Electrical measurements were conducted after rinsing. COOH beads were physisorbed onto both regions because of the lack of blocking on the device, although COOH beads were adsorbed more on the GFETs in Region 1 due to the attractive electrostatic forces. The corresponding plots in Fig. (a) and (b) show that the values of VDP for the GFETs in Region 1 shifted up, which was attributed to the negative charge of COOH. In contrast, those in Region 2 remained relatively unchanged, as shown in Fig. ). The results demonstrated that bare GFETs did not exhibit a response to COOH beads, despite the adsorption of the charged target onto the graphene channel. The limited response stems from the strong nonlinear screening caused by the large ψ0 of the bare graphene.