NATURAL MATERIALS FOR SUSTAINABLE ELECTRONICS
Our research group investigates naturally occurring, bio-sourced organic materials (e.g. melanins, tannins, chlorophylls and waste biomass) for sustainable electronics, with particular emphasis on their electrical, electrochemical, and optical properties. These materials offer opportunities for developing functional devices that may be biocompatible, biodegradable, and less dependent on fossil-derived resources.
A major research focus is eumelanin, a naturally occurring brown-black pigment whose unusual electrical properties have attracted scientific interest since the late 1960s. We investigate charge transport in eumelanin, focusing on the interplay between proton migration, electronic transport, and interfacial charge-transfer (redox) processes at metal electrodes. Understanding these phenomena is essential for exploring eumelanin's potential in organic electronics, sensors, and energy-storage devices.
AN INTRODUCTION TO MELANIN
Melanins are a family of naturally occurring pigments found across a wide range of organisms. In humans, they contribute to skin, hair, and eye pigmentation and provide protection against ultraviolet (UV) radiation. Their biological functions extend beyond photoprotection and include roles in metal-ion homeostasis and oxidative stress regulation.
Two major types of melanin are eumelanin, a brown-black pigment, and pheomelanin, a yellow-red pigment. Eumelanin occurs in mammals, reptiles, amphibians, fish, and invertebrates, including cuttlefish and insects. Sepia melanin, obtained from cuttlefish ink, is a widely studied natural source of eumelanin.
Eumelanin exhibits a remarkable combination of physicochemical properties, including broadband UV–visible light absorption, hydration-dependent electrical conductivity, metal-ion binding, redox activity, and free-radical scavenging. These properties make it particularly interesting for applications at the interface of electronics, electrochemistry, and biology.
Despite decades of research, understanding eumelanin's structure–property relationships remains challenging because of its chemical heterogeneity, structural disorder, and limited solubility in common solvents.
Melanins also play important biological roles. For example, interactions between iron and neuromelanin, a complex pigment present in specific regions of the human brain, have been investigated in relation to Parkinson's disease. Furthermore, the strong adhesion of certain melanin-inspired materials to diverse surfaces, including metal oxides, has stimulated research into applications in biomedicine, water treatment, and energy technologies.
Ion-gated transistors exploit electrical double layers, forming at the interface between an ion-including medium and a semiconductor, to modulate the electrical conductance in the transistor channel. The high capacitance (typically 10~40 μFcm-2) of thin electrical double layers (ca 2-4 nm-thick) permits to reach high charge carrier density in semiconducting channels (ca 1015 cm-2), at low operating voltages (as low as 0.1-0.5 V). Ion-gated transistors based on channels of 2D materials, metal oxides and organic molecular semiconductors, making use of different gating media, such as ionic liquids, ion gels and saline aqueous solutions, have been reported.
Ion-gated organic transistors
Among such channel materials, organic semiconductors are interesting for applications in bioelectronics as well as flexible and printable electronics. The working principle of ion-gated transistors depends on several factors, such as the nature of the transistor channel material (permeable or not to the ions of the gating medium) and the gating medium (e.g. its conductivity and viscosity). It is reasonable to hypothesize that the softness and the redox activity featured by organic semiconductors render them prone to a bulk (three dimensional) electrochemical doping, where charge carrier injection/extraction from metal electrodes (source and drain) to the semiconductor is assisted by the incorporation/removal of ions in the channel. Nevertheless, specific contributions pertaining to intermolecular interactions between the ions of the gating medium and the organic semiconducting molecules of the transistors channel, as well as the size of the ions, can dramatically affect the effectiveness of the doping and its extension in the film (three vs two dimensional). Therefore, such specific contributions have to be considered to properly describe the operation of organic ion-gated transistors and are the focus of our efforts in the field.
Ion-gated metal oxide transistors
Metal oxides are transistor channel materials interesting for their processability in air, at low temperature. Among metal oxides, tungsten oxide stands out for its electrochromic, gas sensing and photocatalytic properties. We study ion-gated tungsten oxide transistors and phototransistors working in different ion gating media, such as one hydrophobic ionic liquid and an aqueous electrolyte, fabricated both on rigid and flexible substrates. We are currently investigating ion-gated tungsten oxide phototransistors operating in aqueous media could be used as photocatalytic sensors in portable applications.
Municipal solid waste incineration generates several byproducts, including bottom ash, fly ash, and air pollution control residues. Developing sustainable valorization routes for these materials is essential to reduce landfilling and promote circular resource use.
The presence of metal oxides and carbon-rich regions in incineration ash offers opportunities for their valorization as electroactive functional materials, including components of battery electrodes. However, their chemical and structural heterogeneity calls for a comprehensive understanding of their properties.
Our research combines elemental analysis (INAA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS), and carbon quantification (CHNS/O) to correlate ash composition, structure, chemical states, and electrical response.
XPS revealed metal oxides and sp²-hybridized carbon in carbon-rich regions, while electrical measurements on pressed ash pellets yielded an estimated conductivity of approximately 10⁻⁷ S cm⁻¹ under ambient conditions.
These findings highlight the importance of understanding not only the elemental and mineralogical composition of incineration ash but also its chemical states and electronic structure to guide its valorization into sustainable electroactive materials.
Biosourced, potentially biodegradable, redox-active organic materials, such as quinone-based compounds, offer promising opportunities for sustainable electrochemical energy storage in mild-pH aqueous electrolytes. However, poor electrical contact between organic materials and carbon electrodes can limit rate capability and cycling stability.
Our research focuses on engineering organic material–carbon interfaces to improve electrochemical performance. Chemical treatment of carbon paper can enhance its surface area, porosity, and electrolyte wettability, while introducing oxygen-, nitrogen-, sulfur-, and phosphorus-containing surface functionalities that can contribute to faradaic charge storage.
Using treated carbon paper as a current collector, we developed electrodes incorporating biosourced materials such as Sepia melanin, obtained from cuttlefish ink, and catechin combined with tannic acid. The latter acts as a hydrogen-bonding binder to limit catechin dissolution in aqueous electrolytes. These electrodes exhibited two- to threefold higher capacitance than untreated organic-material-free treated carbon paper, without requiring additional conductive additives or synthetic binders.
Symmetric electrochemical capacitors based on Sepia melanin achieved an areal capacitance of 1,355 mF cm⁻² (452 F g⁻¹), approximately 100% coulombic efficiency, and nearly 100% capacitance retention after 50,000 cycles. Catechin/tannic acid devices reached 898 mF cm⁻² (300 F g⁻¹), approximately 100% coulombic efficiency, and nearly 100% capacitance retention after 10,000 cycles.
These findings demonstrate the potential of interface engineering to develop high-performance electrochemical capacitors using naturally derived organic materials. Our ongoing research extends this approach to other biosourced materials and explores hydrogel electrolytes as alternatives to liquid electrolytes for mechanically robust and environmentally responsible energy-storage devices.
