Transparent and flexible structurally colored biological nanofiber films for visual gas detection

Structurally colored films hold promise as cheap platforms for use in visual detection. However, achieving accurate stimuli-responsive functions that can be distinguished by the naked eye in a single thin, robust, and practically useful material remains a challenge. Herein, we demonstrate flexible and self-standing structurally colored films fabricated by filtrating sustainable hydrophilic nanofibrillated cellulose through a porous hydrophobic membrane, followed by natural drying. 480–644 nm thick and composed of densely packed, high-aspect-ratio nanofibers entangled in web-like structures, the films are transparent, tailorable into a variety of shapes, and easily foldable. They exhibit iridescence owing to thin-film interference and reversibly change color in seconds when exposed to various moist gases and liquids. We validate the visual gas-detection capabilities of the films by measuring the ethanol concentration in the breath of an inebriated participant and envisage that these materials will find use in automated and broad-spectrum gas-detection devices.