Bilal Thesis

7/26/2019 Bilal Thesis

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Synthesis and Characterization of Novel

Organic Ionophores: Application as Selective

Electrochemical Sensors

By

Bilal Khalid

CIIT/FA14-MSCHEM-008/LHR

MS Thesis

COMSATS Institute of Information Technology

Lahore, Pakistan

Spring, 2016


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Synthesis and Characterization of Novel Organic

Ionophores: Application as Selective


A Thesis Presented to

COMSATS Institute of Information Technology, Lahore

In partial fulfillment

of the requirement for the degree of

Master of Science

in

Chemical Engineering

By

Bilal Khalid


Spring, 2016

COMSATS Institute of Information Technology


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A Graduate Thesis submitted to the Department of Chemical Engineering as

partial fulfillment of the requirement for the award of Degree of Master of

Science (MS) in Chemical Engineering.

Name Registration Number

Bilal Khalid CIIT/FA14-MSCHEM-008/LHR

Supervisor

Dr. Mazhar Amjad Gilani

Associate Professor

Department of Chemical Engineering

COMSATS Institute of Information Technology (CIIT)

Lahore

June, 2016


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Final Approval

This thesis titled




Bilal Khalid


Has been approved

In COMSATS Institute of Information Technology, Lahore

External Examiner: __________________________________________

Supervisor: ________________________________________________

Dr. Mazhar Amjad Gilani (Associate Professor)

Department of Chemical Engineering CIIT Lahore, Pakistan

Co-supervisor: ____________________________________________

Dr. Abdur Rahim (Assistant Professor)

IRCBM, CIIT Lahore, Pakistan

Head of Department: _______________________________________

Prof.Dr. Asad Ullah Khan

Department of Chemical Engineering, CIIT Lahore, Pakistan


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Declaration

I Bilal Khalid Registration # CIIT/FA14-MSCHEM-008/LHR hereby declare that Ihave produced the work presented in this thesis, during the scheduled period of study.

I also declare that I have not taken any material from any source except referred to

wherever due that amount of plagiarism is within acceptable range. If a violation of

HEC rules on research has occurred in this thesis, I shall be liable to punishable action

under the plagiarism rules of the HEC

Date: _________________ Signature of the student: ___________________

Bilal Khalid



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Certificate

It is certified Bilal Khalid (CIIT/FA14-MSCHEM-008/LHR) has carried out all the

work related to this thesis under my supervision at the Department of Chemical

Engineering, COMSATS Institute of Information Technology, Lahore and the work

fulfills the requirement for award of MS degree.

Date: _____________ ________________

Supervisor

Dr. Mazhar Amjad Gilani

Associate Professor

Head of Department:

________________

Prof. Dr. Asad Ullah Khan

Head & Chairman, Department of Chemical Engineering

CIIT, Lahore


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DEDICATIONS

I would like to dedicate my thesis

To my respected

Teachers and

parents for

their love,

prayers endless

support


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ACKNOWLEDGEMENTS

I am very thankful toALLAHs AlMIGHTY,WHO blessed me with power, potential

and capability to complete my MS Chemical Engineering work. Love for HOLY

PROPHET MUHAMMAD (SAW) who delivered the message of ALLAH and

enlighten us with the light of knowledge and the holy Book Quran.

I am highly indebted to pay my cordial gratitude to my supervisor who has given me

chance to work in his supervision as a Research Assistant under HEC NRPUproject.

I am obliged to Higher Education Commission, Pakistan for financial support under

project number 2985. I express my fervent sense of thankfulness to my supervisor

Associate Prof. Dr. Mazhar Gilani, Chemical Engineering department, COMSATS

Institute of Information Technology CIIT, Lahore, and my co-supervisor Asst. Prof.

Dr. Abdur Rahim, IRCBM, CIIT, Lahore, for their generous help, teaching attitude,

guidance and dedicated supervision. This thesis was made possible by their support,

patience and persistence.

A special thanks to Asst. Prof Dr Sobia Tabassum for her support and help in the

absence of my supervisor.

I am highly indebted to pay my cordial gratitude to Dr. Asad ullah Khan(Head of

Department Chemical Engineering), CIIT, Lahore, for providing laboratory facilities

during my research work in COMSATS. I am also pleased to pay my thanks to Asst.

Prof. Dr. Shahid Nazir Department of Chemical Engineering, CIIT, Lahore for

support, help and nice command. I am thankful to Asst. Prof. Dr. Asim Laeeq khan

for providing chemicals and lab ware for research purpose.

Thanks is also extended to Asst. Prof. Dr. Nawshad for their help and all faculty

members and research associates of Department of Chemical Engineering, Lahore aswell as of IRCBM for their help, support and guidance.I want to pay my gratitude and

thanks to all members of my family and specially my fatherfor their support, affection,

Love, prayers and special care. Special thanks to Ghayur Abbaswho helped me a lot,

being a good friend and teacher, in material synthesis. A special word of gratitude to

my friends Kamran Alifor his motivation, Hafiz Atiffor his worthy advices and all

my colleagues for helping me.

Bilal Khalid CIIT/FA14-MSCHEM-008/LHR


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Abstract

Synthesis and Characterization of Novel Organic Ionophores:

Application as Selective Electrochemical Sensors

Carbon ceramic sensor was fabricated using Silica-graphite (SiO2/C). Copper oxide

(CuO) was immobilized in situon (SiO2/C) matrix. SiO2/C/CuO was modified with

benzimidazolium-1-acetate ionic liquid by ultra-sonication. Imidazole group present in

ionic liquid acts as ionophore. Surface area (SBET = 99.87 m2/g) and pore volume

(0.3104 cm/g) of the material was calculated from BET analyzer. Scanning electron

microscopy (SEM) images showed materials compactness having no phasesegregation within the magnification used. These materials were used in the fabrication

of working electrodes i.e. SiO2/C, SiO2/C/CuO, SiO2/C/CuO/IL for the selective

electrochemical determination of catechol (analyte). Secondly CP/Cu/FA, chemically

modified carbon paste electrode was prepared using copper formamidine complex

[Cu/FA] because formamidine also behaves as ionophore in the selective

electrochemical determination of different analytes. Electrochemical impedance

spectroscopy has revealed that the SiO2/C/CuO/IL has the lowest charge transfer

resistance and it assists the charge transfer because of its ionic nature. SiO2/C/CuO/IL

electrode exhibits excellent sensitivity, linear response range and low limit of detection

(LOD) which are 0.678 nA dm3mmol-1 cm-2, 0.2 mM10 mM and 6.76 x 10-6mol dm-

3respectively, as compared to other sensor CP/Cu/FA. The sensor also shows very good

sensitivity for the determination of catechol in environmental samples taken from

Monnoo Yarn Dyeing industry.


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Table of Content

Declaration ................................................................................................v

Certificate ................................................................................................ vi

ACKNOWLEDGEMENTS................................................................. viii

1 Introduction ..........................................................................................2

1.1 Sensors ....................................................................................................... 2

1.2 Types of sensors ......................................................................................... 2

1.2.1 Physical sensors .......................................................................................3

1.2.2 Chemical sensors .....................................................................................3

1.2.3 Biosensors ................................................................................................3

1.3 Literature review ........................................................................................ 3

1.3.1 Catechol Optical Sensors: ........................................................................4

1.3.2 SiO2/C-graphite based Selective Electrochemical Sensors......................5

1.3.3 CuO based Selective Electrochemical Sensors ........................................7

1.3.4 Ionic Liquid modified selective catechol sensors ....................................7

1.3.5 Formamidine modified selective electrochemical sensor ........................8

1.4 Scope and Objectives ................................................................................. 8

2 Material and Methods ....................................................................... 11

2.1 Materials ................................................................................................... 11

2.1.1 TEOS (Tetraethylorthosilicate) ..............................................................11

2.1.2 Ethanol ...................................................................................................11

2.1.3 Acetic Acid ............................................................................................11

2.1.4 Benzimidazole........................................................................................11

2.1.5 Nitric Acid .............................................................................................11

2.1.6 Hydrofluoric Acid ..................................................................................11

2.1.7 Graphite..................................................................................................11


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2.1.8 Copper Nitrate ........................................................................................12

2.1.9 Ethylene Glycol .....................................................................................12

2.1.10 Sodium Hydroxide .................................................................................12

2.1.11 Uric Acid ................................................................................................12

2.1.12 Ascorbic Acid ........................................................................................12

2.1.13 4 Amino Phenol .....................................................................................12

