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Bioinspired Heterogeneous Structural Color Stripes from Capillaries Ze Zhao, Huan Wang, Luoran Shang, Yunru Yu, Fanfan Fu, Yuanjin Zhao,* and Zhongze Gu* Such materials are attractive for controlling and manipulating the transmission of light, including lenses, waveguides, reflective mirrors, color pigments, and numerous other optical components.[20–24] In particular, by integrating responsive polymers into the highly ordered colloidal crystals, the PBGs or structural colors of the composite materials could be tuned by swelling or shrinking these polymers with different stimulations.[25–27] These imparted important applications to the colloidal crystal materials in sensors and dynamic optics fields. Despite significant progress having been made in the bioinspired colloidal crystal structural color materials, their main uses are still limited to the homogeneous photonic nanostructures, as most of the self-assembly approaches could only achieve consecutive structural colors on flat substrates.[15,24,28] In contrast, heterogeneous structural colors, such as stripe colors, widely exist in creatures, which play an indispensable role in camouflage and individual recognition.[29–31] In addition, although many kinds of tunable structural color materials have been developed,[3,32–34] it is still a challenge to find a simple and effective way to control the change of structural colors remotely. Thus, heterogeneous structural color materials with novel responsive capability are still anticipated for filling the gaps of biomimetic structural color materials. Graphene, as an emerging multifunctional material, has attracted interest in nearly all fields of materials science because of its extraordinary physical and chemical properties.[35–37] When it integrated with intelligent polymers, such as hydrogels, the composite materials were endowed with several new properties, like extremely high electrical conductivity, high thermal conductivity, excellent mechanical flexibility, and large specific surface area.[38–41] However, the current graphene hydrogels have never been introduced into the colloidal crystals for constructing of intelligent structural color materials. In this paper, we present a new strategy for the formation of the desired structural color materials by fast self-assembly of colloidal nanoparticles in capillaries, as schemed in Figure 1a,b. Because of the nonsynchronous process of the solid–liquid– air interface falling and colloidal assembly, these nanoparticles could form a heterogeneously annular stripe pattern on the inner surface of the capillaries. The width and spacing of the structural color stripe pattern could be precisely tailored

As an important characteristic of many creatures, structural colors play a crucial role in the survival of organisms. Inspired by these features, an intelligent structural color material with a heterogeneous striped pattern and stimuli-responsivity by fast self-assembly of colloidal nanoparticles in capillaries with a certain diameter range are presented here. The width, spacing, color, and even combination of the structural color stripe patterns can be precisely tailored by adjusting the self-assembly parameters. Attractively, with the integration of a near-infrared (NIR) light responsive graphene hydrogel into the structural color stripe pattern, the materials are endowed with lightcontrolled reversible bending behavior with self-reporting color indication. It is demonstrated that the striped structural color materials can be used as NIR-light-triggered dynamic barcode labels for the anti-counterfeiting of different products. These features of the bioinspired structural color stripe pattern materials indicate their potential values for mimicking structural color organisms, which will find important applications in constructing intelligent sensors, anti-counterfeiting devices, and so on.

Structural colors, originating from periodic ordered nanostructures, are widespread in nature and have become a crucial multifunctional component for the survival of many organisms.[1–5] Inspired by the dazzling colors of these creatures, much effort has been made toward the construction of artificial structural color materials through the top-down approaches of photolithography, inkjet printing, computer-assisted laser ablation or etching,[6–10] and the bottom-up approaches of self-assembling different building blocks,[11–14] and so on. Among these methods, the self-assembling of monodispersed colloidal nanoparticles has become an effective route for the construction of different structural color materials because it can be simply carried out under a mild environment.[15–19] The periodic variation in refractive index of the assembled colloidal crystals creates a photonic band gap (PBG) for the materials, which reflects light within a specific range of wavelengths and transmits all others. Dr. Z. Zhao, Dr. H. Wang, Dr. L. R. Shang, Dr. Y. R. Yu, Dr. F. F. Fu, Prof. Y. J. Zhao, Prof. Z. Z. Gu State Key Laboratory of Bioelectronics School of Biological Science and Medical Engineering Southeast University Nanjing 210096, China E-mail: [email protected]; [email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201704569.

