Full-Color Biomimetic Photonic Materials with Iridescent and Non-Iridescent Structural Colors

Enhancing color saturation in photonic glasses through optimized absorption

Photonic glasses, isotropically assembled nanoparticles with short-range correlation, can produce angle independent structural colors. They show broader reflectance spectra and lower saturated colors, compared to photonic crystals. Low color saturation creates barriers for photonic glasses to be used for coatings, cosmetics, and colors. Broadband absorbing materials are commonly used to absorb incoherently scattered light to enhance the saturation. However, there is limited understanding on how the absorption quantitatively affects the colors of photonic glasses. To this end, we here use a validated Monte Carlo-based multiple scattering model to investigate how absorption impacts the reflectance spectra in photonic glasses. We show that the color saturation can be maximized with an optimal level of absorption regardless of sample thickness or refractive index contrast between particles and matrix. We quantitatively demonstrate that the multiple scattering is largely reduced with the optimal absorption level and the reflectance is dominantly contributed by the single scattering. The optimal absorption occurs when the sample absorption mean free path is comparable to the transport mean free path, which offers a guidance on how much absorbing material is needed for creating highly saturated photonic glasses. This work will not only pave ways for pushing applications of angle-independent structural colors, but also improve our understanding of light scattering and absorption in short-range correlated disordered systems.

Melanin-Ce6-loaded polydopamine nanoparticles-based enhanced phototherapy for B16 melanoma cancer cells

Melanoma is one of the most aggressive and lethal types of cancer owing to its metastatic propensity and chemoresistance property. An alternative therapeutic option is photodynamic and photothermal therapies (PDT/PTT), which employ near-infrared (NIR) light to generate heat and reactive oxygen species (ROS). As per previous reports, Melanin (Mel), and its synthetic analogs (i.e. polydopamine nanoparticles) can induce NIR light-mediated heat energy, thereby selectively targeting and ameliorating cancer cells. Similarly, chlorin e6 (Ce6) also has high ROS generation ability and antitumor activity against various types of cancer. Based on this tenet, In the current study, we have encapsulated Mel-Ce6 in a polydopamine (PDA) nanocarrier (MCP NPs) synthesized by the oxidation polymerization method. The hydrodynamic diameter of the synthesized spherical MCP NPs was 139 ± 10 nm. The MCP NPs, upon irradiation with NIR 690 nm laser for 6 min, showed photothermal efficacy of more than 50 °C. Moreover, the red fluorescence in the MCP NPs due to Ce6 can be leveraged for diagnostic purposes. Further, the MCP NPs exhibited considerable biocompatibility with the L929 cell line and exerted nearly 70% ROS-mediated cytotoxicity on the B16 melanoma cell line after the laser irradiation. Thus, the prepared MCP NPs could be a promising theranostic agent for treating the B16 melanoma cancer.

Point-of-care dengue detection: polydopamine-modified electrode for rapid NS1 protein testing for clinical samples

Early diagnosis of dengue infection by detecting the dengue virus non-structural protein 1 (DENV-NS1) is important to the patients to initiate speedy treatment. Enzyme-linked immunosorbent assay (ELISA)-based NS1 detection and RT-PCR are time-consuming and too complex to be employed in remote areas of dengue-endemic countries. Meanwhile, those of NS1 rapid test by lateral flow assay suffer from low detection limit. Electrochemical-based biosensors using screen-printed gold electrodes (SPGEs) have become a reliable detection method to convey both ELISA's high sensitivity and rapid test portability. In this research, we developed an electrochemical biosensor for DENV-NS1 detection by employing polydopamine (PDA)-modified SPGE. The electrodeposition of PDA on the surface of SPGE serves as a bioconjugation avenue for anti-NS1 antibody through a simple and low-cost immobilization procedure. The biosensor performance was evaluated to detect DENV-NS1 protein in PBS and human serum through a differential pulse voltammetric (DPV) technique. The developed sensing platform displayed a low limit of detection (LOD) of 1.63 pg mL-1 and a wide linear range of 10 pg mL-1 to 1 ng mL-1 (R2 ∼ 0.969). The sensing platform also detected DEV-NS1 from four different serotypes in the clinical samples collected from dengue patients in India and Indonesia, with acceptable sensitivity, specificity, and accuracy values of 90.00%, 80.95%, and 87.65%, respectively. This result showcased the facile and versatile method of PDA coating onto the surface of screen-printed gold electrodes for a miniaturized point-of-care (PoC) detection device.

