Structural colors: from natural to artificial systems.

Structural color (SC), generated by light interacting with nanostructured materials, are responsible for the brightest and most vivid coloration in nature. Despite being widespread within the tree of life, there is little knowledge of the genes involved. Partial exceptions are some colonies of Flavobacteriia in which genes involved in a number of pathways, including gliding motility and polysaccharide metabolism, have been linked to SC. A previous genomic analysis of SC and non-SC bacteria suggested that the pterin pathway is involved in the organization of bacteria to form SC. Thus here, we focus on the moeA molybdopterin molybdenum transferase. When this gene was deleted from Flavobacterium IR1, the knock- out mutant showed a strong blue shift in SC of the colony, different from the green SC of the wild-type. The moeA mutant showed a particularly strong blue shift when grown on kappa- carrageenan and was upregulated for starch degradation. To further analyze the molecular changes, proteomic analysis was performed, showing the upregulation of various polysaccharide utilization loci, which supported the link between moeA and polysaccharide metabolism in SC. Overall, we demonstrated that single-gene mutations could change the optical properties of bacterial SC, which is unprecedented when compared to multicellular organisms where structural color is the result of several genes and can not yet be addressed genetically.

Rich iridescent structural colors in nature, such as peacock feathers, butterfly wings, beetle scales, and mollusc nacre, have attracted extensive attention for a long time and they generally result from the interaction between light and periodic structures. However, non-iridescent structural colors, such as silvery structural colors, have received relatively little attention, and they usually result from non-periodic structures. Here, using optical microscopy, fiber-optic spectrometry, field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and laser Raman spectroscopy, we investigate the origin of a novel structural color occurring at the edge of a bivalve shell (i.e., an otter shell). We find that: (1) the structural colors are observed to be uniform metallic silvery when viewed with the naked eye; (2) they are surprisingly multicolored with various colorful pixels juxtaposed together when viewed with an optical microscope; (3) each individual pixel shows a single color originating from a periodic, multilayered organic film with definite spacing (d); and (4) different pixels vary significantly in size, shape, and color with different d values (202–387 nm). Finally, we confirm that the macroscopic silvery color results from the pointillistic mixing of nano-to microscale iridescent pixels. We also discuss the special photonic structure responsible for the silvery color. We hope that this work can not only accelerate our comprehension of photonic materials, but also provide new inspiration for the synthesis of silvery white materials.

Both superwettability and structural colours have attracted considerable attention in recent years. In addition, the combination of structural colours and superwettability could endow materials with broader application prospects. The combination provides a new strategy to design novel functional materials, and there are many studies pertaining to these materials that have been reported in recent years. Herein, a polysulfide (PSF) superhydrophobic coating was synthesized successfully. The PSF superhydrophobic coating possesses excellent superhydrophobicity, oleophobicity for diesel and macroscopic structural colour variation when wetted. The colour is changed when the coating is wetted and it returns to its original colour after drying. In addition, the surface presents better reusability and thermostability which satisfies various daily needs. The PSF superhydrophobic coating can be considered as an excellent candidate for designing wetting responsive materials, and it has enormous application potential in the fields of detection, sensing, anti-counterfeiting and security. For the first time, we present a novel and low-cost strategy to fabricate materials with both superhydrophobicity and structural colour, offering significant insights into the practical application of these functional materials.

Green photonic biosensing: Approaching sustainability in point-of-care diagnostics

Inspired by photonic nanostructures in nature, such as the hair-like chaetae on the body of sea mice, inverse opal photonic crystals films were fabricated with chitosan, a kind of biomacromolecule found in nature. First, monodispersed polystyrene (PS) colloidal crystal templates with different particle sizes were prepared. The inverse opal films (IOFs) were fabricated through in situ cross-linking of the PS templates. The IOFs contain periodically ordered interconnecting pores that endow the films with photonic stop bands and structural colors, which are visible to the naked eye. The IOFs exhibit rapid reversible changes in their structural colors and reflectance peaks in response to alcohols and phenols. Possible mechanisms for the shifts in the IOF's reflectance peaks are proposed. The changes in the IOFs in response to alcohols and phenols provide a potential way to visually detect these organic solvents.

