Ion Sensing With Solution-Gated Graphene Field-Effect Sensors in the Frequency Domain

Here, we examine the concept of frequency domain sensing with solution-gated graphene field-effect transistors, where a sine wave of primary frequency (1
f
) was applied at the gate and modulation of the power spectral density (PSD) of the drain-source current at 1
f
, 2
f
, and 3
f
was examined as the salt in the gate electrolyte was switched from KCl to CaCl
₂
, and their concentrations were varied. The PSD at 1
f
, 2
f
, and 3
f
increased with the concentration of KCl or CaCl
₂
, with the PSD at 1
f
being the most sensitive. We further correlated these changes to the shift in Dirac point. Switching the graphene substrate from oxide to hexagonal boron nitride, led to an improved device-to-device reproducibility and a significant reduction of noise, which translated to a higher signal-to-noise ratio and resolution in sensing salt concentrations. The signal-to-noise ratio at 1
f
was found to be a logarithmic function of KCl or CaCl
₂
concentration in the 0.1 to 1000 mM range.