Structural Color Research Articles

Thin‐film iridescence in the eggshell of a stick insect (Myronides glaucus)

AbstractColours in nature can be pigmentary, structural or a combination of both. The prevalence, function and nanostructural origin of structural coloration in eggs is largely unknown. Stick and leaf insect eggs display a wide variety of colours, most of which are produced by pigments. The eggs of Myronides glaucus (Phasmida: Lonchodidae; Hennemann, 2021), however, show a clear purple to green iridescence. Here, we use micro‐spectrophotometry, Fourier‐transform infrared reflectance, transmission‐ and scanning electron microscopy, atomic force microscopy, finite‐difference time‐domain optical simulations and experimental approaches to elucidate the mechanism for iridescence in M. glaucus eggshells, which together reveal that iridescence is caused by thin‐film interference by a 200‐ to 450‐nm‐thick outermost layer. These results highlight the diversity of phasmid eggs and the need to study the different mechanisms and functions of structural coloration.

Size-Dependent Magnetic Responsiveness of a Photonic Crystal of Graphene Oxide Nanosheets.

A magnetically responsive photonic crystal of colloidal nanosheets can exhibit a controllable structural color, offering diverse potential applications. In this study, we systematically investigated how the lateral sizes of graphene oxide (GO) nanosheets affect their magnetic responsiveness in a photonic system. Contrary to the prediction that larger lateral sizes of nanosheets would be more responsive to an applied magnetic field based on the magnetic energy of anisotropic materials, we discovered that GO nanosheets with larger lateral sizes in the photonic system scarcely responded to a 12 T magnetic field. The lack of magnetic response may be due to the strongly restricted rotational motion of GO nanosheets by mutual electrostatic forces. In contrast, GO nanosheets with medium lateral sizes readily responded to the 12 T magnetic field, forming a uniaxially oriented structure that resulted in a vivid structural color. However, smaller GO nanosheets displayed a less vivid structural color, possibly because of less structural ordering of GO nanosheets. Finally, we found that the photonic crystal of GO nanosheets with optimized lateral sizes responded effectively to the 12 T magnetic field across various GO concentrations, resulting in a vivid and tunable structural color.

Brush-Like Polymeric Gels Enabled Photonic Crystals toward Ultrasensitive Cosolvent Chromism.

Responsive photonic crystals (RPCs) exhibit dynamic chromism upon trigger by various solvents, showing potential applications in qualitative identification and quantitative analysis of multicomponent solvents. However, distinguishing similar solvents, especially traces of cosolvents, remains challenging due to the limited sensitivity of RPCs. To address this, we herein introduce brush-like polymeric gels inside photonic crystals, forming a brush-like polymeric photonic gel (BPPG) that can trace tiny component changes. In this BPPG system, the acrylate backbones and polyethylene glycol (PEG) side-chains stretch incrementally due to the cosolvency of ethanol-water mixtures, resulting in highly sensitive chromatic responses within ethanol-rich concentrations. With water content varying slightly from 0 to 1 vol %, the reflection wavelength of BPPG can sharply redshift over 30 nm, leading to very noticeable changes in structural color. This enhanced sensitivity makes BPPG suitable for real-time, in situ purity monitoring of absolute ethanol during storage, transportation, and other applications.

Dynamic Wavelength-Selective Diffraction and Absorption with Direct-Patterned Hydrogel Metagrating.

Hydrogel nanophotonic devices exhibit attractive tunable capabilities in structural coloration and optical display. However, current hydrogel-based tunable strategies are mostly based on a single physical mechanism, and it remains a challenge to merge multiple mechanisms for active devices with integrated functionalities. Here, a hydrogel metagrating combining Fabry-Pérot (FP) resonance and diffraction effects is proposed for achieving tunable absorption and dynamic wavelength-selective beam steering. Through exploiting hydrogel shrinkage under electron-beam exposure, a hydrogel nanocavity composed of Ag/Hydrogel/Ag three-layer films can be directly printed with arbitrary patterns, enabling the direct-pattering technique of metagrating. The hydrogel nanocavity performs as an FP-type absorber, and its absorption peak rapidly shifts with humidity variation due to the hydrogel layer scaling. The response speed is <320ms, and the absorption peak shift range is >150nm. It is further demonstrated that the hydrogel metagrating exclusively deflects light at the resonance wavelength, and its operating wavelength can be actively switched by regulating ambient humidity. The proposed tunable hydrogel metagrating can promote new technologies of tunable metasurfaces for optical filtering, gas sensing, and optical imaging.

