Bioinspired Noniridescent Structural Color with Hidden

Subscriber access provided by READING UNIV

Article

Bioinspired Noniridescent Structural Color with Hidden Patterns for AntiCounterfeiting Panmiao Liu, Wenya Chang, Lingsha Ju, Lanling Chu, Zhuoying Xie, Jialun Chen, and Jianjun Yang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b01218 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 31, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Nano Materials

Bioinspired Noniridescent Structural Color with Hidden Patterns for Anticounterfeiting Panmiao Liu,† Wenya Chang,‖Lingsha Ju,† Lanling Chu,§ Zhuoying Xie, *‡ Jialun Chen‡ and Jianjun Yang*†

Department of Anesthesiology, The First Affiliated Hospital of Zhengzhou University, Zhengzhou, China, 450052 ‡State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing, China, 210096 §Faculty of Food Science and Engineering, School of Light Industry and Food Engineering, Nanjing Forestry University, Jiangsu Province, Nanjing, China, 210037 ‖Department of Pharmaceutical Engineering, School of Engineering, China Pharmaceutical University, Nanjing, China, 210009 †

* Corresponding author: [email protected]; [email protected]

KEYWORDS: structural color, amorphous arrays, noniridescence, polydopamine, anticounterfeiting

ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Noniridescent structural colors, by virtue of their pigment-like features and simple preparation techniques, have pivotal roles in color-related applications. However, due to the wide bandwidth resulting from coherent scattering, noniridescent structural colors lack the angle-altered color feature, which plays a critical role in angle-altered colored identification of structural color. Therefore, developing a novel strategy for noniridescent structural colors in security applications remains a challenge. Inspired by Diphylleia grayi, we developed a thickness-controlled silica@polydopamine (SiO2@PDA) amorphous arrays anticounterfeiting system. Owing to the melanin absorption of PDA, the SiO2@PDA amorphous arrays exhibit visual noniridescent structural colors in air, but the brownish-black color of PDA in water. More importantly, when the arrays’ thickness exceeds 9 m, the SiO2@PDA amorphous arrays display thickness-independent structural color in air but thickness-dependent PDA (brownish-black) color in water. Based on this finding, any designs of patterns with uniform or multicolored structural color can be encrypted by employing the distinct thicknesses and selectively disclosed in a liquid solvent environment. We believe these simple, user-interactive noniridescent structurally colored arrays are appealing for various security-related applications, such as anticounterfeiting.


Page 2 of 24

Page 3 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION

Structural color is an optical effect caused by the submicroscopic structure reflecting or scattering selected wavelengths of light. [1-4] Such colors exhibit excellent antiphotobleaching as well as unique color tunability, which are remarkable differences from conventional chemical pigments or dyes. These inimitable features make structural colors appealing for various security-related applications.[5-12] For example, a hologram fabricated by laser ablation is a kind of structural color from one-dimensional (1-D) ordered striated structure. The holographic images selectively show the predesigned hidden images by changing the observation angle, which was a widely applied technique for various anticounterfeiting labels of products in the last century. [13,14] In addition, by virtue of the angle-dependent color display of structural color, 1-D ordered layered structures were further developed and applied in anticounterfeiting inks for the identification of banknotes and passports.[15] Furthermore, structural colors from three-dimensional (3-D) periodic structures were recently exploited to enhance the imitational difficulties of security materials. Combinational features of angle-dependent color, periodic distance and lattice refractive index tenability, 3-D periodic optical structures are introduced into more responsive abilities, such as angle-controlled pattern, selective chemical wetting (or soaking) and deformation for advanced security. [16-23] Although conventionally ordered microstructures can exhibit vivid structural color, the fabrication of artificial long-range-order structures usually employs rigorous technology design, such as top-down (lithography) and bottom-up (self-assembly) strategies to arrive at the periodically ordered structure, especially for 3-D structures. [24-28]

Unlike conventional 3-D colloidal regular structures, 3-D colloidal amorphous

photonic

structures

featuring

short-range-order

(not

completely

random)

microstructures show unique angle-independent or noniridescent coloration. The uniform noniridescent structural coloration allows a color closer to the actual dyes or pigments, which significantly broadens the application of structural color in wider ﬁelds. Importantly, compared with 3-D colloidal regular structures, 3-D colloidal amorphous structures are easier to prepare using a relatively simple method, such as a



direct-spraying method, thus providing a widespread application in pigment, colored displays and printings. [29-36] However, because of the broad bandwidth resulting from the coherent scattering of 3-D amorphous structures, noniridescent colors lack the angle-altered color property, which plays a critical role in angle-altered colored identification of structural color. Therefore, developing a novel strategy for noniridescent structural colors to permit security applications remains a challenge. Diphylleia grayi, a group of small herbs in the Berberidaceae family, showed a fantastic phenomenon in which the original white petals in air turn translucent with rain and when dry, they revert to white, as shown in Figure 1a.[37] As reported, the white color of the petals of Diphylleia grayi results from the diffuse scattering generated in the interface between the numerous air-filled lacunae and intercellular spaces and colorless cytolymphs, thus the petals appear as a white color on a sunny day. However, in the rain, water enters the lacunae and intercellular spaces to replace the air, which forms a homogeneous liquid–liquid (water–cytolymph) interface. Because the cytolymph and water have comparable refractive indexes, the diffuse scattering significantly diminishes, so the light transmission is accordingly increased. Therefore, the white petals turn transparent.[38] This interesting intelligent fluid sensor above inspired us to adjust the light transmission of structural color material using a liquid to achieve anticounterfeiting applications. Inspired by the Diphylleia grayi, we constructed 3-D amorphous arrays by silica@polydopamine (SiO2@PDA) particles to simulate the microstructure of Diphylleia grayi. As in our previous reports, the SiO2@PDA amorphous arrays exhibit highly visible noniridescent structural color. Similar to Diphylleia grayi, the SiO2@PDA arrays lose the original structural color after water wetting and exhibit a dark-brown color of PDA owing to the good light transmittance of silica and high light absorption of PDA. More importantly, SiO2@PDA amorphous arrays of different thicknesses in air display constant physical colors through calculation by CIE’s color matching functions. We speculate that the thickness-independence of SiO2@PDA amorphous structural color is due to the absorption by PDA. In air, the SiO2@PDA amorphous arrays show a thickness-independent structural color. Owing


