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Green synthesized graphene quantum dots (GQD) have been doped with nitrogen in an attempt to boost their optical characteristics and application sectors. In the present investigation, the blue luminescent nitrogen-doped GQDs (N-GQDs) were synthesized by single-step hydrothermal synthesis using tamarind shell powder as a precursor. The particle size and zeta potential of N-GQDs were found to be 11.40 nm and be –35.53 mV, respectively. A quantum yield as high as 23.78 % was accomplished at an excitation wavelength of 330 nm at neutral pH. It gets quenched sensitively in the existence of uric acid (UA) combining static quenching, electron transfer, and an inner filter effect mechanism. A linear range was obtained for UA from 10 µM to 100 µM, with a limit of detection (LOD) of 401.72 ± 0.04 pM. Additionally, the N-GQDs were selective toward UA in presence of metal ions and biomolecules that indicated its impending use to monitor UA in clinical samples. In conclusion, this work demonstrates that the N-GQDs as a sensing probe for UA recognition with notable advantages including socioeconomic, simple, and less time-consuming methods as compared to other methods. In the future, it can be potentially explored as a biosensor for UA detection in clinical samples.

Presented in this manuscript are our new research results regarding the fabrication of nitrogen-doped (N-doped) and sulfur-doped (S-doped) graphene quantum dots (GQDs) from citric acid (CA), using the microwave–hydrothermal method. This is a method that uses microwave energy for pyrolysis and carbonization of the precursors of carbon and doped element, followed by a hydrothermal process in NaOH, to yield doped GQDs. Studies using high-resolution transmission electron microscopy (HR-TEM) and Raman spectroscopy have allowed the confirmation of the fabricated products to be GQDs. Detailed studies of absorption, photoluminescence (PL), PL excitation (PLE) spectra, emission–excitation mapping (EEM) images, and PL spectra at different excitation wavelengths have allowed us to find the best excitation wavelength to obtain the highest PL quantum yield (QY) value from the sample, an example being the QY of the S-GQD samples reaching 40.1%. The absorption mechanism, vibration relaxation process, and non-radiative and radiative electron transition are also explained. [ABSTRACT FROM AUTHOR]

A straightforward methodology for obtaining Nitrogen-doped Graphene Quantum Dots (N -GQDs) with aggregation-induced emission (AIE) using salting-out purification processes is presented in this work. Various salts, including NaCl at concentrations of 50 mM, were found to be effective for stratification of graphene-based materials (GBMs) by salting-out treatments. These salts were shown to be safer, more cost-effective, and environmentally sustainable alternatives to (NH 4) 2 SO 4 reported in previous works. Remarkably, this method demonstrated the ability to promote the self-assembly of N -GQDs into stable J-aggregates-like arrays with an average fluorescence decay time of ca. 4 ns and absolute quantum yield with values of 3.5 % and 6.9 % for 100 mM and 2 M, respectively. In contrast to a typical quenching effect caused by π-stacking, these arrays exhibited aggregation-induced emissions, as evidenced by the presence of new excitation and emission bands at 532 and 600 nm, respectively. The scalability of the process, along with the absence of additional purification steps, renders these results of great scientific and technological interest. Furthermore, the modulable fluorescence, size, and stability of these GBMs suggest their potential application in energy cells, sensors, and biomedical imaging. [Display omitted] [ABSTRACT FROM AUTHOR]