Digital multimeter-based immunosensing strategy for sensitive monitoring of biomarker by coupling an external capacitor with an enzymatic catalysis

• We introduce a new digital multimeter-based immunoassay for detection of biomarker.
• Digital multimeter was used for signal readout.
• Enzyme immunoassay was utilized for signal amplification.

A new digital multimeter (DMM)-based immunosensing system was designed for quantitative monitoring of biomarker (prostate-specific antigen, PSA used in this case) by coupling with an external capacitor and an enzymatic catalytic reaction. The system consisted of a salt bridge-linked reaction cell and a capacitor/DMM-joined electronic circuit. A sandwich-type immunoreaction with target PSA between the immobilized primary antibody and glucose oxidase (GOx)-labeled detection antibody was initially carried out in one of the two half-cells. Accompanying the sandwiched immunocomplex, the conjugated GOx could catalyze the oxidation of glucose, simultaneously resulting in the conversion of [Fe(CN)6]3− to [Fe(CN)6]4−. The difference in the concentrations of [Fe(CN)6]3−/[Fe(CN)6]4− in two half-cells automatically produced a voltage that was utilized to charge an external capacitor. With the closing circuit switch, the capacitor discharged through the DMM, which could provide a high instantaneous current. Under the optimal conditions, the resulting currents was indirectly proportional to the concentration of target PSA in the dynamic range of 0.05–7 ng mL−1 with a detection limit (LOD) of 6 pg mL−1. The reproducibility, precision, and selectivity were acceptable. In addition, the methodology was validated by analyzing 12 clinical serum specimens, receiving a good accordance with the referenced values for the detection of PSA.