Page 4 of 24

Page 5 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to the absorption by PDA, the coherently scattered light generated by the SiO2@PDA arrays weakens during light penetrating into the arrays. When the light reaches 9 m thickness of the arrays, the coherently scattered light disappears. So, when the thickness exceeds 9 m, the coherent scatter (structural color) does not change. In water, SiO2@PDA arrays show a constantly deepening dark brown of PDA as the thickness increases. This is due to the voids between the particles being replaced by water, which reduces the refractive index difference between SiO2@PDA particles and voids. The low refractive index difference weakens the coherent scatter of the arrays and increases the transmission, thereby highlighting the dark-brown color of the PDA. According to this interesting finding, two different thicknesses of SiO2@PDA arrays can be patterned into single films. The films are capable of encrypting a graphical code, which is invisible in air but revealed with water. Scheme 1 presents the detailed experimental procedure of the SiO2@PDA patterning. Noticeably, the SiO2@PDA particles, which have been developed to create highly visible structural color materials with controlled color display and strong adhesive property, play a key role in this encryption system.[31,39,40] The admirable hygroscopicity of PDA melanin attracts the water to penetrate rapidly into the different thickness regions and generates a distinct PDA color discrepancy by the naked eye.[41] This fast water-absorbing high colorimetric structural color material is difficult to achieve in traditional noniridescent structural color composed of particles and melanin addition because the common doped black materials are hydrophobic and it is hard to combine the properties of water absorption and broad light absorption simultaneously. Because PDA has similar units to the adhesive plaque of mussels (mussels attach to the surface of substrates by the byssus which is distally tipped by a flared adhesive plaque), PDA has been confirmed to achieve strong adhesion with most substrates.[42-44] Compared with our previous report, the novelty and advances of this work are focusing on the applications of SiO2@PDA for anticounterfeiting as well as more detailed analysis of the optical properties that combine concepts of structural color, low-angle dependence of this color due to the amorphous nature of the particle array, regular absorptive coloration (from the PDA), and contrast



matching to remove the scattering effects that lead to structural color. Therefore, we believe that the selectively patterned display of the noniridescent structural color caused by SiO2@PDA amorphous arrays is potentially useful for security applications, such as anticounterfeiting Scheme 1 Synthesis and detailed patterning encryption procedure of SiO2@PDA particles

RESULTS AND DISCUSSION

The PDA melanin-like particles of selective absorption are synthesized by a protocol we previously reported; [31] dopamine molecules are oxidatively polymerized onto the surface of SiO2 particles, adhered by hydrogen bonding, as shown in Figure S1a. The SiO2 particles, having a particle diameter of 241  2.2 nm, were coated with oxidized dopamine to obtain SiO2@PDA particles with a particle diameter of 295 nm, as shown in the transmission electron microscopy image of Figure S1b. The strong bands at 1355 and 1579 cm–1 in the Raman spectrum and the deepening of color from SiO2 to SiO2@PDA particles both indicate that the PDA was coated on SiO2 particles (Figure S1c, d).[45,46] The aqueous dispersions of SiO2@PDA particles with a diameter of 259 nm formed into structurally colored arrays by a dry-spraying process, as shown


Page 6 of 24

Page 7 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in Figure S2a. Figure S2b shows that the arrays display low-angle-dependent dark red, of which a low peak appears at a wavelength of 650 nm by detecting the reflection, as illustrated in the insert image. The SiO2@PDA particles in the structurally colored arrays are an amorphous arrangement with short-range order, as proved with a scanning electron microscopeimage and two-dimensional Fourier analyses in Figure 1b. The SiO2@PDA amorphous arrays generate constant structural color in air, caused by the coherent scattering of the short-range ordered particles, as shown in Figure 1c.[47,48] As a previous study reported, the perceived color generated by the SiO2@PDA amorphous arrays is independent of their thicknesses in air.[31] Inspired by Diphylleia grayi, we speculate that the SiO2@PDA amorphous arrays after water infiltration will lose the original structural color and exhibit the dark-brown color of PDA, as illustrated in Figure 1-D. Furthermore, the SiO2@PDA amorphous arrays in water will display different PDA colors on varying the arrays’ thickness. To confirm the hypothesis, we prepared SiO2@PDA amorphous structural colored arrays with thicknesses ranging from 3 m to 40 m by varying the volume from 5 to 100 L of the dispersions containing 295  1.7 nm-sized SiO2@PDA particles at a concentration of 0.07 g/mL. The thickness of the arrays increases linearly with dispersion volume, as shown in Figure S3. As expected, the physiological perceived colors of these different thickness amorphous arrays are both dark red after the thickness exceeds 9 m, while the SiO2@PDA arrays after water wetting show a deeper dark-brown color as the arrays’ thickness increases (Figure 1e). Therefore, the SiO2@PDA amorphous arrays display thickness-independent structural color in air but thickness-dependent PDA (brownish-black) color in water when the thickness exceeds 9 m.



