Structural Color Materials Research Articles

Magnetic photonic crystals for biomedical applications.

Magnetic photonic crystals (PhCs), as a representative responsive structural color material, have attracted increasing research focus due to merits such as brilliant refraction colors, instant responsiveness, and excellent manipuility, thus having been widely applied for color displaying, three-dimensional printing, sensing, and so on. Featured with traits such as contactless manner, flexible orientations, and adjustable intensity of external magnetism, magnetic PhCs have shown great superiority especially in the field of biomedical applications such as bioimaging and auxiliary clinical diagnosis. In this review, we summarize the current advancements of magnetic PhCs. We first introduce the fundamental principles and typical characteristics of PhCs. Afterward, we present several typical self-assembly strategies with their frontiers in practical applications. Finally, we analyze the current situations of magnetic PhCs and put forward the prospective challenges and future development directions.

Multidimensional dynamic regulation of cellulose coloration for digital recognition and humidity response

Structural color is an eye-catching phenomenon in nature, which originates from the synergistic effect of cholesteric structure inside living organisms and light. However, biomimetic design and green construction of dynamically tunable structural color materials have been a great challenge in the field of photonic manufacturing. In this work, the new ability of L-lactic acid (LLA) to multi-dimensionally modulate the cholesteric structures constructed from cellulose nanocrystals (CNC) is revealed for the first time. By studying the molecular-scale hydrogen bonding mechanism, a novel strategy that electrostatic repulsion and hydrogen bonding forces jointly drive the uniform arrangement of cholesteric structures is proposed. Due to the flexible tunability and uniform alignment of the CNC cholesteric structure, different encoded messages were developed in the CNC/LLA (CL) pattern. Under different viewing conditions, the recognition information of different digits will continue to reversibly and rapidly switch until the cholesteric structure is destroyed. In addition, the LLA molecules facilitated the more sensitive response of the CL film to the humidity environment, making it exhibit reversible and tunable structural colors under different humidity. These excellent properties provide more possibilities for the application of CL materials in the fields of multi-dimensional display, anti-counterfeiting encryption, and environmental monitoring.

Preparation of robust fully encapsulated structural color fabrics and their application to display

Building photonic crystals on the surface of flexible textiles is a common way to obtain structural color fabrics. However, strict requirements for substrate fabric properties, poor stability of photonic crystals, and the lack of new application scenarios have hindered the promotion and use of structural color materials. In this study, we immobilized polystyrene photonic crystals in a fully encapsulated form on the surface of flexible white cotton fabric. These photonic crystals exhibited bright and iridescent structural colors. The fully encapsulated structure imparted high mechanical stability to the photonic crystals. After conducting mechanical and chemical resistance tests, the structural color fabric maintained an intact photonic crystal structure and a significant structural color effect. We also used the prepared structural color fabric to make adhesive tapes, which could easily adhere to surfaces of different materials (e.g., glass, metals, polymers, etc.) and showed good adhesive properties. The findings of our study not only addressed the limitations of the previous methods of preparing structural color fabrics but also provided a new to use these fabrics in practical situations.

Bacterial nanocellulose membrane as novel substrate for biomimetic structural color materials: Application to lysozyme sensing

The development of optical biosensors based on structural colors generated by short-range ordered colloidal particles is attracting growing interest due to their non-iridescent and non-fading features. In this study, a biomimetic approach using biopolymers for the various steps of sensor construction is presented. Bacterial nanocellulose (BNC) has many foreseen applications in biomedical engineering because of its biocompatibility, good mechanical strength, and large modifiable surface area. Herein, a novel approach is taken by using functionalized BNC as a substrate to build a molecularly imprinted photonic sensing layer. BNC was modified with polydopamine (PDA), which improved the adhesion and mechanical properties of the BNC substrate while providing simultaneously a black background for color saturation. A molecularly imprinted polymer (MIP) also made of PDA was used to create the recognition sites for the biomarker lysozyme. A monodisperse colloidal suspension of silica particles was first synthesized and used as core of the MIP shell, and then the photonic structure was assembled on the PDA-BNC membrane. The biosensor showed a detection limit of about 0.8 nmol L−1 of lysozyme in spiked human serum and demonstrated to be selective against cystatin C. These properties, combined with biocompatible, eco-friendly, and low-cost materials, offer a sustainable sensing platform with great potential for healthcare applications.

