The synthesis of sulfonated graphene is performed by several steps as described in Figure 1 A. Graphene oxides are prepare from graphite by using a modifi ed Hummers method,and then the prereduction of graphene oxide is performed with sodium borohydride at 80 ° C for 1 h to remove the majority of the oxygen functionality. The sulfonation of graphene oxide is then carried out with the aryl diazonium salt of sulfanilic acid in an ice bath for 2 h, followed by post-reduction with hydrazine (at 100 ° C for 24 h) to remove any remaining oxygen functionality. [ 6 ] The obtained products of sulfonated graphene are shown in Figure 1 B and C. It is obvious that sheets of sulfonated graphene could be produced in large size and over a large area. Moreover, the thickness of the sulfonated graphene is only ∼ 3 nm, i.e., three or four-layered graphene sheets