of P(NIPAM) is not sufficient to escape from the CLCsolid shell; therefore, pristine P(NIPAM) single droplets could not be produced.


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When the NIPAM mixture (see the Experimental Section) is encapsulated by the CLC solid shell template, the swelling of P(NIPAM-co-AA) is sufficiently large to rupture the CLCsolid shell producing the cross-linked P(NIPAM-co-AA) single droplets. The cross-linked P(NIPAM-coAA) single droplets have diameters of 433  6 m and a narrow size distribution (Figure 6c). Figure 6d shows the diameter of the P(NIPAM-co-AA) droplets as a function of temperature cycle, for a temperature range of 85 C to 25 C. The size ranges between 237 m (at 85 C) and 432 m (at 25 C) for up to 5 cycles, indicating that the sensitivity to temperature is reversible (Figure 6d). The P(NIPAM-co-AA) droplets become turbid at 85 C as it is above the lower critical solution temperature of P(NIPAM-co-AA). P(NIPAM-co-AA) also exhibits a pH response attributable to the AA component. Figure 6e plots the ratio of the diameter (d/d0) of the produced P(NIPAM-co-AA) hydrogel droplets as a function of pH, with a reference diameter (d0) at pH = 12. A similar pH response to that of the PAA droplets (Figure 6a) (i.e., a step-wise increase of the size of the PAA hydrogel droplets beginning at pH = 6) is observed, indicating that the P(NIPAM-co-AA) droplets are dually responsive smart droplets. Thus, a new tool for producing monodisperse droplets of pH and temperature responsive smart hydrogels with CLCsolid shells is successfully demonstrated. This approach to the production of monodisperse droplets has several advantages, as the monodisperse droplets can be prepared when required using the CLCsolid shells as a template, without specialized equipment. Additionally, the CLCsolid shell can act as a label, where the reflected color identifies the type of droplet.


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Figure 7. (a) TOM and (b) ROM images of the cross-linked PAA patch containing the hexagonally packed CLCsolid-Fe3O4 shells with focal planes at the (i) top, (ii) middle, and (iii) bottom; the inset in (a) is a low-magnification photograph of the film; (r, r) = (118, 15.5) m.

Application of asymmetric CLCsolid shells as anti-counterfeiting patches Control of the positions of the shells is important in order to manipulate the optical properties, especially the Rcomm, of an assembly of CLCsolid shells. The pristine CLCsolid shells were difficult to manipulate; therefore, Fe3O4 nanoparticles were introduced. Figure S6 in the Supporting Information shows the SEM micrograph of the Fe3O4 nanoparticles inside of the CLCsolid shells ( = 42 vol%). Uniform Fe3O4 nanoparticles, with diameters of 80–150 nm, are observed after the dried CLCsolid-Fe3O4 shells were fractured by finger pressure, indicating successful encapsulation of the nanoparticles by the CLCsolid shells. Figure 7a presents the TOM image of a cross-linked PAA patch containing hexagonally packed CLCsolid-Fe3O4 shells, with a lowmagnification photograph inset. Figure 7b shows ROM images of the photonic CLCsolid-Fe3O4


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shell patch, with different focal planes. The Rcent, Rcomm, and Rin reflection modes are observable. The images are different depending on the focal plane. For example, the R comm and Rin are more visible when the focal plane is at the top (Figure 7b i) and bottom (Figure 7b iii), respectively. Thus, results are similar to those obtained for the individual CLCsolid shells in a solvent (Figure 3). This solid photonic patch would be very difficult to counterfeit due to the three different reflection colors. The Rin pattern is dependent on the orientation of the CLCsolid-Fe3O4 shells, which makes the patch more difficult to copy. There are several advantages to using the solidstate patch compared with those previously reported, as the CLC shell and matrix are all solidstate, and thus the photonic CLCsolid-Fe3O4 shell patch is stable for transport and long-term storage, and similar to anti-counterfeiting patch technology already in use.

