High conductivity Sepia melanin ink films for environmentally benign printed electronics

Ultrathin and Smooth Pheomelanin-like Photoconductive Film

This study introduces a facile method for the substrate-independent deposition of pheomelanin-like films, revealing unique and promising electrical characteristics. The conventional darkening of a dopamine solution at a basic pH was significantly delayed by the addition of l-cysteine, resulting in a distinctive temporal pattern: an initial quiescent period without apparent color change followed by an abrupt and explosive burst. Surprisingly, within the quiescent period, the deposition of ultrathin and smooth pheomelanin-like films was observed, in addition to rough and thick films formed after the burst. Regardless of thickness or texture, these films exhibited common chemical properties, including moisture-capturing capability and dark- and bright-state conductivities. Particularly noteworthy were consistent photocurrent responses under bias voltage across various pheomelanin-like films, which were not observed in polydopamine films, highlighting the influential role of l-cysteine addition. These findings present a novel avenue for the potential application of pheomelanin-like films in bioelectronics, emphasizing their distinct electrical characteristics and prompting further exploration into their intricate conductive mechanisms. The study contributes to advancing our understanding of melanin-based materials and their potential in diverse scientific and technological domains.

Electrochemical Doping Effect on the Conductivity of Melanin-Inspired Materials

Eumelanin is a natural pigment that can be particularly valuable for sustainable bioelectronic devices due to its inherent biocompatibility and hydration-dependent conductivity. However, the low conductivity of eumelanin limits its technological development. In this research, electrochemical doping was proposed as an alternative route to increase the electronic conductivity of synthetic eumelanin derivatives. Thin films of sulfonated eumelanin were deposited on platinum interdigitated electrodes and electrochemically treated by using cyclic voltammetry and chronoamperometry treatments. X-ray photoelectron spectroscopy analysis confirmed ion doping in sulfonated melanin. Current-voltage, current-time, and electrochemical impedance measurements were used to investigate the effect of different aqueous electrolytes (including KCl and LiClO4) treatments on the charge transport of sulfonated eumelanin. We show that the conductivity depends on the type and size of the anion used and can reach 10-3 S·cm-1. Additionally, depending on the electrolyte, there is a change in charge transport from mixed ionic/electronic to a predominantly electronic-only conduction. Our results show that the chemical nature of the ion plays an important role in the electrochemical doping and, consequently, in the charge transport of eumelanin. These insights serve as inspiration to explore the use of alternative electrolytes with different compositions further and develop eumelanin-based devices with tunable conductivities.

Emergent properties of melanin-inspired peptide/RNA condensates

Most biocatalytic processes in eukaryotic cells are regulated by subcellular microenvironments such as membrane-bound or membraneless organelles. These natural compartmentalization systems have inspired the design of synthetic compartments composed of a variety of building blocks. Recently, the emerging field of liquid-liquid phase separation has facilitated the design of biomolecular condensates composed of proteins and nucleic acids, with controllable properties including polarity, diffusivity, surface tension, and encapsulation efficiency. However, utilizing phase-separated condensates as optical sensors has not yet been attempted. Here, we were inspired by the biosynthesis of melanin pigments, a key biocatalytic process that is regulated by compartmentalization in organelles, to design minimalistic biomolecular condensates with emergent optical properties. Melanins are ubiquitous pigment materials with a range of functionalities including photoprotection, coloration, and free radical scavenging activity. Their biosynthesis in the confined melanosomes involves oxidation-polymerization of tyrosine (Tyr), catalyzed by the enzyme tyrosinase. We have now developed condensates that are formed by an interaction between a Tyr-containing peptide and RNA and can serve as both microreactors and substrates for tyrosinase. Importantly, partitioning of Tyr into the condensates and subsequent oxidation-polymerization gives rise to unique optical properties including far-red fluorescence. We now demonstrate that individual condensates can serve as sensors to detect tyrosinase activity, with a limit of detection similar to that of synthetic fluorescent probes. This approach opens opportunities to utilize designer biomolecular condensates as diagnostic tools for various disorders involving abnormal enzymatic activity.

A nanoscale study of the structure and electrical response of Sepia eumelanin

Eumelanin, the brown-black member of the melanin biopigment family, is a prototype material for sustainable (green) organic electronics. Sepia eumelanin (Sepia) is a type of biosourced eumelanin extracted from the ink sac of cuttlefish. Electron microscopy and scanning probe microscopy images of Sepia show distinguishable near spherical granules with diameters of about 150-200 nm. We have recently reported on predominant electronic transport in printed films of Sepia formulated inks including the (insulating) binder Polyvinyl-butyral (PVB). In that work, we proposed that inter-granular percolative transport, observed for micrometric interelectrode distances, is promoted by the confining action of the PVB binder on the Sepia granules. Considering that inter-granular transport implies intra-granular transport, in this work we proceeded to a nanoscale study of Sepia granules by High Resolution Atomic Force Microscopy (HR-AFM) and Conductive-AFM (c-AFM). We have observed protrusions on the surface of the Sepia granules, suggesting sub-granular structures compatible with the hierarchical development of Sepia, as proposed elsewhere. For films of Sepia formulated inks deposited on gold-coated substrates, c-AFM revealed, for the very first time, a nanoscale electrical response. Nanoscale studies provide the key to structure-property relationships in biosourced materials strategic for sustainable organic electronics.