Chitosan Infiltrated TiO2 Nanocrystal Composite Optical Metasurfaces for Colorimetric Leaf Sensing

We report high figure-of-merit optical leaf sensors based on dielectric metasurfaces and stimuli-responsive polymers. The metasurfaces have narrowband resonances and are broadly transparent across the visible spectrum to allow photosynthesis for unobstructive monitoring of crop health. These sensors are fabricated at scale using direct nanoimprint lithography into UV curable-TiO2 nanocrystal (NC) inks to structure optical metasurfaces, followed by a room-temperature ligand exchange process to create a nanoporous TiO2 scaffold. The nanoporous TiO2 metasurfaces are then infiltrated by a stimuli-responsive polymer while retaining their geometry and optical quality. As a proof of concept, we demonstrate optical humidity sensors by incorporating the moisture-responsive chitosan biopolymer into the nanoporous TiO2 metasurfaces, resulting in a 430 % improvement in sensitivity compared to conventional RI sensors where the polymer surrounds the metasurface. The sensors are mounted on leaves and the highly reflective resonances, which are readily distinguishable from the leaf background, respond dynamically to leaf water stress, enabling passive, battery-free monitoring of leaf surface humidity.