Gold nanoring core-shell satellites with abundant built-in hotspots and great analyte penetration: An immunoassay platform for the SERS/fluorescence-based detection of carcinoembryonic antigen

We designed an ingenious plasmonic core-shell-satellite nanostructure (HPGNPs@mSiO₂-AuNR) including a hollow porous gold nanoshell (HPGNP) core, a protective silica shell with mesoporous (mSiO₂) interlayer, and a high level of gold nanoring (AuNR) satellite coverage by in situ etching. The HPGNP core modified with 4-mercaptobenzoic acid (4-MBA) possessed relatively strong local-field enhancement. The mSiO₂ coating acted as a continuous interior nanogap and protective shell, and the variable external satellites in the morphology had tunable optical responses and designed “hotspot” regions. Specifically, AuNR satellites exhibited superior plasmonic properties, intense core-satellite plasmon coupling with the HPGNP core, and favorable biocompatibility. Meanwhile, the nanoassemblies had larger surface-to-volume ratios for molecular modification and were penetrable for analyte. With their abundant built-in hotspots and less hindrance for analyte penetration into the hollow interior, the labeled nanoassemblies were successfully used in SERS/fluorescence sensing system for carcinoembryonic antigen (CEA) detection. These immunoprobes with high SERS activity could be used in SERS-based immunoassay for CEA detection with a linear response ranging from 1 pg/mL to 1000 pg/mL. CEA detection in the fluorescence sensing system showed that the fluorescence quenching efficiency gradually decreased as the CEA level increased, with a linear response of 0.5–400 pg/mL. This SERS/fluorescence-based immunoassay had the same CEA detection limit of 0.1 pg/mL. The fine-engineered penetrable nanoassemblies with Au ring satellites can provide an insight into the rational and optimal design of multifunctional nanostructures. They can also broaden the field of vision for the design of multifunctional sensing probes in biochemical detection.