The interplay of chemical structure, physical properties, and structural design as a tool to modulate the properties of melanins within mesopores

Abstract

The design of modern devices that can fulfil the requirements for sustainability and renewable energy applications calls for both new materials and a better understanding of the mixing of existing materials. Among those, surely organic–inorganic hybrids are gaining increasing attention due to the wide possibility to tailor their properties by accurate structural design and materials choice. In this work, we’ll describe the tight interplay between porous Si and two melanic polymers permeating the pores. Melanins are a class of biopolymers, known to cause pigmentation in many living species, that shows very interesting potential applications in a wide variety of fields. Given the complexity of the polymerization process beyond the formation and structure, the full understanding of the melanins' properties remains a challenging task. In this study, the use of a melanin/porous Si hybrid as a tool to characterize the polymer’s properties within mesopores gives new insights into the conduction mechanisms of melanins. We demonstrate the dramatic effect induced on these mechanisms in a confined environment by the presence of a thick interface. In previous studies, we already showed that the interactions at the interface between porous Si and eumelanin play a key role in determining the final properties of composite materials. Here, thanks to a careful monitoring of the photoconductivity properties of porous Si filled with melanins obtained by ammonia-induced solid-state polymerization (AISSP) of 5,6-dihydroxyindole (DHI) or 1,8-dihydroxynaphthalene (DHN), we investigate the effect of wet, dry, and vacuum cycles of storage from the freshly prepared samples to months-old samples. A computational study on the mobility of water molecules within a melanin polymer is also presented to complete the understanding of the experimental data. Our results demonstrate that: (a) the hydration-dependent behavior of melanins is recovered in large pores (≈ 60 nm diameter) while is almost absent in thinner pores (≈ 20 nm diameter); (b) DHN-melanin materials can generate higher photocurrents and proved to be stable for several weeks and more sensitive to the wet/dry variations.

Exploring the Reactivity of Melanins as Photocatalysts for Reductive Dehalogenations

“…[4] Melanins are pigments widely diffused in nature that can be found in mammals (eumelanins, pheomelanins and neuromelanins), in cephalopods (sepiomelanin), in plants and fungi (allomelanins), and in bacteria (pyomelanins). [5] Thanks to a peculiar combination of different levels of disorder, ranging from structural to electronic and supramolecular, [6] melanins can benefit from a unique set of chemical-physical properties that have prompted their application in different sectors, from (bio)electronics [7] to energy conversion and storage, [8] tissue engineering, [9] therapeutics [10] and functional materials. [11] On the other hand, the use of melanins as photoactive materials is set as a very recent application.…”

Bioactive Composite Methacrylated Gellan Gum for 3D-Printed Bone Tissue-Engineered Scaffolds

“…Eumelanins are the dark insoluble pigments found in mammalian skin, eyes and hair [ 30 , 31 , 32 ]. Thanks to a unique set of physico-chemical properties, eumelanins have found many applications as a functional biomaterial in different sectors, from organic (bio)electronics [ 33 , 34 , 35 , 36 ] to energy storage [ 37 ], photo- and radio-protection and regenerative medicine [ 38 , 39 ]. Novel insights indicate that eumelanin can specifically support bone tissue engineering [ 40 , 41 ].…”

Eumelanin from the Black Soldier Fly as Sustainable Biomaterial: Characterisation and Functional Benefits in Tissue-Engineered Composite Scaffolds

“…Eumelanins exhibit a peculiar set of physico-chemical [ 8 ] such as: (1) a broadband and featureless UV-visible absorption profile responsible for their dark colour; (2) a persistent free radical character denoted by an intense signal in the electron paramagnetic resonance (EPR) spectrum; (3) a water-dependent hybrid ionic−electronic conductor behaviour; (4) having multiple redox states available. Overall, these properties have stimulated the application of eumelanins in different sectors including organic electronics, with the design of the eumelanin-based organic thin film transistor (O-TFT) [ 9 ], organic photovoltaic (OPV) devices [ 10 ], organic light-emitting diodes (OLEDs) [ 11 ] and supercapacitors [ 12 ]; regenerative biomedicine, with the design of eumelanin-based bio-interfaces promoting the adhesion, proliferation and differentiation of several cell lines and stem cells [ 9 ]; and remediation processes, with the design of eumelanin-based devices able to sequester metal ions, drugs and pollutants [ 13 ].…”