Additive Mixing and Conformal Coating of Noniridescent Structural Colors with Robust Mechanical Properties Fabricated by Atomization Deposition

Interactions of Melanin with Electromagnetic Radiation: From Fundamentals to Applications

Melanin, especially integumentary melanin, interacts in numerous ways with electromagnetic radiation, leading to a set of critical functions, including radiation protection, UV-protection, pigmentary and structural color productions, and thermoregulation. By harnessing these functions, melanin and melanin-like materials can be widely applied to diverse applications with extraordinary performance. Here we provide a unified overview of the melanin family (all melanin and melanin-like materials) and their interactions with the complete electromagnetic radiation spectrum (X-ray, Gamma-ray, UV, visible, near-infrared), which until now has been absent from the literature and is needed to establish a solid fundamental base to facilitate their future investigation and development. We begin by discussing the chemistries and morphologies of both natural and artificial melanin, then the fundamentals of melanin-radiation interactions, and finally the exciting new developments in high-performance melanin-based functional materials that exploit these interactions. This Review provides both a comprehensive overview and a discussion of future perspectives for each subfield of melanin that will help direct the future development of melanin from both fundamental and applied perspectives.

Facile Access to High Solid Content Monodispersed Microspheres via Dual-Component Surfactants Regulation toward High-Performance Colloidal Photonic Crystals

Monodispersed microspheres play a major role in optical science and engineering, providing ideal building blocks for structural color materials. However, the method toward high solid content (HSC) monodispersed microspheres has remained a key hurdle. Herein, a facile access to harvest monodispersed microspheres based on the emulsion polymerization mechanism is demonstrated, where anionic and nonionic surfactants are employed to achieve the electrostatic and steric dual-stabilization balance in a synergistic manner. Monodispersed poly(styrene-butyl acrylate-methacrylic acid) colloidal latex with 55 wt% HSC is achieved, which shows an enhanced self-assembly efficiency of 280% compared with the low solid content (10 wt%) latex. In addition, Ag-coated colloidal photonic crystal (Ag@CPC) coating with near-zero refractive index is achieved, presenting the characteristics of metamaterials. And an 11-fold photoluminescence emission enhancement of CdSe@ZnS quantum dots is realized by the Ag@CPC metamaterial coating. Taking advantage of high assembly efficiency, easily large-scale film-forming of the 55 wt% HSC microspheres latex, robust Ag@CPC metamaterial coatings could be easily produced for passive cooling. The coating demonstrates excellent thermal insulation performance with theoretical cooling power of 30.4 W m-2, providing practical significance for scalable CPC architecture coatings in passive cooling.

Angle-Independent Color Change in Thermoresponsive Gel-Immobilized Colloidal Amorphous Film Attached to PET Substrate

Gel-immobilized colloidal amorphous structures comprise short-range-ordered monodisperse submicrometer particles embedded into a soft polymer gel. They exhibit an angle-independent structural color that is tunable in response to external stimuli via a volume change in the gel, which has significant potential for the development of sensors that respond to stimuli via angle-independent color changes. In this study, the amorphous structure of a charged colloidal suspension in water was immobilized in a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) gel film and simultaneously attached to a polyethylene terephthalate (PET) substrate. The gel film exhibited a uniform angle-independent color that changed in response to changes in temperature (i.e., thermosensitivity). Attachment to the PET substrate suppressed changes in the gel film area and film distortion, despite significant volume changes in the gel. Consequently, the degree of thermosensitivity was enhanced. The PET-attached gel-immobilized colloidal amorphous film was easy to handle and had excellent flexibility, allowing it to wrap around the surfaces of curved objects. These features are advantageous for sensor applications.

Non-iridescent Structurally Colored Pigments Based on CB@SiO2@TiO2 Core-Bishell Nanospheres with Enhanced Color Stability and Excellent Photocatalytic Activity

In recent years, artificial amorphous photonic structure (APS) materials with high color saturation and angle independence have been competitively reported. However, there is a lack of research into their functionalization and application in practical environments. Here, with practical applications in mind, we prepared APS pigments based on CB@SiO2@TiO2 core-bishell nanospheres and demonstrated high color saturation, enhanced color stability, and excellent photocatalytic activity. SiO2 effectively protected the carbon black particles from ablation during the calcination process. Paints composed of ethanol, ethyl cellulose (EC), and pigments could be spray-coated on any substrate to prepare a structurally colored coating without limitation. The coatings show good mechanical stability and photothermal stability. The color of the structurally colored pigments can be easily changed by adjusting the sizes of the CB@SiO2@TiO2 nanospheres. The photocatalytic activity of the pigments on formaldehyde (HCHO) and methylene blue (MB) solution and reaction kinetics of their degradation were studied by experiment. The results showed that the photocatalytic activity of the pigments increased with the increase of the TiO2 loading, and the degradation rate of HCHO reached 96.7% for 3 h and that of MB reached 97.9% for 60 min when the TiO2 shell thickness was 40 nm. The structurally colored pigments based on CB@SiO2@TiO2 nanospheres effectively solve the environmental problems caused by the application of pigments and have a promising future in the fields of color decoration, display, and painting.

Hierarchical Assembly of Patternable Chiroptical Biotextiles with Extreme Environment Stability

Flexible photonic textiles constructed by sustainable cholesteric organization are very promising to achieve a combination of chiroptical structural colors, mechanical robustness, sustainability, and environment stability. However, the efficient assembly of well-ordered cholesteric nanoarchitectures on flexible textiles in a scalable and patternable manner remains a grand challenge. In this study, we develop an efficient and scalable approach to construct large area chiroptical biotextiles using renewable and bioenabled cellulose nanocrystals (CNCs) as building blocks. This hierarchical assembly enables cholesteric photonic CNCs "cast" in situ, in a seamlessly tessellated design, onto topography-tailored textiles to form a strong interlocked multilayered structure. The resulting hierarchical architecture not only comprises strong photonic-photonic coupling to synergistically enhance the chiroptical properties with tunable wavelengths but also leads to impressive mechanical and optical stability against external mechanical forces and extreme environments. More importantly, through regulating the localized photonic band of the preformed chiroptical textiles by small molecules (e.g., water and glucose), customized colored patterns can be easily generated in large scale that are highly responsive to multistimuli, including chiral polarized light, view angle, and solvent. This chiroptical biotextile is a promising next-generation biomimetic photonic material for defense, aviation, and marine and aerospace special applications.

