A Predictive Design Framework for Tunable Structural Colour Inspired by Morpho Nanostructures

The structural blue color of a Morpho butterfly originates from the diffraction of light and interference effects due to the presence of the microstructures on the wing of the butterfly. Structural color on the surface of a damselfish reversibly changes between green and blue. Inspired by these creatures, we have been trying to prepare high-quality and functional structural color films. We describe our efforts in this Account. A useful technique to prepare such structural color films in colloidal solution is a "lifting" method, which allows us to quickly fabricate brilliant colloidal crystal films. The thicknesses of the films can be controlled by precisely adjusting the particle concentration and the lifting speed. Moreover, in order to prepare a complicated structure, we have used template methods. Indeed, we have successfully prepared the inverse structure of the wing of a Morpho butterfly with this technique. Initially, however, our structural color films had a whitish appearance due to the scattering of light by defects in the colloidal crystal film. Later, we were able to prepare a non-whitish structural color film by doping an appropriate dye in the colloidal particles to absorb the scattering light. In addition to the structural blue color, the wing of the Morpho butterfly has superhydrophobic properties. According to Wenzel's equation, the hydrophobic and hydrophilic properties are enhanced when the roughness of the hydrophobic and hydrophilic surface is increased, respectively. Based on this mechanism, we have successfully prepared structural color films with superhydrophobic properties, as well as with superhydrophilic properties. Another important property that can be seen in nature is tunable structural color, such as the color change that can be seen on the surface of a damselfish. In order to mimic such color change, we have developed several tunable structural color films. In particular, we have successfully prepared phototunable photonic crystals using photoresponsive azobenzene derivatives. In order to apply these structural color films, we developed a technique for patterning them by taking advantage of the wettability of the substrate surface. These materials can be used in the future for self-cleaning pigments and tunable photonic crystals.

Cephalopods achieve their vivid, dynamic structural coloration through reconfigurable lamellar reflectors, acting as a powerful inspiration for adaptive optical materials. However, synthetic systems that replicate this functionality typically require complex material architectures or multistep fabrication processes. Here, a straightforward strategy is presented for producing magnetically responsive photonic microparticles using linear poly(styrene)‐ b ‐poly(2‐vinylpyridine) block copolymers. Self‐assembly in emulsion droplets yields ellipsoids with stacked lamellar domains that exhibit structural colors spanning the entire visible spectrum. Blending block copolymers with different molecular weights enables precise, continuous control over lamellar periodicity and photonic bandgap spectral position, eliminating the need for chemical modification or the use of swelling agents. Notably, incorporating superparamagnetic iron oxide nanoparticles imparts rapid and reversible magnetic alignment of the microparticles. This orientation aligns the lamellar stacks perpendicular to the magnetic field, resulting in Bragg reflection and angle‐dependent coloration. The reflected wavelength can be tuned by changing the viewing angle, which mimics the dynamic optical responses of biological iridophores. Compared to prior systems, our approach reduces synthetic complexity while maintaining high color intensity and angular tunability. Thus, this work introduces a bio‐inspired materials concept that offers a simple route toward dynamically tunable optical materials for adaptive camouflage, smart coatings, and next‐generation photonic devices.

Structural colors are widespread in nature and have become an important component of the lives. Considerable efforts have been made toward reversible color tuning, which underpins intriguing applications, including color displays, anti‐counterfeiting, and information encryption. However, the limited size, complicated fabrication processes, and low modulation speeds of structural colors are the main obstacles to their further development. Herein, a facile method to realize dynamically tunable structural colors is presented, which are enabled by the pixelated programming of soft materials on thickness. Pixelated photoresist microarrays with different heights are obtained using a digitalized lithography technique, enabling delicate control over the thickness of the liquid crystal (LC) layers. Stimuli‐responsive LCs endow structural colors with dynamic and reversible tunability and exhibit remarkable switching speeds with external stimuli. The proposed strategy sheds new light on dynamically tunable structural colors and promotes the development of optical anti‐counterfeiting, thermal sensors, and advanced information encryption.

Artificial Structural Colors and Applications

Photonic nanostructures with widely, rapidly and reversibly tunable diffraction in the visible and near-infrared spectrum can be created through a magnetic field assisted assembly strategy. We first describe the mechanism for the formation of dynamic photonic chains and the tuning of the diffraction colors using an external magnetic field. Then we discuss the fixation of photonic chains in a solid polymer matrix through combining instant magnetic assembly with a rapid UV polymerization process which allows us to confirm the chaining structures. Finally, we demonstrate several applications of magnetically tunable photonic nanostructures for security and sensing devices, high resolution patterning of multiple structural colors.

