Carbon-Fiber Electrodes

3 Carbon-Fiber Electrodes The growing interest in ultramicroelectrodes (Section 4-5.4) has led to widespread use of carbon fibers in electroanalysis. Such materials are produced, mainly in connection with the preparation of high-strength composites, by high-temperature pyrolysis of polymer textiles or via 

FIGURE 4-11 Scanning electron image of a carbon-fiber electrode. 

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Figure 17.6 Redox hydrogel approach to immobilizing multiple layers of a redox enzyme on an electrode, (a) Structure of the polymer, (b) Voltammograms for electrocatalytic O2 reduction by a carbon fiber electrode modified with laccase in the redox hydrogel shown in (a) (long tether) or a version with no spacer atoms in the tether between the backbone and the Os center (short tether). Reprinted with permission fi om Soukharev et al., 2004. Copyright (2004) American Chemical Society.

Figure 17.17 Schematic representation of a single-compartment glucose/02 enzyme fuel cell built from carbon fiber electrodes modified with Os -containing polymers that incorporate glucose oxidase at the anode and bilirubin oxidase at the cathode. The inset shows power density versus cell potential curves for this fuel cell operating in a quiescent solution in air at pH 7.2, 0.14 M NaCl, 20 mM phosphate, and 15 mM glucose. Parts of this figure are reprinted with permission from Mano et al. [2003]. Copyright (2003) American Chemical Society.

Figure 1 Electrochemical detection of catechol, acetaminophen, and 4-methyl catechol, demonstrating the selectivity of differential pulse detection vs. constant potential detection. (A) Catechol, (B) acetaminophen, and (C) 4-methylcatechol were separated by reversed phase liquid chromatography and detected by amperometry on a carbon fiber electrode. In the upper trace, a constant potential of +0.6 V was used. In the lower trace, a base potential of +425 mV and a pulse amplitude of +50 mV were used. An Ag/AgCl reference electrode was employed. Note that acetaminophen responds much more strongly than catechol or 4-methylcatechol under the differential pulse conditions, allowing highly selective detection. (Reproduced with permission from St. Claire, III, R. L. and Jorgenson, J. W., J. Chromatogr. Sci. 23, 186, 1985. Preston Publications, A Division of Preston Industries, Inc.)...

Surface modified NO sensors incorporate an electrode surface that has been modified or treated in some way so as to increase the selectivity of the sensor for NO and promote catalytic oxidation of NO. An early example of such a sensor was presented by Malinski and Taha in 1992 [27], In this publication an —500nm diameter carbon fiber electrode was coated with tetrakis(3-methoxy-4-hydroxyphenyl)porphyrin, via oxidative polymerization, and Nation. This electrode was shown to have a detection limit of — lOnM for NO and great selectivity against common interferences. However, recently it has been shown that this electrode suffers severe interference from H202 [28],... 

M.N. Friedemann, S.W. Robinson, and G.A. Gerhardt, o-phenylenediamine-modified carbon fiber electrodes for the detection of nitric oxide. Anal. Chem. 68, 2621—2628 (1996). 

R. Cespuglio, H. Faradji, Z. Hahn, and M. Jouvet, Voltammetric detection of brain 5-hydroxyindo-lamines by means of electrochemically treated carbon fiber electrodes chronic recordings for up to one month with movable cerebral electrodes in die sleeping or waking rat, in Measurements of Neurotransmitters Release in Vivo (C.A. Marsden, ed.), Wiley, Chichester (1984). 

Nogami, T., Nawa, M. and Mikawa, H., Lightweight, stable and rechargeable battery with an activated carbon fiber electrode, J. Chem. Soc., Chem. Commun., 1982, (20), 1158 1 159. 

A schematic diagram of the cation flow method for generating N-acyliminium ion 2 is shown in Fig. 5. A solution of carbamate 1 is introduced into the anodic compartment of electrochemical microflow cell, where oxidation takes place on the surface of a carbon fiber electrode. A solution of trifluoromethanesulfonic acid (TfOH) was introduced in the cathodic compartment, where protons are reduced to generate dihydrogen on the surface of a platinum electrode. A-Acyliminium ion 2 thus generated can be analyzed by an in-line FT-IR analyzer to evaluate the concentration of the cation. The solution of the cation is then allowed to react with a nucleophile such as allyltrimethylsilane in the flow system to obtain the desired product 3. 

