pH sensing properties of graphene solution-gated field-effect transistors

The use of graphene grown by chemical vapor deposition to fabricate solution-gated field-effect transistors (SGFET) on different substrates is reported. SGFETs were fabricated using graphene transferred on poly(ethylene 2,6-naphthalenedicarboxylate) substrate in order to study the influence of using a flexible substrate for pH sensing. Furthermore, in order to understand the influence of fabrication-related residues on top of the graphene surface, a fabrication method was developed for graphene-on-SiO2 SGFETs that enables to keep a graphene surface completely clean of any residues at the end of the fabrication. We were then able to demonstrate that the electrical response of the SGFET devices to pH does not depend either on the specific substrate on which graphene is transferred or on the existence of a moderate amount of fabrication-related residues on top of the graphene surface. These considerations simplify and ease the design and fabrication of graphene pH sensors, paving the way for developing low cost, flexible, and transparent graphene sensors on plastic. We also show that the surface transfer doping mechanism does not have significant influence on the pH sensing response. This highlights that the adsorption of hydroxyl and hydronium ions on the graphene surface due to the charging of the electrical double layer capacitance is responsible for the pH sensing mechanism. (C) 2013 AIP Publishing LLC.