Highly Invisible Photonic Crystal Patterns Encrypted in an Inverse Opaline Macroporous Polyurethane Film for Anti-Counterfeiting Applications

Preparation and application of invisible print based on patterned photonic crystals

How to establish a fast, remarkable, and different external stimulus response mechanism between similar "patterns" and "backgrounds" is a key issue in the design and preparation of invisible printed fabric based on photonic crystals. SiO2 and P(St-MAA) were used to construct photonic crystals, which were used as "patterns" and "backgrounds" to prepare patterned invisible printed fabric. It also reveals the relationship between the size of the structural unit and the reflection wavelength of the photonic crystals and adjusts the structure of the two parts of the photonic crystals by controlling the particle size of the microspheres so that the "pattern" can achieve the effect of "invisible in the air, visible in the water." It has a simple external stimulus, fast response speed, and remarkable response, and it is easy to observe and difficult to copy. It is expected to be further expanded in combination with inkjet printing technology and to obtain promising applications in textile anticounterfeiting and identification marking.

Bioinspired Multifunctional and Dynamic Color-Tuning Photonic Devices

Tremendous progress has been achieved in comprehending the scientific principles governing nature and translating them into practical applications. Among the areas of interest within photonic systems, bioinspired color-tuning devices originating from physical structure modulation hold significant importance. This Review provides an overview of cutting-edge advancements in bioinspired structural color-tuning photonic devices across various applications. First, we delve into the origins, design principles, and fundamental physics underlying bioinspired structural color systems by showcasing diverse living species found in nature. Subsequently, we explore various photonic nano/microscale building blocks, including one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) structures, along with their fabrication techniques. Additionally, we present various biomimetic material systems and strategies for fabricating dynamic structural color devices. Furthermore, we discuss recent breakthroughs in biomimetic photonic systems across key application areas including sensing, interactive soft robotics, digital information encryption/decryption, dynamic displays, and energy. In summary, this Review provides a comprehensive analysis of bioinspired color-tuning photonic devices, shedding light on their intricate structure-function relationships, and aims to inspire the adoption and development of advanced color-tuning strategies. Finally, we address current challenges and offer insights into potential groundbreaking developments in the field of bioinspired optical materials.

Solvent/temperature dual-responsive photonic crystal structural-color films through double inverse opal structure

A solvent/temperature dual-responsive P(NIPAM-AM) hydrogel is proposed. The free energy model of the hydrogel microstructure and the transformation of the properties of the hydrogel chain segment during deformation were further analyzed. The resultant structural-color hydrogel film was applied to on-demand patterning.

Template-based synthesis of novel polymeric colloidal photonic crystals

Here, we devise a universal method for preparing colloidal photonic crystals (CPCs) composed of novel building blocks, using inverse opal materials as templates. This method not only enables the preparation of three-dimensional (3D) CPCs composed of hard sphere building blocks, but also, facilitates the creation of two-dimensional(2D) CPCs composed of chitosan and 3D CPCs composed of polydimethylsiloxane(PDMS) elastic soft spheres.

Full-Spectrum Mechanochromic Photonic Films with Large Interparticle Distance

Non-close-packed crystalline arrays of colloidal particles in an elastic matrix exhibit mechanochromism. However, small interparticle distances often limit the range of reversible color shifts and reduce reflectivity during a blueshift. A straightforward, reproducible strategy using matrix swelling to increase interparticle distance and improve mechanochromic performance is presented. Photonic composites are initially prepared with silica particle arrays embedded in an elastomer matrix at volume fractions of 0.35-0.5. To increase interparticle distance, the composites are immersed in an elastomer-forming monomer, causing the matrix to swell, followed by photopolymerization, thereby producing liquid-free composites. The degree of swelling is controllable up to 3.16, depending on monomer choice, matrix volume fraction, and crosslinking density. The process can be repeated to further increase swelling up to 10.36. This method can reduce the volume fraction of silica particles from 40% to 3.8%, while interparticle distance increases from 53 to 257 nm. The swollen photonic composites exhibit a full visible spectrum under compression, while minimizing reflectivity loss. This allows red-colored photonic composites to be transformed into vivid multicolor patterns when compressed with stamps featuring spatial height variations.

Ethanol-responsive structural colors with multi-level information encryption based on the patterned three-layer inverse opal photonic crystal

Stimulus-responsive inverse opal photonic crystals (IOPCs) with tunable structural colors show significant promise in information security. To improve upon the traditional bilayer structure with limited color information and single decoding mode, this work developed an ethanol-responsive structure with multi-level information encryption ability by inserting a functional layer into two shielding layers (red Layer A with a photonic stop band (PSB) at 640 nm and green Layer C with a PSB at 530 nm). The functional layer was composed of colorless Layer B, a quick response (QR) code pattern made of TiO2 nanoparticles, and a dense polymer. Due to the isolation of distinct layers, different reflectance values, and different PSB positions of the three-layer IOPC, the structural color of Layer B could only be "turned on" by wetting the entire structure when its PSB redshifted from 360 nm to 460 nm. Specifically, when either side was individually wetted, the PSB of Layer A or C redshifted to 825 nm or 685 nm, and the color of the QR code was dominated by the unwetted red or green layer. After the entire structure had been soaked, the blue QR code was decoded. Meanwhile, when the detecting angle increased from 5° to 60°, the PSBs of Layers B and C in the wetted three-layer IOPC blueshifted from 460 nm to 365 nm and from 685 nm to 540 nm, respectively, which resulted in a cascade decoding process with a single- or mixed-color output. This structure provides a good foundation for multi-level information encryption.

