PLATINUM NANODENDRITES MODIFIED ELECTRODE FOR LEAD (Pb) DETECTION
NURUL HIDAYAH BT RAMLI
UNIVERSITI SAINS MALAYSIA
2020
SCHOOL OF MATERIALS AND MINERAL RESOURCES ENGINEERING UNIVERSITI SAINS MALAYSIA
PLATINUM NANODENDRITES MODIFIED ELECTRODE FOR LEAD (Pb) DETECTION
By
NURUL HIDAYAH BT RAMLI
Supervisor: Prof. Dr. Khairunisak Abdul Razak
Dissertation submitted in partial fulfilment of the requirement for the degree of Master of Science (Material Engineering)
Universiti Sains Malaysia
SEPTEMBER 2020
i
DECLARATION
I hereby declare that I have conducted, completed the research work and written the dissertation entitled “ Platinum nanodendrites modified electrode for lead (Pb) detection”. I also declare that it has not been previously submitted for the award of any degree or diploma or other similar title of this for any other examining body or university.
Name of student: Nurul Hidayah binti Ramli Signature:
Date:14th September 2020
Witnessed by
Supervisor: Prof. Dr Khairunisak Abdul Razak Signature:
Date: 14th September 2020
ii
ACKNOWLEDGEMENT
First and foremost, I would like to thank Allah for I could finish this thesis writing in a given time. Special thanks to my supervisor Prof. Dr Khairunisak Abdul Razak for her guidance, patience, motivation, enthusiasm, immense knowledge, and advice throughout this research project have helped me accomplished my research project.
Thanks to the School of Materials and Minerals Resources Engineering and Institute for Research in Molecular Medicine (INFORMM), Universiti Sains Malaysia for allowing me to utilize their laboratories and providing necessary apparatus, chemicals, and services required to complete my research project. A special thanks to the research officer at INFORMM, Ms. Nor Dyana Zakaria for her technical assistant. Special thanks to the technical assistant for their cooperation and guidance during this research. My deepest gratitude to Dr. Noorhashimah for her help and guidance throughout my research. Last but not least, a special thanks to my family, my father Ramli bin Abdul Wahab and my mother Khadijah Binti Mustapha for their support and motivation. A lot of thanks to all the fellow lecturers and students that also contribute during this research.
iii
TABLE OF CONTENT
DECLARATION ... i
ACKNOWLEDGEMENT ... ii
TABLE OF CONTENT ... iii
LIST OF TABLES ... vi
LIST OF FIGURES ... vii
LIST OF SYMBOL ... xi
LIST OF ABBREVIATION ... xii
ABSTRAK ... xiv
ABSTRACT ... xv
CHAPTER 1 ... 1
INTRODUCTION ... 1
1.1 Introduction ... 1
1.2 Problem statement... 5
1.3 Objectives ... 7
1.4 Scope of study ... 7
1.5 Thesis Outline ... 8
CHAPTER 2 ... 9
LITERATURE REVIEW ... 9
2.1 Heavy metal ... 9
2.1.1 Heavy metal pollution of the river in Malaysia ... 10
2.2 Types of heavy metal ... 14
2.1.2 Lead ... 14
2.2.1 Cadmium ... 15
2.2.2 Arsenic ... 15
2.2.3 Mercury ... 16
2.2.4 Chromium ... 16
2.2.5 Other heavy metal elements ... 16
2.3 Method for heavy metal detection ... 17
2.3.1 Atomic Absorption Spectroscopy (AAS)... 17
2.3.2 Inductive Couple Plasma-Optical Emission Spectroscopy (ICP-OES)19 2.3.3 Ultraviolet-visible spectroscopy (UV-Vis) ... 20
iv
2.3.4 Sensor for heavy metal detection ... 20
2.4 Electrochemical sensor... 25
2.4.1 Potentiometric technique ... 26
