Structural Color Tuning: Mixing Melanin-Like Particles with Different Diameters to Create Neutral Colors

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.

Tuning disorder in structurally colored bioinspired photonic glasses

Colloidal crystals, such as opals, display bright and iridescent colors when assembled from submicron particles. While the brightness and purity of iridescent colors are well suited for ornaments, signaling, and anticounterfeiting, their angle dependence limits the range of their applications. In contrast, colloidal glasses display angle-independent structural color that is tunable by the size and local arrangement of particles. However, the angle-independent color of colloidal photonic glasses usually yields pastel colors that are not vivid due to the disorder in the particle assembly. Here, we report an electrophoretic assembly platform for tuning the level of disorder in the particle system from a colloidal crystal to a colloidal glass. Altering the electric field in our electrophoretic platform allows for deliberate control of the assembly kinetics and thus the level of order in the particle assembly. With the help of microscopy, X-ray scattering, and optical characterization, we show that the photonic properties of the assembled films can be tuned with the applied electric field. Our analyses reveal that angle-independent color with optimum color brightness can be achieved in typical colloidal suspensions when the range of order is at ∼3.2 particle diameters, which is expected at a moderate electric field of ∼15 V mm-1.

Coloration on Bluish Alginate Films with Amorphous Heterogeneity Thereof

Using sodium alginate (Alg) aqueous solution containing indigo carmine (IdC) at various concentrations we characterized the rippled surface pattern with micro-spacing on a flexible film as intriguing bluish Alg-IdC iridescence. The characterization was performed using Fourier-transform infrared spectroscopy, ultraviolet-visible spectroscopy, field emission scanning electron microscopy, atomic force microscopy, electron microscopy, differential scanning calorimetry, thermogravimetric analysis, X-ray diffraction analysis, and photoluminescence detection. The edge pattern on the film had a maximum depth of 825 nm, a peak-to-peak distance of 63.0 nm, and an average distance of 2.34 nm. The center of the pattern had a maximum depth of 343 nm and a peak-to-peak distance of 162 nm. The pattern spacing rippled irregularly, widening toward the center and narrowing toward the edges. The rippled nano-patterned areas effectively generated iridescence. The ultraviolet absorption spectra of the mixture in the 270 and 615 nm ranges were the same for both the iridescent and non-iridescent film surfaces. By adding Ag+ ions to Alg-IdC, self-assembled microspheres were formed, and conductivity was improved. Cross-linked bluish materials were immediately formed by the addition of Ca2+ ions, and the film was prepared by controlling their concentration. This flexible film can be used in applications such as eco-friendly camouflage, anti-counterfeiting, QR code materials for imaging/sensing, and smart hybrid displays.

Structural Color Mixing in Microcapsules through Exclusive Crystallization of Binary and Ternary Colloids

Colloidal crystals are designed as photonic microparticles for various applications. However, conventional microparticles generally have only one stopband from a single lattice constant, which restricts the range of colors and optical codes available. Here, photonic microcapsules are created that contain two or three distinct crystalline grains, resulting in dual or triple stopbands that offer a wider range of colors through structural color mixing. To produce distinct colloidal crystallites from binary or ternary colloidal mixtures, the interparticle interaction is manipulated using depletion forces in double-emulsion droplets. Aqueous dispersions of binary or ternary colloidal mixtures in the innermost droplet are gently concentrated in the presence of a depletant and salt by imposing hypertonic conditions. Different-sized particles crystallize into their own crystals rather than forming random glassy alloys to minimize free energy. The average size of the crystalline grains can be adjusted with osmotic pressure, and the relative ratio of distinct grains can be controlled with the mixing ratio of particles. The resulting microcapsules with small grains and high surface coverage are almost optically isotropic and exhibit highly-saturated mixed structural colors and multiple reflectance peaks. The mixed color and reflectance spectrum are controllable with the selection of particle sizes and mixing ratios.

