Electroanalysis in nanoparticle assemblies

This thesis is concerned with the deposition of nanoparticle films onto boron-doped diamond and tin-doped indium oxide (lTO) surfaces and the characterisation of the films using electron microscopy, powder diffraction methods and quartz crystal microbalance (QCM) data. The redox behaviour of the porous films was examined using cyclic voltammetry in various media to investigate potential electroanalytical applications. TiOz (anatase) mono-layer films were immobilised onto an inert boron-doped diamond substrate. Cyclic voltammetry experiments allowed two distinct steps in the reduction - protonation processes to be identified that are consistent with the formation of Ti(III) surface sites accompanied by the adsorption of protons. Preliminary data for electron transfer processes at the reduced TiOz surface such as the dihydrogen evolution process and the 2 electron - 2 proton reduction of maleic acid to succinic acid are discussed. Novel multi-layer TiOz films were deposited with a variety of organic binder molecules onto ITO substrates. The redox reactivity of Cuz+ with 1,4,7,10- tetraazacyclododecane- 1,4,7, IO-tetrayl- tetrakis (methyl-phosphonic acid) in solution and immobilised on an electrode surface are investigated. The influences of film thickness, scan rate, and pH on the electrochemistry of immobilised pyrroloquinoline quinone was investigated with two possible electron transport processes observed. The thickness of TiOz phytate films was found to change the shape of the resulting cyclic voltammograms dramatically. Computer simulation and impedance spectroscopy allowed insights into the diffusion of electrons to be obtained. 1, 1 ~Ferrocenedimethanol was employed as an adsorbing redox system to study the voltammetric characteristics of carboxymethyl-y-cyclodextrin films and evidence for two distinct binding sites is considered. The apparent transport coefficients for dopamine and Ru(NHJ)6J+ are estimated for TiOz Nafion® films. The electrochemical processes in biphasic electrode systems for the oxidation of water-insoluble N,N-didodecyl-N;N~diethyl-benzene-diamine (DDPD) pure and dissolved in di-(2-ethyl-hexyl)phosphate (HDOP) immersed in aqueous electrolyte media are described. Transfer of the anion from the aqueous electrolyte phase into the organic phase accompanies the oxidation of pure DDPD. In the presence of HOOP, oxidation is accompanied by proton exchange. The electrochemically driven proton exchange process occurs over a wide pH range. Organic microdroplet deposits of OOPD in HDOP at basal plane pyrolytic graphite electrodes are studied using voltammetric techniques and compared to the behaviour of organic microphase deposits in mesoporous Ti02 thin films. Two types of Ti02 thin film electrodes were investigated, (i) a 300-400 nm film on ITa and (ii) a 300-400 nm film on ITa sputter-coated with a 20 nm porous gold layer. The latter biphasic design is superior. Titanium carbide (TiC) nanoparticies were deposited onto ITa electrodes. Partial anodic oxidation and formation of novel core-shell TiC-Ti02 nanoparticies was observed at applied potentials positive of 0.3 V vs. SCE. Significant thermal oxidation of TiC nanoparticies by heating in air occurs at 250 °c leading to coreshell TiC-Ti02 nanoparticies, then Ti02 (anatase) at ca. 350 °c, and Ti02 (rutile) at temperatures higher than 750 °c. The electrocatalytic properties of the core-shell TiC-Ti02 nanoparticulate films were surveyed for the oxidation of hydroquinone, ascorbic acid, dopamine and nitric oxide (NO) in aqueous buffer media. Mono- and multi-layer Ce02 deposits on ITa are shown to be electrochemically active. A reduction assigned to a Ce(IV/III) process has been observed and followup chemistry in the presence of phosphate discovered. The interfacial formation of CeP04 has been proven and effects of the deposit type, pH and phosphate concentration on the process analysed. The electrochemistry of multi-layer Ce02 nanoparticulate films in organic solvent is shown to be more stable.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Electrochemical reactivity of TiO2 nanoparticles adsorbed onto boron-doped diamond

