MODULATION OF ACOUSTIC COUPLING BY PHOTO-OXIDATION OF SELF-ASSEMBLED MONOLAYERS

A thickness-shear mode acoustic wave device operated in a flow-injection configuration has been used to study the effects of oxygen on thiol-based self-assembled monolayers at the liquid-sensor interface when exposed to UV light. The films were composed of dodecanethiol or undec-5-yne-1-thiol. Piezoelectric AT-cut quartz sensors with gold electrodes were submerged in thiol solutions with varying amounts of oxygen present in the solution by altering the amount of time that the solutions were exposed to nitrogen. Oxygen was also removed or incorporated into the system by sparging the liquid buffer used in the flow system with helium gas. When exposed to UV radiation, the device with thiol monolayers in place exhibited reversible changes in series renounces frequency and motional resistance only in the presence of oxygen. This result is ascribed to a photochemically induced oxidation of the sulfur moiety at the device-liquid interface. The response of the device under these conditions is governed by acoustic coupling phenomena and not mass effects. Finally, the in situ cross-linking of undec-5-yne-thiol does not result in any additional changes in response other than those indicated for dodecanethiol confirming that thickness viscoelastic changes at the monolayer level are not relevant to device response.