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

Abstract

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.

In Situ Monitoring of the Polymerization Kinetics of Organic Pollutants during Persulfate-Based Advanced Oxidation Processes Using Plasmonic Colorimetry

“…24,28 According to our previous study, SO 4 −• are readily generated by breaking the S−S bond of PDS through UV photolysis under the illumination conditions employed in this study. 25 Since AuNPs exhibit weak absorption in the UV range (Figure S2a), their influence on PDS photolysis is likely negligible. Given that SO 4 −• often initiates polymerization, we expect it to aid in the formation of 2,4-D polymeric layers on the AuNP surfaces.…”

“…In the presence of SO 4 −• , the AuNP colloid rapidly collapsed due to the oxidation of the citrate stabilizing layer on AuNP surfaces. 25 However, when aromatic compounds, such as 2,4dichlorophenoxyacetic acid (2,4-D), bisphenol A (BPA), and diclofenac sodium (DIC), were introduced, the AuNP colloid was stabilized due to the formation of polymeric layers on AuNP surfaces. TEM images confirmed polymer formation, while electron energy loss spectroscopy (EELS) characterization provided insights into the polymer compositions.…”

Yolk–shell-type gold nanosphere-encapsulated mesoporous silica for catalytic oxidation of organic pollutants in the presence of persulfate

“…The decrease in the catalytic activity may be due to a reduction in the number of active sites due to removal of the stabilizing agent (citric acid) during repeated reactions. 50 Although over-aggregation of the AuNSs was not observed in the TEM image after the fifth cycle (Fig. S5 †), it is assumed that repeated PDS oxidation led to minor changes in the stability of the AuNSs inside the shell.…”

“…Recently, Wang et al reported that UV-activated PDS causes severe aggregation of AuNS due to the destruction of the citric acid layer on the surface of AuNS. 50 In contrast, AuNS@ y SiO 2 did not show any noticeable color change or precipitation after PDS addition (Fig. S3e and f †) because the mesoporous shell physically blocks over-aggregation of the AuNSs.…”

Rapid Prescreening of Trace Imidacloprid in Drinking Water via Concentration-Dependent Surface-Enhanced Raman Spectroscopic Patterns

“…In addition, metal film over nanoparticles (MFON) can be used as an alternative substrate that can enhance the sensitivity of the proposed sensing system. , MFON is a novel metal film coated on a self-assembled nanoparticle layer through the physical vapor deposition (PVD) process. The presence of sharp crevices and nanoscale roughness on the MFON surface serves as effective hot spots .…”