Synthesis of Graphene Oxide (GO). All the glassware was washed
by aqua regia (HNO3/HCl = 1:3, v/v) and then rinsed by Milli-Q
water at least twice (Caution!: aqua regia is a quite strong
oxidant and should be handled with great care). Singlelayer
GO sheets were synthesized from natural graphite by
a modified Hummers method.The as-prepared GO sheets
were dispersed into ethanol or water with a concentration
of 0.2 mg mL1 by sonication.