UNIVERSITI PUTRA MALAYSIA

MODIFICATION OF SCREEN PRINTED ELECTRODE USING REDUCED GRAPHENE OXIDE-GOLD NANOPARTICLES FOR VOLTAMMETRIC

DETECTION OF DIURON AND FENITROTHION

NAFISEH SHAMS

ITMA 2016 20

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MODIFICATION OF SCREEN PRINTED ELECTRODE USING REDUCED GRAPHENE OXIDE-GOLD NANOPARTICLES FOR VOLTAMMETRIC

DETECTION OF DIURON AND FENITROTHION

By

NAFISEH SHAMS

Thesis Submitted to the School of Graduate Studies, Universiti Putra Malaysia,In Fulfilment of the Requirements for the Degree of Doctor of Philosophy

November 2016

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All material contained within the thesis, including without limitation text, logos, icons, photographs and all other artwork, is copyright material of Universiti Putra Malaysia unless otherwise stated. Use may be made of any material contained within the thesis for non-commercial purposes from the copyright holder. Commercial use of material may only be made with the express, prior, written permission of Universiti Putra Malaysia.

Copyright © Universiti Putra Malaysia

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DEDICATION

I dedicate this thesis to my beloved husband for braving the many ups and downs with

me during the trying times, steadily and stoically

You are indeed my pillar of strength

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Abstract of thesis presented to the Senate of Universiti Putra Malaysia in fulfilment of the requirement for the degree of Doctor of Philosophy

ABSTRACT

MODIFICATION OF SCREEN PRINTED ELECTRODE USING REDUCED GRAPHENE OXIDE-GOLD NANOPARTICLES FOR VOLTAMMETRIC

DETECTION OF DIURON AND FENITROTHION

By

NAFISEH SHAMS

November 2016

Chair: Hong Ngee Lim, PhDFaculty: Institute of Advanced Technology

In recent years, increasing attention has been given to the distortion of the aquatic ecosystem around the world. The risk of chemical pollution in water bodies is getting worst day by day. One of the possible sources of these pollutants is pesticides. Thus, the development of an on-line selective and sensitive monitoring system needs to be given priority. The use of electrochemical sensor is the most promising monitoring system due to its low instrumentation cost, simple operation, short response time, highly sensitivity and selectivity compared to other conventional monitoring systems. Recently, the use of nanomaterials such as reduced graphene oxide (rGO) and gold nanoparticles (AuNPs) have been given more attention among researchers around the world. They have been extensively used as sensing materials due to their outstanding properties in terms of conductivity, effective surface area, stability and catalytic effect. In this project, two different approaches have been used to fabricate the electrochemical sensor materials based on rGO/AuNPs nanocomposite for detection of diuron and fenitrothion as herbicide and insecticide respectively in natural waters.

In the first work, rGO/AuNPs nanocomposite was synthesized based on electrochemical co-reduction of graphene oxide and chloroauric acid via cyclic voltammetry technique on the surface of screen printed electrode (SPE). The fabricated sensor (rGO-AuNPs/SPE) showed higher sensitivity towards diuron and its cathodic peak current was directly correlated to the diuron concentration. The field emission scanning electron microscopy (FESEM) image shows the uniform distribution of AuNPs on the surface of rGO nanosheets. The presence of rGO nanosheets was further proven by Raman Spectroscopy. Under optimized conditions, the cathodic peak current was proportional to the diuron concentration over a wide range between 0.5 to 30.0 µg mL-1 with the detection limit of 0.125 µg mL-1 (S/N=3). The proposed diuron electrochemical sensor also exhibited a relative standard deviation of 4.25% for six replicate analysis of 10.0 µg mL-1 diuron and the response of the electrode was declined up to 20% after keeping for 30 days in ambient temperature. In addition, the sensor was successfully employed for the determination of diuron in real natural water samples including lake and sea water.

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In spite of the fact that rGO-AuNPs/SPE sensor (via electrochemical reduction of GO) is environmentally green technique, nevertheless, the modified substrate is polarized in a short potential range due to incomplete reduction of GO functional groups. Therefore, in the second study, a new synthesis procedure was reported for the preparation of rGO/AuNPs nanocomposite using ethylenediamine (en) as a cross- linker for chemical reduction of GO functional groups in order to fabricate a polarized sensor. The constructed nanocomposite (AuNPs/rGO-en) was homogenized in dimethylformamide (DMF) and drop-casted on a SPE to fabricate an electrochemical sensor (AuNPs/rGO-en/SPE) which was sensitive to fenitrothion. The nanocomposite electrode was characterized with FESEM, X-Ray diffraction (XRD) spectroscopy, Fourier transform infrared spectroscopy (FTIR), electrochemical impedance spectroscopy (EIS), Raman spectroscopy and cyclic voltammetry (CV). The anodic peak current at around 0.06 V was proportional to fenitrothion concentration over a wide range of 0.1 to 6.25 ng mL-1

with the limit of detection of 0.036 ng mL-1 (S/N=3). Moreover, in order to evaluate the repeatability of the AuNP/en-rGO/SPE, the peak currents of fenitrothion at two different concentrations (0.5 ng mL-1 and 4.0 ng mL-1) were determined successively under optimum conditions for six times with the same modified sensor, and the RSD values were found to be 4.1% and 4.3%, respectively, exhibiting good repeatability. The stability study showed that the oxidation peak current of fenitrothion at 4.0 ng mL-

1 decreased 2.2% after recording 50 successivecyclic voltammograms. In addition, the proposed senor was successfully employed for the determination of fenitrothion residue in the natural water samples including tap and lake water. The validity of the response was checked with gas chromatography as a standard method and the result was in agreement with constructed sensor.

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Abstrak tesis yang dikemukakan kepada Senat Universiti Putra Malaysia sebagai memenuhi keperluan untuk Ijazah Doktor Falsafah

ABSTRAK

PENGUBAHSUAIAN ELEKTROD CETAK SKRIN MENGGUAN-NANOBAHAN EMAS UNTUK PENGESANAN VOLTAMMETRIK DIURON

DAN FENITROTHION

Oleh

NAFISEH SHAMS

November 2016

Pengerusi : Hong Ngee Lim, PhDFakulti : Institut Teknologi Maju

Dalam tahun-tahun kebelakangan ini, peningkatan penumpuan terhadap herotan akuatik ekosistem diseluruh dunia telah diberikan. Risiko pencemaran kimia dalam badan-badan air telah menjadi teruk dari hari ke hari. Salah satu sumber utama pencemaran ini ialah racun perosak. Oleh itu, pekembangan secara dalam talian bagi pengawasan system yang memilih dan peka perlu diberikan keutamaan. Penggunaan sensor elektrokimia adalah pengawasan system yang lebih menjanjikan disebabkan oleh peralatan kos rendah, mudah untuk dikendalikan, sambutan masa yang singkat, lebih peka dan pemilihan berbanding pengawasan sistem konvensional yang lain. Baru-baru ini, penggunaan bahan nano seperti grafin (rGO) dan nanopartikel emas (AuNPs) telah diberikan perhatian antara penyelidik diseluruh dunia. Ianya telah digunakansecara meluas sebagai bahan pengesanan disebabkan sifatnya yang cemerlang dan had pengesanan sasaran analit yang rendah telah direkodkan. Dalam projek ini, dua pendekatan yang berbeza telah digunakan untuk membikin bahan sensor elektrokimia berasaskan rGO/AuNPs kompositnano untuk pengesanan diuron dan fenitrothion, masing-masing sebagai racun herba dan rerun serangga dalam air semula jadi.

