Abstract: The size and surface chemistry of graphene oxide (GO) dispersed in an aqueous solution are tuned by adjusting the pH value of the parent GO hydrosol. This method is based on the protonation of the carboxyl groups on GO nanosheets (GONs) in an acidic environment and the partial removal of oxygen-containing functional groups in strong basic conditions. GONs with a high electrochemical activity can be obtained by tuning the pH-dependent sheet sizes and the fraction of functional groups. It is found that the functional groups of a GON are more crucial in providing a high electrochemical activity than are the plane edges, and the size of a GON is also a key factor to influence the chemical activity. GONs with smaller sheet sizes but similar functional groups show a weaker electrochemical activity than those with larger sizes. In addition, GONs possess a high activity towards H₂O₂ detection and hence are promising for use as an electrode of biosensors.