Metal binding by melanins: studies of colloidal dihydroxyindole-melanin, and its complexation by Cu(II) and Zn(II) ions

Multifunctional Natural and Synthetic Melanin for Bioelectronic Applications: A Review

Emerging material interest in bioelectronic applications has highlighted natural melanin and its derivatives as promising alternatives to conventional synthetic conductors. These materials, traditionally noted for their adhesive, antioxidant, biocompatible, and biodegradable properties, have barely been used as conductors due to their extremely low electrical activities. However, recent studies have demonstrated good conductive properties in melanin materials that promote electronic-ionic hybrid charge transfer, attributed to the formation of an extended conjugated backbone. This review examines the multifunctional properties of melanin materials, focusing on their chemical and electrochemical synthesis and their resulting structure-property-function relationship. The wide range of bioelectronic applications will also be presented to highlight their importance and potential to expand into new design concepts for high-performance electronic functional materials. The review concludes by addressing the current challenges in utilizing melanin for biodegradable bioelectronics, providing a perspective on future developments.

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.

Novel melanin-derived stationary phase for immobilized metal ion affinity chromatography in recombinant His-tagged protein purification

The matrix of the stationary phase is a crucial element in affinity chromatography for protein purification. Various materials, including polymer or magnetic materials, have been employed as the matrix in the purification of His-tagged protein. Here, for the first time, we utilized a combination of melanin and alginate, both natural polymer materials, to synthesize Ni-melanin/alginate (Ni-M/A) beads for His-tagged protein purification. We investigated the binding of His-tagged Mpro on the Ni-M/A beads, referred to as Ni-M/A-Mpro, and assessed the elution efficiency of Mpro from the beads. Our examination involved FTIR, EDS, XRD, SDS-PAGE, and Western blotting methods. FTIR spectra revealed notable changes in the stretching patterns and intensities of hydroxyl, amine, carbonyl, imine and amide chemical groups, when Mpro protein was present in the Ni-M/A sample. XRD spectra demonstrated the occurrence of two Nickel peaks at 35-40 deg and 40-45 deg in Ni-M/A, but only one nickel peak at 35-40 deg in Ni-M/A-Mpro, indicating the binding of Mpro on the Nickel ions. EDS analysis reported a decrease in the concentration of Nickel on the surface of Ni-M/A from 16% to 7% when Mpro protein was loaded into the stationary phase. Importantly, our data indicated that the purity of the His-tagged protein Mpro after purification reached 97% after just one-step purification using the Ni-M/A stationary phase. Moreover, the binding capacity of Ni-M/A for Mpro was approximately 5.2 mg/g with recovery efficiency of 40%. Our results suggested Ni-M/A as a highly potential solid phase for affinity chromatography in the purification of His-tagged protein.

Redox modulation of oxidatively-induced DNA damage by ascorbate enhances both in vitro and ex-vivo DNA damage formation and cell death in melanoma cells

Elevated genomic instability in cancer cells suggests a possible model-scenario for their selective killing via the therapeutic delivery of well-defined levels of further DNA damage. To examine this scenario, this study investigated the potential for redox modulation of oxidatively-induced DNA damage by ascorbate in malignant melanoma (MM) cancer cells, to selectively enhance both DNA damage and MM cell killing. DNA damage was assessed by Comet and ɣH2AX assays, intracellular oxidising species by dichlorofluorescein fluorescence, a key antioxidant enzymatic defence by assessment of catalase activity and cell survival was determined by clonogenic assay. Comet revealed that MM cells had higher endogenous DNA damage levels than normal keratinocytes (HaCaT cells); this correlated MM cells having higher intracellular oxidising species and lower catalase activity, and ranked with MM cell melanin pigmentation. Comet also showed MM cells more sensitive towards the DNA damaging effects of exogenous H2O2, and that ascorbate further enhanced this H2O2-induced damage in MM cells; again, with MM cell sensitivity to induced damage ranking with degree of cell pigmentation. Furthermore, cell survival data indicated that ascorbate enhanced H2O2-induced clonogenic cell death selectively in MM cells whilst protecting HaCaT cells. Finally, we show that ascorbate serves to enhance the oxidising effects of the MM therapeutic drug Elesclomol in both established MM cells in vitro and primary cell cultures ex vivo. Together, these results suggest that ascorbate selectively enhances DNA damage and cell-killing in MM cells. This raises the option of incorporating ascorbate into clinical oxidative therapies to treat MM.

