No description available.

No description available.

No description available.

The work described in this thesis employs 'dual activation' methodologies and in particular sonovoltammetry to study a variety of electrochemical systems. First, the effect of 20 kHz power ultrasound on the electrochemistry of simple redox systems in both water and acetonitrile is explored and characterised for different cell geometries. A simple Nernst diffusion layer model is validated. Second, the use of insonation for efficient pre-concentration of target species in anodic stripping voltammetry (ASV) is reported. 'Sonotrodes', where the electrodes are incorporated into the ultrasonic horn tips are shown to offer particularly high rates of mass transport and additionally cause ablation of mercury from plated platinum electrodes. The key benefits of sonication are shown to be electrode depassivation and extraction plus rapidity and ease of determination, coupled with the lack of any sample pre-treatment for the detection of lead in both wine and petrol by ASV. Lead levels determined by this method and calibrated by use of standard microaddition are in excellent agreement with those obtained 'blind' by atomic absorption spectroscopy performed at independent laboratories. Third, the novel use of ultrasonically formed emulsions is examined for both analytical and synthetic purposes. The possibility of extracting species both out of an organic phase for analysis and into an organic phase for the synthesis of water-insoluble redox products is realised. Fourth, the sonovoltammetric determination of ascorbic acid in both aqueous solution and the fruit drink Ribena®, is compared with results obtained by laser activated electroanalysis, where low level laser illumination of an electrode is employed to maintain a fresh electrode surface. Small amounts of laser-induced thermal convection at the electrode result in steady-state voltammetry. The level of agreement is excellent between the two methods and also agrees very well with independent chemical analysis. Fifth, laser activated voltammetry (LAV) is further explored and a simple mass transport model verified. Damage caused by high power laser ablation is evidenced by atomic force microscopy (AFM). Applications of the technique for depassivation are illustrated by the reduction of toluidine blue dye and oxidation of ferrocyanide in the presence of blood proteins. The technique is further employed under channel flow conditions to elucidate an unambiguous mechanism for aqueous iodide oxidation at platinum electrodes in the absence of the usual build up of bulk iodine (as evidenced by AFM). Finally, the reduction of methylene green dye at platinum electrodes is used to compare and contrast sonovoltammetry and LAV with thermal and microwave dual activation techniques. The build-up of the passivating reduced form of the dye on the electrode surface, again seen by AFM, is removed by sono-emulsion and laser activation but not by microwave heating. However, greater conductivity is observed at higher temperatures and an electron-hopping mechanism is postulated.

This thesis describes the development of an electroanalytical technique for the estimation of compounds of biochemical interest. The technique involves titration with hypobromite using double electrode systems - the rotating ring disc electrode (RRDE) and the flow-through tubular double electrode (TDE). Hypobromite is continuously electregenerated at the upstream disc (generator) electrode and is transported by convection and diffusion to the downstream ring (detector) electrode where it is detected amperometrically. The presence of a reactive substrate in solution decreases the amount of titrant which reaches the ring, and from measurements of the generating and detecting currents the bulk concentration of substrate may be estimated. At pH 9.2 the amine group of amino acids reacts with two hypobromite molecules and some side chains (e.g. cystinyl, tyrosinyl, tryptophanyl) will also react. The reactivity of these side chains permits the titration of proteins. The titration response may be described theoretically which enables the calculation of the number of hypobromite molecules which react with one protein molecule ( ~500 for haemoglobin). With this chemical amplification the detection limit of the technique is ~10^-8 g ml^-1 for proteins and ~10^-8 M for amino acids. An electronic circuit has been developed which enables the detector electrode to control the rate at which titrant is generated. This autotitrator produces a generator current which is proportional to substrate concentration, and with the TDE should enable the continuous estimation of proteins as they are eluted from a chromatographic column. Proteins have been titrated at pH 9.2 and pH 5; the ratio of a protein's titration responses reflects its amino acid composition and, when combined with the extinction coefficient, enables the identification of proteins. Patent applications for this method of identification and for the autotitration technique have been submitted.