A Rule for Response Sensitivity of Structural-Color Photonic Colloids

To mimic natural photonic crystals having color regulation capacities dynamically responsive to the surrounding environment, periodic assembly structures have been widely constructed with response materials. Beyond monocomponent materials with stimulus responses, binary and multiphase systems generally offer extended color space and complex functionality. Constructing a rule for predicting response sensitivity can provide great benefits for the tailored design of intelligently responsive photonic materials. Here, we elucidate mathematical relationships between the response sensitivity of dynamically structural-color changes and the location distances of photonic co-phases in three-dimensional Hansen space that can empirically express the strength of their interaction forces, including dispersion force, polarity force, and hydrogen bonding. Such an empirical rule is proven to be applicable for some typical alcohols, acetone, and acetic acid regardless of their molecular structures, as verified by angle resolution spectroscopy, in situ infrared spectroscopy, and molecular simulation. The theoretical method we demonstrate provides rational access to custom-designed responsive structural coloration.

Construction of Steady Amorphous Colloidal Array Patterns via Infiltration-Driven Assembly of Core-Shell Microparticles followed by Short-Time Heating

Although core-shell microparticles with a hard core and soft shell are often used to fabricate photonic crystal films, they are rarely applied to construct steady amorphous colloidal array (ACA) patterns. In this work, a series of monodisperse core-shell microparticles with a polystyrene (PS) core and poly(methyl methacrylate-butyl acrylate) (P(MMA-BA)) shell have been successfully synthesized, and the glass transition temperatures (Tg) of the shell layer have been well regulated. The synthesized core-shell microparticles were then used to fabricate ACA patterns via a convenient infiltration-driven assembly method. The results showed that the Tg of the shell significantly affected the microstructure of the amorphous colloidal arrays (ACAs). During the assembly process, the microparticles quickly contacted each other and the lower-Tg shells could merge with each other to form a continuous film. In this situation, the PS core was embedded and ranked in the P(MMA-BA) film, and both the refractive index contrast and order degree of the colloidal array became relatively low, resulting in a poor structural color. However, when the Tg of the shell layer was moderately high, a short-range ordered array was prepared via infiltration-driven assembly, thereby displaying a bright structural color. More importantly, the shell layers could merge with each other to some extent after short-time heating, resulting in fine mechanical stability. In brief, this study provides a facile and environmental approach to construct steady ACA patterns, which is promising in printing and painting industries.

Stimulus-responsive nonclose-packed photonic crystals: fabrications and applications

Stimulus-responsive photonic crystals (PCs) possessing unconventional nonclosely packed structures have received growing attention due to their unique capability of mimicking the active structural colors of natural organisms (for example, chameleons' mechanochromic properties). However, there is rarely any systematic review regarding the progress of nonclose-packed photonic crystals (NPCs), involving their fabrication, working mechanisms, and applications. Herein, a comprehensive review of the fundamental principles and practical fabrication strategies of one/two/three-dimensional NPCs is summarized from the perspective of designing nonclose-packed structures. Subsequently, responsive NPCs with exciting functions and working mechanisms are sorted and delineated according to their diverse responses to physical (force, temperature, magnetic, and electric fields), chemical (ions, pH, vapors, and solvents), and biological (glucose, organophosphate, creatinine, and bacteria) stimuli. We then systematically introduced and discussed the applications of NPCs in sensors, printing, anticounterfeiting, display, optical devices, etc. Finally, the current challenges and development prospects for NPCs are presented. This review not only concludes the design principle for NPCs but also provides a significant basis for the exploration of next-generation NPCs.

Versatile Double Bandgap Photonic Crystals of High Color Saturation

Double bandgap photonic crystals (PCs) exhibit significant potential for applications in various color display-related fields. However, they show low color saturation and inadequate color modulation capabilities. This study presents a viable approach to the fabrication of double bandgap photonic inks diffracting typical secondary colors and other composite colors by simply mixing two photonic nanochains (PNCs) of different primary colors as pigments in an appropriate percentage following the conventional RGB color matching method. In this approach, the PNCs are magnetically responsive and display three primary colors that can be synthesized by combining hydrogen bond-guided and magnetic field (H)-assisted template polymerization. The as-prepared double bandgap photonic inks present high color saturation due to the fixed and narrow full-width at half-maxima of the parent PNCs with a suitable chain length. Furthermore, they can be used to easily produce a flexible double bandgap PC film by embedding the PNCs into a gel, such as polyacrylamide, facilitating fast steady display performance without the requirement of an external magnetic field. This research not only presents the unique advantages of PNCs in constructing multi-bandgap PCs but also establishes the feasibility of utilizing PNCs in practical applications within the fields of anti-counterfeiting and flexible wearable devices.

Construction of cellulose structural-color pigments with tunable colors and iridescence/non-iridescence

Structural colorations have been recognized as a significant way to replace conventional organic dyes for paints, inks, packaging, and cosmetics because of brilliant colors, high stability, and eco-friendliness. However, most current structural-color pigments present an iridescent appearance, and it remains difficult to mitigate a trade-off between lowering the iridescence effect and maintaining the color saturation and brightness. Here, we demonstrate a universal yet economical approach to prepare cellulose structural-color pigments with different sizes. A combined ultrasonication and grinding treatment is explored to adjust the pigment colors as well as control the iridescence-to-non-iridescence transition that depends on the pigment size. The cellulose pigments can be applied on irregular and curved surfaces, having high water-, chemical-, and mechanical-resistances. With humidity-sensing behaviors, the pigments can be further integrated into monitoring systems for environmental management. Such a preparation strategy overcomes the limitation of controlling iridescent and non-iridescent structural colors without sacrificing color properties, which may bring more opportunities to develop new eco-friendly pigments for wide applications.

