Sensing relative humidity with a fluorescent seed-like biodegradable flier

Plant-inspired soft robots enable distributed environmental monitoring. Fliers, <i>i.e.</i> soft robots that are carried passively by the wind, can be effectively deployed and cover large areas and distances. State-of-the-art fliers for humidity sensing are largely composed of electronic components, which increase cost and generate electronic waste. Here, we introduce self-deployable and biodegradable fliers inspired by natural <i>Ailanthus altissima</i> seeds. These artificial fliers are composed of fluorescent, cellulose-based composites with sensing capabilities. The material is shaped into artificial seeds using scalable 3D extrusion processing. Red-emitting Mn²⁺-doped Er³⁺, Yb³⁺:NaYF₄ nanoparticles in the composite provide a strong optical emission upon excitation at 980 nm wavelength. The cellulose matrix absorbs water, which quenches the intensity of fluorescence of the nanoparticles. Increasing humidity thus changes the color of the fluorescence emission from red to green. We used ratiometric sensing to detect the humidity of the surroundings.