Sub-micron silk fibroin film with high humidity sensibility through color changing

Use of structural colors for humidity sensors has great potential owing to their not being power driven and having distinct stimulus/color variation properties, but unfortunately most 1D, 2D or 3D photonic crystals have subtle nanostructures which are difficult to fabricate. Here we report a one-layer sub-micron thin film with bright color and high sensitivity to humidity, by spin coating of silk fibroin (SF) solution. The optical properties of the SF film caused by thin film interference can be easily tuned by the coating rates. Due to the high hydrophilicity of SF, the film exhibits fast responses with evident color variation in 5 s. And combined with the large peak red-shifts for more than 130 nm, such a thin film is superior to many other multilayered or photonic crystal based humidity sensors. Considering the good reversibility and durability, these low-cost but highly efficient SF spin coating sensors can realize colorimetric detection of humidity like pH indicator papers, and may have great potential in applications for anti-counterfeit labeling.