Unclonable Encryption-Verification Strategy Based on Bilayer Shape Memory Photonic Crystals

The ability to reversibly exhibit structural color patterns has positioned photonic crystals (PCs) at the forefront of anti-counterfeiting. However, the security offered by the mere reversible display is susceptible to illicit alteration and disclosure. Herein, inspired by the electronic message captcha, bilayer photonic crystal (BPC) systems with integrated decryption and verification modules, are realized by combining inverse opal (IO) and double inverse opal (DIO) with polyacrylate polymers. When the informationized BPC is immersed in ethanol or water, the DIO layer displayed encrypted information due to the solvent-induced ordered rearrangement of polystyrene (PS) microspheres. The verification step is established based on the different structural colors of the IO layer pattern, which result from the deformation or recovery of the macroporous skeleton induced by solvent evaporation. Moreover, through the evaporation-induced random self-assembly of PS@SiO2 and SiO2 microspheres, unclonable structurally colored identifying codes are created in the IO layer, ensuring the uniqueness upon the verification. The decrypted code in the DIO layer is valid only when the IO layer displays the pattern with the predetermined structural color; otherwise, it is a pseudo-code. This structural color-based "decryption-verification" approach offers innovative anti-counterfeiting applications in nanophotonics.

Structural Color Enhancement through Synchronizing Natural Convection and Marangoni Flow in Pendant Drops

Structural color, renowned for its enduring vibrancy, has been extensively developed and applied in the fields of display and anticounterfeiting. However, its limitations in brightness and saturation hinder further application in these areas. Herein, we propose a pendant evaporation self-assembly method to address these challenges simultaneously. By leveraging natural convection and Marangoni flow synchronization, the self-assembly process enhances the dynamics and duration of colloidal nanoparticles, thereby enhancing the orderliness of colloidal photonic crystals. On average, this technique boosts the brightness of structural color by 20% and its saturation by 35%. Moreover, pendant evaporation self-assembly is simple and convenient to operate, making it suitable for industrial production. We anticipate that its adoption will remarkably advance the industrialization of structural color, facilitating its engineering applications across various fields, such as display technology and anticounterfeiting identification.

Miniaturized spectrometers based on graded photonic crystal films

Miniaturized spectrometers have become increasingly important in modern analytical and diagnostic applications due to their compact size, portability, and versatility. Despite the surge in innovative designs for miniaturized spectrometers, significant challenges persist, particularly concerning manufacturing cost and efficiency when devices become smaller. Here we introduce an ultracompact spectrometer design that is both cost-effective and highly efficient. The core dispersion element of this new design is a graded photonic crystal film, which is engineered by applying gradient stress during its fabrication. The film shows bandstop transmission spectral profiles, akin to a notch filter, enhancing light throughput compared to conventional narrowband filters. The spectral analysis, with a resolution of 5 nm and operating within the wavelength range of 450-650 nm, is conducted by reconstructing the spectrum from a series of such notch transmission profiles along the graded photonic crystal film, utilizing a sophisticated algorithm. This approach not only reduces manufacturing costs but also significantly improves the sensitivity (with a light throughput efficiency of 71.05%) and overall performance of the limitations of current technology, opening up new avenues for applications in diverse fields.

Electrically triggered photonic crystal anti-counterfeiting tags with multi-level response fabricated by regioselective modification of ITO electrode surface

Anticounterfeiting materials based on the photonic crystal (PC) have attracted great interest due to their unique visual effects originating from the changeable structural colors under various external stimuli. However, there still are challenges to improving the anticounterfeiting performance by enhancing the complexity and diversity of the color changes. Here, we fabricated an electrically triggered anticounterfeiting tag by encapsulating the responsive PC with the surface-modified and patterned ITO electrode. The degree of Au deposition or chemical etching in different regions of the ITO was precisely controlled to achieve multi-level differentiated electrical responses, which made the invisible pattern of the tag at 0 V be "revealed in multicolor form" or "gradually revealed" under increasing voltages. The tag possessed two working modes, more diversified visual effects, good usability, and reversibility, which let it become a potentially useful material for anti-counterfeiting applications in the future.

3D-Printed Biomimetic Structural Colors

Resolution control and expansibility have always been challenges to the fabrication of structural color materials. Here, a facile strategy to print cholesteric liquid crystal elastomers (CLCEs) into complex structural color patterns with variable resolution and enhanced expansibility is reported. A volatile solvent is introduced into the synthesized CLC oligomers, modifying its rheological properties and allowing direct-ink-writing (DIW) under mild conditions. The combination of printing shear flow and anisotropic deswelling of ink drives the CLC molecules into an ordered cholesteric arrangement. The authors meticulously investigate the influence of printing parameters to achieve resolution control over a wide range, allowing for the printing of multi-sized 1D or 2D patterns with constant quality. Furthermore, such solvent-cast direct-ink-writing (DIW) strategy is highly expandable and can be integrated easily into the DIW of bionic robots. Multi-responsive bionic butterfly and flower are printed with biomimetic in both locomotion and coloration. Such designs dramatically reduced the processing difficulty of precise full-color printing and expanded the capability of structural color materials to collaborate with other systems.

