Year

2010

Degree Name

Doctor of Philosophy

Abstract

Electrode platforms based on carbon nanotubes (CNTs) or their composites have been extensively investigated over the last two decades since their discovery. CNT‐based electrode platforms have been intensively researched in the electroanalytical and electronic fields. CNTs offer many excellent properties to create electrochemical devices and also improve the properties of composites made from them. Consequently, the theme of this thesis utilises one kind of CNT architecture, which to date has not been successfully investigated for use in electrochemical sensing applications. This architecture, CNT paper or CNT Buckypaper (BP), can be easily prepared and processable by a vacuum‐assisted filtration of well‐dispersed CNT material. Historically, BP has so far not been suitable as an electrochemical sensing platform, as they generally reveal a high background current and low signal to noise ratio (S/N ratio) or low Faradaic response to background charging current ratio. The advantages of BP are that they are readily prepared (cheap), flexible,highly conducting and all carbon in nature.

Therefore the objective of this thesis is to successfully design and develop a novel superior CNT electrochemical platform from a BP architecture with the specific aim to lower the charging effects and therefore improve redox responses. This infers a better S/N ratio suitable for use as a sensing platform. These platforms were fabricated by the intercalation of insulating polymers (IPs) such as poly(styrene‐β‐isobutylene‐β‐styrene) (SIBS), polystyrene (PS), polyisobutylene (PIB), polyurethane‐diol (PU), poly(DL‐lactic acid‐coglycolic acid) copolymers (75:25) (PLA‐PLGA), poly(L‐lactic acid) (PLA), and the inherently conducting polymers (CPs) such as poly(3‐octyl pyrrole) (POP), poly(2‐methoxyaniline‐5‐ sulfonic acid) (PMAS), and poly((E)‐4,4’’‐didecoxy‐3'‐styryl[2,2':5',2'']terthiophene) (PDSTTP). It was found that significant differences in redox behaviour of the bare and V intercalated BPs were found with five of the polymers tested (SIBS, PS, PIB, PDSTTP, and POP). The details of the thesis were summarised as follows.

Firstly, Chapter 3 reveals the screening and investigation of polymer intercalation in terms of the improvements in properties of BP structures which are related to the type of polymer; SIBS, PS, PIB, PLA, PLA‐PLGA and PU improve the mechanical properties; PLA, PLA‐PLGA, PDSTTP, POP and PMAS improve thermal stability; POP and PMAS improve electrical conductivity; SIBS, PS, PIB, POP and PDSTTP improve the electrochemical properties and redox behaviour. The improvement of the electrochemical properties at the novel composite platforms by measurements of reduction of the peak‐to‐peak separation (ΔEP), reduction of double layer capacitance (Cdl), enhancement of the S/N ratio and generation of the highest AC harmonic response resulted in the development of better electrochemical sensing BP platforms.

Full details of the electrochemical studies of IP/SWCNT platforms (Chapter 4) and of CP/SWCNT platforms (Chapter 5) show that the novel polymer‐BP structure relates to a randomly ordered micro‐/nano‐electrode array possessing faster electron transfer (ET) and surface heterogeneity, when compared to the raw BP structure, which contributes to the overall enhanced voltammetric response and emphasises the benefit for electroanalysis.

Direct current (DC) and large‐amplitude Fourier transform alternating current (FT‐AC) cyclic voltammetric (CV) techniques have been employed in the investigation of solutionphase electrochemistry at the five novel BP platforms. The high power FT‐AC technique shows the systematic measurement of readily accessible components; DC component and fundamental, second, third and higher harmonics. Modelling and simulation have been used with all relevant electrochemical parameters, to compare with the experimental data to elucidate the details of the electrode reaction mechanism and electrode surface structure. Both DC and AC techniques proved these intercalated BP electrodes to be superior sensing materials as compared to raw BP materials. Three standard redox probes (ferricyanide [Fe(CN)6]3‐, ferrocenemonocarboxylic acid FMCA0 and ruthenium (III) hexamine [Ru(NH3)6]3+) were employed to evaluate capability in use of intercalated BPs as electrodes having fast ET rate, significantly improved Faradaic response, reduced background charging current, increased S/N ratio and generated higher AC harmonics. Furthermore, the edge‐plane defects of CNTs would be predominant in the novel polymer‐ SWCNT BP composite electrodes.

Finally, for practical use in electrochemical sensing applications, the novel platform (SIBSBP) was successfully demonstrated as a chemical sensor in Chapter 6. This platform requires no further treatment or modifications such as polishing or electrochemical activation which are generally employed for standard electrodes such as glassy carbon electrode (GCE), edge plane pyrolytic graphite electrode (EPPGE), and basal plane pyrolytic graphite electrode (BPPGE) in detection of dopamine (DA), an important neurotransmitter, with the presence of coexistent interferences, such as ascorbic acid (AA) and uric acid (UA). The novel platform reveals selectivity to such analytes with high voltammetric resolution.

The knowledge achieved above during the course of this study could form the basis of novel freestanding superior electrode materials for use in the electroanalytical, bio‐sensing or environmental monitoring fields.

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