Dalam kerja yang pertama, rGO/AuNPs kompositnano telah disintesis berasaskan elektrokimia bersama pengurangan grafin oksida (GO) dan acid chloroauric melalui teknik voltammetri berkitar pada skrin elektrod bercetak (SPE). Penghasilan sensor (rGO/AuNPs/SPE) menunjukkan kepekaan yang tinggi cenderung terhadap diuron dan puncak arus katodenya adalah berkolerasi secara terus dengan kepekatan diuron. Imej mikroskop elektron pengimbas pancaran medan (FESEM) menunjukkan kehadiran AuNPs yang sama rata di atas permukaan nano kepingan rGO. Raman spektrometri membuktikan lagi pengurangan GO kepada nano kepingan rGO melalui pengurangan elektrokimia. Di bawah keadaan optimum, puncak arus katode adalah berkadaran dengan kepekatan diuron di atas julat yang luas di antara 0.5 dan 30.0 µg mL-1 dengan

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had pengesanan 0.125 µg mL-1 (S/N=3). Sensor elektrokimia yang dicadangkan juga mempamirkan relatif sisihan piawai sebanyak 4.25% berdasarkan enam analisis yang direplikakan bagi 10.0 µg mL-1 diuron dan sambutan bagi elektrod menurun 20% selepas disimpan selama 30 hari dalam keadaan sekitaran. Tambahan pula, sensor ini telah berjaya bekerja untuk penentuan diuron dalam air semula jadi nyata termasuk tasik dan air laut.

Meskipun secara faktanya sensor rGO-AuNPs/SPE (melalui elektrokimia pengurangan GO) adalah kaedah hijau persekitaran, namun substrat yang diubah suai adalah berkutub dalam julat potensi yang pendek disebabkan oleh pengurangan yang tidak lengkap GO kumpulan berfungsi. Oleh itu, dalam kajian kedua, satu tatacara sintesis baru telah dilaporkan bagi penyediaan rGO-AuNPs kompositnano menggunakan ethylenediamin (en) sebagai pemaut silang untuk pengurangan kumpulan kefungsian GO secara kimia bagi pembikinan sensor berkutub. Penghasilan kompositnano (AuNPs/en-rGO) telah dihomogenkan dalam dimethylformamide (DMF) dan kaedah ‘drop-casted’ di atas SPE bagi pembikinan sensor elektrokimia yang peka terhadap fenitrothion. Elecktrod kompositnano telah dicirikan dengan FESEM, EDX, Spektroskopi Pembiasan X-Ray (XRD), Spektroskopi inframerah transformasi Fourioer (FTIR), Spektroskopi elektrokimia impedan (EIS), Spektroskopi Raman and voltammetri berkitar (CV). Puncak arus anodal pada kira-kira 0.06V adalah berkadaran pada kepekatan fenitrothion di atas julat yang luas di antara 0.1 to 6.25 ng mL-1, dengan had pengesanan 0.036 ng mL-1 (S/N=3). Tambahan pula, untuk menilai kebolehulangan AuNP/en-rGO/SPE, puncak arus fenitrothion pada dua kepekatan yang berbeza (0.5 ng mL-1 and 4.0 ng mL-1) telah berjaya ditentukan di bawah keadaan optimum enam kali dengan pengubah suaian sensor yang sama, dan nilai RSD didapati masing-masing 4.1% dan 4.3%, mempamerkan kebolehulangan yang baik. Kajian kestabilan menunjukkan bahawa pengoksidaan puncak arus fenitrothion pada 4.0 ng mL-1

menurun 2.2% selepas mencatat 50 kitaran voltammetri berturut-urut. Tambahan pula, sensor ini telah berjaya bekerja untuk penentuan sisa fenitrothion dalam pelbagai sampel air termasuk air paip dan air tasik. Kesahan sambutan telah diperiksakan dengan gas chromatografi sebagai keadah piawaian dan keputusan adalah dalam persetujuan dengan sensor yang dihasilkan.

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ACKNOWLEDGEMENTS

This thesis will not be successful without sincere help from others. As such, it is only appropriate that I express my sincere gratitude to them at this juncture.

I would like to express my deep and sincere gratitude to my supervisor Associate Professor Dr. Lim Hong Ngee for her expertise, kindness, valuable guidance and above all for her patience and optimism which helped a lot in instilling confidence in me. I sincerely thank her for supervising me in writing up this thesis and giving me a constant encouragement and advice. She was very affectionate and was ready to help me even in the midst of her busy schedule and without her guidance, this thesis would have been a distant possibility.

The immense moral support I received from my husband Reza. He has been always ready with practical suggestions to help me. Thank you for solid support, help and advice for matters that boggle my mind. You are indeed my beacon of hope, like the light at the end of a pitch-dark tunnel. Words cannot describe my gratitude towards your understanding, guidance, patience, love and care whenever I am in a dire need of help and support. Without you, my dream of earning this Ph.D. degree would not materialize.

To my cute son Radman, just his smile is enough to make me forget the darkest moment in my life. I love you and thank you for being a good boy when Mama was rushing to complete her assignments.

Let me also take this opportunity to thank my family for their incessant love. My Papa, Mummy and my dearest brothers, Kamran and Keyvan who were always being by my side with a lot of patience and prayerful support.

I would like to thank Prof. Nor Azah, Dean of Institute of Advanced Technology, for providing me with the necessary facilities in the institute. I also would like to thank all of my co-supervisors, Prof. Nor Azah, Dr. Jaafar and Dr. Yusran for their cooperation and support.

My labmates who were always been a source of support to me. I am extremely thankful to Addy, Wawa, Asilah, Fathin, Celine, Chi Huey, Siaw Cheng, Keng Fuong, Oxalato, Hamra and Braedan for their kindness and affection. The fond memories of our times together will always remain with me. My sincere thanks to all my friends of the FDL lab. Thanks to Aliu, Ben, Dayah, Hedayah, Saman and Mimi for their wonderful friendship.

