Relevant Degree Programs
Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - Nov 2019)
A major goal of the Magnetic Resonance Imaging (MRI) community is quantifying myelin in white matter. MRI contrast depends on tissue microstructure, so quantitative models require detailed understanding of Nuclear Magnetic Resonance (NMR) physics in white matter's complex, heterogeneous environment. In this thesis, we study the underlying physics behind two different ¹H contrast mechanisms in white and grey matter tissue: T₁ relaxation and the recently developed inhomogeneous Magnetization Transfer (ihMT).Using ex-vivo white and grey matter samples of bovine brain, we performed a comprehensive solid-state NMR study of T₁ relaxation under six diverse initial conditions. For the first time, we used lineshape fitting to quantify the non-aqueous magnetization during relaxation. A four pool model describes our data well, matching with earlier studies. We also show examples of how the observed T₁ relaxation behaviour depends upon the initial conditions.ihMT's sensitivity to lipid bilayers, like those in myelin, was originally thought to rely upon hole-burning in the supposedly inhomogeneously-broadened lipid lineshape. Our work shows that this is incorrect and that ihMT only requires the presence of dipolar couplings, not a specific kind of line broadening. We developed a simple explanation of ihMT using a spin-1 system. Using solid-state NMR, we then performed measurements of ihMT and T₁D (dipolar order relaxation time) on four samples: a multilamellar lipid system (Prolipid-161), wood, hair, and bovine tendon. ihMT was observed in all samples, even those with homogeneous broadening (wood and hair). Moreover, we saw no evidence of hole-burning.Lastly, we present results from ihMT experiments with CPMG acquisition on the bovine brain samples. We show that myelin water has a higher ihMT signal than water outside the myelin. It was determined that this was due to the unique thermal motion in myelin lipids. In doing so, we developed a useful metric for determining the relative contributions from magnetization transfer and dipolar coupling to ihMT. Also, we applied a qualitative four pool model with dipolar reservoirs. Together, our results are consistent with myelin lipids having a T₁D which is appreciably longer than the T₁D of non-myelin lipids, despite recent measurements to the contrary.
Master's Student Supervision (2010 - 2018)
Nanostructured Ni(OH)₂ electrodes grown on metalized, electro-spun nanofibers have been developed and characterized. On an active mass basis these electrodes, measured using an Ag/AgCl reference electrode, showed excellent specific capacitance of 2,123 F/g and specific capacity of 283 mA•h/g compared to the theoretical limits of 2190 F/g and 289 mA•h/g respectively. These electrodes were also able to achieve a maximum specific energy of 90 W•h/kg at a specific power of 727 W/kg and maintain a specific energy of 32 W•h/kg at a power density of 10 kW/kg. When optimized, the nanostructured electrodes had an internal surface area 5 orders of magnitude greater than an equivalent flat plate electrode and achieved a surface area to volume ratio of 2.5x105 cm-¹. Electrical equivalent circuit models were developed to understand device performance and showed a reasonable accuracy when compared with experimental data. Ultimately, device power density was limited by a combination of ohmic resistance and kinetic polarization. Significant mass transport polarization was suppressed due to a combination of low Ni(OH)₂ nanostructure thickness and high electrode porosity. This device architecture features an integrated current collector and the fabrication process does not require any high temperature or electrochemical processing, opening avenues for both cost reduction and manufacturing simplification. Possible applications of this technology are in advanced nickel metal hydride batteries or Ni(OH)₂ asymmetric storage devices.
Whelks (sea snails) deposit their eggs into tough protein capsules that protect the eggs from the harsh marine environment. The mechanical properties of the protein capsule are fascinating. At low strain the capsule material is stiff but elastic, while at higher strain the modulus decreases and the material stretches dramatically. X-ray diffraction measurements and Raman spectroscopy suggest that the capsules’ high extensibility is due to a reversible phase transition of component protein building blocks from a compact ɑ-helical conformation to a more extended softer conformation called β*. In this work, a variety of Nuclear Magnetic Resonance (NMR) spectroscopy experiments were performed to study the microscopic basis of the egg capsules’ structure and mechanical properties under extension. We have used deuterated probe molecules to study the molecular alignment of whelk egg capsule (WEC). ²H residual quadrupolar splitting measurements are consistent with a two component model for the molecular alignment of WEC, in agreement with optical measurements showing a cross-plied fibre structure. Measurements of ¹H dipolar linewidth and T₁ time as functions of strain are consistent with a previous model that proposed a gradual transition from ɑ-helical coiled-coils to poorly structured worm-like chains with strain. Residual quadrupolar couplings of absorbed water are not strongly affected by strain. Overall, results obtained from these experiments provide useful information to understand the transition mechanism, and may contribute to the development of WEC-like materials for the applications such as new bioencapsulants for delicate tissue implants.
