Humidity-Responsive Photonic Crystals with pH and SO₂ Gas Detection Ability Based on Cholesteric Liquid Crystalline Networks

Abstract

Dynamic photonic crystals with tunable structural colors have been a hot topic in the research of anticounterfeiting devices, decoration, and detection. In this work, we prepared cholesteric liquid crystalline network (CLCN)-based photonic crystals that present humidity- and SO2 gas-responsive behaviors. The covalently cross-linked CLCN film presents humidity-responsive color changes due to the swelling/deswelling of the matrix under different humidity conditions. When treating the CLCN film with SO2 gas, the carboxylic salt converted to the acid and the film was not able to respond to the humidity change anymore. The mechanism of the SO2 gas-gated humidity responsiveness of the CLCN film was characterized. It was found that the acidic gas caused changes of pH, resulting in the conversion of the salt to acid and alteration of the surface property. The influence of concentration of SO2 gas and pH on humidity responsiveness of the CLCN film was investigated. We hope that this method provides inspirations for the design and fabrication of visualized pH and acidic gas detectors.

Robust and Reliable Fabrication of Gelatin Films Containing Micropatterned Opal Structures via Evaporative Deposition and Thermal Gelation

“…These results illustrate the potential of our micropatterned CS-opal gelatin films in environmental monitoring applications. [8][9][10]12 Facile Routes for the Production of CS-Opal Microparticles via Removal of Gelatin. One of gelatin's key advantages over synthetic materials is the ability to easily degrade via a variety of simple treatments, including chemical and enzymatic means, making gelatin a promising sacrificial scaffold material.…”

“…The need for timely monitoring of environmental conditions has become critical with the rise of various global concerns such as unbalanced pH, , increased toxicity of various water sources, − and drastic humidity fluctuations. , Among the classes of various materials for monitoring the environmental conditions, stimuli-responsive materials with optical properties have shown significant potential. − One such optically tunable material is artificial opals that are formed via controlled assembly of nanospheres. , These opal structures exhibit brilliant “structural” colors due to diffraction of white light through the regularly ordered lattices of the nanospheres. , A number of studies have demonstrated scalable production techniques of materials containing artificial opals, including melt-shear organization and electrospinning. , Particularly, hydrogels containing artificial opal structures have gained attention due to the nature of hydrogels to respond to various stimuli by swelling or deswelling, leading to changes in color via change in the lattice parameter and/or refractive indices in the opal structures. − These color changes in response to stimuli allow the hydrogels containing opal structures to be used as simple visual monitoring devices without the need for complex circuits or machinery. , …”

Scalable and Sensitive Humidity-Responsive Polymer Photonic Crystal Films for Anticounterfeiting Application

“…Usually, PCs with a tunable structural color triggered by external stimuli are called responsive PCs . So far, scientists have studied responsive PCs that respond to various external stimuli, such as temperature, − mechanical force, − light, , electricity, − humidity, − magnetic field, − chemicals, , etc. The unique and diverse response behaviors allow responsive PCs to be widely used for anticounterfeiting applications, such as solvent-responsive PC patterns, thermochromic PC stickers, and so on.…”

Solvent-Responsive Invisible Photonic Patterns with High Contrast for Fluorescence Emission Regulation and Anti-Counterfeiting

“…to change their structural colors. − Among these RPCs patterned PCs with different responsiveness in patterns and background regions to external stimuli can be obtained by local modification of RPCs. RPCs can store more information and can be used in the fields of visual sensing, multiple-mode anti-counterfeiting, and code recognition. ,− In some cases, the patterns can be hidden under specific environmental conditions and displayed under external stimuli due to different structural color changes of patterns and background regions, namely, invisible photonic patterns (IPPs). IPPs, which can hide patterns, are more attractive, especially in high-security anti-counterfeiting applications. − …”

“…Photonic crystals (PCs) are optical materials formed by periodic arrangements of media with different refractive indices, showing a photonic band gap (PBG). − When the PBG is located in the visible range, the light overlapping with this wavelength range cannot be propagated in the PCs, and accordingly, the PCs present the color visible to the naked eye, namely, the structural color. − Responsive photonic crystals (RPCs), by combining colloidal crystals with responsive polymers, can change their lattice spacing or refractive index in response to external stimuli (e.g., mechanical force, magnetic field, temperature, humidity, pH, solvent, etc.) to change their structural colors. − Among these RPCs patterned PCs with different responsiveness in patterns and background regions to external stimuli can be obtained by local modification of RPCs. RPCs can store more information and can be used in the fields of visual sensing, multiple-mode anti-counterfeiting, and code recognition. ,− In some cases, the patterns can be hidden under specific environmental conditions and displayed under external stimuli due to different structural color changes of patterns and background regions, namely, invisible photonic patterns (IPPs).…”