Ferritin-Triggered Redox Cycling for Highly Sensitive Electrochemical Immunosensing of Protein

Electrochemical immunoassay amplified with redox cycling has become a challenging topic in highly sensitive analysis of biomarkers. Here a ferritin-triggered redox cycling is reported by using a highly outersphere reaction-philic (OSR-philic) redox mediator ruthenium hexamine (Ru­(NH₃)₆³⁺) to perform the OSR-philic/innersphere reaction-philic (ISR-philic) controlled signal amplification. The screened mediator can meet the needs of lower <i>E</i>⁰′ than ferritin, low reactivity with ISR-philic species, and quick electron exchange with ferritin redox couple. The ferritin-labeled antibody is first bonded to immunosensor surface by recognizing the target antigen capured by the immobilized primary antibody. The ferritin then mediates OSR-philic/ISR-philic transfer from Ru­(NH₃)₆^3+/2+/immunosensor to ferritin-H₂O₂ redox system. The fast mediation and excellent resistant of highly OSR-philic Ru­(NH₃)₆³⁺ against radical oxygen species lead to highly sensitive electrochemical readout and high signal-to-background ratio. The proposed redox cycling greatly enhances the readout signal and the sensitivity of traditional ferritin-labeled sandwich immunoassay. Using Enteropathogenic coli (<i>E. coli</i>) antigen as a model analyte, the developed method shows excellent linearity over the concentration range from 10.0 pg/mL to 0.1 μg/mL and a detection limit of 10.0 fg/mL. The acceptable accuracy, good reproducibility, and selectivity of the proposed immunoassay method in real samples indicate the superior practicability of the ferritin-triggered redox cycling