CONCLUSION Asymmetric photonic CLCsolid shells were successfully fabricated using a microfluidic method and a reactive CLC mesogen mixture. The resulting CLCsolid shells have both an asymmetric shape and an asymmetric pitch. The CLCsolid shells have excellent stability in organic solvents, which is due to their solid state. The prepared CLCsolid shells exhibit Rcent, Rcomm, and Rin reflection modes. Solvent-induced swelling causes a change in the reflected color depending on the solubility parameter; hence, the solubility parameter of a solvent can be determined by color difference, using the naked eye. The use of all three reflection modes broadens the scope for visible light detection. For instance, when Rcent lies outside of the visible range, the comparatively shorter wavelengths of Rin and Rcomm remain visible. The CLCsolid shells were applied as the templates for producing monodisperse single PAA and P(NIPAM-co-AA) droplets. This application was possible as the extraction of the chiral dopant forms the pore of a


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membrane through which cross-linkable monomers can penetrate. The CLCsolid shells assembled in a polymer matrix have a complex and unduplicable pattern comprising R cent, Rcomm, and Rin, which was used as an anti-counterfeiting patch. Thus, the photonic asymmetric CLCsolid shells, with reflections from outer and inner surfaces, have new unique applications which are only possible for a solid-state photonic shell structure with high stability.

EXPERIMENTAL SECTION Materials:

RMM727

(reactive

mesogen

mixture,

Merck,

UK),

(S)-4-cyano-4´-(2-

methylbutyl)biphenyl (CB15, Synthon, Germany), iron oxide (Fe3O4, 50–100 nm in diameter, Sigma-Aldrich, USA), poly(vinyl alcohol) (PVA, Yakuri, Japan), trichloro(1H, 1H, 2H, 2Hperfluorooctyl)silane (PFOTS, 97%, Sigma-Aldrich, USA), tetraethyl orthosilicate (TEOS, TCI chemicals, Japan), citric acid (Sigma-Aldrich, USA), tri-sodium citrate (Fisher chemical, USA), 3-(trimethoxysilyl)propyl

methacrylate

(TMSPMA,

98%,

Sigma-Aldrich,

USA),

poly(vinylpyrrolidone) (PVP, Sigma-Aldrich, USA), bare glass slides (Marienfeld, Germany), Norland optical adhesive 65 (NOA65, Norland Products, USA), micro-pearls (Sekisui, Japan), silicone oil 1000 CS (KF-96, Shin-Etsu Chemical, Japan), acrylic acid (AA, Junsei, Japan), Nisopropylacrylamide (NIPAM, TCI chemicals, Japan), Irgacure 500 (photoinitiator, Ciba®, Switzerland), pH buffer solutions (Samchun, Korea), toluene (Duksan, South Korea), tetrahydrofuran (THF, Duksan, South Korea), pyridine (Duksan, South Korea), ethanol (Duksan, South Korea), methanol (Junsei, Japan), and acetone (Duksan, South Korea) were used as received. Deionized (DI) water was purified using a reverse osmosis system (Pure RO, Romax, South Korea). Predetermined amounts of RMM727 and CB15 were mixed at 70 C for 12 h with magnetic stirring. The content of CB15 in the mixture with RMM727 is denoted as . The 


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value was fixed at 26 wt% unless otherwise specified. The transparent chiral solution became milky after completion of stirring and cooling to 25 C. Preparation of the CLCsolid shells: A micro-capillary device with combined co-flow and flow focusing geometry was used to prepare the CLC shells from RMM727 doped with CB15. A schematic of the device is shown in Figure S7a in the Supporting Information, where the detailed experimental method can also be found. A photograph of the glass capillary showing the production of the CLC shell can be found in Figure S7b in the Supporting Information. The CLC shells were cured by UV irradiation at λ = 365 nm at a distance of 12 cm for 10 min, using a UV curing machine (Inno-Cure 100N, Lichtzen, South Korea). The same UV curing machine and conditions were used in all experiments. The cured CLC shells were washed with acetone 10 times to extract the chiral dopant. Density determination of shells using a floating method: The density of CLC shells at a particular  was determined using a floating method in which density of the medium was controlled using the glycerol/water mixing ratio. Figure S1 in the Supporting Information shows the density of the CLC shells at different values of . Encapsulation of water-dispersible Fe3O4 particles: Fe3O4 particles were dispersed in a density-matching glycerol/water solution containing 1 wt% PVP, by sonication for 60 min, using a sonicator (5800 Ultrasonic Cleaner, Bransonic, USA). The pH of the prepared Fe3O4 dispersion solution was controlled at 6.3 with a citrate buffer, which was prepared with citric acid (1.6 mg) and tri-sodium citrate (0.8 mg) in DI water (50 mL).23-25. The density-matching water/glycerol mixture containing the water-dispersible Fe3O4 particles (0.004 g/mL) was used as the inner phase in the microfluidic fabrication of the Fe3O4 particle-encapsulating CLCsolid (CLCsolid-Fe3O4) shell. The Fe3O4 particle size was large enough to prevent escape from the CLCsolid shells.


