By using PVP as the dispersant and reducing agent,Fe_3O_4@SiO_2/ Ag nanocomposites are synthesized via a simple and one-step method in the present of Fe_3O_4. The nanocomposites are acted well as a SERS-active substrates for Rh B. TheSiO_2 shell between Fe_3O_4 nanoparticles and Ag nanoparticles is used to avoid the agglomeration of Ag nanoparticles and stabilize metal phase. The unique Raman enhancement effect of Fe_3O_4@SiO_2/ Ag substrates indicates that Rh B can be detected at a concentration as low as ...