Water-soluble metasurface labels for environmentally sustainable humidity monitoring, origin authentication, and anticounterfeit packaging

Anticounterfeiting and environmental sustainability have become key priorities in modern packaging and product labeling. Although metasurface-based optical security systems offer ultrathin profiles and strong resistance to forgery, their adoption has been limited by expensive, low-throughput fabrication methods. At the same time, growing environmental awareness has increased demand for biodegradable materials and sustainable manufacturing techniques. Here, we present a water-soluble metasurface label that integrates full-color structural coloration, ultraviolet-to-visible holography, and humidity-responsive sensing within a single compact device. Although hydroxypropyl cellulose alone has a low refractive index and typically requires additional post-processing to achieve functional optical properties, we enhance the effective refractive index significantly by incorporating titanium dioxide nanoparticles. This enhancement allows direct fabrication of high-performance optical nanostructures through a single-step nanoimprint lithography process, offering a cost-effective and scalable solution for diverse packaging substrates. To enable simultaneous spatial and spectral light modulation, we employ a supercell design that decouples geometric phase control from resonant color generation, allowing vivid color and holographic images to coexist. In addition, the hygroscopic nanostructures undergo irreversible deformation after prolonged exposure to high humidity, providing a visual indication of compromised storage conditions. This multifunctional and water-soluble label offers a cost-effective and eco-friendly approach to next-generation optical platforms for anticounterfeiting, environmental monitoring, and sustainable consumer applications.