2D Information Security System Based on Polyurethane Inverse Photonic Glass Structure

Information security has become a major global problem in recent years. Thus, people continue to exert much effort in developing new information security technologies based on encryption and storage. In this study, a 2D information security technology based on polyurethane optical devices with inverse photonic glass structure (PU-IPG) is introduced. Based on 1) the swelling and plasticizing effects of various solvents on PU-IPG and 2) the capillary force that can produce geometric deformation on micro/nanostructures when solvents evaporate, a 2D information security system with two modules of decryption (structural color information display) and anticounterfeiting (structural color transformation) is successfully constructed. The spraying method adopted can be simple and fast and can provide a large area to build photonic glass templates, which greatly improves the capacity and category of information in the encryption system. The prepared PU-IPG optical devices can produce large-area multicolor output capability of information. These devices also have excellent mechanical properties, strong cycle stability, environmental friendliness, and low price. Therefore, the preparation strategy has great reference value and application prospects in the field of information security.

Auxetic Photonic Patterns with Ultrasensitive Mechanochromism

Photonic crystals with mechanochromic properties are currently under intensive study to provide intuitive colorimetric detection of strains for various applications. However, the sensitivity of color change to strain is intrinsically limited, as the degree of deformation determines the wavelength shift. To overcome this limitation, auxetic photonic patterns that exhibit ultra-sensitive mechanochromism are designed. These patterns have a regular arrangement of cuts that expand to accommodate the strain, while the skeletal framework undergoes torsional deformation. Elastic photonic crystals composed of a non-close-packed array of colloidal particles are embedded in the cut area of the auxetic patterns. As the cut area amplifies the strains, the elastic photonic crystals show significant color change even for small total strains. The degree of local-strain amplification, or sensitivity of color change, is controllable by adjusting the width of cuts in the auxetic framework. In this work, a maximum sensitivity of up to 60 nm/% is achieved, which is 20 times higher than bulk films. It is believed that the auxetic photonic patterns with ultra-sensitive mechanochromism will provide new opportunities for the pragmatic use of mechanochromic materials in various fields, including structural health monitoring.

Stimulus-responsive nonclose-packed photonic crystals: fabrications and applications

Stimulus-responsive photonic crystals (PCs) possessing unconventional nonclosely packed structures have received growing attention due to their unique capability of mimicking the active structural colors of natural organisms (for example, chameleons' mechanochromic properties). However, there is rarely any systematic review regarding the progress of nonclose-packed photonic crystals (NPCs), involving their fabrication, working mechanisms, and applications. Herein, a comprehensive review of the fundamental principles and practical fabrication strategies of one/two/three-dimensional NPCs is summarized from the perspective of designing nonclose-packed structures. Subsequently, responsive NPCs with exciting functions and working mechanisms are sorted and delineated according to their diverse responses to physical (force, temperature, magnetic, and electric fields), chemical (ions, pH, vapors, and solvents), and biological (glucose, organophosphate, creatinine, and bacteria) stimuli. We then systematically introduced and discussed the applications of NPCs in sensors, printing, anticounterfeiting, display, optical devices, etc. Finally, the current challenges and development prospects for NPCs are presented. This review not only concludes the design principle for NPCs but also provides a significant basis for the exploration of next-generation NPCs.

Dual-mode security authentication of SrAl2 O4 :Eu,Dy phosphor encapsulated in electrospun cellulose acetate nanofibrous films

Photochromic inks have been an attractive authentication strategy to improve the anti-counterfeiting efficiency of commercial products. However, recent reports have shown significant disadvantages with photochromic inks, including poor durability and high cost. In this context, we developed novel photochromic nanofibres for advanced anti-counterfeiting applications. Lanthanide-doped strontium aluminate (LdSA) nanoparticles (NPs) were prepared and immobilized into electrospun cellulose acetate nanofibres (CANF). Authentication materials immobilized with inorganic photochromic agents can warranty durability and photostability. Therefore, the ultraviolet-stimulated photochromism of LdSA-encapsulated cellulose acetate nanofibres (LdSA@CANF) demonstrated high reversibility and photostability. A broad range of cellulose acetate nanofibres with unique emission characteristics was developed when applying different ratios of LdSA NPs. LdSA@CANF appeared colourless under visible daylight, whereas a green emission was monitored under ultraviolet-light illumination. The shape and chemical content of the photochromic fibrous films were examined using various analytical techniques. The mechanical characteristics of LdSA@CANF-coated paper were investigated. The emission wavelength was detected at 514 nm to designate green colour, whereas the excitation wavelength was detected at 369 nm to indicate transparency. The prepared cellulose acetate nanofibrous film can be described as an efficient strategy for the anti-counterfeiting of commercialized items.