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Preparation of the photonic anti-counterfeiting patch: A patch with hexagonally assembled CLCsolid-Fe3O4 shells ( = 22 wt%) in a cross-linked PAA matrix was prepared. The CLCsolid-Fe3O4 shells were mixed in the liquid-state into an AA/TPGDA/irgacure 500 (89/10/1 wt%) mixture and allowed to stand for 24 h at 60 C. The mixture was cast into a film on PFOTS-coated glass, under a magnet. The film containing hexagonally packed CLCsolid-Fe3O4 shells was UV-cured, washed with water to remove unreacted monomer, and detached from the glass to make a photonic patch. Preparation of the hydrogel droplets: The AA/TPGDA/Irgacure 500 (98.5/0.5/1 wt%) and NIPAM/AA/TPGDA/Irgacure 500 (72.4/24.1/2.5/1 wt%) mixtures in pyridine (40 wt%) (AA and NIPAM mixtures, respectively) were encapsulated into the CLCsolid shells. The CLCsolid shells in acetone were used to preserve the perfect swelled spherical shape. The acetone medium was exchanged with the AA (or NIPAM) mixture. The CLCsolid shells in the AA (or NIPAM) mixture were preheated at 70C for 6 h on a hot plate to facilitate the incorporation of the AA (or NIPAM) mixture into the core of the CLCsolid shell. The excess AA (or NIPAM) mixture was replaced with silicon oil before UV curing. The highly viscous silicon oil is immiscible with the AA (or NIPAM) mixture and prevented its escape from inner phase of the CLC solid shell. The CLCsolid shells encapsulating AA (or NIPAM) mixture, in silicone oil, were UV-cured, and washed with toluene to remove the silicone oil. The pristine monodispersed cross-linked PAA and P(NIPAM-co-AA) droplets were prepared by immersion in methanol (good solvent for PAA and P(NIPAM-co-AA) droplets and poor solvent for the CLCsolid shell) and sonication for 2 h. Measurements: The CLC shells were observed using a charge-coupled device (CCD) camera (STC-TC83USB, Samwon, South Korea) on an optical microscope (ANA-006, Leitz, Germany) in transmission and reflection modes. The side view of CLC shells in a rectangular capillary


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(Vitrocom, USA, 0.9 mm inner diameter (ID) × 0.180 mm wall thickness) was observed with a portable optical microscope (FXC500DE, Fixcope, South Korea). The photonic structure of the CLCsolid shells was investigated using scanning electron microscopy (SEM, S-4800, Hitachi, Japan) after platinum coating of the knife-cut surfaces of the cross-linked PAA film containing the CLCsolid shells used for the photonic anti-counterfeiting patch. UV intensity was measured with two silicon sheets (total thickness is 13.8 mm) on the UV meter (UV-340A, Lutron, USA) because UV was so intense that it was saturated without attenuation. Thus, measured intensity is relative. The UV–vis spectra of CLC films in the range of 300 to 850 nm were obtained using a UV–vis spectrometer (UV-2401PC, Shimadzu, Japan) with the film oriented perpendicularly to the UV–vis beam.

