Dopamine paste electrode

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.)... 

A second surface modification has been reported by Yamamoto et al. These workers added stearic acid to their carbon paste mixture. This produced an electrode which was relatively insensitive to ascorbic acid and DOPAC relative to dopamine. It is theorized that this electrode works because of electrostatic repulsion of the anionic ascorbate and DOPAC by surface stearate groups. Ionic repulsion has also been employed by covering the surface of the working electrode with an anionic polymer membrane. Gerhardt et al. used Nafion, a hydrophobic sulfonated perfluoro-polymer, to make a dopamine selective electrode. This electrode exhibited selectivity coefficients as large as 250 1 for dopamine and norepinephrine over ascorbic acid, uric acid, and DOPAC. 

The first CNT-modified electrode was reported by Britto et al. in 1996 to study the oxidation of dopamine [16]. The CNT-composite electrode was constructed with bro-moform as the binder. The cyclic voltammetry showed a high degree of reversibility in the redox reaction of dopamine (see Fig. 15.3). Valentini and Rubianes have reported another type of CNT paste electrode by mixing CNTs with mineral oil. This kind of electrode shows excellent electrocatalytic activity toward many materials such as dopamine, ascorbic acid, uric acid, 3,4-dihydroxyphenylacetic acid [39], hydrogen peroxide, and NADH [7], Wang and Musameh have fabricated the CNT/Teflon composite electrodes with attractive electrochemical performance, based on the dispersion of CNTs within a Teflon binder. It has been demonstrated that the electrocatalytic properties of CNTs are not impaired by their association with the Teflon binder [15]. 

CNT randomly dispersed composites Many soft and rigid composites of carbon nanotubes have been reported [17]. The first carbon-nanotube-modified electrode was made from a carbon-nanotube paste using bromoform as an organic binder (though other binders are currently used for the paste formation, i.e. mineral oil) [105]. In this first application, the electrochemistry of dopamine was proved and a reversible behavior was found to occur at low potentials with rates of electron transfer much faster than those observed for graphite electrodes. Carbon-nanotube paste electrodes share the advantages of the classical carbon paste electrode (CPE) such as the feasibility to incorporate different substances, low background current, chemical inertness and an easy renewal nature [106,107]. The added value with CNTs comes from the enhancement of the electron-transfer reactions due to the already discussed mechanisms. 

Lyne and O Neil [117] reported the in vivo detection of dopamine using stearate-modified carbon-Nujol paste electrodes. Prior to their work, the detection of dopamine by voltammetric techniques was hindered primarily due to the coexisting ascorbic acid in the extracellular fluid of the mammalian brain. Ascorbic acid oxidizes at electric potentials similar to that of dopamine on many electrode materials. These authors found that the use of stearate-modified carbon-Nujol paste electrodes retards the electro-oxidation of anionic species (such as ascorbate) to such an extent that the cationic dopamine species could be detected in their presence. 

Wang, J., and Walcarius, A. 1996. Zeolite-modified carbon paste electrode for selective monitoring of dopamine. Journal of Electroanalytical Chemistry 407, 183-187. 

Sidwell and Rechnitz (1985) placed a slice of banana pulp tissue on the gas-permeable membrane of a Clark-type oxygen electrode. The banana tissue contains polyphenol oxidase which catalyzes the oxidation of dopamine to dopamine quinone and further to melanin at the expense of oxygen. Wang and Lin (1988) integrated this biocatalytic phase in the electrode body of a membrane-free carbon paste electrode and measured the formation of dopamine quinone at a potential of -0.2 V. This arrangement permitted selective determination of dopamine in the presence of ascorbic acid with a response time of only 12 s. It seems likely that this improved performance of a tissue-containing sensor could be extended to other analytes. 

Forzani ES, Rivas GA, Solis VM. Amperometric determination of dopamine on an enzymatically modified carbon paste electrode. J Electroanal Chem 1995 382 33-40. 

Fig. 5. Two chromatograms obtained by HPLC and electrochemical detection. Left norepinephrine (ME) 85 pg and dopamine (DA) ISO pg, eluted with 0,1 M perchloric acid from a cation-exchange column, detected with a thin-film cell with carbon paste electrode (redrawn). Right perphenazine (PPZ), clopenthixol (CLO), fluphenazine (FPZ) and flupenthixol (FLU), 0.7 ng each, eluted from a methyl-bonded silica column with methanol-aqueous phosphate buffer (pH 7,4), detected with a thin-film cell with glassy carbon electrode. ...

O Neill, R.D. (2005) Long-term monitoring of brain dopamine metabolism in vivo with carbon paste electrodes (a review). Sensors, 5, 317 -342. 

Fig. 7.9 (a) Differential pulse voltammograms for a zeolite-modified electrode (ZME) after deposition of 1 ppm Ag" solution for different pre-concentration times (a) 1 (b) 2 (c) 3 (d) 5 (e) 10 min (Reproduced from Ref. [132] with permission of Elsevier), (b) Cyclic voltammograms of 2 X lO " M (a) dopamine and (b) ascorbic acid at (1) pure and (2) 10 wt.% zeolte-modified carbon paste electrodes (Reproduced from Ref. [133] with the permission of Elsevier)... 

Fig. 47. Background-corrected chronoamperometric measurements of the oxidation of dopamine, DA (0.1 mM) alone and its mixture with 1 mM ascorbic acid, AA. n pp-values at a carbon paste electrode (full lines) were calculated using the ratio Icat/Id Eq. (89) results at carbon fiber electrodes of tip radius, a, (dashed lines) were calculated according to Eq. Uapp = [(kf ) + D /a][(7ct)" + D /a] valid for kft > 5 experimental conditions ...

