An NMR Investigation into the Influence of Phase Transfer Catalyst Aggregation on Brust-Schiffrin Nanoparticle Final Size

Nanoparticles exhibit size-dependent optical, material, and catalytic properties. Of particular interest is the ability of gold nanoparticles to facilitate reactions at the nanoscale, especially when compared to gold’s inert bulk-scale properties. Gold nanoparticles are novel catalytic materials with size-dependent tunable properties.  An ability to synthesize monodisperse nanoparticles is crucial in order to efficiently use the size-dependent properties of gold nanoparticles. Creating a monodisperse population of gold nanoparticles still remains a technological challenge.

The Brust-Schiffrin synthesis methods is a highly praised procedure for synthesizing gold and other noble metal nanoparticles at room temperature and atmospheric conditions. Our focus is to improve the monodispersity of the synthesized nanoparticles. This involves investigating potential sources of polydispersity within the Brust-Schiffrin synthesis.  The first step of the Brust-Schiffrin synthesis is phase transferring an aqueous metallic salt to the organic phase via tetraoctylammonium bromide (TOA-Br), a quaternary ammonium phase transfer catalyst. Currently, there is a debate regarding the morphology of the quaternary ammonium phase transfer catalyst following the transport of gold ions into the organic phase via one of two pathways: (1) the formation of reverse micelles or (2) the  phase transfer catalyst forming ion-ion aggregates.  The overall hypothesis of this research is that the selection of the tetraoctylammonium anion can control the aggregation of the quaternary ammonium salt to influence the size of the nanoparticles produced by the Brust-Schiffrin synthesis.

We have utilized a combination of quantitative ¹H Nuclear Magnetic Resonance Spectroscopy and Diffusion Oriented Nuclear Magnetic Resonance Spectroscopy (DOSY-NMR) to characterize the extent and type of aggregation of tetraoctylammonium salts. Research by other groups has found a change in the chemical shift of water and attributed it to the formation of a reverse micelle. Our analyses of the chemical shift of water revealed that the water is in proximity to halide anions with higher electronegativity resulting in higher chemical shifts. The concentration of water was found to relate to the size of the anion on the quaternary ammonium salt. Quantitative ¹H NMR demonstrated that small anions corresponded to greater amounts of water in the organic phase. Further, the ratio of the concentration of water to the concentration of the phase transfer catalysts in combination with the sizes of the average aggregates indicated that there is not sufficient water to establish an aqueous core.

Diffusion coefficients describing the ammonium phase transfer catalyst were translated into hydrodynamic radii using the Stokes-Einstein Equation. We found that the hydrodynamic radius ranged from 7Å to 19Å depending on the concentration of ammonium salt. Large anions such as AuX₄, where X is Cl or Br, aggregated more than smaller halides Cl^- and Br^-. It was found that the phase transfer of bromoauric acid resulted in larger aggregates than the phase transfer of chloroauric acid. Identical syntheses comparing the size of nanoparticles produced with bromoauric or chloroauric acid revealed that the increase in aggregation corresponded to greater nanoparticle sizes.