Cetyltrimethylammonium bromide-modified spherical and cube-like gold nanoparticles as extrinsic Raman labels in surface-enhanced Raman spectroscopy based heterogeneous immunoassays

This paper reports on the characterization and preliminary comparison of gold nanoparticles of differing surface modification and shape when used as extrinsic Raman labels (ERLs) in high-sensitivity heterogeneous immunoassays based on surface enhanced Raman scattering (SERS). ERLs are gold nanoparticles coated with an adlayer of an intrinsically strong Raman scatterer, followed by a coating of a molecular recognition element (e.g., antibody). Three types of ERLs, all with a nominal size of similar to 30 nm, were fabricated by using spherical citrate-capped gold nanoparticles (sp-cit-Au NPs), spherical CTAB-capped gold nanoparticles (sp-CFAB-Au NPs), or cube-like CTAB-capped gold nanoparticles (cu-CFAB-Au NPs) as cores. The performance of these particles was assessed via a sandwich immunoassay for human IgG in phosphate buffered saline. The ERLs fabricated with sp-CTAB-Au NPs as cores proved to be more than 50 times more sensitive than those with sp-cit-Au NPs as cores; the same comparison showed that the ERLs with cu-CTAB-Au NPs as cores were close to 200 times more sensitive. Coupled with small differences in levels of nonspecific adsorption, these sensitivities translated to a limit of detection (LOD) of 94, 2.3, and 0.28 ng/mL, respectively, for the detection of human IgG in the case of sp-cit-Au NPs, sp-CTAB-Au NPs, and cu-CTAB-Au NPs. The LOD of the cu-CTAB-Au NPs is therefore similar to 340 times below that for the sp-cit-Au NPs. Potential applications of these labels to bioassays are briefly discussed.