2.1.14 L- Cysteine .............................................................................................12

2.1.15 Dopamine ...............................................................................................13

2.2 Instrumentation and characterization techniques ..................................... 13

2.2.1 Scanning Electron Microscope (SEM) ..................................................13

2.2.2 BET multipoint Technique ....................................................................13

2.2.3 Universal testing machine (UTM) .........................................................13

2.2.4 Fourier Transform Infrared Spectroscopy (FTIR) .................................13

2.2.5 Nuclear Magnetic Resonance ................................................................13

2.2.6 Melting Point Apparatus ........................................................................14

2.2.7 Rotary Evaporator ..................................................................................14

2.2.8 Potentiostat Machine:.............................................................................14

2.3 Synthetic procedures for the material preparation ................................... 14

2.3.1 Synthesis of SiO2/C by the solgel method ..........................................14

2.3.2 Synthesis of CuO on SiO2/C matrix.......................................................15

2.3.3 Synthesis of Ionic Liquid .......................................................................16

2.3.4 Synthesis of formamidines .....................................................................16

2.3.5 Fabrication of carbon ceramic electrode ................................................17

2.3.6 Fabrication of carbon paste electrodes ...................................................18

3 Results and Discussion ..................................................................... 20

3.1 Synthesis .................................................................................................. 20

3.1.1 Synthesis of formamidine ......................................................................20

3.1.2 Synthesis of benzimidazolium-1-acetate ...............................................20

3.2 SEM Analysis........................................................................................... 20


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3.2.1 SiO2/C-graphite matrix ..........................................................................20

3.2.2 SiO2/C/CuO Material .............................................................................23

3.2.3 SiO2/C/CuO/IL Material ........................................................................24

3.3 Energy Dispersive X-rays Spectroscopy (EDX) ...................................... 26

3.3.1 EDX analysis of SiO2/C/CuO ................................................................26

3.3.2 EDX color mapping of material SiO2/C/CuO ........................................27

3.4 BET (Brunauer Emmett Teller) surface area and porosity analysis ........ 28

3.5 Fourier transform infrared spectroscopy (FTIR) study of [BzIm]OAc ... 29

3.6 Nuclear Magnetic Resonance (NMR) ...................................................... 30

3.7 Electrochemical analysis of carbon ceramic electrodes: .......................... 31

3.7.1 Cyclic voltammetry:...............................................................................31

3.7.2 Amperometry .........................................................................................34

3.7.3 Calculations............................................................................................37

3.7.4 Interference study and selectivity: .........................................................38

3.7.5 Real Samples Analysis:..........................................................................38

3.7.6 Electrochemical Impedance Spectroscopy: ...........................................39

3.8 Electrochemical Analysis of Chemically modified carbon paste electrodes

(CMCPE- CP/Cu/FA) ............................................................................................ 40

3.8.1 Cyclic voltammetry of CMCPE- CP/Cu/FA..........................................40

3.8.2 Amperometry of CMCPE- CP/Cu/FA ...................................................41

3.8.3 Calculations............................................................................................42

3.9 Conclusion................................................................................................ 42

4 References ......................................................................................... 45


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List of Figures:

Figure 2-1Scheme of synthesis of SiO2/C by Sol Gel Method ................................... 15

Figure 2-2 Synthesis of Benzimidazolium-1-acetate ................................................... 16

Figure 2-3 Schematics of carbon ceramic electrode of 6 mm surface area ................. 17

Figure 2-4 Schematic of carbon paste electrode of 3 mm diameter of hole ................ 18

Figure 3-1 SEM image of SiO2/C-graphite with the magnification of 5 kx ................ 21

Figure 3-2 SEM image of SiO2/C-graphite with the magnification of 10 kx .............. 22

Figure 3-3 SEM image of SiO2/C-graphite with the magnification of 25 kx and 50 kx

.............................................................................................................................. 22

Figure 3-4 SEM image of SiO2/C/CuO with the magnification of 5 kx ...................... 23

Figure 3-5 SEM image of SiO2/C/CuO with the magnification of 10 kx .................... 24

Figure 3-6 SEM image of SiO2/C/CuO with the magnification of 25 kx and 50 kx .. 24

Figure 3-7 SEM image of SiO2/C/CuO/IL with the magnification of 5 kx ................. 25

Figure 3-8 SEM image of SiO2/C/CuO/IL with the magnification of 10 kx ............... 25

Figure 3-9 SEM image of SiO2/C/CuO/IL with the magnification of 25 kx and 50 kx

.............................................................................................................................. 26

Figure 3-10 EDX spectrum of SiO2/C/Cu ................................................................... 26

Figure 3-11 Weight percentage of elements present in Sensor.................................... 26

Figure 3-12 Color mapping of Cu, Si, O and C as in (a) (b) (c) and (d) respectivlely 27

Figure 3-13 BET nitrogen adsorption-desorption isotherm of SiO2/C-graphite matrix

.............................................................................................................................. 28

Figure 3-14 BET pore size distribution of SiO2/C-graphite material .......................... 29

Figure 3-15 IR spectrum of [BzIm]OAc ...................................................................... 30

Figure 3-16 Cyclic voltammograms f working electrodes in 0.1M NaOH at scan rate

of 20 mV sec-1against SCE. Inset shows the cyclic voltammogram of SiO2/C ... 32

Figure 3-17 Cyclic voltammograms of working electrodes by adding 4mM Catechol

in 0.1 M NaOH at scan rate of 20 mV sec-1against SCE ..................................... 33


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Figure 3-18 Cyclic voltammograms of working SiO2/C/CuO/IL based electrode

obtained at different catechol concentrations (0, 2, 4, 6 mM) in 0.1 M NaOH at

scan rate of 20 mV sec-1....................................................................................... 34

Figure 3-19 Amperogram of SiO2/C/CuO/IL electrode at 0.6 V in 0.1M NaOH

solution with successive addition of 0.2 mM Catechol after every 50 sec. Insetshows the calibration curve for the concentration range of 0.22.4 mM ........... 35

Figure 3-20 Amperogram of SiO2/C/CuO/IL electrode at 0.6V in 0.1 M NaOH

solution with successive addition of 1 mM Catechol after 50 sec. Inset figure

shows the calibration curve from 1- 10 mM......................................................... 36

Figure 3-21 Amperogram of successive addition of 0.2 mM catechol upto750 sec and

after that addition of 0.38 mM catechol upto 1700 sec in 0.1 M NaOH. Inset

shows the calibration curves before and after 750 s respectively, for a total range

of concentration 0.2- 9.64 mM ............................................................................. 37

Figure 3-22 Amperometric response of SiO2/C/CuO/IL sensor with successive

addition of 3mM ( CC, 4APh, L-Cys, NaNO2, UA, DA, AA ) and again 5mM CC

in 0.1M NaOH solution after 50 sec. .................................................................... 38

Figure 3-23 Catechol determination in effluent water from Monnoo Yarn Dyeing

with successive addition of 0.5 mL effluent water in 50 mL of 0.1 M NaOH ... 39

Figure 3-24 Nyquist plots for SiO2/C, SiO2/C/CuO, SiO2/C/CuO/IL electrodes with

4mM catechol in 0.1 M NaOH solution. Inset figure shows the zoom image of

SiO2/C/CuO and SiO2/C/CuO/IL.......................................................................... 40

Figure 3-25 CV of CMCPE- CP/Cu/FA with increasing conc. of CC from 0 to 6 mM

.............................................................................................................................. 41

Figure 3-26 Amperogram of CMCPE-CP/Cu/FA with successive addition of 1 mM

catechol (CC) in 0.1 M NaOH solution ................................................................ 41

List of Abbreviations:

AA Ascorbic Acid

BET BrunauerEmmettTeller

CC Catechol

CCC Carbon Ceramic Composite


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CCE Carbon Ceramic Electrode

CA Chronoamperometry

CV Cyclic Voltammetry

DA Dopamine

DPV Differential Pulse Voltammetry

NN Sodium Nitrate

HQ Hydroquinone

L- Cys

LOD

L Cysteine

Limit of detection

Ox Oxidation

Red Reduction

Rpm Revolution per minute

SEM Scanning Electron Microscopy

TEOS Tetraethylorthosilicate

UA Uric Acid

4 APh 4 Amino Phenol

EIS Electrochemical Impedance spectroscopy

SiO2/C Silica Graphite

CMCPE Chemically modified carbon paste electrode

CP/Cu/FA Carbon paste electrode modified with copper formamidine complex

SiO2/C/CuO CuO synthesized on silica graphite support

SiO2/C/Cu/IL CuO synthesized on silica graphite support, modified with ionic liquid


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Chapter 1

Introduction


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1 Introduction

1.1 Sensors

A sensor is a device that senses and responds to input from environment. The

specific input in the form of light, impact, heat, motion, humidity, force, etc. The output

is converted into human-readable electrical display [1]. Human five senses also act as

sensors which are smell, taste, sound, vision and touch. Litmus paper is the simplest

sensor which is used for the detection of acids and bases in laboratory [2].

