In this study, a new, simple, and sensitive method for voltammetric detection of silver nanoparticles was developed. This method was successfully used for fabricating a highly sensitive voltammetric immunosensor on the basis of a silver-nanoparticle-labelled sandwich-type immunoassay for human chorionic gonadotropin (hCG) used as a model protein. This immunosensor comprises a primary antibody immobilized on the surface of a screen-printed carbon strip and a silver-nanoparticle-labelled secondary antibody. After an immunoreaction, silver nanoparticles were captured on the electrode surface, and their quantity was immediately measured electrically, as follows. First, the silver nanoparticles were electrically oxidized to silver ions, and then, silver ions were electrodeposited on the electrode surface. Finally, the amount of deposited silver was determined using differential pulse voltammetry. These detection processes of silver nanoparticles were carried out on the same surface as that used for the immunoassay. Under optimized conditions, a linear relation between logarithmic current peak and logarithmic concentration of hCG was obtained in the range 10–1000 pg mL−1 in a 3 µL analyte sample. The detection limit for hCG was evaluated to be 7.2 pg mL−1 (122 fM, 7.2×10−5 IU cm−1, S/N=3). The proposed detection method has a wide variety of promising applications in electrochemical analysis.