Recent advances in the anti-counterfeiting applications of long persistent phosphors

Counterfeit products have infiltrated numerous regions worldwide, causing substantial damage to the financial interests of individuals, businesses, and countries. Moreover, counterfeit goods can pose a severe risk to human health. Therefore, it is crucial to develop effective anti-counterfeiting methods and authentication technologies. Persistent luminescence (PersL) materials show great potential for anti-counterfeiting applications due to their distinctive spatial and temporal dynamic spectrum performance. The unique luminescence properties of PersL materials enable the creation of optical codes with high capacity. In this perspective, we provide a summary of the latest advancements in anti-counterfeiting technology using long persistent phosphors. We discuss the various construction strategies of optical codes for anti-counterfeiting, which include multicolor luminescence, orthogonal luminescence, dynamic luminescence, and stimulus-response luminescence. In addition, we explore the mechanisms of PersL-based anti-counterfeiting materials and consider potential areas for future development to expand the applications of persistent phosphors.

Portable Comestible-Liquid Quality Test Enabled by Stretchable and Reusable Ion-Detection Photonic Papers

Currently, there have been widespread investigation conducted into responsive photonic crystal hydrogels (RPCHs) characterized by high selectivity and sensitivity for colorimetric indicators and physical/chemical sensors. In spite of this, it remains challenging to use RPCHs for sensing due to their limited mechanical property and molding capability. In the present study, a double-network structure is proposed to design highly stretchable, sensitive, and reusable ion-detection photonic papers (IDPPs) for assessing the quality of visual and portable comestible liquids (e.g., soy sauce). It is constructed by integrating polyacrylamide and poly-methacryloxyethyl trimethyl ammonium chloride with highly ordered polystyrene microspheres. The double-network structure improves the mechanical properties of IDPPs with their elongation at break increasing from 110 to 1600%. Meanwhile, the optical properties of photonic crystals are retained. The IDPPs achieve a fast ion response by applying control on the swelling behavior of the hydration radius of the counter ions through ion exchange. Given a certain concentration range (0.01-0.10 M), chloride ions can be detected fast (3-30 s) by exchanging ions with a small hydration radius through an IDPP, which is clearly observable. Due to the improvement of mechanical properties and the reversible exchange of ions derived from IDPPs, their reusability is significantly enhanced (>30 times). Characterized by a simple operation, high durability, and excellent sustainability, these IDPPs are promising for practical application in food security and human health assessment.

Novel Green Reversible Humidity-Responsive Hemiaminal Dynamic Covalent Network for Smart Window

Recently, smart windows have attracted widespread attention on account of their unique features, yet traditional smart windows still rely on external energy support to accomplish dynamic reversible switching, which not only confines usage but also causes waste of energy. For this purpose, we have prepared hemiaminal dynamic covalent network (HDCN) film with outstanding flexibility and strength by a simple and low-cost method, in which the modulus is 206.28 MPa and the elongation at break is 39.02%. Additionally, the transition from a transparent to an opaque state is achieved when the film is stimulated by humidity, and the dynamic transformation of the film to different phases of transparency is obtained when the film is exposed to different relative humidities (60-99%). Most importantly, HDCN film fulfills the modern green requirements and enables complete dissolution in a certain mildly acidic solution, avoiding environmental pollution when the material is discarded due to loss of function. The dynamic tunability of HDCN film demonstrates great advantages and potential in smart windows and anticounterfeiting.

Structural Color Ionic Hydrogel Patches for Wound Management

Ionic hydrogels have attracted extensive attention because of their wide applicability in electronic skins, biosensors, and other biomedical areas. Tremendous effort is dedicated to developing ionic hydrogels with improved detection accuracy and multifunctionality. Herein, we present an inverse opal scaffold-based structural color ionic hydrogel with the desired features as intelligent patches for wound management. The patches were composed of a polyacrylamide-poly(vinyl alcohol)-polyethylenimine-lithium chloride (PAM-PVA-PEI-LiCl) inverse opal scaffold and a vascular endothelial growth factor (VEGF) mixed methacrylated gelatin (GelMA) hydrogel filler surface. The scaffold imparted the composite patches with brilliant structural color, conductive property, and freezing resistance, while the VEGF-GelMA surface could not only prevent the ionic hydrogel from the interference of complex wound conditions but also contribute to the cell proliferation and tissue repair in the wounds. Thus, the hydrogel patches could serve as electronic skins for in vivo wound healing and monitoring with high accuracy and reliability. These features indicate that the proposed structural color ionic hydrogel patches have great potential for clinical applications.

Bioinspired Colloidal Photonic Composites: Fabrications and Emerging Applications

Organisms in nature have evolved unique structural colors and stimuli-responsive functions for camouflage, warning, and communication over millions of years, which are essential to their survival in harsh conditions. Inspired by these characteristics, colloidal photonic composites (CPCs) composed of colloidal photonic crystals embedded in the polymeric matrix are artificially prepared and show great promise in applications. This review focuses on the summary of building blocks, i.e., colloidal particles and polymeric matrices, and constructive strategies from the perspective of designing CPCs with robust performance and specific functionality. Furthermore, their state-of-the-art applications are also discussed, including colorful coatings, anti-counterfeiting, and regulation of photoluminescence, especially in the field of visualized sensing. Finally, current challenges and potential for future developments in this field are discussed. The purpose of this review is not only to clarify the design principle for artificial CPCs but also to serve as a roadmap for the exploration of next-generation photonic materials.

