Chapter ONE
Introduction
This research work sought to look into new methods of planing C based detectors that could be used as the footing of a various sensing system for conventional research lab based instrumentality but which could besides be readily transferred to emerging micro-nano graduated table devices.
Carbon based electrodes are widely used in electroanalytical applications because of their low cost, big potency window, low background current, various surface chemical science, fast negatron transportation dynamicss and suitableness for assorted feeling applications. The effect of which has led to the development of different types of C based electrodes in usage today for electroanalytical applications, dramatically altering the range of electrochemical methods in the measuring of diverse marks like ions, gases and biological markers ( 1-2 ) .
Carbon exists in assorted physical signifiers – most of which have been extensively studied as electrode stuffs over the old ages. With the exclusion of diamond, most signifiers of C can be easy functionalized through a assortment of reaction paths or through physical/mechanical interventions. These signifiers include a broad scope of graphitic C such as: pyrolytic black lead ( PG ) , formless C, glassy C ( GC ) , C black, C fiber, powdered black lead, and extremely ordered pyrolytic black lead ( HOPG ) . Each is capable of showing alone chemical and physical belongingss ( 3 ) . The most commonly employed C electrode stuffs tend to be those made of glassy C as it serves as a robust, all purpose conductive substrate that can be used for analyzing both oxidization and decrease procedures. Carbon movies or other C complexs like black lead edge with epoxy, wax, mineral oil or parafin are besides used but these tend to be employed for more specific applications ( 2 ) . Several other composite electrode stuffs have besides been reported and a few illustrations include: C nanotubes ( CNT ) , which has gained much prominence since its re-discovery by Iijima in 1991 ( 4 ) and polymer modified C electrodes which include those based on Nafion® and polypyrrole ( 5-7 ) .
The find of C can be dated back to the antediluvian times when it was known as carbon black and wood coal to the earliest human civilisation. Diamonds were likely known every bit early as 2500BCE in ancient China, while C in signifier of wood coal was made around Roman times by the same chemical science as it is today, by heating wood in a clay oven to except air. The historical development of C and timeline for the development of C engineering is given in the figure 1 below.
Carbon STRUCTURE
Most C based electrode stuffs were made entirely of sp2 hybrized C possessing the two-dimensional graphite sheet as their structural edifice block. The structural model is a hexangular lattice consisting of three co-ordinate C atoms in which the intra-planar C-C bond length varies from 1.39 & A ; Aring ; in benzine to about 1.42 & A ; Aring ; in black lead ( 2, 3 ) .
The intra-planar microcrystallite size or sidelong grain size ( La ) is the mean size of the microcrystallite along the x-axis and this fluctuation in La histories for the assortment of signifiers the stuff can take. The graphite microcrystallite size or sidelong grain size ( La ) can run from 3 & A ; Aring ; ( size of a benzine molecule ) to being really big – as in the instance of a macro individual crystal of black lead. Amorphous C, glassy C and C black are known to hold the smallest La values and these can be every bit low as 10 & A ; Aring ; . In contrast, C fibers and pyrolytic black leads have La values in the scope of 100 & A ; Aring ; and 1000 & A ; Aring ; severally ( 3, 8 ) .
The planes of graphite stack in an ABAB… sequence and the distance along the perpendicular axis, perpendicular to the plane of stack is known as interplanar microcrystallite size Lc ( as shown in Figure 1 ) . Other planar stacking sequences have besides been reported by McCreery in the instance of trigonal black lead which has an ABCABC… stacking sequence but this a rare occurence ( 3 ) . The interplanar microcrystallite size ( Lc ) scope from 10 & A ; Aring ; in extremely broken C, as in the instance of C black, to arround 10 µm in natural individual black lead crystals. The fluctuations in La and Lc history for the alteration in belongingss of C usually experienced in graphitized C. Graphitization of C occurs when C ( with little Lc ) is heated above 2000oC in order to increase the Lc size. This procedure makes the sp2 C planes stack parallel to each other thereby increasing Lc, doing the planes glide over one another ( which makes graphitized C a good lubricator ) and the stuff softer ( 3 ) . The Interplanar spacing d002 ( shown in Figure 1 ) varies from 3.354 & A ; Aring ; in individual crystal black lead to 3.6 & A ; Aring ; in less ordered sp2 C and can be larger ( ca. 10 & A ; Aring ; ) in intercalated black lead. The term d002, which was derived from x-ray pulverization diffraction appellation, is used to bespeak the contemplation matching to the interplanar spacing. The d002 value can be sligthly higher in most man-made turbostratic black lead ( 3 ) .