Crack-Free Structural Color Materials Prepared without Disrupting the Particle Arrangement by Controlling the Internal Stress Relaxation and Interactions of the Melanin Particles

In fabricating structural color materials with assembled colloidal particles, there is a trade-off between the internal stresses acting on the particles and the interactions between the particles during solvent volatilization. It is crucial to fabricate crack-free materials that maintain the periodic arrangements of the particles by understanding the mechanism for crack initiation. Here, we focused on the composition and additives of melanin particle dispersions to obtain crack-free structural color materials without disturbing the particle arrangements. The use of a water/ethanol mixture as a dispersant effectively reduced the internal stresses of the particles during solvent evaporation. Furthermore, the addition of low-molecular-weight, low-volatility ionic liquids ensured that the arrangement and interactions of the particles were maintained after solvent volatilization. Optimization of the composition and additives of the dispersion made it possible to achieve crack-free melanin-based structural color materials while maintaining vivid, angular-dependent color tones.

Mechanism of structural colors in binary mixtures of nanoparticle-based supraballs

Inspired by structural colors in avian species, various synthetic strategies have been developed to produce noniridescent, saturated colors using nanoparticle assemblies. Nanoparticle mixtures varying in particle chemistry and size have additional emergent properties that affect the color produced. For complex multicomponent systems, understanding the assembled structure and a robust optical modeling tool can empower scientists to identify structure-color relationships and fabricate designer materials with tailored color. Here, we demonstrate how we can reconstruct the assembled structure from small-angle scattering measurements using the computational reverse-engineering analysis for scattering experiments method and use the reconstructed structure in finite-difference time-domain calculations to predict color. We successfully, quantitatively predict experimentally observed color in mixtures containing strongly absorbing nanoparticles and demonstrate the influence of a single layer of segregated nanoparticles on color produced. The versatile computational approach that we present is useful for engineering synthetic materials with desired colors without laborious trial-and-error experiments.

Computational Reverse-Engineering Analysis for Scattering Experiments (CREASE) with Machine Learning Enhancement to Determine Structure of Nanoparticle Mixtures and Solutions

We present a new open-source, machine learning (ML) enhanced computational method for experimentalists to quickly analyze high-throughput small-angle scattering results from multicomponent nanoparticle mixtures and solutions at varying compositions and concentrations to obtain reconstructed 3D structures of the sample. This new method is an improvement over our original computational reverse-engineering analysis for scattering experiments (CREASE) method (ACS Materials Au2021, 1 (2 (2), ), 140-156), which takes as input the experimental scattering profiles and outputs a 3D visualization and structural characterization (e.g., real space pair-correlation functions, domain sizes, and extent of mixing in binary nanoparticle mixtures) of the nanoparticle mixtures. The new gene-based CREASE method reduces the computational running time by >95% as compared to the original CREASE and performs better in scenarios where the original CREASE method performed poorly. Furthermore, the ML model linking features of nanoparticle solutions (e.g., concentration, nanoparticles' tendency to aggregate) to a computed scattering profile is generic enough to analyze scattering profiles for nanoparticle solutions at conditions (nanoparticle chemistry and size) beyond those that were used for the ML training. Finally, we demonstrate application of this new gene-based CREASE method for analysis of small-angle X-ray scattering results from a nanoparticle solution with unknown nanoparticle aggregation and small-angle neutron scattering results from a binary nanoparticle assembly with unknown mixing/segregation among the nanoparticles.

Co-Assembly of Colloids and Eumelanin Nanoparticles in Droplets for Structural Pigments with High Saturation

Colloidal crystals have been used to develop structural colors. However, incoherent scattering causes the colors to turn whitish, reducing the color saturation. To overcome the problem, light-absorbing additives have been incorporated. Although various additives have been used, most of them are not compatible with a direct co-assembly with common colloids in aqueous suspensions. Here, the authors suggest eumelanin nanoparticles as a new additive to enhance the color chroma. Eumelanin nanoparticles are synthesized to have diameters of several nanometers by oxidative polymerization of precursors in basic solutions. The nanoparticles carry negative charges and do not weaken the electrostatic repulsion among same-charged polystyrene particles when they are added to aqueous suspensions. To prove the effectiveness of eumelanin as a saturation enhancer, the authors produce photonic balls through direct co-assembly of polystyrene and eumelanin using water-in-oil emulsion droplets, while varying the weight ratio of eumelanin to polystyrene. The high crystallinity of colloidal crystals is preserved for the ratio up to at least 1/50 as the eumelanin does not perturb the crystallization. The eumelanin effectively suppresses incoherent scattering while maintaining the strength of structural resonance at an optimum ratio, improving color chroma without compromising brightness.

