Methods of designing carbon based sensors

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CHAPTER ONE

INTRODUCTION

This research work sought to investigate new methods of designing carbon based sensors that could be used as the basis of a versatile detection system for conventional laboratory based instrumentation but which could also be readily transferred to emerging micro-nano scale devices.

Carbon based electrodes are widely used in electroanalytical applications because of their low cost, large potential window, low background current, versatile surface chemistry, fast electron transfer kinetics and suitability for various sensing applications. The consequence of which has led to the development of different types of carbon based electrodes in use today for electroanalytical applications, dramatically changing the scope of electrochemical methods in the measurement of diverse targets like ions, gases and biological markers (1-2).

Carbon exists in various physical forms - most of which have been extensively studied as electrode materials over the years. With the exception of diamond, most forms of carbon can be easily functionalized through a variety of reaction routes or through physical/mechanical treatments. These forms include a wide range of graphitic carbon such as: pyrolytic graphite (PG), amorphous carbon, glassy carbon (GC), carbon black, carbon fibre, powdered graphite, and highly ordered pyrolytic graphite (HOPG). Each is capable of demonstrating unique chemical and physical properties (3). The most commonly employed carbon electrode materials tend to be those made of glassy carbon as it serves as a robust, all purpose conductive substrate that can be used for studying both oxidation and reduction processes. Carbon films or other carbon composites like graphite bound with epoxy, wax, mineral oil or parafin are also used but these tend to be employed for more specific applications (2). Several other composite electrode materials have also been reported and a few examples include: carbon nanotubes (CNT), which has gained much prominence since its re-discovery by Iijima in 1991 (4) and polymer modified carbon electrodes which include those based on Nafion® and polypyrrole (5-7).

The discovery of carbon can be dated back to the ancient times when it was known as soot and charcoal to the earliest human civilization. Diamonds were probably known as early as 2500BCE in ancient China, while carbon in form of charcoal was made around Roman times by the same chemistry as it is today, by heating wood in a clay oven to exclude air. The historical evolution of carbon and timeline for the development of carbon technology is given in the figure 1 below.

CARBON STRUCTURE

Most carbon based electrode materials were made exclusively of sp2 hybrized carbon possessing the planar graphite sheet as their structural building block. The structural framework is a hexagonal lattice consisting of three coordinate carbon atoms in which the intra-planar C-C bond length varies from 1.39Å in benzene to about 1.42Å in graphite (2, 3).

The intra-planar microcrystallite size or lateral grain size (La) is the average size of the microcrystallite along the x-axis and this variation in La accounts for the variety of forms the material can take. The graphite microcrystallite size or lateral grain size (La) can range from 3Å (size of a benzene molecule) to being very large - as in the case of a macro single crystal of graphite. Amorphous carbon, glassy carbon and carbon black are known to have the smallest La values and these can be as low as 10Å. In contrast, carbon fibres and pyrolytic graphite have La values in the range of 100Å and 1000Å respectively (3, 8).

The planes of graphite stack in an ABAB...sequence and the distance along the vertical axis, perpendicular to the plane of stack is known as interplanar microcrystallite size Lc (as shown in Figure 1). Other planar stacking sequences have also been reported by McCreery in the case of rhombohedral graphite which has an ABCABC... stacking sequence but this a rare occurence (3). The interplanar microcrystallite size (Lc) range from 10Å in highly disordered carbon, as in the case of carbon black, to arround 10 µm in natural single graphite crystals. The variations in La and Lc account for the change in properties of carbon normally experienced in graphitized carbon. Graphitization of carbon occurs when carbon (with small Lc) is heated above 2000oC in order to increase the Lc size. This process makes the sp2 carbon planes stack parallel to each other thereby increasing Lc, making the planes glide over one another (which makes graphitized carbon a good lubricant) and the material softer(3). The Interplanar spacing d002 (shown in Figure 1) varies from 3.354Å in single crystal graphite to 3.6Å in less ordered sp2 carbon and can be larger (ca. 10Å) in intercalated graphite. The term d002, which was derived from x-ray powder diffraction designation, is used to indicate the reflection corresponding to the interplanar spacing. The d002 value can be sligthly higher in most synthetic turbostratic graphite (3).