Scalable Structural Coloration of Carbon Nanotube Fibers via a Facile Silica Photonic Crystal Self-Assembly Strategy

The coloration of carbon nanotube (CNT) fibers (CNTFs) is a long-lasting challenge because of the intrinsic black color and chemically inert surfaces of CNTs, which cannot satisfy the aesthetic and fashion requirements and thus significantly restrict their performance in many cutting-edge fields. Recently, a structural coloration method of CNTFs was developed by our group using atomic layer deposition (ALD) technology. However, the ALD-based structural coloration method of CNTFs is expensive, time-consuming, and not suitable for the large-scale production of colorful CNTFs. Herein, we developed a very simple and scalable liquid-phase method to realize the structural coloration of CNTFs. A SiO₂/ethanol dispersion containing SiO₂ nanospheres with controllable sizes was synthesized. The SiO₂ nanospheres could self-assemble into photonic crystal layers on the surface of CNTFs and exhibited brilliant colors. The colors of SiO₂ nanoparticle-coated CNTFs could be easily changed by tuning the sizes of SiO₂ nanospheres. This method provides a simple, effective, and promising way for the large-scale production of colorful CNTFs.

The Fabrication of Full Chromatography SiO2@PDA Photonic Crystal Structural Colored Fabric with High Thermal Stability

Traditional textile dyeing and finishing industries are two of the most important sources of high pollution, high energy consumption, and high emissions. Structural color, as a clean ecological staining method that does not require any dye or pigment, has received extreme attention from researchers. In this study, core-shell structures of SiO2@PDA microspheres were prepared by coating polydopamine (PDA) formed by rapid polymerization of dopamine (DA) on the surface of SiO2 microspheres. Moreover, the structural colors of full chromatography were successfully prepared by vertical self-assembly on silk. The morphology and chemical structure of the prepared SiO2@PDA microspheres were studied by SEM and FT-IR, and the morphology and optical properties of the structured colored fabrics were characterized by SEM and material microscope. The different structural colors of the entire visible region were obtained by controlling the particle size of SiO2@PDA microspheres and the viewing angle of the SiO2@PDA photonic crystal, which are consistent with Bragg’s diffraction law. Since the SiO2@PDA photonic crystal has thermal stability, the prepared structural color fabric could remain highly saturated in color at temperatures up to 200 °C. This has a previously unreported high thermal stability on structural colors of silk. Therefore, the research work will demonstrate a structural color fabric that can prepare full chromatography with high thermal stability.

Reconfigurable Mechanochromic Patterns into Chameleon-Inspired Photonic Papers

Photonic crystal (PC) patterns have shown wide applications in optical devices, information encryption, anticounterfeiting, etc. Unfortunately, it is still a great challenge to reconfigure the PC patterns once fabricated. Herein, a new strategy is presented to reconfigure self-recordable PC patterns by printing local patterns into the chameleon-inspired PC papers using the phase change material (PCM) as ink and then erasing the patterns in ethanol. Multicolor and high-resolution (25 and 75 μm for dot and lines, respectively) patterns can be efficiently and repeatedly reconfigured. In addition, the photonic patterns based on the PC paper and PCM combinations are gifted with mechanochromic characteristics and can show programmable and reversible color change under pressure. The high melting point of the ink, nonclosely packed structures of the PC paper, and the similar solubility parameter of PC paper, PCM, and ethanol are the keys for all these characteristics. This work offers a simple, flexible, efficient way to reconfigure PC patterns with mechanochromic properties and could open up exciting applications for novel hand-operation-based anticounterfeiting and optical devices.

Understanding the Electrophoretic Deposition Accompanied by Electrochemical Reactions Toward Structurally Colored Bilayer Films

Safe, low-cost structurally colored materials are alternative colorants to toxic inorganic pigments and organic dyes. Colloidal amorphous arrays are promising structurally colored materials because of their angle-independent colors. In this study, we focused on precise tuning of the chromaticity by preparing bilayer colloidal amorphous arrays through electrophoretic deposition (EPD). Systematic investigations with various EPD conditions clarified the contributions of each condition to the EPD process and the competing electrochemical reactions, which enabled us to prepare well-colored coatings. EPD films composed of colloidal amorphous array bilayers were successfully synthesized with controlled film thickness. Chromaticity of the films was found to be precisely controlled by the EPD duration. We believe that this understanding of the EPD process and its application to synthesis of structurally colored bilayer films will bring structurally colored materials closer to practical industrial use.

The Development of New Catalytic Pigments Based on SiO2 Amorphous Photonic Crystals via Adding of Dual-Functional Black TiO2-x Nanoparticles

Biomimetic synthesis of amorphous photonic crystals (APCs) is an effective approach to obtaining non-iridescent structural colors. However, the structural colors of artificially prepared APCs are dim or even white due to the influence of incoherent scattering. In this paper, we present a novel method to combine APCs with black TiO_2-x to construct a noniridescent structural color pigments with high visibility and photocatalytic activity. Due to the absorption of incoherently scattered light by black TiO_2-x , the color saturation of structural colors has been significantly increased. In addition, the utilization rate of photogenic carriers was effectively enhanced by the slow light effect generated from the pseudoband gap of SiO₂ APCs with TiO_2-x absorbed full spectrum. The tone and color saturation of catalytic pigments is controlled by the diameter of SiO₂ nanospheres and the ratio of TiO_2-x nanoparticles, which provides a controllable application study in color-related fields as artwork, environmental coatings, and textiles.

Rapid Assembly of Magnetoplasmonic Photonic Arrays for Brilliant, Noniridescent, and Stimuli-Responsive Structural Colors

There are usually trade-offs between maximizing the color saturation and brightness and minimizing the angle-dependent effect in structural colors. Here, a magnetic field-induced assembly for the rapid formation of scalable, uniform amorphous photonic arrays (APAs) featuring unique structural colors is demonstrated. The magnetic field plays a fundamental role in photonic film formation, making this assembly technology versatile for developing structural color patterns on arbitrary substrates. The synergistic combination of surface plasmonic resonance of the Ag core and broadband light absorption of high refractive index (RI) Fe₃ O₄ shell in hybrid magnetoplasmonic nanoparticles (MagPlas NPs) enables breaking the trade-offs to produce brilliant, noniridescent structural colors with high tunability and responsiveness. These features enable the fabrication of various types of highly sensitive and reliable colorimetric sensors for naked-eye detection without sophisticated instruments. Furthermore, large-scale structural color patterns are effortlessly achieved, demonstrating the high potential of the present approach for full-spectrum displays, active coatings, and rewritable papers.

