DESIGN, SYNTHESIS, AND SUPRAMOLECULAR SURFACE CHEMISTRY OF
BI- AND TRIDENTATE SURFACE ANCHORS FOR NANOSCIENCE AND
NANOBIOTECHNOLOGY
A Dissertation
Presented to
The Graduate Faculty of the University of Akron
In Partial Fulfillment
Of the Requirements for the Degree
Doctor of Philosophy
Hui Wang
August, 2007 DESIGN, SYNTHESIS, AND SUPRAMOLECULAR SURFACE CHEMISTRY OF
BI- AND TRIDENTATE SURFACE ANCHORS FOR NANOSCIENCE AND
NANOBIOTECHNOLOGY
Hui Wang
Dissertation
Approved: Accepted:
______Advisor Department Chair Dr. Jun Hu Dr. Kim C. Calvo
______Committee Member Dean of the College Dr. Gerald F. Koser Dr. Ronald F. Levant
______Committee Member Dean of the Graduate School Dr. David A. Modarelli Dr. George R. Newkome
______Committee Member Date Dr. Claire A. Tessier
______Committee Member Dr. Robert R. Mallik
ii ABSTRACT
This dissertation describes the design, synthesis, and supramolecular surface chemistry of bi- and tri-dentate surface anchors for nanoscience and nanobiotechnology. Molecular electronic device candidates, based on the tridentate surface anchor 2,4,9-trithia-tricyclo[3.3.1.13,7]decane, were used to bridge two ruthenium metal clusters. These well-designed ruthenium complexes were used as nanometer-sized molecular connector/metal cluster models to investigate the surface binding characteristics of tridentate surface anchor-metal junctions.
Bi-dentate surface anchors, 1,4-dimercapto-2,3-dimethyl-butane- 2,3-diol and
4,5-dimethyl-2-(4-vinyl-phenyl)-[1,3,2]dioxaborolane-4,5-dithiol, were synthesized.
They were used as ligands for stabilizing gold nanoparticles by two methods: direct reduction reaction, and ligand exchange reaction with triphenylphosphine-stabilized gold nanoparticles. The orientation of the bi-dentate surface anchor-based self-assembled monolayers (SAMs) on the flat gold surface was studied by
Polarization Modulation Fourier Transform Infrared Reflection Absorption
Spectroscopy (PM-FTIRRAS), which showed freestanding surface binding capability of the bi-dentate surface anchors.
New methods to conjugate carbohydrates on the surfaces of gold nanoparticles by the tridentate surface anchor, 2,4,9-trithia-tricyclo[3.3.1.13,7]decane, were studied.
iii Several derivatives of 7-substituted-2,4,9-trithia-tricyclo[3.3.1.13,7]decane were designed and synthesized for this purpose. Two different functional groups, methoxyamino group and terminal alkyne group, can be used to bind to reductive sugars and azido-sugars respectively. These compounds were used as ligands for stabilizing gold nanoparticles by ligand exchange reaction with triphenylphosphine-stabilized gold nanoparticles. The PM-FTIRRAS characterizations of the tri-dentate surface anchor SAM on flat gold surfaces were also studied.
iv DEDICATION
To my parents and my wife
v ACKNOWLEDGEMENTS
I would like to thank my advisor, Dr. Jun Hu, for his guidance and patience
during my doctoral study. I would like also to thank my dissertation committee
members, Dr. Gerald F. Koser, Dr. David A. Modarelli, Dr. Claire A. Tessier, and Dr.
Robert R. Mallik for their comments and suggestions. Dr. Ziegler and Youngs research groups are acknowledged for obtaining and solving the X-ray crystallography presented in this work. Dr. Venkat Dudipala is acknowledged for the
2D gHSQC NMR measurement. Dr. Mallik’s group is acknowledged for obtaining the conductance-voltage data of Al/CdS/tridentate surface anchor/Pb tunnel junctions.
I also want to thank Yubiao Liu, Chalermchai Khemtong, Serhan Boduroglu,
Debanjan Sarkar, Jacob Weingart, and all members of my research group. They have created an effective working atmosphere and helped me in different ways.
I want to thank my parents and family members for their love, support and
encouragement. I also want to express my special thanks to my wife, Yao, for her love,
patience, and everything she has done for me.
vi
TABLE OF CONTENTS
Page LIST OF FIGURES………………………………………………………………. xiv LIST OF SCHEMES…………….……………………………………………..… xvii LIST OF TABLES…………….………………………………………………… xviii
CHAPTER
I INTRODUCTION……………………………………………………………... 1 1.1 Background of Molecular Surface Anchors…………………..………….. 1
1.2 Plan of Research …..………………………………………………….….. 3
1.3 Layout of Dissertation .……………………………………………….….. 5
II CONSTRUCTION OF TRIPODAL MOLECULAR SURFACE ANCHOR- BASED MOLECULAR TUNNELING JUNCTIONS…………………………. 6
2.1 Introduction…………………….…………………………………….…… 6
2.1.1 Design of the Nanometer-Sized Molecular Connector/Metal Cluster System………………………………………………...….. 9
2.1.2 Electronic Properties of the Nanometer-Sized Molecular Connector………………………………………………………..... 10
2.1.3 Ruthenium Clusters at the End of the Nanometer-Sized Molecular Connector………………………………………………………..... 15
2.2 Results and Discussion…..………………………………………………... 16
2.2.1 Synthesis of Molecular Devices…….…….…………………..….. 16
2.2.2 Fabrication of Nanometer-Sized Molecular Connector-Metal Cluster System………………………………….……………..….. 20
vii
2.2.3 Characterizations of Nanometer-Sized Molecular Connector- Metal Cluster System……………….…….…………………..….. 21
2.2.4 NMR and UV-Vis Analysis of Nanometer-Sized Molecular Connector-Metal Cluster System…….…….……………………... 30
2.3 Experimental Section……………………………………………………... 34
2.3.1 General Procedures………………………. .……………………... 34
2.3.2 Synthesis of 2,4,9-Trithia-tricyclo[3.3.1.13,7]decane-7-yl- methanol…………….……………………. .……………………... 34
2.3.3 Synthesis of 2,4,9-Trithia-tricyclo[3.3.1.13,7]decane-7- carbaldehyde…………………..…………. .……………………... 35
2.3.4 Synthesis of 2,4,9-Trithia-tricyclo[3.3.1.13,7]decane-7-yl- ethyne……………….……………………. .……………………... 36
2.3.5 Synthesis of 1,4-Bis((7-2,4,9-trithiaadamantyl)ethynyl) Benzene…………………………………... .……………………... 37
2.3.6 Synthesis of 1,4-Bis(7-2,4,9-trithiaadamantyl) Butadiyne…...... 37
2.3.7 Preparation of Complex 5 ………………………………………... 38
2.3.8 Preparation of Complex 7 ………..…………………………….... 38
2.3.9 Preparation of Complex 9 ……………………………………...... 39
2.4 Conclusions………………………………………………………………... 39
III STABLIZE GOLD NANOPARTICLES WITH DITHIOL LIGANDS………... 40
3.1 Introduction..……………………………………………………………… 40
3.1.1 Stabilization of Metal Nanoparticles…………………………….. 40
3.1.1.1 Charge Stabilization…………………………………. 41
viii
3.1.1.2 Steric Stabilization……….……………………………. 42
3.1.1.3 Electrosteric Stabilization ….…………………..……... 43
3.1.1.4 Stabilization by Ligands………………………………. 44
3.1.2 Preparation and Purification of Gold Nanoparticles……………… 45
3.1.2.1 Triphenylphosphine-Stabilized Gold Nanoparticles...... 45
3.1.2.2 The Brust-Schiffrin Method………………………...... 46
3.1.2.3 Gold Nanoparticles Stabilized by Other Sulfur- Containing Ligands……………………….………...... 47
3.1.2.4 Purification of Gold Nanoparticles………………...... 48
3.1.3 The Surface Plasmon Band of Gold Nanoparticles and Ultraviolet- Visible Spectroscopy…………………...... 49
3.1.4 Size, Shape and Size Distribution of Gold Nanoparticles………… 50
3.1.5 Polarization Modulation Fourier Transform Infrared Reflection Absorption Spectroscopy (PM-FTIRRAS) Characterization of Self-Assembled Monolayers (SAMs) on the Gold Surface……..… 50
3.1.6 Preparation and Characterization of Dithiol Ligand-Stablized Gold Nanoparticles……………………………………………..……..… 52
3.2 Results and Discussion .…………………………………………………… 54
3.2.1 Synthesis of 1,4-Dimercapto-2,3-dimethyl-butane-2,3-diol.……… 54
3.2.2 Synthesis of 4,5-Dimethyl-2-(4-vinyl-phenyl)-[1,3,2] dioxaborolane-4,5-dithiol…………………………………………. 57
3.2.3 Preparation and PM-FTIRRAS Characterization of Self-Assemble Monolayers (SAM) of 1,4-Dimercapto-2,3-dimethyl-butane-2,3- diol (dioldithiol) on the Flat Gold Surface.……………………….. 59
ix
3.2.4 Ligand Exchange Reaction Between 1,4-Dimercapto-2,3- dimethyl-butane-2,3-diol (dioldithiol) and Triphenylphosphine- stabilized Gold Nanoparticles…………..…………………………. 61
3.2.5 Preparation of Gold Nanoparticles Stabilized by 1,4-Dimercapto- 2,3-dimethyl-butane-2,3-diol …...………………………………… 67
3.3 Experimental Section……………………………………………………… 70
3.3.1 General Procedures ………..…………………………….……... 70
3.3.2 Synthesis of 1,4-dibromo-2,3-dimethyl-2-butene …….………... 70
3.3.3 Synthesis of 2,2’-Dimethyl-[2,2’]-bioxiranyl ………...………... 71
3.3.4 Synthesis of S-(4-Acetylsulfanyl-2,3-dihydroxy-2,3- dimethylbutyl) Thioacetate …………………………….……... 72
3.3.5 Synthesis of 1,4-Dimercapto-2,3-dimethylbutane-2,3-diol ….…. 73
3.3.6 Synthesis of 4,5-Dimethyl-[1,2]dithiane-4,5-diol ………………. 74
3.3.7 Synthesis of S-[5-Acetylsulfanyl-4,5-dimethyl-2-(4- vinylphenyl)-[1,3,2]dioxaborolan-4-yl] Thioacetate ………….. 75
3.3.8 Synthesis of 4,5-Dimethyl-2-(4-vinylphenyl)- [1,3,2]dioxaborolane-4,5-dithiol …………………………….... 76
3.3.9 General Procedure of Preparing Self-Assemble Monolayers (SAM) on the Flat Gold Surface ………………...... 77
3.3.10 PM-FTIRRAS Characterization of 1,4-Dimercapto-2,3- dimethyl-butane-2,3-diol SAM on the Gold Surface …...... 78
3.3.11 Ligand Exchange Reaction Between 1,4-Dimercapto-2,3- dimethyl-butane-2,3-diol and Triphenylphosphine-stabilized Gold Nanoparticles ……………...... 78
3.3.12 Preparation of 1,4-Dimercapto-2,3-dimethyl-butane-2,3-diol- Stabilized Gold Nanoparticles by Direct Reduction Reaction ….. 78
x
3.3.13 Ultraviolet Visible Spectroscopy of Gold Nanopartcles ……… 79
3.4 Summary …..…………………………………………………………… 79
IV GOLD NANOPARTICLES WITH MOLECULAR RECOGNITION SITES ON THE SURFACE………………………………………………………….. 80
4.1 Introduction .……………………………………………………………. 80
4.1.1 Immobilization of Carbohydrates on the Gold Surface ………... 81
4.1.2 Gold Nanoparticles with Molecular Recognition Sites on the Surface …………………………………..………………..……. 83
4.2 Results and Discussion………………………………………………….. 90
4.2.1 Synthesis of Methyl 2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7- methanoate .…..…………………………………..…………….. 90
4.2.2 Synthesis of 2-{2-[2-(2-Methoxyamino-ethoxy)-ethoxy]- ethoxy}-ethyl 2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7- methanoate ………………….………………………………… 93
4.2.3 Preparation and PM-FTIRRAS Characterization of Self- Assemble Monolayers (SAM) on the Flat Gold Surface …..…... 96
4.2.4 Ligand Exchange Reaction Between 2-{2-[2-(2- Methoxyamino-ethoxy)-ethoxy]-ethoxy}-ethyl 2,4,9- Trithiatricyclo[3.3.1.13,7]decane-7-methanoate and Triphenylphosphine-stabilized Gold Nanoparticles .…………... 98
4.2.5 Synthesis of 2-[3-Oxo-6-(2-{2-[2-(2-prop-2-ynyloxy-ethoxy)- ethoxy]-ethoxy}-ethoxy)-3H-xanthen-9-yl]-5-(3-{6-[(2,4,9- trithia-tricyclo[3.3.1.13,7]decane-7-carbonyl) -amino]-hexyl}- thioureido)-benzoic Acid …………………………………...… 101
4.2.6 Synthesis of 2-{2-[2-(2-Prop-2-ynyloxy-ethoxy)-ethoxy]- ethoxy}-ethyl 6-[(2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7- carbonyl)-amino]-hexanoate .………………….…………….... 108
xi
4.3 Experimental Section …………………………………..………………. 110
4.3.1 General Procedures ………………...……………………..….. 110
4.3.2 Synthesis of Dimethyl 2-Allyl-Malonate and Diallyl Dimethyl Malonate ...…………………………………………………..... 111
4.3.3 Synthesis of Methyl 2-Allyl-4-pentenoate…………….…….... 112
4.3.4 Synthesis of Methyl 2,2-Diallyl-4-pentenoate …………..……. 112 4.3.5 Synthesis of Methyl 2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7- methanoate ……...……………………………….………….... 113
4.3.6 Synthesis of 2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7- methanoic Acid .………………………..…………………...… 114
4.3.7 Synthesis of 2-{2-[2-(2-Methoxyimino-ethoxy)-ethoxy]- ethoxy}-ethyl 2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7- methanoate …………………..……….……………………….. 115
4.3.8 Synthesis of 2-{2-[2-(2-Methoxyamino-ethoxy)-ethoxy]- ethoxy}-ethyl 2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7- methanoate ………..………………………………………….. 116
4.3.9 General Procedure of Preparing Self-Assemble Monolayers (SAM) on the Flat Gold Surface …………………………….. 117
4.3.10 PM-FTIRRAS Characterization of TP-PEG-CH=NOMe SAM on the Gold Surface ………………….……………………….. 117
4.3.11 Preparation of TP-PEG-CH2-NHOMe-Stabilized Gold Nanoparticles By Ligand Exchange Reaction Between TP- PEG-CH2-NHOMe Ligand and Ph3P-Stabilized Gold Nanoparticles …..………………….………………………..… 118
4.3.12 Ultraviolet Visible Spectroscopy of Gold Nanopartcles …...… 118
4.3.13 Synthesis of 2,4,9-Trithia-tricyclo[3.3.1.13,7]decane-7- carboxylic Acid (6-amino-hexyl)-amide …………………..… 118
xii
4.3.14 Synthesis of 2-(6-Hydroxy-3-oxo-3H-xanthen-9-yl)-5-(3-{6- [(2,4,9-trithia-tricyclo[3.3.1.13,7] decane-7-carbonyl)-amino]- hexyl}-thioureido)-benzoic acid …..………………………..… 119
4.3.15 Synthesis of 2-{2-[2-(2-Prop-2-ynyloxy-ethoxy)-ethoxy]- ethoxy}-ethyl Methanesulfonate ………………..………….… 120
4.3.16 Synthesis of 2-[3-Oxo-6-(2-{2-[2-(2-prop-2-ynyloxy-ethoxy)- ethoxy]-ethoxy}-ethoxy)-3H-xanthen-9-yl]-5-(3-{6-[(2,4,9- trithia-tricyclo[3.3.1.13,7] decane-7-carbonyl)-amino]-hexyl}- thioureido)-benzoic Acid …………………….…..………….… 121
4.3.17 Synthesis of 6-[(2,4,9-Trithia-tricyclo[3.3.1.13,7]decane-7- carbonyl)amino]hexanoic Acid …………....…..………….… 122
4.3.18 Synthesis of 2-{2-[2-(2-Prop-2-ynyloxy-ethoxy)-ethoxy]- ethoxy}-ethyl 6-[(2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7- carbonyl)-amino]-hexanoate ………...……….…..…………… 123
4.4 Summary ……………….……………………………..………………. 124
V CONCLUSIONS AND FUTURE PLANS ………………………………….... 125
5.1 Conclusions …..………………….…………………………………….... 125
5.2 Future Plans .…………………….…………………………………….... 127
REFERENCE………………………….…………………………………….... 128
xiii
LIST OF FIGURES
Figure Page
1.1 Preparation of self-assembled monolayers (SAMs) of alkanethiols on gold surfaces ……………..…………………………………………..………….. 2
1.2 Free-standing surface anchors developed by Fox’s group ………..……….. 3
2.1 An archetypal freestanding metal/molecule/metal junctions ………..…….. 9
2.2 The Al/CdS/tripodal surface anchor/Pb tunnel junctions ………………….. 12
2.3 MO diagram of interaction between HOMO (surface states) of CdS and LUMO of the tripodal surface anchor …………………………..………….. 14
2.4 Orbitals of the molecular device: 1,4-bis((7-2,4,9-trithiaadamantyl)- ethynyl) benzene (DFT calculation: B3LYP/6-311G(d)) ……..………….. 15
2.5 Thermal ellipsoid plot of the structure of complex 5. ………………….….. 22
2.6 Thermal ellipsoid plot of the structure of complex 7. ………………….….. 24
2.7 Thermal ellipsoid plot of the structure of complex 9. ……………...…….. 26
2.8 FT-IR (ATR) spectra: ν(CO) absorption bands of complex 5 and complex 7. …..…………………………………………………………………...….. 28
2.9 FT-IR (ATR) spectra: ν(CO) absorption bands of complex 5 and complex 9. …..…………………………………………………………………...….. 29
13 o 2.10 C NMR spectra of complex 5 (CD3Cl3 as the solvent, 25 C). ……... 31
13 o 2.11 C NMR spectra of complex 5 (CD3Cl3 as the solvent, -55 C). …….. 32
2.12 UV-Vis spectra of the triruthenium cluster complexes ………...………….. 33
3.1 Charge stabilization of metal nanoparticles …………….……..………….. 42
3.2 Steric stabilization of metal nanoparticles ……………..……...………….. 43
3.3 Ligand stabilization of metal nanoparticles ………….………..………….. 44
xiv
3.4 Surface selection rule on the metal surface ……………………..………….. 52
3.5 A Presentation of nanocomposite sensing interfaces with biocompatible polymer coating and gold nanoparticle-cores ………………..…………….. 54
3.6 (a) FTIR spectrum of bulk dioldithiol ligand, and (b) PM-FTIRRAS spectrum of dioldithiol SAMs on the gold surface ……….……..…………. 60
3.7 1H NMR spectra of ligand exchange reaction between triphenylphosphine- stabilized gold nanoparticles and dioldithiol ligand; (a) triphenylphosphine- stabilized gold nanoparticles, (b) the reaction after 1 hour, (c) the reaction after 16 hours, and (d) the reaction after 24 hours ……………..…………. 64
3.8 1H NMR spectrum of dioldithiol ligand …………………..……………….. 65
3.9 FTIR spectra of (a) bulk dioldithiol ligand, (b) ligand exchange reaction between Ph3P-stabilized gold nanoparticles and dioldithiol ligand after 1 hour, (c) the ligand exchange reaction after 16 hours, and (d) PM- FTIRRAS spectrum of dioldithiol SAMs on the gold surface …………….. 66
3.10 UV-Vis absorption spectrum of gold nanoparticles stabilized by 1,4- dimercapto-2,3-dimethylbutane-2,3-diol ……………………..………….. 69
3.11 Structure of 2,2’-dimethyl-[2,2’]-bioxiranyl …………………..………….. 72
3.12 Structure of S-(4-acetylsulfanyl-2,3-dihydroxy-2,3-dimethylbutyl) thioacetate ………………………….………………..…………………….. 73
3.13 Structure of 1,4-dimercapto-2,3-dimethylbutane-2,3-diol …..……..……… 74
3.14 Structure of 4,5-dimethyl[1,2]dithiane-4,5-diol ……………..……..…….. 75
3.15 Structure of 4,5-dimethyl-2-(4-vinylphenyl)-[1,3,2]dioxaborolane-4,5- dithiol ……………..…………………………………………..…..……….. 77
4.1 Attaching biomolecules to gold surfaces by methoxyamino-PEG-tripod surface anchors ……………..………………..…..……………………..….. 84
4.2 Ligand exchange reaction between TP-TEG-CH2NHOMe ligand and triphenylphosphine-stabilized gold nanoparticles …………..……………… 85
xv
4.3 Protein (CD22 of B Cells)-glycon binding probe ……………..…………… 86
4.4 New methods for oligosaccharide modified multifunctional nanoparticles...... 88
4.5 (a) FTIR spectrum of bulk TP-TEG-CH=NOMe ligand, and (b) PM- FTIRRAS spectrum of TP-TEG-CH=NOMe SAMs on the flat gold surface ……………..…………………………………………..………….. 97
4.6 1H NMR spectra of ligand exchange reaction between triphenylphosphine- stabilized gold nanoparticles and TP-TEG-CH2NHOMe ligand; (a) triphenylphosphine-stabilized gold nanoparticles, (b) the reaction after 2 h, and (c) the reaction after 24 h. ……………..……………………..……….. 99
4.7 UV-Vis absorption spectrum of TP-TEG-CH2NHOMe ligand-stabilized gold nanoparticles prepared by ligand exchange reaction …………………. 100
4.8 2D gHSQC NMR spectrum of TP-CONH-C6H12-NHCSNH-Fluorescein- TEG-CCH ………………………………………………………………….. 106
4.9 UV-Vis absorption spectrum of TP-CONH-C6H12-NHCSNH-Fluorescein- TEG-CCH ………………………………………………………………….. 107
xvi
LIST OF SCHEMES
Scheme Page
2.1 Synthesis of 2,4,9-trithia-tricyclo[3.3.1.13,7]decane-7-yl-ethyne …………. 16
2.2 Synthesis of complex 5 ………………………………….…..…………….. 17
2.3 Synthesis of complex 7 …………………………………..…..……………. 18
2.4 Synthesis of complex 9 ………………………………………………..….. 19
3.1 Synthetic procedure of 1,4-dimercapto-2,3-dimethylbutane-2,3-diol ……... 56
3.2 Synthetic procedure of 4,5-dimethyl-2-(4-vinyl-phenyl)-[1,3,2] dioxaborolane-4,5-dithiol ……..………………………………………..….. 58
3.3 Preparation of gold nanoparticles stabilized by dioldithiol and VBE dithiol ………………………..………………………………………….….. 62
3.4 Preparation of dioldithiol-stabilized gold nanoparticles by direct reduction …………………………………...…………………..………….. 68
4.1 Synthesis of methyl 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-methanoate…. 92
4.2 Synthesis of 2-{2-[2-(2-methoxyamino-ethoxy)-ethoxy]-ethoxy}-ethyl 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-methanoate …………………..…... 95
4.3 Synthesis of 2-[3-oxo-6-(2-{2-[2-(2-prop-2-ynyloxy-ethoxy)-ethoxy]- ethoxy}-ethoxy)-3H-xanthen-9-yl]-5-(3-{6-[(2,4,9-trithiatricyclo- [3.3.1.13,7]decane-7-carbonyl)amino]hexyl}thioureido)-benzoic acid …….. 103
4.4 Synthesis of 2-{2-[2-(2-prop-2-ynyloxy-ethoxy)-ethoxy]-ethoxy}-ethyl 6- [(2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-carbonyl)-amino]-hexanoate …... 109
xvii
LIST OF TABLES
Table Page
2.1 Crystal data and structure refinement for complex 5 .…………………...…. 23
2.2 Crystal data and structure refinement for complex 7 …..………….……….. 25
2.3 Crystal data and structure refinement for complex 9 ……………………… 27
xviii CHAPTER I
INTRODUCTION
1.1. Background of Molecular Surface Anchors
The surface chemistry and nanotechnology of soft metals has become one of the
hottest research areas in modern chemistry. Because of their special photonic,
electronic, and chemical properties, metal nanoparticles have become of significant
interest.1 Much effort has been put into the applications of metal nanoparticles,
especially gold nanoparticles, in biological fields. For example, different biomolecular
sensors, such as DNA sensors based on gold nanoparticles, have been developed.2
As a key topic in this area, the surface modification and stabilization of metal
nanoparticles remains challenging. Synthetic capabilities of organic chemistry
increase the development of this field. Various stable molecular layers prepared by
the self-assembling method have been applied in the stabilization of metal
nanoparticles. One of the common examples of self-assembled monolayers (SAMs) is
based on thiols with long alkane chains, originally developed by Nuzzo and Allara.3
The thiols act as surface anchors which can be strongly attached on Au(111) surfaces.
