xx, 242 p. : ill. (some col.) / Applications of ligand-protected nanoparticles have increased markedly in recent years, yet their controlled synthesis remains an under-developed field. Nanoparticle syntheses are highly specialized in their execution and often possess significant limitations. For example, the synthesis of thiol-stabilized gold nanoparticles (AuNPs) with core diameters greater than 5.0 nm is difficult to achieve using existing methods. This dissertation describes the development of a synthetic strategy for thiolate-stabilized AuNPs over a wide range of core sizes using alkyl thiosulfates (Bunte salts) as ligand precursors. The use of Bunte salts permits the synthesis of larger AuNPs than can be achieved using thiols by allowing the AuNP cores to grow to larger diameters before the formation of the thiolate ligand shell. Chapter II details the development of a direct synthesis strategy using Bunte salts as ligand precursors that produces AuNPs with diameters up to 20 nm. Chapter III describes an investigation of the ligand shell formation that occurs during these syntheses. The ligand shell formation involves the adsorption of the Bunte salt to the AuNP surface, where it is converted to the thiolate. This conversion requires an excess of sodium borohydride in the synthesis of >5 nm AuNPs, but not for the synthesis of smaller AuNPs. This synthetic strategy was adapted for use in flow reactors to attain simultaneous AuNP synthesis and characterization. Chapter IV demonstrates that thiol-stabilized AuNPs can be synthesized in a microfluidic device with product monitoring provided by UV-vis absorbance spectroscopy. The development of a capillary flow reactor that permits the incorporation of new monitoring techniques is presented in Chapter V. The incorporation of Small-Angle X-ray Scattering (SAXS) analysis provides quantitative <italic>in situ</italic> determinations of AuNP diameter. The combination of synthetic control and monitoring makes capillary flow reactors powerful tools for optimization of NP syntheses and monitoring NP growth. In Chapter VI, the capillary flow reactor is used in an investigation of AuNP core growth. We also review AuNP growth mechanisms and show how to differentiate these using SAXS and UV-vis analysis. In these studies, AuNP growth is unexpectedly shown to involve a coalescence mechanism. This dissertation includes previously published and co-authored material. / Committee in charge: Victoria De Rose Chairperson; James E. Hutchison, Advisor; Catherine Page, Member; Darren W. Johnson, Member; Miriam Deutsch, Outside Member

Colloidal semiconductor nanocrystals (NCs) have attracted a great deal of interest as bright and stable chromophores for a variety of applications. Their superior physicochemical properties depend on characteristics of the inorganic core, as well as on the chemical nature and structure of the stabilizing organic ligand shell. To evaluate the promising material, a thorough knowledge of structure-property relationships is still demanded. The present work addresses this challenge to three water-soluble NC systems, namely thiol-capped CdTe, thiol-capped CdHgTe, and DNA-functionalized CdTe NCs with special emphasis on the investigation of structure, modification, and influence of the ligand shell. Remarkably, CdTe NCs show bright emission in the visible spectral region and can be synthesized in high quality directly in water. It was shown that the aqueous synthesis also facilitates the preparation of strongly near-infrared (NIR) emitting CdHgTe NCs. The current work presents a detailed study on parameters, by which the emission can be tuned, such as the growth time, the initial Cd : Hg ratio, and the choice of ligand. These insights contribute to the knowledge, which is essential for the design of highly emissive and long-term stable NIR emitting NCs. Further variations of the NC/ligand system include the modification of the ligand shell of CdTe NCs with oligonucleotides based on the strong attachment of DNA molecules to the NC. The successful functionalization of NCs with single-stranded DNA molecules is very promising for the precise and programmable assembly of NCs using DNA origami structures as