2.4.2 Conductometry technique ... 28
2.4.3 Amperometry technique ... 29
2.4.4 Voltammetry technique ... 29
2.5 The material used for electrode modification in heavy metal sensor... 37
2.5.1 Bulk/ Thin film electrode ... 37
2.5.2 Modification of the substrate electrode ... 39
2.5.3 Nanomaterials modified electrodes ... 41
2.6 Platinum as nanomaterials for electrode modification ... 48
2.6.1 Effect of nafion on the properties of modified electrodes... 51
2.6.2 Fabrication of working electrode ... 52
2.7 Summary ... 54
CHAPTER 3 ... 56
METHODOLOGY ... 56
3.1 Introduction ... 56
3.2 Raw materials and chemicals ... 56
3.3 Stage 1: Synthesis and characterization of platinum nanodendrites ... 58
3.3.1 Synthesis of PtNDs ... 58
3.3.2 Characterization of platinum nanodendrites ... 60
3.4 Stage 2: Preparation of platinum nanodendrites modified on the ITO electrode by drop-cast techniques. ... 61
3.4.1 Cleaning process of ITO electrode ... 61
3.4.2 Deposition of PtNDs onto ITO electrodes ... 61
3.4.3 The characterization of PtNDs modified electrode ... 62
3.5 Electrochemical analysis ... 62
3.5.1 Apparatus ... 62
3.5.2 Cyclic voltammetry analysis ... 63
3.5.3 Heavy metal detection using PtNDs/ITO modified electrode... 64
3.5.4 The selectivity of the modified electrode ... 64
3.5.5 Application to the real water sample ... 64
CHAPTER 4 ... 66
RESULTS & DISCUSSION ... 66
v
4.1 Introduction ... 66
4.2 Properties of as synthesized PtNDs... 67
4.2.1 X-ray Diffraction (XRD)... 67
4.2.2 Transmission electron microscope (TEM) ... 68
4.2.3 Inductive Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) 69 4.3 Physical properties of PtNDs/ITO electrode ... 70
4.3.1 Surface Contact Angle ... 70
4.3.2 Distribution of PtNDs particles on the ITO electrode ... 72
4.4 Electrochemical analysis of PtNDs modified ITO electrode ... 73
4.4.1 Cyclic Voltammograms ... 73
4.4.2 Detection of the Pb2+ by DPASV analysis ... 77
4.4.3 Interference Study ... 84
4.4.4 Application in seawater sample ... 85
CHAPTER 5 ... 87
CONCLUSION AND SUGGESTIONS FOR FUTURE WORK ... 87
5.1 Conclusion ... 87
5.2 Suggestions for future works ... 88
REFERENCE ... 89
APPENDIX ... 99
vi
LIST OF TABLES
Table 2.1: The concentration of Heavy Metal contamination river in Malaysia:
Semenyih river, Selangor and adjacent mining ponds, Perak river and Penang river (Badaii and Othman, 2016),(Daniel and Kawasaki, 2016),(Ahmad and Kadir, 2019) and (Alsaffar et al., 2016) ... 12 Table 2.2: Effects of heavy metal to the human health and environmental pollution 14 Table 2.3: Types of analytical equipment used for detection of the heavy metal element ... 18 Table 2. 4: Types of the optical sensor, principle for every type of sensors, published works, and materials used for the sensor... 21 Table 2. 5: The types of electrochemical sensors, principles, and examples of the fabricated sensor ... 27 Table 2. 6: Types of voltammetry technique (Joshi and Sutrave, 2018)... 32 Table 2.7: Heavy metal ions detection using nanomaterials modified electrode in electrochemical analysis ... 42 Table 2.8: Examples of Pt nanoparticles used for electrode modification ... 49 Table 3.1: Materials and chemicals used in this study ... 58 Table 4.1: Concentration of Pt in PtNDs measured using ICP-OES spectrometer .... 70 Table 4.2: Surface contact angle for different cleaning method apply to the ITO:
