Ag@C Core-Shell Colloidal Nanoparticles Prepared by the Hydrothermal Route and the Low Temperature Heating-Stirring Method and Their Application in Surface Enhanced Raman Scattering

Silver nanoparticles (AgNPs) with diameters in the range 40-80 nm have been synthesized by the hydrothermal route and the low temperature heating-stirring method. The influences of reaction time, reactant concentration, and temperature on the AgNP growth have been systemically studied. Experimental surface enhanced Raman scattering (SERS) results show that AgNPs prepared under different temperature and time exhibit a large difference in enhanced SERS signals for rhodamine 6G (10(-6) M). Ag@C core-shell NPs have been proved to be formed by using the above two methods, and the carbon shell thickness is gradually increased with increasing reaction time and temperature. It is found that Ag@C NPs with a thick shell (more than 3 nm) have a very low SERS activity, while those with an ultrathin film (less than 1 nm) have a high SEAS activity, indicating that carbon shell thickness is a key factor affecting the SERS, which has also been evaluated by finite-difference time-domain simulation. The existence of an ultrathin carbon shell around the AgNP can decrease its surface electric property; then Ag@C NP aggregates are easily formed which may produce the higher hot spots than the bare AgNPs. In addition, this kind of Ag@C NPs exhibits a long SERS-active shelf life (6 months), because the carbon shell can protect AgNPs from oxidation.