The largest graphite single crystals are found in commercial grade hopg (ZYA and SPI 1 grade hopg) and these can be between 1-10 mm in size (3, 9). The grain boundaries where individual graphite monocrystals meet are poorly defined and, therefore, suffer from surface defects when exposed. In the case of pyrolytic graphite, the individual graphite crystallite lies along the same plane making the carbon surfaces significantly less prone to defects. Such surface defects in hopg can be as low as 0.2% in coverage and this is a consequence of the large lateral grain size (3, 10). Electrochemical performance, background current and electron transfer kinetics have all been attributed to structural defects on material surfaces - irrespective of type. Reports have been presented relating surface characteristics to electrochemical behaviour, capacitance and adsorption for many electrode materials. The distribution of edge and basal planes on material surfaces has been the special focus for electrochemists with reports indicating more electrochemical (and chemical) reactivity at the edge plane sites than the basal plane sites. The edge plane sites of a hopg have been reported to exhibit faster electron transfer, strong adsorption and low capacitance (1-10).

The structure-reactivity relationship of electrode materials is a very important focus point for biochemical studies. Biologically active molecules like dopamine were shown (Britto and co-worker, 1996) to be successfully oxidized at the edge sites of a carbon nanotube modified electrode. (11). Several other reports have been presented on the chemical reactivity, fast electron transfer kinetics, enhanced sensitivities, lower detection limit, low surface fouling, low background current and electrocatalysis occurring at the edge plane site of CNT and pyroytic graphite (1,12-13). Wang`s group showed that the electrocatalytic activities, background currents and electroanalytical performances of CNT modified electrodes are strongly dependent on edge plane-like site/defect found at the ends of the tube structure (14). Compton et al extensively studied the origins of previously reported electro-catalytic responses of CNT modified electrodes alongside edge plane pyrolytic graphite (EPPG). The catalytic response was subsequently attributed to edge plane sites occurring at the ends of the open tubes and along the tube axis (1, 8, 12). The edge plane sites have been reported to exhibit faster electrode kinetics in comparison to the basal plane and in many instances, an electrode consisting mainly of edge plane (as in eppg electrode) will show a nearly reversible voltammogram. In contrast, a bppg electrode tends to show irreversible behaviour depending on the amount of edge sites present on the electrode. (12).

The electronic structure and chemical reactivity of the edge plane defect sites on graphite can be better understood in terms of the band theory of solids. The HOMO and LUMOs in graphite (assuming an infinite graphite sheet) are closely spaced and as a result they overlap to form the valence and conduction bands which are separated by small energy gap, thus imparting semi-metallic conductive properties to the material. The conductive property of the material is greater along the basal plane than the edge plane due to due to the ability of electrons to hop between graphene sheets in the latter case. When the graphene sheet terminates at the edge plane site, the band structure at the edge plane site also terminates abruptly and this causes the energy of the electron in the valence band to rise sharply. Thus making the edge plane defect site high energy defects and sites at which electron transfer takes place and more chemically reactive than the basal plane site (47, 8).

CHEMISTRY OF CARBON SURFACE

Carbon as a bulk material is relatively inert but the presence of intrinsic reactive surface defects can be exploited and manipulated to impart certain physicochemical properties. The intelligent manipulation of the surface chemistry of carbon can be achieved in a number of ways either through functionalisation or modification of species already present on the surface or simply introduction of new species or more commonly exfoliation of the intrinsic reactive surface defects.

