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

Metal-organic framework photonic crystals with bidisperse particles-based brilliant structural colors and high optical transparency for elaborate anti-counterfeiting

Photonic crystals (PCs) have attracted great interest and wide applications in displays, printing, anti-counterfeiting, etc. However, two main challenges significantly hinder their applications: 1) the tradeoff between high optical transparency across the whole visible range and brilliant colors requiring a large refractive index contrast (Δn), and 2) the way of regulating structural colors by altering tens of different sizes. To address these issues, a new type of metal-organic framework (MOF)-based transparent photonic crystal (TPC) has been fabricated through self-assembling MOF particles into three-dimensional ordered structures which were then infiltrated by polydimethylsiloxane (PDMS). Compared to conventional PCs, these TPCs exhibit 1) both brilliant forward iridescent structural colors and high transmittance (>75 %) across the whole visible spectra range, and 2) conveniently adjustable colors based on bidisperse particles. The unique color-generating mechanism of the light diffraction by each plane lattice and the small Δn between MOF particles and PDMS are the keys to TPCs' characteristics. Moreover, the prepared invisible anti-counterfeit labels can reversibly hide-reveal patterns with elaborate and exchangeable color contrast in a non-destructive way, showing potential applications in anti-counterfeiting, information encryption, and optical devices.

Bioinspired electrically stable, optically tunable thermal management electronic skin via interfacial self-assembly

The skin is the largest organ in the human body and serves vital functions such as sensation, thermal management, and protection. While electronic skin (E-skin) has made significant progress in sensory functions, achieving adaptive thermal management akin to human skin has remained a challenge. Drawing inspiration from squid skin, we have developed a hybrid electronic-photonic skin (hEP-skin) using an elastomer semi-embedded with aligned silver nanowires through interfacial self-assembly. With mechanically adjustable optical properties, the hEP-skin demonstrates adaptive thermal management abilities, warming in the range of +3.5°C for heat preservation and cooling in the range of -4.2°C for passive cooling. Furthermore, it exhibits an ultra-stable high electrical conductivity of ∼4.5×104 S/cm, even under stretching, bending or torsional deformations over 10,000 cycles. As a proof of demonstration, the hEP-skin successfully integrates stretchable light-emitting electronic skin with adaptive thermal management photonic skin.

Smart colloidal photonic crystal sensors

Smart colloidal photonic crystals (PCs) with stimuli-responsive periodic micro/nano-structures, photonic bandgaps, and structural colors have shown unique advantages (high sensitivity, visual readout, wireless characteristics, etc.) in sensing by outputting diverse structural colors and reflection signals. In this review, smart PC sensors are summarized according to their fabrications, structures, sensing mechanisms, and applications. The fabrications of colloidal PCs are mainly by self-assembling the well-defined nanoparticles into the periodical structure (supersaturation-, polymerization-, evaporation-, shear-, interaction-, and field-induced self-assembly process). Their structures can be divided into two groups: closely packed and non-closely packed nano-structures. The sensing mechanisms can be explained by Bragg's law, including the change in the effective refractive index, lattice constant, and the order degree. The sensing applications are detailly introduced according to the analytes of the target, including solvents, vapors, humidity, mechanical force, temperature, electrical field, magnetic field, pH, ions/molecules, and so on. Finally, the corresponding challenges and the future potential prospects of artificial smart colloidal PCs in the sensing field are discussed.

Controllable hydrophilic/superhydrophobic patterned coatings for optical information encryption/decryption based on water-triggered opaque to translucent transition

Anti-counterfeiting technologies are crucial for securing the authenticity and proof of commodities, in which the optical information encryption/decryption has attracted extensive attention for its overriding advantages of visibility and convenience. Inspired by the unique transparency transformation phenomenon of Diphylleia grayi petals, a controllable hydrophilic/superhydrophobic patterned coating with water-triggered opaque to translucent transition is proposed through the construction of a superhydrophobic coating, subsequent air plasma etching under a mask, and final hydrophilic modification to introduce stable invisible patterns. The superhydrophobic region exhibits great water repellency with a water contact angle (WCA) at 157°, while the hydrophilic region quickly absorbs water with a WCA at 61°. The patterned coating presents an opaque state for the serious light scattering induced by the rough microstructure and large refractive index difference between the coating and air, while the hydrophilic patterns on the coating transform to translucent after water infiltration for the reduced roughness and close refractive indexes of the coating and water. The information revealing is rapid and reversible, and demonstrates heat and long-term stability and great reusability. The findings conceivably stand out as a new methodology to fabricate controllable superwettable coatings with optical information encryption/decryption capability for application in anti-counterfeiting.