The largest graphite individual crystals are found in commercial class hopg ( ZYA and SPI 1 class hopg ) and these can be between 1-10 millimeter in size ( 3, 9 ) . The grain boundaries where single black lead monocrystals meet are ill defined and, hence, suffer from surface defects when exposed. In the instance of pyrolytic black lead, the single black lead crystallite lies along the same plane doing the C surfaces significantly less prone to defects. Such surface defects in hopg can be every bit low as 0.2 % in coverage and this is a effect of the big sidelong grain size ( 3, 10 ) . Electrochemical public presentation, background current and negatron transportation dynamicss have all been attributed to structural defects on stuff surfaces – irrespective of type. Reports have been presented associating surface features to electrochemical behavior, electrical capacity and surface assimilation for many electrode stuffs. The distribution of border and basal planes on stuff surfaces has been the particular focal point for electrochemists with studies bespeaking more electrochemical ( and chemical ) responsiveness at the border plane sites than the basal plane sites. The border plane sites of a hopg have been reported to exhibit faster negatron transportation, strong surface assimilation and low electrical capacity ( 1-10 ) .
The structure-reactivity relationship of electrode stuffs is a really of import focal point point for biochemical surveies. Biologically active molecules like Dopastat were shown ( Britto and colleague, 1996 ) to be successfully oxidized at the border sites of a C nanotube modified electrode. ( 11 ) . Several other studies have been presented on the chemical responsiveness, fast negatron transportation dynamicss, enhanced sensitivenesss, lower sensing bound, low surface fouling, low background current and electrocatalysis occurring at the border plane site of CNT and pyroytic black lead ( 1,12-13 ) . Wang`s group showed that the electrocatalytic activities, background currents and electroanalytical public presentations of CNT modified electrodes are strongly dependent on border plane-like site/defect found at the terminals of the tubing construction ( 14 ) . Compton et al extensively studied the beginnings of antecedently reported electro-catalytic responses of CNT modified electrodes alongside border plane pyrolytic black lead ( EPPG ) . The catalytic response was later attributed to inch plane sites happening at the terminals of the unfastened tubings and along the tubing axis ( 1, 8, 12 ) . The border plane sites have been reported to exhibit faster electrode dynamicss in comparing to the basal plane and in many cases, an electrode dwelling chiefly of border plane ( as in eppg electrode ) will demo a about reversible voltammogram. In contrast, a bppg electrode tends to demo irreversible behavior depending on the sum of border sites present on the electrode. ( 12 ) .
The electronic construction and chemical responsiveness of the border plane defect sites on black lead can be better understood in footings of the set theory of solids. The HOMO and LUMOs in black lead ( presuming an infinite graphite sheet ) are closely separated and as a consequence they overlap to organize the valency and conductivity sets which are separated by little energy spread, therefore leaving semi-metallic conductive belongingss to the stuff. The conductive belongings of the stuff is greater along the basal plane than the border plane due to due to the ability of negatrons to skip between graphene sheets in the latter instance. When the graphene sheet terminates at the border plane site, the set construction at the border plane site besides terminates suddenly and this causes the energy of the negatron in the valency set to lift aggressively. Therefore doing the border plane defect site high energy defects and sites at which negatron transportation takes topographic point and more chemically reactive than the basal plane site ( 47, 8 ) .
CHEMISTRY OF CARBON SURFACE
Carbon as a majority stuff is comparatively inert but the presence of intrinsic reactive surface defects can be exploited and manipulated to leave certain physicochemical belongingss. The intelligent use of the surface chemical science of C can be achieved in a figure of ways either through functionalisation or alteration of species already present on the surface or merely debut of new species or more commonly exfoliation of the intrinsic reactive surface defects.