TiO2 (anatase) nanoparticles of ca. 6–10 nm diameter are adsorbed from acidic aqueous solution onto polycrystalline industrially polished boron-doped diamond electrode surfaces. After immobilisation at the electrode surface, TiO2 nanoparticles are imaged in vacuum by electron microscopy (FEGSEM) and when immersed in a liquid film of aqueous 12 M LiCl by in situ scanning tunnelling microscopy (STM). Mono-layer films of TiO2 particles are studied voltammetrically in different electrolyte media. Boron-doped diamond as an inert substrate material allows the reduction of TiO2 particles in phosphate buffer solution to be studied and two distinct steps in the reduction–protonation process are identified: (i) a broad reduction signal associated with the binding of an outer layer of protons and (ii) a sharper second reduction signal associated with the binding of an inner (or deeper) layer of protons. Voltammetric experiments in aqueous 0.1 M NaClO4 with variable amounts of HClO4 suggest that the reduction of TiO2 particles is consistent with the formation of Ti(III) surface sites and accompanied by the adsorption of protons. Saturation occurs and the total amount of surface sites can be determined. Preliminary data for electron transfer processes at the reduced TiO2 surface such as the dihydrogen evolution process and the two-electron–two-proton reduction of maleic acid to succinic acid are discussed

Underpotential surface reduction of mesoporous CeO2 nanoparticle films

The formation of variable-thickness CeO2 nanoparticle mesoporous films from a colloidal nanoparticle solution (approximately 1–3-nm-diameter CeO2) is demonstrated using a layer-by-layer deposition process with small organic binder molecules such as cyclohexanehexacarboxylate and phytate. Film growth is characterised by scanning and transmission electron microscopies, X-ray scattering and quartz crystal microbalance techniques. The surface electrochemistry of CeO2 films before and after calcination at 500 °C in air is investigated. A well-defined Ce(IV/III) redox process confined to the oxide surface is observed. Beyond a threshold potential, a new phosphate phase, presumably CePO4, is formed during electrochemical reduction of CeO2 in aqueous phosphate buffer solution. The voltammetric signal is sensitive to (1) thermal pre-treatment, (2) film thickness, (3) phosphate concentration and (4) pH. The reversible ‘underpotential reduction’ of CeO2 is demonstrated at potentials positive of the threshold. A transition occurs from the reversible ‘underpotential region’ in which no phosphate phase is formed to the irreversible ‘overpotential region’ in which the formation of the cerium(III) phosphate phase is observed. The experimental results are rationalised based on surface reactivity and nucleation effects

Underpotential surface reduction of mesoporous CeO 2 nanoparticle films

This article was published in the Journal of Solid State Electrochemistry, [© Springer] and the original publication is available at www.springerlink.comThe formation of variable-thickness CeO2 nanoparticle mesoporous films from a colloidal nanoparticle solution (approximately 1–3-nm-diameter CeO2) is demonstrated using a layer-by-layer deposition process with small organic binder molecules such as cyclohexanehexacarboxylate and phytate. Film growth is characterised by scanning and transmission electron microscopies, X-ray scattering and quartz crystal microbalance techniques. The surface electrochemistry of CeO2 films before and after calcination at 500 °C in air is investigated. A well-defined Ce(IV/III) redox process confined to the oxide surface is observed. Beyond a threshold potential, a new phosphate phase, presumably CePO4, is formed during electrochemical reduction of CeO2 in aqueous phosphate buffer solution. The voltammetric signal is sensitive to (1) thermal pre-treatment, (2) film thickness, (3) phosphate concentration and (4) pH. The reversible ‘underpotential reduction’ of CeO2 is demonstrated at potentials positive of the threshold. A transition occurs from the reversible ‘underpotential region’ in which no phosphate phase is formed to the irreversible ‘overpotential region’ in which the formation of the cerium(III) phosphate phase is observed. The experimental results are rationalised based on surface reactivity and nucleation effects