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APPROVAL

I certify that a Thesis Examination Committee has met 2016to conduct the final examination of Nafiseh Shams on her thesis entitled “Modification of Screen Printed Electrode using Reduced Graphene Oxide-Gold Nanoparticles for Voltammetric Detection of Diuron and Fenitrothion” in accordance with the Universities and University Colleges Act 1971 and the Constitution of the Universiti Putra Malaysia [P.U. (A) 106] 15 March 1998. The Committee recommends that the student be awarded the Doctor of Philosophy.

Members of the Thesis Examination Committee were as follows:

Suraya Abdul Rashid, PhDAssoc. Prof.Faculty of Institute of Advanced TechnologyUniversiti Putra Malaysia(Chairwoman)

S. ABRAHAM JOHN, PhDProfessorCentre of Nanoscience & NanotechnologyGandhigram Rural Umiversity, India(External Examiner)

Zulkarnain Zainal, PhDProfessorFaculty of Institute of ScienceDepartment of Chemistry, Universiti Putra Malaysia(Internal Examiner)

Mohd Nizar Hamidon, PhDAssoc. Prof. Dr.Faculty of Institute of Institute of Advanced TechnologyUniversiti Putra Malaysia(Internal Examiner)

___________________________NOR AINI AB. SHUKOR, PhDProfessor and Deputy DeanSchool of Graduate StudiesUniversiti Putra Malaysia

Date: 28 February 2017

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This thesis was submitted to the Senate of Universiti Putra Malaysia and has been accepted as fulfilment of the requirement for the degree of Doctor of Philosophy. The members of the Supervisory Committee were as follows:

Hong Ngee Lim, PhDAssociate ProfessorFaculty of Institute of Science Universiti Putra Malaysia(Chairman)

Nor Azah Yusof, PhDProfessor Faculty of Institute of Science Universiti Putra Malaysia(Member)

Jaafar Abdullah, PhDSenior Lecturer Faculty of Institute of Science Universiti Putra Malaysia(Member)

Yusran Sulaiman, PhDSenior Lecturer Faculty of Institute of Science Universiti Putra Malaysia(Member)

_____________________________ROBIAH BINTI YUNNUS, PhDProfessor and DeanSchool of Graduate StudiesUniversiti Putra Malaysia

Date:

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Declaration by graduate student DECLARATION

I hereby confirm that: � this thesis is my original work; � quotations, illustrations and citations have been duly referenced; � this thesis has not been submitted previously or concurrently for any other

degree at any other institutions; � intellectual property from the thesis and copyright of thesis are fully-owned by

Universiti Putra Malaysia, as according to the Universiti Putra Malaysia (Research) Rules 2012;

� written permission must be obtained from supervisor and the office of Deputy Vice-Chancellor (Research and Innovation) before thesis is published (in the form of written, printed or in electronic form) including books, journals, modules, proceedings, popular writings, seminar papers, manuscripts, posters, reports, lecture notes, learning modules or any other materials as stated in the Universiti Putra Malaysia (Research) Rules 2012;

� there is no plagiarism or data falsification/fabrication in the thesis, and scholarly integrity is upheld as according to the Universiti Putra Malaysia (Graduate Studies) Rules 2003 (Revision 2012-2013) and the Universiti Putra Malaysia (Research) Rules 2012. The thesis has undergone plagiarism detection software.

Signature: _______________________ Date: ______________________

Name and Matric No.: _________________________________________

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Declaration by Members of Supervisory Committee

This is to confirm that: � the research conducted and the writing of this thesis was under our

supervision; � Supervision responsibilities as stated in the Universiti Putra Malaysia

(Graduate Studies) Rules 2003 (Revision 2014-2015) are adhered to.

Signature: ________________________ Name of Chairman of Supervisory Committee: _______________________

Signature: ________________________ Name of Member of Supervisory Committee: _______________________

Signature: ________________________ Name of Member of Supervisory Committee: _______________________

Signature:________________________ Name of Member of Supervisory Committee:_______________________

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TABLE OF CONTENTS

Page

ABSTRACT iABSTRAK iiiACKNOWLEDGEMENTS vAPPROVAL viDECLARATION viiiLIST OF TABLES xivLIST OF FIGURES xvLISTOFABBREVIATIONS xxii

CHAPTER1 1

1 INTRODUCTION 11.1 Background of study 1 1.2 Problem statement 2 1.3 Hypothesis of study 3 1.4 Research objective 4 1.5 Scope and limitation 4 1.6 Thesis outline 4

2 LITERATURE REVIEW 62.1 Pesticides 6

2.1.1 Diuron 7 2.1.2 Fenitrothion 8

2.2 Determination of diuron and fenitrothion in water 8 2.3 Sensors 10 2.4 Types of chemical sensors 10 2.5 Voltammetric sensors 11

2.5.1 Conventional electrodes 11 2.5.2 New generation electrodes 12

2.6 Electrochemical sensing techniques 14 2.7 Molecular reaction mechanisms for electrochemical sensing 14

2.7.1 Electrochemical reaction 14 2.7.2 Chemical reactions coupled to electron transfer 15

2.8 Nanomaterials 15 2.8.1 Graphene and its derivatives 16 2.8.2 Gold nanoparticles (AuNPs) 16 2.8.3 rGO-AuNPs nanocomposite 17

2.9 Synthesis methods of nanomaterials 17 2.9.1 Reduced graphene oxide 17 2.9.2 Synthesis of rGO-AuNPs nanocomposite 17

2.10 Voltammetric sensors for detection of pesticides 20 2.10.1 Diuron determination 20 2.10.2 Fenitrothion determination 22

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3 DIURON SENSOR 273.1 Experimental Methods 28

3.1.1 Materials 28 3.1.2 Instrumentation 28 3.1.3 Diuron solution 29 3.1.4 Preparation of phosphate buffer solution (PBS) 29 3.1.5 Preparation of GO 29 3.1.6 Fabrication of rGO modified SPE 30 3.1.7 Fabrication of rGO-AuNPs/SPE modified electrode 30 3.1.8 Electrochemical sensing 31 3.1.9 Effect of amount of HAuCl4 on the surface of GO 31 3.1.10 Electro-active surface area 31

3.2 Optimization procedures 32 3.2.1 Effect of pH 32 3.2.2 Effect of supporting electrolyte 32 3.2.3 Effect of rGO-AuNPs nanocomposite film thickness 32 3.2.4 Effect of GO concentration 32 3.2.5 Effect of scan rate 33

3.3 Calibration curve 33 3.3.1 Selectivity and interference study 33 3.3.2 Precision and stability 33 3.3.3 Sample preparation in real natural water 34