In 1976, Adams published a pioneering article on the voltammetric technique of in vivo determination of electrochemically active compounds in the brain [33]. It describes notably successful measurements on neurotransmitters such as dopamine (DA) and seretonin (5-HT) using carbon paste electrodes with tip diameters from 50 to 200 m implanted in the brain tissue of a rat. Thereafter, in vivo voltammetric technique attracted much attention from neuroscientists and electrochemists, and many papers have been published in the field [30, 34-47]. In this section microelectrodes suitable for the detection of neurotransmitters, the operation techniques for positioning electrodes, and some results are described. 

Other examples of important biomolecules that can be detected using carbon paste electrodes modified with phthalocyanines are the neimotransmitters dopamine, serotonin, and epinephrine. Oni et al. [37] described the utilization of CPE containing iron(II) phthalocyanine (PePc), and iron(II) tetrasulfonated phthalocyanine ([FeTSPc]" ]) for the detection of dopamine and serotonin. The presence of ascorbic acid did not interfere with the determination of both species, individually or in a mixture. Shahrokhian et al. [38] performed epinephrine determinations in pharmaceutical and clinical samples through voltammetric techniques with high sensitivity and selectivity, low detection limit (sub-micromolar), and high reproducibility. 

Oni J, Nyokong T (2001) Simultaneous voltammetric determination of dopamine and serotonin on carbon paste electrodes modified with iron(II) phthalocyanine complexes. Anal Chim Acta 434 9-21... 

Zhou, Y.Z., et al.. Electroanalysis and simultaneous determination of dopamine and epinephrine at poly(isonicotinic acid)-modified carbon paste electrode in the presence of ascorbic acid. Chin. Chem. Lett., 2009. 20 p. 217-220. 

Corona-Avendano S, Ramirez-Siiva MT, Romero-Romo M, Rojas-Hemandez A, Palomar-Pardave M (2013) Influence of the HCI04 concentration on the p-CD electropoiimerization over a carbon paste electrode and on dopamine s electrochemical response. Electrochim Acta 89 854-860... 

Lyne PD, O Neill RD (1990) Stearate-modified carbon paste electrodes for detecting dopamine in vivo decrease in selectivity caused by lipids and other surface-active agents. Anal Chem 62 2347-2351... 

Tissue and Bacteria Electrodes The limited stability of isolated enzymes, and the fact that some enzymes are expensive or even not available in the pure state, has prompted the use of cellular materials (plant tissues, bacterial cells, etc.) as a source of enzymatic activity (48). For example, the banana tissue (which is rich with polyphenol oxidase) can be incorporated by mixing within the carbon paste matrix to yield a fast-responding and sensitive dopamine sensor (Fig. 6.14). These biocatalytic electrodes function in a manner similar to that for conventional enzyme electrodes (i.e., enzymes present in the tissue or cell produce or consume a detectable species). 

Fig. 11.7.2. HPLC separation of catecholamines and serotonin. Chromatographic conditions column, Vydac-CX (35 fim) (500 x 2 mm I.D.) mobile phase, 27.4 mM citrate, 50 mM sodium acetate, 18.4 mM acetic acid, 60 mM so um hydroxide, adjusted to pH 5.3 flow rate, 0.6 ml/min temperature, ambient detection, electrochemical, carbon paste working electrode, electrode potential -1-0.55 V vs. an Ag-AgCl reference electrode. Peaks NA, noradrenaline (80 pg) A, adrenaline (100 pg) DA, dopamine (4(X) pg) a-MDA, a-methyidopamine (internal standard) 5-HT, serotonin (200-250 pg). Reproduced from Patthy and Oyenge (1984), with permission.

Fig. 10. Measurement of norepinephrine and epinephrine in human plasma. Explanation of traces from left to right. A (1) Norepinephrine standard (10 pmol injected) (2) epinephrine standard (10 pmol injected) (3) dopamine standard (10 pmol injected). B (1) Plasma (1.1 ml) collected during a normoglycemic baseline period. Dopamine (10 pmol) added as internal standard prior to aluminum oxide adsorption. Sensitivity changes from 100 nA to 1 nA full scale deflection immediately after elution of the solvent front. Note the small norepinephrine (0.57 nmol/liter) and epinephrine (0.74 nmol/liter) peaks (2) plasma (1.1 ml) from the same subject, insulin-induced hypoglycemia. Note the marked increase in the epinephrine (5.16 nmol/liter) peak and the small rise in norepinephrine (0.75 nmol/liter). Dopamine (10 pmol) added as internal standard. Chromatographic conditions column, Nucleosil (10 [x), 30 cm X 2.1 mm mobile phase, acetate/citrate, 0.1 M, pH 5.2 flow rate, 1.2 ml/min electrode potential, +0.65 V (carbon paste) volume injected, 100 xl plasma extraction, alumina adsorption (Fig. 8).

The most elementary biosensors are fruit pulps or slices which have been combined with amperometric electrodes. A well-known example is the ba-nanatrode (Wang and tin 1988). This sensor, most useful for demonstration experiments, contains a paste mix of banana pulp, nujol and carbon powder which has been pressed into a glass tube with an electric contact (Fig. 7.39). The mass contains the enzyme polyphenolase, which catalyses the oxidation of polyphenols, among them important biological messengers like dopamine. The sensor can be tested by means of simple compounds like catechol, which can be detected in beer. As a result of air oxidation, o-quinone is formed. The latter is an electrochemicaUy active compound which can be detected e.g. by differential-pulse voltammetry. 

Besides the classical CPE, there are almost infinite possibilities related to modifications of the paste, as also related to the inert mineral oil. Recent papers describe the utilization of modified electrodes with porphyrins, able to detect 1.6 x 10 mol dopamine in pharmaceuticals or even 1.1 x lO" " mol L for ascorbic acid in beverages and pharmaceutical samples. ... 

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