A conductive hydrogel acting as a redox-active species was synthesized by crosslinking phytic acid as a ligand and lead(II) as the metal ion. The resulting gelatine-like material displays excellent redox activity and facilitates the transport of electrons and ions. Gold nanoparticles were electrochemically deposited on the hydrogel film, and antibody against cytokeratin antigen was immobilized thereon. An amperometric immunosensor for cytokeratin antigen 21–1 (CYFRA21-1), a kind of biomarker of lung cancer, was obtained by deposition of the composite on a glassy carbon electrode. If operated at − 0.58 V (vs. Ag/AgCl), the sensor exhibits a linear detection range that extends from 50 fg mL−1 to 100 ng mL−1 CYFRA21-1, and a 38 fg mL−1 detection limit (at a signal to noise ratio of 3). The quantitation of CYFRA21-1 in (spiked) human serum samples showed satisfactory accuracy compared to an ELISA.