The Application Of Ag_coreAu_shell Bimetallic Nanoparticles In Surface-Enhanced Raman Immunoassay Analysis

Surface enhanced Raman immunoassay analysis (SERIA) is a new detective technique that combines the advantages of surface enhanced Raman spectroscopy and the immunoassay analysis. However, whether SERIA can be really employed in clinic depends on its detection sensitivity. In previous researches, Au nanoparticles were mainly used as the Raman enhancer for immunoassay due to its biocompatibility and easy preparation, but Au nanoparticles show a lower enhancement in the visible light region in comparison with that of Ag. The motivation of the present study is to prepare the AgcoreAushell bimetallic nanoparticles with unique optical and chemical properties for immunoassay. It is expected that on one hand the Au layers on the nanoparticle surface are biocompatible and on the other hand, the underneath Ag core may provide extra enhancement. In a typical experiment, layered core-shell bimetallic silver-gold nanoparticles were prepared by coating Au layers over Ag seeds using a seed-growth method. With the increase of the Au molar fraction, some pinholes appeared on the nanoparticle surface, which was then coated with a complete Au shell. Correspondingly, the SERS intensity of adsorbed probe molecules (TP and PATP) enhanced first and then weakened, with the maximal intensity being 10 times higher than that on Ag. This unexpected result could be attributed to the presence of some pinholes that act as hot spots for the electromagnetic field enhancement. The SERS active bimetallic nanoparticles were then modified with probe molecules and antibodies to form a reporter-labeled AgcoreAushell nanoparticles and used for immunoassay analysis. Parallel to this, the antibodies were immobilized on a solid substrate for capturing antigens from solution. The captured antigens could then bind the reporter-labeled AgcoreAushell nanoparticles modified with same antibodies, forming a sandwich type complex"capture antibody substrate/antigen/reporter labeled immuno-AgcoreAushell bimetallic nanoparticles". The binding specificity was successfully demonstrated by the detection of characteristic Raman bands of the probe molecules. This primary study demonstrates that the AgcoreAushell bimetallic nanoparticles labeled by the monoclonal antibodies and SERS probes could be used for immunoassay analysis.