The chemistry of graphitic carbon can have a remarkable influence on the electrochemical performance of the material. As previously noted, structural defects on the surface of the material, viz. the basal plane and edge plane site demonstrate some chemical reactivities of their own with edge plane site more chemically reactive than the basal plane site. These highly reactive edge plane defect sites on graphitic carbon can react with atmospheric oxygen and/or water resulting in the surface being functionalize with a variety of oxygen containing group which includes quinonyl, hydroxyl, phenol and carboxylic acid. The surface oxides on graphitic carbon have an enormous effect on the chemistry of the material (3, 47). The surface oxides present on carbon surface have been classified by some authors as acidic, basic or neutral according to their reactivity with known acids and bases (48, 49). The type and quantity of the oxide groups available on carbon varies considerably with the materials and pretreatment history (3). The numbers of synthetically useful functional groups on carbon surface particularly carboxyl, hydroxyl and quinones can be greatly enhanced by a series of chemical pretreatment steps. A lot of techniques have been proposed by different researchers in the past and I can`t be exhaustive enough in this report. The most common of these pretreatment techniques is stirring carbon nanotubes (CNT) in a mixture of strong oxidizing acids such as H2SO4 and HNO3 (50-51) and the effect of this method on the quantity and distribution of carboxyl group on graphitic carbon have been extensively studied (53-55). Mao et al (52) showed that electrocatalytical detection of molecules like thiols were made possible by the inherent redox properties of surface functional groups on SWCNTs. This was achieved by immobilizing ortho-quinone derivatives onto SWCNTs in other to demonstrate how these surface functional groups can interact and enhance sensing applications.

MODIFICATION OF CARBON SURFACE

Several techniques have been developed in modifying the surface of graphitic carbon in recent years and can be broadly classified into mechanical, chemical (34, 35, 36), electrochemical (8, 13, 37, 38), thermal (39, 56-57) and plasma (40) modifications. Such surface modifications can either involve modification of species already present on the surface by increasing the numbers of surface functional group. The attachment of exogenous species onto the electrode surface or the intercalation of species between the graphene layers of the base substrate. The latter can be achieved through electrochemical pre-treatment processes as a result of carbon fracturing (41-42).

Mechanical Modification

Mechanical activation of electrode surface is a method that described a range of superficial processes such as eliminating impurities on the electrode surface, creation of new electrode surface and exposure of surface functional groups. The technique has been shown to be a versatile means of enhancing electrode performance and improving sensitivity through roughening of surface by abrasive polishing (12, 32, 33), Mechanical activation of this nature is normally done on a glassy carbon and bppg electrode where abrasive polishing and/or sonication is known to increase the number of edge sites on the electrode surface in other to improve electroanalytical performance. (2, 21, 43).

Thermal and Plasma Modification

Thermal and plasma modification are other techniques that has been extensively employed in carbon surface modification. In thermal oxidation of carbon, carbon fibres are heated in air to a temperature of between 400-600deg Celsius (39, 56-57). The thermal treatment of carbon fibres increases the adhesion properties, wettability and surface roughness of the material among other effects and creating new edge plane sites on carbon surface. In plasma modification, carbon fibres are continuously treated with dielectric discharge plasma at atmospheric pressure in various gas conditions (40). This technique of carbon surface modification has been shown to increase the number oxygen on the surface of carbon fibre material.

Chemical Modification

Chemical modification of carbon surface represents a modern approach to surface manipulation and this could be achieved by placement of chemical components onto the electrode surface (adsorption, covalent binding, polymer film deposition, polymer coatings) to impart the properties (chemical, electrochemical, optical and other properties) of that component to the modified surface (2). The selection of the immobilized chemical components are based on known or desired properties such as rapid outer-sphere electron agents, chiral centers, electron transfer mediator-catalysts for valuable substrate reaction, functionalities capable of scavenging for trace molecules or ions from solutions, corrosion inhibitors and so on (58). This method of surface modification offers greater control over the electrochemical behaviour given the greater variety of functionality that can be introduced onto the electrode surface and thus providing solutions to host of electroanalytical problems. Chemical modification is a far better approach than the activation of intrinsic functionalities such as quinone and hydroxyl groups. There are 3 directions in which chemical modification is plausible: the formation of covalent bonds (36, 44-45), irreversible adsorption (46), and electrochemically induced functionalisation of surfaces by exogenous species. The latter provides effective alternative to pure chemical methods. The mechanistic approach to covalent modification of carbon surface has been presented in some literatures and a well studied system is the covalent modification of CNT surface with aryl diazonium salts. This scheme involves the direct electrochemical activation of aryl diazonium salts leading to the formation of covalent bond between the aryl group and the MWCNT through a one-step electron reduction at the electrode surface with subsequent loss of N2 and the formation of the corresponding aryl radical which then forms a covalent bond with the material surface (59,60). Compton et al proposed chemical method rather than electrochemical activation of aryl diazonium salts through reduction with hypophosphorous acid (8). In this method, MWCNT was covalently derivatised with 1-anthraquinonyl or 4-ntrophenyl group from the corresponding diazonium salt by stirring MWCNTs in 5mM solution of either anthraquinone-1-diazonium chloride or 4-nitrobenzenediazonium tetrafluoroborate, followed by addition of hypophosphorous acid (50% w/w in water). The mechanism for this chemical immobilization of aryl diazonium salt onto the electrode surface via reduction with hypophosphorous acid is shown in figure 4.

