Despite precautions, thermal reduction of GO can still occur at temperatures as low as 50 o C, as a result of the reaction of carboxyl groups with water, releasing CO2 . Hence, one could expect that longer sonication times would yield smaller but slightly more reduced GO flakes. Moreover, longer sonication times could increase the density of defects across the graphene lattice. We therefore tested the impact of sonication on various physicochemical features including the surface chemistry and crystal structure of the resulting materials, namely s-GO and us-GO. All GO materials exhibited similar optical properties (Figure ), which could be associated to similar abundance of C=O bonds from carbonyl and carboxyl groups, as the oxygen atoms provide free electron pairs to perform n → π* transitions, linked to the absorbance at 300 nm (Figure ), and confer photoluminescence to GO (Figure ). The photoluminescence of GO could be studied using an excitation wavelength around 525 nm, which could be linked to π → π* transitions in the sp 2 hybridised aromatic network of graphene . However, we have also identified multiple excitation wavelengths that replicate the broad fluorescence emission band peaking around 600 nm . They have been associated with the variety of oxygen functionalities in GO, which provide free electrons that contribute for the electronic transitions required for fluorescence emission .