1 The long alkane chains can form highly ordered arrays on the gold surface due to the interchain van der Waal interactions (Figure 1.1).
R
R R R R R
HS
+ S S S S S
Au (111) Surface
Au (111) Surface
Figure 1.1. Preparation of self-assembled monolayers (SAMs) of alkanethiols on gold surfaces
However, SAMs based on alkanethiols have their own disadvantages. The
thermal and air instability is one of the major drawbacks of the thiols.4 The thiols
are susceptible to oxidation and hard to handle. Various sulfur-based functional
groups, such as thioethers, disulfides, dialkyl sulfides, thioacetate groups, and
tetradentate thioether ligands, have been studied as alternatives to thiols for the
preparation of SAMs on gold surfaces.5
2 O SH H2N NH OH
HS SH S SH SS SH HS SH
Figure 1.2. Free-standing surface anchors developed by Fox’s group5
1.2. Plan of Research
The aim of this dissertation is to design, synthesize, and study the applicability of novel bi- and tridentate molecular surface anchors which can be used as general surface anchors chemisorbing on Au (111) and semiconductor surfaces. Ideally, such molecular surface anchors would free-stand on the target surfaces and self-assemble to form well-defined interfacial structures. They should be able to stabilize gold nanoparticles (ideally 50-70 nm) in organic or aqueous solution and allow the formation of stable self-assembled monolayers (SAMs) on flat Au (111) surfaces.
2,4,9-Trithia-tricyclo[3.3.1.13,7]decane derivatives, which were designed and synthesized in our laboratory, were chosen as the tridentate molecular surface anchors.6 These molecules are predicted to have strong interactions with the metal surfaces, and can be used as a general surface anchor chemisorbing on the surfaces of metals and semiconductors, e.g. CdS.7 The rigid and chemically stable structure of this molecule allows the formation of stable SAMs on the metal surface. In
3 addition, different functional groups can be readily introduced to the 7-position of
2,4,9-trithia-tricyclo[3.3.1.13,7] decane. This provides an effective way to
immobilize biomolecules on gold surfaces.
1,4-Dimercapto-2,3-dimethyl-butane-2,3-diol is chosen to be the archetype of
the bi-dentate molecular surface anchors. This molecule can strongly bind to the
metal surfaces via chelating effect. The diol of this molecule can bind to the aryl
boronic acid via reversible covalent complexation.8 Therefore, aryl boronic acid,
which provides sugar recognition site, can be introduced to the surfaces of gold
nanoparticles or flat gold surfaces. The two methyl groups of the ligands can help to
prevent the possible hydrolysis of the dithiol-boronic acid complexes, and they can
also help the molecule to free-stand on the gold surface.
The bi- and tridentate surface anchors-stabilized gold nanoparticles and
self-assembled monolayers (SAMs) on flat Au (111) surfaces can be used in
different areas, such as supramolecular surface chemistry and nanobiotechnology.
One of the important applications is to immobilize biomolecules, such as
carbohydrates, on the gold surfaces. The carbohydrates provide multivalent molecular recognition sites on the gold surfaces. The carbohydrates-attached gold
nanoparticles and flat gold surfaces can be used as potential biomolecular imaging
probes for Polarization Modulation Fourier Transform Infrared Reflection
Absorption Spectroscopy (PM-FTIRRAS) and Surface Enhanced Raman
Spectroscopy (SERs). In addition, the bi- and tridentate surface anchors are
supposed to provide enough chemical and biological stability for bioassays.
4
1.3. Layout of Dissertation
Chapter II focuses on the supramolecular chemistry of tripodal molecular surface
anchors. This chapter discusses the synthesis and characterization of tripodal
molecular surface anchor-based ruthenium complexes, which act as mimics to the bulk metal binding in organic-metal junctions. This section also describes the possible application of tripodal molecular surface anchors in the area of molecular electronics. Chapter III discusses the design and synthesis of dithiol derivatives
(bi-dentate surface anchors), as well as their application in the stabilization of gold
nanoparticles and formation of self-assembled monolayers (SAMs) on flat gold
surfaces. Chapter IV describes tripodal molecular surface anchor-stabilized gold nanoparticles with multivalent molecular recognition sites on the surface.
5 CHAPTER II
CONSTRUCTION OF TRIPODAL MOLECULAR SURFACE ANCHOR-BASED
MOLECULAR TUNNELING JUNCTIONS
2.1. Introduction
As one of the most active fields in chemistry, the studies of molecular electronics are very important in both application and theoretical research.
In application, more and more molecular-level machines based on molecular wires and/or molecular electronic devices, for example, photochemically driven molecular machines, have been designed.9 Balzani and coworkers have developed some light-driven molecular machines based on pseudorotaxane and Ru/Re photosensitizer. 10 -14 Similar technology can be used in other fields such as electrophotography and solar energy conversion. 11 -14 In addition, molecular wires and/or molecular electronic devices are also very useful in the design and fabrication of chemosensors and biosensors. One good example is Swager’s work. 12 These workers have used molecular wires to amplify the signals of chemosensors. The ultimate goal in the field of molecular electronics includes the replacement of the silicon-based electronic elements by molecular electronic devices.16,26 In general, micrometer level is the limitation of the sizes of basic electronic devices. Molecular wires and molecular electronic devices are measured in nanometer, which are
6 promising candidates to replace present-day’s electronic devices.16,26 In addition, the
oxide layers of the traditional electronic devices can result in considerable charge
leakage.16,26 Well designed molecular wires/molecular electronic devices do not have
this disadvantage.
Many chemists are interested in the studies of electron and energy transfer
across a single molecule.13,14 Considering the fact that the intramolecular electron transfer is fast, simple in mechanism, and easy to measure, the studies of electron transport in molecular wires and molecular electronic devices become more and more important.
Generally, molecular wires have two important kinds of features: the structural features and the functional ones.15 Structural features can be simply described as the wire-like shapes of the molecules. Most molecular wires consist of the following components: a bridge, special functional groups which can connect to electrodes or an electron donor/acceptor pair. Some molecular wires may not have the typical electrode-bridge-electrode or donor-bridge-acceptor systems, but they also have similar structures. Functional features means that energy and/or electron can be transported through the molecular wires.15
Normally, structural features of molecular electronic devices are more
complicated than those of molecular wires. They are molecules which have two or
more termini connected by bridges. Normally, they should show nonlinear current-
voltage responses which have special application in molecular electronics.16,26 For
example, the group of Reed and Tour inserted methylene fragments and porphyrin
into the conjugated oligo(phenylene ethynylene) building blocks to tune the current-
voltage responses of the molecules.16,17
7 More and more molecular wire and molecular electronic device units have
been synthesized, which can be used as nanometer-sized synthons for the fabrication
of supramolecular assemblies and devices.12,26 Much work has been focus on the
constructions of those molecular wires and molecular electronic devices which can
span nanometer-sized junctions. Ideally, such units would bind to metal or
semiconductor surfaces which can act as an electrode, and form a well-defined
interfacial structure, which is known as self-assembled monolayers (SAMs). The
study of self-assembled monolayers (SAMs) on metal and semiconductor surfaces has
become an area of intense efforts.18 SAMs are generally highly ordered arrays of
molecules, often with long alkane chains, and specific head groups which can strongly chemisorb on the solid surfaces. A well-known example is the interaction between thiols and Au(111) surfaces.19 A major disadvantage of thiols is their air and thermal
instability.9 Various sulfur-based functional groups, such as thioethers, disulfides,
dialkyl sulfides, thioacetate groups, and tetradentate thioether ligands, have been
studied as alternatives to thiols for the preparation of SAMs on gold surfaces.20
With the rapid progresses of constructing atomic-scale nanostructure devices, understanding of electron transport between electrodes becomes an important problem.20 We have developed a novel tripodal molecular surface anchor, 7-
substituted-2,4,9-trithiatricyclo[3.3.1.13,7]decane.21 Previous work has shown that this
molecule can be used as a general surface anchor chemisorbing on the surfaces of
metals,12 as well as semiconductors, e.g. CdS. 22 In addition, the presence of the
tripodal molecular surface anchor layer remarkably modifies the conductance of CdS
films by the coupling between HOMOs (highest occupied molecular orbital) and
LUMOs (lowest unoccupied molecular) of the surface states on the semiconductor
8 and the adsorbed tripodal surface anchor.22 This will be discussed further in the next section.
2.1.1. Design of a Nanometer-Sized Molecular Connector/Metal Cluster System
A challenging topic in this area is the surface binding characteristics of the tripodal surface anchor-metal junctions. A well-designed nanometer-sized molecular connector/metal cluster has been synthesized. This system consists of a conjugated phenylene ethynylene building block and tripodal surface anchors at the ends which connect with ruthenium clusters. The three ruthenium atoms are attached to one another, which mimic bulk metal surfaces. Each sulfur atom coordinates to one ruthenium atom. The tripodal molecular surface anchors have been recognized for their freestanding surface binding capability and high symmetry.
metal metal
metal/molecule/metal junctions O OC CO O OC OC Ru S S Ru CO OC Ru S S Ru CO CO O O O O OC Ru S S Ru CO
CO OC
Figure 2.1. An archetypal freestanding metal/molecule/metal junctions.
9 The tripodal surface anchor is particularly suitable for constructing single
molecular junctions for the following reasons: First, the divalent sulfur atom is
chemically more stable than thiolates under oxidative conditions and its metal binding
capability is ensured by the chelating effect. 23 Second, the rigid structure of the
tripodal molecular surface anchors allows the construction of devices that are less
susceptible to heat and temperature fluctuation.24 Third, the high symmetry and well-
defined molecular geometry render the molecular tripod most promising for the
formation of well-defined metal-molecule coordination interfaces that are suitable to
be characterized and modeled. Finally, its compact structure should allow the formation of efficient junctions because the conductance for a coherent none resonance junction increases exponentially as the distance between the metal contacts
decreases.25
2.1.2. Electronic Properties of Nanometer-Sized Molecular Connectors
As mentioned before, the nanometer-sized molecular connector-metal cluster
system consists of two parts: (1) a conjugated phenylene ethynylene building block,
and (2) tripodal surface anchors which connect with ruthenium clusters. The
electronic properties of these two parts will be discussed separately.
Phenylene ethynylene, as well as oligo(phenylene ethynylenes), are normally
considered as prototypes of molecular wires. 26 For example, in the structures of traditional DBA (electron donor-bridge-electron acceptor) molecular wires, which consist of an electron donor (D), a bridge (B), and an electron acceptor (A), phenylene ethynylene and oligo(phenylene ethynylenes) act as the bridge.20 Electrons can be transported from the electron donor to the electron acceptor through the bridge. Many different kinds of molecular wires, which consist oligo(phenylene ethynylenes)
10 building block and sulfur-containing end groups, have been synthesized and studied.17
Those sulfur-containing end groups, which are often thiols, act as anchors between oligo(phenylene ethynylenes) and metal/semiconductor surfaces.
As to the electronic properties of tripodal surface anchors, self-assembled monolayers (SAMs) of 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-yl-ethyne (tripodal surface anchor) on the surface of ultra-thin films of semiconductor material (CdS) has been reported recently.22 The tripodal surface anchor-modified CdS films were used
in the fabrication of Al/CdS/tripodal surface anchor/Pb junctions. Temperature
dependent conductance-voltage data of the tunnel junctions were recorded. In the presence of tripodal surface anchor monolayers, the electronic properties of the tunnel junctions significantly changed, in comparison to the typical Al/CdS/Pb junctions.22
The tripodal surface anchors remarkably increased the conductance of the tunnel junctions.
11
Pb
S S S S S S SSSSSSSSSSSS CdS film
Al
Figure 2.2. The Al/CdS/tripodal surface anchor/Pb tunnel junctions.22
In general, there are two basic mechanisms of the electron transfer within molecular wires/molecular devices: tunneling and hopping, which were suggested by
Davis and his coworkers in 1998.18-27 Electron tunneling is a quantum mechanical effect in which electrons are considered as waves rather than classical particles. For a electron in a potential energy well, the wave function describes that the electron can extend out of the potential energy well. That is, the electron has the possibility of being found outside the potential energy well even if its energy is less than the potential energy barrier. Electrons can be transferred through a classically-forbidden energy state via this mechanism. For example, in a traditional DBA molecular wire, electrons can be directly transferred from the electron donor (D) to the electron acceptor (A) via electron tunneling if the distance between the electron donor and the 12 electron acceptor is short enough. In the process of electron hopping, electrons are
temporarily transferred to the bridge from the electron donor, and then to the acceptor.
The conduction mechanisms of a specific molecular wire depend on many factors,
including the energy levels of the highest occupied molecular orbital (HOMOs) and
lowest unoccupied molecular orbital (LUMOs) of the bridge, the electron donor and
the electron acceptor (or the electrodes).18,19
According to the temperature dependent conductance-voltage data, the conduction mechanisms of Al/CdS/tripodal surface anchor/Pb junctions include tunneling and possibly hopping, depending on temperature, current, and voltage.22 A reasonable model to explain the remarkable conductance increase of the junctions suggests that the LUMO of the tripodal surface anchor interact with the HOMO of the n-type semiconductor, forming a new LUMO and a new HOMO surface state. The energy of the new surface states is lower than that of the old ones.22 The new states
act as new conducting channels, and therefore increase the conductance of the junctions. In a p-type semiconductor, the energy of the surface states can be reduced in a similar way.
13
HOMO of the CdS modified by LUMO of the semiconductor tripodal surface anchor tripodal surface anchor (CdS)
Figure 2.3. MO diagram of interaction between HOMO (surface states) of CdS and
LUMO of the tripodal surface anchor.22
In summary, the self-assembled monolayers (SAMs) of 2,4,9-
trithiatricyclo[3.3.1.13,7]decane-7-yl-ethyne (tripodal surface anchor) on the surface of semiconductor film can significantly increase its conductance via the coupling between HOMO and LUMO of the surface states on the semiconductor and the adsorbed tripodal surface anchor. In other words, the conductance of tripodal surface anchor/semiconductor junction is higher than that of the semiconductor without surface modification. It is reasonable to suggest that the tripodal surface anchor is useful in the fabrication of molecular electronic devices. The nanometer-sized molecular connector-metal cluster system can be considered as a molecular device candidate.
14
2.1.3. Ruthenium Clusters at the End of the Nanometer-Sized Molecular Connector
Among all kinds of molecular wires and molecular devices, transition metal
complex-based molecular wires and molecular devices are one of the most important
families. They can be considered as electrode/molecular bridge/electrode junctions
which are the models of simple molecular electronic devices.28 In other words, the transition metal atoms here can somewhat represent the bulk metals connected by molecular electronic devices. The orbitals of the transition metals can readily couple with the organic part of the molecular devices. In addition, d-orbitals of the transition metals make them suitable to donate and/or accept electrons in the molecular devices.
15
2.2. Results and Discussion
2.2.1. Synthesis of Molecular Devices
O Cl 1. OH OOMe OH 1. DIBAL-H Cl O 2. MeOH DMSO S S S SS SS SS 2. Et3N 3 2 1 Swern Oxidation
O O OMe P OMe OH N K2CO3 N
S S SS Ohira-Bestmann SS 3 4
Scheme 2.1. Synthesis of 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-yl-ethyne (TPCCH,
4)27
16
O OMe
OOMe
Ru3(CO)12 O S O SSOC S Heptane C C Ru SS reflux, 30 min Ru Ru OC C CO CO CO CO O
15
Scheme 2.2. Synthesis of complex 5 (Ru3(CO)6(μ-CO)3(TPCOOMe))
17
I
/ Pd(PPh3)4 S S I S S S S CuI/piperidine SS S 4 6
O CO CO CO OC C CO Ru Ru Ru C C CO O S S S O
S Ru3(CO)12 S S S S THF S reflux 6
O S O SSOC C C Ru Ru Ru OC C CO CO CO CO O
7
Scheme 2.3. Synthesis of complex 7 (Ru3(CO)6(μ-CO)3(TPCCBzCCTP)Ru3(CO)6-
(μ-CO)3)
18
S Pd(PPh3)2 S S S S S CuI/diisoproylamine SS S
4 8
O CO CO CO OC C CO Ru Ru Ru C C CO O S S S O
S Ru3(CO)12 S S S S THF S reflux 8
O S O SSOC C C Ru Ru Ru C OC CO CO CO CO O
9
Scheme 2.4. Synthesis of complex 9 (Ru3(CO)6(μ-CO)3(TPCC-CCTP)-
Ru3(CO)6(μ-CO)3)
19
The synthetic route for 1,4-bis((7-2,4,9-trithiaadamantyl)ethynyl) benzene
(TPCC-Bz-CCTP, 6) was originally developed by Khemtong in our lab.36,38 Methyl
2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-methanoate (TPCOOMe, 1) was converted to
2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-carbaldehyde (TPCHO, 3) by sequential
DIBAL reduction29 and Swern oxidation.30 The Ohira-Bestmann reagent was found to
be effective for converting 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-carbaldehyde to
2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-yl-ethyne (TPCCH, 4). 31 This alkyne
underwent facile Sonogashira cross-coupling with diiodobenzene to furnish the
prototypical molecular connector candidate, 1,4-bis((7-2,4,9-trithiaadamantyl)ethynyl)
benzene (TPCC-Bz-CCTP, 6), in high yield. 32 1,4-Bis(7-2,4,9-trithiaadamantyl)-
butadiyne (TPCC-CCTP, 8) was synthesized via a similar method. This synthetic
protocol is highly amenable to variation, and a variety of bridging units can be
incorporated into the scheme to examine structural and electronic effects on the
molecular connectors.
2.2.2. Fabrication of Nanometer-Sized Molecular Connector-Metal Cluster System
1,4-Bis((7-2,4,9-trithiaadamantyl)ethynyl) Benzene (TPCC-Bz-CCTP, 6) was allowed to react with Ru3(CO)12 in refluxing THF for 3 h, Complex 7 (Ru3(CO)6(μ-
CO)3(TPCCBzCCTP)Ru3(CO)6(μ-CO)3) was produced in good isolated yield.
Similarly, the corresponding cluster complexes can be produced in comparable yields.
After purification by silica gel column chromatography, the complexes were
recrystalized from THF/hexanes to produce single crystals that were suitable for X-
ray structure elucidation. Complex 5 (Ru3(CO)6(μ-CO)3(TPCOOMe)) and complex 9
(Ru3(CO)6(μ-CO)3(TPCC-CCTP)Ru3(CO)6(μ-CO)3) was prepared via similar method.
20
2.2.3. Characterizations of Nanometer-Sized Molecular Connector-Metal Cluster
System
The X-ray crystal structures of the complexes (Scheme 2.5-2.7) revealed that
the tripodal ligand coordinates to one face of the triruthenium cluster. The three sulfur
atoms in the tripod bind axially to each ruthenium center in a mode similar to that
observed in the trithane ligand of Ru(CO)6(μ-CO)3(μ-S3C3H6) and the benzene ring of
2 2 2 33 Ru(CO)6(μ3:η :η :η -C6H6). The size of the tripod ligand matches very well with the
triruthenium unit (Figure 2.5); the spacing between sulfur atoms in both compounds is
~3.05 Å, whereas the metal-metal bond distance is 2.85 Å.34 The spacing of metal
atoms in the cluster is very similar to that observed in bulk ruthenium metal, ~2.7 Å,
so we believe that these assemblies mimic bulk metal binding of the nanoclusters. The
small bend in complex 7 (Ru3(CO)6(μ-CO)3(TP-CC-Bz-CC-TP)Ru3(CO)6(μ-CO)3) is
due to solid state packing in the crystal. The metal cluster units exhibit three carbonyl
environments: terminal axial, terminal equatorial and bridging equatorial. The
presence of bridging carbonyls, which are more efficient π acceptors than terminal
carbonyls, most likely is due to the significantly lower π acidity of the tripod ligand
than CO. The mean Ru-Ru bond lengths of each complex is slightly shorter than that
35 of Ru3(CO)12. This is due to the relatively higher π acidity of the bridging carbonyls in comparison to the terminal carbonyls.
The FT-IR (ATR) spectra of complex 5, 7 and 9 are in a good agreement with the observed CO coordination modes.