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.

Waterborne Polymer Coatings with Bright Noniridescent Structural Colors

Structural color pigments offer an efficient, sustainable, and environmentally friendly approach to obtain waterborne polymer coatings. We developed polymer-based spherical photonic pigments to incorporate in aqueous dispersions of polymer nanoparticles used to obtain waterborne polymer films. Our spherical photonic pigments are assembled from polymer nanoparticles and are highly stable in water dispersion, maintaining their optical properties in the final polymer films. Unlike conventional dyes and pigments, which are prone to photobleaching because they are based on the absorption of light, photonic pigments rely on the selective reflection of light by their nanostructure and therefore are not photodegraded. Furthermore, different colors can be obtained from the same materials, changing only their nanostructure, in this case, the size of the polymer nanoparticles. Our novel spherical photonic pigments are noniridescent and can be incorporated in aqueous polymer nanoparticle dispersions without deteriorating their structure to produce waterborne polymer coatings with structural color. This approach for structural colored waterborne polymer coatings is efficient, simple, and environmentally friendly, offering excellent prospects for application in paints and coatings.

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.

Correlative chemical and elemental nano-imaging of morphology and disorder at the nacre-prismatic region interface in Pinctada margaritifera

Understanding biomineralization relies on imaging chemically heterogeneous organic-inorganic interfaces across a hierarchy of spatial scales. Further, organic minority phases are often responsible for emergent inorganic structures from the atomic arrangement of different polymorphs, to nano- and micrometer crystal dimensions, up to meter size mollusk shells. The desired simultaneous chemical and elemental imaging to identify sparse organic moieties across a large field-of-view with nanometer spatial resolution has not yet been achieved. Here, we combine nanoscale secondary ion mass spectroscopy (NanoSIMS) with spectroscopic IR s-SNOM imaging for simultaneous chemical, molecular, and elemental nanoimaging. At the example of Pinctada margaritifera mollusk shells we identify and resolve ~ 50 nm interlamellar protein sheets periodically arranged in regular ~ 600 nm intervals. The striations typically appear ~ 15 µm from the nacre-prism boundary at the interface between disordered neonacre to mature nacre. Using the polymorph distinctive IR-vibrational carbonate resonance, the nacre and prismatic regions are consistently identified as aragonite ([Formula: see text] cm-1) and calcite ([Formula: see text] cm-1), respectively. We observe previously unreported morphological features including aragonite subdomains encapsulated in extensions of the prism-covering organic membrane and regions of irregular nacre tablet formation coincident with dispersed organics. We also identify a ~ 200 nm region in the incipient nacre region with less well-defined crystal structure and integrated organics. These results show with the identification of the interlamellar protein layer how correlative nano-IR chemical and NanoSIMS elemental imaging can help distinguish different models proposed for shell growth in particular, and how organic function may relate to inorganic structure in other biomineralized systems in general.

3D printing of non-iridescent structural color inks for optical anti-counterfeiting

In this work, structural color inks with practical significance in anti-counterfeiting applications have been successfully manufactured by facilely mixing SiO2@PDA@PHEMA hybrid colloidal particles with the mediated molecules of HEMA. The appropriate rheological properties of these photonic inks provide high viscosity and self-supporting performance, ensuring sufficient interaction between particles to form short-range ordered arrays during the mixing and shearing process and thus generating non-iridescent colors. The strong and broad uniform light absorption capabilities of polydopamine (PDA) not only suppress the incoherent multiple scattering of the photonic inks, but also impart surprising optical anti-counterfeiting properties, i.e. black color under ambient illumination and dazzling reflective coloration under strong illumination. With the 3D printing technique, complicated angle-independent patterns with visualization and high fidelity are expected to be fabricated with the as-prepared photonic inks for real-life applications in smart anti-counterfeiting labels, thus encoding encrypted information and selective color rendering accessories.

