Controlled Spread\nof a Ag Layer from the Core to the\nTip along the Branches of AuAg Nanostars for Improved SERS Detection\nof Okadaic Acid in Shellfish

Plasmonic Au–Ag nanostars are excellent surface-enhanced\nRaman scattering (SERS) probes due to bimetallic coupling and the\ntip effect. However, the existing preparation methods of AuAg nanostars\ncannot achieve controlled growth of the Ag layer on the branches of\nnanostars and so cannot display their SERS to the maximum extent,\nthus limiting its sensitivity in biosensing. Herein, a novel strategy\n“PEI (polyethylenimine)-guided Ag deposition method”\nis proposed for synthesizing AuAg core–shell nanostars (AuAg@Ag\nNS) with a tunable distribution of the Ag layer from the core to the\ntip, which offers an avenue for investigating the correlation between\nSERS efficiency and the extent of spread of the Ag layer. It is found\nthat AuAg@Ag NS with a Ag layer coated the whole branch has the strongest\nSERS performance because the coupling between the tips and Ag layer\nis maximized. Meanwhile, as a completely closed core–shell\nstructure, AuAg@Ag NS can confine and anchor 4-ATP inside the Ag layer\nto avoid an unstable SERS signal. By connecting the aptamer, a reliable\ninternal standard nanoprobe with a SERS enhancement factor (EF) up\nto 1.86 × 10<sup>8</sup> is prepared. Okada acid is detected\nthrough competitive adsorption of this SERS probes, and the detection\nlimit is 36.6 pM. The results gain fundamental insights into tailoring\nthe nanoparticle morphologies and preparation of internal standard\nnanoprobes and also provide a promising avenue for marine toxin detection\nin food safety.