Empirical structural design of core@shell Au@Ag nanoparticles for SERS applications

In this work, we synthesized a series of core@shell Au-2r@Ag-t nanoparticles (CS Au-2r@Ag-t NPs) with defined but varied Au core diameter (2r) and Ag shell thickness (t) via overgrowth of Ag on preformed Au NPs at room temperature. We demonstrate that the surface enhanced Raman scattering (SERS) activity of as-prepared Au-2r@Ag-t NPs is dependent on the Ag shell thickness (t). The critical t value (t(c)), above which Au-2r@Ag-t NPs reach the maximal SERS activity, can be empirically correlated with the r values as t(c) = 0.301.r + 0.695 when the diameters of Au cores (2r) are smaller than 42 nm, the mean free path of bulk gold, while the t(c) is fixed at about 3 nm when 2r > 42 nm. The simple empirical rule should be very useful for the design of Au@Ag NPs for SERS applications, which is hardly discussed in the literature.