The rGO was printed by the Epson inkjet printer between the IDEs to form the graphene sensing channels. As reported, the ink showed satisfactory performances to form uniform conductive film with the room temperature properties such as viscosity at ~10 mPa.s, the surface tension at 30 dyn/cm, and small particle size less than 3 µm to prevent nozzle clogging. Antibody immobilization was conducted by drop casting 10 µL 6mM PBASE on the sensor and incubated for 30 min with subsequent washing to remove free linkers. Then the film was N2-dried for the 1h-long room temperature incubation of Anti-L solution. Anti-L was diluted in PBS with different concentrations. Then the sensor surface was blocked with 10 µL 10mM ethanolamine, 10 µL 0.1 wt% Tween-20 and 10 µL 0.5 mg/mL BSA both in PBS sequentially with washing to quench and block the non-specific sites. Samples of lactate was prepared with PBS at various concentrations.