Electrochemical Immunoassay of Antibodies Using Freshly Prepared and Aged Conjugates of Silver Nanoparticles

In this work, silver nanoparticles (AgNPs) were conjugated with immunoglobulins G using a simple passive adsorption method and the composition and properties of freshly prepared (Ab@AgNPs) and aged (Ab@AgNP/ Ag2SNPs) conjugates were investigated for the first time by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and voltammetry. It was found that the state of the AgNPs in the conjugates changed during storage. In the freshly prepared Ab@AgNPs conjugates, silver is present in the metallic state only, while the aged Ab@AgNP/Ag2SNPs conjugates are characterized by the presence of a metallic core of AgNPs coated with a "corona" of Ag2S. This affects the selection of voltammetric conditions for recording an analytical signal from the silver label in the conjugates. The Ab@AgNPs and Ab@AgNP/Ag2SNPs conjugates were successfully tested in the electrochemical immunoassay of dog immuno-globulins as a model. A solid-phase indirect non-competitive assay was performed using a 96-well ELISA plate. To record intense analytical signals from the silver conjugates by linear scan anodic stripping voltammetry (LSASV) on a carbon composite electrode (CCE), they were preliminarily dissolved in acidic solutions. In the case of the Ab@AgNPs conjugate, a nitric acid solution (1 mol L-1) was used to dissolve silver conjugates, while in the case of Ab@AgNP/Ag2SNPs, a 1:1 mixture of 1 mol L-1 CH3COOH and 1 mol L-1 HNO3 was used. The CH3COOH dissolved the Ag2S "corona", and HNO3 dissolved the silver core of the Ab@AgNP/Ag2SNPs conjugate.This work opens up the possibility of the prolonged use of silver conjugates in electrochemical immunoassay, which can be used in commercial test systems for immunological studies as an alternative to conjugates labelled with enzyme labels.