Electrochemical self-assembly of melanin films on gold

Characterization of melanin from Exophiala mesophila with the prospect of potential biotechnological applications

Introducion

Fungal melanin is an underexplored natural biomaterial of great biotechnological interest in different areas. This study investigated the physical, chemical, electrochemical, and metal-binding properties of melanin extracted from the metallotolerant black fungus Exophiala mesophila strain IRTA-M2-F10.

Materials and methods

Specific inhibitory studies with tricyclazole and biochemical profiling of whole cells by synchrotron radiation-based Fourier-transform infrared spectral microscopy (SR-FTIRM) were performed. An optimized extraction protocol was implemented, and purified fungal melanin was characterized using an array of spectrophotometric techniques (UV-Vis, FTIR, and EPR) and by cyclic voltammetry (CV) experiments. The metal-binding capacity of melanin extracts was also assessed by using Cr(VI) as a model heavy metal.

Results

Inhibitory studies indicated that 1,8-dihydroxynaphthalene may be the main precursor molecule of E. mesophila melanin (DHN-melanin). The biochemical characterization of fungal melanin extracts were benchmarked against those from two melanins comprising the precursor molecule L-3,4-dihydroxiphenylalanine (DOPA-melanin): extracts from the ink of the cephalopod Sepia officinalis and DOPA-melanin synthesized in the laboratory. The CV results of melanin extracts incubated with and without cell suspensions of the electroconductive bacterium Geobacter sulfurreducens were indicative of novel semiquinone/hydroquinone redox transformations specific for each melanin type. These interactions may play an important role in cation exchange for the adsorption of metals and in microbial interspecies electron transfer processes.

Discussion

The obtained results provided further evidence for the DHN-nature of E. mesophila melanin. The FTIR profiling of melanin extracts exposed to Cr(VI), compared to unexposed melanin, resulted in useful information on the distinct surface-binding properties of fungal melanin. The parameters of the Langmuir and Freundlicht isotherms for the adsorption of Cr(VI) were determined and compared to bibliographic data. Altogether, the inherent properties of fungal melanin suggest its promising potential as a biomaterial for environmental applications.

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, the What, the Why and the How: An Introductory Review for Materials Scientists Interested in Flexible and Versatile Polymers

Today, western society is facing challenges to create new medical technologies to service an aging population as well as the ever-increasing e-waste of electronic devices and sensors. A key solution to these challenges will be the use of biomaterials and biomimetic systems. One material that has been receiving serious attention for its biomedical and device applications is eumelanin. Eumelanin, or commonly known as melanin, is nature's brown-black pigment and is a poly-indolequinone biopolymer, which possess unique physical and chemical properties for material applications. Presented here is a review, aimed at polymer and other materials scientists, to introduce eumelanin as a potential material for research. Covered here are the chemical and physical structures of melanin, an overview of its unique physical and chemical properties, as well as a wide array of applications, but with an emphasis on device and sensing applications. The review is then finished by introducing interested readers to novel synthetic protocols and post synthesis fabrication techniques to enable a starting point for polymer research in this intriguing and complex material.

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.

Nature-derived materials for the fabrication of functional biodevices

Nature provides an incredible source of inspiration, structural concepts, and materials toward applications to improve the lives of people around the world, while preserving ecosystems, and addressing environmental sustainability. In particular, materials derived from animal and plant sources can provide low-cost, renewable building blocks for such applications. Nature-derived materials are of interest for their properties of biodegradability, bioconformability, biorecognition, self-repair, and stimuli response. While long used in tissue engineering and regenerative medicine, their use in functional devices such as (bio)electronics, sensors, and optical systems for healthcare and biomonitoring is finding increasing attention. The objective of this review is to cover the varied nature derived and sourced materials currently used in active biodevices and components that possess electrical or electronic behavior. We discuss materials ranging from proteins and polypeptides such as silk and collagen, polysaccharides including chitin and cellulose, to seaweed derived biomaterials, and DNA. These materials may be used as passive substrates or support architectures and often, as the functional elements either by themselves or as biocomposites. We further discuss natural pigments such as melanin and indigo that serve as active elements in devices. Increasingly, combinations of different biomaterials are being used to address the challenges of fabrication and performance in human monitoring or medicine. Finally, this review gives perspectives on the sourcing, processing, degradation, and biocompatibility of these materials. This rapidly growing multidisciplinary area of research will be advanced by a systematic understanding of nature-inspired materials and design concepts in (bio)electronic devices.

