Functionalized Gold Nanorod Probes: A Sophisticated Design of SERS Immunoassay for Biodetection in Complex Media

Surface-enhanced Raman scattering (SERS) probes offer considerable opportunities in label-based biosensing and analysis. However, achieving specific and reproducible performance, where low detection limits are needed in complex media, remains a challenge. Herein, we present a general strategy employing gold nanorod SERS probes and rationally designed surface chemistry involving protein resistant layers and antibodies to allow for the selective detection of species in complex media. By utilizing the ability of gold nanorods for selective surface modification, Raman reporters (4-mercaptobenzoic acid) were attached to the tips. Importantly, the sides of the nanorods were modified using a mixed layer of two different length stabilizing ligands (carboxyl-terminated oligo ethylene glycols) to ensure colloidal stability, while antibodies were attached to the stabilizing ligands. The nanoparticle interfacial design improves the colloidal stability, unlocks the capability of the probes for targeting biomolecules in complex matrices, and gives the probes the high SERS efficiency. The utility of this probe is demonstrated herein via the detection of Salmonella bacteria at the single bacterium level in complex food matrices using an anti-Salmonella IgG antibody-conjugated probe. The modular nature of the surface chemistry enables the SERS probes to be employed with a molecularly diverse range of biorecognition species (e.g., antibodies and peptides) for many different analytes, thus opening up new opportunities for efficient biosensing applications.