Effect of chemisorbed thiophenols with an electron donating group on surface-enhanced Raman scattering of gold nanorods†
Surface-enhanced Raman scattering (SERS) is a powerful technique to amplify the weak Raman scattering intensity by molecules chemisorbed on a metallic surface. Herein, we present the interfacial electronic effect of para-substituted aromatic thiophenols (TPs) with an electron donating group (EDG) on SERS of anisotropic gold nanorods (AuNRs) under resonant conditions. Probe molecules with an EDG showed great SERS enhancement in AuNRs at the resonant excitation wavelength. We found that the SERS enhancement with an EDG is caused by the formation of aggregates through intermolecular interactions among probe molecules, such as dimerization with hydrogen bonding via an amino group (–NH2) of p-aminothiophenol (p-ATP) and hydroxyl group (–OH) of p-mercaptophenol (p-MP), resulting in hot-spots between AuNRs. Furthermore, SERS having a stronger EDG (–NH2, p-ATP) with the Hammett constant of −0.66 exhibited greater enhancement than p-MP having hydroxyl (–OH) groups with the Hammett constant of −0.37. We found that the greater enhancement is ascribed to the temporary formation of a positively charged electron withdrawing group (–NH3+) in p-ATP, unlike p-MP, via the interaction of the lone pair of the amino group (–NH2) with ethanol. Therefore, this investigation provides new insightful experimental observations on SERS enhancement of probe molecules with an EDG.