Bioinspired Supramolecular Photonic Composites: Construction and Emerging Applications

Natural organisms have evolved fascinating structural colors to survive in complex natural environments. Artificial photonic composites developed by imitating the structural colors of organisms have been applied in displaying, sensing, biomedicine, and many other fields. As emerging materials, photonic composites mediated by supramolecular chemistry, namely, supramolecular photonic composites, have been designed and constructed to meet emerging application needs and challenges. This feature article mainly introduces the constructive strategies, properties, and applications of supramolecular photonic composites. First, constructive strategies of supramolecular photonic composites are summarized, including the introduction of supramolecular polymers into colloidal photonic array templates, co-assembly of colloidal particles (CPs) with supramolecular polymers, self-assembly of soft CPs, and compounding photonic elastomers with functional substances via supramolecular interactions. Supramolecular interactions endow photonic composites with attractive properties, such as stimuli-responsiveness and healability. Subsequently, the unique optical and mechanical properties of supramolecular photonic composites are summarized, and their applications in emerging fields, such as colorful coatings, real-time and visual motion monitoring, and biochemical sensors, are introduced. Finally, challenges and perspectives in supramolecular photonic composites are discussed. This feature article provides general strategies and considerations for the design of photonic materials based on supramolecular chemistry. This article is protected by copyright. All rights reserved.

Photo-Luminescent Photonic Crystals for Anti-Counterfeiting

The conventional photonic crystals (PCs) are usually prepared by the self-assembly of silica or polystyrene particles. However, their applications are limited significantly due to the lack of the functions of the building blocks. Here, a new kind of photo-luminescent photonic crystals (PLPCs) with brilliant PL and structural colors were prepared by the self-assembly of dye-doped silica particles. The PL and structural colors of PCs can be well-controlled by altering the species of dyes and the size of the particles, respectively. Based on these advantages, PLPC patterns with encrypted information were fabricated through the combination of PLPCs and PCs with similar structural colors but diverse PL colors. These patterns can reversibly hide and display the encrypted information under sunlight and UV illumination, respectively. This work paves a new way for constructing functional PCs and will promote their applications in anti-counterfeiting, smart labels, and optical devices.

Structural Colored Fabrics with Brilliant Colors, Low Angle Dependence, and High Color Fastness Based on the Mie Scattering of Cu2O Spheres

Compared with conventional textile coloring with dyes and pigments, structural colored fabrics have attracted broad attention due to the advantages of eco-friendliness, brilliant colors, and anti-fading properties. The most investigated structural color on fabrics is originated from a band gap of multilayered photonic crystals or amorphous photonic structures. However, limited by the nature of the color generation mechanism and a multilayered structure, it is challenging to achieve structural colored fabrics with brilliant noniridescent colors and high fastness. Here, we propose an alternative strategy for coloring a fabric based on the scattering of Cu₂O single-crystal spheres. The disordered Cu₂O thin layers (<0.6 μm) on the surface of fabrics were prepared by a spraying method, which can generate vivid noniridescent structural color due to the strong Mie scattering of Cu₂O single-crystal spheres. Importantly, the great mechanical stability of the structural color was realized by firmly binding Cu₂O spheres to the fabric using a commercial binder. The structural color can be tuned by changing the diameter of Cu₂O spheres. Furthermore, complex patterns can be easily obtained by spray coating Cu₂O spheres with different particle sizes using a mask. According to color fastness test standards, the dry rubbing, wet rubbing, and light fastness of the structural color on fabric can reach level 5, level 4, and level 6, respectively, which is sufficient to resist rubbing, photobleaching, washing, rinsing, kneading, stretching, and other external mechanical forces. This coloring method may carve a practical avenue in textile coloring and has potentials in other practical applications of structural color.

Structural Coloration of Polyester Fabrics with High Colorfastness by Copolymer Photonic Crystals Containing Reactive Epoxy Groups

Structural color as a revolutionary coloration strategy has been proposed to replace the traditional dyeing and printing process. However, the poor colorfastness and easy crack formation of structural colors on textile fabrics restrict their practical application at present. In this study, poly (tert-butyl acrylate-co-glycidyl methacrylate) (P(t-BA-co-GMA)) copolymers containing reactive epoxy groups with different mass ratios of tert-butyl acrylate (t-BA) and glycidyl methacrylate (GMA) were successfully synthesized, which were used to create structural colors on black polyester fabrics. The results showed that P(t-BA-co-GMA) nanospheres could form crack-free structural colors on polyester fabrics, and the colors vary with the mass ratio of t-BA and GMA to obtain five different colors. The different particle sizes of the different P(t-BA-co-GMA) nanospheres with different refractive indexes and the arrangement of short-range ordered and long-range disordered in microstructures may be the reason of different angle-independent structural colors on polyester fabrics. The P(t-BA-co-GMA) nanosphere structural colors on polyester fabrics possess good abrasion and washing colorfastness. This research provides the experimental basis for the development of crack-free amorphous photonic crystal structural color on fabrics with high colorfastness to promote the practical application of structural color in textile coloration.

Noniridescent structural color from enhanced electromagnetic resonances of particle aggregations and its applications for reconfigurable patterns

HYPOTHESIS

The conventional noniridescent structural colors refer to the coherent scattering of visible light by the short-range ordered structures assembled from the small colloids (100-250 nm). Our hypothesis is that noniridescent structural color can be generated by the random aggregations of large silica particles through the enhanced electromagnetic resonances.

EXPERIMENTS

The random aggregations of large silica particles (350-475 nm) were prepared through the infiltration of silica particles solution with the porous substrate. The mechanism of the structural color is investigated. Reconfigurable patterns are prepared.