The chemical science of graphitic C can hold a singular influence on the electrochemical public presentation of the stuff. As antecedently noted, structural defects on the surface of the stuff, viz. the basal plane and border plane site show some chemical responsivenesss of their ain with border plane site more chemically reactive than the basal plane site. These extremely reactive border plane defect sites on graphitic C can respond with atmospheric O and/or H2O ensuing in the surface being functionalize with a assortment of O incorporating group which includes quinonyl, hydroxyl, phenol and carboxylic acid. The surface oxides on graphitic C have an tremendous consequence on the chemical science of the stuff ( 3, 47 ) . The surface oxides present on C surface have been classified by some writers as acidic, basic or impersonal harmonizing to their responsiveness with known acids and bases ( 48, 49 ) . The type and measure of the oxide groups available on C varies well with the stuffs and pretreatment history ( 3 ) . The Numberss of synthetically utile functional groups on C surface peculiarly carboxyl, hydroxyl and quinones can be greatly enhanced by a series of chemical pretreatment stairss. A batch of techniques have been proposed by different research workers in the past and I can`t be thorough plenty in this study. The most common of these pretreatment techniques is stirring C nanotubes ( CNT ) in a mixture of strong oxidising acids such as H2SO4 and HNO3 ( 50-51 ) and the consequence of this method on the measure and distribution of carboxyl group on graphitic C have been extensively studied ( 53-55 ) . Mao et Al ( 52 ) showed that electrocatalytical sensing of molecules like thiols were made possible by the built-in oxidation-reduction belongingss of surface functional groups on SWCNTs. This was achieved by immobilising ortho-quinone derived functions onto SWCNTs in other to show how these surface functional groups can interact and heighten detection applications.
MODIFICATION OF CARBON SURFACE
Several techniques have been developed in modifying the surface of graphitic C in recent old ages and can be loosely classified into mechanical, chemical ( 34, 35, 36 ) , electrochemical ( 8, 13, 37, 38 ) , thermic ( 39, 56-57 ) and plasma ( 40 ) alterations. Such surface alterations can either affect alteration of species already present on the surface by increasing the Numberss of surface functional group. The fond regard of exogenic species onto the electrode surface or the embolism of species between the graphene beds of the base substrate. The latter can be achieved through electrochemical pre-treatment procedures as a consequence of C fracturing ( 41-42 ) .
Mechanical Alteration
Mechanical activation of electrode surface is a method that described a scope of superficial procedures such as extinguishing drosss on the electrode surface, creative activity of new electrode surface and exposure of surface functional groups. The technique has been shown to be a various agencies of heightening electrode public presentation and bettering sensitiveness through roughening of surface by scratchy shining ( 12, 32, 33 ) , Mechanical activation of this nature is usually done on a glassy C and bppg electrode where scratchy shining and/or sonication is known to increase the figure of border sites on the electrode surface in other to better electroanalytical public presentation. ( 2, 21, 43 ) .
Thermal and Plasma Modification
Thermal and plasma alteration are other techniques that has been extensively employed in C surface alteration. In thermic oxidization of C, C fibers are heated in air to a temperature of between 400-600deg Celsius ( 39, 56-57 ) . The thermic intervention of C fibers increases the adhesion belongingss, wettability and surface raggedness of the stuff among other effects and making new border plane sites on C surface. In plasma alteration, C fibers are continuously treated with dielectric discharge plasma at atmospheric force per unit area in assorted gas conditions ( 40 ) . This technique of C surface alteration has been shown to increase the figure O on the surface of C fiber stuff.
Chemical Modification
Chemical alteration of C surface represents a modern attack to come up use and this could be achieved by arrangement of chemical constituents onto the electrode surface ( surface assimilation, covalent binding, polymer movie deposition, polymer coatings ) to leave the belongingss ( chemical, electrochemical, optical and other belongingss ) of that constituent to the modified surface ( 2 ) . The choice of the immobilized chemical constituents are based on known or desired belongingss such as rapid outer-sphere negatron agents, chiral centres, negatron transportation mediator-catalysts for valuable substrate reaction, functionalities capable of scavenging for hint molecules or ions from solutions, corrosion inhibitors and so on ( 58 ) . This method of surface alteration offers greater control over the electrochemical behavior given the greater assortment of functionality that can be introduced onto the electrode surface and therefore supplying solutions to host of electroanalytical jobs. Chemical alteration is a far better attack than the activation of intrinsic functionalities such as quinone and hydroxyl groups. There are 3 waies in which chemical alteration is plausible: the formation of covalent bonds ( 36, 44-45 ) , irreversible surface assimilation ( 46 ) , and electrochemically induced functionalisation of surfaces by exogenic species. The latter provides effectual option to pure chemical methods. The mechanistic attack to covalent alteration of C surface has been presented in some literatures and a good studied system is the covalent alteration of CNT surface with aryl diazonium salts. This strategy involves the direct electrochemical activation of aryl diazonium salts taking to the formation of covalent bond between the aryl group and the MWCNT through a one-step negatron decrease at the electrode surface with subsequent loss of N2 and the formation of the corresponding aryl group which so forms a covalent bond with the stuff surface ( 59,60 ) . Compton et Al proposed chemical method instead than electrochemical activation of aryl diazonium salts through decrease 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 add-on of hypophosphorous acid ( 50 % w/w in H2O ) . The mechanism for this chemical immobilisation of aryl diazonium salt onto the electrode surface via decrease with hypophosphorous acid is shown in figure 4.