Genetic risk prediction of atrial fibrillation

Background—Atrial fibrillation (AF) has a substantial genetic basis. Identification of individuals at greatest AF risk could minimize the incidence of cardioembolic stroke. Methods—To determine whether genetic data can stratify risk for development of AF, we examined associations between AF genetic risk scores and incident AF in five prospective studies comprising 18,919 individuals of European ancestry. We examined associations between AF genetic risk scores and ischemic stroke in a separate study of 509 ischemic stroke cases (202 cardioembolic [40%]) and 3,028 referents. Scores were based on 11 to 719 common variants (≥5%) associated with AF at P-values ranging from <1x10-3 to <1x10-8 in a prior independent genetic association study. Results—Incident AF occurred in 1,032 (5.5%) individuals. AF genetic risk scores were associated with new-onset AF after adjusting for clinical risk factors. The pooled hazard ratio for incident AF for the highest versus lowest quartile of genetic risk scores ranged from 1.28 (719 variants; 95%CI, 1.13-1.46; P=1.5x10-4) to 1.67 (25 variants; 95%CI, 1.47-1.90; P=9.3x10-15). Discrimination of combined clinical and genetic risk scores varied across studies and scores (maximum C statistic, 0.629-0.811; maximum ΔC statistic from clinical score alone, 0.009-0.017). AF genetic risk was associated with stroke in age- and sex-adjusted models. For example, individuals in the highest versus lowest quartile of a 127-variant score had a 2.49-fold increased odds of cardioembolic stroke (95%CI, 1.39-4.58; P=2.7x10-3). The effect persisted after excluding individuals (n=70) with known AF (odds ratio, 2.25; 95%CI, 1.20-4.40; P=0.01). Conclusions—Comprehensive AF genetic risk scores were associated with incident AF beyond associations for clinical AF risk factors, though offered small improvements in discrimination. AF genetic risk was also associated with cardioembolic stroke in age- and sex-adjusted analyses. Efforts are warranted to determine whether AF genetic risk may improve identification of subclinical AF or help distinguish between stroke mechanisms

Challenges in quantifying changes in the global water cycle

Human influences have likely already impacted the large-scale water cycle but natural variability and observational uncertainty are substantial. It is essential to maintain and improve observational capabilities to better characterize changes. Understanding observed changes to the global water cycle is key to predicting future climate changes and their impacts. While many datasets document crucial variables such as precipitation, ocean salinity, runoff, and humidity, most are uncertain for determining long-term changes. In situ networks provide long time-series over land but are sparse in many regions, particularly the tropics. Satellite and reanalysis datasets provide global coverage, but their long-term stability is lacking. However, comparisons of changes among related variables can give insights into the robustness of observed changes. For example, ocean salinity, interpreted with an understanding of ocean processes, can help cross-validate precipitation. Observational evidence for human influences on the water cycle is emerging, but uncertainties resulting from internal variability and observational errors are too large to determine whether the observed and simulated changes are consistent. Improvements to the in situ and satellite observing networks that monitor the changing water cycle are required, yet continued data coverage is threatened by funding reductions. Uncertainty both in the role of anthropogenic aerosols, and due to large climate variability presently limits confidence in attribution of observed changes

Reduced level of arousal and increased mortality in adult acute medical admissions: a systematic review and meta-analysis

Abstract Background Reduced level of arousal is commonly observed in medical admissions and may predict in-hospital mortality. Delirium and reduced level of arousal are closely related. We systematically reviewed and conducted a meta-analysis of studies in adult acute medical patients of the relationship between reduced level of arousal on admission and in-hospital mortality. Methods We conducted a systematic review (PROSPERO: CRD42016022048), searching MEDLINE and EMBASE. We included studies of adult patients admitted with acute medical illness with level of arousal assessed on admission and mortality rates reported. We performed meta-analysis using a random effects model. Results From 23,941 studies we included 21 with 14 included in the meta-analysis. Mean age range was 33.4 - 83.8 years. Studies considered unselected general medical admissions (8 studies, n=13,039) or specific medical conditions (13 studies, n=38,882). Methods of evaluating level of arousal varied. The prevalence of reduced level of arousal was 3.1%-76.9% (median 13.5%). Mortality rates were 1.7%-58% (median 15.9%). Reduced level of arousal was associated with higher in-hospital mortality (pooled OR 5.71; 95% CI 4.21-7.74; low quality evidence: high risk of bias, clinical heterogeneity and possible publication bias). Conclusions Reduced level of arousal on hospital admission may be a strong predictor of in-hospital mortality. Most evidence was of low quality. Reduced level of arousal is highly specific to delirium, better formal detection of hypoactive delirium and implementation of care pathways may improve outcomes. Future studies to assess the impact of interventions on in-hospital mortality should use validated assessments of both level of arousal and delirium

