Carbon quantum dots (CQDs) are novel nanostructures that have great potential as fluorescent markers due to their multi-fluorescence, down and up converted emission, resistance to photobleaching, and biocompatibility. Here, we report the synthesis of fluorescent CQDs by the submerged arc discharge in water method. We discuss the method's simplicity, natural phases’ separation, and scalability. The produced CQDs size distribution was in the range of 1–5 nm. High-resolution transmission electron microscopy images and their fast Fourier transformation allowed the analysis of the CQDs’ internal structure. The absorption and fluorescence spectra of the as-produced CQDs were analyzed. The UV-Vis spectrum shows a single band with a maximum located at 356 nm. The photoluminescence emission presents two consistent bands with maxima located in the ranges of 320–340 nm (band A) and 400–410 nm (band B). To these emission bands correspond two bands in the excitation spectra located at 275 nm (band A) and 285 nm (band B). The fluorescence quantum yield was assessed as ∼16% using Rhodamine 6G as reference. The capabilities of the produced CQDs as fluorescent markers for in vitro studies were also evaluated by setting them in contact with a cell culture of L929 murine fibroblasts. Control and CQD-treated cell cultures were visualized under a fluorescence microscope. Finally, the mechanism of formation of these nanostructures by top-down methods is discussed, and a general model of formation is proposed.
Nanoparticles toxicity and cellular uptake are substantial factors that regulate the utility of nanoparticles for biomedical applications in diagnostic imaging and therapy. In this work, the cellular uptake of pristine carbon nano-onions (CNOs) produced by submerged arc discharge was confirmed by transmission electron microscopy (TEM) analysis. Carbon nano-onions were localized within early and late endosomes of human keratinocyte cells. TEM images suggest the possible degradation of these structures, in late endosomes of keratinocyte cells after 24 h. Proteomic shotgun analysis revealed that carbon nano-onions cause statistically significant protein abundance changes in a total of 168 proteins. Most of the differentially expressed proteins are involved in biological processes such as metabolic, cellular and immune system processes. Treated cells showed increased expression of clathrin heavy chain 1, calnexin and isoforms β/α, ε and σ 14-3-3 protein, which are related to clathrin-mediated endocytosis. Thus changes in protein abundance concerning clathrin-mediated endocytosis may suggest this as a plausible way by which the carbon nano-onions intracellular traffic occurs.
Carbon nano-onions (CNO) are versatile carbon nanomaterials with many potential biomedical applications. In this work, the interaction of submerged arc discharge in water (SADW) produced CNOs with the neutral red (NR) dye was studied. This dye is used in the in vitro toxicity NR assay, one of the most commonly used dye-based procedures to determine cell viability. Firstly the NR assay was carried out in murine fibroblast cell cultures exposed to CNOs. It was demonstrated that this assay produced invalid results due to the strong adsorption of NR on the CNOs. Fourier transform infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS) studies confirmed the effective adsorption of the NR on CNOs and π-π stacking as the main interaction between them. The adsorption of NR on the CNOs was evaluated by studying the decrease of the dye solution absorbance. The influence of different experimental conditions such as pH and CNOs dosage was evaluated: absorbance was found to diminish with the CNO dosage. For the maximum dosage used of 240 μg ml−1, the highest absorbance drops of −85% at pH 7 and −78% at pH 4 were registered. The adsorption process was found to be described best by a pseudo-first order (PFO) kinetics model (R 2 = 0.99), with a kinetic adsorption constant of k 1 = 0.02 min−1 and achieving an estimated sorption capacity of 3866 mg of dye per gram of CNOs. This is one of the highest values ever reported for dyes’ adsorption on carbon materials. Lastly, density functional theory (DFT) calculations were carried out to gain further insights into the interaction. These studies suggest a CNO highest occupied molecular orbital (HOMO)/NR lowest unoccupied molecular orbital (LUMO) electron density transfer as the main orbital interaction.
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