Polymer Coatings
One of the most various attacks for integrating a qualifier onto the electrode surface is immobilisation of chemical species in a polymer movie. Modifiers immobilised by polymer onto the electrode surface can be achieved through polymer coating of the electrode surface with solution incorporating the dissolved polymer or via electropolymerization in the presence of dissolved monomer. In polymer coated electrode, the polymer movies contain electrochemically and/or chemical reactive centres which can undergo negatron transportation reaction with the electrode. Since the movies by and large contain every bit much monomolecular layers` worth of electroactive sites, electrochemical sensitivenesss are greater than those of immobilised molecular bed. The concentration of electroactive sites in footings of volume in polymer movie is high and can run from 0.1-5M while the measure of electroactive centres can run from ca.10-10 to ca. 1 ten 10-5 or from less than 1 to greater than 20,000 monolayers and this could act upon the chemical responsiveness of the site as their dissolver and ionic environment are different from homogeneous solutions ( 58 ) . The polymers available for functionalising electrodes surface can be grouped under three different classs, which are carry oning polymers, nonconductive polymers and oxidation-reduction gels.
Conducting polymers or inherently carry oning polymers ( ICPs ) such as polypyrrole, polythiophene, polyaniline, poly-indole-carboxylic acids to name of few have all been greatly used in functionalising C surface and the chemical science and mechanism of these carry oning polymers good understood. They possess the ability to exchange, reversibly between their positively charged carry oning provinces to a impersonal insulating signifier ( passivate ) and incorporate and throw out anionic species from reaching solution during redox rections. In most instances, polymers can be doped and the dopant anions serves to compensate the polymer`s positively charged anchor and maintains electrical neutrality. The oxidization and decrease alterations in polymers is non localised at a specific electroactive centre but delocalised over a figure of carry oning polymer. The ability of the pi-electron system to conjugate creates a molecular orbital which extends the full polymeric concatenation and the electrical conduction of the polymer movie ensuing from the electronic construction of the polymeric anchor varies with applied potency. This value ( electronic conduction ) is dependent on the sum of bearers ( negatrons or holes ) created in the polymer concatenation and the mobility of the bearer through the polymer. The polymer concatenation is determined by the measure of dopant nowadays in the polymer and the dopant could run from big organic anions to DNA. These dopants are strongly attached to the polymer web and are hard to take. Therefore, when the polymeric concatenation is in the impersonal or decreased province, the negative charge of the anionic dopant entrapped in the polymeric web is balanced by interpolation of the electrolyte cation ( 2, 58 ) .
The readying of inherently carry oning polymers usually Begins by unmoved electrochemical polymerisation from the monomer solution. The first measure is frequently electro-oxidative formation of cationic group from the get downing monomer, followed by dimerization procedure and farther oxidization. The concluding measure is the matching reaction taking to the the formation of strongly adhering polymeric movie on the surface of the electrode by either of potentiometry, galvanostatic or multiscan method ( 2, 61 ) . By attaching assorted biological and chemical species to the monomer prior to electropolymerization, alterations in the physical belongingss of carry oning polymer can be induced. Properties like molecular acknowledgment and electrocatalytic action through add-on of functional dopants ( complexing agent or negatron transportation go-between ) can be imparted, therefore moving as efficient molecular interfaces between acknowledgment elements and electrode transducers. Conducting polymers offer diverse electrochemical applications including fuel cell, corrosion inhibitors, batteries and chemical detection, owing to their alone physical and chemical belongingss, chiefly the governable and dramatic alteration in electrical conduction and ability to throw out dopant ions. Such chemical feeling include solid-state gas detection ( 62 ) , entrapment and stabilisation of biological molecules, direct detection of DNA hybridisation ( 61 ) and antibody-antigen binding to call a few.
