The morphology of GO
and lyophilized GO/PEI hydrogels was characterized by
using both field-emission scanning electron microscopy
(FE-SEM, S-4800II, Hitachi, Japan) with an accelerating
voltage of 5–15 kV and transmission electron microscopy
(TEM, HT7700, Hitachi High-Technologies Corporation)
with commercial 300-mesh copper grids. Before SEM investigations,
the prepared samples were coated with copper
foil fixed by a conductive adhesive tape and covered
with gold nanoparticles to make them more conductive