3.4 Result and Discussion 34 3.5 Characterization of rGO-AuNPs/SPE modified electrode 34

3.5.1 FESEM study of rGO-AuNPs/SPE modified electrode 36 3.5.2 EDX study of rGO-AuNPs/SPE modified electrode 37 3.5.3 Raman spectroscopy of rGO-AuNPs/SPE modified electrode 37 3.5.4 Calculation of surface area for rGO-AuNPs/SPE modified

electrode 39 3.6 Electrochemical behavior of diuron at rGO-AuNPs/SPE 43 3.7 Optimization of rGO-AuNPs/SPE diuron sensor 44

3.7.1 Effect of pH 44 3.7.2 Effect of buffer solution 45 3.7.3 Effect of rGO-AuNPs nanocomposite film thickness 46 3.7.4 Effect of GO concentration 48 3.7.5 Effect of scan rate 49 3.7.6 Calculation of electron transfer coefficient (α) 52 3.7.7 Calculation of diffusion coefficient (D) 53

3.8 Mechanism of the reaction 55 3.9 Analytical performance 56

3.9.1 Linear calibration range and limit of detection 56 3.9.2 Selectivity and interference study 61 3.9.3 Precision and stability 62

3.10 Applicability of rGO-AuNPs nanocomposite-modified SPE 62 3.11 Summery 62

4 FENITROTHION SENSOR 644.1 Experimental methods 65

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4.1.1 Materials 65 4.2 Instruments 65 4.3 Chemical preparation 65

4.3.1 Fenitrothion solution 65 4.3.2 Preparation of phosphate buffer solution (PBS) 66 4.3.3 Preparation of GO 66

4.4 Preparation of en-rGO nanocomposite 66 4.5 Preparation of AuNPs/en-rGO nanocomposite 66 4.6 Fabrication of AuNPs/en-rGO/SPE 68 4.7 Preparation of sample for characterization 69

4.7.1 Field emission scanning electron microscopy 69 4.7.2 Fourier transform infrared spectroscopy 69 4.7.3 X- ray diffraction (XRD) 69 4.7.4 Raman spectroscopy 69

4.8 Electrochemical studies 70 4.8.1 Electro-active surface area 70 4.8.2 EIS 70

4.9 Preliminary study of diuron on AuNPs/en-rGO/SPE sensor 71 4.10 Optimization procedure 72

4.10.1 Effect of pH 72 4.10.2 Effect of drop casting volume of AuNPs/en-rGO nanocomposite

72 4.10.3 Effect of HAuCl4 Concentration 72 4.10.4 Effect of type of buffer 72 4.10.5 Effect of accumulation potential 72 4.10.6 Effect of accumulation time 73 4.10.7 Effect of scan rate 73 4.10.8 Reproducibility, stability and repeatability studies 73 4.10.9 Selectivity studies 74

4.11 Sample preparation in real natural water 74 4.12 Electrochemical sensing 74 4.13 Preparation of real samples for GC analysis 74 4.14 Result and Discussion 75 4.15 Characterization of AuNPs/en-rGO nanocomposite 75

4.15.1 FESEM morphology of AuNPs/en-rGO nanocomposite 75 4.15.2 FTIR study of AuNPs/en-rGO nanocomposite 76 4.15.3 XRD study of AuNPs/en-rGO nanocomposite 77 4.15.4 Raman spectroscopy of AuNPs/en-rGO nanocomposite 78 4.15.5 EIS study of AuNPs/en-rGO/SPE modified electrode 79

4.16 Calculation of surface area for AuNPs/en-rGO/SPE 80 4.17 Electrochemical behavior of fenitrothion at AuNPs/en-rGO/SPE 82 4.18 Optimization of AuNPs/en-rGO/SPE fenitrothion sensor 85

4.18.1 Effect of pH 85 4.18.2 Effect of drop casting volume of AuNPs/en-rGO nanocomposite

88 4.18.3 Effect of HAuCl4 Concentration 89 4.18.4 Effect of type of buffer 90 4.18.5 Effect of accumulation potential 91

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4.18.6 Effect of accumulation time 92 4.18.7 Effect of scan rate 93

4.19 Electrochemical mechanism of fenitrothion sensor 96 4.20Analytical performance of fenitrothion sensor 97 4.21 Reproducibility, stability, and selectivity of fenitrothion sensor 101 4.22 Selectivity study 101 4.23 Applicability of the proposed fenitrothion sensor 102 4.24 Summery 109

5 CONCLUSIONSAND RECOMMENDATIONS 1105.1 Conclusion 1105.2 Recommendations for future research 111

REFERENCES 113APPENDICES 131BIODATA OF STUDENT 148LIST OF PUBLICATIONS 149

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LIST OF TABLES

Table Page

2.1 Different electrochemical sensors for the detection of diuron. 24

2.2 Different electrochemical sensors for determination of fenitrothion.

26

3.1 Comparison of the performances of different electrochemical sensors for the detection of diuron.

60

3.2 Interference study for determination of 10.0 µg mL-1 diuron in the present of 0.1 mol L-1 KCl and 0.1 mol L-1 phosphate buffer (pH 5.5; scan rate 0.06 Vs-1).

61

3.3 Determination of diuron in water samples (N = 3). 62

4.1 Comparative study of performances of different electrochemical sensors for detection of fenitrothion.

100

4.2 Determination of fenitrothion in some water samples using the proposed sensor (AuNPs/en-rGO/ SPE) and GC analysis under optimum conditions.

103

A1 Criteria of being reversible, quasi-reversible or irreversible reactions according to CV results for redox reaction.

136

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LIST OF FIGURES

Figure Page

2.1 Diuron structure 7

2.2 Fenitrothion structure 8

2.3 Ideal working electrode (A) and ideal reference electrode (B)(Bard et al., 1980)

12

2.4 SPE include of working electrode (WE), counter electrode (C.E.) and reference electrode (R.E.) on a ceramic substrate(M. Li et al., 2012)

13

2.5 A typical electrochemical reaction on electrode surface 14

2.6 The structure of Graphene (A) and graphene oxide (B)(Dreyer et al., 2010)

16

3.1 Dropsense used for electrochemical studies. 29

3.2 One step electrochemical preparation of rGO-AuNP/SPE-modified electrode

31

3.3 Cyclic voltammetry obtained on SPE after drop-casting with (a) 80 µL of GO-HAuCl4 suspension and (b) 0.01 mol L-1 PBS (pH 9.18) at scan rate of 50 mV s-1

35

3.4 Successive CVs (n=15) after drop casting of GO-HAuCl4 suspension (80 µL) on SPE .

36

3.5 FESEM images of (A) rGO sheets and (B) rGO-AuNPs nanocomposite

37

3.6 EDX spectrum of rGO-AuNPs 37

3.7 Raman spectra of GO, GO-AuNPs, and rGO-AuNPs after modification on SPE electrode

38

3.8 CVs obtained for (a) bare SPE, (b) rGO/SPE, and (c) rGO-AuNPs/SPE electrodes in 1.0 mmol L-1 K3[Fe(CN)6] as redox probe in 0.1 mol L-1 KCl and scan rate of 50 mV s-1.