Recently developed carry oning polymer nanowires are characterized by governable size and shape-dependent chemical and physical belongingss and high surface to volume ratio compared to conventional carry oning polymer movie. They hold promising hereafter for chemiresistor and field consequence transistor ( FET ) and molecular electronic device ( 63, 64 ) . They are easy prepared by template-directed electrochemical synthesis affecting electrodeposition into the pores of a membrane templet or within the microchannels in contact with next microelectrode ( 65 ) .
Exfoliation OF CARBON SURFACE
Exfoliation of C surface represents an alternate attack to creative activity of reactive border plane defect on C surface and effect addition in surface functional ( oxygen incorporating ) groups. While the former tends to be the chief ground behind exfoliation of C surface, other grounds include production of graphene sheet ( EG ) , graphite oxide ( EGO ) and scattering of individual walled C nanotubes. Several methods have been proposed for the exfoliation of C surface and they include laser extirpation ( 66 ) , electron beam irradiation ( 67 ) and micro-cook irradiation ( 68 ) and electrolytic exfoliation utilizing poly ( sodium-4-styrenesulfonate ) as an effectual electrolyte. These methods of C surface exfoliation and many more other methods non mentioned here have all been extensively studied by different research workers in electrochemical applications.
CARBON ELECTRODES
Practically, all the signifiers of graphitic and formless C known to electrochemists have been extensively used as electrode stuffs in other to impact characteristic physicochemical belongingss and better electroanalytical public presentation. A few of the signifiers of C are discussed below.
Glassy Carbon Electrode
Glassy C electrodes are widely used in electrochemical applications because of its first-class mechanical and electrical belongingss, broad potency window, chemical inertness ( solvent opposition ) , and comparative consistent public presentation ( 2 ) . Glassy C is a type of non-graphitizing C as it can non transformed into crystalline black lead at higher temperatures. ( 15-16 ) . Other belongingss of glassy C that have been exploited for electroanalytical applications include thermic stableness, utmost opposition to chemical onslaught and really low dissolver permeableness. Glassy C has a lower rate of oxidization in O, C dioxide or H2O vapour than any other allotrope of C except diamond ( 17 ) . It is unaffected by intervention with concentrated sulfuric acid and azotic acid unlike black lead which is reduced to pulverize by a mixture of concentrated sulphuric and azotic acid ( 18 ) . The construction of glassy C was first put frontward by Jenkins and co-workers ( 17,19-20 ) and the theoretical account bears some similarities to the construction of a polymer. It was assumed that the molecular orientation of the polymeric precursor stuff is retained to some extent after carbonisation therefore bring forthing a construction in which the ‘fabril ‘ are really narrow, curved and distorted threads of graphitic C. Several reappraisals have provided handful information on the physical and electrochemical belongingss of glassy C electrode. A extremely porous reticulated vitreous C ( RVC ) , which portions the same physicochemical belongingss with glassy C, is used in flow analysis and spectroelectrochemistry ( 21 ) .
Carbon Fibre Electrodes
Carbon fiber electrode represent a coevals of ultra-microelectrodes and their usage has gained widespread application since first described in 1979 by Ponchon et Al. ( 22 ) for the electrochemical finding of catecholamines utilizing normal pulse polarography. These electrodes had a long open C tip typically less than 200µm. That same twelvemonth, Amstrong-James and Miller developed a new short signifier of CFEs ( electrode tip typically range from 10-30µm ) , for usage in extracellular spike recordings in the CNS combined with micro-iontophoresis ( 23 ) . Carbon fiber electrodes can be classified in to 3 classs depending on their fabrication procedure and they are low- , medium- , and high-modulus type ( 2 ) . The high-modulus type is largely used for electrochemical surveies because of its well-ordered graphite-like stenosis and low porousness. The crisp pointed conelike C tip of CFEs were usually produced by flicker etching method and this methods of tip readying is known to bring forth the best SNR and causes less harm to neural tissue during incursion when used for neurotransmitter spike recordings ( 24, 25 ) . Most electroanalytical applications rely on fibers of 5-20µm diameter that provide the coveted radial diffusion. Such fibers are typically inserted into a borosilicate capillary glass and held in topographic point by epoxy gum ( 2, 24, 25 ) . There are two types of CFEs, the individual and multi-carbon fiber microelectrodes and they differ the in Numberss of C fibers used in the microelectrode.
