A Dissertation entitled Application of Mass Spectrometry to the Characterization of Core and Ligand Shell Modifications of Silver Molecular Nanoparticles by Aydar Atnagulov Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Chemistry ________________________________________ Dr. Terry P. Bigioni, Committee Chair ________________________________________ Dr. Cora Lind-Kovacs, Committee Member ________________________________________ Dr. Dragan Isailovic, Committee Member ________________________________________ Dr. Nikolas Podraza, Committee Member ________________________________________ Dr. Amanda Bryant-Friedrich, Dean College of Graduate Studies The University of Toledo August 2017 An Abstract of Application of Mass Spectrometry to the Characterization of Core and Ligand Shell Modifications of Silver Molecular Nanoparticles by Aydar Atnagulov Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Chemistry The University of Toledo August 2017 Small silver nanoparticles, also called molecular nanoparticles (MNPs) or nanoclusters, are of great research interest due to potential applications in valuable fields such as biomedicine and catalysis. In the past decade, several groups reported successful formulae determination and crystal structures of the various species in this class of materials. Knowing such information is of crucial importance to start testing how slight modifications of both metal core and ligand shell affect the stability and properties of molecular nanoparticles. Among the techniques employed for characterization of MNPs, mass spectrometry plays a vital role. Soft ionization techniques such as matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI), developed primarily for bioanalytical applications, became indispensable for the analysis of MNPs, most of which are labile during the ionization process. Mass spectrometry also provides the high precision required to measure the precise numbers of metal atoms, ligands and charges and thereby determine the formulae of these molecules. While modified samples often resemble a statistical distribution of a product mixture, individual products can be iii successfully discriminated by their mass using tandem mass spectrometry and singled out such that their behavior in the gas phase can be studied. The all-silver M4Ag44(p-MBA)30 cluster was used as a model system, where M is a monocationic counterion and p-MBA is para-mercaptobenzoic acid, which serves as a protecting ligand. Metal core modifications were carried out by substituting gold for silver to form M4AuxAg44-x(p-MBA)30 clusters. Gold was chosen as a second metal due to some of its properties being similar to those of silver, including its electronic structure and atomic size. Co-reduction and galvanic exchange were the two methods used for the preparation of the bimetallic product. The range of product composition was determined, and the most thermodynamically favorable sites of heteroatoms within the nanoparticle structure were established. Moreover, chemical properties such as stability, reactivity, and fragmentation were studied as a function of product composition. Thermodynamic and kinetic barriers were evaluated for the aforementioned reactions. The role of ligands in nanoparticle stability and structural outcome of the synthesis was investigated using the same model system, M4Ag44(p-MBA)30. Several parameters including the length of an aliphatic chain between a sulfur atom and a phenyl ring, introduction of bulky groups, and aromaticity, were varied aiming to find the characteristics required for synthesis of M4Ag44(SR)30 (SR = thiolate). It has been found that ligands can be divided into two categories that produce orthogonal sets of nanoparticles. Such a division is based on the aliphatic or aromatic character of the carbon atom directly bonded to the sulfur atom of the thiol group. The outcomes obtained by the method of direct synthesis have been confirmed by using a ligand exchange, which is a softer modification technique. The limits of miscibility of ligands belonging to two iv different classes were tested on M4Ag44(SR)30 nanoparticle using p-MBA and glutathione. Mass spectrometry provides means to control the purity of the synthetic product and to identify potentially interesting by-products. When M4Ag44(SR)30 was synthesized using para-tert-butylbenzenethiol (TBBT), another MNP, with the molecular formula of M3Ag17(TBBT)12, was identified in the product. The synthesis was further optimized to yield primarily M3Ag17(TBBT)12. The small size of the nanoparticle, structural analysis of other silver and gold MNPs, and use of computational chemistry allowed for the rational prediction of the structure of this nanoparticle. The structure prediction was supported using heteroatom substitution as a structural probe. The inorganic part of the predicted structure was later found to be identical to that of the experimentally determined structure. v Acknowledgements First and foremost, I would like to express gratitude to my advisor, Dr. Terry Bigioni for his guidance during my graduate studies and helping to develop the essential skills of a good scientist. Second, special thanks go to both Dr. Wendell Griffith and Dr. Jingshu Guo for the countless hours of mass spectrometry training that made this work possible. Third, I would like to acknowledge my labmates Dr. Brian Conn, Badri Bhattarai, and Sameera Wickramasinghe for fruitful collaborations and their contributions to this work. Dr. Brian Conn and Badri Bhattarai contributed to co- reduction and galvanic exchange studies described in chapter 2, respectively. Syntheses described in chapters 3 and 4 were performed by Sameera Wickramasinghe. Next, I would like to thank Dr. Uzi Landman and his group for supporting our work with molecular modeling and calculations. My sincere thanks go to both Dr. Dragan Isailovic and Dr. Xiche Hu for classes on mass spectrometry and computational chemistry, respectively. I would also like to thank Dr. Cora Lind-Kovacs and Dr. Nikolas Podraza for taking their time to be part of the dissertation committee. Last, but not the least, I would like to acknowledge the Department of Chemistry and Biochemistry of the University of Toledo for giving me the opportunity to pursue my doctoral degree here and National Science Foundation for financial support of the research. vi Table of Contents Abstract .............................................................................................................................. iii Acknowledgements ............................................................................................................ vi Table of Contents .............................................................................................................. vii List of Figures .................................................................................................................... xi List of Abbreviations ....................................................................................................... xiv List of Symbols ................................................................................................................ xvi 1 Introduction ..............................................................................................................1 1.1 History of Nanoparticles ....................................................................................1 1.2 Development of Precious Metal Molecular Nanoparticles Field .......................3 1.3 Electronic and Geometric Shell Closing Rules. .................................................7 1.4 Mass Spectrometry in Molecular Nanoparticles Research. .............................10 1.5 Dissertation Overview. ....................................................................................12 2 Molecular Metallurgy of M4AuxAg44-x(SR)30 Bimetallic Nanoparticles ...............14 2.1 Introduction ......................................................................................................14 2.2 Experimental ....................................................................................................16 2.2.1 Chemicals ..........................................................................................16 2.2.2 Synthesis of M4AuxAg44-x(p-MBA)30 by Co-reduction ....................17 2.2.3 Synthesis of M4AuxAg44-x(p-MBA)30 by Galvanic Exchange ..........17 2.2.4 Thermal processing ...........................................................................19 vii 2.2.5 Optical measurements .......................................................................19 2.2.6 Electrospray Ionization Mass Spectrometry (ESI-MS) ....................20 2.2.7 Crystallization ...................................................................................21 2.2.8 Single-Crystal X-Ray Diffraction and Analysis of M4Au12Ag32(p-MBA)30 .....................................................................21 2.3 Results and Discussion. ...................................................................................23 2.3.1 Synthesis ...........................................................................................23 2.3.2 Thermal stability ...............................................................................27 2.3.3 Location of Au heteroatoms ..............................................................30 2.3.4 Thermodynamics of galvanic exchange