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@mSiO(2)@AgNP) that consists of a gold nanostar (AuNS) core, a mesoporous silica (mSiO(2)) shell and a layer of silver nanosphere (AgNP) satellites. The core-shell-satellite nanostructure was formed by growing branches in the mSiO(2) shell of mSiO(2) coated gold nanospheres (AuNP@mSiO(2)) using AuNPs as the seeds, and the subsequent deposition of AgNPs on the mSiO(2) surface. The growth of AuNSs in mSiO(2) 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, mSiO(2) 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@mSiO(2)@AgNP was shown to have excellent selectivity and SERS activity which is significantly higher than that of AuNS@ mSiO(2), mSiO(2)@AgNP and AuNP@mSiO(2)@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.