RCA method, cleaned with ethanol and uncleaned ITO electrode ... 71 Table 4.3: The value of anodic current of PtNDs/ITO vs Nafion / PtNDs /ITO ... 77 Table 4.4: Comparison of previous works and current work on detection of Pb(II) comparison;linear range, sensititvity and LOD... 83 Table 4.5: Recovery value for accuracy study for ICP-OES and DPASV for Pb (II) detection. ... 84
vii
LIST OF FIGURES
Figure 2.1: Illustrations on how heavy metals can enter the environment from industrial waste, mining, agricultural activities, urbanization and wastewater treatment to the seawater or river (Masindi and Muedi, 2018) ... 10 Figure 2.2: The map of Malaysia with an overview of the polluted river in Malaysia (Afroz et al., 2014) ... 11 Figure 2.3: The illustration of biological and chemical FET sensor (Kaisti, 2017)... 24 Figure2.4: The chemical recognition probe and transducer part in the electrochemical sensor (Coyle et al., 2014) ... 26 Figure 2.5: The elements and selected components of the biosensor (Gan et al., 2014) ... 26 Figure 2.6: The conductometry gas sensor and its sensing mechanism (Chavali and Nikolova, 2019) ... 29 Figure 2.7: The three-electrode component of voltammetry sensor (Zhang and Hoshino, 2019) ... 30 Figure 2. 8: The branch of electrolysis (Barón-Jaimez et al., 2013) ... 32 Figure 2. 9: The simplified circuit of cyclic voltammetry (Joshi and Sutrave, 2018) 33 Figure 2.10: The cyclic voltammogram (Joshi and Sutrave, 2018) ... 34 Figure 2.11: The example operation of anodic stripping voltammetry: potential vs time (top) and result of stripping voltammogram (bottom) (Wang, 2007) ... 35 Figure 2.12: Typical stripping voltammograms for the detection of heavy metals in drinking water using standard addition method under optimized condition. The concentration of Cd(II) and Pb(II) for each addition is 50µg/L. Inset: corresponding calibration curve (Xuan et al., 2016) .... 36
viii
Figure 2.13: The figure a) glass carbon by Jenkins Kawamura model, b) graphite c) carbon nanotube, d)graphite powder, e)carbon fibers and f) boron- doped diamond (Cavalheiro et al., 2012) and g)unmodified glass carbon electrode (Lima et al., 2019) ... 39 Figure 2.14: Schematic of a screen-printed electrode from Dropsens (Cheng et al., 2013) ... 41 Figure 2.15: (a) Corresponding correlation plot of differential pulse anodic stripping voltammetry in 0.1 M acetate buffer at pH 5.5 as supporting electrolyte using RGO/Bi composite modified carbon paste electrode. Solution concentrations of Cd+2 and Pb+2 were increased by 20µg/L (ppb) in every addition (b) TEM image of Bi RGO/Bi nanocomposite (Sahoo et al., 2013) ... 45 Figure 2.16: SWASV response of 30 Cd (II) AND Pb (II) in 0.1 mol/L acetate buffer solution on BI/SPCE, BI/IL/SPCE, BI/MWNT/SPCE and BI/MWNT-IL/SPCE (Wang et al., 2017) ... 46 Figure 2.17: SWASV for simultaneous detection of Cd+2, Pb+2 and Cu+2 with the modified AuNPs/CNFs/GCE, CNFs/GCE and bare/GCE (Zhang et al., 2016) ... 47 Figure 2.18: DPASV at different concentration of Pb2+ and Cd2+ ions at the PtNFs/GCE (Nguyen et al., 2019b) ... 50 Figure 2.19: TEM image of PtNDs synthesized by chemical reduction method (Song et al., 2006) ... 52 Figure 2.20: The illustration of drop-casting technique (Wu et al., 2020) ... 54 Figure 3. 1: Overview of the research flow for Pb2+ detection using PtNDs modified