A sensor is a device which provides real information about test samples. It

transforms information about test sample into useful signals or data. A sensor can also

be defined as an instrument that responds or detects any physical stimulus. For example,thermometer, microphone and seismometer are used to measure temperature, sound and

motion respectively [3].

Sensitivity, selectivity and stability are three important parameters for a good

sensor. Conventional sensors were synthesized from macro-materials. These sensors

had some limitations such as low precision, sensitivity and larger size [4]. To overcome

these problems nanotechnology is the best option. Due to small size and high surface

area to volume ratio, nanotechnology has many advantages over micro and macro

technology such as low response time, selectivity, portability, stability, and very high

sensitivity. Due to these advantages, nanomaterial based sensors have vast applications

in daily life and their development is hot topic of research in the field of sensors [5].

A nanosensors can be defined as, A device constructed by a material in which at least

one component is in nanoscale range and detects any physical or chemical change[6].

1.2 Types of sensors

There are three major types of sensors

1. Physical sensors

2. Chemical sensors

3. Biosensors


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1.2.1

Physical sensors

Physical sensor is a device that senses and responds to input from the physical

world. The specific input may be light, heat, humidity, impact, moisture, pressure etc.

The output is converted into a signal. The nature of physical phenomena includes

thermal, magnetic, electrical, mechanical, atomic and nuclear, each represent the

specific properties of a body or a system. Effects of these properties are recorded in

useful signals by an instrument. For example: speed of a car recorded by speedometer

and speed of air measured by anemometer are converted into digital signals etc.

Physical sensors have many applications in daily life [7].

1.2.2

Chemical sensors

A device that converts the chemical information about composition and concentration

of analytes (particular elements, ions, compounds, or biomolecules) into an output

(current) signal is called chemical sensor. Calibration curve is normally used to check

the linear response of concentration vs current. Sensitivity of the sensor is a measure of

its ability to distinguish among little changes in the concentration of analyte molecules

[1]. The slope of the calibration curve (current vs concentration) determines sensitivity

which also depends on standard deviation of the measurements. High fluctuations

obviously decrease the sensitivity [8]. Chemical sensors are used for the determination

of analytes, to identify chemical substance, determining of selective ions in the solution

and sensing of harmful gases etc [6].

1.2.3

Biosensors

These sensors are based on biological or biochemical mechanism. These are used for

the identification of biomolecules [1]. These sensors have applications in both clinical

and non-clinical fields by recognition of DNA, lipids, antigens, cholesterol, urea,

glucose etc [9].

1.3 Literature review

Catechol (1,2-dihydroxybenzene) being major phenolic pollutant, is detected in

environmental samples by means of various electrochemical and optical sensors. This

section will cover the recent literature regarding catechol sensors.


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S. Chen et al. reported the synthesis of catechol electrochemical sensors by the

immobilization of silver nanoparticles on carboxy-methyl cellulose (CMC)-modified

cellulose nanofibers. The laccase was physiosorbed on AgNPs-CMC/ cellulose by

electrostatic force of attraction and this modified electrode was used as biosensor for

catechol detection. The cyclic voltammogram revealed that Laccase/AgNPs-

CMC/cellulose/carbon electrode exhibited a detection limit of 1.64l mol dm-3and linear

range from 4.98 x 10-6to 3.65 x 10-3 M [10]. Non-enzymatic amperometric sensor of

catechol and hydroquinone using Pt-Au-organo-SiO2 chitosan composites modified

electrode. Pt/Au/OSi /CS showed excellent electrocatalytic response for the oxidation

and reduction of catechol and a good linear range of 0.0690.98 M was observed [10].

L. Wang et al. reported the fabrication of composite electrode made from silver

nanoparticles, graphene and polydopamine which was used for the amperometric

determination of catechol in environmental samples [11]. J. Sun et al.reported the

carbon modified electrodes synthesized from GO (graphene oxide) and manganese

dioxide nanocomposite which were used for the voltammetric detection of

hydroquinone and catechol simultaneously. The limit of detection for catechol was 10

x 10-9 M [12].

Annamalai Senthil Kumar et al. reported the selective electrochemical sensing ofcatechol and dopamine using ferrocene bound nafion membrane modified electrode.

Linear response range was 250 mM 2.5 mM for catechol and 250 mM 5 mM for

dopamine, with the corresponding detection limit was 10.8 mM and 22.7 mM

respectively. The sensitivity of membrane sensor was 1.1 A mmol L -1 for both

catechol and dopamine [13].

1.3.1

Catechol Optical Sensors:

An optical MEMS (Microelectromechanical systems) sensor for the detection of

catechol was reported by Peter Dykstraa et al. Catechol, on oxidation, gives products

which when react with chitosan film produces a change in absorbance in UV and near

UV region. Taking the advantage of absorbance property, catechol was detected by

measuring the change in the intensity of light at 472 nm through electrodeposition of

chitosan on conductive and transparent film of indium tin oxide. Interferent like

ascorbic acid did not cause an absorbance change. The lowest concentration


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measurements from this device was 1mM while its limit of detection (LOD) was 0.2

mM [14].

Jaafar Abdullah et al. reported that the optical biosensors could be fabricated by using

stacked films of 3-methyl- 2-benzothiazolinone hydrazone (MBTH) immobilized in a

hybrid nafion/sol-gel silicate film and laccase in a chitosan film for the detection of

phenolic compounds. The proposed biosensor exhibited a linear response towards

catechol concentration ranging from 0.5-8.0 mM while the detection limit and response

time calculated were 0.33 mM and 10 min. respectively. The absorption spectrum of

biosensor showed an excellent response at wavelength of 505 nm [15].

1.3.2

SiO2/C-graphite based Selective Electrochemical Sensors

Carbon Ceramic Composites (CCC) is a new class of material having very high stability

and strength. Carbon ceramic electrodes (CCEs) are rigid and porous. They can be

modified with electroactive species and have a renewable surface exhibiting excellent

sensing properties [16]. The surface of electrodes can be regenerated by polishing it

with emery paper [17]. In CCEs, silica is used as reinforcement and provides

mechanically and thermally stable framework and graphite is physiosorbed on silica

backbone which is used for electrical conductance [18]. Literature reveals that

incorporation of different electroactive species on SiO2/C support act as electrocatalysts

and consequently its electrical conductance and sensing properties are increased.

Carbon ceramic material based electrodes have wide range of applications in the field

of catalysis and sensors [19]. Carbon ceramic electrodes have superiority over carbon

paste electrodes and screen printed electrodes because of long durability, regenerative

surfaces, less expensive, easy synthesis and good conductivity [20].

Thiago C. Canevari et al.reported the simultaneous electroanalytical determination of

hydroquinone and catechol on SiO2/C electrode spin-coated with a thin film of Nb2O5.

Differential pulse voltammetry (DPV) was performed to find the oxidation peak of

hydroquinone (HQ) and catechol (CC). Si/C/Nb modified electrode exhibited good

electrochemical response towards catechol [21].

Dopamine (DA, 3,4-dihydroxyphenyl ethylamine) is a catecholamine. It plays an

important role in the functioning of cardiovascular as well as central nervous and


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hormonal system. The abnormal level of dopamine causes brain diseases like

Schizophrenia and Parkinson. The rapid and precise sensing of dopamine is very

mandatory. Abdur Rahim et al. used SiO2/C/Cu(II)phthalocyanine for the fabrication

of electrochemical sensor for the detection of dopamine. The electrode showed an

excellent response against analyte with sensitivity of 0.63 (0.006) nA dm3 mol1 cm2

while the LOD was 0.6 x 10 -6 mol dm3[22]. Furthermore, Abdur Rahim et al. reported

a highly sensitive SiO2/C-graphite based sensor for the detection of oxalic acid. Oxalic

acid can be ingested orally as dietary fiber or synthesized as the final product of

metabolic activity performed by amino acids and ascorbic acids present in humans.

