Melanin: A Greener Route To Enhance Energy Storage under Solar Light

Disclosing novel melanogenesis pathways: Formation of unexpected biphenyl-type dimers through radical-radical coupling by solid-state oxidation of the melanin biosynthetic precursor 5,6-dihydroxyindole

Investigation of the oxidation pathway of 5,6-dihydroxyindole (DHI), one of the main biosynthetic precursors of the brown-to-black skin and hair melanin pigments, represents a promising approach for the elucidation of the structure of these pigments in biological systems. We report herein the exploration of DHI oxidation chemistry under conditions so far poorly investigated, i.e. solid-state mechanochemical conditions, mimicking those that could be found in vivo in melanosomes, where melanin growth takes place in a confined space on a solid proteinaceous matrix, that allowed for the isolation and characterization of new dimers. Mechanistic experiments allowed to propose radical-radical coupling as the main dimerization pathway under solid-state conditions preventing ionic polymerization of the 5,6-dihydroxyindole system, indicating that the oxidation chemistry of this melanogenic precursor strongly depends on the reaction environment. The relevance for melanogenesis of the DHI oxidation pathway, disclosed herein, was also demonstrated by ad hoc experiments in which the solid-state reaction was carried out in the presence of proteins. Finally, the chromophores of the species generated by oxidation of the new dimers were investigated with a view to expanding the knowledge on the functional properties of melanin pigments, including mainly photoprotection.

Heterogenous Copper(0)-Assisted Dopamine Oxidation: A New Pathway to Controllable and Scalable Polydopamine Synthesis

In this study, we introduce an approach for synthesizing polydopamine (PDA) through the controlled oxidation of dopamine using metallic copper. Traditional methods of PDA synthesis often encounter challenges such as scalability, reproducibility, and control over polymerization. Our approach utilizes the catalytic properties of metallic copper in the presence of dissolved oxygen to generate reactive oxygen species (ROS) without additional chemicals. This process allows for precise control over dopamine oxidation, leading to reliable, materials and cost-effective upscalable PDA production. We investigated the reaction kinetics and the role of copper and ROS in dopamine oxidation, using several different experimental techniques. Our results demonstrate that, even at low pH, the copper-assisted method produces PDA with properties comparable to those synthesized through conventional means. We propose a mechanism for PDA synthesis that is initiated by oxygen adsorption onto copper surface, leading to the generation of various ROS which act as oxidizing agents in PDA synthesis. This method presents an advancement in the scalable and controlled production of PDA, with potential applications in various scientific and industrial fields.

Melanin: Nature's 4th bioorganic polymer

The pigments known as the melanins are widely recognized for their responsibility in the coloration of human skin, eyes, hair, and minimising the harmful effects of solar ultraviolet radiation. But specialists are aware that the melanins are present in all living kingdoms, barring viruses, and have functionality that extends beyond neutralizing ionising radiation. The ubiquitous presence of melanin in almost all human organs, recognized in recent years, as well as the presence of melanin in organisms that are evolutionarily distant from each other, indicate the fundamental importance of this class of material for all life forms. In this review, we argue for the need to accept melanins as the fourth primordial class of biological polymers, along with nucleic acids, proteins and polysaccharides. We consistently compare the properties of these canonical biological polymers with the properties of melanin and highlight key features that fundamentally distinguish melanins, their function and its mysteries.

First-Principles Investigation of the Optical Properties of Eumelanin Protomolecules

Using first-principles calculations, we investigate the absorption spectra (in the near-infrared, visible, and first UV range) of the two most probable eumelanin tetrameric molecules exhibiting either a linear open-chain or a cyclic porphyrine-like configuration. In order to simulate a realistic molecular system, an implicit solvent model is used in our calculations to mimic the effect of the solvated environment around the eumelanin molecule. Although the presence of solvent is found not to significantly affect the absorption pattern of both molecules, the onset of the spectra are shifted toward higher energies, especially for the linear tetramer. Interestingly, the absorption spectra and optical onsets of the two molecules differ significantly both in a vacuum and in ethanol. However, the two predicted spectra do not allow us to definitely discriminate between the two configurations when comparing the theoretical predictions with the available experimental spectrum. In addition, a mix of the two eumelanin configurations (close to fifty-fifty) leads to a maximum overlap between theoretical and experimental spectra. Consequently, this theoretical research shows that deeper insight can be gained using beyond DFT techniques on the real form of eumelanin protomolecules present in living systems as well as on their possible use in hybrid solar cells.

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.

