raman characterization of colloidal nanoparticles …...furthermore, i would like to thank dr....

Raman Characterization of Colloidal Nanoparticles

using Hollow-Core Photonic Crystal Fibers

by

Jacky Siu Wai Mak

A thesis submitted in conformity with the requirements

for the degree of Master of Applied Science

Graduate Department of Electrical and Computer Engineering

University of Toronto

© Copyright by Jacky Siu Wai Mak 2011

ii

Raman Characterization of Colloidal Nanoparticles using

Hollow-Core Photonic Crystal Fiber

Jacky Siu Wai Mak

Master of Applied Science

Graduate Department of Electrical and Computer Engineering

University of Toronto

2011

Abstract

This Masters thesis investigates the ligand–particle binding interactions in the thiol–capped

CdTe nanoparticles and dye adsorbed gold nanoparticles. In the CdTe nanoparticles, Raman

modes corresponding to the CdTe core, thiol ligand and their interfacial layers were observed

and correlated to the different nanoparticle properties. To the best of our knowledge, this is the

first time that such strong Raman modes of the thiol-capped nanoparticles in aqueous solution

have been reported. In the gold nanoparticle systems, gold–citrate binding interactions were

observed as well as adsorption of the Raman dyes and binding with the polyethyleneglycol

polymer coating and phospholipid coating. These observations coincided with findings from

conventional optical techniques. In addition, gold nanoparticles were found to carbonize at high

pump power and prolonged exposure time. In summary, the two nanoparticle characterizations

demonstrated the high sensitivity and nondestructive nature of the photonic crystal fiber for

Raman spectroscopy.

iii

Acknowledgments

First, I would like to thank my supervisor, Professor Amr S. Helmy, for the opportunity to learn

about Raman spectroscopy during my summer research three years ago and the opportunity to

work on this very exciting project for my Master’s degree. He has continuously supported my

work and given me much freedom and insight to explore the many difficult problems in my

experiments throughout the degree. More importantly, he had provided me with tremendous

support during the difficult times I had with personal problems at home. I truly appreciate all of

his support and efforts throughout these two years.

In addition, I would like to thank numerous members in our group who have also helped me with

my research work. I want to thank Dr. Abdiaziz Farah for synthesizing the CdTe nanoparticles

and aiding me with the many chemistry questions I had in nano-materials. I want to thank Peter

Chan for confirming the Raman results on the CdTe nanoparticles. I want to thank Basil

Eleftheriades for the help with the photonic crystal fiber splicing experiments in measuring the

core diameter with SEM. Furthermore, I would like to thank Dr. Abdiaziz Farah, Dr. Fatima

Eftekhari, Steve Rutledge, Wen Ma and Dr. Rashid Abu-Ghazalah for the many discussions

about my experiments and the many useful comments on my manuscripts. Without the help of

my colleagues, my work would not have been accomplished in such a timely manner.

Secondly, I would like to thank Professor Gang Zheng, Professor Gilbert Walker and their group

members for providing me with the many nanoparticle samples for our study. In particular, I

would like to thank Christina MacLauglin and Glenda Sun in Professor Walker’s group for

providing the pegylated nanoparticles. In addition, I would like to thank Natalie Tam and Sophie

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Wang in Professor Zheng’s group for providing the phospholipid nanoparticles. Furthermore, I

thank Vivo Nano Inc. for providing the polymer encapsulated gold and silver nanoparticles.

Finally, I would like to express my love and affection to my friends, family and my girlfriend,

Sandy Chau, for their continuous love and support throughout my study. The many long hour

chats were very encouraging and have guided me through my tough times. I would like to thank

Sandy in particular for her efforts in helping me relax and enjoy life out of my busy schedule as

well as her assistance in editing my writings. Without everyone’s support, the completion of this

work would have never been possible.

v

Table of Contents

1. Introduction

1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.2 Thesis Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.3 Organization of Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2. Raman Spectroscopy

2.1 Light-Matter Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2 Classical Treatment of Raman Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

1

5

5

8

9

10

2.3 Quantum Mechanical Treatment of Raman Scattering . . . . . . . . . . . . . . . . . . . . . . . .

2.4 Raman System Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.5 Raman Scattering Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3. Photonic Crystal Fibers

3.1 Classifications of Photonic Crystal Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.2 Index Guiding Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.3 Photonic Bandgap Guiding Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

12

15

19

21

23

25

30

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3.4 Hollow-Core Photonic Crystal Fiber for Efficient Raman Scattering . . . . . . . . . . . . .

4. Raman Characterization of Colloidal CdTe "anoparticles

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.2 Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.3 CdTe Core and Te Defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.4 Core-Thiol Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.5 Carboxylate-Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5. Raman Characterization of Metallic "anoparticles

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5.2 Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5.3 Molecular Interactions in Pegylated Gold Nanoparticles . . . . . . . . . . . . . . . . . . . . . .

5.3.1 Citrate Stabilized Gold Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5.3.2 Pegylated Gold Nanoparticles with MGITC Adsorbed . . . . . . . . . . . . . . . . . . .

5.3.3 Pegylated Gold Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5.4 Molecular Interactions in Phospholipid Gold Nanoparticles . . . . . . . . . . . . . . . . . . . .

5.4.1 Citrate Stabilized Gold Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5.4.2 CV Adsorbed Gold Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5.4.3 Phospholipid Gold Nanoparticles Adsorbed with CV . . . . . . . . . . . . . . . . . . . .

5.5 Pump Power and Duration Dependence of Pegylated Gold Nanoparticles . . . . . . . . .

5.6 Pump Power and Duration Dependence of Phospholipid Gold Nanoparticles . . . . . .

5.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

34

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66

70

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6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Appendix A Snell’s Law in Wave Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Appendix B Synthesis Method and Materials for Thiol-Capped CdTe Quantum Dots

Appendix C "anoparticle Signal Enhancement using HC-PCF . . . . . . . . . . . . . . . . . . .

Appendix D Raman Mode Assignments of Thiol-Capped CdTe "anoparticles . . . . . .

Appendix E Raman Mode Assignments of Gold "anoparticle Systems . . . . . . . . . . . .

Appendix F Fiber Splicing Program Optimization for Large Biological Molecules . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

101

101

104

106

108

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117

119

124

131

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List of Tables

Table 2.1: Laser specifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Table 2.2: Output powers of a 17 mW laser passing through the different optical density

filters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Table 2.3: Specifications of the Olympus M PLAN objectives. . . . . . . . . . . . . . . . . . . . . . . 16

Table 2.4: Scanning range and resolution of diffraction gratings. A 1000µm confocal hole

size is used as a reference for resolution comparison. . . . . . . . . . . . . . . . . . . . . . 17

Table 2.5: Approximate order of magnitude for cross-sections (per molecule), σ, of the

different optical processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Table 4.1: Intensity ratios of the Te A1 mode to CdTe LO mode for CdTe QDs capped

with different thiol agents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Table 4.2: The wavenumber separations, ∆, between the symmetric and asymmetric

stretching modes of the carboxylate, and their corresponding carboxylate-metal. 57

Table 4.3: Intensity ratios of the symmetric carboxylate stretch to CdS 2SO mode. . . . . . . 58

Table 5.1: Nanoparticle samples studied for pegylated nanoparticle system and

phospholipid nanoparticle system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

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Table 5.2: Wavenumber position of the symmetric and asymmetric stretching carboxylate

modes of citrate, their intensity ratio to νas(COO), respective wavenumber

differences and proposed complexes for a fresh batch of citrate stabilized gold

nanoparticles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73


modes of citrate, their intensity ratio to νas(COO), respective wavenumber

differences and proposed complexes for a six-month old batch of citrate

stabilized nanoparticles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75


modes, their respective wavenumber differences and proposed complexes in

pegylated gold nanoparticles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80


modes, their respective wavenumber differences and proposed complexes for a

fresh batch of citrate stabilized gold nanoparticles for synthesizing the

phospholipid nanoparticle system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Table C.1: Enhancement factor of MGITC modes using HC-PCF. . . . . . . . . . . . . . . . . . . . 113

Table D.1: Proposed assignment of thiol-capped CdTe nanoparticles. Spectra are shown in

Figure 4.6 for regime between 100 and 200 cm-1

, Figure 4.8 for regime between

200 and 310 cm-1

and Figure 4.9 for regime between 310 and 1750 cm-1

. . . . . . 118

Table E.1: Proposed assignment of citrate stabilized Gold nanoparticles for pegylated

nanoparticle system. Spectrum is shown in Figure 5.5. . . . . . . . . . . . . . . . . . . . . 120

Table E.2: Proposed assignment of pegylated gold nanoparticles. Spectrum is shown in

Figure 5.9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

Table E.3: Proposed assignment of MGITC adsorbed pegylated gold nanoparticles.

Spectrum is shown in Figure 5.7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

x

Table E.4: Proposed Assignment of citrate stabilized Gold nanoparticles for phospholipid

nanoparticle system. Spectrum is shown in Figure 5.11. . . . . . . . . . . . . . . . . . . . 122

Table E.5: Proposed Assignment of CV adsorbed gold nanoparticles. Spectrum is shown in

Figure 5.12b. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .123

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List of Figures

Figure 2.1: Possible outcomes of light-matter interaction. . . . . . . . . . . . . . . . . . . . . . . .

Figure 2.2: Three possible energy transitions of molecule during the light scattering

process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 2.3: Schematic of the LabRam HR 800 Raman system. . . . . . . . . . . . . . . . . . . .

Figure 3.1: Wave interferences in the photonic bandgap of a one dimensional periodic

material. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 3.2: Cross-section of a HC-PCF taken by scanning electron microscope (SEM).

Figure 3.3: Classification of PCFs based on their main light guiding mechanism. . . . . .

Figure 3.4: A ray of light incident at the interface between two dielectric materials and

refracted according to the Snell’s law. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 3.5: Dispersion diagram of PCF showing regions of light guidance by TIR and

photonic bandgap effect. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 3.6: Dispersion diagram of a water-core PCF.Figure 3.6: Dispersion diagram of

a water-core PCF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9

12

16

22

23

25

26

28

30

xii

Figure 3.7: Dispersion diagram of a photonic crystal as a function of (a) out-of-plane

wave vector, kz, and (b) in-plane wave vector at kza/2π = 1.7 in the

irreducible Brillouin zone (inset) for the triangular lattice of air holes with

period a and radius 0.47a in ε = 2.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 3.8: Schematic illustration of solid angles falling within the photonic bandgap

of a photonic crystal with a triangular lattice operating at a fixed frequency.

Figure 3.9: Raman scattering and signal collection in (a) conventional Raman

spectroscopy and (b) Raman spectroscopy using HC-PCF. . . . . . . . . . . . . .

Figure 3.10: Raman intensity of water mode centered at around 3400 cm-1

for varying

lengths of (a) HC-800 PCF and (b) TCT. . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 3.11: Comparison of silica Raman signals between water and thiol-capping

agent when they are filled into the central core of a HC-PCF. . . . . . . . . .

Figure 4.1: (a) Core-shell structure of CdTe QDs. (b) Molecular structure of capping

ligands. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 4.2: Experimental setup of HC-PCF in HR800 micro-Raman system. . . . . . . . .

Figure 4.3: Raman spectrum obtained when the laser light is focused onto the cladding,

holey region and the center of the central core of the HC-PCF when it is

filled with water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 4.4: Optical images of HC-PCF cross-section at the (a) fused end and (b) non-

fused end after the entire central air core is filled with water. . . . . . . . . . . .

Figure 4.5: Raman intensity comparison of the OH modes of water between

conventional Raman spectroscopy using a cover slide, selective filling of

PCF and non-selective filling of PCF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 4.6: Raman spectra of CdTe QDs with different thiol capping agents in the

lower Raman shift regime between 100 and 200 cm-1

. . . . . . . . . . . . . . . . . .

33

33

35

38

39

45

46

46

48

48

50

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Figure 4.7: (a) UV-vis spectra of the CdTe QDs (inset: fluorescent images of the QD

solutions). (b) PL profile of the QDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 4.8: Raman spectra of CdTe QDs with different thiol capping agents in the mid-

Raman shift regime between 200 and 310 cm-1

. . . . . . . . . . . . . . . . . . . . . . .

Figure 4.9: Raman spectra of CdTe QDs with different thiol capping agents in the high

Raman shift regime between 310 and 1750 cm-1

. . . . . . . . . . . . . . . . . . . . . .

Figure 5.1: Schematic of plasmon resonance for a metal sphere. . . . . . . . . . . . . . . . . . .

Figure 5.2: Molecular structure of (a) pegylated gold nanoparticle system (b)

phospholipid gold nanoparticle system. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 5.3: Scheme of the nanoparticle synthesis process for (a) pegylated and (b)

phospholipid gold nanoparticles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 5.4: Raman spectra of water obtained using cover slide, HC-800 and HC-1060.

Enhancement of the OH stretching mode at 3421 cm-1

is about 101 times

and 112 times relative to the conventional technique respectively. . . . . . . .

Figure 5.5: Raman spectrum of a fresh batch of gold nanoparticles stabilized by citrate.

Inset: enlarged spectrum showing carboxylate stretching modes of citrate. .

Figure 5.6: Raman spectrum of six-month-old gold nanoparticles stabilized by citrate.

Inset: enlarge spectrum showing carboxylate stretching modes of citrate. . .

Figure 5.7: Raman spectrum of pegylated gold nanoparticles adsorbed with MGITC

obtained with 0.17 mW. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 5.8: Raman spectrum of pegylated gold nanoparticles with MGITC adsorbed

and pure MGITC solution before background subtraction (a) and after

background subtraction (b). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

52

53

55

60

67

68

70

71

74

76

76

xiv

Figure 5.9: Raman spectrum of pegylated gold nanoparticles. Inset left: Raman mode

of Au–Cl at 259 cm-1

and Au–S at 305 cm-1

. Inset right: Stretching modes

of carboxylates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 5.10: UV–vis (a), TEM (b) and DLS (c) results obtained from a pegylated

nanoparticle system synthesized using the same method and materials. . .

Figure 5.11: Raman spectrum of 60 nm gold nanoparticles stabilized by citrate. Inset:

enlarged spectrum showing carboxylate stretching modes of citrate. . . . .

Figure 5.12: Raman spectrum of CV adsorbed gold nanoparticles and CV solution

without background subtraction (a) and with background subtraction (b).

Figure 5.13: Raman spectrum of CV adsorbed gold nanoparticles before and after

coating of phospholipid layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 5.14: UV–vis spectra of the phospholipid nanoparticles throughout the synthesis

process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 5.15: Raman spectra of MGITC absorbed pegylated gold nanoparticles with

increasing power from 0.17 mW (a) to 1.7 mW (b), 4.3 mW (c), 8.5 mW

(d) and 17 mW (e). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 5.16: Optical image of nanoparticle clusters around the inner wall of the PCF

core after about 1 minute of selective filling. . . . . . . . . . . . . . . . . . . . . . . .

Figure 5.17: Absorption of MGITC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 5.18: Raman spectra of citrate stabilized gold nanoparticles with increasing

power from 0.17 mW (a) to 1.7 mW (b), 4.3 mW (c), 8.5 mW (d) and 17

mW (e). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 5.19: Raman spectra of polymer encapsulated gold and silver nanoparticles

obtained from Vivo Nano Inc. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

78

79

82

83

85

86

88

90

90

92

94

xv

Figure 5.20: Raman spectra of CV adsorbed gold nanoparticles with increasing power

from 0.17 mW (a) to 1.7 mW (b), 4.3 mW (c), and 17 mW (d). . . . . . . . .

Figure 5.21: Raman spectra of citrate stabilized gold nanoparticles, used for

phospholipid nanoparticle synthesis, with increasing power from 0.17

mW (a) to 1.7 mW (b), 4.3 mW (c), and 8.5 mW (d). . . . . . . . . . . . . . . . .

Figure A.1: Directions of the electric field, magnetic field and wave vector when a TE

field is incident at the interface between two dielectric materials. . . . . . . .

Figure C.1: Raman spectrum of citrate stabilized gold nanoparticles obtained using

cover slide and HC PCF with 633 nm laser. . . . . . . . . . . . . . . . . . . . . . . . .

Figure C.2: Raman spectrum of pegylated gold nanoparticles obtained using cover

slide and HC-PCF with 633 nm laser. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure C.3: Raman spectra of MGITC adsorbed gold nanoparticles obtained using

cover slide and HC-PCF with 633 nm laser. . . . . . . . . . . . . . . . . . . . . . . .

Figure C.4: Raman spectra of citrate gold nanoparticles obtained using cover slide and

HC-PCF with 633 nm laser. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure C.5: Raman spectrum of CV adsorbed gold nanoparticles obtained using cuvette

with 785 nm laser, cover slide and HC-PCF with 633 nm laser. . . . . . . . . .

Figure C.6: Raman spectrum of phospholipid gold nanoparticles obtained using cuvette

with 785 nm, cover slide and HC-PCF with 633 nm laser. Phospholipid

was formed using (a) MHPC and DMPC and (b) SHPC and DMPC. . . . . .

Figure F.1: SEM image of the HC-1060 before (a) and after (b) fiber splicing to

collapse the cladding holes. The central core of the PCF is reduced to about

7 µm after the splicing process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure F.2: Comparison of water modes with HC-800, HC-1060, HC19-1550 and

cover slide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

96

98

107

111

112

113

114

115

116

125

125

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Figure F.3: SEM image of PCF facet after splicing with program modified from that

for HC-800. The central core of the PCF after splicing is about 17 – 18 µm

and all of the cladding holes are closed except the three corner holes. . . . .

Figure F.4: SEM image of PCF fused with fiber–electrode distance at (a) 30, (b) 50, (c)

70, (d) 80, and (e) 90 µm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure F.5: SEM image of three PCFs fused HC15-1990 PCFs at 50 µm from

electrode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure F.6: Optical image of HC19-1550 PCF selectively filled with lung cancer cells.

The bright light at the central core shows the cancer cell solution. . . . . . . .

126

128

129

130

xvii

List of Appendices

Appendix A Snell’s Law in Wave Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Appendix B Synthesis Method and Materials for Thiol-Capped CdTe Quantum Dots

Appendix C "anoparticle Signal Enhancement using HC-PCF . . . . . . . . . . . . . . . . . . .

C.1 Citrate Stabilized Gold Nanoparticles for Pegylated System . . . . . . . . . . . . . . . . . .

C.2 Pegylated Gold Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

C.3 Pegylated Gold Nanoparticles with MGITC Adsorbed . . . . . . . . . . . . . . . . . . . . . . .

C.4 Citrate Stabilized Gold Nanoparticles for Phospholipid System . . . . . . . . . . . . . . .

C.5 Gold Nanoparticles with CV Adsorbed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

C.6 Phospholipid Gold Nanoparticles with CV Adsorbed . . . . . . . . . . . . . . . . . . . . . . .

Appendix D Raman Mode Assignments of Thiol-Capped CdTe "anoparticles . . . . . .

Appendix E Raman Mode Assignments of Gold "anoparticle Systems . . . . . . . . . . . . .

E.1 Citrate Stabilized Gold Nanoparticles for Pegylated System . . . . . . . . . . . . . . . . . .

E.2 Pegylated Gold Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

E.3 Pegylated Gold Nanoparticles with MGITC Adsorbed . . . . . . . . . . . . . . . . . .. . . . . .

106

108

110

110

111

112

113

114

115

117

119

119

120

121

xviii

E.4 Citrate Stabilized Gold Nanoparticles for Phospholipid System . . . . . . . . . . . . . . . . . .

E.5 Gold Nanoparticles with CV Adsorbed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

E.6 Phospholipid Gold Nanoparticles with CV Adsorbed . . . . . . . . . . . . . . . . . . . . . . . . . .

Appendix F Fiber Splicing Program Optimization for Large Biological Molecules . . .

122

122

123

124

xix

List of Acronyms

CCD — Charge-Coupled Device

CV — Crystal Violet

CW — Continuous Wave

CZE — Capillary Zone Electrophoresis

DLS — Dynamic Light Scattering

DMPC — 1,2-Dimyristoyl-sn-Glycero-3-Phosphocholine (C36H72NO8P)

EL — Electroluminescence

EM — Electromagnetic

FDTD — Finite-Difference Time-Domain

FT-IR — Fourier-Transform Infrared Spectroscopy

FWHM — Full-Width at Half Maximum

HC–PCF — Hollow-Core Photonic Crystal Fiber

HRTEM — High Resolution Transmission Electron Microscope

KKR — Korringa-Kohn-Rostoker

MGITC — Malachite Green Isothiocyanate

MHPC — 1-Myristoyl-2-Hydroxy-sn-Glycero-3-Phosphocholine (C22H46NO7P)

MOF — Micro-Structured Optical Fiber

MPA — 3-Mercaptopropionic Acid

MSF — Micro-Structured Fiber

NA — Numerical Aperture

xx

PCF — Photonic Crystal Fiber

PEG — Polyethyleneglycol

PL — Photoluminescence/Photoluminescent

QD — Quantum Dot

SC–PCF — Solid-Core Photonic Crystal Fiber

SEM — Scanning Electron Microscope

SERS — Surface-Enhanced Raman Spectroscopy

SHPC — 1-Stearoyl-2-Hydroxy-sn-Glycero-3-Phosphocholine (C26H54NO7P)

SO — Surface Optical

SPP — Surface Plasmon Polariton

TCT — Telfon Capillary Tube

TE — Transverse Electric

TEM — Transmission Electron Microscope

TG — 1-Thioglycerol

TGA — Thioglycolic Acid

TIR — Total Internal Reflection

UV-Vis — Ultraviolet-visible Spectroscopy

XPS — X-Ray Photoelectron Spectroscopy

XRD — X-Ray Diffraction

xxi

List of Publications

Journal Article

(1) J. S. W. Mak, A. A. Farah, F. Chen, A. S. Helmy, “Photonic Crystal Fiber for Efficient

Raman Scattering of CdTe Quantum Dots in Aqueous Solution”, ACS Nano, vol. 5, pp

3823-3830 (2011).

International Conferences

(1) J. S. W. Mak, A. Farah, Feifan Chen, A. S. Helmy, “Efficient Raman Sensor for

Nanoparticles using Hollow Core Photonic Crystal Fiber,” OSA Sensors, Oral

Presentation, Toronto, June 12-15, 2011.