Carbon Nanotube Electrodes
Carbon nanotubes ( CNTs ) have become widely exploited in electrochemical analysis because of their alone geometric, mechanical, electronic and chemical belongingss, since its re-discovery in 1991 by Iijima, ( 4 ) . Carbon nanotubes can be classified into two categories: individual walled ( SWCNTs ) and multi-walled ( MWCNTs ) . The SWCNTs are made up of a individual graphite sheet rolled cleanly bring forthing a tube diameter of 1-2nm while the latter are several homocentric tubings fitted one interior another ( 1 ) . The multi-walled C 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 black lead planes are formed at an angle to the axis of the tubing and eventually, the bamboo MWCNTs are similar to herringbone but the lone difference is along the length of the tubing which is like a stack of paper cones placed one interior another ( 1 ) . CNT modified electrode have been appraised for their low sensing bounds, increased sensitiveness, decreased overpotentials and opposition to come up fouling ( 26 ) . Carbon nanotubes have been suggested to hold similar electrochemical belongingss to hopg with the unfastened terminal likened to inch plane of hopg while the tubing walls holding belongingss similar to the basal plane of hopg ( 27 ) . Electrochemical applications in which CNT electrode or CNT modified electrode was employed have shown enhanced negatron transportation responsiveness compared to other electrode stuffs for a assortment of biological molecules such as NADH ( 28 ) , Dopastat ( 11 ) , cytochrome ( 29 ) , and DNA ( 30 ) to name of few.
Carbon Paste Electrode
Carbon paste electrode ( CPE ) which is a mixture of graphite pulverization and organic binder ( binding/pasting liquids ) , offers a renewable and modified surface in combination with low cost and really low background current. It was foremost devised by Adams in late fiftiess ( 69, 70 ) and its application in electroanalytical surveies has grown of all time since it was invented. The atom size of the multicrystalline graphite pulverization norms between 0.01-0.02mm in Defense Intelligence Agency. with smaller black lead atoms of 0.01mm Defense Intelligence Agency. demoing lower residuary current. A broad scope of adhering liquids can be used but the pick is slightly narrow to few liquids due to low volatility, pureness and cost. Gluing liquids normally used as binder include nujol, paraffin oil, silicone lubricating oil, ceresin wax and tribromomethane or bromonapthalene with nujol holding the best public presentation ( 71 ) . The composing of the pasting liquids can act upon electrochemical response of the electrode. Increasing the pasting liquids decreases the rate of negatron transportation and besides the background current part. The concatenation length of hydrocarbon gluing liquids can besides act upon electrochemical public presentation. Activation of the electrode ( chemical or electrochemical oxidative pre-treatment ) significantly improves the electrode belongingss forcing it towards the dry black lead bound, likely organizing groups with electrocatalytic or electron mediating qualities on the surface. The electrochemistry observed for CPEs has non been to the full understood but it is thought to affect pervasion of the pasting liquid bed by the electroactive species ( solvent extraction ) . Many organic molecules show high affinity towards C paste and due to the hydrophobic nature of the electrode stuff, they can be extracted from solution with considerable easiness. CPEs represent a suited matrix in which appropriate modifying moeties can be incorporated and can be achieved through the followin methods: ( I ) surface assimilation of qualifier, ( two ) covalent binding of molecules, ( three ) disintegration of lipotropic into the binding liquid and ( four ) direct blending particulate affair into the paste. Direct commixture of the qualifier with the graphite/binder paste is the most used of all the methods as it does non necessitate any particular process in the fond regard of the modifier mediety. Though, particular attending in obtaining homogenous mixture is required in order to accomplish duplicability in electrode public presentation which could be affected by incompatibility in the concentration of the active constituents on electrode surface ( 71 ) . Modifiers used in direct commixture with the graphite/binder paste must be such that they are indissoluble in the analyte solution so as to avoid ‘electrode shed blooding ‘ , 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 colleague in 1964 by fade outing electroactive compounds like ferrocene, anthraquinone or 4-aminobenzophenone in the liquid portion of the paste ( 72, 73 ) . This was followed by other research workers like Cheek and Nelson ( 74 ) , who described a multistep alteration process for CPEs that covalently bind complexing amino groups to carbon atoms. Carbon paste electrodes have been extensively used for electroanalytical applications and they include ferrocene-substituted calix [ 4 ] pyrrole modified C paste for the sensing of anions in H2O ( 75 ) . Carbon paste electrode modified with oxovanadium ( IV ) -4-methyl salopen for the finding of nitrite ( 76 ) and electrochemical sensing of Dopastat utilizing pthalic acid and trixton X-100 modified C paste electrode ( 77 ) .
