We carefully analysed the surface chemistry of the produced GO materials using complementary techniques. Despite the similar functionalisation degree evidenced by TGA (Figure ), ζ-potential measurements (Figure ) and FTIR spectroscopy (Figure ) suggested minor changes to the chemistry of us-GO flakes, with potentially lower abundance of carboxyls and hydroxyls or ethers, which could reduce their colloidal stability. Using XPS, we quantitatively assessed those changes, which consisted in a minor decrease of C-O bonds evidenced in high-resolution O1s spectra (Figure ). Taken together, these results indicate that the overall surface chemistry of GO flakes was not significantly affected by sonication. We hypothesise that the observed changes are due to the breakage of ethers, including epoxides, during the sonication process. Epoxides are a particular type of sp 3 defects present in GO, that are attributed with greater strain, compared to other oxygen functionalities, and hence are more prone to break under sonication . As they should populate the basal plane of GO, they offer several hot spots to break down into smaller flakes. Various studies have predicted that consecutive epoxy groups aligned on the basal plane could promote the formation of cracks due to the increased strain exerted on the graphene lattice . This would lead to the separation of C-O-C to form C=O and C-H , which would lead to a more dramatically decreased abundance of C-O bonds in the O1s spectra, as observed here (Figure ). Therefore, our sonication protocol promoted specifically the breakage of larger GO flakes through the already oxidised sp 3 defects, as suggested by the AFM images (Figure ). This phenomenon is also supported by the reduced polydispersity observed between s-GO and us-GO (Figure ). The mild conditions used to sonicate GO could specifically reduce the lateral dimensions to produce different flake sizes whilst conserving all other examined properties. This conclusion was also supported by Raman spectroscopy (Figure ), which indicated that the sonication step did not induce additional defects to the crystal structure of GO. This physical breakage seems to have been accompanied by the hydration of oxygen functional groups at the newly formed edges, which is supported by the maintenance of the relative amount of carboxylic acids in GO, irrespective of lateral dimensions, as shown by XPS. Rearrangement reactions between these newly formed edges in the smaller GO flakes and the adsorbed water molecules could have resulted in the formation of carboxylic acids .