Computational Reverse-Engineering Analysis for Scattering Experiments of Assembled Binary Mixture of Nanoparticles

In this paper, we describe a computational method for analyzing results from scattering experiments on dilute solutions of supraparticles, where each supraparticle is created by the assembly of nanoparticle mixtures. Taking scattering intensity profiles and nanoparticle mixture composition and size distributions in each supraparticle as input, this computational approach called computational reverse engineering analysis for scattering experiments (CREASE) uses a genetic algorithm to output information about the structure of the assembled nanoparticles (e.g., real space pair correlation function, extent of nanoparticle mixing/segregation, sizes of domains) within a supraparticle. We validate this method by taking as input in silico scattering intensity profiles from coarse-grained molecular simulations of a binary mixture of nanoparticles, forming a close-packed structure and testing if our computational method can correctly reproduce the nanoparticle structure observed in those simulations. We test the strengths and limitations of our method using a variety of in silico scattering intensity profiles obtained from simulations of a spherical or a cubic supraparticle comprising binary nanoparticle mixtures with varying chemistries, with and without dispersity in sizes, that exhibit well-mixed to strongly segregated structures. The strengths of the presented method include its capability to analyze scattering intensity profiles even when the wavevector q range is limited, to handily provide all of the pairwise radial distribution functions, and to correctly determine the extent of segregation/mixing of the nanoparticles assembled in complex geometries.

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.

Polydopamine Films with 2D-like Layered Structure and High Mechanical Resilience

Highly oriented, layered, and mechanically resilient films of polydopamine (PDA) have been synthesized from the air/water interface. The films show a unique layered structure, as shown by scanning and transmission electron studies (SEM/TEM) and X-ray diffraction analysis (XRD), which resemble that of 2D layered materials. The films exhibit a composition typical of PDA-based materials, as evidenced by X-ray photoelectron spectroscopy (XPS); moreover, the samples present the distinctive resonance modes of PDA-based nanomaterials in Raman and infrared spectroscopy (FTIR) experiments. The presence of highly ordinated 3-4 protomolecule stacking, taking place at the air/water interface, with a unique eumelanin-like supramolecular arrangement is presented. Moreover, the films show superior mechanical resilience with E = 13 ± 4 GPa and H = 0.21 ± 0.03 GPa, as revealed by nanoindentation experiments, making them highly resilient and easily transferable. Finally, the ordering induced by the interface opens many possibilities for further studies, including those regarding the supramolecular structure on PDA due to their similarity to 2D layered materials.

High Structural Stability of Photonic Crystals on Textile Substrates, Prepared via a Surface-Supported Curing Strategy

Over the past years, photonic crystals (PCs) with a periodically ordered nanostructure have attracted great attention due to their potential as advanced optical materials for structural coloration of textiles. However, the weak structural stability of PCs on flexible textile substrates makes them vulnerable to strong external forces, hampering their large-scale application. In this work, a waterborne polyurethane (wPU) is chosen for enhancing the structural stability of PCs. The composite PCs (PCs/wPU) show both brilliant structural colors and significantly improved structural stability. The structural color produced by the encapsulated PCs is found to depend on the properties of encapsulating agents. The wPU with high surface tension solidifies mainly on the PC surface in the form of a transparent film, protecting the overall structure of PCs. Meanwhile, a small amount of wPU, infiltrating into the interior of PCs, provides strong adhesion and ensures stability among nanospheres. In turn, polydimethylsiloxane (PDMS) with low surface tension is easy to infiltrate into the interior of PCs, forming fully encapsulated PCs. This reduces the brightness of structural color produced by the final PCs/PDMS composite over the original PCs, due to the replacement of air by PDMS, and thus the decrease in the refractive index contrast of PCs. The supported curing strategy using the encapsulating agent with high surface tension is shown to not only improve the structural stability of PCs but also exert almost no influence on the optical properties of PCs, facilitating the practice application of structural coloration in the textile industry.

