The continuous decrease in the availability of fossil resources, along with an evident energy crisis, and the growing environmental impact due to their use, has pushed scientific research towards the development of innovative strategies and green routes for the use of renewable resources, not only in the field of energy production but also for the production of novel advanced materials and platform molecules for the modern chemical industry. A new class of promising carbon nanomaterials, especially graphene quantum dots (GQDs), due to their exceptional chemical-physical features, have been studied in many applications, such as biosensors, solar cells, electrochemical devices, optical sensors, and rechargeable batteries. Therefore, this review focuses on recent results in GQDs synthesis by green, easy, and low-cost synthetic processes from eco-friendly raw materials and biomass-waste. Significant advances in recent years on promising recent applications in the field of electrochemical sensors, have also been discussed. Finally, challenges and future perspectives with possible research directions in the topic are briefly summarized.

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We report the synthesis and the characterization of different multi-walled carbon nanotubes (MWCNTs) linked to natural molecules, 5,7-coumarins and/or oleic acid, obtained from purified pristine MWCNTs by a cascade of chemical functionalization. The activities of these modified MWCNTs were investigated in vitro on human umbilical vein endothelial cells (HUVECs) by evaluating their ability to influence cell viability and to induce cell apoptosis. Our data showed that pristine MWCNTs are markedly cytotoxic; conversely, the carboxylated carbon nanotubes, much more readily dispersed in aqueous solutions and CNT-Link, the key intermediate designed by us for the drug anchorage, are biocompatible at the tested concentrations (1 and 10 μg ml(-1)).

The acidic oxidation with HNO3/H2SO4 mixtures is widely reported as an effective method to functionalize multi-walled carbon nanotubes (MWCNT). Although effective, a bad control of the oxidation conditions frequently cause serious modifications of carbon nanotube network, limiting further potential applications. Investigations about the effect of functionalization operating conditions on the morphological, chemical and chemical-physical properties of MWCNT can be useful for a proper setting of oxidation reactions of MWCNT according to their specific applications. In this work the effect of HNO3/H2SO4 ratio on the morphological and chemical-physical properties and on the degree of functionalization of MWCNT was investigated. Electron microscopy, thermogravimetric, X-ray diffraction, titration and water dispersion analyses clearly revealed that the increase of the amount of concentrated sulphuric acid in the HNO3/H2SO4 mixture lead to an increase of the amount of functional groups on the MWCNT surface but also to an increase of structural damage in terms of tube cutting and generation of additional defects in the graphitic network of pristine

The timely diagnosis of cancer represents the best chance to increase treatment success and to reduce cancer deaths. Nanomaterials-based biosensors containing graphene quantum dots (GQDs) as a sensing platform show great promise in the early and sensitive detection of cancer biomarkers, due to their unique chemical and physical properties, large surface area and ease of functionalization with different biomolecules able to recognize relevant cancer biomarkers. In this review, we report different advanced strategies for the synthesis and functionalization of GQDs with different agents able to selectively recognize and convert into a signal specific cancer biomarkers such as antigens, enzymes, hormones, proteins, cancer related byproducts, biomolecules exposed on the surface of cancer cells and changes in pH. The developed optical, electrochemical and chemiluminescent biosensors based on GQDs have been shown to ensure the effective diagnosis of several cancer diseases as well as the possibility to evaluate the effectiveness of anticancer therapy. The wide linear range of detection and low detection limits recorded for most of the reported biosensors highlight their great potential in clinics for the diagnosis and management of cancer.

Carbon dots (CDs) samples were synthesized from orange peel waste (OPW) via a simple and eco-friendly hydrothermal carbonization (HTC) and electrochemical (EC) bottom-up synthesis integrated approach. The comprehensive chemical-physical characterization of CDs samples, carried out by various techniques such as TEM, EDX, XRD, FT-IR, underlined their morphological and microstructural features. The CDs exhibited attractive electrochemical properties, and thus an electrochemical sensor by modifying a screen printed carbon electrode (CDs/SPCE) for the detection of nitrobenzene (NB) in water was developed. Electroanalytical performances of CDs/SPCE sensor using differential pulse voltammetry (DPV) demonstrated its high sensitivity (9.36 μA μM À 1 cm À 2 ) towards NB in a wide linear dynamic range (0.1-2000 μM) and a low limit of detection (LOD = 13 nM). The electrochemical sensor also shown high selectivity, long-term stability, and repeatability. This paper might open the way to a new synergistic HTC-EC approach for the synthesis of CDs from waste biomass material and their advanced application in highly efficient electrochemical sensors.

The click cycloaddition of azides to alkynes affording 1,2,3‐triazoles is a widely used and effective chemical transformation, applied to obtain relevant products in medicine, biology, and materials science. In this work, a set of natural deep eutectic solvents (NADESs) has been investigated in the copper‐catalyzed azide‐alkyne cycloaddition reactions (CuAAc) as green and catalytic reaction media. The use of these innovative solvents has been shown to improve the reaction effectiveness, giving excellent yields. NADESs proved to be “active” in their reducing capabilities in several cases and for the absence of added bases in all the performed reactions: DFT calculations demonstrated in fact the involvement of H‐bonds between DESs and alkynes, as well as stabilization of copper catalytic intermediates. The green experimental conditions, the absence of a base, the low temperatures, the lowering of reagent amounts, and the possibility of recycling of the solvents, outline the great potential of NADESs for CuAAc.

The concentration of sodium and potassium ions in biological fluids, such as blood, urine and sweat, is indicative of several basic body function conditions. Therefore, the development of simple methods able to detect these alkaline ions is of outmost importance. In this study, we explored the electrochemical and optical properties of graphene quantum dots (GQDs) combined with the selective chelating ability of the crown ethers 15-crown-5 and 18-crown-6, with the final aim to propose novel composites for the effective detection of these ions. The results obtained comparing the performances of the single GQDs and crown ethers with those of the GQDs-15-crown-5 and GQDs-18-crown-6 composites, have demonstrated the superior properties of these latter. Electrochemical investigation showed that the GQDs based composites can be exploited for the potentiometric detection of Na+ and K+ ions, but selectivity still remains a concern. The nanocomposites showed the characteristic fluorescence emissions of GQDs and crown ethers. The GQDs-18-crown-6 composite exhibited ratiometric fluorescence emission behavior with the variation of K+ concentration, demonstrating its promising properties for the development of a selective fluorescent method for potassium determination.