Aggregated Silver Sols as SERS Substrates

The unique plasmonic properties of the noble metal colloidal nanoparticles make them promising enhancement substrates for surface-enhanced Raman scattering. Obtaining of systems that provide ever-increasing enhancement of the Raman scattered light is a big challenge. Silver colloids studied here are prepared by Lee-Meisel's method. Rhodamine 6G and rhodamine B are used as probe adsorbates. The "raw" colloids obtained in this way exhibit a characteristic extinction band proving surface plasmons excitation, but do not enhance the Raman signal. Theoretical calculations indicate that the extremely large electromagnetic field is induced in the junctions between metallic nanostructures so some degree of their aggregation is necessary to achieve a sufficient gain value. The addition of aggregating agents (KCl or HCl) leads to significant changes in the extinction spectrum and to significant increase in the intensity of surface-enhanced Raman scattering. The experiments show that chloride can not only promote the aggregation process but also effectively affect chemical mechanisms contributing to surface-enhanced Raman scattering.