DOI: 10.1002/adma.201704569

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Figure 1. Formation of the striped structural color materials in capillaries. a,b) Schematic illustration of the self-assembly of colloidal crystals with striped pattern in glass capillaries. c) Optical images and d) characteristic reflection peaks of five different structural colors stripes. The scale bar is 500 µm in (c).

by adjusting the self-assembly parameters. Meanwhile, by employing different nanoparticle sizes for the assembly, stripe patterns with different kinds of structural colors or different combinations of colors could be achieved. Attractively, with the integration of a near-infrared (NIR) light-responsive graphene hydrogel into the structural color stripe pattern, the materials were imparted with a self-reporting and reversible bending behavior, like some creatures. Especially, inspired by the camouflage and individual recognition function of stripe colors existing in nature, we have also explored the potential application of these striped structural color materials as NIR-light-triggered dynamic barcode labels for the anti-counterfeiting of different products. These indicate that the bioinspired structural color stripe pattern materials are valuable in practice. In a typical experiment, the structural color materials were fabricated by self-assembling monodisperse silica colloidal nanoparticles. Different kinds of substrates, like planar glass or

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silicon, could be inserted vertically into the solution containing colloidal nanoparticles for static deposition. In general, homogeneous colloidal crystal films were obtained on the substrates by slowly evaporating off the solvent with high concentrations of silica nanoparticles (usually higher than 1–2%). In addition, heterogeneous stripe-patterned colloidal crystal films could be achieved when the concentrations of the colloidal nanoparticles were very low or the meniscus solid–liquid–air interface fell too fast, as shown schematically in Figure S1 (Supporting Information). During this process, the solvent evaporated from the meniscus interface and the dispersed nanoparticles were carried into the meniscus edge by convective flow to form a colloidal crystal line. With the evaporation proceeding, the width of the colloidal crystal line increased steadily while the interface level fell. Because of the low nanoparticle concentration (typically less than a hundredth of normal concentrations), only a small number of nanoparticles were transferred to the contact

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Figure 2. SEM images of the stripe-patterned colloidal crystals and their replicated nanostructures. a) Full view, b) high-magnification view of surface, and c) cross-section of the stripe-patterned colloidal crystals. d) Full view, e) high-magnification view of surface, and f) cross-section of the replicated inverse opal GO hydrogels. Scale bars are 200 µm in (a) and (d) and 1 µm in (b), (c), (e), and (f), respectively.

line by convective flow, and thus the growth rate of the colloidal crystal line was slower than the rate of the interface level fall, resulting in elongation of the meniscus interface. When the thickness of the meniscus interface reached a critical value that cannot maintain its integrity, the meniscus interface ruptured and created a new contact line down to the next position, which was determined by the contact angle of the substrate. Then, the next colloidal crystal line started to assemble. These consecutive events repeat to produce spontaneously a highly periodic colloidal crystal stripe pattern (Figure S2a, Supporting Information). Although colloidal crystal stripe patterns could be achieved on the substrates, the very low concentrations of the colloidal nanoparticles could only form a few thin layers of colloidal crystals, and the fast decline of the meniscus interface by high-temperature evaporation of the solvent caused disordered assembly of the nanoparticles, both of which could decrease the quality of the colloidal crystal-derived structural colors. To solve these problems, we herein moved the colloidal self-assembly into cylindrical glass capillaries, which were dipped vertically into an open glass bottle containing silica nanoparticles dispersed in ethanol with normal concentrations (Figure 1b). During this process, the evaporation of ethanol inside the capillaries is very slow due to its long evaporation route; this formed an ideal annular colloidal crystal structure in the capillaries. However, benefiting from the open environment of the glass bottle, the evaporation of ethanol in the glass bottle is much faster than that inside the capillaries, which could cause a fast falling of the liquid level in the reservoir resulting in a simultaneous fast falling of the meniscus interface level inside the capillaries, and trigger the rupture of the meniscus interface inside the capillaries to form the stripe structures. In addition, for the selfassembly confined inside the glass capillaries, the radius of curvature of the contact line (namely the radius of curvature of the meniscus interface R = r/cos θ, where r is the radius of the capillary and θ is the contact angle) was an order of magnitude smaller than that on a planar surface, which was also greatly conducive to the formation of a stripe pattern. These synergistic

