The GO sheets were synthesized from graphite flake powders using
a modified Hummers method [15]. The obtained GO aqueous solution
was concentrated (to 20 mg/mL) by centrifugation. A self-built 3D
micro-extrusion system was used to print graphene-based interdigitated
electrodes. The printing system consists of a desktop XYZ motor
(Technodigm, Model: DR3331T-EX), a temperature-controlled substrate
holder (100 °C used here), and a high-precision displacement