Control of Structural Coloration by Natural Sunlight Irradiation on a Melanin Precursor Polymer Inspired by Skin Tanning

Natural melanin affects the reflection and absorption of light, and it is known as an important element in producing bright structural colors in nature. In this study, we prepared core-shell particles using a melanin precursor polymer, that is, polytyrosine (PTy), as a shell layer by the oxidative polymerization of tyrosine ethyl ester (Ty) in the presence of cerium oxide (CeO₂) core particles. Inspired by skin tanning, irradiating the CeO₂@PTy core-shell particles with UV or natural sunlight caused melanization by extending the π-conjugated length of PTy, producing colloidal particles with the ability to absorb light. The pellet samples consisting of CeO₂@PTy particles appeared whitish because of multiple scattered light. In contrast, the light absorption capacity of CeO₂@PTy UV or CeO₂@PTy Sun particles after light irradiation suppressed scattered light, dramatically improving the visibility of the structural color of the pellet samples made from these particles. Thus, a new method has been developed to control the visualization of structural colors to the human eye by irradiating the melanin precursor polymer with light.

Designing angle-independent structural colors using Monte Carlo simulations of multiple scattering

Disordered nanostructures with correlations on the scale of visible wavelengths can show angle-independent structural colors. These materials could replace dyes in some applications because the color is tunable and resists photobleaching. However, designing nanostructures with a prescribed color is difficult, especially when the application-cosmetics or displays, for example-requires specific component materials. A general approach to solving this constrained design problem is modeling and optimization: Using a model that predicts the color of a given system, one optimizes the model parameters under constraints to achieve a target color. For this approach to work, the model must make accurate predictions, which is challenging because disordered nanostructures have multiple scattering. To address this challenge, we develop a Monte Carlo model that simulates multiple scattering of light in disordered arrangements of spherical particles or voids. The model produces quantitative agreement with measurements when we account for roughness on the surface of the film, particle polydispersity, and wavelength-dependent absorption in the components. Unlike discrete numerical simulations, our model is parameterized in terms of experimental variables, simplifying the connection between simulation and fabrication. To demonstrate this approach, we reproduce the color of the male mountain bluebird (Sialia currucoides) in an experimental system, using prescribed components and a microstructure that is easy to fabricate. Finally, we use the model to find the limits of angle-independent structural colors for a given system. These results enable an engineering design approach to structural color for many different applications.

Progress in polydopamine-based melanin mimetic materials for structural color generation

Structural color is a color derived from optical interaction between light and a microstructure and is often seen in nature. Natural melanin plays an important role in bright structural coloration. For example, the vivid colors of peacock feathers are due to structural colors. The periodic arrangement of melanin granules inside the feathers leads to light interference, and the black granules absorb scattered light well, resulting in bright structural color. In recent years, polydopamine (PDA) has attracted attention as a melanin mimetic material. This review article summarizes recent research on structural coloration using PDA-based artificial melanin materials. It also outlines possible applications using bright structural colors realized by artificial melanin materials and future perspectives.

Effect of the Polydopamine Composite Method on Structural Coloration: Comparison of Binary and Unary Assembly of Colloidal Particles

Melanin influences light reflection and absorption and is known to be one of the elements producing structural color, such as that in the feathers of birds. In this study, we used polydopamine (PDA), an artificial melanin, as a light-absorbing material and examined in detail the effect of its composite method on the structural color. The following two composite methods were investigated using cerium(IV) oxide (CeO₂) particles as a core particle: binary coassembly of CeO₂ and PDA particles and unary assembly of CeO₂@PDA core-shell particles. Although both methods dramatically improved the visibility of the structural color by suppressing the scattered light owing to the light absorption capability of the PDA, there was a difference in the particle arrangement, angle dependence of the structural color, and color tone change. By selecting the PDA composite method, the guidelines for providing high visibility and the desired structural color were presented.

Printing a Wide Gamut of Saturated Structural Colors Using Binary Mixtures, With Applications in Anticounterfeiting

Use of colloidal suspensions to generate structural colors has the potential to reduce the use of toxic metals or organic pigments in inkjet printing, coatings, cosmetics, and other applications, and is a promising avenue to create large-scale nanostructures that produce long-lasting colors. However, expanded use of structural colors requires a reduction in coffee-ring effects during printing, which currently requires intricately patterned substrates or high particle concentrations, and diversification of colors to compete with conventional printing inks. Here, we treat substrate surfaces with cold plasma to facilitate spontaneous assembly of particles into colloidal nanostructures, reducing the need for highly concentrated particle suspensions. Moreover, by employing binary mixtures, we can tune the lightness of the hue produced or change the hue itself, allowing us to cover wider regions of color space. We demonstrate the use of this cold-plasma approach on a variety of substrates, favoring substrate diversity on which printing can be performed. This methodology enables creation of high-resolution, complex designs and opens a path for extending the limits of anticounterfeiting applications by using binary mixtures.

