Graphene based pH sensing with extremely high sensitivity

Field-effect transistors (FETs) are versatile tools for measuring numerous biomarkers. We demonstrate that their measurement sensitivity and resolution can be improved by using new materials and device designs. Here, we report on the sensitivity and noise performance of dual-gated graphene FETs. As an example, when measuring pH, the devices exhibit a sensitivity of up to 30 V for a unit change in pH, ≈ 500-fold greater than the Nernst value at room temperature, and a noise limited resolution of 2x10^-4 in the biomedically relevant bandwidth between 0.1 Hz to 10 Hz. The results represent a factor of 2 improvement over previously reported values using dual-gate MoS₂ FETs. This exceptional performance arises from a highly asymmetric dual-gate design that utilizes an ionic liquid top-gate dielectric. The high top-gate capacitance when coupled with the large intrinsic quantum capacitance (≈ 15 µC/cm²) of graphene enables a top-gate to back-gate coupling ratio of ≈ 500. The higher sensitivity, combined with improved resolution make graphene dual-gated FETs powerful tools for field monitoring of pH with applications in biomedical research and technology.