Graphene based pH sensing with extremely high sensitivity*

We have reported pH sensors based on two-dimensional (2D) MoS₂ dual-gated field-effect transistors (2D-MoS₂ dual-FETs) with sensitivity that greatly surpasses the Nernst limit of 59 mV/pH [1]. To further improve device performance, we replaced the 2D-MoS₂ channel with graphene which has a higher maximum quantum capacitance, higher current drive, and lower contact resistance. Our graphene dual-FETs featured asymmetric front-gate (ionic liquid) and back-gate (300 nm SiO₂) capacitances which allowed a quantum capacitance limited signal amplification of up to ~500, a record result for a dual-FET with the given back-gate oxides. The low contact resistance of the graphene dual-FETs enables the devices to operate with two orders of magnitude lower drain voltage than comparable MoS₂ devices. We benchmark graphene dual-FETs against 2D-MoS₂ dual-FETs in sensitive pH measurements. The higher sensitivity, combined with a lower limit of detection make graphene dual-FETs a powerful tool for field monitoring of pH with wide ranging applications in biology, environmental monitoring and agriculture.
[1] Son T. Le et al., Nanoscale 11 (33), 15622-15632 (2019).