The Aromatic Amine pKa Determines the Affinity for Citrate-Coated Gold Nanoparticles: In Situ Observation via Hot Spot-Normalized Surface-Enhanced Raman Spectroscopy

Abstract

The in situ characterization of organic pollutant dynamics on engineered interfaces provides fundamental insights about pollutant fate, transport, and toxicity in aquatic systems. In this study, the association between aromatic amines and citrate-coated gold nanoparticles (cit-AuNPs) under different pH conditions was monitored in situ using hot spot-normalized surface-enhanced Raman spectroscopy (HSNSERS). HSNSERS minimizes the uncertainty of SERS measurements that arises from substrate heterogeneity, laser intensity fluctuation, and laser focusing, and its application enabled quantification of the influence of solution pH on the measured SERS signals. Our results demonstrate that the accumulation of chloroanilines on cit-AuNP surfaces is dictated by solution pH and analyte pKa. At circumneutral pH (≈5.6), only the chloroanilines with relatively high pKa values were readily detected by SERS, while those with low pKa values (<3) were undetectable. In contrast, at low pH, only the chloroanilines with low pKa values were detected. These results highlight the importance of electrostatic forces in modulating the accumulation of the pollutant on cit-AuNP surfaces and suggest the potential for selective pollutant detection by solution pH adjustment. HSNSERS, an innovative analytical platform, can be easily extended to in situ monitoring of the accumulation of pollutants onto gold, silver, and copper nanoparticles triggered by different environmental stimuli (e.g., pH).