Precursor Ion–Ion Aggregation in the Brust–Schiffrin Synthesis of Alkanethiol Nanoparticles

Abstract

Tetraoctylammonium bromide is used in the Brust–Schiffrin nanoparticle synthesis to phase-transfer chloroaurate ions from the aqueous phase to the organic phase. While it is established that the quaternary ammonium complex self-associates in the organic phase, the actual self-assembled structure is poorly understood. We have confirmed the presence of ion–ion aggregates through quantitative 1H nuclear magnetic resonance spectroscopy (NMR), pulsed field gradient, diffusion-ordered NMR (DOSY-NMR), and density functional theory (DFT) based NMR chemical shift calculations. Tetraoctylammonium complexes (TOA-X, where X = Br, Cl, AuCl4–x Br x , AuBr4/Br, and AuBr4/Cl/Br) were investigated to measure the extraction of water into deuterated chloroform. 1H NMR- and DFT-based NMR shielding calculations indicated that deshielding of water is due to hydration of the anion and not the formation of the aqueous core of a reverse micelle. DOSY-NMR results were consistent with the formation of small aggregates at typical Brust–Schiffrin synthesis concentrations. The extent of aggregation correlated with the electronegativity of the anion and was analyzed with a modified, isodesmic, indefinite aggregation model. The substitution of bromoauric acid for chlororoauric acid at conditions emulating the Brust–Schiffrin synthesis in chloroform increased the aggregation of the quaternary ammonium complex. The increase in aggregation corresponded with an increase in the size of the produced nanoparticles from 4.3 to 4.6 nm. Understanding the self-assembly and supramolecular structure of precursors in the Brust–Schiffrin synthesis will enable further refinement of models describing the growth of noble metal nanoparticles.

The significance of bromide in the Brust–Schiffrin synthesis of thiol protected gold nanoparticles

“…As can be seen, [AuBr 4 ] – results in a slightly larger deposition product 3.0 nm instead of 2.0 nm which is similar to previously reported data. 15 We suggest that the smaller size of nanoparticles in the Cl system relates to the presence of Au( i )SR species prior to the addition of BH 4 – which enables more rapid passivation of the nanoparticles during growth.…”

“…As an alternative, TOA + Cl – (tetraoctylammonium chloride) may be used to avoid this halide exchange. The phase transfer has been shown to occur through an ion pair process 14 , 15 rather than the previously proposed reverse micelle formation. 16 – 21 The addition of alkane thiol, added as a capping agent, has been shown to cause the reduction of Au( iii ) to Au( i ).…”

Competition of van der Waals and chemical forces on gold–sulfur surfaces and nanoparticles

“…[98][99][100][101][102][103][104] In the two-phase Brust-Schiffrin synthesis, 29 initial reduction of HAuCl 4 by a thiol leads not to Au(I)-thiolates but instead to HAuCl 2 . 99,100,105 Nanoparticles can form without any thiol involvement, with the latter molecules simply added as stabilizers after the fact, 106 somewhat akin to how thiols react with Au(111) surfaces. Analogous reactions to the Brust-Schiffrin syntheses using Se and Te start not with RSeH or RTeH, but instead with RSeSeR and RTeTeR.…”

Ion-specific clustering of metal–amphiphile complexes in rare earth separations

“…Aggregation of surfactants that are frequently used in nanoparticle synthesis, such as quaternary ammonium surfactants, affects the morphology of formed nanoparticles. [23][24][25] Characterization of the nanostructure of organic phase after solvent extraction, a complex fluid with high concentration of polar solutes such as ions and water, amphiphilic extractants and nonpolar diluents, is non-trivial. Small angle X-ray and neutron scattering (SAXS and SANS, respectively) have been the principal techniques in this effort and they show the link between aggregation structures on extraction efficiency and third phase formation (an undesired fluid-fluid phase transition and splitting due to high metal loading) in different extraction systems.…”