In contrast to the high regioselectivity and good yields of electroreductive intramolecular coupling reactions of ketones with multiple bonds shown in Schemes 34 to 37, the yields of interm olecular coupling reactions have been very low until recently. However, by using carbon fiber electrodes, intermolecular coupling reactions have... 

P. Wilde, M. Maendle, J. Steinbart, and H. Leinfelder. Carbon fiber electrode substrate for electrochemical cells. CA2424948 (2003). 

J. T. Gostick, M. W. Fowler, M. D. Pritzker, M. A. loannidis, and L. M. Behra. In-plane and through-plane gas permeability of carbon fiber electrode backing layers. Journal of Power Sources 162 (2006) 228-238. 

Bell, 1989 Rhee and Bell, 1991), random copolymers of methyl acrylate and acrylonitrile were directly polymerized onto the carbon fiber surface. Dimethyl formamide, dimethyl sulfoxide and distilled water proved to be useful as solvents for this process. Polymerization can take place on the carbon fiber electrode, with initial wetting of the fiber surface leading to better adhesion of the polymer formed. The structure and properties of the polymer can be varied by employing different vinyl and cyclic monomers in homopolymerization. Chemical bond can also be formed, such as polymer grafting to the carbon fiber surface. 

Nitric oxide can be assayed directly in tissues by its electrochemical oxidation on electrode surfaces (Shibuki, 1990). The technique was successfully used by Shibuki in cerebellar slices. However, the probe was fabricated in a glass micropipet coated with a thin hydrophobic chloroneoprene membrane, which makes the technique experimentally difficult. More recently, a carbon fiber electrode... 

Figure 17-26 Electron micrograph of the tip of a Nation-coated carbon fiber electrode. The carbon inside the electrode has a diameter of 10 ixm. Nafion permits cations to pass but excludes anions. /Photo courtesy R. M. WigMman. From R. M. Wightman. L. J. May, and A C. Michael. Detection of Dopamine Dynamics in the Brain, Anal. Chem. 1988, 60. 76VA ]...

Carbon electrodes exhibit a wide range of electron transfer rates for benchmark redox systems, depending on carbon material and surface history. Two examples are shown in Figure 10.2, which compares two carbon surfaces with very different k° for Fe(CN) /4. In some cases, the variations in electrode kinetics have been particularly important to analytical applications. For example, carbon paste and carbon fiber electrodes have been used to monitor neurotransmitters in living animal brains [5,6]. The determination of catechol transmitters in the presence of relatively large amounts of interferents (e.g., ascorbate) de-... 

A major advantage of carbon fiber electrodes is the possibility of using fast voltammetric scan rates (> 100 V/s) due to small electrode area [48]. Even though internal resistance can be significant for carbon fibers (see later), the... 

Figure 10.15 Voltammetry at a cylindrical carbon fiber electrode (1) before and (2) after electrochemical pretreatment (A) 0.1 mM dopamine and (B) 1.0 mM ascorbic acid, pH 7 solutions, scan rate = 0.1 V/s. [Adapted from Ref. 6.]...

Figure 12.8 Light micrograph of a bovine adrenal medullary cell in culture with etched and glass-encased carbon-fiber electrodes (r = 5 /xm) placed adjacent to it. Magnification is 450x. [From Ref. 88, reproduced with permission of the copyright holder.]...

Figure 27.20 Schematic drawing of CE with end-column amperometric detection A, capillary B, cathodic buffer reservoir and electrochemical cell C, carbon fiber electrode D, electrode assembly, E, micromanipulator RE, reference electrode. [Adapted with permission from Ref. 49.]...

Most electrode materials that are employed in LCEC can also be used for CEEC. The most commonly employed working electrode is a carbon fiber. Carbon fibers come in many different sizes and can also be etched to smaller diameters. Common applications of CEEC with carbon fiber electrodes are the detection of catecholamines in single neuronal cells and amino acids in brain microdialysis samples following derivatization with NDA/CN. 

Electrochemical detectors for liquid chromatography have reached a level of maturity in that thousands of these devices are used routinely for a variety of mundane purposes. Nevertheless, the technology is advancing rapidly in several respects. Multiple electrode and voltammetric detectors have been developed for more specialized applications. Small-volume transducers based on carbon fiber electrodes are being explored for capillary and micropacked columns. Recently, electrochemical detection has also been coupled to capillary electrophoresis [47]. Finally, new electrode materials with unique properties are likely to afford improved sensitivity and selectivity for important applications. 

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