Graphene quantum dots obtained by unfolding fullerene

Graphene quantum dots (GQDs) have recently been proposed as fluorescent sensors for metal ions dissolved in water. The physical origin of this fluorescence is still not completely clear, therefore further studies on GQDs and their properties are required. Graphene-like quantum dots were prepared by oxidation and unfolding of commercial buckminsterfullerene. This process was carried out by a modified Hummers method. After dialysis, the solution was diluted with deionized water to obtain the reference solution for optical measurements. Part of the solution was dried to obtain the unfolded fullerene powder. The metal ions were allowed to interact with GQDs by adding the metal-salt water solution to the stock solution of unfolded fullerene. The properties of GQDs before and after incorporation of metal ions (Hg, Cd, Pb, Cu, Ni and As) were characterized by using X-ray photoemission spectroscopy (XPS), Auger electron spectroscopy, UV–vis and fluorescent spectroscopies. In XPS, a particular attention was given to the analysis of photoemission C 1 s and Auger C KVV spectra in order to determine the carbon electronic configuration and its changes after incorporation of metal ions into GQDs. Obtained quantum dots were composed of graphene with oxidized borders. Optical measurements showed that the interaction with metal ions was changing the fluorescence intensity and optical absorbance, with different characteristics for each distinct ionic species. This behavior could be used for a selective multiple sensor detecting different heavy metals in water solutions.