FINDINGS

Dissimilar to the conventional noniridescent colors, the angle-independent colors of silica aggregations originate from the enhanced electromagnetic resonances due to the random aggregation of the particles. The colors (blue, green, and red) and corresponding reflection peak positions of the particle aggregations can be well controlled by simply altering the size of the silica particles. Compared to the traditional prints with permanent patterns, reconfigurable patterns with large-area and multicolor can be fabricated by the repeatedly selective spray of water on the substrate pre-coated with noniridescent colors. This work provides new insight and greenway for the fabrication of noniridescent structural colors and reconfigurable patterns, and will promote their applications in soft display, green printing, and anti-counterfeiting.

Lotus Seedpod Inspiration: Particle-Nested Double-Inverse Opal Films with Fast and Reversible Structural Color Switching for Information Security

The integration of novel structures into colloidal crystals provides the possibility of constructing stimuli-responsive photonic materials. However, in most opal and inverse opal structures, replacing the interior air with an infiltrated liquid will cause partial refractive index matching, resulting in the reduction or even disappearance of the photonic band gap. Herein, inspired by the lotus seedpod, an innovative particle-nested double-inverse opal film with fast and reversible structural color switching (≈1 s) is first fabricated by introducing polystyrene (PS) spheres into an inverted opal backbone. Importantly, refractive index matching can be effectively avoided due to the existence of internal PS spheres, and optical switching from diffusive to photonic behavior is achieved by a liquid with low surface tension for the response. Furthermore, a reversible ethanol stimuli-response bilayer double-inverse opal film with multistate switching for information encryption is proposed by combining optical scattering and diffraction. The scattered light from the top layer caused by the randomly distributed and weakly scattering PS spheres within the pores makes the pattern at the bottom invisible. Simultaneously, the display and discoloration of the pattern can be realized instantaneously by ethanol response. Thus, this new preparation strategy exhibits great potential in the security fields.

Functional Fibers and Fabrics for Soft Robotics, Wearables, and Human-Robot Interface

Soft robotics inspired by the movement of living organisms, with excellent adaptability and accuracy for accomplishing tasks, are highly desirable for efficient operations and safe interactions with human. With the emerging wearable electronics, higher tactility and skin affinity are pursued for safe and user-friendly human-robot interactions. Fabrics interlocked by fibers perform traditional static functions such as warming, protection, and fashion. Recently, dynamic fibers and fabrics are favorable to deliver active stimulus responses such as sensing and actuating abilities for soft-robots and wearables. First, the responsive mechanisms of fiber/fabric actuators and their performances under various external stimuli are reviewed. Fiber/yarn-based artificial muscles for soft-robots manipulation and assistance in human motion are discussed, as well as smart clothes for improving human perception. Second, the geometric designs, fabrications, mechanisms, and functions of fibers/fabrics for sensing and energy harvesting from the human body and environments are summarized. Effective integration between the electronic components with garments, human skin, and living organisms is illustrated, presenting multifunctional platforms with self-powered potential for human-robot interactions and biomedicine. Lastly, the relationships between robotic/wearable fibers/fabrics and the external stimuli, together with the challenges and possible routes for revolutionizing the robotic fibers/fabrics and wearables in this new era are proposed.

Graphene Oxide/Reduced Graphene Oxide Enhanced Noniridescent Structural Colors Based on Silica Photonic Spray Paints with Improved Mechanical Robustness

In contrast to traditional pigment colors, structural colors have developed a great potential in practical applications, thanks to their unique nonfading and color tunable properties; especially amorphous photonic structures with noniridescent structural colors have attracted considerable attention and their applications have expanded to more fields. Herein, graphene oxide (GO) and reduced graphene oxide (RGO) enhanced noniridescent structural colors with excellent mechanical robustness were established by a time-saving approach named spray coating, which allows for rapid fabrication of angular independent structural colors by spraying different photonic spray paints (PSPs) to ensure color multiplicity that was adjusted by the silica nanoparticles (SiO2 NPs) sizes onto the substrates. The incorporation of poly(methyl methacrylate-butyl acrylate) (PMB) improved the adhesion existing among SiO2 inter-nanoparticles and between SiO2 NPs and the substrates, taking advantages of the low glass transition temperature (Tg) of butyl acrylate derivative polymer and made PMB embedded PSPs coated patterns being imparted with good mechanical robustness and abrasive resistance. The peculiar light adsorption of GO and RGO across visible light spectrum facilitate higher color saturation. The improvement in color saturation of GO and RGO doped PSPs is expected to boost the promising applications in structurally colored paintings, inks and other color-related optical fields.

Simple and efficient fabrication of multi-stage color-changeable photonic prints as anti-counterfeit labels

Color changeable photonic prints (CCPPs) show their potential applications in high-level information storage and anti-counterfeiting, but usually suffer from the complex fabrication process and limited color variation. Here, a simple and efficient method is developed to generate CCPPs with multilevel tunable color contrasts by packing the solvent responsive photonic crystals with diverse cross-linking degrees and desired way. The key to the successful fabrication is to create and control over the optical response of each part of the CCPPs through altering the cross-linking degree of PCs and thus the affinity between the CCPPs and solvents. A CCPPs based anti-fake label with the encrypted information functionality which originates from reversible color change between dried state and swelling with the mixture of acetic acid and ethanol is investigated. Compared with conventional CCPPs, the as-prepared CCPPs can reveal multistage information depending on the volume fraction of ethanol. This work provides a new insight for the simple fabrication of CCPPs and will facilitate their applications in the information protection and high-level anti-counterfeiting.

Robust, Portable, and Specific Water-Response Silk Film with Noniridescent Pattern Encryption for Information Security

In modern days, information is a key resource for accelerating the development of society, economy, and culture. Thus, information security has always been a high priority for any country, business, and department. Herein, a simple and effective strategy for preparing an independent optical device for information security is proposed by using silk fibroin materials with a quasiamorphous inverse structure. Given the reversible hydrogen bonds between silk fibroin materials and water molecules, a multicolor high-resolution pattern with a variable color can be obtained by using a simple spray coating method. Furthermore, a reversible water stimulus-response silk film with a laminated structure that consists of hidden and patterned layers and carries quick response (QR) code information is prepared. This device effectively hides (encryption) the QR code pattern in a normal environment and quickly displays the information (decryption) in water. Simultaneously, the silk film shows good mechanical strength, excellent biocompatibility, long-term structural stability, and a unique response mechanism, which make it a suitable carrier of optical information. Thus, this new preparation strategy of an optical device has a potential application value and is an important reference in the fields of information security and functional materials.

