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The chemistry of actinides in solution is a very important aspect of the nuclear fuel cycle, especially as the energy needs of the world continue to increase. However, the radio-active nature of the actinides makes experimentation very difficult and dedicated expensive instruments are required. In addition, the disposal of radio-active waste materials requires a proper understanding of their chemistry at a molecular level. To tackle the problem, and to underpin the experimental studies, in this thesis we have studied the redox chemistry and disproportionation mechanism of actinyl complexes in solution using state-of-the art computational methods. Reduction potentials of actinyl complexes in solution have been estimated in solution using density functional theory (DFT) approaches. Solvation effects were included in the quantum chemistry calculations with the conductor like polarisable continuum model (CPCM) solvation method. First of all, we have validated our computational method by studying a variety of solute cavity definitions within the CPCM solvation model and assessed the performance of a range of DFT functionals to suitable to accurately describe the actinide chemistry in solution. Penta-valent uranyl(V) ions are unstable and readily disproportionate; in this study we have explored outer-sphere electron transfer and disproportionation mechanisms to determine the stability of these ions in solution. We have found that the process of outer-sphere disproportionation is unlikely to occur in non-aqueous solutions, such as DMSO, DMF, DCM, acetonitrile and pyridine, when the uranyl(V) ion is bound with a multi-dentate organic ligand. However, our computational results hypothesise that the presence of a trace of water in the experimental conditions can promote a disproportionation reaction by protonating the uranyl(V) ‘yl’ oxygen atoms and then the electron transfer process would proceed through either inner or outer sphere mechanism. In addition, the effect of alkali metal cations on the outer-sphere disproportionation mechanisms was also studied. Overall it has been shown that DFT can be used to accurately predict the redox properties of actinyl complexes in solution and thus contributing for an effective and efficient design of nuclear material separations, proper as well as safer radioactive waste disposal.

This dissertation is focused on the application of electrochemistry for the fundamental understanding, development, and application of electrochemical sensors. In particular, my research focused on the development and understanding of reference electrodes and ion-selective electrodes for potentiometric sensing applications. Recently, following the needs of point-of-care and wearable sensors, electrode designs have transitioned from bulky devices with an aqueous inner filling solution (e.g. pH electrodes) to planarizable solid-contact electrodes. However, unless their polymeric sensing and reference membranes are held in place mechanically, delamination of the physically adhering membranes limits sensor lifetime, as even minimal external mechanical stress or thermal expansion can result in membrane delamination and, thereby, device failure. To address this problem, we designed a sensing platform based on inexpensive polymers to which membranes are attached covalently through photopolymerization. Even extreme mechanical stress does not result in the delamination of the sensing and reference membranes from the underlying polymer, which results in electrodes that exhibit much improved long-term performance and greatly reduced size. This method of sensor preparation is broadly applicable to a wide range of electrode types and allows for long-term measurements of numerous ions that are of environmental and medical significance. Moreover, the applicability of these ion-selective electrodes for long-term measurements requires reference electrodes that also provide stable responses. Reported here are two types of improved reference electrodes based on capillaries and ionic liquids.