Polysaccharides are a class of biopolymers that are widely exploited in living organisms for a diversity of applications, ranging from structural reinforcement to energy storage. Among the numerous types of polysaccharides found in the natural world, cellulose is the most abundant and widespread, as it is found in virtually all plants. Cellulose is typically organized into nanoscale crystalline fibrils within the cell wall to give structural integrity to plant tissue. However, in several species, such fibrils are organized into helicoidal nanostructures with a periodicity comparable to visible light (i.e., in the range 250-450 nm), resulting in structural coloration. As such, when taking bioinspiration as a design principle, it is clear that helicoidal cellulose architectures are a promising approach to developing sustainable photonic materials. Different forms of cellulose-derived materials have been shown to produce structural color by exploiting self-assembly processes. For example, crystalline nanoparticles of cellulose can be extracted from natural sources, such as cotton or wood, by strong acid hydrolysis. Such "cellulose nanocrystals" (CNCs) have been shown to form colloidal suspensions in water that can spontaneously self-organize into a cholesteric liquid crystal phase, mimicking the natural helicoidal architecture. Upon drying, this nanoscale ordering can be retained into the solid state, enabling the specific reflection of visible light. Using this approach, colors from across the entire visible spectrum can be produced, alongside striking visual effects such as iridescence or a metallic shine. Similarly, polymeric cellulose derivatives can also organize into a cholesteric liquid crystal. In particular, edible hydroxypropyl cellulose (HPC) is known to produce colorful mesophases at high concentrations in water (ca. 60-70 wt %). This solution state behavior allows for interesting visual effects such as mechanochromism (enabling its use in low-cost colorimetric pressure or strain sensors), while trapping the structure into the solid state enables the production of structurally colored films, particles and 3D printed objects. In this article, we summarize the state-of-the-art for CNC and HPC-based photonic materials, encompassing the underlying self-assembly processes, strategies to design their photonic response, and current approaches to translate this burgeoning green technology toward commercial application in a wide range of sectors, from packaging to cosmetics and food. This overview is supported by a summary of the analytical techniques required to characterize these photonic materials and approaches to model their optical response. Finally, we present several unresolved scientific questions and outstanding technical challenges that the wider community should seek to address to develop these sustainable photonic materials.

Currently, one of the most important problems is water scarcity due to increasing population and environmental factors. Humankind can overcome this problem by recycling polluted water. The structural colors obtained from photonic crystal structures draw attention with fadeless bright color, combined with low toxicity and eco-friendliness. In this study, different etching/anodizing processes were applied to obtain Fabry-Perot and Photonic Crystal Ti-TiO2 structures. Structural colors owing to the morphology of the anatase phase on the surface of the samples etched with hydrochloric, sulfuric, and hydrofluoric acid-based solutions were obtained. The structural color of the formation on the titanium surfaces is related to the Fabry-Perot structures, while variations were correlated with Photonic Crystal surface morphologies. Because the high reflectance values contributed to the structural color formation, the photocatalytic efficiency of the samples etched with acid-based solutions was found to be lower than the samples etched with basic sodium and potassium hydroxide solutions. High-efficiency structural color reactors can be obtained by shifting the reflected wavelength range from the absorption wavelength range of the pollution material.

Structural colours, the most intense, reflective and pure colours in nature, are generated when light is scattered by complex nanostructures. Metallic structural colours are widespread among modern insects and can be preserved in their fossil counterparts, but it is unclear whether the colours have been altered during fossilization, and whether the absence of colours is always real. To resolve these issues, we investigated fossil beetles from five Cenozoic biotas. Metallic colours in these specimens are generated by an epicuticular multi-layer reflector; the fidelity of its preservation correlates with that of other key cuticular ultrastructures. Where these other ultrastructures are well preserved in non-metallic fossil specimens, we can infer that the original cuticle lacked a multi-layer reflector; its absence in the fossil is not a preservational artefact. Reconstructions of the original colours of the fossils based on the structure of the multi-layer reflector show that the preserved colours are offset systematically to longer wavelengths; this probably reflects alteration of the refractive index of the epicuticle during fossilization. These findings will allow the former presence, and original hue, of metallic structural colours to be identified in diverse fossil insects, thus providing critical evidence of the evolution of structural colour in this group.

This research is an investigation of the preparation conditions for para-aramid aerogel fibers that are structurally colored by Rayleigh scattering, an angle-independent structural color. It is well known that structural color has the angle dependence of colors. However, there is currently a desire for materials with angle-independent structural color. During the fibers’ spinning process, the microstructures of the fibers are generated. The effect of spinning conditions is investigated by measuring the optical properties of the wet gel fibers. The neutralization speed of negatively charged PPTA fibrils has a significant impact on the formation of the dense microstructure. Spinning under the fast neutralization speed condition makes it possible to achieve the blue-colored fiber derived from Rayleigh scattering. The influence of neutralization speed on the microstructure of the fibers is confirmed by FE-SEM characterization. Furthermore, spinning using a narrow inner diameter needle decreases the whiteness component derived from Mie scattering of the fibers, resulting in a more vivid blue color. The angle-independent structurally colored fibers hold great promise for application in clothing, interiors, and industrial materials.