40

3.9 CVs obtained for rGO-AuNP-modified electrode in the presence of 1.0 mmol L-1 K3[Fe(CN)6] and 0.1 mol L-1 KCl at various scan rates: (a) 50, (b) 60, (c) 70, (d) 80, (e) 90, (f) 100 (g) 110, (h) 120, and (i) 130 mV s-1.

41

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3.10 Dependence of the redox peak currents on the square root of the scan rates for rGO-AuNPs/SPE in 1.0 mmol L-1

K3[Fe(CN)6] and 0.1 mol L-1 KCl

42

3.11 The plot of anodic peak currents trough square root of scan rate for (a) bare SPE and (b) rGO-AuNPs/SPE electrodes in 1.0 mmol L-1 K3[Fe(CN)6] and 0.1 mol L-1 KCl On the surface of bare electrode

42

3.12 Cyclic voltammograms of diuron in 0.1 mol L-1 KCl and 0.01 mol L-1 PBS (pH 5.5) on the surface of (a) bare SPE, (b) rGO/SPE and (c) rGO-AuNPs/SPE. (d) Cyclic voltammetry of rGO-AuNPs/SPE in 0.1 mol L-1 KCl and 0.01 mol L-1 PBS(pH 5.5), Scan rate 50 mV s-1, diuron 10 µg mL-1

43

3.13 Effects of pH, on reduction peak current of 10 µg mL-1 diuron for rGO-AuNPs/SPE in 0.1 mol L-1 KCl and 0.01 mol L-1 PBS (pH 5.5)

45

3.14 Signal to background ratio on reduction peak current of 10 µg mL-1 diuron for rGO-AuNPs/SPE in 0.1 mol L-1 KCl and 0.01 mol L-1 of different buffers (pH 5.5)

46

3.15 Effect of number of CV cycles on S/B ratio of 10 µg mL-1

diuron on rGO-AuNPs/SPE in 0.1 mol L-1 KCl and 0.01 mol L-1 PBS (pH 5.5)

47

3.16 Effect of HAuCl4 concentration in the fabrication of rGO-AuNPs/SPE on reduction peak current of 10 µg mL-1 diuron in 0.1 mol L-1 KCl and 0.01 mol L-1 PBS (pH 5.5)

48

3.17 Effect of GO concentration in co-reduction of rGO-AuNPs nanocomposite on the peak current of 10 � g mL-1 diuron. Conditions: 0.1 mol L-1 KCl and 0.01 mol L-1 phosphate buffer (pH 5.5), HAuCl4 concentration 150 µmol L-1

49

3.18 Cyclic voltammetry of rGO-AuNPs/SPE in the presence of diuron at different scan rates. Conditions: 10 µg mL-1 diuron, 0.1 mol L-1 KCl and 0.01 mol L-1 phosphate buffer (pH 5.5), scan rates (10, 20, 30, 40, 50, 60, 70, 80 and 90 mV s-1)

50

3.19 Plot of dependence of peak current on the root of the scan rate, ν0.5. Conditions: 10 μg mL-1 diuron, 0.1 mol L-1 KCl and 0.01 mol L-1 phosphate buffer (pH 5.5) at different scan rates (10, 20, 30, 40, 50, 60, 70, 80 and 90 mV s-1)

51

3.20 The relationship between Ep and ln ν. Conditions: 10 μg mL-1

diuron, 0.1 mol L-1 KCl and 0.01 mol L-1 phosphate buffer (pH 5.5) at different scan rates (10, 20, 30, 40, 50, 60, 70, 80 and 90 mV s-1)

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3.21 logarithm of intercept of the Tafel plot from CV versus logarithm of different concentration of fenitrothion

53

3.22 Chronoamperometry plot of 10.0 mg mL-1 diuron in PBS (0.01 mol L-1, pH 5.5) on the surface of rGO-AuNPs/SPE

54

3.23 Cottrell plot obtained from the chronoamperometry. All conditions are as Figure 4.19

55

3.24 Mechanism of diuron detection at rGO-AuNPs 56

3.25 LSV for diuron at rGO-AuNPs at different diuron concentrations: (a) 0.5, (b) 2.0, (C) 3.0, (d) 5.0, (e) 8.0, (f) 10.0, (g) 15.0, (h) 20.0, (i) 25.0, and (j) 30.0 µg mL-1

57

3.26 Calibration curve for diuron in 0.1 mol L-1 KCl and 0.01 mol L-1 phosphate buffer (pH 5.5)

58

3.27 Response of rGO-AuNPs/SPE in blank solution (0.1 mol L-1

KCl and 0.1 mol L-1 phosphate buffer, pH 5.5)63

4.1 Construction process of nanocomposite AuNPs/en-rGO 67

4.2 Engage of GO functional groups by ethylenediamine 67

4.3 Fabrication process of AuNPs/en-rGO nanocomposite using ethylenediamine as a cross-linker

68

4.4 Nyquist diagram and its equivalent circuit 70

4.5 Electrochemical response of AuNPs/en-rGO/SPE in (a) blank solution and (b) 18 �g mL-1 of diuron

71

4.6 FESEM images of (A) chemically and (B) electrochemically reduction of GO to rGO

76

4.7 FESEM images of (A) AuNPs/en-rGO and (B) The average particle size

76

4.8 FTIR spectra of GO (a), en-rGO (b), and AuNPs/en-rGO (c) 77

4.9 XRD patterns of GO (a) and AuNPs/en-rGO (b) 78

4.10 Raman spectra of en-rGO (a) and AuNPs/en-rGO (b) 79

4.11 Nyquist diagrams of bare SPE (a), GO (b, inset), en-rGO (c) and AuNPs/en-rGO/SPE (d) in 5.0 mmol L-1 K3[Fe(CN)6] and 0.1 mol L-1 KCl

80

4.12 CVs obtained for bare SPE (a), en-rGO/SPE (b) and AuNP/en-rGO/SPE modified electrode (c) in 1.0 mmol L-1 K3[Fe(CN)6]

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and 0.1 mol L-1 KCl at various scan rates (50–130 mV s-1)

4.13 Anodic peak currents of the bare SPE (a)en-rGO/SPE (b) and AuNP/en-rGO/SPE modified electrode (c) versus square root of scan rates in 1.0 mmol L-1 K3[Fe(CN)6] and 0.1 mol L-1

KCl.

