Substantial Enhancement toward the Photocatalytic Activity of CdS Quantum Dots by Photonic Crystal-Supporting Films

Enhanced Fluorescence Based on Slow Light Effect of ZIF-8 Photonic Crystals for Trace 2,4,6-Trinitrophenol Detection

In response to growing concerns about public safety and environmental conservation, it is essential to develop a precise identification method for trace explosives. To improve the stability and detection sensitivity of perovskite quantum dots (PQDs) and address the issue of low porosity in traditional polymer-based photonic crystals (PhCs), this study proposed a PQD photoluminescence (PL) enhancement strategy based on the slow light effect of ZIF-8 PhCs for highly sensitive, selective, and convenient detection of 2,4,6-trinitrophenol (TNP). The slow light effect at the photonic band gap edge is the basis of amplifying the PL signal. PhCs were fabricated by the evaporation-induced self-assembly method. The diffraction wavelength overlapping the whole visible region was designed to match the emission wavelength of PQDs. Results showed that PhCs matching the PBG edge with PQDs' emission peak amplified the PL signal 11.3 times, significantly improving sensitivity for trace TNP detection with a limit as low as 2.52 nM. Moreover, there was a 13.3-fold enhancement of PQDs' fluorescence lifetime when the emission wavelength fell in the PBG range. The hydrophobic surface of ZIF-8 PhCs enhanced the PQDs' stability and moisture resistance. Furthermore, the selective quenching mechanism of TNP by the sensor was photoinduced electron transfer (PET) verified by DFT calculations and time-resolved PL decay dynamics measurements. This study demonstrated great potential for manipulating light emission enhancement by PhCs in developing efficient fluorescent sensors for trace environmental pollutant detection.

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.

High-Precision Colorimetric Sensing by Dynamic Tracking of Solvent Diffusion in Hollow-Sphere Photonic Crystals

Expensive instruments and complicated data processing are often required to discriminate solvents with similar structures and properties. Colorimetric sensors with high selectivity, low cost, and good portability are highly desirable to simplify such detection tasks. Herein, we report the fabrication of a photonic crystal sensor based on the self-assembled resorcinol formaldehyde (RF) hollow spheres to realize colorimetric sensing of polar solvents, including homologs and isomers based on the saturated diffusion time. The diffusion of solvent molecules through the photonic crystal film exhibits a unique three-step diffusion profile accompanied by a dynamic color change, as determined by the physicochemical properties of the solvent molecules and their interactions with the polymer shells, making it possible to accurately identify the solvent type based on the dynamic reflection spectra or visual perception. With its superior selectivity and sensitivity, this single-component colorimetric sensor represents a straightforward tool for convenient solvent detection and identification.

The Development of New Catalytic Pigments Based on SiO2 Amorphous Photonic Crystals via Adding of Dual-Functional Black TiO2-x Nanoparticles

Biomimetic synthesis of amorphous photonic crystals (APCs) is an effective approach to obtaining non-iridescent structural colors. However, the structural colors of artificially prepared APCs are dim or even white due to the influence of incoherent scattering. In this paper, we present a novel method to combine APCs with black TiO_2-x to construct a noniridescent structural color pigments with high visibility and photocatalytic activity. Due to the absorption of incoherently scattered light by black TiO_2-x , the color saturation of structural colors has been significantly increased. In addition, the utilization rate of photogenic carriers was effectively enhanced by the slow light effect generated from the pseudoband gap of SiO₂ APCs with TiO_2-x absorbed full spectrum. The tone and color saturation of catalytic pigments is controlled by the diameter of SiO₂ nanospheres and the ratio of TiO_2-x nanoparticles, which provides a controllable application study in color-related fields as artwork, environmental coatings, and textiles.

Metal-Organic Framework-Coated Photonic Crystals for High-Performance Thin-Layer Chromatography

Traditional SiO₂ gel-based thin-layer chromatography (TLC) is unable to separate substances with close structures, and its detection requires dyeing when the sample has neither fluorescence nor ultraviolet (UV) absorption signals. Here, high-performance TLC was developed based on metal-organic framework-coated photonic crystal (MOF/PC) films. Using the MIL-100/PC as the TLC plate, the cresol isomers can now be separated in a 2 cm development distance due to the enhanced adsorption-desorption and the good selectivity from the micropores. Meanwhile, the cresol changes the refractive index of MOF/PC, which produces color changes for sample recognition. Compared to the traditional TLC, the MOF/PC TLC shows much higher selectivity toward the analytes, which fully compensates its limited plate numbers and ensures a high-resolution separation. It also provides a convenient solution for chemical analysis as the detection based on structural color change does not require UV irradiation or dyeing treatment.