21
Complex 5
Figure 2.5. Thermal ellipsoid plot of the structure of Complex 5
36 (Ru3(CO)6(μ-CO)3(TPCOOMe)). Hydrogen atoms are omitted for clarity.
Note: Structural elucidation credited to Ziegler’s group, University of Akron
22 Table 2.1. Crystal data and structure refinement for complex 5
36 (Ru3(CO)6(μ-CO)3(TPCOOMe)).
Identification code Ru3S3
Empirical formula C18H12O11Ru3S3 Formula weight 803.67 Temperature 100(2) K Wavelength 0.71073 Å Crystal system Monoclinic Space group P2(1)/c Unit cell dimensions a = 27.679(5) Å α = 90°. b = 11.4525(19) Å β = 112.784(3)°. c = 28.092(5) Å γ = 90°. Volume 8210(2) Å3 Z 12 Density (calculated) 1.951 Mg/m3 Absorption coefficient 1.913 mm-1 F(000) 4656 Crystal size 0.10 x 0.10 x 0.03 mm3 Theta range for data collection 1.46 to 28.29°. Index ranges -36<=h<=36, -14<=k<=14, -37<=l<=36 Reflections collected 58347 Independent reflections 17410 [R(int) = 0.1010] Completeness to theta = 28.29° 85.4 % Absorption correction None Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 17410 / 0 / 949 Goodness-of-fit on F2 0.582 Final R indices [I>2sigma(I)] R1 = 0.0441, wR2 = 0.1028 R indices (all data) R1 = 0.0985, wR2 = 0.1175 Largest diff. peak and hole 0.731 and -0.615 e.Å-3
Note: Structural elucidation credited to Ziegler’s group, University of Akron
23
Complex 7
Figure 2.6. Thermal ellipsoid plot of the structure of Complex 7
(Ru3(CO)6(μ-CO)3(TPCCBzCCTP)-Ru3(CO)6(μ-CO)3). Hydrogen atoms are omitted
for clarity.36
Note: Structural elucidation credited to Ziegler’s group, University of Akron
24 Table 2.2. Crystal data and structure refinement for complex 7
36 (Ru3(CO)6(μ-CO)3(TPCCBzCCTP)Ru3(CO)6(μ-CO)3).
Identification code TPCCRu
Empirical formula C42H22O18Ru6S6•2C4H8O•1/6(C6H14) Formula weight 1771.67 Temperature 100(2) K Wavelength 0.71073 Å Crystal system Triclinic Space group P-1 Unit cell dimensions a = 16.567(5) Å α = 65.113(5)°. b = 19.233(6) Å β = 80.002(5)°. c = 19.315(6) Å γ = 65.786(5)°. Volume 5092(3) Å3 Z 4 Density (calculated) 1.734 Mg/m3 Absorption coefficient 1.549 mm-1 F(000) 2599 Crystal size 0.50 x 0.20 x 0.15 mm3 Theta range for data collection 1.16 to 28.34°. Index ranges -21<=h<=22, -25<=k<=24, -24<=l<=24 Reflections collected 45360 Independent reflections 23517 [R(int) = 0.0303] Completeness to theta = 28.34° 92.6 % Absorption correction None Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 23517 / 0 / 973 Goodness-of-fit on F2 0.970 Final R indices [I>2sigma(I)] R1 = 0.0378, wR2 = 0.0899 R indices (all data) R1 = 0.0495, wR2 = 0.0937 Largest diff. peak and hole 1.631 and -0.608 e.Å-3
Note: Structural elucidation credited to Ziegler’s group, University of Akron
25
Complex 9
Figure 2.7. Thermal ellipsoid plot of the structure of Complex 9
36 (Ru3(CO)6(μ-CO)3(TPCC-CCTP)Ru3(CO)6(μ-CO)3).
Note: Structural elucidation credited to Ziegler’s group, University of Akron
26 Table 2.3. Crystal data and structure refinement for complex 9
36 (Ru3(CO)6(μ-CO)3(TPCC-CCTP)Ru3(CO)6(μ-CO)3).
Identification code Rudimer2
Empirical formula C36H18O18Ru6S6 · 1.7(C4H8O) Formula weight 1659.68 Temperature 120(2) K Wavelength 0.71073 Å Crystal system Monoclinic Space group P2(1)/n Unit cell dimensions a = 13.258(3) Å α = 90°. b = 15.457(3) Å β = 107.049(3)°. c = 15.623(3) Å γ = 90°. Volume 3060.7(10) Å3 Z 2 Density (calculated) 1.800 Mg/m3 Absorption coefficient 1.702 mm-1 F(000) 1611 Crystal size 0.25 x 0.14 x 0.13 mm3 Theta range for data collection 1.78 to 25.00°. Index ranges -15<=h<=15, -17<=k<=18, -18<=l<=18 Reflections collected 21531 Independent reflections 5383 [R(int) = 0.0544] Completeness to theta = 25.00° 99.8 % Absorption correction None Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 5383 / 0 / 298 Goodness-of-fit on F2 0.992 Final R indices [I>2sigma(I)] R1 = 0.0497, wR2 = 0.1264 R indices (all data) R1 = 0.0738, wR2 = 0.1341 Largest diff. peak and hole 1.526 and -0.753 e.Å-3
Note: Structural elucidation credited to Ziegler’s group, University of Akron
27
Figure 2.8. FT-IR (ATR) spectra: ν(CO) absorption bands of Complex 5
(Ru3(CO)6(μ-CO)3(TPCOOMe)) and Complex 7 (Ru3(CO)6(μ-CO)3(TPCC-Bz-
CCTP)Ru3(CO)6(μ-CO)3).
28
Ru-TPCO Me Complex 2 0.30 Ru-TPCC-CCTP Complex
0.25
0.20
0.15
Ester 0.10 Absorbance 0.05
0.00
-0.05 2100 2000 1900 1800 1700 Wavenumbers (cm-1)
Figure 2.9. FT-IR (ATR) spectra: ν(CO) absorption bands of Complex 5
(Ru3(CO)6(μ-CO)3(TPCOOMe)) and Complex 9 (Ru3(CO)6(μ-CO)3(TPCC-
CCTP)Ru3(CO)6(μ-CO)3).
29
2.2.4. NMR and UV-Vis Analysis of Nanometer-Sized Molecular Connector-Metal
Cluster System
The structure of complex 5 was confirmed by 1H, 13C-NMR, and UV-Vis
spectra. The resonances of the 2,4,9-trithiaadamantane part of complex 5 are observed
at 2.73 and 2.78 ppm in 1H NMR spectrum, and at 44, 36 and 31 ppm in 13C NMR,
respectively. The 13C NMR spectrum of complex 5 displays a single metal carbonyl
resonance at 197 ppm due to the rapid pseudo rotation between the terminal and
bridged ligands. This single carbonyl resonance splits into two peaks at 197 and 195
ppm at -55 oC.
According to the 1H NMR spectrum, complex 7 is metastable in good solvents,
1 e.g. CD3OD. The H NMR spectrum displays weak resonances of free ligand 6 when
CD3OD is used as a solvent. The resonances of the 2,4,9-trithiaadamantane part of complex 7 are observed at 2.90 and 2.93 ppm in 1H NMR spectrum, which are similar
to complex 5. The phenyl resonance of complex 7 is not observed in 1H NMR
spectrum, and its ethynyl resonance is not observed in 13C NMR spectrum. A reasonable explanation is that the Ru3 clusters coordinate with the phenylene
ethynylene part of the ligand in CD3OD.
All the complexes display a UV-Vis absorption band at about 370 nm for the
37 Ru3 metal cluster, which is blue-shifted from the 404 nm observed for Ru3(CO)12.
30
13 Figure 2.10. C NMR spectra of complex 5 (Ru3(CO)6(μ-CO)3 (TPCOOMe))
o (CD3Cl3 as the solvent, 25 C).
31
13 Figure 2.11. C NMR spectra of complex 5 (Ru3(CO)6(μ-CO)3 (TPCOOMe))
o (CD3Cl3 as the solvent, -55 C).
32
Figure 2.12. UV-Vis spectra of the triruthenium cluster complexes (1x10 M-5 in
CH2Cl2).
33
2.3. Experimental Section
2.3.1. General Procedures
1H NMR (300 MHz) and 13C NMR (75 MHz) spectra were recorded on a
Varian Gemini-300 spectrometer. All chemical shifts are reported in parts per million
(ppm) and are referenced to the residual solvent resonances. IR spectra were recorded
on a Nicolet NEXUS 870 FT-IR spectrometer equipped with a Thunderdone
Attenuated Total Reflectance (ATR) accessory. UV-Vis absorption spectra were
recorded on an OceanOptics PC2000 spectrometer. X-ray data were recorded at 100K
on a Bruker SMART APEX CCD-based X-ray diffractometer system equipped with a
Mo-target X-ray tube (λ= 0.71037 Å) operated at 2000 W. Details are given in Tables
2.1-2.3.
All the materials were obtained from Aldrich, Acros’s or Fisher Scientific and used without further purification unless otherwise noted. Tetrahydrofuran was distilled from sodium benzophenone ketyl under argon. Methylene chloride was
distilled from calcium hydride under argon. Glassware used was flame-dried or oven-
dried at ~120 oC and cooled in the desiccators. All the reactions were monitored by
thin-layer chromatography (TLC). TLC was performed with 0.2 mm precoated silica
gel with UV 254 on polyester backed plates (Sorbent Technologies). EM Science
silica gel 60 Å (35-75 um) was used in flash chromatography.
3,7 2.3.2. Synthesis of 2,4,9-Trithiatricyclo[3.3.1.1 ]decane-7-yl-methanol (TPCH2OH,
2)29,38
34 To a flame-dried round bottom flask, a solution of methyl 2,4,9-
trithiatricyclo[3.3.1.13,7]decane-7-methanoate (TPCOOMe, 250 mg, 1.0 mmol) in 6
mL of dry toluene was added under an argon atmosphere. The solution was cooled to
0 oC, and a solution of diisobutyl aluminum hydride in hexane (1.7 mL, 1.5 M, 2.5
mmol) was slowly added. The reaction mixture was stirred at 0 oC for 2 h. After the completion of the reaction as indicated by TLC, the solution was quenched by 3 mL of methanol and allowed to warm to room temperature. The reaction mixture was filtered through a Celite® pad. The volatile components were removed via vacuum to give 211 mg white solid. Yield: 92 %. M.P. 197-200 oC (lit. 199-200 oC). 1H-NMR
(300 MHz, CDCl3): δ (ppm) 1.55 (s, 1H, OH), 2.59 (d, 6H, J = 3 Hz, CH2), 3.39 (s,
13 2H, OCH2), 4.35 (br s, 3H, CH). C-NMR (75 MHz, CDCl3): δ (ppm) 74, 43, 40, 34.
FTIR (ATR): 2869, 2920, 3318 cm-1.
2.3.3. Synthesis of 2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7-carbaldehyde (TPCHO,
3)30,38
In a round bottom flask, oxalyl chloride (0.07 mL, 0.8 mmol) was dissolved in
dry methylene chloride (3 mL). Under vigorous stirring, dimethyl sulfoxide (0.14 mL,
2.0 mmol) was added dropwise at -78 oC. The reaction mixture was stirred at -78 oC
for additional 10 min. A solution of 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-yl- methanol (210 mg, 0.965 mmol) in 5 mL of dichloromethane was added dropwise.
The reaction mixture was stirred at -78 oC for 1 h. After the reaction was completed as
indicated by TLC, 5 mL of triethylamine was added. The reaction mixture was
allowed to warm to room temperature. The reaction was quenched with water (25 mL)
and extracted with dichloromethane (3 × 10 mL). The volatile components were
35 removed via vacuum to give the crude product. The compound was used in the next step without further purification due to its chemical instability. 1H-NMR (300 MHz,
CDCl3): δ (ppm) 2.79 (d, 6H, J = 3 Hz, CH2), 4.40 (br s, 3H, SCH)), 9.33 (s, 1H,
CHO). FTIR (ATR): 1720, 2760, 2852, 2921, 2952 cm-1.
2.3.4. Synthesis of 2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7-yl-ethyne (TPCCH, 4)31,38
Sodium hydride (60 mg, 1.5 mmol, 60 % dispersed in mineral oil) was
dispersed in a mixed solvent of dry toluene (10 mL) and dry THF (1 mL). The
reaction mixture was cooled to 0 oC. Under vigorous stirring, dimethyl (2- oxopropyl)phosphonate (250 mg, 1.5 mmol) in dry toluene was added slowly. The solution was stirred for 2 h. A solution of methanesulfonyl azide (180 mg, 1.5 mmol) in dry toluene was added. The reaction mixture was allowed to warm up to room temperature and stirred for additional 3 h. The reaction mixture was filtered through a
Celite® pad. The volatile components were removed via vacuum to give dimethyl (1-
diazo-2-oxopropyl)phosphonate as yellow oil. The compound was used for the next
step without purification.
2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7-carbaldehyde and potassium
carbonate (275 mg, 2.0 mmol) were dispersed in 5 mL of dry methanol, the solution
of dimethyl (1-diazo-2-oxopropyl)phosphonate (1.5 eq) in dry methanol (5 mL) was
added. The reaction mixture was stirred for 18 h at room temperature. The reaction
mixture was extracted with dichloromethane (3 × 20 mL). The combined organic
layers were washed with aqueous solution of sodium bicarbonate (20 mL, 5 %). After
rotary evaporation, the crude product was purified by silica gel column
chromatography (ethyl acetate:hexane = 1:3 (v/v)) to give 133 mg white solid. Yield:
36 o o 1 66 % (2 steps). M.P. 271-273 C (lit. 273-274 C). H-NMR (300 MHz, CDCl3): δ
13 (ppm) 2.28 (s, 1H, CCH), 2.90 (d, 6H, J = 3 Hz, CH2), 4.31 (br s, 3H, SCH). C-
NMR (75 MHz, CDCl3): δ (ppm) 28, 40, 43, 71, 90. FTIR (ATR): 2109, 2911, 2950,
3263 cm-1.
2.3.5. Synthesis of 1,4-Bis((7-2,4,9-trithiaadamantyl)ethynyl) Benzene
(TPCCBzCCTP, 6)32,38
To a round bottom flask, 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-yl-ethyne (50
mg, 0.23 mmol), diiodobenzene (40 mg, 0.12 mmol) and tetrakis(triphenylphosphine)
palladium(0) (10 mg, 10 μmol) were added. The reaction mixture was degassed via
vacuum. In another round bottom flask, 5 mL of piperidine was degassed by three
freeze-pump-thaw cycles and transferred to the above reaction mixture under an argon atmosphere. The reaction mixture was stirred at 60 oC for 2 h. The reaction mixture
was cooled to room temperature and quenched with 10 mL of saturated aqueous solution of ammonium chloride. The mixture was extracted with dichloromethane (3
× 20 mL). The combined organic layers were dried over anhydrous Na2SO4. After
rotary evaporation, the crude product was purified by recrystallization in
dichloromethane to give 35 mg slightly yellow solid. Yield: 61 %. 1H-NMR (300
MHz, CDCl3): δ (ppm) 2.98 (d, 6H, J = 3 Hz, CH2), 4.34 (br s, 3H, SCH), 7.38 (s, 2H).
13 C-NMR (75 MHz, CDCl3): δ (ppm) 28, 40, 43, 81, 95, 121, 132. FTIR (ATR): 2240,
2920, 2940 cm-1.
2.3.6. Synthesis of 1,4-Bis(7-2,4,9-trithiaadamantyl) Butadiyne (TPCC-CCTP, 8)32,38
A solution of 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-yl-ethyne (50 mg, 0.23 mmol), bis(triphenylphosphine)palladium(II) (10 mg, 10 μmol) and CuI (2 mg, 10
37 μmol) in THF (5 mL) was cooled to 0 oC. Diisoproylamine (0.5 mL) was added
slowly. The reaction mixture was stirred at room temperature for 30 min and refluxed
for an additional 2 h. Then the reaction mixture was cooled to 0 oC and treated with a
saturated aqueous solution of ammonium chloride (10 mL). The reaction mixture was
diluted with water (50 mL) and extracted with dichloromethane (3 × 20 mL). The
combined organic layers were washed with a brine solution and dried over anhydrous
Na2SO4. After rotary evaporation, the crude product was purified by recrystallization in dichloromethane to give 31 mg slightly yellow solid. Yield: 63 %. 1H-NMR (300
13 MHz, CDCl3): δ (ppm) 2.92 (d, 6H, J = 3 Hz, CH2), 4.30 (br s, 3H, SCH). C-NMR
-1 (75 MHz, CDCl3): δ (ppm) 28, 40, 45, 67, 87. FTIR (ATR): 2861, 2950 cm .
2.3.7. Preparation of Complex 5 (Ru3(CO)6(μ-CO)3(TPCOOMe))
To a mixture of 62.6 mg methyl 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-
methanoate (0.252 mmol) suspended in dry n-heptane (25 mL), 161.6 mg of
Ru3(CO)12 (0.253 mmol) was added. The reaction mixture was stirred and refluxed for
about 30 min. After the reaction was completed as indicated by TLC, the solvent was
removed via vacuum. The solid residue was purified by silica gel column
chromatography (THF:hexane = 1:1 (v/v)) to give 78 mg brown solid. Yield: 38 %.
1 13 H-NMR (300 MHz, CDCl3): δ (ppm) 2.73 (s, 3H), 2.78 (s, 6H), 3.75 (s, 3H). C-
NMR (75 MHz, CDCl3): δ (ppm) 197, 172, 54, 44, 36, 31. FTIR (ATR): 2958, 2051,
2002, 1973, 1949, 1860, 1791, 1733 cm-1.
2.3.8. Preparation of Complex 7 (Ru3(CO)6(μ-CO)3(TPCCBzCCTP)Ru3(CO)6(μ-CO)3)
To a mixture of 5.4 mg 1,4-bis((7-2,4,9-trithiaadamantyl)ethynyl)benzene
(0.01038 mmol) suspended in dry tetrahydrofuran (15 mL), 13.8 mg of Ru3(CO)12
38 (0.0216 mmol) was added. The reaction mixture was stirred and refluxed for about 3 h.
After the reaction was completed as indicated by TLC, the solvent was removed via vacuum. The solid residue was purified by silica gel column chromatography
(THF:hexane = 1:2 (v/v)) to give 11.3 mg brown solid. Yield: 68 %. 1H-NMR (300
13 MHz, CD3OD): δ (ppm) 2.90 (s, 3H), 2.93 (s, 6H). C-NMR (75 MHz, CD3OD): δ
(ppm) 199, 133, 45, 40. FTIR (ATR): 2955, 2053, 2003, 1975, 1951, 1863, 1790 cm-1.
2.3.9. Preparation of Complex 9 (Ru3(CO)6(μ-CO)3(TPCC-CCTP)Ru3(CO)6(μ-CO)3)
To a mixture of 7.4 mg 1,4-bis(7-2,4,9-trithiaadamantyl)butadiyne (0.0173 mmol) suspended in dry tetrahydrofuran (15 mL), 22.2 mg of Ru3(CO)12 (0.0347 mmol) was added. The reaction mixture was stirred and refluxed for about 24 h. After the reaction was completed as indicated by TLC, the solvent was removed via vacuum.
The solid residue was purified by silica gel column chromatography (THF:hexane =
1:1 (v/v)) to give 10 mg brown solid. Yield: 38 %. FTIR (ATR): 2955, 2052, 2004,
1976, 1954, 1859, 1785 cm-1.
2.4. Conclusions
A new type of bridged metal cluster complexes has been synthesized. The
bridge was a novel tripodal molecular surface anchor based on 7-substituted-2,4,9- trithiatricyclo[3.3.1.13,7]decane, which can be candidates for use in molecular electronic device fabrication. The triruthenium cluster mimics the (111) surface of a
bulk metal, which provide a promising way to investigate the surface binding of the
tripodal surface anchor-metal junctions.
39 CHAPTER III
STABLIZED GOLD NANOPARTICLES WITH DITHIOL LIGANDS
3.1. Introduction
Metal nanoparticles have gained much interest because of their special
photonic, electronic and chemical properties.1 The applications of metal nanoparticles, especially gold nanoparticles in this case, have been widely studied. One of the hottest
topics in this area is the fabrication of different biomolecular sensors using gold nanoparticles as templates. For example, different biosensors based on antibody- attached gold nanoparticles have been developed.39,,40 41 The strong surface plasmon
band of gold nanoparticles has been applied for bioassay.39,40,41
3.1.1. Stabilization of Metal Nanoparticles
The unique physical and chemical properties of metal nanoparticles are
strongly related with their size and shape.42 However, it is well known that metal
nanoparticles are not stable in both aqueous and organic solutions. They tend to
aggregate and precipitate, and van der Waals interactions between metal nanoparticles
can force them to coagulate together. Eventually, the metal nanoparticles will
aggregate into bulky metal particles and precipitate in solution. As a result, they will
lose their special properties which are associated with their size and shape. Therefore,
40 different methods have to be used to overcome the van der Waals force between metal
nanoparticles in order to stabilize them in the solutions.
Generally, there are two types of strategies to stabilize metal nanoparticles:
charge stabilization and steric stabilization. The mechanism of these two types of
stabilization will be discussed here separately.
3.1.1.1. Charge Stabilization
Charge stabilization is also known as electrostatic stabilization. This method is
used mainly in aqueous solution. The positive charge on the surface of the metal
nanoparticles can adsorb different ionic compounds from the solution to form
electrical double-layers. If the Coulombic repulsion from the same electronic charge
of the layers around the metal nanoparticles is strong enough, it will overcome the van
der Waals interaction between metal nanoparticles. Therefore, ionic compounds, e.g.
carboxylates, in the aqueous solution are helpful in preventing the metal nanoparticles
from aggregation and participation.43 However, this type of stabilization has to be controlled carefully. A small change of the ionic strength of the solution may disturb
the electrical layers around the metal nanoparticles, therefore the stabilization of the
nanoparticles may become less effective.1
41
Figure 3.1. Charge stabilization of metal nanoparticles.
3.1.1.2. Steric Stabilization
Macromolecules can also be used to stabilize the metal nanoparticles.
