Changing Faces will hydrogel! Transparent – can quickly switch between the color

Colloidal particles spontaneously forming the lattice and structural motifs can be used as a photonic crystal. Wherein the refractive index of the periodic regulation would provide a photonic stop band structure appears in the form of emitted color (structural colors) of (photonic stopband), in the visible range, the color of the stop band is more obvious. Typically by adjusting the lattice constant and refractive index, can dynamically adjust the color of the structure, and the structure as long as there has been regular, the color will not disappear. The colloidal particles of the polymer matrix is ​​removed, the holes can be formed orderly, thereby fabricate ordered structure materials. In most of the sensor array composed of the colloid, and the condition of the color change is linearly proportional to the external conditions, although a wide range of measurable stimulation, but limited sensitivity. Conventional Colloidal arrays often time consuming, elaborate evaporation-induced self-assembly (evaporation-induced self-assembly) production formula. That is, at the same time to achieve high sensitivity to external stimuli, fully encrypted photon mode, as well as a simple and practical application of preparation scheme sensor security materials, remains a challenge.

[research]

Recently, KAIST Chemical and Biomolecular Engineering Task Force Shin-Hyun Kim prepared by two-step solvent-responsive hydrogel large Kongguang Zi (themacroporousphotonic hydrogels). The authors used a simple method for the preparation of macroporous hydrogels, the hydrogels can be achieved between the wet film under dry transparent film and the color conversion Reversible swollen state, and different proportions of water – ethanol Ming mixture of highly emotional. Under dry conditions, irregular cavities collapse membrane (Cavities) occurs by capillary action (capillary force), low degree of crosslinking of the matrix PEGDA lost long-range order structure, whereby a high degree of transparency. The transparent film in the swollen water or ethanol can be recovered quickly regular pore structure, resulting stopband red shift, and the color emitted, and therefore, can be used as a colorimetric sensor to detect a water – ethanol mixture ratio. Preparation of macroporous hydrogel only thin film casting and rapid photopolymerization, and the two step etching silicon nanoparticles, without the need for evaporation based colloids assembly, which makes the preparation protocol may have goodRepeatability. The research results entitled \”MacroporousHydrogels forFast and Reversible Switching between Transparent and Structurally Colored States\” paper published in the \”Advanced Functional Materials\” (see text after text link).

[Detailed graphics]

1 big Kong Guangzi hydrogel prepared:

Large Kong Guangzai hydrogel (macroporousphotonic hydrogel) than the oxide of the first volume silicon particles dispersed in the PEGDA, then add a photoinitiator. Silica beads silanol groups capable of forming hydrogen bonds between PEGDA, the solvent layer is formed, so that repulsion between particles has, so that even at very high concentrations, high dispersion stability can be maintained. After dispersing between glass plates by capillary action, and UV cross-linking of PEGDA used, thereby preparing a film. After ultraviolet irradiation interstitial silicon particles pPEGDA permanently fixed to the substrate. The slide was removed from the composite membrane was immersed in aqueous hydrofluoric acid solution to etch away the silica particles pPEGDA substrate to form regular holes. Figure 1 shows the two states before and after the immersion, electron microscopy, reflection and transmission spectra.

2 PEGDA molecular weight of the transition state Effect of

of four different molecular weights PEGDA chosen as the base, constructed of different crosslinking density hydrogels. PEDGA higher molecular weight, the smaller the proportion of acrylic acid ester group-containing, the lower the crosslink density. FIG Figure2 a, PEGDA molecular weight of 250, the regular section of the film structure, since highly crosslinked structure capable of holding the substrate pPEGDARegularity during drying mechanical rigidity sufficient to maintain the capillary action, a large deformation does not occur. Crosslinked state of molecular weight of 400 and 575, only the long-range order structure can be maintained locally. Molecular weight is 700, the color structure (structural color) completely disappeared, long-range order structure is completely destroyed. Figure2b c further study four different reflection film and an optical transmission properties of the transmission spectrum and reflection spectrum utilization.

3. Rapid reversible conversion

As shown in FIG Figure3, when the transparent film is immersed in water, no color started, gradually turns red, continued to increase brightness and saturation. Probably because macroporous hydrogel during swelling, the lattice spacing is gradually increased, so the structure of the color red shift occurs. During swelling reflectance spectrum shown Figure3b, variation with time, while the peak of the reflection intensity increases gradually narrowed, the peak wavelength of the center remains almost unchanged, which is consistent with the changes in brightness and saturation of Figure3 a FIG. This shows that the crystal lattice near the film surface in the early macropores greater than the equilibrium swelling state, and the center of the lattice is smaller than in the early swelling equilibrium. Thereafter, the crystal lattice near the surface is gradually reduced, while further expansion of the center of the lattice, eventually forming a homogeneous crystal lattice on the film. In the absence of structural resonance, the reflectance of about 12% dry, wet reflectance with the development of a photonic stop band is increased to about 73%. In the 20 cycles, the reflectance values ​​of the dry and wet almost unchanged; standard differential dry and wet state was 0.69% and 1.04%, respectively.

4. response of water and ethanolAs shown in

FIG Figure4a, transparent film produced in the water changes to orange to green in ethanol in the stop band does not change linearly with the color mixture in a concentration of the components. When the ethanol concentration from 0 to 10%, from orange to red color. When the ethanol concentration of 80%, no significant change. When the concentration is further increased to 90% and 100%, the color changes to orange and green, respectively. That is, a large difference in the mixing ratio of the mixture, the change is small in the content of the fine component concentration occurs, there will be a large red shift amplitude stopband. Stopband in water 580nm, in ethanol is 559 nm, the lattice described swelling in water easily. When the alcohol concentration with 600 nm increased slowly from 0% to 10%, the stopband from 580 is 593 nm, until after the 60%, 70%, and a concentration of 80%, the stopband gradually decreased to 598 and 594 nm; when the concentration of 90% and 100%, and a sudden drop in the 585 559 nm.

5 encrypting security color pattern

FIG figure5 a multi-step photolithography is prepared micro-pattern (micropatterns) FIG. After the silica particle dispersion, can be cured by irradiation through a photomask regioselectivity, and then after washing off the uncured dispersion can be obtained with the same mask pattern in a transparent window composite pattern. This can photolithography, a plurality of areas on a substrate used in the specific requirements dispersion to give a unique pattern. As shown in FIG Figure5 d, different solvents, the background color of the membrane, \”photonic\” and \”crystal\” vary in color. Reflection spectrum shown in Figure5 e further revealed differences in the optical encoder of the three liquids.

Conclusion

Study Design to quickly respond to a macroporous hydrogel membrane, which is transparent, under the wet condition in the dry state the structure of the color). Film obtained by two-step process, the preparation of fast, without the harsh experimental conditions. Since the holes collapse and transparent state is displayed, but also to recover the wet state and display the color ordered pores, this conversion can be completed within tens of seconds, no hysteresis, while according to different water in the dry state – ethanol to show a different oscillation wavelength structure. Macroporous hydrogel may be further refined as fine particulate, including a micro plate, beads and other shapes, it is possible to expand the application, without being limited to sensors and safety materials. Original link: https: //onlinelibrary.wiley.com/doi/10.1002/adfm.202001318