ing is a dipole layer placed in between graphene and the back gate. In moist air and without additional treatment of the silicon oxide substrate (a common insulator for a GFET) this dipole layer exists as adsorbed water molecules at room temperature [@Moser; @Lafkioti] or ordered ice at low temperature[@Wang2010; @Wehling]. The capacitive coupling mechanism is also dominant in electrolyte-gating devices, via ions in the electrical double layer [@Wang2010]. The positive direction of hysteresis is caused by a charge trapping mechanism. Accumulated charge in trap centers will start screening the electric field of the back gate. One of the examples of trap centers are surface states in between SiO$_2$ and graphene [@Wang2010; @Romero2008; @Liao; @Shin]. In case of graphene based FET traps in bulk SiO$_2$ or SiO$_2$/Si interface were excluded in a recent report by Lee et al.[@Lee2011], who measured time scales which were too fast for these types of trapped centers.

A separate charge transfer mechanism which was observed for the hydrogenated surface of diamond [@Chakrapani], carbon nanotubes [@Aguirre] and graphene based FETs [@Sabri2009; @Levesque; @Sidorov2011], is the dissociation of adsorbed water and oxygen on the carbon surface. Since water in equilibrium with air is slightly acidic (pH=6), the electrochemical potential of the carbon surface is higher than that of the solution, resulting in electron transfer from graphene. Therefore, a graphene FET possesses a net p-doping in moist air. The electron transfer is mediated by oxygen solvated in water and can occur in opposite direction with increasing pH. This redox can therefore influence the dynamic response of graphene devices under an applied back gate and cause a positive hysteresis.

A recent report by Fu et al.[@Fu] opened the discussion whether graphene pH sensitivity is caused by charge transfer directly between graphene and the solution [@Ang2008; @Ohno; @Heller] or if the sensitivity is mediated by a layer on top or next to graphene (either oxide or poly