Self-assembled monolayers of thiophenol on gold as a novel substrate for surface-enhanced infrared absorption

A unique method of obtaining surface-enhanced infrared absorption (SEIRA) spectra for chemicals that will not chemically attach to a metal surface has been investigated, Surface enhancements are greatest for molecules that bind to metals. In order to achieve greater enhancement for those analytes that do not bind to SEIRA metals, we have investigated self-assembled monolayers as a means of linking analytes to a gold substrate. Monolayers of thiophenol were formed onto sputter-coated gold-silicon substrates. Analytes were deposited onto the thiophenol-coated gold-silicon wafers, and external reflection SEIRA spectra were then measured. Enhancement factors as high as 30-fold compared to those for conventional SEIRA substrates are demonstrated. The self-assembled monolayers on gold substrates are shown to change both relative intensities and band positions of the adsorbed analyte. These intensity changes and frequency shifts show strong interaction of selected analytes with the self-assembled monolayer. This study of phthalates and nitrosubstituted aromatic compounds demonstrates the usefulness of the technique. Spectral changes evident through the use of the thiophenol monolayer are discussed.