Figure 1 (a) Diphylleia grayi before and after wetting. (b) FESEM image of the amorphous SiO2@PDA arrays with an average diameter of 295 nm and its two-dimensional Fourier analyses (the insert image). (c) Diagram of SiO2@PDA amorphous arrays in air. (d) Diagram of SiO2@PDA amorphous arrays in water. (e) Photographs of structurally colored arrays fabricated by 295 nm SiO2@PDA particles exposed in the air and water, respectively. To analyze the discrepant colored display of different thickness SiO2@PDA arrays in air and water, we first studied the thickness-independent structural color of SiO2@PDA amorphous arrays in air (Figure 2a). Figure 2b shows the absolute reflection spectrum of each SiO2@PDA colored arrays with different thicknesses. The reflectance spectrum shows that the reflection peak undergoes a red-shift from 632 nm to 649 nm before the arrays’ thickness reaches 9 m, while remaining stable at 649 nm once thickness reaches 9 m. The observed blue-shift of reflection when the arrays are below 9 m is attributed to the generated scattering partially permeating from the particle layers into the substrate. Because the glass substrates have a lower


Page 8 of 24

Page 9 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


refractive index as well as strong reflection, the reflected spectrum appears as blue-shifted with a broader coherent scattering peak when the coated SiO2@PDA arrays are thin. A colored measurement is further conducted by the absolute reflectance for all the SiO2@PDA arrays. Calculated by CIE’s color matching functions, we found that the three attributes’ values (hue, luminance and saturation) of these SiO2@PDA structural colors no longer changed when their thicknesses reached 9 m (Figure 2c, S4 and Figure S4).[49] The results demonstrate that these SiO2@PDA arrays display the same physical color when the thickness exceeds 9 m. To explore the mechanism of the thickness-independent color of SiO2@PDA amorphous arrays in air, we obtained the absorption integral area (Figure S5) and the corresponding absorption spectrum (Figure 2d) of SiO2@PDA arrays with different thicknesses through measuring the absolute reflection and transmission spectra. The correlation curves of absorption integral area ratio and thicknesses in Figure 2e show that the SiO2@PDA arrays exhibit an increased light absorption with increasing thickness until the thickness reaches 9 m, and then the light absorption tends to be stable until the thickness reaches 40 m. We deduce that the thickness-independent color of SiO2@PDA arrays may arise from the melanin absorption of PDA.[50] The SiO2@PDA arrays generate structural color by the coherent scattering of short-range-ordered particles. Owing to the melanin absorption of PDA, the scattered light inside the SiO2@PDA arrays weakens and disappears as the thickness reaches 9 m. Thus, the SiO2@PDA arrays generate invariant dominating coherent scattering (structural color) when the thickness exceeds 9 m. The hypothesis is strengthened by the SiO2 arrays of different thicknesses prepared under the same conditions with SiO2@PDA arrays. As shown in Figure S6c, d, in the absence of PDA, the SiO2 arrays, on the one hand, exhibit an unnoticeable structural color, and on the other hand, present a continuously increasing reflection intensity when the arrays thicken. These results clearly show that the thickness-independent color of SiO2@PDA arrays larger than 9 m thickness in air is due to the melanin absorption of PDA.



Figure 2 (a) Schematic of the thickness-independent structural color of SiO2@PDA arrays in air. (b) Reflection spectrum of structurally colored arrays fabricated by 295 nm SiO2@PDA particles with thicknesses of 3 μm, 5 μm, 9 μm, 16 μm, 23 μm, 31 μm, 40 μm in air. (c) The value of the three attributes (hue, luminance and saturation) of SiO2@PDA structurally colored arrays with different thicknesses in air. (d) The absorption spectrum of SiO2@PDA structurally colored arrays with different thicknesses in air. (e) The correlation curves of absorption integral area ratio and thicknesses of SiO2@PDA structurally colored arrays in air. We further studied the thickness-dependent color of SiO2@PDA amorphous arrays in water, as illustrated in Figure 3a. After soaking in water, the SiO2@PDA arrays present the dark-brown color of PDA, and this brownish-black gradually deepens as the thickness increases. Theoretically, the appearance of brownish-black after the air in the SiO2@PDA particles’ gaps are replaced by water derives from the weakened scattering effect of the SiO2@PDA arrays induced by the closer refractive index difference between water and particles, thereby increasing the proportion of absorption effect of PDA on the exhibited color (Figure S7). To confirm this, SiO2@PDA amorphous structurally colored arrays with thickness ranging from 3 m to 40 m were soaked in water for the study of the thickness-dependent color. The absolute reflection spectrum (Figure 3b) and reflection intensity (Figure 3c) of these soggy arrays demonstrate that the SiO2@PDA arrays with different thicknesses in water have a very weak reflection, as well as the intensity gradually reduces as thickness increases, which indicates the arrays display an increased light absorption or