Green and Efficient Inkjet Printing of Cotton Fabrics Using Reactive Dye@Copolymer Nanospheres

Digital inkjet printing of textiles possesses great advantages like high efficiency and flexible production, but the challenges like the risk of causing serious environmental problems due to the large usage of dyes and chemicals still remain a matter of concern. In response to this problem, herein, a novel kind of reactive dye@copolymer nanosphere was prepared through the adsorption of C. I. Reactive Red 218 dyes (RR218) onto cationic poly(styrene-butyl acrylate-vinylbenzyl trimethylammonium chloride) (PSBV) nanospheres and applied in inkjet printing on woven cotton fabric. Results show that the prepared RR218@PSBV nanospheres possessed homogeneous size and good stability for ink preparation. In comparison with the original RR218 solution, the color depth of RR218@PSBV-printed fabric increased by 1.4 times and the dye residues in the printing effluent were reduced by about 45%. Meanwhile, the consumptions of sodium carbonate and urea in conventional inkjet printing were reduced by about 3.3 and 22.8 mg/cm², respectively, and the printing process was simplified with 30% energy saving. Furthermore, the mechanism of the color enhancement by nanospheres was revealed by the calculation of absorption and scattering coefficients based on the Kubelka-Munk function. This work provides a potential application of dye@polymer nanospheres to promote the optimization of the textile inkjet printing technique and alleviates the environmental impact of conventional textile coloration.

Highly Efficient Detection of Homologues and Isomers by the Dynamic Swelling Reflection Spectrum

Precise and efficient detection of solvents with similar refractive index is highly desired but remains a big challenge for the conventional opal because the shift of its reflection wavelength only depends on the refractive index of the solvent to be detected. Here, homologues (alcohols, acids, alkalis, esters, and aromatic hydrocarbons), isomers, and other solvents with similar refractive index and structures were precisely distinguished through the dynamic swelling reflection spectrum (DSRS) pattern based on the different swelling behavior of swellable photonic paper in solvents. The one reflection signal of photonic paper will split into two reflection peaks, which then tend to merge together during the swelling process. The variation of the reflection signals and merging time are highly sensitive to the polarity and refractive index of the solvent, and the differences can be significantly amplified in DSRS, resulting in the distinction of the solvent from its unique geometric pattern. Moreover, the variation tendency of the reflectance provides an additional parameter in recognition of the solvent, which can be explained by calculation and comparison of the practical volume ratio of the solvent swelled into the photonic paper and the corresponding critical volume ratio of the solvent determined by its refractive index.

Robust Structurally Colored Coatings Composed of Colloidal Arrays Prepared by the Cathodic Electrophoretic Deposition Method with Metal Cation Additives

Structurally colored coatings composed of colloidal arrays of monodisperse spherical particles have attracted great attention owing to their versatile advantages, such as low cost, resistance to fading, and low impacts on the environment and human health. However, the weak mechanical stability is considered to be a major obstacle for their practical applications as colorants. Although several approaches based on the addition of polymer additives to enhance the adhesion of particles have been reported, the challenge remains to develop a strategy for the preparation of structurally colored coatings with extremely high robustness using a simple process. Here, we have developed a novel approach to fabricate robust structurally colored coatings by cathodic electrophoretic deposition. The addition of a metal salt, i.e., Mg(NO₃)₂, to the coating dispersion allows SiO₂ particles to have a positive charge, which enables the electrophoresis of SiO₂ particles toward the cathode. At the cathode, Mg(OH)₂ codeposits with SiO₂ particles because OH^- ions are generated by the decomposition of dissolved oxygen and NO₃^- ions. The mechanical stability of the colloidal arrays obtained by this process is remarkably improved because Mg(OH)₂ facilitates the adhesion of the particles and substrates. The brilliant structural color is maintained even after several cycles of the sandpaper abrasion test. We have also demonstrated the coating on a stainless steel fork. This demonstration reveals that our approach enables a homogeneous coating on a complicated surface. Furthermore, the high durability of the coating is clarified because the coating did not peel off even when the fork was stuck into a plastic eraser. Therefore, the coating technique developed here will provide an effective method for the pervasive application of the structural color as a colorant.

Fabrication of Structural-Coloured Carbon Fabrics by Thermal Assisted Gravity Sedimentation Method

Structural-coloured poly(styrene-methyl methacrylate-acrylic acid) (Poly(St-MMA-AA)) deposited carbon fabrics (Poly(St-MMA-AA)/PCFs) with fascinating colours (salmon, chartreuse, springgreen, skyblue, mediumpurple) changing with the (Poly(St-MMA-AA) nanoparticle sizes can be facilely fabricated by the thermal-assisted gravity sedimentation method that facilitates the self-assembly of Poly(St-MMA-AA) colloidal nanoparticles to generate photonic crystals. The particle sizes of Poly(St-MMA-AA) copolymer with core/shell structure varying from 308.3 nm to 213.1 nm were controlled by adjusting the amount of emulsifier during emulsion polymerisation. The presence of the intrinsic chemical information of Poly(St-MMA-AA) copolymer has been ascertained by Raman and Fourier Transform Infrared (FT-IR) Spectroscopy analysis. Colour variation of the as-prepared structural-coloured carbon fabrics (Poly(St-MMA-AA)/PCFs) before and after dipping treatment were captured while using an optical microscope. The structural colours of Poly(St-MMA-AA)/PCFs were assessed by calculating the diffraction bandgap according to Bragg's and Snell's laws. The Poly(St-MMA-AA) photonic crystal films altered the electrical properties of carbon fabrics with the resistivity growing by five orders of magnitude. The differential electrical resistivity between Poly(St-MMA-AA)/PCFs and wet Poly(St-MMA-AA)/PCFs combined with the corresponding tunable colours can be potentially applied in several promising areas, such as smart displays, especially signal warning displays for traffic safety.

