Preparation of graphene quantum dots from pyrolyzed alginate

Pyrolysis at 900 °C under an inert atmosphere of alginate, a natural widely available biopolymer, renders a graphitic carbon that upon ablation by exposure to a pulsed 532 nm laser (7 ns, 50 mJ pulse(-1)) in acetonitrile, water, and other solvents leads to the formation of multilayer graphitic quantum dots. The dimensions and the number of layers of these graphitic nanoparticles decrease along the number of laser pulses from 100 to 10 nm average and from multiple layers to few layers graphene (1-1.5 nm thickness), respectively, leading to graphene quantum dots (GQDs). Accordingly, the emission intensity of these GQDs increases appearing at about 500 nm in the visible region along the reduction of the particle size. Transient absorption spectroscopy has allowed detection of a transient signal decaying in the microsecond time scale that has been attributed to the charge separation state.