Bioinspired multichannel colorful encryption through kirigami activating grating

Optical encryption, exploiting degrees of freedom of light as parameters to encode and decode information, plays an indispensable role in our daily life. Responsive structural color materials can give real-time visible feedback to external stimuli and provide ideal candidates for optical encryption. However, the development of existing responsive structural color materials is hindered by poor repeatability and long feedback time. Meanwhile, there are only few strategies to exploit structural colors in multichannel information encryption. Herein, bioinspired by the structural color variation due to a change in angle arising from the movement of animal’s scales or feathers, we developed a general multichannel information encryption strategy using a two-dimensional deformable kirigami arranging orientations of the grating arrays by design. The kirigami grating sheet shows rapid, repeatable, and programmable color change. This strategy utilizes the topological space deformation to guide the change of optical property, which suggests new possibilities for spatial and spectral encryption as well as mechano-sensing and camouflage.

Rotary Structural Color Spindles from Droplet Confined Magnetic Self-Assembly.

Structural colors materials are profoundly explored owing to their fantastic optical properties and widespread applications. Development of structural color materials bearing flexible morphologies and versatile functionalities is highly anticipated. Here, a droplet-confined, magnetic-induced self-assembly strategy for generating rotary structural color spindles (SCSPs) by fast solvent extraction is proposed. The as-prepared SCSPs exhibit an orderly close-packed lattice structure, thus appearing brilliant structural colors that serve as encoding tags for multiplexed bioassays. Besides, benefitting from the abundant specific surface area, biomarkers can be labeled on the SCSPs with high efficiency for specific detection of analytes in clinical samples. Moreover, the directional magnetic moment arrangement enables contactless magnetic manipulation of the SCSPs, and the resultant rotary motions of the SCSPs generates turbulence in the detection solution, thus significantly improving the detection efficiency and shortening the detection time. Based on these merits, a portable point-of-care-testing strip integrating the rotary SCSPs is further constructed and the capability and advantages of this platform for multiplexed detection of tumor-related exosomes in clinical samples are demonstrated. This study offers a new way for the control of bottom-up self-assembly and extends the configuration and application values of colloidal crystal structural colors materials.

Self-growing photonic composites with programmable colors and mechanical properties

Many organisms produce stunning optical displays based on structural color instead of pigmentation. This structural or photonic color is achieved through the interaction of light with intricate micro-/nano-structures, which are “grown” from strong, sustainable biological materials such as chitin, keratin, and cellulose. In contrast, current synthetic structural colored materials are usually brittle, inert, and produced via energy-intensive processes, posing significant challenges to their practical uses. Inspired by the brilliantly colored peacock feathers which selectively grow keratin-based photonic structures with different photonic bandgaps, we develop a self-growing photonic composite system in which the photonic bandgaps and hence the coloration can be easily tuned. This is achieved via the selective growth of the polymer matrix with polymerizable compounds as feeding materials in a silica nanosphere-polymer composite system, thus effectively modulating the photonic bandgaps without compromising nanostructural order. Such strategy not only allows the material system to continuously vary its colors and patterns in an on-demand manner, but also endows it with many appealing properties, including flexibility, toughness, self-healing ability, and reshaping capability. As this innovative self-growing method is simple, inexpensive, versatile, and scalable, we foresee its significant potential in meeting many emerging requirements for various applications of structural color materials.

Photonic Lignin with Tunable and Stimuli-Responsive Structural Color.

Due to the growing sustainability and health requirements, structural color materials fabricated with functional natural polymers have attracted increasing attention in advanced optical and biomedical fields. Lignin has many attractive features such as great biocompatibility, ultraviolet resistance, antioxidant property, and thermostability, making it a promising natural resource to be fabricated as functional structural color materials. However, to date, the utilization of lignin as the building block for structural color materials is still a challenge due to its disordered structure. Herein, we present a strategy to transform disordered lignin into ordered "photonic lignin", in which monodisperse lignin colloidal spheres are prepared via solvent/antisolvent self-assembly, and then the periodic structure is constructed by centrifugal effect. The photonic lignin exhibits structural colors that are tunable by modulating the diameter of lignin colloidal spheres. We further demonstrate the application of photonic lignin as a natural polymer-based coating that shows bright, angle-independent, and stimuli-responsive structural colors. Moreover, the cytotoxicity assay indicates the excellent biocompatibility of photonic lignin with human skin, blood vessels, digestive systems, and other tissues, which demonstrates the great potential of photonic lignin in the applications such as implanted/wearable optical devices, advanced cosmetics, and smart food packaging.

Responsive Structural Colors Derived from Geometrical Deformation of Synthetic Nanomaterials

Structural Colors Responsive structural colors generated by the geometrical deformation of synthetic nanomaterials with periodic skeletons are recognized as extremely valuable mediums of information conversion. In article number 2200101, An-Hui Lu, Shufen Zhang and Yong Qi, illustrates the stimulus deformation of a typical micro-nano structure under different conditions (humidity/dry) and the structural color change caused by the size effect, intelligently decrypting different information stored in the structural color materials.

