Physical sciences; Chemistry
Thesis or dissertation
- © 2009 Carlos Lledo-Fernandez. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.
This thesis discusses and demonstrates the utility and advantages of redox catalytic reactions and small scale synthetic applications. In particular electro-catalytic reactions are investigated along with electrogenerated chemiluminescence (ECL). ECL is used both as a probe for investigating the redox reactions and as an analytical method. The first chapter reviews the current techniques and achievements found in the area of dynamic electrochemistry and electrogenerated chemiluminescence and discusses occurrence, mechanism and potentially useful reactions worthy of further investigation for analytical applications.
In Chapter 3 the electrocatalytic ECL reaction of Ru(bpy)32+ with tertiary amines in particular with codeine as a model compound was investigated. The aim of the work was to develop a portable drug testing device, that gave good sensitivity and reproducibility. The Ru(bpy)32+ was immobilized within the sol-gel matrix to develop a sensor which could be used directly for measuring tertiary amine containing drugs in buffer solution without the need to add reagents. A system of three electrodes was used in which Ru(bpy)32+ was immobilised on to a glassy carbon working electrode. Initial work involved physical entrapment of the Ru(bpy)32+ within the sol-gel matrix however, problems occurred with leaching of the reagent from this matrix. This problem was overcome by the covalent attachment of a novel Ru(bpy)32+ derivative to the sol-gel matrix. Using this approach a calibration was obtained using ECL for the determination of codeine over the range 1E-3 to 1E-7 M in aqueous buffer with a limit of detection of 2.65E-6 M for codeine. Covalent attachment, as compared to the physical entrapment of CL reagent, was advantageous as it ensured homogeneous distribution of the reagent within the matrix and prevented leaching. This reduced analysis costs, extended sensor lifetime and gave a reproducible analyte responses. This method was, however, only suitable for drugs that were soluble in aqueous solution and an alternative approach was needed for insoluble compounds.
Chapter 4 describes an alternative approach for ECL reaction of tris(2,2’-bipyridyl)ruthenium(II) with tertiary amine compounds. In this approach the electrode was modified using microdroplets of a highly hydrophobic tertiary amine (trioctyamine). As well as allowing for the analysis of compounds that were insoluble in aqueous solution, this setup allowed the investigation of the electron transfer reaction occurring on liquid|liquid interfaces. This mechanism was studied in fully protonated and deprotonated conditions. The extent of the electrochemiluminescence production was shown to be dependent on the degree of the interfacial protonation. Moreover, the
data obtained enabled the estimation of the biphasic pKa, which was found to be approximately 10.8. Furthermore, the mechanism was studied in fully deuterated and dedeuterated conditions, in order to investigate the effect of the deuterium on the biphasic pKa. Results suggest that the pKa increases to 13.18 when deuterium is used instead of protons.
In Chapter 5 a further electrocatalytic reaction was investigated. The reaction was that of vitamin B12a with trans 1,2-dibromocyclohexane (DBCH) in a homogeneous dimethylformamide media. The reaction was studied by cyclic voltammetry. Four peaks were seen due to the two chemically reversible redox (two electron process) couples for vitamin B12a/B12r (Co(III)/CO(II)L) and B12r/B12s (Co(II)L/Co(I)). When the bulk electrocatalytic reaction was tried in a “one pot” system, the reduction could not, however, be achieved; the cathodically synthesised Co(I)L was thought to be reoxidised at the anode. A “two pot” system did not have sufficient potential control and, therefore, chemical reduction was investigated instead.
Four reducing agents (Na/amalgam, NaBH4/NaOH, DL-cysteine/alkali solution and Zn dust/NH4CL) were studied to reduce B12 to B12s for a simple biphasic batch reaction of vitamin B12s with DBCH. The mild reducing agent Na/amalgam was not successful but the other three methods were shown to give 100% yield if the reaction vials were rigorously shaken. This simple type of green, surfactant free reaction has not been previously reported.
The reaction was then investigated in a microfluidic system. For this work the vitamin B12 was reduced before being introduced into the microfluidic device. NaBH4/NaOH and DL-cysteine/alkali solution were selected as the most compatible reducing agents for the microfluidic device. Two types of microfluidic device were employed, one with a T-shape channel, and one with a serpentine channel. The conversions obtained with the microfluidic device were much lower (approximately 10%) than for the simple batch reactions (100%). The yield increased as the flow rate decreased, and the residence time increased. Using the serpentine channel made no noticeable difference to the conversion rate. Problems were also seen due to the use of excess reducing agent prior to introduction of the reduced vitamin B12 into the microfluidic device, as this could cause blockages and bubbles. The main problem was, however, the lack of mixing in the device. One way to overcome this would be to use an ultrasonic transducer with the microfluidic device but although preliminary experiments were carried out, there was not time to fully investigate this approach.
- Department of Physical Sciences, The University of Hull
- Greenway, Gillian M.; Wadhawan, Jay
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