Structural color from pigment-loaded nanostructures

Color can originate from wavelength-dependence in the absorption of pigments or the scattering of nanostructures. While synthetic colors are dominated by the former, vivid structural colors found in nature have inspired much research on the latter. However, many of the most vibrant colors in nature involve the interactions of structure and pigment. Here, we demonstrate that pigment can be exploited to efficiently create bright structural color at wavelengths outside its absorption band. We created pigment-enhanced Bragg reflectors by sequentially spin-coating layers of poly-vinyl alcohol (PVA) and polystyrene (PS) loaded with β-carotene (BC). With only 10 double layers, we achieved a peak reflectance over 0.8 at 550 nm and normal incidence. A pigment-free multilayer made of the same materials would require 25 double layers to achieve the same reflectance. Further, pigment loading suppressed the Bragg reflector's characteristic iridescence. Using numerical simulations, we further show that similar pigment loadings could significantly expand the gamut of non-iridescent colors addressable by photonic glasses.

Influence of Core Type and Shell Thickness on Avian-Inspired Structural Colors Produced from Melanin Nanoparticle Assemblies

Hollow melanosomes found in iridescent bird feathers, including violet-backed starlings and wild turkeys, enable the generation of diverse structural colors. It has been postulated that the high refractive index (RI) contrast between melanin (1.74) and air (1.0) results in brighter and more saturated colors. This has led to several studies that have synthesized hollow synthetic melanin nanoparticles and fabricated colloidal nanostructures to produce synthetic structural colors. However, these studies use hollow nanoparticles with thin shells (<20 nm), even though shell thicknesses as high as 100 nm have been observed in natural melanosomes. Here, we combine experimental and computational approaches to examine the influence of the varying polydopamine (PDA, synthetic melanin) shell thickness (0-100 nm) and core material on structural colors. Experimentally, a concomitant change in overall particle size and RI contrast makes it difficult to interpret the effect of a hollow or solid core on color. Thus, we utilize finite-difference time-domain (FDTD) simulations to uncover the effect of shell thickness and core on structural colors. Our FDTD results highlight that hollow particles with thin shells have substantially higher saturation than same-sized solid and core-shell particles. These results would benefit a wide range of applications including paints, coatings, and cosmetics.

In Situ Synthesis of Keratin and Melanin Chromophoric Submicron Particles

In humans, melanin plays an esthetic role, dictating hair and skin color and traits, while keratin is the protein that comprises most of the epidermis layer. Eumelanin and pheomelanin are types of melanin synthesized from the same building blocks via enzymatic oxidation. Pheomelanin has an additional building block, cysteine amino acid, which affects its final structure. Keratin contains high cysteine content, and by exploiting free thiols in hydrolyzed keratin, we demonstrate the formation of keratin-melanin (KerMel) chromophoric submicron particles. Cryo-TEM analyses found KerMel particle sizes to be 100-300 nm and arranged in the form of a central keratin particle with polymerized l-dopa chains. Attenuated total reflection (ATR)-FTIR, UV-vis, and fluorescence measurements identified new chemical bonds, indicating the formation of KerMel particles. Finally, KerMel replicated natural skin tones and proved cytocompatibility for human epidermal keratinocytes at concentrations below 0.1 mg/mL. Taken together, KerMel is a novel, tunable material that has the potential to integrate into the cosmetic industry.

Nanoparticle-Based Tough Polymers with Crack-Propagation Resistance

Although thin elastomer films of polymer nanoparticles are regarded as environmentally friendly materials, the low mechanical strength of the films limits their use in various applications. In the present study, we investigated the fracture resistance of latex films composed of acrylic nanoparticles where a small quantity of a rotaxane crosslinker was introduced. In contrast to conventional nanoparticle-based elastomers, the latex films composed of the rotaxane-crosslinked nanoparticles exhibited unusual crack propagation behavior; the direction of crack propagation changed from a direction parallel to the crack to one perpendicular to the crack, resulting in an increase in tear resistance. These findings will help to broaden the scope of design of new types of tough polymers composed of environmentally friendly polymer nanoparticles.

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.

Quantitative Optical and Structural Comparison of 3D and (2+1)D Colloidal Photonic Crystals

Colloidal crystals are excellent model systems to study self-assembly and structural coloration because their periodicities coincide with the wavelength range of visible light. Different assembly methods inherently introduce characteristic defects and irregularities, even with nearly monodisperse colloidal particles. Here, we investigate how these imperfections influence the structural coloration by comparing two techniques to obtain colloidal crystals. 3D colloidal crystals produced by convective assembly are well-ordered and periodically arranged but show microscopic cracks. (2+1)D colloidal crystals fabricated by stacking individual monolayers show a decreased hexagonal order and limited crystal registration between single monolayers in the z-direction. We investigate the optical properties of both systems by comparing identical numbers of layers using correlative microspectroscopy. These measurements show that the less ordered (2+1)D colloidal crystals exhibit higher reflected light intensities. Macroscopic reflection integrating all angles shows that the reflected light intensity levels out with an increasing number of layers, whereas incoherent scattering increases. Although both types of colloidal crystal show similar angle-dependent color shifts in specular reflection, the less-ordered structure of the (2+1)D colloidal crystal scatters light within a larger angular range under diffusive illumination. Our results suggest that structural coloration is surprisingly robust toward local defects and irregularities.

