Ascorbic acid 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]. 

Ascorbic acid, acetaminophen/tablets LSV Working electrode carbon paste electrode Reference electrode Ag/AgCl Supporting electrolyte acetate buffer (pH 4.7) Dissolving in water LOD -LOQ - [89]... 

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. 

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. 

Linear sweep and cyclic voltammetric methods have been employed for numerous basic studies of electrochemical systems and for analytical purposes. For example, the technique can be used for in vivo monitoring of substances in the kidney or brain (19) a typical example that employed a miniature carbon paste electrode to study ascorbic acid in a rat brain is illustrated in Figure 6.6.5. These techniques are especially powerful tools... 

Figure 6.6.5 Application of cyclic voltammetry to in vivo analysis in brain tissue, (a) Carbon paste working electrode, stainless steel auxiliary electrode (18-gauge cannula), Ag/AgCl reference electrode, and other apparatus for voltammetric measurements, (b) Cyclic voltammogram for ascorbic acid oxidation at C-paste electrode positioned in the caudate nucleus of an anesthetized rat. [From P. T. Kissinger, J. B. Hart, and R. N. Adams, Brain Res., 55, 20 (1973), with permission.]...

PANI/DBSA nanoparticles have also been studied for their oxidation of ascorbic acid [82]. The nanoparticles were drop-coated onto screen-printed carbon paste electrodes. The peak current response was achieved at approx. 250 mV vs. Ag/AgCl. However, the device also showed sufficient catalysis at 0.0 V vs. Ag/AgCl to allow measurements to be made at this potential where it was capable of detection of 8.3 pM ascorbic acid and was linear in the range of 0.5 to 8 mM. 

Ascorbic acid Cabbage, cucumber, yeiiow squash Ascorbic acid oxidase (AAO) Oxygen electrode/carbon paste electrode... 

Integrated systems refer to heterogeneous films on electrodes designed to contain a mixture of constituents with different functions. For instance, electrocatalysis of organics were successfully achieved with the incorporation of catalytic monomers in carbon paste matrix. Examples are oxidations of alcohol by ruthenium 0x0 complex [59] and ascorbic acid by catechol or aminophenol [60], p-phenylenediamine, or tetramethyl-p-phenylenediamine [61], and Prussian blue [62]. A CME prepared... 

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 ...

A Nafion coating incorporating Os(bpy)3 (with Os in 2) catalyzed the electrochemical oxidation of ascorbate present in the solution The reaction between the catalyst and the substrate proceeded only within the outermost layer of the coating. In order to overcome this problem a polymeric coating with high porosity (copolymer gels of acrylamide/vinylpyridine) was used as an electrode film, and ferri-cyanide-mediated oxidation of ascorbate was studied A carbon paste electrode composed of polyvinylpyridine (PVP) and a graphite powder/Nujor oil matrix bound Fe(CN)g under acidic conditions, which then catalyzed the oxidation of ascorbic acid... 

Now the next question Is, what substances are causing the different peaks Many workers have assumed that peak I Is caused by the oxidation of catecholamines such as noradrenaline. However there Is a problem In that the oxidation potentials for ascorbic acid and the catecholamines are not very different. A further problem we have found Is that oxidation potentials measured In the brain and measured In vitro can be very different. Exposing a carbon paste electrode to rats brain makes It more active. In Table 4 we give results for the... 

Figures Steady state response of the NJ4 -(4-nitrobenzj4)vidogen-glucose oxidase-carbon paste electrode to (a) IQmM ascorbic add and (b) 0.5niM uric acid at several applied potentials. E h point is the mean result fro five electrodes. Repa oduced witii permission from reference 17.

Goreti, M., F. Sales, M. S. A. Castanheira, R. M. S. Ferreira, M. Carmo, V. G. Vaz, and C. Delerue-Matos. 2008. Chemically modified carbon paste electrodes for ascorbic acid determination in soft drinks by flow injection amperometric analysis Portugaliae. Electrochim. Acta 26 147-157. 

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. 

Shahrokhian S, GhaUdiani M, Amini MK (2009) Application of carbon-paste electrode modified with iron phthalocyanine for voltammetric determination of epinephrine in the presence of ascorbic acid and uric acid. Sens Actuators B Chem 137 669-675... 

---