A new protein A assay based on Raman reporter labeled immunogold nanoparticles

A unique, sensitive, highly specific, and photobleaching-resistant immunoassay system utilizing gold nanoparticles and surface-enhanced Raman scattering (SERS) is described. This new system, featuring a capability of bifunctional analysis, is manufactured by chemisorption of antibody immunoglobulin G (IgG) on gold nanoparticles (AuNP), followed by coupling the Raman-active reporter molecule, 5,5′-dithiobis(2-nitrobenzoic acid) (DTNB) to the surface of IgG–AuNP. The adsorbed DTNB molecules exhibit strong Raman signals via both electromagnetic and chemical enhancement. The narrow spectral widths and high photostability assure the system to be an excellent detection label. This SERS-based immunoassay is applied to the detection of protein A, which is a specific surface antigen of Staphylococcus aureus. A working curve is obtained by plotting the intensity of the SERS signal of symmetric NO2 stretching of DTNB at 1333 cm−1 versus the concentration of the analyte (antigen). A dynamic range of two to three orders of magnitude and a detection limit of 1 pg/mL of protein A are achieved.