(2) J. S. W. Mak, A. Farah, Feifan Chen, Amr S. Helmy, “Probing Quantum Dot Cores,

Their Interfaces and Thiol Cappings Non-Destructively in Dilute Solution using Raman

Scattering in Hollow Core Photonic Crystal Fiber,” OSA/IEEE CLEO-QELS, Oral

Presentation, Baltimore, May 2-5, 2011.

(3) J. S. W. Mak, Abdiaziz A. Farah, Feifan Chen, A. S. Helmy, “Photonic Crystal Fiber for

Efficient Raman Scattering of Thiol-capped Quantum Dots in Aqueous Solution,” MRS

Spring Meeting, Oral Presentation, San Francisco, April 25-29, 2011.

xxii

Local Conferences

(1) J. S. W. Mak, F. Eftekhari, Abdiaziz A. Farah, S. Rutledge, A. S. Helmy, “Raman

Spectroscopy of Colloidal CdTe Quantum Dots using Teflon Capillary Tube,” Third

BiopSys All Network Meeting (not disclosed to public), Poster presentation, Toronto,

January 8-9, 2010.

(2) J. S. W. Mak, Abdiaziz A. Farah, F. Eftekhari, A. S. Helmy, “Raman Spectroscopy of

Thiol-Capped Colloidal CdTe Quantum Dots Using Capillary Tubes,” Fourth BiopSys

All Network Meeting (not disclosed to public), Poster presentation, Toronto, May 26-29,

2010.

(3) J. S. W. Mak, Abdiaziz A. Farah, F. Chen, A. S. Helmy, “Probing Quantum Dot Cores,

Their Interfaces and Thiol Cappings Non-Destructively in Dilute Solution using Raman

Scattering in Hollow Core Photonic Crystal Fiber,” Fifth BiopSys All Network Meeting

(not disclosed to public), Poster presentation, Toronto, January 6-7, 2010.

(4) J. S. W. Mak, N. Tam, S. Wang, G. Zheng, A. S. Helmy, “Low Power Raman Detection

of Lung Cancer Cells using Hollow-Core Photonic Crystal Fiber with Functionalized

Gold Nanoparticles,” Sixth BiopSys All Network Meeting (not disclosed to public),

Poster presentation, Toronto, June 16-17, 2010.

1

Chapter 1

Introduction

(1.1) Motivation

Nanotechnology has advanced significantly in the past several decades enabling innovative

solutions for many challenging problems in a wide field of applications. Colloidal nanoparticles

whose dimension in the nanometer range are one of the most innovative material in

nanotechnology over the past two decades as they exhibit unique material, optical, electrical and

thermal characteristics that are tuneable with the size, shape and composition of the particles.1

The surface of particles can also be functionalized with different dyes, polymers or lipids to

provide the necessary functions and properties required; for instance, water solubility, reduced

toxicity, signalling, biocompatibility and bioconjugation. Due to the large design variability,

many efforts have been put into improving the different synthesis methods or on designing the

particular nanoparticle system for an application. However, there have been little efforts in

improving the tools to characterize these novel systems in the nanometer scale.

2

One of the most important technological challenges in quantum dot (QD) advancement is the

development of a cost-effective, reliable and sensitive optical monitoring system to control the

physical, chemical and size-dependent properties of QDs before, during and after their

fabrication on a nanometer scale. Currently, many analytical techniques have been employed but

they rely on changes in the nanoparticles’ material compositions and the overall nanoparticle

properties. For example, photoluminescence (PL), electroluminescence (EL), and ultraviolet–

visible spectroscopy (UV-vis) provide changes in the luminescence and absorption of the

nanoparticles. Imaging techniques such as transmission electron microscope (TEM) or high

resolution transmission electron microscope (HRTEM) provides changes in the physical

geometry and composition. TEM, HRTEM, UV-vis as well as dynamic light scattering (DLS)

and capillary zone electrophoresis (CZE) provide quantitative information in the nanoparticles

size and size distributions. Other techniques such as X-ray diffraction (XRD), and X-ray

photoelectron spectroscopy (XPS) provide information only in the crystallinity and compositions

of the nanoparticles. None of these techniques is capable of describing the binding interactions of

molecules at the nanometer scale. Therefore, the impact of the interactions on the overall

molecular structure, molecular complex, and different QD properties remain unclear. In situ

monitoring of the nanoparticle synthesis also remains challenging. Consequently, this limits our

capability to improve the properties and functionalities further from what has been achieved

today. Complex QD designs for increasing performance and functionalities in different

applications remain very challenging as a result.

To observe detail binding interactions at the molecular level, conventional techniques involve

using Fourier-transform infrared spectroscopy (FT-IR) and Raman spectroscopy whose signals

contain “fingerprints” of the molecular structures. However, strong and broad vibrational modes

3

(signals) of water in FT-IR interfere with the weak vibrational modes of nanoparticles which

severely limit its capability to obtain information on the nanoparticles and the ligands. On the

other hand, water signal is very weak in the complementary technique, Raman spectroscopy.

However, Raman signals are also extremely weak in aqueous or dilute solutions. In situ

monitoring of the nanoparticles using conventional Raman spectroscopy therefore remained very

challenging because of the solution-based environments in which nanoparticles are produced.

Surface enhanced Raman spectroscopy (SERS) can be used to enhance the nanoparticle signals;

however, metallic nanoparticles have to be mixed with the QD solution, which could alter the

structure and properties of the QDs from its as-synthesized state.

In the past decade, many fiber based platforms have been developed to enhance the sensitivity of

conventional Raman spectroscopy for particle or molecule analysis through efficient induction

and generation of Raman scattering signals. For example, Teflon Capillary Tube (TCT) was

demonstrated to enhance the detected Raman signal of aqueous based solutions, such as dilute

toluene,2 sodium carbonate,

2 benzene

2 and creatinine solutions,

3 through confining the pump

laser in the same fiber core with the solution by total internal reflection (TIR). Hollow-core silica

optical fiber was demonstrated to enhance the Raman mode of dilute carbon tetrachloride

solutions also using the same confinement mechanism.4 In addition to fiber platforms that utilize

TIR for light confinement, photonic crystal fibers (PCFs) have been utilized to enhance the

Raman sensitivity in which light is being confined by the photonic bandgap effect. In particular,

solid-core PCF (SC-PCF) was utilized in which the sample solution is confined in the cladding

holes while the pump laser is confined weakly in the solid core. Raman signals are generated by

the evanescent wave that is overlapped with the solution. 4-Mercaptobenzoic acid,5 acetonitrile

6

and Rhodamine 6G solutions7 were characterized using this technique. However, there are

4

several drawbacks found including weaker interactions with evanescence wave, smaller cladding

holes limiting particle sizes and strong signal interferences from the silica fiber. Another type of

PCF is named hollow-core PCF (HC-PCF) in which the core of the PCF is hollow allowing

solutions to be filled inside and interact with the pump laser directly. Rhodamine B was tested

using this technique.8 The solution was filled into both the core and the cladding holes of the

PCF which shifted the bandgap of the fiber and guided the pump laser weakly by the photonic

bandgap effect. It is important to mention that the characterization of nanoparticles using these

techniques were not achieved due to the low Raman cross-section of nanoparticles and the

limited sensitivity of these fiber based platforms for Raman spectroscopy.

To enable molecular level monitoring of the nanoparticles, a novel technology utilizing HC-PCF

for sensitive Raman spectroscopy has recently been developed in our research group.9, 10

The

novelty of our technique is that we can selectively fill the sample solution into only the central

core of the PCF. This enables both the pump laser and the Raman scattering signal to be guided

by both TIR and photonic bandgap effects which allow efficient induction and collection of

Raman scattering signal from every single drop of solution available. This technique is capable

of enhancing the detected Raman scattering signal for about 2 – 3 orders of magnitude without

the need of adding or mixing extra chemicals or materials with the analyte for enhancements,

which is at least an order of magnitude greater than the other fiber based Raman platforms

described. Moreover, this technique retains both the nondestructive property of Raman

spectroscopy and the high flexibility and compactness of optical fibers which make it most

suitable for remote sensing and in-situ detections of nanoparticles in their native states. This

technique has been utilized successfully for characterizing ZnO and CdTe nanoparticles post-

synthesized with rapid thermal annealing.9, 10

5

(1.2) Thesis Objectives

The goal of this thesis is two-fold. First, it is to show that the use of HC-PCF with Raman

spectroscopy can enhance the Raman scattering signal obtained from a wider variety of colloidal

nanoparticles when compared to conventional Raman Spectroscopy. Second, with enhancements

of the detected Raman signal from the novel PCF platform, we want to improve our

understanding of semiconductor and metallic nanoparticles and their binding interactions with

different ligands at the molecular level.

In particular, this thesis aims to confirm the ligand-particle binding speculated in the thiol-

capped CdTe quantum dots and correlate this information to the different properties reported in

the literature. In addition, this thesis aims to study the ligand-particle binding in different gold

nanoparticle systems throughout the entire synthesis process, which has never been achieved

before with Raman spectroscopy due to the limited sensitivity of conventional Raman technique.

Furthermore, this thesis aims to achieve a real time Raman study of the gold nanoparticles at

different measurement conditions to show laser induced effects that limit many nanoparticle

studies with Raman spectroscopy.

(1.3) Organization of Thesis

The structure of this thesis is mainly consisted of two parts. The first part involves the

fundamentals and theoretical aspects of Raman spectroscopy (Chapter 2) and PCF (Chapter 3).

In chapter 2, classical and quantum mechanical treatments of Raman spectroscopy are reviewed

to offer insights into the important parameters governing the Raman effects. In particular, the

efficiency of the Raman scattering signal is discussed and compared with other optical effects.

These analyses demonstrate an important point that the detectable Raman scattering signal is an

6

extremely weak signal compare to other optical events. Thus, an effort to enhance the detected

Raman scattering signal is required to ultimately take advantages of its “fingerprint” features for

molecular analysis and identification. To realize the many efforts that companies have made to

achieve sensitive Raman spectroscopy up until today, the design and operation of one of the most

sensitive Raman system built by Horriba, namely the LabRAM HR800, is also discussed.

Chapter 3 covers the fundamentals and guiding mechanism in PCF. Both the index and bandgap

guiding mechanisms are discussed in detail to provide a theatrical understanding of the PCF

operation. In addition, section 3.4 presents our novel technique to enhance the detected Raman

signal utilizing the hollow core type of PCF. The improved light collection efficiency and

increased light-matter interactions from conventional Raman spectroscopy are analyzed

theatrically and experimentally to demonstrate their different contributions to the detected

Raman signal enhancement. The detected Raman signal enhancements with the PCF are also

compared to the liquid core fiber, namely TCT, to demonstrate the unique advantage of PCF’s

light guiding mechanisms. Furthermore, the advantages of fiber-based detection and metal-free

detection are briefly discussed.

The second part of this thesis discusses the experimental details and results on characterizing

thiol-capped CdTe nanoparticles (Chapter 4) and metallic nanoparticle systems (Chapter 5).

Chapter 4 presents the experimental setups and results on the Raman analysis of thiol-capped

CdTe nanoparticles. One very important feature of our PCF platform described in this chapter is

the technique to selectively fill the central core of the PCF with the liquid sample of interest.

This technique enables the pump laser light and the Raman scattering signals to be guided by

both the index and photonic bandgap confinement; which subsequently lead to an enhancement

7

of the detected Raman signal. A comparison of the Raman signal between conventional

techniques, non-selectively filled PCF and selectively filled PCF shown in Figure 4.7

demonstrates the large sensitivity improvement with our selective filling technique on PCF. This

improved sensitivity enables detailed molecular bindings between the ligand and the nanoparticle

core to be observed as well as vibrational modes of the CdTe core and its contained defects.

These modes are further correlated to the QD’s PL quantum efficiency, stability, water solubility

and bio-conjugation capability. To the best of our knowledge, this is the first time that such

strong Raman modes of the thiol-capped CdTe QDs in aqueous solution have been reported.

Chapter 5 presents the characterization results of two metallic nanoparticle systems for cancer

detection. Raman spectra of pegylated and phospholipid gold nanoparticles obtained at different

stages of the synthesis process are presented and correlated to the structural changes of the

nanoparticles. These results confirmed the stability of the stabilizer, binding of the Raman dye

and coating of the polymer and phospholipid on the nanoparticle surface. Further, these results

demonstrate that efficient Raman scattering of the PCF enables metallic nanoparticle systems to

be characterized through the entire synthesis process; thus, allowing nanoparticles to be

monitored in situ for quality control. As these nanoparticles are designed for cancer detections

with Raman imaging, this chapter also investigates, in real time, the stability of the two

nanoparticle systems under different pump power exposure and exposure durations. Structural

changes in the nanoparticle system are observed with the enhanced Raman signal using HC-PCF.

These changes show the power and exposure time limit in which the pump laser can be focused

onto the nanoparticle systems for non-destructive measurements.

Finally, chapter 6 summarizes the work discussed in this thesis and provides future directions.

8

Chapter 2

Raman Spectroscopy

Raman spectroscopy is a rapid and non-destructive means of probing molecular vibrations

optically through inelastic scattering. This technique provides the unique vibrational modes or

“fingerprints” of the molecules that enable similar molecular structures to be unambiguously

distinguished and identified. This high specificity of the vibrational information to chemical

bonds and structures has led Raman to become a powerful tool in chemistry and medicine since

its first discovery in 1928 by C. V. Raman and K. S. Krishnan.11

Since Raman is an optical

measuring technique, it would involve light interacting with matter. Therefore, we will start this

chapter by discussing the physics behind light-matter interaction; this will lead to the discussion

of the Raman effect in two scales – macroscopically through classical physics, and

microscopically through quantum mechanics. We will then describe the experimental aspect of

Raman spectroscopy and examine the setup and operation of the Raman spectrometer used in

this experiment. Finally, the efficiency of Raman scattering will be discussed to provide an

9

overview of the Raman signal strength compared to other possible optical processes in

spectroscopy and between various molecules of interest in nanotechnology.

(2.1) Light-Matter Interaction

In physics, light is an electromagnetic (EM) radiation that lies in the range of the infrared, visible

light or ultraviolet in the EM spectrum. When light interacts with matters, it can be reflected,

absorbed, scattered, or passed straight through the material without any interaction (Figure 2.1).

If light is scattered by the material, it can be scattered either elastically or inelastically. An

elastically scattered EM radiation is generally called Rayleigh scattering. In Rayleigh scattering,

the EM radiation is scattered by particles that are much smaller than the EM radiation

wavelength upon interaction. The scattered light will have a wavelength that is identical to the

incident light; thus, energy is conserved during the scattering process. If the scattered light has a

wavelength that is different than the incident light, then there is an energy exchange between the

incident light and the material during the interaction. Energy of the incident light could be gained

or lost through the vibrational motion of the molecules in the material causing inelastic scattering

of light. This process is called Raman scattering. In the next two sections, we will look at this

Raman scattering process macroscopically using classical theory of physics and microscopically

using quantum mechanics.

Figure 2.1: Possible outcomes of light-matter interaction.

10

(2.2) Classical Treatment of Raman Scattering

On the macroscopic level, Raman scattering can be viewed as light emitted from oscillating

dipoles in a material induced from EM radiation. Theoretically, this phenomenon can be

described using classical physics as follows;11, 12

consider a monochromatic light (or an EM

wave) propagating in the z-direction with an oscillating electric field in the x-direction. The

strength of the electric field (Ex) at any time (t) can be expressed as:

= cos2 (2.1)

where is the maximum amplitude of the electric field and is the frequency of the

monochromatic light. When this light interacts with a material, its electric field will polarize the

cloud of electrons around each of the molecules in the material and displace them from their

equilibrium position. These electron displacements will oscillate with the electric field and

behave as an oscillating electric dipole. The oscillating electric dipole then emits light (the

scattered light) due to its oscillating electric current. Since the scattered light is dependent on the

oscillation of the electric dipole, changes of the dipole polarity will indicate how frequent the

dipole is oscillating; hence, the frequency of the radiating light. The polarity of a dipole is

determined by its dipole moment (µ):

= = cos2 (2.2)

where α is the polarizability tensor that describes the tendency of electron clouds distortion in the

presence of an external electric field. If the molecule is vibrating with a natural frequency (,

the nuclear displacement q can be written as:

= cos2 (2.3)

where is the vibrational amplitude. For small amplitude of vibration, α can be expressed as a

linear function of q using Taylor series:

11

= +

+ ⋯ (2.4)

Combining (2.2) with the first two terms of (2.4), the dipole moment can be expressed as:

= cos2 = cos2 +

cos2 = cos2 +

cos2 cos2

= cos2

+

cos!2 + " + cos!2 − "$ (2.5)

According to classical theory, the first term in (2.5) represents an oscillating dipole that radiates

light at the frequency which is the same frequency as the incident light; therefore this term

corresponds to Rayleigh scattering. In contrast, the second and third term represents an

oscillating dipole that radiates light at the frequency + and − respectively. These

radiating lights have frequency that is different than the frequency of the incident light; thus, they

are inelastically scattered or Raman scattered. The radiating light with frequency higher than that

of the incident light (i.e. + ) is referred to as anti-Stokes scattering. The radiated light

with frequency lower than that of the incident light (i.e. − ) is referred to as Stokes

scattering. It is important to note that both the Raman scattering terms can vanish if is zero

in certain vibrations. These vibrations are not Raman-active and cannot yield Raman scattering.

Thus, the rate of change of the polarizability must not be zero for vibrations to be Raman-active.

This sets the selection rule for determining which modes of vibration in a molecule yields the

Raman effect. It is not obvious from the equation, but is also implicitly smaller than ;

therefore, Raman scattering is much weaker than Rayleigh scattering, generally by three orders

of magnitude as we will discuss in section 2.5.

12

(2.3) Quantum Mechanical Treatment of Raman Scattering

On the microscopic level, Raman scattering can be described using a semi-phenomenological

model of quantum mechanics called the energy transfer model.11, 13

In this model, light can be

considered as a collection of a basic unit or “particle” called photon. The energy (E) of a photon

is proportional to the frequency of the monochromatic light (v) as follows:

= ℎ (2.6)

where h is the Planck’s constant. Molecules of the material that is interacting with light are

considered systems with many different energy states. The lowest possible state is named the

ground state, whereas all the other higher energy states are considered as the excited states.

When a collection of photons is incident onto a material and being scattered, molecules of the

material are virtually excited to a higher energy state from its initial energy state by the amount

of energy equal to the incident photon. Instantaneously, these excited molecules would return to

a lower state and emit a photon (the scattered light) with an energy that is lower, higher or equal

to the energy of the incident photon. Figure 2.2 shows the schematic of the three possible energy

transitions in the scattering phenomenon. Assuming the molecule interacting with light is

initially sat in its ground state. The molecule will be excited to a virtual state, an energy state that

lies between the electronic states of the molecule which does not actually exist, and descend

instantaneously to the lower energy states emitting a photon with energy that is equal to the

difference between the two energy states. If this lower energy state is the same as the initial state

before the transition occurred (i.e. the ground state in this case), then the emitted photon will

have the same energy as the initial photon; thus, the frequency of the scattered light is equal to

that of the incident light, namely the Rayleigh scattering. If the molecule returns to a energy state

higher than the initial state (i.e. an excited state rather than the ground state in this case), then the

13

emitted photon will have energy less than that of the initial photon; hence, scattering light with

frequency lower than that of the incident light which correspond to the Stokes lines of Raman

scattering. In order for the molecule to return to its initial equilibrium state or ground state, the

molecule will emit a phonon, which is a basic unit of vibrational energy, with energy equal to the

difference between the excited state and the final equilibrium state. The energy of the phonon is

also proportional to the vibrational frequency of the molecule by Planck’s constant (h) like

photon:

'() = ℎ'() (2.7)

If the molecule is initially sat in an excited state, the molecule might return to a state lower than

that of the initial state after it has been excited. In this case, the molecule will emit a photon with

energy greater than that of the incident photon; hence, the scattered light will have a frequency

greater than that of the incident light which corresponds to the anti-Stokes lines of Raman

scattering. The greater energy in the scattered photon is gained from the phonon that is initially

present in the molecule and retaining the molecule at an excited state primarily.

Figure 2.2: Three possible energy transitions of molecule during the light scattering process.

14

In the viewpoint of the energy transfer model, two characteristics of Raman scattering can be

deduced: first, the frequency of the Raman scattered light is relative to the electronic states of the

molecule. Hence, any perturbation field such as strain field, thermal excitation or chemical

potentials resulting from the surrounding environment can affect the Raman peak position.

Second, the number of photons scattered (also corresponding to the intensity of the scattered

light) in the Stokes and anti-Stokes lines are related to the initial state that the molecule is at.

According to Boltzmann’s distribution, most molecules are likely to be resided in the ground

vibrational state at room temperature prior to light interaction. Higher energy states will be

populated with an exponentially decreasing number of electrons as the energy of the state

increases. This implies that majority of the Raman scattered photons will be Stokes scattering

and so most Raman scattered photon will have a lower energy (and lower frequency) than the

incident photon.

It is important to emphasize that the molecule is only virtually excited and the excited state by

the incident photon is also a virtual state in scattering. The word virtual is used because such

excitation of the molecule never existed and the incident photon is never actually absorbed since

energy cannot conserve upon absorption of the incident photon when there is no actual excited

electronic state present in the molecule with energy equal to that of the incident photon. One may

also consider this process as an excitation with a zero lifetime in the virtual state; thus the

molecule will descend to a lower state instantaneously. However, if the incident photon energy

approaches electronic transition energy, a more intensity scattered light would be observed.

15

(2.4) Raman System Setup

The Raman system used in this thesis is a high resolution LabRam HR 800 system manufactured

by Horiba Jobin Yvon. This is a confocal Raman system that collects Raman spectra in the back-

scattering geometry. The main components of the system are as follows:

Laser An internal HeNe laser is equipped in the LabRam Raman system for sample excitation.

The specifications of the laser are summarized in the following table:

Table 2.1: Laser specifications.

Parameters Value

Wavelength 632.817 nm

Max. Power 17 mW

Polarization ratio 500:1

Density Filter Wheel A wheel of five different optical density (OD) filters is installed to

reduce the laser power at different levels. The OD of the filters ranges from 0.3 to 4. An option

of no filter is also available in the system. The power of the laser passing through an OD filter

(Pout) can be determined as follow:

*+, = *(-1001 (2.8)

where Pin is the laser power entering the OD filter. The output powers to the different OD filters

with the equipped 17 mW HeNe laser are listed in the table below:

16

Table 2.2: Output powers of a 17 mW laser passing through the different optical density filters.