Page 10 of 24

Page 11 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


decreased transmission in water as thickness increases. To confirm this, the transmission spectrum of these wetted arrays was measured using a spectrometer with a light source and optical fiber (Figure S8a). The correlation curves of transmission ratio and thicknesses of SiO2@PDA structurally colored arrays at 450 nm of reflection wavelength, shown in Figure S8b, demonstrate that the SiO2@PDA arrays display a declining light transmission in water as thickness increases. Hence, the gradually weakened reflection of SiO2@PDA arrays in water is not ascribed to the transmission increasing rather than the light absorption increasing. Thus, a light absorption analysis from these absolute reflection and transmission spectrum of the wetted SiO2@PDA arrays with thickness ranging from 3 to 40 m was obtained and shown in Figure 3d and Figure S9. Figure 3e shows the correlation curves of absorption integral area ratio and thicknesses of SiO2@PDA structurally colored arrays in water, which are obtained from Figure S9. As shown in Figure 3e, the ratio of absorption integral area increases from 0.4% to 86% as the thickness increases, which validates that the gradually weakened reflection of SiO2@PDA arrays results from the increased absorption as thickness increases. Furthermore, to affirm that the thickness-increased absorption effect of the SiO2@PDA arrays in water is caused by the melanin absorptivity of PDA, we compared the SiO2@PDA arrays with SiO2 arrays under the same conditions. As shown in Figure S10a, unlike the wetted SiO2@PDA arrays, the SiO2 arrays after wetting show a gradual whitening as the thickness increases, resulting from the enhanced scattering of the SiO2 arrays with increased thickness in water. The absorption integral area of SiO2 arrays in Figure S10b indicates that SiO2 arrays present a little light absorption at each thickness in the water. Figure S10c shows that the integrated absorption area of wetted SiO2 arrays is only half of the wetted SiO2@PDA arrays at the same thicknesses. The result convincingly proves that the high absorption of SiO2@PDA arrays is caused by the addition of PDA, also the absorption gradually increases with the thickness. Therefore, the thickness-increased absorptivity of wetting SiO2@PDA arrays primarily results from the melanin absorption of PDA.



Figure 3 (a) Schematic of the thickness-dependent structural color of SiO2@PDA arrays in water. (b) Reflection spectrum of structurally colored arrays fabricated by 295 nm SiO2@PDA particles with thicknesses of 3 μm, 5 μm, 9 μm, 16 μm, 23 μm, 31 μm, 40 μm in water. (c) The correlation curves of reflected intensity with thicknesses of SiO2@PDA structurally colored arrays in water. (d) The absorption spectrum of SiO2@PDA structurally colored arrays with different thicknesses in water. (e) The correlation curves of absorption integral area ratio and thicknesses of SiO2@PDA structurally colored arrays in water. The SiO2@PDA arrays can be patterned by employing selective spray coating techniques.[31,51] A selective spray on white paper through a mask causes regioselective coating of SiO2@PDA particles, thereby resulting in a pattern of SiO2@PDA structurally colored arrays. The visibility of the SiO2@PDA arrays effectively improves color display in the white paper. As shown in Figure S11, different colored patterns with 1  1 cm2 area fabricated by different size of 217 nm, 232 nm, 263 nm and 295 nm SiO2@PDA particles show a distinctly visual color in white paper. A SiO2@PDA pattern can be further coated on the SiO2@PDA structurally colored arrays by the spray process. For example, the pattern of a “pentagram” is prepared on the SiO2@PDA dark red arrays of the 1  1 cm2 area, as illustrated in Figure 4a. According to the diagram in Figure 4b, the resulting structurally colored arrays with patterned images exhibit a consistent dark red because the bottom SiO2@PDA colored arrays are 10 m thickness. Figure 4c confirms this by the obtained pattern-encrypted SiO2@PDA dark red arrays, where particles with a


Page 12 of 24

Page 13 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


diameter of 259 nm are used. In accordance with the steps of Figure 4d and the principles of Figure 4e, the pattern-encrypted SiO2@PDA arrays are visualized (decrypted) by water wetting, of which the process is presented in Movie S1. This is further confirmed in Figure 4f that a dark-colored “pentagram” pattern emerges on the arrays, contrasting sharply with the surrounding light brown after water wetting. To verify the universality of selective appearanceof SiO2@PDA structural color through melanin absorption, we chose four liquids with different refractive indexes to wet our samples—water (1.33), ethanol (1.36), dimethylformamide (DMF) (1.43) and toluene (1.49). As shown in Figure 4g, the “pentagram” pattern appears to be incrementally more inconspicuous as the refractive index of the wetting liquids increases. According to the theoretical analysis for the wetting and nonwetting transition of pattern-encrypted SiO2@PDA arrays, the impaired decryption is attributed to the reduction in the difference in refractive index between the SiO2@PDA particles and the wetting liquid. Moreover, Figure 4g also shows that the SiO2@PDA arrays still exhibit a constant color and intact shape after soaking in a series of organic agents. The excellent stability is due to the mussels-inspired adhesion of PDA, which can effectively improve the applicability of the colored pattern-encrypted paper. Moreover, PDA, SiO2 and paper all have good biocompatibility. Therefore, the encrypted paper composed of SiO2@PDA arrays can be used in high-safety fields such as medicine and food safety.



Figure 4 (a-c) Preparation diagram, schematic and photograph of encrypted SiO2@PDA structurally colored arrays. (d-f) Diagram, schematic and photograph of decryption of SiO2@PDA structurally colored arrays. (g) Photographs of SiO2@PDA amorphous arrays after wetting by water, ethanol, dimethyl formamide and toluene, respectively. In this study, we propose that SiO2@PDA amorphous structural colors can be used as a camouflage color while producing different shades of black when encountering solvent. Therefore, our technology can produce Chinese landscape paintings that are concealed under the intensely colored picture. Figure 5 shows the Chinese landscape paintings produced by SiO2@PDA amorphous arrays of uniform green color (Figure 5a) or multicolors (Figure 5b) before and after water wetting. The results indicate that whether it is a uniform color ora multi-color images produced by the SiO2@PDA amorphous arrays, the designed patterns could appear after the water immersion induced by the difference of the thickness .