Cobalt phthalocyanine (CoPc) was in situ immobilized in the mesoporous SiO2/C-

graphite matrix. Detection of oxalic acid is important because, in human body, reactions

of oxalic acid with alkali and alkaline earth metals are responsible for the formation of

stones in Kidney. The limit of detection was LOD=5.8107molL1[23].

Mirian P. dos Santos et al. reported a novel SiO2/C-graphite based sensor for the

detection of dopamine (DA, 3,4-dihydroxyphenyl ethylamine). The material was

modified with ensal functional group on which copper was physiosorbed by dissolving

0.01 M copper acetate solution in it. The sensor was highly selective and sensitive for

target analyte with the sensitivity of 21.2 (0.003) nA dm3

mol1

cm2

. The presenceof interfering species like ascorbic acid , NADH and uric acid did not affect the

sensitivity of the electrode [24]. Similarly, Abdur Rahim et al. reported SiO2/C-graphite

based sensing material on which cobalt phthalocyanine (CoPc) was in-situ

immobilized. They used SiO2/C/CoPc based electrode for the detection of dissolved

oxygen. The sensor showed a good sensitivity of 2.16 A L mg1 against dissolved

oxygen [25].

Abdur Rahim et al. used a mesoporous SiO2/C/MnPc composite material based

electrochemical sensor for the determination of nitrite in flesh and water. In-situ

immobilization of manganese(II) phthalocyanine on silica-graphite matrix increased

the sensitivity of electrode towards nitrite ions which is 17.31 A dm3 umol-1 [26].

Sergio B.A. Barros et al. presented a SiO2/C/NiPc based sensor for the simultaneous

determination of ascorbic acid (AA) and dopamine (DA, 3,4-dihydroxyphenyl

ethylamine). Nickel (II) phthalocyanine was immobilized in situ on mesoporous


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SiO2/C-graphite. The sensor showed sensitivities of 53.02 A mmol dm3 towards

ascorbic acid and 104.17 A mmol dm3against dopamine respectively [27].

1.3.3 CuO based Selective Electrochemical Sensors

J. Lei et al.reported that CuO doped with mesoporous silica hybrid for amperometric

detection of phenolic compounds like catechol. CuO incorporated on mesoporous silica

showed a good linear response from 1.2 x 10-9to 3.0 x 10-5 M for the determination of

catechol [28]. Y. W. Hsu et al.used CuO/graphene as a non-enzymatic glucose sensor.

A sensitivity of 1.065 nA dm3mmol1cm2 was calculated [29]. X. Song et al.reported

that cupric oxide adsorbed on reduced graphene oxide CuO/rGO composite showed

remarkable electrocatalytic activity of sensor towards catechol oxidation and used thatsensor for highly sensitive determination of catechol [30].

Metal oxides based sensors are inexpensive, rapid and highly sensitive. Nanostructure

transition metals such as Ag, Au, Ni, Cu, Zn, Ti and Pt oxides and composites have vast

applications in sensors [31]. Among these metal oxides, copper oxide has many

advantages such as natural abundance, low production cost, good catalytic and

electrochemical properties [32]. Among the numerous metal oxides available, copper

oxide (CuO), a semi-conducting material with electric as well as photocatalytic

properties, is being used as an electrocatalyst in electrochemical reaction at the surface

of electrodes [33]. Due to attractive features of copper oxide, it is being thought the best

material for sensing purposes. According to the literature, catechol sensors based on

copper oxide nanostructure have excellent stability, selectivity and sensitivity [34].

1.3.4 Ionic Liquid modified selective catechol sensors

Y. Wang et al. reported that poly (crystal violet) functionalized graphene modified

carbon ionic liquid electrode showed good electrochemical response towards the

detection of hydroquinone and catechol simultaneously. The electrode exhibited good

sensitivity with increased conductivity and decreased resistances [35]. A. Ivaska et al.

reported that ionic liquids which were composed of two asymmetrical ions i.e. bulky

cyclic and acyclic cations and smaller organic and inorganic anions showed high

conductivity, non-volatility, less toxicity, large electrochemical linear response and


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good electrochemical stability when used for the selective electrochemical

determination of different analytes [36].

1.3.5 Formamidine modified selective electrochemical sensor

N,N-bis (benzophenone imine)formamidine (b-BPEF) was synthesized and used as an

ionophore for the determination of silver ion. (b-BPEF) [37]. Linear response range

was 2.2 107to 2.2 102 mol L1with a detection limit of 5.0 108mol L1and a

slope of 59.54 0.2 mV decade1over a wide pH range (2.09.0) [37].

1.4 Scope and Objectives

The detection of phenolic compounds like catechol is of great interest because of its

toxic nature for humans as well as for environment [38]. A recent report shows that the

phenolic organic compounds are carcinogenic [39]. Environmental transformation

results in conversion of phenol into catechol and chlorophenols into chlorocatechols

[40]. These phenolic compounds are used in many industrial processes, e.g., in the

manufacturing of plastics, paints, dyes, drugs, insecticides, pesticides, and antioxidants

[41]. Skin contact with catechol causes eczematous dermatitis in humans. Large doses

of catechol, taken orally, can cause depression of the central nervous system (CNS) as

well as the rise of blood pressure in humans [42]. Catechol causes eye burn and woundsproduced by it are not easy to heal. Severe exposure to catechol causes fits. It has high

solubility in water i.e. 430 g/L [43]. This high solubility of catechol in water is due to

the polar nature of molecule which is because of hydroxyl groups present on it. Effluent

water from pharmaceutical, dyeing and insecticides industries contains certain amount

of dissolved catechol because they are using catechol for specific purposes or it is

produced as a byproduct. IARC (International Agency for Research on cancers) has

classified catechol (CAS # 120-80-9) as a Group 2B, a possibly carcinogenic chemical

to humans. Catechol produces glandular stomach tumors in rodents [39]. To get away

from cancer, because of its dreadful nature, one should take precautionary measures for

handling those chemicals that are possibly carcinogenic. For this purpose, detection of

phenolic compounds and, specially, catechol is a hot topic of research nowadays. The

maximum acceptable limit (MAL) of phenolic compounds, mainly, catechol in sewage

and industrial effluents is in the range of 1-2 mg/L [15]. A rapid, accurate, cheap and

highly sensitive and selective electrochemical sensor is required for accurate


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9

determination of catechol in environmental samples [30]. According to literature,

different types of materials have been reported for the fabrication of catechol sensors

including metals, metal oxides, biomolecules and it has already been reported in a

patent that imidazole group, present in ionic liquid, acts as an ionophore for the

selective electrochemical sensing of analytes. Different types of electrodes and

ionophores were used for the selective electrochemical determination of catechol.

Our aim is to synthesize two types of electrochemical sensors: (1) carbon ceramic novel

selective electrochemical sensor based on SiO2/C (support), copper oxide (electroactive

specie) and ionic liquid (ionophore), for the detection of catechol in industrial effluent

water samples. (2) Chemically modified carbon paste electrode based on carbon paste

and formamidine Cu complex [Cu/FA] for the selective electrochemical detection ofcatechol in environmental samples. Response of each sensor will be measured in term

of selectivity, sensitivity and linear limit of detection.


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Chapter 2

Material and Methods


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2 Material and Methods

This section will cover the experimental details, synthetic procedure and

characterization techniques for the materials preparation.

2.1 Materials

This section will cover the materials required for the synthesis of working electrodes

2.1.1 TEOS (Tetraethylorthosilicate)

Analytical grade TEOS purity of 99% was used for the synthesis of SiO2 /C as a

precursor to silica. It is a colorless liquid of density 0.933 g/mL by Merck.

2.1.2 Ethanol

It was 99.8% pure and purchased from Merck chemicals. It is available as

colourless solvent having density 0.789 g/mL

2.1.3

Acetic Acid

It has a density of 1.05 g/mL provided by Merck. It is present in glacial form. It

is used as anion source in the formation of ionic liquid

2.1.4 Benzimidazole

It is provided by Alfa Aesar 99 % in 25 g bottle having melting point of 157 C

and insoluble in water but easily soluble in methanol.