Selective sensing of catechol based on a fluorescent nanozyme with catechol oxidase activity

Nanozymes, an unusual category of nanomaterials possessing enzymatic properties, and have generated considerable interest regarding their application feasibilities on several important fronts. In the present work, an innovative sensing device for catechol was established ground on a fluorescent nanozyme (Cu-BDC-NH2) that exhibited catechol oxidase activity. The fluorescent nanozyme combines both functions of catechol recognition and response signal output, and can realize the sensing of catechol without the addition of other chromogenic agents. In the existence of Cu-BDC-NH2, catechol can be oxidized efficiently to produce quinones or polymers with strong electron absorption capacity, which immediately results in efficient fluorescence quenching of Cu-BDC-NH2. However, other common phenolic compounds, such as phenol, the other two diphenols (hydroquinone and resorcinol), phloroglucinol, and chlorophenol, do not result in efficient fluorescence quenching of Cu-BDC-NH2. The method shows a nice linear relationship between catechol concentration prep the fluorescence intensity of Cu-BDC-NH2 in the scope of 0-10 μM, with a detection limit of 0.997 μM. The detection of catechol in actual water samples has also achieved the satisfactory consequences, which provides a new strategy for the convenient and selective detection of catechol.

Comparative analysis of the jaw apparatus of three marine annelids using scanning electron microscopy: Microstructure and elemental composition

Polychaeta are highly diversified invertebrates that inhabit marine, brackish or freshwater environments. They have acquired a unique range of adaptative features for securing food. However, the jaw apparatus may reveal not only defence and predation mechanisms, but also its relation to environmental chemistry. The present work compared the structure and chemical profile of the jaws of different estuarine Polychaeta: Nephtys hombergii (Nephtyidae), Hediste diversicolor (Nereididae) and Glycera alba (Glyceridae) using Scanning Electron Microscopy (SEM) and Scanning Electron Microscopy with Energy Dispersive X-Ray (SEM-EDX). Analyses revealed that N. hombergii possesses a muscular jawless proboscis with terminal sensorial papillae for detecting prey, whereas the G. alba proboscis exhibits four delicately sharp jaws with perforations for venom delivery and H. diversicolor bears two blunt denticulated jaws to grasp a wide variety of food items. Melanin and metals like copper provide hardness to the slender jaws of Glycera, while, in the absence of heavier metallic elements, halogens contribute to H. diversicolor jaws robustness. The more specific chemistry of the jaws of glycerids is associated with its more refined venom injection, whereas Hediste is an opportunistic omnivore and Nepthys an agile forager. Altogether, the chemistry of jaws is an adaptive feature for feeding, locomotion and even resilience to complex and often adverse chemical profiles of estuaries.

Recent Advances and Progress on Melanin: From Source to Application

Melanin is a biological pigment formed by indoles and phenolic compounds. It is widely found in living organisms and has a variety of unique properties. Due to its diverse characteristics and good biocompatibility, melanin has become the focus in the fields of biomedicine, agriculture, the food industry, etc. However, due to the wide range of melanin sources, complex polymerization properties, and low solubility of specific solvents, the specific macromolecular structure and polymerization mechanism of melanin remain unclear, which significantly limits the further study and application of melanin. Its synthesis and degradation pathways are also controversial. In addition, new properties and applications of melanin are constantly being discovered. In this review, we focus on the recent advances in the research of melanin in all aspects. Firstly, the classification, source, and degradation of melanin are summarized. Secondly, a detailed description of the structure, characterization, and properties of melanin is followed. The novel biological activity of melanin and its application is described at the end.

Iron and copper ions accelerate and modify dopamine oxidation to eumelanin: implications for neuromelanin genesis

Dopamine (DA) is a precursor of neuromelanin (NM) synthesized in the substantia nigra of the brain. NM is known to contain considerable levels of Fe and Cu. However, how Fe and Cu ions affect DA oxidation to DA-eumelanin (DA-EM) and modify its structure is poorly understood. EMs were prepared from 500 µM DA, dopaminechrome (DAC), or 5,6-dihydroxyindole (DHI). Autoxidation was carried out in the absence or presence of 50 µM Fe(II) or Cu(II) at pH 7.4 and 37 ℃. EMs were characterized by Soluene-350 solubilization analyzing absorbances at 500 nm (A500) and 650 nm (A650) and alkaline hydrogen peroxide oxidation (AHPO) yielding various pyrrole carboxylic acids. Pyrrole-2,3,4,5-tetracarboxylic acid (PTeCA) served as a molecular marker of cross-linked DHI units. Importantly, Fe and Cu accelerated DA oxidation to DA-EM and DHI oxidation to DHI-EM several-fold, whereas these metals only weakly affected the production of DAC-EM. The A500 values indicated that DA-EM contains considerable portions of uncyclized DA units. Analysis of the A650/A500 ratios suggests that Fe and Cu caused some degradation of DHI units of DA-EM during 72-h incubation. Results with AHPO were consistent with the A500 values and additionally revealed that (1) DA-EM is less cross-linked than DAC-EM and DHI-EM and (2) Fe and Cu promote cross-linking of DHI units. In conclusion, Fe and Cu not only accelerate the oxidation of DA to DA-EM but also promote cross-linking and degradation of DHI units. These results help to understand how Fe and Cu in the brain affect the production and properties of NM.