Polyurethane-polypyrrole hybrid structural color films for dual-signal mechanics sensing

The monitoring of mechanical indexes involved in body movement has attracted immense interest in the diagnosis of neurodegenerative diseases. Here, we present a hybrid flexible conductive structural color (SC) film with the capability of dual-signal mechanics screening. The film is constructed by oxidatively polymerizing pyrrole on the surface of an inverse opal polyurethane (IPU) membrane, which can be utilized to measure the mechanical indexes through resistance change. Owing to the inverse opal structure, the film shows visual structural color change when stretched and released according to the body movement. Additionally, the highly uniform ordered porous structure endows the conductive film with a lower coefficient of variance on relative resistance change. Benefiting from these features, we have demonstrated that such a flexible conductive SC film could monitor Parkinson's disease (PD) by detecting mechanical indexes simultaneously via dual signals. These features indicate the great value of the stretchable conductive SC films in mechanics sensing applications.

Solvent-Responsive Invisible Photonic Patterns with High Contrast for Fluorescence Emission Regulation and Anti-Counterfeiting

Invisible photonic patterns (IPPs) are photonic materials that can display hidden patterns under external stimulation and are attractive in anti-counterfeiting devices and information storage. In this work, we report a solvent-responsive invisible photonic pattern (SRIPP) with high contrast by polymerizing two monomers of acrylamide (AAm) and poly(ethylene glycol) methacrylate (PEGMA) with different solubility parameters in different regions of poly(hydroxyethyl methacrylate) photonic gels. The two regions with different solvent responsiveness can shrink and swell in the same environment, thus causing the colors of different regions of photonic gel to shift in opposite directions from the initial state. As a result, the contrast of photonic patterns is significantly improved, increasing naked-eye visualization. In addition, by introducing fluorescent substances into the photonic gel and adjusting the photonic band gap (PBG) of photonic gels, we realize the regulation of fluorescence emission and display of fluorescence patterns by utilizing different PBGs on the SRIPP. Dynamic solvent responsiveness patterns and fluorescence patterns are integrated into a photonic gel, showing great potential in information storage and multiple-mode anti-counterfeiting applications.

Preparation of photoluminescent nanocomposite ink for detection and mapping of fingermarks

Simple and efficient detection and mapping method based on a strong turn-on fluorescent pigment was developed for fingerprint analysis. We present a phosphor powder characterized by strong emission which is useful to achieve better fingerprint detection on multicolored or photoluminescent surfaces, such as currency notes characterized by optically changeable inks and highly fluorescent positions, because it offers better contrast and reduce the difficulty of background interference. Novel photochromic ink was prepared to establish a fingerprinted colorless film onto cellulose documents with green emission for anticounterfeiting applications as illustrated by photoluminescence spectra. Inorganic/organic nanoscale composite ink was prepared from rare-earth doped aluminate phosphor nanoparticles (PNPs; 27-49 nm) dispersed in a polyacrylic acid binding agent. PNPs were dispersed efficiently in polyacrylic acid to generate a colorless mark. The produced photochromic inks were spray-coated onto off-white paper sheets enclosing invisible fingermarks, and then exposed to thermofixation. Photochromic film was detected on paper surface presenting a transparent appearance under visible daylight and switchable to green under UV light. The CIE Lab parameters and photoluminescence spectra were studied under visible light and ultraviolet irradiation. The fingerprinted sheets showed fluorescence band at 517 nm upon excitation at 366 nm, showing a bathochromic shift and reversible photochromism without fatigue. The morphologies of pigment phosphor particles and fingerprinted sheets were inspected. The rheological properties of ink and mechanical behavior of the fingerprinted paper samples were explored. HIGHLIGHTS: Novel smart ink with alkaline-earth aluminate and polyacrylic acid was developed. Dual-mode fluorescent photochromism was presented for latent fingerprint analysis. Off-white fingerprinted films under daylight showed color change to green under UV. Fluorescence band monitored at 517 nm upon excitation at 366 nm. Fluorescent fingermark on paper sheets demonstrated good photostability.

Low-Angle-Dependent Anticounterfeiting Label Decoded by Alcohol Tissue Wiping Based on a Multilayer Photonic Crystal Structure

The application of photonic crystals (PCs) as anticounterfeiting materials has been widely investigated because of their tunable photonic stop band and corresponding changeable structural colors. In this work, we designed a composite PC structure including an information CdS PC layer at the bottom and a polymer-based layer composed of an inverse opal PC (IOPC) layer and a disordered porous layer on the top, which can be decoded by an alcohol tissue. The high refractive index of the bottom patterned CdS PC layer provides the structure with a vivid low-angle-dependent structural color in the decoded mode, which ensures the stability of the information conveyed by this label. When the incident angle changed from 5 to 45°, the structural color of the patterned CdS layer changed slightly. In the hidden mode, the low transmittance shields the structural color of the CdS layer. When the structure was wiped with the alcohol tissue, the transmittance of the upper IOPC layer could be increased quickly due to the similar refractive indexes of the used polymer and alcohol, and the pattern of the CdS layer was decoded. Thus, the designed composite PC can act as an anticounterfeiting label, in which the encrypted pattern can be decoded by alcohol tissue wiping and shows a vivid low-angle-dependent structural color. To enhance the anticounterfeiting ability of the designed structure, a double-sided label with different encryption patterns on both sides was designed. Based on the simple reversible encryption and decryption process as well as the color stability, the label shows great application potential in the daily anticounterfeiting field.