82

4.14 Cyclic voltammogram of 4 ng mL-1 fenitrothion at AuNPs/en-rGO/ SPE, scan rate 10 mV s-1, phosphate buffer 0.01 mol L-1

(pH 2)

83

4.15 EIS response of the AuNPs/en-rGO modified- SPE platform towards blank solution containing 0.1 mol L-1 KCl (a) and in different concentrations of fenitrothion (1µM (b), 1.5 µM (c) and 2 µM (d))

84

4.16 DPV of AuNPs/en-rGO/ SPE in the present of 4.0 ng mL-1

fenitrothion at bare SPE (a), GO-modified SPE (b), en-rGO/ SPE (c), AuNPs/en-rGO (d) and in the absent of fenitrothion (e)

85

4.17 DPV voltammograms of 4.0 ng mL-1 fenitrothion at various pHs (1.6, 2, 3, 4, 5, 6) for AuNP/en-rGO/ SPE

86

4.18 The plot of peak potential versus pH in PBS (0.1 mol L-1)containing 4.0 ng mL-1 fenitrothion at AuNPs/en-rGO

87

4.19 The plot of peak current versus pH in PBS (0.1 mol L-1)containing 4.0 ng mL-1 fenitrothion at AuNPs/en-rGO/SPE

88

4.20 Effect of different volumes of AuNPs/en-rGO nanocomposite on on the peak current of fenitrothion (4.0 ng mL-1) in PBS (0.1 mol L-1, pH 2)

89

4.21 Effect of HAuCl4 concentration on the peak current of 4.0 ng mL-1 fenitrothion at AuNPs/en-rGO/ SPE

90

4.22 Effect of kinds of buffer on the oxidation peak current of 4 ng mL-1 fenitrothion using AuNPs/en-rGO/SPE in 0.01 mol L-1

PBS (pH=2)

91

4.23 Effect of accumulation potential on the oxidation peak current of fenitrothion (4 ng mL-1) using AuNPs/en-rGO/SPE in 0.01 mol L-1 PBS (pH=2.0)

92

4.24 Effect of accumulation time on the oxidation peak current of 4 ng mL-1 fenitrothion for AuNPs/en/rGO /SPE in 0.01 mol L-1

PBS (pH=2) electrolyte

93

4.25 Cyclic voltammetry of 4 ng mL-1 fenitrothion at different scan rates on the surface of AuNPs/en-rGO/SPE. Conditions:

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Phosphate buffer (0.01 mol L-1, pH 2)

4.26 The plot of peak current against root of scan rate for fenitrothion (4 ng mL-1). Conditions: Phosphate buffer (0.01 mol L-1, pH 2), accumulation potential, -0.4 V and accumulation time 50 s

94

4.27 Cyclic voltammogram of fenitrothion at 4 ng mL-1 to draw Tafel plot using AuNPs/en/rGO/ SPE. Conditions: Phosphate buffer (0.01 mol L-1, pH 2).

95

4.28 Tafel plot derived from Figure 4.26 in the presence of 4 ng mL-1 fenitrothion

96

4.29 (A) Mechanism for detection of fenitrothion in pre-concentration step and (B) striping step and the related volramogram

97

4.30 DPV curves for fenitrothion on AuNP/en-rGO/SPE at different concentrations: 0.1 (a), 0.2 (b), 0.3 (C), 0.5 (d), 1.0 (e), 1.5 (f), 2.0 (g), 2.5 (h), 3.0 (i), 3.5 (j), 4.0 (k), 5.0 (l), and 6.2 (m) ng mL-1

98

4.31 Calibration curve of fenitrothion in 0.01 mol L-1 PBS (pH 2.0) on AuNPs/en-rGO/SPE (a), bare SPE (b), en-rGO/SPE (c) and AuNPs/SPE (d) with accumulation time of 50.0 s, accumulation potential of -0.4 V, and scan rate of 0.05 V s-1

99

4.32 Amperometric response of AuNPs/en-rGO/SPE upon subsequent additions of 20 µM of fenitrothion (a), 10 µM of fenitrothion (b), 40 µM of Simetryn (c), 40 µM of Chloropham (d), 40 µM of Prometryn (e), 40 µM of Fenuron (f), 40 µM of Diuron (g) 30 µM of fenitrothion (h), and 30 µM of fenitrothion (i) in 0.01 M PBS solution (pH 2.0). Applied potential -0.6 V

102

4.33 Figure 4.32. GC Spectrum of 0.1 ng mL-1 fenitrothion. 103

4.34 GC Spectrum of 1 ng mL-1 fenitrothion 104

4.35 Figure 4.34. GC Spectrum of 5 ng mL-1 fenitrothion. 104

4.36 GC Spectrum of 10 ng mL-1 fenitrothion. 105

4.37 Working curve for fenitrothion in GC analysis as a function of peak area versus concentration in the range of 0.1-10 ng mL-1

fenitrothion. FID detector, column (L=15 m, I.D. 0.32 mm, HP-5), injection volume 10 μL, injection temperature 210 °C, column programming temperature: initial temperature of 185 °C (for 15 min) and increase up to 225 °C at 10 °C min-1.

105

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4.38 Figure 4.37. Response of GC to tap water. 106

4.39 Response of GC to lake water 106

4.40 GC spectrum of tap water in presence of 1ng mL-1 fenitrothion 107

4.41 GC spectrum of tap water in presence of 2 ng mL-1

fenitrothion108

4.42 Figure 4.41. GC spectrum of lake water in presence of 1 ng mL-1 fenitrothion.

108

4.43 Figure 4.42. GC spectrum of lake water in presence of 2 ng mL-1 fenitrothion

109

A1 Family tree highlighting a number of interfacial electrochemical techniques. Red are specific techniques, blue shows experimental conditions and green are the analytical signals (Kissinger & Heineman, 1996).

132

A2 Potential- time changes in LSV (A) and typical LSV voltammogram (B) (Nadjo & Savéant, 1973).

133

A3 Potential changes with time in CV (A) and a typical cyclic voltammogram (B) (Noel & Vasu, 1990).

135

A4 The potential waveform in NPV (A) and typical normal pulse voltammogram (B) (Ewing et al., 1981)

137

A5 Potential-time program in DPV (A) and typical normal pulse voltammogram (B) (Kawagoe et al., 1993).

138

A6 The trend of potential changes with time in SWV (A) and typical Square-Wave Voltammetry peak (B) (Kawagoe et al., 1993).

138

A7 The principle of ASV (A) and voltammogram resulted from stripping step (B) (J. Wang, 1985).

140

B1 Simulated cyclic voltammetry for ErCi mechanism at different scan rates.

143

B2 The relationship between ratio peak currents and scan rate for ErCi and the ErCr mechanisms.

144

B3 The simulated cyclic voltammetry for ErCi' mechanism at different kc values (0.1, 1, 5 and 10 s-1).

145

B4 The simulated cyclic voltammetry for CrEr mechanism at different K values (0.1, 1, and 10) and (B) the simulated voltamograms at K=10 and different scan rates (0.01, 0.1 and 1

146

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Vs-1).

B5 The simulated cyclic voltammetry for ErCiEr mechanism at different k values (0.01, 0.1, 1.0 and 10 s-1) and scan rate of 0.1 Vs-1.