Fast and Reliable Synthesis of Melanin Nanoparticles with Fine-Tuned Metal Adsorption Capacities for Studying Heavy Metal Ions Uptake

Purpose

Adsorption and uptake of heavy metals by polymeric nanoparticles is driven by a variety of physicochemical processes. In this work, we examined heavy metal uptake by synthetic melanin nanoparticles and analyzed physicochemical properties that affect the extent of metal uptake by the nanoparticles.

Methods

Eumelanin nanoparticles were synthesized in a one-pot fast process from a 5,6-diacetoxy indole precursor that is hydrolyzed in situ into dihydroxy indole (DHI). The method allows the possibility of changing the level of sodium ions that ends up in the nanoparticles. Two variants of synthetic DHI–melanin (low-sodium and high sodium variants) were evaluated and demonstrated different relative adsorption efficiencies for heavy metal cations.

Results and Discussion

For the low-sodium DHI–melanin and in terms of percentages of metal ion removal, the relative order of extraction from 50 ppm solutions was Zn²⁺ > Cd²⁺ > Ni²⁺ > Co²⁺ > Cu²⁺ > Pb²⁺, with the extraction percentages ranging from 90% down to 76%, for a 30-minute adsorption time before equilibrium. The lower-sodium DHI–melanin consistently removed more Zn²⁺ than the higher-sodium variant. Electron microscopy (SEM) showed an increase in melanin particle size after metal ions uptake. In addition, X-ray photoelectron spectroscopy (XPS) of DHI–melanin particles with depth profiling after Zn ions uptake supported particle swelling and ion transport within the particles.

Conclusion

These initial studies showed the potential of this straightforward synthesis to obtain synthetic DHI–melanin nanoparticles similar to those from biological sources with the possibility to fine-tune their metal adsorption capacity. These synthetic nanoparticles can be used either for the removal of a variety of metal ions or to mimic and study mechanisms of metal uptake by melanin deriving from biological sources, with the potential to understand, for instance, differential heavy metal uptake by various melanic pigments.

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.

Influence of secondary metabolites on surface chemistry and metal adsorption of a devitalized lichen biomonitor

Despite the broad use of lichens as biomonitors of airborne trace elements, the surface chemistry and metal adsorption parameters of these organisms are still poorly known. The current investigation is aimed at (i) quantifying the acid-base surface properties and the first-order physical-chemical parameters of Cu2+ and Zn2+ adsorption of devitalized Pseudevernia furfuracea, a lichen commonly used in biomonitoring of airborne trace elements, and (ii) comparing the results with those available for moss biomonitors. Equilibrium constants and metal-binding site concentrations were calculated with a thermodynamic model by taking into account the presence/absence of ancillary extracellular cell wall compounds, namely melanin and acetone-soluble lichen substances. An acid-base titration experiment performed in the pH range of 3-10 showed that melanised and non-melanised P. furfuracea samples have lower pHPZC (3.53-3.99) and higher metal-binding site concentrations (0.96-1.20 mmol g-1) compared to that of the mosses investigated so far at the same experimental conditions. Melanin biosynthesis increased the content of carboxyl and phosphoryl groups and reduces that of amine/polyphenols. Cu2+ and Zn2+ adsorption was unaffected by the degree of melanisation while the removal of extracellular lichen substances slightly decreased Zn2+ adsorption. Although Cu2+ and Zn2+ adsorption parameters related to P. furfuracea surfaces were 3 times lower than in the mosses, lichen samples adsorbed the same amount of Cu2+ and 30% more Zn2+. The present study contributes in understanding the role of ancillary cell wall compounds in Cu2+ and Zn2+ adsorption in a model lichen. It also provides a first comparison between the surface physico-chemical characteristics of lichens and mosses frequently used as biomonitors of trace elements.