Polymers or oligomers in the solution can be adsorbed to the surface of metal
nanoparticles. Therefore, a protective layer is formed to prevent the nanoparticles
from aggregation and participation.1 This method is named steric stabilization. The
motion of the adsorbed macromolecules is restricted. When the distance between two
nanoparticles is decreased, the entropy of the system will be decreased, and the free
energy will be increased. That is, the decrease of interparticle distance is a
thermodynamically unfavorable procedure. In addition, when two nanoparticles move
close to each other, the concentration of the adsorbed macromolecules between the
nanoparticles will be increased. The solvent will dilute the local concentration of the macromolecules to reestablish the equilibrium. Therefore, an osmotic repulsion is provided, and the nanoparticles will be separated. Another advantage of steric stabilization is that it takes places in both aqueous and organic solution, while electrostatic stabilization only happens in aqueous solution. The thickness of the 42 marcromolecular protective layers around the nanoparticles can be easily modified by using different types of molecules or changing their length. Hence, the stability of the nanoparticles can be controlled. This is one of the reasons why steric stabilization is widely applied in the stabilization of metal nanoparticles.
Metal Metal Nanoparticle Nanoparticle
Figure 3.2. Steric stabilization of metal nanoparticles.
3.1.1.3. Electrosteric Stabilization
Metal nanoparticles can also be stabilized by ionic surfactants. Ionic surfactants provide both charge and steric stabilizations.1 The hydrophilic groups of the ionic surfactants, e.g. the ammonium groups, will form electrical double-layers around the metal nanoparticles which provide Coulombic repulsion to stabilize the nanoparticles. The hydrophobic part of the surfactants, e.g. the long alkyl chains, will generate steric repulsion.
43 3.1.1.4. Stabilization by Ligands
Many types of traditional ligands can strongly coordinate to mental
nanoparticles. Therefore, the metal nanoparticles form stable colloidal solutions in
suitable solvents.2 Different types of ligands, such as thiols, phosphines, and amines, can act as stabilizer of the metal nanoparticles.44, 45 , 46 , 47 , 48 , 49 , 50 The possible ionic
groups and hydrophobic alkyl chains in the structures of the ligands can provide
Coulombic and steric repulsions respectively, which are helpful in stabilization of the
metal nanoparticles. Ligand-stabilization is especially useful in the preparation and
stabilization of gold nanoparticles. More details will be discussed in the next section.
Metal Nanoparticles
Figure 3.3. Ligand stabilization of metal nanoparticles.
44 3.1.2. Preparation and Purification of Gold Nanoparticles
There are many different methods can be used to prepare metal nanoparticles.
The resulting metal nanoparticles are different in size, shape, and surface properties.
The preparation of metal nanoparticles usually discussed in terms of two general categories: physical methods and chemical ones.1 In short, the physical method means mechanically subdividing bulk materials into nanometer-sized particles. The size distribution of the resulting nanoparticles is usually broad. The physical and chemical properties of the nanoparticles are hard to control, and usually irreproducible.51 On the other hand, the chemical method for metal nanoparticles means nucleation and growth of metal atoms, for example, the reduction of transition metal salts by different reducing agents, or by photochemical reduction methods. By using these methods, the size and shape of the obtained metal nanoparticles are relatively easy to control. In addition, the surface properties of the resulting metal nanoparticles can be modified and tuned by changing the structures and/or compositions of the capping ligands around the nanoparticles.
Several methods of gold nanoparticle preparation will be discussed here. All these methods follow the same strategy: chemical reduction of gold salt in an organic, aqueous, or two-phase solution with capping ligands.
3.1.2.1. Triphenylphosphine-Stabilized Gold Nanoparticles
This method was first developed by Schmid and his coworkers.52,,53 54 In short,
the original procedure is carried out in three steps. First, the reaction of hydrogen
tetrachloro aurate(III) (HAuCl4) with triphenylphosphine (Ph3P) in methanol provides triphenylphosphinechloro aurate(I) (Ph3PAuCl) qualitatively. Second,
triphenylphosphinechloro aurate(I) is reduced by gaseous diborane (B2H2) to give
45 triphenylphosphine-stabilized gold nanoparticles (Schmid’s cluster, Au55(PPh3)12Cl6).
The size of obtained gold nanoparticles is about 1.4 nm in diameter with a narrow size
distribution. Finally, after the ligand-exchange reaction of Schmid’s cluster with water
soluble ligands, such as P(PhSO3Na)3, water soluble phosphane-stabilized gold
nanoparticles can be obtained. The sizes of this type of water soluble gold
nanoparticles range from 15 to 20 nm.
In 2000, an improved one-pot method in preparation of triphenylphosphine-
stabilized gold nanoparticles was reported.55 The technique is similar to the Brust-
Schiffrin method which will be discussed in the next section. In an aqueous/organic
two-phase solution, hydrogen tetrachloro aurate(III) (HAuCl4) and tetraoctyl
ammonium bromide (TOAB), which acts as the phase transfer catalyst, are dissolved.
Triphenylphosphine (Ph3P) is added to the reaction mixture to give
triphenylphosphinechloro aurate(I) (Ph3PAuCl). Then the reducing agent, such as sodium borohydride is added rapidly. The size of resulting triphenylphosphine- stabilized gold nanoparticles (Au101(PPh3)21Cl5) is about 1.5 nm in diameter with a narrow size distribution. Comparing with the original Schmid’s method, this technique is much safer and easier to handle.
3.1.2.2. The Brust-Schiffrin Method
The Brust-Schiffrin method can prepare thiol-stabilized gold nanoparticles in a
one-step, two-phase reaction. 56 ,,57 58 The gold nanoparticles obtained have a quite
narrow size distribution. Because of the self-assembled structures of the alkanethiol
ligands around the gold nanoparticles, the resulting nanoparticles are remarkably
stable. They can be stored at room temperature for a couple of months without
decomposition. They can also be precipitated, isolated, dried, and redissolved in
46 organic solution conveniently. In addition, they can carry out chemical reactions if the
thiols carrying reactive groups. 59 , 60 Generally, this method is carried out in an
aqueous/organic two-phase solution. The gold salt, usually hydrogen tetrachloro
aurate(III) (HAuCl4), in the aqueous can be introduced to the organic solution by a
phase transfer catalyst. The organic solution, e.g. toluene, contains thiol ligands.
When the reducing reagent, such as sodium borohydride, is added into the solution,
the gold salt is reduced rapidly, and the thiol-protected gold nanoparticles are formed
in the organic solution.
A similar procedure can be used to make gold nanoparticles stabilized by
quaternary ammonium salt.61 The gold salt is reduced in an aqueous/organic two-
phase solution in the present of quaternary ammonium salt. The quaternary
ammonium salt, such as tetraoctyl ammonium bromide (TOAB), acts not only as the
phase transfer catalyst, but also as the capping ligands. The quaternary ammonium
salt-stabilized gold nanoparticles can be obtained conveniently. Different thiol-based ligands can be introduced to the surface of the gold nanoparticles via ligand-exchange reactions.
3.1.2.3. Gold Nanoparticles Stabilized by Other Sulfur-Containing Ligands
Different sulfur-containing ligands can be used to stabilize gold nanoparticles.
For example, disulfides can be used as the capping ligands of small-sized gold nnoparticles. 62 , 63 Dialkyl sulfides are also effective stabilizers for gold nanoparticles.64 Zhong and his coworkers prepared tetradentate thioether-stabilized
gold nanoparticles.65
Previous work of our research group shows that 2,4,9-trithia-adamantane
derivatives can be used as effective tripodal surface anchors on flat gold surfaces as
47 well as gold nanoparticles.66,67 The 2,4,9-trithia-adamantane derivatives are thermally
and chemically stable. In addition, they are much easier to handle, comparing with
other sulfur-containing ligands, especially different thiols. The long alkyl chains
which connected to the tripodal surface anchors can be well-packed to form self-
assemble monolayers (SAM) on the gold surfaces. It is found that the 2,4,9-trithia-
3,7 tricyclo[3.3.1.1 ]decane-7-carboxylic acid octadecylamide (TPCONH-C18H37) can serve as a good stabilizer for gold nanoparticles. Using this method, large-sized gold nanoparticles can be easily obtained. The average diameters of the nanoparticles are around 85 to 88 nm.
3.1.2.4. Purification of Gold Nanoparticles
The gold nanoparticles, especially those prepared by two-phase reduction method, need to be purified carefully. The phase transfer catalysts, e.g. quaternary ammonium salts, as well as the excess capping ligands, may cause problems in the characterization of the gold nanoparticles. In addition, they may affect the properties of the gold nanoparticles.
The most convenient way to purify the gold nanoparticles is precipitation and redispersion. When a poor solvent is added to the solution of gold nanoparticles, the
Coulombic and/or steric repulsions provided by the protective layers will be affected.
If the repulsions can not overcome the van der Waals interaction between the nanoparticles, the nanoparticles will aggregate and precipitate. The precipitates can be collected by filtration or centrifuging. The impurities will be leaved in the solution.
The solid gold nanoparticles can be redissolved in suitable solution if the protective layers/ligands are still bound to their surface tightly. Highly pure gold nanoparticles
48 can be obtained by repeating the procedure of precipitation and redispersion. Column
chromatography is another method of the purification of gold nanoparticles.68
3.1.3. The Surface Plasmon Band of Gold Nanoparticles and Ultraviolet-Visible
Spectroscopy
Metal nanoparticles have unique optical properties. For example, gold
nanoparticles, especially those in the diameter rang of 1 to 10 nm, show strong
Ultraviolet-Visible absorption bands that are not present in the bulk gold metal as well
as the individual atoms. This phenomenon is known as surface plasmon resonance,
which happens when light is reflected off the surfaces of gold nanoparticles.69
The free conduction electrons and ionic cores on the surfaces of gold
nanoparticles form plasma states. When external electromagnetic fields, e.g. visible
lights, are applied to the gold nanoparticles, the delocalized electrons keep moving
beyond the neutral states and returning back to the neutral states and so on. This
motion of the electrons is called the plasma oscillation. In another word, a fraction of
the light energy will interact with the free electrons on the surfaces of gold
nanoparticles.70
Usually, the surface plasmon band of gold nanoparticles appears at around 530
nm, which is in the visible spectrum. So the solution of gold nanoparticles normally
shows the deep ruby red color. As other optical properties of metal nanoparticles, the
intensity and frequency of surface plasmon band is strongly depend on the size of
gold nanoparticles. Generally, when the size of gold nanoparticle decreases, the
intensity of the surface plasmon band will decrease.71 The plasmon bandwidth and the
position of the absorption maximum are also strongly affected by the size of
nanoparticles.72 In short, the surface plasmon band, which can be determined by UV-
49 Vis absorption spectrum, can be used to characterize the gold nanoparticles. In
addition, this property of gold nanoparticles allows the development of different
biosensors based on absorbance measurements.73
3.1.4. Size, Shape and Size Distribution of Gold Nanoparticles
As it is mentioned before, the size, shape and size distribution are important
properties of the gold nanoparticles. Transmission electron microscopy (TEM) is a
direct imaging method which can provide first hand information of the size, shape and size distribution of the gold nanoparticles. Therefore, it is widely used in the characterization of gold nanoparticles. Dynamic light scattering (DLS) and small- angle X-ray scattering (SAXS) can provide statistically evident information of nanoparticles, therefore can be used to determine the size distribution of gold nanoparticles.74
3.1.5. Polarization Modulation Fourier Transform Infrared Reflection Absorption
Spectroscopy (PM-FTIRRAS) Characterization of Self-Assembled Monolayers
(SAMs) on the Gold Surface
Infrared reflection absorption spectroscopy (IRRAS) has been widely used in
the in situ characterization of self-assembled monolayers (SAMs) on metal surfaces.75,76 It is proven to be one of the best methods to monitor the chemical
structure and molecular orientation of the SAMs after the research work of Greenler’s
group in the 1980s.73
In general, the Polarization Modulation Fourier Transform Infrared Reflection
Absorption Spectroscopy (PM-FTIRRAS) technique uses a polarized infrared light,
which is generated by a photo elastic modulator (PEM), at grazing angle of incidence
50 to study the SAMs. The electric vector of the polarized light can be resolved into two components: the vector which is perpendicular to the polarized radiation, and the one parallel to the polarized radiation. As the polarized light hits the metal surface, the phase of the former vector will change about 180o after the reflection, and is not depend on the incidence angle. Therefore, the net intensity of the IR signal is cancelled out. On the other hand, the phase change and intensity of the latter vector depend on the incidence angle highly. Normally, the optimal incidence angle on the gold surface is around 80o. 77 Usually, the electric vector which is parallel to the polarized radiation can reach its maximal intensity at this angle of incidence.
The electric vector parallel to the polarized radiation can be further resolved into two components which are parallel (Ex) and perpendicular (Ez) to the metal surface, respectively.78 As the polarized light hits the metal surface, the metal surface will be polarized by the two components. The perpendicular component (Ez) will be enhanced after the interaction with the dipole induced on the metal surface. Therefore, the IR absorption corresponding to the perpendicular vector (Ez) will be increased on the IRRAS spectra. On the other hand, the parallel component (Ex) will be cancelled after the interaction with the dipole induced on the metal surface. Therefore, the parallel vector (Ex) shows little contribution to the PM-FTIRRAS spectra, because the net change of the dipole movement is almost zero. That is, the IR absorption corresponding to the parallel vector (Ex) will be significantly decreased.
51
Ez E
Ez : Enhanced Ex Ex : Cancelled
Metal Surface Metal Surface Metal Surface
Figure 3.4. Surface selection rule on the metal surface.
Later, new techniques, such as polarization modulation technique, lock-in
amplifier technique and real time synchronous sampling technique, have been
introduced into the area of IRRAS. These improvements make the IRRAS more and
more efficient in the characterization of self-assembled monolayers (SAMs) on the
gold surface.
3.1.6. Preparation and Characterization of Dithiol Ligand-Stablized Gold
Nanoparticles
Different dithiol ligands, including 1,4-dimercapto-2,3-dimethyl-butane-2,3-
diol (dioldithiol) and 4,5-dimethyl-2-(4-vinylphenyl)-[1,3,2]dioxaborolane-4,5-dithiol
(VBE dithiol), have been synthesized. Gold nanoparticles stabilized by dioldithiol ligands have been prepared by two methods: direct reduction reaction and ligand
exchange reaction with triphenylphosphine-stabilized gold nanoparticles. The
resulting gold nanoparticles are proven to be highly stable in different organic solvent.
This may be due to a chelating effect because the S-S distance (~3.6 Å) in dioldithiol ligand is close to the expected Au-Au distance on the surface of gold nanoparticles.
52 The hydroxyl groups of the dioldithiol ligand can be easily modified to form different biomolecular recognition sites on the surface of gold nanoparticles. For example, the
dioldithiol ligands can act as sugar models to react with boronic acid derivatives. The
diol of this molecule can bind to the aryl bronic acid via reversible covalent
complexation. 79 The hydroxyl groups of the dioldithiol ligand can also be easily
activated by 1,1’-carbonyl diimidazole forming imidazole-carbamate groups. The
imidazole-carbamate groups can be linked with different oligonucleotides. The two
methyl groups of the dioldithiol ligands can help to prevent the possible hydrolysis of
the dioldithiol-bronic acid complexes, and they can also help the molecule to free-
stand on the gold surface. The formation and PM-FTIRRAS spectra of dioldithiol
Self-Assemble Monolayers (SAMs) on the flat gold surface have also been studied.
The long-term aim of this research project is to develop an optical real-time
blood glucose sensor. Biocompatible nanocapsules with polymer coating and gold
nanoparticle-cores will be fabricated. The core-shell materials will be prepared by
controlled radical polymerization using cross-linkable Self Assemble Monolayers
(SAMs) of boronate-dithiol ligand complex on gold nanoparticles as templates. The
dithiol linker can be removed from gold surface by photooxidation. 80 The biomolecular recognition sites are boronic acids which are widely used as receptors for sugars.81 They will be organized within the inner surface of the polymer shell.
Because Raman signals near the gold surface can be significantly enhanced, the template gold nanoparticles can be used for optical detection such as Surface
Enhanced Raman scattering (SERs).82 Jouliana El Khoury in our research group has
studied the synthesis of nanocomposite polymer gel using controlled radical
polymerization.
53
Biocompatible Polymer
B(OH)2 B(OH)2 (HO)2B
B(OH)2 Au (HO)2B
(HO)2B B(OH)2 (HO)2B B(OH)2
Figure 3.5. A presentation of nanocomposite sensing interfaces with biocompatible
polymer coating and gold nanoparticle-cores.
3.2. Results and Discussion
3.2.1. Synthesis of 1,4-Dimercapto-2,3-dimethyl-butane-2,3-diol (dioldithiol)
1,4-Dimercapto-2,3-dimethyl-butane-2,3-diol (14) can be prepared in four
steps. The synthetic procedure is shown in Scheme 3.1.
2,2’-Dimethyl-[2,2’]-bioxiranyl (12) is the key intermediate of the synthesis.
There are several different method to prepare this molecule, including the oxidation of
2,3-dimethylbuta-1,3-diene in present of meta-chloroperbenzoic acid (MCPBA).
However, this method gives several byproducts which need carefully purification.
Eventually, a two-step procedure, which is based on the method in the reference 45 with small modifications, is selected as shown in the Scheme 3.1. This procedure can produce 2,2’-dimethyl-[2,2’]-bioxiranyl (12) in high overall yield, and the following
54 workup and purification is relatively easy. The crude product is dissolved in diethyl
ether, dried with anhydrous MgSO4, and then filtered through a Celite®-silica gel pad.
After removal of the volatile components, quite pure 2,2’-dimethyl-[2,2’]-bioxiranyl
(12) is produced. The resulting product is directly used in the next step because of its
chemical instability. No further purification is needed.
S-(4-Acetylsulfanyl-2,3-dihydroxy-2,3-dimethylbutyl) thioacetate (13) is
prepared by treating 2,2’-dimethyl-[2,2’]-bioxiranyl (12) with 3-5 equivalents
potassium thioacetate and acetic acid. After silica gel column chromatography
purification, S-(4-acetylsulfanyl-2,3-dihydroxy-2,3-dimethylbutyl) thioacetate (13) is
produced as white needle crystals. The yield is about 60 %.
S-(4-Acetylsulfanyl-2,3-dihydroxy-2,3-dimethylbutyl) thioacetate (13) can be hydrolyzed in a dilute sodium hydroxide in ethanol. In order to produce 1,4-
dimercapto-2,3-dimethyl-butane-2,3-diol (14), the trace of oxygen have to be
carefully removed via three freeze-pump-thaw cycles in order to prevent the possible
oxidation of the thiol group. 1,4-Dimercapto-2,3-dimethyl-butane-2,3-diol (14) can be
obtained as white crystals in 76 % yield. On the other hand, 4,5-dimethyl-
[1,2]dithiane-4,5-diol (15) can be obtained by hydrolysis of S-(4-acetylsulfanyl-2,3-
dihydroxy-2,3-dimethylbutyl) thioacetate (13) in a solution of sodium hydroxide in
ethanol with O2 bubbled through it. 4,5-Dimethyl-[1,2]dithiane-4,5-diol (15) is given
as white crystals in 81 % yield.
55
Br H H O H3C Br2, CH2Cl2 KMnO4, (BnEt3N)Cl
NaOH (aq, 30 %) O CH3 H Br H 10 11 12
O
S K
CH3COOH
O
S
SH OH
OH 1. NaOH (1 % ) in Ethanol HO
2. HCl (aq, 1 M) S HO O HS
14 13
OH OH 1. 1% NaOH in Ethanol, O2
2. HCl 1M SS 15
Scheme 3.1. Synthetic procedure for 1,4-dimercapto-2,3-dimethyl-butane-2,3-diol
(dioldithiol, 14)
56 3.2.2. Synthesis of 4,5-Dimethyl-2-(4-vinylphenyl)-[1,3,2]dioxaborolane-4,5-dithiol
(VBE dithiol)
Two different methods can be used to prepare 4,5-Dimethyl-2-(4-
vinylphenyl)-[1,3,2]dioxaborolane-4,5-dithiol (17). The synthetic procedure is shown
in Scheme 3.2.
S-[5-Acetylsulfanyl-4,5-dimethyl-2-(4-vinylphenyl)-[1,3,2]dioxaborolan-4-yl]
thioacetate (16) is prepared by heating S-(4-acetylsulfanyl-2,3-dihydroxy-2,3- dimethylbutyl) thioacetate (13) and vinyl phenyl boronic acid in dry toluene at about
80 oC. A small amount of 2,6-di-tert-butyl-4-methylphenol is added as an inhibitor to prevent the possible polymerization. After silica gel column chromatography
purification, S-[5-acetylsulfanyl-4,5-dimethyl-2-(4-vinylphenyl)-[1,3,2]dioxaborolan-
4-yl] thioacetate (16) is produced as slight yellow oil in 79 % yield. The compound is
hydrolyzed in a dilute sodium hydroxide in ethanol under argon atmosphere. Oxygen
is carefully removed via three freeze-pump-thaw cycles to prevent the possible
oxidation. 4,5-Dimethyl-2-(4-vinylphenyl)-[1,3,2]dioxaborolane-4,5-dithiol (17) is
obtained as slightly yellow oil in 67 % yield.
Another method for 4,5-dimethyl-2-(4-vinylphenyl)-[1,3,2]dioxaborolane-4,5-
dithiol (17) is heating 1,4-dimercapto-2,3-dimethyl-butane-2,3-diol (14) and vinyl phenyl boronic acid in the present of 2,6-di-tert-butyl-4-methylphenol as an inhibitor.3
The yield is about 85 %. The product is kept as a toluene solution and used without further purification.
57 O S OH B B HO OH OO HO Dry toluene S o stir, 80 C S S O inhibitor O O
13 16
1. NaOH (1 % ) in Ethanol
2. HCl (aq, 1 M)
B OO
SH HS
17
SH OH B B HO OH OO HO HS Dry toluene stir, 80 oC SH HS 14 inhibitor 17
Scheme 3.2. Synthetic procedure for 4,5-dimethyl-2-(4-vinylphenyl)-
[1,3,2]dioxaborolane-4,5-dithiol (VBE dithiol, 17)
58 3.2.3. Preparation and PM-FTIRRAS Characterization of Self-Assemble Monolayers
(SAM) of 1,4-Dimercapto-2,3-dimethyl-butane-2,3-diol (dioldithiol) on the Flat Gold
Surface
A freshly cleaned gold thin film on a glass slide was immersed into a dilute
solution of dioldithiol ligand in ethanol to produce a SAM of dioldithiol on the gold
surface. The two thiol groups of the dioldithio ligand can easily formed a highly
stable SAM on the gold surface because of the chelating effect.