Filter Output Power

(mW)

D0.3 8.5

D0.6 4.25

D1 1.7

D2 0.17

D3 0.017

D4 0.0017

Microscope Objectives Three Olympus M PLAN objectives are equipped in the Raman system

to focus the excitation light onto the sample. The magnifications, numerical apertures (NA), and

working distances of the three objectives are listed in the table below:

Table 2.3: Specifications of the Olympus M PLAN objectives.

Magnification NA Working Distance

(mm)

10x 0.25 10.6

50x 0.7 0.38

100x 0.9 0.21

"otch Filter A holographic notch filter is used to reject the excitation line < 120 cm-1

in the

Stokes edge.

Confocal Hole A confocal hole adjustable between 0 and 1000 µm is available through the

LabRam software to maximize the Raman scattering signal and adjust the field-of-depth in a

Raman measurement.

Spectrograph The HR800 is equipped with an asymmetric Czerny Turner spectrograph of two

different diffraction gratings (1800 g/mm and 600 g/mm) that separates an incoming light into a

frequency spectrum. The 1800 g/mm grating provides a higher wavenumber resolution in a

17

Raman measurement than the 600 g/mm grating, but also a relatively shorter scanning range per

scan. The scanning range and resolution of the two gratings are listed in the table below:

Table 2.4: Scanning range and resolution of diffraction gratings. A 1000µm confocal hole size

is used as a reference for resolution comparison.

Grating

(g/mm)

Possible Scanning

Range (nm)

Scanning Range

per Scan (cm-1

)

Resolution

(cm-1

)

600 0-2850 1241.2 ±2

1800 0-950 321.6 ±1

Detector A standard air cooled silicon charge-coupled device (CCD) detector is equipped in the

Raman system. The resolution of the detector is 1024 x 256 pixels of 26 microns.

Figure 2.3: Schematic of the LabRam HR 800 Raman system.14

18

The setup of the Raman system is illustrated in Figure 2.3. The control and operation of the main

system components are mainly done through the provided software called LabSpec. When

acquiring a Raman spectrum, the excitation laser is first reflected by a mirror to the density filter

wheel for power adjustment. The output laser is then reflected by two more mirrors to the notch

filter at an angle that enables all of the excitation light to be reflected and focused onto the

sample through a microscope objective. The scattered light from the sample will then be

collected back by the same objective and directed through the notch filter to remove all the

excitation light and Rayleigh scattered light. The remaining Raman scattered light will then be

reflected with the forth mirror to the spectrograph through a confocal hole, two focusing lens and

an entrance slit. The Raman scattered light will be separated into the different frequency

components in the spectrograph and the Raman spectrum will be outputted through the CCD

detector.

(2.5) Raman Scattering Efficiency

When light is incident on a material, various interactions can take place simultaneously as

discussed in section 2.1. Some of the light may be absorbed by the material; others may be

Rayleigh scattered. Some of it will even cause fluorescence, which emits light after absorbing the

incident light. In most cases, only a very small portion of the incident light will be Raman

scattered. In order to compare the efficiency of the Raman scattering process with other optical

processes or among the different molecules of interest, a common measure is often desired.

Commonly, the cross-section, σ, or the differential cross-section, dσ/dΩ, is used.13, 15

The cross-

section of an optical process is defined as the ratio of the rate of photon in the optical process in

all directions to the rate of photons striking a molecule. The unit of σ is in m2 or cm

2; therefore, it

19

can be interpreted as the area of molecule that produces a particular optical process. For

example, in Raman scattering, we can define the Raman cross-section (σRS) as follows:

234 = 5345

= *34

(2.9)

where IRS and PRS are the total Raman scattered light average all over random molecular

orientations in photons/s and in W respectively, and I0 and E0 are the flux (photons/s/m2) and

irradiance (W/m2) of the incoming photons. The Raman cross-section, σRS, can be described as

the area of a molecule that produces Raman scattering when the incident light has a photon flux

of Io. The differential Raman scattering cross-section will simply be the variations of the Raman

cross-section with respect to the solid angle. Theoretically, the Raman cross-section is also

proportional to the polarizability derivative for vibrational transition from state m to state n and

the scattering frequency as follows:

234 = 6789|′-| (2.10)

where < = , the polarizability derivative derived in section 2.2. ωs is the frequency of the

scattered light and C is a numerical constant.

Typical values for cross-sections of the different optical processes are shown in Table 2.5. A

comparison of the cross-section for Raman scattering with absorption, fluorescence and Rayleigh

scattering indicates that Raman scattering is the least efficient among the different optical

processes. This shows that the Raman technique is inherently insensitive. Moreover, the cross-

section for Raman scattering is 10 orders of magnitude weaker than fluorescence and 3 orders of

magnitude weaker than Rayleigh scattering. Significant interference from these processes

prevents the Raman technique from being useful in many applications. Nevertheless, significant

enhancement in the Raman cross-section can be achieved if the excitation laser resonates with

20

the electronic transition state of the molecule, or surface enhancement is provided by metallic

particles.

Table 2.5: Approximate order of magnitude for cross-sections (per molecule), σ, of the different

optical processes.15

Process Cross-section of σ (cm2)

Absorption Ultraviolet 10-18

Absorption Infrared 10-21

Emission Fluorescence 10-19

Scattering Rayleigh Scattering 10-26

Scattering Raman Scattering 10-29

Scattering Resonance Raman 10-24

Scattering Surface Enhanced Raman Scattering 10-16

21

Chapter 3

Photonic Crystal Fibers

In the last few decades, a new frontier of research has been conducted to explore ways of

engineering optical properties of materials to control light propagation. One solution to the

problem is found to be photonic crystal, which is a periodic structure of different dielectric

constants or refractive indices on the scale of the wavelength of light. This periodic structure

produces a so-called photonic bandgap, in analogy to the electronic bandgap, in which some

frequencies of light are completely reflected in a certain direction and cannot propagate through

the structure.16

This phenomenon is caused by the constructive and destructive interferences of

the incident and reflected waves from the periodic layers of the structure.17

At each layer of the

periodic structure, light is partially reflected and partially transmitted. If the spacing between

adjacent layers is periodic at the wavelength of light, the multiple reflections can interfere

destructively to eliminate the forward propagating wave and constructively to form the reflected

wave (Figure 3.1). Hence, this structure can act like a mirror which reflects light at the desired

wavelengths by controlling the refractive index of materials in the structure and the spacing

22

between each layer. In two and three dimensional systems, a similar photonic crystal can also be

designed with the appropriate lattice structure and refractive index contrast to achieve a photonic

bandgap.

Figure 3.1: Wave interferences in the photonic bandgap of a one dimensional periodic material.

(1) Wave incident on the material, (2) wave partially reflected off each layer of the structure, (3)

the reflected wave combined with the incident wave to produce a standing wave that does not

travel through the material.17

In 1996, Russell et al. utilized this concept and demonstrated the first fiber with a photonic

crystal cladding.18

This new type of fiber is named the photonic crystal fiber (PCF) (Figure 3.2).

Often, they are also referred to as the micro-structured fiber (MSF), micro-structured optical

fiber (MOF) or holey fiber. Unlike conventional optical fiber which guides light by the TIR

effect of light, PCF can guide light using the photonic bandgap formed in the cladding that

features a two-dimensional periodic triangular structure of large air holes. The core of the PCF in

which light can propagate in is usually a defect formed by removing several air holes in the

center of the periodic structure that runs along the entire fiber length. This defect can be a solid

material like silica, or a large air core that allows gas or liquid to be filled inside while light is

propagating through. In the past decade, many different configurations of the PCF have been

23

designed for various applications of light delivery such as high power, high non-linearity, large

mode area, high birefringent, and high NA. In this chapter, we will first classify the different

types of PCF that has been designed since the first demonstration of PCF. Then, we will discuss

the two possible guiding mechanisms in these PCFs and the different regimes that they can

operate in. Finally, we will focus on a particular type of PCF, named the Hollow-core PCF (HC-

PCF), and discuss how it can be utilized to enhance the Raman scattering signal from aqueous

and biological solutions, which forms the basis of this thesis.

Figure 3.2: Cross-section of a HC-PCF taken by scanning electron microscope (SEM). The

cladding is composed of a holey region which is a two-dimensional photonic crystal surrounded

by a normal silica coating. The core is a defect formed by removing seven holes in the center of

the photonic crystal.

(3.1) Classifications of Photonic Crystal Fibers

PCF is a general term used to describe fibers that utilize a periodic distribution of holes or

dielectric rods in the cladding to form the waveguiding properties of fiber. Although the term

“photonic crystal” refers to structures that form a photonic bandgap in which certain wavelength

of light cannot propagate through, the main guiding mechanism of a PCF does not necessarily

utilize this photonic bandgap effect. If the refractive index of the fiber core is designed to be

24

greater than that of the cladding, TIR is still mainly responsible for the guidance of light even

though the cladding is composed of a periodic structure. At certain propagation directions where

light is not guided by TIR, the photonic bandgap effect may still guide the light if the photonic

bandgap exist. For example, a solid-core PCF, in which the core of the fiber is made out of silica

and the cladding is formed by a periodic distribution of air holes, would guide light in the silica

core with TIR because the average refractive index of the air hole cladding is less than that of the

silica core. In contrast, a HC-PCF in which the same cladding structure is used as the solid-core

PCF but the central core is changed to air would guide light in the central air core due to the

existence of photonic bandgaps in the fiber cladding. However, if the central air core of a HC-

PCF is filled with some liquid or gas, light can also be guided by both TIR and photonic bandgap

effects. Figure 3.3 classifies some of the different designs of PCF based on their main guiding

mechanism.

In the next two sections, we will describe the physical mechanisms of both TIR and photonic

bandgap effect using the wave theory of light in PCF. The different regimes of light propagation

will be discussed in detail.

25

Figure 3.3: Classification of PCFs based on their main light guiding mechanism.19

(3.2) Index Guiding Mechanism

Similar to conventional optical fibers, PCF can guide light using TIR when the refractive index

of the core is higher than the average refractive index of the cladding. This phenomenon is

usually described using Snell’s law in ray optics (Figure 3.4):

= sin @ = = sin @ (2.1)

26

where = is the refractive index of the material where the incident ray is in and @ is the angle the

incident ray makes with the plane normal to the interface. = is the refractive index of the

material where the transmitted ray is in and @ is the angle that the transmitted ray makes with

the normal to the interface. When a ray of light is incident at an interface between two materials,

the angle of the transmitted ray follows the relation of Snell’s law. As @ increases, @ also

increases. At the critical angle (@A, @ = @A = sinC= =⁄ , @ reaches a 90o angle in which

the transmitted ray parallels the interface if = < =. At this angle or greater (i.e. @ ≥ @A), the

incident ray is interpreted to be totally reflected at the interface. This means that light can

propagate through a high index medium, such as water or silica, when it is surrounded by a lower

index medium, such as webs of air holes.

Figure 3.4: A ray of light incident at the interface between two dielectric materials and refracted

according to Snell’s law.

Although Snell’s law is able to describe how light can confine and propagate inside a fiber, its

description is limited in the ray optics regime in which it is only valid when light is propagating

through and around materials in the length scale much greater than the wavelength of light.

Hence, it is unable to explain phenomenon that are resulted from the wave properties of light,

27

such as interference. Since the photonic bandgap effect of a periodic material is a result of the

constructive and destructive interferences of the incident and reflected waves from each layer of

the structure, Snell’s law is not sufficient to explain its effects. In order to realize the different

regimes in which TIR and photonic bandgap guide light in a PCF structure, we will need to

generalize Snell’s law to explain both of the effects.20

One way to accomplish this is to utilize the

wave theory of light which is modeled by Maxwell’s equations.

In the wave model of light, Snell’s law is simply the conservation of both the light frequency, ω,

and the component of the wave vector that is parallel to the interface of the two materials, k||, as a

result of phase matching the incident, reflected and transmitted electric field of the wave due to

boundary condition (Appendix A). This implies that we can study band structures of EM modes

(i.e. the dispersion diagram or band diagram), relating ω and k|| (the two conserved quantities), to

determine regimes that TIR can operate in PCF. At the same time, energy gaps resulted from the

periodic structure of the photonic crystal cladding can also be determined. Let us consider a PCF

composed of a high-index material, nh, and a low-index material, nl, such as silica glass and air.

The dispersion relation of the two materials, 7 = A-G

H|| and 7 = A-I

H||, divides the dispersion plot

into three regions, independent of the geometry of the core and cladding, as shown in figure

3.5.21

The wave properties in each of the three regions can be determined by the transverse wave

vector (i.e. the component of the wave vector that is perpendicular to the interface), HJ, where

HJ = KLA = − H||.

Figure 3.5: Dispersion diagram of PCF showing regions of light guidance by TIR and photonic

bandgap effect. H|| is the component of the wave vector in the invariant direction.

of the periodic air holes in the photonic crystal.

For H|| M LA =N, the only solutions exist are those with

OPKLA = − H|| ). This means that the wave is evanescent in both high and low materials; thus,

no guiding mode exists and light cannot be guided inside the fiber.

For H|| < LA =Q, HJ can take any real values which means that waves can propagate freely in both

materials. Thus, light is not confined by either material unless there is a photonic bandgap in

which certain energy cannot propagate in the material.

For LA =N M H|| M LA =Q, HJ can take any real values in the high index material but is imaginary in

the low index material. This means that the wave can propagate inside the high index material

28

Dispersion diagram of PCF showing regions of light guidance by TIR and photonic

is the component of the wave vector in the invariant direction.

of the periodic air holes in the photonic crystal.

, the only solutions exist are those with HJ imaginary (i.e HJ =

). This means that the wave is evanescent in both high and low materials; thus,

no guiding mode exists and light cannot be guided inside the fiber.

an take any real values which means that waves can propagate freely in both


which certain energy cannot propagate in the material.

can take any real values in the high index material but is imaginary in


Dispersion diagram of PCF showing regions of light guidance by TIR and photonic

is the component of the wave vector in the invariant direction. a is the period

=

). This means that the wave is evanescent in both high and low materials; thus,

an take any real values which means that waves can propagate freely in both


can take any real values in the high index material but is imaginary in


29

while it is evanescent in the low index material. Therefore, if we have a PCF or any fiber where

the index of the core is higher than the average index of the cladding, light has to be confined in

the high index core where the light wave can propagate. The wave would not be able to

propagate through the cladding as it becomes evanescent in the low index material. Hence, light

is guided by TIR within this region. However, photonic bandgap can also exist in this region as

well; thus, photonic bandgap confinement is also possible in this regime.

Figure 3.6 shows a more concrete example of a dispersion diagram for the case of a liquid filled

HC-PCF. In this example, the HC-PCF has a cladding of periodic air holes in silica and a central

core that is filled with water. The dispersion relations of the four different refractive indices in

the fiber (i.e. air, silica, photonic crystal, and water) divides the plot into five different regimes in

which light can propagate in different set of materials or structure. In regime 1 and 2, light can

propagate in both the photonic crystal and water; thus, light can only be guided in the water core

by photonic bandgaps. On the contrary, in regime 3, light can only be guided in water but not in

the photonic crystal cladding; thus, light can be guided by total internal reflections due to the

refractive index difference between water and the photonic crystal cladding. In regime 4 and 5,

light cannot propagate in both the water core and the photonic crystal cladding. Therefore, no

guiding mode is possible in these two regimes. If a narrowband light wave with frequency

centered at the white line in Figure 3.6 is launch into the water-core PCF, this light would

propagate through the water core in three different ways: 1) through translation symmetry along

the fiber axis, 2) through photonic bandgap guidance at the red dot and 3) through TIR along the

red line. This demonstrates the different modes of operation in a liquid core PCF.

30

Figure 3.6: Dispersion diagram of a water-core PCF. In regime 1 and 2, light is free to

propagate in both the water core and the photonic crystal cladding except at the photonic

bandgaps indicated by black lines. In regime 3, light is free to propagate in the water core but

evanescent in the photonic crystal cladding; thus, light can be guided in the water core by TIR. In

regime 4 and 5, light is evanescent in both the water core and the photonic crystal cladding;

therefore, no guidance mode is possible.

(3.3) Photonic Bandgap Guiding Mechanism

In addition to the conventional way of light guidance by TIR, a new type of guiding mechanism

is introduced by the PCF, namely the photonic bandgap guidance. This mechanism is based on

the use of a photonic crystal to prohibit the existence of a certain energy in the cladding of the

PCF; thus, preventing a certain wavelength of light from escaping the core of the fiber. The

region where light cannot exist in the dispersion plot of a photonic crystal is called the photonic

bandgap. If we operate within the photonic bandgap regime of the photonic crystal cladding,

light can propagate through the central core of the PCF even if the core index is less than that of

31

the cladding (i.e. when light guidance with TIR is not possible). This implies that light can now

be guided in an air, gas, or liquid core of a PCF with proper design of the PCF structure.

Theoretically, the photonic bandgap of a photonic crystal can be studied through the dispersion

diagram discussed in section 3.2. The dispersion relation can be determined by first deriving a

translational relation between electric field of two adjacent unit cells through imposing

continuity at the interfaces. According to the Bloch theorem, the electric field vector of a normal

mode of propagation in a periodic medium (i.e. medium periodic in the z-direction by

convention) takes the form

R = RSTUC(VWU(X,CVYCVZ[ (2.2)

where K is the Bloch wavenumber and EK(z) is a periodic function with period Λ such that EK(z)

= EK(z + Λ). This periodic condition yields an eigenvalue problem with the unit-cell translation

of the electric field. The dispersion relation of the photonic crystal can then be obtained through

the eigenvalue of the unit-cell relation. To derive the dispersion relation of a two dimensional

photonic crystal, such as the PCF, a numerical technique would generally be required because a

closed-form relation does not generally exist in most cases. Nowadays, many numerical methods

have been developed to determine dispersion relation of photonic crystals with complex

geometries such as plane wave expansion method, finite-difference time-domain (FDTD)

method, finite element method, order-n spectral method, and Korringa-Kohn-Rostoker (KKR)

method. The details of these numerical techniques are out of the scope of this thesis and

therefore will not be discussed here.

For a photonic crystal with a triangular lattice of air holes in silica, which is typically used in the

cladding of a PCF, the in-plane and out-of-plane dispersion diagram are shown in Figure 3.7.

32

The white-coloured “fingers” in the out-of-plane dispersion diagram are the bandgaps of the

photonic crystal. The large white triangular area under the red air dispersion line is the region

where light is evanescent (i.e. propagation is turned off in the photonic crystal). This domain

corresponds to the guidance of light by TIR. Note that in the out-of-plane propagation, a

complete bandgap does not exist because of the low index contrast between air and silica.22

This

means that light propagating at some angles to the photonic crystal plane is not supported.

Nevertheless, a complete bandgap is not necessary in the PCF because fibers have a continuous

translational symmetry in the z direction. Any propagation along the fiber axis (i.e. kz ≈ 0) will

be conserved as a result of the symmetry. In fact, as a result of the symmetry, the kz component

of the wave vector, is significantly higher than the transverse components (i.e. kx and ky). This

means that only a small portion of the modes need to be guided by the structure. Nevertheless,

light propagating in the plane of the photonic crystal cladding (i.e. in-plane propagation at kz = 0)

or at some angle to the photonic crystal plane will also be guided by the bandgap of the photonic

crystal as demonstrated by the two shaded rings in Figure 3.8. The ring at the waist of the sphere

indicates the in-plane photonic bandgap at kz = 0. The ring at the upper pole of the sphere

denotes the out-of-plane bandgap shown as the white “fingers” in Figure 3.7a which only guide

light for certain values of kz for a particular frequency. In sum, the photonic bandgap guidance of

light in PCF is achieved by a complex solid angle of photonic bandgaps in the cladding of the

fiber. The translational symmetry of the fiber provides light propagation in the central core

without an omni-directional bandgap, which eliminates the need for a high index material to

open up an omni-directional photonic bandgap.

Figure 3.7: Dispersion diagram of a photonic crystal as a function of (a) out

vector, kz, and (b) in-plane wave vector at k

for the triangular lattice of air holes with period

Figure 3.8: Schematic illustration of solid angles falling within the photonic band

photonic crystal with a triangular lattice operating at a fixed frequency. k

wave vectors in the plane of the photonic crystal in which the structure varies periodically. k

denotes the wave vector in the direction with translational symmetry.

33

Dispersion diagram of a photonic crystal as a function of (a) out

plane wave vector at kza/2π = 1.7 in the irreducible Brillouin zone (inset)

for the triangular lattice of air holes with period a and radius 0.47a in ε = 2.1.

Schematic illustration of solid angles falling within the photonic band

photonic crystal with a triangular lattice operating at a fixed frequency. kx and k

wave vectors in the plane of the photonic crystal in which the structure varies periodically. k

denotes the wave vector in the direction with translational symmetry.22

Dispersion diagram of a photonic crystal as a function of (a) out-of-plane wave

a/2π = 1.7 in the irreducible Brillouin zone (inset)

in ε = 2.1.23

Schematic illustration of solid angles falling within the photonic bandgap of a

and ky denote the

wave vectors in the plane of the photonic crystal in which the structure varies periodically. kz

34

(3.4) Hollow-Core Photonic Crystal Fiber for Efficient Raman Scattering

In nature, Raman scattering is an extremely rare event (roughly only 1 photon in 10 million

incident photons is Raman scattered). In fact, Raman scattering is much weaker than most other

optical processes without surface enhancement or being in resonance with the pump laser as

discussed in section 2.5, which makes it difficult to detect in most analytes. Although many

commercialized Raman systems built with low loss optical components and highly sensitive

detectors are readily available in the market now, their conventional way of inducing and

collecting Raman scatterings are still very inefficient which highly limits their system sensitivity,

particularly for aqueous and diluted solutions.