electrodes ... 57
ix
Figure 3. 2: Synthesis of platinum nanodendrites routes ... 59
Figure 3.3: The electrochemical cell setup used in this study... 63
Figure 4.1: The XRD pattern of PtNDs using chemical reduction method ... 68
Figure 4.2: TEM image of as synthesized PtNDs ... 69
Figure 4.3: Particle size distribution of PtNDs ... 69
Figure 4.4: The tabletop SEM image of PtNDs particle distribution on the ITO electrode varied in the cleaning method (a) unclean ITO, (b) clean with ethanol and (c) RCA wash under 1.0 K magnification. Inset in each figure was surface contact angle image... 73
Figure 4.5: Cyclic voltammograms of PtNDs/ITO with varying numbers of PtNDs layers on ITO electrode in 5 x 10-3 mol/L [Fe(CN)6]-3 in 0.1 M of KCl solution (pH 7.0) with scan rate of 0.05 V/s ... 74
Figure 4.6:Effect of varying number of PtNDs layer of on the electrical conductivity of PtNDs/ITO electrode in 5 x 10-3M of [Fe(CN)6]3- in 0.1M of KCl solution (pH 7.0) ... 75
Figure 4.7: Cyclic voltammograms of Nafion/PtNDs/ ITO with varying numbers of PtNDs layers on ITO electrode in 5 x 10-3 mol/L [Fe(CN)6]-3 in 0.1 M of KCl solution (pH 7.0) with scan rate 0.05 V/s ... 76
Figure 4.8: Effect of different layer of PtNDs on the electrical conductivity of Nafion/PtNDs/ITO electrode in 5 x 10-3M of [Fe(CN)6]3- in 0.1M of KCl solution (pH 7.0) ... 77
Figure 4.9: The performance of PtNDs/ITO for Pb detection by DPASV with concentration of Pb(II) ranging from 0.1 ppm to 1 ppm ... 79
Figure 4.10: Linear calibration of PtNDs/ITO for Pb detection ... 79
x
Figure 4.11: DPASV of Nafion /PtNDs /ITO with different concentration of Pb2+
ranging from 0.1 ppm to 1 ppm... 80 Figure 4.12: Linear calibration plot for Nafion/PtNDs/ ITO detection of Pb2+ with varying Pb(II) concentration ranging from 0.1 ppm to 1ppm ... 81 Figure 4.13: Stripping peak of Pb(II) in ICP Multi-elements solution ... 85 Figure 4.14: DPASV curves stripping response of seawater sample collected near Pantai Jerejak, Penang area with and without Pb(II) spiked ... 86 Figure 4.15: The linear calibration plot for detection of Pb(II) in seawater spiked with 10 ppb, 50 ppb and 100ppb ... 86
xi
LIST OF SYMBOL
A Ampere
C Celsius
g Gram
Hz Hertz
L Litter
M Molarity
m Milli
s Second
V Voltage
θ Theta
µ Micro
xii
LIST OF ABBREVIATION
AAS Atomic absorption spectroscopy
AC Alternating current
Ag Silver
AgNPs Silver nanoparticles
Al Aluminium
ASV Anodic stripping voltammetry
Au Gold
AuNPs/CNF Gold nanoparticles carbon nanotubes
AuNPs Gold nanoparticles
Bi Bismuth
Cd Cadmium
Cr Chromium
CSV Cathodic stripping voltammetry
CV Cyclic voltammetry
DPASV Differential pulse anodic stripping
voltammetry
DPV Differential pulse voltammetry
Fe Iron
FET Field effect transistor
GCE Glassy carbon electrode
Hg Mercury
HMDE Hanging mercury drop electrode
ICP/AES Induced coupled plasma atomic
xiii
emission spectroscopy
ICP/MS Inductively couple plasma mass
spectroscopy
ITO Indium tin oxide
LOD Limit of detection
