Electrochemical immunosensors based on the solubility difference of electroactive probe and the dual signal amplification of nanocarrier plus redox cycling

This work reported a redox cycling system for the design of electrochemical immunosensors by using pyrroloquinoline quinone (PQQ) to promote the oxidation of tris(2-carboxyethyl)phosphine (TCEP). The consumption of TCEP was monitored with ferrocenium (Fc⁺) as the electroactive probe, which was based on the difference in the solubility of Fc⁺ with its reduced format (ferrocene, Fc). Metal–organic framework (MOF) was used as the nanocarrier to load biotinylated recognition antibody and PQQ with recombinant streptavidin as the linker. In the absence of target, TCEP could reduce Fc⁺ into insoluble Fc aggregates, thus leading to the decrease in the electrochemical signal. Capture of target allowed for the attachment of antibody-modified MOF-PQQ on the sensing electrode, thus promoting the oxidation of TCEP by O₂ through the redox cycling. In this case, the reduction of Fc⁺ into insoluble Fc aggregates was limited, and Fc⁺ remained in the solution exhibited a high electrochemical signal. The peak current was linearly proportional to the target concentration in the range of 0.001–1 ng/mL with prostate specific antigen as an example. The work should be useful for the design of novel biosensors through the signal amplification of redox cycling.