  The electrical properties of graphene are known to be modified by chemical
species that interact with it. We investigate the effect of doping of
graphene-based devices by toluene (C6H5CH3). We show that this effect has a
complicated character. Toluene is seen to act as a donor, transferring
electrons to the graphene. However, the degree of doping is seen to depend on
the magnitude and polarity of an electric field applied between the graphene
and a nearby electrode. This can be understood in terms of an electrochemical
reaction mediated by the graphene crystal.
