1.5. Aims of this Thesis

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A Thesis Submitted for the Degree of PhD at the University of Warwick

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Developments of New Techniques
for Studies of Coupled Diffusional and Interfacial
Physicochemical Processes

By

Tahani Mohammad Bawazeer

A thesis submitted for the degree of
Doctor of Philosophy

Department of Chemistry
United Kingdom

December 2012
For
my parents…
my husband…
and my Children…
I would never have
reached this point without you,
and I will never be able to thank you enough
Table of Contents

Table of Contents ii
List of Illustrations v
List of Tables xii
List of Abbreviations xiii
List of Symbols xv
Acknowledgment xvii
Declaration xviii
Abstract xix

Chapter 1 Introduction 1
1.1 Fluorescence Confocal Laser Scanning Microscopy (CLSM) 1
1.1.1 Literature Survey 1
1.1.2 CLSM Principle 4
1.1.3 Fluorescence 6
1.1.3.1 Fluorescein 8
1.2 Electrochemistry and Microelectrodes 11
1.2.1 Introduction to electrochemistry 11
1.2.2 Dynamic electrochemistry 12
1.2.3. Mass transport 15
1.2.3.1 Diffusion 16
1.2.3.2 Convection 18
1.2.3.3 Migration 18
1.2.4 Ultramicroelectrodes (UME) 19
1.2.5 Linear Sweep Voltammetry and Cyclic Voltammetry 22
1.2.6 Scanning electrochemical microscopy (SECM) 25
1.2.6.1 Negative Feedback 26
1.2.6.2 Positive Feedback 28
1.4 Finite Element Modelling (FEM) 28
1.5 Aims of the Thesis 31
1.6 References 33

Chapter 2 Experimental Methods 37
2.1 Sample Preparation 37
ii
Table of Contents

2.2 Device Microfabrication and Characterisation 39
2.2.1 Pt Ultramicroelectrode (UME) Fabrication and Characterisation 39
2.2.2 Optical Transparent Single-Walled Carbone Nanotubes 42
Ultramicroelectrodes (OT-SWNTs-UMEs) mat Fabrication
2.3 CLSM Experiments 47
2.3.1 Visualisation of Electrochemical Processes at OT-SWNTs-UMEs 48
2.3.2 Visualization of Proton Diffusion at active and Modified Surfaces 51
2.4 Additional Instrumentation 58
2.4.1 Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) 58
2.4.2 Fluoride Ion Selective Electrode (FISE) Studies 59
2.5 Finite Element Modelling 59
2.6 Solution Preparation 59
2.7 References 61

Chapter 3 Visualisation of Electrochemical Processes at Optically 62
Transparent Single-Walled Carbon Nanotubes
Ultramicroelectrodes (OT-SWNTs-UMEs)
3.1 Overview 62
3.2 Introduction to Carbone Nanotube 69
3.2.1 Characterisation of SWNTs ultrathin films 73
3.2.2 Characterisation of SWNTs disc OT-UMEs 74
3.3 Visualisation of the ORR at SWNTs disc OT- UME 76
3.3.1 Dynamic Visualisation of Ru(bpy) 2+ concentration during CV 79
3
measurements at SWNTs disc OT- UME.
3.3.2 Three dimensional concentration profiles of Ru(bpy) 2+at the 81
3
steady-state current.
3.4 Conclusion 85
3.5 References 87

Chapter 4 Transient Interfacial Kinetics, from Confocal Fluorescence 90
Visualisation: Application to Proton Attack the Treated
Enamel Substrate
4.1 Overview 90
iii
Table of Contents

4.1.1 Enamel Structure 92
4.1.2 Enamel Dissolution 93
4.1.3 Enamel Dissolution Inhibitors 94
4.2 Proton Distribution at Disc UME 95
4.3 CLSM Visualisation of Proton Flux during Enamel Dissolution 96
4.3.1 Experimental Results and Analysis 97
4.4 Finite Element Model 103
4.4.1 Diffusion Coefficients 107
4.4.2 Insights from Simulations 109
4.5 Amount of Zinc and Fluoride Uptake on Treated Enamel 113
4.6 Conclusions 114
4.7 References 116

