Hydrogel-driven tunable photonic crystal grating for gas detection

Tunable gratings dynamically alter diffraction conditions under external stimuli, holding significant application value in many fields. However, in gas detection, existing approaches based on MEMS mechanical tuning, thermo-optic/electrooptic refractive index modulation, or liquid crystals are often constrained by high driving voltages, large volumes, or limited tuning ranges. To address these limitations, we present a hydrogel-driven tunable photonic crystal (PC) grating. By integrating into a miniature spectrometer, we successfully applied it to gas detection. Specifically, by combining polystyrene (PS) microspheres with humidity-sensitive hydrogel and SiO₂ microspheres with pH-responsive hydrogel, we constructed two dynamically tunable PC gratings with adjustable lattice constants. As ambient water vapor or CO₂ concentration increases, hydrogel swelling expands the PC lattice constant, inducing a red shift in the diffraction wavelength. Integrating tunable PC grating into a Czerny-Turner (C-T) spectrometer system transforms gas concentration into precise wavelength shifts, enabling high-accuracy gas detection. Experimental results demonstrate excellent linear responses for both tunable PC gratings across 24-94% relative humidity (RH) and 200-50,000 ppm CO₂ concentrations, alongside robust cycling stability and long-term reliability. we provide a novel and effective approach for the application of tunable gratings in gas detection.