Synthetic melanin nanoparticles as peroxynitrite scavengers, photothermal anticancer and heavy metals removal platforms

Melanin is a ubiquitous natural polyphenolic pigment with versatile applications including physiological functions. This polymeric material is found in a diversity of living organisms from bacteria to mammals. The biocompatibility and thermal stability of melanin nanoparticles make them good candidates to work as free radical scavengers and photothermal anticancer substrates. Research studies have identified melanin as an antioxidative therapeutic agent and/or reactive oxygen species (ROS) scavenger that includes neutralization of peroxynitrite. In addition, melanin nanoparticles have emerged as an anticancer photothermal platform that has the capability to kill cancer cells. Recently, melanin nanoparticles have been successfully used as chelating agents to purify water from heavy metals, such as hexavalent chromium. This review article highlights some selected aspects of cutting-edge melanin applications. Herein, we will refer to the recent literature that addresses melanin nanoparticles and its useful physicochemical properties as a hot topic in biomaterial science. It is expected that the techniques of Dynamic Light Scattering (DLS), Scanning Electron Microscopy (SEM), and time-resolved Electron Paramagnetic Resonance (EPR) will have a strong impact on the full characterization of melanin nanoparticles and the subsequent exploration of their physiological and chemical mechanisms.

Melanin-based nanomaterials: The promising nanoplatforms for cancer diagnosis and therapy

Melanin-based nanoplatforms are biocompatible nanomaterials with a variety of unique physicochemical properties such as strong photothermal conversion ability, excellent drug binding capacity, strong metal chelation capacity, high chemical reactivity and versatile adhesion ability. These innate talents not only make melanin-based nanoplatforms be an inborn theranostic nanoagent for photoacoustic imaging-guided photothermal therapy of cancers, but also enable them to be conveniently transferred into cancer-targeting drug delivery systems and multimodality imaging nanoprobes. Due to the intriguing properties, melanin-based nanoplatforms have attracted much attention in investigations of cancer diagnosis and therapy. This review provides an overview of recent research advances in applications of melanin-based nanoplatforms in the fields of cancer diagnosis and therapy including cancer photothermal therapy, anticancer drug delivery, cancer-specific multimodal imaging and theranostics, etc. The remaining challenges and prospects of melanin-based nanoplatforms in biomedical applications are discussed at the end of this review.

Copper and Melanin Play a Role in Myxococcus xanthus Predation on Sinorhizobium meliloti

Myxococcus xanthus is a soil myxobacterium that exhibits a complex lifecycle with two multicellular stages: cooperative predation and development. During predation, myxobacterial cells produce a wide variety of secondary metabolites and hydrolytic enzymes to kill and consume the prey. It is known that eukaryotic predators, such as ameba and macrophages, introduce copper and other metals into the phagosomes to kill their prey by oxidative stress. However, the role of metals in bacterial predation has not yet been established. In this work, we have addressed the role of copper during predation of M. xanthus on Sinorhizobium meliloti. The use of biosensors, variable pressure scanning electron microscopy, high-resolution scanning transmission electron microscopy, and energy dispersive X ray analysis has revealed that copper accumulates in the region where predator and prey collide. This accumulation of metal up-regulates the expression of several mechanisms involved in copper detoxification in the predator (the P_1B-ATPase CopA, the multicopper oxidase CuoA and the tripartite pump Cus2), and the production by the prey of copper-inducible melanin, which is a polymer with the ability to protect cells from oxidative stress. We have identified two genes in S. meliloti (encoding a tyrosinase and a multicopper oxidase) that participate in the biosynthesis of melanin. Analysis of prey survivability in the co-culture of M. xanthus and a mutant of S. meliloti in which the two genes involved in melanin biosynthesis have been deleted has revealed that this mutant is more sensitive to predation than the wild-type strain. These results indicate that copper plays a role in bacterial predation and that melanin is used by the prey to defend itself from the predator. Taking into consideration that S. meliloti is a nitrogen-fixing bacterium in symbiosis with legumes that coexists in soils with M. xanthus and that copper is a common metal found in this habitat as a consequence of several human activities, these results provide clear evidence that the accumulation of this metal in the soil may influence the microbial ecosystems by affecting bacterial predatory activities.

Ultrastable Lubricating Properties of Robust Self-Repairing Tribofilms Enabled by in Situ-Assembled Polydopamine Nanoparticles

Aqueous lubrication in nature is attracting increasing attention in the tribological fields for reducing friction energy consumption and improving anti-wear durability. Generally, adding nanolubricant additives is one of the most important strategies to effectively enhance the interface performance under boundary lubrication via the formation of a protective tribofilm on rubbing surfaces. However, the adsorbed tribofilms are unstable and are prone to failure during friction, and the interaction mechanism between the tribofilms and frictional interfaces is partly disclosed. In this study, inspired by mussels, an in situ-assembled polydopamine (PDA) tribofilm is achieved with PDA nanoparticles as aqueous lubricant additives, which shows excellent lubrication properties. The coefficient of friction is interface-independent and is reduced by as much as 83%. The results show that the PDA tribofilm can not only form chemical bonding with metal interfaces but also present a synergistic lubrication effect with the upper ceramic surface. Especially, a self-repairing effect of the PAD tribofilm is observed, by which the ultrastable lubricating properties can be achieved during friction, and thus, the friction and wear can be effectively controlled. This work provides an effective method for improving the interface stability of friction pairs under aqueous lubrication and also shows great meaning for industrial applications.

