Calixarenes as potential lonophores for Thallium lonselective electrodes
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The first part of this thesis reports the synthesis of five calixarene molecules and their incorporation into ISEs as thallium(I) selective ionophores for use in clinical and environmental analysis. Four calixarene molecules were successfully synthesized. Two were isolated in the cone conformation with iso-propyl groups attached to the lower rim. The other two were in the 1,3-alternate conformation with allyl groups attached to the phenolic oxygen. The final calixarene synthesized was a calixarene that was partially substituted with iso-propyl groups on the lower rim. The parallel orientation of the aromatic rings was confirmed using single crystal X-ray structure determination. Nuclear Magnetic Resonance (NMR) complexation studies were conducted on the four calixarene derivatives and showed that thallium binds within the aromatic cavity, and that the complexation/decomplexation kinetics and molecule flexibility are affected by the incorporated ion and the attached groups on the calixarene. Once the binding characteristics of the synthesized calixarenes had been examined they were then incorporated into plasticized PVC together with an ion-exchanger to make ISEs capable of determining thallium activities in solutions. Three calixarenes were successfully incorporated into the membranes and produced Nernstian responses over the concentration range 10-2-10-6 M TlNO3. The lower detection limit of the above electrodes lies in the micro-molar range, which is a common characteristic of the experimental setup used. Since the electrodes behaved ideally with respect to thallium(I), it was then decided to test the complex formation constants and selectivities of the three ionophores.The complex formation constant (Log βILn) of two of the calixarene derivatives with thallium(I) were determined to be 6.44 and 5.85 respectively, through the use of the sandwich membrane technique. The selectivities were measured with a new protocol, whereby the electrode had not previously come into contact with the primary ion. This helps to remove ion fluxes of the primary ion and subsequent biased selectivity coefficient of highly discriminated ions. The three ionophores showed excellent selectivity against Zn2+, Ca2+, Ba2+, Cu2+, Cd2+ and Al3+, and moderate selectivity against Pb2+, Li+, Na+, H+, K+, NH4+ and Cs+. Silver was the only common high interferent in all three ionophores tested. As the detection limits of current thallium(I) ISEs in the literature would be insufficient in practical samples, attempts were made to lower the detection limits of the above ISEs with the application of relatively new experimental techniques. The lower detection limit of the three ISEs was successfully lowered by an order of magnitude from the original values through the use of an EDTA-buffered inner filling solution. The lowest achieved detection limit was obtained with the iso-propyl functionalized calixarene, which reached a value of 8.32 nM (IUPAC definition).The second part of the thesis investigated the incorporation of one of the calixarenes into a solid-contact ISE (SC-ISEs), which are seen as the future in this field due to their potential for miniaturisation and use in lab-on-a-chip applications. Four different solid-contact designs were tested to evaluate which was the best to pursue for future testing. The chosen calixarene was successfully incorporated into all four designs with Nernstian responses recorded in each case. The best response was recorded for an electrode which had a solid gold substrate, poly(3-octylthiophene) (POT) intermediate layer and a methyl methacrylate/decyl methacrylate (MMA-DMA) co-polymer membrane. This electrode exhibited a slope of 58.4 mV decade-1 and a lower detection limit of 30.2 nM. The other three solid-contact electrodes, which consisted of a graphite contact, a plasticised PVC membrane on a gold substrate, and a plasticized PVC membrane on a gold substrate with a polypyrrole intermediate layer, exhibited detection limits that were inferior to the MMA-DMA/POT SC-ISE. Further tests were used to assess one of the main problems associated with SC-ISEs, being the presence of water layers of droplets between the membrane and the solid substrate. Potential tests, electrochemical impedance spectroscopy, small angle neutron scattering and the electrode’s reactivity to changes in the concentration of dissolved oxygen were used to study water uptake and the concomitant formation of water layers in solid-contact ISEs. Water was confirmed at the surface of the membrane that consisted only of the membrane and gold substrate, but was not confirmed for the other three electrode designs.
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