Bio-inspired robust non-iridescent structural color with self-adhesive amorphous colloidal particle arrays

Abstract

Here we propose a new method for constructing highly color fast non-iridescent structural color materials by assembling self-adhesive poly-dopamine coated SiO₂ nanoparticles (PDA@SiO₂) for amorphous colloidal arrays through a “spraying” process. Simply by alkaline vapor treatment, the adhesive forces and fastness of the amorphous colloidal arrays were significantly improved. This was demonstrated by lap shear tests of tape tearing and cohesive failure as well as a series of fastness tests like sandpaper abrasion, finger wiping and ultrasonic cleaning. Besides, the strengthening fastness reaction could occur on different substrates, including glass, metals, polymers and paper, regardless of their chemical and physical properties. Moreover, the structural color of the PDA@SiO₂ arrays was bright due to the broadband absorption of PDA, and was tunable according to the size, PDA content and arrangement of the PDA@SiO₂ arrays.

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Non-iridescent structural colored materials, in which the color arises from coherent-light scattering of amorphous colloidal nanoparticle arrays, have garnered ever-increasing scientific interest during the last decade.^1–3 The amorphous structure of colloidal arrays endows the structural materials with angle-independent color like dyes or pigments, while it avoids the problem of chemical/photo bleaching which widely exists in dye/pigment coatings.⁴ The unique properties of the structural color materials can lead to various applications including building textiles, display boards, print media, colorimetric sensors, and optical devices.^5–9 However, the amorphous structure gives rise to a strong incoherent-light scattering across the entire visible spectrum, which causes color whitening problems in human color vision. To address this challenge, black materials^10–13 such as carbon nanoparticles/nanotubes and cuttlefish ink have been used to improve the visibility of non-iridescent structural color by doping. However, these materials do not contribute to the fastness of structural color arrays, which is a crucial factor for practical applications. Here we report the construction of highly color fast amorphous colloidal arrays by self-adhesive building blocks, poly-dopamine coated SiO₂ nanoparticles (PDA@SiO₂). Poly-dopamine (PDA) is a common synthetic melanin produced by the autoxidation of dopamine (DA). Owing to its catechol units, PDA can strongly adhere to most substrates, and this has been applied for the modification of various substrates, and the preparation of functional materials by simple chemistry.¹⁴ Besides, PDA can suppress the incoherent-light scattering of amorphous colloidal arrays due to its broad absorption of light.¹⁵ For example, Xiao et al.¹⁶ and Kohri et al.^17,18 respectively utilized PDA and PDA@PS (polystyrene) nanoparticles to fabricate bright and high-visibility non-iridescent structural colors. However, all of these works only focused on improving color visibility and did not develop adhesion properties for structural color materials. In this study, we propose a robust non-iridescent structural color prepared by spraying PDA@SiO₂ amorphous colloidal arrays and subsequently treating them with alkaline vapor. The adhesive forces both between particle and particle (P–P) and particle and substrate (P–S) were significantly improved, as demonstrated by lap shear tests of tape tearing and cohesive failure. The tests, including sandpaper abrasion, finger wiping and ultrasonic cleaning, further demonstrated the high color fastness of the PDA@SiO₂ amorphous colloidal arrays. Besides, the PDA@SiO₂ had higher photo/thermal stability than PDA@polymer and could be mass produced with facile adjustability of light absorption and particle size by changing the thickness of the PDA coating and silica core diameter, respectively. These features of PDA@SiO₂ amorphous colloidal arrays promote more extensive practical applications for multiple non-iridescent structural color patterns.

The building blocks, monodispersed PDA@SiO₂, were synthesized by polymerizing DA onto spherical silica nanoparticles in 1 mM (pH 7–7.5) Tris-buffer (Fig. 1a). Fig. 1b shows the transmission electron microscopy (TEM) image of the PDA@SiO₂ particles with an average diameter of 295 nm. The strong bands at 1355 and 1579 cm⁻¹ in the Raman spectra indicate that the PDA was coated on the SiO₂ nanoparticles (Fig. S1b†). The thickness of the PDA shell could be adjusted by the initial concentration of DA. As shown in Fig. S1c,† the thickness of the PDA shell increased with the concentration of DA, which resulted in the increase of the PDA@SiO₂ diameter (Fig. S1d†). The corresponding color of the PDA@SiO₂ suspension gradually became dark with the increment of PDA shell thickness (Fig. S1e†). Non-iridescent structural color was fabricated by spraying an aqueous suspension of monodispersed PDA@SiO₂ onto the substrate using a spraying–drying method (Fig. 1c and Movie S1†).^19,20Fig. 1d shows bright green films with a thickness of 16 μm on glass, fabricated from monodispersed PDA@SiO₂ particles with average diameters of 232 nm (Fig. S1a†). The color in ambient light was invariable when changing viewing angle. The scanning electron microscopy (SEM) images and two-dimensional Fourier analyses showed that the film was formed by amorphous PDA@SiO₂ arrays without long-range order (Fig. S2e†). The bright non-iridescent structural color arose from the scattering of coherent-light and absorption of scattered incoherent-light by the amorphous PDA@SiO₂ arrays. In the reflection spectra, a broad reflection peak emerged at 520 nm and its position had a low-dependence on view angle (Fig. S2a and b†). Furthermore, the PDA coated on the SiO₂ nanoparticles absorbed incoherent-light scattering and improved the visibility of the non-iridescent structural color (Fig. S3†). As shown in Fig. 1e and f, the amorphous PDA@SiO₂ arrays exhibited 80–93% absorptivity, while amorphous SiO₂ arrays only had an absorptivity of 30–60%.

