Multi-branch Au/Ag bimetallic core-shell-satellite nanoparticles as a versatile SERS substrate: The effect of Au branches in a mesoporous silica interlayer

We synthesized a novel bimetallic core-shell-satellite surface-enhanced Raman scattering (SERS) substrate (AuNS@mSiO₂@AgNP) that consists of a gold nanostar (AuNS) core, a mesoporous silica (mSiO₂) shell and a layer of silver nanosphere (AgNP) satellites. The core-shell-satellite nanostructure was formed by growing branches in the mSiO₂ shell of mSiO₂ coated gold nanospheres (AuNP@mSiO₂) using AuNPs as the seeds, and the subsequent deposition of AgNPs on the mSiO₂ surface. The growth of AuNSs in mSiO₂ leads to the formation of a large number of closely spaced branches which provide enormous hotspots; and by varying the length of the branches, the nanogap distance between the AuNS core and the AgNP satellites can be carefully adjusted. Besides, mSiO₂ could allow the infiltration of the analyte into the nanogap area that has high SERS enhancement. To demonstrate the feasibility of the SERS substrate, the bimetallic core-shell-satellite nanostructures were used for the label-free detection of a pesticide named thiram. The AuNS@mSiO₂@AgNP was shown to have excellent selectivity and SERS activity which is significantly higher than that of AuNS@mSiO₂, mSiO₂@AgNP and AuNP@mSiO₂@AgNP. This method exhibits a good linear response when the thiram concentration ranges from 10^-9 M to 10^-3 M, and the limit of detection (LOD) is 10^-9 M, showing that the bimetallic core-shell-satellite nanostructure is a promising SERS substrate.