Polymer Coatings

One of the most versatile approaches for incorporating a modifier onto the electrode surface is immobilization of chemical species in a polymer film. Modifiers immobilised by polymer onto the electrode surface can be achieved through polymer coating of the electrode surface with solution containing the dissolved polymer or via electropolymerization in the presence of dissolved monomer. In polymer coated electrode, the polymer films contain electrochemically and/or chemical reactive centers which can undergo electron transfer reaction with the electrode. Since the films generally contain as much monomolecular layers` worth of electroactive sites, electrochemical sensitivities are greater than those of immobilised molecular layer. The concentration of electroactive sites in terms of volume in polymer film is high and can range from 0.1-5M while the quantity of electroactive centers can range from ca.10-10 to ca. 1 x 10-5 or from less than 1 to greater than 20,000 monolayers and this could influence the chemical reactivity of the site as their solvent and ionic environment are different from homogenous solutions (58). The polymers available for functionalising electrodes surface can be grouped under three different categories, which are conducting polymers, non-conducting polymers and redox gels.

Conducting polymers or inherently conducting polymers (ICPs) such as polypyrrole, polythiophene, polyaniline, poly-indole-carboxylic acids to name of few have all been greatly used in functionalising carbon surface and the chemistry and mechanism of these conducting polymers well understood. They possess the ability to switch, reversibly between their positively charged conducting states to a neutral insulating form (passivate) and incorporate and expel anionic species from contacting solution during redox rections. In most cases, polymers can be doped and the dopant anions serves to counterbalance the polymer`s positively charged backbone and maintains electrical neutrality. The oxidation and reduction changes in polymers is not localised at a specific electroactive center but delocalised over a number of conducting polymer. The ability of the pi-electron system to conjugate creates a molecular orbital which extends the entire polymeric chain and the electrical conductivity of the polymer film resulting from the electronic structure of the polymeric backbone varies with applied potential. This value (electronic conductivity) is dependent on the amount of carriers (electrons or holes) created in the polymer chain and the mobility of the carrier through the polymer. The polymer chain is determined by the quantity of dopant present in the polymer and the dopant could range from large organic anions to DNA. These dopants are strongly attached to the polymer network and are difficult to remove. Thus, when the polymeric chain is in the neutral or reduced state, the negative charge of the anionic dopant entrapped in the polymeric network is balanced by insertion of the electrolyte cation (2, 58).

The preparation of inherently conducting polymers normally begins by in-situ electrochemical polymerization from the monomer solution. The first step is often electro-oxidative formation of cationic radical from the starting monomer, followed by dimerization process and further oxidation. The final step is the coupling reaction leading to the the formation of strongly adhering polymeric film on the surface of the electrode by either of potentiometry, galvanostatic or multiscan method (2, 61). By attaching various biological and chemical species to the monomer prior to electropolymerization, changes in the physical properties of conducting polymer can be induced. Properties like molecular recognition and electrocatalytic action through addition of functional dopants (complexing agent or electron transfer mediator) can be imparted, thus acting as efficient molecular interfaces between recognition elements and electrode transducers. Conducting polymers offer diverse electrochemical applications including fuel cell, corrosion inhibitors, batteries and chemical sensing, owing to their unique physical and chemical properties, mainly the controllable and dramatic change in electrical conductivity and ability to expel dopant ions. Such chemical sensing include solid-state gas sensing (62), entrapment and stabilization of biological molecules, direct sensing of DNA hybridization (61) and antibody-antigen binding to name a few.

