Functionalized gold nanoparticles (AuNPs) are of interest for their optical and electrical properties, specifically the surface plasmon resonance phenomenon and potential applications for nano-circuitry. In this study, the ligand composition of small (~5nm diameter) thiol-functionalized AuNPs with mixed-ligand monolayers was investigated to better understand how the molar feed ratio of ligands used during their synthesis influences the composition and distribution of ligands on the particle surface . Surface chemistry has been shown not only to affect stability; which environments are amenable for the particle, but morphology also serves as a means to control reactivity and interactions with other media. The system under study was a combination of two ω-functionalized thiols, mercapto ethoxy ethoxy ethanol (MEEE) displays strong hydrogen bonding and a negative charge whereas mercapto pentane trimethyl ammonium chloride (MPTMA) is hydrophobic, positively charged, and bears a large ammonium head. UV/Visible spectrophotometry and electron microscopy were utilized to determine particle size, while 1H-NMR, ζ-potential analysis, FTIR spectroscopy were used to probe monolayer composition and surface charge. Alkyl halide ligand precursors, purified ligand, and a battery of mixed-monolayer AuNPs were assessed via 1H-NMR to determine the relative composition of the monolayer. Furthermore, the gold cores were digested with iodine to better elucidate the ligand composition without electronic baseline broadening which accompanies electromagnetic assessment of metal nanoparticles. Preliminary 1H-NMR data suggests that the distribution of functionalized thiols on the AuNP surface is not directly indicative of ligand feed ratio stoichiometry but thiol-thiol interactions on the particle surface lead to the minority thiol being over-representative in the mixed-monolayer.

Mixed-monolayer gold nanoparticles: Compositional analysis with an internal standard 1H-NMR experiment

ACS Spring 2022

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