2.1.5 Nitric Acid

Nitric acid (65%) was purchased from Merck chemicals. It is available as

colorless liquid with pungent smell

2.1.6

Hydrofluoric Acid

Technical grade (38-40%) hydrofluoric Acid was supplied by Merck chemicals.

It was used as catalyst during gelation process of sol gel method.

2.1.7 Graphite

Graphite powder was 99.99% pure with particle size smaller than 125 microns

and mesh size of 120 mesh. It was used as a conducting material in SiO2/C matrix and

was purchased from sigma Aldrich.


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2.1.8

Copper Nitrate

Copper nitrate is available as blue colored powder. It was used for in situ

synthesis of CuO nanoparticles on SiO2/C matrix. It was supplied by Merck chemicals.

2.1.9

Ethylene Glycol

It was purchased from Daejung chemicals Korea with purity 99%. It is available as a

colorless liquid with density of 1.11 g/cm3

2.1.10Sodium Hydroxide

It is available in the form of pallets. It is a strong alkali. It was used in synthesis process

to maintain the pH of solution during in-situimmobilization of copper oxide CuO and

was also used as supporting electrolyte for electrochemical oxidation of catechol.

Analytical grade sodium hydroxide was provided by Sigma Aldrich chemicals.

2.1.11Uric Acid

It is white crystalline solid. It is used for the interference study of catechol with

modified electrode. It was purchased from BDH chemicals with purity 98%.

2.1.12Ascorbic Acid

It is commercially available as white solid. It was purchased from Daejung with purity

of 99%. It was also used for interference study to measure selectivity of proposed

electrode.

2.1.134 Amino Phenol

It is present in reddish black solid which is used in the interference study of electrodes

as a phenol. Its molar mass was 109 g/mol with a density of 1.13 g/cm provided by

Sigma- Aldrich.

2.1.14L- Cysteine

It is purchased from sigma Aldrich and available in white crystalline material. It is used

in interference study of modified electrodes


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2.1.15

Dopamine

It is available in liquid state and dissolved in (HCl) 40 mg /5 mL which is purchased

from CLINIX, The Medicine store. It was used as an interferent to determine the

selectivity of modified electrode towards target analyte i.e. Catechol

2.2 Instrumentation and characterization techniques

Characterization of the samples was carried out by following techniques

2.2.1

Scanning Electron Microscope (SEM)

The morphology of the prepared samples was analyzed by scanning electron

microscope VEGA3 TESCAN in IRCBM. The instrument was operated at 10 kV. SEM

images were recorded in the range of 900 nm - 5 m. The porous nature was verified

by SEM images.

2.2.2 BET multipoint Technique

BET surface area and porosity analyzer (Micromemitics TriStar II 3020) was used for

calculating the surface area, porosity and pore size distribution.

2.2.3

Universal testing machine (UTM)

Pallets of synthesized material were prepared by universal testing machine (Testometric

UTM M500-50AT). 60mg of material was pressed under 6000-pound force in a 6 mm

diameter die. Disk was prepared using UTM.

2.2.4 Fourier Transform Infrared Spectroscopy (FTIR)

Functional group analysis was performed using FTIR. Thermo-Nicolet 6700 P FTIR

Spectrometer (USA) Range: 4000 to 400cm-1. Attenuated Total Reflectance (ATR) was

used for our liquid sample of ionic liquid.

2.2.5

Nuclear Magnetic Resonance

For structural analysis, 1H NMR and 13C NMR was performed on a 500 MHz NMR

spectrometer from Malaysia.


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2.2.6

Melting Point Apparatus

Melting point meter (MPM) was used to determine the melting point of ionic liquid in

capillary tubes. Model MPM-H2 was used having upper limit of melting point

calculation as 360 C.

2.2.7 Rotary Evaporator

BUCHI Rotavapor R -215 has been used to evaporate solvent under reduced pressure

and elevated temperature.

2.2.8

Potentiostat Machine:

Sensing properties of material were determined by potentiostat machine (GamryReference-3000). Cyclic voltammetry was used for the measurement of oxidation

potential of sensor against catechol. By using amperometric technique, response of

sensor was recorded at different concentrations of catechol while electrochemical

impedance spectroscopy was performed to check the charge transfer resistance.

2.3 Synthetic procedures for the material preparation

2.3.1

Synthesis of SiO2/C by the solgel method

SiO2/C is prepared by following a reported method [23].The sol gel method was used

to prepare SiO2/C having amount of SiO2and C in 2:1 ratio. Tetraethylorthosilicate

(TEOS) and ethanol were mixed in proportion 1:1 (v/v) TEOS/ethanol to make 50 ml

solution. Then 4.0 mL distilled water and 0.1 mL conc. HNO3 were added in the

solution and let it stir gently at a temperature of 80 oC for 3 hours. After that, graphite

3.3g, 4mL distilled water and 0.1 mL HF catalyst were added to pre-hydrolyzed TEOS

solution [44]. The amount of graphite added in the reaction mixture was 50% (w/w) of

SiO2, mixture was sonicated until gelation of the material occurred and then allowed to

rest. The rate of evaporation of solvent determines the distribution of porosity in the

xerogel.

The gel obtained was dried at room temperature slowly for about 15 days and

then ground to powder and washed thoroughly, initially with deionized water and then

in soxhlet extractor for 3 h. Finally, the product was dried in oven at 393 K for 4 h [19].


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Figure 2-1 Scheme of synthesis of SiO2/C by Sol Gel Method

2.3.2

Synthesis of CuO on SiO2/C matrix

Copper oxide nanoparticles were synthesized in situon mesoporous SiO2/C-

graphite to form SiO2/C/CuO composite material by following a reported method [29]

. For this purpose, 0.35 g of copper nitrate was dissolved into 50 mL ethylene glycol in

a round bottom flask. 0.1M Sodium hydroxide (NaOH) solution was added dropwise,

if needed, to maintain the pH of solution at 9. Digital pH meter was used for monitoring

the pH. Then 5g SiO2/C powder was added into solution and let it stir for 30 min at arate of 300 rpm at room temperature so that the ingredients could be mixed properly.

Then the reaction mixture was heated at a temperature of 190 oC for 3 hours. After that

the reaction mixture was cooled down to room temperature and filtered. Blackish solid

product was dried at 150 oC. Finally, the product was annealed in air environment for

30 min to get mesoporous SiO2/C/CuO nanocomposite material as required product

[45].


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2.3.3

Synthesis of Ionic Liquid

For the synthesis of ionic liquid, reported method was followed by a little modification

of reaction conditions [46]. Benzimidazolium-1-acetate was synthesized by mixing 2

grams of benzimidazole (16.9 mmol) in 10 mL of methanol in a round bottom flask.

The temperature was kept 80 oC so as to achieve refluxing. Equimolar acetic acid

volume (0.965 mL) was added in 15 minutes at a rate of 3 drops / min. The reaction

mixture was left for 24 hrs under reflux [47]. After that the reaction mixture was heated

at 100 oC at pressure of 50 mbar rotavapor. The product was obtained as yellow

transparent liquid. It is a room temperature ionic liquid.

Figure 2-2 Synthesis of Benzimidazolium-1-acetate

2.3.4 Synthesis of formamidines

Formamidines have been used as ionophores for the detection of many analyte

molecules. The condensation reaction between 4-aminophenazone and

triethylorthoformate took place to synthesize formamidine using reported method [37].

The reaction mixture was left for stirring at a rate of 220 rpm at a refluxing temperature

of 160 C for 12 hr. The progress of reaction was checked after every 2 hours using

TLC (90 % EtOAC and 10 % methanol). After the completion of reaction as verified

by TLC. To further verify the spots, a solution was made by adding 200 mg ninhydrin

in 5 % v/v acetic acid and 95 % v/v n- butane. Purplish spots appeared by dipping TLC

cards in the above solution. The reaction mixture was cooled down, washed with hexane

to remove any impurity. A yellow crystalline product was obtained after

recrystallization with ethanol. This formamidine was used for the electrochemical

determination of catechol.


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Apart from this, other formamidines were also prepared following the reported method

[48] by changing the amines like aniline and picolyl amine. The ratio of molar

concentration amines and triethylorthoformate used was in 2:1. A maximum yield was

obtained with 10 mol% cerium ammonium nitrate as catalyst in 20 mL water as solvent.

Solid products were obtained using all these amines which were separated by filtration.

