Reversible Light-Controlled Sol-Gel Transition of an Ionic Hydrogel

The understanding of light-mediated control in advanced materials critically depends on the deployment of highly efficient azobenzene-based photoswitches. A key challenge lies in optimizing their light reactivity and developing water-soluble variants that can function as hydrogel cross-linkers. This study represents the design and synthesis of a water-soluble, cationic azobenzene derivative, which serves as a versatile cross-linker to develop a photoresponsive azobenzene composed of sodium salt of poly-2-acrylamido-2-methylpropanesulfonic acid-based [Azo(+)@pSAMPS(-)] hydrogels. Hydrogels have been synthesized by combining hydrogen bonding, electrostatic interactions, and π-π stacking in a way that works well together. The light-induced behavior of the azobenzenes, specifically their trans-to-cis isomerization, has been studied via UV-vis and ¹H NMR spectroscopies, demonstrating efficient conversion within seconds under 365 nm UV irradiation. Comprehensive characterizations via UV-vis, FESEM, and rheological studies confirm the hydrogel's photoisomerization and structural properties. The system successfully encapsulates Doxorubicin hydrochloride (DOX) and demonstrates as a photoresponsive vehicle, enabling on-demand drug release for precise therapeutic control. Besides, this work focuses on simplifying the azobenzene synthesis, aiming to reduce production costs and enhance scalability. This simple approach facilitates the development of advanced light-responsive materials for different biomedical applications, overcoming the challenges associated with more traditional and complex methods.