AC-mode Graphene Field-Effect Transistors for Sensing Stress Biomarkers in Physiologically Relevant Fluids

Graphene field-effect transistor (GFET) based biosensors are suitable for integrating into wearable sensor technology that could demonstrate the sensitivity and selectivity for real-time detection and monitoring of biomarkers. Previously reported GFET biosensors are operated in dc-mode and have shown high sensitivity for sensing biomarkers in solutions with a low salt concentration. However, the sensitivity of the DC-mode GFET biosensors decreases significantly for sensing operation in a physiological fluid, like sweat and interstitial fluid. In these physiological fluids, the salt concentration is high (>100 mM), and the Debye screening length is short (~ 1 nm), which limits the sensitivity of biosensors.

To overcome the Debye screening length limitation, we developed alternating current (ac)-mode GFET biosensors for sensing neuropeptide-Y (NPY), a key stress biomarker, in artificial sweat at physiologically relevant ionic concentrations. The ac-mode sensor responses are several times larger than that measured in dc-mode classically used for testing GFET sensors, as the Debye screening length limitation is significantly relaxed in the ac-mode sensing. The maximum sensor response occurs at ac-voltage carrier frequency of ~500 kHz for the salt concentration of 100 mM in the AS, and the maximum frequency decreases with decreasing the salt concentrations. A simple analytical model can well explain the frequency and salt concentration-dependent sensor response.