Magnetically Assembled Photonic Crystal Gels with Wide Thermochromic Range and High Sensitivity

Abstract

Thermochromic poly(N‐isopropyl acrylamide) (PNIPAM) photonic crystal gels based on 1D magnetically assembling colloidal nanocrystal clusters have attracted much attention due to its convenient preparation process, striking color response, and good mechanical strength. However, there remain challenges to broaden the thermochromic range and improve the sensitivity for their iridescent color display. Here, a PNIPAM photonic gel with wide thermochromic range and high sensitivity is prepared by using four‐arm star poly(ethylene glycol) acrylamide (PEGAAm) as cross‐linker at appropriately reduced magnetic field strength as well as cross‐linker content. PEGAAm improves the homogeneity of the microstructure in PNIPAM photonic gel and thus maintains the structure colors at a wide temperature range from room temperature to 44 °C. The reduced magnetic field strength of 70 Gs and low cross‐linker content (the molar ratio of monomer to cross‐linker of 300:1) lead to a large initial lattice spacing of the photonic gel and thus wide diffraction wavelength migration of 194 nm. This optimized PNIPAM gel exhibits vivid iridescent colors from orange–red to indigo blue as temperature increases from 20 to 44 °C with satisfactory repeatability. Therefore, it may be an ideal candidate for temperature sensors and displays with utility and accuracy such as low‐temperature burns.

Designable Micro‐/Nano‐Structured Smart Polymeric Materials

“…Besides the responsive volume changes, with flexible use of functional nanomaterials, smart hydrogels can achieve stimuli-responsive changes of physico-chemical properties such as conductivity and color. [48][49][50] For example, with exfoliated laponite clay nanosheets as nano-crosslinkers to create poly(N,N-dimethylacrylamide) (PDMAA) networks (Figure 3e), smart hydrogels with high stretchability and hydroacoustic-sensitive conductivity can be developed. [48] This PDMAA hydrogel soaked with LiCl solution allows storage of Li + ions in the clay nanosheets due to electrostatic adsorption and cation exchange.…”

“…Moreover, smart hydrogels with responsive color change usually can be obtained by engineering colloidal nanocrystal structures in their polymeric networks. [50] The stimuli-responsive volume change of the hydrogel can vary the nanocrystal structures as well as the diffraction property to display stimuliinduced colors.…”

Metal–organic framework-based photonic crystal platforms for gas sensing: a review

“…1-D PCs (Bragg-Stacks) are multiple-layer structures assembled by alternating layers of two materials with high and low refractive indices. It means that 1-D PCs have a regularly variational refractive index in one direction but isotropic in the other two directions 62,63 . 2-D PCs have periodically varied refractive indices in two directions but homogenous in the third direction.…”

“…This means that 1-D PCs show a regular variation in their refractive index in one direction but are isotropic in the other two directions. 62,63 2-D PCs have periodically varied refractive indices in two directions but homogenous in the third direction. 3-D PCs are structures in which the refractive index regularly varies in all three directions.…”

A novel ion-responsive photonic hydrogel sensor for portable visual detection and timely removal of lead ions in water

“…Photonic crystals (PCs) are a class of novel optical materials with periodic micro-/nano-structures that possess a photonic bandgap to control and manipulate the flow of light in the visible region and thus exhibit bright structural colors. [14][15][16][17] Fixing colloidal PCs, such as monodisperse polystyrene (PS), [18][19][20][21] silica (SiO 2 ), 22,23 and superparamagnetic colloidal nanoparticles, [24][25][26] in responsive polymers brings about responsive photonic hydrogels (RPHs), which have the capability of adjusting the lattice spacing among the embedded PC particles in response to the volume change of hydrogels (swelling or shrinkage) caused by various external stimuli (temperature, humidity, pressure, pH, ions, molecules, and bacteria), [27][28][29][30][31][32][33][34] thus generating various brilliant colors. Such RPHs with embedded PCs do not suffer from photobleaching, and their striking advantage of having colourfast, highly stable, bright colors enable them to be ideal colorimetric detection devices compared with other sensing platforms.…”