Nanoscale phase segregation of mixed thiolates on gold nanoparticles

Abstract

Phase segregation and domain formation is observed within the protecting monolayer of gold nanoparticles (AuNPs) using ion mobility-mass spectrometry, a two-dimensional gas-phase separation technique. Experimental data is compared to a theoretical model that represents a randomly distributed ligand mixture. Deviations from this model provide evidence for nanophase separation resulting in anisotropic AuNPs.

Figure 1

Typical workflow for experiments presented here. Mixed-ligand AuNPs with unknown levels of nanophase separation are analyzed by MALDI-IM-MS. The MALDI process leads to the fragmentation of protecting gold-thiolate complexes from the AuNP surface. The gold-thiolate ions undergo gas-phase separation from organic ions. The Au₄L₄ ion species are extracted from the data using software, and their abundances are compared to a theoretical model based on the binomial distribution. Deviations indicate nanophase separation in the AuNP monolayer.

Figure 2

A comparison of experimental and theoretical ligand distributions for free gold-thiolate complexes and three mixed-ligand AuNPs obtained by ligand-exchange reactions of tiopronin AuNPs with free glutathione, 11-mercaptoundecanoic acid (MUA), or mercaptoundecyltetraethylene glycol (MUTEG). Deviation from the theoretical model indicates the presence of phase-segregated gold-thiolate monolayers on AuNPs. Various ligand mixtures yield different degrees of nanophase separation.

Figure 3

A comparison of residual sums of squares (log r, ordinate) for various mixed-ligand AuNPs (above) at different ligand:ligand ratios (SR:SR′, abscissa). Specific ligand mixtures are indicated by color and shape; open and filled symbols indicate mixed-ligand syntheses and ligand exchange reactions, respectively. Abbreviations in the legend are noted in the text. Dashed lines indicate preliminary qualitative assessments of the r values associated with the different types of monolayer structures on AuNPs. The only AuNPs exhibiting phase-segregated monolayers were formed by ligand exchange. Error bars (±1σ, n = 2) are shown for some Tio:GS and OT:DT AuNPs. Deviations are normally within ~3% ligand abundance and ~10⁻³ for r.