Carbon dots (CDs) due to their unique optical features, chemical stability and low environmental hazard are applied in different fields such as metal ion sensing, photo-catalysis, bio-imaging and tribology, among others. The aims of the present research were to obtain CDs from vegetable wastes (tea and grapes) as carbon sources and to explore their potential properties as radical scavengers. CDs from glutathione/citric acid (GCDs) were synthetized for comparison purposes. The CDs were investigated for their chemical structure, morphology, optical and electronical properties. The antioxidant activity has been explored by DPPH and Folin-Ciocelteau assays in aqueous media. Due to their solubility in oil, the CDs prepared from tea wastes and GCDs were assayed as antioxidants in a mineral oil lubricant by potentiometric determination of the peroxide value. CDs from tea wastes and GCDs exhibited good antioxidant properties both in aqueous and oil media. Possible mechanisms, such as C-addition to double bonds, H-abstraction and SOMO-CDs conduction band interaction, were proposed for the CDs radical scavenging activity. CDs from natural sources open new application pathways as antioxidant green additives.

Lipids are widely distributed in nature and are one of the most important components of natural foods, synthetic compounds, and emulsions. To date, there is a strong social demand in the industrial sector for the use of sustainable products with a minimal environmental impact. Depending on their origin and composition, lipids can be employed as a plausible alternative as biodegradable lubricants in order to reduce the use of conventional mineral oil lubricants and mitigate their environmental impact. This perspective provides an overview of the advantages and constrains of vegetal oils under different lubrication regimes and the tribochemical reactions that can take place. Also, the different factors and pathways that influence their oxidation, the key role of moisture, and the changes of physical properties under pressure and temperature are reviewed. Special emphasis is devoted to the oxidation instability of fatty acids and vegetal oils and the physical and chemical approaches to improve oxidative and thermal stability are described in detail.

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Since their discovery in 2004, carbon dots have attracted strong interest in the scientific community due to their characteristic properties, particularly their luminescence and their ease of synthesis and derivatization. Carbon dots can be obtained from different carbon sources, including natural products, resulting in a so-called ’green synthesis’. In this work, we obtain carbon dots from tomato juice in order to obtain nanoparticles with the antioxidant capabilities of the natural antioxidants present in that fruit. The obtained material is characterized regarding nanoparticle size distribution, morphology, surface functional groups and optic properties. Antioxidant properties are also evaluated through the DPPH method and their cytotoxicity is checked against human dermal fibroblast and A549 cell-lines. The results indicate that carbon dots obtained from tomato have a higher antioxidant power than other already-published antioxidant carbon dots. The bandgap of the synthesized materials was also estimated and coherent with the literature values. Moreover, carbon dots obtained from tomato juice are barely toxic for healthy cells up to 72 h, while they induce a certain cytotoxicity in A549 lung carcinoma cells.

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Nowadays, proteins and polysaccharides play a fundamental role in the manufacturing of biocompatible materials applied in food packaging. The resulting films have, however, limits associated with the resistance to mechanical stress; therefore, it is important to reinforce the initial mixture with additives that promote the development of stronger molecular links. Carbon dots (CDs) are excellent candidates for this purpose due to the presence of surface functional groups that determine the formation of numerous intramolecular bonds between the charged biopolymers. The present research aims to evaluate the effect of CDs on the mechanical properties of biopolymer films obtained from sodium caseinate (CAS), high methoxyl pectin (HMP) and glycerol used as plasticizers. Green carbon dots (gCDs) were obtained from natural organic sources by green synthesis. The effects of gCDs on the flow behavior and viscoelastic properties of mixed biopolymer dispersions and the thermophysical properties of the corresponded films were evaluated by steady and unsteady shear rheological measurements and differential scanning calorimetry (DSC) tests, respectively. The dynamic mechanical measurements were realized taking into account the parameters of temperature and relative humidity. The results indicate a significant change in the viscosity of the protein–polysaccharide dispersions and the thermomechanical properties of the corresponding film samples reinforced with higher amounts of gCDs.

The deterioration of oil-based products during processing, distribution and storage has a major negative impact on the industry from an economic point of view. The spoilage of oil is mainly due to its oxidation which can be triggered by various factors, such as UV light, heating or the presence of impurities that result in the formation of radical species. In this context, several packaging alternatives have recently been developed with the aim to protect and extend the shelf life of oil-based products. This work aimed to study the antioxidant properties of bio-polymer-based films (BPFs) obtained from high methoxylated pectin (HMP) and sodium caseinate (CAS) and enriched with different concentrations of green carbon dots (gCDs), 0.25%, 0.50 and 1% w/w, obtained from apple pomace (APCDs) and rosemary powder (RCDs). The resulting films (gCDs-BPFs) have shown that the presence of gCDs not only modified the surface roughness of the films, but also positively affected their antioxidant properties. The addition of gCDs enhanced the radical inhibiting capacity of the raw BPFs by 42 and 62% for the films containing 1% RCDs and 1% APCDs, respectively. As a proof of the concept, two oil samples (edible and cosmetic) were treated with the obtained antioxidant films, and the results demonstrated that in both types of samples the oxidation process was minimized during the five days of the experiment. These results are promising and suggest that the antioxidant bio-polymer-based films could be excellent candidates for further production of active packaging.

The pollution caused by heavy metals (HMs) may occur through both natural processes and anthropogenic activities and is found in complex media. The purpose of this review is to summarize the state-of-art of fluorescent CDs and the sensing applications in a systematic manner. This review intends to provide clues on the origin on the observed selectivity in chemiluminiscence sensors, which was until now a stated but unaddressed question, and still remains open for debate. Indeed, it is tempting to think that CDs possessing functional groups with soft bases at the surface are able to detect soft metal acids, while the opposite is to be suspected for hard acid-base pairs. However, the literature shows several examples where this trend does not hold. We found that such observation is explained by the involvement of dynamic quenching, which does not involve the formation of a nonfluorescent complex, as in the case of static quenching. We have provided an interpretation of published data that was not provided by the original authors and offer guidelines to enable the design of CDs to target ions in solution.