Synthesis and characteristics of bimetallic core-shell Ag-Au nanoflowers as SERS-tag

Abstract

Bimetallic nanoparticles (NPs) have received considerable attention for their unique optical, magnetic and other properties. Their composition, architecture, shape and size can be controlled for achieving desired properties for specific applications. There is a great interest in surface enhanced Raman scattering (SERS) due to its high sensitivity and the ability of multiplex detection. Among various SERS tags, flowerlike core-shell Ag-Au NPs are very attractive because an intensity enhancement factor of 104–106 may be achieved, which will be sufficient for obtaining Raman signals large enough to enable single molecule detection. In our research, we developed an effective method to synthesis flowerlike Ag-Au NPs with diameters of 90 nm, which consisted of an Au core and a highly branched Ag shell and could incorporate the Raman reporter RhB. The flowerlike core-shell Ag-Au NPs were initiated by the formation of Au nanoaggregates, serving as in situ seeds for the subsequent deposition of Ag shell. The shape and thickness of the Ag shell could be altered by changing the Ag precursor amount, which is very important for tuning the optical property. Time-course measurements by UV-vis spectroscopy, HRTEM and STEM-EDX were made to follow the reaction progress and the evolution of the flower-like shape. For one reaction, the Au:Ag atomic ratio was found to be 5:1 in nanoparticles, which agreed well with their atomic ratio (1:0.2) in the initial reaction solution. The Ag-Au NPs exhibited strong SERS as compared to simple Au or Ag NPs. A folic acid-chitosan biopolymer shell could also be formed on RhB@Ag-Au NPs for further bioconjugation and biodetecion. These nanoparticles are a new class of SERS tags and can have other biomedical applications.