Funciones del Director de CEBA.

Since the first attempt at using diamond as an electrode in 1983 by Iwaki and Sakairi, conducting diamond has been employed for a wide range of electroanalytical applications: from heavy metal detection to in-vivo neurotransmitter detection.138-139 Compared to that of common metal electrodes: e.g. gold and platinum; and common carbon electrodes: e.g. glassy carbon and carbon fibre; diamond electrodes have an extremely wide potential window; lower background currents; higher chemical and mechanical stability; resistance to fouling; and controllable surface termination.128, 140-142

The electrochemical properties of pBDD can be influenced by surface termination.129, 143 _As

grown, CVD diamonds posses a non-polar hydrogen terminated surface due to the presence of hydrogen gas during the cooling stage at the end of the CVD growth process.140, 143 _This

CHAPTER 1:INTRODUCTION

surface termination is unstable and can change during anodic polarisation to contain a range of oxygen functional groups: hydroxyl, ether and carbonyl.144-146 _{Oxygen-terminated surfaces}

display greater stability under potential control and have been found to provide a wider potential window and lower background currents than hydrogen terminated diamond.147-149

The surface termination can also be modified to be completely oxygen terminated via several methods, including subjecting the material to oxygen plasma or boiling in acid.148, 150-152

The surface of diamond is relatively inert compared to other carbon electrodes, particularly with regard to the adsorption of species. Therefore, electrode reactions that require the adsorption of species/intermediates are strongly inhibited on diamond and require high overpotentials in order to drive the electrochemical reaction.153-155 _{On the other hand, the}

inability of electroactive species to adsorb gives rise to a resistance of an electrode towards surface deactivation from fouling by adsorbed species; especially biological species such as dopamine, making it an ideal material for the detection of biomolecules.141, 156-159 _The

electrolysis of water is an inner-sphere reaction, requiring the adsorption of intermediates in order for the evolution of oxygen (water oxidation) and hydrogen (water reduction) to take place.154, 160-161 _{The inhibited (dissociative) adsorption at pBDD means the reaction is}

inhibited; hence, the requirement of large over-potentials to drive hydrogen and oxygen evolution.154, 162 _{The wider potential window enables the detection of a greater range of}

electroactive species, previously impossible to measure due to the limitations of commonly used electrodes.163-164 _{Diamond electrodes have been found to produce potential windows of}

3-4 V in aqueous solution.128-129, 143, 165-166

Low background currents are another example of a superior characteristic demonstrated by pBDD electrodes. Low capacitance is due to the low DOS near the Fermi level.167 _This

results in the capacitative currents being dominated by a space charge layer; where electrons move from the electrolyte solution to the electrode in order to achieve equilibrium, creating a depletion layer of holes at the surface.168-169 _{The chemical inertness of the}

electrode surface also contributes to the low background currents; surface processes (surface oxide formation and reduction) that would contribute to background currents are negligible.165 _{The result is in an improved electroanalytical sensitivity compared to}

conventional electrodes.

These remarkable chemical and physical properties depend greatly on the quality of the diamond; the presence of any sp2 _{carbon both diminishes the potential window as well as}

making the electrode more reactive and less resistant towards surface blocking.143, 170 _sp2

CHAPTER 1:INTRODUCTION

19 adsorption of species via a range of intermolecular forces.10 _{Lower quality pBDD, containing}

a significant amount of graphitic carbon thus has a potential window similar to that of GC and HOPG. The presence of graphitic carbon can be determined with Raman spectroscopy, which can also give an indication of the level of boron doping.171-172

The surface of pBDD has been studied using a wide range of techniques where it has been shown that the heterogeneous uptake of boron results in heterogeneous electrochemical and electrical properties. For example it has been found that the (111) crystal face may incorporate up to 10 times more boron than the (100) face during synthesis.135-137 _Using

cathodoluminescence, field emission scanning electrochemical microscopy (FE-SEM) and Fluorescence Laser Scanning Confocal Microscopy studies have shown that different crystal facet orientations incorporate boron at different rates (Figure 1. 15).173-176 _{Conducting atomic}

force microscopy (C-AFM) has been used to study the local resistivity of the pBDD surface and electrochemical imaging techniques such as scanning electrochemical microscopy (SECM) and scanning electrochemical cell microscopy (SECCM) have been employed to study the local electrochemical properties of pBDD (Figure 1. 15).173, 176 _{These studies have}

shown that crystals of different orientations, and hence different boron concentration, which impacts on ET kinetics, more importantly these studies have shown that electrochemical activity occurs over an entire grain and is not limited to grain boundaries.173-176

Figure 1. 15 (a) (i) Cathodoluminescence mapping; (ii) C-AFM image and (iii) FE-SEM image of a pBDD surface. Adapted from reference 173. (b) (ii) SECCM and FE-SEM image of a pBDD surface. Adapted from reference 176.

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1.4 PROJECT AIM

It is evident from the overview that carbon electrodes have key attributes for use in electrochemical sensing and analysis, yet fundamental aspects of their behaviour are unclear. It is the hypothesis of this work that an improved understanding of carbon electrodes would aid in the design and optimisation of carbon electrodes particularly for electroanalysis.

Chapters 3 and 4 are concerned with understanding the electrochemical properties and surface effects of pyrolytic graphite.

Chapter 5 explores the reactivity of the neurotransmitter, DA, on the basal plane of HOPG and how it compares to GC, pBDD, BPPG and EPPG is examined in Chapter 6.

Chapter 7 investigates the relationship between the adsorption of 2,6-AQDS to step defect density of graphite surfaces in order to determine whether it is an appropriate measure for determining the quality of a carbon surfaces.

Chapter 8 explores the use of pBDD to study the neurotransmitter, 5-HT, and Chapter 9 studies its suitability for the development of a glucose sensor.

CHAPTER 1:INTRODUCTION

21