Lanthanide-Doped Upconversion Nanoparticle-Cross-Linked Double-Network Hydrogels with Strong Bulk/Interfacial Toughness and Tunable Full-Color Fluorescence for Bioimaging and Biosensing

The design and fabrication of tough and fluorescent hydrogels still remains as a challenging problem due to the poor mechanical property and water-induced luminescence quenching effect. Here, a new strategy for developing tough and fluorescent hydrogels was proposed by incorporating 3-(trimethoxysilyl)propyl methacrylate (MPS)-functionalized upconversion (UC) fluorescent NaREF4:Ln3+@NaYF4 core–shell nanoparticles (MPS-CSNPs) into agar/poly(N-(hydroxyethyl)acrylamide) (agar/pHEAA) double-network hydrogels, producing agar/pHEAA@MPS-CSNPs gels. Three typical agar/pHEAA@MPS-CSNPs gels with RGB-emitting fluorescence were fabricated to exhibit a combination of extraordinary mechanical properties, including high strength (fracture stress of 2.4 MPa), extensibility (fracture strain of 5.6), stiffness (elastic modulus of 1.9 MPa), and toughness (tearing energy of 11000 J/m2), fast self-recovery (47%/23% toughness/stiffness), excellent self-healing property (self-healing tensile stress/strain of 0.9 MPa/0.9), and superior interfacial toughness of 1100–2000 J/m2 on various solid surfaces. In parallel, the full-color UC fluorescence of the hydrogels can be readily tuned from primary RGB colors to any secondary color (even to white color) by simply changing the types and relative ratios of single or multiple MPS-CSNPs. Agar/pHEAA@MPS-CSNPs hydrogels can also retain long fluorescence stability up to 60 days in a dry storage state. Mechanical enhancement and tunable fluorescence in agar/pHEAA@MPS-CSNPs gels are attributed to hybrid cross-linking effects from covalent bonds between abundant vinyl groups on MPS-CSNPs and the second pHEAA network and noncovalent bonds between and within both agar and pHEAA networks. Taking advantage of highly tough, surface adhesion, and self-healing properties, we further fabricated an agar/pHEAA hydrogel film containing fluorescence-responsive patterns for potential anticounterfeiting. We also envision that the agar/pHEAA@MPS-CSNPs hydrogels hold great potential for developing next-generation tough and fluorescent hydrogels for imaging, biosensing, and other optical applications.