Fig. 1 (a) Schematic of the synthesis of PDA@SiO₂ and (b) TEM image of PDA@SiO₂ particles of 295 nm size (comprising 288 nm silica cores and 7 nm PDA shells). (c) Schematic of the fabrication of amorphous PDA@SiO₂ arrays. (d) Array images of 232 nm PDA@SiO₂ particles at various viewing angles. TEM transmission, absorption, and reflection of (e) PDA@SiO₂ arrays and (f) SiO₂ arrays with 16 μm thickness detected at normal incidence.

The self-adhesion of the amorphous colloidal nanoparticle arrays was inspired by mussels. Mussels attach to the surface of substrates by the byssus which is distally tipped by a flared adhesive plaque.^21,22 The plaque is assembled from proteins stockpiled in the foot of the mussel. The adhesive protein subtypes of Mytilus edulis foot protein, Mefp-3 and Mefp-5 are firstly squirted onto the surface of the substrate by the foot and localized near the interface between the adhesive pad and the substrate. Mefp-3 and Mefp-5 have the highest 3,4-dihydroxyphenylalanine (DOPA) contents of 20 and 30 mol% respectively,^23,24 which are believed to contribute to adhesion (Fig. 2a).²⁵ Because PDA has similar catechol units to DOPA, PDA has been confirmed to achieve strong adhesion with most substrates. Here we reinforced the PDA@SiO₂ arrays and the adhesion to substrates via ammonia vapor treatment because the PDA shells are further oxidized and cross-linked in an alkaline atmosphere (Fig. 2b). To confirm the high color fastness of the arrays, we investigated the damage resistance performance of PDA@SiO₂ arrays treated with ammonia vapor (PDA@SiO₂-AM) as well as that of bare SiO₂ arrays and untreated PDA@SiO₂ arrays. Fig. 2c shows the samples on glass substrates before and after fastness tests, including sandpaper abrasion (Fig. S4†), finger wiping and ultrasonic cleaning. After 10 cycles of abrasion by sandpaper, the retained area was zero for SiO₂ arrays, 45% for PDA@SiO₂ arrays and 100% for PDA@SiO₂-AM arrays. Fig. S5† shows the SEM images of these samples after abrasion. Only some traces along the abrasion direction were observed in the PDA@SiO₂-AM arrays, but a little peeling and a lot of peeling were presented respectively in PDA@SiO₂ arrays and SiO₂ arrays. Besides, the color of the PDA@SiO₂-AM arrays was unchanged after sandpaper abrasion, which was also revealed by the reflection spectra in Fig. S5.† Furthermore, during the finger-wipe test, 200 wipes removed SiO₂ arrays completely, while it removed only 20% of the PDA@SiO₂ arrays and almost none of the PDA@SiO₂-AM arrays. During the ultrasonic cleaning test, which was conducted with a power of 90 W at a distance of 2 cm from the bottom of the ultrasonic instrument, the retained area was 10% for SiO₂ arrays, 35% for PDA@SiO₂ arrays and 65% for PDA@SiO₂-AM arrays after 30 minutes. The curves of retained area for these tests are shown in Fig. 2d. The shedding speed of the SiO₂ arrays was always faster than that of the PDA@SiO₂ arrays, which was faster than that of the PDA@SiO₂-AM arrays for all fastness tests. As a result, compared to the initial SiO₂ arrays, the PDA@SiO₂ arrays exhibited increased resistance to abrasion (45%), wiping (20%) and ultrasonic treatment (25%). However the PDA@SiO₂-AM arrays presented remarkably improved resistance to abrasion (100%), wiping (100%) and ultrasonic treatment (55%). To further investigate the adhesion energy of the PDA@SiO₂-AM arrays, the tensile stress derived from tape tearing was examined by lap shear testing, as shown in Fig. 2e. The measured lap shear force of the SiO₂ arrays was close to 0.001 N (line a), while that for PDA@SiO₂ arrays was 0.32 N (line b). The residue of a thin layer of particles on the tape shown in Fig. S6† revealed that the lap shear forces by tape tearing for SiO₂ arrays and PDA@SiO₂ arrays were primarily attributed to the P–P adhesion. However, the PDA@SiO₂-AM arrays were almost completely peeled off from the substrates and stuck on the tape during the tape tearing process because of the high P–P adhesion. Thus the stable tension values of around 0.55 N for the PDA@SiO₂-AM sample (line c) were mostly attributed to P–S forces. To measure the P–P force of PDA@SiO₂-AM, we stuck the PDA@SiO₂-AM arrays on the surface of tape. As expected, the arrays torn from the P–P interface presented an average of 0.88 N tension (line d), close to threefold that of the PDA@SiO₂ sample. Further, the lap shear test of vertical uniaxial tension was used to measure cohesive failure tension using a 1 cm × 1 cm area tape cured onto the arrays, as illustrated in Fig. 2f. The displacement-force curves displayed that the PDA@SiO₂ samples suffered cohesive failure at a tension of 7.2 N between the particles and tape surface, which was double that of the SiO₂ samples (3.97 N). The samples of PDA@SiO₂-AM fractured between the substrates and tape surface after 13 N tension but the cohesion between the arrays and tape surface was still tight (Fig. S7†). This indicated that the cohesive failure tension of PDA@SiO₂-AM between particles and particles exceeded 13 N, which was approximately fourfold higher than that for the SiO₂ samples. Therefore, the mechanical strength and adhesion were highly improved by the PDA shells and the binding was further reinforced by alkaline treatment.²⁶

