An electrochemical immunosensor for the detection of a cancer biomarker protein in serum at femtomolar concentrations with ultralow background response has been developed, which consists of (i) a hydrophilic polyacrylic acid brush-modified indium tin oxide substrate as an antifouling substrate and (ii) a graphene-quantum dots-antibody 'bionanoconjugate' as a signal amplification label in voltammetric detection of targets in a glassy carbon electrode.