Facile and simultaneous synthesis of graphene quantum dots and reduced graphene oxide for bio-imaging and supercapacitor applications

A simple and facile method for the simultaneous preparation of water soluble, fluorescent graphene quantum dots (GQDs) and reduced graphene oxide (RGO) is proposed and demonstrated in the present work. This particular method involves constant current electrolysis of a graphite rod either in an aqueous solution or in absolute ethanol medium consisting of sodium methoxide as an electrolyte. The process of electrolysis is carried out at a fixed current density for varied durations. Further chemical reduction of the resultant electrochemically exfoliated graphitic solution produces a clear supernatant solution and a black precipitate. Depending upon the solvent medium, duration of electrolysis and nature of the reducing agents, the size and in turn the emission colour of GQDs could be altered. In this work specifically GQDs that emit blue, green and yellow colours have been prepared. Structural, morphological and chemical characteristics of these GQDs are analyzed by using microscopic [scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM)], spectroscopic (UV-Vis spectroscopy, Fourier transform infrared (FTIR), X-ray diffraction (XRD), Raman) and electrochemical techniques. Microscopic images obtained using HRTEM analysis clearly reveal the formation of lattice fringe patterns for these multi-coloured GQDs along with uniform particle size distribution. Further these fluorescent GQDs are utilized for bio-imaging applications and cytotoxicity studies using Staphylococcus aureus and Escherichia coli bacteria. Similarly, the resultant black precipitate is identified as a few layered reduced graphene oxide and the structural and morphological characterization is carried out using SEM, FTIR, XRD and Raman spectroscopic studies. Further electrochemical investigation indicates the possible application of such layered RGO as a supercapacitor electrode material possessing a fast response time in the order of seconds for delivering the stored power.