Identification of chemical features. All three GO types exhibited comparable colloidal stability in water, according to ζ-potential measurements (Figure ). In addition, the Raman spectra obtained upon the irradiation with a 633 nm laser indicated the appearance of defects in the crystal structure of graphene (Figure ), as indicated by the emergence of a typical D band (~1338 cm -1 ) as opposed to the G band (~1590 cm -1 ). The crystal structure of GO was quantitatively studied by calculating the ratio between the Raman intensities of the D and G bands (ID/IG). This ratio did not significantly differ among the three GO materials.