Development of electrochemical (Bio)sensors for the detection of dopamine

The development of dopamine (DA) electrochemical sensors and biosensors is described in this thesis. The aim was to develop (bio)sensors that would continuously and specifically detect DA over a wide concentration range, with high sensitivity and low detection limits. The electrode surfaces used were platinum (Pt), boron-doped-diamond (BDD), and carbon-paste (CP). The first two were used to construct DA sensors, while Pt was shown to be superior for the construction of biosensors. The DA biosensor configurations were based on the use of a Pt electrode as a transducer surface, polyphenol oxidase (PPO) as an enzyme, ferrocene (Fc) as a mediator, and polypyrrole (PPY) and Nafion as membranes. A range of electrochemical techniques were employed in this research.It was determined that the oxidation of DA on bare Pt is a surface-controlled reaction, occurring at low overpotentials. The reaction is electrochemically reversible, involving the spontaneous adsorption of DA on the electrode surface. The Pt and BDD sensors were efficiently used to determine DA in aqueous solutions. In order to increase their resistance to the ascorbic acid (AA) interference, the sensor surfaces were modified by a thin Nafion film. This configuration was shown to selectively detect DA even when AA was present with DA at a 1000-time larger concentration. The lowest DA detection limit was achieved using the unmodified BDD sensor, 50 nM. Nevertheless, both the unmodified and Nafion-modified Pt and BDD sensors were suitable for monitoring of DA at concentration levels typical for urine samples.It was shown that the sensitivity and detection limit of the developed Pt-based biosensors depend on the amount of PPO and Fc incorporated into the PPY membrane, and also on their ratio. The modification of the biosensor by a Nafion membrane offered three benefits: an increase in sensitivity, an improvement in detection limit, and a significant minimization of the AA interference. An optimum biosensor architecture was made by polymerizing PPY for 40 minutes from a pyrrole solution containing 2,400 U mL-1 of PPO and 10 mM of Fc, on top of which a thin Nafion film was formed. Using chronoamperometry as a detection technique, this biosensor yielded a DA detection limit of 20 nM, which makes it suitable for monitoring DA levels in brain. Even a lower detection limit, 10 nM, and higher sensitivity were achieved by using electrochemical impedance spectroscopy (EIS) as a detection technique. Unfortunately, the developed biosensor lacked operational stability, predominately due to the leakage of PPO and Fc into the storage solution