Chitosan-Infiltrated TiO2 Nanocrystal Composite Optical Metasurfaces for Colorimetric Leaf Sensing

We report the design and assembly of geometrically and compositionally complex dielectric metasurfaces, termed nanocrystal-adaptive polymer composite optical metasurfaces (NCOMs), architected from nanocrystal (NC) building blocks and stimuli-responsive polymers to enable high figure-of-merit optical leaf sensors. These NCOM sensors are fabricated at scale by direct nanoimprint lithography into UV-curable TiO2 NC inks to structure optical metasurfaces in the geometry of surface relief gratings atop waveguides that support high reflectivity, narrowband guided-mode resonances. A room-temperature ligand exchange process creates a nanoporous TiO2 scaffold that is subsequently infiltrated with a stimuli-responsive polymer while retaining the metasurface geometry and optical quality. As a proof of concept, we demonstrate optical humidity sensors by incorporating a moisture-responsive biopolymer chitosan into the nanoporous TiO2 metasurfaces, resulting in a 430% improvement in sensitivity compared to conventional refractive-index sensors where the polymer is adjacent to the metasurface. The NCOM geometry is engineered to position its resonance at wavelengths above 750 nm, enabling detection with standard VIS-NIR Si photodetectors while maintaining broadband transparency across the photosynthetically active 400-700 nm wavelength range for unobstructive monitoring of crop health. NCOM sensors are mounted face-down on leaves, and the highly reflective resonances are readily distinguished from the leaf background and respond dynamically to leaf water stress, enabling passive, battery-free monitoring of leaf surface humidity.