Biomimetic Cooling: Functionalizing Biodegradable Chitosan Films with Saharan Silver Ant Microstructures.

The increasing occurrence of hot summer days causes stress to both humans and animals, particularly in urban areas where temperatures can remain high, even at night. Living nature offers potential solutions that require minimal energy and material costs. For instance, the Saharan silver ant (Cataglyphis bombycina) can endure the desert heat by means of passive radiative cooling induced by its triangular hairs. The objective of this study is to transfer the passive radiative cooling properties of the micro- and nanostructured chitin hairs of the silver ant body to technically usable, biodegradable and bio-based materials. The potential large-scale transfer of radiative cooling properties, for example, onto building exteriors such as house facades, could decrease the need for conventional cooling and, therefore, lower the energy demand. Chitosan, a chemically altered form of chitin, has a range of medical uses but can also be processed into a paper-like film. The procedure consists of dissolving chitosan in diluted acetic acid and uniformly distributing it on a flat surface. A functional structure can then be imprinted onto this film while it is drying. This study reports the successful transfer of the microstructure-based structural colors of a compact disc (CD) onto the film. Similarly, a polyvinyl siloxane imprint of the silver ant body shall make it possible to transfer cooling functionality to technically relevant surfaces. FTIR spectroscopy measurements of the reflectance of flat and structured chitosan films allow for a qualitative assessment of the infrared emissivity. A minor decrease in reflectance in a relevant wavelength range gives an indication that it is feasible to increase the emissivity and, therefore, decrease the surface temperature purely through surface-induced functionalities.

Filaments for 3D Printing of Iridescent Structural Colors

Filaments for 3D Printing of Iridescent Structural Colors

Polymerization-induced ultrafast and massive growth of colloidal photonic crystals on hydrogel surface

Structural color materials based on colloidal photonic crystals (CPCs) have been extensively investigated since they can be prepared efficiently. Commonly, photonic hydrogels are prepared by a polymerizing pre-gel solution, in which colloids are self-assembled by passive methods. Except for magnetic colloids, the process of forming colloidal photonic crystals in a pre-gel solution requires hours to days, and it is almost impossible to assemble the colloids directly in hydrogels. Here, we present a mild strategy to prepare hydrogels with surface structural color, ordered colloidal arrays are formed at the air-hydrogel interface actively during the polymerization process, and bright structural color forms in minutes on a large scale. Polymer exclusion is the key driving force for the assembly of the colloids, other factors, such as evaporation flux, can also promote the colloids to assemble. Significantly, heterogeneous structural color patterns can be produced easily by exposing the hydrogel surface to air sequentially using a mask during the polymerization process. The simple and fast method provides a general route for preparing structural colored surfaces, which may be suitable for various applications.

Engineering Fano resonances in plasmonic metasurfaces for colorimetric sensing and structural colors

In this paper, we present the design and fabrication of a plasmonic metasurface based on titanium dioxide (TiO2) nanowire arrays integrated with plasmonic layers. The structure is engineered to produce Fano resonances within the visible spectrum, resulting from the coupling of localized surface plasmon resonances, lattice modes, and nanowire's optical modes. Experimentally, we show that by tuning the geometrical features of the metasurface, such as the length, diameter, and period of the nanowires, a high-quality factor single peak can be achieved in the reflection spectra, resulting in vivid structural colors in bright field. To our knowledge, this is the first demonstration of such vivid colors with nanowire arrays in bright field reflections. When characterized by refractive index fluids around the refractive index of water, the plasmonic metasurface also showed great potential for biochemical colorimetric sensing. The best design demonstrated a bulk sensitivity of 183 nm/RIU with high Q resonance features and linear changes in color values using image processing.

