Synthesis, Characterization and Application of Water-soluble Gold and Silver Nanoclusters

The term ‘nanotechnology’ has emerged as a buzzword since the last few decades. It has found widespread applications across disciplines, from medicine to energy. The synthesis of gold and silver nanoclusters has found much excitement, due to their novel material properties. Seminal work by various groups, including ours, has shown that the size of these clusters can be controlled with atomic precision. This control gives access to tuning the optical and electronic properties.

The majority of nanoclusters reported thus far are not water soluble, which limit their applications in biology that requires water-solubility. Going from organic to aqueous phase is by no means a simple task, as it is associated with many challenges. Their stability in the presence of oxygen, difficulty in characterization, and separation of pure nanoclusters are some of the major bottlenecks associated with the synthesis of water-soluble gold nanoclusters. Watersoluble gold nanoclusters hold great potential in biological labeling, bio-catalysis and nanobioconjugates.

To overcome this problem, a new ligand with structural rigidity is needed. After considering various possibilities, we chose Captopril as a candidate ligand. In my thesis research, the synthesis of Au₂₅ nanocluster capped with captopril has been reported. Captopril-protected Au₂₅ nanocluster showed significantly higher thermal stability and enhanced chiroptical properties than the Glutathione-capped cluster, which confirms our initial rationale, that the ligand is critical in protecting the nanocluster. The optical absorption properties of these Au₂₅ nanoclusters are studied and compared to the plasmonic nanoparticles.

The high thermal stability and solubility of Au₂₅ cluster capped with Captopril motivated us to explore this ligand for the synthesis of other gold clusters. Captopril is a chiral molecule with two chiral centers. The chiral ligand can induce chirality to the overall cluster, even if the core is achiral. Therefore, to obtain Au₃₈ clusters as an enantiomer, the ligand employed should be chiral. The enantioselective synthesis of Au₃₈ capped with different chiral ligands has been reported and their chiroptical properties have been compared.

The synthesis of a series of water-soluble Au nanoclusters has motivated us to study the effect of capping ligands and the core-size on their steady-state and time-resolved fluorescence properties, since the photoluminescence properties are particularly important for bioimaging and biomedical applications of nanoclusters. To gain fundamental insights into the origin of luminescence in nanoclusters, the effect of temperature on the fluorescence properties of these clusters has also been studied.

The different sized nanoclusters ranging from a few dozen atoms to hundreds of atoms form a bridge between discrete atoms and the plasmonic nanocrystals; the latter involves essentially collective electron excitation-a phenomenon well explained by classical physics as opposed to quantum physics. The central question is: at what size does this transition from quantum behavior to classical behavior occur? To unravel this, we have successfully synthesized a series of silver nanoclusters. The precise formula assignment and their structural determination are still ongoing. We have successfully demonstrated the application of these water-soluble Au nanoclusters in photodynamic therapy for the treatment of cancer. We have successfully demonstrated that Au nanocluster system can produce singlet oxygen without the presence of any organic photosensitizers. In a collaborative project with Dr. Peteanu’s group, the quenching efficiency of organic dyes by these water soluble nanoclusters is studied in different systems.

Overall, this thesis outlines the successful synthesis of a family of water-soluble nanoclusters, their optical, chiroptical and fluorescence properties, as well as some applications of these nanoclusters.