PCF can be a means of enhancing the detected Raman signals from aqueous or diluted solutions,

without the need of additional metallic nanoparticles for surface enhancements.24, 25

In particular,

HC-PCF, whose PCF featuring a central air core and a high air-filling fraction cladding formed

by a periodic air hole array in silica, enables the pump laser to be confined inside the central core

of the PCF through both photonic bandgap effects and total internal reflections. This is a

distinguishing feature of our technique as we can utilize both guiding mechanisms by selectively

filling the central air core with analyte to maintain the photonic bandgap effects of the cladding.

By confining both the pump laser and the analyte along the length of the PCF, a larger volume

for light-matter interaction is achieved compared to a conventional Raman spectroscopy scheme.

Figure 3.9 compares the conventional Raman spectroscopy scheme with the one using HC-PCF

as an interaction medium for Raman spectroscopy. In conventional Raman spectroscopy (Figure

3.9a), the pump laser is focused directly into the analyte to generate Raman scattering. Most of

the Raman signal detected is scattered from the beam waist of the pump laser in which the pump

laser is most intense. In this case, the pump laser and the analytes are only interacting in the

35

volume limited by the spot size of the pump laser in the lateral direction and the depth of field in

the axial direction. Since Raman signals are scattered in omni directions, only a fraction of the

scattered signal is collected by the objective and detected by the detector due to the limited NA

of the objective. In the case of an aqueous QD solution, the amount of scattered signals detected

from the small quantity of QDs dispersed in water is very minimal.

Figure 3.9: Raman scattering and signal collection in (a) conventional Raman spectroscopy and

(b) Raman spectroscopy using HC-PCF.

By employing HC-PCF as the medium for light-matter interactions (Figure 3.9b), both the pump

laser and the analyte can be confined within the central core of the HC-PCF. Subsequently, the

confined laser power within the core induces more interaction with the analyte that is filled

inside. Thus, Raman signals are scattered throughout the entire length of the fiber as opposed to

just the depth of field of the objective in the conventional scheme. Since Raman signals are

mostly shifted by less than tenths of nanometer from the wavelength of the pump laser, they will

also be confined inside the central core of the PCF and collected throughout the fiber. As a

36

result, the output signal from the fiber end will be collected more efficiently by the objective for

detection. With the use of HC-PCF, the detected Raman signal can be about 2 to 3 orders of

magnitude greater than that of the conventional Raman technique in practice.

Although it is not trivial to completely decouple the enhancement due to increased collection

efficiency throughout the fiber from that due to increased interaction length, the order of

magnitude increased in the detected Raman signal due to the two different components was

studied. This was carried by measuring the enhancement of the detected Raman signal with

increasing PCF and Teflon capillary tube (TCT) length (Figure 3.10). The same pump power,

coupling objective, and setup were used for all experimental points in the figure. Hence, detected

signal enhancement with increasing fiber length at a short distance is mostly contributed by the

increased light-matter interactions. For an increased PCF length from 4 cm to 11 cm, the Raman

signal is enhanced by about 3 to 4 times. The experimental points are also fitted to a model for

calculating Raman intensity in a liquid core waveguide in a backscattering configuration as

follows:26

5\]^ = *2 _2`a1 − UC\ (2.3)

where Po is the fraction of laser power that is coupled into the waveguide, α is the loss

coefficient of the waveguide, 4A is the numerical aperture of the waveguide, and L is the length

of the waveguide with solution filled inside. ρ and σ are the Raman cross-section and the number

of scattering centers per unit volume of the sample solution respectively. The experimental data

collected were fitted to the model using α and κ ≡ Poρσπ4A.27

According to our model, if we

further increase the length of the PCF and TCT beyond the experimental data points, the

37

enhancement will reach a plateau where it will not increase further because of the absorption and

attenuation loss of the fibers.

If we compare the normalized Raman intensity between PCF and TCT at a fixed fiber length,

PCF achieved a greater enhancement because of the greater collection efficiency in PCF. At a

fiber length of ~10 cm, the normalized Raman intensity from PCF is about 20 times greater than

that from TCT. This signal increase is contributed to the improved collection efficiency of the

PCF relative to the TCT. The greater collection efficiency in PCF can be understood

mathematically from the solid angle that collects Raman scattering inside a liquid core fiber at a

fiber section according to:26

bA-c = =Adc − =cee (2.4)

where bcone is the solid angle that collects Raman scattering. ncore and neff are the refractive index

of the fiber core and cladding respectively. Consider a water core fiber where ncore = 1.33. Ωcone

would be 0.33 in the case of TCT since neff of Teflon is 1.29. In the case PCF whose cladding is

made up of air holes with over 90% air filling fraction, neff would be close to 1; thus, Ωcone would

be greater than 0.33 (i.e. larger collection angle than that of the TCT). In fact, neff of the HC-

PCFs used for this thesis (i.e. HC-800 and HC-1060 from NKT Photonics) are 1.14 and 1.17

respectively according to Eftekhari et al.27

They correspond to Ωcone of value 1.47 and 1.25,

which are about 4 times larger than that of the TCT. This shows that a greater collection angle is

obtained with PCF which resulted in an increased Raman signal in comparison to the TCT.

38

Figure 3.10: Raman intensity of water mode centered at around 3400 cm-1

for varying lengths

of (a) HC-800 PCF and (b) TCT. The intensity is normalized with respect to the intensity

obtained with a cuvette. The coupling objective, pump power and setup were kept the same for

all experimental points. Insets: Predicted plateau of intensity with length up to 500 cm.

In addition to the detected Raman signal enhancement, it is important to note that the pump laser

is confined inside the “liquid” core of the PCF; thus, interaction between the pump laser and the

glass wall of the fiber is minimal and the resulted interference is very limited in the presence of

other vibrational modes and backgrounds. Figure 3.11 compares the Raman signal of silica with

that of water and a thiol-capping agent that is filled into the central core of a HC-PCF. The silica

Raman modes are very weak in the water spectrum when there is no Raman mode or background

signal present between 300 and 500 cm-1

. In the spectrum of the thiol-capping agent, no silica

Raman modes are observed when there are vibrational modes present between 300 and 500 cm-1

.

This shows that the Raman signal interference from the silica wall of the HC-PCF is insignificant

when the central core is filled with analyte.

39

Figure 3.11: Comparison of silica Raman signals between water and thiol-capping agent when

they are filled into the central core of a HC-PCF. Weak silica modes are observed in the water

spectrum when no Raman mode is present between 300 and 500 cm-1

. When Raman modes are

present as shown in the spectrum of the thiol-capping agent, no silica mode is observed.

Furthermore, unlike SERS, whose detected Raman signal enhancements require metallic

nanoparticles to be absorbed or in close proximity with the analyte, the detected signal

enhancement by efficiently inducing and collecting Raman scatterings with HC-PCF completely

eliminates the possibility of altering analyte structure or properties due to conformational

changes when interacting with the metallic nanoparticles. However, if conformational changes of

analyte are not a concern, surface enhancements can also be obtained in addition to the efficient

Raman scattering of HC-PCF by incorporating metallic nanoparticles into the analyte or central

core of the HC-PCF.

In the essence of combining HC-PCF with Raman spectroscopy, the practical advantages of

optical fibers are also added to the conventional scheme. This includes the high flexibility and

compactness of optical fibers for remote sensing and in-situ detections, sampling versatility, and

potential for low-cost analysis as the cost of HC-PCF drops dramatically through economies of

40

scale. In addition, HC-PCF requires only a very small sample volume for detection, and it is

capable of detecting multiple compounds in parallel. This is crucial in many biological,

environmental and forensic applications where sample analysis with only nanoliter volume is

required.

It is important to mention that this novel PCF platform for Raman spectroscopy is capable of

enhancing the detected Raman signal for solutions with many particles (hundreds or more) as

well as solutions with a single particle theoretically. In solutions with many particles, the

confinement of both the pump laser and the sample solution in the PCF core enable more

particles to generate detectable Raman scattering signals. Moreover, these confinements enable

more photons to interact with each particle to generate more signals due to the high photon

density in the PCF core as a result of the strong light confinements. Although there are only one

Raman scatterer in single particle solutions, more Raman scattering signal can still be generated

compared to conventional Raman spectroscopy due to the increased photon density in the PCF

core which allows a greater number of photons to interact with the one particle to generate more

Raman scattering signals. However, there are also other technique that are developed to provide

greater Raman scattering signal for single particle applications such as SERS and Raman

tweezers.28, 29

The development of the PCF platform is currently focused on the multi-particle

systems in which the ensemble averaged interactions and structures of the nanoparticles can be

monitored before, during and after their synthesis.

In the next three chapters, we will discuss how the HC-PCF can be used in practice to detect

molecular bindings in nanostructures including semiconductor QDs and metallic nanoparticles.

Experimental details and results from each of these studies will be presented and discussed.

41

Chapter 4

Raman Characterization of

Colloidal CdTe "anoparticles

(4.1) Introduction

Colloidal QDs are suspensions of semiconductor nanoparticles that offer both the intriguing

optical properties of quantum-confined particles, and the practical advantages of solution-based

processing.30-32

In the past two decades, aqueous synthesis of colloidal QDs has gained much

popularity and evolved tremedously.33-37

This simple and cost-effective technique allows very

small (2-6 nm), monodisperse, and highly water-soluble QDs to be synthesized in gram

quantities. QD capping plays a pivotal role in the properties and utility of the material; the use of

short-chain thiols, such as thioglycolic acid (TGA), as capping agents have shown to greatly

improve the PL quantum efficiency of as-synthesized QDs to values of 40-60%.33

3-

mercaptopropionic acid (MPA) has also shown to offer a larger range of size and PL tunability,

42

and a longer emission decay time.33

Other thiols, such as 1-thioglycerol (TG), are found to be

more suitable for synthesizing stable core-shell QDs.36

The unique properties of the different

colloidal QDs have potentials for a wide field of novel applications ranging from photovoltaics38

and optoelectronics,39

to biosensing,40

bioimaging41

and even cancer treatments.42

One of the most important technological challenges in QD advancement is the development of a

cost-effective, reliable and sensitive optical monitoring system to control the physical, chemical

and size-dependent properties of QDs before, during and after their fabrication on a nanometer

scale. To measure and compare these properties between the different QDs, many analytical

techniques have been employed including but not limited to PL, EL, UV-vis, TEM or HRTEM,

CZE, XRD, and XPS. These techniques provide valuable information on the composition and

properties of the QDs; yet, none of them describes how the capping agents binds with the core of

the QDs. Their impact on the overall molecular structure, molecular complex, and different QD

properties also remain unclear. Consequently, this limits our capability to improve the quantum

efficiency, stability, and bioconjugating ability further from what has been achieved today.

Complex QD designs for increasing performance and functionalities in different applications

remain very challenging.

FT-IR can be an alternative for determining the binding interactions between the QDs and their

capping agents. However, the strong and broad absorption bands of water often overlap with

those from the QDs and stabilizing agents. This limits the number of vibrational modes that can

be resolved.

A complementary technique to FT-IR is Raman spectroscopy, which is a rapid and non-

destructive means of probing molecular vibrations optically through inelastic scattering. Raman

43

provides the unique vibrational modes or “fingerprints” of the molecules that enable similar

molecular structures to be unambiguously distinguished and identified. More importantly, the

vibrational modes of water are inherently weaker in Raman scattering than in FT-IR. Distinctive

Raman modes of colloidal QDs in their native and dilute aqueous environment can be readily

obtained without significant interference. However, Raman scattering has not been successful in

characterizing QDs because it is generally very weak compared to elastic scattering, and it is

further weakened; therefore, it has only been used in characterizing QDs coated on films or in

powder form.37, 43-46

SERS has also been used to obtain an enhanced Raman signal from

QDsolutions.24, 25, 47

However, metallic nanoparticles have to be mixed with the QD solution,

which could alter the structure and properties of the QDs from its as-synthesized state due to

conformational changes when interacting with metallic nanoparticles.

In this chapter, we demonstrated the use of the novel HC-PCF as a means of enhancing the

detected Raman signals from aqueous or diluted solutions, without the need of additional

metallic nanoparticles.9, 10

In previous works of our research group, HC-PCF demonstrated

potentials for low-cost in-situ monitoring of QD structure during the synthesis process. The

usefulness of HC-PCF for Raman spectroscopy has recently been demonstrated in the studies of

ZnO nanoparticles synthesized through base hydrolysis method,10

and colloidal CdTe

nanoparticles post-synthesized using rapid thermal annealing.9 In these studies, Raman modes of

the ZnO and CdTe nanoparticles were obtained owing to the detected signal enhancements

provided by the HC-PCF.

In this chapter, we report the enhanced detected Raman signals from aqueous solutions of thiol-

stabilized CdTe QDs through the use of HC-PCF. Strong and well-resolved vibrational modes

44

ascribed to the CdTe semiconductor core, capping agents, and their interface were obtained for

the first time in the aqueous medium. The observed Raman vibrational modes are divided into

three regimes. The modes in the low Raman shift regime reveal the low crystallinity of CdTe

QDs capped with TGA due to a large inclusion of Te compounds and surface defects. The modes

in the mid and high Raman shift regime reveal the formation of thiolates, and Cd-S bonds

between the thiolate and the CdTe core, which stabilize the QD through structured CdSxTe1-x

ternary compound. MPA is also found to promote the formation of unidentate and chelating

bidentate complexes with the CdTe core. These complexes further passivate the QD and

potentially improve its stability at the expense of its solubility and bioconjugating ability. This

experiment substantiates the promise of HC-PCF in enhancing the Raman scattering signal of

nanoparticles in aqueous solutions and enables possible studies of molecular structures relating

to the different properties of QDs.

(4.2) Experimental Details

The thiol-capped CdTe QDs used in this experiment were synthesized with minor modification

to literature procedure.48, 49

The resulting solid CdTe QD samples were then weighted and

dispersed in deionised water to prepare aqueous solutions of 2 mg/mL for Raman measurements,

0.04 mg/mL for UV-vis and 0.4 mg/mL for PL. For detailed synthesis methods and materials of

the CdTe QDs, please refer to Appendix B.

The structure of the highly fluorescent CdTe QDs and the three different ligands used for

stabilization are shown in Figure 4.1. Room temperature PL measurements were performed using

a Perkin-Elmer Luminescence Spectrophotometer in aqueous solution. UV-vis measurements

were performed using an Infinite M1000 TECAN system also in aqueous solution.

45

Figure 4.1: (a) Core-shell structure of CdTe QDs. (b) Molecular structure of capping ligands.

Raman spectra were acquired using a Horiba Jobin Yvon HR800 micro-Raman system equipped

with a CW 632.8 nm HeNe laser in the range of 2 – 5 mW. The spectral resolution of the

spectrometer was about 1 cm-1

. To incorporate HC-PCF onto the stage of the Raman system for

measurements, the fiber setup shown in Figure 4.2 was used. In this setup, HC-PCF is placed

inside a fiber chuck which is held with a chuck holder on an aluminum block that is placed on

the stage of the Raman system. To measure a Raman spectrum, the laser light was focused into

the central core of the HC-PCFs using a 100x objective to maximize the Raman signal from the

analyte while minimizing those from the fiber background. Figure 4.3 compares the Raman

spectra of water that is filled inside the central core of a HC-PCF when the objective is focused

on different spots of the PCF cross-section. The strongest water signal and the weakest

background are obtained when the laser light is focused into the central core of the PCF where

water is filled.

46

Figure 4.2: Experimental setup of HC-PCF in HR800 micro-Raman system.

Figure 4.3: Raman spectrum obtained when the laser light is focused onto the cladding, holey

region and the center of the central core of the HC-PCF when it is filled with water.

0 500 1000 1500 2000 2500 3000 3500 4000

0

1000

2000

3000

4000

5000

6000

Raman Intensity (a.u.)

Raman Shift (cm-1)

Cladding

Holey Region

Core Center

47

Each spectrum of the QDs was averaged over 40 measurements with an exposure time of 30 s.

This long averaging and exposure time were to maximize the signal-to-noise and signal-to-

background of the QDs. An increased laser exposure on the QDs sample allowed some water to

be evaporated. This reduced the water signal to a level lower than that of the fluorescent signal

which eliminated Raman mode interferences. Since CdTe nanoparticles do not evaporate, the

CdTe QD signals also increased relative to that of the water.

The HC-PCFs used in this experiment were obtained from NKT Photonics, HC-800-01. Each

piece was segmented, stripped and cleaved into ~6 cm long. The core of the HC-PCF was

selectively filled, using a technique developed by Irizar et al.,10

to allow enhancement of the

detected Raman signal through bandgap confinement of the pump laser. This technique involved

splicing one end of the HC-PCF with a conventional fiber splicer to collapse all the cladding

holes while leaving the central core open (Figure 4.4a). Then, the spliced end of the PCF was

submerged into the analyte to fill the PCF’s central air core by capillary effect. Figure 4.4b

shows an optical image of the opposite end of PCF after its entire core is filled with analyte.

The intensity of the OH modes of water between different techniques is compared in Figure 4.5.

By selectively filling the entire central air core with water, the intensity of the OH stretching

mode at 3427 cm-1

is about 101 times greater than that of the conventional Raman spectroscopy

with a cover slide. This enhancement is attributed to the combination of the TIR and photonic

bandgap guidance inside the liquid core. Compared to non-selective filling (i.e. filling both the

central air core and the cladding holes with water) in which there is only the photonic bandgap

effect with the bandgap shifted to the lower wavelength due to the filling of the air cladding

holes, the intensity of the OH stretching mode at 3427 cm-1

is 73 times greater in the selective

48

filling case. This shows that our selective filling technique of the PCF gives the greatest

enhancement of the detected Raman signal compared to conventional techniques and other

analyte filling methods of the PCF.

Figure 4.4: Optical images of HC-PCF cross-section at the (a) fused end and (b) non-fused end

after the entire central air core is filled with water.

Figure 4.5: Raman intensity comparison of the OH modes of water between conventional

Raman spectroscopy using a cover slide, selective filling of PCF and non-selective filling of

PCF. Inset: OH stretching mode of water obtained from non-selectively filled PCF and

conventional Raman spectroscopy.

49

Raman spectra obtained were baseline removed and fitted with Gaussian or Gaussian-Lorentzian

mixed functions to determine their peak positions, amplitudes, and full-width at half-maximums

(FWHMs).

(4.3) CdTe Core and Te Defects

Raman spectra of aqueous CdTe QDs from 100 to 1750 cm-1

were obtained. The spectra are

divided into three regimes in which the peaks are correlated to the vibrational modes in the CdTe

semiconductor core, capping ligands, and their interfacial structures. Some of the modes are

broad because they are overlapped with multiple weak Raman modes that are located in

proximity. These modes might be originated from different functional groups, different

conformations of the same functional groups, or both. The limited sensitivity of the system

prevented them from being resolved.

In the low Raman shift regime, between 100 and 200 cm-1

as depicted in Figure 4.6, Raman

modes corresponding to the crystalline core and Te defects of the CdTe QDs are observed. The

peaks at ~120, ~140 and ~160 cm-1

are attributed to the Te A1, Te E and CdTe TO, and CdTe LO

modes of the QDs, respectively.50-54

These modes are observed in all three spectra indicating that

both the crystalline CdTe core and Te defects are presented in the QDs regardless of the type and

structure of the capping agent used. In particular, the CdTe LO modes are shifted to the lower

wavenumber by ~10 cm-1

compared to the LO mode of bulk CdTe crystal reported at ~170

cm-1

.51, 55

These shifts indicate the presence of phonon localization due to quantum confinement

effects, which suggests that the expected zero-dimensional CdTe structures are indeed in place.

In addition, the similar shifts of the CdTe LO mode among the three spectra show that similar

crystalline sizes are formed with the three different capping ligands used.

50

Figure 4.6: Raman spectra of CdTe QDs with different thiol capping agents in the lower Raman

shift regime between 100 and 200 cm-1

.

Moving toward the lower Raman shifts at ~120 and ~140 cm-1

, two Te modes are observed in the

three spectra. The two Te modes indicate the presence of Te crystals, either within the center of

the QD or the surface defects at the boundary of the QDs.53, 54, 56

QDs with surface defects are

known to have lower crystallinity and introduce trap states which reduces the PL quantum

efficiency.57

However, the amount of Te inclusions or surface defects cannot be quantified from

the spectra as the Raman cross-sections of Te modes are 75 times larger than that of the

crystalline CdTe modes.54

Because of the close proximity and broadness of the Te E and CdTe

TO vibrational mode at 140 cm-1

, precise contribution of the Te E modes from QDs with

different caps cannot be determined from their spectra. Nevertheless, we can calculate the

intensity ratios of the Te A1 mode to CdTe LO mode to compare the crystallinity of these QDs.

As shown in Table 4.1, the ratio is highest for TGA-capped QDs, followed by those for TG and

then MPA. In our previous study, we have shown that an increase in the ratio indicates an

51

increased in Te inclusion or surface defects relative to the amount of CdTe crystals formed in the

QDs.9 This suggests that crystallinity improved from TGA-capped QDs to TG-capped QDs and

is greatest with MPA-capped QDs. The improved crystallinity contributes to a greater PL

efficiency with fewer trap states that leads to non-radiative processes; thus, a brighter QD is

synthesized with MPA. This result correlates to the relative PL quantum efficiency estimated

from the UV-vis and PL spectra shown in Figure 4.7. The PL quantum efficiency can be

calculated as follows:

gh1 = gdceadceah1

=h1=dce

ih1idce

(2.1)

where g is the PL quantum efficiency, A is the absorbance at the excitation wavelength, D is the

integral area under the PL curve. The subscripts QD and ref correspond to the QD and the

reference sample (typically Rhodamine 6G with a refractive index of 1.359 and a quantum

efficiency of 95% excited at 400 nm), respectively. Since the QD concentrations are the same for

the three different capping agents, their refractive indices are also the same. However, MPA-

capped QDs give weaker absorbance compared to those capped with TGA and TA in the UV-vis

spectra at 400 nm. MPA-capped QDs also have the largest integral area in the PL spectra out of

the three QD types. The weaker absorbance and larger integral area of MPA-capped QDs

correspond to a higher PL quantum efficiency which agrees with the Raman result obtained.

Table 4.1: Intensity ratios of the Te A1 mode to CdTe LO mode for CdTe QDs capped with

different thiol agents.

Capping Agents TGA TG MPA

Te A1 / CdTe LO 2.949 0.901 0.499

52

Figure 4.7: (a) UV-vis spectra of the CdTe QDs (inset: fluorescent images of the QD solutions).