Polydopamine Nanoparticles Prepared Using Redox-Active Transition Metals

Autoxidation of dopamine to polydopamine by dissolved oxygen is a slow process that requires highly alkaline conditions. Polydopamine can be formed rapidly also in mildly acidic and neutral solutions by using redox-active transition-metal ions. We present a comparative study of polydopamine nanoparticles formed by autoxidation and aerobic or anaerobic oxidation in the presence of Ce(IV), Fe(III), Cu(II), and Mn(VII). The UV-vis spectra of the purified nanoparticles are similar, and dopaminechrome is an early intermediate species. At low pH, Cu(II) requires the presence of oxygen and chloride ions to produce polydopamine at a reasonable rate. The changes in dispersibility and surface charge take place at around pH 4, which indicates the presence of ionizable groups, especially carboxylic acids, on their surface. X-ray photoelectron spectroscopy shows the presence of three different classes of carbons, and the carbonyl/carboxylate carbons amount to 5-15 atom %. The N 1s spectra show the presence of protonated free amino groups, suggesting that these groups may interact with the π-electrons of the intact aromatic dihydroxyindole moieties, especially in the metal-induced samples. The autoxidized and Mn(VII)-induced samples do not contain metals, but the metal content is 1-2 atom % in samples prepared with Ce(IV) or Cu(II), and ca. 20 atom % in polydopamine prepared in the presence of Fe(III). These differences in the metal content can be explained by the oxidation and complexation properties of the metals using the general model developed. In addition, the nitrogen content is lower in the metal-induced samples. All of the metal oxidants studied can be used to rapidly prepare polydopamine at room temperature, but the possible influence of the metal content and nitrogen loss should be taken into account.

Development of Cu-Modified PVC and PU for Catalytic Generation of Nitric Oxide

Nitric oxide (NO) generating surfaces are potentially promising for improving haemocompatibility of blood-contacting biomaterials. In the present report, Cu-modified poly(vinyl chloride) (PVC) and polyurethane (PU) were prepared via polydopamine (pDA)-assisted chelation. The copper content on the PVC and PU modified surfaces, assessed by inductively coupled plasma - optical emission spectrometry (ICP-OES), were about 3.86 and 6.04 nmol·cm−2, respectively. The Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) data suggest that copper is attached to the polymer surface through complex formation with pDA. The cumulative leaching of copper from modified PVC and PU during the five day incubation in phosphate buffered saline (PBS), measured by inductively coupled plasma mass spectrometry (ICP-MS), was about 50.7 ppb and 48 ppb, respectively which is within its physiological level. Modified polymers were tested for their ability to catalytically generate NO by decomposing of endogenous S-nitrosothiol (GSNO). The obtained data show that Cu-modified PVC and PU exhibited the capacity to generate physiological levels of NO which could be a foundation for developing new biocompatible materials with NO-based therapeutics.

Polymer grafted on carbon nanotubes as a flexible cathode for aqueous zinc ion batteries

We have developed a free-standing, flexible cathode for an aqueous zinc ion battery by grafting cross-linked polydopamine on carbon nanotubes. The cathode is highly stable, with little capacity degradation for 500 cycles. The flexible cathode is environmentally benign and biocompatible which can enable applications from biomedical devices to grid storage.

Macroscale Biomolecular Electronics and Ionics

The conduction of ions and electrons over multiple length scales is central to the processes that drive the biological world. The multidisciplinary attempts to elucidate the physics and chemistry of electron, proton, and ion transfer in biological charge transfer have focused primarily on the nano- and microscales. However, recently significant progress has been made on biomolecular materials that can support ion and electron currents over millimeters if not centimeters. Likewise, similar transport phenomena in organic semiconductors and ionics have led to new innovations in a wide variety of applications from energy generation and storage to displays and bioelectronics. Here, the underlying principles of conduction on the macroscale in biomolecular materials are discussed, highlighting recent examples, and particularly the establishment of accurate structure-property relationships to guide rationale material and device design. The technological viability of biomolecular electronics and ionics is also discussed.

