Fabrication of tough photonic crystal patterns with vivid structural colors by direct handwriting

Chameleon-inspired active tunable structural color based on smart skin with multi-functions of structural color, sensing and actuation

Tunable structural color has many potential applications in artificial camouflage, mechanical sensors, etc. Despite the extensive efforts to develop efficient tunable structural color, there is still a wide gap between the existing "passive" tuning methods and the "active" strategy found on organisms such as chameleons that can change color according to the environment. Inspired by the active tunable color system of chameleons, we propose a smart skin comprising a nanoscale hole array of photonic crystals, carbon nanotube coatings, and liquid crystal elastomers, to integrate multiple functions, i.e., structural color tunability, sensing, and actuation, in one structure. The smart skin was further coupled with an image acquisition unit (which mimics eyes to obtain colors from the environment) and a controller (which mimics the brain to process the signals transmitted from the image acquisition unit to the smart skin), to construct an active tunable structural color system. The proposed system autonomously modulates the color according to the environmental color. To validate the color tuning, color scanning from red to green to blue or vice versa is demonstrated in this work, which could certainly open up new paths to create active tunable structural color systems, and thus, push the development of structural color-based devices and systems.

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.

Color-Tuning Mechanism of Electrically Stretchable Photonic Organogels

In contrast to nano-processed rigid photonic crystals with fixed structures, soft photonic organic hydrogel beads with dielectric nanostructures possess advanced capabilities, such as stimuli-responsive deformation and photonic wavelength color changes. Recenlty, advanced from well-investigated mechanochromic method, an electromechanical stress approach is used to demonstrate electrically induced mechanical color shifts in soft organic photonic hydrogel beads. To better understand the electrically stretchable color change functionality in such soft organic photonic hydrogel systems, the electromechanical wavelength-tuning mechanism is comprehensively investigated in this study. By employing controllable electroactive dielectric elastomeric actuators, the discoloration wavelength-tuning process of an electrically stretchable photonic organogel is carefully examined. Based on the experimental in-situ response of electrically stretchable nano-spherical polystyrene hydrogel beads, the color change mechanism is meticulously analyzed. Further, changes in the nanostructure of the symmetrically and electrically stretchable organogel are analytically investigated through simulations of its hexagonal close-packed (HCP) lattice model. Detailed photonic wavelength control factors, such as the refractive index of dielectric materials, lattice diffraction, and bead distance in an organogel lattice, are theoretically studied. Herein, the switcing mechanism of electrically stretchable mechanochromic photonic organogels with photonic stopband-tuning features are suggested for the first time.

Toward High Sensitivity: Perspective on Colorimetric Photonic Crystal Sensors

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

Iridescent Colloidal Crystals Composed of SiO2 Porous Hollow Sphere for SERS Application

In recent years, the application of low-refractive-index materials in the optical field has attracted considerable attention due to it high transmittance and high optical sensitivity. In this study, we synthesized SiO₂ porous hollow spheres (SPHS) with an ultralow refractive index (n = 1.05) by using a templating method. Their refractive indices could be easily controlled from 1.05 to 1.08 by tuning the thickness of shell. In addition, a droplet coatings method is proposed for SPHS colloidal crystal (CC) by controlling the temperature and humidity. The SPHS CCs displayed distinct structural colors when the incident angle was adjusted and demonstrated high angular resolution. Moreover, the iridescent color changes could be observed with the naked eye. For surface-enhanced Raman scattering application, more analyte could be absorbed by the porous shells, and metal nanoparticles were coated on the SPHSs surface to increase the hot spot density for improving the SERS intensity.

Laser-Induced Patterned Photonic Crystal Heterostructure for Multimetal Ion Recognition

In this work, a new method of direct laser writing patterned photonic crystal heterostructure on a glass surface is proposed. A multi-heterostructure photonic crystal (MHPC) is predeposited on the glass surface and then the laser spot is focused on it and moves according to the set program, leading to the formation of patterned MHPC. Scanning electron microscopy (SEM) and finite element simulation show that the patterning is caused by the local thermal annealing of the polymer colloidal spheres through the thermal conduction effect of the substrate on the laser energy. The patterned area presents a function of the water confinement effect and can be used as a high-performance droplet analysis chip. By integrating the patterned MHPC array and seven fluorescent dyes, nine metal ions can be successfully recognized and discriminated. This approach is quite facile and fast for designing colloidal photonic crystals with controllable patterns. Moreover, it is of considerable significance for the practical application of photonic crystal heterostructure in the detection, sensing, anti-counterfeiting, and display fields.

Bioinspired structural color nanocomposites with healable capability

This minireview summarizes the recent development of healable structural color nanocomposites from the perspective of the construction strategies.

