ZnO nanowire microelectrode arrays for integration with neuronal networks

<p>Microelectrode arrays (MEAs) have been shown as a successful approach for neuroscientists to monitor the signal communication within the neuronal networks for understanding the functionality of the nervous system. However, using conventional planar MEAs is shown to be incapable of precise signal recording from neuronal networks at single-cell resolution due to low signal-to-ratio (SNR). This thesis looks at developing an electronic platform that comprises of zinc oxide nanowires (ZnO-NWs) on MEAs as a future device to record action potential (AP) signals with high SNR from human neuronal networks at single-cell resolution. Specifically, I studied the controlled growth of ZnO nanowires with various morphologies at exact locations across the substrate. I then investigated the biocompatibility of ZnO nanowires with different morphology and geometry for interaction with human NTera2.D1 (hNT) neurons. Finally, I examined the electrical characteristics of MEAs that were integrated with ZnO nanowires and metal encapsulated ZnO nanowires in comparison to the planar MEAs. The hydrothermal growth of ZnO nanowires is thoroughly investigated as a technique to allow synthesis of the nanowires at a low temperature (95°C) with a low cost and high scalability that can also be applied on flexible substrates. The morphology of the ZnO nanowires was varied (diameters of 20–300 nm, lengths of 0.15–6.2 µm, aspect ratios of 6–95 and densities of 10–285 NWs/µm²) by controlling the critical growth parameters such as the precursor concentration (2.5–150 mM), growth time (1–20 h) and additive polyethylenimine (PEI) concentration (0–8 mM). The diameter and length of the ZnO nanowires were increased by increasing the precursor concentration and growth time. Using the standard precursor concentration of 25 mM, growth times of up to 4 h were found effective for the active growth of the nanowires due to the consumption of the precursor ions and precipitation of ZnO. The addition of 6 mM PEI to the growth solution was shown to mediate the growth solution, allowing the extension of the nanowire growth to 20 h or longer. The PEI molecules were also attached to the lateral faces of the nanowires that confined their lateral growth and promoted their axial growth (enhanced aspect ratio from 12 ± 3 to 67 ± 21). Standard photolithography techniques were also introduced to selectively grow ZnO nanowires on exact locations across the substrates. The role of the ZnO seed layer geometry, seed layer area and gap, on the growth of ZnO nanowires was also investigated. Despite using the constant growth parameters (25 mM of precursor concentration with 4 h of growth time) changing the seed line widths (4 µm–1 mm) and the gap between the seed lines (2 µm–800 µm) resulted in the morphology of the nanowires to vary across the same substrate (diameters of 50–240 nm, lengths of 1.2–4.6 µm, aspect ratios of 9–34 and densities of 28–120 NWs/µm²). The seed area ratio of 50% was determined as a threshold to influence the nanowire morphology, where decreasing the seed area ratio below 50% (by increasing the adjacent gap or decreasing the seed layer area) increased the growth rate of the nanowires. The biocompatibility of ZnO nanowires with human hNT neurons was investigated in this work for the first time. The adhesion and growth of hNT neurons on the arrays of ZnO nanowire florets were determined to be influenced by both geometry and morphology of the nanowires. The growth of the hNT neurons was promoted by 30% compared to the control Si/SiO₂ substrate surface when ZnO nanowires with lengths shorter than 500 nm and densities higher than 350 NWs/µm² were grown. The hNT neurons on all nanowires were also demonstrated to be functionally viable as they responded to the glutamate stimulation. ZnO nanowires were shown to improve the electrical properties of the MEAs by reducing the electrochemical impedance due to the increased 3D surface area. The ZnO nanowires that were grown with 50 mM of precursor concentration for 4 h of growth time lowered the impedance from 835 ± 40 kΩ of planar Cr/Au MEAs to 540 ± 20 kΩ at a frequency of 1 kHz. In contrast, the ZnO nanowires that were grown with PEI for 35 h showed that despite the increased surface area by a factor of 45× the impedance was found to be quite high, 2.25 ± 0.2 MΩ at 1 kHz of frequency. The adsorption of PEI molecules to the lateral surfaces of the nanowires was thought to behave as a passivation layer that could have restricted the charge transfer characteristics of the ZnO-NW MEAs. Encapsulation of the pristine ZnO nanowires that were grown with standard precursor concentration of 25 mM for 4 h of growth time with different metallic layers (Cr/Au, Ti and Pt) further improved the electrical characteristics of the MEAs. The ZnO nanowires that were encapsulated with a 10 nm thin layer of Ti and Pt achieved the lowest electrochemical impedance of 400 ± 25 kΩ at 1 kHz in this work. The robustness of the Ti encapsulated ZnO nanowires were also improved in comparison to the PEI ZnO nanowires. The improved electrochemical characteristics and mechanical stability of the MEAs integrated with metal encapsulated ZnO nanowires have shown a great promise for improving the SNR of recording signals from neuronal cells for long term measurements. This work concludes that both pristine ZnO nanowire MEAs and metal encapsulated ZnO nanowire MEAs will be capable of recording AP signals from human neuronal networks at single-cell resolution. However, further optimisation and extensions of the work are required to record AP signals from human neuronal cells.</p>