The work described in this thesis deals with the use of (^29)Si and (^27)Al NMR to obtain information about the chemical structure of aqueous silicate and aluminosilicate solutions. This has extended the knowledge gained in previous studies. A wide range of alkaline and tetraalkylammonium hydroxide silicate and aluminosilicate solutions (mostly also containing sodium) has been examined. Such solutions are shown to contain a large range of anions. The first highly-resolved (^27)Al NMR spectra of alkaline aluminosilicate solutions are presented and discussed. The linewidths and number of resolved lines are shown to depend critically on several factors, especially the pH and Si:Al ratio, as well as the concentration of various components. At least thirteen separate peaks or bands are observed at the optimum conditions of pH~10.35 and Si:Al = 1. Tentative assignments of some bands are presented, based on (^27)Al and (^29)Si shift omparability, (^27)Al linewidths and (^27)Al spin-lattice relaxation measurements. Relative intensities of the various (^27)Al signals are given for the pH ~10.35 solution. A correlation of (^27)A1 and (^29)Si chemical shifts has been established. Dilution effects and dynamic equilibria are considered for the aluminosilicate solutions. Trends in the spectra with changing concentrations of the various components have been identified. Under certain conditions, additional peaks are observed. Two of these are tentatively assigned to aluminium nuclei in "three-membered" rings. Moreover, a few very sharp hues (i.e. with very low electric field gradients), currently of unknown origin, are observed in several spectra. At certain concentrations chemical exchange can be shown to take place. The effects of tetraalkylammonium (TAA) and alkali metal cations on the equilibrium distribution of aluminosilicate oligomers in aqueous alkaline aluminosilicate solutions have been investigated using evolution with time of Al-27 NMR spectra. The results indicate that there are no observable differences in the initial distribution of aluminosilicate species (i.e. immediately following solution mixing) involving a mixture of TAA and Na cations and those involving alkali metal cations alone. However, in the latter case, this distribution, in contrast to those for TAA/Na solutions, is not stable, the species quickly re-equilibrating, giving broad signals of q(^3) and perhaps q(^4) type. The effect of Al and Si concentration on the formation of gel in?aluminosilicate solutions is also investigated with ^^Al NMR spectra. It is shown that the gel time strongly depends on the Al concentration and the temperature. A graph of Al and Si concentrations with gel time has been established.Silicon-29 NMR spectra have been obtained for more than 20 aqueous alkaline silicate solutions containing methanol. A signal assigned to CH(_3)OSi(OH)(_3) or one of its deprotonated congeners is studied in detail for the first time for the solution conditions involved. Its appearance has been monitored as a function of the solution composition. The equilibrium constant for its formation is of the order of 0.65. The effects of alcohol, silicate and TAAOH concentration and of the nature of the alkylammonium base on this reaction have been investigated.

Increasing concern about the rapid escalation of environmental pollution has led to strong legislation to ensure, for example, that the emission of pollutants from vehicles and industries is controlled to an acceptable level. As a consequence, there has been a rapid expansion of research into developing more efficient and low-cost gas monitoring systems. Currently, commercial solid-state atmospheric gas detection systems are based on one sensor for each gas, while research systems are an array of sensors for the detection of multiple gases. In this research, techniques are developed whereby more than one gas is detected using a single resistive gas sensor. A novel modulated temperature technique was used to enhance the selectivity of the resistive SnO2 gas sensor. Fast Fourier transforms was used to extract the Fourier coefficients. These in turn were used as input to neural networks for training and subsequently for prediction purposes. The result has shown that a single doped SnO2 resistive microsensor can be used to classify binary gas mixture in air. The research objectives have been fulfilled in that a novel way in detecting the components and the concentration level of a binary gas mixture was developed. Additionally, a low-cost low-power intelligent gas monitoring system was designed. This included the design of a novel temperature/thermometer circuit.

No description available.

