N-Doped Carbon Dot Hydrogels from Brewing Waste for Photocatalytic Wastewater Treatment

The brewery industry annually produces huge amounts of byproducts that represent an underutilized, yet valuable, source of biobased compounds. In this contribution, the two major beer wastes, that is, spent grains and spent yeasts, have been transformed into carbon dots (CDs) by a simple, scalable, and ecofriendly hydrothermal approach. The prepared CDs have been characterized from the chemical, morphological, and optical points of view, highlighting a high level of N-doping, because of the chemical composition of the starting material rich in proteins, photo-luminescence emission centered at 420 nm, and lifetime in the range of 5.5-7.5 ns. With the aim of producing a reusable catalytic system for wastewater treatment, CDs have been entrapped into a polyvinyl alcohol matrix and tested for their dye removal ability. The results demonstrate that methylene blue can be efficiently adsorbed from water solutions into the composite hydrogel and subsequently fully degraded by UV irradiation

Synthesis of Carbon Dots for medical and photocatalytic applications

Carbon dots (CDs) are a novel carbon-based nanomaterial that draws a great attention in the last decade. The easy way of synthesis and the cheapest nature of the precursors used, stimulate a lot of scientist to propose new way of synthesis and applications. Notwithstanding the number of paper published, a systematic and reproducible way of synthesis is not yet been achieved, as well as a common definition of the resulting nanomaterials and their properties. In this thesis, the synthetic protocols and the characterization procedures used for the CDs will be deeply investigated in order to apply the nanomaterials in different applications. In particular, carbon dots synthesized from fully biocompatible precursors such as glucose, fructose and ascorbic acid, are characterized and employed for drug loading applications. The study reveals that the choice of the precursors is a crucial step because it affects the structural properties of the nanomaterials and the biological properties, revealing an unexpected toxicity for the fructose derived CDs, ascribable to its thermal degradation pathway. Furthermore, the drug loading capabilities were found to be correlated to the morphology of the nanoparticles, revealing the crucial role of the \u3c0-\u3c0 interactions in achieving a loading up to 28 %wt for the glucose-based CDs. A second study was conduct on citric acid based CDs, in order to determine the best synthetic approach for photocatalytic applications. The nanoparticles synthesized both by hydrothermal and pyrolysis treatment, from sole citric acid and in combination with a nitrogen doping agent, were deeply characterized. The analysis reveals the difference in chemical, structural, and optical properties between the two synthetic methods. The photoreduction of methylviologen (MV) was used as model reaction to study the photocatalytic ability of the CDs, and the results reveal a relationship between the synthetic methods employed, the structural and optical properties of the CDs and their ability to act as a sensitizer. The amorphous nitrogen doped CDs reveals to be the best choice for this application with an initial rate conversion comparable to other reported in literature. The same CDs were also tested for the photocatalytic cleavage of C-O bonds in activated esters without the use of metals. The study reveals that CDs can successfully catalyze the reaction, with complete conversion and almost total selectivity. In addition, the CDs employed, shows different reactivity depending on the precursors and synthetic methods employed for their synthesis, with the same trend displayed for the MV photoreduction. This study highlights the ability of CDs to act as photosensitizer, without the addition of metals, in an organic reaction, opening a new scenario in the use of this nanomaterial in photocatalytic applications

New perspectives in lignin valorization: Lignin-derived nanostructures

Lignin is undoubtedly one of the most interesting biomasses in terms of high added-value materials obtainable from its valorization. As by-product of pulp and paper industries and biorefineries, it is produced in large volumes and is readily available at low cost. The unlocking of its full potential is therefore of crucial importance in view of a sustainable growth based on circular economy paradigms. In this context, the generation of lignin-based nanomaterials is attracting considerable attention as the self-assembly characteristics of this biopolymer can be easily exploited, thus avoiding time- and resource-consuming functionalization or purification steps and, most importantly, preserving all its peculiar and unique features. In the last years, many researchers have devoted their efforts toward the development of more efficient and sustainable procedures for the synthesis of lignin-derived nanomaterials, also expanding the possible applications thanks to the easy tunability of their functional properties. In this contribution, the most important synthetic procedures for the obtainment of lignin nanoparticles, nanocapsules and nanofibers are critically revised and discussed, and the range of uses they have been tested for is presented

The Laccase-Lig Multienzymatic Multistep System in Lignin Valorization

A laccase-Lig multienzymatic multistep system for lignin depolymerization was designed and developed. Studies were performed on pristine and fractionated lignins (Kraft and Organosolv) using a specific cascade of enzymes, that is, laccases from Bacillus licheniformis and from Funalia trogii, respectively for Kraft and Organosolv lignin, followed by the Lig system from Sphingobium sp. SYK-6 (β-etherases Lig E and Lig F, glutathione lyase Lig G). Careful elucidation of the structural modifications occurring in the residual lignins associated with the identification and quantification of the generated low-molecular-weight compounds showed that (i) the laccase-Lig system cleaves non-phenolic aryl glycerol β-O-4 aryl ether bonds, and (ii) the overall reactivity is heavily dependent on the individual lignin structure. More specifically, samples with low phenolic/aliphatic OH groups ratio undergo net depolymerization, while an increased phenolic/aliphatic OH ratio results in the polymerization of the residual lignin irrespective of its botanical origin and isolation process