Recently developed conducting polymer nanowires are characterized by controllable size and shape-dependent chemical and physical properties and high surface to volume ratio compared to conventional conducting polymer film. They hold promising future for chemiresistor and field effect transistor (FET) and molecular electronic device (63, 64). They are easily prepared by template-directed electrochemical synthesis involving electrodeposition into the pores of a membrane template or within the microchannels in contact with adjacent microelectrode (65).

EXFOLIATION OF CARBON SURFACE

Exfoliation of carbon surface represents an alternative approach to creation of reactive edge plane defect on carbon surface and consequence increase in surface functional (oxygen containing) groups. While the former tends to be the main reason behind exfoliation of carbon surface, other reasons include production of graphene sheet (EG), graphite oxide (EGO) and dispersion of single walled carbon nanotubes. Several methods have been proposed for the exfoliation of carbon surface and they include laser ablation (66), electron beam irradiation (67) and microwave irradiation (68) and electrolytic exfoliation using poly(sodium-4-styrenesulfonate) as an effective electrolyte. These methods of carbon surface exfoliation and many more other methods not mentioned here have all been extensively studied by different researchers in electrochemical applications.

CARBON ELECTRODES

Practically, all the forms of graphitic and amorphous carbon known to electrochemists have been extensively used as electrode materials in other to impact characteristic physicochemical properties and improve electroanalytical performance. A few of the forms of carbon are discussed below.

Glassy Carbon Electrode

Glassy carbon electrodes are widely used in electrochemical applications because of its excellent mechanical and electrical properties, wide potential window, chemical inertness (solvent resistance), and relative reproducible performance (2). Glassy carbon is a type of non-graphitizing carbon as it cannot transformed into crystalline graphite at higher temperatures. (15-16). Other properties of glassy carbon that have been exploited for electroanalytical applications include thermal stability, extreme resistance to chemical attack and very low solvent permeability. Glassy carbon has a lower rate of oxidation in oxygen, carbon dioxide or water vapour than any other allotrope of carbon except diamond (17). It is unaffected by treatment with concentrated sulphuric acid and nitric acid unlike graphite which is reduced to powder by a mixture of concentrated sulphuric and nitric acid (18). The structure of glassy carbon was first put forward by Jenkins and colleagues (17,19-20) and the model bears some similarities to the structure of a polymer. It was assumed that the molecular orientation of the polymeric precursor material is retained to some extent after carbonization thus producing a structure in which the 'fabril' are very narrow, curved and twisted ribbons of graphitic carbon. Several reviews have provided handful information on the physical and electrochemical properties of glassy carbon electrode. A highly porous reticulated vitreous carbon (RVC), which shares the same physicochemical properties with glassy carbon, is used in flow analysis and spectroelectrochemistry (21).

Carbon Fibre Electrodes

Carbon fibre electrode represent a generation of ultra-microelectrodes and their use has gained widespread application since first described in 1979 by Ponchon et al. (22) for the electrochemical determination of catecholamines using normal pulse polarography. These electrodes had a long exposed carbon tip typically less than 200µm. That same year, Amstrong-James and Miller developed a new short form of CFEs (electrode tip typically range from 10-30µm), for use in extracellular spike recordings in the CNS combined with micro-iontophoresis (23). Carbon fibre electrodes can be classified in to 3 categories depending on their manufacturing process and they are low-, medium-, and high-modulus type (2). The high-modulus type is mostly used for electrochemical studies because of its well-ordered graphite-like stricture and low porosity. The sharp pointed conical carbon tip of CFEs were normally produced by spark etching method and this methods of tip preparation is known to produce the best SNR and causes less damage to neuronal tissue during penetration when used for neurotransmitter spike recordings (24, 25). Most electroanalytical applications rely on fibres of 5-20µm diameter that provide the desired radial diffusion. Such fibres are typically inserted into a borosilicate capillary glass and held in place by epoxy glue (2, 24, 25). There are two types of CFEs, the single and multi-carbon fibre microelectrodes and they differ the in numbers of carbon fibres used in the microelectrode.