Biosourced quinones for high-performance environmentally benign electrochemical capacitors via interface engineering

Biosourced and biodegradable organic electrode materials respond to the need for sustainable storage of renewable energy. Here, we report on electrochemical capacitors based on electrodes made up of quinones, such as Sepia melanin and catechin/tannic acid (Ctn/TA), solution-deposited on carbon paper engineered to create high-performance interfaces. Sepia melanin and Ctn/TA on TCP electrodes exhibit a capacitance as high as 1355 mF cm-2 (452 F g-1) and 898 mF cm-2 (300 F g-1), respectively. Sepia melanin and Ctn/TA symmetric electrochemical capacitors operating in aqueous electrolytes exhibit up to 100% capacitance retention and 100% coulombic efficiency over 50,000 and 10,000 cycles at 150 mA cm-2 (10 A g-1), respectively. Maximum power densities as high as 1274 mW cm-2 (46 kW kg-1) and 727 mW cm-2 (26 kW kg-1) with maximum energy densities of 0.56 mWh cm-2 (20 Wh kg-1) and 0.65 mWh cm-2 (23 Wh kg-1) are obtained for Sepia melanin and Ctn/TA.

Influence of a Polyneurotransmitter on DNA-Mediated Förster-Based Resonance Energy Transfer: A Path Leading to White Light Generation

The physiologically important biomolecule, dopamine (DA), shows strong self-oxidation and aggregation behaviors, which have been controlled and modulated to result in fluorescent polydopamine (F-PDA) nanoparticles. On the other hand, the simultaneous binding of two diverse deoxyribonucleic acid (DNA) binding probes, 4',6-diamidino-2-phenylindole dihydrochloride (DAPI) and ethidium bromide (EtBr), has been elaborately established to follow the Förster-based resonance energy transfer (FRET) pathway. The comparative understanding of this DNA-mediated FRET in three media, phosphate buffer saline (PBS) of pH 7.4, DA, and F-PDA, has concluded that the FRET efficiency in the three media follows the order: PBS > DA > F-PDA. This controlled FRET in the fluorescent F-PDA matrix serves a pivotal role for efficient white light (WL) generation with excellent Commission Internationale de l'Eclairage (CIE) parameters that match well with that of pure WL emission. The obtained WL emission has been shown to be very specific with respect to concentrations of different participating components and the excitation wavelength of the illuminating source. Furthermore, the optical properties of the WL emitting solution have been observed to be retained excellently inside the well-known agarose gel matrix. Finally, the mechanistic pathway behind such a FRET-based WL generation has been established in detail, and to the best of our knowledge, the current study offers the first and only report that discloses the influence of a fluorescent polyneurotransmitter matrix for successful generation of WL emission.

Solid-State Electrochemical Energy Storage Based on Soluble Melanin

Biocompatible and biodegradable powering materials are appealing systems for biomedical and electronic devices. Melanin is a natural and multifunctional material with redox capability, which is of great interest in electrochemical energy storage functionalities. In our work, we explored the use of soluble melanin derivatives as active materials for symmetric solid-state supercapacitors operating in the dark and under illumination. We observed that our devices were photo-pseudocapacitive. Additionally, under illumination, our best device showed a specific capacitance of 57.7 mFg−1 at a scan rate of 0.01 Vs−1, with a decrease of 53% in resistance compared to that in the dark. Our outcome suggests that soluble melanin is a promising material for solid-state powering elements in wearable and environmentally friendly devices.

Micropatterned Silk-Fibroin/Eumelanin Composite Films for Bioelectronic Applications

There has been growing interest in the use of natural bionanomaterials and nanostructured systems for diverse biomedical applications. Such materials can confer unique functional properties as well as address concerns pertaining to sustainability in production. In this work, we propose the biofabrication of micropatterned silk fibroin/eumelanin composite thin films to be used in electroactive and bioactive applications in bioelectronics and biomedical engineering. Eumelanin is the most common form of melanin, naturally derived from the ink of cuttlefish, having antioxidant and electroactive properties. Another natural biomaterial, the protein silk fibroin, is modified with photoreactive chemical groups, which allows the formation of electroactive eumelanin thin films with different microstructures. The silk fibroin/eumelanin composites are fabricated to obtain thin films as well as electroactive microstructures using UV curing. Here, we report for the first time the preparation, characterization, and physical, electrochemical, and biological properties of these natural silk fibroin/eumelanin composite films. Higher concentrations of eumelanin incorporated into the films exhibit a higher charge storage capacity and good electroactivity even after 100 redox cycles. In addition, the microscale structure and the cellular activity of the fibroin/eumelanin films are assessed for understanding of the biological properties of the composite. The developed micropatterned fibroin/eumelanin films can be applied as natural electroactive substrates for bioapplications (e.g., bioelectronics, sensing, and theranostics) because of their biocompatible properties.

