Quantum Dot Redox Sensors: Understanding Excited State Dynamics

Semiconductor quantum dots (QDs) are a quintessential example of nanotechnology; their useful optoelectronic properties (including bright photoluminescence) that distinguish them from bulk semiconductors arise primarily due to their nanoscopic size, and the discrete, quantum mechanical nature of matter.
The “redox environment”, defined as the general tendency of molecules to be reduced or oxidised in a given microenvironment, is one of the central concepts of the field of redox biology. Current methods of measuring the redox environment within living cells are unsatisfactory. To address this, a number QD-based redox sensors have been developed, however, there are still open questions about the physics of such sensors, particularly with respect to their excited-state electron dynamics. This thesis details the excited state dynamics of QD-based redox sensors, as well as their application to biology. Chapter 1 contains an overview of the photophysics of such QD biosensors, as well as a review of the relevant literature.
Chapter 3 details electron microscopy-based studies of the internal structure of CuInS2 QDs, aimed at understanding their defect-related excited state dynamics, with a view to their application as less toxic biosensors. It was concluded that the emissive transition in CuInS2 QDs is associated to an electronic state that arises due to large, polydisperse defect clusters that exist within the CuInS2 lattice.
Chapter 4 details ultrafast spectroscopic studies of a QD redox sensor that consists of a CdTe/CdS core/shell QD coupled with a quinone-derived electron acceptor (Q2NS), which acts as a redox-switching, photoluminescence-quenching electron acceptor. It was found that the comparatively efficient switchable quenching is due to an ultrafast trapping scheme, involving an electron energy state associated with a surface-based lattice defect. Application of the redox sensors to biological cells was then studied, particularly with respect to the mechanism by which cells internalise the QDs, and the resulting QD microenvironment. It arises that endocytosis and subsequent compartmentalisation of QDs by cells presents a significant challenge to the application of QDs as intracellular biosensors.