Self-Assembled Cysteamine Reporter Ligands for SERS Nitrate Detection in Continuous Flow

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 <i>gauche</i> to <i>trans</i> conformation ratio, were dependent upon the cysteamine protonation state. The terminating amine of cysteamine was Raman active when protonated near the expected SAM p<i>K</i>_a and the gold-thiol bond was increasingly Raman active above the expected p<i>K</i>_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 10¹ to 10³ nanomolar (10⁰ to 10² 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.