A series of fundamental studies are presented of the components of a prototypical electrochemical quartz crystal microbalance (EQCM)-based sensor for metal ions. Designed for use in the nuclear industry, the sensor consists of a gold (Au) piezoelectrode modified by a Nafion® polymer layer impregnated with a cationic hydroxainic acid (HA), a ligand showing oxidation state specific selectivity towards actinide 4+ ions and a non-radioactive demonstrator ion, Fe3+ In its final configuration, the polymer layer will also contain a redox mediator, methylene blue (MB), to facilitate electrochemical conditioning of the analyte. The following results have been obtained from EQCM and microelectrode voltammetry studies of each of the component parts. The Gold Piezoelectrode - The dissolution and redeposition processes of Au in HNO3 solution at pH I & 0 were investigated as a function of sweep rate (v). At pH I and v < 10 mV s-1 , Au dissolves through a 3e’ oxidation at E > +1.00 V (vs. SCE), a process that, at v ≤ 10 mV s-1, is kinetically hindered with respect to under potential deposition (UPD) of OH surface sublattices and subsequent 2-D-phase oxide formation. The inhibition arises from a closed packed layer of nitrate ions at the gold surface, although the layer is disrupted and the inhibition removed by the gold dissolution and reprecipitation processes that occur during potential cycling at low v. The Hydroxamic Acid Ligand - Study of the oxidation of acetohydroxamic acid (AHA) in nitric acid revealed that three processes occur at Au electrodes at potentials in the vicinity of AHA oxidation at pH 0 & I: (a) oxidation of the Au surface; (b) oxidation of the AHA; (c) complexation of the AHA with electrogenerated Au3+ . The last process inhibits oxide formation and associated electrode passivation, so promoting the occurrence of the first two. The two current peaks associated with AHA oxidation show no associated mass change, indicating that AHA oxidation is controlled by semi-infinite diffusion. The waves correspond to I and 2 electron processes with current onsets of +0.55 V and ̴̴̴+0.80 V (vs. SCE) respectively. Oxidation of AHA appears to be irreversible and blocked at electrodes with minimal microscopic roughness by close packing of nitrate ions at the surface. The Polymer Layer and the Demonstrator Ion, Fe 3+ - Microelectrogravimetric studies of the Fe2+/3+ couple were conducted at bare Au, Nafion® modified Au and Nafion® modified Au impregnated with the hydrophobic cationic HA ligand, desferrioximine (DFA+) electrode. In background nitric acid (pH 1 & 0), the electrochemistry of Nafion ® layer partitioned Fe 2+/3+ ions exhibits thin layer cell behaviour with the polymer layer increasing in mass during reduction of Fe(III) due to an influx of highly hydrated H+ ions necessary to maintain electroneutrality. Studies on Nafion® layers impregnated with DFA+ and Fe3+ show that while Fe3+ complexes with DFA+, Fe2+ does not and that, upon reduction of the complexed ferric ion, the resultant Fe(II)-DFA+ complex dissociates suggesting that electrochemical expulsion of the analyte is possible. The Redox Mediator - At pH 7.9, the reduction of methylene blue (MB) to leucomethylene blue (LMB), occurs in two steps: (i) a reversible net 1.5 e transfer to form a solid charge transfer complex (LMB/LMB+) (ii) an irreversible reduction of LMB/LMB+ to LMB, the onset of which can be directly observed as an inflection in the voltamassogram, the first time that this has been reported. The reversibility of the MB to LMB reduction within the context of charge transfer complex electrogeneration suggests that MB may be a suitable mediator for actinide cation reduction within the polymer layer of the proposed sensor. It is known that the pendular amino-groups of LMB are protonated below pH 1, so rendering the resultant LMBH22+ soluble in aqueous solution. This means (a) difficulties associated with the irreversibile reoxidation of the solid LMB generated at pH 7.9 will be obviated at pH I; and (b) being a large cation, we could expect LMBH22+ to be retained within the Nafion® layer.

The aim of this study was to exploit the sensitivity of the chemiluminescent ruthenium tris(2,2'-bipyridme) redox system for analytical purposes by producing electrochemically regenerable electrodes modified with a polymer derivative of this complex. Ruthenium tris(4-meth)i-4' -vinyl-2,2' -bipyridine) bis(hexafluorophosphate) was synthesised and subsequently polymerised onto an electrode surface using the technique of cyclic voltammetry. A potential was applied to the resulting Chemically Modified Electrode (CME) and upon reaction of the ruthenium centres with an analyte in solution, chemiluminescence was observed. Measurement of the emission intensity was used for the determination of the analyte concentration The ruthenium centres were subsequently regenerated by the reversal of the applied potential Very littie environmentally unfriendly and expensive waste results from this process. This is in direct contrast with current methods which traditionally use the ruthenium tris(2,2'-bipyndine) complex either in solution or immobilised within a membrane for analyses of this type. Attention was focused upon detection of the oxalate ion and the ruthenium tris(2,2'-bipyridine)/oxalate redox system reported in the literature was used as a model to investigate the capabilities of the CME's produced. A scan rate of 10 V/s between the potential limits of + 0.5 to + 1.5 V at pH 6.5 was established as the optimum conditions. A linear working range for oxalate was observed from 1.9 x 10'^ M to the limit of detection, 1.1 x 10'^ M. The lifetime of a CME was investigated and although the chemiluminescent signal diminished with time, the electrode was still functioning after 24 weeks, a total of over 200 regenerations. A series of amines and valine were also successfully detected using CME's. In addition, a similar sensor was fabricated from the electropolymerization of a novel ruthenium complex, ruthenium tris(4-methyl-4'-(£-prop-2-enyl>2,2'-bipyridine) bis(hexafluorophosphate). This was also shown to be capable of chemiluminescence emission and was successfully regenerated Two electrochemically regenerable CME's were therefore produced and this research is a valuable advance in the field of chenuluminescent detectors.