LSV Linear sweep voltammetry
MFE Mercury film electrode
MWCNT Multiwall carbon nanotubes
NFC Nanofibrillated cellulose
Ni Nickel
NPV Normal pulse voltammetry
Pb Lead
Pt Platinum
PtNDs Platinum nanodendrites
rGO reduced graphene oxide
SEM Scanning electron microscope
SERS Surface-enhanced raman scattering
SPCE Screen printed carbon electrode
SPE Screen printed electrode
SPR Surface plasma resonance
SWASV Square-wave anodic stripping
voltammetry
SWV Square-wave voltammetry
TRGO Thermally reduced graphene oxide
UV-Vis Ultraviolet-visible spectroscopy
xiv
PENGUBAHSUAIAN ELEKTROD PLATINUM NANODENDRIT UNTUK PENGESANAN PLUMBUM (Pb)
ABSTRAK
Pencemaran logam berat di persekitaran telah menyebabkan pengumpulan logam berat dalam rantai makanan dan mengakibatkan masalah kesihatan yang serius kepada manusia seperti kecederaan otak dan kegagalan buah pinggang. Oleh itu, pengesanan logam berat adalah penting. Dalam penyelidikan ini, platinum nanodendrit (PtNDs) telah disintesis menggunakan kaedah penurunan kimia Sifat- sifat PtNDs telah dikaji dengan menggunakan pembelau sinar-X (XRD), mikroskop transmisi elektron (TEM) dan plasma induksi optik pelepasan spektrometri (ICP- OES). Analisa XRD menunjukkan fasa yang terdapat dalam sampel adalah platinum.
Analisis TEM menunjukkan struktur yang dimiliki oleh platinum adalah nanodendrit dengan saiz partikel nano ~35 nm. Analisa dari ICP-OES spektrometer menunjukkan kepekatan koloid Pt ialah 6.77 ppm. PtNDs di letakkan di atas elektrod ITO dengan jumlah lapisan yang berbeza. Nafion seterusnya di letakkan di atas lapisan terakhir elektrod PtNDs/ITO. Elektrod yang diubah suai telah dikaji menggunakan analisa voltametri berkitar (CV) dan voltammetri pelucutan anodik denyut pembezaan (DPASV). Elektrod yang diubah menunjukkan prestasi yang baik dalam pengesanan Pb (II) dengan julat linear 0.1-1 ppm. Nilai had pengesanan (LOD) untuk PtNDs/ITO dan Nafion/PtNDs/ITO ialah 0.877 dan 0.827 ppm dan sensitiviti elektrod ialah 234.11µA / ppm dan 159.89 µA / ppm. Analisis interferens menunjukkan elektrod yang diubah sangat selektif terhadap pengesanan Pb (II).
Kebolehgunaan elektrod yang diubah dikaji dalam air laut untuk pengesanan Pb (II) adalah hampir sama dengan analisa menggunakan spektrometer ICP-OES. Hasil kajian mendapati elektrod PtNDs/ITO boleh digunakan sebagai pengesan plumbum.
xv
PLATINUM NANODENDRITES MODIFIED ELECTRODE FOR LEAD (Pb) DETECTION
ABSTRACT
Heavy metal pollution in the environment has caused accumulation of heavy metal ions in the food chain and this has led to serious health issues for humans such as brain injury and kidney failure. Thus, the detection of heavy metal is important. In this research, the platinum nanodendrites (PtNDs) were synthesized using chemical reduction method and used for fabrication of Indium Tin Oxide (ITO) modified electrode for heavy metal detection. The properties of PtNDs of was characterized using X-ray diffraction (XRD), transmission electron microscope (TEM) and inductive coupled plasma-optical emission spectroscopy (ICP-OES). XRD analysis revealed the phase present in the sample was platinum. The TEM analysis showed the structure of platinum was nanodendrites with a mean particle size of ~ 35 nm.