2.3.5 Fabrication of carbon ceramic electrode

For the preparation of working electrodes of SiO2/C, SiO2/C/CuO and SiO2/C/CuO/IL

materials, pallets of each material were prepared with UTM (Universal Testing

Machine) by pressing 60 mg of each sample at a rate of 20 mm/ min under the force of

6000-pound force. The resultant pallets, having dimensions of 6 mm dia, 1.5 mm

thickness and 0.28 cm2of surface area of disk were glued at one end of glass tube

(diameter 6 mm, length 15.4 cm) using UHU and joint was wrapped with Teflon tape

to prevent the entry of solvent into the glass tube. Electrical connections were made by

inserting a copper wire in the glass tube with a small quantity of graphite powder so

that the Cu wire might be contacted with disk properly.

Figure 2-3 Schematics of carbon ceramic electrode of 6 mm surface area


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2.3.6

Fabrication of carbon paste electrodes

First of all, carbon paste was prepared by mixing carbon- graphite powder for

conductance and silicon oil as binder. Copper was chemisorbed on formamidine by the

method reported [24]. Briefly 50 mg of formamidine was added in 0.01 mM solutionof CuCl2 in ethanol and let the solution to evaporate so that copper, which is an

electroactive specie, could form a complex with formamidine.

For the fabrication of CMCPE, carbon paste and formamidine complex were mixed in

agate mortar and pestle in a ratio of 4:6. After that the mixture was inserted in 3 mm

hole present in carbon paste electrode (ALS, the electrochemical company, Japan. Then

the electrode was left for drying with downside up for 24 hrs so that material might not

leach out of electrode during selective electrochemical study.

Figure 2-4 Schematic of carbon paste electrode of 3 mm diameter of hole


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Chapter 3

Results and Discussion


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3 Results and Discussion

3.1 Synthesis

3.1.1 Synthesis of formamidine

The formamidine was synthesized by modifying the reported method [37]. 4-Amino

phenazone and triethylorthoformate were refluxed with an addition of few drops of

acetic acid. The progress of the reaction was monitored by thin layer chromatography.

Melting point of the product was measured to be 178 oC.

3.1.2

Synthesis of benzimidazolium-1-acetate

The reaction was performed between benzimidazole and acetic acid at a refluxing

temperature of 80o

C. The results were tremendous as we obtained a room temperature

ionic liquid after the evaporation of methanol in rotary evaporator. Acetic acid

protonated the benzimidazole and also acted as anion source.

3.2 SEM Analysis

3.2.1 SiO2/C-graphite matrix

Surface morphology of synthesized material was examined by SEM.Figure 3-1

andFigure 3-2 show the SEM images of SiO2/C-graphite at 10 kV with magnification

of 5kx and 10kx respectively. It showed granular and flakes structures.

SEM images of matrix at higher magnification also illustrated the flakes on the

surface of material as shown inFigure 3-3.It displayed the micrograph at 10 kV with

magnification of 25 and 50 kx. These images showed the high compactness of material

and small gaps between flakes.


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Figure 3-1 SEM image of SiO2/C-graphite with the magnification of 5 kx


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Figure 3-2 SEM image of SiO2/C-graphite with the magnification of 10 kx

Figure 3-3 SEM image of SiO2/C-graphite with the magnification of 25 kx and 50 kx


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3.2.2

SiO2/C/CuO Material

Figure 3-4 toFigure 3-6 showed SEM images of SiO2/C/CuO material obtained at 10

kV with magnification of 5-50 kx. These images showed the high compactness of

material and small gaps between some flakes. There was no phase segregation there.These images exhibited high strength because of composite nature of material. The pore

size was confirmed by BET multipoint analyzer and homogeneity of material was

confirmed by EDX elemental analysis.

Figure 3-4 SEM image of SiO2/C/CuO with the magnification of 5 kx


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Figure 3-5 SEM image of SiO2/C/CuO with the magnification of 10 kx

Figure 3-6 SEM image of SiO2/C/CuO with the magnification of 25 kx and 50 kx

3.2.3 SiO2/C/CuO/IL Material

Figure 3-7 toFigure 3-9 show the SEM images obtained at 10 kV with magnification

of 5-50 kx. Ionic Liquid was added in 25 % by weight of SiO2/C/CuO. Small flakes on

some micro sized agglomerates showed the physisorption of ionic liquid on

SiO2/C/CuO surfaces. These images showed the compactness of material and small

pores in between.


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Figure 3-7 SEM image of SiO2/C/CuO/IL with the magnification of 5 kx

Figure 3-8 SEM image of SiO2/C/CuO/IL with the magnification of 10 kx


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Figure 3-9 SEM image of SiO2/C/CuO/IL with the magnification of 25 kx and 50 kx

3.3 Energy Dispersive X-rays Spectroscopy (EDX)

3.3.1

EDX analysis of SiO2/C/CuO

Synthesis of copper oxide nanoparticles was confirmed by EDX analysis.Figure 3-10

showed high intensity peaks at 0.1, 0.6 and 1.7 kV. These peaks were assigned to

graphite (C), oxygen (O) and silicone (Si) in the matrix respectively. One high intensity

peak at 1 kV and low intensity peak of Cu appeared at 8 kV confirmed the presence of

Cu. This study provided the information about weight % of these elements present in

material.

Figure 3-10 EDX spectrum of SiO2/C/Cu

Table of Composition of Elements in SiO2/C/CuO

Element Weight% Atomic%

C 45.83 56.58

O 37.91 35.13

Si 15.27 8.06

Cu 0.98 0.23

Total 100.00 100.00

Figure 3-11 Weight percentage of elements present in Sensor


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3.3.2

EDX color mapping of material SiO2/C/CuO

Figure 3-12 shows the EDX color mapping. It can be concluded from these figure that

Si, O, C and Cu particles are homogeneously dispersed in the matrix. Copper is in

smaller amount. The uniform dispersion and distribution of elements is associated tothe compactness of material.

Figure 3-12 Color mapping of Cu, Si, O and C


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3.4 BET (Brunauer Emmett Teller) surface area and porosity

analysis

The pore size, pore volume and surface area of SiO2/C-graphite matrix was investigated

by BET multipoint surface area and porosity analyzer.Figure 3-13 shows adsorption-

desorption isotherm of nitrogen. The isotherm shows hysteresis adsorption and

desorption which is typical for mesoporous materials. The shape of isotherm is similar

to H3-type hysteresis loop as defined by International Union of Pure and Applied

Chemistry (IUPAC) in 1985. The surface area and pore volume of material was

calculated by software itself by BJH method. The surface area (SBET) and pore volume

(pv) of material is 99.87 m/g and 0.3104 cm/g respectively.Figure 3-14 shows pore

size distribution of material. Pore distribution region of sample is maximum at 14.15

which depicts that the material is in mesoporous range i.e. 2 - 50 nm.

Figure 3-13 BET nitrogen adsorption-desorption isotherm of SiO2/C-graphite matrix


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2 4 6 8 10 12 14 16 18 20 22

0.0

0.1

0.2

0.3

0.4

0.5

0.6

dv/dlog(w)(cm

3/g)

Pore Width (nm)

Figure 3-14 BET pore size distribution of SiO2/C-graphite material

3.5 Fourier transform infrared spectroscopy (FTIR) study of

[BzIm]OAc

Function group analysis was performed using FTIR- ATR. The IR spectrum of

Bendimidazolium-1-acetate is shown inFigure 3-15.Less intensive peak at 665.05

showed out of plane bending of CC bond, strong peak at 738.023, 862.13 and weak

peak at 936.28 cm-1showed CH out of plane bending. While peak at 1004.74 cm-1

showed the trigonal bending of CC. Peak at 1147.45 cm-1 showed CH in plane

bending while peak at 1238.09 cm-1showed CC stretching and peak at 1363.44 cm-

1CN stretching, peak at 1620.90 showed C= C stretching, peak at 1687.43 showed

C=N which are typical for benzimidazolium group as verified from literature [49]. The

characteristic peak of carboxylic group appears at 1697.59 cm -1 while CH

characteristic peak appeared at 2902.38 and 2964.57 cm-1 [50].


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4000 3500 3000 2500 2000 1500 1000

0.0

0.2

0.4

0.6

0.8

665

.3315

741

.5072

860

.1098

881

.3233

936

.2855

956

.5347

1004

.747

1049

.103

1147

.456

1238

.095

1363

.448

1395

.268

1454

.087

1620

.902

1651

.758

1687

.435

17

00

.935

2360

.481

2902

.389

2973

.743

73

8.

