ABSTRACT FUNCTIONALIZING Au25 NANOCLUSTERS WITH CROWN ETHER LIGANDS FOR THE DETECTION OF DISSOLVED HEAVY METALS Gold nanoclusters are an intermediate form between molecular and bulk gold. In some respects, they retain a physical appearance to gold nanoparticles but possess molecular-like properties unique to them. One notable example is the highly stable and symmetrical Au25(SCH2CH2Ph)18 (Au25) nanocluster, which has captivated many owing to its rich electronics and optical properties. Like their nanoparticle counterparts, gold nanoclusters are capable of undergoing surface modifications by ligand exchange. Here, we attempt to tailor Au25 through ligand exchange with crown ether ligands for the detection of bismuth(III), cadmium(II), lead(II), mercury(II), and thallium(III) by ion recognition methods. Crown ethers are well known for their chelating properties, in some cases generating “sandwich” complexes with appropriate ions. Here we attempt to exploit this property on the crown ether functionalized nanoclusters to induce aggregations, detectable by UV- Vis. Instead, we report the unusual outcomes of the ligand exchange reactions and unexpected reactions between the Au25 clusters and the active metal ions. In addition to these studies, we explore diffusion-ordered 1H-NMR spectroscopy (DOSY) as an alternative to transmission electron microscopy (TEM), an important technique used for determining particle diameter. DOSY addresses some of the limitations of TEM while providing equally, if not more, precise measurements. Randy Espinoza May 2018 FUNCTIONALIZING Au25 NANOCLUSTERS WITH CROWN ETHER LIGANDS FOR THE DETECTION OF DISSOLVED HEAVY METALS by Randy Espinoza A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Chemistry in the College of Science and Mathematics California State University, Fresno May 2018 © 2018 Randy Espinoza APPROVED For the Department of Chemistry: We, the undersigned, certify that the thesis of the following student meets the required standards of scholarship, format, and style of the university and the student's graduate degree program for the awarding of the master's degree. Randy Espinoza Thesis Author Jai-Pil Choi (Chair) Chemistry Alam Hasson Chemistry Joy J. Goto Chemistry For the University Graduate Committee: Dean, Division of Graduate Studies AUTHORIZATION FOR REPRODUCTION OF MASTER’S THESIS X I grant permission for the reproduction of this thesis in part or in its entirety without further authorization from me, on the condition that the person or agency requesting reproduction absorbs the cost and provides proper acknowledgment of authorship. Permission to reproduce this thesis in part or in its entirety must be obtained from me. Signature of thesis author: ACKNOWLEDGMENTS This work was not possible on my own. First, I would like to thank my mother and stepfather, family, and friends for providing support from home. In the lab, I’d like to thank the Der, Garret, Misk, and other undergraduate students for providing assistance and Monika and Logan for constant feedback and new insight related to research. Many mistakes were made along the way, and many experiments did not always go as planned, but in all cases, I had an advisor with great patience to help fix them, thank you, Dr. Choi. Along the journey there were many troughs along the way, some of which I discouraged me, thank you, Dr. Krishnan, for providing the much needed encouragement during those tough times. I’d also like to thank Dr. Ghosh and her group in UC Merced for much inspiration, the Bridges to Doctorate program and everyone in it for providing the tools necessary to get through this, my fellow peers who provided assistance, small or large, and the whole Chemistry’s staff, you are the best. TABLE OF CONTENTS Page LIST OF TABLES ................................................................................................. vii LIST OF FIGURES ............................................................................................... viii INTRODUCTION .................................................................................................... 1 Gold Nanoparticles ............................................................................................ 1 Diffusion-Ordered 1H-NMR Spectroscopy Theory ........................................ 11 METHODS AND MATERIALS ........................................................................... 15 Synthesis of Crown Ether-Based Ligands ...................................................... 15 Synthesis of Au25(SCH2CH2Ph)18 ................................................................... 17 Ligand Exchange Reactions ............................................................................ 