(b) PL profile of the QDs.

Overall, both the crystalline CdTe core and defects are observed in QDs capped with TGA, MPA

and TG. However, different crystalline cores are obtained with MPA-capped QDs being the most

crystalline among the three solutions.

(4.4) Core-Thiol Interface

In the mid-Raman shift regime, between 200 and 310 cm-1

, Raman modes are attributed to the

binding interactions between the QD core and the thiol capping agents as shown in Figure 4.8.

The two peaks at ~291 and ~260 cm-1

shown in this regime are ascribed to the surface optical

phonon (SO) mode of the CdS compound43, 58

and CdS-like LO phonon mode of the ternary

CdSxTe1-x compound, respectively.59, 60

The presence of the two CdS related modes denotes the

formation of CdS compounds on the surface of the QDs. This suggests that Cd ions on the

surface of the QD core are bonded with the S ions in the thiol terminus of the capping agents for

stabilization. This hypothesis is supported by the absence of the S-H stretching modes, in the

range of 2560 - 2590 cm-1

, in all three spectra (not shown), which shows that S-H bonds in the

thiol groups are broken and have then formed thiolates. These thiolates have perhaps binded with

53

Cd ions and formed CdS compounds around the CdTe core. Several other studies have also

proposed the same binding interaction between the thiol group and the core of the QDs.37, 61, 62

Since no visible CdS LO mode at ~300 cm-1

is observed, a CdS shell has not formed around the

CdTe cores in the three solutions. Instead, a CdSxTe1-x ternary structure might have formed

around the CdTe core, which is commonly formed upon heating processes.63, 64

It is known that

the position of CdS-like LO mode shifts from 305.6 (at x = 1) to 258.7 cm-1

(at x = 0) as S

content decreases in CdSxTe1-x compounds.59

Using the experimentally observed wavenumbers

and their corresponding CdSxTe1-x alloy compositions from Fischer et al.,

59 a composition around

CdS0.7Te0.3 is estimated for all three QD samples capped with the different thiol agents. The rich

S content in the alloys indicate that many thiolates are coupled with the CdTe cores and formed a

system close to the CdS shell. This strongly indicates that thiolates are strongly passivating the

surface of the QDs and thereby preventing the CdTe core from exposing and interacting with the

environment. Strong surface passivation also contributes to a greater PL efficiency through

elimination of surface defect states.

Figure 4.8: Raman spectra of CdTe QDs with different thiol capping agents in the mid-Raman


.

54

It is important to note that both the CdS SO and the CdS-like LO modes are observed in all three

spectra with the different capping agents used. The peak position of the two modes are similar

among the three spectra, suggesting that the QD core binds similarly with the three thiol agents,

thus achieving the same alloy composition. Since the three thiol agents have different chain

lengths, this suggests that the degree of surface passivation might be unrelated to the length of

the thiol chain bonded to the QD surface. This is in agreement with the finding from Algar et al.,

in which a reduction of non-radiative decay rate with increasing alkyl thiol chain length was not

observed; thus, there was no improvement on the surface passivation of the QDs with lengthy

thiol agents.65, 66

In fact, it was suggested that the longer alkyl thiol chain of MPA and TG

excludes water from the QD surface, resulting in a higher quantum yield than the ones capped

with TGA (Figure 4.7). Previously, it was reported that photo-oxidation of the surface thiol

agents could catalyze the formation of disulfides through breaking the Cd-S bonds and reacting

with a neighbouring thiol group.67, 68

However, no S-S vibrational modes in the region between

480 and 510 cm-1

were observed in any of the three spectra. This indicates the absence of

disulfide bonds and confirms that photo-oxidization has not occurred in our solutions. The strong

surface passivation and the absence of photo-oxidization observed from the Raman spectra

further demonstrate that the CdTe QDs capped with TGA, MPA and TG are all highly stable in

their native aqueous environment.

(4.5) Carboxylate-Metal Complexes

In the high Raman shift regime, between 310 and 1750 cm-1

, vibrational modes of the thiol

capping agents are observed as shown in Figure 4.9. Notably, the peaks between 1330 and 1500

cm-1

and between 1570 and 1580 cm-1

are attributed to the symmetric and asymmetric stretching

modes of the carboxylates, respectively.37, 62, 69

These modes are only observed in TGA, and

55

MPA-capped QDs, in which one end of the thiol chain is terminated with a carboxylic acid

group. These modes indicate that the carboxylic acid groups are absorbed onto the QD surfaces

as carboxylates, most likely by forming bonds with the Cd ions on the CdTe core. Though one-

phonon Raman modes of CdO are not allowed due to the selection rules, no well defined

vibrational modes can be extracted from the Raman spectra.70

Nevertheless, it is known that the

carboxylic acid group can be absorbed on metals as carboxylates after deprotonation.37, 62, 71, 72

Multiple shifted modes of carboxylate symmetric stretches might also have risen if multiple

complexes of the carboxylate would have been formed. As such, the two symmetric stretching

modes observed are a result of two carboxylate-metal formations.

Figure 4.9: Raman spectra of CdTe QDs with different thiol capping agents in the high Raman


.

On the basis of previous work, carboxylate ions may coordinate to a metal in one of the three

modes: unidentate, bridging bidentate, and chelating bidentate.69, 71

These complexes can be

identified through the wavenumber separations, ∆, between the symmetric and asymmetric

56

stretches of the carboxylate. Unidentate complexes exhibit ∆ values (200 – 470 cm-1

) that are

much greater than the ionic complexes. Conversely, chelating bidentate complexes exhibit ∆

values (<110 cm-1

) that are much less than the ionic complexes. Bridging bidentate complexes

exhibit ∆ values (140 – 190 cm-1

) that lie in between the chelating bidentate and ionic complexes.

As shown in Table 4.2, both TGA and MPA-capped QDs have two carboxylate-metal

complexes. Both TGA and MPA show the formation of chelating bidentate complexes. In

addition, TGA formed the bridging bidentate complexes while MPA formed the unidentate

complexes. It was recently reported that the carboxylate-metal complex changes from unidentate

to bridging bidentate when the average size of the QDs increases from 8 to 20 nm.37

However,

the average particle sizes of our TGA and MPA-capped QDs calculated are very similar (i.e. 3.20

nm for TGA-capped QDs and 3.07 nm for MPA-capped QDs using their first absorption

maximum at 546 and 536 nm, respectively).73

The small size difference between the two QD

solutions is unlikely to cause a difference in the carboxylate-metal complex. This suggests that

the length of the alkyl thiol chain might have influenced not only the type of complexes formed

but also the amount of complexes formed. To compare the quantity of carboxylate-metal

complex formed between the different bindings interactions and QD solutions, the intensity

ratios of the COO symmetric stretches to the CdS 2SO mode are calculated. As shown in Table

4.3, the two ratios are much greater than 1 for MPA-capped QDs and less than 1 for TGA-

capped QDs. The large ratio implies that a large quantity of carboxylate ions is coordinated with

the Cd ions. This is not only suggesting that the formation of carboxylate-metal complexes is

much more favourable with MPA than TGA, but also that the surface of the QD is more

passivated by carboxylates in the MPA chain.74, 75

As a result, fewer defect sites are present in

MPA-capped QDs, which corresponds to a higher PL quantum efficiency, however, in the

57

sacrifice of losing its solubility potentially due to the reduction of free carboxylate ions.

Bioconjugation to the QDs might also be limited with a reduced number of thiol terminuses on

its surface. Since the chelating bidentate bindings of the carboxylates-metal complex are stronger

than the unidentate bindings of the thiolates-metal complex, the larger quantity of chelating

bidentate complexes formed in MPA-capped makes it more stable than the TGA-capped ones.

Altogether, both TGA and MPA-capped QDs form carboxylate-metal complexes with the CdTe

core, but the longer alkyl chain in MPA enabled a larger quantity of the complexes to be formed

with it rather than with TGA; thus potentially permitting a higher stabilized QD to be

synthesized with MPA.

Table 4.2: The wavenumber separations, ∆, between the symmetric and asymmetric stretching

modes of the carboxylate, and their corresponding carboxylate-metal.

Capping

Agent

Separation

between

νs(COO)1 and

νas(COO), ∆1 Structure

Separation

between

νs(COO)2 and

νas(COO), ∆2 Structure

TGA 187 cm-1

bridging bidentate 70 cm-1

chelating bidentate

MPA 215 cm-1

unidentate 102 cm-1

chelating bidentate

1 symmetric stretches between 1300 – 1400 cm-1


Table 4.3: Intensity ratios of the symmetric carboxylate stretch to CdS 2SO mode.

Intensity Ratio

Capping

Agent

νs(COO)1 /

CdS 2SO

νs(COO)2 /

CdS 2SO

TGA 0.205 0.547

MPA 12.803 5.390



58

(4.6) Summary

In summary, we demonstrated the use of HC-PCF to obtain efficient Raman scattering of the

different thiol-capped CdTe QDs in aqueous environment. Strong and well-resolved Raman

modes of the CdTe semiconductor core, capping ligands, and their interfacial structures were

successfully observed and compared without integrating any metallic nanoparticles for

enhancement. These modes were also correlated to the QD’s PL quantum efficiency, stability,

water solubility and bio-conjugation capability. To the best of our knowledge, this is the first

time that such strong Raman modes of the thiol-capped CdTe QDs in aqueous solution have been

reported. The enhanced detected Raman signals were achieved through increased light-matter

interaction and efficient accumulation of the Raman scattering signal along the entire length of

the HC-PCF.

59

Chapter 5

Raman Characterization of

Metallic "anoparticles

(5.1) Introduction

Metallic nanoparticles (of size 1 – 100 nm in diameter) synthesized with noble metal, such as

silver and gold, are of great interest in the past decade across a number of disciplines in science,

engineering, and medicine due to their unique and remarkable capability of enhancing optical

properties in the visible and near-infrared range (~400 – 1000 nm).76-79

Optical properties,

including absorption, Rayleigh scattering and Raman scattering, are enhanced because metallic

nanoparticles produce a plasmon resonance in the presence of an external EM wave of light. This

plasmon resonance introduces a range of wavelength in which the optical effects are increased by

orders of magnitude. In particular, Raman scattering enhancement can be induced by an

enhanced EM fields at or near the surface of the metallic nanoparticles. This local EM field

60

enhancement is introduced by an oscillating cloud of electrons at the particle surface, namely,

surface plasmon resonance, when the particle is irradiated by light (Figure 5.1). In the absence of

an external EM field, electrons at the surface of the metallic nanoparticles are free to oscillate

and move around due to the conductive nature of noble metals. However, in the presence of an

external EM wave of light, electron cloud around the particle surface oscillates coherently with

the external EM field, producing an overall EM field outside the particle that is stronger than

incident one. To particles or molecules that are adsorbed or in close proximity (< 10 nm) to the

particle surface, this is seen as one strong EM field with amplitude summing the applied field

and that resulted from the surface plasmon resonance. This strong EM wave resulted from the

surface plasmon resonance is called surface plasmon polariton (SPP). In medical diagnostic and

sensing, SPP from metallic nanoparticles, particularly gold nanoparticles, significantly enhances

the sensitivity of many conventional spectroscopic techniques such as fluorescence and Raman

spectroscopy, enabling a new world of biological materials and systems to be explored.

Figure 5.1: Schematic of plasmon resonance for a metal sphere. Electron clouds of metal

spheres are displaced relative to the nuclei in the presence of an electric field, inducing coherent

oscillations with the incident electric field.76

In the field of cancer diagnostics, metallic nanoparticles are utilized to target and locate

malignant cells in the human body.80-83

When functional metallic nanoparticles are bind to cancer

61

cells in the tissues or organs, they enhance optical signals from cancer indicators adsorbed on the

particle surface to indicate the presence of cell abnormality. The large signal enhancement from

the metallic nanoparticles enables cancers at their early stages to be diagnosed accurately and

quickly, allowing cancer treatments to be much more effective. In fact, metallic nanoparticles

can also be used simultaneously as agents for cancer therapy through ablation of tumors with

rapid conversion (~1ps) of absorbed light into heat.84-90

Typically, gold nanoparticles are used for diagnostics instead of silver or other cadmium–

containing semiconductor quantum dots because gold has little or no long term toxicity or other

adverse effects that can damage or destroy cells during diagnostics.91, 92

Gold is also more

chemically stable compared to copper. Furthermore, spherical gold colloidal nanoparticles can

easily be synthesized in a wide range of sizes (2 – 100 nm) through reduction of gold ions in

solutions.93-96

Nevertheless, previous research has shown that gold nanoparticles with a core size

of ~60 – 80 nm are most efficient for Raman signal enhancements in the red (630 – 650 nm) and

near-infrared (785 nm) those light absorption and autofluorescence is minimal.82, 97, 98

To locate and detect malignant cells, gold nanoparticles are tagged with dyes to give a strong

spectroscopic signal when attached to a malignant cell. Fluorescence dyes can be used to give

bright spectral signals when enhanced by the gold nanoparticles; however, fluorescent peaks

have a large spectral width. The FWHM of a fluorescent peak is typically around 50 – 100 nm,

which limits the number of different dyes that can be multiplexed together for simultaneous

detection of different cells.99-101

Fluorescent signals are also often bleached rapidly by the optical

probe.102-104

Moreover, autofluorescence from biological species interferes with the actual signal

from the dye, making signal detection more difficult.105

Conversely, Raman dyes give narrow

62

spectral features with FWHM of just a few nm, allowing multiple different dyes to be

multiplexed spectrally.106, 107

The narrow linewidth of the Raman signal also differentiates itself

easily from the broad autofluorescence. Further, Raman signals do not photobleach.108

However,

Raman scattering signal are typically much weaker than fluorescence, making cancer cells in

small quantities difficult to detect. Nevertheless, recent experiments demonstrated that SERS

signal can be comparative to fluorescence.80, 81, 109

In addition to dye adsorption, nanoparticles are often coated with a protective layer of polymer or

lipid for improved stability, biocompatibility and circulation in the biological system. Two types

of protective layers are commonly used, particularly for Raman imaging:83

(1) thiol-modified

polyethyleneglycol (PEG) and (2) phospholipid. Thiol-modified PEG coated nanoparticles have

shown to greatly improved the stability of the SERS signal under strong acid, strong base,

concentrated salts and different organic solvents.83

They also prevent biofouling (undesired

accumulation of biological matters on nanoparticles) and allow efficient conjugation of tumor

targeting antibodies.110-112

Recently, phospholipid coated nanoparticles incorporating a Raman

dye is demonstrated.113, 114

Phospholipids maximize the biocompatibility of the nanoparticles as

they mimic membranes of the human cells. They also provide greater stability on the adsorbed

Raman dyes than the thiol-modified PEG and more resistant to aggregation than citrate coated

gold nanoparticles.114

Lastly, for cancer detection, nanoparticles have to be conjugated with antibodies for identifying

and binding to receptors or antigens overexpressed on cancer cells.

Functional gold nanoparticles, with Raman dye and protective coating, for cancer diagnostics

have been demonstrated for different types of cancers in the literature including leukemia,112, 115,

63

116 cervical cancer,

117 head and neck cancer,

83 breast and lung cancer

115 and kidney cancer.

118

These nanoparticle systems are commonly characterized by several techniques to determine their

optical characteristics and to confirm the addition of the functions throughout and after the

synthesis:

1. UV–vis spectroscopy

UV–vis spectroscopy is utilized to confirm the stability of the nanoparticles after

adsorbing the Raman dye and coating the nanoparticle surface. The absorption of

the nanoparticles should remain unchanged or with a minor shift of only a few

nanometers after the synthesis process. A minor shift might be induced by the

addition of a Raman dye or coating material whose absorption peak is higher or

lower than the ones of the nanoparticles. However, if a larger shift (~10 – 100

nm) of the absorption peak is observed, it would indicate that the size of the

metallic nanoparticle is changed. An increase in the particle size could mean that

the surface chemistry of the nanoparticle is changed during the synthesis process

causing further growth or aggregation of the nanoparticles. Broadening or

emerging of new peaks in the spectrum further indicates that the nanoparticles are

aggregated, possibility due to changes in the ion quantity (i.e. Na+ and Ca

2+) and

acidity of the colloidal solution.

2. DLS

DLS provides size distribution of the nanoparticles. It is used to confirm that the

protective layer of the nanoparticle is coated properly in all or most of the

nanoparticles. The addition of a protective layer around the nanoparticle causes an

64

up shift of the size distribution in DLS. Due to the small size of the dye, the

measured particle size normally remains unchanged after dye adsorption.

3. TEM

TEM is used to provide physical images of the nanoparticle structure through the

use of an electron beam. TEM allows a more accurate conformation of the

nanoparticle size and coating of the particles with polymer and phospholipid. A

bright diffuse corona can be observed around the dark coloured nanoparticles in

the TEM image after the nanoparticles are successfully coated with PEG or

phospholipid.

4. Raman Spectroscopy

Raman spectroscopy is used to ensure that Raman dyes are adsorbed on the

nanoparticle surface and generate a SERS signal. In most cases, a strong Raman

dye signal obtained from the nanoparticle solution would consider that the dye

adsorption is successful as the enhanced Raman signal can only be achieved

through the gold nanoparticles.

Although these techniques provide logical evidences of the nanoparticle functionalization, direct

evidences on the actual molecular binding interaction as well as the type of bindings are not

available. For example, UV–vis spectroscopy, DLS and TEM only provide information on the

nanoparticle stability, size and size distributions. Although, shifting of the particle size

distribution after coating the particle is logically reasonable, the structure of the coating layer and

its binding interaction with nanoparticle core is undetermined. Moreover, these techniques do not

describe the stability of the bare nanoparticle (i.e. nanoparticle without any conjugation and

functionalization) which is governed by the binding interaction of the stabilizer and the

65

nanoparticle core. Raman scattering contains bonding specific information between the

stabilizer, Raman dye, polymer or lipid, and the nanoparticles, but the low sensitivity of

conventional Raman spectroscopy prevents these information to be observed. Currently there is

no conventionally technique available to confirm the binding of the functional molecules to the

nanoparticle core at the different synthesis stages. Since the nanoparticle interface provides the

nanoparticle its sensitivity, stability, solubility and capability of interacting with the

biomolecules, it is important that we understand the actual binding between the particle core and

the different functional molecules at the molecular level. The lack of understanding in the actual

particle-molecule binding limits the way which we can diagnosis any practical issues and further

improve the nanoparticle systems to the extent that it can be fully realized for cancer detection.

In the cost perspective, it is very expensive to acquire all four systems for nanoparticle

characterization if a research group or company is to purchase them or utilized facilities from

labs that have them. Therefore, there is an emerging need to development a technique which

enables monitoring of the molecular bindings between the nanoparticles and the functional

molecules.

In this chapter, effort to achieve monitoring of molecular binding interaction at the nanoparticle

surface is established using Raman spectroscopy. We will demonstrate the use of the PCF-

enhanced Raman spectroscopy for characterizing two gold nanoparticles during the different

stages of the synthesis process, namely, the pegylated nanoparticle system and the phospholipid

nanoparticle system. Binding interactions between the nanoparticle core, and the different

functional molecules essential for target-based cancer diagnostics, including the stabilizer,

Raman dye and PEG coating, were observed. In addition, no binding was observed between the

phospholipid coating and gold nanoparticle core which supported that lipid encapsulations are

66

electrostatically governed. Furthermore, the stability of the two nanoparticle systems was tested

under different laser operation for Raman spectroscopy. Degradation of the nanoparticle system

with increasing laser power and exposure time was observed. Carbonization of the nanoparticle

solution was detected at 8.5 – 17 mW of power with a 633 nm laser for exposure duration longer

than 9 minutes. This suggests that localized heating of the gold nanoparticle can alter its binding

interactions with functional molecules which destroy the nanoparticle system, rendering it

nonfunctional for cancer detections or lead to faulty information in medical diagnostics,

particularly in Raman imaging.

(5.2) Experimental Details

Two different gold nanoparticle systems were synthesized for this study, namely, the pegylated

gold nanoparticles tagged with malachite green isothiocyanate (MGITC) dye and the

phospholipid gold nanoparticles tagged with crystal violet (CV). The schematic of the

nanoparticles and the molecular structures of the stabilizer, dye, polymer and lipids are shown in

Figure 5.2. Both nanoparticle systems utilized citrate stabilized colloidal gold particles with a

core diameter of 60 nm. The pegylated and phospholipid gold nanoparticles were provided by

Dr. Gilbert Walker’s group in the Chemistry Department at the University of Toronto and Dr.

Gang Zheng’s group in the Medical Biophysics Department also at the University of Toronto,

respectively. These pegylated and phospholipid nanoparticles were designed and synthesized for

the detection of leukemia and lung cancer cells using the imaging technique with Raman

spectroscopy, respectively. The general scheme for synthesizing the two nanoparticle systems is

shown in Figure 5.3. In general, gold nanoparticles stabilized with citrate were first coated with

the Raman dye. Then, they were coated with either the PEG polymer or phospholipid layer for

further stability. The synthesis details for the nanoparticles systems can be found in literature of

67

Nguyen et al.112

for the pegylated gold nanoparticles and Tam et al.113

for the phospholipid gold

nanoparticles.

Figure 5.2: Molecular structure of (a) pegylated gold nanoparticle system (b) phospholipid gold

nanoparticle system.

68

Figure 5.3: Scheme of the nanoparticle synthesis process for (a) pegylated and (b) phospholipid

gold nanoparticles.

To study the binding interaction between the Raman dye, PEG, phospholipid and the gold

nanoparticle core, nanoparticles from different stages of the synthesis process were obtained and

studied using Raman spectroscopy. Table 5.1 shows the nanoparticle samples obtained for

studying the two nanoparticle systems. Raman characterizations were done using the new HC-

PCF modeled HC-1060 purchased from NKT Photonics. The HC-1060 has approximately the

same central hollow core size (~10 µm) as the HC-800 used for the CdTe study in Chapter 4. The

new HC-1060 was used because it provides greater Raman signal enhancement than the HC-800

due to the larger effective index of the PCF cladding.27

Figure 5.4 shows the Raman spectra of

69

water obtained using HC-800, HC-1060 and conventionally with a cover slide. Intensity of the

OH stretching mode at 3427 cm-1

is about 10 % greater with the use of HC-1060 than HC-800.

