Fast inverse opal humidity sensor based on acrylamide/AMPS hydrogel

A new fast humidity sensitive inverse opal hydrogel was fabricated through the copolymerization of acrylamide, 2-acrylamido-2-methylpropane sulfonic acid (AMPS), and N,N'-Methylene-bis-acrylamide within the interstitial space of a highly monodisperse polystyrene opal template, followed by template etching. Ultraviolet-visible is the spectroscopy results of the sensor in different relative humidity (RH) levels demonstrated various optical reflection peaks within the visible wavelength range so that the Bragg diffraction peak of inverse opal hydrogel (IOH) red-shifted 88 nm from 426 nm (violet) to 514 nm (green) by increasing RH from 20 to 90 %. Using the optimum amount of AMPS as a functional comonomer in the hydrogel structure, the sensitivity of the IOH humidity sensor (Delta lambda) increased considerably about 90 nm instead of previous IOH humidity sensors reported by Barry et al. In addition, the comparison between IOH humidity sensors with and without AMPS represented that the first group has more speed in responding to stimuli. Further, the sensor had highly good adhesion to the glass substrate and could be used as a pH paper. Furthermore, it changed its optical reflection peak by increasing environmental RH within a few seconds (less than 2 s) and quickly returned to its original state as humidity indicated a decrease. Finally, RH increase/decrease cycles revealed that the sensor has a long lifetime (more than one year) without response time degradation or reproducibility.