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

Abstract

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 Ln³⁺ 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 (∼500 kPa), stretchability (∼7 times the original length), toughness (∼1 MJ m⁻³), and high compressive strength (∼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.