Surface-enhanced infrared absorption of DMIP on gold-germanium substrates coated by self-assembled monolayers

Gold islands coated with self-assembled monolayers of three enough different thiols-thiophenol, propanethiol and octadecanethiol-are shown to enable the detection of molecules without polar groups, such as dimehylisophthalate (DMIP), using Surface enhanced infrared absorption (SEIRA). Binding of thiols was studied using SERS and SEIRA. The careful comparison of the transmission and p-polarized external reflection SEIRA spectra of DMIP taken using the modified substrates, with the corresponding spectra taken on the uncoated gold-Ge and bare Ge and CaF2 surfaces, has allowed to elucidate the role played by the underlayer substrate and the different thiols in the total observed enhancement effect. Furthermore, the DMIP orientation with respect to the gold surface for each thiol-coating has also been deduced, confirming that the surface selection rule in our case applies both to reflection and transmission SEIRA spectra. No SERS spectra of DMIP, either on the modified or on the uncoated gold substrates, were revealed. (C) 2003 Elsevier B.V. All rights reserved.