ASSOCIATED CONTENT Supporting Information. Schematic and real photographic images of experimental setup; microfluidic setup for producing monodisperse CLC shells; density of the CLC shells as a function of ; TOM and ROM images of the CLC single droplet and shells with shell thicknesses; TOM and ROM images of the CLCsolid shells prepared at different UV intensity; ROM images of the CLCsolid shells in water; ROM images and UV spectrum of the CLCsolid shells in other solvents; SEM micrograph of the Fe3O4 nanoparticles inside of the CLCsolid shells AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (2017R1A2B2006818) ABBREVIATIONS AA, acrylic acid; Rin, reflection of inner; Rcomm, reflection of communication; Rcent reflection of central. REFERENCES (1) Xu, Q.; Hashimoto, M.; Dang, T. T.; Hoare, T.; Kohane, D. S.; Whitesides, G. M.; Langer, R.; Anderson, D. G., Preparation of Monodisperse Biodegradable Polymer Microparticles Using a Microfluidic Flow-Focusing Device for Controlled Drug Delivery. Small 2009, 5, 1575-1581. (2) Karnik, R.; Gu, F.; Basto, P.; Cannizzaro, C.; Dean, L.; Kyei-Manu, W.; Langer, R.; Farokhzad, O. C., Microfluidic Platform for Controlled Synthesis of Polymeric Nanoparticles. Nano Lett. 2008, 8 , 2906-2912. (3) Champion, J. A.; Katare, Y. K.; Mitragotri, S., Particle shape: a New Design Parameter for Micro- and Nanoscale Drug Delivery Carriers. J. Control Release 2007, 121, 3-9. (4) Chen, L. J.; Gong, L. L.; Lin, Y. L.; Jin, X. Y.; Li, H. Y.; Li, S. S.; Che, K. J.; Cai, Z. P.; Yang, C. J., Microfluidic Fabrication of Cholesteric Lliquid Crystal Core-Shell Structures Toward Magnetically Transportable Microlasers. Lab Chip 2016, 16, 1206-1213. (5) Geng, Y.; Jang, J.-H.; Noh, K.-G.; Noh, J.; Lagerwall, J. P. F.; Park, S.-Y., Through the Spherical Looking-Glass: Asymmetry Enables Multicolored Internal Reflection in Cholesteric Liquid Crystal Shells. Adv. Opt. Mater. 2018, 6. (6) Robbie, K.; Broer, D. J.; Brett, M. J., Chiral Nematic Order in Liquid Crystals Imposed by an Engineered Inorganic Nanostructure. Nature 1999, 399, 764-766. (7) Lee, J. H.; Kamal, T.; Roth, S. V.; Zhang, P.; Park, S. Y., Structures and Alignment of Anisotropic Liquid Crystal Particles in A Lliquid Crystal Cell. Rsc. Adv. 2014, 4, 40617-40625.


