Natural lignin nanoparticles: a promising nano-crosslinker for constructing fluorescent photoswitchable supramolecular hydrogels

Photoresponsive supramolecular hydrogels using nanomaterials as crosslinkers with reversible sol–gel transition are attractive because their mechanical properties can be switched in a wide range by controlling light irradiation. Until now, most nano-crosslinkers for supramolecular hydrogels are limited to inorganic nanoparticles or synthetic micelles. In this work, we demonstrated the application of natural lignin nanoparticles as nano-crosslinkers to fabricate multifunctional photoresponsive supramolecular hydrogels. Photoswitchable and photoluminescent ternary supramolecular hydrogels were fabricated using polyacrylic acid (PAA), azobenzene guanidine (Azo-Gu) and α-cyclodextrin grafted cellulolytic enzyme lignin nanoparticles (α-CD@CEL-NPs). The aggregation-induced emission active fluorescence of α-CD@CEL-NPs makes the hydrogels strongly fluorescent under UV light excitation. Moreover, due to the photoresponsive host–guest interaction between Azo-Gu and α-CD@CEL-NPs, the ternary supramolecular hydrogels showed reversible sol–gel transition by controlling UV and blue light irradiation. This work may encourage the fabrication of more multifunctional supramolecular hydrogels with various functionalities in the future.