Photodynamic Lignin Hydrogels: A Versatile Self-Healing Platform for Sustained Release of Photosensitizer Nanoconjugates

A multifunctional hydrogel that combines the properties of pH-responsiveness as well as controlled release holds enormous potential for antimicrobial photodynamic therapy through the development of wound dressings and coatings. Utilization of lignin (a biodegradable and cost-effective biopolymer) could be advantageous as a sustainable alternative over the conventional hydrogels for photodynamic therapy, which is an underexplored area. In this work, the lignin-based hydrogel was developed, which was found to have remarkable self-healing properties. Moreover, the lignin-based photodynamic hydrogels designed via in situ and ex situ methods were found to be transparent enough (through the variation of lignin concentration) to be utilized for photodynamic therapy applications. The in situ method is an example of lignin hydrogel synthesis which is advantageous as it saves multiple reaction steps of nanoparticle synthesis and incorporation into the hydrogel. However, for the demonstration of photodynamic effect, the lignin-based hydrogels were doped with a photosensitizer (Rose Bengal, RB) and also with RB-conjugated lignin-derived silver nanocomplexes (RB@L-AgNCs). The developed lignin-based nanocomposite hydrogels were characterized through various methods including UV–vis spectroscopy, HRTEM, BET, XRD, FESEM, rheology, and FTIR to determine their material properties. Interestingly, the lignin hydrogels were found to possess pH-responsive properties for controlled release of the incorporated nanoagents. Antimicrobial photodynamic therapy studies were performed using lignin-based nanocomposite hydrogels as well as bare lignin hydrogels through utilization of a green laser for a short time (3 min). Mechanistically, it was observed that the nanocomposite-doped hydrogel worked much better as compared to the native lignin hydrogel due to higher reactive oxygen species (ROS) generation. Furthermore, the antimicrobial photodynamic activity of the developed lignin-based nanocomposite hydrogels was validated through fluorescence microscopy studies (live–dead cell imaging). These developed biodegradable lignin-based nanocomposite hydrogels can be efficiently used for the development of wound dressings and nanocoatings over various surfaces for stimuli responsive antimicrobial effect.