A Flexible and Robust Structural Color Film Obtained by Assembly of Surface-Modified Melanin Particles

In this study, core-shell-hairy-type melanin particles surface modified with a polydopamine shell layer and a polymer brush hairy layer were fabricated and assembled to readily obtain bright structural color films. The hot pressing of freeze-dried samples of melanin particles decorated with a hydrophilic, low glass transition temperature polymer brush results in films that exhibit an angle-dependent structural color due to a highly periodic microstructure, with increased regularity in the arrangement of the particle array due to the fluidity of the particles. Flexible, self-supporting, and easy-to-cut and process structural color films are obtained, and their flexibility and robustness are demonstrated using compression tests. This method of obtaining highly visible structural color films using melanin particles as a single component will have a significant impact on practical materials and applications.

Biomimetic Chiral Photonic Materials with Tunable Metallic Colorations Prepared from Chiral Melanin-like Nanorods for UV Shielding, Humidity Sensing, and Cosmetics

Many biological species combine the helical organization of cellulose or chitin microfibrils with broadband light absorption of black melanin to produce brilliant structural colors with metallic and glossy effects and other diverse functions. In this work, based on core-shell CNC@PDA chiral nanorods consisting of cellulose nanocrystals (CNCs) as the core and melanin-like polydopamine (PDA) as the shell that can form well-defined chiral liquid crystal phases, we report chiral photonic materials that closely mimic the unique coloration mechanisms and functionalities mastered by several biological species. The photonic films formed by such single CNC@PDA nanorods have brilliant iridescent structural colors originating from selective reflection of circularly polarized lights by the helical organization of CNC@PDAs across the films. Furthermore, the colors of such films have background-independent brightness, high visibility, and metallic effects that arise from the light absorption of the PDA component. Especially, the color ranges and metallic effects of the films can be conveniently tuned by varying the thickness of the PDA shell. In addition, the UV absorption and hygroscopic properties of PDA endow these CNC@PDA films with efficient broadband UV shielding and sensitive humidity-induced dynamic color changes. Due to the mussel-like superior adhesion of PDA, CNC@PDA-based photonic coatings can be formed conformably onto diverse kinds of substrates. A shiny eye shadow with viewing angle-dependent colorful patterns was used to demonstrate the potential applications. With combinations of multiple unique properties in one photonic material fabricated from a single building block, these CNC@PDA-based films are expected to have potential applications in cosmetics, UV protection, anticounterfeiting, chiral reflectors, etc.

Cephalopod-Mimetic Tunable Photonic Coatings Assembled from Quasi-Monodispersed Reflectin Protein Nanoparticles

The remarkable dynamic camouflage ability of cephalopods arises from precisely orchestrated structural changes within their chromatophores and iridophores photonic cells. This mesmerizing color display remains unmatched in synthetic coatings and is regulated by swelling/deswelling of reflectin protein nanoparticles, which alters platelet dimensions in iridophores to control photonic patterns according to Bragg's law. Toward mimicking the photonic response of squid's skin, reflectin proteins from Sepioteuthis lessioniana were sequenced, recombinantly expressed, and self-assembled into spherical nanoparticles by conjugating reflectin B1 with a click chemistry ligand. These quasi-monodisperse nanoparticles can be tuned to any desired size in the 170-1000 nm range. Using Langmuir-Schaefer and drop-cast deposition methods, ligand-conjugated reflectin B1 nanoparticles were immobilized onto azide-functionalized substrates via click chemistry to produce monolayer amorphous photonic structures with tunable structural colors based on average particle size, paving the way for the fabrication of eco-friendly, bioinspired color-changing coatings that mimic cephalopods' dynamic camouflage.

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.

