Synthesis and SERS activity of super-multibranched AuAg nanostructure via silver coating-induced aggregation of nanostars.

The super-multibranched AuAg bimetallic nanostructures are synthesized due to the aggregation of Au nanostars in the process of silver coating. The super-multibranched bimetallic nanostructures with different silver coating thickness are obtained by changing the concentration of silver nitrate and ascorbic acid. It has been found that the formation of these nanostructures is due to the stacking of several nanostars during the process of silver coating. By comparing the silver coating process of gold nanostars with different branch lengths, we found that the nanostars with longish branches are easy to aggregate and form the super-multibranched nanostructures in the process of silver coating. In the AuAg bimetallic nanostructures, the silver layer is mainly covered on the surface of the cores and the thickness increases with the increasing of the AgNO3 , which leads to the change of the surface-enhanced Raman scattering (SERS) activity. It has been found that the SERS activity is stronger when the silver layer is thin and the Au branches are still exposed to the outside of the Ag shell. The sample with the strongest SERS activity has been used to detect thiram with different concentrations. The Raman intensity increases linearly with the logarithmic concentration of thiram ranging from 10-3 to 10-7  M with a detection limit of 6.3 × 10-7 M. These experimental results show that the super-multibranched bimetallic nanostructures have a broad application prospect in molecular detection and biologic sensing based on SERS.