Hollow mesoporous cubic silica self-assembling into photonic crystals with rhombohedral lattices and vivid structural colors for anti-counterfeiting

Colloidal photonic crystals (PCs) feature face-centered cubic (FCC) lattices since spherical particles are usually used as building blocks; however, constructing structural colors originating from PCs with non-FCC lattices is still a big challenge due to the difficulty in preparing non-spherical particles with tunable morphologies, sizes, uniformity, and surface properties and assembling them into ordered structures. Here, uniform, positively charged, and hollow mesoporous cubic silica particles (hmc-SiO2) with tunable sizes and shell thicknesses prepared by a template approach are used to self-assemble into PCs with rhombohedral lattice. The reflection wavelengths and structural colors of the PCs can be controlled by altering the sizes or the shell thicknesses of the hmc-SiO2. Additionally, photoluminescent PCs have been fabricated by taking the advantage of the click chemistry between amino silane and isothiocyanate of a commercial dye. The PC pattern achieved by a hand-writing way with the solution of the photoluminescent hmc-SiO2 instantly and reversibly shows the structural color under visible light but a different photoluminescent color under UV illumination, which is useful for anticounterfeiting and information encryption. The non-FCC structured and photoluminescent PCs will upgrade the basic understanding of the structural colors and facilitate their applications in optical devices, anti-counterfeiting, and so forth.

Lattice Transformation-Induced Retroreflective Structural Colors

Retroreflective structural colors can usually be achieved based on interference combined with a total internal reflection mechanism or diffraction of a monolayer hexagonal two-dimensional (2D) colloidal array. Here, a novel retroreflective structural color was generated based on a hexagonal-parallelogram lattice transformation by stretching 3D photonic crystals with nonclosely packed long-range order. Compared to previous retroreflective colors, this new retroreflective color exhibits two unique off/on color switches: (1) a strain-dependent off/on color switch along the stretching direction and (2) a sample horizontal rotation angle-dependent off/on color switch under the fixed strain. These strain-responsive retroreflective colors are ideal candidates for visually sensing kinesio tapes' strain in practical applications and anticounterfeiting. This work reveals a new structural color regulation mechanism and will advance potential applications in anticounterfeiting, sensing, displays, etc.

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.

Rewritable Structurally Colored Paper Based on Hollow SiO2-Polyurethane Composite Photonic Crystal Film

Rewritable paper, which can be used multiple times as an effective solution for sustainable development and lessen the heavy environment pollution, has received widespread attention. A photonic crystal with dye-free character and tunable structure color has attracted significant interest in this area. Generally, handwriting on the photonic crystal structure containing a responsive polymer or hydrogel ingredient was based on the change of lattice spacing. It is necessary to enrich the diversities of color adjustment mechanism for further application. Herein, a flexible rewritable photonic crystal structurally colored paper with excellent mechanical strength based on the hollow SiO2 (h-SiO2) particle and polyurethane was developed. Owning to the varied optical response of h-SiO2 photonic crystal film in different solvents, handwriting on this paper was realized by applying polarity solvents such as EG as colorless ink directly, which could also be erased by resoaking the film in water. Writing and erasing on this paper were totally reversible. The color adjustment mechanism and the realization of handwriting on this paper are totally different from those of the previous reported photonic crystal-based rewritable paper. The combination of quick handwriting and flexibility is significant for potential application as rewritable paper.

Bio-Inspired Highly Brilliant Structural Colors and Derived Photonic Superstructures for Information Encryption and Fluorescence Enhancement

Inspired by the brilliant and tunable structural colors based on the large refractive index contrast (Δn) and non-close-packing structures of chameleon skins, ZnS-silica photonic crystals (PCs) with highly saturated and adjustable colors are fabricated. Due to the large Δn and non-close-packing structure, ZnS-silica PCs show 1) intense reflectance (maximal: 90%), wide photonic bandgaps, and large peak areas, 2.6-7.6, 1.6, and 4.0 times higher than those of silica PCs, respectively; 2) tunable colors by simply adjusting the volume fraction of particles with the same size, more convenient than the conventional way of altering particle sizes; and 3) a relatively low threshold of PC's thickness (57 µm) possessing maximal reflectance compared to that (>200 µm) of the silica PCs. Benefiting from the core-shell structure of the particles, various derived photonic superstructures are fabricated by co-assembling ZnS-silica and silica particles into PCs or by selectively etching silica or ZnS of ZnS-silica/silica and ZnS-silica PCs. A new information encryption technique is developed based on the unique reversible "disorder-order" switch of water-responsive photonic superstructures. Additionally, ZnS-silica PCs are ideal candidates for enhancing fluorescence (approximately tenfold), approximately six times higher than that of silica PC.

