A new label-free electrochemical immunosensor is constructed for the selective and sensitive determination of the clinically relevant biomarker receptor tyrosine kinase (AXL) in human serum. The disposable immunosensing platform is prepared by immobilization of the specific anti-AXL antibody onto amine functionalized graphene quantum dots (fGQDs)-modified screen-printed carbon electrodes (SPCEs). The affinity reactions were monitored by measuring the decrease in the differential pulse voltammetric (DPV) response of the redox probe Fe(CN)63−/4−. All the experimental variables involved in the preparation of the modified electrodes and in the immunosensor performance were optimized. The as prepared immunosensor exhibits an improved analytical performance with respect to other electrochemical immunosensors reported so far, with a wider range of linearity and a lower detection limit, 0.5 pg mL−1, which is more than one hundred thousand times lower than the established cut-off value for heart failure (HF) diagnosis in serum (71 ng mL−1). The developed immunosensor was successfully applied to the determination of the endogenous content of AXL in serum of HF patients without any matrix effect observed after just a sample dilution. Display omitted •Sensitive label-free voltammetric immunosensor for receptor tyrosine kinase (AXL) determination.•Antibody immobilization via Schiff reaction on amine functionalized graphene quantum dots.•LOD of 0.5 pg mL−1 far below the cut-off value for heart failure diagnosis in serum.•Accurate determination of AXL in heart failure patients' sera without matrix effect after just a dilution.