Preparation and setup of the HC-1060 for Raman experiments were the same as HC-800

discussed in Chapter 4. However, the fiber splicing program was modified to ensure all cladding

holes were collapsed while leaving the central core open to its maximum for solution filling. For

all measurements, continuous wave (CW) 633 nm HeNe was used with 0.17 mW of power and

averaged over 2 spectra unless otherwise specified.

Table 5.1: Nanoparticle samples studied for pegylated nanoparticle system and phospholipid

nanoparticle system.

Samples

Pegylated

Nanoparticles Citrate Stabilized Gold Nanoparticles

Pegylated Gold Nanoparticles with

MGITC Adsorbed

Pegylated Gold Nanoparticles without

MGITC Adsorbed

Phospholipid

Nanoparticles Citrate Stabilized Gold Nanoparticles

Citrate Stabilized Gold Nanoparticles

with CV Adsorbed

Phospholipid Coated Gold

Nanoparticles with CV Adsorbed

70

Figure 5.4: Raman spectra of water obtained using cover slide, HC-800 and HC-1060.

Enhancement of the OH stretching mode at 3421 cm-1

is about 101 times and 112 times relative

to the conventional technique respectively. Spectra are obtained with 0.17 mW of power with a

HeNe (633 nm) laser.

(5.3) Molecular Interactions in Pegylated Gold "anoparticles

(5.3.1) Citrate Stabilized Gold 4anoparticles

Raman spectra of the pegylated nanoparticles were obtained at the different stages of the

synthesis process. In the first stage, gold nanoparticles were synthesized and stabilized by

citrates to maintain their size and shape for a controlled surface plasmon effect for Raman

enhancements. Although gold nanoparticles used in this experiment, and that of the phospholipid

nanoparticles, were purchased commercially from Ted Pella Inc., these nanoparticles degrade

after a period of storage time, which cause failures of dye adsorption if they are used for

functionalization. Figure 5.5 shows the Raman spectrum obtained from a fresh batch of citrate

stabilized gold nanoparticles. In particular, stretching modes of the carboxylates between 1380

and 1600 cm-1

are of interest here. These Raman modes are associated with the three carboxylic

71

acid groups of the citrate. These carboxylic acid groups are known to form complexes with the

gold surface which stabilizes the nanoparticle core after deprotonation.119, 120

The Raman modes

at 1381, 1445 and 1544 cm-1

are assigned to the symmetric stretches of the carboxylates. The

mode at 1597 cm-1

is assigned to the asymmetric stretch of the carboxylates. Similar to the CdTe

nanoparticles discussed in section 4.3, the wavenumber difference between the symmetric and

asymmetric stretch of the carboxylates mode indicates any complexes formed between the metal

and the carboxylates (i.e. the gold-carboxylate complex in this case).

Figure 5.5: Raman spectrum of a fresh batch of gold nanoparticles stabilized by citrate. Inset:

enlarged spectrum showing carboxylate stretching modes of citrate.

As a reminder to the reader, if the wavenumber difference between the symmetric and

asymmetric modes of the carboxylate is greater than those of the ionic values, a unidentate

complex is formed. Conversely, if the wavenumber difference is much lower than the ionic

values, then a bridging bidentate complex is formed. Lastly, if the wavenumber difference is

greater than that of the bridging bidentate and closes to the ionic values, then a chelating

72

bidentate complex is formed. For liquid solutions similar to citrate, the symmetric stretches of

carboxylate anion are located around 1414 – 1425 cm-1

where the asymmetric stretch of the

carboxylate anion is located around 1560 – 1580 cm-1

.121

Therefore, the wavenumber difference

between the ionic carboxylate symmetric and asymmetric stretching mode is about 145 – 164

cm-1

. With 100 mW of power using an Argon ion laser (514.5 nm), Munro et al. also observed

carboxylate symmetric stretching modes between 1400 and 1575 cm-1

from citrate stabilized

silver nanoparticles.119

Table 5.2 shows the wavenumber position of the symmetric and asymmetric stretching modes of

carboxylate, their respective intensity ratio to the carboxylate asymmetric stretch, wavenumber

differences and proposed complexes according to the discussion above. The table shows that one

carboxyl group of the citrate is ionic which means that it did not form a complex with the

nanoparticle and was exposed on the surface. In addition to the carboxylate anions, citrate

formed unidentate and bridging bidentate complexes with the nanoparticle core. This finding

correlates with the proposed interactions of Munro et al. in which two of the three carboxyl

groups interact with the citrate reduced colloid and the third group is exposed on the surface

observed from his surface enhanced resonant Raman spectroscopy.119

Therefore, fresh gold

nanoparticles used in this experiment were stabilized strongly by citrates through both bidentate

and unidentate bindings. In addition, Table 5.2 shows that the intensity ratio of the symmetric

mode at 1544 cm-1

to the asymmetric mode at 1597 cm-1

is greater than one while the ratio of the

other two symmetric modes are less than one. This shows that more bridging bidentate

complexes were formed than unidentate and ionic complexes which suggests that the bridging

bidentate complex is more favourable in stabilizing the nanoparticle core, probably due to the

stronger bonding strength of the bidentate complex than the unidentate.

73


modes of citrate, their intensity ratio to νas(COO), respective wavenumber differences and

proposed complexes for a fresh batch of citrate stabilized gold nanoparticles.

νs(COO) νas(COO)

Intensity Ratio

of νs(COO) to

νas(COO)

Wavenumber

Separation between

νs(COO) and νas(COO)

Proposed

Structure

1381 1597 0.474 216 cm-1

Unidentate

1445 1597 0.867 152 cm-1

Ionic

1544 1597 1.703 53 cm-1

Bridging Bidentate

To compare the structural difference between the degraded nanoparticles and the fresh ones,

Raman spectrum of an old batch (~ 6 months old) of citrate stabilized nanoparticles stored in the

dark at room temperature was obtained (Figure 5.6). The Raman modes at 1386 and 1454 cm-1

are assigned to the symmetric stretches of the carboxylates. Although the water mode at ~1652

cm-1

is strong, the carboxylate stretching modes at 1553 cm-1

and 1597 cm-1

were determined

through fitting three Gaussian functions to the broad peak at ~1600 cm-1

. The Raman mode at

1553 cm-1

is assigned to the symmetric stretch while the 1597 cm-1

is assigned to the asymmetric

stretch of the carboxylates. Table 5.3 shows the carboxylate-metal complexes determined from

the carboxylate stretching modes. The same three complexes were found in the degraded

nanoparticle solution. However, the intensity ratio of the unidentate and ionic complexes are

greater than one instead of less than one in the fresh nanoparticles. The ratio of the bidentate

complexes is also reduced to less than one. This suggests that some of the bidentate complexes in

the nanoparticle solution were changed to unidentate complexes, ionic complexes and probably

even detached from the gold core. Due to the lost of surface enhancement from the gold

nanoparticles, free flowing citrates could not be detected for comparison. Nevertheless, the

reduction of bidentate complexes suggests that stabilization of the gold cores was weakened

74

through long storage time at room temperature. Furthermore, the lost of citrate anions might

reduce the adsorption strength of the Raman dye as well as the phospholipid coating (in the case

of the phospholipid nanoparticles system) through electrostatic interactions.

Figure 5.6: Raman spectrum of six-month-old gold nanoparticles stabilized by citrate. Inset:

enlarge spectrum showing carboxylate stretching modes of citrate.


modes of citrate, their intensity ratio to νas(COO), respective wavenumber differences and

proposed complexes for a six-month old batch of citrate stabilized nanoparticles.

νs(COO) νas(COO)

Intensity Ratio

of νs(COO) to

νas(COO)

Wavenumber

Separation between


Proposed

Structure

1386 1597 1.157 211 cm-1

Unidentate

1454 1597 1.209 143 cm-1

Ionic

1553 1597 0.658 44 cm-1

Bridging Bidentate

75

(5.3.2) Pegylated Gold 4anoparticles with MGITC Adsorbed

The next step in the synthesis process was to adsorb the Raman dye, MGITC, on to the gold

nanoparticle. However, without attaching PEG polymers to the nanoparticles, MGITC might

detach quickly. Therefore, we obtained Raman spectrum of the pegylated gold nanoparticles with

MGITC adsorbed instead (Figure 5.7). The nanoparticle spectrum shows many intense and

narrow (small FWHM) Raman peaks between 200 – 1630 cm-1

. These peaks correspond to the

vibrational modes of MGITC adsorbed on to the gold nanoparticle surface. Figure 5.8 compares

the Raman spectrum of pure MGITC solution with the pegylated nanoparticles before

background subtraction. The MGTIC spectrum before background subtraction not only contains

a broad background from the fluorescence, the intensity of the Raman modes is also much lower

than those obtained from the pegylated nanoparticles. This is because the strong fluorescence of

MGITC is quenched by the enhanced local EM field at the gold surface in the pegylated

nanoparticles. Since Raman modes do not photobleach (i.e. cannot be quenched by strong EM

field), they retained in the spectrum. In fact, the surface enhancement of the gold nanoparticles

increased the Raman scattering signal from MGITC, resulted in a much intensified Raman

spectrum of MGITC. The matching Raman modes between the two spectra further confirms that

the intense Raman modes in the nanoparticles spectrum are those of MGITC. Therefore, the

intensified Raman signal of MGITC in the nanoparticle spectrum was due to surface

enhancement of the gold nanoparticles; hence indicating that MGITC was adsorbed on to the

nanoparticles surface.

76

Figure 5.7: Raman spectrum of pegylated gold nanoparticles adsorbed with MGITC obtained

with 0.17 mW.

Figure 5.8: Raman spectrum of pegylated gold nanoparticles with MGITC adsorbed and pure

MGITC solution before background subtraction (a) and after background subtraction (b). Spectra

obtained with HeNe laser (633 nm) at 0.17 mW.

77

(5.3.3) Pegylated Gold 4anoparticles

It is believed that the sulphur atom in the thiol-modified PEG binds to the gold nanoparticles

core after replacing the citrate as the stabilizer because of the strong binding affinity of the

sulphur atom.122-125

However, Raman modes of citrate are much weaker than MGITC (as shown

when we compare the intensity counts of the MGITC modes in Figure 5.7 and the citrate modes

in Figure 5.5) due to the strong Raman cross-section of the phenyl rings in MGITC.126

Therefore,

the absence of citrate modes in the pegylated nanoparticle spectrum in Figure 5.7 does not

indicate that the citrates were desorbed from the nanoparticles surface. If citrates were still

stabilizing the gold nanoparticle core, the strong MGITC modes would most likely be overlapped

by the carboxylate stretching modes of citrate since they overlap between the 1390 – 1600 cm-1

regions of the spectrum. Similarly, Au-S stretching modes at the lower Raman shift regime

around 300 cm-1

would also be overlapped by the strong MGITC modes. Thus, it is difficult to

determine the bindings between the PEG, citrate and the nanoparticle core after the Raman dye is

adsorbed.

To determine the binding interaction between thiol-modified PEG and the nanoparticles, we

obtained Raman spectrum of the pegylated gold nanoparticles without any Raman dye adsorbed

(Figure 5.9). At a higher pump power (i.e. 8.5 mW), the spectrum shows stretching mode of Au-

S bonds at 305 cm-1

in addition to the carboxylate stretching modes from gold-citrate bindings.

Stretching modes of Au-S are typically reported to be between 200 – 240 cm-1

.127

However,

when the sulphur atom is attached to a long polymer chain, this mode is shifted to the Raman

shift region between 300 – 310 cm-1

.128

Nevertheless, this mode would have been overlapped by

the MGITC mode at 233 cm-1

in the MGITC adsorbed nanoparticle sample; thus, it was not

observed in the spectrum of the nanoparticles with MGITC adsorbed. Nevertheless, the Raman

78

mode at 305 cm-1

observed with the pegylated nanoparticles without MGITC indicates that thiol-

modified PEG polymers were bind to the surface of the gold nanoparticles. Moreover, we have

confirmed that the binding was made through the gold-sulphur bond. Figure 5.10 shows the UV–

vis spectra, TEM and DLS results of the same pegylated nanoparticles obtained from a different

batch of synthesis. The UV–vis spectra show that absorption of the nanoparticles remained the

same after pegylation which means that the size of the gold core did not changed and the desired

surface plasmon effects were maintained after pegylation. The bright “white” ring around the

nanoparticle in the TEM image shows that PEG polymers were functionalized on the

nanoparticle surface, indicating that pegylation was successful. Moreover, the size up-shift in the

DLS histogram shows that the average nanoparticle sizes were increased in the system. This

further confirms that the nanoparticles were functionalized with PEG polymers as we have

observed with the Raman spectra. Although these results were obtained from a different batch of

nanoparticles, they indicate that the pegylation method used in this experiment for Raman

characterization works and that our analysis of the Raman results are promising.

Figure 5.9: Raman spectrum of pegylated gold nanoparticles. Inset left: Raman mode of Au–Cl

at 259 cm-1

and Au–S at 305 cm-1

. Inset right: Stretching modes of carboxylates.

79

Figure 5.10: UV–vis (a), TEM (b) and DLS (c) results obtained from a pegylated nanoparticle

system synthesized using the same method and materials.112

To our surprise, the carboxylate stretching modes are still observed in addition to the Au-S

stretching mode. This suggests that only some of the citrates were replaced with the Au-S bonds

from pegylation. The wavenumber separations between the symmetric and asymmetric mode are

shown in Table 5.4. The carboxylate-gold complexes determined in the table show that citrates

are still stabilizing the gold nanoparticles through both the unidentate and bridging bidentate

complexes after pegylation. Thus, the strong binding affinity of the sulphur atom did not

completely replace the citrate as the stabilizer for the gold nanoparticles in this case, might be

80

due to the lack of PEG polymers. The intensity ratios of the complexes show that most of the

carboxylate–gold bindings are unidentate instead of the stronger bridging bidentate, which

suggests that many of the bridging bidentate complexes were removed or changed to unidentate

complexes through breaking one of the Au-O bonds as a result of the strong affinity of the Au–S

bindings.


modes, their respective wavenumber differences and proposed complexes in pegylated gold

nanoparticles.

νs(COO) νas(COO)

Intensity Ratio

of νs(COO) to

νas(COO)

Wavenumber

Separation between


Proposed

Structure

1375 1595 1.366 220 cm-1

Unidentate

1465 1595 0.750 130 cm-1

Ionic

1539 1595 0.525 56 cm-1

Bridging Bidentate

It is important to note that the carboxylate stretching modes of the citrate stabilized nanoparticles

and the Au-S stretching mode of the pegylated nanoparticles were not observed using

conventional Raman spectroscopy when the nanoparticle solutions were studied on a cover slide

(see Appendix C.1 and C.2). The background-to-signal ratio of the MGITC modes from MGITC

adsorbed nanoparticles was also enhanced with HC-PCF compared to those obtained using the

conventional Raman techniques (Appendix C.3). Approximate enhancements of the detected

Raman scattering signal from the different nanoparticle solutions using the HC-PCF platform is

calculated in Appendix C. The enhanced Raman signal can be attributed to the efficient Raman

scattering of the particles using the HC-PCF platform. Furthermore, the short exposure time (2 s)

and low power exposure (hundreds of microwatts to a few milliwatts) of the pump laser ensured

81

that the characteristics of the nanoparticle system were most representative to its as-synthesis

state due to the highly non-destructive nature of the PCF platform.

(5.4) Molecular Interactions in Phospholipid Gold "anoparticles

(5.4.1) Citrate Stabilized Gold 4anoparticles

Citrate stabilized gold nanoparticles and CV adsorbed gold nanoparticles, before and after

coating with phospholipids, were obtained for Raman characterizations. First, commercially

synthesized 60 nm gold nanoparticles stabilized by citrate was characterized using Raman

spectroscopy with HC-PCF (Figure 5.11). Similar to the citrate stabilized gold nanoparticles used

for synthesizing the pegylated nanoparticles, carboxylate stretching modes of the citrates is

observed between 1390 and 1600 cm-1

. The Raman modes at 1337, 1446, and 1573 cm-1

are

attributed to the symmetric stretching modes of carboxylate. The Raman mode at 1608 cm-1

is

attributed to the asymmetric stretching modes of carboxylate. Table 5.5 shows the proposed

carboxylate-metal complexes derived from the symmetric and asymmetric stretches of the

carboxylate Raman modes. The same three complexes (unidentate, bridging bidentate and ionic)

were formed as the citrate stabilized nanoparticles studied in the pegylated nanoparticle system.

This is reasonable as both citrate stabilized gold nanoparticles are 60 nm in diameter and they

were both purchased from Ted Pella Inc. However, the intensity ratios of the three complexes

measured in this experiment are similar which differs from the pegylated nanoparticle system in

which more bidentate complexes were presented. This difference could be related to the different

batches of the nanoparticle purchased or the different duration and condition of the storage time.

Nevertheless, the similar structures formed between the citrate and the gold core shows that both

nanoparticle systems were stabilized similarly at the time of synthesis.

82

Figure 5.11: Raman spectrum of 60 nm gold nanoparticles stabilized by citrate. Inset: enlarged

spectrum showing carboxylate stretching modes of citrate.


modes, their respective wavenumber differences and proposed complexes for a fresh batch of

citrate stabilized gold nanoparticles for synthesizing the phospholipid nanoparticle system.

νs(COO) νas(COO)

Intensity Ratio

of νs(COO) to

νas(COO)

Wavenumber

Separation between


Proposed

Structure

1337 1608 0.717 271 cm-1

Unidentate

1446 1608 0.392 162 cm-1

Ionic

1573 1608 0.556 35 cm-1

Bridging Bidentate

(5.4.2) CV Adsorbed Gold 4anoparticles

The next step in the synthesis process is to attach the Raman dye (i.e. CV) onto the nanoparticle

surface to provide a Raman signal. Raman spectrum of the CV adsorbed nanoparticles is shown

in Figure 5.12 along with the spectrum of CV solution before and after background subtraction.

Similar to MGITC solution, Raman spectrum of the CV solution contains a broad fluorescent

signal overlapping Raman modes of CV as shown in the raw Raman spectrum. In comparison,

83

Raman spectrum of the CV adsorbed gold nanoparticles shows enhanced Raman modes of CV.

This enhancement can be attributed to the surface enhanced Raman scattering due to the gold

nanoparticles after CV was adsorbed on it. Therefore, this enhancement confirms that the CV

dye was successfully adsorbed onto the gold nanoparticles surface. After the removal of

fluorescent signal (Figure 5.12b), it can be seen that the Raman modes from CV adsorbed gold

nanoparticles match closely with those from the CV solution. This further confirms that the

enhanced Raman modes are CV Raman modes; thus, CV dyes were adsorbed onto the gold

surface successfully.

Figure 5.12: Raman spectrum of CV adsorbed gold nanoparticles and CV solution without

background subtraction (a) and with background subtraction (b).

84

(5.4.3) Phospholipid Gold 4anoparticles Adsorbed with CV

The final step in preparing the phospholipid nanoparticles is to coat the CV adsorbed gold

nanoparticles with phospholipid layers for strong biocompatibility. Typically, TEM images and

DLS are used to confirm the effectiveness of the coating after the synthesis process. Raman

spectroscopy is difficult to obtain much information about the coating as phospholipid layers do

not actually bond with the gold nanoparticles core, citrate or the Raman dye. Therefore, we

expect to see no new emerging or disappearances of Raman modes to show that no bonding has

actually formed. However, in conventional Raman spectroscopy, even if Raman spectra of the

phospholipid nanoparticles resemble those of the CV adsorbed nanoparticles without the

phospholipid, the low sensitivity of conventional systems might have prevented weak changes

from being observed. With the use of HC-PCF, the results hold more promises as it provides

higher sensitivity than conventional techniques. It is important to note that although an extra C-C

bond is formed between DMPC (C36H72NO8P) and MHPC (C22H46NO7P) lipids that forms the

phospholipid coating, the addition of one C-C stretch mode is difficult to observe in the

spectrum. This is mainly because of two reasons. First, C-C stretching mode is weak relative to

the phenyl rings of CV. Second, the phospholipid layer is isolated from the gold nanoparticle

surface by the citrate due to their electrostatic interactions; therefore, Raman signals of the lipids

are not enhanced more than the Raman dye. As a result, the strong CV modes would have

overlapped the C-C stretches of the lipids, preventing it from being observed.

Figure 5.13 compares the Raman spectrum of CV adsorbed gold nanoparticles with and without

the phospholipid coating. Both of the Raman spectra contain the same Raman modes which are

those of the CV dye as we expect due its strong Raman cross-sections. More importantly,

additional C-C stretching mode or mode shifting, broadening and relative intensity changes of

85

the CV modes are not observed. This provides supporting evidence that the phospholipid layer

did not form bonds with the gold core or the Raman dye and potentially binding with the gold

nanoparticles core through electrostatic interaction. In addition, intensity of the CV modes from

phospholipid nanoparticles is found to be reduced compared to that without the coating. This is

probably due to the higher light absorption of the phospholipid at the 633 nm laser line and the

scattered Raman signal which confirms the presence of the phospholipid. UV–vis spectra

throughout the synthesis process are shown in Figure 5.14. These spectra were obtained from a

different batch of nanoparticles synthesized using the same recipes as this experiment. UV–vis

spectra show that the absorption peak of the nanoparticles remained unchanged after the addition

of CV and phospholipid coating. This confirms that the surface plasmon resonance of the

nanoparticles was retained. In addition, it is important to note that the absorption peak of the

nanoparticles after coating with the phospholipid is shifted slightly to the higher wavelength.

This correlates with our speculation that the absorption of the phospholipid is higher in the red

part of the spectrum which reduced the Raman scattering of the Raman dye. Thus, this confirms

the presence of the phospholipid layer and that it did not bond with the nanoparticles core to

form the lipid coat.

Figure 5.13: Raman spectrum of CV adsorbed gold nanoparticles before and after coating of

phospholipid layer. 0.17 mW of CW HeNe laser was used with 2 s exposure time.

86

Figure 5.14: UV–vis spectra of the phospholipid nanoparticles throughout the synthesis

process.113

The use of HC-PCF enhanced the detected Raman signal enabling Raman modes of citrates to be

determined. Raman modes of CV from the nanoparticles were also enhanced compared to

conventional technique. A comparison of the Raman signals between the use of a HC-PCF and a

cuvette from phospholipid nanoparticles at the different stages are shown in Appendix C.4 – C.6.

