Lignin Upconversion by Functionalization and Network Formation.

Lignin, a complex and abundant biopolymer derived from plant cell walls, has emerged as a promising feedstock for sustainable material development. Due to the high abundance of phenylpropanoid units, aromatic rings, and hydroxyl groups, lignin is an ideal candidate for being explored in various material applications. Therefore, the demand on lignin valorization for development of value-added products is significantly increasing. This mini-review provides an overview of lignin upconversion, focusing on its functionalization through chemical and enzymatic routes, and its application in lignin-based polymer resins, hydrogels, and nanomaterials. The functionalization of lignin molecules with various chemical groups offers tailored properties and increased compatibility with other materials, expanding its potential applications. Additionally, the formation of lignin-based networks, either through cross-linking or blending with polymers, generates novel materials with improved mechanical, thermal, and barrier properties. However, challenges remain in optimizing functionalization techniques, preserving the innate complexity of lignin, and achieving scalability for industrial implementation. As lignin's potential continues to be unlocked, it is poised to contribute significantly to the shift towards more eco-friendly and resource-efficient industries.

Development of a light activatable lignin nanosphere based spray coating for bioimaging and antimicrobial photodynamic therapy.

Many coating materials are commercially available to combat microbial infections. However, these coatings are difficult to synthesize, and are mostly composed of toxic chemicals. Lignin is an under-explored natural biopolymer with multifaceted potential. Lignin, with adhesive, UV resistant, and antimicrobial properties, is a suitable candidate to develop coating materials. Here we report a smart method to fabricate a sustainable nanospray coating from lignin which does not require any toxic chemicals or additives during synthesis. Initially, we have developed stable lignin nanospheres in a single step in aqueous medium, which were later utilized as a lignin nanospray (LNSR). The LNSR was characterized by dynamic light scattering, scanning electron microscopy, FTIR and other analytical techniques. This LNSR showed remarkable UV blocking, antioxidant and light-activated antimicrobial properties. Interestingly, for the first time, the LNSR demonstrated photoluminescence, making it useful for bioimaging. Moreover, singlet oxygen generation potential was observed in the LNSR, which could render it useful in phototheranostic applications (i.e. light assisted imaging and photodynamic therapy). Further, the LNSR was directly utilized to fabricate a sustainable coating. The nanospray coating exhibited maximum light-induced cell killing when applied to common microbes as detected by live-dead cell imaging. Taken together, the lignin nanospray coating developed via a direct pathway holds great promise to disinfect microbes in the presence of light.

Lignin-Bimetallic Nanoconjugate Doped pH-Responsive Hydrogels for Laser-Assisted Antimicrobial Photodynamic Therapy.

Bioinspired nano-antimicrobials stand out in terms of cost effectiveness and scalability when compared to their chemically synthesized counterparts. There is limited efficacy of current antibiotics due to their interference with the immune system as well as development of antibiotic resistance. Lignin, which is a naturally abundant polyphenol-rich biopolymer, can be utilized for the fabrication of sustainable antimicrobial materials. In the present work, development of stable nanocomposite hydrogels embedded with lignin-based photodynamic nanoconjugates has been described. This could lead to complete eradication of microbial infection upon laser exposure. For designing such hydrogels, initially photosensitizer decorated lignin-metallic and lignin-bimetallic nanoconjugates were developed utilizing simple and nontoxic methods. These photodynamic nanoconjugates were then characterized and doped into a poly(acrylic acid)-based hydrogel in order to achieve efficient pH-triggered controlled release. The nanocomposite hydrogels allowed maximum transmission of light, promoting their applicability in antimicrobial photodynamic therapy. Utilization of hydrogel helped in better retention of nanoconjugates, maintaining their antimicrobial photodynamic efficacy as validated via IC50 measurement and live-dead cell imaging. The biocompatible pH-responsive photodynamic antimicrobial hydrogels developed herein could be potentially applicable in controlled drug delivery through the construction of wound dressings, as well as for developing antifungal, antibacterial, or antiviral nanocoatings.

Development of agri-biomass based lignin derived zinc oxide nanocomposites as promising UV protectant-cum-antimicrobial agents.

Agri-biomass derived lignin is one of the most abundant natural nontoxic organic polymers. However, a major portion of lignin is underutilized, which is being left in the fields or thrown into rivers, causing waste accumulation. Utilization of the low-cost biomass-derived lignin serves a dual purpose by reducing agri-waste and by converting it into value-added materials. Here we describe the valorization of agri-biomass based lignin via its direct utilization in the green and one-step synthesis of zinc oxide nanocomposites. Lignin offered an easy and sustainable synthesis of nanocomposites in a water-ethanol mixture without the need to use any toxic material. The lignin derived nanomaterials showed excellent potential as antioxidant, UV-blocking and antimicrobial agents due to the synergistic effect of lignin and zinc oxide. Further, these nanocomposites were incorporated as an additive into a commonly used body cream to impart UV-blocking and antimicrobial properties. This one-step, cost-effective and green synthesis technique of lignin derived zinc oxide nanocomposites not only contributes to agro-waste reduction, but at the same time helps in the production of value-added materials.

Engineering Lignin Stabilized Bimetallic Nanocomplexes: Structure, Mechanistic Elucidation, Antioxidant, and Antimicrobial Potential.

Lignin, being a natural antioxidant and antimicrobial underutilized biopolymer derived mainly from agro-waste, is a material of great interest. In this study, lignin was chosen as a matrix to synthesize silver-gold bimetallic and monometallic nanocomplexes to explore the synergistic antioxidant and antimicrobial properties of the lignin stabilized nanoagents. The synthesis of the nanocomplexes was carried out using a one pot method, utilizing lignin as the sole source for reducing, capping, and stabilizing the nanoagents. Further, characterization studies were performed to determine the exact structure of the nanocomplexes. The developed nanocomplexes were found to possess substantial phenolic and flavonoid contents, which contributed to their high antioxidant activity. Further, the antioxidant and antimicrobial activity of the lignin-bimetallic and monometallic nanocomplexes was evaluated and compared with pristine lignin. Moreover, the mechanism behind the antimicrobial activity of the nanocomplexes was elucidated through various methods, namely, reactive oxygen generation, nucleic acid leakage, and DNA cleavage studies. The obtained results were greatly supported by scanning electron microscopy, transmission electron microscopy, and live-dead cell imaging techniques. This study is a contribution in converting waste to value added functional nanomaterials for potential antioxidant and antimicrobial applications.