Preparation of biomimetic non-iridescent structural color based on polystyrene-polycaffeic acid core-shell nanospheres

Caffeic acid (CA), as a natural plant-derived polyphenol, has been widely used in surface coating technology in recent years due to its excellent properties. In this work, caffeic acid was introduced into the preparation of photonic band gap materials. By controlling the variables, a reasonable preparation method of polystyrene (PS) @polycaffeic (PCA)–Cu(ii) core–shell microspheres was achieved: 1 mmol L⁻¹ cupric chloride anhydrous (CuCl₂), 3 mmol L⁻¹ sodium perborate tetrahydrate (NaBO₃·4H₂O), 2 mmol L⁻¹ CA and 2 g L⁻¹ polystyrene (PS) were reacted at 50 °C for 10 min to prepare PS@PCA–Cu(ii) core–shell microspheres through rapid oxidative polymerization of CA coated PS of different particle diameters. The amorphous photonic crystal structure was self-assembled through thermal assisted-gravity sedimentation, resulting in structural color nanomaterials with soft and uniform color, no angle dependence, stable mechanical fastness and excellent UV resistance.

Caffeic acid (CA), as a natural plant-derived polyphenol, has been widely used in surface coating technology in recent years due to its excellent properties.

Enhancing the Refractive Index of Polymers with a Plant-Based Pigment

Polymers are essential components of many nanostructured materials. However, the refractive indices of common polymers fall in a relatively narrow range between 1.4 and 1.6. Here, it is demonstrated that loading commercially-available polymers with large concentrations of a plant-based pigment can effectively enhance their refractive index. For polystyrene (PS) loaded with 67 w/w% β-carotene (BC), a peak value of 2.2 near the absorption edge at 531 nm is achieved, while maintaining values above 1.75 across longer wavelengths of the visible spectrum. Despite high pigment loadings, this blend maintains the thermoforming ability of PS, and BC remains molecularly dispersed. Similar results are demonstrated for the plant-derived polymer ethyl cellulose (EC). Since the refractive index enhancement is intimately connected to the introduction of strong absorption, it is best suited to applications where light travels short distances through the material, such as reflectors and nanophotonic systems. Enhanced reflectance from films is experimentally demonstrated, as large as sevenfold for EC at selected wavelengths. Theoretical calculations highlight that this simple strategy can significantly increase light scattering by nanoparticles and enhance the performance of Bragg reflectors.

Non-close-packed arrangement of soft elastomer microspheres on solid substrates

Unlike rigid microparticles, soft and deformable elastomer (rubber) microspheres were found to exhibit a non-close-packed arrangement on solid substrates after the evaporation of water from their dispersions. The microscopic observation revealed that individual microspheres are ordered in regular intervals at the air/water interface of a sessile droplet and remain fixed on the substrate without being affected by the capillary forces during evaporation due to their deformability. Moreover, using the Langmuir-Blodgett method, thin films of non-close-packed structures could be successfully generated over large areas. Our findings may potentially help to control the arranged structures of elastomer microspheres, which can be expected to improve the nano-science and technology for the precise control for e.g. surface patterning.

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.

Parameterization of the optical constants of polydopamine films for spectroscopic ellipsometry studies

Bio-inspired polydopamine coatings offer vast possibilities for surface modification of materials. The thickness of such nanometric coatings is usually estimated based on ellipsometry measurements. However, the complex light-absorbing nature of polydopamine is often overlooked when analyzing such data, which can result in inaccurate estimations of the coating thickness as well as the optical properties. In this study, we prepared and characterized three polydopamine coatings where the film thickness and surface roughness are systematically varied. For each case, we developed suitable optical models and showed how an inappropriate optical model can provide inaccurate estimates of the coating properties. AFM height profiles were obtained from scratched areas of each sample to verify the thickness values estimated by ellipsometry. The results confirm that polydopamine coatings, depending on the oxidation conditions, can possess different structural and optical properties, and thus require unique optical models for the ellipsometry analysis.

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.

