Gold Nanoparticle Aggregation Pathways Probed by Evanescent Wave Cavity Ringdown Spectroscopy: The Role of Guanidine...

Gold nanoparticles (GNPs) are widely utilized in biomedical sensing, environmental monitoring, and imaging due to their tunable localized surface plasmon resonance, which is highly sensitive to aggregation dynamics. Understanding the factors governing GNP aggregation is crucial for optimizing plasmonic sensors and enhancing detection methodologies. Guanidine hydrochloride (GdnHCl), a strong chaotropic agent, influences nanoparticle stability and aggregation, yet its direct role in modulating GNP assembly remains largely unexplored. While guanidine-functionalized GNPs have been studied for controlled aggregation, the impact of free GdnHCl in solution has not been systematically investigated. In this study, we employ evanescent wave cavity ringdown spectroscopy to probe GdnHCl-induced GNP aggregation kinetics in real-time with high sensitivity. Additionally, we explore the influence of l-tryptophan (l-Trp) on this aggregation process, revealing a distinct phenomenon of "frustrated aggregation", where the interplay between guanidinium ions and aromatic amino acids alters the conventional aggregation pathway. This study provides critical insights into the physicochemical interactions governing nanoparticle stability, which can inform the development of advanced nanobiosensors. Our findings contribute to a deeper understanding of GNP aggregation mechanisms and offer new perspectives for designing responsive plasmonic systems for biomedical and analytical applications.