Synthese von Gold-Komplexen und deren Reduktion zu Gold-Nanopartikeln mit Natriumborhydrid
In this work, the gold(I) complexes and the gold nanoparticles were synthesized and analyzed. The aim of this work is to investigate the connection between gold(I) complexes and gold nanoparticles, and to explain the line broadening and the splitting of the signals in NMR spectra of gold nanoparticles.
The work consists of three parts. In the first part, two homoleptic gold(I) complexes Na5[Au(TPPTS)2] and Na8[Au(TPPTS)3] with high purity were successfully synthesized, characterized by using different analytical methods and compared. By optimization of the molar ratio, the purity of Na8[Au(TPPTS)3] was doubled as the result of early works. The 1H-DOSY studies and the 31P-NMR measurement showed that these two gold(I) complexes behaved as monomer in solution at room temperature. The one-dimensional NMR study revealed the rapid ligand exchange between the two homoleptic gold(I) complexes at room temperature. It was subsequently demonstrated that the Na8[Au(TPPTS)3] complex was unstable. In addition, the Analysis with XPS showed interesting results: the gold atom at Na8[Au(TPPTS)3] in solid had the state of Au(I), in comparison, the gold atom in Na5[Au(TPPTS)2] solid showed the mixed valence state of Au(I) and Au(III). In this part, the results showed two reasons for the line broadening of NMR signals, which were decoordination and fast ligand-exchange.
In the second part of the work, the TPPTS-stabilized ultrasmall gold nanoparticles were synthesized, characterized and compared with the two gold(I) complexes. The influences of precursors, the amounts of reducing agent and the purification methods were examined. Upon analysis with NMR, the results clearly showed that the gold(I) complexes always existed as a byproduct in gold nanoparticles, although the gold(I) complexes were used as precursors and the reducing agent was used sufficiently. The comparison of the NMR spectra of gold nanoparticles with ligands and gold(I) complexes showed that all 1H- , 13C- and 31P-NMR-signals of gold nanoparticles were broadened. The reason for the line broadening of NMR-signals of purified gold nanoparticles was due to the size distribution of nanoparticles. For the non-pure gold nanoparticles, the broad signals could be caused by the impurities of gold(I) complexes in nanoparticles. To distinguish the signals from gold nanoparticles and gold complexes, the coordination shifts in 31P-NMR spectra provided supportive informations.
In the third part of this work, the chiral gold nanoparticles were synthesized and characterized. When chiral ligands (L-, D-Cysteine) were used as a stabilizer for the synthesis of gold nanoparticles, the produced gold nanoparticles exhibited the chirality. This did not influence the line broadening of the NMR-signals. By increasing the temperature, the chirality of gold nanoparticles was reduced. After ligand exchange with achiral ligand (thioglycolic acid), the gold nanoparticles still showed chirality. It was suggested that this chirality was related to the surface and the core of gold nanoparticles. The interesting phenomena observed after ligand exchange were the chiral inversion and the chirality enhancement of gold nanoparticles, the reasons for these phenomena were explained.