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Modulation of the Redox Activity of Colloidal Quantum Dots by Tuning Nanoscale Electrostatic Interactions

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This thesis describes a series of fundamental studies that address the role of electrostatic interactions in modulating i) the permeability of the ligand shell of a colloidal quantum dot (QD) to an anionic redox probe; ii) the resulting yield of photoinduced electron exchange within the QD ‒ redox probe complex; and iii) how this yield governs the optical properties of the QD ensemble. The probability of adsorption of a molecular redox probe to a water-soluble QD reflects the magnitude of electrostatic repulsion or attraction between them. The change in Gibbs free energy of this adsorption reaction is therefore correlated, via an electrostatic double layer model, with the charge density at the interface between the QD ligand shell and the solvent. The organic counterions within an aqueous QD dispersion also play a nontrivial role in screening the electrostatic potential at this interface. This screening effect is dependent on the steric bulk of the counterion, and this dependence is dominated by the van der Waals attraction between the QD ligand shell and these counterions. The protonation equilibrium of a histamine-derivatized dihydrolipoic acid ligand allows for reversible modulation of the electrostatic potential at the QD ligand shell/solvent interface and cycles the fluorescence intensity of a QD ‒ redox probe mixture. The research described in this thesis furthers the study of photoinduced electron transfer as a probe to more completely understand QD ‒ molecule hybrid systems useful in QD-based photocatalytic and biosensing platforms.

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