Designing macroporous hydrogels with polar-solvents-sensitive photonic property

Abstract

Photonic crystals have nanostructure in which the refractive index is changed periodically and show unique optical properties by selectively reflecting the light of a specific wavelength. The optical properties of photonic crystals are preserved as long as the nanostructure is not changed, so photonic crystals have the potential to substitute dyes or pigments. In particular, colloidal photonic crystals can be fabricated on a large scale by the self-assembly of colloid particles, and its structural color can be easily tuned by an external stimulus or viewing angle. Thus, colloidal photonic crystals can be used for display, sensors, and anti-counterfeiting materials. In this study, we propose the application of photonic crystals composed of macroporous hydrogels. We fabricate film type colloidal photonic crystals by dispersing monodispersed silica particles in a photocurable resin and photopolymerization. In the case of the low cross-linking density of hydrogels constituting the film, the film is transparent due to the collapsed structure when the silica particles are selectively removed. However, when the transparent hydrogel is exposed to polar solvents such as water or ethanol, the porous structure is restored by swelling of hydrogels, resulting in showing structural color. The structural color change is reversible and repetitively, so the transparent film can be used as sensors to detect polar solvents. Also, the transparent film containing micropattern fabricated by photolithography has the potential to use as anti-counterfeiting patches due to showing different structural color patterns depending on the type of solvent.