Ligand Exchange Equilibrium at Quantum Dot Surfaces in Polar and Aqueous Solvent Environments

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With the support of the Macromolecular, Supramolecular, and Nanochemistry Program in the Division of Chemistry, Dr. Andrew Greytak of the University of South Carolina will investigate the chemistry at the surface of quantum dots (QDs) in polar solvents such as water to increase their utility in biological applications. QDs are nanometer-scale particles composed of semiconducting compounds, that are typically stabilized in solution by layers of organic molecules attached to their surfaces. QDs often exhibit bright and size-tunable fluorescence that has led to their use in imaging applications, but their utility is limited by unknown governing principles. Dr. Greytak and his group are connecting emerging analytical methods and physical models to chemical reactions that yield water-soluble QDs to better understand these limitations. Broader impacts involve mentorship of graduate and undergraduate students; introduction of new experimental modules into a physical chemistry lab course; coordination of a research and practice showcase with the University of South Carolina Office of Sustainability that provides an opportunity to discuss the role of QDs and other advanced materials in lighting, display, and solar energy applications; and outreach through a local science and engineering fair for junior and senior high school students.The research employs isothermal titration calorimetry (ITC) in combination with NMR techniques to investigate the relative importance of surface coordination and inter-ligand interactions in governing the binding strength and exchange rate of ligands on quantum dots (QDs) in polar solvent environments. In the last few years, the Greytak group has demonstrated that isothermal titration calorimetry (ITC) can be an important technique for characterizing QD-ligand interactions, with success in measuring ligand association, ligand exchange, and sequential exchange and binding processes at chalcogenide QD surfaces. This proposal will extend and go beyond these achievements by applying ITC to new classes of nanocrystal reactions in polar solvents, especially those that probe the formation and behavior of water-soluble QDs suitable for bio-imaging applications. Because ITC can distinguish sequential binding to various sites based on differences in their enthalpy of binding, this approach is expected to lead to improved understanding of colloidal QD surface chemistry. Comparisons are to be made among representative small molecule anionic ligands in aqueous buffers. Additional investigations seek to clarify the nature of binding in emerging block copolymer ligands based on imidazoles and analogous heterocyclic.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.