A new electrochemical immunoassay protocol for attomolar detection of proteins has been developed by the use of the dendritic immunocomplex of protein A- and antibody-modified colloidal gold labels for the primary amplification and the catalytic enlargement of these multiple colloidal gold nanoparticles for the secondary amplification. Such use of double amplification procedure is combined with a sensitive stripping analysis of the dissolved gold tags to achieve a dramatic enhancement of the antigen-antibody binding response. The efficient magnetic separation of the immunocomplex can additionally minimize nonspecific adsorption effects. Factors affecting the performance have been optimized. Such amplified electrical transduction is demonstrated for a near-homogeneous suspension immunoassay of the human IgG model analyte with the detection limit of 5.2 fg•mL^－1 or 34.7 amol/L (S/N＝3). This new colloidal gold-based metalloimmunoassay approach holds great promise for highly sensitive detection of target proteins in various fields such as biodiagnostics, environmental monitoring and biodefense.

Key words: electrochemical immunoassay, signal amplification, multiple colloidal gold label, stripping analysis