Fig. 3. Characterization and quality assessment of the on-chip GFET sensing platform: (A) the design of on-chip integrated sensor array outperforms the conventional GFET single sensor which uses an external bulky gate electrode (e.g. Ag/AgCl) and has minimal detection throughput; (B–D) electrochemistry of the device in a three-electrode set-up with an Ag/AgCl reference electrode and carbon foil counter electrode in 0.001 M PBS solution of pH 7.4; (B) the cyclic voltammogram of the device contacted at the gold/aluminum oxide junction in a scan rate range of 1000–1000 mV s⁻¹; (C) cyclic voltammogram of the device contacted on gold showing a stability test done at a 1000 mV s⁻¹ over 200 cycles; (D) the chronoamperometry experiment contacted on gold showing the current measured in response to the open circuit potential and applied potential; (E) AFM images showing the successful functionalization/binding of the PBASE, the antibody, and the target protein, with scale bar: 1 μm; (F) the Raman spectroscopy before and after PBASE functionalization on graphene surface; (G) I–V curves showing changes in the Dirac point after each surface functionalization step, where electrical measurement (I–V curves) confirmed the successful functionalization/binding of each molecule on the graphene surface (330 fM of spike proteins); (H) monitoring of I–V curves over time on the same device (33 pM of spike proteins). V_sd = 20 mV, indicating good stability of the data produced by each device over time; (I) I–V curves indicating good data repeatability and the signal stability of multiple sensing devices (n = 4) at 330 pM of spike proteins in d1000 PBS.