New insight into melanin for food packaging and biotechnology applications

Melanin is a dark brown to black biomacromolecule with biologically active multifunctional properties that do not have a precise chemical structure, but its structure mainly depends on the polymerization conditions during the synthesis process. Natural melanin can be isolated from various animal, plant, and microbial sources, while synthetic melanin-like compounds can be synthesized by simple polymerization of dopamine. Melanin is widely used in various areas due to its functional properties such as photosensitivity, light barrier property, free radical scavenging ability, antioxidant activity, etc. It also has an excellent ability to act as a reducing agent and capping agent to synthesize various metal nanoparticles. Melanin nanoparticles (MNP) or melanin-like nanoparticles (MLNP) have the unique potential to act as functional materials to improve nanocomposite films' physical and functional properties. Various food packaging and biomedical applications have been made alone or by mixing melanin or MLNP. In this review, the general aspects of melanin that highlight biological activity, along with a description of MNP and the use as nanofillers in packaging films as well as reducing and capping agents and biomedical applications, were comprehensively reviewed.

Melanins as Sustainable Resources for Advanced Biotechnological Applications

Melanins are a class of biopolymers that are widespread in nature and have diverse origins, chemical compositions, and functions. Their chemical, electrical, optical, and paramagnetic properties offer opportunities for applications in materials science, particularly for medical and technical uses. This review focuses on the application of analytical techniques to study melanins in multidisciplinary contexts with a view to their use as sustainable resources for advanced biotechnological applications, and how these may facilitate the achievement of the United Nations Sustainable Development Goals.

Probing the heterogeneous structure of eumelanin using ultrafast vibrational fingerprinting

Eumelanin is a brown-black biological pigment with sunscreen and radical scavenging functions important to numerous organisms. Eumelanin is also a promising redox-active material for energy conversion and storage, but the chemical structures present in this heterogeneous pigment remain unknown, limiting understanding of the properties of its light-responsive subunits. Here, we introduce an ultrafast vibrational fingerprinting approach for probing the structure and interactions of chromophores in heterogeneous materials like eumelanin. Specifically, transient vibrational spectra in the double-bond stretching region are recorded for subsets of electronic chromophores photoselected by an ultrafast excitation pulse tuned through the UV-visible spectrum. All subsets show a common vibrational fingerprint, indicating that the diverse electronic absorbers in eumelanin, regardless of transition energy, contain the same distribution of IR-active functional groups. Aggregation of chromophores diverse in oxidation state is the key structural property underlying the universal, ultrafast deactivation behavior of eumelanin in response to photoexcitation with any wavelength.

Electronic Transport in the Biopigment Sepia Melanin

Eumelanin is the most common form of the pigment melanin in the human body, with diverse functions including photoprotection, antioxidant behavior, metal chelation, and free radical scavenging. Melanin also plays a role in melanoma skin cancer and Parkinson's disease. Sepia melanin is a natural eumelanin extracted from the ink sac of cuttlefish. Eumelanin is an ideal candidate to eco-design technologies based on abundant, biosourced, and biodegradable organic electronic materials to alleviate the environmental footprint of the electronics sector. Herein, the focus is on the reversible electrical resistive switching in dry and wet Sepia eumelanin pellets, pointing to the possibility of predominant electronic transport satisfying conditio sine qua non to develop melanin-based electronic devices. These findings shed light on the possibility to describe the transport physics of dry eumelanin using the amorphous semiconductor model. Results are of tremendous importance for the development of sustainable organic electronics.

Relation between Local Structure, Electric Dipole, and Charge Carrier Dynamics in DHICA Melanin: A Model for Biocompatible Semiconductors

Eumelanins are a family of natural and synthetic pigments obtained by oxidative polymerization of their natural precursors: 5,6-dihydroxyindole and its 2-carboxy derivative (DHICA). The simultaneous presence of ionic and electronic charge carriers makes these pigments promising materials for applications in bioelectronics. In this computational study we build a structural model of DHICA melanin considering the interplay between its many degrees of freedom, and then we examine the electronic structure of representative oligomers. We find that a nonvanishing dipole along the polymer chain sets this system apart from conventional polymer semiconductors, determining its electronic structure, reactivity toward oxidation and localization of the charge carriers. Our work sheds light on previously unnoticed features of DHICA melanin that not only fit well with its radical scavenging and photoprotective properties but also open new perspectives toward understanding and tuning charge transport in this class of materials.