Nanocellulose/melanin-based composites for energy, environment, and biological applications

Melanins, a class of brown-black pigments present in many natural sources, exhibit unique physical and chemical properties such as photoprotection, photoconductivity, metal ion chelation, free radical quenching, proton conductivity, and significant antimicrobial properties. These properties could be leveraged in various environmental and energy applications. However, their limited solubility makes them less amenable to processing. Over the last decade, there has been tremendous interest in developing synthetic analogs of melanin that are easy to dissolve and process. Nonetheless, melanins or their synthetic analogs cannot form mechanically robust free-standing films and fibers. This drawback could be partly circumvented by creating composites of melanin with nanocellulose, a nanomaterial derived from cellulosic biomass. Composites of melanins and nanocellulose offer an opportunity to utilize the functional aspects of melanin-like polymers in a mechanically stable nanocellulose network. This review attempts to capture the recent developments on this topic with special emphasis on technologically relevant applications.