A sandwich-type electrochemiluminescence (ECL) immunosensor for the N-terminal brain natriuretic peptide (NT-proBNP) is described. The assay is based on the quenching of the ECL of graphite-like carbon nitride (g-CN) by polydopamine (PDA). Two-dimensional g-CN is grown in-situ on titanium dioxide nanoflowers (TiO NFs). The macroporous structure of the NFs enhances the interfacial stability of g-CN, and also promotes the ECL reaction of g-CN with the co-reactant. The introduction of gold nanoparticles into the matrix further enhances the ECL and facilitates the immobilization of capture antibodies. The nanoquencher used to label the secondary antibody is synthesized by catalytic polymerization of dopamine in the presence of bimetallic NiPd nanoparticles. The nanoquencher preserves the high reactivity of polydopamine and quenches the ECL of the g-CN/TiO system. Compared to other methods, the detection limit for NT-proBNP is decreased to 50 fg∙mL. Graphical abstract Schematic presentation of the electrochemiluminescence (ECL) process of the immunosensor: titanium dioxide nanoflowers@graphite-like carbon nitride-gold nanoparticles (TiO NFs@g-CN-Au) as luminophor, and polydopamine (PDA) as nanoquencher.