pH-Dependent Foam Formation Using Amphoteric Colloidal Polymer Particles

Near-monodispersed micrometer-sized polystyrene (PS) particles carrying amidino and carboxyl groups on their surfaces were synthesized by soap-free emulsion polymerization using an amphoteric free radical initiator. The resulting amphoteric PS particles were characterized in terms of diameter, morphology, disperibility in aqueous media and surface charge using scanning electron microscopy (SEM), optical microscopy (OM), sedimentation rate and electrophoretic measurements. At pH 2.0, where the amidino groups are protonated (positively charged), and at pH 11.0, where the carboxyl groups are deprotonated (negatively charged), the PS particles were well dispersed in aqueous media via electrostatic repulsion. At pH 4.8, where the surface charges are neutral, the PS particles were weakly aggregated. Furthermore, it was confirmed that the PS particles can function as a pH-sensitive foam stabilizer: foamability and foam stability were higher at pH 2.0 and 4.8, where the PS particles can be adsorbed to the air–water interface, and lower at pH 11.0, where the PS particles tend to disperse in bulk aqueous medium. SEM and OM studies indicated that hexagonally close-packed arrays of PS particles were formed on the bubble surfaces and moiré patterns were observed on the dried foams. Moreover, the fragments of dried foams showed iridescent character under white light.

Positive and negative birefringence in packed films of binary spherical colloidal particles

We have investigated the birefringence in packed films of binary spherical colloidal particles. Particulate films were obtained by drying a mixed suspension of colloidal particles with two different diameters. We observed positive and negative birefringence depending on the diameters and volume ratios of the large and small particles. When the diameters of the large and small particles were similar, the films showed positive birefringence. However, negative birefringence or weakening of positive birefringence was observed in films with a large diameter ratio and an optimal volume fraction of large particles. The large particles were embedded in packed small particles in the negative and weakened positive birefringent films. We propose a packing structure in which a single shell layer of small particles formed around a large particle. Using this model, we estimated the required volume ratio of large particles, and it was in good agreement with the optimal volume fraction. The relation between the packing structure of the binary colloidal particles and the birefringence is discussed.

We have investigated the birefringence in packed films of binary spherical colloidal particles.

Polydopamine as a stable and functional nanomaterial

The mussel inspired chemistry of dopamine leading to versatile coatings on the surface of all kinds of materials in a one pot process was considered as the unique aspect of catecholamine for a long time. Only recently, research has been undertaken to valorize the simultaneous oxidation and colloid formation in dopamine solutions in the presence of an oxidant. This mini review summarizes the synthesis methods allowing to get controlled nanomaterials, either nanoparticles, hollow capsules or nanotubes and even chiral nanomaterials from dopamine solutions. Finally the applications of those nanomaterials will be described.

Experimental and theoretical evidence for molecular forces driving surface segregation in photonic colloidal assemblies

Surface segregation in binary colloidal mixtures offers a simple way to control both surface and bulk properties without affecting their bulk composition. Here, we combine experiments and coarse-grained molecular dynamics (CG-MD) simulations to delineate the effects of particle chemistry and size on surface segregation in photonic colloidal assemblies from binary mixtures of melanin and silica particles of size ratio (D large /D small) ranging from 1.0 to ~2.2. We find that melanin and/or smaller particles segregate at the surface of micrometer-sized colloidal assemblies (supraballs) prepared by an emulsion process. Conversely, no such surface segregation occurs in films prepared by evaporative assembly. CG-MD simulations explain the experimental observations by showing that particles with the larger contact angle (melanin) are enriched at the supraball surface regardless of the relative strength of particle-interface interactions, a result with implications for the broad understanding and design of colloidal particle assemblies.

Bioinspired Production of Noniridescent Structural Colors by Adhesive Melanin-like Particles

Structural color printing of colloidal photonic films with tunable structures or optical properties is of great importance owing to their practical applications. In this article, we present a general method for the fabrication of colloidal particle films with tailored packing geometries by self-assembly of adhesive melanin-like polydopamine (PDA)-coated particles. The adhesion of particles is controlled by varying the thickness of the PDA coating, making it possible for dip coating of colloidal crystals, partly ordered or amorphous colloidal arrays (ACAs) with a tunable degree of order. We further studied the structural color printing of adhesive particles by infiltration-assisted or standard inkjet printing methods. Compared with bare particles, PDA-coated particles not only allow for control over color brightness/angle dependence of the photonic films but also significantly enhance the color quality of ACAs, both of which are useful for display, anticounterfeiting, or sensing applications. Owing to the inherent strong adhesiveness of PDA to virtually all types of surfaces, this PDA-based methodology can be potentially extended to a diverse range of colloidal particles toward the development of photonic devices.

