School of Physics - Theses
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ItemFlexible electrodes for neural recording, stimulation and neurochemical sensing( 2020)This thesis focuses on the development of implantable neural interfaces to perform multifunctional neural recording, neural stimulation and biochemical sensing. Neural interfacing devices using penetrating electrodes have emerged as an important tool in both neuroscience research and medical applications. These implantable electrodes enable communication between man-made devices and the nervous system by detecting and/or evoking neuronal activities. Recent years have seen a rapid development of electrodes fabricated using flexible, ultrathin microwires/microfibers. Compared to the arrays fabricated with rigid materials and larger cross sections, these microwires/microfibers have been shown to reduce foreign body response after implantation, with improved signal-to-noise ratio for neural recording and enhanced resolution for neural stimulation. Carbon fibers (CFs) are considered for implant into particular tissue types since they have small size, cause less tissue damage, and are flexible. CF recording electrodes have shown promise as recording electrodes and have the properties necessary to form sensing electrodes. Micron-scale electrodes such as CFs are expected to evoke localized neural responses due to localized electric fields. CFs are traditionally used with fast-scan cyclic voltammetry to study rapid neurotransmitter changes in vivo and in vitro, as they allow real-time detection of catecholamines with high sensitivity and selectivity. However, they possess narrow usable voltage range, which limits their application for neural stimulation. Additionally, surface fouling occurs with certain neurochemicals potentially obstructing further neurotransmitter adsorption onto the electrode surface. Therefore, they need to be coated with other materials to boost their electrochemical properties for neural stimulation. In this thesis, diamond and diamond-like materials, in particular nitrogen doped ultrananocrystalline diamond (NUNCD) hybrid and boron doped carbon nanowall (B-CNW) are considered as coatings for CFs to enhance properties towards neural interface applications. A focus is finding acceptable properties for recording, stimulation and neurochemical sensing. Novel fabrication techniques were developed to deposit the films onto the surface of CFs. Firstly, the surface of CFs was amine-functionalised and covalent bonds were formed with oxygen terminated nanodiamonds. Films were grown on the treated/seeded fibers using plasma-assisted chemical vapor deposition. To fabricate single fiber electrodes, individual fibers were insulated with capillary glass with 100 micrometer of fiber exposed. The physical and chemical properties of NUNCD hybrid and B-CNW were characterized and studied. The results from electrochemical characterization, in conjunction with both in vitro and in vivo assessments, suggest that these electrodes offer a highly functional alternative to conventional electrode materials for both recording and stimulation, yielding safe charge injection capacities up to 25.08 +-12.37 mC/cm2. To test the capability of electrodes for neural stimulation in vitro, explanted wholemount rat retina was used. The electrodes could elicit localized stimulation responses in the explanted retina. These electrodes with micron -scale cross sections have the potential to improve the spatial resolution for stimulation while minimizing axon bundle activation. In vivo and in vitro single-unit recording showed that the electrodes could detect signals with high signal-to-noise ratios up to 8.7. NUNCD hybrid coated CFs were able to electrochemically detect dopamine with high sensitivity and selectivity. Such electrodes are needed for the next generation of miniaturized, closed-loop implants that can self-tune therapies by monitoring both electrophysiological and biochemical biomarkers.
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ItemNano-electronics at graphene-electrolyte interfaces for biological application( 2017)Research of graphene for life science has attracted significant attention from not only the scientific community but also the general public. Graphene possesses many exciting properties which are beneficial for biological applications, such as biocompatibility, tunable chemical properties, atomic thickness, among others. Detection of biomolecules in liquids, such as the electrolytes encountered in biological environments, is a key application. However, the physical processes at the graphene-electrolyte interface are not fully understood. This thesis presents a theoretical and experimental study of this system, and demonstrates its application in bio-sensing. Electrical double layer (EDL) is a critical component necessary for the investigation of the processes at solid-electrolyte interfaces. A new ionic vibration model for the EDL as an improvement to the conventional Gouy-Chapman-Stern (GCS) theory is proposed in chapter 3, as the GCS fails to describe the EDL capacitance over a wider range of electrode potentials. This new theory elucidates the mechanism behind the ionic dynamics at the interface of solid-electrolyte systems, as ions are found to vibrate near the electrode surface in response to the applied electric field. The calculated results are consistent with measurements, and this provide useful insight not only important for biosensing (as we may need to change the potential on graphene over a wide range), but also for energy storage, neuron stimulation, among others. Since some of these applications (e.g.: neuron stimulation) require large capacitance, and this theory indicates that the EDL capacitance decreases with the electrode potential, one of the approaches is to increase the surface area of the electrode material (graphene). Conformally coating graphene onto a nanoporous three-dimensional (3D) structure gives 3D graphene with very large surface area. A novel method of directly fabricating 3D graphene by plasma assisted graphitization is presented in chapter 4. The obtained material shows very large electrochemical capacitance values, as high as 2.1 mF for a sample of 10 mm$^3$. It also exhibits excellent chemical stability, providing a good platform for electrochemical applications (e.g.: energy storage, neuron stimulation, etc.). This sample is then used to capacitively detect a protein (bovine serum albumin) as a non-destructive label-free method. The results can inspire the work on developing bio-sensors which are able to work as bio-filters simultaneously in future, due to the nano-porous structure. However, the unclear sensing mechanism (whether EDL capacitance or quantum capacitance is dominant), low sensitivity, and long response time of the sensor raise the task for searching for different sensing mechanisms. Based on the discussion of electron transport of graphene in solution in chapter 5, a new method for the detection of biomolecules at very low concentrations using Hall effect measurements on liquid gated graphene based devices is proposed in chapter 6. The results of L-histidine detection suggests high sensitivity in the pM range, exceeding the performance of the conventional amperometric and potentiometric techniques under the same conditions. Through the determination of changes of the mobility and the charge carrier density in graphene we provide new clues for understanding different mechanisms of molecular detection on graphene-based devices in liquids. Theoretical modeling of the experiments indicates that the asymmetric electron-hole mobilities can explain the enhanced detection sensitivity. Quantum capacitance is only dominant near the "Dirac" point, while EDL capacitance dominates at higher potential. This explains the drastic changes of the measured electronic properties close to the "Dirac" point. The detailed theoretical and experimental studies presented in this thesis provide a deep understanding for graphene-electrolyte systems, which are important not only for biological applications (such as biosensing, neuron stimulation and recording, bionic implants, etc), but also for energy storage systems.