  A major bottleneck in the fabrication of efficient bio-organic nanoelectronic
devices resides in the strong charge recombination that is present at the
different interfaces forming the complex system. An efficient way to overcome
this bottleneck is to add a self-assembled monolayer (SAM) of molecules between
the biological material and the electrode that promotes an efficient direct
electron transfer whilst minimising wasteful processes of charge recombination.
In this work, the presence of a pyrene-nitrilotriacetic acid layer carrying
different metal centers as SAM physisorbed on graphene is fully described by
mean of electrochemical analysis, field emission scanning electron microscopy,
photoelectrochemical characterisation and theoretical calculations. Our
multidisciplinary study reveals that the metal center holds the key role for
the efficient electron transfer at the interface. While Ni2+ is responsible for
an electron transfer from SAM to graphene, Co2+ and Cu2+ force an opposite
transfer, from graphene to SAM. Moreover, since Cu2+ inhibits the electron
transfer due to a strong charge recombination, Co2+ seems the transition metal
of choice for the efficient electron transfer.
