Comparative investigation of NADH electrooxidation at graphite electrodes modified with two new phenothiazine derivatives

doi:10.1016/j.jelechem.2011.07.046

Journal of Electroanalytical Chemistry

Volume 661, Issue 1, 1 October 2011, Pages 192-197

https://doi.org/10.1016/j.jelechem.2011.07.046 Get rights and content

Abstract

Two new phenothiazine derivatives, 3,7-di(m-aminophenyl)-10-ethyl-phenothiazine (DAEP) and 3,7-di(m-hydroxyphenyl)-10-ethyl-phenothiazine (DHEP), were used to modify solid graphite electrodes by adsorption from DMSO solution. Cyclic voltammetry and rotating disc electrode measurements, performed under different experimental conditions (pH, potential scan rate, rotation speed and NADH concentration) showed that: (i) both compounds undergo a quasi-reversible one electron / one proton redox process (k_s = 0.73 s⁻¹ for DAEP and 0.38 s⁻¹ for DHEP), occurring at a formal standard potential which is not affected by the nature of substituent at positions 3 and 7; (ii) the pK_a values of the adsorbed mediators are slightly different (6.9 for DAEP and 7.8 for DHEP); (iii) the second order rate constant for NADH electrocatalytic oxidation is significantly larger for DAEP (k_obs,[NADH]=0 = 6014 M⁻¹ s⁻¹) than for DHEP (k_obs,[NADH]=0 = 134 M⁻¹ s⁻¹). Considering the mediators’ structure, we made an attempt to explain the higher electrocatalytic efficiency of DAEP mediator.

Highlights

► Modified electrodes with two new phenothiazine derivatives. ► The voltammetric response was not affected by the substituent at positions 3 and 7. ► The modified electrodes showed electrocatalytic activity for NADH oxidation. ► Besides the thermodynamic driving force the surface coverage strongly influenced the electrocatalytic efficiency.

Introduction

β-Nicotinamide adenine dinucleotide (NADH) is a coenzyme for more than 350 dehydrogenases, being involved in a wide range of enzymatic reactions. In living cells, NADH is involved in the production of adenosine triphosphate, compound that regulates the energy release from cells. For these reasons the electrochemical oxidation of NADH is a central topic in a wide range of bioelectrochemical studies [1], [2], [3], [4], [5], [6], [7]. Although the formal potential of NADH/NAD⁺ couple is relatively low (−0.56 V vs. SCE at pH 7 and 25 °C), a high overpotential is required to oxidize NADH at bare conventional electrodes [1], [2], [3]. Moreover, the direct electrochemical oxidation of NADH is usually accompanied by low sensitivity and electrode fouling.

One way to overcome these problems is the use of redox mediators due to their ability to promote the electron transfer process. The mediators for NADH electrochemical oxidation were immobilized on the electrode surface by: (i) covalent attachment [8], [9]; (ii) electrochemical polymerization [10], [11]; (iv) via entrapment in polymeric matrices [12]; incorporation in carbon paste [13]; (v) adsorption [14], [15], [16]; (vi) self-assembling [17], [18], [19], [20]. Among these techniques, the adsorption of electrocatalytical active species onto carbonaceous electrode materials represents a simple and inexpensive procedure, leading to modified electrodes highly suitable for NADH electrocatalytic oxidation.

So far, several classes of compounds were used as mediators for NADH oxidation, such as quinone derivatives, phenylenediamines, alkyl-phenazines and redox dyes (phenoxazine and phenothiazine derivatives) [1], [2], [21]. Nevertheless, for various reasons (stability of the adsorbed mediator, fast heterogeneous electron transfer between the electrode and mediator, fast chemical redox reaction between the adsorbed mediator and dissolved NADH) phenoxazine and phenothiazine derivatives are considered as being among the most suitable mediators to obtain modified electrodes for electrocatalytic oxidation of NADH. Consequently, a wide variety of these mediators was investigated, including Meldola Blue [22], Nile Blue [23], Toluidine Blue [24], Methylene Green [25], and similar compounds as well [16], [26], [27], [28], [29], [30]. Although many of already investigated redox mediators present interesting electrochemical characteristics, there is a permanent search for new compounds with improved electrocatalytic behavior. Moreover, only few papers deal with correlations between the mediator’s structure and its electrocatalytic activity [21], [31].

In this context, here we present a detailed electrochemical study of two new phenothiazine derivatives, namely 3,7-di(m-aminophenyl)-10-ethyl-phenothiazine (DAEP) and 3,7-di(m-hydroxyphenyl)-10-ethyl-phenothiazine (DHEP) as well as their electrocatalytic behavior towards NADH oxidation. Based on the mediators’ structures we have attempted to formulate an explanation for the differences observed between their electrocatalytic behaviors.

