The redox response of chlorogenic acid solution at an inactivated glassy carbon electrode was investigated and an ECE mechanism was proposed for the electrode process. It has been shown that the oxidation of chlorogenic acid at an activated glassy carbon electrode leads to the formation of a deposited layer of about 4.5×10−10 mol cm−2 at the surface of the electrode. Cyclic voltammetry was used for the deposition process and the resulting modified electrode retains the activity of the quinone/hydroquinone group anticipated for a surface-immobilized redox couple. The properties of the electrodeposited films, during preparation under different conditions, and the stability of the deposited film were also examined. The pH dependence of the redox activity of these films was found to be 57 mV per pH unit, which is very close to the anticipated Nernstian dependence of 59 mV per pH unit. The modified electrode exhibits potent and persistent electrocatalysis for NADH oxidation in phosphate buffer solution (pH 7.0) with a diminution of the overpotential of about 430 mV and an increase in peak current. The electrocatalytic current increases linearly with NADH concentration from 0.1 to 1.0 mM. The apparent electron transfer rate constant, ks, and the heterogeneous rate constant for electrooxidation of NADH, kh, were also determined using cyclic voltammetry and rotating disk electrode voltammetry, respectively.
"However, the remainder of the measurements for the detection of DA were carried out at pH 7.0, which is close to the physiological pH of human body fluid, even though the peak current obtained at this pH was $85% of that obtained at pH 5.0. In addition, the slope of À55.4 AE 0.003 mVÁpH À1 (inset of Fig. 4(B)) revealed that the same number of electrons and protons were involved in the oxidation of DA  , which is a two-electron and two-proton transfer process corresponding to the oxidation of DA to dopaminequinone and the subsequent reduction of dopaminequinone to DA, as shown in Scheme 2 and Scheme S1   . 3.4. "
[Show abstract][Hide abstract] ABSTRACT: A highly sensitive and selective electrochemical method based on a poly(chromotrope 2B)-modified
anodized glassy carbon electrode (PCHAGCE) was developed for the determination of dopamine (DA) in
the presence of uric acid (UA) and ascorbic acid (AA). The PCHAGCE sensor exhibited excellent electron-mediating behavior towards the oxidation of DA in 0.1 M phosphate buffer solution (PBS) (pH 7.0). It was
found that the electrocatalytic activity was significantly dependent on the charge status and molecular
structure of the target molecules. Differential pulse voltammetry (DPV) measurements revealed
oxidation signals for DA, UA, and AA that were well-resolved into three distinct peaks with AA–DA, DA–
UA, and AA–UA peak potential separations (DEp) of 172,132, and 304 mV, respectively. A detection limit of
0.04 0.001 mM (S/N = 3) and a quantification limit (S/N = 10) of 0.149 0.03 mM were obtained for DA
sensing in a linear range of 1 to 40 mM in PBS (pH 7.0) with a very high sensitivity of 1.522 0.032 mA�mM�1. The DA concentrations in human urine samples were also successfully determined with recoveries of 94.0–98.0%. This approach provides a simple, easy, sensitive, and selective method to detect DA in the presence of AA and UA.
"), bis- phenothiazin-3-yl methane (Gligor et al. 2009b), catechol (Bai et al. 2010), neutral red (Lu et al. 2010), phenosafranine (Saleh et al. 2011b), azure I (Cai and Xue 1997a), Nile blue (Cai and Xue 1997b), Meldola's Blue (Bala et al. 2004), chlorogenic acid (Zare and Golabi 1999) Entrapping/incorporation Meldola's Blue (Doaga et al. 2009; Gründig et al. "
[Show abstract][Hide abstract] ABSTRACT: The applicability of dissolved redox mediators for NAD(P)(+) regeneration has been demonstrated several times. Nevertheless, the use of mediators in solutions for sensor applications is not a very convenient strategy since the analysis is not reagentless and long stabilization times occur. The most important drawbacks of dissolved mediators in biocatalytic applications are interferences during product purification, limited reusability of the mediators, and their cost-intensive elimination from wastewater. Therefore, the use of immobilized mediators has both economic and ecological advantages. This work critically reviews the current state-of-art of immobilized redox mediators for electrochemical NAD(P)(+) regeneration. Various surface modification techniques, such as adsorption polymerization and covalent linkage, as well as the corresponding NAD(P)(+) regeneration rates and the operational stability of the immobilized mediator films, will be discussed. By comparison with other existing regeneration systems, the technical potential and future perspectives of biocatalytic redox reactions based on electrochemically fed immobilized mediators will be assessed.
"However, the most frequent solution is the use of electron transfer mediators to improve the charge transfer kinetics. Modified electrodes using quinones   , catechols  , phenazines , phenoxazines  , and phenothiazines      as electron transfer mediators have been described. Several electropolymerized films of redox    or conducting    polymers also show electrocatalytic effect on the oxidation of NADH and NADPH. "
[Show abstract][Hide abstract] ABSTRACT: A carbon paste electrode modified with the adsorbed products of the electrochemical oxidation of adenosine triphosphate is described. The electrode was applied to the amperometric electrocatalytic detection of the reduced form of both nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate. The catalytic oxidation current shows a linear dependence on the concentration of the reduced form of nicotinamide adenine dinucleotide up to 1x10(-4)M, with a detection limit of 5x10(-9)M. Modified carbon paste electrodes were coated with an electrogenerated film of nonconducting poly(o-phenylenediamine) to obtain a stable amperometric response for at least 150h. In addition to static measurements, determination of both reduced cofactors was carried out in a flow injection analysis system with a thin-layer amperometric detection cell. The electrocatalytic monitoring of reduced nicotinamide adenine dinucleotide phosphate was applied to flow injection measurement of isocitrate dehydrogenase activity in serum. The results were in good agreement with those for the standard spectrophotometric test kit. The proposed method consumed less time and reagents and provided better precision than the standard method.
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