Patterned SiO2/Polyurethane Acrylate Inverse Opal Photonic Crystals with High Color Saturation and Tough Mechanical Strength

Patterned structural color photonic crystals (PCs) based on periodic photonic nanostructures have attracted great interest in developing high-performance sensors and other smart optical materials as well as tunable structurally colored fashion textiles. However, previously reported patterned PCs with both high color saturation and tough mechanical strength were difficult to achieve, which restricts their practical applications. Herein, arbitrarily patterned silica/polyurethane acrylate (SiO₂/PUA) inverse opal photonic crystals (IOPCs) with high color saturation and tough mechanical strength were innovatively designed and fabricated by writing with photopolymerizable PUA "ink" on a self-assembled hollow SiO₂ PC template. The high color saturation of the prepared SiO₂/PUA IOPCs originated from the high refractive index contrast between the encapsulated air-filled core and the SiO₂/PUA composite skeleton. The cross-linked flexible PUA matrix tightly warped the self-assembled hollow SiO₂ nanospheres together, endowing the obtained SiO₂/PUA IOPCs a structural color pattern with tough mechanical strength. The structural colors of SiO₂/PUA IOPCs could be finely tuned by regulating their basic parameters, and a redshift in the resultant structural color was observed due to an increase in the lattice constant when increasing the core size and/or shell thickness of the hollow SiO₂ nanospheres.

Preparation of Noniridescent Structurally Colored PS@TiO2 and Air@C@TiO2 Core-Shell Nanoparticles with Enhanced Color Stability

Natural amorphous photonic crystals benefit from reflectance at selective wavelengths in some specific existing natural systems. Noniridescence from natural organisms has also attracted great interest for various examples in bionic colors, pigments, and paintings. Here, Air@C@TiO₂ sphere was obtained by the first calcination of PS@TiO₂ core-shell nanoparticles in nitrogen to ensure the integrity of the shell structure followed by low-temperature calcination to obtain the appropriate color saturation. We demonstrate that, compared with the prepared colored PS@TiO₂/carbon black (CB) pigments, angle-independent hollow Air@C@TiO₂ nanoparticles have enhanced color stability under the action of in situ synthesized carbon black (CB). Our results suggest that it is easy to change the color of these Air@C@TiO₂ spheres by adjusting the sphere structure sizes, which have the potential to show visual signaling.

Hand Painting of Noniridescent Structural Multicolor through the Self-Assembly of YOHCO3 Colloids and Its Application for Anti-Counterfeiting

YOHCO₃ colloidal particles with tunable size, composition, and optical properties were prepared, and they were used for the fabrication of amorphous photonic crystals? (APCs) patterns through direct hand painting. YOHCO₃ colloids were synthesized by a seeding growth method, in which the colloid size could be controlled by altering the seed amounts and the composition and optical properties can be altered via the doping of Eu³⁺. APCs? films with bright, permanent, and tunable structural colors were prepared by the self-assembly of YOHCO₃ colloids of different sizes. Multicolor patterns can be obtained quickly and efficiently by hand painting with the dispersion of YOHCO₃ colloids as ink. An APCs? pattern assembled from YOHCO₃:Eu colloids is also fabricated, and the pattern shows blue structural color under natural light and bright red colors under illumination of UV light. The facile synthesis procedure, simple assembly process, and unique optical properties of the APCs make it valuable for practical applications such as structural color-based printing and anticounterfeiting.

Self-assembled colloidal arrays for structural color

Structural color materials that are colloidally assembled as inspired by nature are attracting increased interest in a wide range of research fields. The assembly of colloidal particles provides a facile and cost-effective strategy for fabricating three-dimensional structural color materials. In this review, the generation mechanisms of structural colors from colloidally assembled photonic crystalline structures (PCSs) and photonic amorphous structures (PASs) are first presented, followed by the state-of-the-art and detailed technologies for their fabrication. The variable optical properties of PASs and PCSs are then discussed, focusing on their spatial long- and short-order structures and surface topography, followed by a detailed description of the modulation of structural color by refractive index and lattice distance. Finally, the current applications of structural color materials colloidally assembled in various fields including biomaterials, microfluidic chips, sensors, displays, and anticounterfeiting are reviewed, together with future applications and tasks to be accomplished.

Fabrication and Characterization of Angle-Independent Structurally Colored Films Based on CdS@SiO2 Nanospheres

It is easy for chemical pigments produced from organic chemicals to disappear when exposed to light over time. Recently, structurally colored pigments produced by materials with high indices of refraction such as TiO₂ or ZnS have attracted great attention. This study presents that CdS@SiO₂ core-shell nanospheres were synthesized through a homogeneous deposition method followed with a modified Stöber method and a calcination process. Colored film assembled by pigments shows low angle dependence with high stability against degradation under environmental factors. Moreover, the structural color of CdS@SiO₂ arrays was bright and tunable according to the size without changing the overall material design. Compared with the conventional method, the addition of black substances in colloidal spheres is the generally used method to realize angle-independent structural coloration. However, black materials (such as carbon blacks and acetylene black) are not stable because of the high surface energy, and usually reunite together easily, and then lead to a nonuniform distribution and significant decrease in brightness. Thus, we report self-assembly colored films with great low angle dependence but not any black substances. Moreover, the refractive index of CdS is higher than generally used PS, PMMA, and SiO₂, and the SiO₂ shell is poisonless. CdS@SiO₂ structurally colored films have promising nonbleaching pigments and have potential applications for displays, colorimetric sensors, colorful decoration, and pigments.