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

Engineered covalent triazine framework inverse opal beads for enhanced photocatalytic carbon dioxide reduction

The development of highly ordered covalent triazine framework (CTF) materials with tailored structures is crucial for advancing functional material applications. Herein, we introduce a novel approach to fabricate covalent triazine framework inverse opal (CTF-IO) photonic crystal beads via a microfluidic-assisted assembly method and pore-confined polymerization. The polymerization process occurs within the interstitial voids of SiO2 nanoparticles (NPs) photonic crystals, where spatial confinement dictates the growth and arrangement of the CTF framework, resulting in a robust and precisely ordered inverse opal (IO) structure. The pore sizes, governed by the packing geometry of SiO2 NPs, are theoretically estimated to highlight the role of confinement in achieving structural fidelity. The unique slow-light effect of the CTF-IO structure enhances light absorption and charge transport, offering a versatile platform for light-driven CO2 conversion applications. As a demonstration, the optimized CTF-240 exhibit superior photocatalytic performance in CO2 reduction, achieving a yield of 118.69μmol g-1h-1 and a selectivity of 97.25 % without sacrificial agents or co-catalysts, significantly outperforming bulk CTF. This work underscores the potential of photonic crystal-guided framework design for diverse advanced applications, providing insights into the interplay between spatial confinement, structural engineering, and functional performance.

Self-flickering bioinspired actuator with autonomous motion and structural color switching

Color-tunable actuators with motion and color-changing functions have attracted considerable attention in recent years, yet it remains a challenge to achieve the autonomous regulation of motion and color. Inspired by Apatura ilia butterfly with dynamic structural color and Pelargonium carnosum plant with moisture responsive bilayer structure, an automatic color-tunable actuator is developed by integrating photonic crystals layer and hygroscopic layer. Taking advantage of the asymmetric hygroscopicity between two layers and the angle-dependent structural color of photonic crystals, this actuator can continuously self-flicker in humid environment by visual switching in structural color due to automated cyclic motion. The actuator is assembled into the self-flapping biomimetic butterfly with switchable color and the self-reporting information array with dynamic visual display, demonstrating its autoregulatory motion and color. This work provides a new strategy for developing automatic color-tunable actuator and suggests its potential in the intelligent robot and optical display.

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.

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.

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.

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.

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.

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.

Scalable Structural Coloration of Carbon Nanotube Fibers via a Facile Silica Photonic Crystal Self-Assembly Strategy

The coloration of carbon nanotube (CNT) fibers (CNTFs) is a long-lasting challenge because of the intrinsic black color and chemically inert surfaces of CNTs, which cannot satisfy the aesthetic and fashion requirements and thus significantly restrict their performance in many cutting-edge fields. Recently, a structural coloration method of CNTFs was developed by our group using atomic layer deposition (ALD) technology. However, the ALD-based structural coloration method of CNTFs is expensive, time-consuming, and not suitable for the large-scale production of colorful CNTFs. Herein, we developed a very simple and scalable liquid-phase method to realize the structural coloration of CNTFs. A SiO₂/ethanol dispersion containing SiO₂ nanospheres with controllable sizes was synthesized. The SiO₂ nanospheres could self-assemble into photonic crystal layers on the surface of CNTFs and exhibited brilliant colors. The colors of SiO₂ nanoparticle-coated CNTFs could be easily changed by tuning the sizes of SiO₂ nanospheres. This method provides a simple, effective, and promising way for the large-scale production of colorful CNTFs.

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.

Preparation of biomimetic non-iridescent structural color based on polystyrene-polycaffeic acid core-shell nanospheres

Caffeic acid (CA), as a natural plant-derived polyphenol, has been widely used in surface coating technology in recent years due to its excellent properties. In this work, caffeic acid was introduced into the preparation of photonic band gap materials. By controlling the variables, a reasonable preparation method of polystyrene (PS) @polycaffeic (PCA)–Cu(ii) core–shell microspheres was achieved: 1 mmol L⁻¹ cupric chloride anhydrous (CuCl₂), 3 mmol L⁻¹ sodium perborate tetrahydrate (NaBO₃·4H₂O), 2 mmol L⁻¹ CA and 2 g L⁻¹ polystyrene (PS) were reacted at 50 °C for 10 min to prepare PS@PCA–Cu(ii) core–shell microspheres through rapid oxidative polymerization of CA coated PS of different particle diameters. The amorphous photonic crystal structure was self-assembled through thermal assisted-gravity sedimentation, resulting in structural color nanomaterials with soft and uniform color, no angle dependence, stable mechanical fastness and excellent UV resistance.

Caffeic acid (CA), as a natural plant-derived polyphenol, has been widely used in surface coating technology in recent years due to its excellent properties.