The resulting SAM of dioldithiol on the gold surface was characterized by
PM-FTIRRAS (Figure 3.6). The spectrum was obtained at a lock-in frequency at 3000
cm-1 with 4000 scans. For the spectrum of dioldithiol SAM, two absorptions at 2963
-1 and 2900 cm are assigned to the νaCH3 and νsCH3 modes respectively. For the spectrum of bulk dioldithiol ligand, the absorptions at 2984, 2942, 2918, and 2849
-1 cm are assigned to the νaCH3, νaCH2, νsCH3 and νsCH2 modes respectively. After
the formation of dioldithiol SAM on the gold surface, the νaCH3 absorption shifted
-1 -1 -1 from 2984 cm to 2963 cm , and the νsCH3 absorption shifted from 2918 cm to
2900 cm-1. The absorptions are not broadened comparing with the spectrum of bulk
dioldithiol ligand, which indicates that dioldithiol SAM is well-packed on the gold
surface. No νaCH2 and ν sCH2 absorptions were detected in the spectrum of
dioldithiol SAM on the gold surface. The reason is that the C-H bond is supposed to
be parallel to the gold surface after the formation of dioldithiol SAM. According to
the metal surface selection rules, the IR absorption will be decreased if the dipole
59 moment is parallel to the metal surface. No S-H absorption was detected in the
spectrum of dioldithiol SAM because of the interaction of thiol group with the gold.
Figure 3.6. (a) FTIR spectrum of bulk dioldithiol ligand, and (b) PM-FTIRRAS spectrum of dioldithiol SAMs on the gold surface
60
61
3.3.4. Ligand Exchange Reaction Between 1,4-Dimercapto-2,3-dimethyl-butane-2,3- diol (dioldithiol) and Triphenylphosphine-stabilized Gold Nanoparticles
1,4-Dimercapto-2,3-dimethyl-butane-2,3-diol (dioldithiol)-stabilized gold nanoparticles were prepared by ligand exchange reaction. To a solution of triphenylphosphine (Ph3P)-stabilized gold nanoparticles, 1,4-dimercapto-2,3- dimethyl-butane-2,3-diol (dioldithiol ligand) was added. The displacement of the capping ligands occurred, because of the stronger affinity of gold toward sulfur comparing with phosphorus.
62
H SH OH H H HO HS H Au
Gold Nanoparticles Stabilized by Gold Nanoparticles or SAM Triphenylphosphine (Ph3P) or Stabilized by Dioldithiol Tetraoctylammonium bromide (TOAB)
B OO
Au SH HS
Gold Nanoparticles Stabilized by Gold Nanoparticles or SAM Triphenylphosphine (Ph3P) or Stabilized by VBE thiol Tetraoctylammonium bromide (TOAB)
Scheme 3.3. Preparation of gold nanoparticles stabilized by dioldithiol and VBE dithiol
The reaction was monitored by 1H NMR. From the 1H NMR spectra (Figure
3.7), it is clear that the broad peak at around 7 ppm of triphenylphosphine-stabilized
gold nanoparticles became significantly decreased (after 1 h) and eventually
disappeared on the base line (after 16 h) after the dioldithiol ligand was added into the
gold nanoparticles solution. The peak of free triphenylphosphine at 7.6 ppm became
63 more intense. These are evidences of the ligand exchange reaction. The peak of
methylene group (Ha, Hb) of dioldithiol ligand is an AB system, which shows two
doublets of doublet at 2.65 and 3.13 ppm respectively. Comparing with the 1H NMR
spectrum of free dioldithiol (Figure 3.8), the signal of methylene group of dioldithiol in the reaction mixture was significantly broadened, and almost disappeared in the baseline after 24 h. In addition, the aliphatic proton signal at 1.22 ppm is also broadened. All these evidences clearly show the formation of dioldithiol-stabilized gold nanoparticles. The detailed mechanism of the interaction of dioldithiol ligand with the gold surface has been studied by El Khoury in our research group.87
The dioldithiol-stabilized gold nanoparticles were characterized by FTIR
(Figure 3.9). After the dioldithiol ligand was added into the gold nanoparticles
solution, the S-H absorption at around 2557 cm-1 became significantly decreased and eventually disappeared on the base line because of the interaction between thiol and
-1 gold. Two absorptions at 2986 and 2937 cm are assigned to the νaCH3 and νaCH2 modes respectively. The absorption at around 3057 cm-1 is assigned to the aromatic C-
H stretching of free triphenylphosphine which is significantly increased as a function
of time. This is also an evidence of the ligand exchange reaction between
triphenylphosphine-stabilized gold nanoparticles and dioldithiol ligand.
Comparing with the spectrum of dioldithiol SAMs on the flat gold surface, the
metal surface selection rules shown on the spectrum of dioldithiol-stabilized gold
nanoparticles is not distinctive. One possible reason is that the surface of gold nanoparticles is much rougher than that of a flat gold surface. It may cause somewhat disorganization of the dioldithiol layer packed on the surface of gold nanoparticles.
64
Figure 3.7. 1H NMR spectra of ligand exchange reaction between triphenylphosphine- stabilized gold nanoparticles and dioldithiol ligand; (a) triphenylphosphine-stabilized gold nanoparticles, (b) the reaction after 1 h, (c) the reaction after 16 h, and (d) the reaction after 24 h.
65
Figure 3.8. 1H NMR spectrum of dioldithiol ligand
66
Figure 3.9. FTIR spectra of (a) bulk dioldithiol ligand, (b) ligand exchange reaction between Ph3P-stabilized gold nanoparticles and dioldithiol ligand after 1 h, (c) the ligand exchange reaction after 16 h, and (d) PM-FTIRRAS spectrum of dioldithiol
SAMs on the gold surface
67 3.3.5. Preparation of Gold Nanoparticles Stabilized by 1,4-Dimercapto-2,3-dimethyl-
butane-2,3-diol (dioldithiol)
1,4-Dimercapto-2,3-dimethylbutane-2,3-diol (dioldithiol)-stabilized gold
nanoparticles were also prepared by direct reduction of aurate salt in the presence of
dioldithiol, which is know as the Brust-Schiffrin method. The aurate(III) salt was transferred to an organic solvent by a phase transfer reagent, such as tetraoctylammonium bromide (TOAB). The dioldithiol ligand was then added into the organic solution of aurate and tetraoctylammonium bromide (TOAB) to give an orange solution. After the mixture was thoroughly mixed by vigorous stirring for 30 min, a freshly prepared sodium borohydride solution was added quickly. A black solution of dioldithiol-stabilized gold nanoparticles was obtained in 10 min. The solution was kept stirring overnight to complete the reaction. The organic solution of dioldithiol-stabilized gold nanoparticles was separated, concentrated and precipitated in hexane. The solid gold nanoparticles were centrifuged and washed with hexane and small amount of water respectively to remove the phase transfer reagent.
68
H SH OH H H HO HS H HAuCl4 Au
NaBH4
Gold Nanoparticles or SAM Stabilized by Dioldithiol
Scheme 3.4. Preparation of dioldithiol-stabilized gold nanoparticles by direct reduction
The resulting gold nanoparticles can be dissolved in different organic solvent, such as diethyl ether, methylene chloride, ethanol and methanol, to form a very stable solution. They can also be redispersed several times without any decomposition. The
UV-Vis absorption spectrum of the dioldithiol-stabilized gold nanoparticles (Figure
3.10) shows an intense surface plasmon band with the maximum at around 530 nm.
69
Figure 3.10. UV-Vis absorption spectrum of gold nanoparticles stabilized by 1,4- dimercapto-2,3-dimethylbutane-2,3-diol
70 3.3. Experimental Section
3.3.1. General Procedures
1H NMR (300 MHz) and 13C NMR (75 MHz) spectra were recorded on a
Varian Gemini-300 spectrometer. All chemical shifts are reported in parts per million
(ppm) and are referenced to the residual solvent resonances. IR spectra were recorded
on a Nicolet NEXUS 870 FT-IR spectrometer equipped with a Thunderdone
Attenuated Total Reflectance (ATR) accessory. UV-vis data were recorded on an
OceanOptics S2000 spectrometer at 298 K. High resolution mass spectra were obtained at the Mass Spectrometry and Proteomics Facility at The Ohio State
University. Gold films were produced by sequential thermal deposition of Ti (2000 Å)
and 99.99% pure gold (1800 Å) on glass slides. The gold films consist exclusively of
polycrystalline Au[111] surface with the surface roughness factor determined to be
1.1.
All the materials were obtained from Aldrich, Acros’s or Fisher Scientific and
used without further purification unless otherwise noted. Tetrahydrofuran was distilled from sodium benzophenone ketyl under argon. Methylene chloride was
distilled from calcium hydride under argon. Glassware used was flame-dried or oven-
dried at ~120 oC and cooled in the desiccators. All the reactions were monitored by
thin-layer chromatography (TLC). TLC was performed with 0.2 mm precoated silica
gel with UV 254 on polyester backed plates (Sorbent Technologies). EM Science
silica gel 60 Å (35-75 um) was used in flash chromatography.
3.3.2. Synthesis of 1,4-dibromo-2,3-dimethyl-2-butene (11)83
The procedure was followed according to ref. 83 with modifications.
71 In a 250 mL round bottom flask, 2,3-dimethylbuta-1,3-diene (3.65 g, 44.5 mmol) was dissolved in dry methylene chloride (45 mL), and the solution was cooled to -78 oC. Under vigorous stirring, a solution of bromine (7.03 g, 44.0 mmol) in dry methylene chloride (30 mL) was added dropwise. The reaction mixture was stirred at
-78 oC for 5 h. The reaction mixture was warmed to 0 oC. The reaction was quenched with a saturated sodium bicarbonate solution (20 mL). The organic layer was washed with ice-cold water (3 × 20 mL), and then dried with anhydrous MgSO4. The volatile components were removed via vacuum evaporation. The crude product was carefully washed with small amount of cold hexane to give 6.39 g pale green crystals. Yield: 60
1 %. H-NMR (300 MHz, CDCl3): δ (ppm) 1.89 (s, 6H, CH3), 4.00 (s, 2H, CH2Br). The crude product was used in the next step without further purification.
3.3.3. Synthesis of 2,2’-Dimethyl-[2,2’]-bioxiranyl (12)84
The procedure was followed according to ref. 45 with modifications. In a round bottom flask, 1,4-dibromo-2,3-dimethyl-2-butene (2.50 g, 10.3 mmol) was dissolved in methylene chloride (10 mL). Then, the solution was cooled to 0 oC.
Sodium hydroxide aqueous solution (10 mL, 30 %) and benzyltriethylammonium chloride (0.1 g, 0.44 mmol, as the phase transfer agent) were added to the solution.
Under vigorous stirring, pulverized potassium permanganate (2.62 g, 16.5 mmol) was added in small potions to the reaction mixture over a period of 48 h at 0 oC. After the reaction was completed as indicated by TLC, the reaction mixture was warmed to room temperature. The reaction mixture was diluted with diethyl ether (30 mL) and stirred vigorously. The organic layer was separated, and the dark brown viscous aqueous solution was washed with diethyl ether (3 × 20 mL). The combined organic
solution was dried with anhydrous MgSO4, and filtered through a Celite®-silica gel 72 pad. The volatile components were removed via vacuum to give 0.995 g 2,2’-
1 dimethyl-(2,2’)-bioxiranyl. Yield: 85 %. Colorless oil. H-NMR (300 MHz, CDCl3):
a b δ (ppm) 1.38 (s, 3H, CH3), 2.59 (d, 2H, J = 5.4 Hz, H ), 2.81 (d, 2H, J = 5.4 Hz, H ).
The product was used in the next step without further purification because of its
chemical instability.
Ha Hb O H3C
O CH3 Ha Hb
Figure 3.11. Structure of 2,2’-dimethyl-[2,2’]-bioxiranyl
3.3.4. Synthesis of S-(4-Acetylsulfanyl-2,3-dihydroxy-2,3-dimethylbutyl) Thioacetate
(thioacetate diol, 13)
In a round bottom flask, 2,2’-dimethyl-[2,2’]-bioxiranyl (0.50 g, 4.39 mmol)
was dissolved in dry tetrahydrofuran (25 mL). Under vigorous stirring, potassium
thioacetate (1.43 g, 12.5 mmol) and acetic acid (1.48 g, 24.6 mmol) were added
slowly. The reaction mixture was stirred at room temperature for 6 h. After the
reaction was completed as indicated by TLC, the reaction mixture was filtered, and
the solid was washed with tetrahydrofuran (30 mL). The combined organic solution
was dried with anhydrous MgSO4. After evaporation of the volatile components, the
crude product was purified by silica gel column chromatography (ethyl acetate:hexane
= 35:100 (v/v)) to give 0.62 g white crystal. Yield: 59 %. M.P. 100-102 oC. 1H-NMR
(300 MHz, CDCl3): δ (ppm) 1.23 (s, 6H, CH3), 2.41 (s, 6H, CH3CO), 3.06 (d, 2H, J =
73 a b 13 14.0 Hz, H ), 3.52 (d, 2H, J = 14.0 Hz, H ). C-NMR (75 MHz, CDCl3): δ (ppm)
21.21, 30.71, 37.22, 76.81, 197.39. FTIR (ATR): 1060, 1135, 1223, 1355, 1381, 1688,
2985, 3469 cm-1.
O
Ha S
HO Hb
Ha HO
S Hb
O
Figure 3.12. Structure of S-(4-acetylsulfanyl-2,3-dihydroxy-2,3-dimethylbutyl) thioacetate
3.3.5. Synthesis of 1,4-Dimercapto-2,3-dimethylbutane-2,3-diol (dioldithiol, 14)
In a flame-dried two-neck round bottom flask, equipped with a dropping
funnel, a solution of S-(4-acetylsulfanyl-2,3-dihydroxy-2,3-dimethylbutyl) thioacetate
(0.10 g, 0.37 mmol) in 5 mL ethanol was added under argon atmosphere. Then the
solution was cooled to -78 oC. A solution of 1 % (wt) sodium hydroxide in ethanol
(10 mL) was added dropwise. After three freeze-pump-thaw cycles, the reaction
mixture was stirred at room temperature for 5 h. After the reaction was completed as
indicated by TLC, the reaction mixture was acidified to pH 5-6 by aqueous
hydrochloric acid (1 M). The ethanol and other volatile compounds were removed via
vacuum evaporation. The aqueous solution was extracted with methylene chloride (3 74 × 10 mL). The combined organic solution was dried with anhydrous MgSO4. After
evaporation of the volatile components, 52 mg of white crystals were obtained. Yield:
o 1 76 %. M.P. 97-100 C. H-NMR (300 MHz, CDCl3): δ (ppm) 1.18 (s, 6H, CH3), 1.48
(t, 2H, J = 9.0 Hz, SH), 2.62 (dd, 2H, J = 13.0, 9.0 Hz, Ha), 3.07 (dd, 2H, J = 13.0, 9.0
b 13 Hz, H ). C-NMR (75 MHz, CDCl3): δ (ppm) 20.63, 33.20, 75.41. FTIR (ATR):
-1 +• 1064, 1220, 1347, 1377, 1456, 2563, 2984 cm . HRMS (EI): calcd for C6H14 O2S2
182.04297, found 182.0409.
Ha SH
OH Hb
Ha HO
HS Hb
Figure 3.13. Structure of 1,4-dimercapto-2,3-dimethylbutane-2,3-diol
3.3.6. Synthesis of 4,5-Dimethyl[1,2]dithiane-4,5-diol (15)
In a round bottom flask, S-(4-acetylsulfanyl-2,3-dihydroxy-2,3-dimethylbutyl)
thioacetate (0.05 g, 0.19 mmol) and a solution of 1 % (wt) sodium hydroxide in
ethanol (10 mL) were added. The reaction mixture was stirred vigorously for 8 h
under O2 atmosphere. After the reaction was completed as indicated by TLC, the
reaction mixture was acidified to pH 5-6 with aqueous hydrochloric acid (1 M). The ethanol and other volatile compounds were removed via vacuum evaporation. The aqueous solution was extracted with methylene chloride (3 × 10 mL). The combined organic solution was dried with anhydrous MgSO4. After evaporation of the volatile
components, 27 mg of white crystals were obtained. Yield: 81 %. 1H-NMR (300 MHz,
75 a CDCl3): δ (ppm) 1.33 (s, 6H, CH3), 2.47 (d, 2H, J = 13.8 Hz, H ), 2.91 (br s, 2H, OH),
b 13 3.54 (d, 2H, J = 12.9 Hz, H ). C-NMR (75 MHz, CDCl3): δ (ppm) 23.46, 42.45,
78.25. FTIR (ATR): 937, 1086, 1203, 1235, 1338, 1375, 1409, 1456, 2910, 2939,
2980, 3460 cm-1.
OH OH
Hb Hb
Ha SSHa
Figure 3.14. Structure of 4,5-dimethyl-[1,2]dithiane-4,5-diol
3.3.7. Synthesis of S-[5-Acetylsulfanyl-4,5-dimethyl-2-(4-vinylphenyl)-
[1,3,2]dioxaborolan-4-yl] Thioacetate (16)
In a round bottom flask, S-(4-acetylsulfanyl-2,3-dihydroxy-2,3-dimethylbutyl)
thioacetate (0.96 g, 0.27 mmol) was dissolved in 15 mL dry toluene. Then vinyl
phenyl boronic acid (40.0 mg, 0.25 mmol), and small amount of 2,6-di-tert-butyl-4-
methylphenol (inhibitor, prevent the possible polymerization) and anhydrous MgSO4 were added. Under vigorous stirring, the reaction mixture was heated to 80 oC for 2 h.
After the reaction was completed as indicated by TLC, the reaction mixture was
filtered. After evaporation of the volatile components, the crude product was purified
by silica gel column chromatography (ethyl acetate:hexane = 35:100 (v/v)) to give 70
1 mg product. Yield: 79 %. Slightly yellow oil. H-NMR (300 MHz, CDCl3): δ (ppm)
1.45 (s, 6H, CH3), 2.34 (s, 6H, CH3CO), 3.26 (br, 4H, CH2), 5.33 (d, 1H, J = 11.0 Hz,
-CH=CH2), 5.85 (d, 1H, J = 17.0 Hz, -CH=CH2), 6.77 (dd, 1H, J = 11.0, 11.0 Hz, -
13 CH=CH2), 7.42 (d, 2H, J = 8.1 Hz, ArH), 7.72 (d, 2H, J = 8.1 Hz, ArH). C-NMR (75
76 MHz, CDCl3): δ (ppm) 20.71, 30.69, 37.37, 85.35, 115.36, 125.77, 135.34, 136.99,
140.77, 194.88. FTIR (ATR): 1088, 1132, 1320, 1359, 1398, 1609, 1695, 2940, 2990 cm-1.
3.3.8. Synthesis of 4,5-Dimethyl-2-(4-vinylphenyl)-[1,3,2]dioxaborolane-4,5-dithiol
(VBE dithiol, 17)
In a flame-dried two-neck round bottom flask, equipped with a dropping
funnel, a solution of S-[5-acetylsulfanyl-4,5-dimethyl-2-(4-vinylphenyl)-
[1,3,2]dioxaborolan-4-yl] thioacetate (0.10 g, 0.26 mmol) in 5 mL ethanol was added
under argon atmosphere. Then the solution was cooled to -78 oC. A solution of 1 %
(wt) sodium hydroxide in ethanol (10 mL) was added dropwise. After three freeze-
thaw cycles, the reaction mixture was stirred at room temperature for 2 h. After the
reaction was completed as indicated by TLC, the reaction mixture was carefully
acidified to pH 5-6 by aqueous hydrochloric acid (1 M). The ethanol and other
volatile compounds were removed via vacuum evaporation. The aqueous solution was
extracted with methylene chloride (3 × 10 mL). The combined organic solution was
dried with anhydrous MgSO4. After evaporation of the solvent, 52 mg of product
1 were obtained. Yield: 67 %. Slightly yellow oil. H-NMR (300 MHz, CDCl3): δ (ppm)
1.52 (s, 6H, CH3), 1.64 (dd, 2H, J = 6.0, 10.0 Hz, SH), 2.68 (dd, 2H, J = 14.0, 10.0 Hz,
a b H ), 3.03 (dd, 2H, J = 14.0, 6.0 Hz, H ), 5.32 (d, 1H, J = 10.0 Hz, -CH=CH2), 5.83 (d,
1H, J = 18.0 Hz, -CH=CH2), 6.74 (dd, 1H, J = 18.0, 10.0 Hz, -CH=CH2), 7.43 (d, 2H,
13 J = 8.0 Hz, ArH), 7.79 (d, 2H, J = 8.0 Hz, ArH). C-NMR (75 MHz, CDCl3): δ (ppm)
20.36, 33.39, 77.23, 115.12, 128.29, 135.39, 136.98. FTIR (ATR): 1088, 1358, 1397,
1453, 1514, 1610, 2853, 2973 cm-1. The product was kept as a toluene solution and
used without further purification due to its chemical instability.
77
B OO
Hb Hb
Ha Ha SH HS
Figure 3.15. Structure of 4,5-dimethyl-2-(4-vinylphenyl)-[1,3,2]dioxaborolane-4,5- dithiol
3.3.9. General Procedure of Preparing Self-Assemble Monolayers (SAM) on a Flat
Gold Surface
The 100 mL glass bottles and the gold slides were soaked in piranha solution
(sulfuric acid (concentrated):H2O2 (30 %) = 7:3 (v/v)) (Caution: the piranha solution reacts violently with organic compounds)85 and rinsed with distilled water and absolute ethanol, respectively. The glass bottles and the gold slides were blown with nitrogen until dry. The clean gold slides were characterized by PM-FTIR as background.
All the SAMs were prepared in glass bottles at room temperature. The clean gold slides were immersed into a 1 mM methylene chloride or ethanol solution of the anchor molecules for 1 to 2 days. The resulting SAMs were rinsed with water and then blown with nitrogen until dry before characterization by PM-FTIR.
78 3.3.10. PM-FTIRRAS Characterization of 1,4-Dimercapto-2,3-dimethyl-butane-2,3-
diol (dioldithiol) SAM on the Gold Surface
Polarization modulation infrared reflection absorption spectra (PM-FTIRRAS)
were recorded by a nitrogen-purged, custom-modified Nicolet Nexus 870 Fourier
transform infrared spectrometer equipped with a liquid nitrogen-cooled MCT detector
and a Hinds Intruments PEM-90 photoelastic modulator operating at 100 kHz.86 The incoming polarizer placed before the sample was oriented to give p-polarization. The incoming infrared radiation was reflected from the sample. The angle of incidence was about 80o to obtain the best signals. The spectral resolution was 4 cm-1. All the
spectra were collected over 4000 scans.
