Natural melanin pigments and their interfaces with metal ions and oxides: emerging concepts and technologies
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Prospective Article
Natural melanin pigments and their interfaces with metal ions and oxides: emerging concepts and technologies Eduardo Di Mauro, Ri Xu, Guido Soliveri, and Clara Santato, Department of Engineering Physics, Polytechnique Montréal, C.P. 6079, Succ. Centre-ville, Montréal, Québec, H3C 3A7, Canada Address all correspondence to Clara Santato at [email protected] (Received 1 March 2017; accepted 2 May 2017)
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.
Introduction The migration of human beings from the tropics to the rest of the globe has been marked by variation in the production of the brown–black melanin pigment of the skin. While hominins near the equator developed dark photoprotective melanin-rich pigmentation due to high ultraviolet (UV) radiation exposure, those settling in low-UVB (λ = 280–315 nm) environments developed depigmented skin with facultative pigmentation (tanning) to sustain the photosynthesis of vitamin D3. Melanin is thus a key factor in one of the most noticeable human polymorphisms: skin color.[1] Humans lacking melanocytes (melanin-producing cells) in the ear and animals with albinism (a deficit or absence of melanin) display hearing conditions as in domestic cats with white fur and blue eyes, thus suggesting a biofunctional role of melanin beyond photoprotection.[2, 3] In the melanin biopigments family, eumelanin is a brown– black type found in the human body, other mammals, reptiles, amphibians, and fishes as well as in invertebrates, such as cuttlefish and insects; Sepia melanin is a type of natural eumelanin extracted from the ink sac of cuttlefish. Pheomelanin is a yellowish-red melanin.[4, 5] Eumelanin has been intensively studied in recent decades for functional properties, such as UV–vis absorption, metal chelation, and free radical scavenging. It also features an antioxidant behavior. The limited solubility of eumelanin in most organic solvents has rendered challenging the understanding of its physicochemical properties.[4, 6] Here, after a brief introduction of molecular structural aspects of eumelanin biopigments and their effect on the properties of the pigment, we critically review the binding
properties of eumelanin toward metal cations and eumelanin– metal oxide interfaces. Eumelanin is involved in the accumulation and release of metal cations in the human body.[7
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