Full-color mechanical sensor based on elastic nanocomposite hydrogels encapsulated three-dimensional colloidal arrays

A label free and easy-to-prepare optical sensor with high mechanical strength for the fast and visible response to tension and pH variables was reported. An elastic nanocomposite hydrogel was successfully fabricated by in situ free-radical co-polymerization of N,N-dimethylacrylamide (DMAA), acrylic acid (AA) and aluminum oxide nanoparticles (Al₂O₃ NPs). DMAA and AA could form effective cross-linkage with Al₂O₃ NPs via hydrogen binding, electrostatic interaction and bidentate bridging. By encapsulating polymethyl methacrylate (PMMA) colloidal arrays into the hydrogels, the optical nanocomposite hydrogels (ONHs) can be easily obtained. This three-dimensionally ordered array exhibits a stop band, and the optical hydrogels can tune the bright color from blue to cyan, green, yellow green, yellow, orange and then finally red by mechanical stimulation of uniaxial tension within 50% strain following Bragg's law. The mechanical and optical properties can be adjusted via changing the fraction of AA and polymerization time. Meanwhile, the mechanical and optical responses were reversible for more than 10 times without deterioration. Besides, this material has rapid response to the changing of pH and the full-color change was observed. Owing to the high biocompatibility of nanocomposite hydrogel (NC gel) system and the excellent mechanical strength and water-stability of these ONHs, this optical sensor has high potential to be applied in biomedical field as tension or pH sensor for the intraocular pressure measurement and tonometry and blood gas analysis.