Engineering Raspberry-like Plasmonic Nanoclusters as Tags in Surface-Enhanced Raman Scattering-Based Immunoassays

Surface-enhanced Raman scattering (SERS) is highly attractive with the advantages of non-destructive performance, high specificity, and ultra-sensitivity. However, it is still a great challenge to design SERS tags with strong and undisturbed SERS signals via a simple method for an SERS-based immunoassay. Herein, a simple one-pot method was developed for the fabrication of SERS nanotags with interior reporters located at the nanogaps between plasmonic structures. Benefiting from the reducibility and easy-to-polymerize properties of aniline, Au3+ ions were reduced and grew into small-sized Au nanoparticles with a thin layer of polyaniline (PANI) by using aniline as the reductant. Following the continuous polymerization and the reduction reactions, PANI-coated Au nanoparticles were assembled into a nanocluster with sub 5 nm gaps, and PANI located at these gaps were used as interior reporters in SERS tags. As proof-of-concept, a histidine-tagged antigen was used as the model analyte for the SERS-based immunoassay. The proposed sensing platform showed the response to the histidine-tagged antigen ranging from 0.1 to 1000 ng mL−1 with a detection limit of 0.01 ng mL−1. The remarkable and undisturbed SERS signals make the proposed SERS tags feasible for detections of biomarkers with a low concentration in complex biological samples.