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effects allowed the formation of colloidal crystal stripe patterns with ideal structural colors, as shown in Figure 1c. The microstructures of the colloidal crystal stripe patterns from the improved method were characterized by scanning electron microscopy (SEM), as shown in Figure 2a–c. It was found that an obvious colloidal crystal structure with a striped pattern was formed on the inner surface of the glass capillary (Figure 2a). From a high-magnification view of a single strip in close-up images of the surface and cross-section, it could be observed that the silica nanoparticles in the strip all selfassembled into a close-packed face-centered-cubic (fcc) array with fewer defects and many stacked layers (Figure 2b,c), which were better than the colloidal crystal stripes on a planar substrate with poorly ordered structures and limited thin layers (Figure S2b,c Supporting Information). Thus, similar to the traditional self-assembled homogeneous colloidal crystal films, the colloidal crystal stripe patterns in the glass capillary also have an ideal 3D ordered microstructure. Because of the highly ordered spatial periodicity in their microstructure, the heterogeneous annular colloidal crystal stripes exhibited a characteristic reflection spectrum with a wavelength peak in a certain PBG. In general, the main characteristic reflection peak position can be estimated by Bragg’s equation for a normal incident beam onto the (111) plane of the fcc structure: λ = 1.633dn, Where d is the center-to-center distance between two neighboring nanoparticles and n is the average refractive index of the colloidal crystals. Therefore, by employing silica nanoparticles within a certain diameter for the self-assembly, annular stripe-patterned colloidal crystals with dazzling structural colors in the visible light spectrum from blue to red could be achieved, as shown in Figure 1c,d. A prominent feature of our self-assembly method is that the width and spacing of the formed structural color stripes could be precisely tailored by varying self-assembly parameters, such as the inner diameter of the glass capillary and the concentration of the silica nanoparticles. It was found that by increasing the capillary diameter from 0.3 to 1.1 mm, the width of the structural color stripes had a slowly increasing trend from about

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Figure 3. Structural color stripes with different widths, spacings, colors, and combinations. a) The optical image and b) curve chart showing the variation of the width of the stripe and spacing with the change of capillary inner diameter. The scale bar is 500 µm. c) The optical image and d) curve chart showing the variation of the width of the stripe and spacing with the change of silica solution concentration. The scale bar is 500 µm. All results took average over five independent measurements and error bars were standard deviations. The combination between stripe patterns of different width of stripes and spacing in e,f) same and g,h) different color. The scale bar is 300 µm.

50 to 180 µm, and the width of their spacing slowly decreased with a sharp decrease from the original 500 to about 150 µm, as shown in Figure 3a,b. When fixing the capillary diameter and changing the nanoparticle concentration, the width of the stripe and spacing all increased with increasing concentration, where the latter increased more, from about 30 to 330 µm (Figure 3c,d). These tailoring parameters were also modified for striped colloidal crystals with other structural colors (Figure S3, Supporting Information). In addition to diameter and concentration, varying other parameters, such as the evaporation temperature and the glass capillary length, would also modulate the width and spacing of the stripes. Because of fast self-assembly process, the falling of solution level inside the capillary is mainly caused by the falling of the liquid level of the reservoir in the glass bottle. As a result, the volume of the liquid column in the capillary maintain relatively constant and nanoparticles, due to gravity sedimentation, cannot diffuse from the reservoir into the capillary in time. Also, the self-assembled nanoparticles only account for very small fraction of the total nanoparticles in the solution inside the capillary, which will only cause a negligible effect of decrease in nanoparticle concentration. Therefore, the nanoparticle concentration in the solution inside the capillary keep relatively stable during the process, which ensures the width uniformity of the stripes and the spacing of the formed structural color patterns as shown in Figure 1c. This feature facilitated the design and formation of various flexible structural color stripe patterns in a single capillary. To demonstrate this concept, a glass capillary with striped structural color assembled for the first time was reused as the substrate for the second selfassembly process, where both patterns were produced in an in situ combination (Figure 3e–h).

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Theoretically, the nanoparticles are charge-repelled and stably dispersed in solution. While, upon the assembly of these nanoparticles on the inner surface of the capillaries, they gain the interparticle van der Waals forces. As a result, the highly ordered microstructures formed by nanoparticles self-assembly are very stable, which is difficult to be broken by general force, such as capillary force and electrostatic repulsion force among the nanoparticles. Therefore, during the second deposition, the solution in the resulting glass capillary would not disturb or re-suspend the self-assembled nanoparticles from the previous depositions. It was found that the combined structural color stripe patterns could be achieved only when the cascade self-assemblies were in a special sequence (Figure S4, Supporting Information), in which the narrow stripes should form first. Thus, in the first self-assembly, a relatively low concentration of silica nanoparticles solution was employed to make relatively narrow stripes and spacing, after which wide stripes were added to the original pattern by utilizing a relatively high concentration. Through this successive self-assembly, colloidal crystal stripe patterns with the same structural colors but different stripes and spacings could be generated (Figure 3e,f). In addition, by employing different silica nanoparticles for the cascade self-assemblies, complex stripe patterns with distinct widths of stripes and spaces, and even different structural colors, could also be created, as shown in Figure 3g,h. As far as we know, this self-assembling method and its derived heterogeneous striped structural color pattern materials have not been reported previously, which have potential for security code applications. To give the structural color stripe patterns the feature of dynamic regulation, responsive graphene hydrogel was integrated into the patterns and employed as a scaffold

