Measurement of the hysteresis change of the device after exposure to enzyme solutions. (a) ΔVDirac change and gate effect of an initial graphene transistor (i), after the functionalization of graphene surface with functional polypeptide linker molecule (ii), after NP assembly on the graphene surface (iii), and after exposure to 1 μM enzyme solution for 30 sec, 5 min, 1 hr and 50 hrs (iv ~vii). (b) Changes in ΔVDirac of graphene device (i), after functionalization with linker molecules (ii), after NP assembly (iii), after the exposure to Carboxypeptidase B in PBS solution with the concentration of 10 nM (iv), 100 nM (v), 1 μM (vi) and 10 μM (vii) for 3 hours.