Inertial microfluidics: A method for fast prediction of focusing pattern of particles in the cross section of the channel

Inertial microfluidics is utilized as a powerful passive method for particle and cell manipulation, which uses the hydrodynamic forces of the fluid in the channel to focus particles in specific equilibrium positions in the cross section of the channel. To achieve high performance manipulation, knowledge of focusing pattern of particles in the cross section of channel is essential. In this paper, we propose a method to address this important issue. To this end, firstly inertial microfluidics is analyzed in rectangular cross section channels. The results indicate that fluid flow velocity and channel's cross-sectional profiles have great impacts on the forces exerted on particles. Next, these results are utilized to propose a method to predict equilibrium positions in non-rectangular cross section channels through some simple calculations. This method is based on approximating the velocity profile of a non-rectangular cross section channel by utilizing portions of velocity profiles of different rectangular cross section channels. To analyze the method's performance, results obtained from the proposed method are compared with Direct Numerical Simulation (DNS) and experimental studies of seven non-rectangular channels. It is observed that the proposed approach accurately predicts particles trajectories and their equilibrium positions in the cross section of channels.

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.

Solution-Processable Photonic Inks of Mie-Resonant Hollow Carbon-Silica Nanospheres

Hollow carbon-silica nanospheres that exhibit angle-independent structural color with high saturation and minimal absorption are made. Through scattering calculations, it is shown that the structural color arises from Mie resonances that are tuned precisely by varying the thickness of the shells. Since the color does not depend on the spatial arrangement of the particles, the coloration is angle independent and vibrant in powders and liquid suspensions. These properties make hollow carbon-silica nanospheres ideal for applications, and their potential in making flexible, angle-independent films and 3D printed films is explored.

Designing Structural-Color Patterns Composed of Colloidal Arrays

Structural coloration provides a great potential for various applications due to unique optical properties distinguished from conventional pigment colors. Structural colors are nonfading, iridescent, and tunable, which is difficult to achieve with pigments. In addition, structural color is potentially less toxic than pigments. However, it is challenging to develop structural colors because elaborate nanostructures are a prerequisite for the coloration. Furthermore, it is highly suggested the nanostructures be patterned at various length scales on a large area to provide practical formats. There have been intensive studies to develop pragmatic methods for producing structural-color patterns in a controlled manner using either colloidal crystals or glasses. This article reviews the current state of the art in the structural-color patterning based on the colloidal arrays. We first discuss common and different features between colloidal crystals and glasses. We then categorize colloidal arrays into six distinct structures of 3D opals, inverse opals, non-close-packed arrays, 2D colloidal crystals, 1D colloidal strings, and 3D amorphous arrays and study various methods to make them patterned from recent key contributions. Finally, we outline the current challenges and future perspectives of the structural-color patterns.

Ellipsoidal Artificial Melanin Particles as Building Blocks for Biomimetic Structural Coloration

Inspired by the structural coloration of anisotropic materials in nature, we demonstrate the preparation of structural color materials by the assembly of anisotropic particles. Spherical artificial melanin particles consisting of a polystyrene core and polydopamine shell were stretched asymmetrically to form uniform-sized ellipsoidal particles with different aspect ratios. The aspect ratio and assembly method of the ellipsoidal particles influence the structural coloration, indicating that the particle shape is one of the important parameters for controlling the structural coloration. The discovery of a method to control the structural color using ellipsoidal particles is useful in basic research on structural colors in nature and provides flexibility in material design and extends the application range of structural color materials.

