Pyridine-Dicarbohydrazone-Based Polyacrylate Hydrogels with Strong Mechanical Property, Tunable/Force-Induced Fluorescence, and Thermal/pH Stimuli Responsiveness

The smart fluorescent hydrogels exhibiting tunable fluorescence have been used as biosensors, probes, light-emitting devices, and information storage and confidentiality. However, they usually show single stimuli responsiveness and possess weak mechanical properties, which limit their applications. Herein, we report a pyridine-dicarbohydrazone-based polyacrylate hydrogel (the As-prepared hydrogel and Eu-hydrogel) showing strong mechanical property, tunable fluorescence ability, and thermal and pH stimuli responsiveness. The As-prepared hydrogel is prepared by light-induced micellar copolymerization, and Eu-hydrogel is obtained after the formation of Eu3+-(pyridine-dicarbohydrazone) coordination. The As-prepared hydrogel could be made in large size (side length of 15 cm), exhibiting a tensile strength of 1.55 MPa and a fracture tensile strain of 800%. The fluorescence intensity of Eu-hydrogel is tunable by [Eu3+] (approaching 655% of the minimum state at [Eu3+] = 0.1 mol). It is demonstrated that Eu-hydrogels display force-induced fluorochromic response during stretching. The transmittance temperature of the As-prepared hydrogel ranges from 36 to 42 °C. Reversible Eu3+-(pyridine-dicarbohydrazone) coordination enables Eu-hydrogels to have excellent fatigue resistance. The pH stimuli responsiveness of the As-prepared hydrogel and Eu-hydrogel is triggered by switchable covalent hydrozone bonds. In terms of the shape-memory ability, the As-prepared hydrogel could be reconstructed by Eu3+ and Eu-hydrogel could be deformed by H+. This mechanically strong (pyridine-dicarbohydrazone)-based polyacrylate hydrogel with multistimuli responsive behaviors is promising to be used in information transmission, smart windows, load-bearing devices, energy-absorbing materials, and soft wearable devices.