3.3.11. Ligand Exchange Reaction Between 1,4-Dimercapto-2,3-dimethyl-butane-2,3-
diol (dioldithiol) and Triphenylphosphine-stabilized Gold Nanoparticles
Triphenylphosphine-stabilized gold nanoparticles (10 mg) were dissolved in
dueterated dichloromethane (0.7 mL) in a 5 mL NMR tube. 1H NMR spectra were recorded as a standard. 1,4-Dimercapto-2,3-dimethyl-butane-2,3-diol (1.0 mg) was then added to the NMR tube and dissolved in the solution. 1H NMR spectra were
recorded at 1, 8, 16, and 24 h respectively.
3.3.12. Preparation of 1,4-Dimercapto-2,3-dimethyl-butane-2,3-diol (dioldithiol)-
Stabilized Gold Nanoparticles by Direct Reduction Reaction87
In a round bottom flask, aqueous solution of HAuCl4·3H2O (0.015 M, 15 mL)
was mixed with a solution of tetraoctylammonium bromide (TOAB) in toluene. The
reaction mixture was stirred vigorously for 0.5 h. The organic layer was separated and
washed with water (2 × 10 mL). Under vigorous stirring, 0.09 mmol of 1,4-
79 dimercapto-2,3-dimethyl-butane-2,3-diol in methylene chloride (60 mM) was added
quickly to the organic solution. The solution was stirred for 0.5 h. Then NaBH4 aqueous solution (0.2 M, 12 mL) was added quickly. The solution was stirred overnight at room temperature. The organic layer was separated and concentrated via vacuum. The gold nanoparticles were precipitate in hexane, centrifuged and washed with hexane and small amount of water, respectively.
3.3.13. Ultraviolet Visible Spectroscopy of Gold Nanopartcles
Solid gold nanoparticles were dissolved in methylene chloride to obtain 0.2
μM gold nanoparticle solutions. UV-Vis spectra were recorded on an OceanOptics
PC2000 spectrometer. The spectra were plotted via the data analysis software Origin
(MicrocalTM software, Inc.).
3.4. Summary
1,4-Dimercapto-2,3-dimethyl-butane-2,3-diol (dioldithiol) and 4,5-dimethyl-2-
(4-vinylphenyl)-[1,3,2]dioxaborolane-4,5-dithiol (VBE dithiol) have been synthesized.
They are found to be effective stabilizers for dispersing and stabilizing gold
nanoparticles. In addition, the functional groups of the dioldithiol and VBE dithiol ligand can be easily modified to form different biomolecular recognition sites on the
surface of gold nanoparticles. Gold nanoparticles stabilized by dioldithiol ligands
have been prepared by two methods: direct reduction reaction and ligand exchange
reaction with triphenylphosphine-stabilized gold nanoparticles. The PM-FTIRRAS
characterization of dioldithiol SAM on the flat gold surface was also studied.
80 CHAPTER IV
GOLD NANOPARTICLES WITH MOLECULAR RECOGNITION SITES
ON THE SURFACE
4.1. Introduction
The biological applications of functionalized gold nanoparticles have been
widely studied because of their special photonic, electronic and chemical
properties. 88 , 89 , 90 A large number of the fabrications of different biomolecular
recognition methods based on chemical or biological modified gold nanoparticles
have been reported. In most cases, the gold nanoparticles are used as probes and templates attached to the biomolecular recognition moieties. Molecules with sulfur- containing surface anchors, such as thiols, disulfides and thioethers, are readily assembled on the gold surface. Some of the examples will be discussed in this section.
One of the pioneer works is reported by Alivasatos, et al. in 1996.91 They
developed a DNA sensor using gold nanoparticles as templates. The chemical and
biological properties of the DNAs were retained after being attached to the surfaces of
gold nanoparticles. The DNAs on the gold surface can hybridize with their
complementary single-stranded DNA templates and showed potential application in
DNA sensing. Another important pioneering work of the functionalization of gold
nanoparticles with thiol-modified DNA was reported by Mirkin, et al.92 The DNA
81 modified gold nanoparticles aggregated reversibly after complementary DNA strands
were added into the system. This aggregation resulted in the shift of the surface
plasmon band of gold nanoparticles, therefore it can be used for selective detection of
DNA.93, 94, 95 Later, peptide nucleic acids were also used to be self-assembled on the surface of gold nanoparticles instead of DNA.96 Moreover, different biosensors based
on antibody-attached gold nanoparticles have been developed.97,98,99 The interactions
between the antibodies and their conjugate antigen provide potential uses in immuno-
sensing. The strong surface plasmon band of gold nanoparticles has been applied for bioassay. Recently, the immobilization of different carbohydrates on gold
nanoparticles as well as their bioassay applications has been a subject of sustained
interest. 100 In 2001, de la Fuente, et al. first reported the preparation of gold nanoparticles protected by self-assembled monolayers of carbohydrate antigens.101
This type of multifunctional glyconanoparticles can be used in the study of carbohydrate–carbohydrate interaction mediated biological processes,102 and can be
used as a platform for potential carbohydrate-based anticancer vaccines.103
Much effort has been put into this field and remarkable progress has been
made. However, the application of biomodified nanoparticles in the biological fields
is still in its infancy.104 One of the major challenges is the delicate procedure of
synthesizing and manipulating biomodified gold nanoparticles. In this chapter, the
synthesis of gold naonoparticles with molecular recognition sites on the surface will
be discussed.
4.1.1. Immobilization of Carbohydrates on the Gold Surface
Carbohydrates play important roles in the intracellular signaling. Carbohydrate
binding proteins mediate cellular processes central to immune regulation, cell
82 trafficking, endocytosis and human disease.105 The studies of carbohydrate-related
interactions provide new insights into the corresponding biological processes, and
provide new possibilities for drug development.106 Multivalent interactions between
carbohydrates and cell surface receptors attract special interest because of their
importance in different biological processes including viral/bacterial infection and the
inflammatory response and relatively high binding affinity.107,108,109
One important example in this field is the sialic acid containing glycans. The
Siglec family binds sialic acid containing glycans on glycoproteins and glycolipids as
ligands, and is differentially expressed on various cells in the immune system, such as
CD22 in B cells.110 CD22 is a regulator of B-cell receptor signaling, which prevents aberrant immune response and autoimmune disease by regulating the B-cell receptor
activation.111,112 Much effort has been put into the understanding of the intercellular signaling in the immune system. Paulson et al have developed high affinity glycan ligands to CD22.113,114 Probes comprising the free sialic acids, mono-valent sialosides
and multivalent polyacrylamide polymers have been synthesized and studied. The
multivalent glycoprotein interactions in biological systems are most difficult to
emulate with existing materials and methods.
Among different biomolecule-conjugated gold nanoparticles, carbohydrate-
modified gold nanoparticles attract great interest recently. Carbohydrate-modified
gold nanoparticles can be used to fabricate useful tools to mimic the behavior of
biomolecules in cells, therefore have different potential applications.115 Recently, the
multivalent binding between carbohydrate-based nanoparticles and lectins has also
been studied.116 However, complexity of carbohydrate-modified gold nanoparticle- based probes and the complexity and heterogeneity of cell surfaces remain big
challenges.
83 Moreover, gold thin films with biomolecules, such as different carbohydrates, self-assembly attached on the surface can also act as potential sensitive probes which allow surface-based real-time, label-free analysis, such as Polarization Modulation
Fourier Transform Infrared Reflection Absorption Spectroscopy (PM-FTIRRAS),
Surface Enhanced Raman Spectroscopy (SERs), Surface Plasmon Resonance (SPR) and Quartz Crystal Microbalance (QCM).117 The self-assembled monolayer (SAM) of carbohydrates has been recently applied in the study of multivalent carbohydrate- related interactions.118,119
4.1.2. Gold Nanoparticles with Molecular Recognition Sites on the Surface
We have developed two different strategies to conjugate carbohydrates on the surfaces of gold nanoparticles. Both of the strategies are based on the design and synthesis of 7-substituted-2,4,9-trithiatricyclo[3.3.1.13,7]decane (tripod surface anchors) which can be used as general surface anchors for self-assembling of various biological molecules on gold surfaces.21 The tripod surface anchors provide exceptional chemical and biological stability for these probes, therefore are suitable for in vivo assays.
Methoxyamino group (N(OMe)), which can be prepared form O-methyl- oxime group, can react with reductive sugars to form β-N(OMe)-linked glycosides via an intermediate oxyiminium ion.120 Therefore, the terminal N(OMe) groups of the surface anchors on the surfaces of gold nanoparticles or flat gold surfaces can be used to form self-assembled monolayers of carbohydrates on the gold surface.
Another strategy to form self-assembled monolayers of carbohydrates on the surfaces of gold nanoparticles or flat gold surfaces is based on “click chemistry”.121,122
84 The terminal alkyne groups of the surface anchors on the gold surfaces can react with azido sugars via Cu(I)-catalyzed cycloaddition reactions in aqueous media.123
O O O O N C 3
S SS Au
O O O O O O N O N C H O 3 C 3
sugar S S SS SS Au Au OH HO O HO OH OH
OH HO OH HO O OHH
Figure 4.1. Attaching biomolecules to gold surfaces by methoxyamino-PEG-tripod surface anchors
85
O O O O N C 3 H
S SS Au
Gold Nanoparticles Stabilized by Triphenylphosphine (Ph3P)
Figure 4.2. Ligand exchange reaction between TP-TEG-CH2NHOMe ligand and triphenylphosphine-stabilized gold nanoparticles
The long-range aim of this project is to fabricate a multifunctional molecular imaging probe with a sialoside analog as the CD22 homing ligand, fluorocene, and a metal surface anchor 7-substituted-2,4,9-trithiatricyclo[3.3.1.13,7]decane. The
compounds can be self-assembled to form SAMs of the glycoconjugate on gold
surfaces and provide multivalent molecular recognition sites on the surfaces of gold
nanoparticles. The multivalent sugar-coated gold nanoparticles can be used as ideal
virus mimics for immunology studies. The tripod surface anchor allows convenient
synthesis of highly concentrated colloidal solutions of large gold nanoparticles (50-60
nm), which are similar to the sizes of flu virus particles, and are readily for molecular
imaging and Surface Enhanced Raman Spectroscopy (SERs) optical sensing. The
fluorophore part provides sensitive method for tracing the nanoparticles in vivo
investigations. The oligo(ethylene glycol) chain can be used to increase the water-
solubility as well as biocompatibility of the gold nanoparticles. The adjustable
hydrophobic chain is supposed to help the molecule tightly pack on the gold surface
86 to form self-assemble monolayers. In addition, the tripod surface anchors provide exceptional chemical and biological stability for the probes.
carbohydrate ligand
O NH COOH OH HO O adjustable HO AcHN OH O HO hydrophobic O O O O O HO chain HO NHAc OH
HO2C O N N N S H S N O O N N O O O S H S O H anchor fluorophor
Figure 4.3. Protein (CD22 of B cells)-glycon binding probe
87
adjustable Hydrophobic O Chain
HO2C O
S H S N N N OH S H S O H anchor fluorophor
carbohydrate ligand
O NH COOH OH HO O HO AcHN OH O HO O O O O HO HO NHAc OH N N N S H O N O O S O O O S O anchor adjustable Hydrophobic Chain
Figure 4.3. Protein (CD22 of B cells)-glycon binding probe (continued)
88 Au
Gold Nanoparticles
Sugar N3
Click reaction
Au
O O O O 4
CO2H
HN S
HN
C6H12
O NH
S SS
H N OH O ONa
O HO OH O O HN O HO O HO O HO Sugar N O 3 N O HO NH 3 HO O
Figure 4.4. New methods for oligosaccharide modified multifunctional nanoparticles
89 F
F
Au
F Gold Nanoparticles
Sugar N3
Click reaction
F
Au
F
HO O O
O O CO H O 4 2 F
HN S
HN HN O C6H12
O NH S S S
S SS
H N OH O ONa
O HO OH O O HN O HO O HO O HO Sugar N O 3 N O HO NH 3 HO O
Figure 4.4. New methods for oligosaccharide modified multifunctional nanoparticles
(continued)
90
4.2. Results and Discussion
4.2.1. Synthesis of Methyl 2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7-methanoate
(Tripod surface anchor, 1)
7-Substituted-2,4,9-trithiatricyclo[3.3.1.13,7]decane derivatives were originally
synthesized by Lingrend in 1976 124 and Kittredge et al in 2002. 125 Chalermchai
Khemtong in our research group improved the synthetic procedure of these species
with major modifications in 2004.21,39 According to the optimized synthetic route,
methyl 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-methanoate (tripod surface anchor, 1)
can be prepared in six steps. The synthetic procedure is shown in Scheme 4.1.
Triallyl methyl acetate (21) is the key intermediate of the synthesis. It can be
synthesized via four steps from diethyl malonate which is commercially available.
After the allylation reaction with allyl bromide as a reagent and sodium ethoxide as a
base, an allyl group was added to the α-carbon from the carbonyl group. The second
allyl group was added in the same way to give diallyl dimethyl malonate (19). The
overall yield of these two steps was about 77 %. After the decarboxylation reaction
with refluxing aqueous sodium chloride solution and dimethyl sulfoxide, diallyl
methyl acetate (20) was obtained in about 63 % yield. The resulting diallyl methyl
acetate (20) was then reacted with lithium diisopropylamide as a base and allyl
bromide as a reagent to give triallyl methyl acetate (21) in high yield (about 92 %).
Methyl 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-methanoate (tripod surface
anchor, 1) was prepared from triallyl methyl acetate (21) in two steps. The ozonolysis
reaction of the triallyl methyl acetate (21) gave corresponding trialdehydes after treating with dimethyl sulfide. The resulting reaction mixture was then directly treated with thionation reagent to obtain the tripod surface anchor (1) in about 30-40 % yield.
91 Both Lawesson’s reagent 126 and phosphorus pentasulfide worked well in the thionation/cyclization reaction. A mixture of phosphorus pentasulfide (P2O5) and
alumina (Al2O3) was eventually chosen as the thionation reagents because of the following reasons. First, the reaction can reach completion in a relatively short time.
Second, the workup and column chromatography purification of the resulting crude
product was relatively easier. In addition, this thionation reagent was less expensive
than Lawesson’s reagent.
92
MeONa in MeOH
OO Br OO
OO OO 18
MeONa in MeOH
Br
NaCl in H2O DMSO OO O reflux H OO O
20 19
LDA, HMPA in THF Br
O
O
21
Scheme 4.1. Synthesis of methyl 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-methanoate
21 (tripod surface anchor, TCOOCH3, 1)
93 OO 1. O3, CH2Cl2 -78 oC O O O 2. (CH3)2S O O
21 22
P2S5/Al2O3 CH3CN reflux
O OMe
S SS
1
Scheme 4.1. Synthesis of methyl 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-methanoate
21 (tripod surface anchor, TCOOCH3, 1) (continued)
4.2.2. Synthesis of 2-{2-[2-(2-Methoxyamino-ethoxy)-ethoxy]-ethoxy}-ethyl 2,4,9-
3,7 Trithiatricyclo[3.3.1.1 ]decane-7-methanoate (TP-TEG-CH2NHOMe, 27)
2-{2-[2-(2-Methoxyamino-ethoxy)-ethoxy]-ethoxy}-ethyl 2,4,9-trithiatricyclo-
3,7 [3.3.1.1 ]decane-7-methanoate (TP-TEG-CH2NHOMe, 27) was synthesized from
3,7 methyl 2,4,9-trithiatricyclo[3.3.1.1 ]decane-7-methanoate (TPCOOCH3, 1) in four
steps. The procedure is shown in Scheme 4.2.
The tripod surface anchor (1) was hydrolyzed with lithium hydroxide in a mixed solvent of tetrahydrofuran, methanol and water. 2,4,9-Trithia-
tricyclo[3.3.1.13,7]decane-7-methanoic acid (TPCOOH, 23) was obtained in about 90
94 % yield after the acidification and concentration of the reaction mixture. The resulting
compound 7 was dried and then reacted with thionyl chloride in refluxing methylene
chloride to give 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-carbonyl chloride (TPCOCl,
24). Freshly prepared compound 24 should directly be used in the next step because
of its instability. The reaction of compound 24 and {2-[2-(2-hydroxy-ethoxy)-
ethoxy]-ethoxy}-acetaldehyde O-methyl-oxime (abbrev. TEG-O-methyl-oxime) was
carried out at 5 oC in present of 5.0 equivalent of anhydrous pyridine. 2-{2-[2-(2-
Methoxyimino-ethoxy)-ethoxy]-ethoxy}-ethyl 2,4,9-trithia-tricyclo[3.3.1.13,7]decane-
7-methanoate (TP-TEG-CH=NOMe, 26) was obtained in about 49 % yield as a yellow oil after careful purification. This product was a mixture (trans:cis = 1.51:1) according to the 1H NMR spectra. After the reduction reaction of compound 26 with
sodium cyanoborohydride in acetic acid at room temperature, 2-{2-[2-(2-
methoxyamino-ethoxy)-ethoxy]-ethoxy}-ethyl 2,4,9-trithiatricyclo[3.3.1.13,7]decane-
7-methanoate (TP-TEG-CH2NHOMe, 27) was finally synthesized in 73 % yield.
95 O OMe O OH 1. LiOH
MeOH/THF/H2O
S 2. HCl (aq.) S SS SS
1 23
O OH O Cl
SOCl2, dry CH2Cl2 reflux
S S SS SS 23 24
O O HO O NOMe
25 Pyridine
dry CH2Cl2
O O N O O 3
S SS
26
H ONO O O N O O O O 3 3
NaCNBH3 acetic acid S S SS SS
26 27
Scheme 4.2. Synthesis of 2-{2-[2-(2-methoxyamino-ethoxy)-ethoxy]-ethoxy}-ethyl
3,7 2,4,9-trithiatricyclo[3.3.1.1 ]decane-7-methanoate (TP-TEG-CH2NHOMe, 27)
96 4.2.3. Preparation and PM-FTIRRAS Characterization of Self-Assemble Monolayers
(SAM) on the Flat Gold Surface
A freshly cleaned gold thin film on a glass slide was immersed into a dilute solution of TP-TEG-CH=NOMe ligand (26) in methylene chloride to produce a SAM on the gold surface. The three sulfur atoms of the tripod surface anchor can easily formed a well-packed SAM on the gold surface with high stability.
The resulting SAM of TP-TEG-CH=NOMe on the gold surface was characterized by PM-FTIRRAS (Figure 4.6). The spectrum was obtained at a lock-in frequency at 3000 cm-1 with 4000 scans. For the spectrum of the SAM, two
-1 absorption peaks at 2927 and 2850 cm are assigned to the νaCH2 and νsCH2 modes, respectively. Compared with the spectrum of bulk TP-TEG-CH=NOMe ligand, the
intensity of the νaCH2 and νsCH2 peaks significantly increased after the formation of
SAM on the flat gold surface. One reason is that the C-H bonds of the CH2 of the
tripod anchor are supposed to be perpendicular to the gold surface. The IR absorption
will be increased if the dipole moment is perpendicular to the metal surface according
to the metal surface selection rules. The peaks at 1731 and 1625 cm-1 are assigned to
the C=O stretching of the ester and C=N stretching of the oxime, respectively. The
intensity of C=N stretching peak of the TP-TEG-CH=NOMe SAM is significantly
decreased comparing with the spectrum of bulk ligand, which indicates that C=N
bond of the oxime is parallel to the gold surface.
97
Figure 4.5. (a) FTIR spectrum of bulk TP-TEG-CH=NOMe ligand, and (b) PM-
FTIRRAS spectrum of TP-TEG-CH=NOMe SAMs on the flat gold surface
98 4.2.4. Ligand Exchange Reaction Between 2-{2-[2-(2-Methoxyamino-ethoxy)-
ethoxy]-ethoxy}-ethyl 2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7-methanoate (TP-TEG-
CH2NHOMe, 27) and Triphenylphosphine-stabilized Gold Nanoparticles
2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7-carboxylic acid 2-{2-[2-(2-
methoxyamino-ethoxy)-ethoxy]-ethoxy}-ethyl ester (TP-TEG-CH2NHOMe)- stabilized gold nanoparticles were prepared by a ligand exchange reaction. To a solution of triphenylphosphine (Ph3P)-stabilized gold nanoparticles, TP-TEG-
CH2NHOMe was added. The displacement of the capping ligands occurred, because of the stronger affinity of gold toward sulfur comparing with phosphorus.
The reaction was monitored by 1H NMR. From the 1H NMR spectra (Figure
4.5), it is clear that the broad peak at around 7 ppm of triphenylphosphine-stabilized
gold nanoparticles became significantly decreased (after 2 h) and eventually
disappeared on the base line (after 24 h) after the TP-TEG-CH2NHOMe ligand was
added into the gold nanoparticles solution. The peak of free triphenylphosphine at 7.6 ppm became more intense. These evidences showed the occurrence of the ligand exchange reaction.
The resulting TP-TEG-CH2NHOMe-stabilized gold nanoparticles were also characterized by UV-Vis spectroscopy. The UV-Vis absorption spectrum of diluted solution of TP-TEG-CH2NHOMe-stabilized gold nanoparticles in methylene chloride
(Figure 4.6) shows an intense band with the maximum at around 530 nm, which is the
characteristic surface plasmon band of gold nanoparticles.
99
Figure 4.6. 1H NMR spectra of ligand exchange reaction between triphenylphosphine- stabilized gold nanoparticles and TP-TEG-CH2NHOMe ligand; (a) triphenylphosphine-stabilized gold nanoparticles, (b) the reaction after 2 h, and (c) the reaction after 24 h.
100
0.50 Gold Nanoparticles-ligand exchange
0.45
0.40
0.35
0.30
0.25
Absorbance 0.20
0.15
0.10
0.05 300 400 500 600 700 800 Wavelength (nm)
Figure 4.7. UV-Vis absorption spectrum of TP-TEG-CH2NHOMe ligand-stabilized gold nanoparticles prepared by ligand exchange reaction
101 4.2.5. Synthesis of 2-[3-Oxo-6-(2-{2-[2-(2-prop-2-ynyloxy-ethoxy)-ethoxy]-ethoxy}-
ethoxy)-3H-xanthen-9-yl]-5-(3-{6-[(2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-carbonyl)
-amino]-hexyl}-thioureido) -benzoic Acid (TP-CONH-C6H12-NHCSNH-Fluorescein-
TEG-CCH, 28)
2-[3-Oxo-6-(2-{2-[2-(2-prop-2-ynyloxy-ethoxy)-ethoxy]-ethoxy}-ethoxy)-3H-
xanthen-9-yl]-5-(3-{6-[(2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-carbonyl)-amino]-
hexyl}-thioureido)-benzoic acid (TP-CONH-C6H12-NHCSNH-Fluorescein-TEG-CCH,
28) was synthesized from TPCOOCH3 (1) in five steps. The procedure is shown in
Scheme 4.3.
