Plasmon-Enhanced Surface-Enhanced Raman Scattering Mapping Concentrated on a Single Bead for Ultrasensitive and Multiplexed Immunoassay.

A single microbead (MB)-concentrated surface-enhanced Raman scattering (SERS) mapping strategy is proposed for ultrasensitive and multiplexed immunoassay with high precision. In this design, the SERS tags are specifically immobilized on the surface of a plasmonic gold nanoparticle (GNP) layer-coated single MB via target protein-mediated immune coupling. By this means, even ultralow target dosage can bring highly concentrated SERS tags on the confined small zone around the single MB, and the target-induced SERS signals are largely enhanced by the plasmonic layer, endowing the proposed strategy with ultrahigh sensitivity to quantify subpicogram per milliliter levels of proteins. Moreover, the per-pixel averaged SERS intensity is adopted for target quantitation through mapping the SERS signals around the MB's surface, achieving greatly improved reproducibility compared with traditional single-point measurement. Benefiting from the intrinsic merits of SERS mapping, this elegant strategy also enables multiplexed immunoassay on a single MB.