Year

1997

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Abstract

Since the early development of polymer film electrodes, a great deal of research activity has been carried out into their application as sensors. Due to their ability to undergo molecular interactions with particular species of interest, conducting polymers such as polypyrrole, polyaniline and polythiophene constitute a new class of organic polymers with great promise as sensor materials. The properties of conductivity and electroactivity of conducting polymers then enable the electrical signal generated by these interactions to be readily detected.

With the unique feature in that they can be electrochemically switched between the oxidised and the reduced states accompanied with the movement of anion or cation, conducting polymers have been successfully exploited to produce electrochemical signals for the detection of electroinactive anions or cations. Conducting polymers may also be used as a matrix for the immobilisation of specific molecules like complexation agents, enzymes and antibodies. The capability of analyte recognition by the immobilised molecules can then be used to generate analytically useful signals for the corresponding substances such as metals, enzymatic substrates and antigens.

In this work, conducting polymers incorporating different functional groups were used for the detection of proteins, for the prevention of electrode fouling generated by phenol oxidation and for the development of a novel type of electromembrane biosensor for the detection of glucose.

The detection of proteins using conducting polymer sensors incorporating simple molecules rather than biomolecules was investigated in Part1. This section investigates the relatively simple chemistries occurring between analyte proteins and the immobilised molecules, i.e., "surfactant binding", "dye binding" and the electrocatalysis of electron transfer mediators.

To achieve responses for proteins, a range of counterions containing different functional groups, including surfactants, sulphonated dyes and Fe(CN)6 3-, were incorporated into polypyrrole matrices. As well, a composite film consisting PPy/Cl and the inorganic compound prussian blue was studied. The analytical utility was demonstrated using an FLA system. In general, the protein responses were influenced by the polymer composition, the nature of the supporting electrolyte, solution pH and the applied potential or pulsed potential waveform. These factors can be used to modify selectivity and sensitivity.

Finally, the most promising polymers, optimised at macroelectrodes, were individually coated onto each working electrode of a microarray system. Varied selectivity at each electrode was achieved by modifying polymer composition and applied potentials. Combining protein responses with chemometric techniques, protein identification and quantification have been performed. With four-sensor pattern (PPy/MO, PPy/BG, PPy/PTS and PPy/Cl/PB) classification of a range of proteins (HSA, BSA , CYT.C, OVA, α-LAC and MYO ) was achieved. The individual protein in a mixture of two components (HSA and CYT.C) was quantitatively analysed with acceptable errors. In this way, a new concept of synthetic electronic antibody system has been demonstrated for the determination of proteins. Compared with a real antibody containing system, due to the incorporation of the non-biological species, this purely synthetic system possesses advantages in reproducibility, reusability and life time.

Electrode fouling or passivation is a general problem associated with the oxidation of phenol at a solid electrode. In the second part of this work, the feasibility of using a conducting polymer to prevent electrode fouling was addressed by employing polyaniline and polypyrrole systems. The optimisation of the polymer composition was carried out from simple counterions to those with some specific functional groups. The effect of polymer structure and composition was confirmed by UV-vis and FTIR spectroscopy. It was found that the polymer surface containing hydrophilic groups showed an excellent ability to prevent fouling using either polyaniline or polypyrrole. Using an FLA system, with a pulsed potential waveform applied, the substantially stable and reproducible oxidation response of phenol was obtained at a PPy/oASA polymer coated electrode.

In Part 3, a novel type of electromembrane sensor has been developed. In some cases, in the development of n e w sensors, a dichotomy often arises in that the electrochemical conditions for optimal analyte recognition differ from those required for signal generation. In this work, use of an insulating microfiltration membrane coated with platinum on both sides allows independent optimisation of the analyte reaction and signal generation components required for electrochemical sensing. This was successfully demonstrated with a well-defined electrochemical system (ferri/ferrocyanide) as well as a well-established enzymatic system (glucose oxidase).

For the ferri/ferrocyanide system, two independent potentials Er and Ed were applied in the reaction zone and detection zone respectively. With Er applied in reaction zone, analyte Fe (CN)6 3- was reduced to Fe (CN)6 4-, which then passed through the membrane to the detection side. In the detection zone, the positive potential E d was used for the re-oxidation of Fe(CN)6 4- to produce an analytical signal. The independent optimisation of these two potentials ensures optimal performance of the sensor.

For the glucose oxidase (GOD) system, polypyrrole incorporating GOD was coated on the membrane reaction side. Due to separation by the substrate membrane, the molecular recognition (enzymatic reaction) in the reaction zone and the signal generation (H202 oxidation) in the detection zone can be optimised independently to achieve a sensitive and selective response. This PPy/GOD coated membrane system showed excellent sensitivity, reproducibility and life time. Polymer degradation encountered with conventional PPy/GOD sensors, due to the oxidation of generated H202 and polymer overoxidation at the potential necessary for signal generation, can be prevented. A further advantage of this system is that interferents are excluded from the detection zone by a combination of the substrate membrane and the polymer, thus ensuring high selectivity of the sensor.

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