Optimized structure for a moisture-sensitive colorimetric sensor utilizing photonic crystals based on a metal–organic framework

We propose an optimized structure for a humidity-sensing colorimetric sensor that utilizes a metal-organic framework to form a one-dimensional photonic crystal. The structure changes spectral reflectivity by switching its photonic band gap (PBG). We found that, although photonic crystals on a highly reflective substrate increased the peak more than a transparent substrate, the resulting ripples in the wavelength band near the PBG destroy our ability to detect the color conversion. We also show that, despite the blue-shift of the peak PBG wavelength for higher angles of incidence, our sensor can still create significant color conversion at nonzero angles when the environmental humidity changes. Therefore, photonic crystals on a transparent substrate that utilize PBG-switching can operate as moisture-sensitive colorimetric sensors. Our study will help optimize sensors and industrial nano-structures.