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.

The optical and conductometric responses of sol-gel-based ZnO thin films with embedded 5% mol. Au or Pt nanoparticles (NPs) have been compared at different operating temperatures, to assess the effect of noble metal additions on to H ₂ , CO and NO ₂ gases sensors response. The scanning electron microscopy and X-ray diffraction techniques have been utilized for morphological characterization of the crystal structures of the films after annealing at 400°C for 30 min and the effect of Au and Pt NPs additions on crystallite growth is discussed. Au NPs additions is effective to enhance the optical response to H ₂ and CO at 390 nm and in the 500-700-nm wavelength range at 300- °C temperature. No optical response to NO ₂ gas, even at low concentrations has been measured in the optical mode. The catalytic action of Pt NPs additions powerfully improves both the optical to H ₂ and CO at 390 nm, and the conductometric gas responses. Detection limits as low as 20-ppb NO ₂ and 50-ppm H ₂ , in line with the best performances reported to the best of our knowledge in literature for ZnO-based sensors, have been measured in the conduction mode. Response times have been also compared, highlighting the positive effects played by Pt additions.