Soft ligand stabilized gold nanoparticles: incorporation of bipyridyls and two-dimensional assembly.

HYPOTHESIS: The synthesis of gold nanoparticles (AuNPs) in organic solvents without addition of surfactant may be achieved without the use of strong capping agents in the presence of soft ligands from the starting materials. Through post synthesis surface modification with ligands having moderate or strong affinity, the soft ligands should be displaced, allowing for manipulation of the AuNP size, size distribution, as well as the interparticle spacing. EXPERIMENTS: Synthesis of gold nanoparticles was undertaken using the reducing agent 9-borabicyclo[3.3.1]nonane without addition of stabilizing ligands. Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS) were used to characterize the NPs. The interparticle spacing in 2D assemblies of the NPs was correlated with localized surface plasmon resonance (LSPR) shifts exhibited by the NPs in solution. FINDINGS: Soft-ligand-stabilized AuNPs in a size range of 3-5 nm were generated at a slow rate. The soft ligands were found to be chloride (Cl(-)) ions and triethylphosphine (Et3P). Bipyridyl and thiol ligands displaced the weakly adsorbed soft ligands. Thiolate-stabilized AuNPs formed long range 2-D assemblies with regular interparticle spacing (edge-to-edge). UV-vis spectroscopy analysis shows the thiolated AuNPs exhibited an exponential localized wavelength (λmax) decay dependence on the alkylthiolate chain length, indicating an increase in the interparticle spacing.

One-step synthesis of phosphine-stabilized gold nanoparticles using the mild reducing agent 9-BBN.

A simple method to synthesize phosphine-stabilized gold nanoparticles (AuNPs) of narrow size dispersion using the mild reducing agent 9-borabicyclo[3.3.1]nonane (9-BBN) is described. The methodology produces particles 1.2-2.8 nm in size depending on the reaction conditions and the phosphine ligand used. The phosphine-stabilized AuNPs exhibit size dependent localized surface plasmon resonance (LSPR) behavior as measured by UV-visible spectroscopy. (31)P NMR spectroscopy analysis of triphenylphosphine-AuNPs (TPP-AuNPs) shows a peak shift to 63.0 ppm compared to pure TPP at -5.4 ppm which is attributed to adsorption of TPP on the AuNP surface. Synthesis of trioctylphosphine-stabilized AuNPs demonstrates the versatility of the 9-BBN-based method. We present initial investigations of using TPP-AuNPs as precursor materials for nanoparticles functionalized with other ligands through ligand exchange reactions with dodecanethiol (DDT) and 11-mercaptoundecanoic acid (MUA).