Melanins (from the Greek μέλας, mélas, black) are bio-pigments ubiquitous in flora and fauna. Eumelanin is an insoluble brown–black type of melanin, found in vertebrates and invertebrates alike, among which Sepia (cuttlefish) is noteworthy. Sepia melanin is a type of bio-sourced eumelanin that can readily be extracted from the ink sac of cuttlefish. Eumelanin features broadband optical absorption, metal-binding affinity and antioxidative and radical-scavenging properties. It is a prototype of benign material for sustainable organic electronics technologies. Here, we report on an electronic conductivity as high as 10 −3 S cm −1 in flexographically printed Sepia melanin films; such values for the conductivity are typical for well-established high-performance organic electronic polymers but quite uncommon for bio-sourced organic materials. Our studies show the potential of bio-sourced materials for emerging electronic technologies with low human- and eco-toxicity.

Eumelanin, a macromolecular biopigment, is an attractive candidate for sustainable (green) organic electronics. Establishing structure–property relationships in eumelanin films is an essential step to exploit its technological potential. We report on the evolution from the molecular state to film after spin coating on silicon dioxide solutions of (5,6)-dihydroxyindole (DHI) and (5,6)-dihydroxyindole-2-carboxylic acid (DHICA) eumelanin building blocks (monomers). The evolution of the spin-coated films was studied under various environmental conditions, such as ambient vs an ammonia atmosphere, which catalyzes polymerization. Atomic force microscopy images reveal dramatic morphological changes as a function of the environmental conditions. Infrared and UV–vis spectroscopies indicate that these changes are due to a combination of physical (self-assembly) and chemical (polymerization) processes. Preliminary electrical measurements on films were also carried out.

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Global materials’ and energy constraints and environmental issues call for a holistic approach to waste upcycling. We propose a chemically rational, cost-effective and environmentally friendly recovery of non-leaching gold from e-waste using aqueous chemistry with hydrogen peroxide, an environmentally benign oxidant, and lactic acid, a food chain byproduct. The oxidation of the base metals enables the release of gold in its metallic state in the form of flakes subsequently separated via filtration. Our main byproduct is a precursor of Cu2O, a relevant metal oxide for solar energy conversion applications. The recovered gold was characterized by scanning electron microscopy, energy dispersive spectroscopy and X-ray photoelectron spectroscopy to gain insight into the morphology of the flakes and their chemical composition. Furthermore, recovered gold was used to successfully fabricate the source and drain electrodes in organic field-effect transistors.

BACKGROUND Bio‐sourced (natural) organic materials are often chemically and structurally disordered, such that their structure‐property relationships must be explored using model systems. Eumelanin is an interesting candidate among natural organic materials. RESULTS In this work, the locations of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of 5,6‐dihydroxyindole (DHI) and 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA) building blocks (monomers) of the black‐brown biopigment eumelanin, in film form, are studied. The films are fabricated by the spin‐coating technique (i.e., here indicated as DHI‐ and DHICA‐films), which is sometimes followed by ammonia‐induced solid‐state polymerization (i.e., indicated as AISSP‐DHI and AISSP‐DHICA films), as well as by thermal evaporation (i.e., evaporated DHI and DHICA films). From Ultraviolet photoemission spectroscopy (UPS), we deduced the ionization energies (EI) for all DHI‐ and DHICA‐based films to be in the range of 5.34‐5.56 eV and 5.35‐5.80 eV, respectively. The electron affinities (χE) are measurable using inverse photoemission spectroscopy (IPES) for DHI films (3.80 eV) and both evaporated DHI (4.0 eV) and DHICA (3.9 eV) films. On the other hand, the χE values of DHICA, AISSP‐DHI, and AISSP‐DHICA films are estimated with about 0.5 eV of uncertainty. UV‐visible spectroscopy reveals the preferred chromophoric bands for DHICA, AISSP‐DHICA, and DHI are in the range of 300‐330 nm, while AISSP‐DHI exhibits a broadened UV‐visible absorbance. CONCLUSION Our study paves the way for the design of suitable metal‐eumelanin interfaces for electronic applications. © 2021 Society of Chemical Industry (SCI).