Ligand exchange reactions and PEG stabilization of gold nanoparticles

Abstract

Gold nanoparticles (Au NPs) were synthesized by the citrate reduction method. Au NPs were aggregated when mixed with mercaptoethanol (ME), dopamine (DAH) and adenine (AD) as the surface passivating citrate molecules were incompletely but heavily replaced by them as supported by the occurrence of a new peak at a longer wavelength and eventual disappearance of the peak/s in the UV–Visible spectra due to the settlement of the aggregated NPs. Transmission electronic microscopic (TEM) images showed aggregated NPs. AD was increasingly replaced by an increasing concentration of dithiothreitol (DTT) in the range of 20 ​μM–10 ​mM, but an incomplete replacement was resulted even after using highly concentrated DTT (10 ​mM). mPEGSH passivates the gold surface where they bind to the gold surface via a mushroom-like configuration. mPEGSH stabilizes the Au NPs preventing aggregation upon mixing with ME, DAH and AD, as revealed by no change in the position and the absorbance of the localized resonance surface plasmon peak in the UV–Visible spectra and well-dispersed Au NPs in the TEM images. Mushroom-like arrangement of mPEGSH on the Au NPs allow adsorption of ME, DAH and AD as revealed by the surface enhanced Raman spectroscopic data.