Thermo-responsive and reversible colorimetric sensors as fever-checkers using low critical solution temperature polymer-Au nanocomposites

Hydrogels have proven to be among the best substrates for thermo-responsive patches owing to their hydrophilic functional groups. Poly(N-isopropylacrylamide) (PNIPAM) is a common thermo-responsive polymer with a low critical solution temperature (LCST). This polymer changes its phase in response to the solution temperature. In this study, a thermo-responsive colorimetric sensor was fabricated by combining Au nanoparticles (AuNPs) with PNIPAM hydrogels to alter the interparticle spacing of AuNPs contained in the polymer matrix in response to temperature stimuli. The as-prepared PNIPAM@Au hydrogel demonstrated excellent reversible swelling-shrinking capabilities when the temperature changed from 25 degrees to 50 degrees C in 1 min. As the hydrogels shrank, water was released near the shrinking hydrogel. The color of the hydrogel changed from red to purple. This color shift revealed that the localized surface plasmon resonance band red-shifted as the interparticle distance of the AuNPs in the PNIPAM@Au hydrogel was reduced owing to the coupling effect of each AuNP. The thermo-responsive and colorimetric characteristics of the PNIPAM@Au hydrogel may be useful in monitoring body temperature changes in infants with high fever.