The ICP-OES spectrometer analysis revealed the concentration of PtNDs solution is 6.77 mg/L. The as-synthesized PtNDs were drop-casted on the ITO electrode with varying number of layers with and without nafion. The modified electrode was subjected to cyclic voltammetry (CV) and differential pulse anodic stripping voltammetry (DPASV). The modified electrode showed good performance in detection of Pb(II) with linear range 0.1-1 ppm the limit of detections (LOD) were 0.877 and 0.827 ppm, and sensitivities of 234.11µA/ppm and 159.89 µA/ppm for PtNDs/ITO and Nafion/PtNDs/ITO, respectively. The interference analysis showed the modified electrode was highly selective towards detection of Pb (II). The applicability of modified electrode was studied using seawater sample for Pb (II) detection with reading in good agreement with ICP-OES analysis. The findings revealed the PtNDs/ITO electrode can be used as Pb sensor.
1 CHAPTER 1 INTRODUCTION
1.1 Introduction
Heavy metal elements are known for toxicity and may cause serious effect to the environment. Examples of heavy metal elements are mercury (Hg), lead (Pb), cadmium (Cd) and arsenic (As). Pb is among the most toxic elements that is harmful to the human health and environment (Pujol et al., 2014). Pb is stated as the second most toxic material by Agency for Toxic Substances and Disease Registry (ATSDR). Pb is non-biodegradable and can persist in the food chain. This can cause serious threat to human health. Heavy metals tend to accumulate inside the human body and cause various diseases such as cancer, kidney failure, brain damage, and organ failure (Koudelkova et al., 2017). Thus, the detection of heavy metal traces is essential. Detection of heavy metal traces are performed through various methods.
Typical conventional methods that have been used for heavy metal detection are Atomic Absorption Spectroscopy (AAS), Induced Coupled Plasma/ Atomic Emission Spectroscopy (ICP/AES), Inductively Coupled Plasma with Mass Spectrometric (ICP/MS) and X-ray fluorescence (Pujol et al., 2014). However, these conventional methods are expensive, high labor cost, and not suitable for on-field applications. The other method that has been used to detect heavy metal is sensor.
Types of sensors that have been used to detect heavy metal are optical, electrochemical, and field-effect transistor (FET) sensors. The electrochemical sensor does not need a recognition probe as bare electrode can achieve selectivity towards the specified heavy metal ion because heavy metal has defined redox potential (Li et al., 2013). An electrochemical sensor can be divided into
2
amperometry, conductometry, voltammetry, impedemetry and potentiometry (Li et al., 2013).
An electrochemical sensor by anodic stripping voltammetry (ASV) under voltammetry method is suitable for heavy metal detection (Li et al., 2013). This is because ASV method usually has two steps which are electrochemical deposition or accumulation of heavy metal at a constant potential to pre-concentrate analyte onto the electrode surface and follow by stripping or dissolution of deposited analyte from the electrode surface (Li et al., 2013). ASV is one of the most promising technique as it has high sensitivity and low detection limits (LOD) (Han et al., 2015). The pre- concentration step in ASV allows low detection limits for heavy metal detection (Han et al., 2015). ASV technique consists of differential pulse anodic stripping voltammetry (DPASV) and square wave anodic stripping voltammetry (SWASV).
The sensitivity of DPASV was 10 to 20 times higher than normal electrochemical cells (Pujol et al., 2014). Whereas SWASV is the combination of the advantages of other systems and improve the quality of the quantitative information in electrochemical cell (Barón-Jaimez et al., 2013).
Typically, electrochemical sensors for heavy metal detection consists of three electrodes: working electrode, reference electrode and counter electrode. Sensitivity and specificity of electrochemical sensors are highly dependent on a working electrode. Previously, mercury-based electrodes have been used as the working electrode in electrochemical detection of heavy metal owing to their high sensitivity and good for heavy metal detection (Li et al., 2013). Examples of mercury-based electrodes are hanging mercury drop electrode (HMDE), mercury film electrode (MFE) and dropping mercury electrode (DME/SMDE) (Ariño et al., 2017).