Photonics in nature and bioinspired designs: sustainable approaches for a colourful world

Biological systems possess nanoarchitectures that have evolved for specific purposes and whose ability to modulate the flow of light creates an extraordinary diversity of natural photonic structures. In particular, the striking beauty of the structural colouration observed in nature has inspired technological innovation in many fields. Intense research has been devoted to mimicking the unique vivid colours with newly designed photonic structures presenting stimuli-responsive properties, with remarkable applications in health care, safety and security. This review highlights bioinspired photonic approaches in this context, starting by presenting many appealing examples of structural colours in nature, followed by describing the versatility of fabrication methods and designed coloured structures. A particular focus is given to optical sensing for medical diagnosis, food control and environmental monitoring, which has experienced a significant growth, especially considering the advances in obtaining inexpensive miniaturized systems, more reliability, fast responses, and the use of label-free layouts. Additionally, naturally derived biomaterials and synthetic polymers are versatile and fit many different structural designs that are underlined. Progress in bioinspired photonic polymers and their integration in novel devices is discussed since recent developments have emerged to lift the expectations of smart, flexible, wearable and portable sensors. The discussion is expanded to give emphasis on additional functionalities offered to related biomedical applications and the use of structural colours in new sustainable strategies that could meet the needs of technological development.

Angular-Independent Photonic Pigments via the Controlled Micellization of Amphiphilic Bottlebrush Block Copolymers

Photonic materials with angular-independent structural color are highly desirable because they offer the broad viewing angles required for application as colorants in paints, cosmetics, textiles, or displays. However, they are challenging to fabricate as they require isotropic nanoscale architectures with only short-range correlation. Here, porous microparticles with such a structure are produced in a single, scalable step from an amphiphilic bottlebrush block copolymer. This is achieved by exploiting a novel "controlled micellization" self-assembly mechanism within an emulsified toluene-in-water droplet. By restricting water permeation through the droplet interface, the size of the pores can be precisely addressed, resulting in structurally colored pigments. Furthermore, the reflected color can be tuned to reflect across the full visible spectrum using only a single polymer (M_n  = 290 kDa) by altering the initial emulsification conditions. Such "photonic pigments" have several key advantages over their crystalline analogues, as they provide isotropic structural coloration that suppresses iridescence and improves color purity without the need for either refractive index matching or the inclusion of a broadband absorber.

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.

Optical costs and benefits of disorder in biological photonic crystals

Photonic structures in ordered, quasi-ordered or disordered forms have evolved across many different animal and plant systems. They can produce complex and often functional optical responses through coherent and incoherent scattering processes, often too, in combination with broadband or narrowband absorbing pigmentation. Interestingly, these systems appear highly tolerant of faults in their photonic structures, with imperfections in their structural order appearing not to impact, discernibly, the systems' optical signatures. The extent to which any such biological system deviates from presenting perfect structural order can dictate the optical properties of that system and, thereby, the optical properties that system delivers. However, the nature and extent of the optical costs and benefits of imperfect order in biological systems demands further elucidation. Here, we identify the extent to which biological photonic systems are tolerant of defects and imperfections. Certainly, it is clear that often significant inherent variations in the photonic structures of these systems, for instance a relatively broad distribution of lattice constants, can consistently produce what appear to be effective visual appearances and optical performances. In this article, we review previously investigated biological photonic systems that present ordered, quasi-ordered or disordered structures. We discuss the form and nature of the optical behaviour of these structures, focusing particularly on the associated optical costs and benefits surrounding the extent to which their structures deviate from what might be considered ideal systems. Then, through detailed analyses of some well-known 1D and 2D structurally coloured systems, we analyse one of the common manifestations of imperfect order, namely, the extent and nature of positional disorder in the systems' spatial distribution of layers and scattering centres. We use these findings to inform optical modelling that presents a quantitative and qualitative description of the optical costs and benefits of such positional disorder among ordered and quasi-ordered 1D and 2D photonic systems. As deviation from perfectly ordered structures invariably limits the performance of technology-oriented synthetic photonic processes, we suggest that the use of bio-inspired fault tolerance principles would add value to applied photonic technologies.

Three-Dimensional Photonic Crystal Bulks with Outstanding Mechanical Performance Assembled by Thermoforming-Etching Cross-linked Polymer Microspheres

Traditional self-assembly methods for photonic crystals (PCs) limited by poor mechanical performance and microstructure defects make it hard to be directly applied to optical devices, whose performance strongly rely on mechanical performance and microstructure of PCs. Here, a thermoforming-etching strategy combining both traditional processing and nanofabrication is reported to develop cross-linked polystyrene microsphere-based PC bulks with outstanding mechanical performance. It illustrates scientific principles, where surface molecular chains of PS microspheres were activated and entangled with each other under thermoforming conditions (200 °C; 220 MPa), resulting in applicable mechanical strength (hardness and modulus reach 0.12 and 4.12 GPa, respectively). The optimum optical reflectivity of the PS microsphere-based (180 nm) PC bulk is 49.4% at 381 nm. Furthermore, these PC bulks have been successfully written in anti-counterfeiting and realized colorful pattern printing. The innovative method opens a new route for the rapid and simple fabrication of the nanoparticle structure which can be used as various functional devices and directly promotes the industrialization of bulk PC devices, such as optical and display devices, and so forth.

