This thesis describes the study of various grating based optical fibre sensors for applications in refractive index sensing. The sensitivity of these sensors has been studied and in some cases enhanced using novel techniques. The major areas of development are as follows. The sensitivity of long period gratings (LPGs) to surrounding medium refractive index (SRI) for various periods was investigated. The most sensitive period of LPG was found to be around 160 µm and this was due to the core mode coupling to a single cladding mode but phase matching at two wavelength locations, creating two attenuation peaks, close to the waveguide dispersion turning point. Large angle tilted fibre gratings (TFGs) have similar behaviour to LPGs, in that they couple to the co-propagating cladding modes. The tilted structure of the index modulation within the core of the fibre gives rise to a polarisation dependency, differing the large angle TFG from a LPG. Since the large angle TFG couple to the cladding mode they are SRI sensitive, the sensitivity to SRI can be further increased through cladding etching using HF acid. The thinning of the cladding layer caused a reordering of the cladding modes and shifted to more SRI sensitive cladding modes as the investigation discovered. In a SRI range of 1.36 to 1.40 a sensitivity of 506.9 nm/URI was achieved for the etched large angle TFG, which is greater than the dual resonance LPG. UV inscribed LPGs were coated with sol-gel materials with high RIs. The high RI of the coating caused an increase in cladding mode effective index which in turn caused an increase in the LPG sensitivity to SRI. LPGs of various periods of LPG were coated with sol-gel TiO2 and the optimal thickness was found to vary for each period. By coating of the already highly SRI sensitive 160µm period LPG (which is a dual resonance) with a sol-gel TiO2, the SRI sensitivity was further increased with a peak value of 1458 nm/URI, which was an almost 3 fold increase compared to the uncoated LPG. LPGs were also inscribed using a femtosecond laser which produced a highly focused index change which was no uniform throughout the core of the optical fibre. The inscription technique gave rise to a large polarisation sensitivity and the ability to couple to multiple azimuthal cladding mode sets, not seen with uniform UV inscribed gratings. Through coupling of the core mode to multiple sets of cladding modes, attenuation peaks with opposite wavelength shifts for increasing SRI was observed. Through combining this opposite wavelength shifts, a SRI sensitivity was achieved greater than any single observed attenuations peak. The maximum SRI achieved was 1680 nm/URI for a femtosecond inscribed LPG of period 400 µm. Three different types of surface plasmon resonance (SPR) sensors with a multilayer metal top coating were investigated in D shape optical fibre. The sensors could be separated into two types, utilized a pre UV inscribed tilted Bragg grating and the other employed a post UV exposure to generate surface relief grating structure. This surface perturbation aided the out coupling of light from the core but also changed the sensing mechanism from SPR to localised surface plasmon resonance (LSPR). This greatly increased the SRI sensitivity, compared to the SPR sensors; with the gold coated top layer surface relief sensor producing the largest SRI sensitivity of 2111.5nm/URI was achieved. While, the platinum and silver coated top layer surface relief sensors also gave high SRI sensitivities but also the ability to produce resonances in air (not previously seen with the SPR sensors). These properties were employed in two applications. The silver and platinum surface relief devices were used as gas sensors and were shown to be capable of detecting the minute RI change of different gases. The calculated maximum sensitivities produced were 1882.1dB/URI and 1493.5nm/URI for silver and platinum, respectively. Using a DFB laser and power meter a cheap alternative approach was investigated which showed the ability of the sensors to distinguish between different gases and flow rates of those gases. The gold surface relief sensor was coated in a with a bio compound called an aptamer and it was able to detect various concentrations of a biological compound called Thrombin, ranging from 1mM to as low as 10fM. A solution of 2M NaCl was found to give the best stripping results for Thrombin from the aptamer and showed the reusability of the sensor. The association and disassociation constants were calculated to be 1.0638×106Ms-1 and 0.2482s-1, respectively, showing the high affinity of the Aptamer to thrombin. This supports existing working stating that aptamers could be alternative to enzymes for chemical detection and also helps to explain the low detection limit of the gold surface relief sensor.

This thesis describes novel methods for the determination of selenium at the ultratrace level. The introduction reviews the techniques currently employed to determine selenium, and outlines the need for a highly sensitive and precise analytical method. Chapters two and three describe the determination of selenium by differential pulse polarography and differential pulse cathodic stripping voltammetry respectively. The application of these methods to the analysis of soils and plant materials is also reported. The determination of selenium by microwave plasma emission spectroscopy is described in chapter four. However, because emission spectroscopy was not sufficiently sensitive for the low level determination of selenium, electrodeposition and ion exchange techniques were investigated for the preconcentration of selenium. The results of this work are presented in chapter five. Finally, the combination of electrodeposition and microwave plasma emission spectroscopy for the low level determination of selenium is reported.