However, mercury is highly toxic, thus, other electrodes with less toxic becomes of
3
interest. Several works reported used of bismuth film electrode (Hutton et al., 2006) and gold film electrodes for electrochemical sensors (Sun et al., 1997). However, bulk electrode did not satisfy the sensitivity required for heavy metals detection.
Thus, to enhance the electrode performance, the surface modification using nanoparticles on the electrode surface becomes of interest. There are several types of substrate electrodes have been used for modification which are glassy carbon electrode (GCE) (Yang et al., 2014a), screen-printed carbon electrode (SPCE) (Wang and Yue, 2017) and indium-tin-oxide (ITO) electrode (Tang and Cheng, 2013). The method used for nanomaterial synthesis can affect the properties of nanomaterials modified electrodes. Several methods have been used in nanomaterials modified electrodes such as direct electrochemical deposition method (Xing et al., 2011), in-situ synthesis method (Sang et al., 2017), and ex-situ synthesis method such as hydrothermal method (Hoa et al., 2015) and chemical reduction method (Ting et al., 2015). Synthesis method produces specific properties of nanomaterials (Khan et al., 2019). Among all the synthesis method, chemical reduction method is one of the most promising method to synthesis colloidal metal nanoparticles owing to convenient operation and simple equipment (Martínez-Abad, 2011).
There are various nanoparticles that have been used in modifying working electrodes. The advantages of using nanomaterials to modify electrode are high surface area, low solution resistance and can enhance electron transfer (Li et al., 2013). Hence, sensitivity and specificity of heavy metal detection can be enhanced.
Examples of nanomaterials used in modification of working electrode are bismuth (Bi) (Sahoo et al., 2013; Yang et al., 2014b), carbon (C) (Wang and Yue, 2017), silver (Ag) (Xing et al., 2011; Sang et al., 2017) and platinum (Pt) (Nguyen et al., 2019a). Nanomaterials modified electrodes possess strong potential and may
4
improve the sensitivity and selectivity of the electrochemical sensors (Abdel-Karim et al., 2020).
The properties of nanomaterials are dependent on their size, structure, and distribution. Different morphologies of nanomaterials have been used for electrode modification; nanoparticles (Han et al., 2015), nanoflower (Nguyen et al., 2019b), nanotubes (Jeromiyas et al., 2019) and nanostar (Dutta et al., 2019). The size and structure of nanomaterials used for electrode modification affect the number of adsorption sites and surface to volume ratio possess by the nanostructure (Tonelli et al., 2019). Higher surface to volume ratio and large number of adsorption sites can enhance the electrode properties such as sensitivity, selectivity, and efficiency in electrochemical analysis (Tonelli et al., 2019). The modified electrode performance also can be improved with the addition of nafion. Nafion is well known as ionomer membrane that has hydrophobic poly(tetrafluorothylene) component (Xing et al., 2011). The ionomer membrane is a proton-conductive polymer film that allows only protons to cross-over the membrane (Yusoff, 2018). The advantages of nafion is it is chemically and mechanically stable and can prevent nanoparticles leaching from the electrodes surface (Szoke et al., 2019). Most reported work used nafion for electrochemical sensors for glucose detection (Kang et al., 2019), However, limited works can be found on using nafion for heavy metal detection.
5 1.2 Problem statement
Heavy metal that accumulate in the environment and food chain causes exposure to the living organism. The high exposure to heavy metal may lead to serious health effect (Koudelkova et al., 2017). Thus, the detection of heavy metal that accumulated in the environment such as seawater, river and groundwater need to be done. The spectroscopy techniques such as AAS, ICP-MS and ICP-OES have been used in detection of the heavy metal (Li et al., 2013). However, these spectroscopy techniques require high labour cost and not suitable for on field applications (Han et al., 2015). Thus, for detection of heavy metal, sensors are favourable as alternative as it is simple, low-cost, and suitable for field used (Li et al., 2013). The electrochemical sensor is one of the best ways for heavy metal detection.