ZnO nanowire microelectrode arrays for integration with neuronal networks

<p>Microelectrode arrays (MEAs) have been shown as a successful approach for neuroscientists to monitor the signal communication within the neuronal networks for understanding the functionality of the nervous system. However, using conventional planar MEAs is shown to be incapable of precise signal recording from neuronal networks at single-cell resolution due to low signal-to-ratio (SNR). This thesis looks at developing an electronic platform that comprises of zinc oxide nanowires (ZnO-NWs) on MEAs as a future device to record action potential (AP) signals with high SNR from human neuronal networks at single-cell resolution. Specifically, I studied the controlled growth of ZnO nanowires with various morphologies at exact locations across the substrate. I then investigated the biocompatibility of ZnO nanowires with different morphology and geometry for interaction with human NTera2.D1 (hNT) neurons. Finally, I examined the electrical characteristics of MEAs that were integrated with ZnO nanowires and metal encapsulated ZnO nanowires in comparison to the planar MEAs. The hydrothermal growth of ZnO nanowires is thoroughly investigated as a technique to allow synthesis of the nanowires at a low temperature (95°C) with a low cost and high scalability that can also be applied on flexible substrates. The morphology of the ZnO nanowires was varied (diameters of 20–300 nm, lengths of 0.15–6.2 µm, aspect ratios of 6–95 and densities of 10–285 NWs/µm²) by controlling the critical growth parameters such as the precursor concentration (2.5–150 mM), growth time (1–20 h) and additive polyethylenimine (PEI) concentration (0–8 mM). The diameter and length of the ZnO nanowires were increased by increasing the precursor concentration and growth time. Using the standard precursor concentration of 25 mM, growth times of up to 4 h were found effective for the active growth of the nanowires due to the consumption of the precursor ions and precipitation of ZnO. The addition of 6 mM PEI to the growth solution was shown to mediate the growth solution, allowing the extension of the nanowire growth to 20 h or longer. The PEI molecules were also attached to the lateral faces of the nanowires that confined their lateral growth and promoted their axial growth (enhanced aspect ratio from 12 ± 3 to 67 ± 21). Standard photolithography techniques were also introduced to selectively grow ZnO nanowires on exact locations across the substrates. The role of the ZnO seed layer geometry, seed layer area and gap, on the growth of ZnO nanowires was also investigated. Despite using the constant growth parameters (25 mM of precursor concentration with 4 h of growth time) changing the seed line widths (4 µm–1 mm) and the gap between the seed lines (2 µm–800 µm) resulted in the morphology of the nanowires to vary across the same substrate (diameters of 50–240 nm, lengths of 1.2–4.6 µm, aspect ratios of 9–34 and densities of 28–120 NWs/µm²). The seed area ratio of 50% was determined as a threshold to influence the nanowire morphology, where decreasing the seed area ratio below 50% (by increasing the adjacent gap or decreasing the seed layer area) increased the growth rate of the nanowires. The biocompatibility of ZnO nanowires with human hNT neurons was investigated in this work for the first time. The adhesion and growth of hNT neurons on the arrays of ZnO nanowire florets were determined to be influenced by both geometry and morphology of the nanowires. The growth of the hNT neurons was promoted by 30% compared to the control Si/SiO₂ substrate surface when ZnO nanowires with lengths shorter than 500 nm and densities higher than 350 NWs/µm² were grown. The hNT neurons on all nanowires were also demonstrated to be functionally viable as they responded to the glutamate stimulation. ZnO nanowires were shown to improve the electrical properties of the MEAs by reducing the electrochemical impedance due to the increased 3D surface area. The ZnO nanowires that were grown with 50 mM of precursor concentration for 4 h of growth time lowered the impedance from 835 ± 40 kΩ of planar Cr/Au MEAs to 540 ± 20 kΩ at a frequency of 1 kHz. In contrast, the ZnO nanowires that were grown with PEI for 35 h showed that despite the increased surface area by a factor of 45× the impedance was found to be quite high, 2.25 ± 0.2 MΩ at 1 kHz of frequency. The adsorption of PEI molecules to the lateral surfaces of the nanowires was thought to behave as a passivation layer that could have restricted the charge transfer characteristics of the ZnO-NW MEAs. Encapsulation of the pristine ZnO nanowires that were grown with standard precursor concentration of 25 mM for 4 h of growth time with different metallic layers (Cr/Au, Ti and Pt) further improved the electrical characteristics of the MEAs. The ZnO nanowires that were encapsulated with a 10 nm thin layer of Ti and Pt achieved the lowest electrochemical impedance of 400 ± 25 kΩ at 1 kHz in this work. The robustness of the Ti encapsulated ZnO nanowires were also improved in comparison to the PEI ZnO nanowires. The improved electrochemical characteristics and mechanical stability of the MEAs integrated with metal encapsulated ZnO nanowires have shown a great promise for improving the SNR of recording signals from neuronal cells for long term measurements. This work concludes that both pristine ZnO nanowire MEAs and metal encapsulated ZnO nanowire MEAs will be capable of recording AP signals from human neuronal networks at single-cell resolution. However, further optimisation and extensions of the work are required to record AP signals from human neuronal cells.</p>

