A SERS-based immunoassay for the detection of α-fetoprotein using AuNS@Ag@SiO₂ core-shell nanostars

A surface-enhanced Raman scattering (SERS)-based sandwich type immunoassay was developed for the detection of α-fetoprotein (AFP) with high sensitivity, reproducibility and selectivity. In the immunoassay, an antibody immobilized nitrocellulose (NC) membrane was employed to capture AFP and antibody conjugated silica coated gold/silver core-shell nanostars (AuNS@Ag@SiO₂) were used as SERS probes. AuNS@Ag was synthesized by the direct deposition of Ag on the AuNS. The Ag shell grew outward from the center towards the tips and adopted a polyhedral shape. The AuNS@Ag nanoparticles were shown to have higher SERS activity than gold nanostars (AuNSs), and their LSPR wavelengths could be finely tuned across a wide spectrum ranging from visible to near-infrared by simply adjusting the Ag shell thickness. The silica coating improved colloidal stability of the nanostructure and provided functionality for antibody conjugation. By using 4-mercaptobenzoic acid (4-MBA) as a Raman reporter molecule, the proposed SERS sensor is able to detect AFP with a low limit of detection (LOD) of 0.72 pg mL^-1 and a wide and clinically relevant linear detection range from 3 pg mL^-1 to 3 μg mL^-1. Hence, this SERS-based immunoassay provides a strategy for the detection of trace AFP and has potential to assist in the monitoring of prognosis for hepatocellular carcinoma (HCC) patients.