Heterostructured CoOx-TiO2 Mesoporous/Photonic Crystal Bilayer Films for Enhanced Visible-Light Harvesting and Photocatalysis

Heterostructured bilayer films, consisting of co-assembled TiO₂ photonic crystals as the bottom layer and a highly performing mesoporous P25 titania as the top layer decorated with CoO_x nanoclusters, are demonstrated as highly efficient visible-light photocatalysts. Broadband visible-light activation of the bilayer films was implemented by the surface modification of both titania layers with nanoscale clusters of Co oxides relying on the chemisorption of Co acetylacetonate complexes on TiO₂, followed by post-calcination. Tuning the slow photon regions of the inverse opal supporting layer to the visible-light absorption of surface CoO_x oxides resulted in significant amplification of salicylic-acid photodegradation under visible and ultraviolet (UV)–visible light (Vis), outperforming benchmark P25 films of higher titania loading. This enhancement was related to the spatially separated contributions of slow photon propagation in the inverse opal support layer assisted by Bragg reflection toward the CoO_x-modified mesoporous P25 top layer. This effect indicates that photonic crystals may be highly effective as both photocatalytically active and backscattering layers in multilayer photocatalytic films.

Angle-Sensitive Photonic Crystals for Simultaneous Detection and Photocatalytic Degradation of Hazardous Diazo Compounds

Congo Red (CR) and Amido Black 10B (AB-10B) are anionic diazo dyes, which are metabolized to produce a bioaccumulative and persistent carcinogen, benzidine. In this regard, an angle sensitive sensor composed of photonic crystal supported photocatalyst was fabricated for the simultaneous detection and photocatalytic degradation of diazo dyes from aqueous solutions. Reflectance spectroscopy was used in the detection of CR and AB-10B, which was based on the emergence of the incident angle dependent reflection peaks from the TiO2 coated two-dimensional photonic crystal (2D-PhC) surfaces and their subsequent quenching due to the presence of dye molecules whose absorbance peak intensity overlapped the reflection peak intensity of TiO2 at the respective angle. Interestingly, ultraviolet (UV) mediated photocatalytic degradation of CR and AB-10B was achieved using the same TiO2 coated 2D-PhC surfaces. 2D-PhC underneath the TiO2 layer was able to confine and localize the light on the TiO2 coated 2D-PhC surface, which enhanced the light absorption by dye molecules on the TiO2 surface and the photocatalytic efficiency in the degradation of CR and AB-10B. Finally, this proof-of-concept study demonstrated the fabrication of copolymer film based photonic crystal supported photocatalytic device, which can be used for developing miniaturized sensors competent in on-field detection and degradation of pollutants.

Selective and Colorimetric Detection of p-Nitrophenol Based on Inverse Opal Polymeric Photonic Crystals

The detection of p-nitrophenol (PNP) is of great significance for assessment of environment pollution and potential health risks. In this study, based on inverse opal polymeric photonic crystals (IOPPCs), a selective and visual sensor for high-performance PNP detection is developed. Due to their unique optical properties, IOPPCs report events by change of color, which can easily be observed by the naked eye. Hydroxyethyl methacrylate (HEMA) was selected as the functional monomer with which to fabricate the IOPPCs. By precisely adjusting the molar ratio between the functional monomer and the crosslinker, the sensors were only able to be sensitive to a specific solution, thus realizing the visual, selective, and semi-quantitative detection of PNP. When the sensors were immersed in different concentrations of PNP solution, their Bragg diffraction wavelengths showed different redshifts. The color of the IOPPCs changed from green to red as the peak shift of Bragg diffraction occurred. In addition, the IOPPCs displayed good interference immunity and reusability.

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

Invisible photonic crystal (PC) pattern with encrypted and discoverable information is potentially useful for anti-counterfeiting labels, but it is still a big challenge to realize strict invisibility, fast response, and convenient triggering. Here, a new kind of soaking-revealed invisible PC pattern is fabricated by the regional coating of "ethylene glycol-ethanol" ink on a collapsed inverse opaline macroporous polyurethane (IOM-PU) film, followed by a quick thermal treatment. During the above process, wet heating retains the collapsed but recoverable IOM structure, but dry heating disables the recovery of ordered IOM structure due to the adhesion of macropore walls, which render the "pattern" and the "background" with different optical responses to the solvent. In the dry state, the pattern was invisible because both the collapsed IOM-PU film and the adhesive PU film are colorless and transparent. Once the sample is soaked in ethanol-water mixtures, the invisible pattern appears immediately because only the "wet-heated" region recovers the ordered macroporous structure and shows color, which forms a significant contrast in color to the "dry-heated" region. Compared to the previously invisible PC pattern, the current material has many superior properties, such as high invisibility, large color contrast in showing, excellent recyclability, and good toughness in bending and stretching.