A Versatile Strategy for Transparent Stimuli-Responsive Interference Coloration

The bioinspired stimuli-responsive structural coloration offers a wide variety of potential applications, ranging from sensing to camouflage to intelligent textiles. Because of its design simplicity, which does not require multilayers of materials with alternative refractive indices or micro- and nanostructures, thin film interference represents a promising solution toward scalable and affordable manufacturing of high-quality responsive structural coloration systems. However, thin films of polymers with appropriate thickness generally do not exhibit visible structural colors if they are directly deposited on transparent substrates such as glass. In this work, a versatile new strategy that enables transparent stimuli-responsive interference coloration (RIC) in the polymer-metal-substrate system is presented. The key concept is to use an ultrathin metal layer as an optical filter instead of high refractive index substrate or highly reflective substrate. Such an optical filter layer allows tuning of the degree of transparency, the constructive interference reflection light, and complementary destructive interference transmission light via changing the metal layer thickness. Real-time, continuous, colorimetric RIC sensors for humidity, organic vapor, and temperature are demonstrated by using different stimuli-responsive polymers. The transparent RIC film on glass shows strong coupling of constructive interference reflected colors and complementary destructive interference transmitted colors on opposite sides of the film. Such transparent RIC film allows for the proof-of-concept demonstration of a self-reporting, humidity-sensing window.