Following a discussion of -the theoretical concepts involved, the effects of pH and interference of citrate or tartrate in EDTA titrations are. computed from published stability constant data for twenty two common metal cations. Serious interference caused by the simultaneous presence of chromium(ll'l) and tartrate during the titration of cobalt(ll), copper(ll), zinc(ll) and cadmium(ll) with EDTA is attributed to formation of ternary (heteronuclear) tartrate complexes containing chromium(lll) and bivalent metal ion in the ratio 1:1. The properties and conditions for formation of the complexes are described. Among the anomalies described for heteronuclear chromium(lll) citrate complexes formed with bismuth(III), cerium(lll), manganese(ll), tin(n) or lead(ll) is the non-additivity in absorbance in the visible spectra. A structural model for the heteronuclear complexes is advanced to account for the enhanced absorbances and titrimetric errors. Evidence is presented for the existence of many other heteronuclear chromium(lll) citrates and tartrates, and the formation of heteronuclear hydroxycarboxylate complexes is reviewed. Their importance as interferences in analytical chemistry is emphasised. Problems encountered in the complexometric and redox standardisations of chromium(III) solutions are discussed, including formation of inert sulphato complexes and loss of chromium by volatilisation. The novel extraction of chromium(lll) by the high - capacity extractant octanoic acid is described, but applications are severely limited by interference from iron(lll) and nickel(ll). Copper in brass is determined spectrophotoraetrically with citrate, the interference of a heteronuclear copper(ll) citrate complex vath zinc(ll) being counteracted in the preparation of the standards. The fluorimetric determination of traces of succinic acid by synthesis of succinylfluorescein is assessed and modified, and the extension of similar methods to other carboxylic acids is described. The protonation equilibria of succinylfluorescein are investigated.

This thesis describes the experimental investigations concerning the integration and optimisation of a supercontinuum source into a fluorescence lifetime spectrometer. [Fluorescence lifetime spectrometers based on Time Correlated Single Photon Counting (TCSPC) have long suffered from the lack of a compact, broadband excitation source. The source should ideally emit picosecond pulses with a repetition rate adjustable up to megahertz. Supercontinuum sources are an ideal candidate.] Various commercial supercontinuum sources were evaluated. Initial work was carried out on a source with a standard endlessly single-mode photonic crystal fibre (PCF) with a short wavelength limit of 450 nm, followed by later work with a source using high- PCF (a PCF with a large air filling fraction), that had an emission down to 400 nm, and a prototype source with a tapered fibre that had emission to ~320 nm. Key parameters including pulse width, pulse position and pulse height distribution were found to be very wavelength dependent, and their behaviour is explained by theory. The measured pulse widths of the supercontinuum sources were found to be typically ~100 ps, with longer durations found at the blue extreme of the spectra. Analysis of the data showed that this was due to the broadened pulses being a superposition of two pulse sequences with different dispersion characteristics. It was shown that, by taking account of the particular optical and temporal properties of a supercontinuum source, it was possible to make high quality fluorescence lifetime measurements of standard fluorophores such as fluorescein, anthrascene and erythrosine B. A novel device was constructed and evaluated for the wavelength separation of a supercontinuum source based on wedge interference filters. Initial prototypes of the device were able to measure the fluorescence emission spectra of common fluorophores and adequately separate a supercontinuum. Further iterations of the design, employing multiple filters, allowed for the construction of a device that included bandwidth control. The device allowed the transmission bandwidth to be tuned from ~6 nm to >50 nm with a transmission of >70 % for bandwidths >8 nm. The transmission figures achieved are better than any alternative form of wavelength separation, for example devices based on acousto-optic tunable filters (AOTFs) and diffraction gratings. A novel monochromator for fluorescence studies was also constructed using a wedge interference filter simultaneously with a diffraction grating. The design had improved performance compared to a single grating based monochromator in terms of stray light, with only a small drop in throughput and no change in instrument footprint. The stray light performance was found to be comparable to that of a double monochromator over the spectral range of the filter.

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No description available.

No description available.