All‐In‐One Photonic Crystals With Multi‐Stimuli‐Chromic, Color‐Recordable, Self‐Healable, and Adhesive Functions

AbstractFabricating photonic crystals (PCs) with dynamic multi‐stimuli‐responsive structural colors, recordable colors, and self‐healable properties is significant for their emerging applications, yet remains a big challenge. Here, an all‐in‐one multifunctional PC (MFPC) film with outstanding mechanochromic, thermochromic, solvatochromic, color/shape‐recordable, self‐healable, and adhesive functions is designed and prepared by simply non‐close‐assembling silica particles into the unique 2‐[[(Butylamino)carbonyl]oxy]ethyl acrylate (BCOEA) followed by photopolymerization. The success is mainly due to the rational combination of non‐close‐packing structures and BCOEA's characteristics, including its urethane groups with numerous sacrificial hydrogen bonds, temperature‐dependent refractive index, swellable and deformable polymer network, as well as low glass‐transition temperature. The MFPC's hue and color saturation can be reversibly and dynamically modulated by strain/pressure/solvents and temperature, respectively, thereby realizing visually and spectrally sensing these stimuli. More interestingly, the color‐responsiveness combined with other functions endows MFPCs with fascinating emerging applications, including multilayer optical filters, inkless printing, reconfigurable multicolor patterns, stretching‐based anti‐counterfeiting, etc. This work offers a new perspective for designing next‐generation smart photonic materials and will facilitate their all‐round applications.

Smart photonic crystal hydrogels for visual glucose monitoring in diabetic wound healing

Diabetes is a global chronic disease that seriously endangers human health and characterized by abnormally high blood glucose levels in the body. Diabetic wounds are common complications which associate with impaired healing process. Biomarkers monitoring of diabetic wounds is of great importance in the diabetes management. However, actual monitoring of biomarkers still largely relies on the complex process and additional sophisticated analytical instruments. In this work, we prepared hydrogels composed of different modules, which were designed to monitor different physiological indicators in diabetic wounds, including glucose levels, pH, and temperature. Glucose monitoring was achieved based on the combination of photonic crystal (PC) structure and glucose-responsive hydrogels. The obtained photonic crystal hydrogels (PCHs) allowed visual monitoring of glucose levels in physiological ranges by readout of intuitive structural color changes of PCHs during glucose-induced swelling and shrinkage. Interestingly, the glucose response of double network PCHs was completed in 15 min, which was twice as fast as single network PCHs, due to the higher volume fraction of glucose-responsive motifs. Moreover, pH sensing was achieved by incorporation of acid-base indicator dyes into hydrogels; and temperature monitoring was obtained by integration of thermochromic powders in hydrogels. These hydrogel modules effectively monitored the physiological levels and dynamic changes of three physiological biomarkers, both in vitro and in vivo during diabetic wound healing process. The multifunctional hydrogels with visual monitoring of biomarkers have great potential in wound-related monitoring and treatment.Graphical abstract

Laser Control of Specular and Diffuse Reflectance of Thin Aluminum Film-Isolator-Metal Structures for Anti-Counterfeiting and Plasmonic Color Applications

Plasmonic structural color originates from the scattering and absorption of visible light by metallic nanostructures. Stacks consisting of thin, disordered semicontinuous metal films are attractive plasmonic color media, as they can be mass-produced using industry-proven physical vapor deposition techniques. These films are comprised of random nano-island structures of various sizes and shapes resonating at different wavelengths. When irradiated with short-pulse lasers, the nanostructures are locally restructured, and their optical response is altered in a spectrally selective manner. Therefore, various colors are obtained. We demonstrate the generation of structural plasmonic colors through femtosecond laser modification of a thin aluminum film–isolator–metal mirror (TAFIM) structure. Laser-induced structuring of TAFIM’s top aluminum film significantly alters the sample’s specular and diffuse reflectance depending on the fluence value and the number of times a region is scanned. A “negative image” effect is possible, where a dark field observation mode image is a negative of a bright field mode image. This effect is visible using an optical microscope, the naked eye, and a digital camera. The use of self-passivating aluminum results in a long-lasting, non-fading coloration effect. The reported technique could be used in anti-counterfeiting and security applications, as well as in plasmonic color printing and macroscopic and microscopic marking for personalized fine arts and aesthetic products such as jewelry.

