Ligand exchange reactions of 1.5-nm triphenylphosphine-stabilized nanoparticles with omega-functionalized thiols provides a versatile approach to functionalized, 1.5-nm gold nanoparticles from a single precursor. We describe the broad scope of this method and the first mechanistic investigation of thiol-for-phosphine ligand exchanges. The method is convenient and practical and tolerates a surprisingly wide variety of technologically important functional groups while producing very stable nanoparticles that essentially preserve the small core size and size dispersity of the precursor particle. The mechanistic studies reveal a novel three-stage mechanism that can be used to control the extent of ligand exchange. During the first stage of the exchange, AuCl(PPh3) is liberated, followed by replacement of the remaining phosphine ligands as PPh3 (assisted by gold complexes in solution). The final stage involves completion and reorganization of the thiol-based ligand shell.

Purification and size-based separation of nanoparticles remain significant challenges in the preparation of well-defined materials for fundamental studies and applications. Diafiltration shows considerable potential for the efficient and convenient purification and size separation of water-soluble nanoparticles, allowing for the removal of small-molecule impurities and for the isolation of small nanoparticles from larger nanostructures in a single process. Herein, we report studies aimed at assessing the suitability of diafiltration for (i) the purification of water-soluble thiol-stabilized 3-nm gold nanoparticles, (ii) the separation of a bimodal distribution of nanoparticles into the corresponding fractions, and (iii) the separation of a polydisperse sample into fractions of differing mean core diameter. NMR, thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) measurements demonstrate that diafiltration produces nanoparticles with a much higher degree of purity than is possible by dialysis or a combination of solvent washes, chromatography, and ultracentrifugation. UV−visible spectroscopic and transmission electron microscopic (TEM) analyses show that diafiltration offers the ability to separate nanoparticles of disparate core size. These results demonstrate the applicability of diafiltration for the rapid and green preparation of high-purity gold nanoparticle samples and the size separation of heterogeneous nanoparticle samples. They also suggest the development of novel diafiltration membranes specifically suited to high-resolution nanoparticle size separation.

A convenient preparation is reported for a series of functionalized, thiol-stabilized gold particles with subnanometer core diameters (d CORE ) 0.8 ( 0.2 nm). The preparation produces previously inaccessible materials through ligand exchange reactions of a phosphine-stabilized precursor, Au 11 (PPh 3 ) 8 Cl 3 , with ω-functionalized alkanethiols. Convenient access to these new materials is a prerequisite to the detailed study of the electronic and optical properties of subnanometer particles and the investigation of the utility of these building blocks in nanoscale devices. Preliminary investigations of the optical properties of these new materials by UV-vis spectroscopy revealed that the particles have defined optical transitions. These results confirm the presence of discrete energy levels in the electronic structure of these materials that might be expected due to quantum size effects. Initial studies also suggest that the optical properties depend on the nature of the stabilizing ligand shell. The ligand exchange method described is applicable for a diverse family of alkanethiols to produce both organic-and water-soluble particles that show increased stability over the phosphine-stabilized precursor. Extensive characterization indicates that the thiol-stabilized exchange products have well-defined core sizes and dispersities.

Ligand exchange of phosphine-stabilized undecagold precursor particles, Au11(PPh3)8Cl3, with omega-functionalized thiols provides a convenient and general approach for the rapid preparation of large families of thiol-stabilized, subnanometer (dCORE approximately 0.8 nm) particles. The approach permits rapid incorporation of specific functionality into the stabilizing ligand shell, is tolerant of a wide range of functional groups, and provides convenient access to new materials inaccessible by other methods. Mechanistic studies and trapping experiments give insight into the progression of the ligand exchange, providing evidence that the core size of the phosphine-stabilized undecagold precursor particles is preserved during ligand exchange. The optical properties of the thiol-stabilized nanoparticles depend strongly on the composition of the ligand shell, and a series of studies suggests that this dependence is a result of the ligand shell's influence on the electronic structure of the particle core, as opposed to a structural change within the nanoparticle core.

No abstract