Gold-modified silver nanorod arrays for SERS-based immunoassays with improved sensitivity

In sandwich SERS immunoassays, the SERS-active substrate plays a central role in the detection sensitivity. Silver nanorod (AgNR) arrays fabricated by oblique angle deposition (OAD) provide superior analytical qualities and have been demonstrated as excellent SERS substrates. This work reports highly sensitive sandwich immunoassays performed on the OAD-AgNR substrates for the first time, utilizing 4-MBA-labeled immuno-Au nanoparticles (Im-AuNPs) as SERS tags. The aligned AgNR arrays were first deposited on glass slides by OAD, and the surface modified with gold (Au) using chloroauric acid solution via a galvanic replacement reaction. Immunoassays executed on both AgNR arrays and Au-modified AgNR arrays show higher detection sensitivity compared to that performed on self-assembled silver nanoparticles substrates. Though the AgNRs demonstrate better SERS activity than Au-modified AgNRs, the immunoassay performed on the Au-modified AgNRs exhibits higher sensitivity. Further characterization by scanning electron microscopy and fluorescence spectroscopy shows that the improved sensitivity can be attributed to the increased number of Im-AuNPs that are specifically captured on the Au-modified nanorods surface and their more uniform distribution. Concentration-dependent SERS spectra of human IgG on the Au-modified AgNR arrays reveal a good linear response range from 100 fg mL^-1 to 100 ng mL^-1, with an LOD of 2.5 fg mL^-1.