Characterization of Retinal Pigment Epithelial Melanin and Degraded Synthetic Melanin Using Mass Spectrometry and In Vitro Biochemical Diagnostics

With increasing age, there is an observable loss of melanin in retinal pigment epithelial (RPE) cells. It is possible that degradation of the pigment contributes to the pathogenesis of retinal disease, as the cellular antioxidant material is depleted. Functionally, intact melanin maintains protective qualities, while oxidative degradation of melanin promotes reactive oxygen species (ROS) generation and formation of metabolic byproducts, such as melanolipofuscin. Understanding the structural and functional changes to RPE melanin with increasing age may contribute to a better understanding of disease progression and risk factors for conditions such as age-related macular degeneration (AMD). In this study, human donor RPE melanin is characterized using MALDI mass spectrometry to follow melanin degradation trends. In vitro models using ARPE-19 cells are used to assess photo-reactivity in repigmented cells. Significant protection against intracellular ROS produced by blue light is observed in calf melanin-pigmented cells versus unpigmented and black latex bead controls (P < 0.0001). UV-B exposure to aged human melanin-pigmented cells results in a significant increase in nitric oxide production versus control cells (P < 0.001). Peroxide-treated synthetic melanin is characterized to elucidate degradation products that may contribute to RPE cell damage.

Effects of pH and Oxidants on the First Steps of Polydopamine Formation: A Thermodynamic Approach

We present a general thermodynamic top-down analysis of the effects of oxidants and pH on dopamine oxidation and cyclization, supplemented with UV-vis and electrochemical studies. The model is applicable to other catecholamines and various experimental conditions. The results show that the decisive physicochemical parameters in autoxidation are the p K values of the semiquinone and the amino group in the oxidized quinone. Addition of Ce(IV) or Fe(III) enhances dopamine oxidation in acidic media in aerobic and anaerobic conditions by the direct oxidation of dopamine and, in the presence of oxygen, also by the autoxidation of the formed semiquinone. At pH 4.5, the enhancement of the one-electron oxidation of dopamine explains the overall reaction enhancement, but at a lower pH, cyclization becomes rate-determining. Oxidation by Cu(II) at reasonable rates requires the presence of oxygen or chloride ions.

Free-standing polydopamine films generated in the presence of different metallic ions: the comparison of reaction process and film properties

Polydopamine is widely used in surface modification, nanofiltration, photonic devices and drug delivery. The formation mechanism and properties of polydopamine are modified by the experimental conditions. Herein we demonstrated a comparison study of free-standing polydopamine films generated at the air-solution interface and their corresponding nanoparticles in solutions, in the presence of various metallic cations, Na+, Ca2+, Mg2+ and Co2+. Adding metallic ions influenced the intermediates in dopamine polymerization, and in turn the morphology and properties of the produced free-standing polydopamine films. Moreover, we observed that the polymerization process accompanying the stratification determines the formation of free-standing films at the air-solution interface: the fast polymerization of dopamine in a Co2+ environment leads to a rugged film surface and porous film body, whereas the comparatively slow polymerization of dopamine under conditions of other metallic ions results in a smooth and solid film. In addition, the water contact angles of the upper and lower surface of the polydopamine films were different. This investigation enriches our knowledge of dopamine polymerization in different environments, which is particularly useful for further application of free-standing polydopamine films.

Melanin-embedded materials effectively remove hexavalent chromium (CrVI) from aqueous solution

Background

Currently, it is recognized that water polluted with toxic heavy metal ions may cause serious effects on human health. Therefore, the development of new materials for effective removal of heavy metal ions from water is still a widely important area. Melanin is being considered as a potential material for removal of heavy metal from water.

Methods

In this study, we synthesized two melanin-embedded beads from two different melanin powder sources and named IMB (Isolated Melanin Bead originated from squid ink sac) and CMB (Commercial Melanin Bead originated from sesame seeds). These beads were of globular shape and 2–3 mm in diameter. We investigated and compared the sorption abilities of these two bead materials toward hexavalent-chromium (Cr^VI) in water. The isotherm sorption curves were established using Langmuir and Freundlich models in the optimized conditions of pH, sorption time, solid/liquid ratio, and initial concentration of Cr^VI. The FITR analysis was also carried out to show the differences in surface properties of these two beads.

