The introduction of oxygen functionalities across the graphene lattice was confirmed qualitatively through several techniques including ζ-potential, Raman and FTIR spectroscopy measurements, which all indicated the chemical transformation of the starting graphite. All three GO materials evidenced similar colloidal stability in ultrapure water (Figure ). The stability of GO in polar solvents is determined by the presence of oxygen functionalities, particularly carboxyl and hydroxyl groups, which can confer a negative charge to the surface of GO flakes and mediate electrostatic repulsion forces between them, thus preventing aggregation of the materials in water .