SERS of positive silver colloids with addition of aggregating agent

A very strong surface-enhanced Raman scattering(SERS) spectrum was obtained when adding some aggregating agent such as HNO_3 and NaNO_3 in positive silver colloid reduced by citrate using 633 nm radiation.In aid of characterizing the SERS of this process,SERS from positive silver colloid with the addition of H_2SO_3,HCl,and NH_3 recorded.It was found that the SERS come from the citrate when adding HNO_3,NaNO_3 and H_2SO_3,from NH_3 and citrate when adding NH_3,and only from Ag—Cl when adding HCl through comparing the spectra.Furthermore,the surface layer of positive silver colloid particles could be Ag~+,and the diffusion layer could be citrate.