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was expected to have a photothermally responsive performance. To observe this performance more intuitively, NIR light was irradiated from one side of the fiber. Because of the asymmetric light irradiation, the fiber underwent one-sided shrinkage, which led to bending of the fiber toward the light source, and the bending point was at the irradiated site, as shown in Figure 5b. The relationships of the bending angles of fibers and the rGO with four different contents in the fibers were recorded in real time during the NIR irradiation with constant laser power (Figure 5c). It can be observed that all the fibers started to bend immediately when irradiated and reached the final angles in less than 2 min. In general, higher rGO content in the microfibers could convert more heat, and cause a more easily observed bending. However, too high a content of the rGO could also occupy more grids of the hydrogel, which Figure 4. The generation of the GO hydrogel fibers with the striped inverse opal nanostructures. a) Green stripe-patterned colloidal crystals template, b) red hybrid template with PNIPAM/GO would block the activity of the fibers. Therehydrogel infiltration, c) green striped invers opal PNIPAM/GO hydrogel fiber, and d) green fore, under the combined effect of the two striped inverse opal PNIPAM/rGO hydrogel fiber. The scale bar is 500 µm. e,f) Stripe-patterned contradictory roles, there was no significant hydrogel fibers with blue and red structural color made from the templates with corresponding bending difference in these different samples blue and red striped pattern. The scale bar is 500 µm. except for the sample with 4 mg mL−1 GO. In addition, it was found that the higher the laser power, the greater the bending angle in a certain range, as for replication of their microstructures. During this process, shown in Figure S7 in the Supporting Information. These a pregel solution of graphene oxide (GO) dispersed in therresults indicate that the PNIPAM/rGO hydrogel fibers have mosensitive N-isopropylacrylamide (NIPAM) was first poured the capability of converting light into kinetic energy, which into the as-prepared stripe-patterned colloidal crystals template is like the phenomenon called phototropism, where plants (Figure 4a,b). After polymerization of the mixture hydrogels in respond to the stimulus of light and produce a bending toward the capillary, hydrofluoric acid was used as the etching reagent the light source (Figure 5a). to remove the glass capillary and the silica colloidal crystal temIt is worth mentioning that at the same time as bending plate, which resulted in a GO hydrogel fiber with the striped occurred, the shrinkage also drove a decrease in the center-toinverse opal nanostructure (Figure 4c). This nanostructure was center distance between two neighboring macropores of the confirmed by SEM images, as shown in Figure 2d–f. It was striped inverse opal nanostructure (namely d in Bragg’s equaobserved that the GO hydrogel fiber had stripe patterns that tion), which led to a blueshift of the local structural colors exhibited ordered and interconnected multilayer macropores and characteristic reflection peaks of the fibers. Therefore, of inverse opal structures. Because of the ordered arrangeduring the stripe-patterned red structural color fiber bending ment of these macropores, the hydrogel fibers possessed the at its middle part, the red-to-blue transition first took place same bright striped structural colors as their colloidal crystal at the middle and gradually spread to both ends of the fiber templates (Figure 4a,d–f and Figure S5 in the Supporting Infor(Figure 5d and Movie S1, Supporting Information). To invesmation). To pursue better photothermal conversion efficiency, tigate the relationship between the striped structural color patthe hybrid PNIPAM/GO hydrogel was further treated using tern and the bending angle, the first (i), third (iii), fifth (v), and reductant to form reduced graphene oxide (rGO), as confirmed seventh (vii) stripes starting from the fiber front were selected in Figure 4d and Figure S6 (Supporting Information). Due to to record the color change. The resulting four separate curve the increasing contrast of the black background, the striped graphs of the characteristic reflection peak positions varying structural colors of the PNIPAM/rGO hydrogel fibers were with angles are presented in Figure 5e. It was found that due to more vivid. direct exposure to the NIR light, stripe vii quickly changed from To demonstrate the dynamic regulation capability of the red to blue within 10° and then immediately shifted to invisstriped structural color PNIPAM/rGO hydrogel fibers, a parible wavelengths. Afterward, accompanied by a slow increase allel NIR light with a constant radiation radius was employed in the bending angles, the heat converted by rGO propagated to trigger the fiber materials. It has been extensively confirmed toward both ends of the fiber and caused shrinkage at stripes that rGO produces photothermal heating effects due to its v, iii, and i successively. Therefore, at each bending angle, the strong NIR absorption, and PNIPAM hydrogel exhibits temfiber had a corresponding and specific structural color pattern perature-responsive swelling and deswelling behaviors because of stripes. This indicates that by observing the structural color of a shift between hydrophilic and hydrophobic states. Comof the stripe at a particular location, the bending angle of the bining both features, the composite PNIPAM/rGO hydrogel