Diamond Electrode
Diamond electrodes have received much attending since the first work on the electrochemistry of B doped diamonds ( BDD ) electrode was published by Pleskov et Al in 1987 ( 78 ) . There is now a displacement towards diamond based electrodes with several alteration techniques emerging quickly. Diamond as electrode stuff has an outstanding physical, chemical and mechanical belongingss that have been exploited for a assortment of electronic and electrochemical applications. In electrochemical applications, Boron doped diamond electrode offers outstanding belongingss which are different from conventional electrodes such as glassy C, Pt electrode, hopg and pyrolytic black lead. The belongingss include negative negatron affinity ( NEA ) ( 79 ) , broad electrochemical potency window in aqueous and non-aqueous media ( 80-81 ) , highly low capacitive current, high electrochemical stableness, and low background current as a consequence of dual bed electrical capacity ( 82-85 ) . The high stableness of diamond in terrible chemical environment makes it suited as electrode stuff in strongly acidic chloride and fluoride electrolytes. Diamond electrodes have besides been found to be insensitive to fade out O and opposition to come up fouling ( inactivation ) ( 86 ) . Boron doped diamond electrode, due to its inner-sphere mechanism exhibit really high overpotentials for the development of O and H. Diamond is a good dielectric but upon doping, the stuff can be made significantly conductive possessing either semiconducting material or semimetallic electronic belongingss depending on the degree of doping. Doping of diamond can be achieved through chemical vapour deposition ( CVD ) techniques. The add-on of B can be achieved by presenting B2H6 ( 87, ) or B ( OCH3 ) 3 ( 88 ) to the gas watercourse during CVD or by puting B pulverization near the borders of the substrate before interpolation in the CVD chamber ( 89 ) . A few alteration techniques have been proposed for boron-doped diamond ( BDD ) electrodes. Alteration with redox active atoms or compounds can ease negatron transportation between the electrode and the analytes significantly cut downing activation overpotential. A big Numberss of compounds or atoms have been used to intercede negatron transportation through alteration of BDD surface. Platinum implanted BDD electrode for the electrochemical sensing of H perioxide ( 90 ) is a really good illustration of metal atom alteration. Deposition of little sum of IrOx bunchs on BDD surface was demonstrated by Duo and colleague to greatly heighten O development reaction and the oxidization of organic species in possible part similar to the decomposition of H2O ( 91 ) . In the sensing of chemical and biological species, boron-doped diamond electrode is fast going electrode of pick due to its legion advantage showing new chances for electrochemical applications. It has been used in the electrochemical procedure for waste H2O intervention ( 92-93 ) , sensing of dye ( 94 ) , pentachlorophenol ( 95 ) , Dopastat and NADH ( 96 ) to call a few.
Future DIRECTIONS AND CHALLENGES
Carbon in the last two to three decennary has experience enormous growing in several countries of research and applications with scientists seeking more ways to maximising the chances that could be derived from C stuffs to work outing many of life ‘s jobs. As antecedently stated in the different signifiers of C investigated, carbon stuff possesses a great trade of flexibleness in its ability to be manipulated and adapted to a scope of analytical applications compared to conventional metallic stuffs when used as electrodes. The successful execution of several alteration techniques makes C material really robust in different electroanalytical applications. The development of C based electrode from au naturel C substrate to the new coevals CNTs, BDD, hopg, buckminster fullerene and CFEs has opened up more chances for electrochemist in the detection/recognition of complex chemical and biological molecules at highly low concentrations. These new coevals C electrodes have besides offered the chance of enormous size decrease to the nanoscale degree with enhanced electrochemical public presentation and it has revolutionised detector design and applications. There has been several studies in the used of nanostructured C stuffs in the development of multi
AIMS AND OBJECTIVES
This research work sought to look into new methods of planing C based detectors that could be used as the footing of a various sensing system for conventional research lab based instrumentality but which could besides be readily transferred to emerging micro-nano graduated table devices. The chief aims of this undertaking work are:
- To plan and manufacture nanostructured C detector utilizing nanographite pulverization and composite polycarbonate granules.
- To exfoliate the surface of the nanostructured C detector and functionalise the surface of the exfoliated graphitic C with metal nanoparticles and chemical species.
