pH Sensitivity of Edge-Gated Graphene Field-Effect Devices with Covalent Edge Functionalization

We introduce here a strategy for a field-effect device, termed graphene edge field-effect transistor (GrEdge-FET), where a micron-wide graphene monolayer is gated exclusively through its edge in an aqueous environment. This is achieved by passivating the basal plane selectively using photolithography. We observe a field-effect behavior in buffer solutions with an ON/OFF ratio of nearly 10 in a small gate-voltage range (±0.5 V) without any need for complex nanofabrication or specialized electrolytes. We attribute this effect to the electrical double layer capacitance at the edge–electrolyte interface, which efficiently gates the entire graphene sheet although it acts only at the edge. We demonstrate that GrEdge-FET devices find applications as pH sensors. Through diazonium electrochemistry, the edges are functionalized persistently with substituted phenyl moieties, which renders the devices with a higher pH sensitivity than classical graphene FETs. Moreover, since only the edge is modified, the favorable field-effect behavior is preserved, despite the covalent nature of the attachment of the functional groups.