Some studies on electrochemical sensors

SOME STUDIES ON

ELECTROCHEMICAL SENSORS

by

ANITA

THESIS SUBMITTED

IN FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

Department of Chemistry

INDIAN INSTITUTE OF TECHNOLOGY, DELHI

JULY, 1994

.. to my parents, my husband & my son

CERTIFICATE

This is to certify that thesis entitled "SOME STUDIES ON ELECTROCHEMICAL SENSORS" being submitted by Anita for the award of the degree of "Doctor of Philosophy" in Chemistry is a record of bonafide research work carried out by her.

Anita has worked under my guidance and supervision and has fulfilled the requirements for submission of the thesis, which to my knowledge, has reached the requisite standard.

The results contained in this thesis have not been submitted, in part or in full, to any other University or Institute for the award of any degree.

4.4

C

Professor

Department of Chemistry

Indian Institute of Technology, New Delhi-110 016

ACKNOWLEDGMENTS

It is my privilege to express my deep sense of gratitude to my revered guide, Prof.A.S.N.Murthy for his valuable guidance, constant inspiration, continuous encouragement, critical comments and benevolent help during the tenure of the present work. Without his devotion, timely directions and spirit to work, this thesis could never have seen the light of the day.

I am also highly thankful to Prof.G.N.Rao, Head, Department of Chemistry, for providing me facilities and encouraging the present investigations. My thanks are also due to all my affectionate friends, Jyotsna, E.Rajasekaran, Satish and Kaushik for their keen co-operation. I am also indebted to Dr.R.L.Gupta and Dr.Lalitha who helped me a great deal in maintaining the tempo of my research work.

I extend my heartiest thanks to my husband Mr.K.R.Dureja for his co-operation in the completion of the thesis which I can not adequately acknowledge in words. I also owe a debt of gratitude to my friend Dr.Anuradha Singh Verma and her husband, Dr.Sanjay Verma for providing me invaluable help and family atmosphere throughout the investigation.

I also acknowledge my gratitude to Indian Institute of Technology, Delhi for providing me financial assistance.

Care has been taken to give proper credit for the work of other authors in the literature. The author would like to apologise for any unintentional omissions which might have occurred by oversight.

Besides, I should not fail in my duty to mention my thanks to Mr. Samsheer Dagar and Mr. Jagdish Lohia for typing the manuscript and also to Mr. Jagdish Tuteja for the drafting work.

Lastly, , I avail myself of this opportunity to pay great respect to my parents and my in-laws for their profound love and affection for me.

ABSTRACT

Mediated electrochemistry offers a generic route to the development of sensors for different analytes. The use of p- benzoquinone as an electron transfer mediator has been considered in the glucose/glucose oxidase (GOD) coupled reaction in homogeneous solution. At a pyrolytic graphite (PG) electrode [basal plane (BPG) and edge plane (EPG), but edge plane in particular], p-benzoquinone exhibits high reversibility unlike on metal or carbon paste electrodes.

Such an electrochemical reversibility on PG electrodes is advantageous because it lends itself for immobilization either by the mediator or enzyme or both for future sensor development. A second order rate constant for the reaction between p-benzoquinone and GOD has been obtained from cyclic voltammetric data, which is comparable to that reported in the literature for ferrocene derivatives. In solution at a pH 7.0, the measured catalytic current is linearly related to glucose concentration in the range 1-10 mM.

There are a large number of NAD4-/NADH dependent dehydrogenase enzymes which are useful for sensor development specific for different substrates. The oxidation of NADH at metal electrodes requires large overpotentials.

Mediators in solution or mediator modified electrodes could be employed to oxidise NADH electrocatalytically at lower potentials. The thesis describes investigations on the

modification of a EPG electrode by dip coating and electro-deposition procedures. The TCNQ modified electrodes were characterized by cyclic voltammetric technique and found to be quite stable.

TCNQ modified electrodes prepared by dip coating or electro-deposition procedure were found to be electrocatalytic to NADH oxidation..The rate constant data and the calibration plots indicate that TCNQ modified electrode can be effectively used for NADH sensing over an extended concentration range. The results are significant for the development of amperometric enzyme sensors for subrtrates which are enzymatically coupled to NAD4INADH.

The electrocatalytic oxidation of L-ascorbic acid (AH2) on TCNQ modified electrode has also been examined in detail.

The surface coverage was --6.1x10-9 mol cm-2. The electrocatalytic activity of the modified electrode for AH2 oxidation in phosphate buffer (pH 7.0) was shown by cyclic voltammetry at a potential of 0.08V vs. Ag/AgCl. The catalytic current varies linearly with concentration of AH2 in the range 1-10 mM. The estimated rate constant for the reaction between AH2 and immobilized TCNQ is 1.8x106 M-1 s-1. The electro-oxidation of AH2 by tetrathiafulvalene (TTF) has also been examined at carbon electrodes by employing cyclic voltammetric technique. In solution phase,

the oxidation of AH2 was observed at —0.25 V vs. SCE and linearity is seen in the range 0.5-10.0 mM. The rate constant for the reaction between AH2 and TTF+ is estimated to be 1.1x105 M-1 s-1. Electrocatalysis of AH2 was also examined at TTF modified carbon paste electrode at a potential of —0.23 V vs. Ag/AgCl. The modified electrode shows linear variation of current with concentration of AH2 over the range 0.05 to 1.00 mM. Therefore, the method is ideally suited for quantitative determination of AH2.

CONTENTS

Page No.

CERTIFICATE ACKNOWLEDGEMENT ABSTRACT

CHAPTER I INTRODUCTION

I.1 Oxidases 3

1.2 Dehydrogeneses 6

1.3 Mediated Electron Transfer 8 1.4 Direct Electron Transfer 20 1.5 Scope of the Present Work 24

References 26

CHAPTER II MATERIALS AND METHODS

II.1 Materials 32

11.2 Electrochemical Cell Design 33 11.3 Electrode Preparation 36 11.4 Measurement Techniques 41

References. 50

CHAPTER III ELECTROCHEMICAL STUDY OF BENZOQUINONE- MEDIATED GLUCOSE/GLUCOSE OXIDASE REACTION

III.1 Introduction 53

111.2 Experimental 55

111.3 Results And Discussion 56

111.4 Conclusion 81

References 82

CHAPTER IV ELECTROCHEMICAL OXIDATION OF NADH AT 7,7,8,8-TETRACYANOQUINODIMETHANE (TCNQ) MODIFIED ELECTRODE

IV.1 Introduction 85

IV.2 Experimental 90

IV.3 Results And Discussion 92

IV.4 Conclusion 103

References 105

CHAPTER V NADH SENSING WITH ELECTROCHEMICALLY MODIFIED TCNQ ELECTRODE

V.1 Introduction 109

V.2 Experimental 110

V.3 Results And Discussion 111

V.4 Conclusion 121

References 122

CHAPTER VI ELECTROCHEMICAL OXIDATION OF L-ASCORBIC ACID ON 7,7,8,8- TETRACYANOQUINODIMETHANE (TCNQ) MODIFIED ELECTRODE

VI.1 Introduction 123

VI.2 Experimental 127

VI.3 Results And Discussion 128

VI.4 Conclusion 139

References 141

CHAPTER VII TETRATHIAFULVALENE AS A MEDIATOR FOR ELECTROCATALYTIC OXIDATION OF L-ASCORBIC ACID

VII.1 Introduction 145

VII.2 Experimental '147 VII.3 Results And Discussion 149 VII.4 Conclusion 158

References 159

SUMMARY APPENDIX