Carbon Nanotube Electrodes

Carbon nanotubes (CNTs) have become widely exploited in electrochemical analysis because of their unique geometric, mechanical, electronic and chemical properties, since its re-discovery in 1991 by Iijima, (4). Carbon nanotubes can be classified into two classes: single walled (SWCNTs) and multi-walled (MWCNTs). The SWCNTs are made up of a single graphite sheet rolled flawlessly producing a tube diameter of 1-2nm while the latter are several concentric tubes fitted one inside another (1). The multi-walled carbon nanotubes exist in different morphological variations-the 'hollow-tube', 'bamboo-like' or 'herringbone-like' MWCNTs. The hollow-tube MWCNTs has the axis of the graphite planes parallel to the axis of the CNT while in the herringbone morphology, the graphite planes are formed at an angle to the axis of the tube and finally, the bamboo MWCNTs are similar to herringbone but the only difference is along the length of the tube which is like a stack of paper cones placed one inside another (1). CNT modified electrode have been appraised for their low detection limits, increased sensitivity, decreased overpotentials and resistance to surface fouling (26). Carbon nanotubes have been suggested to have similar electrochemical properties to hopg with the open end likened to edge plane of hopg while the tube walls having properties similar to the basal plane of hopg (27). Electrochemical applications in which CNT electrode or CNT modified electrode was employed have shown enhanced electron transfer reactivity compared to other electrode materials for a variety of biological molecules such as NADH (28), dopamine (11), cytochrome (29), and DNA (30) to name of few.

Carbon Paste Electrode

Carbon paste electrode (CPE) which is a mixture of graphite powder and organic binder (binding/pasting liquids), offers a renewable and modified surface in combination with low cost and very low background current. It was first devised by Adams in late 1950s (69, 70) and its application in electroanalytical studies has grown ever since it was invented. The particle size of the multicrystalline graphite powder averages between 0.01-0.02mm in dia. with smaller graphite particles of 0.01mm dia. showing lower residual current. A wide range of binding liquids can be used but the choice is somewhat narrow to few liquids due to low volatility, purity and cost. Pasting liquids commonly used as binder include nujol, paraffin oil, silicone grease, ceresin wax and bromoform or bromonapthalene with nujol having the best performance (71). The composition of the pasting liquids can influence electrochemical response of the electrode. Increasing the pasting liquids decreases the rate of electron transfer and also the background current contribution. The chain length of hydrocarbon pasting liquids can also influence electrochemical performance. Activation of the electrode (chemical or electrochemical oxidative pre-treatment) significantly improves the electrode properties pushing it towards the dry graphite limit, probably forming groups with electrocatalytic or electron mediating qualities on the surface. The electrochemistry observed for CPEs has not been fully understood but it is thought to involve permeation of the pasting liquid layer by the electroactive species (solvent extraction). Many organic molecules show high affinity towards carbon paste and due to the hydrophobic nature of the electrode material, they can be extracted from solution with considerable ease. CPEs represent a suitable matrix in which appropriate modifying moeties can be incorporated and can be achieved through the followin methods: (i) adsorption of modifier, (ii) covalent binding of molecules, (iii) dissolution of lipophilic into the binding liquid and (iv) direct mixing particulate matter into the paste. Direct mixing of the modifier with the graphite/binder paste is the most used of all the methods as it does not require any special procedure in the attachment of the modifier moiety. Though, special attention in obtaining homogeneous mixture is required in order to achieve reproducibility in electrode performance which could be affected by inconsistency in the concentration of the active components on electrode surface (71). Modifiers used in direct mixing with the graphite/binder paste must be such that they are insoluble in the analyte solution so as to avoid 'electrode bleeding', a phenomenon which causes modifier concentration gradients on the electrode surface and electrode-solution interface. The first chemically modified CPEs were studied by Kuwana and co-worker in 1964 by dissolving electroactive compounds like ferrocene, anthraquinone or 4-aminobenzophenone in the liquid part of the paste (72, 73). This was followed by other researchers like Cheek and Nelson (74), who described a multistep modification procedure for CPEs that covalently bind complexing amino groups to carbon particles. Carbon paste electrodes have been extensively used for electroanalytical applications and they include ferrocene-substituted calix [4] pyrrole modified carbon paste for the detection of anions in water (75). Carbon paste electrode modified with oxovanadium (IV)-4-methyl salopen for the determination of nitrite (76) and electrochemical detection of dopamine using pthalic acid and trixton X-100 modified carbon paste electrode (77).