In the electrochemical sensor, the modification of the working electrode is crucial as it affects the sensitivity and selectivity of the electrochemical sensor. The mercury-based electrode has been used as it has high sensitivity but highly toxic. The mercury-based electrode is then replaced with a bismuth electrode that is more environmentally friendly. However, the bulk electrode cannot satisfy the need for high sensitivity detection limit for heavy metal traces. Problem arises from using a bulk electrode is large overpotential during analyte deposition (Li et al., 2013). This problem can be minimized by using nanomaterials modified electrode that requires lower potential for analytes deposition compared to bare electrodes (Abraham et al., 2020).
Modification of the electrode surface with nanomaterials can enhance the sensitivity and selectivity of the electrode in heavy metal detection (Li et al., 2013).
6
Nanomaterials are known for its advantages of enhancing the surface area and can improve the electron transfer rate (Ward Jones and Compton, 2008). Various metallic nanomaterials have been used for Pb detection such as mercury (Hg) nanomaterial (Nagles et al., 2012), bismuth (Bi) nanomaterial (Yang et al., 2014a), silver (Ag) nanomaterial (Pérez-Ràfols et al., 2017), gold (Au) nanomaterials (Wan et al., 2015) and platinum (Pt) nanomaterials (Nguyen et al., 2019b). However, Hg and Ag nanomaterials are toxic compared to Pt (Asharani et al., 2011). Bi nanomaterials also suffer from instability due to its natural oxidization, thus, Pt is more stable compare to Bi nanomaterials (Li et al., 2013). Pt has better electrocatalytic properties compared to Au (Han et al., 2015).
Various Pt nanomaterials have been used for electrode modification such as Pt nanoparticles (Dai and Compton, 2006), Pt nanoflower (Nguyen et al., 2019b), and Pt nanotubes (Xu et al., 2008). The properties of nanoparticles are dependent on the nanostructures of the nanomaterials. To enhance the properties of Pt nanomaterial modified electrode for Pb detection, Pt nanodendrites is used.
Furthermore, there is no reported work on Pt nanodendrites modified electrode performance.
Nafion is known as sulfonated tetrafluoroethylene-based fluoropolymer- copolymer with conductive properties (Yusoff, 2018). Nafion can improve the electrode stability as it has excellent ability in film forming and only enables cations conduction to pass through the membrane and reach the electrode surface thus makes the electrode more selective (Yusoff, 2018). Thus, to improve the selectivity and stability of the modified electrode, nafion is used for the electrode modification.
7
In this work, the Pt nanodendrites was synthesized using chemical reduction method. The PtNDs was characterized with TEM, XRD and ICP-OES spectrometer.
The modification of ITO electrode with PtNDs was performed using drop-cast technique. 1% of nafion was used in fabrication of the PtNDs/ITO electrode. The performance of PtNDs/ITO electrode was measured with cyclic voltammetry (CV) and for Pb detection, differential pulse anodic stripping voltammetry (DPASV). The specificity of the modified electrode was analyzed using multi-element solution and the applicability of the produced electrode was tested in the real sample water.
1.3 Objectives
The main objectives of this research are:
1. To synthesize and investigate the properties of platinum nanodendrites
2. To study the physical properties of platinum nanodendrites modified electrodes
3. To study the electrochemical properties of platinum nanodendrites in heavy metal detection of Pb2
1.4 Scope of study
This study focuses on the synthesis and characterization of PtNDs. PtNDs were synthesized by using chemical reduction method and were characterized by X- ray Diffraction (XRD) for phase identification. Transmission Electron Microscope (TEM) was used to observe the morphology and size of the Inductive Couple Plasma-Optical Emission Spectroscopy (ICP-OES) was used to determine the concentration of the PtNDs solution. The fabrication of modified electrode was performed by drop-cast technique. The number of PtNDs layer was varied; 1-layer, 2-layer, 3-layer, 4-layer, 5-layer, and 6-layer, on the ITO electrode. The physical properties of the modified ITO electrode PtNDs were determined using contact angle