Preparation of Janus structural color sheets with flexibility, stability and low angle dependence based on textile

Structural colors with low angle dependence arising from photonic crystals are of increasing interest in various fields, such as sensing and display. In this study, the composite Poly(styrene-methyl methacrylate-acrylic acid) photonic crystals consisting of amorphous, simple cubic and face-centered cubic structures were facilely constructed by vacuum-assisted filtration self-assembly. Interestingly, this method enables microspheres to be assembled on flexible fabrics in a "bottom-up" manner, and different types of structural color sheets (single-sided coloring, double-sided homochromy or heterochromatic) can be obtained by controlling the dosage and particle size of the microspheres. In particular, the prepared structural color sheets are exceptionally bright and have low angle dependence. Subsequently, the photonic crystals in the flexible fabric were reinforced by the introduction of Poly(methyl methacrylate-butyl acrylate), which resulted in a significant improvement in the mechanical stability, the performance of which was demonstrated by water-rinsing, friction and folding tests. This study provides a novel way to prepare new structural color materials with bright and low angle dependence. In addition, this double-sided heterochromatic structural color sheet has potential applications in the display field.

Responsive Structural Colors Derived from Geometrical Deformation of Synthetic Nanomaterials

Responsive structural colors generated by the geometrical deformation of synthetic nanomaterials with a periodic skeleton corresponding to light wavelength through interference, diffraction, and scattering are recognized as extremely valuable information conversion methods. However, there lacks systematic generalization regarding the responsive structural colors generated from the response deformation of micro–nano structures. In this review, the recent progress on synthetic structural color materials (SCMs) and their size effects on color conversion are summarized and discussed. Different deformable micro–nano structures, such as microspheres, inverse opals, surface wrinkles, nano‐/micropillars, layered stacks, and others, have been summarized. Their common feature is the variation in structural color produced by the deformation of nanostructures, but the difference lies in the deformation mechanism and response to stimuli. It is aimed to figure out the relationship and mechanism between stimuli deformation and the variable structural color of the corresponding nanostructures, inspiring the creation of the next generation of intelligent SCMs. Finally, perspectives on the development of responsive SCMs derived from the geometrical deformation of synthetic nanomaterials are presented. Several potential stimuli‐deformable micro–nano structures are discussed. It is believed that stimuli deformation is a powerful tool for the fabrication of high‐performance responsive SCMs to advance photonic technology.

Research Progress on the Design of Structural Color Materials Based on 3D Printing

AbstractThe structural color is produced by the reflection of light through a periodic dielectric structure with photonic bandgap. Because of its special visual effects, researchers in many fields begin to pay attention to photonic crystals with structural colors. This review focuses on the combination of 3D printing technology and photonic crystal structural color materials, summarizes the influence factors of structural color rendering mechanism and the evaluation of color performance, and summarizes the research of structural color materials in simulation. Finally, the challenges faced by structural color materials are summarized, and the future development trend of structural color materials is prospected.

Toward High Sensitivity: Perspective on Colorimetric Photonic Crystal Sensors.

The sensitivity of colorimetric photonic crystal (PC) sensors have been significantly improved with the advancement of deformable structural color materials, structures design, sensing signal analysis methods, and fabrication strategies. In this perspective, the strategies toward high-sensitivity colorimetric PC sensors are discussed, from the perspectives of molecular design, single sensor construction, and multisensor assembly, which include incorporation of flexible polymer chains, construction of strong sensor-analyte interactions, incorporation of more soft materials, construction of stimuli-angle/orientation relationship, design of colorimetric sensors in series, and assembly of colorimetric PC sensors in parallel. Based on these strategies, progress of high-sensitivity colorimetric PC sensors in recent years is summarized, in terms of mechano-sensors and chemo-/biosensors. Specifically, PC based optical-electrical dual-signal sensing devices are included. Finally, the future development and challenges of high-sensitivity colorimetric PC sensors are presented, in regards to deformable properties, optical properties, analysis methods, and fabrication strategies.

Solvatochromic discrimination of alcoholic solvents by structural colors of polydopamine nanoparticle thin films

Structural color materials are promising sensory materials that exhibit a stable and stimuli responsive reflection color. To fully utilize their optical property as colorimetric sensors, a material having stimuli-sensitive and optically superior properties has to be applied for designing the structural color material. In this work, a solvatochromic structural color material made of multilayered polydopamine nanoparticles (PdNPs) is presented. Because PdNP has a high refractive index in the range of 1.7–2.0 and good swelling capability, it exhibits a notable solvatochromic behavior in various alcoholic solvents. The solvent sensing performance shows good reliability when compared with that of a refractive index sensing based structural color material. The dynamic reflection spectral patterns of the PdNP thin film upon solvent drying provides more information on the diffusion rate and surface tension of each target solvent. These results indicate that the PdNP thin film can be used to differentiate between physicochemically similar solvents.

