N-doped graphene quantum dots as a novel highly-efficient matrix for the analysis of perfluoroalkyl sulfonates and other small molecules by MALDI-TOF MS
N-doped graphene quantum dots (N-GQDs) were synthesized by a facile, rapid method based on a time-efficient bottom-up strategy. In addition, solid-phase extraction (SPE) was, for the first time, applied to the purification of N-GQDs, which was very time-efficient compared to the traditional dialysis method and facilitated the removal of salts and the obtention of N-GQDs with homogeneous size. The as-prepared N-GQDs, with good solubility in both organic solvents and water, were successfully employed as a novel matrix for the analysis of perfluoroalkyl sulfonates (PFSs) and other small molecules by MALDI-TOF MS. Compared with conventional matrices, N-GQDs displayed a superior efficiency, less background interferences, high sensitivity and good repeatability. The presence of polymorphous nitrogen species in GQDs and the π-conjugated structure contributed to an enhanced desorption/ionization efficiency. The homogeneity of the material provided a good repeatability. Furthermore, quantitative detection of perfluorooctane sulfonate (PFOS) in environmental samples was successfully determined by MALDI-TOF MS using N-GQDs; this suggests that N-GQDs have a potential to play an important role in the rapid, sensitive and precise analysis of trace levels of organic pollutants, such as PFSs. Thus, it is of great interest to broaden this method to apply it to the analysis of other small-molecular weight organic pollutants in the future.