Mercury ion (Hg2+) is one of the most toxic pollutants that can exist throughout the environment and be diffused into water, soil, air, and eventually the food chain. Even a very low level of Hg2+ diffused in living organisms can hurt their DNA and cause the permanent damage of the central nervous system and a variety of consequential disorders. Hence, the development of a sensitive and specific method for the detection of Hg2+ at trace ranges is extremely important as well as challenging. Fluorometric detection assays based on graphene quantum dots (GQDs) and carbon quantum dots (CQDs) offer considerable potential for the determination and monitoring of heavy metals due to their fascinating properties. Although the quantum yield of GQDs and CQDs is sufficient for their use as fluorescent probes, doping with heteroatoms can significantly improve their optical properties and selectivity toward specific analytes. This review explores the primary advances of CQDs and GQDs in their great electronic, optical, and physical properties, their synthetic methods, and their use in Hg2+ fluorimetry detection.

Graphene quantum dots (GQDs) are useful nanomaterials of excellent water solubility, biocompatibility and optical stability for various applications. In this study, we achieved the nitrogen fixation activity enhancement of autogenous azotobacters (Azotobacter vinelandii) by GQDs and revealed its mechanism. GQDs (1-10 mg/L) accelerated the logarithmic growth phase of A. vinelandii, making bacteria enter the plateau earlier. Biocompatible GQDs did not cause cell death and oxidative damage of A. vinelandii. The expression levels of metabolism and nitrogen fixation related genes were significantly up-regulated by GQDs, thereby enhancing the activity of nitrogenase. The nitrogen fixation activity of A. vinelandii reached 471.7 % of the control upon the exposure to GQDs. The intracellular polyhydroxyalkanoate vesicles were consumed to provide energy for nitrogen fixation, which involved with a chain reaction of carbohydrate metabolism, lipid metabolism, and energy metabolism pathways accompanied by significant increases in electron transfer rate. The synergistic effect of GQDs on A. vinelandii further facilitated the augmentation of soil nitrogen content in Qinghai-Tibet Plateau. Our results provided an effective approach to enhance the biological nitrogen fixation by GQDs.