Fig. 2 (a) Photograph of a mussel attached to a stone surface and the illustrated mechanism of mussel adhesion.²² (b) Schematic of PDA@SiO₂ arrays treated by ammonia vapor. (c) Photograph of SiO₂, PDA@SiO₂ and PDA@SiO₂-AM samples on glass substrates before and after robustness tests of sandpaper abrasion, finger-wiping and ultrasonic tape-exposure. (d) Retained area ratios during the tests of sandpaper abrasion, finger-wiping and ultrasonic tape-exposure for SiO₂ (square), PDA@SiO₂ (circle) and PDA@SiO₂-AM (triangle) arrays on glass substrates. (e) Schematic of the lap shear test by tape tearing and the obtained displacement-force curves for SiO₂ arrays (P–P force; line a), PDA@SiO₂ arrays (P–P force; line b), PDA@SiO₂-AM arrays (P–S force; line c) and PDA@SiO₂-AM arrays (P–P force; line d). (f) Schematic of the lap shear test of tensile cohesive failure and the obtained displacement-force curves for SiO₂ arrays, PDA@SiO₂ arrays and PDA@SiO₂-AM arrays. The dashed areas show the cohesive failure positions on the curves.

In order to investigate the adhesion mechanism after ammonia vapor treatment on a molecular scale, attenuated total reflection-infrared (ATR-IR) spectroscopy and Raman spectroscopy were conducted for the SiO₂ arrays, PDA@SiO₂ arrays and PDA@SiO₂-AM arrays. Fig. 3a reveals that the absorption of aromatic ring vibration (νC=C) peaks from the PDA structure are located at 1541 cm⁻¹ in the PDA@SiO₂ spectrum, while at 1458 cm⁻¹ for the PDA@SiO₂-AM arrays.²⁷ The significant displacement of this νC=C peak to lower wavenumber for the PDA@SiO₂ arrays can probably be attributed to the crosslinking of aromatic rings after alkaline treatment, which produces a conjugate effect and decreases the absorption energy and absorption frequency (Fig. 3c). The Raman spectra further confirmed that the PDA was distributed in the PDA@SiO₂ and PDA@SiO₂-AM arrays from the PDA characteristic bands appearing in the range of 1379 and 1539 cm⁻¹, as shown in Fig. S9.†²⁸ Among them, the aromatic ring characteristic bands of PDA@SiO₂ also exhibited a lower wavenumber shift from 1410 cm⁻¹ to 1379 cm⁻¹ after ammonia vapor treatment because of the conjugate effect (Fig. 3b). Moreover, the decreased broad absorbance of νOH in the spectral region 3600–2800 cm⁻¹ together with the increased H-bond absorbance peak at 2980 cm⁻¹ (Fig. 3d) demonstrated that more H-bonds composed from surface bound H-bonds and intramolecular H-bonds with phenolic hydroxyl and phenol carbonyl oxygens were generated for the PDA@SiO₂ arrays after alkaline treatment (Fig. 3e).²⁹ Besides, due to the involvement of PDA, the intensity of the SiO₂ characteristic peaks gradually decreased from SiO₂ arrays to PDA@SiO₂ arrays and PDA@SiO₂-AM arrays (Fig. S8†). Hence, the decrement from the PDA@SiO₂ arrays to the PDA@SiO₂-AM arrays also provided evidence for the greater self-polymerization degree of the PDA@SiO₂ arrays after ammonia vapor treatment. These results demonstrated that the ammonia vapor provided a humid alkaline environment which accelerated the oxidative self-polymerization among PDA shells and promoted more covalent and non-covalent bonding.^30,31 On the other hand, it also contributed to the binding between PDA@SiO₂ particles and substrates from the formation of more hydrogen bonds between the OH groups of the catechol and the O atoms in the glass.³²

