Bioinspired stimuli-responsive multilayer film made of silk-titanate nanocomposites

In this work, we report a structurally-colored multilayer film made of a silk-based material, inspired by the cuticle of Hoplia coerulea, a species of beetle able to change its color in the presence of moisture. For the generation of interference color, we combined the regenerated silk fibroin, which has a refractive index of 1.55, with a nanocomposite made of silk and titanate nanosheets, a novel 2D material which presents a high refractive index. The multilayer film was fabricated through a layer by layer deposition from the respective water-based solutions by spin coating. The final structure exhibits an interference peak in the transmittance spectrum centered at 400 nm, whose position responds to changes in the relative humidity of the environment. The stimuli-responsive properties of the film were characterized and a simple optical model for the sensing mechanism was proposed. Specifically, the silk-based multilayer showed a reversible color change when exposed to different relative humidities, and good performances in terms of reproducibility and stability over time.