Using templation with Nanocrystalline Cellulose (NCC), a mesoporous silica and organosilica film with a tunable chiral nematic pore structure and long, narrow pores has recently been developed. This novel material has interesting optical properties: it selectively reflects left-handed polarized light and has an iridescent appearance, with its perceived colour controlled by tuning the pitch of chiral structure. Its possible applications include enantioselective catalysis and filtering, and optical sensors. In this work, a variety of Nuclear Magnetic Resonance (NMR) spectroscopy experiments were run to characterize the films and composite systems. ¹³C and ²⁹Si Magic Angle Spinning NMR spectra confirmed removal of the NCC template via sulphuric acid and showed the process does not cleave organosilica bonds. NMR cryoporometry, which uses the signal from absorbed liquid water, relates freezing point depression to pore size. This method was found to be non-destructive, accurate, and more sensitive and precise than nitrogen sorption to determine pore sizes. The silica films were found to have a smaller (~3 nm) pore width size distribution than the organosilica films (~6-9 nm). Using Pulsed Field Gradient (PFG) NMR, the diffusion of absorbed water was found to be ~2x as fast perpendicular to the surface normal than parallel to it, with diffusion parallel to the pore axis essentially unrestricted. Silica films had overall slower diffusion than organosilica films. Finally, a composite system was made by functionalizing an organosilica film with n-Octyl, enabling it to absorb ¹⁵N-labelled 8CB liquid crystal. Reversible switching of the reflective properties was seen upon heating absorbed liquid crystals to the isotropic phase. ¹⁵N NMR spectra were taken of the sample with different orientations to the field, showing that at room temperature, the 8CB mesogens are on average aligned down the pores, and after melting, they are isotropic. Large, unexplained magnetic susceptibility effects are seen in the room temperature spectra. Overall, these experiments will enable further development of these materials and other composite systems.
Solid polymer electrolytes have the potential to improve manufacturability, performance, and safety characteristics of lithium-ion batteries by replacing conventional liquid electrolytes. Two different solid polymer electrolyte materials were characterized using Nuclear Magnetic Resonance (NMR) techniques. The first material is a result of research efforts on single-ion conducting polymers. The material is intended to combine the high conductivity properties of ionic liquids with lithium cation single-ion conduction. The goal of the synthesis was to produce a polymerized ionic liquid, where crosslinking an anionic monomer (AMLi) with poly(ethylene glycol) dimethacrylate (PEGDM) immobilizes the fluorinated anionic species. Pulsed-field gradient NMR diffusion measurements of the AMLi/PEGDM samples have demonstrated that both the lithium cations and fluorinated anions are mobile and contributing toward conductivity. Therefore, further work is required to successfully immobilize the fluorinated anion in a crosslinked network. The ⁷Li and ¹⁹F diffusion coefficients of the AMLi/PEGDM 40/60 sample were 3.4x10⁻⁸ cm²/s and 2.2x10⁻⁸ cm²/s at 100°C. The second material incorporates a poly(ethylene oxide) (PEO) conductive block and polyethylene (PE) reinforcement block. The PEO/PEO-b-PE/LiClO₄ samples were not intended to be single-ion conducting and materials synthesis aimed to maximize conductivity and mechanical properties. A ⁷Li diffusion coefficient of ~4x10⁻⁸ cm²/s at 60°C was observed. It is expected that the anion would also be mobile and therefore the polymer electrolyte would be a bi-ionic conductor. These samples demonstrated higher ⁷Li diffusion coefficients at a given temperature and superior mechanical properties for a flexible polymer electrolyte compared to the AMLi/PEGDM samples. Practically, the diffusion measurements of the solid polymer samples are extremely challenging, as the spin-spin (T₂) relaxation times are very short, necessitating the development of specialized pulsed-field gradient apparatus. These results provide valuable insight into the conduction mechanisms in these materials, and will drive further optimization of solid polymer electrolytes.
Solid state nuclear magnetic resonance experiments were performed in order to investigate the microscopic properties of three resilin/resilin-like proteins: An16, rec1-resilin, and natural resilin in dragonfly tendons. Three different types of experiments were performed: measurements of chemical shifts in carbon-13 spectra, measurements of residual quadrupole couplings in deuterated water absorbed in the samples, and measurements of proton residual dipole couplings based on the buildup of multiple quantum coherences. The results suggest that the molecular chains in the materials tested are primarily randomly coiled and lacking in regular structure, and are able to easily change between many transient conformations. These conformations can vary significantly in terms of their structural characteristics, resulting in a broad distribution of localized dynamics. When stretched, An16 showed a slightly increased tendency to adopt beta-sheet secondary structure. The natural resilin also exhibited slightly more rigid structure than the other materials, which may be related to greater efficiency in the natural crosslinking process.
Abstract Nanocrystalline cellulose (NCC) shows very unique properties – in suspension, it spontaneously forms a chiral nematic phase and, in high purity, exhibits iridescence. While native cellulose has historically been extensively studied in solution and solid-state NMR with success, the physical structure between NCC nanocrystallites is not fully known. Due to the complex structure of the nanocrystallites, conventional diffraction techniques cannot fully determine the structure. In this work ¹H-²H exchange coupled with the following NMR techniques were used to investigate the crystallite structure of NCC: ¹³C CP/MAS, ¹³C T₁, T₂ and T₁p measurements and ¹³C-²H and ¹³C-³¹P REDOR.Abstract Results suggest a broad distribution of regions possessing varied dynamical and structural properties. Based upon previously assigned peaks arising from crystalline and amorphous regions, approximately 40% of the NCC particles are characteristic of amorphous and/or surface regions. Even though NCC preparation is designed to remove amorphous regions, this result is remarkably similar to native, untreated cellulose. Proton-deuterium exchange experiments suggest an unequal proton exchangeability between the different possible exchange sites, and suggest that the samples consist not of sharply defined exchangeable and unexchangeable regions, rather they are more uniformly partially exchanged. We also describe a method to determine the number of surface phosphate groups remaining after hydrolysis treatment. ¹³C-³¹P REDOR experiments conclude that 2.6 ± 0.2 phosphate groups are attached to C₂ or C₃ per 100 monomers.
No abstract available.