High-sensitivity colorimetric humidity sensor using nanostructured Fano resonator with ultra-wide color gamut

The colorimetric sensor, which transduces physical information into visible colors, is an intriguing sensor. The metal-hydrogel-metal (MHM) structure, based on the Fabry-Perot resonance, has been investigated as colorimetric humidity sensor due to its hygroscopic properties. However, conventional MHM shows low responsiveness because the superstrate metal barrier hinders water-hydrogel interaction. Additionally, the subtractive coloration limits color gamut. Here, we propose a nanostructured Fano resonator colorimetric sensor (nFRCS) that offers fast response and an ultra-wide color gamut through additive coloration. The Fano resonant optical coating, which couples the continuum state of a lossy medium with the discrete state of MHM, generates Lorentzian reflection spectrum. Nanohole arrays in nFRCS provide a direct pathway for water to reach the hydrogel layer and produce plasmonic resonances that suppress undesired color reflection. Roll-to-plate nanoimprint lithography is employed to finely transfer nanohole patterning. Consequently, nFRCS shows 141 % of the standard RGB and 105 % of Adobe RGB. In terms of responsiveness, it demonstrates response times in the millisecond range.