Scalable and Sensitive Humidity-Responsive Polymer Photonic Crystal Films for Anticounterfeiting Application

In this study, we fabricate a new kind of ultrasensitive humidity-responsive photonic crystal (HPC) films based on emulsion polymerization and the open mill and bending-induced ordering technique (OM-BIOT) method, which is simple and scalable. The HPC film senses relative humidity (RH) from 9 to 98% for the polymer matrix swells up in high RH and shrinks in low RH, leading to a large reflectance shift (81 nm) and distinct color change. Based on the double-peak reflective spectra of the HPC film, we confirm the gradient swelling hypothesis and find that the thickness is another important factor for controlling the sensitivity and response rate of the HPC film. Except for static humidity, the HPC film can also respond to the dynamic humid flow of blowing and polar solvents, which broadens its application potential. This kind of HPC film shows a vivid structural color, and the humidity-responsive behavior is quick, distinct, energy-free, and reversible, having a great prospect for anticounterfeiting application.

Bio-inspired shape-memory structural color hydrogel film

Structural colors, derived from existing natural creatures, have aroused widespread attention in the materials regulation for different applications. Here, inspired by the color adjusting mechanism of hummingbird, we present a novel shape-memory structural color hydrogel film by introducing shape memory polymers (SMPs) into synthetic inverse opal scaffold structure. The excellent flexibility as well as the inverse opal structure of the hydrogel films imparts them with stable stretchability and brilliant structural colors. Benefiting from the transient structural anisotropy of copolymers, the hybrid films are possessed with shape-morphing behaviors capability. Based on the shape transformations and color responsiveness performance, we have demonstrated diverse structural color actuators with complex shapes for different tasks. Notably, as the photothermal responsive graphene quantum dots were integrated into the hydrogel, the hybrid films could also be endowed with the feature of light-controlled reversible deformation with synchronous structural color variation. These features demonstrate that the presented shape-memory structural color hydrogel film is valuable for soft robotics with multi-functions of sensing, communication and disguise.

Photo-Luminescent Photonic Crystals for Anti-Counterfeiting

The conventional photonic crystals (PCs) are usually prepared by the self-assembly of silica or polystyrene particles. However, their applications are limited significantly due to the lack of the functions of the building blocks. Here, a new kind of photo-luminescent photonic crystals (PLPCs) with brilliant PL and structural colors were prepared by the self-assembly of dye-doped silica particles. The PL and structural colors of PCs can be well-controlled by altering the species of dyes and the size of the particles, respectively. Based on these advantages, PLPC patterns with encrypted information were fabricated through the combination of PLPCs and PCs with similar structural colors but diverse PL colors. These patterns can reversibly hide and display the encrypted information under sunlight and UV illumination, respectively. This work paves a new way for constructing functional PCs and will promote their applications in anti-counterfeiting, smart labels, and optical devices.

Wide-Gamut Biomimetic Structural Colors from Interference-Assisted Two-Photon Polymerization

Two-photon polymerization (TPP) is an emerging direct laser writing technique for the fabrication of structural colors. However, its coloration ability is suppressed as the vertical resolution is up to several microns. To solve this issue, an interference-assisted TPP technique was employed. Laser interference at a highly reflective interface produced the periodic energy redistribution along the vertical direction, turning the laser voxel into multilayer structures and confirming this technology as a facile and robust method for precise control of its vertical feature size. Biomimetic structural colors (BSCs) inspired from the ridge-lamella configurations in the Morph butterflies were fabricated using this improved TPP technique. The coloration mechanisms of the multilayer interference from the lamella layers, the thin-film interference from the fusion of multilayers, and the hybrid situations were systematically studied. These BSC colors were grouped as pixel palettes with various TPP parameters corresponding to each other, and they spanned almost the entire standard red-green-blue color space. Moreover, under optimized conditions, it was possible to fabricate a 1 cm² area within 2.5 h. These features make interference-assisted TPP an ideal coloration method for practical applications, such as display, decoration, sensing, and so on.

Supramolecular DNA Photonic Hydrogels for On-Demand Control of Coloration with High Spatial and Temporal Resolution

Hydrogels embedded with periodic arrays of nanoparticles display a striking photonic crystal coloration that may be useful for applications such as camouflage, anticounterfeiting, and chemical sensing. Dynamically generating color patterns requires control of nanoparticle organization within a polymer network on-demand, which is challenging. We solve this problem by creating a DNA hydrogel system that shows a 50 000-fold decrease in modulus upon heating by ∼10 °C. Magnetic nanoparticles entrapped within these DNA gels generate a structural color only when the gel is heated and a magnetic field is applied. A spatially controlled photonic crystal coloration was achieved by photopatterning with a near-infrared illumination. Color was "erased" by illuminating or heating the gel in the absence of an external magnetic field. The on-demand assembly technology demonstrated here may be beneficial for the development of a new generation of smart materials with potential applications in erasable lithography, encryption, and sensing.