Observation of Dominant Nuclei and Magic-Sized CdS Nanoparticles in a Single-Phase System

Cadmium sulfide nanoparticles (CdS NPs) were synthesized by using cadmium acetate and thiourea as precursors and sodium oleate as the surfactant under different cadmium acetate concentrations in anhydrous ethanol. Cadmium (Cd) precursor concentration greatly affected the nucleation-growth of CdS NPs. In extremely dilute solution with a Cd precursor concentration of 0.1 mmol · L−1, an overlapped nucleation and growth corresponding to two pronounced absorption peaks at 310 nm and 350 nm, respectively, was observed. Unparalleled nucleation was dominant within very long reaction time until 10 hours. The nuclei and the resulting magic-sized CdS NPs may be used as seeds to prepare size and shape controllable nanoparticles. On the contrary, at a high Cd precursor concentration (5 mmol · L−1), nucleation and growth were separated. Only one first exciton absorption peak standing for the growth of regular CdS NPs appeared at 440 nm. Many techniques including transmission electron microscopy (TEM), X-ray powder diffraction (XRD), ultraviolet-visible (UV-Vis) absorption and photoluminescence (PL) spectrometers were applied to characterize the morphology, crystalline structure, and optical properties of CdS NPs.

Solution-Phase Synthesis of Nanoparticles and Growth Study

<p>This thesis is concerned with solution-phase synthesis of nanoparticles and growth of nanoparticles in solution. A facile synthesis route was developed to produce nanoparticles of iron, iron carbide and ruthenium. In general, the synthesis involved the reaction/decomposition of a metal precursor in solution, in the presence of a stabilising agent, in a closed reaction vessel, under a hydrogen atmosphere. The crystallinity, crystal structure, morphology and chemical composition of the nanoparticles obtained were studied primarily by transmission electron microscopy (TEM), selected area electron diffraction (SAED), powder X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDS). Scanning quantum interference device magnetometry (SQUID) was used to characterise the magnetic properties of iron and iron carbide nanoparticles. In situ synchrotron-based XRD was employed to investigate the growth of platinum nanoparticles of different morphologies. The synthesis of iron and iron carbide nanoparticles was investigated at temperatures 80-160 °C. Syntheses at 130 °C and above produced mainly single-crystal α-Fe nanoparticles, whereas those at lower temperatures yielded products consisting of α-Fe and Fe₃C nanoparticles. Nanoparticles of larger than 10 nm oxidised on the surface leading to core/shell structures, and those of smaller size oxidised completely upon exposure to air. Core/shell nanoparticles of larger than 15 nm were observed to be stable under ambient conditions for at least a year, whereas those smaller in size underwent further oxidation forming core/void/shell structures. The magnetic properties of selected samples were characterised. The core/shell nanoparticles were shown to exhibit ferromagnetic behaviours, and saturation magnetisations were obtained at the range of 100-130 emu g⁻¹. Nanoparticle size and size distribution, and morphology were found to be a result of combined effect of precursor concentration and the relative stabiliser concentration. In general, high-precursor concentration resulted in less controlled reaction and produced large nanoparticle size and size distribution. Under the high-concentration condition, the use of stabilisers in reduced amount then led to a diverse range of morphologies, which include dimer, porous and branched structures. As for the synthesis of ruthenium nanoparticles, reactions of different precursors were investigated at temperatures ranging from room temperature to 140 °C. Highly crystalline ruthenium nanoparticles of different sizes and morphologies were obtained through different experimental conditions. The increase in nanoparticle size was found to be a result of increasing reaction temperature and/or decreasing stabiliser to ruthenium ratio. This trend was observed to be independent of the type of stabilisers and precursors used. The use of stabilisers with different binding characteristics has facilitated the formation of non-spherical nanoparticles; these include rod-like structures with high aspect ratios (of up to 12), hexagonal and truncated triangular plate-like structures, and tripods. The growth of faceted and branched structures of platinum nanoparticles was investigated by employing in situ XRD techniques. TEM was used to examine the intermediate structures. The two different morphologies were previously shown to be governed by precursor concentration. It was found that the growth in the low-concentration reaction was characteristic of a thermodynamically controlled regime, whereas that in the high-concentration reaction occurred at much greater rates under a kinetically controlled regime. Based on the observations obtained, different growth mechanisms were proposed and discussed. The former involved an oriented attachment mechanism, while the latter, a novel mechanism involving selective growth and etching processes. The results are followed by an overall discussion comparing and contrasting the various syntheses involved, and relating the results of syntheses to those of the growth studies.</p>

