Deciphering gold nanotag-induced electron-transfer blockage in competitive electrochemical immunosensor: toward a nanoparticle-sandwiched structure for sensitive SARS-CoV-2 antigen detection

Abstract

Rapid and sensitive detection of SARS-CoV-2 remains essential for infection control and pandemic preparedness, especially with the continuous emergence of new variants. In this study, a competitive electrochemical immunosensor was developed for the detection of the S-protein using a polyethyleneimine-coated gold nanoparticle (PEI-AuNP)-based strategy. The screen-printed carbon electrode (SPCE) was modified with PEI-AuNPs to enhance surface area, conductivity, and biocompatibility, allowing efficient immobilization of the S-protein. Simultaneously, PEI-AuNPs conjugated with anti-S-protein antibodies (anti-S-protein@PEI-AuNPs) were used as nanotags. In the presence of the target antigen, antigen-antibody complexes form in the solution, reducing nanotag binding to the electrode. In contrast, the absence of the target leads to a higher density of nanotags on the electrode surface, which hinders electron transfer of the redox probe [Fe(CN)₆]^3−/4–, resulting in a lower current response. The sensor demonstrated a wide linear detection range from 0.10 to 25 ng mL⁻¹, with a low detection limit of 0.050 ng mL⁻¹. It exhibited excellent sensitivity, selectivity, and stability. Successful detection in human serum, indoor surface swabs, and tap water with high recovery highlights its real-world applicability. This platform can be adapted for emerging pathogens, making it a valuable tool for current and future pandemic surveillance.