Phase-Controlled Synthesis of Ruthenium and Palladium Nanocrystals

The utility of noble-metal nanocrystals in applications ranging from catalysis to biomedical applications has increased with the ability to finely tune their shapes and sizes. In particular, the catalytic activity of the nanocrystals is strongly affected by their shape, as this parameter is directly related to the atomic arrangement on the surface. Another way to alter the surface atomic arrangement is through changing the crystal structure (or phase) of the nanocrystal, a property known as polymorphism. A powerful method to achieve such control over the crystal structure is through template-directed growth to obtain metastable core-shell nanocrystals. In this dissertation, I present a number of studies delving into the mechanistic details behind template-directed phase control for Pd and Ru, alongside an evaluation of their catalytic properties. First, the importance of particle size on successful template-directed deposition was demonstrated through deposition of Ru on 12-, 18-, 22-, and 26-nm Pd nanoplates, where small nanoplates resulted in fcc-Ru shells, while the larger ones gave hcp-Ru overgrowth. The size dependence was ascribed to a trade-off between the bulk and surface energies that changed with particle size. On small nanoplates, the high proportion of total surface area coming from the side faces makes it favorable to grow fcc-Ru, which deposits smoothly on the side facets (low surface energy) at the expense of forming a metastable phase (high bulk energy). For large nanoplates, only a small proportion of the surface area comes from the side, promoting the growth of hcp-Ru as the resulting jagged side faces (high surface energy) could be compensated by the formation of a thermodynamically stable phase (low bulk energy). To further elucidate the mechanistic details involved in phase-controlled synthesis, the influence of the template’s shape was investigated next. When Ru was deposited on 8-25 nm Pd cubic nanocrystals, the Ru shell took an fcc phase, but on 14-26 nm Pd octahedral nanocrystals, the Ru was deposited as fcc on the small templates before reverting to hcp on the larger ones. The {100} facets displayed on cubic templates forced the Ru to take the fcc phase due to a symmetry mismatch between the facets of the fcc¬-Pd template and hcp-Ru, while on octahedral templates, the displayed {111} facets could be symmetrically aligned with either hcp- or fcc-Ru, allowing for the overgrowth of either crystal structure. Thus, on octahedral templates, the crystal structure depends on particle size and it is determined by the balance of surface and bulk energies. With an improved understanding of template-directed phase control, this method could be extended to obtain hcp-Pd deposition on an hcp¬-Ru template. Under careful control of the reaction conditions, Pd could be deposited on an hcp-Ru template in either the standard fcc phase, or in a novel, metastable hcp phase. It was essential to slow down the reduction rate of the Pd precursor in order to obtain phase-controlled Pd. The ability to control the crystal structure of noble-metal nanocrystals, coupled with a mechanistic understanding of this process, will enable the development of nanostructured materials with unique properties through rational and deterministic syntheses.Ph.D

Effect of general anaesthesia on functional outcome in patients with anterior circulation ischaemic stroke having endovascular thrombectomy versus standard care: a meta-analysis of individual patient data

Background: General anaesthesia (GA) during endovascular thrombectomy has been associated with worse patient outcomes in observational studies compared with patients treated without GA. We assessed functional outcome in ischaemic stroke patients with large vessel anterior circulation occlusion undergoing endovascular thrombectomy under GA, versus thrombectomy not under GA (with or without sedation) versus standard care (ie, no thrombectomy), stratified by the use of GA versus standard care. Methods: For this meta-analysis, patient-level data were pooled from all patients included in randomised trials in PuMed published between Jan 1, 2010, and May 31, 2017, that compared endovascular thrombectomy predominantly done with stent retrievers with standard care in anterior circulation ischaemic stroke patients (HERMES Collaboration). The primary outcome was functional outcome assessed by ordinal analysis of the modified Rankin scale (mRS) at 90 days in the GA and non-GA subgroups of patients treated with endovascular therapy versus those patients treated with standard care, adjusted for baseline prognostic variables. To account for between-trial variance we used mixed-effects modelling with a random effect for trials incorporated in all models. Bias was assessed using the Cochrane method. The meta-analysis was prospectively designed, but not registered. Findings: Seven trials were identified by our search; of 1764 patients included in these trials, 871 were allocated to endovascular thrombectomy and 893 were assigned standard care. After exclusion of 74 patients (72 did not undergo the procedure and two had missing data on anaesthetic strategy), 236 (30%) of 797 patients who had endovascular procedures were treated under GA. At baseline, patients receiving GA were younger and had a shorter delay between stroke onset and randomisation but they had similar pre-treatment clinical severity compared with patients who did not have GA. Endovascular thrombectomy improved functional outcome at 3 months both in patients who had GA (adjusted common odds ratio (cOR) 1·52, 95% CI 1·09–2·11, p=0·014) and in those who did not have GA (adjusted cOR 2·33, 95% CI 1·75–3·10, p<0·0001) versus standard care. However, outcomes were significantly better for patients who did not receive GA versus those who received GA (covariate-adjusted cOR 1·53, 95% CI 1·14–2·04, p=0·0044). The risk of bias and variability between studies was assessed to be low. Interpretation: Worse outcomes after endovascular thrombectomy were associated with GA, after adjustment for baseline prognostic variables. These data support avoidance of GA whenever possible. The procedure did, however, remain effective versus standard care in patients treated under GA, indicating that treatment should not be withheld in those who require anaesthesia for medical reasons