Carbon-Paste Electrodes

2 Carbon Paste Electrodes Carbon paste electrodes, which use graphite powder mixed with various water-immiscible nonconducting organic binders (pasting liquids), offer an easily renewable and modified surface, low 

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FIGURE 3-21 Typical amperometric readout during automated flow injection assays of ethanol at an enzyme carbon-paste electrode. Peaks a through h 2 x 10 5 M to 1.6 x 10 4 M ethanol. 

FIGURE 3-27 Three-dimensional chromatogram for oxidizable biological compounds at a multichannel amperometric detection system, consisting of an array of 16 carbon-paste electrodes held at different potentials. AA = ascorbic acid NE = norepinephrine DOPAC = 3,4-dihydroxyphenylacetic acid 5-HIAA = 5-hydroxyindole-3-acetic acid DA = dopamine HVA = homovanillic acid. (Reproduced with permission from reference 68.)... 

FIGURE 4-15 Cyclic voltanmiograms for 1.5 x 10 3 M ribose (a), glucose (b), galactose (c), and fructose (d) recorded at a Ru02-modified carbon-paste electrode. Dotted lines were obtained in carbohydrate-free solutions. (Reproduced with permission from reference 50.)... 

FIGURE 6-24 Response pattern of an amperometric sensor array for various carbohydrates. The array comprised carbon-paste electrodes doped with CoO (1), Cu20 (2), NiO (3) and Ru02 (4). (Reproduced with permission from reference 84.)... 

Preconcentration of metal cations is also achieved by providing ligand binding sites in polymer layers, e.g. 4-methyl-4 -vinyl-2,2 -bipyridine for Fe " and Cu ". Carbon paste electrodes containing dimethylglyoxime or o-phenanthroline... 

Homogeneous electrochemical enzyme immunoassays for both phenytoin and digoxin have been developed. In both cases the label was glucose-6-phosphate dehydrogenase, which catalyzes the reduction of NAD to NADH. The NADH produced was detected by LCEC at a carbon paste electrode. 

If the electrochemical sensor does not require a permselective membrane, immobilization of the enzyme onto the surface of the electrode is possible. Glassy carbon graphite reticulated vitreous carbon and carbon paste electrodes... 

Skotheim et al. [286, 357, 362] have performed in situ electrochemistry and XPS measurements using a solid polymer electrolyte (based on poly (ethylene oxide) (PEO) [363]), which provides a large window of electrochemical stability and overcomes many of the problems associated with UHV electrochemistrty. The use of PEO as an electrolyte has also been investigated by Prosperi et al. [364] who found slow diffusion of the dopant at room temperature as would be expected, and Watanabe et al. have also produced polypyrrole/solid polymer electrolyte composites [365], The electrochemistry of chemically prepared polypyrrole powders has also been investigated using carbon paste electrodes [356, 366] with similar results to those found for electrochemically-prepared material. 

In recent years, the electrodes of similar structure have also found application in the electroanalytical chemistry, for purposes of researching the electrochemical behavior of solid substances, etc. These electrodes are known as the modified carbon paste electrodes or carbon paste electroactive electrodes [2]. 

F.-C. Gong, X.-B. Zhang, C.-C. Guo, G.-L. Shen, and R.-Q. Yu, Amperometric metronidazole sensor based on the supramolecular recognition by metallopor-phyrin incorporated in carbon paste electrode, Sensors, 3 (2004) 91-100. 

A.A. Ciucu, C. Negulescu, and R.P. Baldwin, Detection of pesticides using an amperometric biosensor based on ferophthalocyanine chemically modified carbon paste electrode and immobilized bienzymatic system. Biosens. Bioelectron. 18, 303-310 (2003). 

Z.M. Liu, H.F. Yang, Y.F. Li, Y.L. Liu, G.L. Shen, and R.Q. Yu, Core-shell magnetic nanoparticles applied for immobilization of antibody on carbon paste electrode and amperometric immunosensing. 

V.G. Bonifacio, L.H. Marcolino, M.F.S. Teixeira, and O. Fatibello-Filho, Voltammetric determination of isoprenaline in pharmaceutical preparations using a copper(II) hexacyanoferrate(III) modified carbon paste electrode. Microchem. J. 78, 55-59 (2004). 