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Figure 5. Biomimic regulation of the PNIPAM/rGO hydrogel fibers under NIR light. Schematic illustrations of a) the phototropism of a sunflower and b) hydrogel fiber bending toward the direction of light. c) The bending angles of the hydrogel fiber with four different GO content of 2, 4, 6, and 8 mg mL−1 changes over time. d) A group of Optical images to show a whole bending process of the hydrogel fiber with varying striped structural color pattern at different bending angles. The scale bar is 500 µm. e) Relationship between the bending angles and the characteristic reflection peak positions, namely corresponding structural color for four different stripes at different positions: i, iii, v, and vii (from top to bottom).

fiber could be semiquantitatively determined, which is difficult to achieve on the fibers without or only with homogeneous structural color. The heterogeneously striped pattern-based selfreporting feature makes the structural color stripes good candidates for intelligent software materials. To further demonstrate the potential value of this candidate, the structural color PNIPAM/rGO hydrogel stripes were employed as dynamic barcode labels for the anti-counterfeiting of different products, as schemed in Figure 6a. As shown above, the structural color PNIPAM/rGO hydrogel could be imparted with not only simple uniform width and color stripes, but also complex stripe patterns with multiple widths and colors. These stripes provided much complicated information than the existing barcodes and raised the difficulty for counterfeiters to forge. Attractively, by integrating a NIR light source into the barcode reader, these structural color stripe patterns could show dynamical color shifts under NIR scanning because of their photothermal response (Figure 6b–d and Movie S2, Supporting Information). In typical examples, it could be observed that the monocolor stripes barcode showed a quick and obvious blueshift of the structural color from red to green (Figure 6b) under the NIR scanning. While For the composite bicolor stripes barcode, its red stripes shifted to blue and green stripes became invisible (Figure 6c), which revealed the hidden decoding information with a double variation in both color and composition of the stripes. It was worth mentioning that 2D stripes barcodes with unique hidden encoding

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information could also be obtained by combining two individual hydrogel stripes with respective compositions and colors as a whole barcode (Figure 6d). These results indicated that the versatile barcode labels featured by dynamic encoding information and colors were of great potential for anti-counterfeiting applications. In summary, we have developed an intelligent structural color material with a heterogeneous striped pattern based on a nonsynchronous process of the solid–liquid–air interface falling and colloidal assembly in capillaries. By adjusting the confined colloidal self-assembly parameters such as the diameter of the glass capillary and the concentration of the colloidal nanoparticles, the width and spacing of the structural color stripe pattern were precisely tailored. In addition, stripe patterns with different kinds of structural colors or different combinations of structural colors were generated by employing different sizes of nanoparticles for the self-assembly. The materials were given an NIR-light-controlled reversible bending behavior and self-reporting color indication feature with integration of the graphene hydrogel into the structural color stripe pattern. As a typical anti-counterfeiting application, the striped structural color materials as NIR-light-triggered dynamic barcode labels were demonstrated. These features of the structural color stripe pattern composite materials indicate their potential value for mimicking structural color organisms, constructing intelligent sensors and anti-counterfeiting devices, and so on.

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Figure 6. Anti-counterfeiting applications of the structural color PNIPAM/rGO hydrogel stripes as dynamic barcode labels. a) Schematic illustrations of the structural color stripe patterns showing dynamical color shifts under NIR scanning. Three different stripes barcode: b) monocolor stripes barcode, c) composite bicolor stripes barcode, and d) 2D stripes barcode showing blueshifts of the structural color under NIR scanning.

Supporting Information

Keywords

Supporting Information is available from the Wiley Online Library or from the author.

anti-counterfeiting, colloidal crystals, graphene, responsive, structural color Received: August 11, 2017 Revised: September 1, 2017 Published online:

Acknowledgements Z.Z. and H.W. contributed equally to this work. This work was supported by the National Science Foundation of China (Grant Nos. 21473029, 51522302, and 21327902), the NSAF Foundation of China (Grant No. U1530260), the National Science Foundation of Jiangsu (Grant No. BK20140028), the Program for New Century Excellent Talents in University, and the Scientific Research Foundation of Southeast University.

Conflict of Interest The authors declare no conflict of interest.

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