(a) Bare graphene FETs in 0.1 mM PB buffer at pH 7 with 10 mM KCl (red), 100 mM KCl (blue) and 1 M KCl (green). (b) Graphene FETs coating with SLBs in 0.1 mM PB buffer at pH 7 with 10 mM KCl (red), 100 mM KCl (blue) and 1 M KCl (green). (c) Bare graphene FETs in 0.1 mM PB buffer with 100 mM KCl at pH 4 (red), pH 7 (blue), and pH 10 (green). (d) Graphene FETs coating with SLBs in 0.1 mM PB buffer with 100 mM KCl at pH 4 (red), pH 7 (blue), and pH 10 (green). All measurements are conducted at Vds = 100 mV.