Chapter 5 Combined Confocal Laser Scanning Microscopy - 119
Electrochemical Techniques for the Investigation of Lateral
Diffusion of Protons at Surfaces
5.1 Biological Membranes 119
5.2 Supported Lipid Bilayer (SLB) Synthesis and Characterisation 123
5.3 Modification of Surfaces by Ultrathin Films 126
5.4 Proton Lateral Diffusion 128
5.5 Result and Discussion 132
5.5.1 Characterisation of SLB-Modified Surfaces by AFM 132
5.5.2 Measurement of proton diffusion at modified surfaces 134
5.6 Conclusion 145
5.7 References 147

Chapter 6 Conclusion 151

Appendix 155

iv
List of Illustrations

List of Illustrations
Chapter 1 Introduction 1
Figure 1.1 (a) Full-field illumination (b) Single point illumination 4
Figure 1.2 Schematic diagram of a confocal microscope 5
Figure 1.3 Schematic of 3D z-stack imaging in CLSM 6
Figure 1.4 A Jablonski diagram showing the origin of fluorescence, with the 7
photoexitation of an electron from (1) to (2) and emission light
when the electron moves from (3) to (1)
Figure 1.5 Definition of Stokes shift 8
Figure 1.6 Illustration of pH vs. intensity of fluorescence after fluorescein 9
excitation at 488 nm and detection at 530 nm
Figure 1.7 Molecular structures of neutral fluorescein and its anions 10
Figure 1.8 A diagrammatic representation of the electrical double layer 12
where the red cations are solvated with small blue circles
representing water molecules, and large green anions are
specifically adsorbed onto the electrode surface
Figure 1.9 The effect of the applied potential on the Fermi level 13
Figure 1.10 A schematic representation of a typical electrode reaction 14
Figure 1.11 Different geometries of electrodes and their diffusion fields: (a) 20
Disk electrode (b) Cylinder electrode and (c) Ring electrode
Reproduced from reference
Figure 1.12 (a) Schematic of planar diffusion profile exhibited by a 21
macroelectrode and (b) the hemispherical diffusion at disc UME
Figure 1.13 Showing the RG of an UME where rs is the radius of the whole 22
UME and a is the radius of the metal wire
Figure 1.14 Typical CV responses for (a) a macroelectrode and (b) an 23
ultramicroelectrode
Figure 1.15 (a) Schematic of negative feedback, for an UME near an 27
insulating substrate and (b) Theoretical approach curve for a 25
µm diameter UME with an RG of 10, depicting negative feedback
Figure 1.16 (a) Schematic of positive feedback near a conducting substrate (b) 28
Theoretical approach curve for a 25 µm diameter UME displaying
positive feedback
v
List of Illustrations

Figure 1.17 (a) An example of a simple triangular mesh used for FEM and 30
(b)the same domain where the mesh is finer at two edges

Chapter 2 Experimental Methods 37
Figure 2.1 Conventional light microscope image of the tip of a 25 μm 40
diameter platinum disc UME (a) from the side (b) the electrode
surface
Figure 2.2 Cyclic voltammogram for the oxidation of 1 mM FcTMA at a 25- 41
μm diameter Pt UME. The scan rate was 10 mV s-1
Figure 2.3 SECM experimental approach curve (red line) and the theory 42
(blue line) towards inert glass substrate performed in 0.1 M KNO
3
Figure 2.4 Schematic illustration the procedure for growing carbon nanotube 43
ultrathin mats using catalysed chemical vapour deposition
technique
Figure 2.5 (a) Schematic of the procedure used for fabricating SWNTs 45
network disk UMEs, (b) Geometry of the shadow mask used for
i
microfabrication of Cr/Au bands for disk UME experiments, (b )
ii
Photographs of SWNTs devices used in electrochemical
experiments
Figure 2.6 Micro-Raman spectrum of SWNTs ultrathin mats grown on single 47
crystal quartz
Figure 2.7 Schematic representation of the experimental setup used for 49
visualising OT-SWNTs-UMEs
Figure 2.8 Schematic diagram of the clamp system developed for use with 51
the confocal microscope
Figure 2.9 (a) Rise time for a current of 20 nA using homemade galvanostat 52
and (b) corresponding change of the potectial applied to the UME
working electrode
Figure 2.10 Describes the CLSM-SECM set-up where: (a) is a schematic 53
detailing the components seen in (b); (b) is a photograph showing
the UME and enamel sample inserted in the cell and immersed in
solution

vi
List of Illustrations

Figure 2.11 Experimental set up for SECM tip positioning and CLSM 55
experiments
Figure 2.12 Schematic (not to scale) of experimental arrangement for studies 57
of proton diffusion at a SLB
Figure 2.13 Schematic of experimental arrangement for studies of proton 58
diffusion in a thin layer