Page 14 of 24

Page 15 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5 Photographs of two Chinese landscape paintings produced by SiO2@PDA amorphous arrays of uniform green color (a) or multi-colors (b) before and after water wetting. CONCLUSION

In conclusion, we report a novel anticounterfeiting strategy for noniridescent structural colors using SiO2@PDA amorphous arrays as security materials. The SiO2@PDA amorphous arrays exhibit uniform noniridescent structural colors regardless of their thicknesses as they are exposed to air. The invariant structural colors are verified to be induced by the high absorbance of PDA, which suppresses the multiple scattering to generate the predominant resonant colors when the thickness exceeds 9 m. Inspired by the Diphylleia grayi, these structurally colored arrays display the dark-brown color of PDA after wetting by water. More importantly, the dark-brown color strongly depends on the thickness of the SiO2@PDA arrays, enabling the development of differential brownish-black colors. To use the



SiO2@PDA amorphous arrays as a new type of encrypted ink, colored images with designed patterns are prepared on white papers. The SiO2@PDA amorphous arrays in the white papers also show the thickness-dependent brownish-black colors when they are exposed in a solvent; otherwise, they are muted noniridescent structural colors. Any designs of patterns can be encrypted by employing distinct thicknesses. Under air, the designs are not disclosed because all the arrays in the patterns are even colors. However, the designs can be selectively revealed by a different solvent. We believe these simple, user-interactive structurally colored arrays are appealing for various security-related applications. METHODS Materials. 3-Hydroxytyramine hydrochloride (dopamine hydrochloride) (DA·HCl), tris(hydroxymethyl) aminomethane (Tris) were purchased from Aladdin (Shanghai, China). Ethyl alcohol, dimethyl formamide and toluene were purchased from Sinopharm Chemical Reagent Company (Shanghai, China). SiO2 particles with different diameters were purchased from Nanjing Nanorainbow Biotechnolgoy Co., Ltd. (China). The white papers are sketch paper and were purchased from Tongxiang Yimei Paper Industry Co. (Hunan, China). Glass slide was purchased from Nanjing Wanqing Co. (Nanjing, China). A polypropylene (PP) film (PP2910, 100 μm thick, 210 mm × 297 mm, 3M) was obtained from Aice Instrument. Deionized water (18.0 MΩ cm, Milli-Q gradient system, Millipore) was used in all experiments. All chemical reagents were used without further purification. Synthesis of monodisperse SiO2@PDA nanoparticles. Silica particles (0.42g) were mixed with dopamine hydrochloride (0.05 g) in Tris-buffer (25 mL, 10 mM) for 24 h. The obtained SiO2@PDA particles were washed by repeating centrifugation with dialysis against deionized water and stored in water. Preparation of structural color arrays. Particle solution was prepared by dispersing SiO2@PDA in deionized water and then ultrasonic for 10 min to form a latex suspension containing 0.07 g/ml SiO2@PDA. Glass slides were washed by a 50 wt% alcohol solution in an ultrasonic bath for 15 min. Then these glass slides were taken


Page 16 of 24

Page 17 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


out and wiped by a dust-free paper. The structural colored arrays were fabricated by spraying the 0.07 g/ml SiO2@PDA aqua using an airbrush system with a 0.2-mm bore, at a pressure of 0.2 MPa. The working distance between the bore and substrate was about 5 cm. And the fabrication process of the samples was conducted at temperature of 45 ℃ and humidity of 19%. Preparation of structural color patterns. The structural color patterns were fabricated by spraying SiO2@PDA or SiO2 on the substrate via pattern masks. The pattern masks were designed using AutoCAD and fabricated using a laser cutter (4060, Ketai) with a Polypropylene film. SiO2@PDA amorphous arrays with a particular thickness were obtained by spraying a preset volume of solution from the cavity at the top of the spray gun. The solution in the cavity was premeasured using a pipette. The square SiO2@PDA structural colored arrays with thicknesses of 3, 5, 9, 16, 23, 31 and 40 m were fabricated by varying the volume with 5, 10, 20, 40, 60, 80 and 100 L of the dispersions containing 295  1.7 nm-sized SiO2@PDA particles at a concentration of 0.07 g/mL using a 1 cm  1 cm square pattern mask. The procedure of hidden patterns of SiO2@PDA structural colored arrays is as follows: first, a uniform colored array, of which the thickness exceeds 9 m, was fabricated on white paper; then, a hidden pattern with a certain thickness was sprayed through a predesigned patterned mask covered on these arrays. Characterization. The reflection and transmission spectrum were measured by a spectrometer (QE65000, Ocean Optics) with light source (DH-2000UV-VIS-NIR, Mikropack) and optical fiber (QR200-7-UV-BX, Ocean Optics). The absolute reflectance was measured using a diffuse reflection standard plate (WR-D97-30, Oceanhood) as completely diffuse reflector. The SiO2@PDA images were obtained by Transmission Electron Microscopy (TEM, JEM2100F). The image of the amorphous colloidal arrays was obtained by Field Emission Scanning Electron Microscope (FESEM, Zeiss Ultra Plus). The images of the cross-section of amorphous colloidal arrays were obtained by a scanning electron microscope (SEM, S-3000N, Hitachi). The absorptivity of SiO2@PDA and SiO2 amorphous arrays was compared at a uniform condition.