Results

The optimized conditions for isotherm sorption of Cr^VI on IMB/CMB were set at pH values of 2/2, sorption times of 90/300 min, and solid-liquid ratios of 10/20 mg/mL. The maximum sorption capacities calculated based on the Langmuir model were 19.60 and 6.24 for IMB and CMB, respectively. However, the adsorption kinetic of Cr^VI on the beads fitted the Freundlich model with R² values of 0.992 for IMB and 0.989 for CMB. The deduced Freundlich constant, 1/n, in the range of 0.2–0.8 indicated that these beads are good adsorption materials. In addition, structure analysis data revealed great differences in physical and chemical properties between IMB and CMB. Interestingly, FTIR analysis results showed strong signals of –OH (3295.35 cm^− 1) and –C=O (1608.63 cm^− 1) groups harboring on the IMB but not CMB. Moreover, loading of Cr^VI on the IMB caused a shift of broad peaks from 3295.35 cm^− 1 and 1608.63 cm^− 1 to 3354.21 cm^− 1 and 1597.06 cm^− 1, respectively, due to –OH and –C=O stretching.

Conclusions

Taken together, our study suggests that IMB has great potential as a bead material for the elimination of Cr^VI from aqueous solutions and may be highly useful for water treatment applications.

Functions of fungal melanin beyond virulence

Melanins are ancient biological pigments found in all kingdoms of life. In fungi, their role in microbial pathogenesis is well established; however, these complex biomolecules also confer upon fungal microorganisms the faculty to tolerate extreme environments such as the Earth's poles, the International Space Station and places contaminated by toxic metals and ionizing radiation. A remarkable property of melanin is its capacity to interact with a wide range of electromagnetic radiation frequencies, functioning as a protecting and energy harvesting pigment. Other roles of fungal melanin include scavenging of free radical, thermo-tolerance, metal ion sequestration, cell development, and mechanical-chemical cellular strength. In this review, we explore the various functions ascribed to this biological pigment in fungi and its remarkable physicochemical properties.

Calcium-Mediated Control of Polydopamine Film Oxidation and Iron Chelation

The facile preparation of conformal polydopamine (PDA) films on broad classes of materials has prompted extensive research into a wide variety of potential applications for PDA. The constituent molecular species in PDA exhibit diverse chemical moieties, and therefore highly variable properties of PDA-based devices may evolve with post-processing conditions. Here we report the use of redox-inactive cations for oxidative post-processing of deposited PDA films. PDA films incubated in alkaline CaCl₂ solutions exhibit accelerated oxidative evolution in a dose-dependent manner. PDA films incubated in CaCl₂ solutions exhibit 53% of the oxidative charge transfer compared to pristine PDA films. Carboxylic acid groups generated from the oxidation process lower the isoelectric point of PDA films from pH = 4.0 ± 0.2 to pH = 3.1 ± 0.3. PDA films exposed to CaCl₂ solutions during post-processing also enhance Fe2+/Fe3+ chelation compared to pristine PDA films. These data illustrate that the molecular heterogeneity and non-equilibrium character of as-deposited PDA films afford control over the final composition by choosing post-processing conditions, but also demands forethought into how the performance of PDA-incorporated devices may change over time in salt solutions.

Chelation competition induced polymerization (CCIP): construction of integrated hollow polydopamine nanocontainers with tailorable functionalities

A novel CCIP strategy is exploited to construct integrated hollow polydopamine nanocontainers, and Au NRs@PDA NCTs constructed through this method show excellent performance in cancer eradication.

Oxidative Self-Polymerization of Dopamine in an Acidic Environment

A weak alkaline condition (pH > 8) is a general requirement for oxidative self-polymerization of dopamine. Here, we first demonstrated the generation of polydopamine in an acidic environment via a hydrothermal method. The pH scope of self-polymerization of dopamine is extended to pH ∼ 1 in a hydrothermal process. Polydopamine generated via a hydrothermal method shows similar chemical features and radical scavenging activity with that generated in a basic environment.

Control of heterogeneous nucleation and growth kinetics of dopamine-melanin by altering substrate chemistry

Dopamine-melanin (DM or "polydopamine") can be deposited on virtually any substrate from solution through autoxidation of dopamine. The versatility of this process has allowed surface-mediated assembly of DM for a wide variety of functional coatings. Here we report the impact of well-defined surface chemistries on the nucleation and growth of such films. DM was deposited on silicon dioxide (SiO2) and SiO2 substrates modified with self-assembled monolayers (SAMs) bearing octadecyl (C18), phenethyl, and aminopropyl functional groups. Atomic force microscopy revealed three-dimensional islands whose areal density and surface coverage are lowest on bare SiO2 substrates and highest on the neutral aromatic and aliphatic substrates. Increasing the pH of the solution from 8.2 to 10 dissociates catechol moieties in DM and inhibits adsorption on negatively charged SiO2 substrates. The growth rate of DM films on SAM-modified SiO2 is maximized at pH 9.5 and almost completely abolished at pH 10 because of increased DM solubility. The initial rates of DM adsorption were measured using quartz crystal microbalance with dissipation measurements. The initial adsorption rate is proportional to the nucleation density, which increases as the hydrophobicity of the substrate increases. Taken together, these data provide insight into the rates of heterogeneous nucleation and growth of DM on substrates with well-defined chemistries.

