and electron-transfer, and then controlling these mechanisms for user-defined purposes. For energy applications (e.g., capacitors and batteries), the focus is on storing large quantities of electrons and controlling their discharge. [1][2][3] For information processing applications (e.g., electronics), [4,5] materials are desired that can gate and rectify electron-flow. [6,7] Other applications focus on materials that offer state-dependent properties (e.g., optoelectronics) [8] or memory (e.g., memristors). [9] With the growing emphasis on safety and sustainability, and the expanding interest in life-science applications, there is an increasing interest in the development of electronic materials that function in aqueous systems. [10] In aqueous systems, electron-transfer is constrained by the solvent and typically occurs through at least two distinctly different mechanisms each of which favors different types of materials. [11][12][13] One electron-transfer mechanism is a metal-like conductivity. [14,15] Carbon-based nanomaterials have been especially important for conferring such conductivity [14,16] with benefits that include enhanced double layer charge storage for energy applications [17][18][19][20][21] and electrocatalytic properties for sensing applications. [22][23][24] A second electron-transfer mechanism involves reduction-oxidation (redox) reactions. Redox polymers offer such properties and have been used in applications that include energy Electronic materials that allow the controlled flow of electrons in aqueous media are required for emerging applications that require biocompatibility, safety, and/or sustainability. Here, a composite hydrogel film composed of graphene and catechol is electrofabricated, and that this composite offers synergistic properties is reported. Graphene confers metal-like conductivity and enables charge-storage through an electrical double layer mechanism. Catechol confers redox-activity and enables charge-storage through a redox mechanism. Importantly, there are two functional populations of catechols: conducting-catechols (presumably in intimate contact with graphene) allow direct electron-transfer; and non-conducting-catechols (presumably physically separated from graphene) require diffusible mediators to enable electrontransfer. Using a variety of spectroelectrochemical measurements, that the capacity of the composite for charge-storage increases in proportion to the extent by which the catechol-groups can undergo redox-state switching is demonstrated. To illustrate the broad relevance of this work, how the redoxstate switching can be related to both the charge storage of energy materials and the memory of molecular electronic materials is discussed. The authors believe this work is significant because it demonstrates that: conducting and redox-active components enable distinctly different mechanisms for chargestorage and electron-transfer; these components act synergistically; and mediators provide unique opportunities to extend the capabilities of electronic materials.

We report herein an optimized procedure for preparation of carboxamides of 5,6-dihydroxyindole-2-carboxylic acid (DHICA), the main biosynthetic precursor of the skin photoprotective agents melanins, to get access to pigments with more favorable solubility properties with respect to the natural ones. The developed procedure was based on the use of a coupling agent (HATU/DIPEA) and required protection of the catechol function by easily removable acetyl groups. The O-acetylated compounds could be safely stored and taken to the reactive o-diphenol form just before use. Satisfactorily high yields (>85%) were obtained for all amides. The oxidative polymerization of the synthesized amides carried out in air in aqueous buffer at pH 9 afforded melanin-like pigmented materials that showed chromophores resembling those of DHICA-derived pigments, with a good covering of the UVA and the visible region, and additionally exhibited a good solubility in alcoholic solvents, a feature of great interest for the exploitation of these materials as ingredients of dermocosmetic formulations.

Lignins are phenolic polymers endowed with potent antioxidant properties that are finding increasing applications in a variety of fields. Consequently, there is a growing need for easily available and sustainable sources, as well as for green extraction methodologies of these compounds. Herein, a ball milling/deep eutectic solvent (DES)-based treatment is reported as an efficient strategy for the recovery of antioxidant lignins from the shells of edible nuts, namely chestnuts, hazelnuts, peanuts, pecan nuts, and pistachios. In particular, preliminarily ball-milled shells were treated with 1:2 mol/mol choline chloride:lactic acid at 120 °C for 24 h, and the extracted material was recovered in 19–27% w/w yields after precipitation by the addition of 0.01 M HCl. Extensive spectroscopic and chromatographic analysis allowed for confirmation that the main phenolic constituents present in the shell extracts were lignins, accompanied by small amounts (0.9% w/w) of ellagic acid, in the case of chestnut shells. The recovered samples exhibited very promising antioxidant properties, particularly in the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay (EC50 values ranging from 0.03 to 0.19 mg/mL). These results open new perspectives for the valorization of nut shells as green sources of lignins for applications as antioxidants, e.g., in the biomedical, food, and/or cosmetic sector.

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A straightforward, low-cost, and scalable solid-state mechanochemical protocol for the synthesis of silver nanoparticles (AgNP) based on the use of the highly reducing agri-food by-product pecan nutshell (PNS) is reported herein. Under optimized conditions (180 min, 800 rpm, PNS/AgNO3 ratio = 55/45 w/w), a complete reduction in silver ions was achieved, leading to a material containing ca. 36% w/w Ag0 (X-ray diffraction analysis). Dynamic light scattering and microscopic analysis showed a uniform size distribution (15–35 nm average diameter) of the spherical AgNP. The 2,2-Diphenyl-1-picrylhydrazyl (DPPH) assay revealed lower—although still absolutely high (EC50 = 5.8 ± 0.5 mg/mL)—antioxidant properties for PNS for the further incorporation of AgNP, supporting the efficient reduction of Ag+ ions by PNS phenolic compounds. Photocatalytic experiments indicated that AgNP-PNS (0.4 mg/mL) was able to induce the >90% degradation of methylene blue after 120 min visible light irradiation, with good recycling stability. Finally, AgNP-PNS demonstrated high biocompatibility and significantly light-enhanced growth inhibition properties against Pseudomonas aeruginosa and Streptococcus mutans at concentrations as low as 250 μg/mL, also eliciting an antibiofilm effect at 1000 μg/mL. Overall, the adopted approach allowed to reuse a cheap and abundant agri-food by-product and required no toxic or noxious chemicals, making AgNP-PNS a sustainable and easy-to-access multifunctional material.

The search for new synthetic melanin-related pigments that maintain the antioxidant and photoprotective properties of naturally occurring dark eumelanins, while overcoming their unfavorable solubility, and molecular heterogeneity is presently a very active issue for dermo-cosmetic purposes. In this work, we explored the potential of a melanin obtained from the carboxybutanamide of a major eumelanin biosynthetic precursor, 5,6-dihydroxyindole-2-carboxylic acid (DHICA), by aerobic oxidation under slightly alkaline conditions. Analysis of the pigment by EPR, ATR-FTIR and MALDI MS indicated a substantial structural similarity to DHICA melanin, while investigation of the early intermediates confirmed unchanged regiochemistry of the oxidative coupling. The pigment exhibited a UVA–visible absorption even more intense than that of DHICA melanin, and a noticeable solubility in polar solvents of dermo-cosmetic relevance. The hydrogen- and/or electron-donor ability, and the iron (III) reducing power as determined by conventional assays provided evidence for marked antioxidant properties not merely ascribable to the more favorable solubility profile, while the inhibitory action of the radical- or photosensitized solar light-induced lipid peroxidation was more marked compared to that of DHICA melanin. Overall, these results hint at this melanin, which remarkable properties are, in part, due to the electronic effects of the carboxyamide functionality as a promising functional ingredient for dermo-cosmetic formulations.

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