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Amorphous photonic crystals (APCs) assembled by mesoporous SiO2 particles were first used to develop thin layer chromatography (TLC) with high separation efficiency and convenient colorimetric recognition. The m-SiO2 APC film has a large number of micron-scale pores, which significantly accelerate the solvent diffusion and address the problem of the short-distance development. Meanwhile, the abundant mesopores in m-SiO2 particles intensify the adsorption–desorption between analytes and APC film. Although the critical micron-scale pores were introduced by the sacrifice of long-range ordered structure, the APC film still has uniform structural colors due to its short-range ordered structure. Therefore, the APC film retains the advantage of colorimetric recognition, where the loaded chemicals cause an increased refractive index and a structural-color change. Compared to traditional SiO2 gel TLC, the sample recognition in APC TLC was simplified because chemical compounds, which must be dyed, can be directly recognized now. Meanwhile, the separation resolution achieved by m-SiO2 APC TLC was also improved due to the enhancement of adsorption–desorption.

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.

An invisible photonic crystal (PC) pattern encrypted at the dry state and decrypted by solvent is a new anticounterfeiting material system, but its diversity and complexity in response are still required to be increased to enhance the anticounterfeiting effect. Herein, a triple‐state invisible pattern encrypted in hollow‐silica/polyurethane (h‐SiO2/PU) film with two decryption mechanisms is developed based on the asymmetric solvent response of h‐SiO2/PU with different SiO2 shell thicknesses. The pattern is printed via etching SiO2 shell in the designed region, which creates an etched “pattern” containing thin‐shell h‐SiO2 particles and an unetched “background” containing thick‐shell particles. Different from the traditional invisible PC patterns, the current system possesses triple states, including a “dry” invisible state composed of two close inverse opal structures, a “wet” invisible state composed of inverse opals filled with water, and an ethanol‐soaking visible state due to the prevention of solvent diffusion by the thicker shell. The invisible patterns are reversibly and randomly switched among these three states, which suggest that they are decrypted from the dry invisible state through selective solvent infiltration or from the wet invisible state through selective extraction. Such increased diversity and complexity make this invisible PC pattern ideal material for anticounterfeiting applications.

This work reported a high quality photonic crystal film with an ultra-narrow photonic bandgap obtained via a chemical synthetic route. The bandgap is much narrower than that of traditional colloidal crystals, which makes the film qualified for use in optical devices. The narrow PBG originates from not only the high crystallinity and uniform orientations of microcrystals within the film but also the very close refractive indices between the silica and the polymer matrix. Due to the matching of the refractive index, the amorphous contents of the film are optically transparent and do not interfere with the reflection, so that the photonic crystal film is tolerant of the existence of disordered contents.

Nitrogen adsorption-desorption for mesopore characterization requires the using of expensive instrumentation, time-consuming processes, and the consumption of liquid nitrogen. Herein, a new method is developed to measure the pore parameters through mixing a mesoporous substance with a supersaturated SiO2 colloidal solution at different temperatures, and subsequent rapid measurement of reflection changes of the precipitated liquid photonic crystals. The pore volumes and diameters of mesoporous silica were measured according to the positive correlation between unit mass reflection change (Δλ/m) and pore volume (V), and the negative correlation between average absorption temperature (T) and pore diameter (D). This new approach may provide an alternative method for fast, convenient and economical characterization of mesoporous materials.

A SiO2 colloidal photonic crystal film with a hierarchical porous structure is fabricated to demonstrate an integrated separation and colorimetric detection of chemical species for the first time. This new photonic crystal based thin layer chromatography process requires no dyeing, developing and UV irradiation compared to the traditional TLC. The assembling of mesoporous SiO2 particles via a supersaturation-induced-precipitation process forms uniform and hierarchical photonic crystals with micron-scale cracks and mesopores, which accelerate the diffusion of developers and intensify the adsorption/desorption between the analytes and silica for efficient separation. Meanwhile, the chemical substances infiltrated to the voids of photonic crystals cause an increase of the refractive index and a large contrast of structural colors towards the unloaded part, so that the sample spots can be directly recognized with the naked eye before and after separation.

Traditional photonic sensing based on the change of balanced reflection of photonic structures can hardly distinguish chemical species with similar refractive indices. Here a sensing method based on the dynamic reflection spectra (DRS) of photonic crystal gel has been developed to distinguish even homologues, isomers and solvents with similar structures and physical properties. There are inherent relationships between solvent properties, diffusion behaviour and evolution of reflection signals, so that the geometric characteristics of DRS pattern including ascending/descending, colour changes, splitting/merging and curvature of reflection band can be utilized to recognize different organic solvents. With adequate solvents being tested, a database of DRS patterns can be established, which provide a standard to identify an unknown solvent., Photonic sensing is a method for detecting individual chemical species, but can fail when they are sufficiently similar in physical properties. Here, the authors report a method that can distinguish even very closely related species, such as homologues and chemical isomers.

Efficient and large scale printing of photonic crystal patterns with multicolor, multigrayscale, and fine resolution is highly desired due to its application in smart prints, sensors, and photonic devices. Here, an electric-field-assisted multicolor printing is reported based on electrically responsive and photocurable colloidal photonic crystal, which is prepared by supersaturation-induced self-assembly of SiO2 particles in the mixture of propylene carbonate (PC) and trimethylolpropane ethoxylate triacrylate (ETPTA). This colloidal crystal suspension, named as E-ink, has tunable structural color, controllable grayscale, and instantly fixable characteristics at the same time because the SiO2/ETPTA-PC photonic crystal has metastable and reversible assembly as well as polymerizable features. Lithographical printing with photomask and maskless pixel printing techniques are developed respectively to efficiently prepare multicolor and high-resolution photonic patterns using a single-component E-ink.