The Brust-Schiffin synthesis is a widely used procedure for the production of precursor gold nanoparticles that can be further functionalized for use as sensor devices, catalytic material, or drug delivery systems. The synthesis requires the phase transfer of gold ions from an aqueous phase to an organic phase. The phase transfer is facilitated by a quaternary ammonium phase transfer catalyst, tetraoctylammonium bromide (TOABr). However, the final state of the gold ion in the organic phase is not well understood. There is an ongoing debate in the literature over whether the phase transfer results in the formation of reverse micelles or in a molecular complex.
Other research groups have suggested that reverse micelles are formed because the nuclear magnetic resonance proton shift of water deviates from that of water in an organic solvent,1 while the formation of a molecular complex is supported by conductivity measurements and small angle x-ray measurements.2 We utilized Diffusion Oriented Nuclear Magnetic Resonance Spectroscopy (DOSY-NMR) to quantify the hydrodynamic radius of TOABr during the phase transfer. DOSY-NMR applies a gradient magnetic pulse to spatially map the location of the chemical species through their Larmor frequency and quantifies the diffusion of the chemicals by their signal intensity attenuation following a diffusion delay. The translational self-diffusion coefficients were used to determine the radius of TOABr through a modified Stokes-Einstein relationship. DOSY-NMR couples the accuracy of Small Angle Neutron Scattering (SANS) with the accessibility of Dynamic Light Scattering (DLS) to nondestructively probe the size of chemicals at the Angstrom-scale.
Our results suggest that there is no critical micelle concentration between 2mM and 400mM of TOABr in chloroform saturated with water. Typical syntheses utilize 200 mM of TOABr. A critical micelle concentration would be evident with an increase in the hydrodynamic radius of the phase transfer catalyst. Instead, DOSY-NMR results describe the formation of a molecular complex. Our study suggests that subsequent reduction of the gold ion in the Brust-Schiffin occurs in the organic phase, and not in an aqueous phase within reverse micelles. Additionally, the study indicates that TOABr not only phase transfers gold into the organic phase, but also would necessitate that the borohydride ion be transferred into the organic phase by TOABr. Further insight into the mechanism of phase transfer and particle formation will improve the ability to synthesize monodisperse particles and ultimately lead to facile methodologies of size control.
1. Li, Y., Zaluzhna, O. & Tong, Y. J. Critical Role of Water and the Structure of Inverse Micelles in the Brust–Schiffrin Synthesis of Metal Nanoparticles. Langmuir 27,7366–7370 (2011).
2. Perala, S. R. K. & Kumar, S. On the Mechanism of Phase Transfer Catalysis in Brust–Schiffrin Synthesis of Metal Nanoparticles. Langmuir 29, 14756–14762 (2013).