Electrical Measurements. The devices were characterized using custom-built portable measurement systems (Fig. (a) and (b)). A buffer solution of 0.01×PBS was used for the electrical measurements. A drain-source bias (VDS) of 0.1 V was applied, and a gate-source bias (VGS) was imposed through the solution via an Ag/AgCl electrode. The source-drain current (IDS) was measured while sweeping VGS. VGS at the minimum conductance point (VDP), also known as the charge neutrality point (CNP), was calculated for each GFET using polynomial fitting.