Synthesis of Aucore-Agshell type bimetallic nanoparticles for single molecule detection in solution by SERS method

This paper reports the evolution of a new class of core - shell type, that is, Au-core-Ag-shell bimetallic nanoparticles by seed mediated technique for surface enhanced Raman scattering (SERS) study. Here it is demonstrated how to control the thickness of Ag-shell with the variation of gold seed (similar to 15 nm) to Ag ion concentration which in turn control the particle size in the range from 50 to 100 nm with increase of shell thickness. For 50 nm core - shell particles the thickness of the shell was similar to 17 nm, for 70 nm particles the thickness was similar to 27 nm and for 100 nm the thickness was similar to 42 nm. SERS study was performed on those particles using the analyte crystal violet ( CV) to examine the impact of the size and field effects of the bimetallics on SERS spectra. A surprising finding is that a small particle as low as 50 nm have been found to be highly efficient for SERS, even it enables the detection of a selected dye molecule down to single molecular level. The sensitivity of the SERS detection limit has been improved further with an activating reagent like NaCl. The newly modeled bimetallic system establishes a relationship between the local electromagnetic ( EM) field effect and chemical effect (CE) on the enhancement of SERS spectra, which provides further insight into the enhancement mechanism of SERS.