Fig. 3 (a) ATR-IR spectrum of PDA@SiO₂-AM (line a), PDA@SiO₂ (line b) and SiO₂ (line c) arrays in 1700–1300 cm⁻¹ region. The aromatic ring (νC=C) of PDA@SiO₂-AM located at 1458 cm⁻¹ and PDA@SiO₂ arrays for 1541 cm⁻¹. (b) Raman spectra of PDA@SiO₂-AM (line a) and PDA@SiO₂ (line b) arrays range from 1335 cm⁻¹ to 1470 cm⁻¹. The characterization spectra of aromatic ring (νC=C) for PDA@SiO₂-AM is 1379 cm⁻¹ and for PDA@SiO₂ arrays is 1410 cm⁻¹. (c) Schematic of cross-linking for PDA@SiO₂ after treatment of ammonium vapor and the red line is the generated covalent bond among PDA moleculars. (d) ATR-IR spectrum of PDA@SiO₂-AM (line a), PDA@SiO₂ (line b) and SiO₂ (line c) arrays in 3600–2800 cm⁻¹ region and hydrogen bond located at 2980 cm⁻¹. (e) Schematic of H-bond generation for PDA@SiO₂ after treatment of ammonium vapor and the red dashed line is the generated H-bond between the PDA moleculars or PDA moleculars-substrates.

To better understand the performance of the self-adhered non-iridescent structural color material, we investigated the color fastness of the PDA@SiO₂-AM arrays prepared on various substrates, including plastic, metal and paper. As shown in Movies S4–S7,† the fastness of the colloidal nanoparticle arrays on poly(methylmethacrylate) (PMMA), aluminum (Al) foil and paper, respectively, were examined by sandpaper abrasion. As expected, the PDA@SiO₂-AM arrays exhibited remarkably increased fastness on these substrates, as presented in Fig. 4a. The changing curves of retained area ratio on the PMMA substrate showed that for the SiO₂ arrays and PDA@SiO₂ arrays this decreased to 44% and 80% original area, respectively, at a constant speed, while the PDA@SiO₂-AM arrays remained constant during the whole abrasion process (Fig. S10a†). For the substrate of Al foil, the SiO₂ arrays were removed completely after only five abrasion cycles and the PDA@SiO₂ arrays and PDA@SiO₂-AM arrays were reduced to 26% and 70% after 10 abrasion cycles, respectively (Fig. S10b†). However, the situation was different on paper substrates, on which the nanoparticle arrays were protected by their porous structures, resulting in all samples on paper dropping off only a small amount after 10 cycles of sandpaper abrasion (Fig. S10c†). Furthermore, finger wipe tests showed that the PDA@SiO₂-AM patterns on these substrates did not display any damage after 200 wipes (Fig. S12 and Movie S2†). These results indicated that the PDA@SiO₂-AM arrays showed a remarkable resistance to abrasion on various substrates due to the self-adhesive property of PDA@SiO₂. PDA@SiO₂ relied on different binding mechanisms to adhere to these substrates. On inorganic surfaces, PDA@SiO₂ formed coordination bonds, whereas on organic surfaces, oxidized PDA@SiO₂ was capable of adhering via covalent bond formation.^33,34 Thus bonds between PDA@SiO₂ and organic substrates were stronger than those of PDA@SiO₂-inorganic substrates. As demonstrated in Fig. 2d, Fig. S10a and S10b,† the shedding speeds of PDA@SiO₂ arrays on glass and Al substrates were faster than those on the PMMA substrate. Furthermore, on inorganic surfaces PDA@SiO₂ arrays formed H-bonding and/or multivalent chelating interactions, which strongly depend on the surface wettability.³³ Therefore Al foil (with a water contact angle of 96.5 ± 0.2°, Fig. S11b†) presented a faster shedding speed than glass substrates (with a water contact angle of 36.3 ± 0.1°, Fig. S11a†) for the PDA@SiO₂-AM arrays, as shown in Fig. 2d and Fig. S10b.† Due to the nanoparticle protective effect of a rough porous structure,³⁵ PDA@SiO₂-AM arrays on wettable porous paper presented inconspicuous shedding after 10 cycles of abrasion.