Drug-gene interactions of antihypertensive medications and risk of incident cardiovascular disease: a pharmacogenomics study from the CHARGE consortium

Background Hypertension is a major risk factor for a spectrum of cardiovascular diseases (CVD), including myocardial infarction, sudden death, and stroke. In the US, over 65 million people have high blood pressure and a large proportion of these individuals are prescribed antihypertensive medications. Although large long-term clinical trials conducted in the last several decades have identified a number of effective antihypertensive treatments that reduce the risk of future clinical complications, responses to therapy and protection from cardiovascular events vary among individuals. Methods Using a genome-wide association study among 21,267 participants with pharmaceutically treated hypertension, we explored the hypothesis that genetic variants might influence or modify the effectiveness of common antihypertensive therapies on the risk of major cardiovascular outcomes. The classes of drug treatments included angiotensin-converting enzyme inhibitors, beta-blockers, calcium channel blockers, and diuretics. In the setting of the Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE) consortium, each study performed array-based genome-wide genotyping, imputed to HapMap Phase II reference panels, and used additive genetic models in proportional hazards or logistic regression models to evaluate drug-gene interactions for each of four therapeutic drug classes. We used meta-analysis to combine study-specific interaction estimates for approximately 2 million single nucleotide polymorphisms (SNPs) in a discovery analysis among 15,375 European Ancestry participants (3,527 CVD cases) with targeted follow-up in a case-only study of 1,751 European Ancestry GenHAT participants as well as among 4,141 African-Americans (1,267 CVD cases). Results Although drug-SNP interactions were biologically plausible, exposures and outcomes were well measured, and power was sufficient to detect modest interactions, we did not identify any statistically significant interactions from the four antihypertensive therapy meta-analyses (Pinteraction > 5.0×10−8). Similarly, findings were null for meta-analyses restricted to 66 SNPs with significant main effects on coronary artery disease or blood pressure from large published genome-wide association studies (Pinteraction ≥ 0.01). Our results suggest that there are no major pharmacogenetic influences of common SNPs on the relationship between blood pressure medications and the risk of incident CVD

Layer-by-layer deposition of open-pore mesoporous TiO 2- Nafion® film electrodes

The formation of variable thickness TiO2 nanoparticle-Nafion® composite films with open pores is demonstrated via a layer-by-layer deposition process. Films of about 6 nm diameter TiO2 nanoparticles grow in the presence of Nafion® by “clustering” of nanoparticles into bigger aggregates, and the resulting hierarchical structure thickens with about 25 nm per deposition cycle. Film growth is characterized by electron microscopy, atomic force microscopy, and quartz crystal microbalance techniques. Simultaneous small-angle X-ray scattering and wide-angle X-ray scattering measurements for films before and after calcination demonstrate the effect of Nafion® binder causing aggregation. Electrochemical methods are employed to characterize the electrical conductivity and diffusivity of charge through the TiO2-Nafion® composite films. Characteristic electrochemical responses are observed for cationic redox systems (diheptylviologen2+/+, Ru(NH3)3+/2+6, and ferrocenylmethyl-trimethylammonium2+/+) immobilized into the TiO2-Nafion® nanocomposite material. Charge conduction is dependent on the type of redox system and is proposed to occur either via direct conduction through the TiO2 backbone (at sufficiently negative potentials) or via redox-center-based diffusion/electron hopping (at more positive potentials)