Nanosphere-Aggregation-Induced Reflection and Its Application in Large-Area and High-Precision Panchromatic Inkjet Printing

Artificial structural colors have attracted more and more attention due to their high photostability, low toxicity, and brilliant colors. Inkjet printing of photonic crystals or amorphous photonic structures can realize large-scale structural color patterns, while plasma printing of metals can achieve high-precision color images. However, still no method is available to fabricate structural color patterns on both a large scale and with high precision. Here, nanosphere-aggregation-induced reflection (NAIR) is first theoretically and experimentally demonstrated and vivid full-spectrum structural color can be generated based on NAIR. Dramatically different from photonic crystals, the accumulation of only a few monodisperse dielectric spheres with an appropriate refractive index and diameter can produce bright structural colors, which makes high resolution possible. By introducing commercial inkjet printers, this aggregate structure can be constructed at high speed in a large scale. Importantly, the color mixing is easily performed by simultaneously applying spheres with different sizes, which allow us to sophisticatedly control the generated color. The demonstrated NAIR printing paves the way toward a full-spectrum, large-scale, and high-precision structural color, offering great potential for daily commercial utilization.

Rapid and Tunable Method To Fabricate Angle-Independent and Transferable Structurally Colored Films

The assembly of monodisperse particles into colloidal arrays that diffract visible light through constructive interference is of considerable interest due to their resilience against color fading. In particular, noniridescent structurally colored materials are promising as a means of coloration for paints, inks, cosmetics, and displays because their color is angle independent. A rapid and tunable assembly method for producing noniridescent structurally colored colloidal-based materials that are pliable after fabrication is described. Structurally colored particle arrays were fabricated by centrifuging highly charged silica particles suspended in deionized water. By tuning the particle diameter, the colors displayed by the arrays spanned the visible spectrum while retaining angle-independent structural color. The color of centrifuged colloids of a single particle diameter was precisely controlled within 50 nm by modulating the particle concentration. The peak wavelength diffracted by the material was further tuned by altering the centrifugal rate and assembly time. Centrifugation assembly of particles in a polymer solution also produces noniridescent colloidal films, and the control of their color is reported. Together, these results offer design considerations for the centrifugation-based assembly of colloidal films with tunable structural color that are transferable after fabrication and are angle independent.

Bilayer Heterostructure Photonic Crystal Composed of Hollow Silica and Silica Sphere Arrays for Information Encryption

Utilizing photonic crystals to fabricate information encryption materials has attracted widespread interest due to their tunable optical properties and responsiveness to external stimuli. In most of the previously reported systems, the information is hidden at a specific angle and the angle-dependent invisibility is a limitation. Meanwhile, poor structural stability is still a key issue that needs to be solved for potential applications. In this paper, a bilayer heterostructure photonic crystal containing ordered hollow silica inverse opal arrays, amorphous silica opal arrays, and poly(vinyl alcohol) (adhesive) is successfully constructed. It makes the information highly invisible at any angle and also achieves information encryption. With this strategy, the information can be hidden by the noniridescent structural color derived from the strong scattering effect of light from the top layer of amorphous silica sphere arrays. After wiping with ethanol or a refractive-index-matching solvent, the scattering effect vanishes and the amorphous silica sphere arrays become transparent. The reflected light of the bottom layer caused by the increasing refractive index contrast between the inside and outside of the hollow silica spheres could rapidly reveal the hidden information. The bilayer photonic crystal exhibits robust structural stability, and the hiding/revealing process is completely reversible, which shows great potential applications in steganography and information encryption.

Fabrication of Coatings with Structural Color on a Wood Surface

A facile method for the fabrication of colloidal photonic crystal coatings with tunable structural color on a wood surface was presented. The photonic crystal coatings were formed from monodisperse latex spheres composed of poly(styrene-methyl methacrylate-acrylic acid) (P(St-MMA-AA)). The latex spheres with a hard PSt core and elastomeric P(MMA-AA) shell were prepared using the emulsion polymerization method. The sessile drop method, a rapid single-step self-assembly method through simple evaporation of emulsion, was used to form three-dimensional colloidal crystals. Coatings with brilliant colors and uniform Bragg’s diffraction covering the entire visible region were fabricated by controlling the sphere size. This simple method provided new insight into the development of wood color embellishment.

Structurally colored protease responsive nanoparticle hydrogels with degradation-directed assembly

A tunable protease responsive nanoparticle hydrogel (PRNH) that demonstrates large non-iridescent color changes due to a degradation-directed assembly of nanoparticles is reported. Structurally colored composites are fabricated with silica particles, 4-arm poly(ethylene glycol) norbornene (4PEGN), and a proteolytically degradable peptide. When placed in a protease solution, the peptide crosslinks degrade causing electrostatic binding and adsorption of the polymer to the particle surface which leads to the assembly of particles into compact amorphous arrays with structural color. The particle surface charge and size is investigated to probe their effect on the assembly mechanism. Interestingly, only PRNHs with highly negative particle surface charge exhibit color changes after degradation. Ultra-small angle X-ray scattering revealed that the particles become coated in polymer after degradation, producing a material with less order compared to the initial state. Altering the particle diameter modulates the composites' color, and all sizes investigated (178-297 nm) undergo the degradation-directed assembly. Varying the amount of 4PEGN adjusts the swollen PRNH color and has no effect on the degradation-directed assembly. Taken together, the effects of surface charge, particle size, and polymer concentration allow for the formulation of new design rules for fabricating tunable PRNHs that display vivid changes in structural color upon degradation.

Preparation of Noniridescent Structurally Colored PS@TiO2 and Air@C@TiO2 Core-Shell Nanoparticles with Enhanced Color Stability

Natural amorphous photonic crystals benefit from reflectance at selective wavelengths in some specific existing natural systems. Noniridescence from natural organisms has also attracted great interest for various examples in bionic colors, pigments, and paintings. Here, Air@C@TiO₂ sphere was obtained by the first calcination of PS@TiO₂ core-shell nanoparticles in nitrogen to ensure the integrity of the shell structure followed by low-temperature calcination to obtain the appropriate color saturation. We demonstrate that, compared with the prepared colored PS@TiO₂/carbon black (CB) pigments, angle-independent hollow Air@C@TiO₂ nanoparticles have enhanced color stability under the action of in situ synthesized carbon black (CB). Our results suggest that it is easy to change the color of these Air@C@TiO₂ spheres by adjusting the sphere structure sizes, which have the potential to show visual signaling.