Kraft (Nano)Lignin as Reactive Additive in Epoxy Polymer Bio-Composites

The demand for high-performance bio-based materials towards achieving more sustainable manufacturing and circular economy models is growing significantly. Kraft lignin (KL) is an abundant and highly functional aromatic/phenolic biopolymer, being the main side product of the pulp and paper industry, as well as of the more recent 2nd generation biorefineries. In this study, KL was incorporated into a glassy epoxy system based on the diglycidyl ether of bisphenol A (DGEBA) and an amine curing agent (Jeffamine D-230), being utilized as partial replacement of the curing agent and the DGEBA prepolymer or as a reactive additive. A D-230 replacement by pristine (unmodified) KL of up to 14 wt.% was achieved while KL–epoxy composites with up to 30 wt.% KL exhibited similar thermo-mechanical properties and substantially enhanced antioxidant properties compared to the neat epoxy polymer. Additionally, the effect of the KL particle size was investigated. Ball-milled kraft lignin (BMKL, 10 μm) and nano-lignin (NLH, 220 nm) were, respectively, obtained after ball milling and ultrasonication and were studied as additives in the same epoxy system. Significantly improved dispersion and thermo-mechanical properties were obtained, mainly with nano-lignin, which exhibited fully transparent lignin–epoxy composites with higher tensile strength, storage modulus and glass transition temperature, even at 30 wt.% loadings. Lastly, KL lignin was glycidylized (GKL) and utilized as a bio-based epoxy prepolymer, achieving up to 38 wt.% replacement of fossil-based DGEBA. The GKL composites exhibited improved thermo-mechanical properties and transparency. All lignins were extensively characterized using NMR, TGA, GPC, and DLS techniques to correlate and justify the epoxy polymer characterization results

Carbon dots as photocatalysts for organic synthesis: metal-free methylene–oxygen-bond photocleavage

We report for the first time that irradiation of four different citric acid-derived carbon dots (CDs), in the absence of any other redox mediators, promotes an organic reaction. In this proof-of-concept study methylene-oxygen bond reductive photocleavage in N-methyl-4-picolinium esters is demonstrated. Cyclic voltammetry and UV-Vis spectra of the CDs and of the esters indicate that photocleavage reactivity correlates with the redox properties and the relative energies expressed in the Fermi scale. A photo-fragmentation mechanism is proposed. This study offers a new possibility to employ inexpensive and readily available CDs to promote photo-organic reactions

Carbon-dots from sugars and ascorbic acid: Role of the precursors on morphology, properties, toxicity and drug uptake

There is the need for reproducible, simple, high-yielding synthetic protocols aimed at obtaining Carbon Dots (CDs) with controlled surface properties, fluorescence, photothermal and photochemical behavior, biocompatibility, tumor targeting ability, drug absorption biodistribution and tumor uptake. This paper describes a systematic study on the effect of glucose, fructose and ascorbic acid as starting materials for the preparation of highly luminescent CDs, characterized by a blue emission. Their composition and morphology are investigated by titration of OH surface groups, spectroscopic techniques, high-resolution-transmission electron microscopy (HR-TEM) and their toxicity was tested toward HeLa cells. CDs made using fructose were toxic while CDs made from glucose and ascorbic acid showed good biocompatibility. The reproducible and simple synthetic procedure yields luminescent biomass-derived CDs for combined cancer therapy and diagnostics. Their doxorubicin (DOX) drug loading capabilities were measured by spectrofluorimetry indicating a crucial role of the morphologies of the CDs in controlling DOX uptake. The glucose derived CDs showed up to 28 %w/w of DOX loading

Design of Carbon Dots for Metal-free Photoredox Catalysis

The photoreduction potential of a set of four different carbon dots (CDs) was investigated. The CDs were synthesized by using two different preparation methods-hydrothermal and pyrolytic-and two sets of reagents-neat citric acid and citric acid doped with diethylenetriamine. The hydrothermal syntheses yielded amorphous CDs, which were either nondoped (a-CDs) or nitrogen-doped (a-N-CDs), whereas the pyrolytic treatment afforded graphitic CDs, either non-doped (g-CDs) or nitrogen-doped (g-N-CDs). The morphology, structure, and optical properties of four different types of CDs revealed significant differences depending on the synthetic pathway. The photocatalytic activities of the CDs were investigated as such, that is, in the absence of any other redox mediators, on the model photoreduction reaction of methyl viologen. The observed photocatalytic reaction rates: a-N-CDs ≥ g-CDs > a-CDs ≥ g-N-CDs were correlated with the presence/absence of fluorophores, to the graphitic core, and to quenching interactions between the two. The results indicate that nitrogen doping reverses the photoredox reactivity between amorphous and graphitic CDs and that amorphous N-doped CDs are the most photoredox active, a yet unknown fact that demonstrates the tunable potential of CDs for ad hoc applications