Angle-Independent Color Change in Thermoresponsive Gel-Immobilized Colloidal Amorphous Film Attached to PET Substrate.

Gel-immobilized colloidal amorphous structures comprise short-range-ordered monodisperse submicrometer particles embedded into a soft polymer gel. They exhibit an angle-independent structural color that is tunable in response to external stimuli via a volume change in the gel, which has significant potential for the development of sensors that respond to stimuli via angle-independent color changes. In this study, the amorphous structure of a charged colloidal suspension in water was immobilized in a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) gel film and simultaneously attached to a polyethylene terephthalate (PET) substrate. The gel film exhibited a uniform angle-independent color that changed in response to changes in temperature (i.e., thermosensitivity). Attachment to the PET substrate suppressed changes in the gel film area and film distortion, despite significant volume changes in the gel. Consequently, the degree of thermosensitivity was enhanced. The PET-attached gel-immobilized colloidal amorphous film was easy to handle and had excellent flexibility, allowing it to wrap around the surfaces of curved objects. These features are advantageous for sensor applications.

Amorphous photonic structure in a charged colloidal system showing angle-independent uniform color.

Structural colors derived from the scattering or reflection of light by submicron-sized ordered structures are interesting due to their non-fading and tunable colors. Herein we show a colored angle-independent amorphous phase of monodisperse colloids between the crystalline and random phases in a charged colloidal system. The amorphous photonic structures with low packing density display uniform color over large areas; this is easily altered by adjusting the particle volume fraction. The transmission spectra of amorphous phases display an intersection point at the Bragg wavelength, allowing design of the transmission color. We demonstrate that the aqueous suspended colloidal amorphous structures can be immobilized in hydrogel films, while preserving their spectral properties. The sensitivity of the gel to ethanol allows the color to be tuned by changing the ethanol concentration. This angle-independent uniform color with easy tunability has great potential for application in advanced color materials and sensors.