A Novel SERS-Active Tag Based on Bimetallic Flowerlike Au-Ag Nanoparticles

Bimetallic nanoparticles have received considerable attention due to their unique optical, magnetic and other properties. Their composition, architecture, shape and size can be adjusted relatively easily in order to change their properties, which provide great potential for their diverse applications. Bimetallic Au-Ag nanoparticles are very attractive because they exhibit remarkable optical properties due to their surface plasmon resonance (SPR) absorption. In this investigation, a simple synthesis process for novel, bimetallic flowerlike Au-Ag composite nanoparticles was developed. Biocompatible folic acid-conjugated chitosan with flexible molecular chains were used as the structure-directing agent in nanoparticle synthesis. The structure and properties of Au-Ag nanoparticles were studied using a variety of analytical techniques including high resolution transmission electron microscopy. These nanoparticles contained an Au core and an Ag shell and the shape and thickness of the Ag shell could be easily controlled by varying the synthesis condition. The formation and growth of flowerlike bimetallic Au-Ag composite nanoparticles was investigated and its mechanism proposed. On the basis of flowerlike Au-Ag nanoparticles which can produce surface enhanced Raman scattering (SERS), a novel SERS tag was formed, which consisted of an Au-Ag flowerlike nanoparticle, embedded Rhodamine B (RhB, a Raman reporter molecule), and a folic acid-conjugated chitosan outer layer. This new SERS tag (RhB@Au-Ag tag) exhibited greatly increased SERS intensity of RhB and hence showed great potential for tumor cell targeting and detection.