A. Abbaspour and M.A. Mehrgardi, Electrocatalytic oxidation of guanine and DNA on a carbon paste electrode modified by cobalt hexacyanoferrate films. Anal. Chem. 76, 5690-5696 (2004). 

D. Moscone, D. D Ottavi, D. Compagnone, G. Palleschi, and A. Amine, Construction and analytical characterization of Prussian Blue-based carbon paste electrodes and their assembly as oxidase enzyme sensors. Anal. Chem. 73, 2529—2535 (2001). 

XOD is one of the most complex flavoproteins and is composed of two identical and catalytically independent subunits each subunit contains one molybdenium center, two iron sulfur centers, and flavine adenine dinucleotide. The enzyme activity is due to a complicated interaction of FAD, molybdenium, iron, and labile sulfur moieties at or near the active site [260], It can be used to detect xanthine and hypoxanthine by immobilizing xanthine oxidase on a glassy carbon paste electrode [261], The elements are based on the chronoamperometric monitoring of the current that occurs due to the oxidation of the hydrogen peroxide which liberates during the enzymatic reaction. The biosensor showed linear dependence in the concentration range between 5.0 X 10 7 and 4.0 X 10-5M for xanthine and 2.0 X 10 5 and 8.0 X 10 5M for hypoxanthine, respectively. The detection limit values were estimated as 1.0 X 10 7 M for xanthine and 5.3 X 10-6M for hypoxanthine, respectively. Li used DNA to embed xanthine oxidase and obtained the electrochemical response of FAD and molybdenum center of xanthine oxidase [262], Moreover, the enzyme keeps its native catalytic activity to hypoxanthine in the DNA film. So the biosensor for hypoxanthine can be based on... 

S.Q. Liu and H.X. Ju, Reagentless glucose biosensor based on direct electron transfer of glucose oxidase immobilized on colloidal gold modified carbon paste electrode. Biosens. Bioelectron. 19, 177-183 (2003). 

Q.L. Wang, G.X. Lu, and B.J. Yang, Hydrogen peroxide biosensor based on direct electrochemistry of haemoglobin immobilized on carbon paste electrode by a silica sol-gel film. Sens. Actuators, B Chem. 99, 50-57 (2004). 

U.A. Kirgoz, S. Timur, J. Wang, and A. Teleponcu, Xanthine oxidase modified glassy carbon paste electrode. Electrochem. Commun. 6, 913—916 (2004). 

Carbon paste and graphite epoxy electrodes modified with RUO2 can be used for detection of amino acids and peptides in F1A systems. Optimal conditions are in strongly alkaline solutions at +0.45 V vs Ag/AgCl electrode, with a fast and linear response. Carbon paste electrodes can be modified also with C03O4371. Colorimetric methods for the determination of amino groups attached to a solid support may give erroneous values... 

Amino acids enhance the oxidation peak of Cu(0) obtained with a carbon paste electrode incorporating Cu(II) cyclohexylbutyrate. The increased current is proportional to the amino acid concentration at trace levels in the pM range373. The behavior of such electrodes was investigated for cysteine (115). On scanning potentials in the positive direction, the amino acid is accumulated on the electrode as the Cu(I) complex at +0.90 V vs a standard calomel electrode (SCE), in acetate buffer at pH 4.5 linear range is 2 x 10 9 to 1 x 10-7 M, 1 min accumulation, RSD 3% (n = 5)374,375. 

Carbon electrodes can be made from a number of various crystalline forms of carbon. The two most common versions are the carbon paste electrode and the glassy carbon electrode. In Chapter 11, devoted to the electrochemistry of biological functions, it will be seen that pyrolytic graphite electrodes have also found wide application. Recently, attempts to use carbon-nanotube electrodes have been also proposed.9... 

The carbon paste electrode is suitable for aqueous solutions and is prepared by mixing finely ground carbon with mineral oil (in a typical preparation, 15 g of ground carbon is mixed with 9 ml of Nujol). A paste of butter-like consistency is obtained that can be shaped into small disks. 

Since rusticyanin is positively charged (p/=9.1), its electrochemical response should be tested at negatively charged electrodes (namely, edge-plane pyrolitic graphite or glassy carbon electrodes). As a matter of fact, no positive response is obtained at a carbon paste electrode, but in the presence of a promoter such as 4,4/-bipyridyl a sufficiently resolved cyclic voltammetric response is obtained, Figure 37.67... 

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