Fig. 2. (A) Schematic illustration of the fabrication of SERS tags and photographs of Ag@AuNPs (left), Au@AgNPs mixed with HS-GO for 5 min (middle) and 10 min (right); (B) SERS enhancement of Au@Ag-GO nanocomposites prepared with different Au@AgNPs/GO ratios (mL : mg): from 0.5 : 1 to 15 : 1. Raman spectra from the 4-MPBA adsorbed on the Au@Ag-GO surface; (C–G) TEM images of Au@Ag-GO nanocomposites prepared with different Au@AgNPs/GO ratios (mL : mg): 0.5 : 1 (C), 1 : 1 (D), 5 : 1 (E), 10 : 1 (F) and 15 : 1 (G).