Electroanalytical studies of diamond electrodes
Keywords: carbofuran
beta-lactams
aldicarb
tetracycline
guanosine
carbaryl
quinolones
adenosine
fenobucarb
dopamine
DNA
antibiotics
BDD
carbamate pesticides
malathion
aminofluorenone

The experimental work presented in this thesis explores the suitability of boron-doped diamond (BDD) electrodes for the electrochemical sensing of compounds important to health. Various forms of BDD electrode have found increased application to electroanalysis in the past two decades, but there is still room to explore more complex molecules with synthetic BDD. The aim of this thesis is to examine whether the known chemical and material advantages—wide potential window, low background capacitance, chemical inertness, and physical robustness—can be realised.
Although BDD electrodes generally have higher resistance to fouling by analyte media than other carbon and metallic electrode materials, electrode poisoning can still be very problematic. The first study investigates ways to restore or improve the electrode response reliably and quickly after it has been fouled. The fouling analyte was dopamine (DA), known to form a wide variety of polymers and is one of the most fouling chemicals that exists. After 3 cyclic scans, a thick adsorbed layer was observed by scanning electron microscopy (SEM), which negatively impacted the accuracy of further measurements. In a solution of 1 mM DA, the heavily fouled BDD can be efficiently returned to high activity by using 20 pulses of alternating ± 10 mA/cm^2 current density with 100 ms duration. If only anodic pulses are used, then this is not observed. This treatment in situ increased the quality of the signal to beyond the initial value for an acid-cleaned electrode. This finding provides confidence for exploring other heavily fouling analytes.
The second study examined the voltammetric behaviour of the genotoxic pollutant 2-amino-9-fluorenone (2-AFN) and its interaction with double-stranded DNA (dsDNA). Cyclic voltammetry revealed complex adsorptive behaviour of 2-AFN onto the BDD. Applying 60 pulses of alternating ± 10 mA/cm^2 current density with 100 ms duration in situ gave consistent and reproducible voltammetry. With the aid of SEM and X-ray photoelectron spectroscopy to study the nature of the adsorption, it was shown that 2-AFN diffuses to the electrode and undergoes proton-dependent (below pH 10.0) irreversible oxidation in the anodic region to form dimers or longer polymer chains. These and other monomers can then also take part in the reversible reduction of the carbonyl group. By optimising the parameters of differential pulse voltammetry, the limit of detection of 2-AFN was found to be 10 nM—an order of magnitude lower than previously reported.
After exposing dsDNA to 2-AFN, preferential damage to the guanosine moiety was observed, suggesting the intercalation of 2-AFN into the planar aromatic structure of dsDNA. Extensive square wave voltammetry showed that 2-AFN causes a disproportionate amount of damage to dsDNA, observed as significant deviations from the expected peak currents and potentials.
In the third study, bare BDD electrodes were used in the pulsed voltammetric and amperometric quantification of five pesticides in buffered solutions and in river water: namely, the four carbamates carbaryl (CB), carbofuran (CF), fenobucarb (FC), and aldicarb (AC), along with malathion (MT). After exposure to 1 mM MT, a reduction feature was observed on subsequent scans that is highly sensitive to pH. This is attributed to a complex carbon–oxygen adsorbate that is robust to aggressive cleaning methods and only removable by hydrogen-plasma etching. For the carbamates, detection limits of 1 µM for FC, CB, CF (better than other undecorated planar electrodes), and 2.5 µM for AC were obtained. The same limits and calibration ranges were achieved via amperometric batch-injection in spiked river water for CB, and CF. The detection limit in river water for FC was 5 µM. The oxidised BDD electrode demonstrated linear sensitivity up to 100 µM. Alkaline hydrolysis applied prior to analyte mixing allowed for the simultaneous detection of CF, CB, and FC.
The fourth study turns to a group of compounds that has recently come back into public interest: antibiotics. Amoxicillin (AMX), cefadroxil (CFX), ofloxacin (OFX), and tetracycline (TC) have all been used for growth promotion in industrial farming. These molecules are rarely investigated via electrochemistry, and here BDD electrodes were applied to the analysis of OFX for the first time, achieving a competitive detection limit of 1 µM. Because of peak-definition and sensitivity problems of AMX, this work focused on the other three analytes. Square-wave voltammetry was used to achieve a detection limit of 500 nM for CFX and 1 nM for TC, which is an order of magnitude lower than previously reported. All three exhibited a wide dynamic range, up to 200 µM. A major advantage of using a BDD electrode was realised for this project. Because of the inertness of the diamond surface, adsorption which dominates the electrochemistry of these compounds on non-diamond electrodes did not take place.
The thesis concludes with a summary of key results and suggestions for future research topics.