Computational and experimental evaluation of selective substitution of thiolated coumarin derivatives on gold nanoparticles: Surface enhancing Raman scattering and electrochemical studies

Gold nanoparticles (AuNPs) of various sizes were prepared and treated with a mixture of HS-(CH2)11-NHCO-coumarin (act as Raman reporter) and HS-PEG-(CH2)11COOH (as co-stabilizer and also to attach biomarkers on activated COOH) to produce mixed monolayer protected clusters of gold (AuMMPCs). In this paper, we demonstrate the significance of the HS-(CH2)11-NHCO-coumarin concentration (percentage ratio) on the enhancement factor (EF), and the geometry of the adsorbed ligands on AuNPs of different sizes. The calculated EFs from Raman spectra reveal a significant decrease with an increase in AuNPs sizes. Molecular dynamics calculations were carried out to obtain the adsorption energies for different ratios of HS-PEG-(CH2)11COOH to HS-(CH2)11-NHCO-coumarin. It was found that molecules that adsorb strongly on the surface of the metal, underwent changes in their polarizability and consequently enhanced Raman intensities were observed, and this was in agreement with experimental data.