Capping Ligand Effects on Quick-Freezing-Induced Au Nanoparticle Aggregates (QFIAAs) and Their Application in NIR-SERS Dye Screening

This work extends our previous study of quick freezing-induced gold (Au) nanoparticle aggregates (QFIAAs) as near-infrared (NIR)-surface-enhanced Raman scattering (SERS) substrates. Simple quick freezing of citrate-capped AuNPs produces reproducible NIR-SERS-active aggregates. Here, we examine how the size and charge of three capping ligands─citrate, tannic acid, and polyvinylpyrrolidone (PVP)─affect QFIAA formation and the adsorption behaviors of dyes on the resulting aggregates, which together determine SERS response. The small, negatively charged citrate ligands promote compact aggregation and efficient dye adsorption within hot spots, yielding highly sensitive SERS signals. Citrate vibrations also serve as internal Raman markers to monitor dye-induced secondary aggregation, a key factor since excessive clustering can distort hot-spot density and SERS responses. Dye adsorption on QFIAAs follows the Sips model, indicating heterogeneous adsorption sites within the aggregates. Dye adsorption and signal intensity depend strongly on the dye charge and molecular geometry. Finally, QFIAAs were used to evaluate newly synthesized NIR dyes, revealing that strong electronic absorption alone at the excitation wavelength does not guarantee high SERS activity. These results establish QFIAAs as a quantitative mechanistic platform for NIR-SERS studies.