Surface-enhanced Raman scattering of coumarin 343 on silver colloidal nanoparticles

• Coumarin 343 shows two distinct surface-enhanced Raman spectra on silver colloids.
• Carboxylate and cyclic ester group are pH-dependent surface adsorption sites.
• DFT simulated Raman spectra fairly represent experimental results.

Surface-enhanced Raman scattering (SERS) of coumarin 343 (C343) adsorbed on silver colloidal nanoparticles reduced by sodium citrate was investigated and the surface adsorption geometry of C343 on Ag was sought by optimizing C343-Ag complexes for neutral and deprotonated C343 molecules in the DFT simulations. The SERS of C343 showed a number of spectral changes upon solution pH change. We found that deprotonated C343 adsorbs on the Ag nanoparticles through the carboxylate group keeping a perpendicular geometry to the surface. When protonated, the adsorption geometry of C343 is changed into more or less flat to the surface as the cyclic ester group becomes a preferred surface adsorption site.

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