Fig. 4 (a) SiO₂, PDA@SiO₂ and PDA@SiO₂-AM arrays after 10 cycles treatment with sandpaper abrasion on PMMA, Al foil and paper substrates, respectively. The dashed areas show the original areas of the samples. (b) PDA@SiO₂ arrays on glass treated for different times with 20 wt% ammonia vapor. (c) PDA@SiO₂ arrays with particle sizes of 172, 193, 204, 215, 232 and 252 nm (top line from left to right) and 264, 273, 286, 291, 309 and 323 nm (bottom line from left to right). (d) PDA@SiO₂ arrays with original DA concentrations of 0, 1, 5, 10, 20 mM (formed with 275 nm silica core particles and 2 nm, 7 nm, 12 nm, 20 nm coatings). (e) The multi-color patterns of PDA@SiO₂-AM arrays on glass, PMMA, Al foil and paper.

Since the alkaline treatment caused further polymerization of PDA, its influence on color was investigated. As shown in Fig. 4b, the appearance of the PDA@SiO₂ pattern as well as the color analysis (Fig. S13†) was unchanged with the increase of ammonia vapor treatment time. Since the color multiplicity is necessary for structural color patterns, multiple structural colors were achieved by changing the size of the silica core, PDA content and arrangement of PDA@SiO₂. Firstly, the non-iridescent structural colors with hue ranging from blue to purple-red were obtained by varying the size of PDA@SiO₂, as shown in Fig. 4c. The hue of such structural color was consistent with the dominant reflection wavelength in the reflection spectrum, which linearly increased with the diameter of PDA@SiO₂ (Fig. S14a and b†). Secondly, the luminance of the color was tuned by the content of PDA in PDA@SiO₂ (Fig. 4d). With the increase of PDA content, the film sprayed from 295 nm PDA@SiO₂ particles gradually revealed a red color from coherent-light scattering accompanied with the decrease of reflectance in the whole visible region (Fig. S15a and b†). Since the content of PDA was related to the DA concentration during the growth of the PDA shell on SiO₂, the luminance declined with the increase of DA concentration (Fig. S15c†). Finally, the saturation of color was slightly tuned by the arrangement of PDA@SiO₂. A higher concentration of colloid suspension tended to form a more ordered arrangement, which improved the saturation (Fig. S16†). However the thickness of the sprayed coating was negligible to the non-iridescent structural color when it appeared a certain color (Fig. S17†). Fig. 4e exhibits multi-color PDA@SiO₂-AM patterns of the Chinese-style painting, Peking Opera, on PMMA, glass, paper and Al foil. The vivid color patterns illustrate that the high fastness and bright non-iridescent structural color presents a good alternative for future color painting nanomaterials.

Conclusions

In conclusion, we prepared a robust non-iridescent structural color by spraying PDA@SiO₂ amorphous colloidal arrays. By simple treatment with alkaline vapor, the colloidal arrays presented remarkably higher fastness than bare SiO₂ and PDA@SiO₂ arrays without treatment, as demonstrated by lap shear tests of tape tearing and cohesive failure, and fastness tests of sandpaper abrasion, finger wiping and ultrasonic cleaning. Besides, the strengthening fastness reaction for the PDA@SiO₂ arrays was compatible with various substrates, including glass, plastic, metal and paper. Moreover, the structural color of the PDA@SiO₂ arrays was bright due to the broadband absorption of PDA, and was tunable according to the size, PDA content and arrangement of PDA@SiO₂. This study provided a facile and universal approach for constructing fast non-iridescent structural color and has practical applications in optical display, painting and cosmetics. Due to the good biocompatibility of both SiO₂ and PDA, this kind of structural color also has potential applications for biomedical materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the research Fund of the National Key Research and Development Program of China (No. 2017YFA0205700), the National Natural Science Foundation (No. 21327902, 21635001, 51628102), the Natural Science Foundation of Jiangsu Province (Grant BK20150024), and the Fundamental Research Funds for the Central Universities.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nr08056e

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