Diamond Electrode

Diamond electrodes have received much attention since the first work on the electrochemistry of boron doped diamonds (BDD) electrode was published by Pleskov et al in 1987 (78). There is now a shift towards diamond based electrodes with several modification techniques emerging rapidly. Diamond as electrode material has an outstanding physical, chemical and mechanical properties that have been exploited for a variety of electronic and electrochemical applications. In electrochemical applications, Boron doped diamond electrode offers outstanding properties which are different from conventional electrodes such as glassy carbon, platinum electrode, hopg and pyrolytic graphite. The properties include negative electron affinity (NEA) (79), wide electrochemical potential window in aqueous and non-aqueous media (80-81), extremely low capacitive current, high electrochemical stability, and low background current as a result of double layer capacitance (82-85). The high stability of diamond in severe chemical environment makes it suitable as electrode material in strongly acidic chloride and fluoride electrolytes. Diamond electrodes have also been found to be insensitive to dissolved oxygen and resistance to surface fouling (deactivation) (86). Boron doped diamond electrode, due to its inner-sphere mechanism exhibit very high overpotentials for the evolution of oxygen and hydrogen. Diamond is a good insulator but upon doping, the material can be made significantly conductive possessing either semiconductor or semimetallic electronic properties depending on the level of doping. Doping of diamond can be achieved through chemical vapour deposition (CVD) techniques. The addition of boron can be achieved by introducing B2H6 (87,) or B(OCH3)3 (88) to the gas stream during CVD or by placing boron powder near the edges of the substrate before insertion in the CVD chamber (89). A few modification techniques have been proposed for boron-doped diamond (BDD) electrodes. Modification with redox active particles or compounds can facilitate electron transfer between the electrode and the analytes significantly reducing activation overpotential. A large numbers of compounds or particles have been used to mediate electron transfer through modification of BDD surface. Platinum implanted BDD electrode for the electrochemical detection of hydrogen perioxide (90) is a very good example of metal particle modification. Deposition of small amount of IrOx clusters on BDD surface was demonstrated by Duo and co-worker to greatly enhance oxygen evolution reaction and the oxidation of organic species in potential region similar to the decomposition of water (91). In the detection of chemical and biological species, boron-doped diamond electrode is fast becoming electrode of choice due to its numerous advantage presenting new opportunities for electrochemical applications. It has been used in the electrochemical process for waste water treatment (92-93), detection of dye (94), pentachlorophenol (95), dopamine and NADH (96) to name a few.

FUTURE DIRECTIONS AND CHALLENGES

Carbon in the last two to three decade has experience tremendous growth in several areas of research and applications with scientists seeking more ways to maximizing the opportunities that could be derived from carbon materials to solving many of life's problems. As previously stated in the different forms of carbon investigated, carbon material possesses a great deal of flexibility in its ability to be manipulated and adapted to a range of analytical applications compared to conventional metallic materials when used as electrodes. The successful implementation of several modification techniques makes carbon material very robust in different electroanalytical applications. The evolution of carbon based electrode from bare carbon substrate to the new generation CNTs, BDD, hopg, buckminster fullerene and CFEs has opened up more opportunities for electrochemist in the detection/recognition of complex chemical and biological molecules at extremely low concentrations. These new generation carbon electrodes have also offered the opportunity of tremendous size reduction to the nanoscale level with enhanced electrochemical performance and it has revolutionised sensor design and applications. There has been several reports in the used of nanostructured carbon materials in the development of multi

AIMS AND OBJECTIVES

This research work sought to investigate new methods of designing carbon based sensors that could be used as the basis of a versatile detection system for conventional laboratory based instrumentation but which could also be readily transferred to emerging micro-nano scale devices. The main objectives of this project work are:

  1. To design and fabricate nanostructured carbon sensor using nanographite powder and composite polycarbonate granules.
  2. To exfoliate the surface of the nanostructured carbon sensor and functionalise the surface of the exfoliated graphitic carbon with metal nanoparticles and chemical species.
  3. To carry out characterization of fabricated nanostructured carbon sensor and access the electrochemical performance.
  4. To use the fabricated nanostructured carbon sensor in electrochemical detection of target chemical and biological species.

