Suppression of salt-induced aggregation of 4-mercaptobenzoic acid functionalized gold nanoparticles by aromatic dye molecules.

The colloidal instability of ligand-functionalized gold nanoparticles under electrolyte-rich conditions limits their application in plasmonic sensing. Here, 4-mercaptobenzoic acid-functionalized gold nanoparticles (Au-4MBA) were investigated for their aggregation behavior in NaCl-containing media and stabilization by aromatic dyes. Au-4MBA nanoparticles exhibited an LSPR band at 524 nm, while exposure to 5 mM NaCl induced aggregation, characterized by a secondary plasmon band at ∼670 nm and a visible color change. Congo Red (CR) progressively suppressed this aggregation-associated response with increasing concentration. The colloidal stability was strongly pH-dependent but remained relatively insensitive to temperature over 10-50 °C. Comparative evaluation of structurally distinct dyes revealed different stabilization behaviors, following the order CR ≈ Erythrosine B > Methyl Orange > Alizarin Red S >> Methylene Blue. These differences are consistent with a structure-dependent influence of aromatic dyes on nanoparticle stability. The experimental observations support a proposed electrosteric stabilization mechanism involving possible interfacial association and noncovalent interactions that restrict close nanoparticle approach and plasmonic coupling. These findings provide a simple strategy for improving the electrolyte stability of ligand-functionalized plasmonic nanoparticles.