Water rewriteable double-inverse opal photonic crystal films with ultrafast response time and robust writing capability

Photonic paper, as a crucial branch of the research of structural color materials, has attracted considerable attention. Herein, innovative large-area water rewriteable double-inverse opal (WRDIO) film with ultrafast response time (∼0.8 s) and robust writing capability is fabricated by introducing polyacrylamide gel into the double-inverse opal. The hydrophilic nature of the polyacrylamide gel can induce water to quickly penetrate the internal pores of the inverted opal backbone. Simultaneously, the randomly distributed polystyrene microspheres within the pores become ordered under the action of liquid buoyancy, which eliminates the scattering disturbance and makes the film switch to the structural color state. Importantly, the as-prepared WRDIO film can be erased and rewritten over 500 cycles without significantly reducing the color quality. Besides, the retention time of the written marks can be controlled by writing using water pens with different water outputs. Interestingly, responsive spatio-temporal colorful patterns could be achieved based on the different volatilities of written marks. Furthermore, advanced optical anti-counterfeiting material with multistate switching for information security is prepared based on controlled wettability. Given these superior properties, this novel WRDIO film provides a new solution for future applications of rewriteable paper.

A Biomimetic Human Lung-on-a-Chip with Colorful Display of Microphysiological Breath.

Lung-on-a-chip models hold great promise for disease modeling and drug screening. Herein, inspired by the iridescence phenomenon of soap bubbles, a novel biomimetic 3D microphysiological lung-on-a-chip system with breathing visualization is presented. The system, with an array of pulmonary alveoli at the physiological scale, is constructed and coated with structural color materials. Cyclic deformation is induced by regular airflow, resembling the expansion and contraction of the alveoli during rhythmic breathing. As the deformation is accompanied with corresponding synchronous shifts in the structural color, the constructed system offers self-reporting of the cell mechanics and enables real-time monitoring of the cultivation process. Using this system, the dynamic relationships between the color atlas and disease symptoms, showing the essential role of mechanical stretching in the phenotypes of idiopathic pulmonary fibrosis, are investigated. These features make this human lung system ideal in biological study, disease monitoring, and drug discovery.

Angular-Independent Structural Colors of Clay Dispersions.

Clay mineral nanosheet colloids were found to show angular-independent structural colors after desalting. Naked-eye observation and UV–visible reflectance spectra showed that the color is tuned by varying the average nanosheet size and nanosheet concentration. The low angular-dependence of the structural color was also clarified by these observations, which is the first case for a nanosheet system. The present system is expected as an environmentally benign and low-cost structural color material for various applications.

Monodisperse colloidal spheres with a high refractive index: their synthesis, assembly and application in structural colored materials

The review summarizes recent advances in preparing monodisperse colloidal spheres with a high refractive index, effective methods for assembling these spheres into structural color materials and their intriguing optical properties with related applications.

Quadruple Anti‐Counterfeiting Retroreflective Structural Color Films

AbstractWhile structural color materials are widely exploited in anti‐counterfeiting applications, it still remains a challenge to develop advanced and multifunctional structural colors for new anti‐counterfeiting technologies. Herein, a novel patterned retroreflective structural color film (PRSCF) with quadruple anti‐counterfeiting capacities is reported based on the changes in iridescent and non‐iridescent colors due to total internal reflection and interference mechanisms, reversible thermal‐responsive patterns owing to the shape memory effect, and reversible decryption and encryption of the patterns due to the surface wettability difference. This quadruple anti‐counterfeiting PRSCF greatly improves the security level and may find important potentials in precious artworks and displays.

Thermally Tunable Structural Coloration of Water/Surfactant/Oil Emulsions.

Stimuli-responsive structural color in nature has fascinated scientists, directing them to develop artificial coloration materials that adjust colors in response to external stimuli. Many stimuli-responsive structural color materials have been realized. However, only a few have reported on all-liquid-type materials, which have a particularly desirable feature because they impart their function to the device of any shape. We have previously reported the development of a consistent structural color within a narrow temperature range for all-liquid-type emulsions comprising a long-chain amidoamine derivative (C18AA) and tetraoctylammonium bromide (TOAB). In the present study, we demonstrate that introducing NaCl as an electrolyte affords a highly thermo-sensitive color-changing ability to the emulsions. The structural color of the emulsions can be controlled from red to blue by tuning the temperature. Furthermore, the C18AA and TOAB concentrations can independently regulate the color and coloring-temperature, respectively, realizing that the desired color can develop at a given temperature.