ASSOCIATED CONTENT Supporting Information Available: Diagram of synthesis of SiO2@PDA particles; TEM image of SiO2@PDA particles; Raman spectra of SiO2@PDA particles and SiO2 particles; Photograph of SiO2@PDA and SiO2 suspension; Diagram of fabrication of amorphous SiO2@PDA arrays; Photographs and reflection of arrays at various viewing angles fabricated by 295 nm SiO2@PDA; The insert image Diagram of reflection detection of SiO2@PDA arrays; SEM images of cross-section of SiO2@PDA arrays with 3 μm, 5 μm, 9 μm, 16 μm, 23 μm, 31 μm, 40 μm; The relationship between the thickness and volume of the dispersions containing 295 nm sized SiO2@PDA particles at a concentration of 0.07 g/ml; The results of the three tristimulus values and coordinate values of SiO2@PDA structural colors with different thickness calculated by a CIE co-ordinate calculator software; The absorption integral area of SiO2@PDA structurally colored arrays with different thicknesses in air; Photographs of the SiO2@PDA structurally colored arrays fabricated by SiO2 particles with thicknesses of 3 μm (black line), 5 μm (red line), 9 μm (blue line), 16 μm (pink line), 23 μm (green line), 31 μm (dark blue line), 40 μm (purple line); Reflection spectra of SiO2 arrays before water wetting; Reflection spectra of SiO2 arrays after water wetting; The reflection spectrum of SiO2@PDA arrays of 16 μm thickness before (pink line) and after (black line) water wetting; The absorption integral area of SiO2@PDA structurally colored arrays with different thicknesses in water; Photographs and absorption integral area of the SiO2 arrays with thicknesses of 3 μm, 5 μm, 9 μm, 16 μm, 23 μm, 31 μm, 40 μm after water-wetting; Comparison curves of the absorption integrated area of SiO2@PDA arrays (black line) and SiO2 arrays (grey line) of different thicknesses after immersion in water; Photographs of the structurally colored SiO2@PDA arrays fabricated by diameters of 217 nm, 232 nm, 263 nm and 295 nm in white paper. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Conflict of Interest The authors declare no competing financial interest. Author Contributions P.Liu and W.Chang carried out the experiments. P. Liu, Z. Xie and J. Yang designed the project and wrote the manuscript. L. Ju, L. Chu and J.Chen revised the manuscript. All authors discussed and interpreted results and commented on the manuscript. ACKNOWLEDGEMENTS


Page 18 of 24

Page 19 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


This work was supported by the research Fund of the National Natural Science Foundation (No. 21872026, 81801081), and the Project funded by China Postdoctoral Science Foundation (No. 2019M652588).



Reference 1. Parker, A. R. Colour in Burgess Shale Animals and The Effect of Light on Evolution in The Cambrian. Pro. Roy. Soc. B Biol. Sci 1998, 265, 967-972. 2. Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Single-Crystal Colloidal Multilayers of Controlled Thickness. Chem. Mater. 1999, 11, 2132-2140. 3. Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Monodispersed Colloidal Spheres: Old Materials with New Applications. Adv. Mater. 2000, 12, 693-713. 4. Arsenault, A. C.; Puzzo, D. P.; Manners, I.; Ozin, G. A. Photonic-crystal Full-colour Displays. Nat. Photonics. 2007, 1, 468-472. 5. Kim, H.; Ge, J.; Kim, J.; Choi, S. E.; Lee, H.; Park, W.; Yin, Y.; Kwon, S. Structural Colour Printing Using A Magnetically Tunable and Lithographically Fixable Photonic Crystal. Nat. Photonics 2009, 3, 534-540. 6. Bai, L.; Xie, Z.; Wang, W.; Yuan, C.; Zhao, Y.; Mu, Z.; Zhong, Q.; Gu, Z. Bio-Inspired Vapor-Responsive Colloidal Photonic Crystal Patterns by Inkjet Printing. ACS Nano 2014, 8, 11094-11100. 7. Fang, Y.; Ni, Y.; Leo, S.-Y.; Taylor, C.; Basile, V.; Jiang, P. Reconfigurable Photonic Crystals Enabled by Pressure-responsive Shape-memory Polymers. Nat. Commun. 2015, 6, 7416. 8. Heo, Y.; Kang, H.; Lee, J.-S.; Oh, Y.-K.; Kim, S.-H. Lithographically Encrypted Inverse Opals for Anti-Counterfeiting Applications. Small 2016, 12, 28, 3819-3826. 9. Xiao, M.; Li, Y.; Zhao, J.; Wang, Z.; Gao, M.; Gianneschi, N. C.; Dhinojwala, A.; Shawkey, M. D. Stimuli-Responsive Structurally Colored Films from Bioinspired Synthetic Melanin Nanoparticles. Chem. Mater. 2016, 28, 5516-5521. 10. Cho, S.; Shim, T. S.; Kim, J. H.; Kim, D.-H.; Kim, S.-H. Selective Coloration of Melanin Nanospheres through Resonant Mie Scattering. Adv. Mater. 2017, 1700256. 11. Zhong, K.; Li, J.; Liu, L.; Cleuvenbergen, S. V.; Song, K.; Clays, K. Instantaneous, Simple, and Reversible Revealing of Invisible Patterns Encrypted in Robust Hollow Sphere Colloidal Photonic Crystals. Adv. Mater. 2018, 30, 1707246. 12. Liu, P.; Bai, L.; Yang, J.; Gu, H; Zhong, Q; Xie, Z.; Gu, Z. Self-assembled Colloidal Arrays for Structural Color. Nanoscale Adv. 2019, 1, 1672-1685. 13. Park, I.-Y.; Ahn, S.; Kim, Y.; Bae, H.-S.; Serial Number Coding and Decoding by Laser Interference Direct Patterning on The Original Product Surface for Anti-counterfeiting. Opt. Express 2017, 25,14644-14653. 14. Aggarwal, A. K.; Kaura, S. K.; Chhachhia, D. P.; Sharma, A. K. Concealed Moiré Pattern Encoded Security Holograms Readable by A Key Hologram. Optics & Laser Technology 2006, 38, 117-121. 15. Hu, H.; Zhong, H.; Chen, C.; Chen, Q. Magnetically Responsive Photonic Watermarks on Banknotes. J. Mater. Chem. C 2014, 2, 3695-3702. 16. Nam, H.; Song, K.; Ha, D.; Kim, T. Inkjet Printing Based Mono-layered Photonic Crystal Patterning for Anti-counterfeiting Structural Colors. Sci. Rep. 2016, 6, 30885.


