Graphene transistors are insensitive to pH changes in solution

TL;DR

Graphene transistors show minimal sensitivity to pH changes, unlike silicon-based sensors, and their surface chemistry can be modified to alter this insensitivity, making them suitable as stable reference electrodes.

Contribution

This study demonstrates that pristine graphene transistors are inherently insensitive to pH variations, and surface modifications can further influence their response, clarifying conflicting reports in literature.

Findings

Graphene GFETs exhibit negligible pH-induced gate shifts.

Surface coatings like fluorobenzene reduce pH sensitivity.

Al-oxide layers increase pH sensitivity.

Abstract

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.