Synthesis and characterization of gold nanoparticles capped with tiopronin-derivatives and pyrrolidinium ionic liquids

Abstract

Nanoparticles have widespread applications in many areas and are a new generation of advanced materials. Among the different kinds of nanoparticles, gold nanoparticle is one of the most widely studied and utilized. The easy preparation, higher stability, low toxicity and unique properties make the gold nanoclusters the material of choice in many applications. The objective of this study is to synthesize and characterize a series of gold nanoparticles capped by different ligands. In the first part of the thesis, the preparation and characterization of several tiopronin derivatives and their gold nanoparticles were demonstrated. The tiopronin derivatives were designed by modifying their hydrophobic segment or the amino acid residue based on the parent tiopronin. The phenyl, ethyl, alanine, proline and leucine derivatives were synthesized. All, except the phenyl-, tiopronin derivatives gave stable, water-soluble and very small gold nanoparticles. The gold nanoparticles were characterized by IR, UV-visible, transmission electron microscopy, luminescence spectroscopy and thermogravimetric analysis. In the second part of the thesis, the ligand exchange reaction involving the tiopronin derivatives on the small gold nanoclusters was studied. The difference in structure of the derivatives did not introduce much discrimination on the extent and kinetics of the ligand exchange reactions. However, introduction of negative charges on the ligand monolayer on gold nanoparticles by pH adjustment significantly reduced the extent of exchange reaction at the initial stage and allowed the investigation of ligand exchange occurring at the higher energy metal surface defective sites. A difference in the exchange kinetics among different tiopronin derivatives was observed at pH 6.5. The initial stage for all exchange processes with tiopronin derivatives can generally be described by the second order Langmuir diffusion-limited model. This confirms the diffusion nature of the ligand exchange reaction between tiopronin and its derivatives on gold surface before equilibrium was established. In the third part of the thesis, the preparation of gold nanoparticles by direct reduction with tetrabutylammonium borohydride in neat pyrrolidinium-based bis(trifluoromethanesulfonyl)imide ionic liquids was studied. The average size of the gold nanoparticles prepared was 4 - 8 nm in diameter. The size-dispersity and colloid stability were improved when the nanoclusters were synthesized in longer alkyl chain pyrrolidinium ionic liquids. The presence of bromide only produced spherical nanoparticles while chloride could induce the formation of thread-like structure. A lower reaction temperature generally leads to a wider spread of average particle size of the gold nanoparticles.