Practical Preparation of Elastomer-Immobilized Nonclose-Packed Colloidal Photonic Crystal Films with Various Uniform Colors

Colloidal photonic crystals, which are three-dimensional periodic structures of monodisperse submicron-sized particles, are expected to be suitable for novel photonic applications and color materials. In particular, nonclose-packed colloidal photonic crystals immobilized in elastomers exhibit significant potential for use in tunable photonic applications and strain sensors that detect strain based on color change. This paper reports a practical method for preparing elastomer-immobilized nonclose-packed colloidal photonic crystal films with various uniform Bragg reflection colors using one kind of gel-immobilized nonclose-packed colloidal photonic crystal film. The degree of swelling was controlled by the mixing ratio of the precursor solutions, which used a mixture of solutions with high and low affinities for the gel film as the swelling solvent. This facilitated color tuning over a wide range, enabling the facile preparation of elastomer-immobilized nonclose-packed colloidal photonic crystal films with various uniform colors via subsequent photopolymerization. The present preparation method can contribute to the development of practical applications of elastomer-immobilized tunable colloidal photonic crystals and sensors.

Precisely sensing hydrofluoric acid by photonic crystal hydrogels

It is a great challenge to detect hydrofluoric acid (HF) with high precision, good selectivity, and visual readouts characteristics. Herein, a new photonic crystal (PC) hydrogel HF sensor based on the "selective etching-induced swelling" mechanism has been developed. This HF sensor consisting of silica/water/hydroxyethyl acrylate and non-closely packed structures was fabricated through simple non-close-assembling, photopolymerization, and water swelling processes. Silica slightly etched by HF induces the swelling of PC hydrogel, leading to the variation of reflection wavelength and structural colors, thereby realizing visually and spectrally sensing HF (0-10 mM). The unique structure and compositions of PC hydrogel are the keys to the high sensing precision, outstanding selectivity, and low detection limit (0.1 mM). Furthermore, the sensor possesses tailorable, portable, easy-to-operation, and low-cost (<0.01 $/sensor) advantages. This work provides an efficient and convenient tool for sensing and recognizing HF in the aqueous solution for practical applications and upgrades the basic understanding of the photonic sensing mechanism.

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.

Robust Large-Sized Photochromic Photonic Crystal Film for Smart Decoration and Anti-Counterfeiting

Photochromic materials are widely investigated due to their vivid color transformation for many real applications. In this work, a new kind of multiangle photochromic photonic crystal (PC) material with high robustness and long durability for smart phone decoration and anticounterfeiting features is fabricated. After thermal mixing of spiropyran powder and monodisperse core-interlayer-shell (CIS) particles, a large-area and high-quality photochromic PC film has been prepared by the self-designed bending-induced ordering technique (BIOT). The spiropyran powder can be well dispersed in the order-structured PC system, so the perfect synergistic combination of photochromism and angle-dependent structure colors can be achieved. The color-switching test for the as-prepared photochromic PC film proved its excellent reversibility and stability. Because of the excellent flexibility of the photo-cross-linked PC films, they can be designed and cut into various shapes with high robustness and long durability. Interestingly, a temperature-controlled photochromic effect was found in this photochromic PC system. Therefore, the as-prepared photochromic PC films can play a significant role in the fields of smart decoration and anticounterfeiting by their unique color switching effects under different stimuli. More importantly, our work verified the feasibility of this route to prepare a series of large-sized advanced smart PC devices by adding versatile functional materials.

Photonic Cellulose Films with Vivid Structural Colors: Fabrication and Selectively Chemical Response

Recent advances in structural-color cellulose nanocrystal (CNC) materials have been made toward chemical sensing applications; however, such materials lack sufficient color chroma for naked-eye observation, and their selective recognition to given chemicals as well as the corresponding mechanism has rarely been reported. Here, a dopamine-infiltration and post-polymerization approach is proposed to construct vivid structural-color composite films. The chiral nematic structure of CNC enables the structural coloration, while the strong light absorption of the polymeric co-phase, polydopamine (PDA) enhances the color chroma and visibility. By controlling the PDA amount, the composite films can detect organic solvents quantitatively and selectively via visible color changes. From the viewpoint of the compatibility and similitude principle, notably, a critical solubility parameter distance (R₀) between PDA and "active" solvents is defined with a three-dimensional Hansen solubility sphere; this well constructs a rule for the sensing selectivity of the chemochromic composite films. The findings pave the foundation for the design of colorimetric sensors with specifically testing objects.

Multicolor Light Mixing in Optofluidic Concave Interfaces for Anticounterfeiting with Deep Learning Authentication

Anticounterfeiting technology has received tremendous interest for its significance in daily necessities, medical industry, and high-end products. Confidential tags based on photoluminescence are one of the most widely used approaches for their vivid visualization and high throughput. However, the complexity of confidential tags is generally limited to the accessibility of inks and their spatial location; generating an infinite combination of emission colors is therefore a challenging task. Here, we demonstrate a concept to create complex color light mixing in a confined space formed by microscale optofluidic concave interfaces. Infinite color combination and capacity were generated through chaotic behavior of light mixing and interaction in an ininkjet-printed skydome structure. Through the chaotic mixing of emission intensity, wavelength, and light propagation trajectories, the visionary patterns serve as a highly unclonable label. Finally, a deep learning-based machine vision system was built for the authentication process. The developed anticounterfeiting system may provide inspiration for utilizing space color mixing in optical security and communication applications.

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.

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.