Highly Stretchable and Luminescent Polymer Hydrogels Cross-Linked by a Minute Amount of Water-Soluble Lanthanide Organic Polyhedra

Polymer networks cross-linked by the metal–organic polyhedra have attracted dramatic attention in recent years due to the high degrees of freedom in tuning their structures and functions. Particularly, the 4f electrons from lanthanide ions have offered the lanthanide organic complex with unique magnetic and optical properties. Herein, for the first time, we proposed a facile scheme to construct polymer hydrogels with superior stretchability, strong luminescence, and magnetic resonance relaxation properties via the incorporation of a minute amount of lanthanide organic polyhedra as the cross-links. A bipyridyl bis-tetrazole ligand, bearing an alkene group, was synthesized for coordination with Ln3+ (Eu3+ and/or Gd3+) ions in water to form lanthanide organic polyhedra complexes (K24Ln8L12), which were subsequently used as cross-linkers (0.0083–0.033 mol % of the monomer) for the free radical polymerization with acrylamide monomers to obtain Ln-polyhedra hydrogels. Thanks to the large functionalities and the dynamic feature in lanthanide metal coordination complexes as the network junctions, a tiny amount of cross-linker has rendered these hydrogels superior stretchability exceeding 4200% and tensile stress at break beyond 400 kPa at a stretching rate of 100 mm/min. In addition, Eu3+ and Gd3+ polyhedra can offer hydrogels with a high luminescence quantum yield and efficient magnetic resonance relaxation properties, respectively. In particular, Eu3+ and Gd3+ ions can coassemble with the ligand to form K24EuxGd8–xL12 polyhedra, simultaneously rendering the cross-linked hydrogels superior stretchability, strong luminescence, and fast spin–lattice relaxation rate, and thus a great potential for applications in both luminescent and magnetic resonance imaging.