Electrostatic fence induced assembly of low-concentration colloidal nanospheres to form liquid photonic crystals

Liquid photonic crystals (LPCs) have great application potential in sensors, anti-counterfeiting materials and detection due to their sensitive dynamic responsiveness. Currently, the preparation of LPCs mainly relies on the supersaturation of colloidal nanospheres. However, the supersaturation method usually fails to obtain LPCs based on low-concentration colloidal nanospheres. In turn, the use of high-concentration colloidal nanospheres results in poor mobility of LPCs, which makes them inappropriate for subsequent utilization in responsive systems. In this study, self-assembly of LPCs with vibrant structural colors is achieved through the electrostatic fence effect by introducing an anionic carboxylate-containing polymer as an inducer into the system of low-concentration negatively charged colloidal nanospheres (≥1.25 wt%). It is shown that the anionic carboxylate groups on the inducer molecules and the appropriate molecular chain length are the decisive factors for the inducing effect. The obtained LPCs exhibit a typical non-close-packed structure with a face-centered cubic arrangement of nanospheres. The nearest inter-nanosphere surface distance is 12.10 nm, and the farthest one is as long as 40.30 nm. The LPCs possess good dynamic recovery performance and sensitive optical response characteristics, which are conducive to application in responsive optical sensors directly or after filling.

3D Printing of Hierarchical Structures Made of Inorganic Silicon-Rich Glass Featuring Self-Forming Nanogratings.

Hierarchical structures are abundant in nature, such as in the superhydrophobic surfaces of lotus leaves and the structural coloration of butterfly wings. They consist of ordered features across multiple size scales, and their advantageous properties have attracted enormous interest in wide-ranging fields including energy storage, nanofluidics, and nanophotonics. Femtosecond lasers, which are capable of inducing various material modifications, have shown promise for manufacturing tailored hierarchical structures. However, existing methods, such as multiphoton lithography and three-dimensional (3D) printing using nanoparticle-filled inks, typically involve polymers and suffer from high process complexity. Here, we demonstrate the 3D printing of hierarchical structures in inorganic silicon-rich glass featuring self-forming nanogratings. This approach takes advantage of our finding that femtosecond laser pulses can induce simultaneous multiphoton cross-linking and self-formation of nanogratings in hydrogen silsesquioxane. The 3D printing process combines the 3D patterning capability of multiphoton lithography and the efficient generation of periodic structures by the self-formation of nanogratings. We 3D-printed micro-supercapacitors with large surface areas and a high areal capacitance of 1 mF/cm2 at an ultrahigh scan rate of 50 V/s, thereby demonstrating the utility of our 3D printing approach for device applications in emerging fields such as energy storage.

Mechanochromic 3D Soft Photonic Crystals Enabled Anticounterfeiting and Encryption Information Storage

AbstractInspired by the color‐changing abilities of natural organisms, this research focuses on the design and fabrication of mechanochromic liquid crystal photonic crystal thin films with periodic self‐assembly structures. Benefiting from its unique 3D self‐assembled structure with a body‐centered cubic lattice, BP I film exhibits excellent mechanical properties, characterized by superior elasticity and toughness due to the presence of rigid and flexible microdomains and suitable dislocation line volumes. As the strain increases, the structural color of the BP I film demonstrates a repeatable and rapid shift from red to blue, covering the entire visible light spectrum with high reflectivity. The incorporation of fluorescent molecules further enhances the mechanical properties and endowing the film with circularly polarized luminescence under UV light irradiation. Utilizing different fluorescent dyes as inks, arbitrary patterned writing can be performed on the BPI film, enabling specific information hiding and encryption. The prepared label exhibits excellent environmental stability, presenting promising applications in information storage, anti‐counterfeiting, and flexible 3D displays. The study may contribute to a deeper understanding of the relationship between microstructure, mechanical deformation, and optical properties, potentially advancing the development of innovative responsive materials and multifunctional devices.