Making Photonic Crystals via Evaporation of Nanoparticle-Laden Droplets on Superhydrophobic Microstructures

We employed a convenient evaporation approach to fabricate photonic crystals by naturally drying droplets laden with nanoparticles on a superhydrophobic surface. The final drying morphology could be controlled by the concentration of nanoparticles. A dilute droplet resulted in a torus, whereas a quasi-spherical cap with a bottom cavity was made from a concentrated droplet. Remarkably, the nanofluid droplets maintained high contact angles (≳120°) during the entire evaporation process because of inhomogeneous surface wetting. Bottom-view snapshots revealed that during evaporation the color of the contact area changed sequentially from white to red, orange, yellow, and eventually to green. Scanning electron microscopy and Voronoi analysis demonstrated that nanoparticles were self-assembled to a hexagonal pattern. Finally, based on the effects of particle size, material, and volume concentration on the reflected wavelengths, a model has been developed to successfully predict the reflected wavelength peaks from the contact area of evaporating colloidal droplets. Our model can be easily adopted as a manufacturing guide for functional photonic crystals to predict the optimal reflected color made by evaporation-driven self-assembly of photonic crystals.

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.

Particle Monolayer-Stabilized Light-Sensitive Liquid Marbles from Polypyrrole-Coated Microparticles

Liquid marbles are water droplets coated with solid particles that prevent coalescence and allow storage, transport, and handling of liquids in the form of a powder. Here, we report on the formation of liquid marbles that are stabilized entirely by a single monolayer of solid particles and thus minimize the amount of required solid material. As stabilizing particles, we synthesize relatively monodisperse, 80 μm-sized polystyrene (PS) particles coated with heptadecafluorooctanesulfonic acid-doped polypyrrole (PPy-C₈F) shell (PS/PPy-C₈F particles) by aqueous chemical oxidative seeded polymerization of pyrrole using FeCl₃ as an oxidant and heptadecafluorooctanesulfonic acid as a hydrophobic dopant. We characterize the physicochemical properties of the particles as a function of the PPy-C₈F loading. Laser diffraction particle size analyses of dilute aqueous suspensions indicate that the polymer particles disperse stably in water medium before and after coating with the PPy-C₈F shell. X-ray photoelectron spectroscopy studies indicate the formation of a PPy-C₈F shell around the PS seed particles, which was further supported by deflated morphologies observed by scanning electron microscopy after extraction of the PS component from the PS/PPy-C₈F particles. We investigate the performance of the dried PS/PPy-C₈F particles to stabilize liquid marbles. Stereo- and laser microscope observations, as well as gravimetric studies, confirm that the PS/PPy-C₈F particles adsorb to the water droplet surface in the form of a particle monolayer with the characteristic hexagonal close-packed structure expected for spherical (colloidal) particles. Mechanical integrity of the liquid marble increases with an increase of PPy-C₈F loading of the PS/PPy-C₈F particles. The water contact angle of the PS/PPy-C₈F particles at air-water interface increases from 82 ± 12° to 102 ± 17° with an increase of PPy-C₈F loading. This increase in water contact angle directly correlates with the shape of the LM, with higher contact angles giving more spherical LMs. Furthermore, the broadband light absorption properties of the PPy coating was used to control evaporation rate of the enclosed water phase on demand by irradiation with a near-infrared laser. The evaporation rate could be finely controlled by the thickness of the PPy-C₈F shell of the particles stabilizing the liquid marbles.

