Tyramine-Based Enzymatic Conjugate Repeats for Ultrasensitive Immunoassay Accompanying Tyramine Signal Amplification with Enzymatic Biocatalytic Precipitation

A new impedimetric immunoassay protocol based on enzyme-triggered formation of tyramine-enzyme repeats on gold nanoparticle (AuNP) was designed for highly sensitive detection of carcinoembryonic antigen (CEA, as a model) by virtue of utilizing enzymatic biocatalytic precipitation toward 4-chloro-1-naphthol (4-CN) on anti-CEA antibody (Ab1)-modified immunosensor. Initially, AuNP was functionalized with horseradish peroxidase and detection antibody (HRP-AuNP-Ab2), and then HRP–tyramine conjugate was utilized for the formation of tyramine–HRP repeats through the triggering of the immobilized HRP on the AuNP with the aid of H2O2. In the presence of target CEA, the carried HRP–tyramine repeats accompanying the sandwiched immunocomplex catalyzed the 4-CN oxidation to produce an insoluble precipitation on the immunosensor, thus causing a local alteration of the conductivity. Three signal-transduction tags including HRP-Ab2, HRP-AuNP-Ab2, and HRP-AuNP-Ab2 with HRP–tyramine repeats were employed for target CEA evaluation, and improved analytical properties were achieved by HRP-AuNP-Ab2 with HRP–tyramine repeats. Using the unique signal-transduction tag, the analytical performance of the impedimetric immunoassay was studied in detail. Under the optimal conditions, the impedimetric immunosensor displayed a wide dynamic working range of between 0.5 pg mL–1 and 40 ng mL–1 with a detection limit (LOD) of 0.38 pg mL–1 relative to target CEA. The coefficients of variation (CVs) were ≤9.3% and 13.3% for the intra-assay and interassay, respectively. The levels of CEA in eight clinical serum specimens were measured by using the developed impedimetric immunosensor. The obtained results correlated well with those from the electrochemiluminescent (ECL)-based immunoassay with a correlation coefficient of 0.998.