Kinetics of Melanin Polymerization during Enzymatic and Nonenzymatic Oxidation

Melanin is an abundant biopigment in the animal kingdom, but its structure remains poorly understood. This is a substantial impediment to understanding the mechanistic origin of its observed functions. Proposed models of melanin structure include aggregates of both linear and macrocyclic units and noncovalently held monomers. Both models are broadly in agreement with current experimental data. To constrain the structural and kinetic models of melanin, experimental data of high resolution with chemical specificity accompanied by atomistic modeling are required. We have addressed this by obtaining electronic absorption, infrared, and ultraviolet resonance Raman (RR) spectra of melanin at several wavelengths of excitation that are sensitive to small changes in structure. From these experiments, we observed kinetics of the formation of different species en route to melanin polymerization. Exclusive chemical signatures of monomer 3,4-dihydroxyphenylalanine (dopa), intermediate dopachrome (DC), and early-time polymer are established through their vibrational bands at 1292, 1670, and 1616 cm^-1 respectively. Direct evidence of reduced heterogeneity of melanin oligomers in tyrosinase-induced formation is provided from experimental measurements of vibrational bandwidths. Models made with density functional theory show that the linear homopolymeric structures of 5,6-dihydroxyindole can account for experimentally observed wavenumbers and broad bandwidth in Raman spectra of dopa-melanin. We capture resonance Raman (RR) signature of DC, the intermediate stabilized by the enzyme tyrosinase, for the first time in an enzyme-assisted melanization reaction using 488 nm excitation wavelength and propose that this wavelength can be used to probe reaction intermediates of melanin formation in solution.

Polydopamine and eumelanin models in various oxidation states

The most stable molecular structures of PDA and eumelanin are proposed.

Polydopamine and eumelanin molecular structures investigated with ab initio calculations

A set of computational methods that contains a brute-force algorithmic generation of chemical isomers, molecular dynamics (MD) simulations, and density functional theory (DFT) calculations is reported and applied to investigate nearly 3000 probable molecular structures of polydopamine (PDA) and eumelanin. All probable early-polymerized 5,6-dihydroxyindole (DHI) oligomers, ranging from dimers to tetramers, have been systematically analyzed to find the most stable geometry connections as well as to propose a set of molecular models that represents the chemically diverse nature of PDA and eumelanin. Our results indicate that more planar oligomers have a tendency to be more stable. This finding is in good agreement with recent experimental observations, which suggested that PDA and eumelanin are composed of nearly planar oligomers that appear to be stacked together via π-π interactions to form graphite-like layered aggregates. We also show that there is a group of tetramers notably more stable than the others, implying that even though there is an inherent chemical diversity in PDA and eumelanin, the molecular structures of the majority of the species are quite repetitive. Our results also suggest that larger oligomers are less likely to form. This observation is also consistent with experimental measurements, supporting the existence of small oligomers instead of large polymers as main components of PDA and eumelanin. In summary, this work brings an insight into the controversial structure of PDA and eumelanin, explaining some of the most important structural features, and providing a set of molecular models for more accurate modeling of eumelanin-like materials.

