Comparison of Resonant and Non Resonant Conditions on the Concentration Dependence of Surface Enhanced Raman Scattering from a Dye Adsorbed on Silver Colloid

Abstract

Surface enhanced Raman scattering (SERS) and surface enhanced resonance Raman scattering (SERRS) from a silver colloid suspension are compared using a dye designed to bond strongly to a silver surface. Titration of the dye into a silver colloid suspension caused aggregation in a controlled manner without an aggregating agent being added. Concentrations of dye equivalent to between 5 × 10-9 and 10-3 M in the final dilution before adsorption to the silver were used. The results suggest that monolayer coverage of the surface occurs at approximately 10-6 M. Above this concentration, the suspensions are less stable, and the relationship between intensity and concentration is complex. Below this concentration, three main regions can be identified by electronic absorption spectroscopy. At dye concentrations up to 7.5 × 10-8 M, there is little evidence of aggregation, although there are changes in the spectra ascribed here to surface changes caused by dye adsorption. Between 7.5 × 10-8 M and 2.5 × 10-7 M, a well-defined small aggregate appears to occur and above 2.5 × 10-7 M larger less well-defined aggregates form. SERRS gave linear concentration dependence below 7.5 × 10-8 M suggesting scattering from single particles. A changeover region occurs close to where the first evidence of aggregation is detected by electronic spectroscopy, and at higher concentrations up to about monolayer coverage a second linear region was obtained. SERS below the concentration at which aggregation is detected by electronic spectroscopy was weak and difficult to obtain. At higher concentrations, the SERS gradients are steeper and the maximum enhancement observed is within a factor of 4 of that obtained in SERRS. The study shows that there is a different mechanism in SERRS compared to SERS with single particle enhancement being much greater in SERRS.

Characterization of Novel Ag on TiO2 Films for Surface-Enhanced Raman Scattering

“…It is known that the benzotriazole moiety forms a polymeric silver complex with the surface of silver; indeed, benzotriazole itself is commonly used as an anti-tarnishing agent 28 . Work by our group has established that the attachment of the GM-19 dye onto silver is essentially irreversible and that the dye remains in position over a wide range of experimental conditions and concentrations, allowing highly reproducible analysis [23][24][25][26][27] . The dye is easily applied, typically as a weak methanolic solution (10 -6 M in this work); adsorption is almost 100% and the observed SERS signal is usually significant on SERS active substrates, due to the large spontaneous Raman cross-section of the dye.…”

“…Thus, in a typical experiment, a Ag-coated film under test was placed in a 10 -6 M GM-19 solution of methanol for 15 minutes, removed and allowed to dry in air before being placed under the SERS microprobe microscope. The GM-19 dye has an absorbance maximum at 446 nm 27 , and, therefore, in this work, there is a limited contribution to the observed SERS signal due to surface enhanced resonance Raman spectroscopy (SERRS), since an excitation laser light of wavelength 514.5 nm was used 25 . All Ag films generated as part of this work were tested for SERS activity using the above procedure.…”

“…Research carried out within the group shows that at 10 -6 M, GM-19 typically forms a monolayer on most Ag surfaces 25 . Thus, using the Ag/TiO 2 films at higher concentrations the SERS signals were found to decrease whereas, at lower concentrations, 10 -7 -10 -6 M, the signal was found to be approximately proportional to the concentration of GM-19.…”

“…Thus, using the Ag/TiO 2 films at higher concentrations the SERS signals were found to decrease whereas, at lower concentrations, 10 -7 -10 -6 M, the signal was found to be approximately proportional to the concentration of GM-19. Smith et al 25 have shown that the linear region in a SERS peak signal intensity versus calibration graph using Ag colloids can be extended by an order of magnitude using 457.9 nm laser light, because the spectra are due to SERRS, instead of the less-sensitive process of SERS.…”

Surface-enhanced Raman scattering of silver-gold bimetallic nanostructures with hollow interiors

“…From the originally used metal electrode 15,16 to metal colloid systems, [17][18][19] the researches about SERS-active substrate have been explored extensively to develop much higher enhancement ability and stability. Silver and gold colloids are most frequently employed metallic systems in SERS.…”