Water-soluble graphene quantum dots (GQDs, ca. 10 nm in diameter) that exhibit bright blue photoluminescence (PL) are prepared by hydrothermal (chemical) cutting of oxidized graphene sheets (see figure). The mechanisms of the cutting and luminescence are discussed. his discovery of PL of GQDs may extend T the range of application of graphene-based materials to optoelectronics and biological labeling.