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.

Submerged arc discharge in liquids has shown to be a promising method to synthesize a wide variety of nanomaterials. However, it requires an accurate arc current control to ensure the desired purity and structure of the products. A fluctuating arc current increases the dispersion in size distribution, as well as in the obtained nanoparticles structure pattern. Consequently, the arc current stability is essential to ensure the product homogeneity and quality. A system which ensures high stability of the arc discharge is presented. It has three basic elements: an electrode gap micropositioning system controlled by a feedback arc current measurement, a current’s stabilization element and a data acquisition system to record the magnitudes of the relevant physical parameters. The most suitable algorithm for micropositioning system was determined. The utilization of a step motor gave an additional advantage in measuring the anode displacement. The employed stabilization element improves the current and power stability by 4 and 2.7 times, respectively. The data acquisition system allows taking control and information about the relevant parameters of the process and the interaction between them. This system, based on direct arc current measurement, is superior in terms of achieving higher stability and current sensibility, to the ones based on arc voltage or arc light emission. It is also an adaptable tool to carry on further experiments.

Submerged arc discharge (SAD) is a simple method to produce carbon nanostructures (CNSs). However, its potential cannot be fully exploited because it generates contaminants and unwanted by-products (CUBPs) that are difficult to eliminate. The formation mechanisms of CNSs and CUBPs were investigated by measuring the correlations between the SAD main parameters (current, voltage, power, anode displacement, and sound emission). It was demonstrated that the SAD takes place in a succession of stable and unstable zones that induce homogeneous and heterogeneous nucleation processes, respectively. In the stable zones, carbon vapor jets are generated and induce the appearance of vortices. Both processes stimulate nucleation. From the measurement of the sound emitted by the jets, the dimensions of the discharge channel were determined. These dimensions match the anode crater size measured by scanning electron microscopy. In the unstable zones, vibrations and thermal stress in the anode intensify. Graphite microparticles are released and act as nucleation centers that induce the formation of CUBPs. While most of the discharge elapses in stable zones, the highest fraction of anode erosion occurs in unstable zones. These results made evident that current theoretical models fail to explain the presence of observed impurities because they do not take into account the influence of vibrations and heterogeneous nucleation. The operation of the synthesis device was simulated, and the results obtained reinforce the aforementioned conclusions. The acoustic emission of the SAD allowed obtaining information on the installation operation for the optimization of its design. Based on this information, recommendations were made for the installation design.

Many papers, in which the submerged arc discharge (SAD) method in nanoparticle synthesis was used, reported similar operating parameters, but different electrode erosion rate values, different yields and purities of the obtained nanostructures, and a different sort of contaminants present in the synthesis. Analyzing these articles, we found insufficient attention to ensure the arc power stability, which is a key factor guaranteeing the product homogeneity and quality. This paper presents an analysis of different control strategies, remarks their advantages and drawbacks, and proposes the most appropriate technique to be used in SAD. The most appropriate technique is proposed from the SAD stabilization method analysis.

Submerged electric arc discharge in liquids has shown to be a promising method for synthesizing a wide variety of nanomaterials. However, it requires an accurate current stability control to ensure the desired purity and structure of the products. The discharge stability control through light emission has been previously studied, but still requires further investigation to clarify the influence of some parameters. The present work has studied the solution's transmittance variation over time, the correlation between the arc light emission and the arc current, and the feasibility of controlling the arc current by using a specific wavelength of the arc light spectrum. Several limitations of the optoelectronic control were found at low currents (I < 50 A).