Fabrication of\nLuminescent Triple-Cross-Linked Gelatin/Alginate\nHydrogels through Freezing-Drying-Swelling and Freezing-Thawing Processes

Lanthanide-containing\nluminescent hydrogels have shown\npotential\nfor sensing and imaging applications. Nonetheless, integrating lanthanide\nions or complexes into the polymer matrix often results in the poor\nstability and mechanical strength of the hydrogels. This work presents\nan innovative approach to fabricating luminescent hydrogels with three\ndynamic cross-links: imine bond, boronate ester bond, and metal–ligand\ncoordination. Europium(III) (Eu<sup>3+</sup>) ions are incorporated\ninto a dual-cross-linked matrix composed of phenylboronic acid-polyethylenimine-modified\ngelatin (PPG) and alginate dialdehyde (ADA) through a combined treatment\ninvolving freeze-drying-swelling (FDS) and freeze–thawing (FT)\nprocesses. The FDS process facilitates the formation of additional\neuropium-carboxylate cross-links within the polymeric network to enhance\nits luminescence and stability, while the FT process strengthens the\nnetwork physically. The impact of the FDS-FT cycle number on the microstructures\nand properties of PPG/ADA-Eu<sup>3+</sup> hydrogels is thoroughly\ninvestigated, and their potential for monitoring bacterial growth\nand detecting copper(II) ions is also demonstrated.