Activation of Silver Colloids for Enhancement of Raman Scattering

Obtaining of systems that provide ever-increasing enhancement of the Raman scattered light is a big challenge. The silver colloids obtained by reduction of silver nitrate by trisodium citrate are the most promising enhancement systems. Nanoparticles of silver forming fresh colloid obtained by this way exhibit in extinction spectrum the absorption band proving of the surface plasmons excitation, but do not enhance the Raman scattered light, are not surface enhanced Raman scattering active. Adding of KCl or $HNO_3$ causes the surface enhanced Raman scattering activation, i.e. causes that the conditions for aggregation of nanoparticles and for adsorption on their surface of molecules providing Raman scattering are created. Increasing amount of added KCl or $HNO_3$ leads to significant changes in the extinction spectrum and to significant increase in intensity of surface enhanced Raman scattering. The observed changes in extinction spectrum are mainly due to progressive aggregations of nanoparticles. In junctions between nanoparticles existing inside aggregates the condition for single molecule surface enhanced Raman scattering are fulfilled and therefore aggregation leads to increase in average surface enhanced Raman scattering intensity. Silver colloid activated by using KCl provides about 10-fold higher surface enhanced Raman scattering than that treated with $HNO_3$.