Artificial Structural Colors and Applications

Amorphous photonic structures with non-iridescent and highly-stable structural color were fabricated via a simple one-step spray-coating technique. With this strategy, the obtained films on textile substrates presented short-ordered and amorphous photonic structures (APSs) similar to the amorphous nanostructures of avian feathers. The structural color presented the same hue when viewed at different angles and could be well controlled by varying the diameters of the SiO2 nanospheres. The prepared fabrics with structural color exhibited high color stability due to stability in both the assembled physical structure and the refractive index. The high stability of the assembled physical structure was attributed to the cementing effect of Poly(methylmethacrylate-butylacrylate) P(MMA-BA) existing between textile substrate and SiO2 nanospheres and among SiO2 nanospheres, while the high stability in the refractive index was contributed by the liquid-resistance achieved by both the surface roughness and the low-surface-energy of the as-sprayed APSs. With the resistances to external forces and liquid invasion, the non-iridescent brilliant structural color of the as-prepared fabrics could be kept steady. In this study, an approach of fabricating APSs with non-iridescent and stable structural color was established to enhance its potential application in structural coloration of textiles, and other color-related smart textiles.

A review of bio-based dialdehyde polysaccharides as multifunctional building blocks for biomedical and food science applications.

Following the success of the 1st International Symposium on Functional Materials held in Kuala Lumpur, Malaysia, 5–8 December 2005, the second symposium was held in the beautiful city of Hangzhou, People's Republic of China, 16–19 May 2007. The latter symposium was a gathering of about 200 renowned researchers from 16 countries around the world. The conference consisted of 24 symposia, 5 keynote papers, 21 invited papers, 108 oral presentations and about 160 poster papers covering the frontier areas of materials science and technology of functional materials. They included topics such as energy storage materials, ferroelectric materials, ferromagnetic materials, ferroelectric thin films, applications of functional materials, nanofabrication, computational design, shape memory alloys, application of shape memory materials, ferroelectrics and thermoelectrics, advances in characterizations, magneto-optical materials, Zn and Ti oxides, synthesis of nanopowders and wires, and many other advanced functional materials. With the receipt of more than 396 abstracts, this conference was a gathering of great minds in one place to discuss the research frontiers and discoveries in functional materials.The Organizing Committee would like to express its sincere thanks to the members of the International Advisory Committee for their invaluable contributions to the symposium. The committee is also grateful for the generous support from the many sponsors. A word of sincere thanks needs to go to Professor Roger Wäppling, Editor-in-Chief and the editorial staff of IOP Publishing for the publication of selected papers in this special issue of Physica Scripta. Finally, our deepest gratitude should be directed to the National University of Singapore, Zhejiang University and the General Research Institute for Nonferrous Metals, People's Republic of China for, without their support, the conference would not have been a success.

The photonic nanoarchitectures in the wing scales of butterflies play an important role in the imaginal life of these insects, and they are developed under high evolutionary pressure. These nanocomposites of chitin and air can generate vivid structural colors that are mainly used for sexual communication. In the case of the butterfly species Common Blue (Polyommatus icarus), the nanostructures are reproduced so precisely, that only a ±10 nm peak position variation can be measured in the reflectance spectra of the blue structural color of the wings, which requires nanometer-scale accuracy in the reproduction of the photonic nanoarchitectures. Therefore, these precisely replicated nanoporous structures are promising templates of the future artificial photonic materials and also can be used in the potential applications. In this work, we present the results of our investigations regarding the relationship of the male structural colors with their population genetic structure across the Western Palearctic region. When natural populations were compared to an inbred lineage raised in a custom-made insectarium, variation of the structural color have been discovered which may have genetic background. The population genetic data showed significant differences between the wild European and the inbred lineages while only a minor shift was found in the structural color. This was still in the wavelength range defined by the European populations which is in good agreement with the previous observations that the sexual signaling color is essentially stable over a distance of 1600 km within Europe.

Rapid preparation of structural color coatings on flexible textiles by simple vacuum-assisted filtration self-assembly