Year

2013

Degree Name

Doctor of Philosophy

Department

School of Chemistry

Abstract

Galvanically coupling enzymatic bioelectrodes with drug loaded conducting polymers to develop a controlled drug delivery system is investigated. The system is designed to be a novel and sophisticated approach to in vivo drug delivery for the treatment of epilepsy. There is a suite of anti-epileptic drugs (AEDs) that are extremely effective at suppressing seizure activity however; their use is restricted due to debilitating side effects. As such, it is of interest to develop novel drug release methods to deliver the AEDs directly to the target area in the brain and in small doses to minimise, and ideally eliminate, the side effects. The significance of using an enzymatic power source lies in its ability to operate off simple sugars naturally present in the human body. Additionally, the on/off switching behaviour of the conducting polymer, polypyrrole (PPy), can be accessed to release the drug when the oxidation state of the polymer is changed. By exposing the glucose oxidase (GOx) bioelectrode to glucose in solution, the catalytic current generated can be harnessed to charge the polymer and instigate drug release on cue.

Experiments were carried out to develop and characterise GOx bioelectrodes based on a direct electron transfer (DET) mechanism facilitated by carbon nanomaterials, namely multi-walled carbon nanotubes (MWNT) and reduced graphene oxide (rGO).

Chapter 3 investigates the development of solvated graphene electrodes (SGE), a hydrogel-like system based on graphene, with immobilised GOx. It is proven through biological protein and kinetic assays that the GOx is stable within the graphene structures and the electrodes can be reproducibly fabricated. Through electrochemical techniques, including cyclic voltammetry, impedance, amperometry and fourier transform alternating current voltammetry, it was revealed, however, that the enzyme is not electrically wired to the solvated graphene matrix, and as such, cannot directly transfer the electrons generated from the oxidation of glucose to the nanostructured electrode. As a result of its inability to access the catalytic current, no useable charge is generated and the structures cannot be used to power drug release from a conducting polymer scaffold.

In Chapter 4, aqueous dispersions of rGO and MWNT are fabricated through a modified chemical reduction method. The significant advantage of the method developed is the omission of any stabilizing compound or organic solvent to obtain stable rGO-MWNT dispersions. Significantly biological entities, such as GOx, can be successfully incorporated into the dispersion. These dispersions were characterised using XPS, SEM, zeta potential and particle size measurements which showed that the dispersion stability is not sacrificed with the addition of GOx, and significantly, the electrical properties of the rGO and MWNTs are maintained. In this study, rGO acts as an effective dispersing agent for MWNTs and does not affect the solubility or electroactivity of the GOx. Bioelectrodes fabricated from these rGO-MWNT-GOx dispersions were characterised electrochemically to test their feasibility in facilitating direct electron transfer (DET) from the redox centre of the enzyme to the electrode. The DET results showed that the specific catalytic current generated at an optimized rGO-MWNT-GOx electrode was 72 μA/μg GOx, which is 3 times more efficient than other literature values for similar systems. The remarkable specific catalytic current can be attributed to the use of purified enzyme, the efficiency of charge transfer within the rGO-MWNT composite and the ability of the electrode to facilitate direct electron transfer. The optimised electrode developed and characterised in Chapter 4 is used in Chapter 5 to power the release of drug from PPy.

The release of the anti-inflammatory drug, Dexamethasone-21-phosphate (DEX), and the anti-convulsant drug, Fosphenytoin (FOS), from PPy films are characterised in Chapter 5. This work investigates, for the first time, the controlled release of FOS from a conducting polymer and galvanically connects the optimised rGO-MWNT-GOx bioelectrode (presented in Chapter 4) to drive the release, also the first of its kind. When the two half cells are connected and the conditions are changed in the bioelectrode cell from 0 mM to 150 mM glucose, there is an increase in the charging current generated, as a result of the additional catalytic current. A corresponding increase in the rate of FOS released was measured in the release cell from 24 to 47 ng/mL. This result shows that the concept of galvanically coupling enzyme-based bioelectrodes with drug loaded conducting polymers to fabricate a controlled drug delivery device is valid.

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