Structural Coloration with Nonclose-Packed Array of Bidisperse Colloidal Particles

Colloidal crystals and glasses have their own photonic effects. Colloidal crystals show high reflectivity at narrowband, whereas colloidal glasses show low reflectivity at broadband. To compromise the opposite optical properties, a simple means is suggested to control the colloidal arrangement between crystal and glass by employing two different sizes of silica particles with repulsive interparticle potential. Monodisperse silica particles with repulsive potential spontaneously form crystalline structure at volume fraction far below 0.74. When two different sizes of silica particles coexist, the arrangement of silica particles is significantly influenced by two parameters: size contrast and mixing ratio. When the size contrast is small, a long-range order is partially conserved in the entire mixing ratio, resulting in a pronounced reflectance peak and brilliant structural color. When the size contrast is large, the long-range order is rapidly reduced along with mixing ratio. Nevertheless, a short-range order survives, which causes low reflectivity at a broad wavelength, developing faint structural colors. These findings offer an insight into controlling the colloidal arrangements and provide a simple way to tune the optical property of colloidal arrays for structural coloration.

Adhesion Control of Branched Catecholic Polymers by Acid Stimulation

Biomimetic material design is a useful method for producing new functional materials. In recent years, catecholic polymers inspired from the adhesion mechanism of marine organisms have attracted attention. Here, we demonstrated the preparation of catecholic polymers by reversible addition-fragmentation chain transfer (RAFT) polymerization of an acetonide-protected catecholic monomer, that is, N-(2-(2,2-dimethylbenzo-1,3-dioxol-5-yl)ethyl)-acrylamide (DDEA). By selecting the specific RAFT reagents, well-defined branched PDDEA and linear PDDEA were obtained. These PDDEA samples showed stronger adhesion strength after deprotection by acid stimulation compared with that before deprotection. In addition, we demonstrated the adhesion control of synthetic polymers by photoirradiation in the presence of photoacid generators, which decompose under light and release an acid.

Hairy Polydopamine Particles as Platforms for Photonic and Magnetic Materials

By selecting the core materials and grafted-hair polymers, hairy particles with polymer brushes can create various types of functional materials. In recent years, polydopamine (PDA) particles that are obtained by polymerizing dopamine, which is an amino acid derivative, have attracted attention for various applications. Herein, we present a novel approach for creating photonic and magnetic materials from hairy PDA particles. By grafting a hydrophilic hair polymer, we have succeeded in producing photonic materials capable of structural color changes. Furthermore, we have demonstrated the preparation of magnetic materials by immobilizing holmium, which is one of the lanthanide elements, by electrostatic interactions onto a cationic hair polymer. These results demonstrate the possibility of hairy PDA particles for a wide range of applications, such as for photonic and magnetic materials.

Melanin Precursor Influence on Structural Colors from Artificial Melanin Particles: PolyDOPA, Polydopamine, and Polynorepinephrine

Polydopamine (PDA) is of interest as a mimetic material of melanin to produce structural color materials. Herein, to investigate the influence of the material composition of the artificial melanin particles on structural color, we demonstrated the preparation of core-shell particles by polymerization of norepinephrine or 3,4-dihydroxyphenylalanine, which are melanin precursors similar to dopamine, in the presence of polystyrene particles. It was revealed that the arrays of the prepared particles produce high-visibility structural color because of absorption of scattering light. Although poly(3,4-dihydroxyphenylalanine) showed the same tendency as PDA which was previous studied, polynorepinephrine can easily produce a smooth and thick shell layer compared with that of PDA, and pellets consisting of the particles showed angle-dependent structural color. Therefore, the artificial melanin particles that produce angle-dependent structural color became stable than ever before. These results indicated that material compositions of artificial melanin particles have influence on structural color, and an important finding for application as a coloring material was obtained.

Bioinspired Photonic Pigments from Colloidal Self-Assembly

The natural world is a colorful environment. Stunning displays of coloration have evolved throughout nature to optimize camouflage, warning, and communication. The resulting flamboyant visual effects and remarkable dynamic properties, often caused by an intricate structural design at the nano- and microscale, continue to inspire scientists to unravel the underlying physics and to recreate the observed effects. Here, the methodologies to create bioinspired photonic pigments using colloidal self-assembly approaches are considered. The physics governing the interaction of light with structural features and natural examples of structural coloration are briefly introduced. It is then outlined how the self-assembly of colloidal particles, acting as wavelength-scale building blocks, can be particularly useful to replicate coloration from nature. Different coloration effects that result from the defined structure of the self-assembled colloids are introduced and it is highlighted how these optical properties can be translated into photonic pigments by modifications of the assembly processes. The importance of absorbing elements, as well as the role of surface chemistry and wettability to control structural coloration is discussed. Finally, approaches to integrate dynamic control of coloration into such self-assembled photonic pigments are outlined.