Page 20 of 24

Page 21 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17. Chen, J.; Liu, P.; Du, X.; Xie, Z.; Clickable Colloidal Photonic Crystals for Structural Color Pattern. Langmuir 2018, 34, 13219-13224. 18. Ding, H.; Cheng, Y.; Gu, H.; Zhao, Y.; Wang, B.; Gu, Z. Tunable Fiber Bragg Grating based on Responsive Photonic Crystals. Nanoscale 2013, 5, 11572-11576. 19. Liao, J.; Zhu, C.; Gao, B.; Zhao, Z.; Liu, X.; Tian, L.; Zeng, Y.; Zhou, X.; Xie, Z.; Gu, Z. Multiresponsive Elastic Colloidal Crystals for Reversible Structural Color Patterns. Adv. Funct. Mater. 2019, 1902954. 20. Wang, H.; Xu, Q.; Shang, L.; Wang, J.; Rong, F.; Gu, Z.; Zhao, Y. Boronate Affinity Molecularly Imprinted Inverse Opal Particles for Multiple Label-free Bioassays. Chem. Commun. 2016, 52, 3296-3299. 21. Leverant, C. J.; Leo, S-Y.; Cordoba, M. A.; Zhang, Y.; Charpota, N.; Taylor, C.; Jiang, P. Reconfigurable Anticounterfeiting Coatings Enabled by Macroporous Shape Memory Polymers. ACS Appl. Polym. Mater. 2019, 1, 36-46. 22. Meng, Y.; Qiu, J.; Wu, S.; Ju, B.; Zhang, S.; Tang, B. Biomimetic Structural Color Films with a Bilayer Inverse Heterostructure for Anticounterfeiting Applications. ACS Appl. Mater. Interfaces 2018, 10, 38459-38465. 23. Liu, B.; Wang, K.; Gao, B,; Lu, J.; Li, H.; Zhao, X. TiO2‑Coated Silica Photonic Crystal Capillaries for Plasmon-Free SERS Analysis. ACS Appl. Nano Mater. 2019, 2, 3177-3186. 24. Campbell, M.; Sharp, D. N.; Harrison, M. T.; Denning, R. G.; Turberfield, A. J. Fabrication of Photonic Crystals for The Visible Spectrum by Holographic Lithography. Nature 2000, 404, 53-56. 25. Parker, A. R.; Xia, Y. Macroporous Membranes with Highly Ordered and Three-Dimensionally Interconnected Spherical Pores. Adv. Mater. 1998, 10, 1045-1048. 26. Li, F.; Tang, B.; Wu S.; Zhang, S. Facile Synthesis of Monodispersed Polysulfide Spheres for Building Structural Colors with High Color Visibility and Broad Viewing Angle. Small 2017, 13, 1602565. 27. Gu, Z.-Z.; Fujishima, A.; Sato, O. Fabrication of High-Quality Opal Films with Controllable Thickness. Chem. Mater. 2002, 14, 760-765. 28. Takeoka, Y.; Honda, M.; Seki, T.; Ishii, M.; Nakamura, H. Structural Colored Liquid Membrane without Angle Dependence. ACS Appl. Mater. Interfaces 2009, 1, 982-986. 29. Takeoka, Y.; Yoshioka, S.; Takano, A.; Arai, S.; Nueangnoraj, K.; Nishihara, H.; Teshima, M.; Ohtsuka, Y.; Seki, T. Production of Colored Pigments with Amorphous Arrays of Black and White Colloidal Particles. Angew. Chem. Int. Ed. 2013, 52, 7261-7265. 30. Iwata, M.; Teshima, M.; Seki, T.; Yoshioka, S.; Takeoka, Y. Bio-Inspired Bright Structurally Colored Colloidal Amorphous Array Enhanced by Controlling Thickness and Black Background. Adv. Mater. 2017, 29, 1605050. 31. Liu, P.; Chen, J.; Zhang, Z.; Xie, Z.; Du, X.; Gu, Z. Bio-inspired Robust Non-iridescent Structural Color with Self-adhesive Amorphous Colloidal Particle Arrays. Nanoscale 2018, 10, 3673-3679. 32. Bai, L.; Mai, V. C.; Lim, Y.; Hou, S.; Möhwald, H.; Duan, H. Large-Scale