Melanins and melanogenesis: from pigment cells to human health and technological applications

During the past decade, melanins and melanogenesis have attracted growing interest for a broad range of biomedical and technological applications. The burst of polydopamine-based multifunctional coatings in materials science is just one example, and the list may be expanded to include melanin thin films for organic electronics and bioelectronics, drug delivery systems, functional nanoparticles and biointerfaces, sunscreens, environmental remediation devices. Despite considerable advances, applied research on melanins and melanogenesis is still far from being mature. A closer intersectoral interaction between research centers is essential to raise the interests and increase the awareness of the biomedical, biomaterials science and hi-tech sectors of the manifold opportunities offered by pigment cells and related metabolic pathways. Starting from a survey of biological roles and functions, the present review aims at providing an interdisciplinary perspective of melanin pigments and related pathway with a view to showing how it is possible to translate current knowledge about physical and chemical properties and control mechanisms into new bioinspired solutions for biomedical, dermocosmetic, and technological applications.

Preparation of Thin Melanin-Type Films by Surface-Controlled Oxidation

The preparation of thin melanin films suitable for applications is challenging. In this work, we present a new alternative approach to thin melanin-type films using oxidative multilayers prepared by the sequential layer-by-layer deposition of cerium(IV) and inorganic polyphosphate. The interfacial reaction between cerium(IV) in the multilayer and 5,6-dihydroxyindole (DHI) in the adjacent aqueous solution leads to the formation of a thin uniform film. The oxidation of DHI by cerium(IV) proceeds via known melanin intermediates. We have characterized the formed DHI-melanin films using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), UV-vis spectroscopy, and spectroelectrochemistry. When a five-bilayer oxidative multilayer is used, the film is uniform with a thickness of ca. 10 nm. Its chemical composition, as determined using XPS, is typical for melanin. It is also redox active, and its oxidation occurs in two steps, which can be assigned to semiquinone and quinone formation within the indole structural motif. Oxidative multilayers can also oxidize dopamine, but the reaction stops at the dopamine quinone stage because of the limited amount of the multilayer-based oxidizing agent. However, dopamine oxidation by Ce(IV) was studied also in solution by UV-vis spectroscopy and mass spectrometry in order to verify the reaction mechanism and the final product. In solution, the oxidation of dopamine by cerium shows that the indole ring formation takes place already at low pH and that the mass spectrum of the final product is practically identical with that of commercial melanin. Therefore, layer-by-layer formed oxidative multilayers can be used to deposit functional melanin-type thin films on arbitrary substrates by a surface-controlled reaction.

Evidence of Porphyrin-Like Structures in Natural Melanin Pigments Using Electrochemical Fingerprinting

Eumelanins are extended heterogeneous biopolymers composed of molecular subunits with ambiguous macromolecular topology. Here, an electrochemical fingerprinting technique is described, which suggests that natural eumelanin pigments contain indole-based tetramers that are arranged into porphyrin-like domains. Spectroscopy and density functional theory calculations suggest that sodium ions undergo occupancy-dependent stepwise insertion into the core of porphyrin-like tetramers in natural eumelanins at discrete potentials.

Excitonic effects from geometric order and disorder explain broadband optical absorption in eumelanin

Eumelanin is a ubiquitous biological pigment, and the origin of its broadband absorption spectrum has long been a topic of scientific debate. Here, we report a first-principles computational investigation to explain its broadband absorption feature. These computations are complemented by experimental results showing a broadening of the absorption spectra of dopamine solutions upon their oxidation. We consider a variety of eumelanin molecular structures supported by experiments or theoretical studies, and calculate the absorption spectra with proper account of the excitonic couplings based on the Frenkel exciton model. The interplay of geometric order and disorder of eumelanin aggregate structures broadens the absorption spectrum and gives rise to a relative enhancement of absorption intensity at the higher-energy end, proportional to the cube of absorption energy. These findings show that the geometric disorder model is as able as the chemical disorder model, and complements this model, to describe the optical properties of eumelanin.