Water-Rewritable Structural Color Textiles with Fast Response Speed and Antispreading Capability.

The stimuli-responsive textiles, especially water-responsive textiles, have garnered attention owing to their environmental compatibility. Inspired by the hydrochromic behavior of Diphylleia grayi, water-rewritable structural color (WRSC) textiles exhibiting fast response speed and antispreading capability were fabricated by spraying hollow SiO2 (H-SiO2) microspheres and poly(trifluoroethyl methacrylate-butyl acrylate) [P(TFEMA-BA)]. The water-written textiles exhibited structural color changes in 0.6 s via an increase in the refractive index, driven by water penetrating the gaps between H-SiO2 microspheres. The structural color was restored to the initial state after the water evaporated, allowing multiple cycles of the write-erase-write mode. The hydrophobic P(TFEMA-BA) adhesive was used to construct a stable chromogenic array and endow WRSC textiles with antispreading properties, thereby improving structural stability and achieving clear writing patterns. The prepared WRSC textiles demonstrated high flexibility, structural stability, and water-rewritable properties, providing advanced bionic inspiration and valuable design ideas for rewritable materials and smart textiles.

Calix[6]arene-Functionalized Photonic Hydrogel Biosensor for Naked-Eye Cholesterol Detection Based on Supramolecular Host-Guest Interactions.

Cholesterol (CHO) is an essential constituent of human cellular tissues and a crucial activity indicator for the clinical diagnosis and prevention of various diseases. Abnormal CHO levels can lead to various cardiovascular diseases, including coronary heart disease, cerebral thrombosis, and atherosclerosis. Thus, developing simple and effective methods for CHO detection is of great significance. Herein, a novel calix[6]arene-modified photonic hydrogel biosensor (PAAH@SCX6) was developed for naked-eye monitoring of CHO based on supramolecular host-guest interactions between 4-sulfocalix[6]arene (SCX6) and CHO molecules. This sensor was constructed by embedding Fe3O4 colloidal nanocrystal cluster chains into a poly(acrylamide-co-acrylic acid) smart hydrogel (PAAH), followed by incorporation of plentiful SCX6 units into the PAAH via hydrogen bonding interactions. The specific recognition of SCX6 to CHO leads to the volume expansion of the hydrogel, causing a shift in the photonic band gap and a change in the hydrogel's structural color. The sensor demonstrated a linear detection range of 2.83-5.20 mM, covering the typical CHO levels in the human body. Importantly, the PAAH@SCX6 biosensor showed high selectivity and satisfactory stability, making it highly suitable for practical applications. Such a photonic hydrogel-based biosensor provides a convenient, simple, and efficient strategy for visual CHO detection.

Organic-Inorganic Hybrid Silica Film with a TGBA* Structure.

Twisted grain boundary (TGB) phases exhibit highly frustrated and complex liquid crystal structures, and have attracted enormous interest because of their unique internal structure, textures and properties. However, among the few real concerns related to these interesting structures, applying them to prepare polymer-stabilized colored liquid crystal films has been challenging. Herein, the organic-inorganic hybrid silica (OIHS) films with a TGBA* structure were prepared using two organosilanes and one chiral additive under an acidic condition. The structural color of the films can be adjusted by varying the polycondensation temperature and the concentration of the chiral additive. A structurally colored pattern was prepared by the inject printing, which was suitably applied for decoration and anti-counterfeiting.