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LIST OF ABBREVIATIONS

AdSV Adsorptive Stripping Voltammetry

ASV Anodic stripping voltammetry

C.E Counter electrode

CSV Cathodic stripping voltammetry

CV Cyclic voltammetry

DPV Differential pulse voltammetry

D Diffusion coefficient

DMF Dimethyl formamide

DME Dropping mercury electrode

EIS Electrochemical impedance spectroscopy

EDX Energy-dispersive X-ray spectroscopy

F Faradic constant

FESEM Filed emission scanning electron microscopy

FID Flame ionization detector

FTIR Fourier Transformed Infrared Spectroscopy

GC Gas chromatography

GC-MAS Gas chromatography- mass spectrometry

R Gas constant

GCE Glassy carbon electrode

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GO Graphene Oxide

AuNP/en-rGO Gold nanoparticles/ ethylenediamine-reduced Graphene Oxide

K Kelvin

LOD Limit of detection

LOQ Limit of quantitative

LSV Linear Sweep Voltammetry

NPs Nanoparticles

NPV Normal pulse voltammetry

n Number of electron transfer

PAMAN Polyamidoamine

rGO-AuNP Reduced graphene oxide- gold nanoparticle

rGO Reduced graphene oxide

SEM Scanning electron microscopy

HME Hanging mercury electrode

RE Reference electrode

RSD Relative standard deviation

� Scan rate

SPE Screen-printed electrode

SWV Square wave voltammetry

A Surface area

α transition coefficient

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UV Ultraviolet

W.E Working electrode

XRD X-ray diffractometry

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CHAPTER 1

CHAPTER 1

INTRODUCTION

1.1 Background of study

Undoubtedly, human population around the world is increasing day by day. It is expected that by 2050 the population will reach more than 9 billion and still expected that our population will increase more than 2 billion within the next 40 years (Matlack, 2010). As a result, we will need to double the amount of food we currently produce in order to feed everyone. This has resulted in the maximization of yields of many crops grown under conditions that reduce insect and disease pressure and on soils enriched with inorganic fertilizer. Therefore, the agriculture industry has changed a lot over the past century and too many efforts have been done to improve soil fertility to reach more quantitative and qualitative of agricultural products. One of the major problems in the agricultural sectors is the loss of crop products due to the insect pests, which targeted on the specific hosts and lead serious lesions on fruit and vegetables (Litz & Gray, 1995). Research has shown that since the 1940s, some of the chemicals such as pesticides helps in increasing the crop yield by preventing the destruction of food crops by pests or unpleasant plants and also provides a better plant quality (Grung et al.,2015). Moreover, it was also been reported in the USA, by ceasing the use of pesticides can lead to the reduction of the total output of the crops and livestock about 30% and therefore increase the price of farm products around 50 to 70% (Wolfenburger et al.,2004). As a matter of fact, the pesticides industry has become one of the most important supporting industries in agriculture. For instance, just in Malaysia, the estimation value of herbicides imported in 1995 was 31,708 million Ringgit (Matlack, 2010). Even though pesticides has been considered as the toxic and harmful chemical to the environment, but their contribution in controlling and monitoring the pests, and also deficit alternative, equivalent and effective methods available have made the use of pesticides the best candidate in controlling the pest and thus in overall increasing the crop production.The extensive use of pesticides for agricultural products has resulted in the presence of residues in numerous environmental media. Pesticide contamination on the water surface has been well documented worldwide and constitute a major issue that gives rise to concerns at local, regional, national and global scales (Cerejeira et al.,2003; Huber et al., 2000; Planas et al., 1997). However, since the use of pesticides isnecessary for agricultural industries, the environmental contamination of their residue need to be taken into consideration and appropriate measures should be put in place to prevent their environmental pollution (Hikita et al., 1977).

There has been a great deal of concern and argument for decades over pesticides and their toxicology to human and animals. However pesticide consumption is necessary to improve the agricultural products, but there is a large body of evidence claim that pesticides will cause too many side effects in human and animals due to direct and indirectly environmental contamination. Multiple organ cancer is one of the most crucial problems due to misuse of pesticides (Anwar, 1997). The endocrine

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complication is another problem which is caused by the effects of pesticides (van der Werf, 1996). The potential side effect related to endocrine complication always affected on male hormones such as testosterone which are responsible for healthy male reproductive systems. In addition, infertility and sterility are other reported potential side effects, when an individual is vulnerable to pesticides. Most of these problems commonly affect people who are unprotected against pesticides such as workers in farms or daily consumers of sprayed products. It was reported that most of the gardeners and farmers have higher possibilities of being affected with brain damage, which may result in the lack of ability to talk smoothly, detect words or colours (Edwards, 2013). An even human embryo is also not safe if there is too much exposure to pesticide and this excessive vulnerable could lead to miscarriage and birth defects (van der Werf, 1996). Therefore, exposure to pesticides must be avoided for pregnant mothers. Besides, respiratory disorders such as wheezing, chronic bronchitis and asthma, and skin irritation are the other common diseases occur in humans who are frequently vulnerable to the pesticides in agricultural products (Ündeğer & Başaran, 2005). Aforementioned exposures to pesticides have reported a shocking number of deaths correlated to either chronic kidney sickness or intestinal nephritis in India, and normally victims are persons who worked on farms and exposed to sprayed pesticides. Negative impacts of pesticides not only affecting human health but also affecting on animals, environmental and water as well and this phenomenon was observed to indirectly lead to many forthcoming problems to humans being. Therefore, the environmental monitoring and rapid sensing of these compounds have become important urgency in order to ensure the homeland security and health protection (Arduini et al., 2006; Corley, 2003; Fukata et al., 2005). In conclusion, the necessity to develop simple, selective and sensitive on-site monitoring systems need to be considered and given a major priority.

1.2 Problem statement

Diuron, N’-(3,4-dichlorophenyl)-N, N-dimethylurea (DCMU), is an herbicide derived from urea. It is considered to be highly toxic and persistent when applied in high dosages to the soil, with a half-life of over 300 days (Katsumata et al., 2009). In order to evaluate the impact of diuron on coral reefs, it is important to monitor its concentration in coastal and coral reef waters. Malaysia is the world’s second largest palm oil producer (Ong & Goh, 2002). Previous studies have shown that pesticides such as diuron have played an important role in controlling and preventing pests and harmful plants, and indirectly increase palm oil production (Sapozhnikova, Wirth, Schiff, Brown, & Fulton, 2007). As a result, it is often detected in ground water and surface water. However, it is considered to be a priority hazardous substance by the European Commission (Feng et al., 2009). Diuron kills plants by blocking electron transport at photosystem II, thus inhibiting photosynthesis (R. J. Jones & Kerswell, 2003). Ecotoxicological studies show that diuron has significant impacts on the host and/or symbionts of corals and aquatics (R. Jones, 2005).Moreover, Fenitrothion [O,O-dimethyl O-(4-nitro-m-tolyl) phosphor] is a contact organophosphate insecticide that has been used since 1959 in place of dichlorodiphenyltrichloroethane (DDT) for operational control of insects on rice, cereals, fruits, vegetables, stored grains, cotton, and in forests, and for fly, mosquito, and cockroach control in public health programs (Sreedhar et al. 2011). Generally, organophosphate compounds disrupt the

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cholinesterase enzyme that regulates acetylcholine (Cremisini et al. 1995; Fennouh et al. 1997; Zhang et al. 2001). Although fenitrothion is useful for the treatment of agricultural products, its residue is reported to be very toxic to non-target species such as birds, mammals, and aquatic life (Sreedhar et al. 2015; Story et al. 2011).

