Photonic crystal hydrogels based on highly reproducible molding method.

A lot of sensors using structural color are based on periodic nanostructures, or photonic crystals. So the nanostructures need to be fabricated with high reproducibility so that those sensors can be suitable for practical and commercial applications. Furthermore, achieving the reproducible fabrication is more challenging for hydrogel-based devices with structural color. In this study, we propose a novel molding approach to fabricate photonic crystal hydrogels with high reproducibility. A silicon wafer with a monolayer of self-assembled nanoparticles is used as a mold to transfer nanostructures onto the hydrogel surface. Since the molding technique is sensitive to the mechanical properties of the hydrogel, we optimized these properties by adjusting the monomer-to-crosslinker ratio. The ratio of 50:1 was identified as the optimal composition for the molding method to ensure both mechanical stability and chemical responsiveness. In order to demonstrate reproducibility, the molding processes were performed for over 50 cycles, resulting in hydrogel exhibiting structural colors with optical and mechanical integrity. Additionally, hydrogels showed reversible color changes in response to various solvents. Volume change of the hydrogel caused variation of periodicity of photonic crystal, which led to red-shifted colors upon swelling and blue-shifted colors upon contraction. This study shows that photonic crystal hydrogels can be fabricated with enhanced reproducibility by molding method. And it also shows that they can be applied to structural color-based sensors. The principle of this study can be extended to biosensing and environmental monitoring applications by incorporating selective molecules such as antibodies.