Chapter 3 Visualisation of Electrochemical Processes at Optically 62
Transparent Single-Walled Carbon Nanotubes
Ultramicroelectrodes (OT-SWNTs-UMEs)
Figure 3.1 a) AFM (3 nm × 3 nm) AFM images (full height scale=5 nm) of a 71
SiO surface after cCVD growth of SWNTs. b) associated FE-
2
SEM image of a SWNT network. The scale bar represents 5 μm.
Taken from reference
Figure 3.2 (a) 1 µm × 1 µm tapping mode AFM height images of SWNT 74
networks and ultrathin mats grown on single crystal ST-cut quartz
substrate, using Co as the catalyst, sputter-deposited on the
substrate 20 s prior to growth. (b) Micro-Raman spectrum of
SWNT ultrathin mats grown on single crystal quartz
Figure 3.3 UV-Vis spectra of: blank ST-cut quartz; SWNT mat grown on ST- 75
cut quartz substrate; and ST-cut quartz coated with S1818 positive
photoresist
Figure 3.4 (a) Brightfield mode (b) and fluorescence mode images of an 76
optically transparent carbon nanotube ultramicroelectrode
(zoomed out and zoomed in). The images were recorded in the x-y
plane parallel to the electrode
Figure 3.5 CV of ORR and simultaneous change of solution light intensity as 77
a function of potential, at a SWNT disc OT-UME (scan rate 10
mVs-1) for 0.1 M NaCl aqueous solution with 8 µM fluorescein
and an initial pH 5. The arrows indicate the initial scan direction.
The intensity values were collected from the middle of the UME
surface, from a 35 µm diameters region of interest (ROI). The
fluorescence mode insets illustrate the development of pH
vii
List of Illustrations

gradients as a function of potential. The scale bars are 250 µm
Figure 3.6 (a) Fluorescence intensity at a potential of -1.0 V in 0.1 M NaCl 78
solution with 8 µM fluorescein in the xy, xz and yz planes. The
scale bar in the xy image is 250 µm. The red and white lines in xz
and yz cross sections indicate the photoresist and OT UME
surface. (b) A graph of intensity below (B) and above (A) the OT-
UME plane in 35 µm diameter ROI. The dashed line indicates the
electrode surface
Figure 3.7 (a) CV (red) and simultaneous change of the fluorescence 80
intensity (black: scan rate 5 mV/s) for the oxidation of 10 mM
Ru(bpy) 2+ in aqueous solution in 0.1 M NaCl. The arrows
3
indicate the forward scan direction. The intensity values were
collected from the 35 mm diameter region of interest at the centre
of the UME. Intensity values and current values were normalised
by the initial intensity value at time = 0 s (It=0) and the steady-
state limiting current value (i ), respectively. The insets show
lim
fluorescence profiles near the OT-CNT-UME surface during
Ru(bpy) 2+ oxidation. The scale bars are 25 mm. (b) Relationship
3
between normalised current and fluorescence intensity change for
forward andreverse scan directions in the range of quartile
potentials of the voltammetric signature
Figure 3.8 The change of solution intensity over time during stepping up the 82
potential from 0 V to the potential of the mass transport limiting
current (1.2 V), determined from the CV for 10 mM Ru(bpy) 2+
3
aqueous solution in 0.1 M NaCl. The inlets indicate the formation
of the diffusion zone at the UME surface. The scale bar is 25 µm
Figure 3.9 Cross sectional light intensity change below and above the UME 84
surface before (left) and after (right) the potential step for the 1
mM (a), 5 mM (c), and 10 mM (e) Ru(bpy) 2+ in 0.1 M NaCl (e).
3
All images were collected with the z-volume 150 µm (vertical
white dashed line) and the z-step 2 µm. Z-stack before the
potential step and during the steady state for the 1 mM (b), 5 mM
(d), and 10 mM (f) Ru(bpy) 2+ in 0.1 M NaCl. The scale bars in (a,
3
viii