Year

2006

Degree Name

Doctor of Philosophy

Department

Intelligent Polymer Research Institute, Department of Chemistry - Faculty of Science

Abstract

Synthesis of inherently conducting polymers (ICPs) nanoparticles is an option to improve the processability and conductivity of ICPs. In this thesis, the synthesis and application of ICPs nanoparticles has been demonstrated. Various polymerisation methods, such as emulsion polymerisation, use of steric stabiliser and synthesis in ionic liquid (IL) media, have been used to synthesise polymer nanoparticles. These synthesis methods render the ICPs nanoparticles stable as dispersions which are more processable and contain peculiar and fascinating properties superior to their bulk counterparts. These nanoparticles are further applied as mediators for biosensors. They have been fabricated into sensors using electrodeposition, evaporative casting, or ink-jet printing methods. Electrodeposition method results in formation of ultra thin nanostructured polymeric films that enhance sensor performance. Evaporative casting method is an easy one-step method, but precision is hard to achieve and dense films with rough morphology are formed. Ink-jet printing can be used to produce precise and accurate patterns and also this approach is amenable to mass production. Polyaniline (PANI) nanoparticles; nanoPANI-dodecylbenzene sulphonic acid (DBSA) have been synthesised using emulsion polymerisation (Chapter 3). The nanoPANI-DBSA obtained has a conductivity of 34±7 S/cm with particle size in the range of 10±2 nm. The nanoPANI-DBSA has been used as a mediator layer in biosensor applications as demonstrated in Chapter 7. These nanoparticles were fabricated onto the conductive electrode using an electrodeposition method with subsequent immobilisation of the enzyme horseradish peroxidase (HRP). Sensor performance was examined using amperometric method and HRP/hydrogen peroxide (H2O2) configuration as a model system. The nanodomain of the nanoPANI-DBSA particles contributed to highly ordered nanostructure patterning on the electrode surface. This uniform surface showed improved enzyme deposition characteristics, a lower background signal and better sensor performance at a lower HRP loading when compared to the sensors fabricated from electropolymerisation of the bulk monomer. NanoPANI-DBSA particles aggregate at high concentrations; hence they are not amenable to ink-jet printing. sPANI-DBSA was prepared from centrifugation of the nanoPANI-DBSA dispersions and used as a material for ink-jet printing. HRP was premixed with the sPANI-DBSA nanodispersions before fabrication onto ITO-coated mylar using ink-jet printing. The print quality from the sPANI-DBSA nanodispersions was inconsistent and the catalytic signal of this biosensor was very low. These resulted in no further ink-jet printing work for this material. The PANI-DBSA-rapid mixing (RM) nanodispersions were synthesised using a RM method. These dispersions contained nanometre size PANI particles dispersed in aqueous media. These nanoparticles have been successfully printed using ink-jet printing as outline in Chapter 9. This work has demonstrated the ink-jet printability of conducting polymer nanoparticles and their use as working electrodes for biosensors. The sensor response from these ink-jet printed PANI-DBSA-RM was higher than the sensor response from evaporative casting of poly(2-methoxyaniline-5-sulphonic acid) (PMAS) in Chapter 8. The addition of functional group into PANI nanoparticles was also investigated. Carbolan Blue (CB) dye was incorporated into the PANI backbone using emulsion polymerisation method as demonstrated in Chapter 4. The dye was proved to have strong interaction with PANI backbone using Raman spectroscopy and centrifugation test. The distinct solution colour after a reduction process could lead the PANI-DBSA-CB to be a potential candidate of the material for electrochromic devices. Synthesis of polypyrrole (PPy) nanoparticles is demonstrated in Chapter 5. Poly(vinyl alcohol) (PVA) was used as the steric stabiliser to produce PPy-DS-PVA nanoparticles. These nanoparticles were well dispersed in water with particle size in the order of 52±5 nm. Aggregation was obvious in concentrated solutions and leaded to poor ink-jet printed quality of the PPy-DS-PVA nanoparticles. The water soluble polymer, PMAS, was also used to fabricate biosensors using evaporative casting method in Chapter 8 and ink-jet printing in Chapter 9. In chapter 8, its solubility enabled PMAS to pre-mix with the HRP enzyme prior to complexing with the polycations poly(L-lysine) hydrochloride (PLL) and subsequently casting onto ITO coated mylar substrate. This biosensor format has proven ability to easily fabricate the conducting polymer nanoparticles by one-step evaporative casting. The optimised sensors exhibited good sensor response, high selectivity and very good long-term stability. The ink-jet printed films from PMAS and PLL solutions (Chapter 9) showed better electroactivity compared to the evaporative cast films which could lead to better sensor performance. However, the problem of PLL blocking the print head resulted in the discontinuation of its use. The polyterthiophene (PTTh) aqueous dispersed nanoparticles were also successfully synthesised in the presence of surfactant (DBSA) and in ionic liquid; 1-ethyl- 3-methylimidazolium bis(trifluoromethane-sulfonyl)amide (emiTFSA) as demonstrated in Chapter 6. The dispersion of PTTh-DBSA nanoparticles has shown poor colloidal stability and poor electroactivity. Although the PTTh nanoparticles synthesised in emiTFSA needed 2-3 minutes sonication to be dispersed in water, they have shown good electrochemistry and being test in another study in our laboratories for its use in photovoltaic devices. These processable ICPs nanoparticles are promising materials for biosensor applications, electrochromic devices and solar cells. Assembly of these nanoparticles on to conductive substrates leads to highly ordered nanostructured ICPs on the surface and improves the biosensor performances. Also these nanoparticles prove their ability to be processable in mass production scale.

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