Super Strong and Tough Polybenzimidazole/Metal Ions Coordination Networks: Reinforcing Mechanism, Recyclability, and Anti-Counterfeiting Applications

Nature has provided many delicate strategies for optimizing the structural characteristics of biological materials. One such strategy is the strengthening and toughening of matrix materials by aduandant and hierarchically arranged non-covalent crosslinking. However, efficient strengthening and toughening of high-performance aromatic polymers by non-covalent bonds has rarely been reported yet. Herein, we report the preparation and characterizations of a metal coordination bonds crosslinked polybenzimidazole (PBI) network. By optimizing the synthetic parameters, the strength of copper ion (Cu²⁺ ) crosslinked PBI is improved from 87.8 to 218.4 MPa, and the toughness is increased from 19.4 to 111.9 MJ m^-3 , corresponding to increments of 148.7 % and 476.8 %, respectively, which surpass all previously reported non-covalent bonds crosslinked high-performance polymers. PBI with varied chain flexibility are then synthesized to deeply understand the stregnening and toughening mechanism. In addition, the glass transition temperature of PBI is dramatically increased by 75 °C after Cu²⁺ crosslinking. Moreover, the chemical recycling of PBI from crosslinekd network, and the development of a novel high-temperature resistant or high-temperature rewritable anti-counterfeiting films based on Cu²⁺ crosslinked PBI are also demonstrated. This study is expected to shed light on design principle for future supramolecularly crosslinked and recyclable high-performance polymers.

Colloidal Self-Assembly Approaches to Smart Nanostructured Materials

Colloidal self-assembly refers to a solution-processed assembly of nanometer-/micrometer-sized, well-dispersed particles into secondary structures, whose collective properties are controlled by not only nanoparticle property but also the superstructure symmetry, orientation, phase, and dimension. This combination of characteristics makes colloidal superstructures highly susceptible to remote stimuli or local environmental changes, representing a prominent platform for developing stimuli-responsive materials and smart devices. Chemists are achieving even more delicate control over their active responses to various practical stimuli, setting the stage ready for fully exploiting the potential of this unique set of materials. This review addresses the assembly of colloids into stimuli-responsive or smart nanostructured materials. We first delineate the colloidal self-assembly driven by forces of different length scales. A set of concepts and equations are outlined for controlling the colloidal crystal growth, appreciating the importance of particle connectivity in creating responsive superstructures. We then present working mechanisms and practical strategies for engineering smart colloidal assemblies. The concepts underpinning separation and connectivity control are systematically introduced, allowing active tuning and precise prediction of the colloidal crystal properties in response to external stimuli. Various exciting applications of these unique materials are summarized with a specific focus on the structure-property correlation in smart materials and functional devices. We conclude this review with a summary of existing challenges in colloidal self-assembly of smart materials and provide a perspective on their further advances to the next generation.

Noniridescent structural color from enhanced electromagnetic resonances of particle aggregations and its applications for reconfigurable patterns

HYPOTHESIS

The conventional noniridescent structural colors refer to the coherent scattering of visible light by the short-range ordered structures assembled from the small colloids (100-250 nm). Our hypothesis is that noniridescent structural color can be generated by the random aggregations of large silica particles through the enhanced electromagnetic resonances.

EXPERIMENTS

The random aggregations of large silica particles (350-475 nm) were prepared through the infiltration of silica particles solution with the porous substrate. The mechanism of the structural color is investigated. Reconfigurable patterns are prepared.

FINDINGS

Dissimilar to the conventional noniridescent colors, the angle-independent colors of silica aggregations originate from the enhanced electromagnetic resonances due to the random aggregation of the particles. The colors (blue, green, and red) and corresponding reflection peak positions of the particle aggregations can be well controlled by simply altering the size of the silica particles. Compared to the traditional prints with permanent patterns, reconfigurable patterns with large-area and multicolor can be fabricated by the repeatedly selective spray of water on the substrate pre-coated with noniridescent colors. This work provides new insight and greenway for the fabrication of noniridescent structural colors and reconfigurable patterns, and will promote their applications in soft display, green printing, and anti-counterfeiting.

Core/shell colloidal nanoparticles based multifunctional and robust photonic paper via drop-casting self-assembly for reversible mechanochromic and writing

Photonic crystals film that possesses periodic dielectric structure have shown great prospect in developing environmentally friendly paper alternatives due to the unique properties of dye free and non-photobleaching, but their practical application is limited by the weak interaction between colloidal particles. Although some progress has been obtained, it is still a challenge to develop photonic paper with the desired mechanical and optical properties. Herein, multifunctional hard core/soft shell nanoparticles with controlled size are fabricated by semi-continuous seed emulsion polymerization method. Compared with convention colloidal particles, these core/shell nanoparticles can facile self-assemble into large-scale dense ordered structure film via dried at room temperature due to the relatively low glass transition temperature (Tg) of the shell layers. The facile fabrication route enables the continuous high-through put production of the photonic papers. The as-formed papers not only possess the capacity to solvent (water/ethanol) rewritable and multicolor painting, but also can rapidly reversible mechanochromic. Moreover, due to the good compatibility of core/shell interface, these photonic films possess excellent mechanical properties, demonstrating that this multifunctional film makes the fabrication of novel robust rewritable papers possible and enables visual monitoring of deformation degree.

