The Synthesis and Behavior of Positive and Negatively Charged Quantum Dots THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Andrew Paul Zane Graduate Program in Chemistry The Ohio State University 2011 Master's Examination Committee: Professor Prabir K. Dutta, Advisor Professor Susan V. Olesik Copyright by Andrew Paul Zane 2011 Abstract The synthesis, characteristics, and macrophage uptake of CdSe/ZnS core/shell quantum dots were studied. Insights into the mechanism of nucleation and growth of the quantum dots were gained by performing in-situ fluorescence spectroscopy during a microwave synthesis. The size and surface charge of quantum dots capped by 3- mercaptopropionic acid (3-MPA, negatively charged) and thiocholine (positively charged) were characterized by dynamic light scattering (DLS) and electrophoretic light scattering (ELS). Finally, macrophage uptake studies were performed by Amber Nagy via flow cytometry to determine the level of quantum dot association with murine alveolar macrophages, and to determine a possible uptake pathway into the cells. The mechanism for the CdSe/ZnS synthesis was determined by an in-situ fluorescence experiment. A fast nucleation step occurred, resulting in small CdSe seed nanoparticles which were protected from aggregation by the 3-MPA. Upon microwave heating, these caps were removed from the surface and began to deteriorate. The CdSe cores underwent Ostwald ripening in which smaller particles dissolved and provided free ions to increase the size of the larger particles. After this period, free zinc in the solution reacted with sulfur, freed from MPA decomposition, to form a ZnS shell around the CdSe core. The ZnS shell passivated the surface of the CdSe core, resulting in increased quantum yield. A maximum quantum yield of 22% was attained after 80 minutes of microwave heating, after which it began to decrease. ii Both MPA and thiocholine coated quantum dots were stable in water and in Roswell Park Memorial Institute (RPMI) media. MPA coated quantum dots aggregated in fetal bovine serum and serum free media, but remained stable. In both cases the surface charge was reduced, indicating the association of a species in the media with the quantum dots. We propose this was due to bovine serum albumin. Thiocholine coated quantum dots aggregated significantly in dilute fetal bovine serum and serum free media, and were not stable. In pure fetal bovine serum, the thiocholine quantum dots were stable, but had a negative surface charge. We propose that this sample was stable because the concentration of bovine serum albumin was high enough to fully coat and protect the quantum dots. MPA coated quantum dots quickly associated with macrophages, and followed a scavenger receptor uptake pathway. This association was found to be charge dependent; less negatively charged quantum dots did not associate as strongly with the macrophages. Positive and aggregated quantum dots did not associate as significantly, though some uptake was observed. This uptake was not via the scavenger receptor pathway. iii Dedicated to my parents Betty and Levi Zane iv Acknowledgments I would first like to acknowledge my advisor Dr. Prabir Dutta, who has tried his best to whip me into shape and is responsible for overseeing what I feel has been a time of tremendous personal growth. Each year I can look back and see how far I have gone; this distance would be much shorter without the guidance of Dr. Dutta. I look forward to continuing to work with and learn from him over the next few years. I want to thank my lab-mates and co-workers, especially my friends Dedun Adeyemo, Kevin Cassidy, Betsy Heck, Michael Severance, Suvra Mondal, Prasenjit Kar, and Govindhan Maduraiveeran who have all made my time in the lab and in Columbus better. Also: Jeremy White, Supriya Sabbani, Xiaogan Li, Weizhen Xiong, Brian Peebles, Joselyn Del Pilar, Max Mullen, and Chenhu Sun. I want to thank William Schumacher, who developed the synthesis I use in this thesis and provided helpful guidance in getting started. I would also like to acknowledge Dr. James Waldman and Amber Nagy who are responsible for the cell uptake studies. Last but not least I am grateful for my friends and family, who give me plenty of distractions from chemistry. I especially want to thank my loving and supportive girlfriend Nicolette Hicks for keeping me happy and sane over the last year. I look forward to going home every day to her and our cat Princess. v Vita 1985…………………………………………Born: Cincinnati, Ohio 2003................................................................Graduated Wilmington High School Wilmington, Ohio 2008................................................................B.S. Chemistry, Wright State University Dayton, Ohio 2008 to present ..............................................Graduate Teaching Associate, Department of Chemistry, The Ohio State University Columbus, Ohio Fields of Study Major Field: Chemistry vi Table of Contents Abstract ............................................................................................................................... ii Acknowledgments............................................................................................................... v Vita ..................................................................................................................................... vi List of Tables ...................................................................................................................... x List of Figures .................................................................................................................... xi Chapter 1: Introduction ....................................................................................................... 1 1.1 Quantum Dots ........................................................................................................... 1 1.2 Synthesis of Quantum Dots ....................................................................................... 3 1.3 Particle Stability ........................................................................................................ 4 1.4 Light Scattering Measurements ................................................................................. 7 1.4.1 Dynamic Light Scattering (DLS) ....................................................................... 8 1.4.2 Electrophoretic Light Scattering (ELS) ............................................................ 13 1.5 Quantum Dot Uptake .............................................................................................. 20 1.6 Research Focus ........................................................................................................ 21 Chapter 2: Experimental Description................................................................................ 25 2.1 Synthesizing Negatively Charged Quantum Dots ................................................... 25 vii 2.1.1 Materials ........................................................................................................... 25 2.1.2 Precursor Solutions ........................................................................................... 25 2.1.3 Reaction ............................................................................................................ 27 2.2 Synthesizing Positively Charged Quantum Dots .................................................... 28 2.2.1 Materials ........................................................................................................... 28 2.2.2 Thiocholine Solution ........................................................................................ 28 2.2.3 Ligand Exchange .............................................................................................. 28 2.3 Characterization ...................................................................................................... 29 2.3.1 Optical Characterization ................................................................................... 29 2.3.2 Size and Charge ................................................................................................ 30 2.3.3 pH Titration ...................................................................................................... 30 2.3.4 Behavior in Biological Media ........................................................................... 31 2.4 In-Situ Fluorescence Study of Reaction Mechanism .............................................. 31 2.5 Quantum Dot Association with Macrophages ........................................................ 33 Chapter 3: Results ............................................................................................................. 35 3.2 Optical Properties .................................................................................................... 40 3.3 pH Titration ............................................................................................................. 41 3.4 Size and Charge in Biological Media ...................................................................... 43 3.4.1 Media Only ....................................................................................................... 44 viii 3.4.2 3-MPA Coated Quantum Dots ......................................................................... 47 3.4.3 Thiocholine Coated Quantum Dots .................................................................. 51