Highly Sensitive Diffraction Grating of Hydrogels as Sensors for Carbon Dioxide Detection

A diffraction grating of a stimulus-responsive hydrogel was fabricated for the detection of CO2 with a low detection limit. The hydrogel consisted of dimethylaminopropyl methacrylamide (DMAPMA), methyl methacrylate, and 2-hydroxyethyl methacrylate. The sensing gratings were fabricated through soft-lithographic duplication of photoinduced surface-relief-gratings on azo molecular glass films on the hydrogel surfaces. The detection of an analyte was achieved by the reaction of tertiary amino groups of DMAPMA in the hydrogel with CO2 in water and swelling of hydrogel gratings to cause the variation of the optical-diffraction efficiency (DE). The relationship between DE and CO2 concentration, controlled by the gas-injection rate and volume, was systematically investigated. The performance of continuous detection of CO2 was also evaluated. The sensing device showed high sensitivity, reliable readability, and excellent repeatability for CO2 detection. The CO2-sensing performance of the hydrogel grating was proved to be almost unaffected by relative humidity variations of the environment in a wide range.