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.

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.

Wetting-Enhanced Structural Color for Convenient and Reversible Encryption of Optical Information

Encrypted storage of optical information has attracted increasing interest for anticounterfeiting, information transmission, and military applications. In this study, an inverse opal-structured titanium dioxide/heptadecafluorodecyltrimethoxysilane (IOS-T/F) panel is developed. Based on a unique wetting-enhanced mechanism of structural color vision derived from a reduced light scattering and strengthened effective refractive index, this panel is capable of reversible writing/erasing and encryption/decryption of optical information. Multiple levels of information can be compiled, concealed, and erased simply using controlled ultraviolet irradiation to form patterned hydrophilic/hydrophobic differences, and the process of revealing or concealing the information only requires a few drops of water or evaporation, respectively. Importantly, the functions of the IOS-T/F panel can be well maintained under harsh conditions, including strongly acidic/alkaline environments or extreme temperatures (from -40 to 80 °C), as well as can be recovered after staining by various pollutants. This system provides simple encryption, rapid decryption, and the ability to store multiple sets of information under diverse application scenarios, which represents a novel material design strategy for security-related applications and smart optical systems.

Solution-Processable Photonic Inks of Mie-Resonant Hollow Carbon-Silica Nanospheres

Hollow carbon-silica nanospheres that exhibit angle-independent structural color with high saturation and minimal absorption are made. Through scattering calculations, it is shown that the structural color arises from Mie resonances that are tuned precisely by varying the thickness of the shells. Since the color does not depend on the spatial arrangement of the particles, the coloration is angle independent and vibrant in powders and liquid suspensions. These properties make hollow carbon-silica nanospheres ideal for applications, and their potential in making flexible, angle-independent films and 3D printed films is explored.

Brilliant Structurally Colored Films with Invariable Stop-Band and Enhanced Mechanical Robustness Inspired by the Cobbled Road

Recently, structural colors have attracted great concentrations because the coloration is free from chemical- or photobleaching. However, the color saturation and mechanical robustness are generally competitive properties in the fabrication of PCs (photonic crystals) films. Besides, the structure of PCs and their derivatives are easy to be invaded by liquids and lead to band gap shifts due to the change of refractive index or periodicity. To solve those problems, we infiltrate polydimethylsiloxane (PDMS) into the intervals between regularly arrayed hollow SiO2 nanospheres, inspired by the cobbled road prepared by embedding stone in the bulk cement matrix. Consequently, the as-prepared PCs films show brilliant colors, invariable stop-bands, and excellent mechanical robustness. Moreover, the water contact angle even reached 166° after a sandpaper abrasion test. The combination of brilliant colors, invariable stop-bands, and excellent robustness is significant for potential application in paint and external decoration of architectures.