Solution-Phase Synthesis of Nanoparticles and Growth Study

<p>This thesis is concerned with solution-phase synthesis of nanoparticles and growth of nanoparticles in solution. A facile synthesis route was developed to produce nanoparticles of iron, iron carbide and ruthenium. In general, the synthesis involved the reaction/decomposition of a metal precursor in solution, in the presence of a stabilising agent, in a closed reaction vessel, under a hydrogen atmosphere. The crystallinity, crystal structure, morphology and chemical composition of the nanoparticles obtained were studied primarily by transmission electron microscopy (TEM), selected area electron diffraction (SAED), powder X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDS). Scanning quantum interference device magnetometry (SQUID) was used to characterise the magnetic properties of iron and iron carbide nanoparticles. In situ synchrotron-based XRD was employed to investigate the growth of platinum nanoparticles of different morphologies. The synthesis of iron and iron carbide nanoparticles was investigated at temperatures 80-160 °C. Syntheses at 130 °C and above produced mainly single-crystal α-Fe nanoparticles, whereas those at lower temperatures yielded products consisting of α-Fe and Fe₃C nanoparticles. Nanoparticles of larger than 10 nm oxidised on the surface leading to core/shell structures, and those of smaller size oxidised completely upon exposure to air. Core/shell nanoparticles of larger than 15 nm were observed to be stable under ambient conditions for at least a year, whereas those smaller in size underwent further oxidation forming core/void/shell structures. The magnetic properties of selected samples were characterised. The core/shell nanoparticles were shown to exhibit ferromagnetic behaviours, and saturation magnetisations were obtained at the range of 100-130 emu g⁻¹. Nanoparticle size and size distribution, and morphology were found to be a result of combined effect of precursor concentration and the relative stabiliser concentration. In general, high-precursor concentration resulted in less controlled reaction and produced large nanoparticle size and size distribution. Under the high-concentration condition, the use of stabilisers in reduced amount then led to a diverse range of morphologies, which include dimer, porous and branched structures. As for the synthesis of ruthenium nanoparticles, reactions of different precursors were investigated at temperatures ranging from room temperature to 140 °C. Highly crystalline ruthenium nanoparticles of different sizes and morphologies were obtained through different experimental conditions. The increase in nanoparticle size was found to be a result of increasing reaction temperature and/or decreasing stabiliser to ruthenium ratio. This trend was observed to be independent of the type of stabilisers and precursors used. The use of stabilisers with different binding characteristics has facilitated the formation of non-spherical nanoparticles; these include rod-like structures with high aspect ratios (of up to 12), hexagonal and truncated triangular plate-like structures, and tripods. The growth of faceted and branched structures of platinum nanoparticles was investigated by employing in situ XRD techniques. TEM was used to examine the intermediate structures. The two different morphologies were previously shown to be governed by precursor concentration. It was found that the growth in the low-concentration reaction was characteristic of a thermodynamically controlled regime, whereas that in the high-concentration reaction occurred at much greater rates under a kinetically controlled regime. Based on the observations obtained, different growth mechanisms were proposed and discussed. The former involved an oriented attachment mechanism, while the latter, a novel mechanism involving selective growth and etching processes. The results are followed by an overall discussion comparing and contrasting the various syntheses involved, and relating the results of syntheses to those of the growth studies.</p>