Strain sensing multi-stimuli responsive light emitting lanthanide-based tough and stretchable hydrogels with tunable luminescence and fast self-recovery using metal-ligand and hydrophobic interactions

Lanthanide-based hydrogels have recently attracted massive interest due to their multipurpose applications like in optical sensing, optical image probes, UV OLEDs, electroluminescent devices and biological applications. Here we have developed dual physically cross-linked (metal-ligand and hydrophobic interactions) lanthanide-based hydrogels (in combination with low density chemical cross-links) where acrylamide and methacrylic acid are copolymerized along with a hydrophobic terpyridine based sensitizer (6-(2,2 ':6 ',2 ''-terpyridine-4 '-yloxy)-hexylacrylamide) through a micellar route. In this design, the luminescence of the Ln(3+) ions sequestered in the hydrophobic micellar region of the hydrogel is enhanced by the sensitizer, which also acts as a ligand for metal-ligand cross-linking. The hydrogel material responds to and hence can be utilized to sense various stimuli, such as pH, temperature, metal ions and mechanical strain. By tuning the lanthanide to sensitizer (ligand) molar ratio, the mechanical properties of the hydrogel materials can be modulated. Luminescence tuning over a wide spectrum of emission colours can be achieved by varying the molar ratio of Eu3+/Tb3+ in the hydrogel. The luminescent hydrogels possess a combination of favourable mechanical properties: high tensile strength (similar to 500 kPa), stretchability (similar to 7 times the original length), toughness (similar to 1 MJ m(-3)), and high compressive strength (similar to 3.3. MPa at 90% compressive strain). Because of the presence of dynamic lanthanide-sensitizer (ligand) cross-links that act as sacrificial motifs, the hydrogels showed excellent tensile and compressive self-recovery, along with robust anti-fatigue properties and self-healing characteristics. Applications as a flexible resistive and emissive strain sensor as well as a fluorescent anticounterfeiting ink are also demonstrated.

Automated summary

Lanthanide-based hydrogels have attracted significant attention recently due to their wide range of applications, including optical sensing, electroluminescent devices, and biological applications. In this study, a dual physically cross-linked hydrogel was developed using a micellar route, where a hydrophobic sensitizer enhanced the luminescence of Ln(3+) ions and acted as a ligand for metal-ligand cross-linking. The hydrogel exhibited responsiveness to various stimuli and tunable mechanical properties. The luminescent hydrogels showed excellent self-recovery, anti-fatigue properties, and self-healing characteristics, and demonstrated applications as a strain sensor and an anticounterfeiting ink.