Various quantitative analysis of pesticides have been carried out with different techniques such as high-performance liquid chromatography (HPLC) (Bester et al.,2001; Leandro et al., 2006; Topuz et al., 2005), mass spectrometry (Dallüge et al., 2002), molecular imprinting (Pardieu et al., 2009; Sánchez-Barragán et al., 2007), gas chromatography (Berijani et al., 2006; Eisert & Levsen, 1995; Lehotay et al., 2005),enzyme-linked immunosorbent assay (ELISA) (Ferrer et al., 1997; Liang et al., 2007; Wengatz et al., 1998), biosensors (Liu et al., 2014; Sassolas et al., 2012; Zhang et al.,2015) and electrochemical sensors (Bakas et al., 2014; Rapini & Marrazza, 2015). It is clear that chromatography and mass spectrometry equipment are expensive and require tedious sample treatment including extraction, as well as skilled technicians for operations. On the other hand, ELISA systems require long-time analysis, have low stability, disposable and sometimes have low precision due to the use of biological elements for detection. As a result, the development of rapid, inexpensive, sensitive and on-site analytical strategies for detection of pesticides is desirable. On the other hand, rGO-AuNPs nanocomposite films can generate synergy on electrocatalytic activity and improve the sensitivity of the sensors. The introduction of AuNPs can both improve the conductivity of rGO and prevent the agglomeration process.

In this respect, electrochemical sensors are adequate candidates for quantitative analysis of pesticides due to their great benefits including simplicity, portability and low cost. They give attractive sensing characteristics, accurate results and fast response time with little or no complications of pre-treatment steps, thereby opening up the possibilities of direct on-site analysis with intuitive devices. Hence, they are used as alternative analytical methods for pesticide’s control (Agüı et al., 2002; Kurzawa & Kowalczyk-Marzec, 2004; Ronkainen et al., 2010). In this sense rGO-AuNPsnanocomposites based electrochemical sensors improve the performance of the electrochemical reactions, due to increasing on surface area, as well as achieving faster kinetics (Downard et al., 2006; Welch & Compton, 2006).

1.3 Hypothesis of study

� The combination of rGO and AuNPs has a synergic effect in terms of surface area and conductivity to enhance the performance of electrochemical detection of pesticides.

� Diuron and fenitrothion as two pesticide compounds can reduce/oxidize on the surface of modified electrodes.

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1.4 Research objective

The main objective of this study is to develop simple, rapid, stable, selective and sensitive electrochemical sensors based on reduced graphene oxide/gold nanoparticles for determination of some pesticides in natural water resources. The specific objectives of the research are summarized and listed as follows:

i. To synthesizerGO/AuNPs nanocomposites based on two different reduction methods through electrochemical and chemical methods.

ii. To fabricate electrochemical sensors based on rGO/AuNPs without using any mediator.

iii. To optimize the electrochemical sensors towards diuron and fenitrothion detection.

iv. To evaluate selectivity, stability and precision of the sensors.

v. To evaluate the applicability of the fabricated electrochemical sensors in real natural water samples.

vi. To validate accuracy of the electrochemical sensors with a standard test method.

1.5 Scope and limitation

Electroanalytical sensors can play an important role in monitoring the environmental pollution and thus indirectly protect our green nature to become contaminate and extinct for the next century. The electroanalytical sensor devices have been satisfied many of the requirements for on-site monitoring of priority pollutants. They are inherently sensitive and selective towards electroactive species, fast and accurate, compact, portable and cheap. These advantages provide them to be used as ultra-microelectrodes, highly sensitive chemicals, computerized instrumentations and flow detectors. On the other hand, electrochemical sensors have shown some limitations such as short-term stability, troublesome electron-transfer pathways and electrochemically active interferences in the sample. These kinds of problems become more serious in real sample analysis due to the unknown matrix and in this case, even standard addition method cannot be useful. So the improvement of these drawbacks must be given priority.

1.6 Thesis outline

This thesis contains five chapters including an introduction, literature review, methodology, result and discussion and finally conclusion and recommendation. A brief idea of each chapter is given as bellow.

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Chapter One is divided into six sections. This chapter is started with the background of study and then, the toxicology of pesticide to human and environment is being introduced as the problem statement. In the third section, the hypotheses of the proposed research study are mentioned and the objectives of this research are described in the fourth section. Finally, scope and limitation and thesis outline are being explained in the fifth and sixth section of this chapter, respectively.

Chapter Two highlights the basic principles of this work and detailed literature review of previous techniques based on electrochemical sensors. Diuron and fenitrothion are introduced and after an overview on the determination of diuron and fenitrothion in water through different methods, the classification of sensors is discussed. After that voltammetric techniques are explained as the most sensitive electrochemical techniques. This chapter is followed by an introduction on GO and AuNPs, its unique properties and synthesis methods. Finally, the voltammetric sensors have been reviewed separately for the determination of diuron and fenitrothion.

Chapter Three describes the materials and methods of the diuron sensor. Synthesis of GO, rGO and rGO-AuNPs were illustrated.All instruments used for characterization of nanocomposites such asfield emission scanning electron microscopy (FESEM), and Raman spectrometry are introduced in this chapter. Moreover, electrochemical devices for quantitative analysis of diuron also described in this chapter. Finally, results and discussion were demonstrated and the details of the results are interpreted and discussed.

Chapter Four describes the materials and methods of the fenitrothion sensor. Synthesis of GO, rGO-en and AuNPs/en-rGO were illustrated. All instruments used for characterization of nanocomposites such asfield emission scanning electron microscopy (FESEM), Fourier transform infrared microscopy (FTIR), X-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS) and Raman spectrometry are introduced in this chapter. Moreover, electrochemical devices for quantitative analysis of fenitrothion also described in this chapter. Finally, results and discussion were demonstrated and the details of the results are interpreted and discussed.

The conclusion of the research study is demonstrated in Chapter Five by summarizing the results of the study and given some suggestions for further research efforts.

Some additional subjects are covered in Chapter Six as Appendices.

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