Dually tunable inverse opal hydrogel colorimetric sensor with fast and reversible color changes

Through opal-templated photopolymerization of copolymeric hydrogel consisting of 2-hydroxyethyl methacrylate, N-isopropylacrylamide, acrylic acid, and N,N′-methylenebis(acrylamide), a dually tunable inverse opal sensor by temperature and pH has been successfully synthesized. Temperature drop or pH increase induced hydrogel swelling and subsequent color changes of the sensor respectively due to lower critical solution temperature behavior of N-isopropylacrylamide and the altered Donnan potential by acrylic acid units. The color change driven by hydrogel swelling could be quantitatively measured by reflectance spectra. Increased content of N-isopropylacrylamide in the hydrogel resulted in a larger volume change under the same T-drop experiment. On the contrary, higher crosslinker content led to a less swollen hydrogel. From the kinetic investigations, response time for temperature change was measured to be much faster than that for pH variation (0.5 s vs. 28 s). An exceptionally rapid temperature response could be explained by fast water influx to the thin layers at the top of inverse opal hydrogel after an instantaneous temperature change. A cyclic pH/temperature changes (pH jump–temperature-drop–pH drop–temperature-jump) on a dually tunable sensor showed a perfect restoration of diffraction wavelength as well as the rapid response times for all stimuli. This work facilitates the fabrication of a variety of multi-responsive inverse opal sensors exhibiting rapid response kinetics by simply copolymerizing the individual sensing monomers using the optimized photo-polymerization and the colloidal templating route.