(5.5) Pump Power and Duration Dependence of Pegylated Gold "anoparticles

The second part of this characterization chapter is to study real time changes of the nanoparticle

systems under different pump power and exposure durations. Although Raman spectroscopy is

known to provide non-destructive characterization of the sample species, high power exposure

and long exposure durations on stabilized nanoparticles could still destroy their bindings with the

stabilizer, Raman dye, and coating; thus, deteriorating the nanoparticles system and rendering it

not functional.

To study effects of the laser power and exposure time on nanoparticles, Raman spectra of the

pegylated gold nanoparticles with MGITC adsorbed were obtained continuously under different

87

laser powers (i.e. 0.17, 1.7, 4.3, 8.5 and 17 mW). Beginning with 0.17 mW, Raman spectra were

obtained continuously with 2 s exposure time (averaging over 2 spectra) using CW HeNe (633

nm) laser. Due to the wide scanning range of the spectra, the total laser exposure time on the

nanoparticles was about 30 s per measurement. A 15 s gap with no laser exposure was in place

after each measurements to allow the grating to move back to its initial position before the next

measurement begun. Spectra were taken continuously at the same laser power until the spectrum

was stable (i.e. no significant broadening, intensity drop, or mode shifting) for 2 – 3 spectra.

Figure 5.15 shows the Raman spectra of MGITC adsorbed pegylated gold nanoparticles obtained

with power ranging from 0.17 mW to 17 mW (i.e. the maximum power output of the pump

laser). These spectra show intense Raman modes of MGITC between 200 and 1650 cm-1

as

discussed in section 5.3.2. MGITC modes obtained at 0.17 mW and 1.7 mW are identical

although their intensities are reduced in subsequent spectra at the same laser power (i.e. with

prolonged exposure time at the same power). With power increased to 4.25 mW, all MGITC

modes still retain in the spectra except that the intensity of the modes fluctuate as shown in

spectrum 10 and 11. Mode intensity in spectrum 10 is reduced relative to the previous spectrum

(i.e. spectrum 9). However, the intensity improved again in subsequent spectrum (i.e. spectrum

11). In addition, there are also some changes in the relative intensities of the peaks around 1400

and 1500 cm-1

. These intensity fluctuations and relative intensity changes could be caused by the

ensemble average orientations of the MGITC dye that were changing dynamically relative to the

gold surface during the measurements.83, 129

Since SERS enhancement is greatest when

molecular bonds are perpendicular to the gold surface, orientation changes in the molecular

bonds would lead to changes in the mode intensities and relative intensities. Nevertheless, all

Raman modes of MGITC are present in the spectra (compared to the spectra obtained with

88

Figure 5.15: Raman spectra of MGITC absorbed pegylated gold nanoparticles with increasing

power from 0.17 mW (a) to 1.7 mW (b), 4.3 mW (c), 8.5 mW (d) and 17 mW (e). Spectra

obtained with the same power were taken with increasing numerical value. The pump laser was

exposed for a total of 30 s for each measurement.

89

0.17 mW); therefore, no major structural changes of the nanoparticles system are observed at this

power level. However, at about 8.5 mW, Raman modes at 1447, 1486, 1512 and 1529 cm-1

(the

weak modes between the two strong ones around ~1500 cm-1

) begun to broaden. At the

maximum power of our pump source (i.e. 17 mW), intensities of the MGITC modes begin to

reduce and Raman modes between 1183 and 1627 cm-1

broaden further until only two broad

features, with FWHM of about 150 – 300 cm-1

, center at 1353 and 1583 cm-1

were left after

prolonged measurements. The intensity reduction of the MGITC Raman modes indicates that

MGITC molecules were detaching from the gold nanoparticle core. Since the intense MGITC

Raman modes were result of the surface enhancement of the gold nanoparticles, mode intensities

of the MGITC dye would significantly reduce when some MGITC dyes are detached from the

nanoparticle surface. MGITC detachment could be caused by excessive heating of the

nanoparticles upon long laser exposure time at high power. Metallic nanoparticles can be

efficiently heated by laser illumination at or near their plasmon resonance.130

Govorov et al. has

also reported that the surface of the nanoparticles could even heat up by almost 25 degrees

Kelvin with an array of nanoparticles.131

Furthermore, it was observed that an increased quantity

of nanoparticles would cause a stronger increase in the temperature of the system. Since

nanoparticles tends to cluster around the inner side wall of the PCF core (Figure 5.16), probably

due to electrostatic effect, significant temperature increase might be experienced by the

nanoparticles system causing decomposition and detachment of the MGTIC dye. On the other

hand, MGITC itself might have also decomposed due to strong absorption of the 633 nm pump

laser since the pump wavelength locate at the absorption peak of MGITC solution (Figure 5.17).

90

Figure 5.16: Optical image of nanoparticle clusters around the inner wall of the PCF core after

about 1 minute of selective filling. Bright dots around the PCF core at the inner wall are the

nanoparticles.

Figure 5.17: Absorption of MGITC.132

Red line shows the excitation of the HeNe laser at 633

nm which locates at the absorption peak of the MGITC dye.

The cause of mode broadenings at 1353 and 1583 cm-1

is not as clear. However, this broadening

occurs at the spectral region where the carboxylate stretching modes were observed in the citrate

stabilized nanoparticles. Therefore, we further investigated the molecular changes of the citrate

stabilized gold nanoparticles with increasing laser power and exposure time (Figure 5.18). With

pump laser power between 0.17 mW and 8.5 mW, citrate modes were observed although

91

intensity of the modes around 1600 cm-1

were much more enhanced than the other modes.

Nevertheless, the three symmetric stretching modes and one asymmetric stretching modes of

carboxylate could be observed between 1390 and 1600 cm-1

in these spectra. However, the

Raman mode at 1585 cm-1

broadened when power was increased from 4.3 mW to 8.5 mW. At

maximum power of the pump laser (i.e. 17 mW), Raman modes between 1100 and 1700 cm-1

broadened just like the spectra obtained from the MGITC adsorbed pegylated gold nanoparticles

shown in Figure 5.15e. After prolonged laser exposure, two similar broad peaks at 1353 and

1583 cm-1

were also observed. This suggests that the two broad peaks observed from the MGITC

adsorbed pegylated gold nanoparticle spectra were caused by changes in citrate-gold bindings.

The position and broadness of two peaks at 1353 and 1583 cm-1

find similarities to those

reported for carbon related structures including amorphous carbon film,133

graphite,134, 135

and

carbon nanotubes.136-138

In fact, these signals have been observed for small quantity of ‘graphitic’

carbon in silver (i.e. less than a monolayer of amorphous carbon).139, 140

In these literatures, two

carbon related peaks were reported: (1) between 1280 – 1450 cm-1

and (2) between 1520 – 1600

cm-1

. The former is referred to as the D peak (D for disordered) in which the feature correlates to

the bond angle disorder between carbon atoms. A highly disordered carbon structure, such as

benzene clusters, contributes to an intense D peak in the Raman spectrum.133

The latter carbon

peak is referred to as the G peak (G for graphite) as it is correlated to graphite-like structures.

The G peak may also arise from C=C stretch vibrations of olefinic (unsaturated hydrocarbon

chain having a general formula CnH2n which involves a carbon double bond) or conjugated

carbon chains and aromatic rings such as benzene rings.141-143

Both the D peak and G peak might

have linewidth ranging from 10 to 160 cm-1

depending on the carbon cluster size, distribution,

stress and environment.

92

Figure 5.18: Raman spectra of citrate stabilized gold nanoparticles with increasing power from

0.17 mW (a) to 1.7 mW (b), 4.3 mW (c), 8.5 mW (d) and 17 mW (e). Spectra obtained with the

same power were taken with increasing numerical value. Spectrum 25 and 26 are not shown due

to saturation of the detector over the whole spectrum. The pump laser was exposed for a total of

30 s for each measurement.

93

According to Schwan et al.,133

hydrogenated amorphous carbon film with G linewidth broader

than 45 cm-1

contains mostly small aromatic clusters with cluster sizes less than 1 nm. Although

this result was concluded from data obtained with 514.5 nm laser line, the broad G line in the

MGITC adsorbed gold nanoparticles (FWHM of 124 cm-1

) suggests that the actual structure

formed should not differ by a large extent.

The source of carbon for these aromatic clusters is not straight forward to us. Formation of

amorphous carbon structures were observed in varies conditions with different carbon sources

involved. For example, Raman signal of amorphous carbon was observed in silver island films

prepared by physical vapour deposition at room temperature on sapphire in vacuum below 10-9

Torr in which the only possible source of carbon is CO from the rest gas.144

Carbon

intermediates were also observed by Raman spectroscopy on surface of the silver electrode in

which intermediates were believed to be formed from dissolved CO2.145

Moreover, Otto et al.

reported that G and D peak at 1350 and 1595 cm-1

were observed when a freshly polished silver

sample was dipped into cyanide solution exposed to air.146

Tsang et al. proposed that adsorbed

amorphous carbon with short range order of graphite was formed.139

Tsang et al. further showed

that Raman signal of amorphous carbon signal increased under irradiation with 514.5 nm

radiation of 10 W/cm2 while signal of a monolayer of benzoic acid on a rough silver film

(evaporated on a rough CaF2 substrate film) at room temperature was decreased. This suggested

that amorphous carbon layers were formed at the expense of the benzoic acid monolayer.

Recently, Hore et al. reported that carbon layers could be formed on gold by simply blending

gold oxide with inert aliphatic polymers at 250 oC in which the carbon nanolayers were

stabilized by anionic gold after melting of the polymer.147

In addition, we have also observed the

94

D and G peaks at 1375 and 1580 cm-1

on polymer encapsulated gold and silver nanoparticles

obtained from Vivo Nano Inc. (Figure 5.19).

Figure 5.19: Raman spectra of polymer encapsulated gold and silver nanoparticles obtained

from Vivo Nano Inc.

In our experiment, the possible sources of carbon can be contaminants in the nanoparticle

solutions resulted from its synthesis process or carbon species formed from decompositions of

the citrate ligands during the Raman spectra acquisitions. Decompositions could be possible in

citrate stabilized gold nanoparticles if surface of the gold nanoparticles were oxidized. With

excessive heating, thermal denaturation in citrate could leads to formations of short polymer

chains28

and then amorphous carbon structures. Raman signal of small quantities of amorphous

carbon due to thermal denaturation of free flowing citrates might also be possible since

amorphous carbon signals are very strong in Raman. However, further investigation is required

to confirm the source of carbonization in these citrate stabilized nanoparticles.

95

(5.6) Pump Power and Duration Dependence of Phospholipid Gold

"anoparticles

The effects of pump power and exposure time on CV adsorbed phospholipid nanoparticles are

found similar to the pegylated nanoparticle system. Figure 5.20 shows the Raman spectra of CV

adsorbed phospholipid gold nanoparticles obtained with power ranging from 0.17 mW to 17

mW. The phospholipid layers were formed by SHPC (C26H54NO7P) and DMPC instead of

MHPC and DMPC; however, the absorption and Raman spectra of the nanoparticle system are

the same between the uses of the two different lipid layers. Similar to the pegylated nanoparticle

study, spectra were obtained continuously until spectrum remained stable (i.e. no significant

broadening, intensity drop or mode shifting) for 2 – 3 spectra.

At 0.17 mW and 1.7 mW, strong CV modes are observed indicating that CV molecules were

successfully adsorbed onto the gold nanoparticle surface. For 8 to 10 spectra, CV Raman modes

did not shift nor disappeared; however, intensity of the CV Raman modes drops by about 50 –

60% relative to those obtained in the very first spectrum at each power. This suggests that

prolonged laser exposure might cause minor detachment of the CV molecules, probably due to

increased local temperature from clustering and heating of the gold nanoparticles or

photodecomposition of the CV due to absorption of the 633 nm laser line. In addition, the

relative intensity of the mode at 1625 cm-1

reduced significantly with prolonged laser exposure.

The mode at 1625 cm-1

is attributed to the stretching mode between one of the three phenyl rings

and a nitrogen atom. The intensity reduction of this mode suggests that there is a photo induced

decomposition of the CV dye at the phenyl ring – nitrogen bond. Nevertheless, strong CV

Raman modes retained and stabilized after 8 – 10 measurements.

96

Figure 5.20: Raman spectra of CV adsorbed gold nanoparticles with increasing power from

0.17 mW (a) to 1.7 mW (b), 4.3 mW (c), and 17 mW (d). The pump laser was exposed for a total

of 30 s for each measurement.

97

At 4.25 mW, however, intensity of the Raman modes not only reduced further, spectral region

between 1000 and 1760 cm-1

broadened and transformed into two broad peaks around 1377 and

1583 cm-1

. At maximum power of the pump laser (i.e. 17 mW), the spectrum is further

converged into two broad peaks, at 1353 and 1594 cm-1

, similar to that observed from the

pegylated nanoparticles. This suggests that clusters of carbon structures were formed, most likely

for the same reason as the pegylated nanoparticles. Moreover, narrow CV features have

disappeared which indicate that CV molecules were detached from the gold surface as surface

enhanced signals from the gold are not observed anymore. Furthermore, we have observed

further relative intensity reduction of the phenyl ring–nitrogen bond at 1625 cm-1

which indicates

that more CV dyes were photodecomposed. The decomposed CV dyes might have contributed to

the formation of the amorphous carbon clusters through forming phenyl ring clusters.

To confirm that the two broad peaks at 1353 and 1594 cm-1

were also contributed by citrates

stabilizing the gold nanoparticles through carbonizations as determined from the pegylated

nanoparticle experiment in section 5.5, molecular changes of the citrate stabilized gold

nanoparticles used in this experiment were investigated using Raman spectroscopy with

increasing laser power and exposure time (Figure 5.21). At 4.3 mW, the spectrum of citrate

stabilized gold nanoparticles converged into two peaks at 1353 and 1576 cm-1

. The peak at 1353

cm-1

is attributed to the D peak of carbon structure whereas the peaks at 1575 and 1594 cm-1

are

attributed to the G peak. These pairs of carbon peaks indicate that carbon structures were formed

in the nanoparticle system probably upon thermal denaturation of citrates as discussed at the end

of section 5.5. It is to our surprise that the intensity of these two carbon bands was reduced at 8.5

mW. A possible explanation is that carbon structures were only formed near the beam waist of

98

the laser where the intensity was the greatest. Slight shifting of the carbon structures or the laser

spot might have led to reduced signal of the carbon structures.

It is important to note that although carbonization effect in phospholipid nanoparticles is

observed at 4.25 mW whereas the same effect is observed at 17 mW in pegylated nanoparticles,

the pegylated nanoparticle was exposed to the pump laser for 4 times longer in duration (~120 s)

at 0.17 mW and almost twice longer (~120 s) at 1.7 mW. Therefore, excessive heating of the

nanoparticles at the lower pump power could still lead to carbonization in the nanoparticles

Figure 5.21: Raman spectra of citrate stabilized gold nanoparticles, used for phospholipid

nanoparticle synthesis, with increasing power from 0.17 mW (a) to 1.7 mW (b), 4.3 mW (c),

and 8.5 mW (d). The pump laser was exposed for a total of 30 s for each measurement.

99

solution as observed in the phospholipid nanoparticles. Nevertheless, the appears of the two

broad peaks around 1350 and 1580 cm-1

in the two nanoparticle systems show that both systems

are subject to carbonization at high laser power in short exposure time or low laser power at

prolonged laser exposure time. The detail changes of the nanoparticles system obtained in real

time was enabled by the HC-PCF with its nanoliter sampling volume and high sensitivity.

(5.7) Summary

In this chapter, pegylated and phospholipid gold nanoparticle systems were characterized at the

different synthesis stages using Raman spectroscopy. Bindings between citrate, PEG and gold

nanoparticle surface as well as adsorption of the MGITC and CV dyes were observed. The

complexes formed between carboxylate ions in the citrate and the gold surface as well as

bonding between sulphur atoms in the PEG and the gold core were observed nondestructively

only with the enhanced detected Raman signal using HC-PCF. In addition, no bonding was

observed between the phospholipid coating and the gold core even with the more sensitive HC-

PCF platform. This further supported the electrostatic interaction hypnotized between the two.

Furthermore, the effects of the pump laser power and prolonged exposure time on the two

nanoparticle systems were investigated in real time using HC-PCF. Decomposition of the Raman

dyes and formation of carbon structures were observed in both nanoparticle systems at high

pump power and prolonged exposure time. As these nanoparticles were designed for imaging of

cancer cells using Raman spectroscopy, these results suggest that appropriate laser parameters

should be considered for in vivo imaging studies. Particle stability under different thermal

conditions and laser operations should be characterized for the different nanoparticle designed.

Otherwise, high power density of the pump laser could deteriorate the nanoparticle system,

which not only renders it not functional, but also potentially lead to false imaging information.

100

Raman characterization of functional nanoparticles demonstrated in this chapter is most

promising with HC-PCF for its simplicity, superior sensitivity and truly nondestructive nature

that are unachievable conventionally.

101

Chapter 6

Conclusion

(6.1) Summary

Colloidal nanoparticles have proven to be a promising candidate for a wide variety of

applications including photovoltaics, optoelectronics, medical diagnostics, medical therapy and

many more. The complex requirements of these applications require design and synthesis of the

nanoparticles to be controlled and monitored down to the molecular level where binding

interactions can be observed. However, conventional characterization techniques have been

limited to certain properties of the nanoparticles, their compositions, or their physical structure in

the micrometer scale in which molecular bindings and detailed molecular complex cannot be

obtained. In this work, a new Raman spectroscopy technique utilizing PCF has been

demonstrated to enable sensitive detection of different colloidal nanoparticle systems down to

their molecular world.

102

First, in chapter 3, we demonstrated that the detected Raman signal obtained with HC-PCF is

enhanced by about 2 to 3 orders of magnitude compared to conventional Raman spectroscopy

with a cuvette. In addition, we showed both experimentally and theoretically by adapting to the

theory of liquid core waveguides that the enhancement increases with increasing length of the

PCF. The theoretical results further showed that the enhancement reaches a plateau at about 500

cm. By comparing the detected Raman signal enhancement of the PCF with that from TCT, we

showed that the enhancement in PCF is due to both the increased light–matter interactions and

increased collection efficiency of the signal. Nevertheless, it is not trivial to completely decouple

the enhancement due to each of the two components. However, for PCF lengths that are less than

approximately 10 cm, we roughly estimated that the increased light-matter interactions enhances

the Raman signal by about 3 to 4 times while the increased collection efficiency enhances the

signal by about 20 times.

In chapter 4, we further demonstrated that the detected Raman signal is enhanced by about 1 to 2

orders of magnitudes by selectively filling the PCF to enable light guidance by both TIR and

photonic bandgap effects. By selectively filling the PCF with thiol-capped CdTe nanoparticles,

we demonstrated that strong and well-resolved Raman modes of the CdTe semiconductor core,

capping ligands, and their interfacial structures were successfully observed and compared

without integrating any metallic nanoparticles for enhancement. To the best of our knowledge,

this is the first time that such strong Raman modes of the thiol-capped CdTe QDs in aqueous

solution have been reported. These strong Raman modes enabled many binding interactions and

properties of the QDs to be compared between the uses of different capping ligands. First,

intensity ratios of the Te A1 mode to CdTe LO mode showed that MPA-capped QDs were more

crystalline than that stabilized by TGA and TG which agreed with the higher PL quantum

103

efficiency of MPA estimated from the measured absorbance and PL. Second, the sulphur heavy

CdS0.7Te0.3 interfacial layer observed from the Raman spectra showed that thiol capping agents

stabilized the QDs through their sulphur atom as speculated in many literatures. Moreover, the

strong surface passivation by the CdS0.7Te0.3 component and the absence of photo-oxidization

observed from the Raman spectra demonstrated strong stability of these thiol-capped

nanoparticles. Third, the carboxylate stretching modes observed in the Raman spectra

demonstrated the co-existence of carboxylate–metal complexes. In particular, TGA was found to

form bridging bidentate complex and chelating bidentate complex while MPA was found to form

unidentate and chelating bidentate complexes. Moreover, the longer alkyl chain MPA was found

to enable a larger quantity of the complexes to be formed compared to TGA which stabilized the

CdTe core further but potentially weakened its ability for strong solubility and bioconjugation.

Finally, in chapter 5, we presented the characterization results of a pegylated gold nanoparticle

system and a phospholipid gold nanoparticle system for cancer detection. In both nanoparticle

systems, we observed bindings between the citrate ligand, Raman dye and the gold nanoparticle

core through efficient Raman scattering with the PCF platform. Carboxylate groups in the citrate

stabilizers were found to form ionic, unidentate and strong bridging bidentate complexes with the

gold core which correlates with proposed binding interactions in the literature. Through

comparing the Raman spectrum between fresh and degraded citrate stabilized nanoparticles, we

found that nanoparticle stability were weakened in the degraded nanoparticles as the amount of

strong bridging bidentate complexes were reduced compared to ionic and unidenate complexes.

In the pegylated nanoparticle system, PEG–Au bonds were observed in the Raman spectrum

which confirms the pegylation of the nanoparticles. In the phospholipid system, no modes

correspond to any bond between the phospholipid and gold core was observed which supported

104

the hypothesis that the bindings were governed by electrostatics. These results demonstrated that

efficient Raman scattering of the PCF enables metallic nanoparticle systems to be characterized

through the different stages of the synthesis process; thus, allowing nanoparticles to be

monitored in situ for quality control. In addition, we demonstrated that high pump power or

prolonged measurement time can cause Raman dye decomposition and formation of amorphous

carbon structures in both nanoparticle systems. These observations provided insights on the

power sustainability of the nanoparticles and their limits on the acquisition power and time for

Raman imaging.

In summary, this thesis presents an ultra-sensitive platform for Raman spectroscopy to enable

studies of molecular bindings in nanoparticle systems with low pump power.