Multicolored Photonic Crystal Carbon Fiber Yarns and Fabrics with Mechanical Robustness for Thermal Management

Multicolored photonic crystal carbon fiber (CF) yarns and fabrics with mechanical robustness in a full spectrum are reported. By facilely controlling the thickness of the periodic layer, a series of photonic CF yarns and fabrics with vivid structural colors ranging from purple, green, yellow, orange, to red are obtained. Interestingly, the prepared multicolored CF yarns show anisotropic optical reflection properties because of their unique axisymmetric geometry, while the plain-woven fabrics exhibit vivid colors even under ambient scattering light. Most importantly, they can withstand cyclical mechanical rubbing, laundering, and accelerated light aging, indicating great potential for practical uses. Finally, considering such impressive characteristics as well as reflection in the visible and near-infrared regions, the above photonic crystal microstructure is further used as a new material for the application of outdoor reflective cooling of the textile surface, demonstrating a superior temperature reduction up to ∼12 °C with respect to the control sample.

Different Structural Colors or Patterns on the Front and Back Sides of a Multilayer Photonic Structure

The application of photonic crystals in the field of color display and anticounterfeiting has been widely studied because of their brilliant and angle-dependent structural colors. Most of the research is focused on structural colors on the front side of photonic crystals, and both sides of the crystals usually display the same or similar optical properties. Here, multilayer photonic crystals with different structural colors or different patterns on the front and back sides were designed. In a trilayer photonic structure, an amorphous SiO₂ layer with a thickness of about 10 μm was inserted into two layers of highly ordered photonic crystals with band gaps of 625 and 470 nm. The amorphous SiO₂ layer acts as a gate to prohibit light transmission, and thereby, the structural colors of the two photonic crystals were separated. Hence, the trilayer structure shows red and blue colors on each side. Then, a light window was opened in the disordered layer using a patterned mask; thus, a pattern with a mixed color of both ordered layers was observed on each side in the window field, which was obviously different from the background color. Finally, completely different patterns on each side were also realized by building a multilayer structure. The different structural colors or patterns on each side of the photonic structures provide them with enriched color range and enhanced display or anticounterfeiting ability.

Bio-inspired transparent structural color film and its application in biomimetic camouflage

The transparent wings of insects with intelligent structural colors or good invisibility in different surroundings provide them with unique camouflage capability for protection and information exchange. Inspired by the existence of ordered biological nanostructures on the surface of the wings, freestanding composite photonic crystal (PC) films were prepared by infiltrating polydimethylsiloxane (PDMS, n = 1.41) into the interstices of a highly ordered opal PC using poly(methyl methacrylate) (PMMA, n = 1.49) spheres as building blocks. The appropriate refractive index contrast (Δn = 0.08) endowed the composite film with high transparency and vivid structural colors. Consequently, the PC film was invisible in shaded surroundings and showed brilliant structural color under sunlight. Also, 186, 229 and 257 nm PMMA spheres were used to obtain composite PC films with different structural colors. Moreover, as a proof of concept, a biomimetic dragonfly-shaped film was fabricated using a patterned substrate. When it was placed on a green tree under sunlight, abundant structural colors appeared at different specular viewing angles. However, it camouflaged in the environment when the shadows of the green tree shielded the sunlight or when viewed in non-specular angles with sunshine. This unique property indicated their potential applications in biomimetic camouflage and smart stealth materials for bionic machines.

Large-Area and Water Rewriteable Photonic Crystal Films Obtained by the Thermal Assisted Air-Liquid Interface Self Assembly

Compared with traditional paper, water rewritable photonic crystal (PC) paper is an environmentally friendly and low resource-consuming material for information storage. Although, recently reported PC papers have high-quality structure color showing promising prospect, the paper size, that is within several centimeters, still limits turning it from potential to reality. Here, we present a new water rewritable PC film as large as the A4 size (210 × 300 mm²) with a high-quality structure color. The material is prepared by thermal assisted self-assembly on the air-liquid interface. To fix such a large-area self-assembled PC film, we partially deform and coalesce the self-assembled nanoparticles, which have low glass transition temperature. This process causes the film to be transparent and structural colorless but still keeps the inner 3D-ordered structure. Then, utilizing the hydrophilic nature of the assembled block, the film can be switched to a structural color state by touching water. Diverse brilliant structural colors appear with different assembled particle (poly(butyl methacrylate- co-methylmethacrylate- co-butyl acrylate- co-diacetone acrylamide) named as PBMBD) sizes. The transparency-structural color transition can be performed multiple times reversibly in all or specific regions of the film. It provides a new solution for future applications of rewriteable PC paper.

Hand Painting of Noniridescent Structural Multicolor through the Self-Assembly of YOHCO3 Colloids and Its Application for Anti-Counterfeiting

YOHCO₃ colloidal particles with tunable size, composition, and optical properties were prepared, and they were used for the fabrication of amorphous photonic crystals? (APCs) patterns through direct hand painting. YOHCO₃ colloids were synthesized by a seeding growth method, in which the colloid size could be controlled by altering the seed amounts and the composition and optical properties can be altered via the doping of Eu³⁺. APCs? films with bright, permanent, and tunable structural colors were prepared by the self-assembly of YOHCO₃ colloids of different sizes. Multicolor patterns can be obtained quickly and efficiently by hand painting with the dispersion of YOHCO₃ colloids as ink. An APCs? pattern assembled from YOHCO₃:Eu colloids is also fabricated, and the pattern shows blue structural color under natural light and bright red colors under illumination of UV light. The facile synthesis procedure, simple assembly process, and unique optical properties of the APCs make it valuable for practical applications such as structural color-based printing and anticounterfeiting.