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

Abstract

The design and fabrication of tough and fluorescent hydrogels still remains as a challenging problem due to the poor mechanical property and waterinduced 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 NaREF 4 :Ln 3+ @NaYF 4 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/m 2 ), fast selfrecovery (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/m 2 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.

Stimuli‐Responsive Nanocomposite Hydrogels for Biomedical Applications

“…[ 151,152 ] Besides serving as stimuli transducers, upconversion nanoparticles can also be surface‐engineered with reactive handles, for instance vinyl‐functionalized nanoparticles that can participate in network reinforcing in order to produce tough double‐network nanocomposite hydrogels envisioned for biosensing and bioimaging applications. [ 153 ] From a critical standpoint, future endeavors propelling upconversion nanoparticles in tissue engineering scenarios should alleviate concerns over the in vivo biodistribution and fate of such nanomaterials following administration, as well as potential impact on cellular activities. Furthermore, pioneering studies should outline and promote assembly methodologies that can improve current upconversion optical efficiency to even greater levels, thus decreasing the exposure times of NIR irradiation (i.e., can damage tissues from accumulated temperature build‐up) that is required to activate the implanted hybrid platforms, while also broadening their applicability to lower cost, commercially available in vivo imaging machines with less powerful, and focused NIR irradiation sources.…”

Soft Bioelectronics Based on Nanomaterials

“…An advantage of the mold casting method is that it can be applied to various nanocomposites regardless of their cross-linking time or mechanism (Figure C). ,− In this method, a mold with a desired shape is prepared first. Then the precursor solution is casted into the mold, and removed from the mold after its cross-linking.…”

Recent advances in polysaccharide‐based hydrogels for synthesis and applications

“…[268,269] Fluorescent hydrogels have also attracted tremendous interests due to their significant potential in bioimaging. Generally, luminescent materials (e.g., rare elements, [270,271] quantum dot, [272] and organic fluorescent dyes [273] ) were introduced into hydrogel networks through chemical covalent grafting or physical mixing. All-biomass fluorescent hydrogels were fabricated by in situ crosslinking of alginate or CNFs with the presence of biomass carbon dots.…”

“…Covalently fixing of fluorescents in hydrogels could address those issues. [270,273] Recently, 3-(trimethoxysilyl)propyl methacrylate (MPS)-functionalized rare element nanoparticles serving as crosslinkers were copolymerized with monomers to covalently anchor on polymer network to form double-network polysaccharide-based hydrogels, which showed strong surface adhesion, full-color fluorescence. [270] Furthermore, fluorescent hydrogels with a strong afterglow could be detected both under the skin and in the stomach, making them a promising candidate for bioimaging.…”