02

936

.2855

1004

.747

1049

.103

1

147

.456

1238

.095

1363

.448

145

4.

087

1489

.764

1620

.902

1

687

.435

1

697

.59761

2902

.389

2964

.579

71

665

.0503

862

.1364

Abs

orbance

Wave number (cm-1)

Figure 3-15 IR spectrum of [BzIm]OAc

3.6

Nuclear Magnetic Resonance (NMR)

To confirm the structure of benzimidazolium-1-acetate ionic liquid, NMR was

performed using DMSO-d6 solvent. Its spectroscopy data is1HNMR (500 MHz,

DMSO): = 2.690 (s, 3H), 7.547-7.566 (d, 2H), 7.709-7.771 (d, 2H), 9.166 (s, 1H).

13C NMR (125 MHz, DMSO): 38.444, 112.949, 126.766, 131.002, 131.990, 141.153.


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3.7 Electrochemical analysis of carbon ceramic electrodes:

Electrochemical analysis was done to test the sensing properties of working electrodes.

For this purpose, three electrode system was used. Saturated Calomel electrode (SCE),

Platinum wire electrode and SiO2/C-Graphite, SiO2/C/CuO and SiO2/C/CuO/IL

electrodes were used as reference, auxiliary and working electrodes respectively.

Pressed disks of 6mm diameter, made from synthesized materials i.e., SiO2/C-Graphite,

SiO2/C/CuO and SiO2/C/CuO/IL were used to synthesize the working electrodes.

Sodium Hydroxide (NaOH) 0.1 M was used as supporting electrolyte to carry out

electrochemical reactions throughout this research work. Three techniques which were

used to study electrochemical sensing properties of electrodes as sensor are i.e. cyclic

voltammetry, chronoamperometry, Electrochemical Impedance Spectroscopy (EIS)

3.7.1 Cyclic voltammetry:

Cyclic voltammetry was used to measure the redox potentials of the working electrodes.

Reactions on electrodes usually involve electrons transfer on surface which is

influenced by electrode potential [51]. In cyclic voltammetry, the electrode potential

changes linearly vs time. The rate of voltage change per unit time is known as the scan

rate (mV/s). For our case, it is 20 mV/sec. The potential difference is applied between

the working electrode (WE) and the reference electrode (RE) which is SCE while the

current is measured between the working electrode (WE) and the auxiliary/counter

electrodes (CE) respectively [52]. A wide potential window (-0.2 to 0.8 V) was applied

during electrochemical measurement and changes in current was recorded against

voltage. Figure 3-16 and Figure 3-17 shows the cyclic voltammograms of three

different electrodes i.e. SiO2/C, SiO2/C/CuO and SiO2/C/CuO/IL in the Blank solution

0.1 M NaOH (absence of analyte) and in the presence of 4mM Catechol (CC)

respectively. It can be seen that the SiO2/C/CuO/IL electrode shows good

electrochemical response and higher current as compared to SiO2/C and SiO2/C/CuO

electrodes. SiO2/C /CuO/IL electrode shows an oxidation peak of catechol at 0.6 V

volts. However, SiO2/C-graphite and SiO2/C/CuO electrodes do not show any oxidation

peak for glucose. By comparing these results, it is proved that the material

(SiO2/C/CuO/IL) shows good electrochemical response towards catechol.


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Figure 3-16 Cyclic voltammograms f working electrodes in 0.1M NaOH at scan rate of 20 mV sec -1

against SCE. Inset shows the cyclic voltammogram of SiO2/C

-200 0 200 400 600 800

-1500

-1000

-500

0

500

1000

1500

-200 0 200 400 600 800

-5

0

5

10

15

Cu

rren

t(u

A)

Potential (mV)

SiO2/C

Current(uA)

Potential(mV)

SiO2/C/CuO

SiO2/C/CuO/IL

SiO2/C


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-200 -100 0 100 200 300 400 500 600 700 800 900

-1200

-900

-600

-300

0

300

600

900

1200

1500

1800

Potential (mV)

Current(uA)

SiO2/C/CuO/ILin 4mM cat

SiO2/C/CuO 4mM cat

SiO2/C in 4mM cat

Figure 3-17 Cyclic voltammograms of working electrodes by adding 4mM Catechol in 0.1 M NaOH at

scan rate of 20 mV sec-1against SCE

This result was further verified by adding different concentrations of catechol in 0.1 M

NaOH solution. The cyclic voltammograms obtained with SiO2/C/CuO/IL electrode at

different concentrations of catechol were shown inFigure 3-18.It can be seen that with

the increase of catechol concentration oxidation peak current also increases. So it is

proved that the material SiO2/C/CuO/IL was oxidizing catechol electrochemically.


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-200 -100 0 100 200 300 400 500 600 700 800 900

-1500

-1200

-900

-600

-300

0

300

600

900

1200

1500

1800

2100

2400

2700

Currnet(uA)

Potential (mV)

SiO2/C/CuO/IL Blank

SiO2/C/CuO/IL + 2mM CC



Figure 3-18 Cyclic voltammograms of working SiO2/C/CuO/IL based electrode obtained at different

catechol concentrations (0, 2, 4, 6 mM) in 0.1 M NaOH at scan rate of 20 mV sec -1

3.7.2 Amperometry

The chronoamperometric technique was used to study the effect of concentration versus

current.Figure 3-19 shows the chronoamperometric curves for successive addition of

0.2 mM of catechol solution at a constant applied potential of 0.6 V after each 50 s.

This curve was obtained in low concentration range of catechol from 0.2 - 2.4 mM,

which shows the response of our electrode in very low concentration of catechol in

solution. Throughout the amperometry, we used modified electrode SiO2/C/CuO/IL

because it was showing good response as verified by cyclic voltammetry.


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Figure 3-19 Amperogram of SiO2/C/CuO/IL electrode at 0.6 V in 0.1M NaOH solution with successive

addition of 0.2 mM Catechol after every 50 sec. Inset shows the calibration curve for the concentration

range of 0.22.4 mM

Response of electrode was also checked for higher concentration of catechol to

calculate the linear range.Figure 3-20 shows the amperogram at higher concentration

of catechol. After addition of 1mM catechol in 0.1 M NaOH solution after 50 seconds,

electrode exhibits good linear response. The response of electrode was very excellent

up to 10mM Catechol which proved that synthesized material had very wide linear

range.

0 100 200 300 400 500 600 700100

150

200

250

300

350

400

450

500Equation y = a + b*x

Plot Current

Weight No Weighting

Intercept 146.32727

Slope 155.63636 Residual Sum of 159.12727

Pearson's r 0.99925

R-Square(COD) 0.99851

Adj. R-Square 0.99834

0.0 0.5 1.0 1.5 2.0 2.5

150

200

250

300

350

400

450

500

Current(uA)

Conc (mM)

Current(uA)

Time (sec)


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Figure 3-20 Amperogram of SiO2/C/CuO/IL electrode at 0.6V in 0.1 M NaOH solution with successive

addition of 1 mM Catechol after 50 sec. Inset figure shows the calibration curve from 1- 10 mM

Electrochemical linear response of electrode for long range of concentration from 0.2

mM to 9.64 mM was further verified chronoamperometrically with successive addition

of 0.2 mM catechol upto 750 sec and after that addition of 0.38 mM catechol upto 1700

sec.Figure 3-21 verifies the response of electrode for large time domain.

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000

0

200

400

600

800

1000

1200

1400

16001800

2000

2200

2400Equation y = a + b*

Plot Current

Weight No Weig

Intercept 328.9333

Slope 173.1393

Residual S50992.49

Pearson's 0.98985

R-Square( 0.9798

Adj. R-Squ 0.97727

0 2 4 6 8 10

400

600

800

1000

1200

1400

1600

1800

2000

Current(uA)

Conc (mM)

Current(uA)

Time (sec)


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Figure 3-21 Amperogram of successive addition of 0.2 mM catechol upto750 sec and after that addition

of 0.38 mM catechol upto 1700 sec in 0.1 M NaOH. Inset shows the calibration curves before and after

750 s respectively, for a total range of concentration 0.2- 9.64 mM

3.7.3 Calculations

Sensitivity was calculated from the slope of calibration curve divided by area of

electrode as can be seen inFigure 3-21

Sensitivity = Slope/ Area = 191.87/0.2827 = 0.678 nA dm3mmol-1cm-2

The limit of detection determines the lowest concentration of analyte molecules that

can be detected for a given type electrochemical sensor.