- To transport out word picture of fabricated nanostructured C detector and entree the electrochemical public presentation.
- To utilize the fancied nanostructured C detector in electrochemical sensing of mark chemical and biological species.
Chapter TWO
OVERVIEW OF ELECTROCHEMICAL PROCESSES
Electrochemical procedures trade with the procedures and factors that affect the conveyance of charge across interfaces between chemical stages. In electrochemical procedures, one of the two stages lending to the interface of involvement is the electrolyte which is a stage through which charges are carried by the motion of ions. The electrolyte may be in the liquid stage ( solutions ) or in the solid stage ( fused salts or ionically carry oning solid with nomadic ions ) . The electrode is the 2nd stage of the interface at the boundary and it is the stage through which charges are carried by electronic motion. The electrode can either be solid or liquid and metals or semiconducting materials. In the context of experimental electrochemical procedures, one can non believe about events at a individual interface in isolation but instead, one must believe about the belongingss of aggregation 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 decrease reactions ( besides known as electrochemical reactions or half reactions ) occur via the ingestion of negatrons at one electrode and the supply of negatrons at the other so that there is no net ingestion of negatrons, keeping the jurisprudence of preservation of energy. Whether current is fluxing through the cell or non, there is a mensurable difference in the potency between the two electrodes which is a manifestation of the corporate difference in electric potency between all the different stages in the way of the current. The alteration in electric potency in traveling across different conducting stages occurs at the electrode interface and the self-generated nature of this passage indicates that a really high electric field exists at the interface and it can merely exercise great effects on the kinetic behavior of the charge bearers at the interface part. In add-on, the magnitude of the possible difference at the interface has an consequence on the comparative energies of the bearers in the two stages and therefore controls the way of charge transportation. The chemical alterations happening at the two electrodes is described by two independent half reactions which represents the overall alterations taking topographic point in an electrochemical cell and each half reaction reacts to the possible difference at the corresponding electrode. In most instances, the focal point is ever on one of these half reactions and the electrode at which it is happening is known as the working electrode. In other to concentrate on the working electrode, the other half of the cell needs to be standardized. This can be achieved utilizing an electrode made up of two distinguishable stages holding changeless composing known as mention electrode. Full description and inside informations of the working and mention electrodes will be provided subsequently on in this chapter. Any mensurable measure in the electrochemical cell is so ascribed to the working electrode since the potency of the mention electrode is fixed and non expected to alter. Thus the potency of the working electrode is observed or controlled with regard to the mention electrode. The equivalent is by stating the energy of negatrons within the working electrode is controlled or observed with regard to the mention electrode. By raising the energy degree of the negatrons within the working electrode ( driving the electrode to a more negative potencies ) , the negatrons will make a flat high plenty to busy the vacant provinces on the species in the electrolyte and they will flux from the electrode to the solution. Similarly, when the energy of the negatrons within the electrode is lowered by infliction of a more positive potency, the negatrons on the solute in the electrolyte will flux to the electrode when they find a more favorable energy. The flow of negatrons from within the electrode to the solution and frailty versa is known as decrease and oxidization current severally. The critical potency at which the oxidization and decrease occurs is the standard possible Eo of the chemical species present in the system ( X1, X2 ) .
Electrochemical Cells Design
The design of electrochemical cells depend on whether current fluxing through the electrical circuit is from the happening of self-generated electrode reaction ( voltaic cell ) or from an external beginning which causes reaction to happen at the electrodes ( electrolytic cell ) by altering the negatron energy within the electrodes as discussed above. The external energy beginning can be a variable electromotive force or current and can allow the passage from a voltaic cell to electrolytic cell through the controlled application of variable electromotive force or current. As antecedently stated, the working or index electrode and the half reaction happening at the interfacial part of this electrode is frequently the primary focal point in electrochemical experiments. The potency of this electrode is observed or controlled with regard to the mention electrode which is fixed and does non go through current. Thus the current of the electrochemical cell base on ballss between the working electrode and a 3rd electrode called subsidiary electrode. The electrochemical cell therefore contains three electrode system viz. the working or index electrode, mention electrode and the subsidiary or antagonistic electrode. In some electrochemical research, it might be required that the reaction merchandises from the working electrode and the subsidiary electrode are kept separate, in such state of affairs the electrochemical cell is frequently designed with a centrifuge