The synthesis of N-substituted propionyl- and phenylacetyl- substituted chiral moieties based on the oxazolidin-2-one or 1,3-oxazin-2-one ring systems has been acheived via the use of a variety of different reagents and bases. A ¹H NMR spectroscopic study of the prochiral protons Ha and Hb in the methylene group of these systems has been undertaken at 200, 250 and 360 MHz with the aim of determining the extent, if any, to which the diastereotopicity imparted by the chiral auxiliary in a specific asymmetric transformation is reflected in the NMR parameters. For the N-propionyl derivatives the coupling patterns of the prochiral methylene protons Ha and Hb are almost the same, viz, a doublet of quartets for each proton. The chemical shifts and the coupling constants are very similar and fall in the range 2.63-3.03 ppm, and 16.5-18.3 Hz for J_HaHb and 7.3±0.2 Hz for J_Ha-Hb/Me. The coupling patterns for Haand Hb of the Nphenylacetyl derivatives of most of these chiral auxiliaries are pairs of doublets with a coupling constant JHaHb in the range 15.5-16.2 Hz, whilst some of them have the same chemical shift and therefore appear as a singlet. In these systems no correlation was observed between the diastereotopicity imparted by the chiral auxiliary as measured by the stereochemical outcome for the aldol reactions (and Diels-Alder reactions for the corresponding acrylate derivatives) in terms of diastereomeric excess (d.e.%) and the chemical shift separation of the diastereotopic protons. Attempts to distinguish between the prochiral methylene protons Ha and Hb in the N-propionyl derivatives of the different auxiliaries by ¹H NMR nOe difference spectra failed apart from that based on the bornyl moiety. For this purpose, its titanium-derived complex was prepared by treatment of the N-propionyl derivative with titanium tetrachloride in deuteriated chloroform. Compared to the freely rotating system, both Ha and Hb in the complex had distinct chemical shifts and appeared as a doublet of quartets, which were well separated from each other. In addition, the signals were shifted to higher frequency relative to the free propionate, indicative of electron donation to the titanium. A comparison of the chiral efficiency of both the carbohydrate- and terpenoid-derived auxiliaries found that the measured levels of asymmetric induction imparted by lithium enolate systems, which are generated via lithium diisopropylamide, were markedly different and in favour of the former presumably due to chelation control by ketal and/or pyranose ring oxygens. On the other hand, much higher levels of induction are measured with boron enolates derived from terpenoid-based auxiliaries with di-n-butylboron triflate (Bu2130Tf), and moreover, can bring about the opposite sense of induction compared to the lithium enolate systems. These aspects of stereocontrol are discussed in terms of different transition states. In the presence of excess Bu2BOTf used to promote anti-selectivity in asymmetric aldol reactions with benzaldehyde, a homochiral terpenoid-derived I ,3-oxazin-2-one resulted unexpectedly in ring cleavage by an intramolecular process to form a N-substituted tetrahydro- I ,3-oxazine-2,4-dione in virtually quantitative yield. Mechanistic aspects of this unusual reaction are discussed.

This thesis reports the voltammetric applications and fundamental frequency-dependent properties of carbon-based electrode materials. A range of electrochemical systems hasve been investigated, and new materials have been electrochemically characterised, which will be of use to the field of electrochemistry. In Chapter 3 of this thesis, different graphenes were utilised as electrode composite materials, and their electrochemical behaviour was de-convoluted. It was found that surfactant-free graphenes were useful for the detection of guanine in terms of a reduced activation potential, which is thought to be derived from a pi-pi adsorption mechanism. The oxygen reduction reaction was also focussed upon and it was found that the type of graphene utilised did not affect the electrochemical mechanism in the respective reactions, but the peroxide yields changed. This could have dramatic ramifications for users choosing carbon materials as catalyst supports. Screen-printed electrodes were applied to novel systems including theophylline and creatinine, finding that their use as portable sensors was viable in two ways. For theophylline, a direct oxidation mechanism was useful for the detection of the medicine, while in the case of creatinine, an indirect detection method was found to be effective as creatinine is not electrochemically active. In Chapter 5, the first graphene screen-printed electrodes were developed and characterised. The result was two graphene screen-printed electrodes, with differing electrochemical properties, both of which could be used for different applications. Finally, Chapter 6 focusses upon whether electrochemical impedance spectroscopy is useful for screen-printed electrodes and carbon modifications. The work in this thesis finds that a synergy could potentially be formed, and in particularly, has found that it would be wise to operate screen-printed electrodes around +0.2 V due to this being the point where there is no net charge at the electrode surface under standard conditions.

No description available.