templates. For both, functionality and optical properties, the surface chemistry of the NCs plays a substantial role and was subject to an extensive investigation. As there is no generally applicable technique to determine the amount of stabilizers and the structure of the ligand shell, the presented study is based on a combination of various methods particularly tailored to the analysis of water-soluble CdTe NCs capped by short-chain thiols. CdTe NCs served as a model system for the described analysis of the ligand shell, since they are thoroughly studied regarding synthesis and features of the core. Aiming for the quantification of thiols, a straightforward colorimetric assay, the Ellman\'s test, is for the first time introduced for the analysis of NCs. Accompanied by elemental analysis an approximate number of thiols per NC becomes accessible. Moreover, theoretical calculations were performed to estimate the amount of ligand that would cover the NC in a monolayer of covalently bound molecules. In contrast to these results, the experimental values point to a larger amount of thiols immobilized on the NC. Attempts to remove the ligand indicate the presence of Cd in the ligand shell and thermogravimetric studies show that the ligands are not loosely assembled in the ligand shell. The outstanding conclusion of these findings involves the presence of Cd-thiol complexes in the ligand shell. Further results unambiguously show that the amount of Cd-thiol complexes present in the NC solution strongly influences the concentration-dependent emission yield of the NCs. Additional studies dedicated to the considerable influence of the ligand shell highlight a strong effect of pH, NC concentration, type and purity of the solvent, and the number of precipitation steps on the emission of water-soluble semiconductor NCs. These substantial investigations emphasize the need to carefully control the conditions applied for handling, optical measurements, and application of NCs. In order to gain a deeper insight into the complex structure of the native ligand shell, techniques deliberately chosen for the in situ analysis were applied for thioglycolic acid-capped CdTe NCs. Information from dynamic light scattering (DLS) regarding the stability and the shell thickness are consistent with previous results showing a large ligand network on the NC surface and a decreasing stability of the NCs upon dilution. Importantly, nuclear magnetic resonance (NMR) spectroscopy allows for the distinction of bound and free ligands directly in solution and proves the presence of these species for the NCs studied. In particular, the results indicate that the ligands are not strongly bound to the NC core and that both, free and bound ligand species, consist of modified thiol molecules, such as Cd-thiol complexes. These findings support previous assumptions and allow to establish a distinct picture of the ligand shell of water-soluble semiconductor NCs. Further insights were obtained from small-angle X-ray scattering (SAXS), which facilitates the identification and the determination of the composition of NC core as well as ligand shell. Element-specific SAXS yields the final proof of the presence of Cd in the ligand shell. The model developed for the optimal fitting of the experimental scattering curves additionally confirms the findings from the other methods. In conclusion, the present work contributes to the challenging goal of a comprehensive knowledge of interactions between the NC core and the ligands. The fundamental development of a structural model of water-soluble CdTe NCs including information on stoichiometries is accomplished by the combination of the techniques presented and emphasizes the challenge to assign a clear border between the ligand shell and the Cd-thiol complexes in solution. Altogether, CdTe NCs capped by thioglycolic acid are best described by a crystalline core surrounded by a water-swollen Cd-thiolate shell that considerably affects the optical properties of the system. Notably, the results of the versatile study provide the opportunity to control the overall properties and to evaluate water-soluble semiconductor NCs for particular applications in photonics and optoelectronics.