Constructing an anti-counterfeiting material with non-interference dual optical modes is an effective way to improve information security. However, it remains challenging to achieve multistage secure information encryption due to the limited stimulus responsiveness and color tunability of the current dual-mode materials. Herein, a dual-mode hydrogel with both independently tunable structural and fluorescent colors toward multistage information encryption, is reported. In this hydrogel system, the rigid lamellar structure of poly(dodecylglyceryl itaconate) (pDGI) formed by shear flow-induced self-assembly provides the restricted domains wherein monomers undergo polymerization to form a hydrogel network, producing structural color. The introduction of fluorescent monomer 6-acrylamidopicolinate (6APA) as a complexation site provides the possibility of fluorescent color formation. The hydrogel's angle-dependent structural color can be controlled by adjusting the crosslinking density and water content. Additionally, the fluorescence color can be modulated by adjusting the ratio of lanthanide ions. Information of dual-mode can be displayed separately in different channels and synergistically overlayed to read the ultimate message. Thus, a multistage information encryption system based on this hydrogel is devised through the programed decryption process. This strategy holds tremendous potential as a platform for encrypting and safeguarding valuable and authentic information in the field of anti-counterfeiting.

Hydrogels are ubiquitous in nature, which are three-dimensionally (3D) crosslinked polymer networks with amounts of water inside. Naturally, they can be regarded as semi/solid showing intermediate properties of solid and liquid. Hydrogels have attracted growing interest in diverse applications, due to their excellent biocompatibility, permeability, and 3D network. With the development of polymer synthesis, great progress in tough hydrogels, and continuous emerging of advanced fabrication methods, hydrogels become promising functional materials. To further broaden their practical usages, great efforts have made to hydrogel functionalization, which mainly focused on both polymer network and architectures within hydrogels. Normally, functional hydrogels were fabricated from stimuli-responsive polymers, cleavable bonding, inhomogeneous or aligned microstructures, especially, functionalization by diverse nanoparticle composites. In this study, cellulose nanocrystals (CNCs) as one of the bio-based natural nanoparticles were investigated in hydrogel functionalization owing to their excellent mechanical properties, facile surface modification and unique optical properties. CNCs and surface modified CNCs were introduced into synthetic hydrogels to ensure the implementation of specific functions of hydrogels. CNCs and surface modified CNCs with methyl acrylamide groups (CNCs-MAm) were incorporated into thermal-responsive and solvent-driven bilayer hydrogel actuators (BHAs). The introduction of CNCs and CNCs-MAm significantly improved the mechanical properties of BHAs and ensured largely promoted lifting capability for them. The weight-lifting capability of BHAs was promoted from ~800 wt% to ~1800 wt% of their own polymer weight. Furthermore, the CNCs with numerous carboxyl groups on the surface can apparently increase the spatial distinction of dynamic hydrogels. When ferric ions were introduced along with diffusion from outside to inside of hydrogels, the carboxyl groups would complex with ferric ions. This additional crosslinking retarded the penetration of ferric ions, increasing the spatial crosslinking difference. This promoted spatial distinction assisted in fabricating closed hollow hydrogels with tunable microstructures of the inner and outer walls. CNCs can accelerate the process of spatial separation to form the hollow interior in around 9 days of dialysis, whereas, about 20 days of dialysis for non-composite hydrogels. Distinguished from typical thermal controlled release system, this hollow hydrogel showed unique sustained release of hydrophilic small molecules at higher temperature. It would reach release equilibrium after only ~24 h at 25 oC, while the release equilibrium was largely retarded to ~200 h at 37 oC as comparison. In addition, as one-dimensional nanomaterials, CNCs and gold nanorods (GNRs) are widely used in optical materials due to their respective inherent natures: birefringence with accompanying light retardation in aligned CNCs and surface plasmon resonance (SPR) of GNRs. These properties of both nanorods were combined to generate synergistic and readily tunable structural colors of polymer films. The CNCs and GNRs are embedded either in the same hybrid composite films or in separated films after their unidirectional alignment from dynamic precursor hydrogels. By synergistically leveraging the optical features of CNCs and GNRs with diverse amounts in hybrid films or in stacked individual films, wide-ranging structural colors were realized, which is far beyond the limitation of the same films solely with aligned CNCs or GNRs. Increasing GNRs contents leads to promoted color red with enhanced light absorption at 520 nm and CNCs influence the overall phase retardation, giving distinctively structural colors. Furthermore, with angle adjustment between CNCs films and GNRs films using stacking/rotating technique, we further achieve facile and continuous color manipulation easily for color combinations. In one set of stacked films, light absorption wavelengths can traverse from roughly 500 nm to 650 nm solely by rotating GNRs film (0-180°). Tuning the adjustable synergism of the birefringence of CNCs and SPR of GNRs in one film or separate films provides great potential for structural colors, which enlightens new avenue for optical applications. This thesis is a cumulative work including 3 publications. One of them was already published and two are under submission. The background, the objective of the study, results and discussion of these three publications and the conclusion are presented in Sections 1-4.