Characterization of polydopamine thin films deposited at short times by autoxidation of dopamine

Current interest in melanin films derived from the autoxidation of dopamine stems from their use as a universal adhesion layer. Here we report chemical and physical characterization of polydopamine films deposited on gold surfaces from stirred basic solutions at times ranging from 2 to 60 min, with a focus on times ≤10 min. Data from Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and electrochemical methods suggest the presence of starting (dopamine) and intermediate (C=N-containing tautomers of quinone and indole) species in the polydopamine films at all deposition times. A uniform overlayer analysis of the XPS data indicates that film thickness increased linearly at short deposition times of ≤10 min. At deposition times ≥10 min, the films appeared largely continuous with surface roughness ≈ ≤ 2 nm, as determined by atomic force microscopy (AFM). Pinhole-free films, as determined by anionic redox probe measurements, required deposition times of 60 min or greater.

Biomaterials-based electronics: polymers and interfaces for biology and medicine

Advanced polymeric biomaterials continue to serve as a cornerstone for new medical technologies and therapies. The vast majority of these materials, both natural and synthetic, interact with biological matter in the absence of direct electronic communication. However, biological systems have evolved to synthesize and utilize naturally-derived materials for the generation and modulation of electrical potentials, voltage gradients, and ion flows. Bioelectric phenomena can be translated into potent signaling cues for intra- and inter-cellular communication. These cues can serve as a gateway to link synthetic devices with biological systems. This progress report will provide an update on advances in the application of electronically active biomaterials for use in organic electronics and bio-interfaces. Specific focus will be granted to covering technologies where natural and synthetic biological materials serve as integral components such as thin film electronics, in vitro cell culture models, and implantable medical devices. Future perspectives and emerging challenges will also be highlighted.

Synthesis and assembly of fungal melanin

Melanin is a unique pigment with myriad functions that is found in all biological kingdoms. It is multifunctional, providing defense against environmental stresses such as ultraviolet (UV) light, oxidizing agents and ionizing radiation. Melanin contributes to the ability of fungi to survive in harsh environments. In addition, it plays a role in fungal pathogenesis. Melanin is an amorphous polymer that is produced by one of two synthetic pathways. Fungi may synthesize melanin from endogenous substrate via a 1,8-dihydroxynaphthalene (DHN) intermediate. Alternatively, some fungi produce melanin from L-3,4-dihydroxyphenylalanine (L-dopa). The detailed chemical structure of melanin is not known. However, microscopic studies show that it has an overall granular structure. In fungi, melanin granules are localized to the cell wall where they are likely cross-linked to polysaccharides. Recent studies suggest the fungal melanin may be synthesized in internal vesicles akin to mammalian melanosomes and transported to the cell wall. Potential applications of melanin take advantage of melanin's radioprotective properties and propensity to bind to a variety of substances.

"Naked" gold nanoparticles supported on HOPG: melanin functionalization and catalytic activity

Reductive electrodesorption has been used to produce "naked" gold nanoparticles (AuNPs) 3 nm in size on HOPG from different thiolate-capped AuNPs. The clean AuNPs transform the electrocatalytic inert HOPG into an active surface for hydrogen peroxide electroreduction, causing a lowering of the cathodic overpotential of 0.25 V with respect to the Au(111) surface. Compared to the plain gold substrates, the nanostructures promote only a slight increase in the hydrogen evolution reaction. In a second modification step a ∼1 nm thick melanin-iron coating is electrochemically formed around the AuNPs. This ultrathin melanin-iron coating largely improves the catalytic activity of the bare AuNPs for both hydrogen peroxide electroreduction and hydrogen evolution reaction. This strategy, which integrates electrochemistry and nanotechnology, can be applied to the preparation of efficient "naked" AuNPs and organic-iron capped AuNPs catalysts.

Structural, electrical, electronic and optical properties of melanin films

We present thick, uniform and rather flat melanin films obtained using spray deposition. The morphology of the films was investigated using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Temperature-dependent electrical resistance of melanin thin films evidenced a semiconductor-like character and a hysteretic behavior linked to an irreversible process of water molecule desorption from the melanin film. X-ray Photoelectron Spectroscopy (XPS) was carried out to analyze the role of the functional groups in the primary and secondary structure of the macromolecule, showing that the contribution of the 5,6-dihydroxyindole-2-carboxylic acid (DHICA) subunit to the molecule is about 35%. Comparison of the optical absorption of the thick (800nm) and thin (80nm) films showed a spectral change when the thickness increases. From in vacuum photoconductivity (PC) measured at controlled temperatures, we suggest that the melanin films exhibit a possible charge transport mechanism by means of delocalized pi states along the stacked planar secondary structure.