The preparation of the intermediate, 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-
carbonyl chloride (TPCOCl, 24), was discussed in section 4.2.2. Freshly made
compound 24 should be used to react with hexane-1,6-diamine in the next step. It should be note that the direct monoacylation of symmetrical diamines has proven to be difficult to control.127,128,129 The major product obtained will be diacylated
compound if the diamine is not selectively protected beforehand. One approach to
solve this problem is to treat the symmetrical diamine with 1 equivalent of 9-BBN
before the addition of an acyl chloride, which was reported by Zhang et al. in 2003.130
The choice of the solvent is also important in order to obtain the best result. 2,4,9- trithia-tricyclo[3.3.1.13,7]decane-7-carboxylic acid (6-amino-hexyl)-amide (TP-
CONH-C6H12-NH2, 25) was prepared by this method. Dry tetrahydrofuran was used
as the solvent. The yield of this reaction is about 40 %. The resulting compound 25
was then reacted with fluorescein isothiocyanate isomer I at room temperature in
present of dry triethylamine. The reaction was carried out in dark in order to prevent
the possible decomposition of the fluorescein. 2-(6-Hydroxy-3-oxo-3H-xanthen-9-yl)-
5-(3-{6-[(2,4,9-trithia-tricyclo[3.3.1.13,7]decane-7-carbonyl)amino]hexyl}thioureido)-
102 benzoic acid (TP-CONH-C6H12-NHCSNH-Fluorescein, 26) was obtained as a bright
yellow oil in 51 % yield after careful silica gel column chromatography purification.
The synthetic procedure for 2-{2-[2-(2-prop-2-ynyloxy-ethoxy)-ethoxy]-
ethoxy}-ethyl methanesulfonate (MeSO3-TEG-CCH, 27) was followed according to
ref. 45 with modifications.131 Tetra(ethylene glycol) was treated with potassium t-
butyloxide at 0 oC. Then, 3-bromo-propyne was added to the reaction mixture to give
O-propargyl-tetra(ethylene glycol) as an oil. Then, the resulting O-propargyl- tetra(ethylene glycol) was treated with methanesulfonyl chloride at 0 oC with
warming to room temperature under vigorous stirring to give compound 27. The
overall yield of these two steps was 29 %.
2-[3-Oxo-6-(2-{2-[2-(2-prop-2-ynyloxy-ethoxy)-ethoxy]-ethoxy}-ethoxy)-3H-
xanthen-9-yl]-5-(3-{6-[(2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-carbonyl)amino]-
hexyl}thioureido)benzoic acid (TP-CONH-C6H12-NHCSNH-Fluorescein-TEG-CCH,
28) was obtained by the reaction of compound 26 and compound 27 in dry dimethyl formamide with anhydrous K2CO3 used as a base. The reaction was carried out in the
dark in order to prevent the possible decomposition of the fluorescein. After careful
silica gel column chromatography purification, product 28 was obtained as bright
yellow oil in 23 % yield. However, the drawback of this method is that the product
yield of the reaction is relatively low. And it required about 24 h for the reaction to
reach completion. The temperature of the reaction has to be carefully controlled at
about 35-40 oC to accelerate the reaction. Higher reaction temperature should be
avoided in order to prevent the decomposition of the product.
103
OOMe OOH OCl
LiOH SOCl2
S THF / H2O S CH2Cl2 S SS SS SS
1 23 24
H O N NH 1. 9-BBN (1 eq), dry THF 2 H2N NH2 OCl S 2. SS
25 S SS
3. H2O
HO O O
CO2H O H O N COOH NCS S H NH2 S N C6H12 NH O NH S S O S SS N HO 25 26 25 oC, 24 hr
Scheme 4.3. Synthesis of 2-[3-oxo-6-(2-{2-[2-(2-prop-2-ynyloxy-ethoxy)-ethoxy]- ethoxy}-ethoxy)-3H-xanthen-9-yl]-5-(3-{6-[(2,4,9-trithiatricyclo[3.3.1.13,7]decane-7- carbonyl)amino]hexyl}thioureido)benzoic acid (TP-CONH-C6H12-NHCSNH-
Fluorescein-TEG-CCH, 28)
104
1) LiOH O O O O O S O O O OH O HO O 2) 27 Br
O 3) Cl S O
HO O O O O O O 4
CO2H CO2H
HN S HN S HN 27 HN C6H12 C6H12 O NH K2CO3 O NH 35 OC, 24 hr
S SS S SS
26 28
Scheme 4.3. Synthesis of 2-[3-oxo-6-(2-{2-[2-(2-prop-2-ynyloxy-ethoxy)-ethoxy]-
ethoxy}-ethoxy)-3H-xanthen-9-yl]-5-(3-{6-[(2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-
carbonyl)-amino]-hexyl}-thioureido)-benzoic acid (TP-CONH-C6H12-NHCSNH-
Fluorescein-TEG-CCH, 28) (continued)
The structure of compound 28 was confirmed by 1H NMR. However, the 13C
NMR spectrum was not obtained because of the small amount of the compound. The structure of compound 28 was also analyzed using 2D H-C gHSQC (Heteronuclear
Single Quantum Correlation) NMR (solvent: CD3OD). The spectrum was recorded on
a Varian INOVA 750 spectrometer. The 2D NMR spectrum shows the peaks of 2,4,9- 105 trithiatricyclo[3.3.1.13,7]decane part (δ: 2.92, 43.18; 4.45, 41.77 ppm), fluorescein part
(δ: 7.34, 102.56; 6.58, 105.39; 6.94, 115.28; 7.07, 115.9; 8.44, 121.65; 6.68, 130.13;
5.42, 130.84; 7.2, 132.96 ppm), terminal alkyne group (δ: 4.23, 58.73 ppm, -OCH2-
CC-), and tetra(ethylene glycol) part (δ: 3.39-4.41, 70.04-73.58 ppm). However, the hydrocarbon chain between 2,4,9-trithiatricyclo[3.3.1.13,7]decane part and fluorescein
part was not observed.
106
Figure 4.8. 2D gHSQC NMR spectrum of TP-CONH-C6H12-NHCSNH-Fluorescein-
TEG-CCH (28)
107
UV-Vis spectrum of this compound was also studied. The compound displays strong UV-Vis absorption bands at about 470 and 500 nm, which is blue-shifted from that of fluorescein isothiocyanate isomer I (λex 492 nm, λem 518 nm).
Figure 4.9. UV-Vis absorption spectrum of TP-CONH-C6H12-NHCSNH-Fluorescein-
TEG-CCH (28)
108 4.2.6. Synthesis of 2-{2-[2-(2-Prop-2-ynyloxy-ethoxy)-ethoxy]-ethoxy}-ethyl 6-
[(2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7-carbonyl)-amino]-hexanoate (TP-CONH-
C5H10-COO-TEG-CCH, 31)
2-{2-[2-(2-Prop-2-ynyloxy-ethoxy)-ethoxy]-ethoxy}-ethyl 6-[(2,4,9-trithia-
3,7 tricyclo[3.3.1.1 ]decane-7-carbonyl)-amino]-hexanoate (TP-CONH-C5H10-COO-
TEG-CCH, 31) was synthesized from 6-[(2,4,9-trithia-tricyclo [3.3.1.13,7]decane-7-
carbonyl)-amino]-hexanoic Acid (TP-CONH-C5H10-COOH, 29) and 2-{2-[2-(2-prop-
2-ynyloxy-ethoxy)-ethoxy]-ethoxy}-ethanol (TEG-CCH, 30). The procedure is shown
in Scheme 4.4.
6-[(2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7-carbonyl)-amino]-hexanoic acid
(TP-CONH-C5H10-COOH, 29) was synthesized from TPCOOCH3 (1) in three steps.
Freshly made 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-carbonyl chloride (TPCOCl, 24)
should be used to react with 6-amino-hexanoic acid in a mixed solution of THF and
NaOH aqueous solution. The reaction mixture was acidified by HCl aqueous solution
until pH = 2-3. After careful purification, product 29 was obtained in 83 % yield. The
resulting compound was dried and then reacted with thionyl chloride in refluxing
methylene chloride to give 6-[(2,4,9-trithia-tricyclo[3.3.1.13,7] decane-7-carbonyl)-
amino]-hexanoyl chloride, which should directly be used in the next step because of
its instability.
2-{2-[2-(2-Prop-2-ynyloxy-ethoxy)-ethoxy]-ethoxy}-ethanol (TEG-CCH, 30)
was prepared from tetra(ethylene glycol) via the procedure mentioned in 4.2.5. The
reaction of compound 29 (TP-CONH-C5H10-COOH) and 30 (TEG-CCH) was carried out at room temperature in present of 10.0 equivalent of anhydrous triethylamine.
After careful silica gel column chromatography purification, product 31 (TP-CONH-
C5H10-COO-TEG-CCH) was obtained as yellow oil in 47 % yield.
109
OOMe OOH OCl
LiOH SOCl2
S THF / H2O S CH2Cl2 S SS SS SS 1 23 24
OH H2N O
H2O, NaOH
O S H N S OH S O
29
1) LiOH O O O O O O O OH HO HO 2) Br 30
S O O H 1) SOCl2 S H N OH N O S S O S S 4 O 2) 17 O 31 29
Scheme 4.4. Synthesis of 2-{2-[2-(2-prop-2-ynyloxy-ethoxy)-ethoxy]-ethoxy}-ethyl
6-[(2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-carbonyl)-amino]-hexanoate (TP-CONH-
C5H10-COO-TEG-CCH, 31)
110 4.3. Experimental Section
4.3.1. General Procedures
1H NMR (300 MHz) and 13C NMR (75 MHz) spectra were recorded on a
Varian Gemini-300 spectrometer. All chemical shifts are reported in parts per million
(ppm) and are referenced to the residual solvent resonances. IR spectra were recorded
on a Nicolet NEXUS 870 FT-IR spectrometer equipped with a Thunderdone
Attenuated Total Reflectance (ATR) accessory. UV-Vis absorption spectra were
recorded on an OceanOptics S2000 spectrometer. High resolution mass spectra were
obtained at the Mass Spectrometry and Proteomics Facility at The Ohio State
University. Gold films were produced by sequential thermal deposition of Ti (2000 Å)
and 99.99% pure gold (1800 Å) on glass slides. The gold films consist exclusively of
polycrystalline Au[111] surface with the surface roughness factor determined to be
1.1.
All the materials were obtained from Aldrich, Acros, Fluka, Lancaster or
Fisher Scientific and used without further purification unless otherwise noted.
Tetrahydrofuran was distilled from sodium benzophenone ketyl under argon.
Methylene chloride was distilled from calcium hydride under argon. Glassware used
was flame-dried or oven-dried at ~120 oC and cooled in the desiccators. All the reactions were monitored by thin-layer chromatography (TLC). TLC was performed with 0.2 mm precoated silica gel with UV 254 on polyester backed plates (Sorbent
Technologies). EM Science silica gel 60 Å (35-75 um) was used in flash chromatography.
111 4.3.2. Synthesis of Dimethyl 2-Allyl-Malonate (18) and Diallyl Dimethyl Malonate
(19)21
To the freshly distilled methanol (300 mL) in a flame-dried 500 mL round
bottom flask which was cooled with an ice bath, sodium metal (10.6 g, 0.45 mol) was added slowly. When the sodium was dissolved, dimethyl malonate (40.0 g, 34.6 mL,
0.30 mol) was added into the solution. Under argon atmosphere and vigorous stirring,
allyl bromide (43.9 g, 31.6 mL, 0.33 mol) was added dropwise in a period of 1.5 h at
0 oC. Then the reaction mixture was refluxed for 0.5 h. After completion of the
reaction (monitored by TLC and FTIR), the reaction mixture was cooled to room
temperature. The solution was acidified by glacial acetic acid until pH = 5-7. The
solvent and other volatile compounds were removed via vacuum evaporation. Then
H2O (50 mL) was added to the residue. The aqueous layer was extracted with methylene chloride (3 x 50 mL). The combined organic layer was washed by brine, and then dried over anhydrous sodium sulfate. The methylene chloride was then removed via vacuum evaporation. The crude dimethyl 2-allyl-malonate (18) was obtained as a light yellow oil. This crude product was used directly in the next step without purification. FTIR (ATR): 1153, 1201, 1436, 1735, 2952, 3080 cm-1.
To the freshly distilled methanol (300 mL), sodium metal (10.6 g, 0.45 mol)
was added at 0 oC. When the sodium was dissolved, dimethyl 2-allyl-malonate (60.3 g,
0.35 mol) was added. Under vigorous stirring, allyl bromide (32.5 mL, 0.35 mol) was
added dropwise in a period of 1.5 h at 0 oC. Then the reaction mixture was refluxed
for 0.5 h. After completion of the reaction, the reaction mixture was cooled to room
temperature and acidified by glacial acetic acid until pH = 5-7. The volatile
compounds were removed via vacuum. Then H2O (50 mL) was added to the residue.
The aqueous layer was extracted with methylene chloride (3 x 50 mL). The combined
112 organic layer was washed by brine, and dried by anhydrous sodium sulfate. The
methylene chloride was removed via vacuum evaporation to give 49.4 g diallyl dimethyl malonate (19) as light yellow oil. Yield (2 steps): 77 %. 1H-NMR (300 MHz,
CDCl3): δ (ppm) 2.60 (d, 4H, J = 7 Hz, CH2), 3.68 (s, 6H, OCH3), 5.08 (t, 4H, J = 3
13 Hz, -CH=CH2), 5.60 (m, 2H, J = 3 Hz, -CH=CH2). C-NMR (75 MHz, CDCl3): δ
(ppm) 37, 52, 58, 119, 132, 171. FTIR (ATR): 1218, 1437, 1734, 2954, 3079 cm-1.
4.3.3. Synthesis of Methyl 2-Allyl-4-pentenoate (Diallyl Methyl Acetate, 20) 21
Diallyl dimethyl malonate (19) (19.5g, 0.092 mol) was dissolved in the
mixture of dimethyl sulfoxide (DMSO, 80 mL) and water (6.2 g, 0.345 mol). Sodium
chloride (6.7 g, 0.115 mol) was added to the solution. The reaction mixture was
refluxed for 48 h. After completion of the reaction as indicated by TLC, 150 mL
water was added to the mixture. The steam distillation of the reaction mixture will
give crude product as colorless oil floating at the top of the aqueous layer. The
aqueous layer was extracted with diethyl ether (3 x 20 mL). The combined organic
layer was dried by anhydrous sodium sulfate. The volatile components were removed
via vacuum evaporation to give methyl 2-allyl-4-pentenoate (20) (8.96 g). Yield: 63
1 %. H-NMR (300 MHz, CDCl3): δ (ppm) 2.34 (m, 4H, CH2), 2.54 (t, 1H, CH), 3.66 (s,
3H, OCH3), 5.04 (m, 4H, J = 3 Hz, -CH=CH2), 5.74 (m, 2H, J = 3 Hz, -CH=CH2).
13 C-NMR (75 MHz, CDCl3): δ (ppm) 36, 45, 51, 117, 135, 175. FTIR (ATR): 1193,
1437, 1737, 2951, 3080 cm-1.
4.3.4. Synthesis of Methyl 2,2-Diallyl-4-pentenoate (Triallyl Methyl Acetate, 21) 21
To a solution of diisopropylamine (12.9 g, 17.9 mL, 0.13 mol) in 80 mL dry
tetrahydrofuran, n-butyllithium (1.6 M in hexane, 80 mL, 0.13 mol) was added
113 dropwise at 0 oC. After stirring for 30 min, the solution was cooled to -78 oC. A
solution of methyl 2-allyl-4-pentenoate (13.5 g, 0.085 mol) in 20 mL dry
tetrahydrofuran was added dropwise through a dropping funnel. After stirring for 30
min, a mixture of allylbromide (11.0 mL, 0.13 mol) and hexamethylphosphoramide
(22.0 mL, 0.13 mol) was added dropwise through a dropping funnel. The reaction was
stirred for 2 h at -78 oC, and allowed to slowly warm up to room temperature. The
reaction mixture was quenched by water (30 mL). The aqueous layer was extracted
with methylene chloride (3 x 30 mL). The combined organic layer was washed by
brine, and dried by anhydrous sodium sulfate. The volatile components were removed
via vacuum evaporation. Vacuum distillation gave pure product as colorless liquid
o 1 (15.2 g, 102 C, 5 torr) Yield: 92 %. H-NMR (300 MHz, CDCl3): δ (ppm) 2.35 (d,
6H, J = 7 Hz, CH2), 3.71 (s, 3H, OCH3), 5.08 (m, 6H, -CH=CH2), 5.67 (m, 3H, -
13 CH=CH2). C-NMR (75 MHz, CDCl3): δ (ppm) 39, 49, 51, 118, 132, 175. FTIR
(ATR): 1733, 2951, 3078 cm-1.
4.3.5. Synthesis of Methyl 2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7-methanoate
(TPCOOMe, 1)21
The procedure was followed according to ref. 21 with modifications.
Ozone was bubbled through the solution of methyl triallyl acetate (21) (2.85 g,
13.70 mmol) in methylene chloride (100 mL) at -78 oC ozone from an ozone generator. After the reaction mixture displays a light blue color, argon was bubbled through it for about 10 min to remove excess ozone. Dimethyl sulfide (3.5 mL, 41.10 mmol) was added to the reaction mixture. The reaction mixture was allowed to slowly warm up to room temperature and stirred at this temperature for 4 h. The solvent was removed via vacuum and then dry acetonitrile (200 mL) was added to the residue.
114 While vigorous stirring, a mixture of phosphorus pentasulfide (9 g) and alumina (10 g)
was added slowly to the solution at a rate that the temperature of the mixture was not
over 40 oC. The mixture was refluxed under argon for 16 h. After completion of the
reaction as indicated by TLC, the reaction mixture was cooled to room temperature.
Alumina was removed by filtration and washed with acetonitrile (50 mL). The solvent
was evaporated via vacuum. The resulting mixture was purified by silica gel column
chromatography (ethyl acetate:hexane = 1:3 (v/v)) to give 1.43 g white solid. Yield:
o 1 39 %. M.P. 149-151 C. H-NMR (300 MHz, CDCl3): δ (ppm) 2.92 (d, 6H, J = 3 Hz,
13 CH2), 3.76 (s, 3H, OCH3), 4.33 (br s, 3H, SCHS). C-NMR (75 MHz, CDCl3): δ
(ppm) 38, 40, 41, 52, 175. FTIR (ATR): 1740, 2923, 2944 cm-1. HRMS: calcd for
+• C9H12O2S3Na 270.989158, found 270.98971.
4.3.6. Synthesis of 2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7-methanoic Acid
(TPCOOH, 23)21
The procedure was followed according to ref. 21 with modifications.
To the solution of 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-carboxylic acid
methyl ester (175 mg, 0.66 mmol) in a mixed solvent of tetrahydrofuran (2 mL),
methanol (3 mL) and water (1 mL), lithium hydroxide monohydrate (140mg, 6.6
mmol) was added slowly. The reaction mixture was stirred vigorously at room
temperature for 1 h and then refluxed for 30 min. After completion of the reaction as
indicated by TLC, the reaction mixture was cooled in ice bath and then acidified to
pH 2 by aqueous hydrochloric acid (3 M). The reaction mixture was concentrated to
~4 mL via vacuum and cooled in ice bath for 1 h. The resulting mixture was filtrated,
washed with ~2 mL of ice-cold water, and vacuum dried to give 143 mg of a white
o 1 solid. Yield: 92 %. M.P. 235-237 C. H-NMR (300 MHz, CDCl3): δ (ppm) 2.96 (br d,
115 13 6H, CH2), 4.36 (br s, 3H, CH). C-NMR (75 MHz, CDCl3): δ (ppm) 202, 42, 39, 38.
-1 +• FTIR (ATR): 1695, 2920, 2933, 2700-3150 cm . HRMS: calcd for C8H10 O2S3
233.983741, found 233.9847.
4.3.7. Synthesis of 2-{2-[2-(2-Methoxyimino-ethoxy)-ethoxy]-ethoxy}-ethyl 2,4,9-
Trithiatricyclo[3.3.1.13,7]decane-7-methanoate (TP-TEG-CH=NOMe, 26)
To a flame-dried round bottom flask, 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-
methanoic acid (TPCOOH, 88.2 mg, 0.376 mmol) and freshly distilled anhydrous
methylene chloride (5 mL) was added respectively. Thionyl chloride (0.15 mL, 5.0 eq)
was added dropwise into the solution under vigorous stirring. The reaction mixture
was refluxed for about 1 hour. After the completion of the reaction as indicated by
FTIR monitory, the reaction mixture was cooled to room temperature. The volatile
compounds were removed via vacuum evaporation to give 2,4,9-
trithiatricyclo[3.3.1.13,7]decane-7-carbonyl chloride (24) as a yellow solid. The
product was directly used in the next step without further purification because of its
chemical instability.
The freshly made 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-carbonyl chloride
(24) was dissolved in anhydrous methylene chloride (5 mL). The solution was cooled
to 5 oC by an ice bath. Under vigorous stirring, a solution of {2-[2-(2-Hydroxy-
ethoxy)-ethoxy]-ethoxy}-acetaldehyde O-methyl-oxime (125.0 mg, 0.564 mmol, 1.5
eq) in 2 mL of anhydrous methylene chloride and anhydrous pyridine (0.15 mL, 1.880
mmol, 5.0 eq) were added dropwise, respectively. The reaction mixture was stirred at
5 oC for about 6 h. After the completion of the reaction as indicated by TLC, the solvent and other volatile compounds were removed via vacuum evaporation. The residue was dissolved in methylene chloride (10 mL) and washed with brine (2 x 3
116 mL). The solution was dried over anhydrous Na2SO4. The Na2SO4 was removed by
filtration. The volatile components were removed via vacuum evaporation. The crude
product was purified by silica gel column chromatography (methylene
chloride:methanol = 98:2 (v/v)) to give 80.9 mg yellow oil. Yield: 49.2 %. The
1 product is a mixture (trans:cis = 1.51:1). H-NMR (300 MHz, CDCl3): δ (ppm) 2.90
(d, J = 5.4 Hz, 15H), 3.64-3.72 (m, 31H), 3.85 (d, J = 3.0 Hz, 8H), 4.10 (d, J = 5.8 Hz,
3H), 4.28-4.32 (m, 11H), 6.84 (t, J = 3.5 Hz, cis-CH=NOMe, 1H), 7.43 (t, J = 5.8 Hz,
13 trans-CH=NOMe, 1.5H). C-NMR (75 MHz, CDCl3): δ (ppm) 175.1, 147.2, 70.8,
70.7, 70.6, 70.1, 69.2, 68.1, 65.7, 64.3, 62.3, 62.0, 41.3, 40.1, 38.7. FTIR (ATR):
1557, 1629, 1735, 2851, 2927 cm-1.
