Monodispersed gold nanoparticles (Au NPs) of average size 12 nm were synthesized by the citrate reduction method. They tend to aggregate upon addition of thiol ligand mercaptoethanol, and amine ligands, dopamine, and adenine as indicated by the shift of localized resonance plasmon peak (LSPR) to higher wavelength where the intensity reduced to zero with time and by the transmission electron microscopic images (TEM) where aggregated NPs were observed, due to the partial desorption of surface passivating citrate ions due to the high affinity of thiol and amine groups to the gold surface compared to the oxygen groups. Adenine was increasingly replaced by increasing concentration of mercaptoethanol (28 µM to 1 mM) and 1,4-Dithiothreitol (20 µM to 10 mM) as shown by UV-Visible spectroscopic and Surface enhanced Raman spectroscopic (SERS) analysis, confirming that the thiol groups have more affinity to the gold surface than the amine groups. Au NPs were stabilized by mixing with polyethylene glycol-SH 6000 (PEG-SH) where a small red shift of the LSRP peak with a slight increase in the intensity was observed as PEG-SH has a higher refractive index and dielectric constant than water. No significant aggregation of Au NPs was observed in TEM images upon addition of PEG-SH. PEG-SH adsorption to the gold surface was confirmed by SERS analysis. The stabilization of Au NPs by PEG-SH was studied by the addition of the same ligands to PEG-SH adsorbed AuNPs. There was no shift in the position of LSPR peak and no reduction of intensity with prolonged exposure. TEM images also showed no aggregation of the nanoparticles. However, SERS spectra indicate the adsorption of the above ligands to gold surface even in the presence of PEG-SH due to the mushroom-like structure of PEG-SH, where small ligands can reach the gold surface but in a low quantity.

Ligand exchange reactions and PEG-SH stabilization of gold nanoparticles

ACS Spring 2022

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