Surface templated inverse photonic glass for saturated blue structural color

To substitute conventional pigments, which often are toxic or suffer from fading in ultraviolet light, non-iridescent structural colors should demonstrate high spectral selectivity, while being also mechanically stable. However, conventional photonic glass (PhG) shows low color saturation due to the gradual transition in the reflection spectrum and low mechanical stability due to weak interparticle attachment. Here, a PhG with sharp spectral transition in comparison with the conventional full sphere PhG is designed by a conformal coating via atomic layer deposition (ALD) onto an organic PhG template. The ALD deposition allows to control the film thickness precisely for the highly saturated color. This structure can be described by hollow particle motifs with the effective size larger than the interparticle distance. Such unusual PhG is motivated by the achievable features in the spatial Fourier transform of a disordered assembly of such motifs. The surface-templated inverse PhG shows much higher color saturation than the direct PhG from full spheres. Moreover, the dense and solid connected shell will be beneficial for mechanical stability. These results pave the way for highly saturated structural colors. The demonstrated sharp spectral selection feature can be also considered for many related applications such as sunscreens, photovoltaics and radiative cooling by adjusting the reflection transition to the required wavelength. This can be achieved by proportionally scaling the motif and lattice dimensions as well as the film thickness.

Strong Biomimetic Immobilization of Pt-Particle Catalyst on ABS Substrate Using Polydopamine and Its Application for Contact-Lens Cleaning with H2O2

Polydopamine (PDA)-a known adhesive coating material-was used herein to strongly immobilize a Pt-particle catalyst on an acrylonitrile-butadiene-styrene copolymer (ABS) substrate. Previous studies have shown that the poor adhesion between Pt particles and ABS surfaces is a considerable problem, leading to low catalytic durability for H2O2 decomposition during contact-lens cleaning. First, the ABS substrate was coated with PDA, and the PDA film was evaluated by X-ray photoelectron spectroscopy. Second, Pt particles were immobilized on the PDA-coated ABS substrate (ABS-PDA) using the electron-beam irradiation reduction method. The Pt particles immobilized on ABS-PDA (Pt/ABS-PDA) were observed using a scanning electron microscope. The Pt-loading weight was measured by inductively coupled plasma atomic emission spectroscopy. Third, the catalytic activity of the Pt/ABS-PDA was evaluated as the residual H2O2 concentration after immersing it in a 35,000-ppm H2O2 solution (the target value was less than 100 ppm). The catalytic durability was evaluated as the residual H2O2 concentration after repeated use. The PDA coating drastically improved both the catalytic activity and durability because of the high Pt-loading weight and strong adhesion among Pt particles, PDA, and the ABS substrate. Plasma treatment prior to PDA coating further improved the catalytic durability.

Structural color printing with a dielectric layer coated on a nanotextured metal substrate: simulation and experiment

The printing of plasmonic structural colors relies on noble metal nanostructures fabricated on Si, glass, or plastic substrates. This paper presents a simple surface structure for producing vivid structural colors directly from common metal substrates. The structure is formed by texturing the surface of stainless steel (STS) via imprinting and coating it with a dielectric layer. Diverse colors are generated simply by varying the thickness of the dielectric layer. The colors arise from surface plasmon resonance and guided-mode resonance of the incident light, which are excited on the textured STS surface and inside the dielectric layer, respectively. A finite-difference time-domain simulation shows that 500 nm is the optimum texture periodicity with regard to the tunability and vividness of the colors. This is experimentally verified by printing many differently colored images on the surface of STS substrates with a texture period of 500 nm. The proposed structure/method does not require a nanofabrication technique such as electron-beam lithography or focused ion beam etching. The results of the study provide a facile route for producing vivid structural colors on metals, which may find various applications, including surface decoration, product identification, anti-counterfeiting, and perfect absorbers.

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.

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.

Graphene hybrid colloidal crystal arrays with photo-controllable structural colors

An intelligent structural color hydrogel with photo-controllable capability was developed by adding graphene oxide (GO) into colloidal particle solutions. The high charge characteristic of GO could significantly enhance the electrostatic repulsion effect between adjacent particles and promote the ordered assembly of the colloidal particles. The resultant colloidal crystal arrays (CCAs) with a small amount of GO additive were imparted with vivid angle-dependent structural colors due to the enhanced photon absorption of the hybrid materials, whereas their structural colors became dull and angle-independent with a high GO concentration, which contributes to the isotropic short-range ordered CAA nanostructures. It was demonstrated that the GO hybrid structural color hydrogels with temperature-sensitive polymer components featured photo-responsive properties, which provided remotely controllable dynamic structural colors for different patterns. These features of the GO hybrid structural color hydrogels make them promising for the applications of anti-counterfeiting barcode and other related fields.