Polydopamine-Based 3D Colloidal Photonic Materials: Structural Color Balls and Fibers from Melanin-Like Particles with Polydopamine Shell Layers

Nature creates beautiful structural colors, and some of these colors are produced by nanostructural arrays of melanin. Polydopamine (PDA), an artificial black polymer produced by self-oxidative polymerization of dopamine, has attracted extensive attention because of its unique properties. PDA is a melanin-like material, and recent studies have reported that photonic materials based on PDA particles showed structural colors by enhancing color saturation through the absorption of scattered light. Herein, we describe the preparation of three-dimensional (3D) colloidal photonic materials, such as structural color balls and fibers, from biomimetic core-shell particles with melanin-like PDA shell layers. Structural color balls were prepared through the combined use of membrane emulsion and heating. We also demonstrated the use of microfluidic emulsification and solvent diffusion for the fabrication of structural color fibers. The obtained 3D colloidal materials, i.e., balls and fibers, exhibited angle-independent structural colors due to the amorphous assembly of PDA-containing particles. These findings provide new insight for the development of dye-free technology for the coloration of various 3D colloidal architectures.

Structural Emergence in Particle Dispersions

Particle dispersions, such as pigment-based inks, comprise weakly bound, milled nanoparticles. The properties of these pigments depend on both their chemical composition and a rather complex structural hierarchy which emerges from these dispersions. The emergence of structure under semidilute conditions is related to the structure of the dilute particles, the particle spacing (mesh size), processing history, and the interaction potential. Kinetic simulations could predict such emergence using these input parameters. In this paper, organic pigments are studied as an example of the importance of emergent structure to predict properties such as brilliance and opacity. Organic pigments are used to impart color to commercial inks, plastics, coatings, and cosmetics. In many cases, dilute pigments are mass fractal structures consisting of aggregated nanoparticles held together by weak van der Waals forces. In water, surfactant is added to create a pigment dispersion (an ink). The final properties of a pigment emerge from a complex interplay between aggregation and dispersion of aggregates as a function of concentration. Samples of the organic pigment yellow 14, PY14, were milled to four primary particle sizes to study the effect on structural emergence. The interaction between surfactant-stabilized PY14 aggregates in an aqueous medium was quantified by the second virial coefficient, A₂, which reflects long-range interactions. The degree of aggregation is associated with short-range attractive interactions between primary particles. In this series of pigments, the degree of aggregation increases dramatically with reduction in primary particle size. Concurrently, the second-order virial coefficient, A₂, increases reflecting stronger long-range repulsive interactions with particle size. Structural emergence can be understood through the percolation concentration and the filler mesh size. A₂ is translated into a repulsive interaction potential for use in dissipative particle dynamics simulations to enable predictive modeling. This description of the interactions between dispersed pigment aggregates allows for a more scientific and predictive approach to understand structural emergence.

Wood-mimetic skins prepared using horseradish peroxidase catalysis to induce surface wrinkling of chitosan film upon drying

We previously developed bio-based wrinkled surfaces induced by wood-mimetic skins upon drying in which microscopic wrinkles were fabricated on a chitosan (CS) film by immersing it in a phenolic acid solution, followed by horseradish peroxidase (HRP)-catalyzed surface reaction and drying. However, the detailed structure of the resulting wood-mimetic skins, including crosslinking mode and thickness, has not been clarified due to the difficulty of the analysis. Here, we prepare wrinkled films using ferulic acid (FE), vanillic acid (VA), and homovanillic acid (HO) and characterize their structures to clarify the unknown characteristics of wood-mimetic skin. Chemical and structural analyses of wood-mimetic skins prepared using VA and HO indicate that the crosslinking structure in the skin is composed of ionic bonds between CS and an oligophenolic residue generated by the HRP-catalyzed reaction on the CS surface. Moreover, the quantity of these ionic bonds is related to the skin hardness and wrinkle size. Finally, SEM and TOF-SIMS analyses indicate that the skin thickness is on the submicron order (<200nm).

Liquid-immune structural colors with angle-independence inspired from hollow melanosomes

In this work, hollow polydopamine (PDA) amorphous colloidal structures with liquid-immune and angle-independent structural colors have been successfully fabricated.