Colloidal semiconductor nanocrystals (NCs) have attracted a great deal of interest as bright and stable chromophores for a variety of applications. Their superior physicochemical properties depend on characteristics of the inorganic core, as well as on the chemical nature and structure of the stabilizing organic ligand shell. To evaluate the promising material, a thorough knowledge of structure-property relationships is still demanded. The present work addresses this challenge to three water-soluble NC systems, namely thiol-capped CdTe, thiol-capped CdHgTe, and DNA-functionalized CdTe NCs with special emphasis on the investigation of structure, modification, and influence of the ligand shell. Remarkably, CdTe NCs show bright emission in the visible spectral region and can be synthesized in high quality directly in water. It was shown that the aqueous synthesis also facilitates the preparation of strongly near-infrared (NIR) emitting CdHgTe NCs. The current work presents a detailed study on parameters, by which the emission can be tuned, such as the growth time, the initial Cd : Hg ratio, and the choice of ligand. These insights contribute to the knowledge, which is essential for the design of highly emissive and long-term stable NIR emitting NCs. Further variations of the NC/ligand system include the modification of the ligand shell of CdTe NCs with oligonucleotides based on the strong attachment of DNA molecules to the NC. The successful functionalization of NCs with single-stranded DNA molecules is very promising for the precise and programmable assembly of NCs using DNA origami structures as templates. For both, functionality and optical properties, the surface chemistry of the NCs plays a substantial role and was subject to an extensive investigation. As there is no generally applicable technique to determine the amount of stabilizers and the structure of the ligand shell, the presented study is based on a combination of various methods particularly tailored to the analysis of water-soluble CdTe NCs capped by short-chain thiols. CdTe NCs served as a model system for the described analysis of the ligand shell, since they are thoroughly studied regarding synthesis and features of the core. Aiming for the quantification of thiols, a straightforward colorimetric assay, the Ellman\'s test, is for the first time introduced for the analysis of NCs. Accompanied by elemental analysis an approximate number of thiols per NC becomes accessible. Moreover, theoretical calculations were performed to estimate the amount of ligand that would cover the NC in a monolayer of covalently bound molecules. In contrast to these results, the experimental values point to a larger amount of thiols immobilized on the NC. Attempts to remove the ligand indicate the presence of Cd in the ligand shell and thermogravimetric studies show that the ligands are not loosely assembled in the ligand shell. The outstanding conclusion of these findings involves the presence of Cd-thiol complexes in the ligand shell. Further results unambiguously show that the amount of Cd-thiol complexes present in the NC solution strongly influences the concentration-dependent emission yield of the NCs. Additional studies dedicated to the considerable influence of the ligand shell highlight a strong effect of pH, NC concentration, type and purity of the solvent, and the number of precipitation steps on the emission of water-soluble semiconductor NCs. These substantial investigations emphasize the need to carefully control the conditions applied for handling, optical measurements, and application of NCs. In order to gain a deeper insight into the complex structure of the native ligand shell, techniques deliberately chosen for the in situ analysis were applied for thioglycolic acid-capped CdTe NCs. Information from dynamic light scattering (DLS) regarding the stability and the shell thickness are consistent with previous results showing a large ligand network on the NC surface and a decreasing stability of the NCs upon dilution. Importantly, nuclear magnetic resonance (NMR) spectroscopy allows for the distinction of bound and free ligands directly in solution and proves the presence of these species for the NCs studied. In particular, the results indicate that the ligands are not strongly bound to the NC core and that both, free and bound ligand species, consist of modified thiol molecules, such as Cd-thiol complexes. These findings support previous assumptions and allow to establish a distinct picture of the ligand shell of water-soluble semiconductor NCs. Further insights were obtained from small-angle X-ray scattering (SAXS), which facilitates the identification and the determination of the composition of NC core as well as ligand shell. Element-specific SAXS yields the final proof of the presence of Cd in the ligand shell. The model developed for the optimal fitting of the experimental scattering curves additionally confirms the findings from the other methods. In conclusion, the present work contributes to the challenging goal of a comprehensive knowledge of interactions between the NC core and the ligands. The fundamental development of a structural model of water-soluble CdTe NCs including information on stoichiometries is accomplished by the combination of the techniques presented and emphasizes the challenge to assign a clear border between the ligand shell and the Cd-thiol complexes in solution. Altogether, CdTe NCs capped by thioglycolic acid are best described by a crystalline core surrounded by a water-swollen Cd-thiolate shell that considerably affects the optical properties of the system. Notably, the results of the versatile study provide the opportunity to control the overall properties and to evaluate water-soluble semiconductor NCs for particular applications in photonics and optoelectronics.