Graphene transistors are insensitive to pH changes in solution
  We observe very small gate-voltage shifts in the transfer characteristic of as-prepared graphene field-effect transistors (GFETs) when the pH of the buffer is changed. This observation is in strong contrast to Si-based ion-sensitive FETs. The low gate-shift of a GFET can be further reduced if the graphene surface is covered with a hydrophobic fluorobenzene layer. If a thin Al-oxide layer is applied instead, the opposite happens. This suggests that clean graphene does not sense the chemical potential of protons. A GFET can therefore be used as a reference electrode in an aqueous electrolyte. Our finding sheds light on the large variety of pH-induced gate shifts that have been published for GFETs in the recent literature. 
Wangyang Fu, Cornelia Nef, Oren Knopfmacher, Alexey Tarasov, Markus   Weiss, Michel Calame, and Christian Sch\"onenberger
cond-mat.mes-hall cond-mat.mtrl-sci