Date of Award
1-1-2023
Document Type
Dissertation
Degree Name
Ph.D. in Chemistry
First Advisor
Amal Dass
Second Advisor
Daniell Mattern
Third Advisor
Jonah Jurss
School
University of Mississippi
Relational Format
dissertation/thesis
Abstract
Gold nanomolecules are atomically monodisperse nano-particles having a precise number of gold atoms and thiolate ligands. The 1–3 nm size range exhibit size-dependent molecule-like properties and have gained broad interest due to their applications in imaging, catalytic activity, biosensing, and drug delivery. The capping ligand governs the physio-chemical properties of an AuNM. Based on the C-S bonding, there are three groups of thiols. Aliphatic thiol, such as phenyl ethane thiol, has a sulfur atom directly bonded to an aliphatic group. 4-tert-Butylbenzene thiol is an aromatic ligand as the sulfur atom is directly bonded to an aromatic carbon. The next group is the bulky ligand group, such as tertiary butyl thiol, as the sulfur atom is directly bonded to bulky carbon. The lack of structural information obstructs the physicochemical understanding of AuNMs.
Chapter 2 discusses the X-ray crystal structure of the Au30-xAgx(S-tBu)18 alloy and the effect of the ligand on alloying site preferences. Gold-silver nanoalloys prepared by co-reduction of metal salts are known to have only partial Ag occupancies. Interestingly, Au30-xAgx(S-tBu)18 has 100% Ag occupancy at two sites on the core surface and partial Ag occupancies on the surface, capping, and staples sites. X-ray diffraction and electrospray ionization mass spectrometry studies confirmed the Au30-xAgx(S-tBu)18 (x = 1-5) composition. The effect of the ligand on Ag doping can be seen in the crystal structures of Au36-xAgx(SPh-tBu)24 and Au38-xAgx(SCH2CH2Ph)24 when compared with that of Au30-xAgx(S-tBu)18. Ag is preferentially doped onto the core surface when the ligand is aliphatic, and Ag is doped in both the core surface and staple metal sites when the ligand is aromatic or bulky.
Chapter 3 reports the X-ray crystal structure of a 24-atom gold nanomolecule protected by 16 tert-butyl thiolate ligands. The composition of Au24(S-tBu)16 was confirmed by X-ray crystallography and electrospray ionization mass spectrometry. The nanomolecule was synthesized in a one-phase synthesis and crystallized from hexane–ethanol layered solution. The X-ray structure confirms the 16-atom core is protected by two monomeric and two trimeric staples with four bridging ligands. The Au24(S-tBu)16 cluster follows the shell-closing magic number of 8.
The transformation brought about by ligand exchange is one of the effective methods for synthesizing gold-thiolate nanomolecules. In this method, the AuNMs are treated with an excess exogenous thiol at an elevated temperature. It has been found that the ligand exchange is often accompanied by the conversion of the metal core from a larger size to a smaller size, depending on the type of exogenous capping ligand employed. Chapter 4 discusses the transformation of a smaller-size AuNM (133 Au atoms) to a larger-size AuNM (279 Au atoms). The Au144(SCH2CH2Ph)60 AuNM in the presence of 4-tert-butylbenzenethiol under refluxing conditions first transforms to Au133(SPh-tBu)52, and then with the transformation reaction proceeding to form larger-sized AuNMs, Au191(SPh-tBu)66 and Au279(SPh-tBu)84. The reaction progress was monitored with matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) and UV-vis spectroscopy, and the intermediates and AuNMs were identified with electrospray ionization (ESI) MS. In conjunction with the above experiments, theoretical explorations using density functional theory calculations have been carried out, probing the energetics and thermodynamic stabilities underlying the observed size-changing transformations. It also elucidates the systematic size-dependent trends in the electronic structure of the original 144-gold-atoms-capped AuNM and the transformation products, including analysis of the formation of superatom shells through the use of the core-cluster-shell model.
Chapter 5 discusses the ion mobility-tandem mass spectrometry of four selected AuNMs. These four AuNMs are capped by aliphatic phenylethane thiol (PET), aromatic 4-tert-butyl benzenethiol (TBBT), and bulky tertiary butyl thiol (S-tBu). Unlike conventional mass spectrometry, ion mobility mass spectrometry separates the ions based on their size, shape, and m/z. Au38(PET)24 ionizes in a +2 charge state with high intensity. The ion mobility spectrum of [Au38(PET)24]+2 revealed the dissociation of the parent ion into two bands, Au38 and Au34. But no core fragmentation was observed. [Au36(TBBT)24Cs2]+2 shows fragmentation of the outer staple layer and core. [Au30(S-tBu)18]+1 shows fragmentation similar to Au38(PET)24. It shows two bands, Au30 and Au26, but no core fragmentation was observed. [Au23(S-tBu)16]-1 shows the fragmentation of the outer protecting layer and the core. Interestingly, all four molecules show Au4(SR)4 fragments regardless of the capping ligand type and dissociation pattern.
Recommended Citation
Wijesinghe, Kalpani Eshangi Hirunika, "Gold Nanomolecules: Nanomolecular Metallurgy, Crystal Structures, and Stability" (2023). Electronic Theses and Dissertations. 2596.
https://egrove.olemiss.edu/etd/2596