Competitive electrochemical immunosensors by immobilization of hexahistidine-rich recombinant proteins on the signal labels

Most of proteins in the commercial ELISA kits are recombinant with hexahistidine (His₆) tags. Based on this fact, we proposed a simple procedure for the preparation of signal labels and the design of competitive immunosensors. The signal labels were fabricated through the high-affinity interaction between the His₆ tails on the surface of recombinant proteins and the coordinatively unsaturated copper ions on the surface of pristine Cu-based metal organic framework (Cu-MOF). The recombinant protein-modified MOF could be captured by the sensor electrode, thus producing a strong differential pulse voltammetry signal through the electrochemical reduction of Cu²⁺ ions in MOF. However, the target protein in sample solution could compete with the His₆-tagged protein on the surface of Cu-MOF to bind the antibody attached on the electrode, thus leading to the decrease in the electrochemical signal. The peak current showed an inversely relationship with the concentration of target protein. The competitive immunosensor exhibited a linear range of 1 pg/mL to 1 ng/mL with SARS-CoV-2 nucleocapsid protein (N-protein) as the target analyte. We believe that the method can be used to detect other proteins by utilizing the characteristic of recombinant proteins in commercial kits.