Natural melanin pigments and their interfaces with metal ions and oxides: emerging concepts and technologies

Abstract

Melanin (from the Greek μέλας, mélas, black) is a biopigment ubiquitous in flora and fauna, featuring broadband optical absorption, hydration-dependent electrical response, ion-binding affinity as well as antioxidative and radical-scavenging properties. In the human body, photoprotection in the skin and ion flux regulation in the brain are some biofunctional roles played by melanin. Here we discuss the progress in melanin research that underpins emerging technologies in energy storage/conversion, ion separation/water treatment, sunscreens, and bioelectronics. The melanin research aims at developing approaches to explore natural materials, well beyond melanin, which might serve as a prototype benign material for sustainable technologies.

Macroscale Biomolecular Electronics and Ionics

“…Melanin Composites and Doping : It has been known for several decades that melanins bind metal ions very strongly—particularly transition metals such as copper, iron and europium . There is some evidence that this metal binding could perturb the macroscopic electrical conductance, but this is by no means definitive.…”

Cephalopod‐Derived Biopolymers for Ionic and Protonic Transistors

“…Melanins are ubiquitous natural pigments that serve coloration and photoprotection roles in many animals . Variants of these pigments called eumelanins also have specialized functions within cephalopod ink .…”

“…Melanins are ubiquitous natural pigments that serve coloration and photoprotection roles in many animals . Variants of these pigments called eumelanins also have specialized functions within cephalopod ink . Generally, eumelanins are considered to consist of copolymerized (i.e., crosslinked) 5,6‐dihydroxyindole (DHI) and 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA) building blocks, as illustrated in Figure A .…”

“…Variants of these pigments called eumelanins also have specialized functions within cephalopod ink . Generally, eumelanins are considered to consist of copolymerized (i.e., crosslinked) 5,6‐dihydroxyindole (DHI) and 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA) building blocks, as illustrated in Figure A . When obtained from different sources, these notoriously diverse biopolymers often diverge significantly in their chemical structures and feature distinct physical properties .…”

“…Outside of neuroscience, squid (and other cephalopods) are widely recognized as sources of numerous commercially valuable biopolymers, some of which have been shown to possess interesting electrical properties in vitro. For example, squid glands produce ink clouds, as illustrated in Figure , which help squid to evade potential predators and are composed in part of indole‐based natural pigments called eumelanins (Figure , top left) . Variants of these pigments were initially postulated to behave as electronic conductors over 50 years ago but were subsequently recategorized as mixed electronic–protonic conductors .…”