▎ 摘　　要

Graphene field-effect transistors are widely used for development of biosensors. However, certain fundamental questions about details of their functioning have not been fully understood yet. One of these questions is the presence of gate/leakage currents in the electrolyte-gated configuration. Here, we report our observations and causes of this phenomenon on chemical vapor deposition (CVD)-grown graphene. We observed transistor's gate currents occurring at the surface of graphene exposed to the electrolyte. Gate currents are capacitive when the graphene channel is doped by holes and Faradic when it is doped by electrons in field-effect measurements. We prove that Faradic currents are attributed to the reduction of oxygen dissolved in the aqueous solution and their magnitude increases with each measurement. We employed cyclic voltammetry with a redox probe Fc(MeOH)(2) to characterize changes in the graphene structure that are responsible for this activation. Collectively, our results reveal that through the course of catalytic oxygen reduction on the transistor's surface, its electroactivity toward an out-of-plane heterogeneous electron transfer increases.