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(8) Lee, H. G.; Munir, S.; Park, S. Y., Cholesteric Liquid Crystal Droplets for Biosensors. ACS Appl. Mater Interfaces 2016, 8, 26407-26417. (9) Geng, Y.; Noh, J.; Drevensek-Olenik, I.; Rupp, R.; Lenzini, G.; Lagerwall, J. P., High-fidelity Spherical Cholesteric Liquid Crystal Bragg Reflectors Generating Unclonable Patterns for Secure Authentication. Sci. Rep. 2016, 6, 26840. (10) Kim, J. G.; Park, S. Y., Photonic Spring-Like Shell Templated from Cholesteric Liquid Crystal Prepared by Microfluidics. Adv. Opt. Mater. 2017, 5, 1700243. (11) Noh, K. G.; Park, S. Y., Smart Molecular-Spring Photonic Droplets. Materials Horizons 2017, 4, 633-640. (12) Schwartz, M.; Lenzini, G.; Geng, Y.; Ronne, P. B.; Ryan, P. Y. A.; Lagerwall, J. P. F., Cholesteric Liquid Crystal Shells as Enabling Material for Information-Rich Design and Architecture. Adv. Mater. 2018, 30, e1707382. (13) Kramer, M.; Stumbe, J. F.; Turk, H.; Krause, S.; Komp, A.; Delineau, L.; Prokhorova, S.; Kautz, H.; Haag, R., pH-Responsive Molecular Nanocarriers Based on Dendritic Core-Shell Architectures. Angew. Chem. Int. Ed. Engl. 2002, 41, 4252-4256. (14) Haag, R., Supramolecular Drug-Delivery Systems Based on Polymeric Core-Shell Architectures. Angew. Chem. Int. Ed. Engl. 2004, 43, 278-282. (15) Noh, K. G.; Park, S. Y., Biosensor Array of Interpenetrating Polymer Network with Photonic Film Templated from Reactive Cholesteric Liquid Crystal and Enzyme-Immobilized Hydrogel Polymer. Adv. Funct. Mater. 2018, 28. (16) Stumpel, J. E.; Gil, E. R.; Spoelstra, A. B.; Bastiaansen, C. W. M.; Broer, D. J.; Schenning, A. P. H. J., Stimuli-Responsive Materials Based on Interpenetrating Polymer Liquid Crystal Hydrogels. Adv. Funct. Mater. 2015, 25, 3314-3320. (17) Shiratori, S. S.; Rubner, M. F., pH-Dependent Thickness Behavior of Sequentially Adsorbed Layers of Weak Polyelectrolytes. Macromolecules 2000, 33, 4213-4219. (18) Yang, S. Y.; Rubner, M. F., Micropatterning of Polymer Thin Films with pH-Sensitive and Cross-Llinkable Hydrogen-Bonded Polyelectrolyte Multilayers. J. Am. Chem. Soc. 2002, 124, 2100-2101. (19) Park, H. I.; Park, S. Y., Smart Fluorescent Hydrogel Glucose Biosensing Microdroplets with Dual-Mode Fluorescence Quenching and Size Reduction. ACS Appl. Mater. Interfaces 2018, 10, 30172-30179. (20) Shah, R. K.; Kim, J. W.; Agresti, J. J.; Weitz, D. A.; Chu, L. Y., Fabrication of Monodisperse Thermosensitive Microgels and Gel Capsules in Microfluidic Devices. Soft Matter 2008, 4, 2303-2309. (21) Chen, S. F.; Jiang, L.; Dan, Y., Preparation and Thermal Response Behavior of Poly (Nisopropylacrylamide-co-acrylic acid) Microgels via Soap-Free Emulsion Polymerization Based on AIBN Initiator. J. Appl. Polym. Sci. 2011, 121, 3322-3331. (22) Yuan, M.; Ju, X. J.; Xie, R.; Wang, W.; Chu, L. Y., Micromechanical Properties of Poly(NIsopropylacrylamide) Hydrogel Microspheres Determined Using A Simple Method. Particuology 2015, 19, 164-172. (23) Banerjee, S.; Raja, S. O.; Sardar, M.; Gayathri, N.; Ghosh, B.; Dasgupta, A., Iron Oxide Nanoparticles Coated with Gold: Enhanced Magnetic Moment due to Interfacial Effects. J. Appl. Phys. 2011, 109. (24) Sood, A.; Arora, V.; Shah, J.; Kotnala, R. K.; Jain, T. K., Ascorbic Acid-Mediated Synthesis and Characterisation of Iron Oxide/Gold Core-Shell Nanoparticles. J. Exp. Nanosci. 2016, 11, 370-382.


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(25) Arsalani, N.; Fattahi, H.; Nazarpoor, M., Synthesis and Characterization of PVPFunctionalized Superparamagnetic Fe3O4 Nanoparticles as An MRI Contrast Agent. Express Polym. Lett. 2010, 4, 329-338.

BRIEFS The asymmetric solid-state cholesteric liquid crystal shells were demonstrated for applications to solvent sensors, anti-counterfeiting patches, and labeled templates for monodispersed droplets. CLCsolid Shell in Organic Solvents

50 μm

Water

Acrylic acid

Acetone

Pyridine


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Optical Properties and Applications of Photonic Shells - ACS Applied

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