A durable, high‐performance structural coloration approach is presented using TiON‐based Fabry‐Perot (F‐P) cavities. Conventional metal‐insulator‐metal structures suffer from oxidation and mechanical degradation, limiting their industrial viability. Here, the top metal layer is replaced with TiON, a lossy dielectric known for its corrosion resistance and high hardness. Fabricated TiON/Si 3 N 4 /metal structures achieve vivid, tunable colors across a broad spectrum while maintaining high stability. Their optical, mechanical, and environmental robustness is systematically evaluated through five‐year aging, humidity, and abrasion tests. Reflectance spectra confirm stable near‐perfect light absorption (>90%) with minimal color shifts (≈10 nm). Camera imaging, CIE chromaticity analysis, and SEM investigation validate long‐term color retention and stable surface morphology. The findings demonstrate that TiON‐based F‐P cavities offer a promising solution for industrial applications, overcoming key limitations of conventional structural color materials.

It is broadly observed that graphene oxide (GO) films appear transparent with a thickness of about several nanometers, whereas they appear dark brown or almost black with thickness of more than 1 μm. The basic color mechanism of GO film on a sub-micrometer scale, however, is not well understood. This study reports on GO pseudo-1D photonic crystals (p1D-PhCs) exhibiting tunable structural colors in the visible wavelength range owing to its 1D Bragg nanostructures. Striking structural colors of GO p1D-PhCs could be tuned by simply changing either the volume or concentration of the aqueous GO dispersion during vacuum filtration. Moreover, the quantitative relationship between thickness and reflection wavelength of GO p1D-PhCs has been revealed, thereby providing a theoretical basis to rationally design structural colors of GO p1D-PhCs. The spectral response of GO p1D-PhCs to humidity is also obtained clearly showing the wavelength shift of GO p1D-PhCs at differently relative humidity values and thus encouraging the integration of structural color printing and the humidity-responsive property of GO p1D-PhCs to develop a visible and fast-responsive anti-counterfeiting label. The results pave the way for a variety of potential applications of GO in optics, structural color printing, sensing, and anti-counterfeiting.

Photonic crystals play the vital role in structural color appearance, and they can be fabricated on polymer substrates. In this paper, monodispersed SiO2 microspheres with the average diameter ranging from 150 to 300 nm were prepared by classical Stober method. The spherical size of SiO2 microspheres was regulated by controlling concentrations of ammonia. The tunable structural colors could be changed by modulating the diameters of SiO2 microspheres or viewing angles, which adhered to the law of the Bragg diffraction. As a kind of polymer fabric, the polyester fabrics, smoother than natural fiber fabrics, exhibited bright structural colors from the well-ordered photonic crystal microstructure by vertical deposition self-assembly of SiO2 photonic crystals. Moreover, the result indicated that the difference of fabric-woven structure could affect the lightness of structure color, and the lightness on satin fabric was duller than that of plain fabric. Besides, we have also discussed the influence of fabric structure on the lightness of structure color using the theory of thin film interference. It is believed that the structural color could provide a new strategy for related polymer product coloration without chemical dyes and pigments, and has a potential to reduce the pollution in related polymer materials dyeing and printing processes.

Structural coloration has attracted great interest from scientists and engineers in recent years, owing to fascination with various brilliant examples displayed in nature as well as to promising applications of bio-inspired functional photonic structures and materials. Much research has been done to reveal and emulate the physical mechanisms that underlie the structural colors found in nature. In this article, we review the fundamental physics of many natural structural colors displayed by living organisms as well as their bio-inspired artificial counterparts, with emphasis on their connections, tunability strategies, and proposed applications, which aim to maximize the technological benefits one could derive from these photonic nanostructures. WIREs Nanomed Nanobiotechnol 2016, 8:758-775. doi: 10.1002/wnan.1396 For further resources related to this article, please visit the WIREs website.

Well-resolved oil-soluble Au-doped ZnCdS quantum dots and enhancing doping emission with In-codoping

A photochemically tunable structural color material was prepared by infiltration of the polymer liquid crystal (LC) having azo-chromophores in a SiO 2 inverse opal structure. The SiO 2 inverse opal film infiltrated with the polymer LC reflected a light, which is called a structural color, corresponding to the periodicity as well as the refractive indices of the inverse opal structure. Linearly polarized light irradiation caused the shift of the structural color band to longer wavelength more than 15 nm. This is caused by the formation of uniaxially anistorpic molecular orientation of the polymer LC. The switched state was stable under interior light, and reversible switching of the reflection band can be achieved by the linearly and circularly polarized light irradiation. This photoswitching property will be suitable for various optical materials such as memory, display so on.

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.

Dynamic control of structural colors across the visible spectrum with high brightness has proven to be a difficult challenge. Here, this is addressed with a tuneable reflective nano-optical cavity that uses an electroactive conducting polymer (poly(thieno[3,4-b]thiophene)) as spacer layer. Electrochemical doping and dedoping of the polymer spacer layer provides reversible tuning of the cavity's structural color throughout the entire visible range and beyond. Furthermore, the cavity provides high peak reflectance that varies only slightly between the reduced and oxidized states of the polymer. The results indicate that the polymer undergoes large reversible thickness changes upon redox tuning, aided by changes in optical properties and low visible absorption. The electroactive cavity concept may find particular use in reflective displays, by opening for tuneable monopixels that eliminate limitations in brightness of traditional subpixel-based systems.