19 Metal Reactions with Au25 .............................................................................. 19 Differential Pulse Voltammetry ...................................................................... 20 Diffusion-Ordered 1H-NMR Spectroscopy ..................................................... 20 RESULTS AND DISCUSSION ............................................................................ 22 - Synthesis of Au25(SCH2CH2Ph)18 and Crown Ether Ligands ........................ 22 - Ligand Exchange Reactions of Au25(SCH2CH2Ph)18 with Crown Ether Ligands ................................................................................................. 26 - Metal Reactions with Au25(SCH2CH2Ph)18 .................................................... 31 Diffusion Constants and Size Estimations by Diffusion-Ordered NMR Spectroscopy ........................................................................................ 42 CONCLUSION ...................................................................................................... 52 REFERENCES ....................................................................................................... 55 LIST OF TABLES Page Table 1. Halfway potentials (V) of the Au25 products obtained using DPV. ........ 41 Table 2. Average diffusion coefficients (10-9 m2/s) from DOSY-NMR for Ferrocene (1.8 mM) and Au25 (70 µM) at different temperatures. .......... 49 Table 3. Average diffusion coefficients (10-9 m2/s) from DOSY-NMR for Ferrocene (1.8 mM) and Au25 (140 µM) at different temperatures ........ 49 Table 4. Average diffusion coefficients (10-9 m2/s) from DOSY-NMR for Ferrocene (1.8 mM) and Au25 (270 µM) at different temperatures. ........ 50 Table 5. The diameter of Au25 (nm) for each concentration and temperature calculated from DOSY-NMR using Equation 2. ..................................... 50 LIST OF FIGURES Page Figure 1. TEM image of gold nanoparticles ranging from 10 - 50 nm (left) and 20 nm gold nanoparticles (right). ....................................................... 2 Figure 2. The MALDI mass spectra (A) and UV-Vis (B) of various clusters. ....... 3 Figure 3. The SWV of various clusters. .................................................................. 3 Figure 4. Reaction scheme of synthesis of gold clusters. ........................................ 5 - Figure 5. On the left in blue is the absorbance spectrum of Au25 and in red 0 is the absorbance of Au25 . ........................................................................ 6 Figure 6. Complete ionic structure of [TOA][Au25(SCH2CH2Ph)18] (left.) ............ 6 Figure 7. Ligand exchange can occur at all three possible site. .............................. 7 Figure 8. The energy barriers corresponding to the ligand exchange at site A. ...... 8 Figure 9. From left to right are 2-Hydroxymethyl-12-crown-4 (12C4), 2- Hydroxymethyl-15-crown-5 (15C5), 2-Hydroxymethyl-18-crown- 6(18C6), and 1-Aza-18-crown-6 (A18C6). ............................................ 10 Figure 10. Scheme of the reversible metal-crown ether complex. ........................ 10 Figure 11. A TEM time-lapse of AuNP aggregating. ........................................... 12 + - Figure 12. UV-Vis absorption spectra of [TOA] [Au25(SCH2CH3)18] ................. 23 Figure 13. 1H-NMR of 2-[(6-Mercaptohexyl)oxy]methyl-12-crown-4 in CDCl3. ................................................................................................. 24 Figure 14. 1H-NMR of 2-[(6-Mercaptohexyl)oxy]methyl-15-crown-5 in CDCl3. ................................................................................................. 24 Figure 15. 1H-NMR of 2-[(6-Mercaptohexyl)oxy]methyl-18-crown-6 in CDCl3. ................................................................................................. 25 1 Figure 16. H-NMR of N-(6-Mercaptohexyl) Aza-18-Crown-6 in CDCl3. .......... 25 - Figure 17. (a) The H-NMR spectrum of [Au25(SCH2CH2Ph)18] in acetone-d6. .. 27 1 Figure 18. H-NMR spectrum of Au25 after ligand exchange reaction with 2-[(6-Mercaptohexyl)oxy]methyl-12-crown-4 in CDCl3. ...................... 27 ix ix Page 1 Figure 19. The H-NMR spectrum of Au25 after ligand exchange reaction with 2-[(6-Mercaptohexyl)oxy]methyl-15-crown-5 in CDCl3. .............. 28 1 Figure 20. The H-NMR spectrum of Au25 after ligand exchange reaction with 2-[(6-Mercaptohexyl)oxy]methyl-18-crown-6 in CDCl3. .............. 28 1 Figure 21. The H-NMR spectrum of Au25 after ligand exchange reaction with N-(6-Mercaptohexyl) Aza-18-Crown-6 in CDCl3. ......................... 29 Figure 22. UV-Vis of products from the ligand exchange reactions of Au25 and (A) 12-crown-4, (B) 15-crown-5, (C) 18-crown-6, and (D) aza-18- crown-6 ligands. .....................................................................................