Lotus Seedpod Inspiration: Particle-Nested Double-Inverse Opal Films with Fast and Reversible Structural Color Switching for Information Security

The integration of novel structures into colloidal crystals provides the possibility of constructing stimuli-responsive photonic materials. However, in most opal and inverse opal structures, replacing the interior air with an infiltrated liquid will cause partial refractive index matching, resulting in the reduction or even disappearance of the photonic band gap. Herein, inspired by the lotus seedpod, an innovative particle-nested double-inverse opal film with fast and reversible structural color switching (≈1 s) is first fabricated by introducing polystyrene (PS) spheres into an inverted opal backbone. Importantly, refractive index matching can be effectively avoided due to the existence of internal PS spheres, and optical switching from diffusive to photonic behavior is achieved by a liquid with low surface tension for the response. Furthermore, a reversible ethanol stimuli-response bilayer double-inverse opal film with multistate switching for information encryption is proposed by combining optical scattering and diffraction. The scattered light from the top layer caused by the randomly distributed and weakly scattering PS spheres within the pores makes the pattern at the bottom invisible. Simultaneously, the display and discoloration of the pattern can be realized instantaneously by ethanol response. Thus, this new preparation strategy exhibits great potential in the security fields.

Simple and efficient fabrication of multi-stage color-changeable photonic prints as anti-counterfeit labels

Color changeable photonic prints (CCPPs) show their potential applications in high-level information storage and anti-counterfeiting, but usually suffer from the complex fabrication process and limited color variation. Here, a simple and efficient method is developed to generate CCPPs with multilevel tunable color contrasts by packing the solvent responsive photonic crystals with diverse cross-linking degrees and desired way. The key to the successful fabrication is to create and control over the optical response of each part of the CCPPs through altering the cross-linking degree of PCs and thus the affinity between the CCPPs and solvents. A CCPPs based anti-fake label with the encrypted information functionality which originates from reversible color change between dried state and swelling with the mixture of acetic acid and ethanol is investigated. Compared with conventional CCPPs, the as-prepared CCPPs can reveal multistage information depending on the volume fraction of ethanol. This work provides a new insight for the simple fabrication of CCPPs and will facilitate their applications in the information protection and high-level anti-counterfeiting.

Robust, Portable, and Specific Water-Response Silk Film with Noniridescent Pattern Encryption for Information Security

In modern days, information is a key resource for accelerating the development of society, economy, and culture. Thus, information security has always been a high priority for any country, business, and department. Herein, a simple and effective strategy for preparing an independent optical device for information security is proposed by using silk fibroin materials with a quasiamorphous inverse structure. Given the reversible hydrogen bonds between silk fibroin materials and water molecules, a multicolor high-resolution pattern with a variable color can be obtained by using a simple spray coating method. Furthermore, a reversible water stimulus-response silk film with a laminated structure that consists of hidden and patterned layers and carries quick response (QR) code information is prepared. This device effectively hides (encryption) the QR code pattern in a normal environment and quickly displays the information (decryption) in water. Simultaneously, the silk film shows good mechanical strength, excellent biocompatibility, long-term structural stability, and a unique response mechanism, which make it a suitable carrier of optical information. Thus, this new preparation strategy of an optical device has a potential application value and is an important reference in the fields of information security and functional materials.

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.

Self-Assembled Mechanochromic Shape Memory Photonic Crystals by Doctor Blade Coating

Mechanochromic shape memory photonic crystals can memorize their original structures and recover the inherent structural colors in response to external stimuli; thereby they have rendered various important optical applications. Unfortunately, most existing shape memory polymers are thermoresponsive, and the corresponding mechanochromic characteristics are limited by the heat-demanding programming process. Besides that, a great majority of current fabrication methodologies suffer from low throughput, hindering the practical applications. Herein, a scalable technology is developed to engineer macroporous shape memory photonic crystals by self-assembling silica colloidal crystals in a polyurethane acrylate/polyethoxylated trimethylolpropane triacrylate/poly(ethylene glycol) diacrylate matrix, followed by a wet etching treatment to selectively remove silica colloids. The as-created photonic crystals display a brilliant structural color, which is reversibly tunable with mechanical deformation at ambient conditions. Upon stretching, the reduced interlayer lattice spacing of the photonic crystals leads to a blueshift of the reflection peak position and a significant color change. Importantly, the stretched macroporous film can fix its temporary structures without applying any contact force and simultaneously recover its original configuration and appearance by applying ethanol evaporation-induced capillary pressures. The reversibility and the dependence of templated silica colloid size on mechanochromic characteristics have also been investigated in the research.

Dual stimuli-responsive lanthanide-based phosphors for an advanced full-color anti-counterfeiting system

Dual stimuli-responsive lanthanide (Ln)-based phosphors were prepared for constructing a multi-level full color anti-counterfeiting system. The encrypted information can be visually read out under UV irradiation. Further triggered by water, not only are the fluorescence colors changed, but so are the patterns of the encrypted information. The Ln-based phosphors can be cost-effectively prepared in a simple way, presenting great potential application in constructing an advanced anti-counterfeiting system with a convenient authentication assay.

Dual stimuli-responsive lanthanide-based phosphors were prepared for constructing a multilevel full color anti-counterfeiting system, augmenting the security level for information protection.