Limit of Detection (LOD) = 3*(blank solution) /slope = 6.76x10-6mol dm-3.

General straight line equation Y=mx+c is written in the form.

Y= 191.87 (x conc. in mM of catechol) + 122.92 (I in A) straight line equation of

calibration curve presented above having a correlation coefficient of R2= 0.988

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700

0

200

400

600

800

1000

1200

1400

1600

1800Equation y = a + b*x

Plot Current

Weight No Weighting

Intercept 122.92255 19.089

Slope 191.87101 3.7019

Residual Sum of Squares 110656.96089

Pearson's r 0.99446



0 1 2 3 4 5 6 7 8 9 10 11

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Current(uA)

Conc. (mM)

Current(uA)

Time (sec)

SiO2/C/CuO/IL with 0.2mM & 0.38 mM CC

0.2mM

0.38mM


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3.7.4

Interference study and selectivity:

Interference study was done using other analytes in same concentration. To evaluate

the selectivity of modified electrode sensor interfering substances such as 4-amino

phenol, L-cysteine, uric acid, ascorbic acid, sodium nitrate and dopamine were addedafter 50 sec as shown in self-explanatoryFigure 3-22.The sensor did not show any

response to these interfering species even in the same concentration with catechol.

Hence, modified electrode SiO2/C/CuO/IL showed good selectivity towards catechol.

0 100 200 300 400 500 600

0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

2400

Current(uA)

Time (sec)

CC

CC 4AP Cys NaNO2 UA DA AA

Figure 3-22 Amperometric response of SiO2/C/CuO/IL sensor with successive addition of 3mM ( CC,

4APh, L-Cys, NaNO2, UA, DA, AA ) and again 5mM CC in 0.1M NaOH solution after 50 sec.

3.7.5

Real Samples Analysis:

Response of the proposed sensor was checked towards catechol in the real samples. The

effluent water from pharmaceutical, dyeing and insecticides industry contain certain

amount of dissolved catechol. For this purpose, the effluent water from Monnoo Yarn

Dyeing was collected and 0.5 mL of water was added after every 50 secs in 50 ml of


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0.1 M NaOH. Current vs time response was recorded as shown inFigure 3-23.It is

evident that the current increases with the increase of successive addition of real sample

in equal intervals of time, which shows that the proposed sensor detects catechol in

effluent water from yarn dyeing industry.

0 50 100 150 200 250 300 350 400 450 500 550

0

200

400

600

800

1000

1200

1400

1600

1800

Current(

uA)

Time (sec)

Figure 3-23 Catechol determination in effluent water from Monnoo Yarn Dyeing with successive

addition of 0.5 mL effluent water in 50 mL of 0.1 M NaOH

3.7.6

Electrochemical Impedance Spectroscopy:

EIS can be characterized in two regions i.e. a semicircular part at low frequencies and

a straight line at higher frequencies. The electron transfers resistance (Rct) at the

electrode surface is directly related to the diameter of the semicircle and the inclined

line is related to the diffusion limit step [45]. Here inFigure 3-24, SiO2/C is showing

maximum charge transfer resistance as it has large semicircle diameter.

The order of semi-circle diameter was SiO2/C > SiO2/C/CuO > SiO2/C/CuO/IL which

shows that the maximum electrons transferred from SiO2/C/CuO/IL electrode and

higher current achieved which is specific for ionic liquids i.e. better electrical

conductors [53].


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The order of conduction of electrons is SiO2/C/CuO/IL > SiO2/C/CuO > SiO2/C which

shows that ionic liquids modified electrode exhibits excellent sensitivity and low limit

of detection achieved for this electrode.

Figure 3-24 Nyquist plots for SiO2/C, SiO2/C/CuO, SiO2/C/CuO/IL electrodes with 4mM catechol in

0.1 M NaOH solution. Inset figure shows the zoom image of SiO2/C/CuO and SiO2/C/CuO/IL.

3.8 Electrochemical Analysis of Chemically modified carbon paste

electrodes (CMCPE- CP/Cu/FA)

For electrochemical study of chemically modified carbon paste electrode which is

CP/Cu/FA carbon paste electrode made from copper formamidine complex [Cu/FA],

we performed CV as well as amperometry. But the sensitivity and selectivity were not

good for catechol.

3.8.1

Cyclic voltammetry of CMCPE- CP/Cu/FA

Cyclic voltammetry was performed to find the redox potential of CMCPE, the potential

window (-0.20.8 V), there was a linear change in peak current with the increase in

concentration as illustrated in Figure 3-25.There was no sharp peak observed, just

change in current was seen with increase in concentration. The change in current was

maximum at 0.6 V.

0 2000 4000 6000 8000 10000

0

500

1000

1500

2000

2500

3000

50 100 150 200 250

0

10

20

30

40

50

60

70

80

-Zim

ag(ohm)

Z real (ohm)

-Zimag(ohm)

Z real (ohm)

SiO2/C

SiO2/C/CuO

SiO2/C/CuO/IL


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Figure 3-25 CV ofCMCPE- CP/Cu/FA with increasing conc. of CC from 0 to 6 mM

3.8.2 Amperometry of CMCPE- CP/Cu/FA

Current response vs concentration was observed using amperometry. Potential was

fixed at 0.6 V. There was a little increase in current by adding 1mM CC after 50 sec.

The sensitivity, limit of detection and linear response range were not good.

Figure 3-26 Amperogram of CMCPE-CP/Cu/FA with successive addition of 1 mM catechol (CC) in

0.1 M NaOH solution

0.0 0.2 0.4 0.6 0.8

-100

-50

0

50

100

150

200

0 mM CC

2 mM CC

4 mM CC

6 mM CC

Current(uA)

potential (mV)

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750

30

40

50

60

70

80

90

100

110

120

130

140

Equation y = a + b*x

Plot Current

Weight No Weighting

Intercept 49.30303 5.

Slope 7.17133 0.7

Residual Sum of S 764.71911

Pearson's r 0.95174



0 2 4 6 8 10 12

0

20

40

60

80

100

120

140

Current(uA)

Conc. (mM)

C

urrent(uA)

Time (s)


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3.8.3

Calculations

LOD was calculated by 3 / slope was 0.8 mmol dm-3

while the sensitivity was calculated by

Sensitivity = Slope /area = 7.17 / 0.14 = 50 A/ mmol cm-2

The linear range was form 1- 10 mM but R2 was 0.900 which shows very much

deviation from linearity.

3.9 Conclusion

Two types of novel sensors for the selective electrochemical detection of catechol were

fabricated. Ionophore used in carbon ceramic sensor and carbon paste sensors were

benzimidazolium-1-acetate and formamidine respectively. Both compounds were novel

organic compounds which were used for the electrochemical determination of catechol.

First carbon ceramic electrode was based on the SiO2/C/CuO/IL material. Silica-

graphite matrix was synthesized by sol-gel method. Silica backbone substrate provides

mechanically and thermally stable framework and graphite is used for electrical

conductance. Porosity of the materials plays an important role in the entrapping ofelectroactive species. Diffusion of analytes and their interaction with the electroactive

species also occurred through the pores. To enhance the electrical conductance and

sensitivity, the synthesized material was decorated with copper oxide nanoparticles.

SiO2/C/CuO material was further modified with ionic liquid and used to develop

working electrode as sensor for the detection of catechol. Imidazole group present in

ionic liquid worked as an ionophore and facilitated the transfer of charges which has

already been verified by electrochemical response.

The other sensor was fabricated using copper formamidine complex [Cu/FA] which

was CP/Cu/FA. Carbon paste was used for conductance and binding purpose.

Formamidine acted as facilitator for catechol. The response of electrode was checked

but it was very much less sensitive than SiO2/C/CuO/IL. Comparison showed that

SiO2/C/CuO/IL could be used for selective electrochemical determination of catechol

in environmental samples.


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The direct electro-oxidation of electrode against catechol was characterized by cyclic

voltammetry. The sensing ability of electrode was further verified using amperometry.

EIS exhibited that ionic liquid modified electrode has lowest charge transfer resistance.

The sensor showed excellent sensitivity and detection range for catechol. Moreover, the

synthesized materials were characterized with Scanning Electron Microscopy (SEM)

and BET surface area analyzer, FTIR and NMR.


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Chapter 4

References


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