Bioinspired Melanin‐Based Optically Active Materials

Abstract

the properties and functions of melanin, the synthesis of melanin-like nanoparticles, and its versatile applications in catalysis, energy, and biomedical fields. Over the last 5 years, more than 1000 papers on melanin have been published annually, based on data from the Web of Science. Quite a few of these papers have reviewed melanin's chemical structure, physiochemical properties, and biomedical applications. [4-6] However, a critical review focusing on optical functions of melanin is still lacking, despite tremendous emerging work on melaninbased photonic materials. The purpose of this review is to summarize current advances in bioinspired melanin-based optically active materials. Specifically, we will cover inherent optical properties of melanin, and then summarize melanin's optical functions in nature and its optics-related applications. 2. Melanin Structure and Optical Properties Melanin has complex chemical structures that is not yet fully understood, however, it does not hinder the exploration of their optical properties. In this section, we will first discuss classifications of melanin and then summarize unique optical properties of melanin, which will be important for both understanding its biological function in nature and synthetic applications. 2.1. Classification and Structure Conventionally, all black pigments are called melanin. Recently, d'Ischia et al. suggested that any phenolic polymers that have broadband light absorption, antioxidant, and intrinsic free radical character should be characterized as melanin. [5] Exact chemical structures of melanin remain elusive, mainly due to their insolubility in solvents, close binding with other cellular tissues, and an amorphous structure. Here, we do not elaborate on the complexities of melanin's chemical structures, [7] but rather focus on its classification. Melanin can be categorized as either natural or synthetic depending on whether it was synthesized in vivo or in vitro. Both have similar macroscopic properties, but their structures are likely quite different. Natural melanin, including eumelanin, pheomelanin, and neuromelanin, is produced from a series of enzymatic reactions starting from L-tyrosine in biological systems (Figure 1). [1] In cells (commonly melanocytes), the tyrosinase oxidizes L-tyrosine to dopaquinone (DQ). If cysteine concentration in Melanin is a widespread multifunctional biological pigment that has emerged as a promising platform for applications in coating, catalysis, energy, drug delivery, and medical therapy. Melanin is also a compelling material for photonic applications because of its favorable material characteristics, including broadband absorption, high refractive index, tunable fluorescence, and UV blocking capabilities. However, there is not yet a critical review focusing on optical functions of melanin. This review summarizes current advances in bioinspired melanin-based optically active materials, covering melanin's inherent optical properties and functions both in nature and in optics-related applications. It is ...

Progress in polydopamine-based melanin mimetic materials for structural color generation

“…Artificial melanin materials based on PDA were created by mimicking natural melanin, which is important for vivid structural coloration in nature [ 98 , 122 , 123 ]. By constructing a microstructure with black artificial melanin materials or incorporating them into the microstructure, the reflection and absorption of light were properly controlled, and highly visible structural coloration was realized.…”

Melanins as Sustainable Resources for Advanced Biotechnological Applications

“…Melanins (and analogues thereof) have the potential for involvement in each of the different industry sectors across the world: [ 30,424,426,617–622 ] their production, extraction, and purification (e.g., agricultural/fermentation/insect/marine sources, potentially involving blue/brown/gray/green/white/yellow biotechnologies) [ 619,623 ] is dealt with by the primary economic sector; their use in manufacturing products (e.g., electronics, plastics, textiles, potentially involving red/white/yellow biotechnologies) [ 619,624 ] is dealt with by the secondary economic sector; the tertiary sector involves services (e.g., distribution of melanin‐containing products, healthcare (e.g., diagnosis/therapy), appropriate waste management/recycling of melanin‐containing products, potentially involving red/white/yellow biotechnologies); the quaternary sector (e.g., education (by using melanin in interdisciplinary teaching in higher education), research and development in academic/industry settings, potentially involving all types of biotechnologies); and the quinary sector, specialized services delivered by the highest level of government/industry decision/policy makers (potentially involving purple/violet biotechnologies), such as utilization of natural resources such as melanins to demonstrate commitment to corporate social responsibilities, or to achieve the UN SDGs (potentially involving all forms of biotechnologies). The 17 UN SDGs aim to end poverty, protect the planet, and ensure all people enjoy peace and prosperity by 2030.…”

“…There is also significant potential for melanins as sustainably sourced feedstocks/materials for technical applications, for example, as coatings, [ 37 ] as dyes for textiles, [ 624,632–636 ] for application in environmental remediation (e.g., heavy metal, nuclear contamination/waste, etc. ), [ 629,637 ] and as sustainable components of electronic products (as emphasized in Figure 6) due to their sustainable nature and prospects for biodegradation at the end of their useful lifetime.…”

“…), [ 629,637 ] and as sustainable components of electronic products (as emphasized in Figure 6) due to their sustainable nature and prospects for biodegradation at the end of their useful lifetime. [ 37,424,426,503,617–620,638–641 ] It is noteworthy that in 2017, 94 billion tonnes of resources were extracted worldwide and this is forecast to rise to 184 billion tonnes by 2050, which is 400% more than the Earths capacity. [ 642 ] This overextraction and the associated practices cannot be sustained.…”

“…While the name melanin was initially applied to black pigments, it has subsequently been used to describe pigments of colors from black/brown eumelanins [ 17 ] to red/yellow pheomelanins, [ 18,19 ] all of which play a role in skin pigmentation, [ 20 ] in combination with carotenoids, hemoglobin, etc. [ 17,21 ] Melanins have a variety of functions in nature (from photoprotection to photosensitization, [ 20,22 ] antioxidant defense, and metal/drug binding), [ 23–26 ] which reflect a unique combination of chemical, electrical, optical, and paramagnetic properties, [ 27–30 ] and their properties have resulted in their application in materials science for a range of historical, [ 31 ] medical, and technical applications. [ 32–37 ]…”