Structural colouration in Drosophila wings is thermally plastic and exhibits ecological variation

IntroductionStructural colour patterns and their functions in insect wings are less known. Wing interference patterns (WIPs) are colour patterns apparent when wings are viewed against black backgrounds; the angle of incident light and wing surface structures also influence the resulting wing colour pattern. To date, WIPs are correlated with mate attraction, while the impact of geography and environment on WIPs remains unexplored. We explore WIP variation in Drosophila melanogaster populations collected from three altitudes and also compare WIP variation in sibling species D. melanogaster and D. simulans reared at three different temperatures to understand if local selection pressures could also influence WIPs.MethodologyWings of D. melanogaster males were collected from three different altitudes, and D. melanogaster and D. simulans males reared at three different temperatures were imaged. Images were analysed for their relative red, green and blue content in the RGB colour space. In representative images, wing thickness was assessed using the Newton colour series.ResultsAn altitudinal cline in WIPs was observed in the cosmopolitan D. melanogaster collected from the Western Himalayas. Relative RGB values and increase in altitudes were negatively correlated. Thermal responses in WIPs were parallel for both D. melanogaster and D. simulans. Relative RGB values were negatively correlated with rearing temperature. In both species, wing thickness measurements indicated that the wings of flies reared at low temperatures had greater blueness (cyan and magenta) compared to flies reared at moderate to high temperatures; the latter had more green and yellow content. Wing thickness pattern was also consistent for D. melanogaster flies collected from low versus higher altitudes.DiscussionWe find WIPs to be a plastic trait in response to temperature. WIP response to thermal variation corroborates with the temperature of the geographic origin in D. melanogaster. The adaptive significance of WIP variation and associated wing thickness remains unclear. Future studies could explore the underlying adaptive significance of structural colour patterning under different environmental conditions.

A basic study on sound insulation characteristics of Triply Periodic Minimal Surface by using the finite element method

Triply periodic minimal surfaces (TPMS) have garnered attention in the field of natural science. For example, these structures are observed in insect wing scales, where their intricate periodic structures create structural colors via the reflection and interference of certain wavelengths of light. The objective of this study is to utilize the acoustic analogy of such photonic properties for sound insulation. In a previous report, it was found that the bandgap of G-surface, which is one of TPMS, enabled us to insulate sound effectively at specific frequencies. Furthermore, it was suggested that sound insulation performance can be improved by compressing the structure along the incident direction while maintaining its thickness. In this paper, we focus on the P-surface, which is a typical TPMS other than the G-surface. The sound insulation properties were calculated by numerical analysis and compared with those of the G-surface. As a result, it is found that P- and G-surfaces are effective structures for different frequency bands. In addition, compression of the P-surface along the incident direction in the same manner as in the previous study improves the sound insulation performance without increasing the thickness, as well as G-surface.

Preparation of structural colors from lignin: Improving the homogeneity between different raw materials by solvent precipitation fractionation

Structural color materials prepared by lignin colloidal spheres (LCSs) have attracted great interest due to their biocompatibility, biodegradability, multifunctionality, and ability to weaken incoherent scattering. However, the lignin raw materials originating from different biomass resources have distinct heterogeneity leading to a big difference in the size of resultant LCSs via self-assembly, which is a major challenge for the preparation of stable structural colors. In this work, we employ the solvent precipitation method to fractionate the crude lignin samples obtained from corncob, softwood, hardwood, and bamboo, respectively. After solvent precipitation fractionation, the self-assembly behaviors of different lignin samples become similar. The LCSs prepared by fractionated lignin samples show quite similar sizes (212, 202, 215, and 210 nm) compared with the LCSs prepared by crude lignin samples (341, 83, 128, and 253 nm). The quantitative atomic force microscope force spectroscopy demonstrates the improved homogeneity of LCSs prepared by fractionated lignin samples is attributed to similar long-ranged intermolecular forces in self-assembly. By employing LCSs as building blocks, the lignin-based pigments with stable structural colors are successfully prepared. The presented colors are independent of the biomass resource types, which benefits the product stability and large-scale production of lignin-based structural color materials.