Chemical and structural diversity in eumelanins: unexplored bio-optoelectronic materials

Eumelanins, the characteristic black, insoluble, and heterogeneous biopolymers of human skin, hair, and eyes, have intrigued and challenged generations of chemists, physicists, and biologists because of their unique structural and optoelectronic properties. Recently, the methods of organic chemistry have been combined with advanced spectroscopic and imaging techniques, theoretical calculations, and methods of condensed-matter physics to gradually force these materials to reveal their secrets. Herein we review the latest advances in the field with a view to showing how the emerging knowledge is not only helping to explain eumelanin functionality, but may also be translated into effective strategies for exploiting their properties to create a new class of biologically inspired high-tech materials.

Theoretical models of eumelanin protomolecules and their optical properties

The molecular structure of melanin, one of the most ubiquitous natural pigments in living organisms, is not known and its multifaceted biological role is still debated. We examine structural models for eumelanin protomolecules, based on tetramers consisting of four monomer units (hydroquinone, indolequinone, and its two tautomers), in arrangements that contain an interior porphyrin ring. These models reproduce convincingly many aspects of eumelanin's experimentally observed behavior. In particular, we present a plausible synthetic pathway of the tetramers and their further complexation through interlayer stacking, or through formation of helical superstructures, into eumelanin macromolecules. The unsaturated nature of C-C bonds in indolequinone units and the finite size of protomolecules introduce covalent bond formation between stacked layers. We employ time-dependent density functional theory to calculate the optical absorption spectrum of each molecule along the eumelanin synthesis pathway, which gradually develops into the characteristic broad-band adsorption of melanin pigment due to electron delocalization. These optical spectra may serve as signatures for identifying intermediate structures.

Picogram detection of metal ions by melanin-sensitized piezoelectric sensor

A heavy metal ion sensor was constructed by cross-linking melanin onto the gold electrode of quartz crystal microbalance (QCM). A mercury ion sensitivity of 518+/-37 Hz/ppm was observed, a substantial increase in sensitivity compared to previous reports of 10-50 Hz/ppm with the limit of detection at 5 ppb. Detection of other metal ions including Sn(2+), Ge(4+), Li(+), Zn(2+), Cu(2+), Bi(3+), Co(2+), Al(3+), Ni(2+), Ag(+), and Fe(3+) were also performed. Unexpectedly, binding of Mn(7+), Pb(2+), Cd(2+), and Cr(3+) increased resonant frequencies. The surface profile of melanin thin film upon binding to metal ions was investigated by atomic force microscopy (AFM). Structural change of melanin upon binding to metal ions was characterized by circular dichroism and by infrared spectroscopy. The current study provides the first example of melanin-coated piezoelectric sensor showing high sensitivity and selectivity to metal ions.

Electrocatalytic and Magnetic Properties of Ultrathin Nanostructured Iron–Melanin Films on Au(111)

We have prepared ultrathin, nanostructured melanin films on Au(111) by means of electrochemical self-assembly. These films were characterized by using Auger electron spectroscopy, X-ray absorption near-edge structure spectroscopy, scanning tunneling microscopy, magnetic force microscopy, and electrochemical techniques. Two types of nanostructures are present in the film: melanin nanoparticles and Fe(3)O(4) nanoparticles. The melanin nanoparticles contain Fe bonded to oxygen-containing phenolic groups in an octahedral configuration similar to that found in Fe(2)O(3). The inorganic-organic composite exhibits magnetic properties and catalyzes the electroreduction of hydrogen peroxide in alkaline and neutral electrolyte solutions. The electrocatalytic activity depends on the Fe-bound melanin and appears to be similar to that found for Fe-porphyrins.