Oxidation peak current

The electrochemical response of analytes at the CNT-modified electrodes is influenced by the surfactants which are used as dispersants. CNT-modified electrodes using cationic surfactant CTAB as a dispersant showed an improved catalytic effect for negatively charged small molecular analytes, such as potassium ferricyanide and ascorbic acid, whereas anionic surfactants such as SDS showed a better catalytic activity for a positively charged analyte such as dopamine. This effect, which is ascribed mainly to the electrostatic interactions, is also observed for the electrochemical response of a negatively charged macromolecule such as DNA on the CNT (surfactant)-modified electrodes (see Fig. 15.12). An oxidation peak current near +1.0 V was observed only at the CNT/CTAB-modified electrode in the DNA solution (curve (ii) in Fig. 15.12a). The differential pulse voltammetry of DNA at the CNT/CTAB-modified electrode also showed a sharp peak current, which is due to the oxidation of the adenine residue in DNA (curve (ii) in Fig. 15.12b). The different effects of surfactants for CNTs to promote the electron transfer of DNA are in agreement with the electrostatic interactions... 

It has been demonstrated that the presence of CNTs greatly increases the oxidation peak current of 6-benzylaminopurine. The CNT-modified electrode is suitable for the determination of trace amounts of benzylaminopurine and has the advantages of high sensitivity, quick response, and good stability [86], Wang et al. have studied the electro-catalytic oxidation of thymine at a a-cyclodextrin incorporated CNT coated electrode in an alkaline media. A sensitive detection scheme for thymine has been further developed by using differential pulse voltammetry [87], The electrochemical determination... 

In the 02-saturated buffer solution, an increase in reduction peak was observed, accompanied by a decrease in the oxidation peak current, which demonstrates that GOD in the film nicely catalyzes the oxygen reduction. When glucose is added to the 02-saturated buffer solution, the reduction peak current decreases with the increase of the glucose concentration. (3-(D)-glucose is the substrate of GOD and it will react with the enzyme and decrease the concentration of the oxidized form of GOD on the electrode surface ... 

Takamura et al. have reported a voltammetric method for the determination of chlorpromazine using an anodically oxidized carbon electrode [167]. A vitreous-carbon electrode was maintained at +1.6 V vi. S.C.E. for 2 minutes (in 0.5 M phosphate buffer at pH 6.8). Under these conditions, chlorpromazine gave an oxidation peak current on cyclic voltammograms that varied linearly with concentration over the range of 50 nM to 1 pM. 

Figure 12. Concentration dependence of the responses to 5 -GMP and 5 -AMP of the oriented membranes based on (a) cytosine derivative 10b, (b) thymine derivative 11b, and (c) cytosine derivative 22. The response factor, R(sample), is expressed by the decrease in the oxidation peak current (/e) upon analyte addition [/ (sample) = [/p(blank) - /p(sample)]//p(blank)l. Measured at 20 °C with a 0.1 M phosphate buffer solution (pH 6.0, 20 °C) containing 1.00 mM [FeICNIel .

Next, some typical examples will be presented of how a DNA-electrochemical biosensor is appropriate to investigate the DNA damage caused by different types of substances, such as the antioxidant agent quercetin (Scheme 20.1), an anticancer drug adriamycin (Scheme 20.2) and nitric oxide. In all cases, the dsDNA damage is detected by changes in the electrochemical behaviour of the immobilized dsDNA, specifically through modifications of the purinic base oxidation peak current [3,5,40]. 

The oxidation of adriamycin at the thick-layer DNA-electrochemical biosensor was investigated and the effect of the immersion time of the thick-layer DNA biosensor in 1 iM adriamycin solution (Fig. 20.5) was compared with the results obtained with a bare GCE [37,56,57]. Using the thick-layer DNA-electrochemical biosensor it was possible to preconcentrate adriamycin on the thick layer of DNA and the adriamycin oxidation peak current was found to increase with immersion time and to reach saturation after lh of immersion (see insert, Fig. 20.5A). 

Li et al. [77] reported the electrocatalytic oxidation of norepinephrine at a GCE modified with SWCNTs. The electrode showed a very good reproducibility and stability. A linear relationship was obtained between the oxidation peak current and norepinephrine concentration between 1.0 x 10 and 1.1 x 10" M and the detection limit was 6.0 x 10 M. The electrocatalytic activity of the SWCNTs-modified-GCE was also demonstrated with dopamine, epinephrine and ascorbic acid. 