4.3.8. Synthesis of 2-{2-[2-(2-Methoxyamino-ethoxy)-ethoxy]-ethoxy}-ethyl 2,4,9-
3,7 Trithiatricyclo[3.3.1.1 ]decane-7-methanoate (TP-TEG-CH2NHOMe, 27)
To a solution of 2-{2-[2-(2-methoxyimino-ethoxy)-ethoxy]-ethoxy}-ethyl
2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-methanoate (TP-TEG-CH=NOMe, 31.3 mg,
0.0715 mmol) in acetic acid (2 mL), sodium cyanoborohydride (18.0 mg, 0.286 mmol,
4.0 eq) was added under vigorous stirring. The reaction mixture was stirred at room
temperature for about 4 h. The volatile compounds were removed via vacuum
evaporation. The residue was dissolved in methylene chloride (6 mL) and washed
with saturated NaHCO3 solution (2 x 2 mL). The solution was dried over anhydrous
Na2SO4. The Na2SO4 was removed by filtration. The volatile components were
removed via vacuum evaporation. The crude product was purified by silica gel
column chromatography (methylene chloride:methanol = 95:5 (v/v)) to give 23.0 mg
1 yellow oil. Yield: 73.2 %. H-NMR (300 MHz, CDCl3): δ (ppm): 2.90 (d, J = 5.4 Hz,
6H), 3.10 (t, J = 5.1 Hz, 2H), 3.53 (s, 3H), 3.60-3.74 (m, 12H), 4.29-4.34 (m, 5H).
117 13 C-NMR (75 MHz, CDCl3): δ (ppm) 175.1, 70.9, 70.8, 70.7, 69.2, 67.6, 64.3, 61.7,
51.4, 41.3, 40.1, 38.7, 30.0. FTIR (ATR): 1448, 1547, 1675, 1727, 2917 cm-1.
4.3.9. General Procedure of Preparing Self-Assemble Monolayers (SAM) on the Flat
Gold Surface
The 100 mL glass bottles and the gold slides were soaked in piranha solution
(sulfuric acid (concentrated):H2O2 (30 %) = 7:3 (v/v)) (Caution: the piranha solution reacts violently with organic compounds)85 and rinsed with distilled water and absolute ethanol respectively. The glass bottles and the gold slides were blown with nitrogen until dry. The clean gold slides were characterized by PM-FTIR as background.
All the SAMs were prepared in glass bottles at room temperature. The clean gold slides were immersed into a 1.0 mM methylene chloride of the anchor molecules for 1 to 2 days. The resulting SAMs were rinsed with water and blown with nitrogen until dry before characterization by PM-FTIR.
4.3.10. PM-FTIRRAS Characterization of TP-TEG-CH=NOMe SAM on the Gold
Surface
Polarization modulation infrared reflection absorption spectra (PM-FTIRRAS) were recorded by a nitrogen-purged, custom-modified Nicolet Nexus 870 Fourier transform infrared spectrometer equipped with a liquid nitrogen-cooled MCT detector and a Hinds Intruments PEM-90 photoelastic modulator operating at 100 kHz.132 The incoming polarizer placed before the sample was oriented to give p-polarization. The incoming infrared radiation was reflected from the sample. The angle of incidence
118 was about 80o to obtain the best signals. The spectral resolution was 4 cm-1. All the
spectra were collected over 4000 scans.
4.3.11. Preparation of TP-PEG-CH2-NHOMe-Stabilized Gold Nanoparticles By
Ligand Exchange Reaction Between TP-PEG-CH2-NHOMe Ligand and Ph3P-
Stabilized Gold Nanoparticles
Triphenylphosphine-stabilized gold nanoparticles (8.0 mg) were dissolved in
dueterated dichloromethane (0.7 mL) in a 5 mL NMR tube. 1H NMR spectra were recorded as a standard. Then 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-carboxylic acid
2-{2-[2-(2-methoxyamino-ethoxy)-ethoxy]-ethoxy}-ethyl ester (TP-PEG-CH2-
NHOMe Ligand, 1.0 mg) was added to the NMR tube and dissolved in the solution.
1H NMR spectra were recorded at 2, 16, and 24 h respectively.
4.3.12. Ultraviolet Visible Spectroscopy of Gold Nanopartcles
Gold nanoparticles solution in dueterated dichloromethane was diluted by
methylene chloride to obtain 0.1 μM gold nanoparticle solutions. UV-Vis spectra
were recorded on an OceanOptics PC2000 spectrometer. The spectra were plotted via
the data analysis software Origin (MicrocalTM software, Inc.).
4.3.13. Synthesis of 2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7-carboxylic Acid (6-
Amino-hexyl)-amide (TP-CONH-C6H12-NH2, 25)
2,4,9-Trithia-tricyclo[3.3.1.13,7] decane-7-carbonyl chloride (24) was prepared
from 139.5 mg (0.595 mmol) 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-carboxylic acid
(23) via the procedure mentioned in 4.3.7.
119 To a flame-dried round bottom flask, a solution of hexane-1,6-diamine (83.0
mg, 0.714 mmol, 1.2 eq to compound 24) in anhydrous tetrahydrofuran (10 mL) was
added. Then 9-BBN (1.43 mL, 0.4 M in hexane, 0.71 mmol, 1.2 eq to compound 24)
was added dropwise under vigorous stirred. After stirred at room temperature for 1 h,
a solution of freshly made 2,4,9-trithia-tricyclo[3.3.1.13,7] decane-7-carbonyl chloride
in anhydrous tetrahydrofuran (2 mL) was added dropwise. The reaction mixture was
stirred at room temperature for a further 4 h. The reaction mixture was quenched by a small amount of water. The aqueous layer was extracted with methylene chloride and the combined organic layer was dried over anhydrous Na2SO4. The Na2SO4 was
filtrated and the volatile components were removed via vacuum evaporation. The
crude product was purified by silica gel column chromatography (methylene
chloride:methanol = 95:5 (v/v)) to give 79.9 mg pure product. Yield: 40.4 %. 1H-
NMR (300 MHz, CD3OD): δ (ppm) 4.46 (s, 3H), 3.29 (t, J = 7.2 Hz, 2H), 2.97 (t, J =
7.2 Hz, 2H), 2.92 (d, J = 3.0 Hz, 6H), 1.74-1.46 (m, 8H). 13C-NMR (75 MHz,
CD3OD): δ (ppm) 177.4, 42.3, 40.8, 40.2, 39.9, 38.8, 29.7, 27.9, 26.7, 26.4. FTIR
(ATR): 2967, 2915, 2854, 1620, 1502, 1459, 1437 cm-1.
4.3.14. Synthesis of 2-(6-Hydroxy-3-oxo-3H-xanthen-9-yl)-5-(3-{6-[(2,4,9-trithia-
tricyclo[3.3.1.13,7]decane-7-carbonyl)-amino]-hexyl}-thioureido)-benzoic acid (TP-
CONH-C6H12-NHCSNH-Fluorescein, 26)
The reaction was carried out in dark in order to prevent the possible
decomposition of the fluorescein.
To a solution of compound 25 (116.3 mg, 0.350 mmol) in dry dimethyl
formamide (15 mL), fluorescein isothiocyanate (168.4 mg, 0.418 mmol, 1.2 eq) was
added. Then dry triethylamine (0.24 mL, 1.650 mmol, 5.0 eq) was added dropwise
120 under vigorous stirring at room temperature. The reaction mixture was stirred at this
temperature for 16 h. After the completion of the reaction as indicated by TLC, the
volatile components were removed via vacuum evaporation. The residue was
suspended in methylene chloride (10 mL). Then 1.0 M HCl solution was added to the
reaction mixture until pH = 7. The solution was dried over anhydrous Na2SO4. The
Na2SO4 was removed by filtration. The solvent was removed via vacuum evaporation.
The crude product was purified by silica gel column chromatography (methylene chloride:methanol = 95:5 (v/v)) to give 128.8 mg jelly-like yellow solid. Yield: 51 %.
1 H-NMR (300 MHz, CD3OD): δ (ppm) 8.11-6.50 (m, 9H), 4.32 (s, 3H), 3.59 (m, 2H),
3.20 (m, 2H), 2.82 (d, J = 3.6 Hz, 6H), 1.66-1.24 (m, 8H). 13C-NMR (75 MHz,
CD3OD): δ (ppm) 190.3, 181.9, 180.7, 177.2, 170.9, 160.9, 153.7, 143.1, 141.7, 134.4,
131.1, 129.9, 128.5, 125.3, 122.5, 119.5, 113.4, 111.0, 110.8, 103.4, 103.3, 46.5, 45.0,
42.3, 40.7, 40.1, 40.0, 38.8, 29.8, 29.3, 26.9. FTIR (ATR): 3265, 2933, 1736, 1607,
1502, 1453, 1374, 1315, 1257 cm-1.
4.3.15. Synthesis of 2-{2-[2-(2-Prop-2-ynyloxy-ethoxy)-ethoxy]-ethoxy}-ethyl
Methanesulfonate (MeSO3-TEG-CCH, 27)
The procedure was followed according to ref. 131 with modifications.
A solution of tetra(ethylene glycol) (5.0g, 0.0258 mol) in tetrahydrofuran (100
mL) was cooled to 0 oC. Potassium t-butyloxide (1.15 g, 0.010 mol, 0.4 eq) was added
slowly under vigorous stirring. The reaction mixture was stirred at this temperature
for 0.5 h. 3-Bromo-propyne (0.9 mL, 0.010 mol, 0.4 eq) was added dropwise to the
reaction mixture and stirred for 2 h. Then 1.0 M HCl solution was added to the
reaction mixture until pH = 7. The organic solvent was dried over anhydrous Na2SO4.
The Na2SO4 was removed by filtration. The volatile components were removed via
121 vacuum evaporation to give 2.17 g O-propargyl-tetra-(ethylene glycol) as an oil. The crude product was directly used for next step without further purification. 1H-NMR
(300 MHz, CDCl3): δ (ppm) 4.13 (d, J = 2.4 Hz, 2H), 3.63-3.50 (m, 16H), 2.41 (t, J =
2.1 Hz, 1H).
To a flame-dried round bottom flask, a solution of crude O-propargyl-
tetra(ethylene glycol) (2.17 g) in methylene chloride (100 mL) was added inside and
cooled to 0 oC. Methanesulfonyl chloride (2.47 mL, 0.0319 mol, 3 eq) was added
dropwise under vigorous strring. The reaction mixture was allowed to slowly warm
up to room temperature and stirred at this temperature for 3 h. The volatile
components were removed via vacuum evaporation. The crude product was purified
by silica gel column chromatography (methylene chloride:methanol = 98:2 (v/v)) to
1 give 0.87 g yellow oil. Yield (2 steps): 29 %. H-NMR (300 MHz, CDCl3): δ (ppm)
4.37 (t, J = 4.2 Hz, 2H), 4.20 (d, J = 1.2 Hz, 2H), 3.76-3.64 (m, 14H), 3.08 (s, 3H),
13 2.44 (t, J = 2.1 Hz, 1H). C-NMR (75 MHz, CDCl3): δ (ppm) 79.74, 74.83, 71.49,
70.72, 70.68, 70.53, 69.50, 69.23, 69.15, 61.82, 58.54, 37.85. FTIR (ATR): 3283,
3021, 2879, 2115, 1720, 1454, 1349, 1247, 1173 cm-1.
4.3.16. Synthesis of 2-[3-Oxo-6-(2-{2-[2-(2-prop-2-ynyloxy-ethoxy)-ethoxy]-
ethoxy}-ethoxy)-3H-xanthen-9-yl]-5-(3-{6-[(2,4,9-trithia-tricyclo[3.3.1.13,7] decane-
7-carbonyl)-amino]-hexyl}-thioureido)-benzoic Acid (TP-CONH-C6H12-NHCSNH-
Fluorescein-TEG-CCH, 28)
The reaction was carried out in dark in order to prevent the possible
decomposition of the fluorescein. To a solution of compound 26 (49.8 mg, 0.069
mmol) in dry dimethyl formamide (8 mL), a solution of compound 27 (42.8 mg, 0.138
mmol, 2 eq) in dry dimethyl formamide (2 mL) and anhydrous K2CO3 (95.3 mg,
122 0.690 mmol, 10 eq) were added respectively. The reaction mixture was warmed to 37 oC and stirred at this temperature for 24 h. After the completion of the reaction as indicated by TLC, the reaction mixture was cooled to room temperature. The reaction mixture was filtrated. The solution was neutralized by 1.0 M HCl aqueous solution until pH = 7. The volatile components were removed via vacuum evaporation. The residue was purified by silica gel column chromatography (methylene chloride:methanol = 95:5 (v/v)) to give 14.8 mg bright yellow oil. Yield: 23 %. 1H-
NMR (300 MHz, CDCl3): δ (ppm) 7.27-6.46 (m, 9H), 4.36 (s, 3H), 4.24 (m, 2H), 4.20
(s, 2H), 3.91-3.32 (m, 18H), 2.87 (s, 6H), 2.05 (s, 1H), 1.59-1.26 (m, 8H). FTIR
(ATR): 3295, 2918, 2849, 2113, 1723, 1641, 1596, 1514, 1486, 1254, 1107 cm-1.
4.3.17. Synthesis of 6-[(2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7-carbonyl)amino]- hexanoic Acid (TP-CONH-C5H10-COOH, 29)
2,4,9-Trithia-tricyclo[3.3.1.13,7] decane-7-carbonyl chloride (24) was prepared from 150.0 mg (0.640 mmol) 2,4,9-trithiatricyclo[3.3.1.13,7]decane-7-carboxylic acid
(23) via the procedure mentioned in 4.3.7.
To a solution of 6-amino-hexanoic acid (167.9 mg, 1.280 mmol, 2.0 eq to compound 24) in 3 mL of NaOH aqueous solution (1.27 M), a solution of 2,4,9- trithia-tricyclo[3.3.1.13,7] decane-7-carbonyl chloride (24) in 5.0 mL THF was added dropwise. The reaction mixture was stirred at room temperature for 5 h. The solution was acidified by 1.0 M HCl aqueous solution until pH = 2-3. The solid was collected.
The aqueous solution was extracted by THF (3 x 3 mL). The organic solution was combined with the solid. The volatile components were removed via vacuum evaporation. The residue was purified by silica gel column chromatography
(methylene chloride:methanol = 95:5 (v/v)) to give 185.0 mg of a slightly yellow
123 1 solid. Yield: 83 %. H-NMR (300 MHz, CDCl3): δ (ppm) 5.68 (s, 1H), 4.38 (s, 3H),
3.33 (m, 2H), 2.88 (s, 6H), 2.40 (t, J = 7.5 Hz, 2H), 1.73-1.26 (m, 6H). 13C-NMR (75
MHz, CDCl3): δ (ppm) 200.7, 179.5, 41.9, 40.3, 39.4, 33.6, 29.2, 26.3, 24.2. FTIR
(ATR): 3402, 3323, 2916, 1717, 1633, 1553, 1286 cm-1.
4.3.18. Synthesis of 2-{2-[2-(2-Prop-2-ynyloxy-ethoxy)-ethoxy]-ethoxy}-ethyl 6-
[(2,4,9-Trithiatricyclo[3.3.1.13,7]decane-7-carbonyl)-amino]-hexanoate (TP-CONH-
C5H10-COO-TEG-CCH, 31)
2-{2-[2-(2-Prop-2-ynyloxy-ethoxy)-ethoxy]-ethoxy}-ethanol (TEG-CCH, 30)
was prepared from tetra(ethylene glycol) via the procedure mentioned in 4.3.15.
To a flame-dried round bottom flask, 6-[(2,4,9-trithia-tricyclo[3.3.1.13,7]
decane-7-carbonyl)-amino]-hexanoic acid (29, 155.0 mg, 0.446 mmol) and a mixture
of freshly distilled anhydrous methylene chloride (3 mL) and anhydrous THF (3 mL) was added respectively. Thionyl chloride (0.40 mL, 10 eq) was added dropwise into the solution under vigorous stirring. The reaction mixture was refluxed for about 3 hours. After the completion of the reaction as indicated by FTIR monitory, the reaction mixture was cooled to room temperature. The volatile components were removed via vacuum evaporation to give 6-[(2,4,9-trithia-tricyclo[3.3.1.13,7] decane-
7-carbonyl)-amino]-hexanoyl chloride as a yellow oil. The product was directly used
in the next step without further purification because of its chemical instability.
To the solution of 6-[(2,4,9-trithia-tricyclo[3.3.1.13,7] decane-7-carbonyl)-
amino]-hexanoyl chloride in anhydrous THF (2 mL), a solution of dry 2-{2-[2-(2-
prop-2-ynyloxy-ethoxy)-ethoxy]-ethoxy}-ethanol (TEG-CCH, 30) in anhydrous THF
(4 mL) was added slowly under vigorous stirring. Anhydrous triethylamine (0.62 mL,
10 eq) was added dropwise. The reaction mixture was stirred at room temperature for
124 16 hours. After the completion of the reaction as indicated by TLC, the volatile
components were removed via vacuum evaporation. The residue was purified by
silica gel column chromatography (methylene chloride:methanol = 95:5 (v/v)) to give
1 117.6 mg pure product as an oil. Yield: 47 %. H-NMR (300 MHz, CD3OD): δ (ppm)
4.60 (br s, 2H), 4.38 (s, 3H), 4.15 (d, J = 2.4 Hz, 2H), 3.68-3.60 (m, 14H), 3.17 (m,
2H), 2. 82 (s, 1H), 2.80 (d, J = 3.6 Hz, 6H), 2.32 (t, J = 7.2 Hz, 2H), 1.61-1.31 (m,
13 6H). C-NMR (75 MHz, CD3OD): δ (ppm) 178.1, 175.2, 80.6, 76.2, 71.9, 71.6, 71.4,
71.3, 70.4, 70.1, 69.9, 64.6, 59.1, 43.0, 41.2, 40.5, 39.4, 34.8, 30.0, 27.3, 25.6. FTIR
(ATR): 3364, 2928, 2865, 1731, 1652, 1528, 1446, 1281, 1099 cm-1.
4.4. Summary
A new method for oligosaccharide modified multifunctional nanoparticles was
studied. Several derivatives of 7-substituted-2,4,9-trithiatricyclo[3.3.1.13,7]decane
were designed and synthesized for this purpose. The stable, rigid 2,4,9-trithia-
tricyclo[3.3.1.13,7]decane group was used as a surface anchor which can attach on the surface of gold nanoparticles or flat gold surface. The adjustable oligo(ethylene glycol)
part was used to increase the water-solubility and biocompatibility of the gold
nanoparticles. The adjustable hydrophobic chain can help the molecules tightly
packing on the gold surface to form SAMs. Two different functional groups,
methoxyamino group and terminal alkyne group, can be used to bind to reductive
sugars and azido-sugars respectively. The fluorophore part can be used to provide an
sensitive method for tracing the nanoparticles in vivo investigations.
125 CHAPTER V
CONCLUSIONS AND FUTURE PLANS
5.1. Conclusions
A new type of bridged metal clusters was designed and synthesized. The bridges
were molecular electronic device candidates based on novel tripodal molecular surface anchors, 2,4,9-trithia-tricyclo[3.3.1.13,7]decane. The triruthenium cluster
mimics the surface of a bulk metal. These metal clusters can be used as models to
investigate the surface binding of tripodal surface anchor-metal junctions.
Additionally, they are promising for further studies of electron transfer across a single
molecule.
Two bi-dentate surface anchors, 1,4-dimercapto-2,3-dimethyl-butane-2,3-diol
(dioldithiol) and 4,5-dimethyl-2-(4-vinyl-phenyl)-[1,3,2]dioxaborolane-4,5-dithiol
(VBA dithiol), were designed and synthesized. The methyl groups of the dioldithiol
ligands can help to prevent the possible hydrolysis of the dioldithiol-bronic acid
complexes in aqueous solution, and also provide the freestanding surface binding
capability of the molecule. Both of the molecules were proven to be effective
stabilizers for dispersing and stabilizing gold nanoparticles. Additionally, they can be 126 easily modified to fabricate biocompatible nanocapsules with gold
nanoparticle-cores and polymer shells with sugar recognition sites on the inner
surfaces. The PM-FTIRRAS (Polarization Modulation Fourier Transform Infrared
Reflection Absorption Spectroscopy) characterization of dioldithiol SAM on the flat
gold surface was also studied.
Several compounds based on of tripod surface anchors, 7-substituted-
2,4,9-trithia-tricyclo[3.3.1.13,7]decane, were designed and synthesized in order to
conjugate carbohydrates on the surfaces of gold nanoparticles. The methoxyamino
groups or terminal alkyne groups of the compounds can be used to bind to reductive
sugars and azido-sugars respectively. These compounds were used as ligands for
stabilizing gold nanoparticles by ligand exchange reaction with
triphenylphosphine-stabilized gold nanoparticles. The adjustable oligo(ethylene
glycol) part was used to increase the water-solubility and biocompatibility of the gold nanoparticles. The adjustable hydrophobic chain can help the molecules tightly packing on the gold surface to form SAMs. Fluorophores can also be attached to the surfaces of gold nanoparticles by tripod surface anchors. Therefore, a surface anchor toolkit was prepared to form multivalent sugar-coated gold nanoparticles. In addition, the SAMs of the compounds on flat gold surfaces were also characterized by PM-FTIRRAS.
127 5.2. Future Plans
Suggested future developments of this research project shall include the following:
(a) The fabrication and biological application of multivalent sugar-coated gold
nanoparticles, which can be used as multifunctional molecular imaging probes.
Other biomolecules, e.g. proteins and DNAs, can also be attached to the surface
of gold nanoparticles via similar strategies.
(b) The fabrication of biocompatible nanocapsules with polymer coatings and gold
nanoparticle-cores using cross-linkable SAMs of boronate-dithiol ligand
complex on gold nanoparticles as templates. This type of nanocapsules can be
used as an implantable blood glucose sensor. The biomolecular recognition sites
are boronic acids within the inner surface of the polymer shell. Gold
nanoparticles can be used for Surface Enhanced Raman scattering (SERs).
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