School of Physics - Theses
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ItemQuantum Sensing of Ferritin-Bound Iron Based on the Nitrogen-Vacancy Defect in Diamond( 2023-11)Iron is an essential element for biological homeostasis. Consequently, the identification of abnormal iron status via simple, cheap and easy to administer tests is highly important. Currently, iron deficiency and overload are assessed with a suite of tests including measures of peripheral blood biomarkers and liver imaging. Within this array of assessment techniques, serum ferritin concentration is the most influential. As the iron storage protein, ferritin correlates well with overall iron levels, especially in cases of deficiency. However, in some situations this relationship breaks down. In particular, concomitant inflammation or infection can confound interpretation of serum ferritin levels. The identification of a new, more reliable biomarker, could greatly improve diagnostic outcomes. One candidate which may fulfill this objective is ferritin-bound iron, or the iron cargo housed inside ferritin. Ferritin can store up to 3000 iron atoms, so a direct measurement of how much iron is present may act as a better indicator of overall iron status than ferritin concentration alone. This thesis covers the development of a clinically viable technique for directly measuring ferritin-bound iron via its magnetic behaviour, using T1 relaxometry with the nitrogen-vacancy defect in diamond. This technique can report on the average iron content of a statistically relevant number of ferritin proteins with high sensitivity and a simple set up. To realise this goal, an initial proof-of-principle demonstration was performed using single crystal diamond. A standard curve of T1 relaxation rate versus iron load was created, which established the baseline sensitivity of the test as well as elucidating how the magnetic properties of the ferritin core vary with iron content. The standard curve revealed anomalous magnetic behaviour which was explained using a theoretical model detailing a morphological change to the iron core occurring at relatively low iron loads. This model provided an L^0.35 scaling of the uncompensated Fe spins, in agreement with previous theoretical predictions. Additionally, this method produced a low detection limit (ferritin 3% loaded at a concentration of 7.5x10^(-6) g/mL), although these values were outside the physiological range. Following this proof-of-concept device, a second sensing technique was developed to improve throughput and sensitivity. Reporting from an ensemble of dispersed nanodiamonds, this novel modality drastically reduced the diamond-target interaction time and increased the sensitivity by roughly a factor of 4. It also offered improved user-friendliness, miniaturisation potential, and could be applied to a range of sensing problems outside the scope of this thesis. Next, two techniques for improving the intrinsic properties of the nanodiamonds were trialed. These were small batch fabrication for optimised oxygen-termination, and silica coating. Both interventions resulted in longer T1 relaxation times, as desired, however the silica coating procedure was able to produce nanodiamonds on par with bulk crystal — a result previously observed at the single particle level but never for a nanodiamond ensemble. Unfortunately, despite improving the nanodiamonds’ intrinsic characteristics, silica coating introduced a deleterious standoff at the diamond interface, resulting in a dramatic loss of sensitivity. To better understand the effects of this standoff, a numerical model was developed. Using Monte Carlo initialisation to mimic the distribution of properties found in every nanodiamond sample, this model showed a shell thickness of 1 nm to optimal. Finally, oxygen-terminated nanodiamonds were used to sense ferritin-bound iron. In addition to improved through-put, the new modality was approximately twice as sensitive. Given the assumed charge incompatibility between the nanodiamond and ferritin, the sensitivity is expected to improve significantly with nanodiamonds engineered for specific binding. Taken in aggregate, this thesis has demonstrated the feasibility of T1 relaxometry to quantify ferritin-bound iron, and provides strong groundwork for further development towards a clinically viable test.
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ItemDeveloping and applying quantum sensors based on optically addressable spin defects( 2023-04)Quantum sensing aims to further our understanding of the natural world and support an upcoming technological revolution by exploiting quantum properties or systems to exceed the performance of classical sensing. Owing to their convenient modes of operation and strong room temperature quantum properties, optically active spin defects hosted within solid state materials have come to prominence as one of the foremost tools of choice in this landscape. Many applications now aim to leverage dense ensembles of such defects to boost measurement sensitivity or scale up, which places greater emphasis on the quality of the host material and sensor production methods since cherry-picking individual defects is no longer an option. The prototypical example of such a defect is the nitrogen-vacancy (NV) centre in diamond, which exhibits remarkable room temperature spin coherence, bestowed upon it by diamond's material properties. In this thesis, we first look at optimising the production of NV ensembles for quantum sensing, aiming to efficiently and cost-effectively produce sensors capable of performing high sensitivity measurements in two key regimes that will be central to the experimental applications explored later. The topics examined are hyperpolarisation of a nuclear spin ensemble on the diamond surface through coupling to an ultra-near-surface NV layer, and investigating the properties of a van der Waals antiferromagnet through widefield NV microscopy. The demands placed on the NV layer for these applications are diverse from one another, with charge stability and quantum coherence properties being vital for the former, and the ability to scalably and reproducibly create layers of known thickness crucial to the latter. In light of these studies, we finally consider whether a different spin system housed within an entirely separate materials system, the boron-vacancy defect in hexagonal boron nitride, may be a suitable alternative to the well-established NV diamond system. We find that the distinct properties of the new host material provide both advantages and disadvantages compared to diamond, and that this system could allow quantum sensing to find even broader scope in the future. By investigating the link between host material properties and the suitability of a quantum sensor for given applications, this thesis provides a unique perspective on the future of the field, which will likely demand more highly specialised and varied sensors.
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ItemThe theory of the nitrogen-vacancy colour centre in diamond( 2012)The nitrogen-vacancy (NV) colour centre in diamond is a model system for many quantum technologies including, metrology, information processing and communications. The NV centre is also highly suitable for employment in various nanotechnology applications, such as biological and sub-diffraction limit imaging, and in tests of fundamental physics, such as cavity quantum electrodynamics and the quantum entanglement of mesoscopic systems. The remarkable properties of the centre are however, not currently fully understood, with several unresolved issues limiting the performance of the centre in its many important applications. As the unresolved issues are interrelated and concern different aspects of the centre's properties, they may only be resolved by the development of a single self-consistent theory of the NV centre. The aim of this work has been to develop such a theory. The theory has been developed using a combination of the molecular model of deep level defects in semiconductors, group theoretical methods and ab initio calculations. The highly structured nature of the theory will enable its future use in the systematic identification of other colour centres that possess properties that exceed those of the NV centre.