Femtomolar immunoassay based on coupling gold nanoparticle enlargement with square wave stripping voltammetry

The enhancement in sensitivity for an electrochemical immunoassay by the autocatalytic deposition of Au3+ onto gold nanoparticles has been studied. By coupling the autocatalytic deposition with square-wave stripping voltammetry, enlarged gold nanoparticles labeled on goat anti-rabbit immunoglobudin G (GaRIgG-Au) and, thus, the rabbit immunoglobulin G (RIgG) analyte could be determined quantitatively. A variety of variables, such as concentration of AuCl4-, the reducing agent used, the duration of autocatalytic deposition, and parameters for the stripping analysis were optimized. From a calibration graph over a broad dynamic range of concentrations (1-500pg mL(-1); R-2 =0.9975) a very low detection limit, 0.25 pg mL(-1) (1.6 M), which is three orders of magnitude lower than that obtained by a conventional immunoassay using the same gold nanoparticle labels was obtained; this finding confirms applicability and effectiveness of our method of enhancing the sensitivity of gold nanoparticle label-based sandwich immunoassays. The reliability of this method was confirmed by the rather low values of RSD (2.82%, n = 11; 2.44%, n = 9) obtained for assays of a blank solution and for 0.02 ng mL(-1) RIgG solution, respectively. (c) 2005 Elsevier B.V. All rights reserved.