School of Physics - Theses
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ItemOxygen Terminated Nanodiamond Photoelectrodes for Neuromodulation( 2021)Neuromodulation is used for the treatment of a number of neural impairments that hinder cell activation and proper information transfer. The treatment of psychiatric disorders, Alzheimer’s and Parkinson’s disease, motor function disorders, blindness due to retinal degeneration, dystonia, and epilepsy are some of the examples of clinical applications of neuromodulation. Beyond the widely used approach of using wired electrodes for neuromodulation, various methods have been explored. One of them is to use light to wirelessly control neural networks. When used within safe intensity limits, light can stimulate or modulate the function of neurons. Among the wide variety of optical neuromodulation and stimulation methods is the use of photosensitive thin films or particles as transducers to convert the incident light into electrical signals. It has been claimed that this method can be less invasive than electrically driven stimulation and has the added benefit of scalability in both stimulation location and resolution. Moreover, compared with the optogenetic neuromodulation technique, photoactive surfaces have the capability to eliminate the need for genetic modification to introduce photosensitive proteins to the target neural tissues and possible adverse immune system responses. In this technique, photo-excited charge carriers in an electrode are used to stimulate neural tissue. Various materials have been used for this purpose such as conductive polymers, photoconductive silicon, and semiconducting quantum dots, but many of these materials are not ideal because they lack sufficient biostability and are in some cases toxic. Diamond-based materials are excellent candidates owing to their high corrosion resistance, good biocompatibility, and excellent charge injection properties and recent publications demonstrate their electrical response to illumination. A key feature of diamond-based materials is the ability to control their properties by controlling their surface termination. Surface modification is used to alter nanodiamond properties such as chemical affinity, and electrical and optical properties. In particular, the transfer of photoexcited electrons from the conduction band of the nanodiamond to the adlayer at the nanodiamond surface depends on the surface termination of this material. By tuning these surface properties, a diamond can be an effective photocathode for neural stimulation/modulation. Earlier works have demonstrated that oxygen terminated diamond displays a charge-balanced capacitive charge transfer when it is illuminated in saline solution. In capacitive charge transfer, no photoexcited electrons are transferred across the interface and the diamond/electrolyte adlayer can be modelled as a simple electrical capacitor. On the other hand, hydrogen-terminated diamond exhibits Faradic charge transfer, which may be due to the transfer of photoexcited electrons to the adlayer because of the negative electron affinity of the hydrogen-terminated surface. This suggests that the properties of oxygen terminated diamond are more favourable for neural stimulation/modulation. The capacitive charge transfer mechanism is known to minimize both electrode damage and cell degradation which should be avoided in neural stimulation. This thesis starts by examining the properties of oxygen terminated detonation nanodiamond (O-DND) particles with the size of 20 nm to 170 nm. Earlier works have demonstrated that such particles can be incorporated into living cells. When coupled with the high surface area to volume ratio for nanoparticles, this system is potentially attractive for optically induced cell stimulation/modulation. Whilst the presence of a photogenerated charge accumulation layer on oxygen-terminated diamond films has been demonstrated, this has not been experimentally observed in O-DND particles. In this project, furnace annealing is used to oxygen terminate DND powder and to investigate the effect of light on O-DND particles’ surface charge in an aqueous solution. During these experiments, two new techniques are developed. In the 1st method, the electric double layer around the nanoparticles is probed whilst suspended in saline solution and without the need for any electrical connection to the nanoparticles. To do this a 4-electrode electrochemical impedance spectroscopy technique is used to measure the complex impedance of the O-DND nanoparticles suspended in the saline solution as a function of frequency. This technique shows good sensitivity to the surface termination of DND and using this technique, the capacitance and resistance of the particles suspended in the saline solution were extracted. Building on this method, the effect of light on the EDL of the