Abstract
Elevated nitrate concentrations in aquatic environments can contribute to the formation of harmful algal blooms, which lead to eutrophication. In this work, cysteamine self-assembled monolayers (SAMs) on two-dimensional gold nanostructured substrates were investigated for the capture and detection of nitrate anions by surface enhanced Raman scattering (SERS) under continuous fluid flow. An indirect detection strategy is demonstrated where cysteamine Raman activity and SAM reconfiguration change due to nitrate adsorption. Nitrate adsorption, as well as SAM reconfiguration based on the gauche to trans conformation ratio, were dependent upon the cysteamine protonation state. The terminating amine of cysteamine was Raman active when protonated near the expected SAM pK(a) and the gold-thiol bond was increasingly Raman active above the expected pK(a). Highly charged SAMs (pH 3) were not responsive to nitrate, suggesting that nitrate detection is reliant upon the dynamic interplay between protonation, charge state, and nitrate adsorption. Cysteamine SAMs responded to nitrate concentrations spanning 10(1) to 10(3) nanomolar (10(0) to 10(2) parts per billion), which are considerably lower than those previously reported for direct detection of nitrate using cationic SAMs. This work demonstrates the potential for indirect SERS detection of anionic pollutants using rationally selected capture + reporter ligands.