Noniridescent Structural Color Printing Enabled by Infiltration ‐ Driven Nonequilibrium Colloidal Assembly. Adv. Mater. 2018, 1705667. 33. Arsenault, A. C.; Puzzo, D. P.; Manners, I.; Ozin, G. A. Photonic-crystal Full-colour Displays. Nat. Photonics 2007, 1, 468-472. 34. Zhu, C.; Xu, W.; Chen, L.; Zhang, W.; Xu, H.; Gu, Z.-Z. Magnetochromatic Microcapsule Arrays for Displays. Adv. Funct. Mater. 2011, 21, 2043-2048. 35. Zhang, Y.; Dong, B.; Chen, A.; Liu, X.; Shi, L.; Zi, J. Using Cuttlefish Ink as an Additive to Produce Non-iridescent Structural Colors of High Color Visibility. Adv. Mater. 2015, 27, 4719-4724. 36. Zhang, Y.; Han, P.; Zhou, H.; Wu, N.; Wei, Y.; Yao, X.; Zhou, J.; Song, Y. Highly Brilliant Noniridescent Structural Colors Enabled by Graphene Nanosheets Containing Graphene Quantum Dots. Adv. Funct. Mater. 2018, 28, 1802585. 37. Yong, J.; Chen, F.; Yang, Q.; Du, G.; Shan, C.; Bian, H.; Farooq, U.; Hou, X. Bioinspired Transparent Underwater Superoleophobic and Anti-oil Surfaces. J. Mater. Chem. A 2015, 3, 9379-9384. 38. Cai, G.; Wang, J.; Eh, A. L.-S.; Chen, J.; Qian, K.; Xiong, J.; Thangavel, G.; Lee, P. S. Diphylleia grayi-Inspired Stretchable Hydrochromics with Large Optical Modulation in the Visible-Near-Infrared Region. ACS Appl. Mater. Interfaces 2018, 10, 37685-37693. 39. Bian, F.; Wu J.; Wang, H.; Sun, L.; Shao, C.; Wang, Y.; Li, Z.; Wang, X.; Zhao. Y.; Bioinspired Photonic Barcodes with Graphene Oxide Encapsulation for Multiplexed MicroRNA Quantification. Small 2018, 14, 1803551 40. Heeswijk, E. P. A. van.; Kloos, J. J. H.; Heer, J. de.; Hoeks, T.; Grossiord, N.; Schenning, A. P. H. J. Well-Adhering, Easily Producible Photonic Reflective Coatings for Plastic Substrates. ACS Appl. Mater. Interfaces 2018, 10, 29378-29384 41. Xiao, M.; Li, Y.; Zhao, J.; Wang, Z.; Gao, M.; Gianneschi, N. C.; Dhinojwala, A.; Shawkey, M. D. Stimuli-Responsive Structurally Colored Films from Bioinspired Synthetic Melanin Nanoparticles. Chem. Mater. 2016, 28, 15, 5516-5521. 42. Waite, J. H. Adhesion a` la Moule. Integr. Comp. Biol. 2002, 42, 1172-1180. 43. Höhme, C.; Filiz, V.; Abetz, C.; Georgopanos, P.; Scharnagl, N.; Abetz, V. Postfunctionalization of Nanoporous Block Copolymer Membranes via Click Reaction on Polydopamine for Liquid Phase Separation. ACS Appl. Nano Mater. 2018, 1, 3124-3136. 44. Mandal, K.; Jana, N. R. Galactose-Functionalized, Colloidal-Fluorescent Nanoparticle from Aggregation-Induced Emission Active Molecule via Polydopamine Coating for Cancer Cell Targeting. ACS Appl. Nano Mater. 2018, 1, 3531-3540. 45. Thakur, V. K.; Yan, J.; Lin, M. F.; Zhi, C. Y.; Golberg, D.; Bando, Y.; Sim, R.; Lee, P. S. Novel Polymer Nanocomposites from Bioinspired Green Aqueous Functionalization of BNNTs. Polym. Chem. 2012, 3, 962-969. 46. Yan, D.; Xu, P.; Xiang, Q.; Mou, H.; Xu, J.; Wen, W.; Li, X.; Zhang, Y. Polydopamine Nanotubes: Bio-inspired Synthesis, Formaldehyde Sensing Properties and Thermodynamic Investigation. J. Mater. Chem. A 2016, 4,


Page 22 of 24

Page 23 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3487-3493. 47. Y. Takeoka, Angle-independent Structural Coloured Amorphous Arrays. J. Mater. Chem. 2012, 22, 23299-23309. 48. Noh, H.; Liew, S. F.; Saranathan, V.; Mochrie, S. G. J.; Prum, R. O.; Dufresne, E. R.; Cao, H. How Noniridescent Colors Are Generated by Quasi-ordered Structures of Bird Feathers. Adv. Mater. 2010, 22, 2871-2880. 49. Charles, P. Digital Video and HD - Algorithms and Interfaces. 2012, 25, 275. 50. Takeoka, Y.; Yoshioka, S.; Takano, A.; Arai, S.; Nueangnoraj, K.; Nishihara, H.; Teshima, M.; Ohtsuka, Y.; Seki, T. Production of Colored Pigments with Amorphous Arrays of Black and White Colloidal Particles. Angew. Chem., Int. Ed. 2013, 52, 7261-7265. 51. d’Ischia, M.; Wakamatsu, K.; Napolitano, A.; Briganti, S.; Garcia-Borron, J.; Kovacs, D.; Meredith, P.; Pezzella, A.; Picardo, M.; Sarna, T.; Simon, J. D.; Ito, S. Melanins and Melanogenesis: Methods, Standards, Protocols. Pigm. Cell Melanoma Res. 2013, 26, 616-633.



Table of Contents


Page 24 of 24

Bioinspired Noniridescent Structural Color with Hidden

Recommend Documents