nanoparticles is investigated. Whilst the capacitance and resistance of particles in saline solution is measured, the changes in particle capacitance and resistance due to light illumination are too small to be measured within the standard error range. In the 2nd method, the zeta potential of particles in a solution is measured using the laser doppler electrophoresis technique. The zeta potential directly relates to the surface charge of the nanoparticles, and by measuring it, the light-induced changes in the surface charges may be observed. Here a conventional Zetasizer is modified to allow measurement of the zeta potential while the samples are illuminated with an optical fibre. Using this technique, the changes in surface charge is characterized as a function of different surface terminations, but no changes in the zeta potential under illumination are detected within the sensitivity of the technique. Possible reasons for the lack of observable changes in zeta potential under illumination are discussed. In the 2nd part, this work is focused on the photoresponse of nitrogen-doped ultrananocrystalline diamond, under the assumption that the defect levels created by the nitrogen doping contribute to a photoresponse at longer wavelengths (around 800 nm) which makes it a favourable material for photostimulation. The same surface termination is employed for the nitrogen-doped ultrananocrystalline diamond (N-UNCD) films to evaluate oxygen terminated N-UNCD as a biocompatible photoactive surface for neural stimulation. The oxygen annealing time is optimized to gain the maximum electrochemical capacitance for photoelectrodes, and the electrochemical properties of samples are investigated. Moreover, the electrochemical capacitance of N-UNCD samples oxygen terminated with different techniques is measured and compared with the oxygen annealed sample. The oxygen annealed sample exhibits the greatest electrochemical capacitance and can be optimized to reach a value of about 30 mF cm-2, 6 times higher than other techniques used in this thesis and also previously reported Pt electrodes and comparable to sputtering iridium oxide electrodes. This enhancement is suggested to be due to a combination of factors, including oxygen surface functionalities, graphitic grain boundary etching, and the removal of trans-polyacetylene (TPA) and hydrogen from the sub-surface layer during the oxygen annealing process. N-UNCD exhibits a photoresponse at longer wavelengths, hence it is possible to employ Near-infrared (NIR) light for photoexcitation. NIR light has a higher penetration depth and less phototoxicity than the lower wavelength which makes N-UNCD a favourable material for in vivo photostimulation. The oxygen annealed N-UNCD, which displays a very high surface capacitance, is evaluated in terms of its photoresponse to NIR light. Under optimal conditions, a capacitive photocurrent of 3.7 uA/W is achieved, higher than previously reported photocurrent values of optically driven N-UNCD electrodes. This translates to an approximate 200 times increase in the photocurrent compared with the as-grown sample. Surface sensitive spectroscopy techniques reveal that these orders of magnitude enhancement in photocurrent are due to the formation of a diamond-rich capping layer as the result of preferential etching of graphite at the grain boundaries. It has been suggested that the surface treatments in reactive oxygen resulted in changes in the surface functional groups, which modulate the surface Fermi level. These results hold significance for applications of oxygen terminated N-UNCD photoelectrodes for neuromodulation applications. For neuromodulation, the surface of the electrode must support neuronal growth as well as had perfect biostability. The stability test results show that oxygen annealed N-UNCD photoelectrodes have remarkable stability when stimulated over many cycles in saline. Moreover, the surface of oxygen annealed N-UNCD displays significant biocompatibility and encourages neuron growth without the necessity of promoters, indicating that it is highly biocompatible. When studying the neuronal growth on oxygen annealed N-UNCD surfaces, light stimulation is found to greatly enhance neuronal growth, with better survival rates and improved neurite outgrowth. As the light illumination does not show a significant impact on the control samples, the improved neuronal growth on oxygen annealed N-UNCD films is tentatively concluded to be due to their photoelectric responses. This is the first evidence that implies the potential for using the photoresponse from oxygen terminated N-UNCD to improve neuronal growth. These results suggest that light could be used to direct the growth of specific neural networks. Possibly in the future oxygen terminated N-UNCD photoelectrodes could find applications for neural network regeneration and nervous system repair.