Catechol-mediated reversible binding of multivalent cations in eumelanin half-cells

Electrochemical storage systems that utilize divalent cations such as Mg2+ can improve the volumetric charge storage capacities compared to those that use monovalent ions. Here, a cathode based on naturally derived melanin pigments is used in secondary Mg2+ batteries. Redox active catechol groups in melanins permit efficient and reversible exchange of divalent Mg2+ cations to preserve charge storage capacity in biopolymer cathodes for more than 500 cycles.

Tyrosinase as a multifunctional reporter gene for Photoacoustic/MRI/PET triple modality molecular imaging

Development of reporter genes for multimodality molecular imaging is highly important. In contrast to the conventional strategies which have focused on fusing several reporter genes together to serve as multimodal reporters, human tyrosinase (TYR)--the key enzyme in melanin production--was evaluated in this study as a stand-alone reporter gene for in vitro and in vivo photoacoustic imaging (PAI), magnetic resonance imaging (MRI) and positron emission tomography (PET). Human breast cancer cells MCF-7 transfected with a plasmid that encodes TYR (named as MCF-7-TYR) and non-transfected MCF-7 cells were used as positive and negative controls, respectively. Melanin targeted N-(2-(diethylamino)ethyl)-18F-5-fluoropicolinamide was used as a PET reporter probe. In vivo PAI/MRI/PET imaging studies showed that MCF-7-TYR tumors achieved significant higher signals and tumor-to-background contrasts than those of MCF-7 tumor. Our study demonstrates that TYR gene can be utilized as a multifunctional reporter gene for PAI/MRI/PET both in vitro and in vivo.

Sonochemically produced polydopamine nanocapsules with selective antimicrobial activity

A facile, versatile, and one-pot sonochemical synthesis of polydopamine (PDA)-nanocapsules from dopamine is reported. The nanocapsules (227 ± 25 nm) can encapsulate hydrophobic substances while retaining the reactivity of PDA toward nucleophilic reactions, enabling facile surface modification for different applications. PDA nanocapsules are nontoxic to mammalian cells while Cu-containing PDA capsules demonstrate strong (99.9%) and rapid (15 min) bactericidal activity.

Comparison of synthetic dopamine-eumelanin formed in the presence of oxygen and Cu2+ cations as oxidants

Eumelanin is not only a ubiquitous pigment among living organisms with photoprotective and antioxidant functions, but is also the subject of intense interest in materials science due to its photoconductivity and as a possible universal coating platform, known as "polydopamine films". The structure of eumelanin remains largely elusive, relying either on a polymeric model or on a heterogeneous aggregate structure. The structure of eumelanin as well as that of the closely related "polydopamine films" can be modified by playing on the nature of the oxidant used to oxidize dopamine or related compounds. In this investigation, we show that dopamine-eumelanins produced from dopamine in the presence of either air (O2 being the oxidant) or Cu(2+) cations display drastically different optical and colloidal properties in relation with a different supramolecular assembly of the oligomers of 5,6 dihydroxyindole, the final oxidation product of dopamine. The possible origin of these differences is discussed on the basis of Cu(2+) incorporation in Cu dopamine-eumelanin.

Hydration-controlled X-band EPR spectroscopy: a tool for unravelling the complexities of the solid-state free radical in eumelanin

Melanin, the human skin pigment, is found everywhere in nature. Recently it has gained significant attention for its potential bioelectronic properties. However, there remain significant obstacles in realizing its electronic potential, in particular, the identity of the solid-state free radical in eumelanin, which has been implicated in charge transport. We have therefore undertaken a hydration-controlled continuous-wave electron paramagnetic resonance study on solid-state eumelanin. Herein we show that the EPR signal from solid-state eumelanin arises predominantly from a carbon-centered radical but with an additional semiquinone free radical component. Furthermore, the spin densities of both of these radicals can be manipulated using water and pH. In the case of the semiquinone radical, the comproportionation reaction governs the pH- and hydration-dependent behavior. In contrast, the mechanism underlying the carbon-centered radical's pH- and hydration-dependent behavior is not clear; consequently, we have proposed a new destacking model in which the intermolecular structure of melanin is disordered due to π-π destacking, brought about by the addition of water or increased pH, which increases the proportion of semiquinone radicals via the comproportionation reaction.