Qualitative analysis of some alkanethiols on Au nanoparticles during SERS

Abstract

The surface enhanced Raman spectroscopy enhancement factors (SERS EFs) for different AuNP–surfactant systems are measured and the observed trend is theoretically and qualitatively investigated. Four thiolated coumarin derivatives (HS–(CH₂)₁₁–NHCO–coumarin, HS–(CH₂)₁₁–NHCO–indole, HS–(CH₂)₁₁–triphenylimidazole and HS–(CH₂)₁₁–hydroquinone respectively) together with a stabilizing surfactant, HS–(CH₂)₁₁–PEG-COOH, were adsorbed in ratios of 1% and 50% onto different sized Au nanoparticles. Similar simulation experiments were done to determine the influence of the nanoparticle–surfactant binding energy (E_b) on SERS EFs. Other parameters that were qualitatively investigated were the dependency of the Raman enhancement on (a) the size of Au nanoparticles, (b) the relative surface-to-molecule orientation, (c) the molecular orbitals (HOMOs and LUMOs), (d) the molecule electro-and nucleophilic centres and cross-sectional areas as well as (e) the molecular electrostatic potentials. It was observed that each of these parameters contribute to the observed trend and the final Raman signal enhancement.