CHAPTER TWO

OVERVIEW OF ELECTROCHEMICAL PROCESSES

Electrochemical processes deal with the processes and factors that affect the transport of charge across interfaces between chemical phases. In electrochemical processes, one of the two phases contributing to the interface of interest is the electrolyte which is a phase through which charges are carried by the movement of ions. The electrolyte may be in the liquid phase (solutions) or in the solid phase (fused salts or ionically conducting solid with mobile ions). The electrode is the second phase of the interface at the boundary and it is the phase through which charges are carried by electronic movement. The electrode can either be solid or liquid and metals or semiconductors. In the context of experimental electrochemical processes, one cannot think about events at a single interface in isolation but rather, one must think about the properties of collection of interfaces called electrochemical cells which are systems described by two electrodes (or three electrodes depending on the cell design) separated by at least one electrolyte. It is at these electrodes that oxidation or reduction reactions (also known as electrochemical reactions or half reactions) occur via the consumption of electrons at one electrode and the supply of electrons at the other so that there is no net consumption of electrons, maintaining the law of conservation of energy. Whether current is flowing through the cell or not, there is a measurable difference in the potential between the two electrodes which is a manifestation of the collective difference in electric potential between all the different phases in the path of the current. The change in electric potential in moving across different conducting phases occurs at the electrode interface and the spontaneous nature of this transition indicates that a very high electric field exists at the interface and it can only exert great effects on the kinetic behaviour of the charge carriers at the interface region. In addition, the magnitude of the potential difference at the interface has an effect on the relative energies of the carriers in the two phases and thus controls the direction of charge transfer. The chemical changes occurring at the two electrodes is described by two independent half reactions which represents the overall changes taking place in an electrochemical cell and each half reaction reacts to the potential difference at the corresponding electrode. In most cases, the focus is always on one of these half reactions and the electrode at which it is occurring is known as the working electrode. In other to focus on the working electrode, the other half of the cell needs to be standardized. This can be achieved using an electrode made up of two distinct phases having constant composition known as reference electrode. Full description and details of the working and reference electrodes will be provided later on in this chapter. Any measurable quantity in the electrochemical cell is then ascribed to the working electrode since the potential of the reference electrode is fixed and not expected to change. Thus the potential of the working electrode is observed or controlled with respect to the reference electrode. The equivalent is by saying the energy of electrons within the working electrode is controlled or observed with respect to the reference electrode. By raising the energy level of the electrons within the working electrode (driving the electrode to a more negative potentials), the electrons will reach a level high enough to occupy the vacant states on the species in the electrolyte and they will flow from the electrode to the solution. Similarly, when the energy of the electrons within the electrode is lowered by imposition of a more positive potential, the electrons on the solute in the electrolyte will flow to the electrode when they find a more favourable energy. The flow of electrons from within the electrode to the solution and vice versa is known as reduction and oxidation current respectively. The critical potential at which the oxidation and reduction occurs is the standard potential Eo of the chemical species present in the system (X1, X2).

Electrochemical Cells Design

The design of electrochemical cells depend on whether current flowing through the electrical circuit is from the occurrence of spontaneous electrode reaction (galvanic cell) or from an external source which causes reaction to occur at the electrodes (electrolytic cell) by changing the electron energy within the electrodes as discussed above. The external energy source can be a variable voltage or current and can permit the transition from a galvanic cell to electrolytic cell through the controlled application of variable voltage or current. As previously stated, the working or indicator electrode and the half reaction occurring at the interfacial region of this electrode is often the primary focus in electrochemical experiments. The potential of this electrode is observed or controlled with respect to the reference electrode which is fixed and does not pass current. Thus the current of the electrochemical cell passes between the working electrode and a third electrode called auxiliary electrode. The electrochemical cell thus contains three electrode system viz. the working or indicator electrode, reference electrode and the auxiliary or counter electrode. In some electrochemical research, it might be required that the reaction products from the working electrode and the auxiliary electrode are kept separate, in such situation the electrochemical cell is often designed with a separator

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