Monitoring chemical reactions in aqueous solution is a challenge because most instrumental techniques either are not suited for the rapid timescales, are not sensitive enough to detect products at low concentrations, or do not have sufficient structure-to-spectrum relationships. Raman spectroscopy is a promising method to monitor reactions, as it is fast, dependent on chemical structure, and has little interference from water. However, Raman scattering is generally very weak. Surface-enhanced Raman spectroscopy (SERS) improves the signal strength of Raman spectroscopy by using a metal surface plasmon, or oscillation of the surface’s electrons, to allow for highly selective and sensitive detection and characterization of analyte molecules. An aqueous colloid of partially aggregated gold nanoparticles is an accessible substrate to generate plasmons necessary for SERS. Analyte is adsorbed to the nanoparticles, initiating aggregation. A surfactant (sodium dodecyl sulfate) is added at an optimal time to arrest aggregation, leading to a highly stable substrate. Because of this, SERS enhancement is present over multiple hours under powerful UV light. As peaks in the Raman spectra evolve over time, structural changes can be detected. Paraquat, a common yet dangerous herbicide that is banned in the EU, China, and many other countries, was monitored with SERS in real time as it degrades when exposed to intense light, analogous to sunlight. The photodegradation products have not been well characterized in the past, but methylpyridinium is expected. A reaction mechanism and kinetics can then be determined for this reaction. Gathering better data about paraquat and its degradation in the environment may help users apply paraquat in a manner that minimizes harm to humans and the ecosystem.

Real time monitoring of paraquat photodegradation using colloidal gold surface enhanced Raman spectroscopy

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.