Electrocatalytic detection of aurothiomalate on carbon electrodes, Application as a nonenzymatic label to the quantification of proteins

A new method to determine the concentration of protein solutions, bovine serum albumin and rabbit IgG anti human IgM (RIgG), has been developed in this work. This method is based on the catalytic effect of the electrodeposited gold on glassy carbon electrodes upon silver electrodeposition. Thus, ionic gold was used as an electroactive label of proteins. Proteins were labelled using a sodium aurothiomalate solution (gold(I) complex). Both solutions were prepared in unbuffered solutions of 0.15 M NaCl, pH 7.5, and the binding was carried out at 37 °C for 24 h. Then, free gold was separated from the labelled protein by a dialysis system, against a 0.15 M NaCl, pH 7.5, solution for 24 h at room temperature. The gold(I) bound to the protein, which was previously adsorbed on the electrode surface, was electrodeposited in 0.1 M HCl at -1.00 V for 5 min, and then oxidised in 0.1 M H2SO4 at a fixed potential, for 1 min. Then, silver electrodeposition at an adequate potential for 1 min followed by anodic stripping voltammetry were carried out in aqueous 1.0 M NH3–2.0×10^-4 M AgNO3. Thus, a good linear relationship between the anodic stripping peak and the concentration of labelled protein was found, achieving limits of detection of about 1×10^-8 M. The RIgG gold labelling was also used for monitoring an immunological reaction, against a biotinylated anti rabbit IgG which was immobilised on the electrode surface through the streptavidin:biotin interaction, showing that the labelling of RIgG with gold did not affect the antibody:antigen interaction.