Melanin and Polypyrrole-Coated Nanocellulose Hydrogel Networks for Environmental Sensing and Energy Storage

Melanins are black-brown pigments of a specific class of poly indolequinones found in nature and in the human body. They are responsible for photoprotection, radical scavenging, and metal ion chelation. Recently, there has been significant interest in eumelanin as a functional material due to its macromolecular structure and the exploitation of the quinone-hydroquinone redox equilibrium. While eumelanin can be used in many promising applications, it is insoluble in most solvents, limiting its processing into homogeneous materials and coatings. A promising approach is to use a carrier system to stabilize eumelanin by incorporating cellulose nanofibrils (CNFs), a nanoscopic material that originates from plant biomass. In this work, a flexible network consisting of CNFs coupled with vapor-phase polymerized conductive polypyrrole (PPy) is utilized to form a functional eumelanin hydrogel composite (MelaGel) for environmental sensing and battery applications. Flexible sensors for detecting pH or metal ions made from MelaGel can detect both pH values in a range from 4 to 10 and metal ions like zinc(II), copper(II), and iron(III), paving the way for environmental and biomedical sensor applications. The reduced internal resistance in the MelaGel leads to improved charge storage ability compared to synthetic eumelanin composite electrodes. Other noteworthy advantages of the MelaGel are the amphiphilic nature of PPy and the additionally offered redox centers. Lastly, this material was tested in aqueous electrolyte zinc coin cells, where it was shown to have charge/discharge stability for over 1200 cycles, showcasing this MelaGel composite as a promising eumelanin-based composite hybrid sensor/energy storage material.