Hu et al. [58] reported a well defined peak at 0.68 V for 5.0 x 10 M indole-3-acetic acid (an important hormone present in plants) in pH 2.0 phosphate buffer at GCE modified with MWCNT dispersed in DHP [58]. The oxidation peak current of the indole-3-acetic acid increased gradually with the amount of MWCNT-DHP dispersed at the GCE up to a volume of 15 pL. For higher volumes the thickness of the layer blocked the electron transfer. The oxidation peak current presented a linear relationship with the concentration of the hormone from 1 X 10 M to 5 X 10 M, with a detection limit of 2 x 10 M and a reproducibility of 4.3 % for 36 measurements of 5 x 10 M. The authors extended the use of this sensor to the determination of the hormone in gladiola, apple and phoenix leaves showing a very good agreement with HPLC determinations. 

In the absence of substrate at MU-modified Au electrode oxidation and reduction was measured which corresponded to the Fe /Fe redox couple of the heme site with E° = -120 mV [120]. The heterogeneous electron transfer rate constant is in the order of = 15 s. The oxidation peak current increases proportionally to sulfite concentrations between 10 and 100 /aM. The current increase is approaching saturation at concentrations higher than 3 mM. 

Hernandez et al. developed a mediator modified AChE sensor with SPE and for the detection of OP and CA pesticides in river water samples [22], The SPE surface was modified with a mediator, tetracyanoquinodimethane (TCNQ). In the absence of this mediator, the direct oxidation of thiocholine requires higher working potential (upto 680 mV). However, this working potential has been considerably lowered in the presences of mediator. In prior to each experiment, the SPE surface was coated either with 2 pi of a solution of 0.1 mM of TCNQ in acetonitrile or with 2 pi of a suspension containing 5 pi of 1 mM TCNQ solution in acetonitrile and 50 pi NF. The modified SPE was stored overnight at room temperature, in the presence of desiccant and used. The detection of OP and CA was based on the measurement of the degree of inhibition of AChE. About 50 pi of river water samples were added to 1 ml of 0.1 M PBS pH 7.5 with AChE (1 U ml 1) and incubated for 10 min. Then 20 pi of the 0.5 mM acetylthiocholine (ATCh) solution was added. After 5 min, 100 pi of this solution was directly dropped on the surface of the modified SPE. The oxidation peak current obtained in DPV measurements was calculated as (I2) and later it was compared with the oxidation current obtained without pesticide (IQ. 

Intercalated doxorubicin has been also used as an electrochemical label in the detection of DNA hybridization events in a genosensor built by layer-by-layer covalent attachment of multiwalled carbon nanotubes and Au-NPs [46]. The oxidation peak current obtained by differential pulse voltammetry showed a linear relationship with the logarithm of the target DNA concentration in the range 5.0 x 10 to 1.0 x 10 M, with a detection limit of 6.2 pM. 

Here /free is the electrochemical peak height of a compound obtained at bare electrode, and / bound is the oxidation peak current of a com-... 

Zhu et al. proposed more complicated biosensor construction for selective binding of rhodamine B and 1-aminopyrene [119]. To do this, first GO is electrochemically reduced on the surface of the GCE and then iV-acetylaniline is polymerized on top of it. After this, -CD was covalently attached to the composite surface by electrooxidation. This sensor was used for voltammetric determination of 1-aminopyrene, an electrochemically active polycyclic aromatic hydrocarbon deleterious for living organisms. The authors show that -CD binds rhodamine B selectively, but in the presence of 1-aminopyrene, the rhodamine B molecules are displaced by 1-aminopyrene. This displacement results in a decreased oxidation peak current of rhodamine B and the appearance of an oxidation peak current for 1-aminopyrene. The current is linearly dependent on the 1-aminopyrene concentration between 10-260 nM. 

Fig. 1.5 Methanol oxidation peak currents as a function of Pt coverage (4 nmol as 2D islands is approximately 0.8 of a monolayer), where If and Ib represent peak currents at forward and backward scans, respectively (a) Pt mass activity for methanol oxidation on commercial PtRu and PtML/Ru electrocatalysts (b) [77] (reproduced with permission from J. Electrochem. Soc. 155, B183 (2008). Copyright 2003, The Electrochemical Society)...

LB films of artificial lipids transferred onto electrode substrates could be used as glucose [14] and calcium ion [15] sensors and for hydrogen evoluhon [16]. Okahata etal. coated a Sn02 electrode with LB films of synthetic phospholipids [15]. Oxidation peak currents of a maker ion ([Fe(CN)6] / - ) in the aqueous phase through the hpid films increased with the addition of Ca " " ions only when the LB films were in the fluid hquid crystalline phase above the phase transition... 

Figure 3. Concentrational dependence of O oxidation peak current. T=600 C V=0.1 V s ...

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