(6.2) Future Work

In the future, the use of HC-PCF can be extended to the in situ studies of colloidal nanoparticles

using Raman spectroscopy. Physical changes of the QD structure, such as crystallinity,

interfacial modes and surface passivation, can be monitored and controlled dynamically during

the various synthesis processes for property optimizations. For example, surface chemistry

between QDs and electrode surface can be optimized to achieve efficient sensitized

photocurrents for practical photovoltaic devices. Furthermore, structural dynamics of the QDs

can be determined experimentally in different biological systems to reveal the possible cause of

some undesirable effects (i.e. carbonization, cytotoxicity and photobleaching). Systematic study

of molecular structures will enable us to better understand the basis of different QD properties

which further optimizes our QD designs for different applications. Ultimately, HC-PCF can be

served as a platform for studying the vibrational modes of complex aqueous solutions of relevant

105

biological samples such as DNA, proteins and potentially peripheral blood too. In fact, targeted

detection of lung cancer cells using the pegylated and phospholipid nanoparticles are currently

underway. A suitable PCF model has been chosen and its splicing program to achieve selective

filling has been optimized for the large cell sizes (Appendix F).

The PCF platform can be further developed to achieve greater sensitivities, improved

functionalities and practical usages. For example, the platform can be developed to collect both

the forward and backward scattering to improve the system sensitivity. An enclosed cell can also

be incorporated to achieve a sensitive gas sensor. In many practical applications, selective filling

from the side of the PCF might be required because both ends of the PCF are required for signal

collection and fluidic control. However, mechanical modification of the PCF is not trivial,

therefore further investigation is required.

106

Appendix A

Snell’s Law in Wave Optics

Using Maxwell’s equation, one can describe a plane wave propagating in space using:

R = Rj cos7 − k ∙ m (A.1)

where E is the electric field with magnitude |Eo|, ω is the frequency of light, k is the wave vector,

and r is the position vector. If we consider a transverse electric (TE) field incident at the interface

between two materials as shown in Figure A.1, we can write the following three equations to

describe the electric field that is parallel to the material interface:

Rn = ( coso7( − H(p + H([q rst (A.2)

Rm = d coso7d − Hdp − Hd[qrst (A.3)

Ru = , coso7, − H,p + H,[qrst (A.4)

where the subscripts i, r, t refer to the incident, reflected and transmitted wave respectively. In

order to satisfy boundary conditions, the component of the electric fields that is parallel to the

material interface must be equal and continuous at the interface. This implies that

107

Rnq = 0 + Rmq = 0 = Ruq = 0 (A.5)

or ( cos7( − H(p |[v + d cos7d − Hdp |[v

= , cos7, − H,p|[v (A.6)

This can only be satisfied when the phase of the two terms are matched; hence,

7( − H(p = 7d − Hdp = 7, − H,p (A.7)

which can only be satisfied for all t when

7( = 7d = 7, = 7 (A.8)

and for all x when

H( = Hd = H, (A.9)

Equation (A.8) and (A.9) form the generalized rules for Snell’s law which requires the

conservation of the frequency of light and the component of the wave vector that is parallel to

the interface of the two materials.

Figure A.1: Directions of the electric field, magnetic field and wave vector when a TE field is

incident at the interface between two dielectric materials.

108

Appendix B

Synthesis Method and Materials for

Thiol-Capped CdTe Quantum Dots

The thiol-capped CdTe QDs used in this study were synthesized with minor modifications to

literature procedures.48, 49

Sodium borohydride (NaBH4, 99%), tellurium powder (~200 mesh,

99.8%), cadmium chloride (CdCl2, 99%), TGA (98%), MPA (99%), and TG (99%) purchased

from Aldrich chemicals. All chemicals were used as received. Millipore Q water (18 sm) were

used throughout the nanocrystal synthesis.

Synthesis of TGA-capped CdTe QDs: 300 mg (7.90 mmol) of NaBH4 was dissolved in 10 ml of

water under argon environment and cooled in an ice bath. 400 mg (3.14 mmol) of tellurium

powder was added and the reaction mixture was left stirring for 2 hours to get a deep pinkish-

purple clear solution. The resulting NaHTe solution was kept under argon before use. 1.15 g

(6.28 mmol) of CdCl2 was then dissolved in 70 ml of Millipore water and bubbled with argon for

109

20 mins. 1 mL (15.08 mmol) of TGA was slowly added to this solution which formed white

precipitates due to the formation of Cd-TGA complex in the solution. The pH value of the

solution was adjusted to 11.0 by dropwise addition of 2.0 M NaOH solution with stirring. The

freshly prepared NaHTe solution is then rapidly added to the Cd precursor solution at room

temperature. After continuous stirring at room temperature for 10 mins, the resulting orange

solution is refluxed at predetermined time (2 – 16 hrs) and then cooled to room temperature. The

obtained CdTe NCs were precipitated by the addition of reagent grade acetone and were isolated

and purified by repeated precipitation/centrifugation cycles with acetone/water and dried in a

vacuum overnight. Solid CdTe QD samples were then weighted and dispersed in deionised water

to prepare aqueous solutions of 2 mg/mL for Raman measurements, 0.04 mg/mL for UV-vis and

0.4mg/mL for PL measurements respectively.

MPA-capped CdTe QDs were obtained using 400 mg of tellurium powder, 297 mg of NaBH4,

1.15 g of CdCl2 and 1.15 mL of MPA as described above.

TG-capped CdTe QDs were obtained using 565 mg of tellurium powder, 420 mg of NaBH4,

1.0 g of CdCl2 and 1.62 mL of TG as described above.

110

Appendix C

"anoparticle Signal Enhancement

using HC-PCF

(C.1) Citrate Stabilized Gold "anoparticles for Pegylated System

Figure C.1 compares the Raman spectra of citrate stabilized gold nanoparticles obtained using

conventional Raman spectroscopy with a cover slide and HC-PCF with a CW HeNe (633 nm)

laser. Carboxylate stretching modes between 1370 – 1600 cm-1

are not observed using cover

slide with 0.17 mW and 2 s of exposure time (averaging over 2 spectra). In contrast, carboxylate

stretching modes are observed in the spectrum obtained using HC-PCF with the same power and

exposure time.

111

Figure C.1: Raman spectrum of citrate stabilized gold nanoparticles obtained using cover slide

and HC-PCF with 633 nm laser.

(C.2) Pegylated Gold "anoparticles

Figure C.2 compares the Raman spectra of pegylated gold nanoparticles (without Raman dye

attached) obtained using a cover slide and HC-PCF with a 633 nm laser. Gold-sulphur bond

between thiol-modified PEG and gold nanoparticles core (Au–S stretch at 305 cm-1

) as well as

carboxylate stretching modes between 1380 – 1600 cm-1

are not observed using cover slide with

0.17 mW and 2 s of laser exposure (averaging over 2 spectra). However, both modes are

observed in the spectrum obtained using HC-PCF with the pump power and exposure time as

shown in Figure C.2.

112

Figure C.2: Raman spectrum of pegylated gold nanoparticles obtained using cover slide and

HC-PCF with 633 nm laser. Inset: graph focused on spectrum obtained from direct measurement.

(C.3) Pegylated Gold "anoparticles with MGITC Adsorbed

Figure C.3 compares the Raman spectra of MGITC adsorbed gold nanoparticles obtained using a

cover slide and HC-PCF with 2 s laser exposure using 633 nm laser (averaging over 2 spectra).

MGITC modes are observed using both the cover slide and HC-PCF. Nevertheless, the detected

Raman signal of the MGITC modes are enhanced in the spectrum obtained uisng HC-PCF. Table

C.1 shows the enhancement factor using HC-PCF relative to conventional technique. HC-PCF

enhanced the MGITC modes by about 15-19 times.

113

Figure C.3: Raman spectra of MGITC adsorbed gold nanoparticles obtained using cover slide

and HC-PCF with 633 nm laser. Inset: spectra focused on region between 900 and 1600 cm-1

.

Table C.1: Enhancement factor of MGITC modes using HC-PCF.

Mode

(cm-1

)

Intensity

(Cover

Slide)

Intensity

(PCF) Enhan.

Power

(Cover

Slide)

Power

(PCF) Enhan.

Total

Enhancement

927 1771 5664 3.2 1 mW 0.17 mW 5.9 ~ 19 times

1616 8130 20474 2.5 1 mW 0.17 mW 5.9 ~ 15 times

(C.4) Citrate Stabilized Gold "anoparticles for Phospholipid System

Figure C.4 compares the Raman spectrum of citrate stabilized gold nanoparticles obtained using

a cover slide and HC-PCF with 2 s laser exposure (averaging over 2 spectra). Citrate modes


are only observed with enhancements from the HC-PCF.

114

Figure C.4: Raman spectra of citrate gold nanoparticles obtained using cover slide and HC-PCF

with 633 nm laser.

(C.5) Gold "anoparticles with CV Adsorbed

Figure C.5 compares the Raman spectrum of CV adsorbed gold nanoparticles obtained using a

cuvette using 785 nm laser, cover slide and HC-PCF using 633 nm laser with 2 s laser exposure

(averaging over 2 spectra). Only CV modes are observed in all spectra. However, mode

intensities were enhanced with the use of HC-PCF. Using Raman mode at 1623 cm-1

as a

reference, mode intensity was enhanced about 64 times with HC-PCF relative to the use of a

cover slide.

115

Figure C.5: Raman spectrum of CV adsorbed gold nanoparticles obtained using cuvette with

785 nm laser, cover slide and HC-PCF with 633 nm laser. Inset: spectra focused on region


.

(C.6) Phospholipid Gold "anoparticles with CV Adsorbed

Figure C.6 compares the Raman spectrum of CV adsorbed gold nanoparticles obtained using a

cuvette using 785 nm laser, cover slide and HC-PCF using 633 nm laser with 2 s laser exposure

(averaging over 2 spectra). Similar to CV adsorbed nanoparticles without any coating, only CV

modes are observed on all spectra. However, unlike those without the phospholipid coating,

some CV modes were enhanced with HC-PCF while others were weaker than those obtained

with 785 nm laser. This is probably because of the stronger light guidance of the PCF at the

higher Raman shift regime as it gets closer to the bandgap of the PCF. Nevertheless, all CV

modes were enhanced with HC-PCF relative to those obtained with a cover slide. Using Raman

mode at 1623 cm-1

as a reference, mode intensity was enhanced about ~ 20 and ~ 32 times with

HC-PCF relative to the use of a cover slide in phospholipid nanoparticles using MHPC and

SHPC respectively.

116

Figure C.6: Raman spectrum of phospholipid gold nanoparticles obtained using cuvette with

785 nm, cover slide and HC-PCF with 633 nm laser. Phospholipid was formed using (a) MHPC

and DMPC and (b) SHPC and DMPC. Inset: spectra focused on region between 2000 and 2500

cm-1

.

117

Appendix D

Raman Mode Assignments of Thiol-

Capped CdTe "anoparticles

Abbreviations: δ – in-plane-deformation γ – out-of-plane-deformation ρ – rocking vibration

ν – stretching vibration τ – twisting vibration def. – deformation [subscripts:

s – symmetric as – asymmetric]

Table D.1: Proposed assignment of thiol-capped CdTe nanoparticles. Spectra are shown in

Figure 4.6 for regime between 100 and 200 cm-1

, Figure 4.8 for regime between 200 and 310

cm-1

and Figure 4.9 for regime between 310 and 1750 cm-1

.

Raman shift (cm-1

)

proposed assignment TGA MPA TG

121 124 122 Te A1

140 141 142 Te E / CdTe TO

160 160 159 CdTe LO

263 261 261 CdS-Like LO

(Continued ext Page)

118

Raman shift (cm-1

)

proposed assignment TGA MPA TG

291 293 291 CdS SO

332 335 337 C-S def.

584 586 581 CdS 2SO

447 432 443 δ(C-C-O)

535 544 532 τ(O-H)

629 γ(C-C=O)

680 δ(O=C-O)

683 660 ν(C-S) gauche conformer

718 ρ(CH2)

755 ν(C-S)

752 747 ν(C-S) trans conformer

893 892 895 ν(C-OH)

994 993 ν(C-C)

1055 1055 1059 ν(C-C)

1136 τ(CH2)

1215 1215 1215 τ(CH2)

1363 1356 νs(COO)

1496 1469 νs(COO)

1525 ν(COS-)

1571 νas(COO)

1685 1685 ν(C=O)

119

Appendix E

Raman Mode Assignments of Gold

"anoparticle Systems

Abbreviations: δ – in-plane-deformation γ – out-of-plane-deformation ρ – rocking vibration

ν – stretching vibration I.p. – in-plane O.o.p. – out-of-plane ϕ – phenyl ring

[subscripts: s – symmetric as – asymmetric]

(E.1) Citrate Stabilized Gold "anoparticles for Pegylated System

Table E.1: Proposed assignment of citrate stabilized Gold nanoparticles for pegylated

nanoparticle system. Spectrum is shown in Figure 5.5. Assignments are based on Munro et al.,119

Kerker et al.148

and De Melo et al.149

Raman shift

(cm-1

)

Proposed

Assignment

266 Au Surface Mode

(Au-Cl- or AuO)

422 γ(COO−)

508 δ(CCC)

Raman shift

(cm-1

)

Proposed

Assignment

725 ρ(CH2)

889 νs(CC)

(Continued

ext Page)

120

Raman shift

(cm-1

)

Proposed

Assignment

1020 νs(CC)

1135 νas(CCO)

1291 ν(CO) + δ(OH)

1381 νs(COO−)

1445 νs(COO−)

1544 νs(COO−)

1597 νas(COO−)

Raman shift

(cm-1

)

Proposed

Assignment

2721 Overtones and

Combinations

2824

2855 νs(CH)

2921 νas(CH)

3240 νs(OH)

3401 νas(OH)

(E.2) Pegylated Gold "anoparticles

Table E.2: Proposed assignment of pegylated gold nanoparticles. Spectrum is shown in Figure

5.9. Assignments are based on Munro et al.,119

Kerker et al.,148

De Melo et al.,149

and Al-Sa'ady

et al.128

Raman shift

(cm-1

) Proposed

Assignment

261 Au Surface Mode

(Au-Cl- or AuO)

304 ν(AuS)

443 γ(COO−) + δ(CCC)

887 νs(CC)

940 ν(C–COO) + ρ(CH2)

1071 νs(CC)

1159 νas(CCO)

1246

1292 ν(CO) + δ(OH)

1375 νs(COO-)

1465 νs(COO-)

Raman shift

(cm-1

)

Proposed

Assignment

1539 νs(COO-)

1595 νas(COO-)

2136

2165

2718 Overtones and

Combinations

2827

2859 νs(CH)

2919 νas(CH)

3257 νs(OH)

3407 νas(OH)

121

(E.3) Pegylated Gold "anoparticles with MGITC Adsorbed

Table E.3: Proposed assignment of MGITC adsorbed pegylated gold nanoparticles. Spectrum is

shown in Figure 5.7. Assignments are based on Lueck et al.150

Raman shift

(cm-1

)

Proposed

Assignment

231 C–ϕ3 breathing

354 ϕ–C–ϕ bend

434

450

472

535 I.p. benzene ring

deformation

575

592

632

675


bend, stretch NC

bend, torsion

743

771

811 O.o.p. C–H

845

928

950 N(CH3)2 stretch,

bend

Raman shift

(cm-1

)

Proposed

Assignment

987 I.p. benzene

1001 I.p. benzene

1182 I.p. C–H bend

1197

1228 N–C stretch, NC

bend

1306 I.p. C–C, C–C–H

1347 Combination

1375 N–ϕ stretch

1398 I.p. C–C, C–H

1454 NC bend and rock


1510

1530 ϕ=N+ stretch

1593 I.p. ring stretch,

bend

1625 N–ϕ, C–C stretch

3254 νs(OH)

3410 νas(OH)

122

(E.4) Citrate Stabilized Gold "anoparticles for Phospholipid System

Table E.4: Proposed Assignment of citrate stabilized Gold nanoparticles for phospholipid

nanoparticle system. Spectrum is shown in Figure 5.11. Assignments are based on Munro et

al.,119

Kerker et al.148

and De Melo et al.149

Raman shift

(cm-1

)

Proposed

Assignment

1006

1337 νs(COO-)

1446 νs(COO-)

1573 νs(COO-)

1608 νas(COO-)

2130

Raman shift

(cm-1

)

Proposed

Assignment

2722 Overtones and

Combinations

2824

2856 νs(CH)

2926 νas(CH)

3260 νs(OH)

3408 νas(OH)

(E.5) Gold "anoparticles with CV Adsorbed

Table E.5: Proposed Assignment of CV adsorbed gold nanoparticles. Spectrum is shown in

Figure 5.12b. Assignments are based on Lueck et al.150

Raman shift

(cm-1

) Proposed

Assignment

208 C–ϕ3 breathing

217

260

349 ϕ–C–ϕ bend

433

451

Raman shift

(cm-1

)

Proposed

Assignment

475


deformation

570

616

676

(Continued

ext Page)

123

Raman shift

(cm-1

)

Proposed

Assignment


bend, stretch NC

bend, torsion

754

770

780

811 O.o.p. C–H

926 I.p. benzene

953 N(CH3)2 stretch,

bend

984 I.p. benzene

1002 I.p. benzene

1131

1181 I.p. C–H bend

1232 N–C stretch, NC

bend

Raman shift

(cm-1

)

Proposed

Assignment

1308 I.p. C–C, C–C–H

1344 Combination

1378 N–ϕ stretch

1397 I.p. C–C, C–H



1537 ϕ=N+ stretch

1592 I.p. ring stretch,

bend

1625 N–ϕ, C–C stretch

3248 νs(OH)

3418 νas(OH)

(E.6) Phospholipid Gold "anoparticles with CV Adsorbed

Same as CV adsorbed gold nanoparticles (Table E.5).

124

Appendix F

Fiber Splicing Program Optimization

for Large Biological Molecules

PCFs utilized in the nanoparticle studies are not suitable for studying large biological molecules

with sizes that are in the range of micrometers or tenths of micrometers, such as human cells.

This is because the central hollow core of the PCF is too small by design (only about ~10 µm in

diameter). Moreover, after collapsing the cladding holes of the PCF for selective filling, the

diameter of the central core at the fiber end is reduced to only about 7 µm (Figure F.1). The large

size of many biological molecules would not be able to fill into the central core of the PCF for

measurements.

125

Figure F.1: SEM image of the HC-1060 before (a) and after (b) fiber splicing to collapse the

cladding holes. The central core of the PCF is reduced to about 7 µm after the splicing process.

To enable efficient Raman scattering of large biological molecules with PCF, we obtained PCF

modeled HC19-1550 in which its central hollow core is ~20 µm in diameter. Its core diameter is

twice of the HC-800 and HC1060 utilized in the nanoparticle experiments. HC19-1550 PCF

features the largest central hollow core for PCF commercially available at the time of

experiment. However, the trade off for the large core fiber is the lower enhancement of the

Raman modes with a 633 nm pump laser compared to HC-800 and HC1060 due to their different

cladding designs for different photonic bandgaps (Figure F.2).

Figure F.2: Comparison of water modes with HC-800, HC-1060, HC19-1550 and cover slide.

126

To selectively fill biological molecules into the central hollow core of the HC19-1550, the fiber

splicing program of the HC-800 was modified to collapse all cladding holes of the HC19-1550

PCF while maximizing its central core size after splicing. As human cells are typically 10 – 20

µm in diameter, the central core of the PCF would be best to remain as close to 20 µm as

possible (i.e. leaving the size of the central core almost unchanged). Initial trial and error in

tuning the splice power index led to a core size of about 17 – 18 µm in diameter (Figure F.3). All

of the cladding holes were closed except the three corner holes.

Figure F.3: SEM image of PCF facet after splicing with program modified from that for HC-

800. The central core of the PCF after splicing is about 17 – 18 µm and all of the cladding holes

are closed except the three corner holes.

Further improvements of the splicing program were achieved through varying the distance of the

fiber facet from the electrode. We chose to tune this parameter because we had already

determined a set of parameters that could collapse almost all the cladding holes. Varying the

distance between the fiber and the electrode would allow fine tuning of the hole collapsation.

127

Figure F.4 shows SEM images of the PCF spliced with electrode–fiber distance varying from 30

to 90 µm with precision of ± 5 µm. The core diameter was maximized (core size of ~19 – 20 µm)

with all the cladding holes collapsing at about 50 – 70 µm from the electrode. At a shorter

distance (i.e. 30 µm), all cladding holes were closed tightly but the central core was only about

14 – 15 µm in diameter. At a further distance (i.e. 80 µm), the central hole was enlarged to about

30 – 33 µm. The large core size will cause tapering along the fiber length which affects coupling

of the pump laser to the PCF. Further increasing the distance caused enlargement of the corner

holes around the central core which is also undesirable for light coupling.

To test the variability of the central core size using the determined program, the same parameters

were used to splice three different stings of the HC19-1550 PCF fibers (Figure F.5). The core

diameter fluctuates between 17 to 22 µm. This fluctuation could be caused by the focusing

differences of the PCF as focusing was done through observing the sharpest fiber edge along the

fiber axis with the naked eyes. As a result, large focusing differences could be made between

different fibers. However, this could be improved with practices or with a fiber splicer that has

an automatic focusing feature.

128

Core Diameter: 14 –15 µm Core Diameter: 20 – 21 µm

…

Core Diameter: 19 – 20 µm Core Diameter: 30 – 33 µm

Core Diameter: 21 – 22 µm

Figure F.4: SEM image of PCF fused with fiber–electrode distance at (a) 30, (b) 50, (c) 70, (d)

80, and (e) 90 µm.

129

Finally, a HC19-1550 PCF fused with the determined parameter was submerged into a solution

of lung cancer cells, with sizes ranging approximately between 10 to 15 µm in diameter. Figure

F.6 shows the optical image of the PCF facet from the opposite end. The brightness at the central

core shows the cancer cell solution. The solution was only filled in the central core and not the

claddings; therefore, all the cladding holes were closed successfully. The modified fiber splicing

program for the large core PCF enables large biological molecules, such as cancer cells, to be

studied using Raman spectroscopy with enhanced sensitivity provided by the PCF.

Core Diameter: 21 –22 µm Core Diameter: 17 – 18 µm

Core Diameter: 18 – 19 µm

Figure F.5: SEM image of three PCFs fused HC15-1990 PCFs at 50 µm from electrode.

130

Figure F.6: Optical image of HC19-1550 PCF selectively filled with lung cancer cells. The

bright light at the central core shows the cancer cell solution.

131

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raman characterization of colloidal nanoparticles …...furthermore, i would like to thank dr....

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