Controlled Citrate Oxidation on Gold Nanoparticle Surfaces for Improved Surface-Enhanced Raman Spectroscopic Analysis of Low-Affinity Organic Micropollutants

Surface-enhanced Raman spectroscopy (SERS) provides an ultrasensitive, fast, and inexpensive method for organic micropollutant analysis, but its applications are limited by the low affinity of most organic micropollutants toward plasmonic nanoparticle surfaces. Particularly, the citrate layer on gold nanoparticle (AuNP) surfaces exerts strong resistance to ligand exchange and prevents carboxylic and phenolic pollutants from entering SERS "hot spots". In this study, we aim to extend the application of SERS to low-affinity organic micropollutants by oxidative decomposition of the citrate layer on AuNP surfaces. The kinetics of citrate oxidation were carefully controlled using sulfate radicals that were slowly released from peroxydisulfate photolysis, which guarantees both the stability of AuNP colloid and generation of a high density of SERS hot spots for pollutant analysis. In situ Raman spectroscopic monitoring demonstrates that citrate is first oxidized to di- and monocarboxylate acids and subsequently displaced by guest ligands. This oxidation-induced ligand exchange has been applied for SERS analysis of various low-affinity organic micropollutants, including monochloro-substituted carboxylates and phenols, as well as a widely used herbicide 2.4-dichlorophenoxyacetic acid. This study substantially broadens the library of organic micropollutants for label-free SERS analysis and advances SERS toward a holistic analytical tool for water quality monitoring.