Section snippets

Reagents

DAEP and DHEP were synthesized and characterized [32] by a research group from the Organic Chemistry Department of our faculty and were kindly offered to us by Prof. Ion Grosu.

β-Nicotinamide adenine dinucleotide (reduced form, NADH) was purchased from Sigma (St. Louis, MO, USA) as disodium salt. Phosphate buffer solutions, used as supporting electrolyte, were prepared using K₂HPO₄·3H₂O and KH₂PO₄ (Merck, Darmstadt, Germany). The pH value of the solution was adjusted by using appropriate H₃PO₄

Electrochemical behavior

The voltammetric responses of G/DAEP and G/DHEP modified electrodes, recorded in 0.1 M phosphate buffer solution (pH 7), exhibit a well-defined peak pair (results not shown), characteristic for phenothiazinic derivatives. The redox process associated to this peak pair involves a one-electron transfer (Fig. 1) and leads to a cation radical formation [27], [28], [31], [33].

As can be noticed from Table 1, the nature of the substituent at the 3 and 7 positions does not influence the formal standard

Conclusions

In an attempt to improve the process of mediated NADH electro-oxidation, two new modified electrodes were obtained by adsorption of 3,7-di(m-aminophenyl)-10-ethyl-phenothiazine and 3,7-di(m-hydroxyphenyl)-10-ethyl-phenothiazine on spectrographic graphite. A detailed electrochemical study performed on these electrodes showed that: (i) both compounds undergo a quasi-reversible redox process, involving 1 electron/1 proton; (ii) the voltammetric response occurs at a formal standard potential which

Acknowledgments

The authors thank for financial support to National Council for Scientific Research of Higher Education Romania (CNCSIS, Grant ID-PCCE 140/2008).

Vasilica LATES was born in 1984 in Targu Mures, Romania. She received her Master degree in 2008 from Babes-Bolyai University of Cluj-Napoca, Romania with specialization in applied electrochemistry for development of biosensors and immunosensors. She is currently running PhD joint studies at Babes-Bolyai University of Cluj-Napoca and University Via Domitia from Perpignan (France), in the field of bioanalytical systems applied for biomedical and biotechnological investigations. The PhD fellowship

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Delia GLIGOR was born in 1971 and she received her PhD degree in 2002 in applied electrochemistry, at Babes-Bolyai University of Cluj-Napoca. Her main research interests are in the field of applied electrochemistry (modified electrodes) and electroanalytical chemistry (sensors/biosensors).

Liana Maria Muresan is the head of the Physical Chemistry Department of the Faculty of Chemistry and Chemical Engineering, Babes-Bolyai University, Cluj-Napoca, Romania. She received her PhD degree in applied electrochemistry in 1992. Her main research interests are in the field of chemically modified electrodes and metals / composites electrodeposition / corrosion.

Ionel Catalin POPESCU is the head of the Laboratory for Electrochemical Research, Faculty of Chemistry and Chemical Engineering, Babes-Bolyai University, Cluj-Napoca, Romania. He received his PhD degree in 1976 in electroanalytical chemistry. His main interests are in the field of electroanalytical chemistry (sensors/biosensors) and applied electrochemistry (modified electrodes, electrocatalysis and ion selective electrodes).

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Comparative investigation of NADH electrooxidation at graphite electrodes modified with two new phenothiazine derivatives

Abstract

Highlights

Introduction

Section snippets

Reagents

Electrochemical behavior

Conclusions

Acknowledgments

J. Electroanal. Chem.

Bioelectrochemistry

J. Electroanal. Chem.

Electrochem. Commun.

J. Electroanal. Chem.

J. Electroanal. Chem.

Bioelectrochemistry

Biosens. Bioelectron.

Ethanol and uric acid, Sens. Actuators. B

J. Electroanal. Chem.

J. Colloids Interface. Sci.

J. Electroanal. Chem.

Electrochim. Acta

Electrochim. Acta

J. Electroanal. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

Electrochemistry of NAD(P)+ /NAD(P)H

Catalytic electrooxidation of NADH for dehydrogenase amperometric biosensors

Microchim. Acta

Amperometric biosensors based on NAD(P)-dependent dehydrogenase enzymes

Electroanalysis

Electrochemical regeneration of oxidoreductases for cell-free biocatalytic redox reactions

Biocatal. Biotransform.

NAD(P)-Based Biosensors

Electrochemistry of NAD(P)⁺ /NAD(P)H