Lanthanide-containing
luminescent hydrogels have shown
potential
for sensing and imaging applications. Nonetheless, integrating lanthanide
ions or complexes into the polymer matrix often results in the poor
stability and mechanical strength of the hydrogels. This work presents
an innovative approach to fabricating luminescent hydrogels with three
dynamic cross-links: imine bond, boronate ester bond, and metal–ligand
coordination. Europium(III) (Eu3+) ions are incorporated
into a dual-cross-linked matrix composed of phenylboronic acid-polyethylenimine-modified
gelatin (PPG) and alginate dialdehyde (ADA) through a combined treatment
involving freeze-drying-swelling (FDS) and freeze–thawing (FT)
processes. The FDS process facilitates the formation of additional
europium-carboxylate cross-links within the polymeric network to enhance
its luminescence and stability, while the FT process strengthens the
network physically. The impact of the FDS-FT cycle number on the microstructures
and properties of PPG/ADA-Eu3+ hydrogels is thoroughly
investigated, and their potential for monitoring bacterial growth
and detecting copper(II) ions is also demonstrated.